Transition of Self-assembled Structures in Poly(N- octadecyl acrylamide-co-hydroxylethyl acrylamide) Random Copolymer Films

This paper shows that a simple random copolymer can form self-assembled lamellae, the structure of which depends upon the comonomer composition and annealing temperature. A random copolymer of octadecyl acrylamide and hydroxyethyl acrylamide [p(ODA/HEAm)] was prepared via free racial copolymerization. Thin films of p(ODA/HEAm) were prepared via a spin-coating method and their structures were studied using X-ray diffraction. It was found that copolymers with a HEAm content between 28% and 50% formed self-assembled lamellae upon annealing at 10 ˚C above the glass-transition temperature. The self-assembled form has a “side-chain-mixed” lamellar structure in which the ODA and HEAm side chains are oriented perpendicularly to the lamellar plane composed of the polymer main chain. Interestingly, a copolymer with HEAm content between 36% and 50% transforms from the side-chain-mixed lamellar structure to form a “side-chain-segregated” lamellar structure upon annealing at a much higher temperature (160 ˚C). In this structure, the ODA and HEAm side chains are oriented perpendicularly to the lamellar plane and in opposite directions to each other. We concluded that the lamellar structure that is formed is determined by the strain force generated during self-assembly and by the segregation force between the comonomers.