Dynamic heterogeneities, boson peak, and activation volume in glass-forming liquids

There are various arguments and models connecting the characteristic length associated with the boson peak vibrations xi to the length scale of dynamical heterogeneity L-het. xi is usually defined as the ratio of the transverse sound velocity to the boson peak frequency. Here we present pressure, temperature, and molecular weight dependencies of xi, estimated using light scattering, in a few molecular and polymeric glass formers. These dependencies are compared with respective dependencies of the activation volume Delta V-# in the same materials. Good agreement is found for the pressure and molecular weight dependencies of xi and Delta V-# measured at the glass transition temperature T-g. These results provide more evidence for a possible relationship between the sensitivity of structural relaxation to density (activation volume) and the heterogeneity volume. However, contrary to the expectations for L-het, xi does not decrease with temperature above T-g in most of the studied materials. The temperature dependence of xi is compared to that of L-het in glycerol and orthoterphenyl (OTP) estimated from literature data. The analysis shows a clear difference in the behavior of xi (T) and Delta V-#(T) at temperatures above T-g, although Delta V-#(T)(1/3) and L-het(T) have similar temperature dependence. Possible reasons for the observed difference are discussed.