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-C-8)) brush in place of conventional thermal annealing. Poly(FA-C-8) brush films of 40 nm thickness were prepared by surface-initiated atom transfer radical polymerization. In-situ neutron reflectivity measurements for the poly(FA-C-8) 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-C-8) 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.