Neutron reflectivity study of glassy polymer brushes in density fluctuating supercritical carbon dioxide

By using in situ neutron reflectivity, we measured the swelling behavior of two types of polymer brushes, deuterated polystyrene with a trichlorosilane end group and deuterated polystyrene-block-poly(4-vinylpyridine) block copolymer, in supercritical carbon dioxide (scCO ₂). The measurements were conducted in the pressure range of 0.1-20 MPa at 36°C. The pressure dependence of the brush height clearly showed an anomalous peak at the density fluctuation ridge (pressure = 8.2 MPa) that defined the maximum long-range density fluctuation amplitude in the pressure-temperature phase diagram of carbon dioxide (CO ₂). The density profile of the brush, which could be approximated by a simple step function, and the magnitude of the brush height both indicated that the solvent quality of scCO ₂ for the deuterated polystyrene brushes was still poor even at the density fluctuation ridge. In addition, atomic force microscopy images for the frozen polystyrene brush prepared by the rapid drying of CO ₂ showed a phase-separated structure, as predicted from the numerical calculations of Grest and Murat, as a function of the variable Nσ, where N is the polymerization index and σ is the grafting density.