Confined crystallization of Poly(ethylene glycol) in spherulites of Poly (L-lactic acid) in a PLLA/PEG blend

The intriguing crystallization behavior of poly(ethylene glycol) (PEG) in a blend specimen of PEG with poly(L-lactic acid) (PLLA) was studied by polarized optical microscopy (POM) and differential scanning calorimetry (DSC). The PLLA/PEG (50/50) blend specimen was heated up to 180.0 degrees C and kept at this temperature for 5 min to obtain a complete melt without liquid-liquid phase separation. Then, a two-step temperature-jump was conducted as 180.0 degrees C -> 127.0 degrees C -> 45.0 degrees C. For this particular condition, it was found that PEG can crystallize only in the preformed spherulites of PLLA, as no crystallization of PEG was found in the matrix of the mixed PLLA/PEG amorphous phase. This is because of the depression of the freezing temperature (T-f) of PEG in the complete mixture of the PLLA/PEG (50/50) blend specimen by noting that T-f for the neat PEG specimen is 52.0 degrees C. Upon the PLLA crystallization at 127.0 degrees C, the PEG content in the amorphous region inside the PLLA spherulite was increased because of the formation of the pure solid phase of PLLA (crystalline phase). Namely, the PLLA content was decreased in the amorphous phase of the PLLA/PEG mixture. Then, T-f of PEG increased so that PEG could crystallize but this crystallization was only allowed inside the PLLA spherulite. Thus, the confined crystallization of PEG can be accounted for. It was further found that the ultimate degree of crystallinity (phi(infinity)) of PEG was increased and the induction period of the confined crystallization of PEG was decreased with an increase in the PLLA crystallization time at 127.0 degrees C. This is simply because the PEG content in the amorphous phase was increased due to the increase in the PLLA amount crystallized, which was more clearly recognized as a function of the size of the preformed PLLA spherulite. The direct evidence of the PEG crystallization was obtained by the wide-angle X-ray scattering. The kinetics was also studied for the confined crystallization of PEG. Based on the results of the kinetic study, the effect of the space confinement will be discussed.