Scientific Results

Molecular dynamics during linear chain polymerization from real-time dielectric spectrometry and calorimetry

Year: 1997

Authors: Tombari E., Ferrari C., Salvetti G., Johari G.P.

Autors Affiliation: CNR, IST FIS ATOM & MOL, VIA GIARDINO 7, I-56127 PISA, ITALY

Abstract: To obtain physical insight into the slowing of the molecular dynamics when a macromolecule grows as a result of polymerization, (i) the dielectric relaxation spectra, and (ii) the heat evolved during the growth of a linear chain structure by means of the reaction of diepoxide with a monoamine have been measured simultaneously and continuously over time, at a fixed temperature of 314.2 K, during fixed-rate heating to 341.5 K and thereafter cooling from this temperature. An instrument was designed for the purpose. The studies yield information, almost simultaneously, on the changes in the dc conductivity, on the static and dynamic behaviours of the dipolar relaxation, and on the number of covalent bonds, n, in the states of the structures formed by irreversible polymerization. The dc conductivity decreases with increase in n, but this decrease cannot be attributed entirely to the increase in viscosity. The decrease in the ion population has a significant effect on the change in dc conductivity. Both the equilibrium permittivity and the limiting high-frequency permittivity decrease on polymerization. This decrease is attributed mainly to a decrease in the dipolar orientational correlation factor in the former case, and to a predominant increase in the phonon frequencies in the latter case. It is found that the slowing of the molecular dynamics that occurs here on increase in n is more than compensated by the acceleration of the dynamics on increase in the thermal energy. This effect is interpreted in terms of the changes in the configurational entropy, S-conf, which leads to relations expressing the dependence of S-conf on n, as well as on the temperature. A faster molecular dynamics of the Johari-Goldstein relaxation evolves as n increases. This dynamics does not depend on the decrease in S-conf with decrease in n, but depends only on the change in S-conf with temperature. The dielectric behaviour of the completely polymerized state obtained after repeated thermal cycling of the initially molecular liquid has been studied, and the results are related to the molecular dynamics observed during the growth of the macromolecules.


Volume: 33      Pages from: 7017  to: 7037

DOI: 10.1088/0953-8984/9/33/006

Citations: 49
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