The degradation process observed during step annealing of 73/27 HBA/HNA copolyester

Stepwise annealing near the crystal-liquid crystal transition temperature w as applied to a 73/27 4-hydroxybenzoic acid (HBA)/2-hydroxy-6-naphthoic aci d (HNA) copolyester to increase the melting transition from 281 to 316 degr ees C. An unexpected degradation process was observed when further annealin g was applied at 305 degrees C. An irreversible loss of crystallinity was c onfirmed by differential scanning calorimetry (DSC) and wide-angle X-ray di ffraction (WAXD). The melt rheology of the step ordered 73/27 HBA/HNA copol yester was investigated using a dynamic rheometer. In contrast to the untre ated and ordered sample, the degraded material exhibited an increase in mel t viscosity (0.3 orders) after heating at 370 degrees C for 1 min,indicatin g some form of irreversible reaction. Furthermore, a plot of storage modulu s versus loss modulus showed a completely different pattern for the degrade d material. These observations suggest that a small amount of branching or cross-linking has occurred in the 73/27 HBA/HNA copolyester during step ann ealing at 305 degrees C. An attempt was made to simulate this process by ad ding 1 wt % of a cross-linkable oligomer to the 73/27 HBA/HNA copolyester. From the melt viscosity, it could be concluded that a small. amount of bran ching (below 0.5 wt %) has occurred most likely in the crystalline regions of the ordered copolyester.