Side chain crystallization in microphase-separated poly(styrene-block-octadecylmethacrylate) copolymers

The crystallization behavior of two microphase-separated poly(styrene-b-octadecylmethacrylate) block copolymers with lamellar and cylindrical morphology is studied by DSC. The findings are compared with results for a polyoctadecylmetharcylate (PODMA) homopolymer. The situation in the block copolymers is characterized by the occurrence of a confined side chain crystallization in small PODMA domains surrounded by a glassy polystyrene phase. The strength of confinement effects depends significantly on the block copolymer morphology. The crystallization behavior of PODMA lamellae with a thickness of about 10 nm is only slightly affected and similar to the situation in the homopolymer. In cylindrical PODMA domains with a diameter of about 10 nm strong confinement effects are observed: the degree of crystallinity is 50% reduced and the crystallization kinetics slows down. The Avrami coefficients change from n≈3 for the homopolymer and PODMA lamellae to n≈1 for PODMA cylinders. This observation indicates one-dimensional growth in small cylinders or a change from heterogeneous to homogeneous nucleation. Pros and cons of both approaches are discussed. A speculative picture explaining qualitatively the differences in the crystallization behavior of PODMA lamellae and cylinders in a glassy polystyrene matrix is presented.