Stress–optical behaviour of polyester networks

Thermoelastic networks were prepared by end-linking hydroxyl terminated chains of poly(neopentylglycol hexafluoroglutarate) (PNGHFG) and poly(diethylene glycol terephthalate) (PDET) using, respectively, tri(p-isocyanate-phenyl)-thiophosphate and 2,4-bis(p-isocyanate benzyl)-p-phenylisocyanate as crosslinking agents. The plots of birefringence versus stress for PNGHFG and PDET networks exhibit negative deviations from linearity at elongation ratios greater than 1.9 and 5, respectively. The values of the optical configuration parameter Δa for the former and latter networks are 2.98 A3 at 5 °C and 20.80 A3 at 30 °C, respectively. Theoretical calculations carried out using the conformational energies obtained from the critical analysis of the mean-square dipole moments of diethyl hexafluoroglutarate and poly(neopentylglycol hexafluoroglutarate) give the value of 2.24 A3 at 5 °C for this parameter, in fair agreement with the experimental result. Similar calculations carried out on PDET networks give Δa  = 3.74 A3 at 30 °C, a value significantly lower than the experimental result. The cause of the strong discrepancy between the theoretical and experimental results observed for the optical configuration parameter of PDET is discussed. © 2002 Society of Chemical Industry