Fragility of unsaturated polyester resins cured with styrene: influence ofthe 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 DeltaC(p) = [C-p,(l) - C-p,(g)](T )=(Tg), the values of apparent activation energy Deltah('). and the values of the fragility index rn for each sample. It is found a constant value of DeltaC(p) = 0.29 Jg(-1) K-1 for all the samples while Deltah(*) and m incre ase with the styrene content. These results discussed in regard with the st rong fragile gluss-formers concept indicate a character becoming more fragi le when the amount of styrene increases. Introducing the "bead concept" pro posed by Wunderlich, we show that the average elementary energy barrier tha t the molecular species (or relaxing units) must overstep during their rela xation process is unchanged whatever the content of styrene. The origin of the variations with the styrene content of the fragility behavior is unders tood by the increase of the bead number. (C) 2001 Elsevier Science Ltd. All rights reserved.