The notion of heterogeneous dynamics in glasses, that is, the spatial and temporal variations of structural relaxation rates, explains many of the puzzling features of glass dynamics. The nature and the dynamics of these heterogeneities, however, have been very controversial. Single rhodamine B molecules in poly(vinyl acetate) at the glass transition reorient through sudden jumps. With a statistical search for the most likely break points in the logarithm of the ratio of the two perpendicular fluorescence polarizations, we determine the times of these angular jumps. We interpret these jumps as an indication for individual glass rearrangements in the vicinity of the probe molecule. Time-series analysis of the resulting sequence of waiting times between jumps shows that dynamic heterogeneities in the matrix exist, but are short lived. From the correlation of the logarithm of the waiting time between subsequent jumps, we determine an upper limit for the lifetime of heterogeneities in the sample. The correlation time of τhet = 32 s is three times shorter than the orientational correlation time of the probe molecule, τorient = 90 s, in the sample at this temperature, but 13 times longer than the structural relaxation time, τα = 2.5 s, estimated for this sample from dielectric experiments. We present a model for glass dynamics in which each rearrangement in one region causes a random change in the barrier height for subsequent rearrangements in a neighboring region. This model, which equates the dynamics of the heterogeneities with the dynamics of the glass itself and thus implies a factor of one between heterogeneity lifetime and structural relaxation time, successfully reproduces the statistics of the experimentally observed waiting time sequences.
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14 January 2011
Research Article|
January 11 2011
Single molecules reveal the dynamics of heterogeneities in a polymer at the glass transition
Dieter Bingemann;
Department of Chemistry,
Williams College
, 47 Lab Campus Drive, Williamstown, Massachusetts 01267, USA
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Rachel M. Allen;
Department of Chemistry,
Williams College
, 47 Lab Campus Drive, Williamstown, Massachusetts 01267, USA
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Scott W. Olesen
Scott W. Olesen
Department of Chemistry,
Williams College
, 47 Lab Campus Drive, Williamstown, Massachusetts 01267, USA
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a)
Author to whom correspondence should be addressed. Electronic mail: [email protected].
b)
Present address: San Francisco Estuary Institute, Oakland, California 94621, USA.
J. Chem. Phys. 134, 024513 (2011)
Article history
Received:
February 24 2010
Accepted:
October 26 2010
Citation
Dieter Bingemann, Rachel M. Allen, Scott W. Olesen; Single molecules reveal the dynamics of heterogeneities in a polymer at the glass transition. J. Chem. Phys. 14 January 2011; 134 (2): 024513. https://doi.org/10.1063/1.3516516
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