Pressure-induced suppression of supramolecular dynamics in hydrogen-bonded adamantane derivative

Determining how pressure and temperature compete to govern supramolecular organization in hydrogen-bonded liquids remains a fundamental challenge. Here, we investigate memantine (MEM), a monoamine with a single polar group, which surprisingly exhibits three distinct relaxation modes associated with supramolecular clustering, collective and local dynamics, making it ideal to probe this competition. To this end, we combined differential scanning calorimetry, high-pressure broadband dielectric and Raman spectroscopy, and molecular dynamics (MD) simulations. Our results reveal strongly pathway-dependent phase behavior: isobaric cooling at ambient pressure induces a transition to a plastic crystal, whereas isothermal compression preserves the liquid state. Although both pathways affect hydrogen-bonding interactions, isochronal comparisons indicate that cooling is more effective than compression at promoting supramolecular clustering, as reflected in a higher Debye relaxation amplitude. Notably, even when the structural α- and secondary relaxation times scale at a given temperature-pressure condition, the Debye process does not, indicating that compression reduces the timescale separation between α and Debye dynamics. MD simulations corroborate this picture, revealing a highly associated state upon cooling but a more fragmented hydrogen-bonded organization under compression. Finally, MEM exhibits an unusually high pressure coefficient of the glass transition temperature and activation volume compared with typical amines, which we attribute to the rigid adamantane scaffold that controls the pressure response and restricts cluster formation.