Hybrid Single Crystals of Poly(ε-caprolactone) Homopolymers and Poly(ε-caprolactone)-b-Poly(ethylene oxide) Block Copolymers

Hybrid single crystals (HSCs) of different poly(ε-caprolactone) (PCL) homopolymers with a poly(ε-caprolactone)-b-poly(ethylene oxide) (PCL-b-PEO) block copolymer (BCP) are prepared. The effects of PCL length, PCL/PCL-b-PEO molar ratio, crystallization temperature (T_c), and solvent on crystal morphology are investigated. The optimal T_c for the formation of more perfect HSCs is between those for homocrystals of individual PCL and PCL-b-PEO and roughly increases with the length of PCL and PCL/PCL-b-PEO molar ratio. The chain folding in the HSCs is studied by comparing the experimentally measured heights obtained by atomic force microscopy (AFM) and theoretically calculated ones based on a sandwich structure model. Under most situations, the PCL homopolymers adopt a larger chain folding number in the HSCs than that in their homocrystals, while the chain folding of BCP remains unaltered. However, when both PCL homopolymer and PCL-b-PEO BCP crystallize slowly and the overcrowding of the PEO is effectively alleviated, thicker HSCs can be formed, in which the PCL homopolymer preserves the chain folding in its homocrystals but the BCP adopts a reduced chain folding number as compared with that in its homocrystals. The relative crystallization rate of PCL homopolymer versus BCP also affects the real composition and overall height of the HSCs.