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Article

Preparation of Polymer-Based Nano-Assembled Particles with Fe3O4 in the Core

by
Jian Wang
*,
Wenjie Zhang
,
Yating Zhang
and
Haolin Li
The Department of Materials Engineering, Taiyuan Institute of Technology, Taiyuan 030008, China
*
Author to whom correspondence should be addressed.
Polymers 2023, 15(11), 2498; https://doi.org/10.3390/polym15112498
Submission received: 4 May 2023 / Revised: 25 May 2023 / Accepted: 26 May 2023 / Published: 29 May 2023
Browse Figures
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Abstract

:
Organic–inorganic nanocomposite particles, possessing defined morphologies, represent the next frontier in advanced materials due to their superior collective performance. In this pursuit of efficient preparation of composite nanoparticles, a series of diblock polymers polystyrene-block-poly(tert-butyl acrylate) (PS-b-PtBA) were initially synthesized using the Living Anionic Polymerization-Induced Self-Assembly (LAP PISA) technique. Subsequently, the tert-butyl group on the tert-butyl acrylate (tBA) monomer unit in the diblock copolymer, yielded from the LAP PISA process, was subjected to hydrolysis using trifluoroacetic acid (CF3COOH), transforming it into carboxyl groups. This resulted in the formation of polystyrene-block-poly(acrylic acid) (PS-b-PAA) nano-self-assembled particles of various morphologies. The pre-hydrolysis diblock copolymer PS-b-PtBA produced nano-self-assembled particles of irregular shapes, whereas post-hydrolysis regular spherical and worm-like nano-self-assembled particles were generated. Utilizing PS-b-PAA nano-self-assembled particles that containing carboxyl groups as polymer templates, Fe3O4 was integrated into the core region of the nano-self-assembled particles. This was achieved based on the complexation between the carboxyl groups on the PAA segments and the metal precursors, facilitating the successful synthesis of organic–inorganic composite nanoparticles with Fe3O4 as the core and PS as the shell. These magnetic nanoparticles hold potential applications as functional fillers in the plastic and rubber sectors.

