Enhanced mechanical properties and thermal stability of PSMA by functionalized graphene nanosheets

Abstract

Poly(styrene-co-maleic anhydride) (PSMA) is an important heat boosting additive of some engineering materials. In this study, functionalized graphene nanosheets were developed and introduced to composite with PSMA through a simple physical blending method. To promote a homogeneous dispersion of graphene in PSMA, graphite oxide (GO) was functionalized with 3-aminopropyltriethoxysilane (APTS) before the reduction. The successful functionalization of reduced graphene oxide (RGO) was confirmed by Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analyses (TGA). This modified RGO was introduced into PSMA, and DMA was employed to investigate the mechanical properties. The results showed that the mechanical storage modulus of PSMA was greatly improved, especially at high temperature. Meanwhile, the thermal stability of PSMA/APTS-RGO was also greatly enhanced.

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1 Introduction

Due to the large polarity, high compatibility and reactivity between polymers and fillers, poly(styrene-co-maleic anhydride) (PSMA) is widely used as a compatibility booster and heat boosting additive in many engineering materials.¹ However, when added to resins which are often used under hot environments, the mechanical properties and thermal stability of PSMA decreases dramatically. Thus, to enhance the properties of PSMA is of great significance.

Compositing nanofillers with the polymer by strong interface interactions has proven to be an effective way to substantially improve its performances.² Among them, chemical bonding is the most commonly used method. For example, N-phenylaminomethyl polyhedral oligomeric silsesquioxane (POSS) was introduced as a crosslink agent to enhance the mechanical properties of PSMA through the formation of permanent network.² But the formation of crosslink networks in polymer composites always induced the difficulty in the further processing. Recently, the incorporation of traditional nanoparticles into PSMA without chemical bonding was observed to promote the chain motion and lower the T_g of polymers, resulting in a decrease of mechanical properties and thermal stability. For instance, Orietta et al. prepared PSMA/POSS nanocomposites through grafted mono-functional POSS onto PSMA chains. They found that the unbound POSS act as a plasticizer increased molecular mobility of polymer chains, while the grafted POSS had no significant effects.³ Therefore, to improve the mechanical properties and thermal stability without any loss of process ability of PSMA, new fillers are urgently needed.

As known, when PSMA goes into its rubber state at high temperature, the motion of the polymer chains is much accelerated and polymer becomes soft. Although traditional nanofillers can strengthen PSMA in its glass state, the high temperature led to the decreased mechanical properties of PSMA-based nanocomposites. If the filler can form a network and interpenetrate the PSMA matrix, the collapse of the intrinsic PSMA network will be largely inhibited. To form stable filler network inside the PSMA matrix, nanofillers with large specific surface area and sufficient filler-host matrix interface interaction are needed. However, for most conventional nanofillers, aggregation occurs before sufficient amount being added. The aggregation of the fillers at this stage results in the decreased performance of the composites eventually. Accordingly, it is expected that fillers with large surface area may provide better networks for interfacing with PSMA and sufficient strength for sustaining the PSMA, which can improve the properties of PSMA at high temperature.⁴

Graphene, which consists of a two-dimensional (2D) sheet of covalently bonded carbon atoms, possesses a layered structure with remarkable large surface area (calculated value, 2630 m² g⁻¹),⁵ high Young's modulus (∼1100 GPa)⁶ and thermal stability.^7,8 In recent years, there has been enormous interest in fabricating graphene-based nanocomposites, which resulted in enhanced mechanical, crystal and electrical properties of the host polymers.^9–13 It is supposed that the unique structure can form interpenetrated network with polymers.^14–17 For example, Potts et al. incorporated 0.05 wt% graphene into PMMA through in situ polymerization and observed the enhancement in storage modulus and shift in glass transition temperature for PMMA.¹⁸ In our previous work, we found that functionalized graphene had good biocompatibility with PLLA. These graphene could be easily introduced into PLLA via simple physical blending method, and greatly improved its mechanical properties and heat resistance.⁴ As there was abundant acetic anhydride group in PSMA, the NH₂ functionalized graphene was supposed to have good compatibility with it.

In this study, RGO was introduced to composite with PSMA and the mechanical and thermal properties were investigated. In order to promote better dispersion of RGO in the matrix and the effective interface interaction between polymer and the filler, RGO was chemically functionalized with 3-aminopropyltriethoxysilane (APTS).^19,20 The APTS-RGO was found homogenously dispersed in PSMA matrix, and much interface interaction was generated between PSMA and the layered RGO (Scheme 1). As expected, the resulting nanocomposite via physical blend method exhibited greatly enhanced storage modulus and improved heat distortion resistance.

