Molecular and micro-scale heterogeneities in Raman modes of a relaxing polymer glass

We have used Raman spectroscopy to study relaxation dynamics at two different length scales, molecular level and micro-scale in order to probe the presence of cooperative rearranging regions in a polymer glass. Response to slow thermal cycles and fast quench through the glass transition temperature (T _g) is analyzed for film and unprocessed forms of polyvinyl acetate (PVAc). In PVAc film, enhanced disorder and molecular mobility lead to peak broadening by about a factor of 10 compared to unprocessed PVAc. Thermal cycles (10 K min⁻¹) produce hysteresis in integrated Raman peak intensity (loop area $A_{\text{INT}}^I$). $A_{\text{INT}}^I$ values of film are two orders of magnitude more than unprocessed, indicating more configurational mosaics with higher interfacial energy dissipations. Ageing after 60 K min⁻¹ quench manifests as heterogeneous molecular dynamics of film Raman modes with significant peak-width variations, differentiating high mobility and low mobility modes. Two-dimensional mapping of film Raman modes after quench reveal micro-scale clusters of average size ≈250 molecules having fractal boundaries with fractal dimension d _f = 1.5, resembling d _f of percolation clusters below percolation threshold. During thermal cycling and relaxation after a quench, cooperative segmental dynamics with large correlations between skeletal C–C stretch and side branch modes is observed. The observations are analyzed in the context of the random first order transition theory of glasses, which attributes heterogeneous relaxations in glasses to the presence of clusters of variable configurational states. (paper)