1. Introduction

Poly (acrylic acid) (PAA) and polyacrylate exhibit utility as adsorbents [1], anti-scaling agents [2], dispersants [3] and thickeners [4] in modern industrial production. This versatility arises from the presence of carboxylic acid in the PAA polymer main chain, which facilitates interactions with various particles, surfaces, or small molecules via hydrogen or ionic bonding. However, the “living”/controlled polymerization of acrylic acid monomers was challenging due to the strong complexation of their carboxyl groups [5,6,7,8,9]. Typically, researchers resort to carboxyl-protected acrylate monomers, such as tert-butyl acrylate (tBA) and tert-butyl methacrylate (tBMA), for polymerization. These are subsequently subjected to deprotection reactions, yielding polymers with carboxylic acid, which are then employed in the development of diverse functional materials [10,11,12,13,14,15,16,17,18].
For example, Yoshinaga et al. [19] synthesized poly(tert-butyl acrylate)-b-poly(methyl methacrylate) (PtBA-b-PMMA) diblock copolymer by living anionic polymerization. This was then hydrolyzed to obtain poly (acrylic acid)-b-poly(methyl methacrylate) (PAA-b-PMMA) and doped with titanium dioxide (TiO2) nanoparticles using the sol-gel method. The resulting PAA@TiO2-b-PMMA organic–inorganic composite film material exhibited high transparency in the visible region, and the content of TiO2 in the composite film material was adjusted by changing the ratio of PAA segments, which in turn changed the refractive index of the film material. In a separate study, Ryu et al. [20] synthesized polystyrene-b-poly(2, 2, 2-propenoic acid trifluoroethyl ester) (PS-b-PTFEAs) from polystyrene-b-poly(tert-butyl acrylate) (PS-b-PtBAs) via a high conversion rate ester exchange reaction of tBA monomer units with trifluoroethanol, catalyzed by polyphosphoric acid, which can form ordered film materials. Hvilsted et al. [21] synthesized an amphiphilic diblock copolymer with a sulfhydryl group at the end, polycaprolactone-b-polyacrylic acid (HS-PCL-b-PAA), and introduced gold nanoparticles to form stable and well-dispersed organic–inorganic nanocomposite assembled particles using the complexation of the carboxyl group on the PAA segment with metal ions. Among them, PCL, which constituted the core of the nanoassemblies, demonstrated biocompatibility and exhibited high drug permeability to small drug molecules, while PAA, which constituted the shell, was biocompatible and had mucosal adhesion, and thus could be used as a drug carrier for the treatment of bladder cancer. Chen et al. developed a flexible photonic crystal using a relatively soft hydrophobic monomer tBA. The obtained PtBA photonic crystal has good flexibility, excellent hydrophobic properties, and bright tunable structural color. It was found that a wide color gamut of 112% and 10 times higher luminous intensity could be achieved by placing the PtBA film on the back side of a liquid crystal display [22]. Bettinger and other researchers developed a star-shaped poly (acrylic acid)-b-poly(n-butyl acrylate)/polyaniline (Star-AA20-b-BA80/PANI) conjugated polymer network with good flexibility and electrical conductivity for use as wearable electronics or implantable biomedical devices [23].
Polymerization-Induced Self-Assembly (PISA) was a revolutionary method for producing polymer-based nano-assembled particles [24,25,26,27,28,29]. In the PISA procedure, polymerization and self-assembly may occur concurrently, simplifying the operation steps, and the resulting nanoassemblies have a controllable morphology and high solid content (up to 50%) [30,31,32,33,34,35,36]. Living Anionic Polymerization (LAP) was characterized by relatively simple and easily controlled polymerization rate, high monomer conversion rate (close to 100%), a high activity of polymer living species, and no side reactions [37,38,39,40,41,42,43]. In our group’s previous research work, combining the controllability of LAP and the self-assembly function of PISA, the LAP PISA technique was developed and realized the LAP PISA process based on isoprene, styrene and p-tert-butylstyrene monomers, and the nano-self-assembled particles with different morphologies such as spheres, worms and vesicles were efficiently prepared [44,45]. However, the absence of post-modifiable functional groups on the prepared nano-assembled particles limits the application of the nano-assembled particles obtained by LAP PISA in practice.
In view of the versatility of poly (tert-butyl acrylate), we introduced tert-butyl acrylate functional monomers into the study of LAP PISA in order to expand the scope of LAP PISA monomers. Herein, we successfully synthesized the diblock copolymer PS-b-PtBA nano-self-assembled particles using cyclohexane as the solvent, trace Tetrahydrofuran (THF) as the polarity modifier, n-Butyllithium (n-Bu−Li+) as the initiator, 1,1-Diphenylethylene (DPE) as the active species conversion agent, styrene as the first monomer, and tBA as the second monomer by the LAP PISA technique. Subsequently, the tert-butyl group on the tBA monomer unit in the diblock copolymer was hydrolyzed to the carboxyl group using CF3COOH to obtain PS-b-PAA-based nano-self-assembled particles, and the resulting nano-self-assembled particles were characterized by Gel Permeation Chromatography (GPC), Nuclear Magnetic Resonance (NMR), Transmission Electron Microscopy (TEM) and Dynamic Light Scattering (DLS) to investigate the factors influencing the morphology of the nano-self-assembled particles. Finally, we also used the nano-self-assembled particles with carboxyl groups in the core as polymer templates, using the carboxyl group to complex with the metal precursors, and the organic–inorganic nanocomposite particles with a Fe3O4 core and polystyrene shell were prepared by reduction reaction (Scheme 1).

2. Materials and Methods

2.1. Materials and Chemicals

Styrene (St, 99%), 1,1-Diphenylethylene (DPE, 98%) tert-Butyl acrylate (tBA, 99%), and tert-Butyl methacrylate (tBMA, 98%) were purchased from Adamas Reagent Co., Shanghai, China, stirred and dried with calcium hydride for 20 h, distilled under reduced pressure and then used. Cyclohexane (99.9%) was purchased from Adamas Reagent Co., Shanghai, China, dried with calcium hydride and then distilled at atmospheric pressure. Tetrahydrofuran (THF, Water ≤ 30 ppm (by K.F.), 99.9%, stabilized with BHT, safedry, safeseal), n-Butyllithium (n-Bu−Li+, 1.6 M solution in hexanes, safeseal), FeCl2 (99.5%, powder), FeCl3·6H2O (98%), and Aqueous ammonia (NH3·H2O, 25–28%, AR) were purchased from Titan Scientific Co., Ltd., Shanghai, China, and used directly. Trifluoroacetic acid (TFA, 99%), and Calcium hydride (CaH2, 95%, AR) were purchased from Aladdin Biochemical Technology Co., Ltd., Shanghai, China, and used directly.

