Vitrification and the Glass Transition

Techniques measuring the (thermo)mechanical properties as cure proceeds are very appropriate for the assessment of vitrification. One of the most important changes upon vitrification is the increase in modulus by two or three orders of magnitude (from 10 Pa in the rubbery state to 10 Pa in the glassy state), together with a change in cure shrinkage. 

With thermomechanical analysis (TMA), the expansion or shrinkage of the sample under constant stress is monitored against time or temperature. It is also possible to measure dilatometric changes [53]. Near vitrification, the change in volume contraction versus isothermal reaction time will be reduced due to diffusion control. 

DETA is a complementary dynamic method for the evaluation of vitrification. The advantage of DETA is that the dynamic range is large, e.g. from about 0.1 Hz to 100 kHz. Vitrification can be assessed either as the midpoint or as the inflection point in the stepwise decrease of the permittivity, s or as the peak in the loss factor, s [56,57], 

For the calculation of Tg, measured in cooling or heating experiments by DMTA, TMA or DETA, similar calculation procedures as for the vitrification point are valid [58]. 

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