*Article* **Polyolefin**/**ZnO Composites Prepared by Melt Processing**

**Alojz Anžlovar 1,\*, Mateja Primožiˇc <sup>2</sup> , Iztok Švab <sup>3</sup> , Maja Leitgeb <sup>2</sup> , Željko Knez <sup>2</sup> and Ema Žagar 1,\***


Academic Editors: Marinella Striccoli, Roberto Comparelli and Annamaria Panniello Received: 31 May 2019; Accepted: 28 June 2019; Published: 2 July 2019

**Abstract:** Composites of polyolefin matrices (HDPE and PP) were prepared by melt processing using two commercially available nano ZnO powders (Zinkoxyd aktiv and Zano 20). The mechanical and thermal properties, UV-Vis stability, and antibacterial activity of composites were studied. Tensile testing revealed that both nano ZnO types have no particular effect on the mechanical properties of HDPE composites, while some positive trends are observed for the PP-based composites, but only when Zano 20 was used as a nanofiller. Minimal changes in mechanical properties of composites are supported by an almost unaffected degree of crystallinity of polymer matrix. All polyolefin/ZnO composites exposed to artificial sunlight for 8–10 weeks show more pronounced color change than pure matrices. This effect is more evident for the HDPE than for the PP based composites. Color change also depends on the ZnO concentration and type; composites with Zano 20 show more intense color changes than those prepared with Zinkoxyd aktiv. Results of the antibacterial properties study show very high activity of polyolefin/ZnO composites against *Staphylococcus aureus* regardless of the ZnO surface modification, while antibacterial activity against *Escherichia coli* shows only the composites prepared with unmodified ZnO. This phenomenon is explained by different membrane structure of gram-positive (*S. aureus*) and gram-negative (*E. coli*) bacteria.

**Keywords:** high density polyethylene; polypropylene; ZnO nanoparticles; composites; antibacterial activity; UV-Vis stability; mechanical and thermal properties

#### **1. Introduction**

Polymer nanocomposites with inorganic nanoparticles are propulsive fields of research due to innovative combination of properties, arising from the application of inorganic nanofiller. The introduction of nanofillers into the polymer matrices, in most cases, generates relevant problems in terms of their dispersibility [1]. Since inorganic nanoparticles are mostly hydrophilic, they substantially differ in surface energy from the hydrophobic polymer matrices, causing phase segregation of both composite constituents. Nanoparticles are inclined towards formation of stable agglomerates, which are difficult to break up into individual particles and to disperse them uniformly in the host polymer matrix [2]. Due to a relatively small specific interface surface of agglomerates, they prevent efficient transfer of beneficial properties of the nanofiller, related to its nanoscopic dimension and to the host polymer, resulting in 'nanofilled materials' with properties similar to the traditional microcomposites [3]. In accordance with these considerations, it is clear that controlling the dispersion of

nanoparticles throughout the polymer matrix is highly important to fully exploit a potential of polymer nanocomposites. In this respect, the development of effective mixing and dispersion procedure is crucial in nanocomposite preparation [4]. Various approaches have been proposed for the manufacturing of polymer nanocomposites with homogeneously dispersed inorganic nanoparticles [5,6]. Possible solutions are the chemical modification of nanoparticle surface by various silanes to reduce surface energy or in-situ synthesis of nanoparticles inside the host polymer matrix [7,8].

Zinc oxide (ZnO), particularly its nanostructures, has recently attracted significant attention as a highly promising material for a broad range of applications [9–11]. ZnO is a frequently used semiconductor with high UV absorption, interesting electrical and optical properties, which strongly depend on the particle size and shape [12,13]. Besides, ZnO influences the thermal and mechanical properties as well as the chemical and physical stability of polymer matrices [14]. ZnO is known for its UV protecting capability [15–17], but its impact on the catalytic degradation of polymers has been less explored [18,19].

In the middle of the 1990s, it was discovered that ZnO also shows antibacterial activity against some bacterial strains. There are many studies on the preparation of ZnO/polymer nanocomposites with antibacterial activity [20,21]. For effective antibacterial activity, ZnO has to come into direct contact with the microorganisms. In the case of composites prepared with surface modified ZnO, antibacterial activity is compromised at the expense of better matrix stability.

Many publications reported on polyolefin/ZnO (nano)composites which were prepared by various processes. Here, we focus only on the composites prepared by melt processing (extrusion and injection molding). Some authors disclosed enhanced electrical properties (reduced resistivity for few orders of magnitude) [22–24] or UV stability [15–17] when large quantities of nano ZnO (30 wt% and more) were added to the polyolefin (PE or PP) matrices. Some of them reported on significantly improved antibacterial activity of PE or PP/ZnO nanocomposites [7,20,21,25]. Considering the mechanical properties of polyolefin/ZnO (nano)composites, some authors reported on significantly enhanced properties [15,26,27], while others stated only a minimal effect of nano ZnO addition [28–30]. Because interfacial interaction between ZnO and polyolefins is very weak [31], the surface of ZnO is frequently modified, mostly with silanes, in order to increase the compatibility between both composite constituents [27,30]. Nano ZnO can increase the degree of crystallinity of polyolefins [31,32], although many authors also reported its negligible effect [22,29]. Therefore, there are still many aspects of polyolefin/ZnO nanocomposites formation and properties, as well as potential applications that need to be cleared out.

Here, we report on the preparation of the composite materials of polyolefin matrices (HDPE and PP) and various types of commercially available nano ZnO (unmodified and surface modified with stearic acid, triethoxycaprylylsilane, and [3-methacryloxypropyl]trimethoxysilane) by deposition of nano ZnO on the surface of polyolefin granules and subsequent melt processing. Our goal was to introduce certain functionalities into the studied polymer matrices, such as UV absorption and antibacterial activity. Besides, the impact of ZnO addition on the composites' mechanical and thermal properties, as well as UV-Vis stability, was studied.

#### **2. Results and Discussion**

#### *2.1. Characterization of ZnO Nanofillers*

Samples of commercial zinc oxides (Zinkoxyd aktiv and Zano 20) were characterized before they were applied as the nanofillers. The specific surface area based on the BET method is 42.8 m<sup>2</sup> /g for Zinkoxyd aktiv and 25.1 m<sup>2</sup> /g for Zano 20, while the average pore dimensions are 16.2 nm and 7.6 nm, respectively. SEM micrographs show well-defined ZnO nanoparticles with particle sizes between 20 and 100 nm for both samples (Figure 1). Zano 20 contains a larger fraction of rodlike ZnO structures (Figure 1b), while the degree of particle agglomeration is higher for the Zinkoxyd aktiv. FTIR spectra of ZnO powders (Figure 2A) show characteristic strong and broad absorption

bands between 420 and 450 cm−<sup>1</sup> due to the two transverse optical stretching modes of ZnO [33]. In the FTIR spectrum of Zinkoxyd aktiv, consisting of ZnO nanoparticles with irregular spherical morphology, only broad absorption band with a maximum at 447 cm−<sup>1</sup> is observed (Figure 2A), while the FTIR spectrum of Zano 20 shows broad band with two maxima, one at 447 cm−<sup>1</sup> and the other one at 434 cm−<sup>1</sup> (Figure 2A), which are characteristic of the rod-like ZnO morphology [34]. Besides, Zinkoxyd aktiv shows additional absorption bands at 1508 and 1400 cm−<sup>1</sup> , typical for the organic moieties most probably located on the surface of ZnO particles, indicating that Zinkoxyd aktiv is more organophillic than Zano 20. Photoluminescence spectra of both ZnO powders show the near band edge UV emission from 380 to 400 nm and numerous visible light emission peaks at 423, 448, 461, 485, and 529 nm [10,12] (Figure 2B). Differences between the samples in the visible light emission region are rather small, indicating small differences in the quantity and type of intrinsic defects on the surfaces of both ZnO samples [35]. A larger difference was observed in the near band edge peak, which is located at 381.5 nm and 392 nm for the Zano 20 and Zinkoxyd aktiv, respectively. The intensity of this peak is much higher for the Zano 20 and is most probably related to the rod-like ZnO morphology (thick rods) [36]. XRD diffractograms (Figure 2C) show diffraction maxima that are characteristic of the crystalline ZnO with hexagonalwurtzite structure (JCPDS card no. 01-079-0205) at 2θ values: 31.8, 34.5, 36.2, 47.6, 56.6, 62.9, 66.4, 67.9, 69.1, 72.6, and 76.9 [37]. Calculated average crystallite sizes are 16.4 nm for Zinkoxyd aktiv and 42.7 nm for Zano 20, indicating that the latter contains much larger crystallites than the former one. Based on these results, we conclude that the Zano 20 has larger crystallite size, lower specific surface area, and rod-like morphology, while the Zinkoxyd aktiv has irregular spherical morphology, larger specific surface area, and organic layer on the surface. between 420 and 450 cm−1 due to the two transverse optical stretching modes of ZnO [33]. In the FTIR spectrum of Zinkoxyd aktiv, consisting of ZnO nanoparticles with irregular spherical morphology, only broad absorption band with a maximum at 447 cm−1 is observed (Figure 2a), while the FTIR spectrum of Zano 20 shows broad band with two maxima, one at 447 cm−1 and the other one at 434 cm−1 (Figure 2a), which are characteristic of the rod-like ZnO morphology [34]. Besides, Zinkoxyd aktiv shows additional absorption bands at 1508 and 1400 cm−1, typical for the organic moieties most probably located on the surface of ZnO particles, indicating that Zinkoxyd aktiv is more organophillic than Zano 20. Photoluminescence spectra of both ZnO powders show the near band edge UV emission from 380 to 400 nm and numerous visible light emission peaks at 423, 448, 461, 485, and 529 nm [10,12] (Figure 2b). Differences between the samples in the visible light emission region are rather small, indicating small differences in the quantity and type of intrinsic defects on the surfaces of both ZnO samples [35]. A larger difference was observed in the near band edge peak, which is located at 381.5 nm and 392 nm for the Zano 20 and Zinkoxyd aktiv, respectively. The intensity of this peak is much higher for the Zano 20 and is most probably related to the rod-like ZnO morphology (thick rods) [36]. XRD diffractograms (Figure 2C) show diffraction maxima that are characteristic of the crystalline ZnO with hexagonalwurtzite structure (JCPDS card no. 01-079-0205) at 2θ values: 31.8, 34.5, 36.2, 47.6, 56.6, 62.9, 66.4, 67.9, 69.1, 72.6, and 76.9 [37]. Calculated average crystallite sizes are 16.4 nm for Zinkoxyd aktiv and 42.7 nm for Zano 20, indicating that the latter contains much larger crystallites than the former one. Based on these results, we conclude that the Zano 20 has larger crystallite size, lower specific surface area, and rod-like morphology, while the Zinkoxyd aktiv has irregular spherical morphology, larger specific surface area, and organic layer on the surface.

FTIR spectra of ZnO powders (Figure 2a) show characteristic strong and broad absorption bands

 **Figure 1.** SEM micrographs of (**a**) Zinkoxyd aktiv; (**b**) Zano 20.

**Figure 1.** SEM micrographs of **a**) Zinkoxyd aktiv; **b**) Zano 20.

**Figure 2.** (**A**) FTIR spectra; (**B**) Photoluminescence spectra; (**C**) XRD diffractograms of commercial nano ZnO powders.

**Figure 2. A**) FTIR spectra; **B**) Photoluminescence spectra; **C**) XRD diffractograms of commercial nano

#### ZnO powders. *2.2. Composites of Nano ZnO with HDPE and PP Matrices; Unmodified ZnO and Surface-Modified ZnO with Stearic Acid*

*2.2. Composites of Nano ZnO with HDPE and PP Matrices; Unmodified ZnO and Surface-Modified ZnO with Stearic Acid*  First, we studied the distribution of ZnO in polyolefin matrix by SEM microscopy using a backscattered electron detector. SEM micrographs in Figure S1 A–D show the distribution of ZnO in the HDPE matrix at a concentration of nano ZnO of 1.0 wt%. The dimensions of ZnO are from 1 to 5 μm, indicating that ZnO is predominantly in the aggregated form. A comparison of the micrographs of the HDPE nanocomposites prepared by unmodified ZnO (Figure S1 A and C) and with stearic acid First, we studied the distribution of ZnO in polyolefin matrix by SEM microscopy using a backscattered electron detector. SEM micrographs in Figure S1A–D show the distribution of ZnO in the HDPE matrix at a concentration of nano ZnO of 1.0 wt%. The dimensions of ZnO are from 1 to 5 µm, indicating that ZnO is predominantly in the aggregated form. A comparison of the micrographs of the HDPE nanocomposites prepared by unmodified ZnO (Figure S1A,C) and with stearic acid modified ZnO (Figure S1B,D) indicates that stearic acid coating improves compatibility between the Zano 20 and HDPE, as indicated by a reduced number of large aggregates (Figure S1D).

modified ZnO (Figure S1 B and D) indicates that stearic acid coating improves compatibility between the Zano 20 and HDPE, as indicated by a reduced number of large aggregates (Figure S1 D). Table S1 shows the mechanical properties of HDPE/ZnO composites (unmodified ZnO and with stearic acid (3.0 wt%) modified ZnO: Zinkoxyd aktiv and Zano 20) as a function of nano ZnO concentration. The results show that Zinkoxyd aktiv has no particular effect on the composites' mechanical properties since they remain more or less unchanged with a slight downward trend. An exception is elongation at break, which shows a trend of slight increase (Table S1, Figure S2 A). The HDPE nanocomposites with Zano 20 show reduction in the tensile strength and Young's modulus and a more pronounced increase in elongation at break (Table S1 and Figure 3). It is obvious that the addition of nano ZnO slightly deteriorates the mechanical properties of the HDPE/ZnO nanocomposites. In particular, the decrease in tensile strength and Young's modulus (by 21% and 25%, respectively) was pronounced when 2% by weight of Zano 20 was added (Figure 3 and Figure S2 B). Obviously, such a high concentration of Zano 20 in HDPE is not beneficial. Surface modification Table S1 shows the mechanical properties of HDPE/ZnO composites (unmodified ZnO and with stearic acid (3.0 wt%) modified ZnO: Zinkoxyd aktiv and Zano 20) as a function of nano ZnO concentration. The results show that Zinkoxyd aktiv has no particular effect on the composites' mechanical properties since they remain more or less unchanged with a slight downward trend. An exception is elongation at break, which shows a trend of slight increase (Table S1, Figure S2A). The HDPE nanocomposites with Zano 20 show reduction in the tensile strength and Young's modulus and a more pronounced increase in elongation at break (Table S1 and Figure 3). It is obvious that the addition of nano ZnO slightly deteriorates the mechanical properties of the HDPE/ZnO nanocomposites. In particular, the decrease in tensile strength and Young's modulus (by 21% and 25%, respectively) was pronounced when 2% by weight of Zano 20 was added (Figure 3 and Figure S2B). Obviously, such a high concentration of Zano 20 in HDPE is not beneficial. Surface modification of ZnO with stearic acid shows slightly enhanced compatibility between ZnO and HDPE (Figure S1), but no improvement of composites' mechanical properties was observed (Table S1).

