X-ray photoelectron spectroscopy studies of water-induced surface reorganization of amphiphilic poly(2-hydroxyethyl methacrylate- g -dimethylsiloxane) copolymers using cryogenic sample handling techniques

A series of amphiphilic graft copolymers having a hydrophilic poly(2-hydroxyethyl methacrylate) (HEMA) backbone and hydrophobic polydimethylsiloxane (PDMS) side chains were studied by x-ray photoelectron spectroscopy (XPS) in both dry and hydrated states using cryogenic sample handling techniques. The polymers were synthesized using anionic methods for the PDMS side chains, yielding a tightly controlled PDMS graft chain length in a polymer brush surface configuration. The effects of hydration time and polymer bulk structure (e.g., PDMS graft length) and composition on surface reorganization were examined. XPS studies provided direct quantitative data on the extent of polymer surface reorganization between dry and hydrated states. The dry surface was enriched with PDMS while the hydrated surface was enriched with HEMA. The surface reorganization reaches equilibrium when copolymers were hydrated for 24 h. In the dry state, PDMS graft length and bulk composition have a strong effect on surface composition; while in the hydrated state, the polymer surface composition was nearly independent of bulk structure or composition.