Observations on the Melt Rheology of Thermotropic Aromatic Polyesters

Rheological measurements were made on a variety of substantially aromatic copolyesters. All of them differ from flexible chain polymers in having unusually long relaxation times, estimated from shear rate dependence of viscosity and from melt elasticity. This behaviour is not accounted for by an unusual molecular weight distribution (MWD); limited measurements indicate that the MWD is somewhat narrower than that of conventional condensation polymers. Numerous anomalous flow phenomena have been observed, though not necessarily with all samples. These include the occurrence of a yield stress, large secondary shear stress maximum after start‐up of steady shear, transient negative normal stress, and shear‐thickening of melt viscosity. Although the melts are highly elastic, they do not exhibit swelling after extrusion from a capillary. The melt rheology is often very sensitive to temperature and dependent upon thermal history. In one material this dependence has been linked to the destruction and/or growth of crystallites formed from blocks of one comonomer.