Tuning glass-forming dynamics by modifying hydrogen bonding: From polyalcohols to van der Waals liquids

This work investigates how modifications of molecular structure influence the dynamics of hydrogen-bonded glass-formers, with particular focus on glycerol and sorbitol. By systematically eliminating hydrogen bonding through acetylation-yielding glycerol triacetate and sorbitol hexaacetate-and by comparing with previous studies involving pressure, nanoconfinement, and hyperquenching, we assess the role of hydrogen bonds in governing structural and secondary relaxation processes. Broadband dielectric spectroscopy reveals that the removal of hydroxyl groups leads to a substantial reduction in dielectric strength and notable changes in the structural α-relaxation, including variations in glass transition temperature, fragility, and the non-exponentiality parameter. These changes differ qualitatively between glycerol and sorbitol, indicating that the impact of hydrogen bonding depends sensitively on molecular architecture. In contrast, the acetylated systems behave as van der Waals glass-formers and conform to established correlations between dielectric strength and relaxation shape. Analysis of secondary relaxations shows that the Johari-Goldstein β-relaxation persists across all modifications, with its characteristic timescale remaining closely linked to the primitive relaxation time predicted by the coupling model. External perturbations such as high pressure and hyperquenching modify intermolecular coupling and relaxation dispersion without significantly affecting dielectric strength, while nanoconfinement reduces cooperativity and brings the α-relaxation close to the primitive limit. Overall, the results demonstrate that hydrogen bonding strongly influences intermolecular coupling and dynamic heterogeneity, but the fundamental relation between primary and secondary relaxations remains robust.