2.2. Preparation of the Macromolecular Initiator PS

Firstly, 84.70 mL cyclohexane (66.07 g), 1.1 mL THF (1.00 g) and 7.6 mL styrene (6.93 g) were added to a dry and clean ampoule in turn, stirred and protected by nitrogen; then, n-Bu−Li+ (1.60 mmol/mL) was slowly added dropwise to the mixed system to remove impurities until the color of the mixture changed to golden yellow, indicating that the impurities were removed; finally, 1 mL of n-Bu−Li+ (1.60 mmol/mL), which was used to initiate styrene polymerization, was immediately added to the ampoule, and the reaction was stirred at 0 °C for 30 min to obtain a macromolecular initiator PS with active species at the end, and a 5 mL sample was left for characterization.

2.3. Conversion of the Active Centre of the Macromolecular Initiator PS

First, 5.0 mL of cyclohexane (3.90 g) and 0.8 mL of DPE (1.11 g) were added sequentially to a 25 mL dry, clean, nitrogen-protected round-bottom flask; then, n-Bu−Li+ (1.60 mmol/mL) was slowly added to the flask dropwise with stirring to consume impurities until the color of the mixture changed to orange; finally, the mixture was added to the ampoule of macromolecular initiator PS, the color of the system changed to deep red, and the reaction was stirred for 20 min at 0 °C to prepare the macromolecular initiator PS with DPE-Li as the active center.

2.4. Preparation of the Diblock Copolymer PS-b-PtBA

First, 6.2 mL of tBA (5.54 g) was added to 25 mL of a dry, clean round-bottom flask filled with nitrogen, stirred and DPE-Li was slowly added dropwise to consume impurities until the color of the mixture changed to pale yellow; then, tBA monomer was added to the ampoule after the removal of impurities to start the LAP PISA process, and the red color of the reaction system immediately faded. The system gradually became turbid and the viscosity increased as the polymerization proceeded. After stirring the reaction for 1 h at room temperature, the diblock copolymer PS-b-PtBA was obtained; finally, the reaction system was exposed to air to terminate the polymerization.

2.5. Preparation of PS-b-PAA Nanoself-Assembled Particles

PS-b-PAA was obtained by hydrolyzing the tBA monomer unit on the diblock copolymer PS-b-PtBA obtained by LAP PISA using CF3COOH. Specific experimental steps: the polymer PS-b-PtBA (7.50 g) was added to a 20 mL round bottom flask and stirring was started, 0.6 mL CF3COOH (0.90 g) was slowly added dropwise to the flask with a syringe and the hydrolysis reaction was carried out at room temperature for 24 h. During the hydrolysis, the system was observed to become more turbid, and finally the product PS-b-PAA was obtained and dispersed in cyclohexane for characterization.

2.6. Preparation of PS-b-PAA@Fe3O4 Nanocomposite Particles

Using the carboxyl group on the PAA chain segment to complex with the metal precursors FeCl3·6H2O and FeCl2, Fe3+ and Fe2+ were introduced into the core of PS-b-PAA nano-self-assembled particles, and NH3·H2O was used as the reducing agent to generate PS-b-PAA@Fe3O4 organic–inorganic nanocomposite particles in situ (Figure 1). The specific experimental steps are as follows. First, 0.3012 g of PS34-b-PAA20 diblock copolymer nano-assembled particles were dispersed in 10 mL DMF, 0.3962 g of FeCl2 and 0.8434 g of FeCl3·6H2O (25 equivalents of carboxyl group) were added, and stirred under nitrogen protection for 24 h; subsequently, 9 mL of NH3·H2O (in excess) was added and reacted at 50 ℃ or 30 min, and then aged at 80 °C for 1 h, followed by centrifugation at 1000 rpm for 5 min to remove the larger size of the aggregated nanoparticles to obtain PS34-b-PAA20@Fe3O4 nanocomposite particles.

