Novel analytical approaches for the study of mobility and relaxation phenomena in positional isomers of GABA

Abstract

γ-Aminobutyric acid (GABA), and its positional isomers DL-α-aminobutyric acid (AABA) and DL-β-aminobutyric acid (BABA) have been analysed, in the solid state, using thermally stimulated current (TSC) spectroscopy. Secondary relaxations in these molecules have been detected for the first time. GABA displays two secondary relaxations at 77 ± 2 °C and 114 ± 2 °C, whilst AABA and BABA each display a single secondary relaxation at 109 ± 1 °C and 104 ± 1 °C, respectively. Analysis (decoupling of molecular mobilities) of secondary relaxations using thermal windowing (TW) and relaxation map analysis (RMA) show that the dipole relaxation associated with secondary transitions observed for the aminobutyric acids requires activation energies of 189.9 ± 3.2 kJ mol(-1) (GABA), 142.4 ± 1.4 kJ mol(-1) (AABA) and 195.7 ± 0.8 kJ mol(-1) (BABA). However, the ΔH(≠) values observed were found to exhibit negligible deviations from the zero entropy line. This indicates that the relaxation processes are localised, non-cooperative rotational motions that have a negligible influence on entropy changes of detected molecular mobilities. Furthermore, RMA analysis revealed that AABA and BABA display compensation behaviour i.e., entropy and enthalpy are linearly related to each other, whereas GABA did not demonstrate such behaviour. The coordinates of the compensation point (compensation temperature (Tc) and the compensation relaxation time (τc)) were found to be 214 ± 6 °C and 0.051 ± 0.024 s, respectively for AABA and 153 ± 3 °C and 0.025 ± 0.011 s for BABA. For the molecules investigated the compensation points coincide with the starting temperature of the higher temperature thermal events i.e. sublimation, melt/decomposition, and indicate a correlation between secondary relaxation processes and the main thermal transitions, found via TGA and DSC studies.

Glassy state molecular mobility and its relationship to the physico-mechanical properties of plasticized hydroxypropyl methylcellulose (HPMC) films

“…Details of the TSC technique can be found elsewhere (Turnhout, 1975, Ramos and Mano, 1997, Correia et al, 2000, Owusu-Ware et al, 2013). Briefly, TSC measures currents generated by the movement of molecular dipoles, as a function of temperature, in response to externally applied static electrical field.…”