Continuous fractionation and solution properties of PVC, 6. Pressure dependence of the viscosity — influence of molecular weight and composition

Viscosities were measured as a function of pressure and temperature with solutions of PVC 75 000 in cyclohexanone (CHO) and polymer contents ranging from 0,6 to 12 wt.-%, by means of a Searle-type (≥3 wt.-%) and a rolling-ball viscometer (<3 wt.-%). Furthermore, the influence of molecular weight was determined with solutions of 8 wt.-% of PVC 20 000, PVC 37 000 and PVC 100 000. (The numbers in the codes of the PVC specimens are their approximate molecular weights.) For all concentrations and molecular weights, the viscosity increases in a more or less exponential manner with increasing pressure. The ratio f₁₀₀₀ of the viscosity of the solution at 1 000 and 1 bar can be varied by the change of the polymer content from 2,5 (the value of the pure solvent, index s) to 3,5 (12 wt.-% PVC 75 000) at t = 25°C and from 2,23 to 2,94 at t = 80°C. An increase of the molecular weight of the polymer raises f₁₀₀₀ in a similar manner as the polymer concentration. Using the reduced variables V^≠/V^≠ (ratio of the volumes of activation of the solution and the pure solvent) and c? (product of the polymer concentration and the intrinsic viscosity), all results obtained by variation of T, c and M_w can be represented by a master curve. This means that it is possible to calculate the pressure dependence of a given polymer solution of arbitrary polymer concentration from a mere measurement of the intrinsic viscosity at normal pressure. Criteria are presented which allow a forecast concerning the occurrence of minima in the concentration dependence of the energy of activation of the viscous flow E^≠ and V^≠.