Complex nature of the β relaxation and fragility in aromatic polyesters

The following experimental study shows the dielectric relaxation behavior of random copolyesters formed by butylene terephthalate and butylene isophthalate comonomeric units. The experimental data were analyzed using two relaxation modes describing the sub-glass relaxation, characterized by different activation energies. We propose that the high frequency secondary relaxation β1 is associated to the most flexible part of the repeat unit, whereas the low frequency mode β2 has been assigned to atoms of the ester groups linked to the aromatic rings. A strong correlation between the dynamic fragility of these polymers and the monomer architecture was also demonstrated.