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**Figure 5.** Tensile strength of the blends (see Section 3.2 for definition).

**%OHQGFRPSRVLWLRQFigure6.**Elongationatbreakofthe blends(seeSection3.2fordefinition).

**Figure 7.** Tensile modulus of the blends (see Section 3.2 for definition).

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According to the literature and in agreemen<sup>t</sup> with our tensile results for the ternary systems, it is often observed that the localization of an elastomer copolymer at the interface can convert glassy polymer blends into a much more deformable/ductile material having a distinct phase structure and mechanical properties [22,34]. Based on the results of the interfacial analysis (Table 1) and phase morphology (Figures 2–4), ternary blends based on PEO have a finer morphology, leading to superior tensile properties compared to the blends containing PBAT (Figure 5). For the same blend composition, the higher tensile strength of the PA25/(PEO10) blend (49.6 MPa) compared to that of PA25/(PBAT10)

with 46.8 MPa may be related to the possibility of dipole–dipole interactions between the ester groups of PLA and the ether groups of PEO as well as hydrogen bonding between the amide groups of PA11 and the ether oxygens of PEO, inducing better compatibility between PLA and PA11 and leading to the higher tensile strength of the corresponding sample [35,36]. According to the spreading coefficient predictions and SEM images, PEO is able to spread at the interface in PL50/PA50, providing a soft and deformable interfacial area or surrounding PA11 droplets in PL70/PA30, forming a core–shell structure (PA11 encapsulated by PEO) in the PLA matrix. This action promotes uniform particles distribution and consequently a better mixing state, leading to improved stress transfer between PLA and PA11 under tension (delayed fracture process). The ternary blends containing PEO yield higher deformation than PBAT due to the better interactions between PLA and PA11 and increased PLA chain mobility, as PEO addition resulted in a significant increase (104%) in elongation at break of PA40/(PEO20\*) compared to that of PA40/(PBAT20\*) with an elongation at break of 42.8% and PL50/PA50 with 7.9% (Figure 6). It has been reported that increasing the PLA chain mobility through the plasticization effect of PEO may lead to better interfacial adhesion and a more homogenous phase structure generating smooth stress transfer between PLA and PA11 as well as a homogenous structure with less particles coalescence and smaller size in PLA/PA11 blends. Heshmati and Favis [7] also observed substantial elongation at break increases for a PL50/PA50 blend from 6% to 100% after the addition of 10% PEO copolymer, which was attributed to its plasticization effect and the presence of a more elastic content (elongation at break of PEO = 516.4%, see Table 2) inducing higher deformation/elasticity, which was previously reported in similar works [37]. The decreasing trend of tensile moduli can be ascribed to the weaker intermolecular interactions of PLA upon PEO addition and improved rearrangemen<sup>t</sup> between polymer chains under external force to induce flexibility to PLA [38]. Figure 7 shows a decrease in the tensile modulus of the ternary blends upon addition of the elastomer phase with low inherent modulus (PEO = 73.6 MPa and PBAT = 105 MPa, see Table 2) and their very low glass transition temperatures (PEO = −67 ◦C [39] and PBAT = −30 ◦C [30]), which is typically reported for rubber-toughened plastic blends. For example, increasing the PEO content from 10 to 20 wt % showed a decreasing trend of tensile modulus for PL70/PA30 from 1167 to 1017 MPa and 946 MPa for the ternary systems attributed to the substitution of the rigid components (PLA and PA11) with a soft rubbery component (PEO) of low rigidity. In similar reports, Nofar et al. [40] observed important drops in the tensile modulus of PLA from 1800 to 1200–1250 MPa upon the addition of 25% PBAT or poly(butylene succinate-co-butylene adipate) (PBSA) (minor dispersed phases) due to the very low modulus of these elastomers, which is in line with the findings of Wu and Zhang [41] reporting significant decreases in PLA tensile modulus from 1.8 to 1.1 GPa upon the addition of 10% acrylonitrile–butadiene–styrene (ABS).

