Crystallization behavior of crystalline/crystalline polymer blends under confinement in electrospun nanofibers of polystyrene/poly(ethylene oxide)/poly(ε-caprolactone) ternary mixtures.

We have studied the crystallization behavior of crystalline/crystalline blends of poly(ethylene oxide) (PEO) and poly(ε-caprolactone) (PCL) in electrospun nanofibers fabricated from ternary blends of polystyrene (PS), PEO, and PCL, where PS was present as the majority component. It was demonstrated previously that PEO in PS/PEO binary blend nanofibers with a low PEO weight fraction (≦0.2) crystallized predominantly through homogenous nucleation due to the small PEO domain size which excluded the presence of heterogeneities (Soft Matter, 2016, 12, 5110). Here, it was found that PCL in PS/PCL binary blend nanofibers exhibited similar behavior, but at a much lower weight fraction of PCL (≦0.1) due to the presence of an inherently higher concentration of heterogeneities in the PCL homopolymer. In the PS/PEO/PCL ternary blend nanofibers, where the combined weight fraction of PEO and PCL was kept at 0.2 or less, the crystallization of the two components took place separately through both heterogeneous and homogenous nucleation mechanisms. The phase segregated crystallization behavior was further confirmed by the melting behavior of the blend nanofibers and wide angle X-ray diffraction (WAXD) measurements. Most significantly, the homogenous nucleation of both PEO and PCL was suppressed whereas the heterogeneous nucleation was enhanced in the ternary blend nanofibers even at very low weight fraction of PEO or PCL. This was plausibly attributed to the coupling between the crystallization and the liquid-liquid phase separation (LLPS) of the PEO/PCL mixture dispersed in the PS matrix during non-isothermal cooling of the blend nanofibers. Furthermore, it was observed that thermal treatment of the PS/PEO/PCL blend nanofibers above the glass transition temperature of PS further promoted the heterogeneous nucleation-initiated crystallization of PEO because of a complex interplay between Plateau-Rayleigh instability-induced domain breakup and its further coalescence and demixing within the PEO/PCL domains embedded in the PS matrix.