A polystyrene/silica hybrid nanomatrix formed in natural rubber

Our work aims to investigate the morphology and properties of natural rubber (NR) dispersed in a polystyrene/silica hybrid nanomatrix. The hybrid nanomatrix structure was formed by the graft copolymerization of vinyltriethoxysilane onto NR grafted with styrene. This hybrid nanomatrix structure was composed of nanosilica with a size of less than 100 nm, and the incorporation of polystyrene resulted in outstanding mechanical and viscoelastic properties. The tensile strength of NR with the hybrid nanomatrix structure was 23 MPa, and the storage modulus was 2.54 MPa, which were 2.5 and 15 times higher than those of the silica nanomatrix. The outstanding mechanical and viscoelastic properties of this NR material were attributed to the formation of the hybrid nanomatrix structure. The roles of polystyrene and silica and their synergetic effect were clari ﬁ ed by investigating the morphology and properties of the hybrid nanomatrix after acetone extraction and annealing.


Introduction
A hybrid nanomatrix structure is defined as the hierarchical structure of a multicomponent system. Natural rubber (NR) with a hybrid nanomatrix structure is composed of an NR dispersoid as the major component and a hybrid nanomatrix as the minor component. The hybrid nanomatrix may consist of inorganic components, i.e., silica, and organic components, i.e., polystyrene (PS), which are prepared through the graft copolymerization of vinyltriethoxysilane (VTES) and styrene, respectively. NR materials with hybrid nanomatrix structures may have the potential to be used as novel NR-based nanomaterials due to their excellent mechanical and viscoelastic properties, which are indispensable for highly advanced applications, i.e., heavy-duty applications.
Strategies for preparing NR materials with hybrid nanomatrix structures have been designed to explore new applications of NR since they realizes the properties of various functional components and the outstanding properties of NR [1][2][3]. For instance, the organic component in the nanomatrix may contribute to functionalization and modification, and the inorganic component may be responsible for durability, compressiveness, abrasion resistance and fatigue properties. Moreover, the synergetic effect between these components may enhance the performance of the materials. In previous works [4][5][6], a silica nanomatrix was fabricated through the graft copolymerization of VTES or p-styryltrimethoxysilane (STMS) onto NR in the latex stage. The polymerization of VTES and STMS occurred with vinyl head groups as monomers to form a vinyl polymer, and the organosilane groups were hydrolyzed and condensed to form in situ silica; thus, a nanomatrix composed of vinyl polymer and silica was formed. A high mechanical strength was achieved due to the synergistic effect between the rubber and organic-inorganic nanomatrix. However, the vinyl polymer matrix that formed was too thin, and the size of the silica particles was rather large, i.e., 200-300 nm, when the grafting occurred with VTES. Hence, the enhanced mechanical strength was not significant. Furthermore, there was no attempt to use a suitable organic monomer together with an organosilane monomer in the graft copolymerization system. It is expected that grafting other organic polymers onto NR followed by the formation of nanosilica would result in an NR material with outstanding properties. In our previous work [7], we successfully prepared a filler nanomatrix structure by the graft copolymerization of styrene and VTES onto NR. The graft copolymer had a high tensile strength and storage modulus. Noticeably, nanosilica and PS were found to form a hybrid nanomatrix, in which nanosilica was well incorporated into the PS matrix. Thus, in the present work, we constructed a hybrid nanomatrix structure composed of nanosilica and PS and investigated the relationship between the morphology and properties of the NR material with a hybrid nanomatrix structure.

Experimental
Silica nanomatrix was formed by the graft copolymerization of VTES (1.0 mol/kg-rubber) onto deproteinized NR (DPNR) in the presence of 0.033 mol/kg-rubber of tetraethylenepentamine/tert-butylhydroperoxide (TEPA/TBHP) as initiators for approximately 2 h. The gross latex, after rotary evaporation, was cast into films and dried. In contrast, the PS/silica hybrid nanomatrix was prepared by two consecutive graft copolymerizations. First, styrene monomer (1.0 mol/kg rubber) was graft copolymerized onto DPNR latex for 3 h to form PS-grafted NR. Subsequently, the graft copolymerization of the VTES monomer (1.0 mol/ kg rubber) onto PS-grafted NR gross latex was carried out, followed by rotary evaporation and casting. Each graft copolymerization was carried out using TEPA/TBHP initiators of 0.033 mol/kg rubber. The PS/silica hybrid nanomatrix sample was annealed at 130°C for 30 mins under a pressure of 10 MPa and subsequently cooled slowly at room temperature. Acetone extraction of the PS/silica hybrid nanomatrix was conducted for 24 h under N 2 gas.
Silica content was determined from the weight of the sample before and after incineration at 1000°C for 3 h. Styrene content and styrene conversion were calculated as described in our previous work [8]. The density of the samples was determined according to ASTM D792 using a DH-300 Electronic Densimeter. Tensile properties were measured with a Toyoseiki Strograph VG5E (Japan) according to JIS K6251. Graft copolymer films with a thickness of approximately 1 mm were cut into dumbbell shapes, and tensile measurements were performed at room temperature with a crosshead speed of 200 mm/min. Viscoelastic properties were measured with an MCR 301 (Anton Paar Physica) analyzer using a PP12 measuring system. The frequency-dependence measurement was performed at an initial strain of 1% and an angular frequency of 0.1-100 rad/s at 30°C. The morphology of the unstained samples was observed using a transmission electron microscope with a JEOL JEM-2100 at an accelerating voltage of 200 kV.

