Water modulates the lamellar structure and interlayer correlation of poly(perfluorooctyl acrylate) films: a specular and off-specular neutron scattering study

Abstract

Comb-like polymers with pendant-like perfluorocarbon side chains self-assemble into smectic lamellae and have been extensively used as water-repellent, hydrophobic coating materials characterized by large water contact angles (θ > 120°). As poly(perfluorooctyl acrylate) films are “apparently hydrophobic” (θ > 120°), the interaction of such materials and water molecules has been largely overlooked. To unravel the molecular-level interactions between water and apparently hydrophobic polymers, specular and off-specular neutron scattering experiments were conducted at defined osmotic pressure ΠH2O. The poly{2-[(perfluorooctylethyl)carbamate]ethyl} acrylate (PFAUr-C₈), which had a carbamate linker, transitioned to another lamellar phase at 89 °C. At T = 25 °C; the lamellar periodicity of PFAUr-C₈ slightly increased with decreasing osmotic pressure, while the vertical correlation length increased. However, the poly[(perfluorooctyl)ethyl] acrylate (PFA-C₈) that did not contain a carbamate linker directly transitioned to a disordered phase at 84 °C. The lamellar periodicity of PFA-C₈ was largely independent of the osmotic pressure, suggesting that PFA-C₈ was poorly hydrated. Remarkably, the vertical correlation length decreased with decreasing osmotic pressure. Because hydration facilitated by the linker modulated the smectic lamellae of the poly(perfluoroalkyl acrylate), water molecules could be used to optimize the self-assembly of apparently hydrophobic liquid crystalline polymers.