Dielectric polarization and the stages of a macromolecule's growth
To investigate the change in the orientation polarization, particularly during the growth of a macromolecule by addition reactions in a liquid, dielectric relaxation spectra of diglycidyl ether of bisphenol A have been studied in real time. It is found that in the plot of the dielectric permittivity, epsilon', measured at a fixed frequency against the polymerization time, the first step of the (inverted) sigmoid-shape part of the spectrum resembling a relaxational decrease is a dc conduction effect. After this effect vanishes, the equilibrium permittivity, epsilon(s,) of the liquid increases with time as covalently bonded molecular clusters form at random sites in the bulk liquid, and the dipole orientational correlation increases as a result of hydrogen bonding between the newly formed OH groups. Thereafter, the macromolecule's continuous growth in the liquid decreases the net dipole moment of the covalently bonded network, which decrease epsilon(s). Thus, after the conductance effect has become negligible, a plot of es against the polymerization time shows a maximum. Both the dc conductivity, sigma(0), and the distribution parameter for dielectric relaxation decrease with time, and sigma(0) follows a scaling equation for gelation. The irreversible increase in relaxation time during a macromolecule's growth is explained in terms of a decrease in the configurational entropy, and a relation between the two is provided.
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