Effect of supercritical carbon dioxide on molecular aggregation states of side chains of semicrystalline poly{2-(perfluorooctyl)ethyl acrylate} brush thin films

We report a carbon dioxide-based approach to induce highly ordered molecular aggregation states of perfluoroalkyl (R_f) chains of densely-grafted poly{2-(perfluorooctyl)ethyl acrylate} (poly(FA-C₈)) brush in place of conventional thermal annealing. Poly(FA-C₈) brush films of 40 nm thickness were prepared by surface-initiated atom transfer radical polymerization. In-situ neutron reflectivity measurements for the poly(FA-C₈) brush films under the isothermal condition of T = 309 K, which is below the bulk melting temperature (T_m = 348 K), elucidated large expansion of polymer chains due to sorption of CO₂ molecules. Comparison of the swelling behavior with an amorphous poly{2-(perfluorobutyl) ethyl acrylate} brush thin film clarified that the sorption of CO₂ molecules results in the melting of the semicrystalline poly(FA-C₈) brush at P > 4.1 MPa. In addition, by using out-of-plane grazing incidence wide-angle X-ray diffraction, it was found that subsequent slow quench from P > 4.1 MPa induces rearrangement of the rigid rod-like R_f groups, forming highly ordered molecular aggregation structures similar to those via a conventional thermal process. The appropriate CO₂ process conditions for the effective induction of the highly ordered structures of the rigid R _f groups are discussed in detail.