Scheme 1 Scheme for the preparation of GO, functionalized GO and PSMA/graphene nanocomposites.

2 Results and discussion

The functionalization of APTS-RGO was investigated using FTIR, as shown in Fig. 1a. Compared with GO, the peaks at 3410 cm⁻¹, 1730 cm⁻¹ and 1059 cm⁻¹ in the spectrum of RGO disappeared, indicating successful reduction of GO.^19,21,22 And in the spectrum of APTS-RGO, the newly appeared absorption peaks at 1045 cm⁻¹ (representing the Si–O–C bond) and 1119 cm⁻¹ (representing the Si–O–Si bond) confirmed the successful introduction of APTS.^19,23

Fig. 1 (a) FTIR spectra of GO, RGO and APTS-RGO. (b) TGA curves of GO, RGO and APTS-RGO in N₂ at a heating rate of 20 °C min⁻¹. (c) XPS survey scans of RGO. (d) XPS survey scans of APTS-RGO.

Since the substituents adhered to graphene sheets can be thermally removed under 200–500 °C, TGA is applied to confirm the functionalization of nanoparticles.²⁴ Fig. 1b showed the TGA curves of GO, RGO and APTS-RGO at a heating rate of 20 °C min⁻¹ under N₂. For GO, due to the pyrolysis of the labile oxygen-containing functional groups, major mass loss occurred during 200 °C to 300 °C.²⁵ RGO showed a better thermal stability than GO, because most thermally labile oxygen functional groups had been removed after chemical reduction. Compared with RGO, APTS-RGO showed more weight loss above 200 °C, which was ascribed to the decomposition of APTS on the modified graphene sheets to functionalized RGO.

X-ray photoelectron spectroscopy (XPS) was employed to further analysis the difference between RGO and APTS-RGO. As shown in Fig. 1c, only C 1s and O 1s were detected from the survey of RGO. And the survey of APTS-RGO (Fig. 1d) illustrated a strong band of N 1s and double bands of Si 2p originating from APTS. In addition, the atom compositions of N and Si element in APTS-RGO were 5.74% and 10.82% respectively (Table 1). These supported that the graphene was successfully functionalized by APTS.

Table 1 Elementary composition of APTS-RGO

Element	Peak	Atom composition (%)
C	1s	63.28
O	1s	20.26
N	2p	5.74
Si	1s	10.82

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The good dispersion of graphene in organic solvent is a critical factor for the development of polymer nanocomposites. The dispersion of RGO and APTS-RGO in DMF was shown in Fig. 2. As shown, RGO agglomerated apparently and settled down on the bottom of the glass bottle (Fig. 2a). On the contrary, APTS-RGO was homogeneously dispersed in DMF without precipitate (Fig. 2b). The phenomenon demonstrated that the grafting APTS on the graphene sheets greatly improved its dispersibility in organic solvent, which was beneficial for the subsequent fabrication of PSMA/graphene nanocomposites.

Fig. 2 Photographs of the dispersion of RGO (a) and APTS-RGO (b) in DMF (samples had been kept for 24 h after 30 min ultrasonic treatment), and TEM images of RGO (c) and APTS-RGO (d), photographs of PSMA/RGO-0.2% (e) and PSMA/APTS-RGO-0.2% (f) composite films.

TEM was employed to further investigate the micro-morphologies of RGO and APTS-RGO. As seen in Fig. 2c, the RGO layers tended to tack together because of the strong van der Waals interaction between graphene layers. To the contrast, the APTS-RGO (Fig. 2d) exhibited better dispersion in DMF, and the modified graphene sheets were well exfoliated. The grafted APTS was helpful for the dispersion and exfoliation of graphene sheets, and these were prerequisites for the formation of a graphene network in polymer matrix.^26–28

The PSMA/APTS-RGO nanocomposites were prepared via a simple solution blend method. To study the morphology of the composite, the composites with 0.2 wt% graphene was selected for characterization. Apparently, the graphene sheets aggregated heavily in PSMA/RGO composites (Fig. 2e), while PSMA/APTS-RGO with the same filler ratio showed a homogeneous appearance (Fig. 2f). The comparison suggested that the compatibility between graphene sheets and PSMA matrix was improved with the incorporation of APTS.

