Influence of Semicrystalline Morphology on the Glass Transition of Poly( L ‐lactic acid)

Summary: Semicrystalline specimens of poly(L‐lactic acid) (PLLA) were prepared by isothermal cold‐ or melt‐crystallization over a wide temperature range. The morphologies at different length scales were characterized using polarized optical microscopy, WAXS and SAXS. The glass transition temperature (Tg), determined calorimetrically, exhibited a general decrease with an increase in crystallization temperature (Tc) for either cold‐ or melt‐crystallized specimens. The measurements of the heat capacity increment at Tg indicated that a significant amount of the rigid amorphous fraction coexisted with the crystalline and mobile amorphous phases in semicrystalline PLLA. A three‐phase model (crystalline phase, mobile amorphous phase and rigid amorphous phase) is, therefore, appropriate for the interpretation of the structure of semicrystalline PLLA. According to a one‐dimensional layer stack model, the layer thicknesses of the three phases were further evaluated approximately. The increasing of Tg was found to be correlated to significant decreasing thickness of the crystalline layer, gradual decreasing thickness of the rigid amorphous layer, and a slight increasing trend in the thickness of the mobile amorphous layer. We suggest that the rigid amorphous layer of semicrystalline PLLA may possibly play a role in loosening the constraints imposed by the crystalline layer on the amorphous chain motions inherent to the glass transition. The optical microscopy photograph shows the spherulitic morphologies developed after melt crystallization at 140 °C. magnified image The optical microscopy photograph shows the spherulitic morphologies developed after melt crystallization at 140 °C.