Confinement-Induced Direct Formation of Stable Form I′ in Electrospun iPB-1/iPP Core-Shell Membranes

Isotactic polybutene-1 (iPB-1) crystallizes into metastable Form II upon melt crystallization, which slowly transforms to stable Form I over 7–10 days, resulting in continuous dimensional shrinkage that limits industrial applications. In this work, we demonstrated that physical confinement in combination with epitaxial nucleation at the iPB-1/iPP interface via coaxial electrospinning can directly induce the formation of stable iPB-1 Form I′, eliminating the need for thermal post-treatment. Core-shell fibers were spun via coaxial electrospinning, using iPB-1 as the core and isotactic polypropylene (iPP) as the shell with varying ratios. The core and shell solution flow rates were systematically varied to control the shell thickness. The core-shell fibers with an optimal shell thickness (CS-2, core/shell flow rate ratio = 1:2) revealed significantly higher Form I′ content (XI′ = 0.92) compared to neat iPB-1 fibers without confinement (XI′ = 0.45). Notably, the metastable Form II was significantly suppressed in the core-shell fibers, without any post-treatment. The confinement effect arises due to restricted chain mobility within the nanoscale core and epitaxial nucleation of iPB-1 on the iPP shell, which promotes the 3/1 helical conformation characteristic of Form I′. Moreover, the core-shell membranes also exhibited improved mechanical strength, with CS-2 showing the optimal tensile strength and Young’s modulus. This work provides a scalable, one-step strategy that offers a potential route to dimensionally stable iPB-1 membranes, bypassing prolonged phase transition and any post-processing treatments.