Simultaneously strengthening and toughening soy protein isolate-based films using poly(ethylene glycol)-block-polystyrene (PEG-b-PS) nanoparticles

Abstract

Well-defined, vesicle-like nanoparticles of poly(ethylene glycol)-block-polystyrene (PEG-b-PS) diblock copolymers, synthesized via a macro-RAFT agent of PEG₄₅-TTC (where “TTC” is the RAFT terminal of trithiocarbonate) mediated dispersion polymerization of styrene, were used to prepare enhanced SPI-based nanocomposite films in this study. The uniform dispersion of the PEG₄₅-b-PS₂₇₆ nanoparticles into the SPI matrix was confirmed by transmission electron microscopy and field emission scanning electron microscopy. The simultaneously strengthening and toughening mechanism of the SPI-based nanocomposite films was achieved. This was accomplished by discontinuous filling of nanoparticles into the SPI matrix due to the hydrophobic PS core which served as the hard-domains to strengthen the mechanical properties of the resultant films. Concurrently, the hydrophilic PEG block was conjunct with the SPI chains through hydrogen bonding, increasing the compatibility between nanoparticles and the SPI matrix, ultimately transferring interfacial stress and increasing the elongation of the resulting films. Compared to unmodified SPI film, the tensile strength and elongation at break value of the SPI/PEG-b-PS nanocomposite films were improved by 85.3% and 11.5%, respectively. Further, the total soluble matter of the nanocomposite films was reduced by 59.7% compared to the control. The surface hydrophobicity of the films was also elevated due to the hydrophobic PS core surface-aggregation. The diblock copolymer examined here, as opposed to other nanofillers, may be the first to be successfully introduced into the SPI biopolymer matrix to fabricate high-performance bio-nanocomposite films.

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1 Introduction

In recent years, there has been a rapid and substantial increase in the global demand for green, sustainable, eco-friendly materials.^1,2 Expansive ranges of renewable resources have been studied to construct polymeric composite materials comprised of both biopolymers and biomass (e.g., celluloses, proteins, plant oils).³ Proteins, as amphiphilic biopolymers, are biodegradable, renewable, and biocompatible. Their functional side chains can readily combine with various reagents.⁴ The soy protein form of soy protein isolate (SPI) (≥90% protein) based films, in particular, represents notable potential applications for packaging, drug delivery, tissue regeneration, and other uses.^5,6 However, the fundamental hydrophilicity and strong molecular interactions of soy proteins limits their potencies and gravely restricts their practical application.⁷ Numerous researchers have attempted to employ physical and/or chemical modifications to enhance SPI-based films.^8–10 However, the unbalanced enhancement of these films (e.g., low toughness) remains problematic.

The incorporation of organic/inorganic nanofillers (e.g., starch nanocrystals, cellulose nanocrystals, nano-TiO₂, nanoemulsions) to fabricate SPI-based materials with enhanced mechanical strength has been extensively researched.^11–14 Apart from the superior surface areas of nanofillers, challenges in terms of their dispersion and the compatibility with the polymeric matrices are persistent roadblocks.¹⁵ For example, hydrophilic nanofillers (e.g., starch nanocrystals) can cause irreversible agglomeration during drying because of the formation of additional hydrogen bonds between nanocrystals.¹⁶ Inorganic nanoparticles (e.g., TiO₂, carbon nanotubes) require a priori surface functionalization to make the nanofillers compatible with the hydrophilic polymeric matrices. For nanoemulsions, surfactants are essential to aid the dispersion of nanoparticles.¹⁷

