PHASE-STRUCTURE AND POLYMORPHISM OF HIGHLY SYNDIOTACTIC POLYPROPYLENE

Polymorphs of highly syndiotactic polypropylene were studied by magic angle spinning C-13 NMR. The conversion of one crystalline form into another was observed by the formation of different conformers of the polymer chain. The reversion of the metastable forms to the conventional crystalline phase was followed directly by variable-temperature experiments and the addition of plasticizers. The multiplicity of signals associated with these structures were fully assigned in terms of trans, gauche conformations, by taking into account a sequence of four bonds in the main chain. Additional signals due to the packing of helical chains of opposite chirality are discussed in terms of the packing disorder present in the structure. The spectral pattern due to the amorphous region depends on temperature and thermal history, because the chain sequences can be observed under conditions of static and dynamic disorder as compared with the NMR time-scale. The chains are fairly elongated, because they contain a considerable amount of trans sequences. Phase distribution and intel faces were characterized by hydrogen relaxation times in the rotating frame T1 rhoH. Phase specificity was retained by recovering the magnetization on the C-13 nucleus under high-resolution conditions. Apparently spin diffusion cannot average out the T1 rhoH value over the whole material when the thickness of the crystalline lemellae is increased by the annealing treatment. In the stretched material crystalline and amorphous phases are intimate and the chains in the amorphous phase also appear to be partly extended.