Results and discussion
A novel hybrid nanomatrix structure was designed via the sequential graft copolymerization of first styrene and then VTES onto NR in the latex stage. The conversion and styrene content measured after the graft copolymerization of styrene were 62.65% and 6.3 phr, respectively. Nanosilica was then introduced through the graft copolymerization of the VTES onto the gross latex of PS-grafted NR. The resulting material, the so-called PS/silica hybrid nanomatrix, had a silica content of 4.6 phr. Silica nanomatrix, which was prepared from the graft copolymerization of VTES onto NR, had a silica content of 4.4 phr. To study the effect of morphology on the properties of NR with the PS/ silica hybrid nanomatrix, acetone extraction (AE) and annealing were performed. Figure 1 shows the morphology of the silica nanomatrix, PS/silica hybrid nanomatrix, and the hybrid nanomatrix after AE and annealing. PS and nanosilica were formed between NR particles as a hybrid nanomatrix (Fig. 1b). The PS nanomatrix was thicker than the vinyl polymer matrix formed in the silica nanomatrix (Fig. 1a). Furthermore, the nanosilica formed in the hybrid nanomatrix (~60 nm) was much smaller than that in the silica nanomatrix (~200 nm). The smaller nanosilica and the thicker PS nanomatrix may have contributed to the density of the samples: the density of the PS/silica hybrid nanomatrix was approximately 0.944 g/cm 3 , which was higher than that of the silica nanomatrix (0.917 g/cm 3 ) and that of pure NR (0.886 g/cm 3 ). The presence of grafted PS may have restricted the growth of the silica particles formed in the PS/silica hybrid nanomatrix. This may have occurred because space existed between the rubber particles and large silica particles in the silica nanomatrix, whereas the silica particles were small in the PS/silica hybrid nanomatrix. The tiny silica particles may have filled in the space and contributed to the high density of the PS/silica hybrid nanomatrix. Figure 1c and d shows the morphology of the PS/silica hybrid nanomatrix after AE and annealing. After AE, nanosilica and grafted PS were well dispersed in the hybrid nanomatrix surrounding the NR particles. However, the thickness of the PS nanomatrix decreased due to the removal of free PS, and the nanosilica remained as is. After annealing, the NR particles seemed to merge together, and the silica particles appeared to aggregate. These phenomena may have occurred due to the diffusion of rubber particles and the dislocation of silica particles, respectively. This implies that the morphology of the PS/silica hybrid nanomatrix changed from continuous to discontinuous. Changes in morphology may have affected the mechanical and viscoelastic properties of the materials.  Figure 2 shows the tensile strength of NR with the silica nanomatrix and PS/silica hybrid nanomatrix before and after AE and annealing. The stress at break of the PS/silica hybrid nanomatrix, i.e., 23 MPa, was 2.5 times higher than that of the silica nanomatrix (~9.0 MPa). In contrast, the annealed PS/silica hybrid nanomatrix had a similar stressstrain curve but smaller stress and strain at break compared to the PS/silica hybrid nanomatrix. Thus, the morphology changed from a continuous PS matrix to a discontinuous PS matrix simultaneously resulted in a decreased stress and strain at break of the PS/silica hybrid nanomatrix. This observation differs from what was observed for the PS nanomatrix [9], in that the discontinuous PS nanomatrix exhibited a more rigid and brittle PS matrix. After AE, the stress at break of the hybrid nanomatrix decreased to 10 MPa, i.e., smaller than one-half of the PS/silica hybrid nanomatrix itself. This result indicated that the thickness of the PS nanomatrix contributed to the high mechanical properties of the hybrid nanomatrix. Noticeably, the stress at a similar strain for the acetone-extracted PS/silica hybrid nanomatrix was higher than those for the PS/silica hybrid nanomatrix, annealed PS/silica hybrid nanomatrix, and silica nanomatrix. This suggested that nanosilica may have contributed to the high stiffness or hardness of NR. Table 1 shows the composition and properties of the various nanomatrix structures formed in NR. The PS/silica hybrid nanomatrix achieved a 23 MPa stress at break and 960% strain at break. These values were better than the stress at break of 20 MPa and strain at break of 1750% for the PS nanomatrix [8] and the stress at break of 20 MPa and strain at break of 500% for the nanodiamond nanomatrix [10]. Thus, regardless of the unvulcanized state, the PS/ silica hybrid nanomatrix attained both a high stress and well-suited strain at break of vulcanized rubber, which has never been observed for unvulcanized NR. Noticeably, the PS/silica hybrid nanomatrix contained smaller amounts of PS and silica than the PS nanomatrix and the nanodiamond nanomatrix. Thus, the enhancement in mechanical properties was attributed to the synergetic effect of PS and nanosilica. Figure 3 shows the frequency dependence of the storage modulus (G'), loss modulus (G"), and loss tangent (tan δ) of the silica nanomatrix and PS/silica hybrid nanomatrix. The Nanodiamond nanomatrix [10] 40 wt.% nanodiamond 20 500 value of G' of the PS/silica hybrid nanomatrix was 2.54 MPa, which was 15 times higher than that of the silica nanomatrix, i.e., 0.17 MPa. The G' of the PS/silica hybrid nanomatrix and silica nanomatrix exhibited similar frequency dependences, as did G" and tan δ.