By virtue of biomechanics of hierarchical biological protein/phospholipid materials (e.g., lamins, collagenous tissue) self-assembled block copolymer nanoparticles represent a very attractive approach to the design of novel organic/inorganic nanocomposite materials.¹⁸ Recently, the polymerization-induced self-assembly (PISA) affords the in situ synthesis of block copolymer nano-objects, particularly, through the macromolecular RAFT (macro-RAFT) agent mediated dispersion polymerization that has proven to be remarkably reliable, as it facilitates convenient in situ polymerization and relatively high concentrations of block copolymers.^19,20 In this study, we attempted to design poly(ethylene glycol)-block-polystyrene (PEG-b-PS) nanoparticles synthesized via a macro-RAFT agent of PEG-TTC (TTC represents the RAFT terminal of trithiocarbonate) mediated dispersion polymerization of styrene, to enhance the mechanical properties and water resistance of SPI-based films. The amphiphilic PEG-b-PS nanoparticles could be uniformly and stably dispersed into the SPI solution through hydrogen bonding, which enhanced the compatibility between the diblock polymer and the SPI matrix. The solvophobic PS core served as a hard-domain to strengthen the mechanical properties of the SPI/PEG-b-PS nanocomposite films. Its surface-aggregation, during water evaporation, is also supposed to improve the water resistance and surface hydrophobicity of the SPI/PEG-b-PS nanocomposite films. The contribution of the PEG-b-PS nanoparticles to the microstructure, thermal properties and morphology of the SPI-based films was characterized. The films' mechanical properties, water resistance and water contact angle were also examined and calculated. As opposed to other nanofillers, diblock copolymer nanoparticles showed the ability to simultaneously strengthen and toughen the resultant SPI film. Indeed, this study may have been the first to introduce diblock copolymer nanoparticles into the biopolymer matrices, which has provided a facile and efficient approach to produce these high-performance nanocomposite films.

2 Experimental

2.1 Materials

The poly(ethylene glycol)-block-polystyrene (PEG₄₅-b-PS₂₇₆) diblock copolymer (15.0 wt%) was synthesized by dispersion RAFT polymerization of styrene (St) in an 80/20 methanol/water solvent employing trithiocarbonate-terminated poly(ethylene glycol) monomethyl ether (PEG₄₅-TTC) as a macro-RAFT agent (Scheme 1).²⁰ The GPC and ¹H NMR spectra of PEG₄₅-b-PS₂₇₆ are shown in Fig. S1 and S2.† The polymer molecular weight (M_n,NMR) of the PEG₄₅-b-PS₂₇₆ was 32.1 kg mol⁻¹, which was calculated via the ¹H NMR spectrum, by comparing the integral areas of δ = 6.36–7.08 ppm for the phenyl groups in the PS block and δ = 3.64 ppm for the PEG backbone. A PDI of PEG₄₅-b-PS₂₇₆ at 1.09 was obtained by GPC analysis, and the molecular weight of PEG₄₅-b-PS₂₇₆ (M_n,GPC) was 24.5 kg mol⁻¹. Soy protein isolate (SPI, 95% protein) was purchased from Yuwang Ecological Food Industry Company. Glycerol (99% purity) and other chemical reagents were of analytical grade and/or purified by standard procedures and purchased from Beijing Chemical Reagents Company.

Scheme 1 Synthesis of the PEG-b-PS diblock copolymers by macro-RAFT agent mediated dispersion polymerization.

2.2 Preparation of SPI/PEG₄₅-b-PS₂₇₆ nanocomposite films

For the SPI-based film preparation, a two-step method was demonstrated.⁹ Firstly, SPI film solution was prepared by combining SPI (5 g), glycerol (2.5 g), and distilled water (95 g) by sequentially adding them into a 250 mL beaker, adjusting the pH to 9.0 ± 0.1 (NaOH solution, 5 M), and then heating in a water bath at 85 °C for 30 min. The final SPI film solutions were fabricated by adding the formulated PEG₄₅-b-PS₂₇₆ nanoparticles as shown in Table 1. Secondly, the aforementioned suspension was ultrasonicated for 2 min to remove the bubbles before pouring into the Teflon-coated plate and vacuum dried at 45 °C for 24 h. Films were stripped and placed into a saturated-K₂CO₃-regulated (50 ± 2% relative humidity (RH)) desiccator at room temperature for further analysis.

