Comparison of ensemble and single molecule approaches to probing polymer relaxation dynamics near Tg

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Abstract
Single molecule spectroscopy and polarization holeburning (PHB) are used to measure the dynamics of rotation for rhodamine 6G in poly(methylacrylate). It is found that the ensemble average correlation time obtained from the single molecule experiments is the same as that obtained from bulk fluorescenceanisotropy decay. Both reflect the same underlying dynamics as the mechanically measured α relaxation. Fitting α relaxation and PHB data to the Vogel–Tammann–Fulcher (VTF) equation, it is found that the two measurements follow the same temperature dependence, but with absolute times that differ by nearly four orders of magnitude. This difference is discussed in two contexts. In the first the relative ratio of the correlation and α-relaxation times depend on probe size, in which case the large difference is not unexpected given the size of the probe. In the second, the difference is attributed to specific probe/polymer interactions. In this case, a modified VTF equation that includes an activation energy term can be used to explain this phenomenon; when all parameters but the activation energy are fixed to the α-relaxation values, it is found that an activation energy of 21 KJ/mol could cause the observed slowing of the rhodamine 6G rotational correlation times.