2.7. Characterization Instruments

A 400 MHz Fourier transform nuclear magnetic resonance spectrometer (1H NMR, Bruker-AVANCE III HD, Ettlingen, Germany): the chemical shift internal standard was tetramethylsilane (TMS), and the solvents used for the test were deuterated chloroform (CDCl3), deuterated methanol (CD3OD) and deuterated dimethyl sulfoxide (DMSO-d6).
Gel permeation chromatography (GPC, THF, Agilent-1260, San Diego, CA, USA): The G1310B pump, G1362A differential detector, G1314F UV detector and gel column (500 Å, molecular weight detection range 500 to 4 × 106 Da) were connected. The calibration standard was polystyrene (PS), the test temperature was 35 °C, and the mobile phase was chromatographic grade THF at a flow rate of 0.5 mL/min. The relative molecular weights and distributions of polymers soluble in THF were measured.
Dynamic light scattering (DLS, Malvern Zetasizer Nano ZS90, Sydney, Australia): the measurement range was from 0.3 to 5000 nm and the scattered light detection angle was 173°. The solvents used for the measurements were cyclohexane and THF. The size of the nanoparticles was measured.
Transmission electron microscopy (TEM, JEOL JEM-1230, Tokyo, Japan): accelerating voltage of 80 kV, line resolution of 0.2 nm, and point resolution of 0.36 nm. The diblock copolymer nanoassembled particles were formulated into a mixture with a concentration of 0.1~0.3% w/w. A drop of the mixture was placed on the copper web coated with carbon film and dried in air for 24 h to remove the solvent for TEM characterization. The solvent used was cyclohexane.
Thermogravimetric analyzer (TGA, Pyris 1, East Lyme, CT, USA): temperature range from room temperature to 1000 °C, sample capacity: 60 mL/1.3 g, balance sensitivity: 0.1 mg, temperature rise/fall rate range from 0.1 °C/min to 200 °C/min. The thermal decomposition temperature of the material was characterized in a nitrogen atmosphere at a rate of 10 °C/min from 50 °C to 700 °C.
X-ray Diffractometer (XRD, D/max2200PC, Tokyo, Japan): used for the measurement of the samples. Cu target Kα radiation, tube voltage of 40 kV, tube current of 30 mA, scanning speed of 5 °C/min, and scanning angle of 5 to 80°.

3. Results and Discussion

3.1. Characterization of PS-b-PtBA in LAP PISA

In the LAP PISA based on St and tBA, n-Bu−Li+ was used as the initiator, cyclohexane as the solvent, and trace amounts of THF as the polarity modifier. In the first stage of the polymerization reaction, the St monomer was first polymerized via LAP to produce the macromolecular initiator PS with reactive species at the end. In the second stage of the polymerization, DPE with large site resistance was added for the conversion of the reactive center to reduce the initiation activity of the macromolecular initiator PS, and then the tBA monomer was added to the system for the LAP PISA process. Due to the poor solubility of PtBA segments in cyclohexane, with the polymerization of tBA monomer, the solubility of the resulting diblock copolymer PS-b-PtBA gradually decreased, and the diblock copolymer PS-b-PtBA started to assemble to form nano self-assembled particles, thus realizing the LAP PISA process. A series of LAP PISA formulations based on PS-b-PtBA were designed by varying the ratio of molecular weight of PtBA and PS segments (Mn,tBA/Mn,PS), solids content, and other factors. The assembly morphology of the obtained nano self-assembled particles was investigated, and the polymerization formulations and data were summarized as shown in Table 1.

3.1.1. NMR and GPC Characterization

The crude products of the polymers obtained from the two polymerization stages of the LAP PISA process were characterized using NMR. As shown in Figure 2, in the 1H NMR spectra of the crude product of the macromolecular initiator PS Figure 2a, 6.30–7.30 ppm was the chemical shift of H on the benzene ring (–C6H5) in the PS segment, and in the 1H NMR spectrum of the crude product of the diblock copolymer PS-b-PtBA Figure 2b, 1.50 ppm was the chemical shift of H on the tert-butyl (–C(CH3)3) in the PtBA segment, 2.05–2.35 ppm was the chemical shift of H in the PtBA chain (–OCH2CHO–), and 6.30–7.30 ppm was the chemical shift of H on the benzene ring (–C6H5) in the PS. Apparently, no signals of double bonds on St or tBA monomers were detected in the 1H NMR spectra, which proves that St and tBA monomers have been completely consumed in the polymerization process of LAP PISA.
We performed GPC characterization of the two polymerization stages of the LAP PISA process based on the diblock copolymer PS-b-PtBA. The GPC curves of the macromolecular initiator PS prepared in the first polymerization stage showed a single peak and a narrow molecular weight distribution (Mw/Mn < 1.10), which proved that the macromolecular initiator PS was successfully synthesized (Figure 3a–c). The diblock copolymer PS-b-PtBA was prepared in the second polymerization stage, and the molecular weight and molecular weight distribution of the block polymer could not be characterized by GPC because the resulting PS-b-PtBA was insoluble in THF, N,N-Dimethylformamide (DMF) and other solvents. This can be attributed to the cross-linking side reaction of the tBA monomer during the second stage of LAP PISA.

