Isothermal DSC experiments for polymer chemorheology

In reactive processing, such as reactive extrusion of thermoplastics, the use of DSC may be seen in determining the stability of the polymer formulation by measuring the oxidation induction time (OIT) (Bair, 1997). This is an isothermal experiment in the presence of air 

A broad exotherm is obtained, corresponding to a typical heat of reaction (shown here as AHq) of an epoxy resin of 107 4 kJ/mol epoxide (Prime, 1997b). Also shown is the curve obtained by heating the system as mixed in an oven at 60 C for increasing times and then running the DSC from room temperature to 150 C. Several features may be noted. 

These DSC results may be further analysed to give the extent of reaction as a function of cure time by using the assumption that the total area under the exotherm represents the integrated heat of reaction, AH,., for the reaction of epoxide with amine (typically 107 4 kJ/mol epoxide as indicated above). Thus the reduction in this area on reacting the system for a 

As noted above, several significant features of the network-forming system, which are important for reactive processing, may be learned from this simple analysis. Firstly it may 

The isothermal DSC curve is a plot of AH At against time so that, from Equation (3.4), it may be integrated to determine the extent of reaction and so analyse the kinetics of the system. There is still the necessity to perform a scanning DSC experiment in order to determine the residual exotherm from the sample due to the cessation of reaction at the cure temperature as the Tg of the resin reaches the isothermal cure temperature. This just becomes a correction in the form of a scaling factor for the entire curve. A further requirement in this analysis of the isothermal cure exotherm is the determination of the form of the baseline for the integration. In many cases the simple assumption is to choose a flat baseline. However, this ignores the fact that the heat capacity, Cp, of the system will 

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