Molecular inhomogeneities in thermal expansion and their impact on relaxation time and densification

Structural relaxation in liquids is considered to be directly controlled by density, a link which is the basis of most theoretical approaches to the glass transition. Yet, none of the theoretical concepts can describe the phenomenon comprehensively; particularly hydrogen bonding liquids defy a proper description. To investigate densification on the molecular scale, Infrared spectroscopy is carried out on a series of polyalcohols at temperatures ranging from far above to far below their respective glass transition temperatures. From specific molecular vibrations, the thermal expansion of intramolecular covalent bonds and strong intermolecular hydrogen bridges is quantified. Pronounced differences between intra- and intermolecular expansion verify the dominance of the latter. Surprisingly, the overall thermal expansion (i.e. the cube root of inverse density) is even bigger than that of the strong hydrogen bridges. This suggests that weak hydrogen bridges dominate thermal expansion while the strong hydrogen bridges clearly control the glass transition. These results demonstrate that inhomogeneities on intra- and intermolecular scale can play distinct roles in densification and glassy solidification and require a careful consideration in a comprehensive theoretical description.