Melt Electrospun Fibrillar Microfibers Prepared from Compatibilized Immiscible Polymer Systems

Abstract

Materials based on polymer microfibers, displaying a high surface-to-volume ratio and porosity, are used in applications including filtration membranes, wound dressing materials, textiles, etc., and controlling both their internal morphology and assembly can enhance the resulting properties. However, the full range of accessible morphological states within multiphase polymer microfibers, displaying highly confined lateral dimensions and subjected to significant elongational deformation, remains only partially explored and understood. In this work, the addition of a polystyrene-block-poly(ε-caprolactone) (PS-b-PCL) diblock copolymer to a model system of melt electrospun PS/PCL immiscible binary system, yields rich arrays of complex morphologies and assemblies, including a novel type of self-supporting PS fibrillar microfibers reminiscent of multistrand yarn, microfibers with dense arrays of highly oriented sub-μm channels, and mats composed of sub-μm polymer fibrils. Indeed, compatibilized systems initially displaying a matrix-dispersed phase morphology (PS/PCL 20/80 vol %) show gradually finer PS fibrils dispersed in a PCL sheath with increasing copolymer content. Selectively dissolving the PCL sheath resulted in the formation of mats composed of well separated, very fine sub-μm (down to 60 nm) PS fibrils. In contrast, compatibilized blends initially displaying a cocontinuous morphology (PS/PCL 50/50 vol %) yielded fibrillar/multistrand PS microfibers displaying self-supporting properties once the PCL was extracted. The block copolymer plays a critical role: it significantly suppresses coalescence, yielding a finer morphology; at the same time, its significant dilution at the interface during the stretching process may allow limited coalescence of neighboring PS fibrils due to reduced steric hindrance, creating new junctions, as copolymer areal density calculations support. The resulting PS fibrillar microfibers exhibit dramatically increased specific surface areas compared to the original microfibers—up to over 2 orders of magnitude greater, consistent with geometrical predictions.