Effect of LCST on the swelling of PAAm-NIPA copolymers: a fluorescence study

Abstract

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. N,N′-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, τ1 and the cooperative diffusion coefficients, D0 from fluorescence intensity, weight and volume variations of the copolymers at various temperatures. It was observed that τ1 first increased up to LCST, and then decreased; naturally D0 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.