  The emergence of nanoscience has increased the importance of experiments able
to probe the very local structure of materials, especially for disordered and
heterogeneous systems. This is technologically important; for example, the
nanoscale structure of glassy polymers has a direct correlation with their
macroscopic physical properties. We have discovered how a local, high frequency
dynamic process can be used to monitor and even predict macroscopic behavior in
glassy polymers. Polyvinylethylenes vitrified by different chemical and
thermodynamic pathways exhibit different densities in the glassy state. We find
that the rate and amplitude of a high frequency relaxation mode (the
Johari-Goldstein process involving local motion of segments of the chain
backbone) can either correlate or anti-correlate with the density. This implies
that neither the unoccupied (free) volume nor the configurational entropy
governs the local dynamics in any general sense. Rather it is the magnitude of
the fluctuations in local density that underlie these nm-scale motions. We show
how properties of the dynamics and the density fluctuations can both be
interpreted in terms of an asymmetric double well potential. Finally, since
fluctuations underlie the macroscopic properties, we argue that information
about the latter should be obtainable from characterization of the local
dynamics.
