Study of the glass transition order parameter in amorphous polybutadiene by incoherent neutron scattering

The temperature dependent elastic incoherent scattering from a glass forming polybutadiene was studied using high resolution neutron spectroscopy. This elastic scattering measures directly the non-ergodicity order parameter of the glass transition. We observed an anomalous decrease of this scattering setting in around 30 K below the thermodynamic glass transition, T_g, the temperature dependence of which is in agreement with the square root of T prediction of the mode coupling approach. The critical temperature of 220 K lies about 30 K above T_g. The missing elastic intensity reappears as inelastic scattering in the 1 meV range. Within the μeV resolution of the backscattering spectrometer no quasielastic scattering can be detected up to 20 K above T_g. The observed inelastic scattering may be interpreted as resulting from a continous shift of the density of states towards low frequencies as a consequence of a general softening of the structure.