Self-Assembled Lamellar Films of Comb-Shaped Copolymers by Segregation between Hydrophobic Side Chains and the Main...

Self-assembled lamellar films of poly(N-dodecyl acrylamide-stat-vinyl phosphonic acid) [p(DDA/VPA)] were formed via the segregation between the hydrophilic main chain and VPA and dodecyl side chains. p(DDA/VPA) copolymers were synthesized by free-radical copolymerization of DDA and VPA with VPA molar concentrations of 19% [p(DDA/VPA19)] and 64% [p(DDA/VPA64)]. Both copolymers exhibited a glass-transition temperature (T_g) and melting temperature for p(DDA/VPA19), but no crystalline or liquid-crystalline phase-transition temperatures, which suggests that both copolymers are amorphous. Thin films of the copolymers were prepared by spin coating, and the structure of the films was studied by X-ray diffraction (XRD) measurements. The as-cast films of the copolymers showed broad diffraction patterns, which suggested the formation of alkyl nanodomains similar to that observed in the pDDA homopolymers. On the other hand, the XRD patterns for both copolymer films showed a sharp Bragg diffraction in the low-q region after annealing at 60 °C. Furthermore, the p(DDA/VPA19) film showed first- and second-order Bragg diffractions with a ratio of 1:2. These XRD patterns suggest that the copolymer films form an ordered lamellar structure. We concluded that the main chain became more hydrophilic by the introduction of VPA, resulting in an increased segregation force relative to the hydrophobic dodecyl side chains, which induces the formation of lamellae. Moreover, doping a p(DDA/VPA64) film with imidazole increased the ordering and uniformity of the lamellar structures due to the increased segregation force by the formation of ion pairs in the hydrophilic comonomer. In their entirety, the results show that statistical copolymerization can be used as a new method to create self-assembled structures.