Free volume study of amorphous polymers detected by solid-state C13 NMR linewidth experiments

Temperature dependence of solid-state C13 nuclear magnetic resonance linewidths of six different amorphous polymers was examined around their glass-transition temperatures (Tg) under the conditions of magic-angle spinning and high-power proton dipolar decoupling (DD). The observed temperature dependence was explained by motional averaging over a dispersion of isotropic chemical shifts and an interference between local anisotropic motion and DD. On the assumption that the correlation time of the local anisotropic motion follows the Williams–Landel–Ferry equation, we found that the reference temperature approximately equals Tg+5 K. From this reference temperature, the fractional free volume of amorphous polymers at Tg is estimated to be 4.7%, which is much larger than that obtained from rheological experiments (2.5%). The difference is attributed to differences in the amplitude of motions detected by both measurements.