The results obtained during the isothermal curing of diglycidyl ether of bisphenol‐A‐based thermosets cross‐linked with pure diaminodiphenyl methane and pure diaminodiphenyl sulfone and with their mixtures have been analyzed to determine how the dc conductivity changes with time during the conversion of its liquid to a gel. The complex permittivity data are first analyzed to show that ac measurements can be used to obtain the ionic conductivity over a considerable period of the curing process. The procedure allows one to obtain the dc conductivity without having data as a function of frequency. The shape of the complex plane plots of the electrical modulus are semicircles, but with small deviations that appear at long times during the curing process. The dielectric consequences of the chemical changes with time during the cross‐linking of the thermoset are analogous to the frequency dependence of the complex permittivity of a liquid. The analysis shows that the dc conductivity σo of a thermoset during its cure follows a power law, σo∝ (tg—t)x, where t is the curing time (t < tg). The results can also be described equally well by a new equation, σo ∝ exp[—B/(to—t)], where x, tg, B, and to are empirical constants all of which vary with the temperature of the cure. tg is close to the time for gelation known from independent studies and to is close to but longer than the time for vitrification. These conclusions are discussed in terms of scaling concepts for the gelation phenomenon.

The sub‐Tg relaxations of bisphenol‐A–based thermosets cured with diaminodiphenyl methane and diaminodiphenyl sulfone have been studied by dielectric measurements over the frequency range 12 Hz to 200 kHz from their ungelled or “least” cured states to their fully cured states. Both thermosets show two relaxation processes, γ and β, as the temperature is increased toward their Tgs. In the ungelled states, the γ process is more prominent than the β process. As curing proceeds, the strength of the γ process decreases and reaches a limiting value, while that of the β process initially increases, reaches a maximum value, and then decreases. An increase in the chain iength and the number of crosslinks increases the number of ‐OH dipoles and/or degree of their motions in local regions of the network matrix. This is partly caused by the decreasing efficiency of segmental packing as the curing proceeds. The sub‐Tg relaxations become increasingly more, separated from the α relaxation during curing. Physical aging causes a decrease in the strength of the β relaxation of the thermosets as a result of the collapse of loosely packed regions of low cross‐linking density, and this decrease competes against an increase caused by further crosslinking during the “post‐cure” process.

The dielectric permittivity and loss during the curing of diglycidyl ether of Bisphenol A with a 0.3:0.7 (mole:mole) mixture of diaminodiphenylmethane and diaminodiphenyl sulfone have been measured from their sol to gel to glass formation regions and the effects of physical aging on their sub-Tg relaxations investigated. The permittivity monotonically decreases with the curing, but the loss initially decreases, then increases to a peak value, and finally reaches extremely low values characteristic of a glassy state. The complex permittivity plotted in a complex plane has the shape of a skewed arc similar to that of the Cole-Cole plots, and the dielectric consequences of the chemical changes with time that occur during the cross-linking of the thermoset are phenomenologically analogous to the frequency dependence of the complex permittivity of a chemically stable amorphous solid. The time dependence of the complex permittivity follows a stretched exponential decay, (t) = ß [-( / ) '], where 0 < y < 1. The value of y = 0.4 at 377 K. Amongst the two sub-Tg relaxation processes observed here, the low-temperature, or y, process is initially most prominent, but its strength decreases on physical aging while that of the /3-process increases. The strength of the ß-process reaches a maximum and then decreases on further aging. The -relaxation process increasingly separates from the sub-Tg relaxations during the curing of the thermoset, and its strength decreases. Its distribution parameter decreases from 0.60 to a limiting value of 0.40 as curing proceeds. These are discussed in terms of localized segmental motions. A concept of accumulated equivalent curing time is introduced for use in both theoretical and practical investigations of a thermoset's curing, and a method for obtaining the distribution of relaxation times from limited dielectric data is proposed. The low-temperature sub-Tg relaxation is shown to have a distribution of relaxation times which remains unchanged on curing and aging.

SynopsisThe dielectric permittivity c' and loss c" during the crosslinking or curing of diglycidyl ether of bisphenol-A with diaminodiphenyl methane and diaminodiphenyl sulfone have been measured from the sol to gel to glass-formation regions. The c' monotonically decreases with time and the c" initially decreases, then increases to a peak value and finally reaches extremely low values characteristic of the glassy state. These features shift to shorter times with an increase in the temperature of curing. Complex-plane plots of c' and 6' ' have the shape of an arc skewed a t both the short-time and the long-time intercepts and resemble Cole-Cole plots of c*. Thus the dielectric consequences of the chemical changes with time during the cross-linking of a thermoset are analogous to the frequency dependence of c* in an amorphous solid. The time dependence of e* follows a stretched exponential decay, $( t ) = exp [ -( t / 7 ) 7 where 0 < y < 1. The parameter y is in the range 0.2 to 0.4, and increases with a decrease in the curing temperature. Additional curing at longer times produces polymer segments in the network with a high reorientation rate. This is observed as deviations from the shape of a skewed arc a t the limiting long-time intercept and appears as a secondary relaxation during the curing process, but at very long times. The e' and c" have been analyzed and the roles of dc conductivity, Ac, and y in determining the shape of the curing isotherms have been estimated. A representation of the data in the complex electrical modulus, M*, formalism shows the occurrence of two relaxation processes during the period of a typical isothermal cure. The feature observed at the shortest time is due to conductivity relaxation and the subsequent feature is due to the dipolar relaxation processes during the curing. The time of cure a t which the relaxation rate reaches a fixed value follows an exponential relation with the reciprocal curing temperature with parameters which are characteristic of a thermoset. The decrease in dc conductivity during sol + gel conversion follows the scaling law used in the description of a critical phenomenon.

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