Physics of linear-chain and network polymerization by dielectric spectroscopy

Abstract

Dielectric relaxation spectroscopy has been used to investigate what happens when a monomeric, molecular liquid polymerizes isothermally to form either a linear chain polymer or a network structure. Polymerization was done by using a diepoxide molecule which after reaction with aniline formed a linear chain polymer and a triepoxide which after a similar reaction formed a molecular network structure. The dielectric permittivity and loss were measured as a function of reaction time for 24 ac frequencies (10Hz–10 5Hz) and the data interpolated to obtain the relaxation spectra for several apparently fixed reaction times. These results are then analyzed in terms of the dependence of the dielectric behaviour on the number of covalent bonds formed as polymerization progressed. The dc conductivity for the linear chain polymerization varied with time in a manner quite different from that for network polymerization. In the latter case, the data followed an equation usually seen as a characteristic of gel-formation. The relaxation time determined from the fixed frequency measurement as the reaction progressed agreed with the relaxation time determined from the dielectric spectra interpolated for apparently fixed states at any instant during the polymerization. These results are interpreted in terms of a decrease in the configurational entropy. As polymerization progressed, both the static and the limiting high frequency permittivity decreased and a faster relaxation process evolved as the polymerization approached completion. It is concluded that the number of covalent bonds formed on polymerization and the temperature can be varied in a manner such as to maintain the relaxation time of the state (with same chemical constituents) at the same value. This equivalence of number of bonds and the temperature should be seen as fundamental to the relaxation behaviour.