Molecular weight–viscosity relationships for poly(1,4‐butylene terephthalate)

The correlation between number‐average molecular weight and intrinsic viscosity in 60:40 phenol‐ sym ‐tetrachloroethane at 30°C for poly(1,4‐butylene terephthalate) was established from endgroup determinations as well as by gel permeation chromatography, eqs. (1) and (10a). The GPC data also yielded relationships between weight‐ and z ‐average molecular weight and intrinsic viscosity, eqs. (10b) and (10c). Melt viscosities, corrected for the thermal history of the melt, were measured at shear stresses in the range of 0.02–0.55 MPa. Linear PBT melts were found to become non‐Newtonian at a shear stress of approximately 0.11 MPa, independent of molecular weight within the range studied. Correlations between melt viscosity at low shear stress versus intrinsic viscosity are presented, as well as the dependence of melt viscosity in the non‐Newtonian region on shear stress and low‐stress (Newtonian) melt viscosity.