Table 1 Experimental details and summary of SPI/PEG₄₅-b-PS₂₇₆ nanocomposite films

Entry	SPI (g)	Glycerol (g)	Water (g)	PEG45-b-PS276^a (wt%)	PEG45-b-PS276 (g, 15 wt%)
a The proportion of modifiers to the SPI solid content.
A	5.0	2.5	95	—	—
B	5.0	2.5	95	0.5	0.17
C	5.0	2.5	95	1.0	0.34
D	5.0	2.5	95	2.0	0.67
E	5.0	2.5	95	4.0	1.34
F	5.0	2.5	95	8.0	2.67

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2.3 Characterization

Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR, Nicolet 6700) was employed to characterize the chemical structural changes of the PEG₄₅-b-PS₂₇₆ diblock copolymer and SPI-based films at a wavelength range of 650 to 4000 cm⁻¹ and a total of 32 scans. Spectra were rectified by the ATR advanced correction in OMNIC software. X-ray photoelectron spectroscopy (XPS) was carried out with monochromatic Al Kα radiation (1486.6 eV) using K-Alpha X-ray photoelectron spectrometer. The X-ray beam was a 100 W, 200 mm-diameter beam rastered over a 2 mm by 0.4 mm area of the specimens. Spectra were collected using a pass energy of 50 eV and resolution of 0.1 eV.

X-ray diffraction (XRD, Bruker D8), equipped with a Cu Kα radiation source (λ = 0.154 nm), was employed to further investigate the films' properties with theta scan ranging from 5° to 60° at a 0.02° interval; the diffraction data were analyzed in Jade 5.0. Transmission electron microscopy (TEM, JEM-1010) was conducted at an acceleration voltage of 80 kV. Dilute SPI, PEG₄₅-b-PS₂₇₆ and SPI/PEG₄₅-b-PS₂₇₆ dispersions were dripped onto a piece of carbon-coated copper grids and dried at room temperature prior to imaging. Field emission scanning electron microscopy (FE-SEM, Hitachi SU8010), with an accelerating voltage of 3 kV, was applied to observe the surface fracture morphologies of the SPI-based films. Samples were sputtered with a 10 mm gold layer to avoid charging under the electron beam prior to observation. Thermogravimetric analysis (TGA, Q50) was used to analyze the thermo-stability of the PEG₄₅-b-PS₂₇₆ diblock copolymer and SPI-based films at a temperature ranging from 40 °C to 600 °C with a heating rate of 10 °C min⁻¹ under N₂ atmosphere (100 mL min⁻¹).

The surface hydrophobicity of the SPI-based films was investigated by contact angle (OCA-20) testing. A sessile droplet (3 μL) of distilled water was dropped onto the surface and the angles of both sides were recorded at an interval of 0.1 s for 180 s, with five replicates conducted for each sample.

The total soluble matter (TSM) of the SPI-based films (20 × 20 mm²) was tested based on variations in weight and was calculated according to eqn (1).^9,14 The TSM value of each sample reported below is the mean of five replicates.

where m_a and m_b are the dried weights of the samples before and after 24 h submersion in water, respectively.

The mechanical properties of the SPI-based films were determined on a tensile machine (DCP-KZ300) with a loading speed of 50 mm min⁻¹ and an initial gauge length of 50 mm. The tensile strength (TS), Young's modulus, and elongation at break (EB) values of each sample (10 × 80 mm²) reported below, are the means of five replicates each. The film thickness was measured with a digimatic micrometer (0–25 ± 0.001 mm), with five replicates for each sample.

