Structure of End-Grafted Polymer Brushes in Liquid and Supercritical Carbon Dioxide: A Neutron Reflectivity Study

Abstract

Neutron reflectivity was used to characterize the structure of end-grafted deuterated poly(dimethylsiloxane) (d-PDMS) brushes on SiOx wafers exposed to liquid and supercritical carbon dioxide (CO2). The solvent quality was tuned continuously over a large range from ideal gas conditions to a near-ϑ solvent by varying temperature and CO2 density. Two distinct regions were seen in the segment density profile as a function of distance from the surface: (i) an inner concentrated region near the substrate where the segment density is high due to the strong attractive short- and long-ranged interactions between the d-PDMS and the SiOx substrate and the attractive intra- and interchain interactions and (ii) an outer solvated region that is dilute in polymer due to solvation by CO2. In the outer solvated region, the well-defined block profile at the worst solvent conditions changes to a more parabolic profile with improving solvent quality. The thickness and volume fraction profiles for the outer solvated region change much more with solvent quality than has been seen in previous studies with incompressible solvents, due to the high asymmetry in the intermolecular interactions as well as the large compressibility and free volume differences between the polymer segments and the solvent.