Early Stage Quiescent and Flow-Induced Crystallization of Intercalated Polypropylene Nanocomposites by Time-Resolved Light Scattering

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Abstract

Time-resolved light scattering demonstrates that the early-stage quiescent and flow-induced crystallization kinetics of an intercalated polypropylene clay nanocomposite differs significantly from that of pure polypropylene. The material studied is organophilic montmorillonite clay dispersed at 2.0 wt % by melt mixing in an isotactic polypropylene matrix with maleic anhydride functionalized polypropylene compatibilizer. Characteristic crystallization times are extracted from the time evolution of integral measures of the angularly dependent parallel polarized and cross polarized light scattering intensity. For quiescent isothermal crystallization with undercooling ΔT < 32 °C, measured relative to its melting temperature, the compatibilized, intercalated nanocomposite displays retarded crystallization kinetics compared to that of pure polypropylene. However, the nanocomposite behavior does not differ significantly from that of a polypropylene/maleic anhydride functionalized polypropylene blend within the temperature range for which the characteristic crystallization times of the latter material could be extracted. Debye−Bueche analysis of the polarized light scattering intensity demonstrates significant differences in the time-dependent growth of isotropic morphology in the polymer and the nanocomposite. Marked flow-induced acceleration of crystallization kinetics is observed for the polymer nanocomposite at applied strain rates for which flow has only a modest effect on polypropylene crystallization. By comparing this behavior to that of a blend of polypropylene and compatibilizer, we infer a significant role for the latter species in flow-induced nanocomposite crystallization. We furthermore find that the magnitude of the flow-induced acceleration of crystallization for the polypropylene nanocomposite is a unique function of the applied strain.

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