Insights into\nthe Thermal Expansion of Amorphous Polymers

We investigated the thermal expansion of amorphous polystyrene\n(PS) and poly(methyl methacrylate) (PMMA) homopolymers using the temperature\ndependence of spatial electron-density correlations. The strong and\nbroad X-ray interference maxima arising from inter- and intramolecular\ncorrelation distances were distinct, maintaining their shape during\nthe heating of the samples to 250 °C. Three maxima characteristic\nof electron density correlations between the backbones, substituents\nalong the chain, and repeat-units were observed. A remarkable temperature\ndependence was identified in the largest correlation distances arising\nfrom the intermolecular interactions. Based on the temperature dependence\nof the correlation distances, the coefficients of molecular expansion\nwere very close to the coefficients of thermal expansion. This study\nprovides a simple yet accurate way to correlate macroscopic volume\nexpansions with the molecular expansion obtained from wide-angle X-ray\nscattering (WAXS) data.