LOW-SHEAR-RATE RHEOMETRY AND POLYMER QUALITY CONTROL

Abstract

Possibilities for improved quality control of polymerization reactions arise from the measurement of N1 (the first normal stress difference) as well as the viscosity η. At low shear rates in particular, N₁ and η are associated with different regions of the relaxation spectrum H(τ); and cannot, in general, be calculated from one another. Above a certain polymer concentration, N₁ and η give information about two very different averages of the molecular weight distribution, with N₁ being remarkably sensitive to small changes in the high molecular weight “tail” of the distribution. By considering usage of these and other rheological measurements, together with ranges of shear rate or frequency, a tentative qualitative classification of different levels of rheological discrimination is suggested. Choice of on-line rheological sensor could be influenced in the first instance by choice of level One on-line sensor — a “Stressmeter” — is discussed in detail. Evidence is given to show that this discriminates well between viscous and normal stress properties, and that, for a number of liquids tested, actual values of normal stress differences can be obtained to a useful degree of approximation. Such data for a wide range of liquids are given, including what are thought to be the first published normal stress data on polyethylene solutions and gelatin solutions.