Molecular mobility of hydroxyethyl cellulose (HEC) films characterised by thermally stimulated currents (TSC) spectroscopy

Abstract

Molecular mobility has long been established to relate to textural properties and stability of polymer films and is therefore an important property to characterise to better understand pharmaceutical film formulations. The molecular mobility of solvent cast hydroxyethyl cellulose (HEC) films has been investigated by means of thermally stimulated current (TSC) below the temperature at which the film was formed. Preliminary physical characterisation of the films was performed using XRPD, TGA, DSC and texture analysis (tensile properties). XRPD results showed the films to be completely amorphous with Tg determined by DSC to be 127 ± 1 °C. TGA analysis showed the films to contain 8 ± 1% water and film was dried to only 0.06 ± 0.01% water content when heated to 160 °C. Application of TSC detected molecular mobility in HEC films at sub-zero temperatures. Two motional transitions with average relaxation time of 50 ± 3 s were identified; a β-relaxation at -57 ± 2 °C, attributed to localised non-cooperative orientation of HEC polymer chain ends and the hydroxyethyl side groups and an α-relaxation, originating from cooperative segmental mobility, at -20 ± 2 °C. The tensile properties i.e., elongation, tensile strength and elastic modulus of the HEC film have been related to the molecular relaxation processes detected by TSC.