Shrinkage heating rate dependence

Gas production resulting from the oxidation reaction is not the only reason for the observed phenomenon. Often a small amount of liquid remains condensed inside the smallest pores as a result of capillary condensation. Above a temperature of about 120-150°C, the liquid, for instance water, evaporates and creates a similar effect. Previously oxidized aerogel samples expand when they are heat treated as a consequence of adsorbed water evaporation. They shrink again in a small temperature range because of the adsorption of air moisture when cooled in air. No shrinkage is recorded if cooling is performed under vacuum. Obviously, the expansion is heating rate dependent [11]. 

Region II shrinkage is kinetically limited, and the rate increases with increased excess free volume and decreases with increased condensation. These effects closely compensate for each other, causing no heating-rate-dependence. 

Rate vs. time curves are are presented as per cent C=C conversion per second by using the experimentally determined heat of polymerization of acrylate groups of 78 kJ.Mol (11). Extents of reaction were reproducible to within 0.7%, the accuracy depends on the accuracy of the heat of reaction. The distortion of DSC curves of fast reactions is discussed below in the section on shrinkage and conversion. 

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