but no improvement of composites' mechanical properties was observed (Table S1).

of ZnO with stearic acid shows slightly enhanced compatibility between ZnO and HDPE (Figure S1),

Study of thermal properties (melting temperature and melting enthalpy) of HDPE/ZnO composites (unmodified Zinkoxyd aktiv and Zano 20) as a function of nano ZnO concentration shows that the addition of nano ZnO only slightly affects the melting temperature, ∆Hm, and crystallinity degree of HDPE matrix (Table S1, Figure S3). Only small changes in the degree of HDPE crystallinity, together with a rather high degree of ZnO aggregation (Figure S1), are explanations for a minimal impact of added nano ZnO on the nanocomposites' mechanical properties. Due to a high degree of ZnO aggregation, the interface surface between ZnO and HDPE is rather small. On the other hand, a major mechanism influencing the nanocomposite mechanical properties is that inorganic nanostructures act as the crystallization nuclei, resulting in a higher crystallinity of the polymer matrix and thus in improved mechanical properties. Since, in our case, the crystallinity degree is not affected by ZnO, this explains only the small changes in observed mechanical properties (Table S1). In literature, some authors reported on significant increase in crystallinity by the addition of nano ZnO [31], while others observed no changes [22]. Consequently, some authors reported on improved composite mechanical properties [7,15] and others did not [2,28]. On the other hand, TGA results reveal improved thermal stability of the HDPE/ZnO composites compared to the neat HDPE (Figure S4). composites (unmodified Zinkoxyd aktiv and Zano 20) as a function of nano ZnO concentration shows that the addition of nano ZnO only slightly affects the melting temperature, ΔHm, and crystallinity degree of HDPE matrix (Table S1, Figure S3). Only small changes in the degree of HDPE crystallinity, together with a rather high degree of ZnO aggregation (Figure S1), are explanations for a minimal impact of added nano ZnO on the nanocomposites' mechanical properties. Due to a high degree of ZnO aggregation, the interface surface between ZnO and HDPE is rather small. On the other hand, a major mechanism influencing the nanocomposite mechanical properties is that inorganic nanostructures act as the crystallization nuclei, resulting in a higher crystallinity of the polymer matrix and thus in improved mechanical properties. Since, in our case, the crystallinity degree is not affected by ZnO, this explains only the small changes in observed mechanical properties (Table S1). In literature, some authors reported on significant increase in crystallinity by the addition of nano ZnO [31], while others observed no changes [22]. Consequently, some authors reported on improved composite mechanical properties [7,15] and others did not [2,28]. On the other hand, TGA results reveal improved thermal stability of the HDPE/ZnO composites compared to the neat HDPE (Figure S4).

Study of thermal properties (melting temperature and melting enthalpy) of HDPE/ZnO

**Figure 3.** Mechanical properties of HDPE/ZnO composites as a function of nano ZnO concentration: **a**) Young's modulus and **b**) Tensile strength. **Figure 3.** Mechanical properties of HDPE/ZnO composites as a function of nano ZnO concentration: (**a**) Young's modulus and (**b**) Tensile strength.

.

SEM micrographs in Figures S5 A–D show the distribution of ZnO in the PP matrix at 1 wt% concentration of nano ZnO. The ZnO particles are predominantly present in PP matrix as the aggregates with sizes varying from 1 to 5 μm. A comparison of micrographs (unmodified ZnO - Figure S5 A and C and ZnO modified with stearic acid, Figure S5 B and D) shows that stearic acid primarily improves the compatibility of Zano 20 with PP matrix, as indicated by the smaller number SEM micrographs in Figure S5A–D show the distribution of ZnO in the PP matrix at 1 wt% concentration of nano ZnO. The ZnO particles are predominantly present in PP matrix as the aggregates with sizes varying from 1 to 5 µm. A comparison of micrographs (unmodified ZnO—Figure S5A,C and ZnO modified with stearic acid, Figure S5B,D) shows that stearic acid primarily improves the compatibility of Zano 20 with PP matrix, as indicated by the smaller number of large aggregates (Figure S5D).

of large aggregates (Figure S5 D). The results of mechanical properties testing of PP/ZnO composites (unmodified ZnO and modified with stearic acid: 3.0 wt%) prepared by Zinkoxyd aktiv or Zano 20 nanofillers as a function of nanoparticle concentration are presented in Table S2 and Figure 4. Zinkoxyd aktiv does not have a significant influence on the composites' mechanical properties at concentrations up to 2.0 wt% (Table S2, Figure S6 A). On the other hand, Zano 20 shows a slightly more pronounced effect as indicated by increased tensile strength by 7.3% and Young's modulus by 6.3% (Table S2, Figure S6 B). Additionally, unmodified ZnO slightly increases Young's modulus, while this was not observed The results of mechanical properties testing of PP/ZnO composites (unmodified ZnO and modified with stearic acid: 3.0 wt%) prepared by Zinkoxyd aktiv or Zano 20 nanofillers as a function of nanoparticle concentration are presented in Table S2 and Figure 4. Zinkoxyd aktiv does not have a significant influence on the composites' mechanical properties at concentrations up to 2.0 wt% (Table S2, Figure S6A). On the other hand, Zano 20 shows a slightly more pronounced effect as indicated by increased tensile strength by 7.3% and Young's modulus by 6.3% (Table S2, Figure S6B). Additionally, unmodified ZnO slightly increases Young's modulus, while this was not observed for the ZnO modified with stearic acid, which is attributed to the plasticizing effect of stearic acid.

for the ZnO modified with stearic acid, which is attributed to the plasticizing effect of stearic acid. Table S2 also summarizes the thermal properties (melting temperature and melting enthalpy) of PP/ZnO composites (Zinkoxyd aktiv and Zano 20) depending on the ZnO concentration. Nano ZnO has no significant effect on the melting temperature, ΔHm, and degree of PP crystallinity (Table S2, Figures S7 and S8). The literature dealing with the effect of ZnO on the PP crystallinity are dubious since some authors reported on increased PP crystallinity [26,32], while others observed no changes, or even the opposite effect [29]. Similar to the case of HDPE/ZnO composites, only minimal changes in the degree of PP matrix crystallinity and a rather high degree of ZnO aggregation (Figure 1) are the explanations for the small effect of added nano ZnO on the mechanical properties of PP Table S2 also summarizes the thermal properties (melting temperature and melting enthalpy) of PP/ZnO composites (Zinkoxyd aktiv and Zano 20) depending on the ZnO concentration. Nano ZnO has no significant effect on the melting temperature, ∆Hm, and degree of PP crystallinity (Table S2, Figures S7 and S8). The literature dealing with the effect of ZnO on the PP crystallinity are dubious since some authors reported on increased PP crystallinity [26,32], while others observed no changes, or even the opposite effect [29]. Similar to the case of HDPE/ZnO composites, only minimal changes in the degree of PP matrix crystallinity and a rather high degree of ZnO aggregation (Figure 1) are the explanations for the small effect of added nano ZnO on the mechanical properties of PP nanocomposites

(Table S2). ZnO addition to the PP can cause various effects on its crystallinity, depending on the composite preparation process; consequently, different effects on the mechanical properties are reported. Some authors reported on enhancement of mechanical properties [22,26,27], while others reported on negligible effect of added nano ZnO [29,38]. nanocomposites (Table S2). ZnO addition to the PP can cause various effects on its crystallinity, depending on the composite preparation process; consequently, different effects on the mechanical properties are reported. Some authors reported on enhancement of mechanical properties [22,26,27], while others reported on negligible effect of added nano ZnO [29,38].

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**Figure 4.** Mechanical properties of PP/ZnO composites as a function of nano ZnO concentration: **a**) Young's modulus and **b**) Tensile strength. **Figure 4.** Mechanical properties of PP/ZnO composites as a function of nano ZnO concentration: (**a**) Young's modulus and (**b**) Tensile strength.

Many research groups also studied the effect of ZnO on the UV stability of polymer matrices. Literature reports enhanced polymer UV-stability since ZnO is an excellent UV absorber, however, the absorbed energy can be transferred to the polymer chains, causing their scission. Upon UV light exposure, ZnO is excited, leading to the formation of oxygen-active (hydroperoxide) species in the presence of OH groups on the surface, which can cause scission of polymer chains [17,39]. The first change caused by the UV light is the color or gloss change [40]. Therefore, the color change of polyolefin/ZnO nanocomposites was measured as a function of the exposure time of the composite to the artificial sun light. Many research groups also studied the effect of ZnO on the UV stability of polymer matrices. Literature reports enhanced polymer UV-stability since ZnO is an excellent UV absorber, however, the absorbed energy can be transferred to the polymer chains, causing their scission. Upon UV light exposure, ZnO is excited, leading to the formation of oxygen-active (hydroperoxide) species in the presence of OH groups on the surface, which can cause scission of polymer chains [17,39]. The first change caused by the UV light is the color or gloss change [40]. Therefore, the color change of polyolefin/ZnO nanocomposites was measured as a function of the exposure time of the composite to the artificial sun light.

Figure S9 and S10 show changes in the color (ΔE) of HDPE/ZnO composites (Zinkoxyd aktiv - Figure S9 and Zano 20 - Figure S10), depending on the exposure time to the artificial sunlight. ΔE up to 7 weeks does not exceed the value of 4, meaning that with the naked eye, the color change is barely detectable. After ten weeks of exposure, ΔE reached the values from 8 to 15, meaning that color change was easily detected by naked eye. The change in color also depends on the nano ZnO concentration, since the largest color changes were observed at the highest nano ZnO concentration (2.0 wt%). Surface modification of nano ZnO with stearic acid reduces color changes (Figure S9 and S10). A comparison of both ZnO nanoparticles revealed that Zano 20 has a more pronounced effect on the color change of HDPE/ZnO composites than Zinkoxyd aktiv, which is in line with the presence of organic layer on the surface of Zinkoxyd aktiv. Other authors reported on similar effects of ZnO and TiO2 particles on the UV stability of PE [19,39]. Figures S9 and S10 show changes in the color (∆E) of HDPE/ZnO composites (Zinkoxyd aktiv—Figure S9 and Zano 20—Figure S10), depending on the exposure time to the artificial sunlight. ∆E up to 7 weeks does not exceed the value of 4, meaning that with the naked eye, the color change is barely detectable. After ten weeks of exposure, ∆E reached the values from 8 to 15, meaning that color change was easily detected by naked eye. The change in color also depends on the nano ZnO concentration, since the largest color changes were observed at the highest nano ZnO concentration (2.0 wt%). Surface modification of nano ZnO with stearic acid reduces color changes (Figures S9 and S10). A comparison of both ZnO nanoparticles revealed that Zano 20 has a more pronounced effect on the color change of HDPE/ZnO composites than Zinkoxyd aktiv, which is in line with the presence of organic layer on the surface of Zinkoxyd aktiv. Other authors reported on similar effects of ZnO and TiO<sup>2</sup> particles on the UV stability of PE [19,39].

Figure S11 shows a color change of the PP/ZnO composites as a function of exposure time to the artificial sunlight. ΔE does not exceed 4, meaning only a moderate color change over ten weeks of exposure, which is equal to two years in real sunlight. The color change depends on the nano ZnO concentration, since the largest change was observed at 2.0 wt% of added ZnO. Surface modification of nano ZnO with stearic acid reduces the effect of nano ZnO, similar to the case of HDPEP/ZnO composites (Figure S11). Compared to HDPE, the PP matrix shows a higher stability to the artificial sunlight (Figure S9), indicating that a lifetime of PP/ZnO composites significantly exceeds two years even at 2.0 wt% of nano ZnO (Zinkoxyd aktiv). Figure S11 shows a color change of the PP/ZnO composites as a function of exposure time to the artificial sunlight. ∆E does not exceed 4, meaning only a moderate color change over ten weeks of exposure, which is equal to two years in real sunlight. The color change depends on the nano ZnO concentration, since the largest change was observed at 2.0 wt% of added ZnO. Surface modification of nano ZnO with stearic acid reduces the effect of nano ZnO, similar to the case of HDPEP/ZnO composites (Figure S11). Compared to HDPE, the PP matrix shows a higher stability to the artificial sunlight (Figure S9), indicating that a lifetime of PP/ZnO composites significantly exceeds two years even at 2.0 wt% of nano ZnO (Zinkoxyd aktiv).

HDPE/ZnO and PP/ZnO composites aged under artificial sunlight were studied by FTIR-ATR spectroscopy to perceive possible chemical processes and changes in chemical composition that occurred in the material (Figure S12 and Figure S13). The FTIR spectra in Figure S12 A–C show only minor differences compared to those of neat HDPE treated in the same way, leading to a conclusion that observed color changes (Figure S9 and S10) are not in correlation with the chemical changes HDPE/ZnO and PP/ZnO composites aged under artificial sunlight were studied by FTIR-ATR spectroscopy to perceive possible chemical processes and changes in chemical composition that occurred in the material (Figures S12 and S13). The FTIR spectra in Figure S12A–C show only minor differences compared to those of neat HDPE treated in the same way, leading to a conclusion that observed color changes (Figures S9 and S10) are not in correlation with the chemical changes (degradation) of the

(degradation) of the studied HDPE/ZnO composite samples. A comparison of FTIR spectra of

studied HDPE/ZnO composite samples. A comparison of FTIR spectra of PP/ZnO composites without and with 1.0 wt% of ZnO exposed to sunlight (Figure S13A–C) show the appearance of a new absorption band at 1725 cm−<sup>1</sup> , corresponding to the formation of carbonyl groups. We studied its intensity as a function of composite exposure time (Figure S14). Results reveal increasing intensity of the carbonyl absorption band with time, but differences between the pure PP and PP/ZnO composites (unmodified and modified) are negligible, so no correlation can be established with the ZnO concentration in the PP matrix. We concluded that carbonyl absorption band is not related to the presence of ZnO [40]. the appearance of a new absorption band at 1725 cm−1, corresponding to the formation of carbonyl groups. We studied its intensity as a function of composite exposure time (Figure S14). Results reveal increasing intensity of the carbonyl absorption band with time, but differences between the pure PP and PP/ZnO composites (unmodified and modified) are negligible, so no correlation can be established with the ZnO concentration in the PP matrix. We concluded that carbonyl absorption band is not related to the presence of ZnO [40]. *2.3. Composites of Nano ZnO with HDPE and PP Matrix—ZnO Surface Modified with Silanes* 

#### *2.3. Composites of Nano ZnO with HDPE and PP Matrix—ZnO Surface Modified with Silanes* HDPE and PP composites were prepared also with commercially available nano ZnO powders,

HDPE and PP composites were prepared also with commercially available nano ZnO powders, the surface of which was modified with silanes (Zano 20 Plus: 3.9 wt% of caprylyl silane and Zano 20 Plus 3: 1.0 wt% of methacrylic silane). SEM micrographs in Figure S15 show distributions of silanized ZnO particles in composites based on HDPE and PP matrices. In comparison with composites prepared with unmodified ZnO and ZnO modified with stearic acid (Figures S1 and S5), the micrographs in Figure S15 show the presence of significantly smaller ZnO aggregates (size below 2 µm), confirming that silanization of ZnO considerably improves compatibility between the polyolefin matrix and nano ZnO. the surface of which was modified with silanes (Zano 20 Plus: 3.9 wt% of caprylyl silane and Zano 20 Plus 3: 1.0 wt% of methacrylic silane). SEM micrographs in Figure S15 show distributions of silanized ZnO particles in composites based on HDPE and PP matrices. In comparison with composites prepared with unmodified ZnO and ZnO modified with stearic acid (Figure S1 and S5), the micrographs in Figure S15 show the presence of significantly smaller ZnO aggregates (size below 2 μm), confirming that silanization of ZnO considerably improves compatibility between the polyolefin matrix and nano ZnO.

