Solution-grown crystals of isotactic polystyrene: neutron scattering investigation of the variation of chain trajectory with crystallization temperature

Abstract

Previous neutron scattering investigations carried out on isotactic polystyrene single crystals grown at 130°C from dilute solutions have revealed a sheet-like conformation and hence very regular chain folding. Here we report the same type of experiments performed on samples crystallized at T c = 75, 85, 105 and 115°C. From the results in both the Guinier and the intermediate ranges, we come to the following conclusions: (i) At 115°C, we reproduce the results obtained at 130°C (regular folding in a single sheet). (ii) Decreasing T c leads to a change of conformation for relatively high molecular weights (7 × 10 5 and 5 × 10 5). While crystallization still occurs along an hk0 plane, the regular folding is interrupted, leading both to superfolding and to crystallization in at least two different lamellae. (iii) For lower molecular weights (3 × 10 5 and 1.5 × 10 5) significant deviations from the sheet-like model appear only at T c = 85°C. We conclude that above a critical molecular weight, which is temperature-dependent, irregularities in the folding habit take place. In this respect, polystyrene resembles polyethylene.