The conundrum of polystyrene-clay nanocomposites

Full exfoliation of montmorillonite in polystyrene has not been achieved. Hence, the full realization of the reinforcing benefit of montmorillonite with regard to the mechanical property enhancement of polystyrene has not been exploited. The definitive discussion that relates to the complex and confounding problem relating to the inability to exfoliate montmorillonite into polystyrene is found in the work of Stretz and Paul [31]. The factors that relate to improved dispersion and exfoliation of montmorillonite in polyolefins that are discussed above do not provide a satisfying response with polystyrene. The conclusion found in this work is the result of extensive investigations of acrylonitrile (SAN) and maleic anhydride (SMA) modified polystyrene [32,33,34]. 

The first study [31] surveyed a wide range of different quaternary ammonium ion modifications of montmorillonite with Tyril 100 (manufactured by Dow Chemical Co.). Tyril 100 is polystyrene with approximately 25 wt.% acrylonitrile and a weight-average molecular 

The polymer-montmorillonite composites were produced with the same Haake twin-screw extruder and protocol that was successfully employed with the nylon 6 work described in Chapter 5. The test samples were prepared by injection molding and characterized by the same equipment and protocol described in the successful nylon 6 preparation of polymer-montmorillonite nanocomposites described in Chapter 5. The montmorillonite content in the composites was prepared at approximately 2, 4, and 5.5 wt.%. 

WAXS and TEM indicated that the best dispersions were prepared with the trimethyl octadecyl quaternary ammonium and dimethyl octadecyl ammonium ion exchanged montmorillonite. The number-average particle length varied from 172 to 240 nm the number-average particle thickness varied from 4.19 to 4.5 nm. The Young s modulus increased from approximately 3.5 GPa for the pure polymer to approximately 5.5 GPa for an approximate 5.5% loading of the ammonium ion-exchanged montmorillonite in the SAN. 

The Halpin-Tsai prediction for the modulus at 2 wt.% loading of fully exfoliated montmorillonite with an aspect ratio of 100 in this polymer is duplicated by the ammonium ion-exchanged montmorillonite that has an experimentally determined aspect ratio of approximately 53.3. The enhanced reinforcing efficiency of this organomontmorillonite in SAN as a function of aspect ratio was not anticipated. 

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