3.1.2. DLS and TEM Characterization

In order to demonstrate that the diblock copolymer PS-b-PtBA formed cross-linked nanoparticles during LAP PISA, we dispersed the diblock copolymer PS-b-PtBA nano-self-assembled particles in cyclohexane and THF (a good solvent for PS and PtBA blocks), respectively, for DLS characterization. The DLS results of PS-b-PtBA nano-self-assembled particles dispersed in cyclohexane are shown in Figure 4a. When the solid content was 15% w/w, the molecular weight of the macromolecular initiator PS was kept in the range of 3500~4000 g/mol, and the Mn,tBA/Mn,PS were designed to be 0.4/1, 0.8/1, and 1.2/1 to obtain PS35-b-PtBA11,PS34-b-PtBA20 and PS36-b-PtBA32, respectively. It can be observed in the corresponding DLS curves that the average size of the diblock copolymer nano-self-assembled particles PS-b-PtBA increases with the increase in Mn,tBA/Mn,PS. The DLS results of the diblock copolymer PS-b-PtBA nanoself-assembled particles dispersed in THF are shown in Figure 4b, and the size distribution of the PS-b-PtBA nanoself-assembled particles synthesized by LAP PISA in THF was in the range of 90–1100 nm. Since both PS and PtBA segments have good solubility in THF, the DLS results of PS-b-PtBA nano-self-assembled particles dispersed in THF should theoretically be less than 10 nm, but the PS-b-PtBA nano-self-assembled particles prepared by LAP PISA exhibited larger size in THF, which further proved that the tBA monomer was cross-linked in situ in the second polymerization stage of LAP PISA and formed cross-linked stable nano-assembled particles. The morphology of the diblock copolymer PS-b-PtBA nanoassemblies obtained by LAP PISA was characterized using TEM. As shown in Figure 4c–e, the diblock copolymers PS35-b-PtBA11 and PS34-b-PtBA20 were obtained by the LAP PISA process when the solid content was 15% w/w and the designed Mn,tBA/Mn,PS was 0.4/1 and 0.8/1, and their TEM images were shown in Figure 4c,d. The irregular morphology of the nano-assembled particles can be observed. The TEM image of the diblock copolymer PS36-b-PtBA32 obtained by the LAP PISA process with a fixed solid content of 15% w/w and increasing Mn,tBA/Mn,PS to 1.2/1 was shown in Figure 4e, from which spherical micelles with a diameter of about 300 nm can be observed. It can be concluded that the LAP PISA system based on PS-b-PtBA was prone to the formation of irregularly shaped nano-self-assembled particles, which can be attributed to the lower glass transition temperature of the second segment PtBA (Tg = 40 °C) and the weaker ability of self-assembly to form the stable core [46]. We hydrolyzed the PtBA chain segment in PS-b-PtBA nano-self-assembled particles into the PAA segment to increase the glass transition temperature of the core block.

3.2. Characterization of PS-b-PAA Nanoself-Assembled Particle

The PtBA segments in PS-b-PtBA nano-self-assembled particles were hydrolyzed to prepare nano-self-assembled particles with a core containing carboxyl functional groups. The series of PS-b-PAA were prepared using CF3COOH hydrolysis of the tBA monomer unit on the diblock copolymer PS-b-PtBA obtained from LAP PISA (Table 2).

3.2.1. NMR Characterization

We purified the samples of the macromolecular initiator PS, the diblock copolymer PS-b-PtBA and the diblock copolymer PS-b-PAA obtained by hydrolysis and characterized them using 1H NMR. Figure 5a showed the 1H NMR spectrum of the purified treated macromolecular initiator PS, Figure 5b was the 1H NMR spectrum of the purified treated PS-b-PtBA, and Figure 5c was the 1H NMR spectra of PS-b-PAA after purification treatment. The area of peak ‘j’ at 2.10–2.40 ppm was used as the reference, and the peaks at 1.00–2.00 ppm in the 1H NMR spectra before Figure 5b and after hydrolysis Figure 5c were integrated and compared, respectively, and it was calculated that the PtBA segment had been completely hydrolyzed into the PAA segment, and the hydrolysis conversion rate was 100%.

