Gelation Behaviors of the Blends

From the steady shear sweeps, the gel time of blends from epoxy crosslinldng was identified as a crossover of either G and G , or the maximum peak in tanb Jyo-tishkumar et al. [67] calculated the apparent activation energy ( ) of gelation based on Eq. (4.3). The values of neat epoxy and thermoplastic-modified systems were all close to 65 kj mol , which was quite similar to that of ordinary epoxy resin and rubber-modified systems. Clearly, in these systems the phase separation had been completed before gelation of the epoxy resin, and so had a limited effect on the chain mobility of epoxy during gelation. 

As the first gelation may relate to physical gels, the critical gel theory proposed by Winter and Chambon [75] was reintroduced to explain such double-gelation behavior. At the gel point, the stress relaxation behavior of the network follows a power law [76]  

From Eq. (4.17), the frequency dependence of the dynamic shear modulus at gel point was deduced as [77]  

the gel point can only be accurately determined under special conditions, using the crossover point of G and G . These conditions are that stress relaxation in the gelation follows a power law, and the exponent n must be exactly 0.5. For most network systems, however, although stress relaxation at gel point also 

The effect of thermoplastic molecular weight and curing rate on gelation behavior has also been studied [70]. By using modified DGEBA-anhydride systems with three different molecular weight-types of PES (20 wt%) and different amounts of 

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