Localized motions and the loss of chemical and physical metastabilities during ageing of amorphous polymers studied by dielectric measurements

Abstract

After molecular addition reactions in a liquid have become diffusion controlled, the extent of polymerization may become kinetically frozen-in at isothermal vitrification. This causes the vitreous state to become chemically and physically metastable. The spontaneous loss of the two metastabilities with time can be studied by dielectric measurements. Studies of three thermosetting polymers formed by addition reactions of (i) ethylenediamine, (ii) propylene-1,3-diamine and hexamethylene-1,6-diamine with diglycidyl ether of bisphenol-A have shown that isothermal ageing below the glass-transition temperature first causes both the temperature and the height of the β-relaxation peak to increase as the chemical metastability is predominantly being lost and, thereafter, causes the temperature of the peak to remain constant but its height to decrease as the physical metastability is lost. The dielectric changes observed during the polymerization of a mixture containing half the stoichiometric amount of hexamethylene-1,6-diamine differ from that containing the stoichiometric amount. This results from the slower reaction rate of the secondary amine with the diepoxide after the primary amine has reacted and the reaction has not reached the diffusion-controlled state. Implications of the dielectric changes on isothermal ageing for our understanding of the nature of localized motions are discussed, and the β-relaxation process is found to be attributable to the reorientation of dipoles in volumetrically heterogeneous regions of loose density packing in the solid polymer's covalently bonded, network structure.