Vitrification and Devitrification during the Non‐Isothermal Cure of a Thermoset. Theoretical Model and Comparison with Calorimetric Experiments

Abstract

The processes of vitrification and devitrification that occur during the non‐isothermal cure of a cross‐linking system are studied in terms of their dependence on frequency. Experimental results are obtained by TOPEM, a new multi‐frequency temperature modulated differential scanning calorimetry (TMDSC) technique, and a simulation with MATLAB was used to model the vitrification and devitrification times as a function of the frequency and heating rate. Simulations were made for five different velocities of scan. Other parameters, such as the exponents in the Kamal equation, λ in the DiBenedetto equation, and the activation energy for the frequency dependence of Tg, were also considered. The experimental and simulated results were found to be in good agreement and some advantages of the TOPEM technique are discussed.magnified image

Identification of nanostructural development in epoxy polymer layered silicate nanocomposites from the interpretation of differential scanning calorimetry and dielectric spectroscopy

“…6): the first one, more well-defined and larger, is due to the homopolymerisation of the epoxy, initiated by the DMAP; the second peak could result from a devitrification process or it might be associated with a thermally induced homopolymerisation, catalysed by the ammonium ion of the organically modified clay. In the system ED (without clay), the second peak, which occurs for heating rates below 15 K min −1 , can be ascribed to a phenomenon of vitrification-devitrification of the system [28,29]. In order to explain the origin of the second peak in the exotherms for the nanocomposite samples in which the homopolymerisation was initiated by DMAP, a dielectric analysis has been carried out on the samples ED and EDM2 with the purpose of comparing their responses.…”

“…6(a) for a heating rate of 2 K min −1 . The second peak can be attributed to a devitrification effect of the system [28,29]. Vitrification occurs when T g increases up to the cure temperature.…”

The Use of Matlab in the Study of the Glass Transition and Vitrification in Polymers

“…In Figure 10 this is seen as the crossing point of the cure curve and the heating rate curve, which is located in the region of the cure curve which is almost horizontal. The temperature at which devitrification occurs is therefore a good approximation to that of the fully cured system, and is almost independent of frequency (Fraga et al, 2010a;Fraga et al, 2010b). This is an important observation, as can be illustrated by a good example.…”

“…By conventional DSC these subtle changes in the rate of cure cannot be identified. On the other hand, by TMDSC the changes in the complex heat capacity allow both vitrification and devitrification to be identified, as a sigmoidal decrease and increase, respectively, in C p *, with their separation being greater the slower is the heating rate (Fraga et al, 2010a(Fraga et al, , 2010b. Generally, the heating rates need to be very slow in order to observe these phenomena: with an epoxy-amine system, for example, these rates are from 0.05 to 0.015 K/min.…”

Cure state detection for pre-cured carbon-fibre epoxy prepreg (CFC) using Temperature-Modulated Differential Scanning Calorimetry (TMDSC)

“…Besides sinusoidal temperature modulation, in the method called TOPEM Ò the temperature is varied in the form of stochastic temperature pulses [14,18,19]. The advantage here is that various modulation frequencies in the mHz range can be evaluated and gives the influence of frequency on the glass transition [18,19].…”