Translational and rotational diffusion of probe molecules in polymer films near Tg: effect of hydrogen bonding
Abstract
The effect of hydrogen bonding on the rotational and translational dynamics of two small molecule probes, lophine and S-lophine, in amorphous polymer films near the glass transition temperature, Tg, was studied using second harmonic generation (SHG) and fluorescence nonradiative energy transfer (NRET), respectively. The two probes are nearly identical in size and shape, with the only structural difference being related to an amine functional group in lophine replacing a sulfur atom in S-lophine. The two probes exhibited essentially identical rotational and translational dynamics in polystyrene, which has no polar units allowing for hydrogen-bonding interactions. However, in poly(isobutyl methacrylate), which can participate in hydrogen bonding with amine units, lophine average rotational reorientation times (〈τrot〉) were found to increase, and translational diffusion coefficients (D) were found to decrease by approximately an order of magnitude as compared to those of S-lophine. The magnitude of this effect is much greater than similar hydrogen-bonding effects reported for diffusion in polymer solutions. The hydrogen-bonding effects can be quantitatively taken into account using an interaction energy term with activation energy, Ea = 6 kJ/mol for translation and 9.5 kJ/mol for rotation. The differences in Ea for the two modes of motion are explained in terms of the differing ways in which D and 〈τrot〉 average over the broad distribution of relaxation times present in polymers near Tg. Implications associated with additive migration in polymers are discussed.