Distorted ring-banded spherulites in poly(l-lactic acid)/poly(ε-caprolactone) blends

Abstract

Unique distorted ring bands with wrinkles and concentric structures in blends of poly(L-lactic acid) (PLLA)/poly(ε-caprolactone) (PCL) were obtained by two-step isothermal crystallization and investigated by using polarized optical microscopy (POM) and atomic-force microscopy (AFM). Peculiar morphologies were revealed while the blends were crystallized by two-step crystallization with a fixed first crystallization temperature of 117 °C and different second crystallization temperatures. From the microscopy results, the morphology as the alternating ridges and valleys of wrinkle bands along with concentric ring bands can be resolved. The PLLA crystals could result in the structures of wrinkle bands and small concentric ring bands at the first crystallization temperature. Furthermore, formation of the alternating ridges and valleys of wrinkle bands could be suggested to be attributed to the different amounts of PCL inserted into the pre-formed crystals of PLLA while crystallizing the blends under a lower second crystallization temperature with fast crystallization of PCL. On the other hand at a higher second crystallization temperature, the wrinkle band changed to a branch-like band. PCL chains with high mobility could be sufficiently diffused and uniformly inserted into the pre-formed crystals of PLLA to generate more perfect crystals of PCL at the higher second crystallization temperature. The wrinkle bands with a significant ridge and valley structure vanished by changing the second crystallization to a higher one.