Heat Capacity and T_g of Polycyanurates as a Function of Crosslink Density and Implications for the Kauzmann Paradox

The absolute heat capacity and glass transition temperature (T_g) of a cyanurate trimer, cyanurate oligomers with different crosslink densities, and a fully crosslinked polycyanurate are measured by the step-scan method using conventional differential scanning calorimetry (DSC). The crosslink density (X_D) is varied by changing the monofunctional to difunctional cyanate ester ratio from X_D = 0 to 1. Preliminary results show that as the crosslink density increases, the step change in heat capacity at T_g (ΔC_p) decreases and T_g increases, as expected. Interestingly, the glass and liquid heat capacities of all cyanurates are found to be on the same lines within experimental error, indicating that ΔC_p depends only on T_g. Equations in the literature for the influence of molecular weight and crosslink density on T_g are tested for these series of materials. Furthermore, an extrapolation of the liquid heat capacity of the fully crosslinked polymer to lower temperatures can be performed and a test of the Kauzmann paradox can be made. The results indicate that, similar to our prior results of poly(α-methyl styrene), entropy decreases smoothly and slowly as temperature changes with no discernable transition in the temperature range where the Kauzmann temperature is expected to lie.