Table S3 summarizes the mechanical properties of HDPE/silanized ZnO. When Zano 20 Plus nanofiller was used, a slight increase in tensile strength (by up to 4.7%—Figure 5a, Figure S16A) and elongation at break was observed, while Young's modulus is somewhat reduced. In the case of Zano 20 Plus 3, only elongation at break slightly increased, while tensile strength and Young's modulus were slightly reduced. The reduction of Young's modulus is attributed to the presence of silane, acting as the plasticizer. Overall, silanized ZnO has only a minor effect on the mechanical properties of ZnO composites with HDPE matrix as indicated by enhancement of Young's modulus and tensile strength by 2.0% and 4.7%, respectively. These results are in accordance with the composites' thermal properties, showing no relationship with ZnO concentration (Table S3). Some authors, on the other hand, reported on the enhancement of composites' mechanical properties when ZnO was modified with the silane coupling agents [7,31]. Table S3 summarizes the mechanical properties of HDPE/silanized ZnO. When Zano 20 Plus nanofiller was used, a slight increase in tensile strength (by up to 4.7%—Figure 5 a, Figure S16 A) and elongation at break was observed, while Young's modulus is somewhat reduced. In the case of Zano 20 Plus 3, only elongation at break slightly increased, while tensile strength and Young's modulus were slightly reduced. The reduction of Young's modulus is attributed to the presence of silane, acting as the plasticizer. Overall, silanized ZnO has only a minor effect on the mechanical properties of ZnO composites with HDPE matrix as indicated by enhancement of Young's modulus and tensile strength by 2.0% and 4.7%, respectively. These results are in accordance with the composites' thermal properties, showing no relationship with ZnO concentration (Table S3). Some authors, on the other hand, reported on the enhancement of composites' mechanical properties when ZnO was modified with the silane coupling agents [7,31].

**Figure 5.** Tensile strength of polyolefin/ZnO composites as a function of silanized nano ZnO concentration (Zano 20 Plus): **a**) HDPE and **b**) PP matrix. **Figure 5.** Tensile strength of polyolefin/ZnO composites as a function of silanized nano ZnO concentration (Zano 20 Plus): (**a**) HDPE and (**b**) PP matrix.

Table S4 summarizes the mechanical properties of PP/silanized ZnO composites. When Zano 20 Plus was used, the tensile strength (by up to 8.7%—Figure 5b, Figure S16 B) and elongation at break increased, while Young's modulus slightly decreased. In the case of Zano 20 Plus 3, only elongation at break increases, while tensile strength and Young's modulus are more or less unaffected. Overall, the silanized ZnO has a rather small influence on the mechanical properties of composites based on the PP matrix since the maximal enhancement of tensile strength is 8.7%. Young's modulus is reduced in most cases, which is attributed to the plasticizing effect of silane. The thermal properties of PP/silanized ZnO composites showed a substantial increase (14.3%) in melting enthalpy when Zano 20 Plus was used (Table S4, Figure S17), while composites with Zano 20 Plus 3 showed no significant Table S4 summarizes the mechanical properties of PP/silanized ZnO composites. When Zano 20 Plus was used, the tensile strength (by up to 8.7%—Figure 5b, Figure S16B) and elongation at break increased, while Young's modulus slightly decreased. In the case of Zano 20 Plus 3, only elongation at break increases, while tensile strength and Young's modulus are more or less unaffected. Overall, the silanized ZnO has a rather small influence on the mechanical properties of composites based on the PP matrix since the maximal enhancement of tensile strength is 8.7%. Young's modulus is reduced in most cases, which is attributed to the plasticizing effect of silane. The thermal properties of PP/silanized ZnO composites showed a substantial increase (14.3%) in melting enthalpy when Zano 20 Plus was used (Table S4, Figure S17), while composites with Zano 20 Plus 3 showed no significant changes.

changes. These observations are in accordance with an increase in tensile strength (Table S4) when

These observations are in accordance with an increase in tensile strength (Table S4) when Zano 20 Plus nanofiller was used, confirming that an increase in crystallinity degree is a predominant mechanism of mechanical properties reinforcement. These results are in contrast to those results, reporting on negligible changes of mechanical properties when silane coupling agents were applied [27,30], but in line with those reporting on enhancement of composite mechanical properties when nano ZnO surface modified with methacrylic silane was applied [32].

Figures S18 and S19 show color changes of HDPE/ZnO and PP/ZnO composites (Zano 20 Plus and Zano 20 Plus 3), depending on the composite exposure time to artificial sunlight and ZnO concentration. The change in color strongly depends on the ZnO concentration, since it is the greatest at 2.0 wt% (Figures S18 and S19). Moreover, a thicker layer of silane (Zano 20 Plus—3.9 wt% of caprylyl silane) slightly reduces color change as compared to the thinner layer (Zano 20 Plus 3–1.0 wt% methacrylic silane). A comparison of color changes shows that HDPE is significantly more light-sensitive than PP, since the latter shows only moderate changes in the color metrics after 10 weeks of exposure time; ∆E is below 4 for PP and between 6 and 11 for HDPE (Figures S18 and S19).

#### *2.4. Antibacterial Properties of Polyolefin Composites with ZnO Nanoparticles*

ZnO is well known as a highly efficient antibacterial agent. The exact mechanism of ZnO antibacterial activity is still not completely elucidated. There are many possible physical and chemical mechanisms of the ZnO interaction with the bacterial cells: (a) generation of reactive oxygenated species (ROS = O<sup>2</sup> −•, HO<sup>2</sup> • and HO• ) under UV or visible light illumination; (b) release of Zn2<sup>+</sup> ions due to partial dissolution of ZnO; (c) generation of H2O<sup>2</sup> by photoinduction; (d) disruption of plasma membrane due to interaction with ZnO; (e) internalization (penetration) of ZnO nanoparticles into the bacterial cell; (f) mechanical damage of the cell membrane [41]. The first three interaction mechanisms are of a chemical nature, while the other three are physical. The real effect of ZnO on the bacterial cells is most probably a combination of these mechanisms.

The measurements of antibacterial activity were carried out on the selected samples (Table 1) of ZnO composites with polyolefin matrices (Table 2, Table 3). We selected the samples containing 2.0 wt% of ZnO either Zinkoxyd aktiv or Zano 20, and ZnO of different surface modification. According to ISO 22196: 2007 standard, the log of reduction must be equal to 2 or higher than 2 so that certain material can be accepted as antibacterial [42]. The results show that all composites have excellent antibacterial activity against *S. aureus* (gram-positive bacteria) (Table 3), while the results on antibacterial activity against *E. coli* (gram-negative bacteria) vary significantly (Table 2) [41]. It is known that ZnO shows a more intense effect on the gram-positive bacteria such as *S. aureus* or *Bacillus subtilis* than on the gram-negative bacteria such as *E. coli* or *Aerobacter aerogenes* [25,43–45]. This can be attributed to their structural and compositional differences. Namely, gram-negative bacteria have an additional outer plasma membrane that consists of a thick lipopolysaccharide layer. The overall thickness of the membrane is larger in the gram-negative bacteria than in the gram-positive ones. These structural differences are the most probable reason for higher resistance of gram-negative bacteria towards ZnO [43–45]. Photographs of bacterial colonies on agar (Figure 6), taken after 24 h of contact with HDPE/ZnO composites containing 2.0 wt% of unmodified nano ZnO (Figure 6B,C), present a significant reduction in the number of *E. coli* bacterial colonies.

**Table 1.** Data on samples of HDPE or PP/ZnO composites selected for antibacterial activity testing (the concentration of nano ZnO is 2.0 wt%). Zinkox-aktiv = Zinkoxyd aktiv.



**Table 2.** Determined antibacterial activities and adequate activities scores of HDPE/ZnO and PP/ZnO composites against *E. coli*. Zinkox-aktiv = Zinkoxyd aktiv. species (ROS = O2−•, HO2• and HO•) under UV or visible light illumination; b) release of Zn2+ ions due to partial dissolution of ZnO; c) generation of H2O2 by photoinduction; d) disruption of plasma membrane due to interaction with ZnO; e) internalization (penetration) of ZnO nanoparticles into the

mechanisms of the ZnO interaction with the bacterial cells: a) generation of reactive oxygenated

ZnO is well known as a highly efficient antibacterial agent. The exact mechanism of ZnO

exposure time; ΔE is below 4 for PP and between 6 and 11 for HDPE (Figures S18 and 19).

*Molecules* **2019**, *24*, x FOR PEER REVIEW 8 of 15

mechanism of mechanical properties reinforcement. These results are in contrast to those results, reporting on negligible changes of mechanical properties when silane coupling agents were applied [27,30], but in line with those reporting on enhancement of composite mechanical properties when

Figures S18 and S19 show color changes of HDPE/ZnO and PP/ZnO composites (Zano 20 Plus and Zano 20 Plus 3), depending on the composite exposure time to artificial sunlight and ZnO concentration. The change in color strongly depends on the ZnO concentration, since it is the greatest at 2.0 wt% (Figures S18 and S19). Moreover, a thicker layer of silane (Zano 20 Plus—3.9 wt% of caprylyl silane) slightly reduces color change as compared to the thinner layer (Zano 20 Plus 3–1.0 wt% methacrylic silane). A comparison of color changes shows that HDPE is significantly more lightsensitive than PP, since the latter shows only moderate changes in the color metrics after 10 weeks of

nano ZnO surface modified with methacrylic silane was applied [32].

*2.4. Antibacterial Properties of Polyolefin Composites with ZnO Nanoparticles*

**Table 3.** Determined antibacterial activities and adequate activities scores of HDPE/ZnO and PP/ZnO composites against *S. aureus*. Zinkox-aktiv = Zinkoxyd aktiv. antibacterial activity against *S. aureus* (gram-positive bacteria) (Table 3), while the results on antibacterial activity against *E. coli* (gram-negative bacteria) vary significantly (Table 2) [41]. It is known that ZnO shows a more intense effect on the gram-positive bacteria such as *S. aureus* or *Bacillus* 


**Figure 6.** Determination of *E. coli* cell number (CFU/mL): (**A**) Starting inoculum concentration of *E. coli*; (**B**,**C**) *E. coli* count on two parallel test samples of HDPE/unmodified ZnO composite after 24 h.

Samples of surface unmodified ZnO show good or excellent activities, while those of surface modified ZnO are poor or even bad (Table 2). Comparing both ZnO nanofillers, Zano 20 shows higher antibacterial activity than Zinkoxyd aktiv, but the difference is mainly in the case of *E. coli*. Based on these results, we conclude that for achieving sufficient antibacterial activities towards both types of bacteria, the application of commercial unmodified Zano 20 ZnO in concentration of 2.0 wt% is recommended, although unmodified Zinkoxyd aktiv also show sufficient antibacterial activity with polyolefin matrices. The observed differences between *E. coli* and *S. aureus* in the antibacterial effect of polyolefin/ZnO composites originate from different membrane structure of gram-negative and gram-positive bacteria. Concerning the possible interaction mechanisms between ZnO and bacteria, we assume that chemical mechanisms do not require a physical contact between the ZnO particle and bacterial membrane since ROS, Zn2<sup>+</sup> ions, and H2O<sup>2</sup> can migrate from the surface of ZnO to the surface of bacteria. We conclude that the membrane structure of *S. aureus* allows the penetration of antibacterial active chemical compounds into the cell, thus explaining why, in this case, surface modified ZnO is also effective (Table 2). On the other side, the cell of *E. coli* is most probably damaged

or destroyed only by the physical processes, requiring physical contact with ZnO, which thus explains why, in this case, only the unmodified ZnO is effective against this type of bacteria (Table 2).

#### **3. Materials and Methods**

#### *3.1. Materials*

Commercially available TIPELIN BA 550-13 HDPE and TIPPLEN K-499 PP granules, and ZnO nanopowders: Zinkoxyd aktiv, Lanxsess, Germany and Zano 20, Umicore, Belgium. In addition, commercial silane modified Nano ZnO nanofillers were also applied: Zano 20 Plus, Zano 20 Plus 3, Umicore, Belgija. The Zano 20 Plus was surface modified with 3.9 wt% of triethoxycaprylylsilane caprylylsilane, while the Zano 20 Plus 3 was modified with 1 wt% of [3-methacryloxypropyl]trimethoxysilane methacrylic silane. *Escherichia coli* strain (DSM 498) and *Staphilococcus aureus* strain (DSM 346) were supplied by DSMZ-Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH, Germany.

#### *3.2. Preparation of Polyolefin*/*ZnO Composites*

ZnO nanopowders were applied to the surface of the PE or PP granulate. For this purpose, nano ZnO was first suspended in ethanol and subsequently the PE or PP granulate was added and sonicated. Finally, ethanol was evaporated on the rotary evaporator. The prepared granulates were extruded at 160 ◦C (PE) or 180 ◦C (PP) for 10 min at 50 rpm with Haake MiniLab extruder (Thermo Fischer Scientific, Karlsruhe, Germany). The mixture was added to the extruder in two portions of 3 g. The extruded melt was ejected from the extruder at 100 rpm and it was captured in a heated container (170 ◦C), which was further placed into a Haake Mini Jet molding machine (Thermo Fischer Scientific, Karlsruhe, Germany) to prepare the testing specimens by injection into a suitable mold heated to 70 ◦C at the pressure of 750 bar and time of 10 s, as well as the post pressure of 250 bar and time of 10 s.

