Intrinsic viscosity of polymer solutions at high shear rates

Abstract

AbstractAlthough low‐shear intrinsic viscosity is a well‐accepted tool for polymer characterization, it often happens (particularly with increasing molecular weights) that it is easier to detect the high‐shear (second) Newtonian viscosity η2 rather than its low shear counterpart. It has also been predicted that because of a higher degree of order, due to disentanglement and orientation, high‐shear viscosity data should simplify the prevailing correlations. The possibility of using high‐shear viscometric data for polymer characterization was examined by determining intrinsic viscosities for several polyisobutylene samples through extrapolation of the high‐shear ultimate viscosity numbers, UVN, to zero concentration: [η]2 = lim UVNC → 0 = limC → 0 (η2–ηs)/ηsC where ηs is the viscosity of the pure solvent. Five samples of unfractionated polyisobutylene (molecular weights of 1.1 × 106–6.6 × 106) in toluene, kerosene, decalin, and gas oil at concentrations of 0.05–2.4 g./dl. were studied. Higher dilution was avoided because of the problem of onset of turbulence. The absence of shear degradation was ascertained by measuring low‐shear intrinsic viscosity data before and after the polymer was exposed to high‐shear conditions. The data show two types of behavior: for the lower molecular weight samples in the low‐viscosity solvents the UVN decreases linearly with dilution, and for the higher molecular weights and higher solvent viscosities the UVN increases with high dilution, i.e., shows an upturn effect. The first type of data can be successfully correlated with appropriate molecular weights by using a typical Mark‐Houwink equation. The exponents in these relationships are in the range of 0.28–0.64, increasing systematically with decrease of solvent viscosity and independent of the “goodness” of the latter. The data that show an upturn effect are not currently amenable to reliable extrapolation techniques. The upturn, however, predicts the conformation of very flexible, isolated polymer chains in viscous solvents under conditions of high shear.