3 Results and discussion

3.1 Structural analysis of SPI/PEG₄₅-b-PS₂₇₆ nanocomposite films

The chemical structure of the SPI/PEG₄₅-b-PS₂₇₆ films was investigated by ATR-FTIR from 4000–650 cm⁻¹ infrared region, as shown in Fig. 1. The characteristic peaks of the PEG₄₅-b-PS₂₇₆ diblock copolymer included interim CH₂ stretching at 840 cm⁻¹, C–O stretching at 1025 and 1107 cm⁻¹, C–H stretching at 2870 cm⁻¹ of the PEG block;²¹ as well as aromatic peaks at 1600, 1498, and 1451 cm⁻¹ of the PS block, respectively (Fig. 1A). For the SPI-based films, the amide bands at 1652, 1544, and 1231 cm⁻¹ were attributed to amide I (C=O stretching), amide II (N–H bending), and amide III (C–N and N–H stretching). The peaks at 2932 and 2875 cm⁻¹ were corresponded to methylene group stretching.¹⁴ The broad overlapped band of free and bound O–H and N–H bending vibrations was at 3284 cm⁻¹. Compared to the control, the band at 3284 cm⁻¹ in the SPI film shifted to 3275 cm⁻¹ in the SPI/PEG₄₅-b-PS₂₇₆ films, reflecting a reduction in intensity as PEG₄₅-b-PS₂₇₆ addition increased and further demonstrated the hydrogen bonding interactions between the PEG block and SPI chains, which accounted for the uniform and stable dispersion of PEG₄₅-b-PS₂₇₆ nanoparticles (Scheme 2).²²

Fig. 1 ATR-FTIR spectra of the PEG₄₅-b-PS₂₇₆ block copolymer, SPI film (a), SPI/PEG₄₅-b-PS₂₇₆ nanocomposite films at different weight ratios (b–f) in (A) full wavenumber ranges, and (B) high wavenumber region of 4000–2200 cm⁻¹.

Scheme 2 Schematic of the PEG-b-PS nanoparticles cooperating with SPI chains to form nanocomposite films with enhanced mechanical properties and surface hydrophobicity.

The surface chemical structure of the films was ascertained by comparing the XPS spectra of unmodified and modified surfaces of the films. Fig. 2 shows the XPS wide scan and C 1s core-level spectra of the unmodified film (Film A, Fig. 2A) and SPI/PEG₄₅-b-PS₂₇₆ nanocomposite film (Film D, Fig. 2B). The wide scan spectra of the films showed peak components of C 1s and O 1s ascribed to C and O elements, and it was found that C concentration increased in the SPI/PEG₄₅-b-PS₂₇₆ nanocomposite film, thus, demonstrating the surface stacking of the amphiphilic PEG₄₅-b-PS₂₇₆ nanoparticles, shown in Scheme 2. The C 1s core-level spectrum of Film A was curved-fitted to four peak components with the binding energy (BE) at 284.3 eV for C–C/C–H species, 285.7 eV for C–O species, 287.6 eV for O–C=O species, and 291.8 eV for the π–π* satellite species.²³ During water evaporation, it was presumed that the PS segments were more favored to accumulate on the uppermost surface than the PEG segments due to its lower surface free energy of 39.4 mN m⁻¹ than that of PEG (44.0 mN m⁻¹) at 20 °C.²⁴ Besides, the length of the PS chains was much longer than that of the PEG, contributing to the surface stacking of the PS segments. This inference was verified by the C 1s core-level spectrum of Film D, in which the intensity of the curved-fitted peak at 284.2 eV for C–C/C–H species was greatly increased, whereas that of the peak at 285.5 eV for C–O species dropped, indicating the enrichment of the C–C/C–H bonds of the PS segments located uppermost to the film surface.²⁵ The uppermost location of PS segments in the SPI/PEG₄₅-b-PS₂₇₆ nanocomposite film enhanced the films' hydrophobicity (Scheme 2).

Fig. 2 XPS wide-scan and C 1s core-level spectra of (A1–A2) an untreated SPI film and (B1–B2) a SPI/PEG₄₅-b-PS₂₇₆ nanocomposite film.