3.2.2. TEM and DLS Characterization

The diblock copolymers PS35-b-PtBA11,PS34-b-PtBA20 and PS36-b-PtBA32 were hydrolyzed to obtain PS35-b-PAA11,PS34-b-PAA20 and PS36-b-PAA32 nano-self-assembled particles containing carboxyl groups in their cores, respectively. The morphology of the hydrolyzed nano-self-assembled particles was characterized using TEM. The results were shown in Figure 6. The TEM image of PS35-b-PAA11, from which regular spherical micelles with a size of about 20 nm can be observed in Figure 6a; Figure 6b,b’ was the TEM image of PS34-b-PAA20, from which regular spherical micelles with a size of about 35 nm can also be observed. Increasing the degree of polymerization of PAA, a mixture of short worm-like micelles and spherical micelles with a size of about 50 nm can be observed in the TEM images of PS36-b-PAA32 (Figure 6c,c’). The TEM characterization results illustrate that when the solids content was 15% w/w and the molecular weight of the macromolecular initiator PS was kept in the range of 3500~4000 g/mol, the size of the PS-b-PAA diblock copolymer nanoparticles gradually becomes larger and the morphology changed from spherical micelles to short worm-like micelles as the polymerization degree of the core block PAA increased. The size of the nano-assemblies was characterized using DLS. Figure 6d shows the DLS measurements of the nano self-assembled particles PS-b-PAA obtained after hydrolysis. The average diameter of the nano self-assembled particles was about 20 nm in the DLS curve of the spherical micelle PS35-b-PAA11, about 50 nm in the DLS curve of the spherical micelle PS34-b-PAA20, and the average diameter of the nano-self-assembled particles was about 80 nm in the DLS curves of the mixed system of worm-like micelles and spherical micelles PS36-b-PAA32. These DLS characterization results showed a variation trend that was in good agreement with the TEM results.
Based on the above characterization results, we can conclude that PS-b-PAA was prepared by CF3COOH hydrolysis of the tBA monomer unit on the diblock copolymer PS-b-PtBA obtained from LAP PISA, and due to the high glass transition temperature of PAA (Tg = 106 °C) [47], the diblock copolymer PS-b-PAA can all form regular-shaped nano-self-assembled particles. The assembly morphology of the nano-self-assembled particles obtained from PS-b-PAA could be adjusted by changing the degree of polymerization of PAA at a solid content of 15% w/w, and the molecular weight of the macromolecular initiator PS was kept in the range of 3500–4000 g/mol.

3.3. Characterization of PS-b-PAA@Fe3O4 Nanocomposite Particles

Using the complexation between the carboxyl group on the PAA segment and FeCl3·6H2O and FeCl2 metal precursors, Fe3+ and Fe2+ were introduced into the core region of PS-b-PAA nano-self-assembled particles, and then NH3·H2O was used as the reducing agent to generate PS-b-PAA@Fe3O4 organic–inorganic nanocomposite particles in situ.

3.3.1. TEM and DLS Characterization

Inorganic–organic nanocomposite particles PS34-b-PAA20@Fe3O4, obtained from the hydrolysis product PS34-b-PAA20 with a solid content of 15% w/w as a template, were used as an example. The morphology was characterized by TEM by dispersing the inorganic–organic nanocomposite particles into cyclohexane. Figure 7a shows the TEM image of PS34-b-PAA20, where spherical micelles with an average diameter of about 50 nm can be observed; the TEM image of PS34-b-PAA20@Fe3O4 after modification of Fe3O4 nanoparticles in the core region of the micelles is shown in Figure 7b,b’, and spherical self-assembled particles with a larger size of about 100 nm can be observed. The size of PS34-b-PAA20@Fe3O4 was significantly larger compared to PS34-b-PAA20 without Fe3O4 modification, and the PS shell layer on the surface of the spherical micelles can be observed in the magnified Figure 7b’ of PS34-b-PAA20@Fe3O4. The DLS characterization results are shown in Figure 7c. The DLS curves of PS34-b-PAA20 nano-assembled particles show an average size of about 50 nm, and the DLS curves of PS34-b-PAA20@Fe3O4 nano-assembled particles show an average size of about 100 nm, again demonstrating that the size of the nano self-assembled particles after the modification of Fe3O4 was larger than that before the modification, which was consistent with the TEM measurements.

3.3.2. TGA Characterization

TGA was utilized to separately analyze nano-assembled particles pre- and post-Fe3O4 modification. As shown in Figure 8, the weight loss was 45.2% in the TGA curve of the organic–inorganic nanocomposite particles PS34-b-PAA20@Fe3O4, while the weight loss was 99.7% in the TGA curve of the diblock copolymer nanoassemblies PS34-b-PAA20, which was almost all thermally decomposed at 485 °C. Obviously, this was due to the introduction of Fe3O4 in polymer nanoassemblies.

3.3.3. XRD Characterization

The nanoassemblies before and after modification of Fe3O4 were characterized separately using XRD. As shown in Figure 9, the characteristic peaks of Fe3O4 (2θ = 18.3°, 30.2°, 35.7°, 43.4°, 53.4°, 57.3° and 62.7°) correspond to the diffraction peaks of (111), (220), (311), (400), (422), (511) and (440) crystalline planes of cubic phase Fe3O4, respectively. The XRD spectra of PS34-b-PAA20 and PS34-b-PAA20@Fe3O4 were compared, and the same PS-b-PAA non-crystalline diffraction peaks were found around 20°, while the characteristic absorption peaks of Fe3O4 appeared in the XRD spectra of PS34-b-PAA20@Fe3O4, proving that Fe3O4 was successfully bound to the core of the polymeric nano-assembled particles.

