Thermal and dielectric spectroscopic investigations on tetrahedrally Co-ordinated organic plastic Crystals: Aminomethylpropanediol and pentaglycerol

Abstract

Thermal and dielectric response of two orientationally disordered organic crystals or plastic crystals (PCs) viz. Aminomethylpropanediol (AMP) and Pentaglycerol (PG) was investigated using differential scanning calorimetry and broadband dielectric spectroscopy (10⁻² –10⁷ Hz) from their deep glassy crystalline states up to their melting temperatures. The effort was made to explore and understand the relaxation dynamics in these systems and to correlate with other PCs, which lack translational degrees of freedom while maintaining rotational dynamics, and to get a comprehensive understanding on their glass forming dynamics. Both systems exhibited non-Arrhenius temperature dependence and non-Debye relaxation characteristics. A distinct Johari-Goldstein (JG) secondary relaxation was resolved in PG, while AMP displayed an excess wing attributed to an unresolved JG process. Both compounds were classified as intermediate on Angell's fragility scale, showing a correlation between increasing molecular weight, higher glass transition temperature, and lower fragility. A transition from non-Arrhenius to Arrhenius behavior was linked to enhanced hydrogen bonding. The data on both samples filled the gap for attaining a comprehensive picture on rotational dynamics of tetrahedral co-ordinated polyalcoholic PCs. DFT-based dihedral scans revealed energy barriers consistent with hidden JG relaxations, under the intense structural relaxation and manifested as an excess wing.