The effects of the PEG₄₅-b-PS₂₇₆ nanoparticles on the protein structure conformations were observed via XRD (Fig. 3). The peaks at 2θ = 8.54° and 20.5° were corresponded to α-helix and β-sheet structures of the SPI secondary conformation, respectively. The peak at 8.54° in the unmodified SPI film shifted to 9.02° and decreased in intensity after the incorporation of PEG₄₅-b-PS₂₇₆ nanoparticles (Table 2), resulting from the denaturation of protein molecules.¹⁴ The calculated degrees of crystallinity of the SPI/PEG₄₅-b-PS₂₇₆ nanocomposite films are listed in Table 2. As the addition of the PEG₄₅-b-PS₂₇₆ nanoparticles increased, film crystallinity dropped to 20.5% and reached its minimum (Film E), possibly as a result of the physical crosslinking between the hydrophilic PEG block and SPI matrix. A further increase in the addition of PEG₄₅-b-PS₂₇₆ nanoparticles resulted in a slight increase in film crystallinity.

Fig. 3 X-ray diffraction patterns of the SPI film (Film A) and SPI/PEG₄₅-b-PS₂₇₆ nanocomposite films (Films C, E, and F).

Table 2 Crystallinity parameters of the SPI-based films

Entry	Film A	Film C	Film E	Film F
Crystallinity degree (%)	23.8	20.9	20.5	21.3
Peak 1 (2θ)	8.54	9.02	8.95	8.75
Peak 2 (2θ)	20.5	20.4	20.2	20.3

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The thermal performances of SPI/PEG₄₅-b-PS₂₇₆ nanocomposite films were tested and analyzed as presented in the ESI.†

3.2 Micromorphology of SPI/PEG₄₅-b-PS₂₇₆ nanocomposite films

The dispersion of the PEG₄₅-b-PS₂₇₆ nanoparticles in the SPI film casting solution, and their compatibility were observed by TEM as shown in Fig. 4. The SPI showed a balanced composition of nonpolar, polar, and charged amino acids, after the heat treatment. The SPI chains were unfolded and could continue to aggregate and form micro-/nano-spheres in the aqueous environment.²⁶ As shown in Fig. 4A, the SPI chains were re-aggregated to fabricate bi-phasic, shell-kernel nanoparticles with diameters ranging from 25–50 nm. Fig. 4B shows the TEM image of the vesicle-like PEG₄₅-b-PS₂₇₆ nanoparticles at a size of approximately 90 to 190 nm in the aqueous solution, where the insoluble PS block formed the membrane of vesicles, and the soluble PEG chains were tethered to both the inner/outer sides of the membrane as shown in the inserted schematic graph. Fig. 4C shows the SPI/PEG₄₅-b-PS₂₇₆ nanocomposite solution, and it was found that the PEG₄₅-b-PS₂₇₆ nanoparticles were well-defined and uniformly dispersed in the SPI aqueous solution, in which the corona of the PEG block combined with the polar groups of the SPI molecules via hydrogen bonding.

Fig. 4 TEM images of diluted (A) SPI solution, (B) PEG₄₅-b-PS₂₇₆ nanoparticles (insert is the schematic structure of PEG₄₅-b-PS₂₇₆), and (C) SPI/PEG₄₅-b-PS₂₇₆ nanocomposite solution.

The fracture surface morphologies of the SPI/PEG₄₅-b-PS₂₇₆ nanocomposite films were compared by FE-SEM, as shown in Fig. 5. The unfolded SPI chains after heat treatment self-aggregated (as confirmed by TEM) and fabricated a relatively compact crosslinking network during the drying process, as evidenced by the even and smooth fracture surface shown in Fig. 5A. The river-like pattern indicates a cleavage fracture in the SPI film. In comparison with the unmodified SPI film, the fracture surface morphologies of the SPI/PEG₄₅-b-PS₂₇₆ nanocomposite films were diversified and relatively uneven with several light and spotted points, which we speculate to be the PEG₄₅-b-PS₂₇₆ nanoparticles together with a certain amount of aggregates. The spotted points on the fracture surface may blunt the crack tips and prevent cracks from propagating, thus increasing the film's toughness.²⁷ As shown in Fig. 5B, the addition of 1 wt% PEG₄₅-b-PS₂₇₆ nanoparticles led to a fairly rough surface. When the addition was raised to 4 wt%, the fracture surface of the SPI/PEG₄₅-b-PS₂₇₆ nanocomposite films became dense and smooth corresponding to the TS increment. Interestingly, the nanoparticles were prone to terminate on the horizontal crack lines, which should have dissipated the forces and toughened the film. Unexpectedly, the excessive addition of nanoparticles caused the films to decrease in smoothness, (Fig. 5D) at least partially contributing to increased brittleness.