4. Conclusions

In summary, utilizing the LAP PISA technique, diblock copolymer nano-self-assembled particles PS-b-PtBA were synthesized. The employed methodology incorporated cyclohexane as a solvent, n-Bu−Li+ as an initiator, trace tetrahydrofuran as a polarity modifier, DPE as the active species conversion agent, St as the first monomer and tBA as the second monomer. The nano-assembled particles were insoluble in THF, DMF and other solvents, and the DLS results in THF showed a large size, demonstrating that the tBA monomer undergoes cross-linking side reactions during LAP PISA to form cross-linked stable nanoparticles with irregular morphology. Subsequently, CF3COOH was used to hydrolyze the tert-butyl group on the tBA monomer unit in the diblock copolymer PS-b-PtBA obtained by LAP PISA into the carboxyl group, and the prepared diblock copolymer PS-b-PAA could self-assemble into spherical and short worm-like micelles with the core containing carboxyl functional groups. The morphology of the nano-self-assembled particles formed by PS-b-PAA gradually evolved from spherical micelles to a mixture of spherical micelles and short worm-like micelles as the degree of polymerization of the core block PAA increased. Finally, the nano-self-assemblies containing carboxyl groups were used as polymer templates, and the PS-b-PAA@Fe3O4 organic–inorganic composite nano-assembled particles were prepared by introducing Fe3O4 into the core of the polymer nanoassemblies using the complexation between the carboxyl group and the metal precursors. It is expected that PS-b-PAA@Fe3O4 spherical nanoparticles can be dispersed uniformly in an elastomer and used as magnetic functional fillers to prepare an electromagnetic shielding elastomer.

Author Contributions

Conceptualization, J.W.; data curation, J.W. and Y.Z.; formal analysis, H.L.; methodology, J.W.; software, W.Z.; validation, W.Z. and Y.Z.; writing—original draft, J.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work is financially supported by Fundamental Research Program of Shanxi Province (202103021223348), Doctoral (Postdoctoral) Research Grant Program (2022KJ038), Outstanding Doctoral (Postdoctoral) Research Grants to Work in Jin (2022LJ012).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

There are no conflict to declare.