## 2.3.2. Impact Properties

The impact strength as a function of blend composition is presented in Figure 8. The concentration of elastomer (PEO and PBAT) was changed from 0 to 20 wt % in the ternary blends to examine its effect on interfacial interaction and impact strength. As expected, PLA, as a very brittle polymer, shows low impact strength (15.4 J/m). A partial miscibility of PLA with polyamide is reported in the literature due to hydrogen bonding between the ester groups of PLA and amine groups of polyamides. However, such interactions are not able to induce enough compatibility and good deformation to improve the impact strength of neat PLA/PA11 blends [15]. The blending of PLA and PA11, as very rigid polymers, results in the formation of a rigid interface that cannot smoothly transfer interfacial stresses in PLA/PA11 blends, resulting in easy crack initiation and propagation along these interfaces and leading to the low impact strength of the 70/30 (22.5 J/m) and 50/50 (37.4 J/m) blends. As expected, the notched Charpy impact strength follows the same trend as the elongation at break upon elastomer (PEO or PBAT) addition. The presence of an elastomer phase changes the rigid interface into a much more deformable area if the copolymer is

localized at the PLA/PA11 interface, thereby increasing failure resistance through effective load transfer, especially when PEO is used [21]. The notched Charpy impact strength of PL70/PA30 and PL50/PA50 blends increases by 2.7 fold (from 22.5 to 84.5 J/m) and about 3 fold (from 37.4 to 147 J/m), respectively, with the addition of PEO (20 wt %). When PBAT is added to the PLA/PA11 blend, the ternary systems show slightly lower impact strength improvement with values of 71.4 J/m and 119.3 J/m upon the addition of PBAT (20 wt %) into PLA/PA11 systems at mixing ratios of 70/30 and 50/50, respectively. The theoretical values of interfacial tension measurements (Table 1) and morphological observations (Figures 2–4) support phase debonding and the low increase in impact strength of the ternary blends modified with PBAT. In similar works, Zolali and Favis [30] reported that the impact strength of PLA/PA11 (50/50) increased from 15 to about 50 J/m upon the addition of 10% PBAT, triggering plastic deformation of the PLA and PA11 matrices, while Kanzawa and Tokumitsu [42] claimed that adding 18% PBAT as an impact modifier only increased the impact strength of PLA/polycarbonate (PC) (60/40) from 1.9 to 2.1 kJ/m2. Our results underline the relations between the phase structure and impact properties for the blends, which are directly influenced by the introduction of an elastomer. Here, the PA40/(PEO20\*) system shows an impact strength (147 J/m) over 10 times higher than that of the virgin PLA (15.4 J/m), thus being considered as a super-tough material [43]. This behavior is in line with similar findings reporting a high level of interfacial interactions for PEO with PLA and PA11 leading to a homogenous phase morphology with fibrillated structures and percolation of the stress field around the cavitated PEO contributing to interfacial shear yielding of the matrix and the increased Izod impact strength of PLA/PA11 (50/50) from 17.3 to 58.3 J/m upon the addition of 20 wt % PEO [22].The presence of PEO as a plasticizer promotes PLA chain mobility and plastic deformation of the PLA matrix, thus increasing the Izod impact strength and elongation at break of binary blends by about 263% (from 11 ± 2 to 40 ± 8 J/m) and 15 fold (from 6 ± 1 to 100 ± 20 %), respectively [7].