Fig. 5 FE-SEM images of the fracture surfaces of (A) unmodified SPI film and SPI/PEG₄₅-b-PS₂₇₆ nanocomposite films with the addition of the PEG₄₅-b-PS₂₇₆ nanoparticles at (B) 1 wt%, (C) 4 wt%, and (D) 8 wt%.

3.3 Mechanical properties of SPI/PEG₄₅-b-PS₂₇₆ nanocomposite films

The contributions of PEG₄₅-b-PS₂₇₆ nanoparticles to the mechanical properties (including TS, Young's modulus, and EB) of the SPI-based films are depicted in Fig. 6. The addition of PEG₄₅-b-PS₂₇₆ nanoparticles in the SPI matrix resulted in the simultaneous enhancement of the TS and EB of the modified films. TS values increased from 4.81 MPa (control) to a maximum of 7.06 MPa (Film E) after modification. The TS of the film slightly declined with the addition of excessive amounts of nanoparticles. The EB of Film E increased to 136.20% from 125.10% of the control.

Fig. 6 Mechanical properties (A) TS; (B) Young's modulus; (C) EB of the control and SPI/PEG₄₅-b-PS₂₇₆ nanocomposite films.

The simultaneous modes of enhancement were most likely related to the highly effective dispersion of nanoparticles in the SPI matrix, as evidenced by the TEM results. The Young's modulus of the SPI/PEG₄₅-b-PS₂₇₆ nanocomposite films, which are based on slope calculations of the obtained stress–strain curves, slightly declined in comparison to the control from 241.3 MPa to 209.4 MPa (Film E). The EB values of the SPI/PEG₄₅-b-PS₂₇₆ nanocomposite films were higher than that of the control with the exception of Film B (87.34%), possibly due to the fact that the insufficient addition of PEG₄₅-b-PS₂₇₆ nanoparticles was unable to soften the SPI chains or transfer the impulsive force. The EB value of the SPI/PEG₄₅-b-PS₂₇₆ nanocomposite film reached its maximum at 144.10% (Film D) and then slightly decreased as nanoparticle addition increased. It was found that the water-soluble PEG block could conjoin with the –NH₂/–OH groups of the SPI chains via hydrogen bonding, which both fortified the crosslinking network and facilitated stress transfer in the interfacial region. The nanoscaled-discontinuous filling of the PEG₄₅-b-PS₂₇₆ nanoparticles may also have broadened the glass transition¹⁸ to considerably enhance the plastic deformation of the nanocomposite film.

3.4 Water resistance and surface hydrophobicity of SPI/PEG₄₅-b-PS₂₇₆ nanocomposite films

The water resistance of the film was determined according to a TSM measurement. The effects of the addition of the PEG₄₅-b-PS₂₇₆ nanoparticles on the TSM of SPI/PEG₄₅-b-PS₂₇₆ nanocomposite films are shown in Fig. 7A. The surface hydrophobicity of the modified films was evaluated according to contact angles (which were measured by taking photographs at 120 s), as shown in Fig. 7B. The addition of nanoparticles contributed to a significant enhancement in water resistance and a marked decline in the TMS, from 42.13% (control) to 26.31% (Film E), which ascribed to the hydrogen bonding between the PEG blocks and the SPI matrix (corresponding to denser fractures and a more compact film structure). However, there was no significant reduction in the TSM once the addition of the PEG₄₅-b-PS₂₇₆ nanoparticles reached 8 wt%.

