Modulated DSC experiments for chemorheology

The above example of the determination of the change in Cp as the epoxy resin underwent crosslinking is an example of a reversible thermal event Cp decreases on vitrification) convoluted by a non-reversible chemical change due to the formation of the network by the ring-opening reaction of the epoxy resin (which produced a large exotherm). Experimentally this separation of the events is achieved by applying a small temperature oscillation on top of the isothermal temperature. The conditions are thus quasi-isothermal. The condition may be described (Jones et al, 1997) by 

The frequency of modulation is adjusted so that there are many cycles over the duration of the transition, which in this case is the glass transition and is therefore broad. A typical modulation condition is 0.5 C in a 60-s period for studying cure of an epoxy resin (Van Assche et al, 1997). The MDSC instrument provides a modulated heating programme of 

The combination of Equations (3.5) and (3.6) enables the DSC signal to be separated into an underlying component (the first two terms) and a cyclic component (the second two terms)  

The first two terms are the equivalent of what is measured in a conventional DSC during the cure of an epoxy resin. The software of the instrument enables the non-reversing 

The three curves correspond to cure below, near and above Tg o, and it may be seen that only in the first case is there a clear step-change in heat capacity. The processing of the resin above Tgco (150°C) results in complete chemical reaction since there is no vitrification during cure. 

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