Insights into the nature of phase transitions and relaxation dynamics in memantine – An active substance forming orientationally disordered crystals

Differential scanning calorimetry, together with X-ray diffraction, infrared, Raman, and broadband dielectric spectroscopy were employed to get a comprehensive insight into the nature of phase transitions, as well as the inter- and intramolecular dynamics in active substance - memantine (MEM). Our studies indicated that during cooling, MEM undergoes a transition from the liquid phase to the plastic crystal (PC(I)) phase. Meanwhile, upon the heating cycle, a transition between the PC(I) and PC(II) phases occurs before reaching the melting point. Moreover, calorimetric experiments revealed a glass transition at much lower temperatures during both thermal protocols. Notably, in-depth analysis of infrared and dielectric data measured in a wide temperature range showed that the intra- and intermolecular dynamics of MEM reveal no significant changes during cooling and only slight variations during the heating cycle. Moreover, dielectric studies indicated multiple relaxation modes in the loss spectra of the examined compound, including the presence of a Debye (D)-process, which exhibits a quite notable time-scale separation from the structural relaxation. Keeping in mind the relatively simple chemical structure of MEM, we assigned this mode to the breakup of H-bonds followed by the rotation of the whole molecule and reformation of this kind of specific interaction, which stays in line with the transient-chain model, proposed to explain the molecular nature of D-relaxation in monohydroxy alcohols. Finally, we found that the merging/splitting of structural and secondary processes occurs at particularly long α-relaxation time (τ_α), contrary to typical glass formers, where this phenomenon is observed for much shorter τ_α.