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 ξ to the length scale of dynamical heterogeneity L_het. ξ 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 ξ, estimated using light scattering, in a few molecular and polymeric glass formers. These dependencies are compared with respective dependencies of the activation volume ΔV# in the same materials. Good agreement is found for the pressure and molecular weight dependencies of ξ and ΔV# 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, ξ does not decrease with temperature above T_g in most of the studied materials. The temperature dependence of ξ 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 ξ(T) and ΔV#(T) at temperatures above T_g, although ΔV#(T)1^/3 and L_het(T) have similar temperature dependence. Possible reasons for the observed difference are discussed.