Influence of semicrystalline morphology on the glass transition of poly(L-lactic acid)

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 (T-g), determined calorimetrically, exhibited a general decrease with an increase in crystallization temperature (T-c) for either cold- or melt-crystallized specimens. The measurements of the heat capacity increment at T-g 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 T-g 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.