Effects of Phase Separation on the Crystallization Behavior in a Binary Blend of Poly(ε-caprolactone) Homopolymer and Poly(ε-caprolactone)-block-Polybutadiene Copolymer

The crystallization process of poly(ε-caprolactone) (PCL) chains in a binary crystalline/crystalline blend, PCL homopolymer and PCL-block-polybutadiene diblock copolymer (PCL-b-PB), has been investigated by synchrotron small-angle X-ray scattering (SR-SAXS) as a function of the characteristic length of composition variation ξ formed by the phase separation between PCL and PCL-b-PB. The temperature range of an UCST-type phase separation overlaps with that of crystallization in this blend, so that the phase separation starts in advance to yield various heterogeneous structures when the blend is quenched from a microphase-separated melt into the crystallization temperature of PCL chains. When ξ<500 nm the crystallization rate of PCL is equal to that of PCL blocks in PCL-b-PB though there is a large difference in the crystallization rate between neat PCL and PCL blocks. When ξ>500 nm, on the other hand, the crystallization rate of PCL deviates significantly from that of PCL blocks, and they approach to the values of neat polymers with increasing ξ. The relation between the crystallization rate and the pre-existing composition heterogeneity is quantitatively discussed.