Flow Behavior of Low Molecular Weight Polybutadiene, Carboxyl-Polybutadiene, and Butadiene-Acrylonitrile Copolymers

Abstract

The flow behavior of some low molecular weight polymers was studied as a function of shear rate, shear stress, and temperature. The polymers included polybutadienes, random carboxyl and carboxyl‐terminated polybutadienes, carboxyl‐terminated butadiene‐acrylonitrile, and acrylonitrile‐butadiene copolymers. Measurements were made in the temperature range 3.8–73°C. A cone and plate viscometer and pressure capillary viscometer were used to cover the shear rate range 1−10 5 sec −1 . Measurements were carried out with various capillary radius to length ratios. M̄ w and M̄ n values were calculated from molecular weight distribution data obtained by gel permeation chromatography. Limiting viscosity numbers were also determined. In the range of shear rates studied, all polymers showed a limiting viscosity at low rates of shear and a region of shear rate thinning. The log viscosity‐log shear rateflow curves for each sample at the various temperatures were superimposable by linear shifts. Energies of activation calculated according to the method of Fox and Loshaek were found to have values characteristic of the molecular structure. Normal stresses developed in capillary flow as measured by entrance effects showed some dependence on structure. Flow results were compared with the Bueche‐Harding experimental standard curve and the Bueche theoretical curve. The presence of polar groups in the polymer chain increased the energy of activation and the relative magnitude of the viscosity. The log viscosity‐log shear rateflow curves for all the samples at any constant temperature were superimposable by linear shifts in two distinct classes according to differences in molecular weight distribution.