Effect of LCST on the swelling of PAAm-NIPA copolymers: a fluorescence study
Temperature sensitive copolymers were prepared by free radical crosslinking copolymerization in aqueous solution with different molar percentage of N-isopropylacrylamide (NIPA) and acrylamide (AAm) monomers. NN'-methylenebis (acrylamide) (BIS) and ammonium persulfate (APS) were used as a crosslinker and an initiator respectively. The steady-state fluorescence (SSF) technique was used to determine the low critical phase transition temperature (LCST) for PAAm-NIPA copolymers. Swelling experiments were performed in water at various temperatures by real time monitoring of pyranine (Py) fluorescence intensity I which decreased as swelling proceeded. The Stern-Volmer equation is modified for low quenching efficiencies to interpret the behavior of pyranine intensity during the swelling of PAAm-NIPA copolymers. The Li-Tanaka equation was used to determine the swelling time constants tau (1) and the cooperative diffusion coefficients D (0) from fluorescence intensity weight and volume variations of the copolymers at various temperatures. It was observed that tau (1) first increased up to LCST and then decreasednaturally D (0) decreased up to LCST and then increased upon increasing temperature. It was understood that (LCST) increases as PAAm contents increase in the PAAm-NIPA copolymers.