#### *3.3. Characterization*

Mechanical properties were measured according to ISO 527 standard on the Shimadzu AGS-GX (Shimadzu, Kyoto, Japan) plus a dynamometer with an initial spacing of 58 mm and the stretching speed of 2 mm/min and 200 mm/min.

UV-vis resistance of composites was determined by exposing the samples to artificial sunlight in the Suntest chamber for certain time periods. Color change was measured as a function of time by using an i1 Pro (X-Rite, Grand Rapids, MI, USA) spectrometer, measuring the spectra of reflectivity in a spectral range of 380–730 nm.

Fourier-transform infrared spectroscopy (FTIR) spectra were recorded in a transmittance mode on a FTIR spectrometer Spectrum One (Perkin-Elmer, Waltham, MA, USA) in a spectral range between 360 cm−<sup>1</sup> and 4000 cm−<sup>1</sup> , and a spectral resolution of 4 cm−<sup>1</sup> using KBr tablets (ZnO powders) or in an ATR mode in a spectral range from 650 to 4000 cm−<sup>1</sup> with a 4 cm−<sup>1</sup> spectral resolution (polyolefin/ZnO composites).

Photoluminescence spectra of ZnO powders were recorded on a Perkin Elmer LS-55 spectrometer (Perkin-Elmer, Waltham, MA, USA) in a range from 330 nm to 620 nm using an excitation wavelength of 325 nm.

SEM micrographs of composites were taken on a Zeiss Supra 35 VP field emission electron microscope (Zeiss, Oberkochen, Germany) at a 15 kV acceleration voltage using a back-scattered electron detector at a working distance of 8 mm.

Thermal properties of polyolefin matrices and composites were determined by DSC calorimetry on DSC-1 (Mettler Toledo, Greifensee, Switzerland) in a temperature range from 25 ◦C to 200 ◦C at a heating rate of 10 K/min.

Thermal stability of composite samples was determined by thermogravimetric analysis (TGA) in oxygen atmosphere using a TGA-1 (Mettler Toledo, Greifensee, Switzerland) instrument. The measurements were performed in a temperature range from 30 ◦C to 800 ◦C at a heating rate of 10 K/min.

Antibacterial activity of ZnO/polyolefin composites was determined according to ISO 22196: 2007 standard.

Crystalline fractions of the ZnO powders were characterized by a wide-angle X-ray diffraction (XRD) on an XPert Pro diffractometer (PANalytical, Almelo, Netherlands) with Cu anode as an X-ray source. X-ray diffractograms were measured at 25 ◦C in the 2θ range from 5◦ to 80◦ with a step of 0.033◦ and step time of 100 s. Crystallite sizes were calculated using the Scherrer formula [10] and Si wafer was used to determine the experimental peak broadening.

Nitrogen sorption measurements were performed on a manometric gas sorption analyzer (Micromeritics Instrument Co., Norcross, GA, USA) at −196 ◦C in the range of relative pressure values from 10−<sup>6</sup> to 1. As-prepared samples were degassed at 140 ◦C for 16 h prior to the measurements. The specific surface areas were determined by BET method based on the obtained sorption isotherms.

#### **4. Conclusions**

SEM microscopy revealed agglomerated nano ZnO with sizes from 1 to 5 µm in polyolefin matrices. Results of tensile testing showed that both nano ZnO do not enhance the mechanical properties of HDPE composites, while PP composites show slight enhancement (Young's module by 6% and tensile strength by 7%) when Zano 20 was used as a nanofiller. Measurements of thermal properties revealed only a small effect of nano ZnO on the degree of crystallinity of these composites, which is in accordance with the unchanged mechanical properties. Surface modification of ZnO with stearic acid increases its compatibility with polyolefin matrices, which, however, does not result in improved composite mechanical properties. Color change of composite materials was studied in dependence of ZnO concentration, exposure time to artificial sunlight, and ZnO type. Higher nano ZnO concentrations cause more obvious color changes of HDPE/ZnO composites, while surface modification of ZnO with stearic acid partially reduces this effect. Nevertheless, the PP matrix shows significantly smaller changes (barely visible with naked eye—∆E is below 4) than the HDPE matrix (∆E is between 8 and 15) after 10 weeks of exposure of composites to artificial sunlight. As revealed by TGA, added nano ZnO (both ZnO types—2.0 wt%) also increased the thermal stability of HDPE while no changes were observed for PP matrix.

Silanization of nano ZnO significantly improved compatibility of ZnO with polyolefin matrices as indicated by smaller ZnO agglomerates (1–2 µm). Despite better ZnO distribution in polyolefin matrices, it has only a minor effect on the composites' mechanical properties. Nevertheless, clear positive trends were observed with silanized nano ZnO as indicated by 5–9% enhanced tensile strength when caprylyl silane (3.9 wt%) was applied as the ZnO surface modifying agent in the PP matrix. This was accompanied by 14.3% increase in the PP melting enthalpy, confirming that increased PP crystallinity is responsible for the enhanced composite mechanical properties. Young's modulus is reduced, which is attributed to the plasticizing effect of silane. Silanization also slightly reduces color changes of polyolefin/ZnO composites. Concerning improvement of mechanical properties in general, the use of nano ZnO powders as the reinforcing agents in polyolefins is economically not justified.

The composites with unmodified and surface modified ZnO are highly active against *S. aureus*, while in the case of *E. coli*, only the composites with unmodified ZnO show sufficient antibacterial activity. This difference is attributed to differences in membrane structure between the gram-positive (*S. aureus*) and gram-negative (*E. coli*) bacteria. To achieve sufficient antibacterial activity of polyolefin composites towards both bacteria types, the application of unmodified commercial ZnO (Zano 20) in the concentration of 2.0 wt% is recommended. Incorporation of ZnO into the polyolefin matrices is considered to be a promising way to improve material antibacterial properties for clinical application or applications in the food industry.

**Supplementary Materials:** The following Figures and Tables are available online, Figure S1: SEM micrographs (magnification 1000×, backscattered electron detector) of HDPE/ZnO composites (Zinkoxyd aktiv – 1.0 wt%): (A) unmodified nano ZnO - Zinkoxyd aktiv; (B) nano ZnO - Zinkoxyd aktiv modified with stearic acid; (C) unmodified nano ZnO - Zano 20; (D) nano ZnO-Zano 20 modified with stearic acid, Table S1: Mechanical and thermal

properties of HDPE/ZnO (Zinkoxyd aktiv or Zano 20) composites as the functions of nano ZnO concentration and surface modification of ZnO with stearic acid. The polymer matrix is polyethylene HDPE - H-3581, the addition of stearic acid is 3% by weight, Znoxaktiv is Zinkoxyd-aktiv, Figure S2: Stress-strain curves of HDPE/ZnO composites as a function of ZnO concentration: 0 wt% - black; 0.5 wt% - green; 2.0 wt% blue: (A) Zinkoxyd aktiv; (B) Zano 20, Figure S3: DSC curves of HDPE/ZnO composites as a function of ZnO concentration, Figure S4: TGA curves of HDPE/ZnO composites as a function of ZnO concentration, Figure S5: SEM micrographs (magnification 1000×, backscattered electron detector) of PP/ZnO composites (Zinkoxyd aktiv – 1.0 wt%): (A) unmodified nano ZnO - Zinkoxyd aktiv; (B) nano ZnO - Zinkoxyd aktiv modified with stearic acid; (C) unmodified nano ZnO - Zano 20; (D) nano ZnO-Zano 20 modified with stearic acid, Table S2: Mechanical and thermal properties of composites with PP matrix and ZnO - Zinkoxyd aktiv or Zano 20 (unmodified ZnO and surface modified with stearic acid) depending on the nano ZnO concentration. The polymer matrix is polypropylene PP - K499, the addition of stearic acid is 3.0 wt%, Znox-aktiv is Zinkoxyd aktiv, Figure S6: Stress-strain curves of PP/ZnO composites as a function of ZnO concentration: 0 wt% - black; 0.5 wt% - green; 2.0 wt% blue: (A) Zinkoxyd aktiv; (B) Zano 20, Figure S7: DSC curves of PP/ZnO composites as a function of ZnO concentration, Figure S8: TGA curves of PP/ZnO composites as a function of ZnO concentration, Figure S9: Changes in color of HDPE/ZnO composites (Zinkoxyd aktiv) as the functions of exposure time to artificial sunlight, nano ZnO concentration, and ZnO surface modification with stearic acid, Figure S10: Changes in color of HDPE/ZnO composites (Zano 20) as the functions of exposure time to artificial sunlight, nano ZnO concentration, and ZnO surface modification with stearic acid, Figure S11: Changes in color of PP/ZnO composites (Zinkoxyd aktiv) as the functions of exposure time to the artificial sunlight and ZnO concentration, Figure S12: FTIR spectra of HDPE/ZnO composites prior to exposure to artificial light and after ten weeks of exposure: (A) pure HDPE; (B) HDPE with 1.0 wt% of ZnO (Zinkoxyd-aktiv); (C) HDPE with 1.0 wt% of ZnO (Zano 20), Figure S13: FTIR spectra of PP/ZnO composites prior to exposure to artificial light and after ten weeks of exposure: (A) pure PP; (B) PP with 1.0 wt% of ZnO (Zinkoxyd-aktiv); (C) PP with 1.0 wt% of ZnO (Zano 20), Figure S14: Intensity of the absorption band at 1725 cm−<sup>1</sup> in PP/ZnO composites as a function of exposure time to the artificial sunlight, Figure S15: SEM micrographs (magnification 1000×, backscattered electron detector) of HDPE/ZnO composites with 1.0 wt% of silane modified ZnO: (A) Zano 20 Plus; (B) Zano 20 Plus 3, and PP/ZnO composites: (C) Zano 20 Plus; (D) Zano 20 Plus 3, Table S3: Mechanical and thermal properties of composites of HDPE matrix and silanized nano ZnO (Zano 20 Plus - 3.9 wt% of caprylyl silane and Zano 20 Plus 3 – 1.0 wt% of methacrylic silane), Table S4: Mechanical and thermal properties of composites with PP matrix and silanized nano ZnO (Zano 20 Plus – 3.9 wt% of caprylyl silane and Zano 20 Plus 3 – 1.0 wt% of methacrylic silane), Figure S16: Stress-strain curves of polyolefin/ZnO composites as a function of silanized ZnO concentration (Zano 20 Plus): 0 wt% - black; 0.5 wt% - green; 2.0 wt% blue. (A) HDPE and (B) PP, Figure S17: DSC curves of PP/ZnO composites as a function of silanized ZnO concentration (Zano 20 Plus), Figure S18: Change in color of HDPE/ZnO composites (Zano 20 Plus and Zano 20 Plus 3) depending on the exposure time to artificial sunlight and nano ZnO concentration as well as the type of surface modification, Figure S19: Change in color of PP/ZnO composites (Zano 20 Plus and Zano 20 Plus 3) as the functions of the exposure time to artificial sunlight and nano ZnO concentration as well as the type of surface modification.

**Author Contributions:** Conceptualization, A.A. and I.Š.; Methodology, A.A., I.Š., and M.P.; Validation, A.A., M.P., I.Š., M.L., Ž.K., and E.Ž.; Formal Analysis, A.A., I.Š., M.P.; Investigation, A.A. and M.P.; Data Curation, M.L. and E.Ž.; Writing—Original Draft Preparation, A.A.; Writing—Review and Editing, A.A., M.P., I.Š., M.L., Ž.K., and E.Ž.; Visualization, A.A.; Supervision, M.L., Ž.K., and E.Ž.; Project Administration, M.L., Ž.K., and E.Ž.; Funding Acquisition, Ž.K. and E.Ž.

**Funding:** This research was funded by the SLOVENIAN RESEARCH AGENCY (Research Core Funding No. P2-0145 and P2-0046) and the SLOVENIAN MINISTRY OF EDUCATION, SCIENCE, AND SPORT (Program Martina No. OP20.00369).

**Acknowledgments:** Authors acknowledge the support of Marta Klanjšek Gunde for the access to equipment for color change measurements.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


**Sample Availability:** Samples of the compounds are not available from the authors.

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

### *Article* **Magnetic Polyurea Nano-Capsules Synthesized via Interfacial Polymerization in Inverse Nano-Emulsion**

#### **Suzana Natour <sup>1</sup> , Anat Levi-Zada <sup>2</sup> and Raed Abu-Reziq 1,\***


Academic Editors: Marinella Striccoli, Roberto Comparelli and Annamaria Panniello Received: 17 June 2019; Accepted: 18 July 2019; Published: 23 July 2019

**Abstract:** Polyurea (PU) nano-capsules have received voluminous interest in various fields due to their biocompatibility, high mechanical properties, and surface functionality. By incorporating magnetic nanoparticle (MNPs) into the polyurea system, the attributes of both PU and MNPs can be combined. In this work, we describe a facile and quick method for preparing magnetic polyurea nano-capsules. Encapsulation of ionic liquid-modified magnetite nanoparticles (MNPs), with polyurea nano-capsules (PU NCs) having an average size of 5–20 nm was carried out through interfacial polycondensation between amine and isocyanate monomers in inverse nano-emulsion (water-in-oil). The desired magnetic PU NCs were obtained utilizing toluene and triple-distilled water as continuous and dispersed phases respectively, polymeric non-ionic surfactant cetyl polyethyleneglycol/polypropyleneglycol-10/1 dimethicone (ABIL EM 90), diethylenetriamine, ethylenediamine diphenylmethane-4,40 -diisocyanate, and various percentages of the ionic liquid-modified MNPs. High loading of the ionic liquid-modified MNPs up to 11 wt% with respect to the dispersed aqueous phase was encapsulated. The magnetic PU NCs were probed using various analytical instruments including electron microscopy, infrared spectroscopy, X-ray diffraction, and nuclear magnetic spectroscopy. This unequivocally manifested the successful synthesis of core-shell polyurea nano-capsules even without utilizing osmotic pressure agents, and confirmed the presence of high loading of MNPs in the core.

