Changes in optical texture of α-spherulites of isotactic polypropylene upon partial melting and recrystallization

Abstract

Textural changes in α-phase spherulites of isotactic polypropylene (iPP) in a sequence of thermal events were examined by means of polarized light microscopy (PLM). This sequence of thermal events involves isothermal crystallization (atT_c = 117 to 140 °C), followed by heating (at 5 °C/min) to nearly complete melting, and then recrystallization upon cooling (at -40°C/min) to T_c During isothermal crystallization, the a-spherulites were of mixed birefringenceat T_c = 117 to 127 °C or of negative birefringence at T_c = 140 °C; upon heating towards melting, the spherulitec birefringence consistently truned negative. More interestingly, after recrystallization during cooling back to Tc from nearcomplate melting, all spherulites exhibited positive birefringence. The recrystallization could also result in speckles of positive birefringence when T_c was high or upon slower cooling. The changes in optical texture are explained in terms of contributions from tangential (or, cross-hatched) subsidiary lamellae which (as compared to the radial dominant lamellae) are relatively low-melting but thicken and recrystallize more readily in the present temperature range.