(510k) Swelling Behavior of Poly(N-Cyclopropylacrylamide) Cross-Linked Thin Films

Stimuli responsive polymers exhibit a variation in properties and structure in response to a change in external conditions, such as temperature, pH, or salt. Polymers that respond to a change in temperature are known as thermoresponsive polymers. These polymers are soluble in water at low temperature, but become insoluble above a threshold temperature, called the lower critical solution temperature (LCST). This volume phase transition can be exploited for usage in sensors, actuators etc. Poly(N-cyclopropylacrylamide)(PcPA) is one such thermoresponsive polymer. PcPA, a homologue of poly(N isopropylacrylamide) (PNIPAAm), shows a different phase behavior. Its LCST is around 47°C and its transition is continuous. In contrast, the LCST of PNIPAAm is 32°C and its transition discontinuous. We synthesized cross-linkable copolymers comprising cyclopropylacrylamide (cPA) and 4-methacryloxy benzophenone(MABP). cPA was synthesized by reacting cyclopropylamine with acryloyl chloride at a ratio of 1:2.Triethylamine was used as an acid scavenger and benzene as the solvent. MABP was synthesized by taking equimolar amounts of 4-hydroxybenzophenone and methacryloyl chloride via an esterification reaction in acetone as the solvent and triethyamine as the acid scavenger. Both the structures were confirmed by NMR. The co-polymer of n-cyclopropylacrylamide and benzophenone was made via free radical polymerization using azoisobutyrnitrile (AIBN) as the initiator. The mixture was degassed by freeze thaw cycles. Then the reaction was carried out in a water bath at 60°C for 18 hours. Cross-linked films were made by spin-coating, followed by UV cross-linking. Benzophenone was chosen as it can be easily photo-cross-linked. 365nm UV light was used for photo-cross-linking as it triggers the n, π * transitions in benzophenone groups. The degree of swelling was measured by ellipsometry.