**Keywords:** polyurea nano-capsules; magnetic nanoparticles; nano-emulsions; interfacial polymerization; composite nanomaterials

#### **1. Introduction**

Nano-capsules, composed of liquid or hollow cores, enclosed in a nontoxic polymeric shell, have been widely investigated for the encapsulation of hydrophobic and hydrophilic substances [1]. Different methods for fabricating nano-capsules and nanoparticles have been developed. Typically, they can be synthesized through interfacial polymerization, suspension polymerization, and nanoprecipitation in oil-in-water (O/W), oil-in-oil (O/O), and water-in-oil (W/O) emulsions, as well as mini- and micro-emulsions [2–8]. Interfacial polymerization and polycondensation is one of the most studied methods for fabricating a wide range of functional polymeric nano-capsules. In this process, polymerization occurs at the interface between two immiscible phases, with each phase containing dissolved complementary monomers, resulting in nano-capsules and nanoparticles with sizes on the order of emulsion droplets [9,10].

To encapsulate hydrophilic compounds, an inverse nano-emulsion (W/O system) was utilized [11,12]. Inverse nano-emulsion, also known as inverse miniemulsion, consists of 50–500 nm surfactant-stabilized aqueous droplets dispersed in a hydrophobic organic continuous phase. Nano-emulsions are kinetically stable and their preparation requires high energy, for example, using high shear homogenization and ultra-sonication methods [13,14].

In order to obtain the desired polymeric nano-capsules, stable inverse nano-emulsions must be prepared. This is achieved by using a combination of non-ionic surfactants with low hydrophilic-lipophilic balance (HLB) and osmotic pressure agents (lipophobes) as co-stabilizers. The surfactant sterically stabilizes the droplets and lipophobes within the droplets, and prevents droplet coalescence by suppressing Ostwald ripening [15–17].

Among the nanostructured materials prepared via interfacial polymerization, polyurea has gained the most interest due to having properties such as biocompatibility, high mechanical characteristics, and surface functionality [14,18,19]. Only a few studies have reported the preparation of polyurea nano-capsules (PU NCs) by means of inverse miniemulsion by utilizing lipophobes. In this regard, Landfester and co-workers reported the preparation of hollow polyurea, polythiourea, as well as polyurethane nano-capsules and nanoparticles [20]. They studied the effect of monomers and solvents on the shell thickness and morphology to develop nanoreactors for preparing silver nanoparticles (NPs). In another study, PU NCs were stabilized with an amino-functionalized surfactant and the encapsulation efficiency was examined by a fluorescent dye [19].

Nano-capsules have a high surface area-to-volume ratio, a narrow size distribution, and high encapsulation efficiency. However, the isolation and recovery of such systems is difficult and requires time consuming and tedious procedures. Nevertheless, these efforts can be minimized by encapsulating magnetite nanoparticles (MNPs), which will endow the PU NCs with superparamagnetic properties. This process facilitates their isolation simply by applying an external magnetic field [21–24]. The polymeric shell protects the MNPs from undergoing agglomeration and oxidation, which otherwise leads to a loss of magnetic properties.

Previously, we reported the synthesis of magnetically separable PU nanoparticles formulated using O/O nano-emulsion by employing the interfacial polycondensation reaction between 2,6-diaminopyridine and polymethylene-polyphenyl isocyanate (PAPI 27) in the presence of poly(1-ethenylpyrrolidin-2-one/hexadec-1-ene) (Agrimer AL 22) surfactant [25]. Spherical particles of ~450 nm size were obtained. In line with that research, we proposed that magnetically separable PU NCs be prepared from inverse nano-emulsion. To the best of our knowledge, encapsulation of MNPs within PU NCs (MNPs@PU NCs), prepared from W/O nano-emulsions, has not been thoroughly investigated [19]. We believe that the encapsulated MNPs within polymeric capsules and matrixes, in which the attributes of PU and MNPs are combined, may be of great interest and can be applied in various fields such as biotechnology, medicine, catalysis, magnetic resonance imaging, agriculture, and other environmental and industrial applications [26–29]. Therefore, the aim of this work was to synthesize and provide an elaborative study as well as to thoroughly characterize new MNPs@PU NCs prepared in a facile manner from inverse nano-emulsion. In addition, we focused on studying the effect of different parameters, such as osmotic pressure agents, amine and isocyanate monomers, solvents, and surfactants.

Here, the magnetic polyurea nano-capsules were prepared through interfacial polycondensation in W/O nano-emulsion. The synthesis involved nano-emulsification of an aqueous phase containing ionic liquid (IL) stabilized magnetite nanoparticles, amine monomers, and an oil phase containing a polymeric non-ionic surfactant. This was followed by the addition of diisocyanate monomer to initiate the interfacial polycondensation, forming a polyurea shell and MNPs encapsulated within the core.

Figure 1.

#### **2. Results and Discussions 2. Results and Discussions**

**2. Results and Discussions** 

#### *2.1. The Formation of Polyurea Nano-Capsules from Water-in-Oil (W*/*O) Nano-Emulsion 2.1. The Formation of Polyurea Nano-Capsules from Water-in-Oil (W/O) Nano-Emulsion*

*2.1. The Formation of Polyurea Nano-Capsules from Water-in-Oil (W/O) Nano-Emulsion*

Prior to the encapsulation of the MNPs, an optimal composition for the synthesis of PU NCs was established through interfacial polymerization reactions in inverse nano-emulsions, as illustrated in Figure 1. Prior to the encapsulation of the MNPs, an optimal composition for the synthesis of PU NCs was established through interfacial polymerization reactions in inverse nano-emulsions, as illustrated in Figure 1.

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Prior to the encapsulation of the MNPs, an optimal composition for the synthesis of PU NCs was established through interfacial polymerization reactions in inverse nano-emulsions, as illustrated in

**Figure 1.** Preparation of polyurea nano-capsules from water-in-oil (W/O) nano-emulsion. **Figure 1.** Preparation of polyurea nano-capsules from water-in-oil (W/O) nano-emulsion. relatively condensed interface. Furthermore, an osmotic pressure agent, termed a lipophobe, was mainly used to maintain the stability of the nano-emulsion during the polymerization process [20].

The polyurea nano-capsules were produced in two steps. Briefly, in the first step, the aqueous phase, composed of triple-distilled water (TDW), amine monomer, and lipophobe, was nanoemulsified by homogenization, followed by ultrasonication in an oil phase consisting of a non-polar organic solvent and surfactant. Subsequently, diisocyanate monomer was slowly added to the nanoemulsion system while sonication and the interfacial polycondensation were initiated to form the PU The polyurea nano-capsules were produced in two steps. Briefly, in the first step, the aqueous phase, composed of triple-distilled water (TDW), amine monomer, and lipophobe, was nano-emulsified by homogenization, followed by ultrasonication in an oil phase consisting of a non-polar organic solvent and surfactant. Subsequently, diisocyanate monomer was slowly added to the nano-emulsion system while sonication and the interfacial polycondensation were initiated to form the PU shell. The reaction between the amine and isocyanate monomers is depicted in Scheme 1. This, hinders Ostwald ripening caused by nano-emulsion polydispersity and consequently, osmotic pressure forms inside the aqueous droplets, which reduces the Laplace pressure. Hence, it prevents the formation of aggregates. In the above process, in order to obtain the optimal composition, various parameters in different proportions were tested, such as the type of surfactant, the amine and isocyanate monomers, the organic solvent, and the lipophobe in different ratios was also examined.

shell. The reaction between the amine and isocyanate monomers is depicted in Scheme 1.

organic solvent, and the lipophobe in different ratios was also examined. **Scheme 1**. The formation of polyurea through interfacial polymerization between diethylene triamine (DETA) and diphenylmethane-4,4'-diisocyanat (4,4'-MDI). **Scheme 1.** The formation of polyurea through interfacial polymerization between diethylene triamine (DETA) and diphenylmethane-4,40 -diisocyanat (4,40 -MDI).

In order to attain stable water droplets dispersed in the oil phase, it is necessary to use the right surfactant. Thus, polymeric non-ionic surfactants with low HLB values were found to be most suitable, since they sterically stabilize the nanodroplets to prevent coalescence and provide a relatively condensed interface. Furthermore, an osmotic pressure agent, termed a lipophobe, was mainly used to maintain the stability of the nano-emulsion during the polymerization process [20]. This, hinders Ostwald ripening caused by nano-emulsion polydispersity and consequently, osmotic pressure forms inside the aqueous droplets, which reduces the Laplace pressure. Hence, it prevents the formation of aggregates.

**Scheme 1**. The formation of polyurea through interfacial polymerization between diethylene triamine

In the above process, in order to obtain the optimal composition, various parameters in different proportions were tested, such as the type of surfactant, the amine and isocyanate monomers, the organic solvent, and the lipophobe in different ratios was also examined. phase and 10 wt% TDW as the dispersed phase, and diethylene triamine (DETA) and diphenylmethane-4,4'-diisocyanat (4,4'-MDI) as the amine and isocyanate monomers. Aggregates were formed after adding the diisocyanate monomer 4,4'-MDI to systems containing 1 wt% and 5 wt% of anionic surfactant (sodium 1,4-bis(2-ethylhexoxy)-1,4-dioxobutane-2-sulfonate)

were investigated using inverse nano-emulsions consisting of 90 wt% toluene as the continuous

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#### 2.1.1. Variation of the Type and Amount of Surfactant (AOT), non-ionic surfactants sorbitane monooleate (SPAN80), or polyoxyethylene (2) stearyl ether (Brij 72). Aggregates were also obtained when 1 wt% of amphipathic emulsifier lecithin was used, as

*2.1.1. Variation of the Type and Amount of Surfactant* 

In preliminary experiments, different percentages of various surfactants with low HLB values were investigated using inverse nano-emulsions consisting of 90 wt% toluene as the continuous phase and 10 wt% TDW as the dispersed phase, and diethylene triamine (DETA) and diphenylmethane-4,40 -diisocyanat (4,40 -MDI) as the amine and isocyanate monomers. confirmed by scanning electron microscopy (SEM) analysis (Figure S1a). However, aggregates with some nano-capsules having incomplete polymerization were obtained when 1 wt% nonionic polymeric surfactant Agrimer AL22 was employed (Figure S1b), whereas nano-capsules with coreshell morphology were obtained with 1 wt% cetyl polyethyleneglycol/polypropyleneglycol-10/1

Aggregates were formed after adding the diisocyanate monomer 4,40 -MDI to systems containing 1 wt% and 5 wt% of anionic surfactant (sodium 1,4-bis(2-ethylhexoxy)-1,4-dioxobutane-2-sulfonate) (AOT), non-ionic surfactants sorbitane monooleate (SPAN80), or polyoxyethylene (2) stearyl ether (Brij 72). Aggregates were also obtained when 1 wt% of amphipathic emulsifier lecithin was used, as confirmed by scanning electron microscopy (SEM) analysis (Figure S1a). However, aggregates with some nano-capsules having incomplete polymerization were obtained when 1 wt% nonionic polymeric surfactant Agrimer AL22 was employed (Figure S1b), whereas nano-capsules with core-shell morphology were obtained with 1 wt% cetyl polyethyleneglycol/polypropyleneglycol-10/1 dimethicone (ABIL EM 90) (PU-1, Table S1), as confirmed by SEM and scanning transmission electron microscopy (STEM) analysis (Figure 2). dimethicone (ABIL EM 90) (PU-1, Table S1), as confirmed by SEM and scanning transmission electron microscopy (STEM) analysis (Figure 2). The presence of the nano-capsules strongly indicates that polymeric surfactants are more compatible with the W/O system. This could be attributed to the fact that the polymeric surfactants produce several adsorption sites with negligible desorption from the emulsion interface. As shown by SEM, ABIL EM 90 provided better results than Agrimer AL22, indicating that ABIL EM 90 is more compatible in inverse nano-emulsion systems, presumably due to the enhancement of droplet stability and the strength of the interfacial film. Further optimizations, such as varying the surfactant percentage, the type and ratio of amine and isocyanate monomers, and varying the organic solvents, were carried out with ABIL EM 90 as the surfactant.

**Figure 2.** The morphology of PU NCs prepared using 1 wt% of ABIL EM 90 as the surfactant: (**a**) SEM and (**b**) STEM images. **Figure 2.** The morphology of PU NCs prepared using 1 wt% of ABIL EM 90 as the surfactant: (**a**) SEM and (**b**) STEM images.

The stability of the W/O nano-emulsion as well as the resulting PU NCs depends not only on the type—it also depends on the amount of the applied surfactant. Therefore, different percentages (0.25%, 0.5%, 1%, 2%, 3%, 4%, and 5%) of ABIL EM 90 were employed. SEM analysis clearly indicated the formation of aggregates with less than 1% surfactant (Figure S2a and S2b). Interestingly, the size and polydispersity as well as the stability of the PU NCs were hardly affected by increasing the percentage of ABIL EM 90, however, at 3% and more, the PU NCs became more attached, as observed in SEM (Figure S2e–S2g). This shows that 1% (Figure S2c) and 2% (Figure S2d) of the surfactant afforded the best results. Because it is preferable to employ the minimum amount of surfactant, 1% The presence of the nano-capsules strongly indicates that polymeric surfactants are more compatible with the W/O system. This could be attributed to the fact that the polymeric surfactants produce several adsorption sites with negligible desorption from the emulsion interface. As shown by SEM, ABIL EM 90 provided better results than Agrimer AL22, indicating that ABIL EM 90 is more compatible in inverse nano-emulsion systems, presumably due to the enhancement of droplet stability and the strength of the interfacial film. Further optimizations, such as varying the surfactant percentage, the type and ratio of amine and isocyanate monomers, and varying the organic solvents, were carried out with ABIL EM 90 as the surfactant.

ABIL EM 90 was chosen as the optimal condition and was used for additional optimization steps. *2.1.2. Variation of the Type and Percentage of the Continuous Organic Phase*  Further optimization was carried out by examining the effect of the organic solvent on the stability and morphology of the PU NCs while utilizing the same composition mentioned above with 1 wt% ABIL EM 90, DETA (2.9 mmol), and 4,4′-MDI (2.9 mmol). Aggregates were formed with cyclohexane as the continuous phase (Figure S3a). However, in the presence of heptane, a mixture of PU particles, aggregates, and NCs was obtained (Figure S3b), whereas xylene provided a result similar to toluene (Figure S3c). These diverse morphologies were obtained presumably due to The stability of the W/O nano-emulsion as well as the resulting PU NCs depends not only on the type—it also depends on the amount of the applied surfactant. Therefore, different percentages (0.25%, 0.5%, 1%, 2%, 3%, 4%, and 5%) of ABIL EM 90 were employed. SEM analysis clearly indicated the formation of aggregates with less than 1% surfactant (Figure S2a,b). Interestingly, the size and polydispersity as well as the stability of the PU NCs were hardly affected by increasing the percentage of ABIL EM 90, however, at 3% and more, the PU NCs became more attached, as observed in SEM (Figure S2e–g). This shows that 1% (Figure S2c) and 2% (Figure S2d) of the surfactant afforded the best results. Because it is preferable to employ the minimum amount of surfactant, 1% ABIL EM 90 was chosen as the optimal condition and was used for additional optimization steps.