**Figure 8.** Impact strength of the blends (see Section 3.2 for definition).

More important increases in the impact strength of PLA/PA11/PEO systems can be related to the presence of partially wet PEO droplets spreading at the interface between PLA and PA11 phases modifying the stress state at the interface of the other two phases to locally release the triaxial stresses and contributing to plastic deformation [22]. It is of high importance to create internal rubber cavitation ahead of a crack and involve the matrix in the plastic deformation to delocalize the triaxial stress, leading to more impact energy dissipation, and to obtain rubber toughened plastic blends [18]. The PEO assembled at the interface between PLA and PA11 contributes to internal rubber cavitation; hence, a significant increase in the impact strength of the ternary blends is observed, while less cavitation is expected to occur upon PBAT addition [22,30]. A suitable interfacial adhesion, better wetting of the particles, finer droplets morphology and lower resistance to the cavitation (low moduli of 50 MPa) of PEO localized at the PLA/PA11 interface provides interfacial interactions to improve the shear yielding and postpone phase debonding or interfacial void formation, leading to significant impact strength improvement.

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

## *3.1. Materials*

Table 3 summarizes the grades, melting point ( *Tm*), density at 25 ◦C, melt flow rate (MFI) at 210 ◦C, and chemical structure of all the materials used in this study. In all cases, the resins were used as received. Residual moisture was removed from the samples prior to the experiments using oven drying at 70 ◦C for at least 8 h except for PEO, which was dried at 40 ◦C.


**Table 3.** Main characteristics of the materials used.

## *3.2. Processing*

Binary PLA/PA11 blends and ternary blends of PLA/PA11/elastomer (PEO or PBAT) were prepared via melt mixing in a co-rotating twin-screw extruder Leistritz ZSE-27 with an L/D ratio of 40 and 10 heating zones coupled to a circular die (2.7 mm in diameter). All the samples were prepared with a screw speed of 100 rpm to give a total flow rate of 3 kg/h under a temperature profile of 175/175/180/180/190/190/200/200/200/200 ◦C from the feed hopper to the die. The extrudates were quenched in a cold-water bath and pelletized using a model 304 pelletizer (Conair, Stanford, CT, USA) and dried (60 ◦C for 12 h) to eliminate any residual water before molding. Injection molding was performed using a PN60 (Nissei, Japan) injection molding machine with a temperature profile of 200–200–190–180 ◦C (nozzle, front, middle and rear). The mold had four cavities to directly produce standard geometries (Type IV of ASTM D638) and impact test bars (dimensions

12.7 × 63.5 × 3.2 mm3) for mechanical characterization. The injection pressure was adjusted (45 to 65 MPa) depending on the compound viscosity, while the mold temperature was fixed at 30 ◦C. Table 4 summarizes the samples prepared in this study and the ratio of each component for binary and ternary blends.


**Table 4.** Coding and formulation of the samples produced.

## *3.3. Characterization*

3.3.1. Contact Angle Measurements

An optical contact angle analyzer (OCA 15 Plus, Future Digital Scientific Corp., Westbury NC, USA) was used at room temperature to measure the contact angle of the materials based on the sessile drop method. Water and ethylene glycol were used as liquids to obtain the average values of five replicates for each sample. The surface tension values were calculated using the following equations [26]:

$$(1 + \cos \theta\_{\mathbf{x}}) \gamma\_{\mathbf{x}} = 4 \left( \frac{\gamma\_{\mathbf{x}}^{\mathbf{d}} \gamma^{\mathbf{d}}}{\gamma\_{\mathbf{x}}^{\mathbf{d}} + \gamma^{\mathbf{d}}} + \frac{\gamma\_{\mathbf{x}}^{\mathbf{p}} \gamma^{\mathbf{p}}}{\gamma\_{\mathbf{x}}^{\mathbf{p}} + \gamma^{\mathbf{p}}} \right) \tag{3}$$

$$(1 + \cos \theta\_y) \gamma\_y = 4 \left( \frac{\gamma\_y^{\;d} \gamma^d}{\gamma\_y^{\;d} + \gamma^d} + \frac{\gamma\_y^{\;p} \gamma^p}{\gamma\_y^{\;p} + \gamma^p} \right) \tag{4}$$

where *γd* and *γ<sup>p</sup>* are the dispersion and polar components of surface tension, respectively (*γ = γd + γ<sup>p</sup>*). In addition, *θx* and *θy* are the contact angles of the polymer with water and ethylene glycol, respectively.