Viscoelastic properties
The frequency dependences of the storage modulus (G'), loss modulus (G"), and loss tangent (tan δ) for the annealed PS/silica hybrid nanomatrix and the acetone-extracted PS/ silica hybrid nanomatrix at 30°C are shown in Fig. 4. The value of G' of the annealed PS/silica hybrid nanomatrix was 2.52 MPa, similar to that of the PS/silica hybrid nanomatrix. The value increased to 3.23 MPa after AE. This may have occurred due to the increase in the hardness of the rubber after AE, as discussed in the previous section.
As seen in Fig. 4, the frequency dependence of G' of the acetone-extracted PS/silica hybrid nanomatrix had the lowest slope, and that of the annealed PS/silica hybrid nanomatrix had the highest slope, which corresponded to a weak and strong dependency, respectively. This may have occurred because the hybrid nanomatrix after AE hindered chain movement, leading to a slow response against frequency. In contrast, the fast response of the hybrid nanomatrix after annealing may have been due to the distortion of the nanomatrix. This fast response also resulted in a high energy loss. Hence, the value of G" of the annealed PS/silica hybrid nanomatrix was observed to be the highest and that of the acetone-extracted PS/silica hybrid nanomatrix was the lowest in both high-and low-frequency regions. Accordingly, the tan δ of the annealed PS/silica hybrid nanomatrix was the highest and that of the acetone-extracted PS/silica hybrid nanomatrix was the lowest. The high and low tan δ values of the annealed PS/silica hybrid nanomatrix and acetoneextracted PS/silica hybrid nanomatrix suggested that the PS/silica hybrid nanomatrix may be more viscous after annealing and more elastic after AE, respectively. Furthermore, the acetone-extracted PS/silica hybrid nanomatrix had entropic and energetic elasticity characteristics similar to those of the PS/silica hybrid nanomatrix, while the annealed PS/silica hybrid nanomatrix did not possess this feature due to the distortion of the hybrid nanomatrix.

Conclusions
A PS/silica hybrid nanomatrix was formed by the graft copolymerization of first styrene and then VTES onto NR particles in the latex stage. The NR particles were well dispersed in the nanomatrix consisting of PS and nanosilica, as elucidated by TEM. The synergetic effect of PS and nanosilica on the mechanical properties was clarified; that is, the PS nanomatrix contributed to the high stress and strain at break, and the nanosilica contributed to the hardness of the NR film. The superior properties of the hybrid nanomatrix were concluded to be due to the synergetic effect of PS and nanosilica. The morphology and mechanical properties of the hybrid nanomatrix were maintained after acetone extraction, whereas they were distorted and reduced, respectively, after annealing. The hybrid nanomatrix, thus, is one of the hierarchical structures that can be formed in NR, and it may be beneficial for expanding the range of NR multifunctional applications.

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