Significant increase in the melting temperature of poly(ɛ-caprolactone) blocks confined in the crystallized lamellar morphology of poly(ɛ-caprolactone)-block-polyethylene copolymers

Abstract

The melting temperature of poly(ɛ-caprolactone) (PCL) blocks, Tm,PCL, in PCL-block-polyethylene (PCL-b-PE) is investigated as a function of the volume fraction of PCL blocks in the system, φPCL, and compared with that in PCL-block-polybutadiene (PCL-b-PB), the precursor of PCL-b-PE with an identical PCL molecular weight. The amorphous PCL block in PCL-b-PE is spatially confined within the solid lamellar morphology formed by the advance crystallization of PE blocks (PE lamellar morphology), which will bring about a considerable depression in the conformational entropy of amorphous PCL blocks. On the other hand, PCL-b-PB forms some microdomain structure when the PCL block is amorphous, so that the conformational entropy of amorphous PCL blocks is not so depressed. The value of Tm,PCL for PCL-b-PE is always higher than that for PCL-b-PB, and the difference in Tm,PCL, ΔTm, increases steadily with increasing φPCL. However, ΔTm drops to almost zero when the PCL block in PCL-b-PE crystallizes in the microdomain structure without forming the PE lamellar morphology, which will be ascribed to the recovery of conformational entropy in the amorphous PCL block. The chain stretching of PCL blocks confined in the PE lamellar morphology is discussed on the basis of ΔTm. The melting temperature of poly(ɛ-caprolactone) blocks, Tm,PCL, in poly(ɛ-caprolactone)-block-polyethylene is investigated as a function of the volume fraction of poly(ɛ-caprolactone) blocks in the system, φPCL, and compared with that in poly(ɛ-caprolactone)-block-polybutadiene, the precursor of poly(ɛ-caprolactone)-block-polyethylene with an identical poly(ɛ-caprolactone) molecular weight. Tm,PCL for poly(ɛ-caprolactone)-block-polyethylene is always higher than that for poly(ɛ-caprolactone)-block-polybutadiene, and the difference in Tm,PCL increases steadily with increasing φPCL.