Vitrification, devitrification, and dielectric relaxations during the non‐isothermal curing of diepoxy‐cycloaliphatic diamine

Abstract

ABSTRACT:The curing of an epoxy resin based on diglycidyl ether of bisphenol A (DGEBA) with a diamine based on 4,4Ј-diamino-3,3Ј-dimethyldicyclohexylmethane (3DCM) was analyzed by dielectric relaxation spectroscopy (DRS) between Ϫ100 and 220°C, at heating rates ranging from 0.1 to 2 K min Ϫ1. The permittivity, Ј, and the loss factor, Љ, were measured by DRS in the frequency range between 1 and 100 kHz. The dielectric relaxations were correlated with the relaxations observed previously by temperature modulated differential scanning calorimetry (TMDSC) at the same heating rates and in modulation conditions of amplitude 0.2 K and a period of 60 s, which is equivalent to a measuring frequency of 16.7 mHz. The dielectric measurements showed three frequency-dependent dipolar relaxations and one ionic relaxation, which was independent of the frequency. The dipolar relaxations were associated with the glass transition of the unreacted system and the vitrification and the devitrification processes of the system during the crosslinking reaction, and the ionic relaxation was associated with the beginning of the crosslinking reaction.

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“…Taking into account that the characteristic relaxation time 0 of this dipolar relaxation decreases with increasing frequency when the temperature increases, this shoulder can be associated with the devitrification of the system. This observation is in agreement with the non-isothermal curing of a diepoxy-cycloaliphatic diamine system (3,3 -dimethyl-4,4 -diaminodicyclohexyl-methane, 3DCM) [33] and accords also with the earlier interpretation of the DSC results (Fig. 3).…”

“…The shift of the peak temperature of the ˛1-relaxation to higher temperatures as the frequency decreases is a consequence of an increase of the relaxation time 0 resulting from a higher degree of conversion. At the same time, as shown earlier [33], ε decreases with increasing temperature and with increasing degree of conversion, indicating that this relaxation is associated with the vitrification phenomenon.…”

“…11 for the frequency of 100 Hz, while only the high temperature flank of such a peak is observed for the lower frequencies of 1 and 10 Hz. This peak, whose nature is dipolar, shifts to higher temperature with increasing frequency as a consequence of the decrease of the characteristic relaxation time for molecular segments 0 ( 0 = (2 f) −1 ) [33][34][35], and corresponds to the ˛-relaxation, associated with the glass transition of the uncured epoxy. It finishes when the cross-linking reaction begins at about 50 • C, as shown in the relaxation map in Fig.…”

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“…The dependence of the devitrification time on frequency is also less pronounced than in the case of vitrification, while, as can be seen in Figure 2, the devitrification temperature is almost independent of the underlying heating rate. to the TTT diagram in the isothermal case, can be constructed, [5][6][7][8] and is shown in MATLAB version 7.0 was used to simulate the non-isothermal cure reaction, taking into account similar considerations as for the isothermal case. [14] In the absence of vitrification, the reaction is chemically controlled and the time (t) dependence of the degree of cure (α, 0 ≤ α ≤ 1) during the reaction can be described by the equation of Kamal: [15] …”

“…[1,2] The subsequent process of devitrification occurs when the continually increasing cure temperature, T c , again exceeds the T g of the vitrified system. The vitrification and devitrification processes which occur during non-isothermal cure have been studied by temperature modulated differential scanning calorimetry (TMDSC) [3][4][5][6] and other dynamic techniques such as dielectric relaxation [7] and torsional braid analysis. [8] The advantage of these dynamic techniques is that they permit the observation of vitrification and devitrification in real time during the experiment, for example through the complex heat capacity in TMDSC.…”