Fig. 7 (A) Total soluble matter of the SPI/PEG₄₅-b-PS₂₇₆ nanocomposite films and (B) photographs of water drops on the film surfaces at 120 s.

The compacted structure of the modified SPI chains, in addition to the well-dispersed and discontinuous-filling of the hydrophobic PS blocks, restricted the permeation of water molecules and reduced the TSM of the SPI/PEG₄₅-b-PS₂₇₆ nanocomposite films. Generally, during water evaporation, the SPI chains self-aggregated and readily exposed polar groups to the top or bottom edges, thus resulting in inherently low water resistance and unstable film performance.²⁸ It was found that as the addition of the PEG₄₅-b-PS₂₇₆ nanoparticles increased, the surface contact angles of the SPI/PEG₄₅-b-PS₂₇₆ nanocomposite films increased from 41.16° ± 2.54° (control) up to 51.46° ± 2.45° (Film F). Furthermore, when the weight proportions of PEG₄₅-b-PS₂₇₆ nanoparticles to SPI exceeded 2 wt%, the surface contact angles increased to a greater extent (Film D–F), partly related to the surface-aggregated nanoparticles working with the hydrophobic PS block to reject water infiltration (Scheme 2). The crosslinking of the PEG block and SPI chains via hydrogen bonding also decreased the polar amine acid (e.g., glutamine, lysine) contents and exposed certain nonpolar groups (hydrophobic) to the film surface.²⁸

4 Conclusions

Vesicle-like nanoparticles of poly(ethylene glycol)-block-polystyrene (PEG-b-PS), synthesized via a macro-RAFT agent of PEG₄₅-TTC (TTC represents the RAFT terminal of trithiocarbonate) mediated dispersion polymerization of styrene, were employed to fabricate high-performance SPI-based nanocomposite films in this study. The effective dispersion of the PEG₄₅-b-PS₂₇₆ nanoparticles into the SPI matrix and their compatibility were confirmed by ATR-FTIR, XPS, and XRD analysis, as well as by TEM and FE-SEM observations. The PEG₄₅-b-PS₂₇₆ nanoparticles were indeed uniformly and discontinuously filled into the SPI matrix by the virtue of hydrophobic PS core. The hydrogen bonding interaction between the PEG block conjunct with the –NH₂/–OH groups on the SPI chains greatly contributed to fortifying the crosslinking network and benefiting interfacial stress transfer. As a result, as opposed to other nanofillers, the strength and EB of the resultant films simultaneously increased above those of the control. The surface hydrophobic PS core aggregation of the PEG₄₅-b-PS₂₇₆ nanoparticles also contributed to an increase in the water resistance and surface hydrophobicity of the film. In short, we developed a facile and efficient approach for introducing functionalized diblock copolymer nanoparticles into biopolymer matrices to fabricate high-performance bio-nanocomposite films.

Acknowledgements

This work was partially supported by the National Forestry Public Welfare Industry Major Projects of Scientific Research (201504502), and the Fundamental Research Central Funds for the Universities (BLYJ201624).