#### 2.1.2. Variation of the Type and Percentage of the Continuous Organic Phase

Further optimization was carried out by examining the effect of the organic solvent on the stability and morphology of the PU NCs while utilizing the same composition mentioned above with 1 wt% ABIL EM 90, DETA (2.9 mmol), and 4,40 -MDI (2.9 mmol). Aggregates were formed with cyclohexane as the continuous phase (Figure S3a). However, in the presence of heptane, a mixture of PU particles, aggregates, and NCs was obtained (Figure S3b), whereas xylene provided a result similar to toluene (Figure S3c). These diverse morphologies were obtained presumably due to solubility differences regarding the isocyanate monomer in the aliphatic and aromatic continuous organic phases. In this regard, 4,40 -MDI has better solubility in toluene and xylene, compared with heptane and cyclohexane, hence, aggregates were formed in the aliphatic solvents. An additional parameter affecting the morphology is related to interfacial tension between the aqueous and organic phases. ABIL EM 90, at the specific applied concentration, seems to sufficiently lower the interfacial tension between toluene and water, rather than between cyclohexane or heptane and water, thus enhancing the stability of the nano-emulsion droplets and forming the desired nano-capsules. *Molecules* **2019**, *24*, x 5 of 15 solubility differences regarding the isocyanate monomer in the aliphatic and aromatic continuous organic phases. In this regard, 4,4′-MDI has better solubility in toluene and xylene, compared with heptane and cyclohexane, hence, aggregates were formed in the aliphatic solvents. An additional parameter affecting the morphology is related to interfacial tension between the aqueous and organic phases. ABIL EM 90, at the specific applied concentration, seems to sufficiently lower the interfacial tension between toluene and water, rather than between cyclohexane or heptane and water, thus enhancing the stability of the nano-emulsion droplets and forming the desired nano-capsules. To further examine the influence of the continuous phase on the formation and size distribution of the PU NCs, different percentages of a continuous phase (70%, 80% 85%, and 90%) consisting of toluene and ABIL EM 90 (1 wt%) were investigated. SEM analysis revealed the formation of polydispersed PU NCs systems with all percentages tested (Figure 3).

To further examine the influence of the continuous phase on the formation and size distribution of the PU NCs, different percentages of a continuous phase (70%, 80% 85%, and 90%) consisting of toluene and ABIL EM 90 (1 wt%) were investigated. SEM analysis revealed the formation of polydispersed PU NCs systems with all percentages tested (Figure 3). In addition, dynamic light scattering (DLS) studies revealed an increase in the average PU NCs size, along with a decrease in the percentage of the continuous phase and a simultaneous increase in the amount of the aqueous phase. When 70%, 80%, 85%, and 90% of the continuous phase were used, average sizes of 897 nm, 655 nm, 242 nm, and 270 nm were obtained, respectively (Figure 4).

**Figure 3.** SEM images of PU NCs prepared using different continuous: aqueous phase ratios: (**a**) 70%:30%, (**b**) 80%:20 %, and (**c**) 85 %:15 %. **Figure 3.** SEM images of PU NCs prepared using different continuous: aqueous phase ratios: (**a**) 70%:30%, (**b**) 80%:20 %, and (**c**) 85 %:15 %.

In addition, dynamic light scattering (DLS) studies revealed an increase in the average PU NCs size, along with a decrease in the percentage of the continuous phase and a simultaneous increase in the amount of the aqueous phase. When 70%, 80%, 85%, and 90% of the continuous phase were used, average sizes of 897 nm, 655 nm, 242 nm, and 270 nm were obtained, respectively (Figure 4).

#### 2.1.3. Variation of the Type of the Polyurea (PU) Monomers

*2.1.3. Variation of the Type of the Polyurea (PU) Monomers* 

**Figure 4.** The influence of continuous: dispersed phase ratios of 70%:30%, 80%:20%, 85%:15%, and 90%:10% on the PU NCs size. Initially, DETA and 4,40 -MDI were utilized as the amine and isocyanate monomers. An additional optimization step was conducted by varying the type and ratio of the amine and isocyanate monomers utilizing 1% of ABIL EM 90 and 10% of the aqueous phase containing poly(acrylamide-*co*-diallyldimethylammonium chloride (polyquaternium 7) as the lipophobe. This process appears to be essential, since the shell thickness and flexibility as well as the porosity and permeability of the PU NCs play a key role in the effectiveness and applicability of the system. Therefore, various amine monomers such DETA, ethylenediamine (EDA), 1,6-hexamethylenediamine (HMDA), and isocyanate monomers such as (PAPI 27, 4,40 -MDI, toluenediisocyanate (TDI), 1,6-hexamethylene diisocyanate (HDI), and 4,40 -methylenebis(cyclohexy isocyanate) (HMDI) were tested.

additional optimization step was conducted by varying the type and ratio of the amine and isocyanate monomers utilizing 1% of ABIL EM 90 and 10% of the aqueous phase containing 70%:30%, (**b**) 80%:20 %, and (**c**) 85 %:15 %.

isocyanate) (HMDI) were tested.

solubility differences regarding the isocyanate monomer in the aliphatic and aromatic continuous organic phases. In this regard, 4,4′-MDI has better solubility in toluene and xylene, compared with heptane and cyclohexane, hence, aggregates were formed in the aliphatic solvents. An additional parameter affecting the morphology is related to interfacial tension between the aqueous and organic phases. ABIL EM 90, at the specific applied concentration, seems to sufficiently lower the interfacial tension between toluene and water, rather than between cyclohexane or heptane and water, thus enhancing the stability of the nano-emulsion droplets and forming the desired nano-capsules.

To further examine the influence of the continuous phase on the formation and size distribution of the PU NCs, different percentages of a continuous phase (70%, 80% 85%, and 90%) consisting of toluene and ABIL EM 90 (1 wt%) were investigated. SEM analysis revealed the formation of

In addition, dynamic light scattering (DLS) studies revealed an increase in the average PU NCs size, along with a decrease in the percentage of the continuous phase and a simultaneous increase in the amount of the aqueous phase. When 70%, 80%, 85%, and 90% of the continuous phase were used,

average sizes of 897 nm, 655 nm, 242 nm, and 270 nm were obtained, respectively (Figure 4).

polydispersed PU NCs systems with all percentages tested (Figure 3).

**Figure 4.** The influence of continuous: dispersed phase ratios of 70%:30%, 80%:20%, 85%:15%, and 90%:10% on the PU NCs size. **Figure 4.** The influence of continuous: dispersed phase ratios of 70%:30%, 80%:20%, 85%:15%, and 90%:10% on the PU NCs size. hexamethylenediamine (HMDA), and isocyanate monomers such as (PAPI 27, 4,4′-MDI, toluenediisocyanate (TDI), 1,6-hexamethylene diisocyanate (HDI), and 4,4'-methylenebis(cyclohexy

*2.1.3. Variation of the Type of the Polyurea (PU) Monomers*  Initially, DETA and 4,4′-MDI were utilized as the amine and isocyanate monomers. An additional optimization step was conducted by varying the type and ratio of the amine and isocyanate monomers utilizing 1% of ABIL EM 90 and 10% of the aqueous phase containing The combination of EDA with HMDI (PU-6), PAPI 27 (PU-5), 4,40 -MDI (PU-8), or TDI (PU-7), as well as DETA with TDI (PU-4), PAPI 27 (PU-2), or HMDI (PU-3) (Table S1) afforded either aggregates or an incomplete formation of the PU shell, as observed in SEM (Figure S4). Polydispersed PU NCs with an average size of 259 nm, as revealed by SEM, transmission electron microscopy (TEM), and DLS analyses (Figure 5), were obtained when 4,40 -MDI and a mixture of DETA and EDA were utilized as the PU monomers (PU-9, Table S1). The combination of EDA with HMDI (PU-6), PAPI 27 (PU-5), 4,4′-MDI (PU-8), or TDI (PU-7), as well as DETA with TDI (PU-4), PAPI 27 (PU-2), or HMDI (PU-3) (Table S1) afforded either aggregates or an incomplete formation of the PU shell, as observed in SEM (Figure S4). Polydispersed PU NCs with an average size of 259 nm, as revealed by SEM, transmission electron microscopy (TEM), and DLS analyses (Figure 5), were obtained when 4,4′-MDI and a mixture of DETA and EDA were utilized as the PU monomers (PU-9, Table S1).

**Figure 5.** (**a**) SEM, (**b**) TEM, and (**c**) the size distribution of PU NCs prepared using 4,4′-MDI and a mixture of DETA and EDA as the monomers (PU-9). **Figure 5.** (**a**) SEM, (**b**) TEM, and (**c**) the size distribution of PU NCs prepared using 4,40 -MDI and amixture of DETA and EDA as the monomers (PU-9).

#### *2.1.4. The Influence of the Electrolyte*  2.1.4. The Influence of the Electrolyte

nano-capsule size of 365 nm (Figure 6c).

In order to suppress Ostwald ripening and obtain stable and non-aggregated capsules, an electrolyte is usually added to the aqueous phase. The influence of the electrolyte polyquaternium 7 on the size and morphology of the PU NCs was further studied using the PU-1 system. Varying the percentage of polyquaternium 7, namely, 0% (PU-10), 1% (PU-11), 2% (PU-12), and 3% (PU-13) (Table S1) had no appreciable effect on the morphology of PU NCs, as observed in SEM (Figure S5a–S5d). However, with 1% NaCl (PU-14, Table S1), aggregates and flattened capsules were obtained (Figure S5e). Additionally, DLS measurements revealed that the size distribution of PU-10 and PU-13 (Figure S6a and S6b) were similar to PU-1. These analyses clearly show that PU NCs are not affected by the electrolytes and can be formed even without utilizing them. An additional system with 80%:20% of continuous and aqueous phases respectively, without electrolyte was prepared (PU-15, Table S1). In In order to suppress Ostwald ripening and obtain stable and non-aggregated capsules, an electrolyte is usually added to the aqueous phase. The influence of the electrolyte polyquaternium 7 on the size and morphology of the PU NCs was further studied using the PU-1 system. Varying the percentage of polyquaternium 7, namely, 0% (PU-10), 1% (PU-11), 2% (PU-12), and 3% (PU-13) (Table S1) had no appreciable effect on the morphology of PU NCs, as observed in SEM (Figure S5a–d). However, with 1% NaCl (PU-14, Table S1), aggregates and flattened capsules were obtained (Figure S5e). Additionally, DLS measurements revealed that the size distribution of PU-10 and PU-13 (Figure S6a,b) were similar to PU-1. These analyses clearly show that PU NCs are not affected by the electrolytes and can be formed even without utilizing them. An additional system with 80%:20% of continuous and aqueous phases respectively, without electrolyte was prepared (PU-15, Table S1). In this case, PU

The complete polymerization of the amine with the isocyanate monomers was revealed by Fourier transform infrared (FTIR) analysis (Figure 7). The presence of absorbance peaks above 3,000 cm−1, which correspond to the overlapping stretching vibrations of the N–H and –OH (from water) groups, and an absorption band at 1,659 cm−1 assigned to C=O stretching vibrations, indicate the formation of polyurea [18]. In addition, the absence of the absorbance peak of the isocyanate group (–C=N=O) at 2,260 cm−1 indicates the complete consumption of the isocyanate monomer. NCs with a core shell structure were also formed, as confirmed by SEM and TEM (Figure 6a,b. DLS analysis, in agreement with SEM, revealed a polydispersed system with an average nano-capsule size of 365 nm (Figure 6c). hybridized C–H, and the peak at 3,028 cm−1 is attributed to sp2 C–H stretching vibrations. In addition, absorbance bands at 1,541 and 1,095 cm−1 are ascribed to the secondary N–H bending and the C–N stretching vibrations, respectively. *Molecules* **2019**, *24*, x 7 of 15 Furthermore, absorbance peaks at 1,599 cm−1 and 1,409 cm−1 can be attributed to C=C stretching

vibrations of aromatic rings. The absorbance bands at 2,852 and 2,922 cm−1 are ascribed to sp3-

*Molecules* **2019**, *24*, x 7 of 15

**Figure 6.** (**a**) SEM, (**b**) TEM, and (**c**) the size distribution of PU-15 prepared using 80%:20% of the continuous and dispersed phases, respectively. **Figure 6.** (**a**) SEM, (**b**) TEM, and (**c**) the size distribution of PU-15 prepared using 80%:20% of the continuous and dispersed phases, respectively.

The formation of the PU NCs (PU-15) was further confirmed by solid state carbon nuclear magnetic resonance (13C CP-MAS NMR) (Figure 8). Peaks at 21–48 ppm and 115–135 ppm are attributed to the aliphatic (from the DETA and EDA monomer) and aromatic (from 4,4'-MDI monomer) carbons of the PU shell. The peak at 156 ppm is ascribed to the carbonyl group (C=O) of the urea, indicating the formation of polyurea [18,30]. The complete polymerization of the amine with the isocyanate monomers was revealed by Fourier transform infrared (FTIR) analysis (Figure 7). The presence of absorbance peaks above 3000 cm−<sup>1</sup> , which correspond to the overlapping stretching vibrations of the N–H and –OH (from water) groups, and an absorption band at 1659 cm−<sup>1</sup> assigned to C=O stretching vibrations, indicate the formation of polyurea [18]. In addition, the absence of the absorbance peak of the isocyanate group (–C=N=O) at 2260 cm−<sup>1</sup> indicates the complete consumption of the isocyanate monomer. Furthermore, absorbance peaks at 1599 cm−<sup>1</sup> and 1409 cm−<sup>1</sup> can be attributed to C=C stretching vibrations of aromatic rings. The absorbance bands at 2852 and 2922 cm−<sup>1</sup> are ascribed to sp3-hybridized C–H, and the peak at 3028 cm−<sup>1</sup> is attributed to sp2 C–H stretching vibrations. In addition, absorbance bands at 1541 and 1095 cm−<sup>1</sup> are ascribed to the secondary N–H bending and the C–N stretching vibrations, respectively. **Figure 6.** (**a**) SEM, (**b**) TEM, and (**c**) the size distribution of PU-15 prepared using 80%:20% of the continuous and dispersed phases, respectively. The formation of the PU NCs (PU-15) was further confirmed by solid state carbon nuclear magnetic resonance (13C CP-MAS NMR) (Figure 8). Peaks at 21–48 ppm and 115–135 ppm are attributed to the aliphatic (from the DETA and EDA monomer) and aromatic (from 4,4'-MDI monomer) carbons of the PU shell. The peak at 156 ppm is ascribed to the carbonyl group (C=O) of the urea, indicating the formation of polyurea [18,30].

