Strong enhancement of the twisting frequency of achiral orthorhombic lamellae in poly(ε-caprolactone) banded spherulites via evaporative crystallization

Abstract

Besides the rather poor and irregular bands with large periods formed at high temperatures, banded spherulites were seldom spotted in pure poly(ε-caprolactone) (PCL). Herein, well-ordered extinction banded spherulites with the smallest spacing less than 10 μm are encountered in solution-cast PCL films via evaporative crystallization, and structural analyses unveil that such narrow bands suggest strong twisting of achiral orthorhombic PCL lamellae. It is proposed that solvent evaporation can enhance dramatically the disorder on opposite lamellar surfaces that stems from a slightly asymmetrical structural feature from the disposition of ester groups within a PCL unit cell, which gives rise to a striking enhancement of the unbalanced surface stresses resulting in the intensification of twisting frequency. The dependence of band spacing on crystallization conditions is also investigated to verify this intensifying effect. The present findings are envisaged to improve our understanding of the mechanical origin of unequal surface stresses for periodic twisting of polymer lamellar crystals.