Influence of Crosslink Density on Swelling and Conformation of Surface-Constrained Poly(N-Isopropylacrylamide) Hydrogels

A stimuli-responsive microgel is a three-dimensional polymer network that is able to absorb and expel a solvent (commonly water). These materials are unique in the fact that their sponge-like behavior can be actuated by environmental cues, like temperature, ion concentration, pH, and light. Because of the dynamic properties of these materials they have found applications in drug-delivery systems, micro-assays, selective filtration, artificial muscle, and non-fouling surfaces. The most well-known stimuli-responsive polymer is Poly(N-isopropylacrylamide) or PNIPAAm and it experiences a switchable swelling or deswelling over a critical temperature ( Tc=~32°C). Below the critical temperature, the gel begins mixing with the surrounding solvent and swells; above this temperature, the opposite is true. The unconstrained hydrogel will continue to swell in all directions until equilibrium is established between its propensity for mixing with the surrounding solvent and the elastic restoring forces of the gel matrix. The strength of the elastic restoring forces is dependent on the interconnectedness of the polymer network and is therefore a function of crosslink density. An increase in crosslink density results in a decreased swelling and vice versa. If the hydrogel is mechanically constrained to a surface, it can experience various wrinkling and buckling conformations upon swelling, as the stresses associated with its confinement are relieved. These conformation characteristics are a strong function of geometry (aspect ratio) and extent of swelling (i.e. crosslink density). In order to capitalize on the utility of this material, it is imperative that its volume transition is well characterized and understood. Toward this end, pNIPAAm gels have been created with 1x10-7 to 2x10-³ mol/cm³ crosslink density and characterized. This was done by first examining its bulk, unattached swelling ability and then by evaluating its microscale properties as a surfaceconfined monolithe. The latter was achieved through the use of confocal microscopy and copolymerization with a fluorescent monomer. This method allows for a detail analysis of the deformations experienced (bulk-structural bending and surface undulating) and will ultimately lend itself to the correlation between crosslink density and the onset of mechanical phenomena