**Figure 7. Figure 7.** The FTIR spectrum of PU-15. The FTIR spectrum of PU-15.

IL-C4@PU NCs.

The formation of the PU NCs (PU-15) was further confirmed by solid state carbon nuclear magnetic resonance (13C CP-MAS NMR) (Figure 8). Peaks at 21–48 ppm and 115–135 ppm are attributed to the aliphatic (from the DETA and EDA monomer) and aromatic (from 4,40 -MDI monomer) carbons of the PU shell. The peak at 156 ppm is ascribed to the carbonyl group (C=O) of the urea, indicating the formation of polyurea [ *Molecules*  18,30]. **2019**, *24*, x 8 of 15

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**Figure 8.** The 13C CP-MAS NMR of PU NCs (PU-15); \* indicates the spinning sideband. **Figure 8.** The <sup>13</sup>C CP-MAS NMR of PU NCs (PU-15); \* indicates the spinning sideband. of the nano-catalysts and catalyst nano-supports by applying an external magnetic field. Herein, we

#### *2.2. Encapsulation of Ionic Liquid-Modified Magnetite Nanoparticles (MNPs-IL-C4@PU NCs) 2.2. Encapsulation of Ionic Liquid-Modified Magnetite Nanoparticles (MNPs-IL-C4@PU NCs)* have prepared magnetic PU NCs in an inverse nano-emulsion (Figure 9a), which could have great potential in the above-mentioned fields.

Magnetic polymer nanomaterials that combine the properties of organic and inorganic components have been fabricated for various applications in biomedical, environmental, sensing, Magnetic polymer nanomaterials that combine the properties of organic and inorganic components have been fabricated for various applications in biomedical, environmental, sensing, drug delivery, and magnetic resonance imaging (MRI) [31–36]. Incorporating MNPs within the PU NCs would impart superparamagnetic properties to the NCs, enabling them to be suitable for various applications such as magnetic nano-catalysts, which can be conveniently recovered under an external magnetic field (Figure 9b). We encapsulated

drug delivery, and magnetic resonance imaging (MRI) [31–36]. Magnetic nanoparticles have also been applied in catalysis to facilitate the isolation and recovery of the nano-catalysts and catalyst nano-supports by applying an external magnetic field. Herein, we have prepared magnetic PU NCs in an inverse nano-emulsion (Figure 9a), which could have great potential in the above-mentioned fields. Magnetic nanoparticles have also been applied in catalysis to facilitate the isolation and recovery of the nano-catalysts and catalyst nano-supports by applying an external magnetic field. Herein, we have prepared magnetic PU NCs in an inverse nano-emulsion (Figure 9a), which could have great potential in the above-mentioned fields. different percentages (1–11 wt% with respect to the aqueous phase) of pre-prepared and stabilized MNPs with ionic liquid-based silane, 1-butyl-3-(3-(trimethoxysilyl)propyl)-1H- imidazol-3- chloride (IL-C4) [25], (MNPs-IL-C4) with an average size of 5–20 nm in the core of PU NCs using a composition employed for preparing the PU-15 system (Table 1).

**Figure 9.** (**a**) MNPs-IL-C4@PU NCs as suspension and powder and (**b**) a magnetic separation of MNPs-IL-C4@PU NCs. **Figure 9.** (**a**) MNPs-IL-C4@PU NCs as suspension and powder and (**b**) a magnetic separation of MNPs-IL-C4@PU NCs.

The surface morphology of PU-15a-15g was probed by SEM (Figure 10) and TEM (Figure 11) analyses, which confirmed the formation of PU NCs with MNPs-IL-C4 encapsulated in the core of the NCs. Since MNPs-IL-C4 is hydrophilic, it does not dissolve in the non-polar continuous phase, therefore, non-encapsulated MNPs were not observed. Additionally, the size distribution of MNPs-IL-C4@PU NCs decreased, compared with pure PU NCs, however, increasing the amount of MNPs Incorporating MNPs within the PU NCs would impart superparamagnetic properties to the NCs, enabling them to be suitable for various applications such as magnetic nano-catalysts, which can be conveniently recovered under an external magnetic field (Figure 9b). We encapsulated different percentages (1–11 wt% with respect to the aqueous phase) of pre-prepared and stabilized MNPs with ionic liquid-based silane, 1-butyl-3-(3-(trimethoxysilyl)propyl)-1H- imidazol-3- chloride (IL-C4) [25],

The surface morphology of PU-15a-15g was probed by SEM (Figure 10) and TEM (Figure 11) analyses, which confirmed the formation of PU NCs with MNPs-IL-C4 encapsulated in the core of the NCs. Since MNPs-IL-C4 is hydrophilic, it does not dissolve in the non-polar continuous phase, therefore, non-encapsulated MNPs were not observed. Additionally, the size distribution of MNPs-IL-C4@PU NCs decreased, compared with pure PU NCs, however, increasing the amount of MNPs

**Figure 9.** (**a**) MNPs-IL-C4@PU NCs as suspension and powder and (**b**) a magnetic separation of MNPs-

(MNPs-IL-C4) with an average size of 5–20 nm in the core of PU NCs using a composition employed for preparing the PU-15 system (Table 1). had little effect on the polydispersity and average size distribution (Table 1, Figure S7). Moreover, FTIR analysis of PU-15a-15g exhibited similar results as pure PU NCs (PU-15), confirming the

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**Table 1.** Composition and encapsulation of MNPs-IL-C<sup>4</sup> in PU-15 <sup>a</sup> . formation of polyurea shell (Figure S8).

<sup>a</sup> Continuous phase (40 g, 80%) consisting of toluene (37.5 g) and ABIL EM 90 (2.5 g, 5 wt%). Dispersed phase (10 g, 20%) consisting of TDW (9.53 g), DETA (2.9 mmol), and EDA (2.9 mmol). Isocyanate (5.83 mmol) was dissolved in 10 g of total toluene and slowly added to the nano-emulsion system. <sup>b</sup> The average size was measured by dynamic light scattering (DLS). PU-15e DETA/EDA 4,4′-MDI 7% 198.5 PU-15f DETA/EDA 4,4′-MDI 9% 246.8 PU-15g DETA/EDA 4,4′-MDI 11% 238.7

a Continuous phase (40 g, 80%) consisting of toluene (37.5 g) and ABIL EM 90 (2.5 g, 5 wt%). Dispersed

The surface morphology of PU-15a-15g was probed by SEM (Figure 10) and TEM (Figure 11) analyses, which confirmed the formation of PU NCs with MNPs-IL-C<sup>4</sup> encapsulated in the core of the NCs. Since MNPs-IL-C<sup>4</sup> is hydrophilic, it does not dissolve in the non-polar continuous phase, therefore, non-encapsulated MNPs were not observed. Additionally, the size distribution of MNPs-IL-C4@PU NCs decreased, compared with pure PU NCs, however, increasing the amount of MNPs had little effect on the polydispersity and average size distribution (Table 1, Figure S7). Moreover, FTIR analysis of PU-15a-15g exhibited similar results as pure PU NCs (PU-15), confirming the formation of polyurea shell (Figure S8). phase (10 g, 20%) consisting of TDW (9.53 g), DETA (2.9 mmol), and EDA (2.9 mmol). Isocyanate (5.83 mmol) was dissolved in 10 g of total toluene and slowly added to the nano-emulsion system. b The average size was measured by dynamic light scattering (DLS). STEM/energy dispersive X-ray spectroscopy (EDS) and element mapping analyses were carried out in order to obtain more information about the composition and structure of the MNPs-IL-C4@PU NCs. STEM/EDS analysis (Figure 12a) also confirmed the presence of an iron element content of the MNPs in the core of the PU NCs and showed the existence of an Si element attributed to the silane group of IL-C4 supported on the MNPs and of ABIL EM 90.

**Figure 10.** SEM images of MNPs-IL-C4@PU NCs. (**a**) 1% (PU-15a), (**b**) 2% (PU-15b), (**c**) 3% (PU-15c), (**d**) 5% (PU-15d), (**e**) 7% (PU-15e), (**f**) 9% (PU-15f), and (**g**) 11% (PU-15g) of encapsulated MNPs-IL-C4. **Figure 10.** SEM images of MNPs-IL-C4@PU NCs. (**a**) 1% (PU-15a), (**b**) 2% (PU-15b), (**c**) 3% (PU-15c), (**d**) 5% (PU-15d), (**e**) 7% (PU-15e), (**f**) 9% (PU-15f), and (**g**) 11% (PU-15g) of encapsulated MNPs-IL-C<sup>4</sup> .

*Molecules* **2019**, *24*, x 10 of 15

*Molecules* **2019**, *24*, x 10 of 15

**Figure 11.** TEM images of MNPs-IL-C4@PU NCs. (**a**) 1% (PU-15a), (**b**) 2% (PU-15b), (**c**) 3% (PU-15c), (**d**) 5% (PU-15d), (**e**) 7% (PU-15e), (**f**) 9% (PU-15f), and (**g**) 11% (PU-15g) of encapsulated MNPs-IL-C4. **Figure 11.** TEM images of MNPs-IL-C4@PU NCs. (**a**) 1% (PU-15a), (**b**) 2% (PU-15b), (**c**) 3% (PU-15c), (**d**) 5% (PU-15d), (**e**) 7% (PU-15e), (**f**) 9% (PU-15f), and (**g**) 11% (PU-15g) of encapsulated MNPs-IL-C<sup>4</sup> . The elemental distribution of MNPs-IL-C4@PU NCs was further probed by EDS mapping

(**d**) 5% (PU-15d), (**e**) 7% (PU-15e), (**f**) 9% (PU-15f), and (**g**) 11% (PU-15g) of encapsulated MNPs-IL-C4.

The elemental distribution of MNPs-IL-C4@PU NCs was further probed by EDS mapping analysis (Figure 12b). In agreement with TEM, EDS mapping displayed the distribution of iron (Fe), indicating that the MNPs are located in the core of the PU NCs (Figure 12b, blue map). When comparing the distribution zones of C with the Si and Fe elements, it can be clearly seen that the C element (Figure 12b, yellow map) of the PU skeleton is distributed throughout all the areas of the STEM/energy dispersive X-ray spectroscopy (EDS) and element mapping analyses were carried out in order to obtain more information about the composition and structure of the MNPs-IL-C4@PU NCs. STEM/EDS analysis (Figure 12a) also confirmed the presence of an iron element content of the MNPs in the core of the PU NCs and showed the existence of an Si element attributed to the silane group of IL-C<sup>4</sup> supported on the MNPs and of ABIL EM 90. analysis (Figure 12b). In agreement with TEM, EDS mapping displayed the distribution of iron (Fe), indicating that the MNPs are located in the core of the PU NCs (Figure 12b, blue map). When comparing the distribution zones of C with the Si and Fe elements, it can be clearly seen that the C element (Figure 12b, yellow map) of the PU skeleton is distributed throughout all the areas of the NCs, whereas the Si element (Figure 12b, orange map), which is a component of silane IL-C4 and ABIL EM 90, is more localized in the center.

**Figure 12.** (**a**) STEM/ EDS and (**b**) EDS mapping analyses of MNPs-IL-C4@PU NCs (PU-15b). **Figure 12.** (**a**) STEM/ EDS and (**b**) EDS mapping analyses of MNPs-IL-C4@PU NCs (PU-15b).

**Figure 12.** (**a**) STEM/ EDS and (**b**) EDS mapping analyses of MNPs-IL-C4@PU NCs (PU-15b). The composition of the PU NCs, pure MNPs, and MNPs-IL-C4@PU NCs was also probed by Xray powder diffraction (XRD). The XRD pattern of PU NCs (PU-15) displayed a broad peak in the The composition of the PU NCs, pure MNPs, and MNPs-IL-C4@PU NCs was also probed by Xray powder diffraction (XRD). The XRD pattern of PU NCs (PU-15) displayed a broad peak in the range 2θ = 10–30°, which is attributed to the amorphous polyurea. The XRD of PU-15a, PU-15d, and The elemental distribution of MNPs-IL-C4@PU NCs was further probed by EDS mapping analysis (Figure 12b). In agreement with TEM, EDS mapping displayed the distribution of iron (Fe), indicating that the MNPs are located in the core of the PU NCs (Figure 12b, blue map). When comparing the distribution zones of C with the Si and Fe elements, it can be clearly seen that the C element (Figure 12b, yellow map) of the PU skeleton is distributed throughout all the areas of the NCs, whereas the Si

range 2θ = 10–30°, which is attributed to the amorphous polyurea. The XRD of PU-15a, PU-15d, and

element (Figure 12b, orange map), which is a component of silane IL-C<sup>4</sup> and ABIL EM 90, is more localized in the center.

The composition of the PU NCs, pure MNPs, and MNPs-IL-C4@PU NCs was also probed by X-ray powder diffraction (XRD). The XRD pattern of PU NCs (PU-15) displayed a broad peak in the range 2θ = 10–30◦ , which is attributed to the amorphous polyurea. The XRD of PU-15a, PU-15d, and PU-15g revealed the presence of amorphous polyurea and showed the characteristic peaks of MNPs at 2θ = 18.1◦ , 30.2◦ , 35.6◦ , 43.2◦ , 53.6◦ , 57.2◦ , and 62.8◦ (Figure 13). *Molecules* **2019**, *24*, x 11 of 15 PU-15g revealed the presence of amorphous polyurea and showed the characteristic peaks of MNPs at 2θ = 18.1°, 30.2°, 35.6°, 43.2°, 53.6°, 57.2°, and 62.8° (Figure 13). *Molecules* **2019**, *24*, x 11 of 15 PU-15g revealed the presence of amorphous polyurea and showed the characteristic peaks of MNPs at 2θ = 18.1°, 30.2°, 35.6°, 43.2°, 53.6°, 57.2°, and 62.8° (Figure 13).

**Figure 13.** XRD pattern of (**a**) PU NCs (PU-15) and MNPs-IL-C4@PU NCs and (**b**) pure MNPs. **Figure 13.** XRD pattern of (**a**) PU NCs (PU-15) and MNPs-IL-C4@PU NCs and (**b**) pure MNPs.

