Order-order transitions in poly(N-octadecyl acrylamide-co-hydroxyethyl acrylamide) statistical copolymer films

This study demonstrates that a simple statistical copolymer can form self-assembled lamellae, whose structures depend on both the comonomer composition and the annealing temperature. Statistical copolymers of octadecyl acrylamide and hydroxyethyl acrylamide [p(ODA/HEAm)] were prepared via free-radical copolymerization, and their thermal properties were studied by differential scanning calorimetry. Thin films of p(ODA/HEAm) were prepared via spin-coating, and their structures were analyzed using X-ray diffraction. It was found that copolymers with HEAm contents between 28 and 50% formed self-assembled lamellae upon annealing at a temperature ∼10 °C above the glass-transition temperature. The self-assembled form was found to possess a "side-chain-mixed" lamellar structure, in which the ODA and HEAm side chains are oriented perpendicular to the lamellar plane composed of the polymer main chain. Interestingly, a copolymer with a HEAm content between 36 and 50% transformed from the side-chain-mixed lamellar structure to generate a "side-chain-segregated" lamellar structure upon annealing at a significantly higher temperature (∼50 °C above T_g). In this structure, the ODA and HEAm side chains were found to be oriented in opposite directions but perpendicular to the lamellar plane. The packing of the side chains in the lamellar structures was studied using Fourier-transform infrared spectroscopy. It was concluded that the structures of the self-assembled lamellae are determined by the strain forces generated during self-assembly, and by the segregation forces existing between the comonomers.