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Polymers 15 02498 sch001 550
Scheme 1. Synthesis of PS-b-PAA@Fe3O4 organic−inorganic composite nanoparticles.
Scheme 1. Synthesis of PS-b-PAA@Fe3O4 organic−inorganic composite nanoparticles.
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Polymers 15 02498 g001 550
Figure 1. The synthesis process of nanoassemblies via LAP PISA and the obtained organic−inorganic composite nanoparticles.
Figure 1. The synthesis process of nanoassemblies via LAP PISA and the obtained organic−inorganic composite nanoparticles.
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Figure 2. 1H NMR spectra for crude macromolecular initiator (a) PS and (b) PS-b-PtBA diblock copolymer (in CDCl3).
Figure 2. 1H NMR spectra for crude macromolecular initiator (a) PS and (b) PS-b-PtBA diblock copolymer (in CDCl3).
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Figure 3. GPC traces for macromolecular initiator PS.
Figure 3. GPC traces for macromolecular initiator PS.
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Figure 4. (a,b) DLS results of the nano-objects in cyclohexane and THF, (c–e) TEM images of PS-b-PtBA nano-self-assembled particles.
Figure 4. (a,b) DLS results of the nano-objects in cyclohexane and THF, (c–e) TEM images of PS-b-PtBA nano-self-assembled particles.
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Figure 5. 1H NMR spectra for the purified macromolecular initiator PS (in CDCl3) (a), PS-b-PtBA diblock copolymer (in CDCl3) (b), and PS-b-PAA (in DMSO-d6 and CD3OD) (c).
Figure 5. 1H NMR spectra for the purified macromolecular initiator PS (in CDCl3) (a), PS-b-PtBA diblock copolymer (in CDCl3) (b), and PS-b-PAA (in DMSO-d6 and CD3OD) (c).
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Figure 6. TEM images of PS-b-PAA nano self-assembled particles obtained by hydrolysis of trifluoroacetic acid (a,b,b’,c,c’) and DLS results of the corresponding nano self-assembled particles (d). ((b’) and (c’) represent the enlarged images of (b) and (c), respectively).
Figure 6. TEM images of PS-b-PAA nano self-assembled particles obtained by hydrolysis of trifluoroacetic acid (a,b,b’,c,c’) and DLS results of the corresponding nano self-assembled particles (d). ((b’) and (c’) represent the enlarged images of (b) and (c), respectively).
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Figure 7. TEM images of (a) the nano-self-assembled particles of diblock copolymer PS34-b-PAA20, (b,b’) the corresponding organic–inorganic nanoparticles PS34-b-PAA20@Fe3O4 ((b’) represents the enlarged images of (b)). (c) DLS results of the corresponding nanoparticles (in cyclohexane).
Figure 7. TEM images of (a) the nano-self-assembled particles of diblock copolymer PS34-b-PAA20, (b,b’) the corresponding organic–inorganic nanoparticles PS34-b-PAA20@Fe3O4 ((b’) represents the enlarged images of (b)). (c) DLS results of the corresponding nanoparticles (in cyclohexane).
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Figure 8. TGA curves of the crosslinked nanoassemblies of diblock copolymer PS34-b-PAA20 and the corresponding organic–inorganic nanoparticles PS34-b-PAA20@Fe3O4 (in the air, 10 °C/min).
Figure 8. TGA curves of the crosslinked nanoassemblies of diblock copolymer PS34-b-PAA20 and the corresponding organic–inorganic nanoparticles PS34-b-PAA20@Fe3O4 (in the air, 10 °C/min).
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Figure 9. XRD spectra of Fe3O4,PS34-b-PAA20 and PS34-b-PAA20@Fe3O4.
Figure 9. XRD spectra of Fe3O4,PS34-b-PAA20 and PS34-b-PAA20@Fe3O4.
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Table
Table 1. Polymerization data of PS-b-PtBA diblock copolymers with different molecular weight ratios of Mn,PtBA/Mn,PS.
Table 1. Polymerization data of PS-b-PtBA diblock copolymers with different molecular weight ratios of Mn,PtBA/Mn,PS.
SamplesSolids Content (wt%)Targeted MW Ratio Mn,PtBA/Mn,PSThe First Polymerization StageThe Second Polymerization StageMorphology 4
Mn,PS (g/mol) 1Mw/Mn 1DPPS 2Mn,PS-b-PtBA(g/mol) 3Conv.tBA 3DPPtBA 3
PS35-b-PtBA11150.4/137001.0935520010011Irregular morphology
PS34-b-PtBA20150.8/136001.0834610010020Irregular morphology
PS36-b-PtBA32151.2/138001.0836790010032Spherical micelles
1 The Mn and Mw/Mn were obtained by GPC measurement. 2 DP of macro-initiator PS (DPPS) was calculated according to the Mn,PS from GPC measurement. 3 DP of PtBA (DPPtBA) and the conversion of tBA (Conv.tBA) were calculated according to the DPPS and 1H NMR spectra. 4 The morphology was observed by TEM image.
Table
Table 2. Polymerization data of PS-b-PAA diblock copolymers with different molecular weight ratios of Mn,PAA/Mn,PS.
Table 2. Polymerization data of PS-b-PAA diblock copolymers with different molecular weight ratios of Mn,PAA/Mn,PS.
SamplesSolids Content (wt%)Targeted MW Ratio Mn,PAA/Mn,PSThe First Polymerization StageThe Second Polymerization StageMorphology 4
Mn,PS (g/mol) 1Mw/Mn 1DPPS 2Mn,PS-b-PAA(g/mol) 3Conv. Taa 3DPPtBA 3
PS35-b-PAA11150.4/137001.2135450010011Spherical micelles
PS34-b-PAA20150.8/136001.0934500010020Spherical micelles
PS36-b-PAA32151.2/138001.1536610010032Spherical and short wormlike micelles
1 The Mn and Mw/Mn were obtained by GPC measurement. 2 DP of PS (DPPS) was calculated according to the Mn,PS from GPC measurement. 3 DP of PAA (DPPAA) and conversion of AA (Conv.AA) were calculated according to the DPPS and 1H NMR spectrum. 4 The morphology was observed by TEM image.
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Wang, J.; Zhang, W.; Zhang, Y.; Li, H. Preparation of Polymer-Based Nano-Assembled Particles with Fe3O4 in the Core. Polymers 2023, 15, 2498. https://doi.org/10.3390/polym15112498

AMA Style

Wang J, Zhang W, Zhang Y, Li H. Preparation of Polymer-Based Nano-Assembled Particles with Fe3O4 in the Core. Polymers. 2023; 15(11):2498. https://doi.org/10.3390/polym15112498

Chicago/Turabian Style

Wang, Jian, Wenjie Zhang, Yating Zhang, and Haolin Li. 2023. "Preparation of Polymer-Based Nano-Assembled Particles with Fe3O4 in the Core" Polymers 15, no. 11: 2498. https://doi.org/10.3390/polym15112498

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