Physical aspects of network polymerization from calorimetry and dielectric spectroscopy of a triepoxide reacting with different monoamines

Abstract

Calorimetry and dielectric relaxation spectroscopy were used to study the evolution of molecular dynamics during the isothermal polymerization of two stoichiometric mixtures of a molecule with three epoxide groups with (i) aniline and (ii) 3-chloroaniline, whose dipole moments as well as the degrees of steric hindrance to chemical reactions differ. The heat evolved on polymerization was used to calculate the number of covalent bonds formed at any instant during the polymerization reaction. The approach of the DC conductivity towards a singularity as the reaction progressed agrees with the Flory-Stockmayer theory of connectivity at gelation and not the percolation theory. It is demonstrated that a plot of DC conductivity against the extent of reaction does not have the same shape as the plot against the time of reaction. The permittivity and loss spectra obtained for structural states containing a fixed number of covalent bonds could be described by equations analogous, but not equivalent to, or the same as, the equations used for describing the dielectric properties measured for a fixed frequency during the growth of a macromolecule's network structure. For a fixed temperature, the relaxation time of the structure formed increased as the exponential of the extent of reaction (raised to the power > 1) increased. Comparative parameter-fits to the spectra showed that the DC conductivity and interfacial polarization alter the shape of the dielectric spectra such as to make misleadingly alternative parameter fits possible. The decrease of the equilibrium dielectric permittivity on polymerization is attributed to a decrease in the dipolar orientational correlation as well as the net dipole moment on increase in the number of covalent bonds. The configurational entropy decreased with increase in the number of covalent bonds formed in a manner that differs from the decrease on cooling, and a formalism relating the two effects is given. As the network structure grew isothermally, a second, high-frequency relaxation process came into evidence. This relaxation is attributed to the availability and growth of local regions of low density and high density in the network structure of the macromolecule. A number of issues of a fundamental nature that have risen since our earliest report on this subject have been elaborated and analytically clarified. © 1997 John Wiley & Sons, Inc.