Random‐coil configurations of aromatic polyesters: Birefringence of poly(triethylene glycol terephthalate) networks in elongation

Hydroxyl‐terminatd poly(triethylene glycol terephthalate) was crosslinked with an aromatic triisocyanate. Birefringence–stress–strain experiments, performed on the networks at 70°C, showed an anomalous increase in the modulus and a downturn in the birefringence–strain isotherms at high elongations. These results suggest that crystallinity is not responsible for the non‐Gaussian behavior of the chains at high extension. The same kind of experiments were performed over the range 20–70°C. Values of the optical configuration parameter Δ a of the order of 13.3 × 10 −24 cm 3 with negligible temperature coefficient were found for these networks. The quantities Δ a and d In Δ a/dT were calculated by means of the rotational isomeric state model. Better agreement between the theoretical and experimental values of these parameters was found for poly(triethylene glycol terephthalate) than for poly(diethylene glycol terephthalate). Since the polarities of the two chains are similar, intermolecular interactions involving terephthaloyl residues may be responsible for the discrepancies observed between theory and experiment for Δ a in aromatic polyesters.