Synthesis and crystallization behavior of poly(l-lactide)-block-poly(ϵ-caprolactone) copolymer

Abstract

We investigate, via wide and small angle X-ray scattering methods (WAXS and SAXS), differential scanning calorimetry (DSC), and optical microscopy (OM), the crystallization behavior of poly(l-lactide)-block-poly(ϵ-caprolactone) copolymer (PLLA-b-PCL) which is classified as a semicrystalline–semicrystalline block copolymer. The synthesis of PLLA-b-PCL was done by using a macroinitiator of hydroxy-terminated PCL (PCL-OH) with various molecular weights and followed by ring opening polymerization of l-lactide. The PLLA-b-PCL(H) with the number-average molecular weight (Mn) of 77,000 and the weight fraction of PCL block (wPCL) of 0.32 showed microphase-separated structures at molten state up to 220°C, as determined from rheological measurement. But, the PLLA-b-PCL(L) with Mn=19,000 and wPCL=0.374 became homogeneous at temperatures higher than 175°C.From the crystallization behavior at 110 and 140°C, we conclude that the chain folding of the crystalline PLLA block in PLLA-b-PCL(H) was quite different from that in PLLA-b-PCL(L). This result suggests that the chain segregation between the blocks at molten state affected significantly the chain folding of a semicrystalline block during the crystallization.