Insights into formation and relaxation of shear-induced nucleation precursors in isotactic polystyrene

Rheooptical experiments on narrow isotactic polystyrene (i-PS) fractions have been carried out to investigate temperature and molar mass dependence of the lifetime of shear-induced nucleation precursors. Similar to i-poly(1-butene) and i-polypropylene, the survival of the flow-induced structures lasts very long, even at temperatures well above the measured melting point of the crystals. It has been observed that the decrease of concentration of nucleation precursors follows a first-order kinetics with a strongly temperature-dependent rate constant. The apparent activation energy of the overall relaxation process is around 400 kJ/mol, of the same order of magnitude previously found for other semicrystalline polymers. A comparative analysis of the apparent activation energy data for different polymers, coupled with morphological evidence, suggests that the rate-determining step in the evolution of the system toward the equilibrium state in the melt is the detachment of stems from the lateral surface of flow-induced oriented bundles, present either as isolated entities, in the case of pointlike nuclei, or as rows altemated by disordered nanodomains, in the long threads of the shish. Thanks to the monodisperse character of the investigated samples, the molar mass dependence of lifetime of oriented nucleation precursors has also been established.