Tg depression in poly(L(-)-lactide) crystallized under partially constrained conditions

A significant reduction in the glass transition temperature (T-g) is found for poly(L-(-) lactide) (PLLA), crystallized under partially constrained conditions, where the polymer is prevented from shrinking. It is proposed that the constrained crystallization process results in increasing the net free volume in the amorphous phase. This is a result of the fixed sample volume in which there is an increasing crystal volume fraction at a higher density than the amorphous fraction, depleting polymer mass from the amorphous fraction, which is not able to contract to maintain its nominal density. The Tg depression increases with increasing degree of crystallization or crystallization temperature, T-c. On the other hand, unconstrained PLLA samples (free to shrink during crystallization) exhibit the conventional trend of increasing T-g with crystallinity or T-c. Although the difference in T-g between samples prepared by the two methods could be large, as much as 30 degreesC, the melting point, heat of fusion, and overall degree of crystallinity in PLLA are not influenced. Finally, shear-induced crystallization does not affect any of the physical properties studied here but only modifies the crystallization rates, increasing them by enhancing nucleation.