The thermogravimetric analysis (TGA) of pure PU NCs and MNPs-IL-C4@PU NCs (1–11%) over

The thermogravimetric analysis (TGA) of pure PU NCs and MNPs-IL-C4@PU NCs (1–11%) over a temperature range of 25–950 °C under an inert atmosphere and at a heating rate of 10 °C/min revealed that pure PU NCs (PU-15) have two degradation steps with a total 94.7% weight loss (Figure 14). The first decomposition step was observed at 125–390 °C, which is attributed to the initial decomposition of the PU shell, toluene, and water [30,37]. The second step, which was at a temperature higher than 390 °C, is attributed to the additional decomposition of PU. The TGA curves of MNPs-IL-C4@PU NCs revealed that when the 1, 2, 3, 5, 7, 9, or 11 wt% of MNPs per aqueous phase was added during the encapsulation process, the measured weight percentage of MNPs was 45.02%, 50.59%, 52.63%, 52.79%, 53.03%, 54.85%, or 55.64%, respectively. Moreover, TGA gives an indication of the stability of the PU NCs. It was clearly seen that all MNPs-IL-C4@PU NCs systems exhibited three degradation steps with the initial decomposition temperature at ~220 °C. This indicates that the presence of MNPs-IL-C4 increased the thermal stability of the PU NCs. The degradation steps at 220– 500 °C and >500 °C are attributed to the decomposition of the PU shell and the IL group attached. Theoretically, pure PU should exhibit a 100% weight loss, since it is composed of pure organic material. However, PU-15 revealed the presence of 5.3% of non-decomposable material, which could be attributed to the inorganic surfactant ABIL-EM 90 and some species formed during the heating The thermogravimetric analysis (TGA) of pure PU NCs and MNPs-IL-C4@PU NCs (1–11%) over a temperature range of 25–950 ◦C under an inert atmosphere and at a heating rate of 10 ◦C/min revealed that pure PU NCs (PU-15) have two degradation steps with a total 94.7% weight loss (Figure 14). The first decomposition step was observed at 125–390 ◦C, which is attributed to the initial decomposition of the PU shell, toluene, and water [30,37]. The second step, which was at a temperature higher than 390 ◦C, is attributed to the additional decomposition of PU. The TGA curves of MNPs-IL-C4@PU NCs revealed that when the 1, 2, 3, 5, 7, 9, or 11 wt% of MNPs per aqueous phase was added during the encapsulation process, the measured weight percentage of MNPs was 45.02%, 50.59%, 52.63%, 52.79%, 53.03%, 54.85%, or 55.64%, respectively. Moreover, TGA gives an indication of the stability of the PU NCs. It was clearly seen that all MNPs-IL-C4@PU NCs systems exhibited three degradation steps with the initial decomposition temperature at ~220 ◦C. This indicates that the presence of MNPs-IL-C<sup>4</sup> increased the thermal stability of the PU NCs. The degradation steps at 220–500 ◦C and >500 ◦C are attributed to the decomposition of the PU shell and the IL group attached. Theoretically, pure PU should exhibit a 100% weight loss, since it is composed of pure organic material. However, PU-15 revealed the presence of 5.3% of non-decomposable material, which could be attributed to the inorganic surfactant ABIL-EM 90 and some species formed during the heating process. a temperature range of 25–950 °C under an inert atmosphere and at a heating rate of 10 °C/min revealed that pure PU NCs (PU-15) have two degradation steps with a total 94.7% weight loss (Figure 14). The first decomposition step was observed at 125–390 °C, which is attributed to the initial decomposition of the PU shell, toluene, and water [30,37]. The second step, which was at a temperature higher than 390 °C, is attributed to the additional decomposition of PU. The TGA curves of MNPs-IL-C4@PU NCs revealed that when the 1, 2, 3, 5, 7, 9, or 11 wt% of MNPs per aqueous phase was added during the encapsulation process, the measured weight percentage of MNPs was 45.02%, 50.59%, 52.63%, 52.79%, 53.03%, 54.85%, or 55.64%, respectively. Moreover, TGA gives an indication of the stability of the PU NCs. It was clearly seen that all MNPs-IL-C4@PU NCs systems exhibited three degradation steps with the initial decomposition temperature at ~220 °C. This indicates that the presence of MNPs-IL-C4 increased the thermal stability of the PU NCs. The degradation steps at 220– 500 °C and >500 °C are attributed to the decomposition of the PU shell and the IL group attached. Theoretically, pure PU should exhibit a 100% weight loss, since it is composed of pure organic material. However, PU-15 revealed the presence of 5.3% of non-decomposable material, which could be attributed to the inorganic surfactant ABIL-EM 90 and some species formed during the heating process.

**Figure 14.** TGA analysis of pure PU NCs (PU-15) and MNPs-IL-C4@PU NCs (1%–11%). **Figure 14.** TGA analysis of pure PU NCs (PU-15) and MNPs-IL-C4@PU NCs (1–11%).

#### **Figure 14.** TGA analysis of pure PU NCs (PU-15) and MNPs-IL-C4@PU NCs (1%–11%). **3. Materials and Methods 3. Materials and Methods**

process.

**3. Materials and Methods**  Cetyl polyethyleneglycol/polypropyleneglycol-10/1 di-methicone (ABIL EM 90) was denoted by Sol-Gel Technologies (Ness Ziona, Israel); poly(1-ethenylpyrrolidin-2-one/hexadec-1-ene) (Agrimer Cetyl polyethyleneglycol/polypropyleneglycol-10/1 di-methicone (ABIL EM 90) was denoted by Sol-Gel Technologies (Ness Ziona, Israel); poly(1-ethenylpyrrolidin-2-one/hexadec-1-ene) (Agrimer

Cetyl polyethyleneglycol/polypropyleneglycol-10/1 di-methicone (ABIL EM 90) was denoted by

(Ewing, NJ, USA). FeCl3 × 6H2O, FeCl2 × 4H2O, and ammonium hydroxide 25% were purchased from Acros Fischer Scientific through their distributor in Israel, Holland Moran LTD (Yehud, Israel). (3 chloropropyl) trimethoxysilane and 1-butyl imidazole were purchased from Sigma Aldrich (Rehovot, Israel). All amine and isocyanate monomers were purchased from Sigma-Aldrich or Acros Fischer.

chloropropyl) trimethoxysilane and 1-butyl imidazole were purchased from Sigma Aldrich (Rehovot, Israel). All amine and isocyanate monomers were purchased from Sigma-Aldrich or Acros Fischer.

AL 22) and polymethylene-polyphenyl isocyanate (PAPI 27) were contributed by FMC Corporation (Ewing, NJ, USA). FeCl<sup>3</sup> × 6H2O, FeCl<sup>2</sup> × 4H2O, and ammonium hydroxide 25% were purchased from Acros Fischer Scientific through their distributor in Israel, Holland Moran LTD (Yehud, Israel). (3-chloropropyl) trimethoxysilane and 1-butyl imidazole were purchased from Sigma Aldrich (Rehovot, Israel). All amine and isocyanate monomers were purchased from Sigma-Aldrich or Acros Fischer.

Scanning electron microscopy (SEM) was utilized to determine the morphology of the PU NCs. The SEM analyses were carried out using a high-resolution scanning electron microscope (HR SEM) Sirion (FEI Company, Hillsboro, OR, USA) using a Schottky-type field emission source and a secondary electron detector. The images were scanned at a voltage of 5 kV. Transmission electron microscopy (TEM) and scanning transmission electron microscopy/energy dispersive X-ray spectroscopy (STEM/EDS) were performed with (S) TEM Tecnai F20 G2 (FEI Company, Hillsboro, OR, USA) operated at 200 kV. The Fourier transform infrared spectra (FTIR) were recorded at room temperature in transmission mode using a Perkin Elmer spectrometer 65 FTIR instrument (Waltham, MA, USA). Thermogravimetric analysis (TGA) was performed on a Mettler Toledo TG 50 analyzer (Greifensee, Switzerland). Measurements were carried out over a temperature range of 25–950 ◦C, at a heating rate of 10 ◦C/min under nitrogen. Dynamic light scattering (DLS) was utilized to determine the size distribution of the PU-NCs. These measurements were performed on a Nano Series instrument of model Nano-Zeta Sizer (Malvern Instruments, Worcestershire, United Kingdom) model ZEN3600. Powder X-ray diffraction (XRD) measurements were performed on a D8 Advance diffractometer (Bruker AXS, Karlsruhe, Germany) with a goniometer radius of 217.5 mm, a secondary graphite monochromator, with 2◦ Sollers slits, and a 0.2 mm receiving slit. Low-background quartz sample holders were carefully filled with the powder samples. XRD patterns within the range 2θ = 1 ◦ to 90◦ were recorded at room temperature using CuKα radiation (λ = 1.5418 Å) with the following measurement conditions: a tube voltage of 40 kV, a tube current of 40 mA, step-scan mode with a step size of 2θ = 0.02◦ , and a counting time of 1 s/step. A solid-state <sup>13</sup>C NMR spectrum was recorded with a Bruker DRX-500 instrument (Rheinstetten, Germany).

#### *3.1. Synthesis of 1-Butyl-3-(3-(Trimethoxysilyl)Propyl)-1H-Imidazol-3-Cholride (IL-C4)*

IL-C<sup>4</sup> was synthesized using procedures reported previously [25]. Briefly, 3-chloropropyltrimethoxysilane (14.2 g, 114.3 mmol) and 1-butylimidazole (22.73 g, 114.3 mmol) were stirred under an inert atmosphere at 120 ◦C for 24 h. The mixture was cooled to room temperature to obtain a yellow-orange viscous liquid (35.32 g, 96% yield).

#### *3.2. Preparation of Magnetite Nanoparticle-Supported IL-C<sup>4</sup> (MNPs-IL-C4)*

MNPs-IL-C<sup>4</sup> was synthesized according to the procedure reported earlier [25].

#### *3.3. Preparation of Polyurea Nano-Capsules (PU NCs)*

The PU NCs were prepared in a typical procedure through interfacial polymerization in W/O nano-emulsion. The optimal PU NCs were prepared as follows: the continuous phase (organic phase, 40 g, 80%) consisted of toluene (37.5 g) and ABIL EM 90 (2.5 g, 5 wt%) was homogenized at 10,000 rpm for 30 s. A dispersed phase (aqueous phase, 10 g, 20%) consisted of triple-distilled water (TDW, 9.53 g), diethylene triamine (DETA, 2.9 mmol), and ethylenediamine (EDA, 2.9 mmol), which were then rapidly added during homogenization. The emulsification process was carried out for a further 1.5 min at 10,000 rpm, followed by sonication for 10 min using an ultrasonic cell disrupter with an output of 130 Watt and 20 KHz. Eventually, diphenylmethane-4,40 -diisocyanate (4,40 -MDI, 5.83 mmol), dissolved in 10 g total toluene, was slowly added to the nano-emulsion system during sonication. The mixture was then stirred for 3 h at room temperature. The resulting PU NCs were collected by centrifugation at 11,000 rpm for 15 min, washed two times with toluene, and finally re-dispersed in toluene to reach a 10 g suspension total.

#### *3.4. Preparation of Magnetic PU NCs (MNPs-IL-C4@PU NCs)*

Magnetic PU NCs were prepared by encapsulation of MNPs-IL-C<sup>4</sup> in the PU NCs. The desired percentages of MNPs-IL-C<sup>4</sup> were dispersed in the aqueous phase and sonicated until all MNPs were fully dispersed. The encapsulation process was achieved by following the same procedure and using the same amount of surfactant and components as described in the preparation of PU NCs. Finally, the reaction mixture was mechanically stirred at room temperature for 3 h. The resulting magnetic PU NCs were separated by an external magnetic field and re-dispersed in toluene.

#### **4. Conclusions**

PU NCs with magnetite nanoparticles encapsulated within the aqueous core were prepared via interfacial polycondensation in inverse nano-emulsion.

Various parameters such as surfactant, solvents, monomers, and lipophobes were thoroughly examined in different ratios and compositions. The obtained systems were characterized for their morphology, chemical composition, thermal stability, size, and encapsulation efficiency. Among the examined parameters and conditions, it was found that the polymeric non-ionic surfactant with a low HLB value, ABIL EM 90, proved to be the best stabilizer for the inverse nano-emulsion and hence, for the established NCs. The desired PU NCs were obtained with DETA, EDA, and 4,40 -MDI as the PU monomers.

The morphology and size distribution of PU NCs were not affected by increasing the percentage of polyquaternium 7, indicating that the NCs can be formulated even without employing electrolytes as osmotic pressure agents. The encapsulation efficiency of PU NCs was examined by encapsulating up to 11% of MNPs-IL-C4. The presence of the MNPs, regardless of the percentage, resulted in increased thermal stability of the PU NCs, as confirmed by TGA analysis. SEM and TEM analyses confirmed that the MNPs-IL-C<sup>4</sup> were not adsorbed on the capsules' shell but rather, were encapsulated in the aqueous core. Nonetheless, the average size distribution of the magnetic PU NCs decreased when compared with pure PU NCs. This could be due to the imidazolium group on the MNPs causing a reduction of Ostwald ripening. Owing to the facile synthesis and biocompatibility of PU, the magnetic properties of MNPs, and the expeditious magnetic separation of the system, the proposed magnetic PU NCs systems may be utilized in various applications such as catalysis, targeted delivery of hydrophilic drugs, and in other biomedical applications both in academia and industry.

**Supplementary Materials:** The following are available online. Figure S1–S5: SEM images, Table S1: System composition for preparing PU NCs, Figure S6–S7: Size distribution of pure PU NCs and MNPs-IL-C4@PU NCs, Figure S8: FTIR spectra.

**Author Contributions:** S.N. prepared the nano-capsules. R.A.R and A.L.-Z. conceptualized the project, R.A.-R. supervised the experiments, R.A.-R and A.L.-Z. acquired the funding, S.N. prepared the original draft of the manuscript, and all coauthors contributed to writing the manuscript.

**Funding:** This research was funded by the Israel Ministry of Agriculture, [grant number 131-1595].

**Acknowledgments:** This work was supported by the Chief Scientist, the Israel Ministry of Agriculture grant # 131-1595. We are also grateful to the Ministry of Science, Technology, and Space for the fellowship of Suzana Natour. We thank Inna Popov and Vladimir Uvarov for helping with the TEM and XRD analysis. Suzana Natour thanks Rajashekharayya Sanguramath for productive discussions.

**Conflicts of Interest:** The authors declare no conflicts of interest. The funders had no role in the design of the study, nor in the collection, analyses, or interpretation of the data, in the writing of the manuscript, or in the decision to publish the results.

#### **References**


**Sample Availability:** Samples of the compound 1-butyl-3-(3-(trimethoxysilyl)propyl)-1*H*-imidazol-3-cholride and MNPs-IL-C4@PU NCs are available from the authors.

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

*Article*