Notes and references

1. P. Gupta and K. K. Nayak, Polym. Eng. Sci., 2015, 55, 485–498.
2. R. R. Koshy, S. K. Mary, S. Thomas and L. A. Pothan, Food Hydrocolloids, 2015, 50, 174–192.
3. Y. Xu, L. Yuan, Z. Wang, P. A. Wilbon, C. Wang, F. Chu and C. Tang, Green Chem., 2016 10.1039/c6gc00859c.
4. A. González, M. C. Strumia and C. I. Alvarez Igarzabal, J. Food Eng., 2011, 106, 331–338.
5. S. Wang, M. F. Marcone, S. Barbut and L.-T. Lim, Food Res. Int., 2012, 49, 80–91.
6. K. B. Chien and R. N. Shah, Acta Biomater., 2012, 8, 694–703.
7. F. Song, D. L. Tang, X. L. Wang and Y. Z. Wang, Biomacromolecules, 2011, 12, 3369–3380.
8. E. Klüver and M. Meyer, Polym. Eng. Sci., 2015, 55, 1912–1919.
9. F. Xu, Y. Dong, W. Zhang, S. Zhang, L. Li and J. Li, Ind. Crops Prod., 2015, 67, 373–380.
10. L. Ma, Y. Yang, J. Yao, Z. Shao and X. Chen, Polym. Chem., 2013, 4, 5425.
11. A. P. Kumar, D. Depan, N. Singh Tomer and R. P. Singh, Prog. Polym. Sci., 2009, 34, 479–515.
12. S.-Y. Wang, B.-B. Zhu, D.-Z. Li, X.-Z. Fu and L. Shi, Mater. Lett., 2012, 83, 42–45.
13. A. González and C. I. Alvarez Igarzabal, Food Hydrocolloids, 2015, 43, 777–784.
14. Y. Li, H. Chen, Y. Dong, K. Li, L. Li and J. Li, Ind. Crops Prod., 2016, 82, 133–140.
15. N. Lin and A. Dufresne, Macromolecules, 2013, 46, 5570–5583.
16. X. Li, C. Qiu, N. Ji, C. Sun, L. Xiong and Q. Sun, Carbohydr. Polym., 2015, 121, 155–162.
17. C. G. Otoni, R. J. Avena-Bustillos, C. W. Olsen, C. Bilbao-Sáinz and T. H. McHugh, Food Hydrocolloids, 2016, 57, 72–79.
18. J. Jancar, J. F. Douglas, F. W. Starr, S. K. Kumar, P. Cassagnau, A. J. Lesser, S. S. Sternstein and M. J. Buehler, Polymer, 2010, 51, 3321–3343.
19. N. J. Warren, O. O. Mykhaylyk, D. Mahmood, A. J. Ryan and S. P. Armes, J. Am. Chem. Soc., 2014, 136, 1023–1033.
20. X. Shen, F. Huo, H. Kang, S. Zhang, J. Li and W. Zhang, Polym. Chem., 2015, 6, 3407–3414.
21. Y. Deng, J. Li, T. Qian, W. Guan, Y. Li and X. Yin, Chem. Eng. J., 2016, 295, 427–435.
22. X. Fan, Y. Su, X. Zhao, Y. Li, R. Zhang, T. Ma, Y. Liu and Z. Jiang, J. Membr. Sci., 2016, 499, 56–64.
23. M. Ma, Z. He, J. Yang, Q. Wang, F. Chen, K. Wang, Q. Zhang, H. Deng and Q. Fu, Langmuir, 2011, 27, 1056–1063.
24. L. B. Feng, S. X. Zhou, B. You and L. M. Wu, J. Appl. Polym. Sci., 2007, 103, 1458–1465.
25. B. Xu, C. Feng, J. Hu, P. Shi, G. Gu, L. Wang and X. Huang, ACS Appl. Mater. Interfaces, 2016, 8, 6685–6692.
26. Z. Teng, Y. Luo and Q. Wang, J. Agric. Food Chem., 2012, 60, 2712–2720.
27. W. Thitsartarn, X. Fan, Y. Sun, J. C. C. Yeo, D. Yuan and C. He, Compos. Sci. Technol., 2015, 118, 63–71.
28. Y. Li, F. Chen, L. Zhang and Y. Yao, Mater. Lett., 2015, 149, 120–122.

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Footnote

† Electronic supplementary information (ESI) available: The characterization description of the ¹H NMR and GPC analysis; the Fig. S1 is the ¹H NMR spectrum of the PEG₄₅-b-PS₂₇₆ nanoparticles, and the Fig. S2 is the GPC trace of the PEG₄₅-b-PS₂₇₆ nanoparticles. Fig. S3 and Table S1 were the data for films' thermal analysis. See DOI: 10.1039/c6ra17051j

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