Effects of hard and soft confinement on crystal polymorphism and crystal development in electrospun core‐sheath fibers

This study employs coaxial electrospinning to fabricate core‐sheath fibers composed of crystalline poly(L‐lactide) (PLLA) in the core and amorphous polystyrene (PS) in the sheath. The fibers undergo cold crystallization at temperatures either below (hard confinement) or above (soft confinement) the glass transition temperature of PS. These conditions are employed to investigate the crystallization behavior of PLLA under multiple confinement types. The electrospinning of PLLA fibers results in the development of coexisting α‐ and α′‐form crystals. The coaxial electrospinning process enhances the extent of chain stretching of PLLA through the PS solution in the outer layer, thereby promoting the close packing of PLLA chains and facilitating the formation of α crystals. Crystallization within soft confinement enables PLLA chains to relax, leading to an increased population of α crystals; this phenomenon is not observed in electrospun single PLLA fibers. A comparison of the crystal parameters in core‐sheath and single PLLA fibers reveals that the crystallite size and the degree of crystallinity are considerably higher in unconfined single PLLA fibers than in confined core‐sheath fibers. The encapsulation of PLLA into the fiber core strongly inhibits the chain movement of PLLA, resulting in a decrease in PLLA crystallizability.