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CPP: Fachverband Chemische Physik und Polymerphysik
CPP 50: Glasses and Glas Transition II
CPP 50.3: Talk
Thursday, March 30, 2023, 12:45–13:00, ZEU 255
When molecular heterogeneities matter: thermal expansion and relaxation time in polyalcohols — •Martin Tress1, Jan Gabriel2, and Friedrich Kremer1 — 1Universität Leipzig, Leipzig, Germany — 2Roskilde University, Roskilde, Denmark
Structural relaxation in liquids is considered to depend directly on the available free volume. Many theoretical concepts of liquid dynamics and the glass transition approximate this molecular quantity using macroscopic density. However, particularly hydrogen (H-) bonding liquids defy proper description by these approaches (e.g. the failure of density-scaling in such materials). To study densification on molecular scale, we use infrared spectroscopy on a series of polyalcohols. By analyzing specific molecular vibrations and correlating them with interatomic bond lengths, the thermal expansion of several types of intramolecular covalent bonds and intermolecular H-bridges is quantified. Pronounced differences between intra- and intermolecular expansion verify the dominance of the latter. Surprisingly, the overall thermal expansion (i.e. cube root of inverse density) is even bigger than that of the strong H-bridges. This suggests that weak H-bridges dominate thermal expansion while strong ones must control structural relaxaztion, i.e. the glass transition. The method is validated by successfully describing the density of water based on extracted H-bridge lengths. Consequently, inhomogeneities on intra- and intermolecular scale can play distinct roles in densification and orientational relaxation and require a careful consideration in a comprehensive theoretical description. [Gabriel, Tress et al. J Chem Phys 154 (2021) 024503]