In order to investigate the distribution of APTS-RGO in PSMA matrix, the fractured surface of the cross-section was further study by SEM. As shown in Fig. 3, lots of particles in the fractured surface of PSMA were observed, which were supposed to be the junctions of PSMA segments.² With the incorporation of APTS-RGO, the surface roughness increased with more graphene loadings. Besides the junctions, numerous strips were grown at the fractured surface of PSMA/APTS-RGO nanocomposites (Fig. 3a–c), which were supposed to be the wrinkle surface of the graphene sheets. The APTS-RGO sheets were well-dispersed in the PSMA matrix, well-wrapping in or at the surface of PSMA matrix.⁹ The TEM images of PSMA/APTS-RGO nanocomposites also showed the uniform distribution of graphene nanosheets in PSMA (Fig. S1†). Homogeneous dispersion of RGO in PSMA and sufficient graphene–PSMA interaction could promote the formation of a stable filler network inside the polymer, which could preserve the mechanical properties of PSMA better in high temperature region and greatly enhance the mechanical properties.²⁹

Fig. 3 SEM images of (a) neat PSMA and composites containing 0.2 wt% (b), 0.5 wt% (c) and 1.0 wt% (d) of APTS-RGO loadings.

To investigate the effects of the functionalized RGO on the mechanical properties of PSMA, especially in the high temperature region, dynamic mechanical tests for neat PSMA and the nanocomposites containing 0.2, 0.5, 1.0 wt% of APTS-RGO were performed (Fig. 4a). For neat PSMA, the storage modulus started to decrease when the temperature was higher than 156 °C, which was the glass transition temperature of PSMA. For the PSMA/APTS-RGO nanocomposites, the storage moduli were similar to that of neat PSMA in the glassy states, while it showed higher storage modulus in the high temperature region. For example, when 0.2 wt% APTS-RGO added, the storage modulus in the high temperature region increased by one order of magnitude compared to neat PSMA. The greatly enhancement of the storage modulus might be contributed by the well-formed graphene network. Fig. 4b showed the tan δ ∼ temperature curves of neat PSMA and nanocomposites with different graphene loadings. The T_g of the nanocomposites shifted gradually with the increasing amount of APTS-RGO, indicating an enhanced thermal heat stability.

Fig. 4 (a) Storage modulus (a) and tan δ (b) curves for PSMA and PSMA/APTS-RGO, (c) TGA results for neat PSMA and PSMA/APTS-RGO nanocomposites, (d) frequency dependence (ω) of the dynamic storage modulus of PSMA/APTS-RGO.

Fig. 4c showed the TGA results of neat PSMA and PSMA/APTS-RGO nanocomposites. The temperature for 5% weight loss of composites exhibited a respectable increase (14 °C) by addition of 1 wt% APTS-RGO, and the char yield of the nanocomposites was also improved. The strong interaction between the PSMA matrix and APTS-RGO sheets restricted the chain mobility of polymer near the graphene surface. Moreover, the APTS-RGO with high aspect ratio prevented the permeation of oxygen and delayed the escape of volatile degradation products and also chars formation.^30,31 The significantly improved mechanical properties and heat stability could be ascribed to the introduction of finely dispersed functionalized RGO, which formed a filler network in the matrix and had a better preservation of the mechanical properties for PSMA in the high temperature region.

In order to determine whether the graphene network was formed in PSMA matrix, rheology experiment was employed to investigate the microstructure of the composites. Fig. 4d showed the variation in storage modulus (G′) as a function of angular frequency (ω) for the PSMA nanocomposites with different APTS-RGO loadings. For all the samples, the slope of G′/ω gradually decreased and G′ gradually increased with more APTS-RGO amounts, which was typical for all the particles-filled polymers.³² A much stronger increase in G′ was observed at low frequencies, which suggested that a graphene network was gradually developed as graphene content increased. When the graphene content reached 1 wt%, a nearly G′ plateau appeared indicating the formation of a three-dimensional percolation network through the sample. This graphene network could effectively preserve the mechanical properties of PSMA at high temperature, and thus, enhanced the storage modulus and heat stability.^4,33

3 Conclusion

In this work, a novel nanocomposite was developed by simple physical blending of PSMA with APTS-RGO. The mechanical properties and heat stability of the composites were largely increased in the high temperature region. 0.2 wt% addition of APTS-RGO dramatically increased the storage modulus of PSMA in the high temperature region by one order of magnitude compared with neat PSMA. Meanwhile, the heat stability of PSMA/APTS-RGO was greatly improved as well. Homogeneous dispersion of graphene and effective interfacial interaction between graphene sheets and PSMA, a graphene–polymer network had been formed in the composites. This work provides a method to greatly improve the properties of polymers via simply physical blending way, with no sacrifice of their process ability.

Acknowledgements

The authors gratefully acknowledge support from the Fundamental Research Funds for the Central Universities (1104020505), National Natural Science Foundation of China (51403003), and National Science Foundation of Jiangsu Province (BK2011413). We also thank Dr Xiaoliang Wang (School of Chemistry and Chemical Engineering, Nanjing University) for the rheology analysis.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra00034g

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