Molecular heterogeneities in the thermal expansivity of polyalcohols

Density is the key quantity for nearly all the numerous theories of the (dynamic) glass transition of supercooled liquids and melts. As mean field quantity, it is used to describe correlations and heterogeneities between regions consisting of several molecules. In contrast, the question how density is created by the interactions (i.e., bonds) within a molecule and to its nearest neighbors is almost unexplored. To investigate this for the example of a homologous series of polyalcohols (glycerol, threitol, xylitol, and sorbitol), Fourier-Transform InfraRed (FTIR) spectroscopy is carried out in a wide range of temperatures from far above to far below the calorimetric glass transition T_g. This enables us to determine the potentials and hence the bond lengths of specific intramolecular and intermolecular interactions. While the former has an expansion coefficient of (∼0.1 pm/100 K) with only smooth changes, the latter shows a 30-40 times stronger response with pronounced kinks at T_g. A comparison with the overall expansion based on mass density reveals that one has to separate between strong (OH⋅⋅⋅O) and weak (CH⋅⋅⋅O) intermolecular hydrogen (H)-bridges. Despite the fact that the latter dominates glassy dynamics, their expansivity is 5 times smaller than that of the weak H-bridges. It is to be expected that such heterogeneities on intramolecular and intermolecular scales are a general phenomenon in liquids and glassy systems demonstrating especially the necessity of atomistic simulations.