## 3.3.2. Morphological Observation

An Inspect F50 scanning electron microscope (SEM) (FEI, Hillsboro, OR, USA) was used at 15 kV to take micrographs and observe the interfacial adhesion quality between all the phases. The cryogenically fractured specimens in liquid nitrogen were coated with gold/palladium to be observed at different magnifications.

## 3.3.3. Mechanical Testing

An Instron (Norwood, MA, USA) universal mechanical tester model 5565 was used to perform tensile tests according to ASTM D638 using a 5 kN load cell at a rate of 10 mm/min and room temperature. The average values of the tensile strength ( *<sup>σ</sup>Y*), Young's modulus (*E*) and elongation at break (*<sup>ε</sup>b*) were reported for five dog bone specimens (type IV) with 3 mm thickness for each formulation.

The notched Charpy impact strength was measured on a Tinius Olsen (Horsham, PA, USA) model 104 at room temperature according to ASTM D256 using 10 samples (60 × 12.7 mm2) for each composition. Before testing, all the samples were automatically V-notched on a Dynisco (Franklin, MA, USA) model ASN 120 m sample notcher 24 h before testing.

## **4. Conclusions**

This work underlined the importance of the presence of an intermediate elastomeric phase to modify the interfacial area and interactions in multiphase systems and improve their properties. In this particular study, the mechanical properties (elasticity and toughness) of brittle PLA-PA11 systems were investigated by the addition of PBAT and PEO.

The results clearly indicated the importance of interfacial interaction and morphology development to produce super-tough PLA-based materials. The morphological results for the ternary PLA/PA11/elastomer blend strongly correlate with the theoretical prediction of interfacial values and spreading coefficients. A positive spreading coefficient of *λPA11/PEO/PLA* = 0.56 mN/m and a lower interfacial tension of polymer pairs in PLA/PA11/PEO compared to PLA/PA11/PBAT suggested a complete wetting of the interface by PEO compared to the partial wetting of PBAT droplets. PEO tended to completely wet the interface of PLA and PA11, thus generating a soft and deformable interfacial area. It was found that PEO addition generated a more homogenous structure with few voids/defects at the PLA/PA11 interface, while the addition of PBAT led to poorly distributed droplets which did not prevent pull-out of the dispersed particles and the presence of a large number of interfacial voids due to the weak interfacial adhesion and poor wetting ability of PBAT.

A brittle-to-ductile transition was achieved for fully biobased PLA/PA11 blend upon the addition of PEO (20 wt %) between PLA and PA11 resulting in a smooth stress transfer at the interface, thus improving shear yielding and more deformation/elasticity as well as leading to improved energy absorption/dissipation before complete parts failure. The addition of PEO (up to 20 wt %) to the PLA/PA11 (70/30) and (50/50) blends increased the elongation at break of up to 45.6% and 104%, while the impact strength was improved by 2.7 fold (from 22.5 to 84.5 J/m) and 3 fold (from 37.4 to 147 J/m), respectively.

Finally, PEO was more effective than PBAT in modifying the interfacial interactions, phase morphology and mechanical properties of the immiscible PLA/PA11 blend because of its partial miscibility with PLA and better affinity toward PA11 as well as increased PLA chain mobility, which further enhanced the interfacial interactions with PA11.

**Author Contributions:** Conceptualization, A.F. and D.R.; methodology, A.F.; formal analysis, A.F.; resources, D.R.; writing—original draft preparation, A.F.; writing—review and editing, D.R.; supervision, D.R.; project administration, D.R.; funding acquisition, D.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC) project RGPIN-2016-05958.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors acknowledge the technical support of the Research Center on Advanced Materials (CERMA).

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

**Sample Availability:** Not available.
