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Changes In Optical Texture Of α-spherulites Of Isotactic Polypropylene Upon Partial Melting And Recrystallization

C. Yeh, A. Su, M. Chen, R. Sugimoto
Published 1995 · Materials Science

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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 (atTc = 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 Tc During isothermal crystallization, the a-spherulites were of mixed birefringenceat Tc = 117 to 127 °C or of negative birefringence at Tc = 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 Tc 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.
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