Viscosities of dilute aqueous solutions of a partially quaternized poly‐4‐vinylpyridine at low gradients of flow

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Abstract
The viscosity of a partially (60.8%) quaternized polyvinylpyridine with n‐butyl bromide has been investigated in dilute‐water and sodium chloride solutions, in a rotation (Couette) viscometer, at low gradients of flow. It was possible to extrapolate the ηsp/c against concentration plot in water by working at very low concentrations of the polymer. At about 5 × 10−5 g./ml. the curve reaches a maximum and decreases sharply upon dilution to about one‐sixth the value at the maximum. The shape and size of the equivalent ellipsoid corresponding to the 60.8% polyvinylpyridinium bromide (DP = 7660) ions was estimated from the limiting viscosity number ([η] = 9500 ml./g.) and birefringence (extinction angle) data. The length of the stretched molecule was estimated to be 7420 A., corresponding to < half the hydrodynamic length of the extended macromolecule. Extrapolations in sodium chloride solutions have been obtained at constant concentrations (up to 10−3 M) of the simple electrolyte. Small amounts of added simple electrolyte decrease the viscosity numbers in solutions at finite concentrations of the polymer to a large extent, whereas the limiting viscosity numbers are affected to a far smaller degree. This phenomenon has qualitatively been attributed to the pronounced decrease in the intermacromolecular electrostatic interactions at finite concentration of the polymer upon the addition of a relatively small amount of mobile electrolyte; at the same time the intramacromolecular interactions are affected to a much smaller extent.