Chemical metastability loss and molecular dynamics by dielectric relaxations during the catalytic polymerization of a diepoxide

• Published in: Chemical physics 1997-11, Vol.223 (2), p.313-322
• Main Authors: Wasylyshyn, D.A., Johari, G.P., Tombari, E., Salvetti, G.
• Format: Article
• Language: English
• ISSN: 0301-0104
• DOI: 10.1016/S0301-0104(97)00231-0

To investigate the effects of chemical reaction on the evolution of relaxation processes on polymerization, the dielectric properties of a diepoxide with a 5% tertiary amine acting as a catalytic agent were studied: (i) for a fixed frequency over the 77–300 K range before and after polymerization for different periods at 335.4 K and, (ii) in the 1 MHz – 10 GHz frequency range during the course of polymerization for 70 ks at 335.4 K. The reaction time data are then converted to the number of covalent bonds, n, formed by a further study by calorimetry. At 335.4 K all polarization relaxes by a single, relatively broad process. As n increases and consequently the liquid becomes more viscous, the strength of the GHz frequency relaxation decreases towards almost extinction, and a second dynamics, namely the α-process, evolves with a relaxation time that increases as the configurational entropy, S conf, decreases. This leads to vitrification, isothermally. Simultaneously, a third relaxation of intermediate characteristic time evolves and grows in strength. Its characteristic time is also independent of the spontaneous decrease in S conf. The evolution of the α-relaxation and decrease in the strength of the GHz frequency relaxation process remarkably resembles the phenomena observed when the viscosity of a chemically stable liquid is increased on supercooling or pressurizing. The results show that the evolution of molecular dynamics is similar whether a liquid approaches its most stable state by chemical reaction alone, or by structural relaxation alone. This in turn leads us to suggest the concept of a chemical fictive temperature for polymers for which polymerization is rarely complete before vitrification occurs.