Frustrated crystallization behavior under hierarchical confinement in electrospun fibers of poly(styrene)/poly (ethylene oxide)/poly(L-lactide) ternary mixtures

The crystallization behavior of binary crystalline mixtures composed of poly(ethylene oxide) (PEO) and poly(L-lactide) (PLLA) was investigated in the electrospun nanofibers fabricated by the ternary blends of polystyrene (PS), PEO, and PLLA. The weight fraction of PEO/PLLA mixture was kept at =0.2 such that PS formed the matrix phase. The preferred mixing of PEO and PLLA resulted in the formation of mixed PEO/PLLA dispersed domains in the PS matrix. The small diameter of the electrospun nanofibers restricted the size of dispersed domain. During cooling from melt, PLLA crystallized first resulting in crystallization induced phase separation and entrapment of PEO within the intracrystalline regions of PLLA crystals. Hence, whereas the PLLA crystallized within the soft PS matrix, the crystallization of PEO occurred within the hierarchical confinement imposed by the PLLA crystals as well as the glassy PS matrix. Interestingly, it was found that the confinement favored the formation of alpha crystals phase in PLLA even at very low crystallization temperatures. Furthermore, due to plausible entrapment of PEO segments within the PLLA crystals, significant depression in PLLA melting temperature was observed. Moreover, the PEO melting temperature also depressed by more than 30 degrees C due to the frustrated crystallization within the hierarchical confinement.