Probe translational and rotational diffusion in polymers near T_g: Roles of probe size, shape, and secondary bonding in deviations from Debye-Stokes-Einstein scaling

Rotational and translational dynamics of a variety of probes are compared in several polymers near the glass transition temperature, T_g. Second harmonic generation was used to measure the rotational relaxation distribution and average rotational relaxation time, 〈τ〉, and fluorescence nonradiative energy transfer was used to measure D_trans, the translational diffusion coefficient. D_trans is affected greatly by probe size and shape, typically with a temperature dependence in the rubbery state near T_g which violates Debye-Stokes-Einstein (DSE) scaling and an apparent enhancement in translational diffusion; in contrast, 〈τ〉 is largely unaffected by probe size and shape (for the ranges studied), as long as the probe is sufficiently bulky to have its dynamics coupled to the polymer α-relaxation, and follows DSE scaling. These effects are associated with the short-time side of the distribution of reorientation relaxation times being sensitive to probe size and aspect ratio and the fact that D_trans is dominated by short-time relaxations while 〈τ〉 is dominated by long-time relaxations. With hydrogen bonding between probe and polymer, both D_trans and 〈τ〉 may be affected.