Laser Heating Dynamics of Poly(methyl methacrylate) Films Doped with Aromatic Molecules as Revealed by Analysis of Diffusion of Triplet States

The 248 nm laser heating dynamics of poly(methyl methacrylate) (PMMA) doped with biphenyl or p-terphenyl was studied quantitatively by the analysis of temperature-dependent T1-T1 annihilation, which is a result of the thermally-activated diffusion of the dopants. The values of the second-order rate constant k2 (M^-1 s^-1) for the annihilation were obtained at several temperatures (293-363 K) under weak laser excitation (2-4 mJ cm^-2). Arrhenius plots of k2 for both films resulted in straight lines that were used to estimate the rate constant at higher temperatures for the simulation of T1-T1 annihilation. Under intense excitation (100-190 mJ cm^2), the T1 decays of both films occurred faster with increased laser fluence. The comparison between these observed T1 decays and the simulated ones revealed the following: (1) most of the absorbed photon energy, at least 70%, is used to increase the temperature of the polymer, (2) the cooling of the polymer, mainly due to heat transfer into the quartz substrate, slows down the diffusion of the dopants and, accordingly, the T1-T1 annihilation, and (3) the temperature at the polymer-quartz interface after laser excitation remains almost constant up to at least a few microseconds.