Abstract:
Rotational-translational decoupling, in which translational motion is apparently enhanced over rotational motion in violation of Debye-Stokes-Einstein predictions, has been observed in a wide variety of materials near their glass transition temperatures (Tg). This has been posited to result from ensemble averaging in the context of dynamic heterogeneity. In this work, single fluorescent probe molecules are tracked rotationally and translationally to interrogate this explanation. In one study, ensemble and single molecule experiments are performed in parallel on the ideal fluorescent probe N,N’-dipentyl-3,4,9,10-perylenedicarboximide (pPDI) in high molecular weight polystyrene near its Tg. Ensemble results show decoupling onset at approximately 1.15Tg, increasing to over three orders of magnitude at Tg. Single molecule measurements also show a high degree of decoupling, with typical molecules at Tg showing translational diffusion coefficients nearly 400 times higher than expected from Debye-Stokes-Einstein predictions. These results are corroborated with supplemental studies on another microscope with lower localization accuracy. Additionally, simulations of translational diffusion are performed and, taken together with experimental results, these reveal that the majority of rotational-translational decoupling in glassy systems occurs through dynamic exchange consistent with wide underlying distributions of diffusion coefficients and exchange coupled to local spatiotemporal dynamics.
