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 prepared by free-radical copolymerization of methyl (methacrylate) (MMA) and ethylene glycol dimethacrylate at 80°C. The FTRF technique, which measures lifetimes, is more powerful in gel swelling experiments than the steady-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 monitor during in situ swelling experiments. Double exponential fits were performed to measure the long (τ2) and short (τ1) components of Py lifetimes which belong to the Py molecules inside and outside the PMMA gels. It was observed that τ2 values inside the gel decrease as swelling proceeds; however, τ1 lifetimes outside the gel stay constant during slow release. An equation is derived for low-quenching efficiencies to interpret the behavior of Py lifetimes inside the gel during swelling. The Li–Tanaka equation was used to determine the cooperative, Dc diffusion coefficients at various temperatures. It was observed that the Dc value increases as the temperature is increased. The activation energy ΔE was measured for the gel swelling process, and found to be 22 kcal mol−1.