Fragility of unsaturated polyester resins cured with styrene: influence of the styrene concentration

Unsaturated polyester resins cured with varying styrene content (20–50% (w/w)) has been studied by calorimetry (DSC). We have evaluated the values of the heat capacity at the glass transition ΔCp=[Cp,l−Cp,g]T=Tg, the values of apparent activation energy Δh*, and the values of the fragility index m for each sample. It is found a constant value of ΔCp=0.29 J g−1 K−1 for all the samples while Δh* and m increase with the styrene content. These results discussed in regard with the strong fragile glass-formers concept indicate a character becoming more fragile when the amount of styrene increases. Introducing the “bead concept” proposed by Wunderlich, we show that the average elementary energy barrier that the molecular species (or relaxing units) must overstep during their relaxation process is unchanged whatever the content of styrene. The origin of the variations with the styrene content of the fragility behavior is understood by the increase of the bead number.