Single molecule studies of heterogeneous dynamics near the glass transition

Abstract

Single molecule spectroscopy is used to investigate the spatial heterogeneities that cause the non-exponential relaxation dynamics observed in glass forming materials near the glass transition. By following the rotational motions of spatially isolated fluorescent probe molecules in real time, single molecule spectroscopy reveals a broad distribution of spatially varying dynamics. On short time scales, molecular rotation occurs through a Brownian diffusional mechanism, however on longer time scales, these normal diffusional motions are punctuated by abrupt environmental changes result in diffusional motion on a different time scale. These two characteristic time scales, the rotational correlation time and the environmental exchange time, are found in both the polymeric glass formers poly(methyl acrylate) and poly(n-butyl methacrylate) and the small molecule supercooled liquid ortho-terphenyl. In all cases, the average vii rotational correlation time reproduces the ensemble behavior measured in bulk experiments and obeys the predicted bulk temperature dependence. Environmental exchange times are much longer than rotational correlation times, but follow the same temperature dependence. The distributions of correlation and exchange times become broader as the temperature is cooled toward the glass transition, reflecting the increasing heterogeneity that accompanies the approach of Tg. The observed normal diffusional behavior disrupted by sudden changes in local environments compares well with several theories of molecular motion near the glass transition.