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
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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.
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