Thermal stability polystyrene/clay nanocomposites

Samakande A, Juodaityte JJ, Sanderson RD, et al. (2008) Novel cationic RAFT-mediated polystyrene/clay nanocomposites synthesis, characterization, and thermal stability. Macromol Mater Eng 293 428 37... 

A. Samakande, J. J. Juodaityte, R. D. Sanderson, and P. C. Hartmann, Novel cationic raft-mediated polystyrene/clay nanocomposites Synthesis, characterization, and thermal stability. Macromolecular Materials and Engineering, 293 (2008), 428-37. 

Wang, J., Dua, J., Zhu, J., Wilkie, C.A., An XPS study of the thermal degradation and flame retardant mechanism of polystyrene-clay nanocomposites . Polymer Degradation and Stability, 2002, 77, 249-252. 

The addition of nanoparticles to synthetic rubber resulting in enhancement in thermal, stiffness and resistance to fracture is one of the most important phenomena in material science technology. The commonly used white filler in mbber industry are clay and silica. The polymer/clay nanocomposites offer enhanced thermo mechanical properties. Bourbigot et al. observed that the thermal stability of polystyrene (PS) is significandy increased in presence of nanoclay [75]. Thermal and mechanical properties of clays multiwalled carbon nanotubes reinforced ethylene vinyl acetate (EVA) prepared through melt blending showed synergistic effect in properties [76]. 

The authors observed that the inclusion complex of CPC with cyclodextrin decomposed at higher temperatures then the pure CPC. A decomposition temperature of 284 °C was observed for this complex, compared to 220 °C for pure CPC. This confirmed the attainment of better thermal stability of the pyridinium modification by complexing with cyclodextrin. The thermal behavior of the polystyrene nanocomposites synthesized using both CPC and the inclusion complex of CPC with cyclodextrin has been demonstrated in Figure 1.18 and is compared with that of the pure polymer. Both the nanocomposites had better thermal stability than the pure polymer however, the thermal performance of inclusion complex-intercalated clay nanocomposites was better than that of the CPC-intercalated clay nanocomposites. The temperature at 5% weight loss was observed to be 33 °C higher for the inclusion complex-intercalated clay nanocomposites as compared to the pure polymer, whereas this increase was 18 °C for the CPC-intercalated clay nanocomposites. 

Other thermally stable surfactants have been used in the preparation of organoclays to produce PS nanocomposites [57]. Triphenylhexadecylstibonium trifluoromethylsulfonate (Table 3.4) was prepared with sodium montmorillonite to prepare polystyrene nanocomposites by bulk polymerization. The organoclay was not uniformly distributed throughout the polymer matrix, but there was evidence of polymer intercalation and a small amount of clay exfoliation (Figure 3.13). The nanocomposite showed enhanced thermal stability. 

Styryltropylium (a carbo-cation) was used to modify clay for producing PS nanocomposites by in situ emulsion polymerization (Table 3.4) [60]. The resulting nanocomposites exhibited a mixture of intercalated and exfoliated structures. The nanocomposites exhibited improved thermal stability and fire retardancy. Halloysite nanotubes (HNT) are a kind of aluminosilicate clay with a hollow nanotubular structure (about 20-50 nm in diameter and several hundred nanometers in length) [61]. HNT was modified with y-methacryloxypropyltrimethoxysilane to produce a nanoclay (Table 3.4). HNT/PS nanocomposites were prepared by in situ bulk polymerization. The thermal stability of the HNT/PS nanocomposites was better than that of the pure polystyrene. 

Two polymerizable cationic surfactants were synthesized to produce thermally stable organoclays (ll-acryloyloxyundecyl)dimethyl(2-hydroxyethyl)ammonium bromide (called hydroxyethyl surfmer), and (ll-acryloyloxyundecyl)dimethylethylammonium bromide (called ethyl surfmer (Table 3.5) [63]. PS nanocomposites were produced by bulk polymerization and by free radical polymerization using these organoclays. Exfoliated structures were obtained with the ethyl surfmer-modified clay, whereas a mixed exfoliated/ intercalated structure was obtained using the hydroxyethyl surfmer-modified clay. The nanocomposites exhibited enhanced thermal stability and an increase in the glass transition temperature, in addition to improved mechanical properties relative to polystyrene. However, intercalated structures were obtained when nanocomposites were prepared in solution, because of competition between the solvent molecules and monomer in penetrating the clay galleries. Enhanced thermal stability was also obtained in the solution polymerization case. 

Several successful strategies are available in the literature [14] that increase the thermal stability of organic molecules. Full utilization of these strategies for the preparation of surface treatments of layered silicates with enhanced thermal stability for the development of polymer-clay nanocomposites has yet to be realized. An example of an effective strategy is the utilization of quaternary ammonium and phosphonium functional polystyrene as a surface treatment for montmorillonite that is employed to prepare polymer-clay nanocomposites [15]. TGA indicated a significant increase in the thermal stability of the organoclay and the polymer-clay nanocomposite. Imidazolium functional surface modifier for montmorillonite demonstrated a significant increase in the thermal stability of ABS terpolymer-clay nanocomposite when compared to the pure polymer and polymer-clay nanocomposites where the surface modification of the montmorillonite was produced with traditional quats [16]. These experiments were via TGA measurements. 

Figure 12.8 shows the TGA results for PS-DCTBAB-MMT and PS-PCDBAB-MMT nanocomposites and Figure 12.9 shows those for PS-co-BA-PCDBAB-MMT and PS-co-BA-DCTBAB-MMT. Only a slight improvement in the thermal stability of PCNs was observed above 50% degradation, relative to the neat polystyrene (see Figure 12.8). Jan et al. also reported that epoxy-clay nanocomposites only showed enhanced thermal stability from 40 to 50% weight degradation. The thermal stability of PS-CNs was also found to increase slightly when the clay loading increased. This has been a characteristic feature of different PCNs, irrespective of their preparation route.The formation of clay char, which acts as a mass transport barrier and insulator between the polymer and the superficial zone where the polymer decomposition takes place, is the cause of the improvements in the thermal stability of Concurrently, the restricted thermal motion...

Zanetti, M., Camino, G., Thomann, R., Mulhaupt, R., Synthesis and thermal behaviour of layered silicate/EVA nanocomposites . Polymer, 2001, 42, 4501 507. Zanetti, M., Bracco, P., Costa, L., Thermal degradation behaviour of PE/clay nanocomposites . Polymer Degradation and Stability, 2004, 85, 657-665. Bourbigot, S., Gilman, J.W., Wilkie, C.A., Kinetic analysis of the thermal degradation of polystyrene montmorillonite nanocomposite . Polymer Degradation and Stability, 2004, 84, 483-492. 

Several efforts have been reported to synthesize new organoclays to improve organoclay thermal stability, while enhancing the interaction between polystyrene and the modified clay. Organoclays prepared from phosphonium, stibonium or imidazolium cations have been employed in such studies [3-5]. Although evidences suggests that clay dispersion and the improvement in nanocomposite properties depend on the surface... 

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