The effect of phase heterogeneity on the rheological properties of melted copolyesters and structural-mechanical parameters of related extruded materials

The flowability diagrams of homogeneous and heterogeneous melts of the copolyesters between poly(ethylene terephthalate) and acetoxybenzoic acid were studied. The results confirmed that there are two temperature regions on the diagrams, corresponding to different structures of the copolyester melts: a low-temperature region, with high-melting crystallites occurring in a nematic liquid-crystal (LC) phase, and a high-temperature region, corresponding to the formation of a homogeneous nematic LC melt, It is shown that an anomaly in the log(gamma) over dot-log tau plot and a temporal instability of the viscosity of the heterogeneous melt are caused by the development of processes involving crystallization and misorientation of the structural elements, The flow curves of the LC melt are typical of the viscoplastic systems and show a tendency to limited flowability with decreasing temperature. Materials extruded from a homogeneous melt exhibit a much higher level of molecular orientation and strength as compared to those of the extrudates obtained from a heterophase melt. The effect of high-melting crystallites, leading to the structural disordering and decreasing strength of the extruded material, increases with the proportion of mesogenic fragments in the chain.