Temperature effect on gel swelling: a fast transient fluorescence study

Fast transient fluorescence (FTRF) technique was employed for studying the swelling of disk shaped poly(methyl methacrylate) (PMMA) gels which were pr epared by free-radical copolymerization of methyl (methacrylate) (MMA) and ethylene glycol dimethacrylate at 80 degreesC. The FTRF technique, which me asures lifetimes, is more powerful in gel swelling experiments than the ste ady-state fluorescence technique where fluorescence intensity is measured. Swelling experiments were performed by using pyrene (Py) doped PMMA gels in chloroform at various temperatures. Decay curves of Py were used to monito r during in situ swelling experiments. Double exponential fits were perform ed to measure the long (tau (2)) and short (tau (1)) components of Py lifet imes which belong to the Py molecules inside and outside the PMMA gels. It was observed that tau (2) values inside the gel decrease as swelling procee ds; however, tau (1) lifetimes outside the gel stay constant during slow re lease. An equation is derived for low-quenching efficiencies to interpret t he behavior of Py lifetimes inside the gel during swelling. The Li-Tanaka e quation was used to determine the cooperative, D-c diffusion coefficients a t various temperatures. It was observed that the D-c, value increases as th e temperature is increased. The activation energy Delta E was measured for the gel swelling process, and found to be 22 kcal mol(-1). (C) 2001 EIsevie r Science Ltd. All rights reserved.