8 Pages

How long do regions of different dynamics persist in supercooled o-terphenyl?

I. Introduction The fundamental origin of the characteristic nonexponential

relaxation processes in supercooled liquids and amorphous polymers has been in dispute for decades. Interest in this issue was revived recently as new experimental techniques have been developed to provide more explicit investigations.1-7 Nonex­ ponential relaxation can be interpreted in two fundamentally different ways. One possibility is that a heterogeneous set of environments exists in supercooled liquids; relaxation within a single environment is essentially exponential, but the relaxation time varies significantly among environments. Alternatively, environments are homogeneous in supercooled liquids and each molecule relaxes nearly identically in an intrinsically nonex­ ponential manner. Most recent experiments indicate that the heterogeneous explanation is more nearly correct,2-7 although this view is not unanimous.8 The size of heterogeneous dynamic regions is estimated to be 2-3 nm,9-11 and the width of the distribution of relaxation times is proposed to be 2-3 orders of magnitude.9·12 However, this heterogeneity cannot last forever; i.e., exchange must occur between domains of different dynamics since supercooled liquids are ergodic. Is this exchange time much longer than or comparable to the ensemble-averaged relaxation time? We will focus on the issue of the lifetime of the heterogeneous dynamic regions in this report.