Probe Translational and Rotational Diffusion in Polymers near Tg: Role 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, Tg. 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 Dtrans, the translational diffusion coefficient. Dtrans is affected greatly by probe size and shape, typically with a temperature dependence in the rubbery state near Tg 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 Dtrans is dominated by short-time relaxations while 〈τ〉 is dominated by long-time relaxations. With hydrogen bonding between probe and polymer, both Dtrans and 〈τ〉 may be affected.