Lower critical solution temperature (LCST) polymers experience a volume-phase transition when subjected to small perturbations in external stimuli. For example, poly(N-isopropylacrylamide) (poly(NIPAAm)) undergoes a hydrophilic/hydrophobic transition at roughly 32^oC. The critical point of poly(NIPAAm) occurs at a temperature of 29.5 ^oC and a volume fraction, c, of 0.43. As the polymer concentration is reduced towards zero, the demixing temperature approaches 34 ^oC. To explain this phenomenon, the effective Flory interaction parameter c must be expanded in powers of f, with a minimum of three terms, wherein each term is a function of temperature.

A key question concerns the use of a concentration dependent c parameter derived from the solution phase diagram to describe the behavior of constrained systems, including end-tethered or cross-linked polymers. We report on the swelling of surface-tethered poly(NIPAAm) networks as characterized by neutron reflectivity. In this study, surface-tethered networks were prepared from photo-cross-linkable poly(NIPAAm) copolymers with benzophenone-pendant monomers. Ultraviolet radiation (£f = 350 nm) triggers the n,p* transition in the benzophenone moieties leading to a biradicaloid triplet state that abstracts a hydrogen from a neighboring aliphatic C-H group, forming a stable C-C bond. Neutron reflection reveals that the discontinuity in the volume transition of the surface-tethered networks coincides with the miscibility gap of non-cross-linked linear poly(NIPAAm). This result signifies that the concentration dependent c interaction parameter is unaffected by cross-linking and can be used to model volume phase transitions in constrained systems.