Origin of enhanced cold crystallization rate for freeze-dried poly(L-lactide) from solutions

摘要

Poly(L-lactide) (PLLA) freeze-dried from dilute 1,4-dioxane solutions exhibited very porous structure composed of thin membranes of which the mean thickness was estimated to be 104 – 135 nm. Heating measurements of differential scanning calorimetry (DSC) showed that the freeze-dried PLLA (FDPLLA) exhibits an exothermic peak due to cold crystallization at 78 – 81°C, which is at least 20 K lower than that for a quenched amorphous bulk PLLA. In accord with this, the overall rate of isothermal cold crystallization was revealed to be greater for the FDPLLA than that for the bulk. The origin of such high crystallizability of FDPLLA is attributed to its large surface area where the chain mobility is greater than in the bulk PLLA. The exothermic peak in the DSC trace shifted to a further lower temperature when the FDPLLA is immersed in ligroin (non-solvent), which also suggests a major role of the free surface in enhancing the cold crystallization rate. On the other hand, the density and the chain conformational feature of the FDPLLA were revealed to be identical to the bulk PLLA.