DSC investigation of chain cooperativity rearrangements and critical length scale of polybutadiene networks in glass transition range

The cooperativity at glass transition of the 1,2-polybutadiene (PB) chains in a series of polymer networks was evaluated by both the standard DSC analysis and the StepScan DSC analysis. Results confirmed by standard DSC analysis showed an increase in the glass transition value simultaneously with the heat capacity step decreasing as the crosslinking degree increased in PB networks. In order to evaluate the size, volume of cooperative rearranging region (CRR) as well as the number of monomer units in the cooperative rearranging region and mean temperature fluctuation in the glass transition range, the StepScan DSC method was used. The values found for these cooperativity parameters decreased with crosslinking degree increasing in PB networks, which claimed that the presence of crosslinks in the structure actually limited the cooperativity rearrangement motions in the glass transition range. It was estimated also the critical length scale of a PB chain into a CCR, proving to be higher than the cooperative domain size of CCR and decreased as crosslinking degree increased in these networks. More than, the increasing thermal fluctuation at the glass transition supported these results and the assumption that the configurational entropy of the smallest CCR in the PB network systems increased with crosslinking degree. The results of this study was in accordance with the conclusion that an overall increases in crosslink junctions in these networks could be correlated with a reduction in chain dimension up to a certain critical length which limits the PB chain rearrangements at glass transition cooperativity.