ISSN:
0022-3832
Keywords:
Chemistry
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
For a soft free-draining coil that changes its shape quite easily in flow, intrinsic viscosity does not depend on velocity gradient. The deformation and extension of the coil in flow, which increases the contribution of the single molecule, is just compensated for by the decrease in number of molecules crossing the flow plane and thus actually contributing to the additional shear stress. The finite valence angle and the restricted rotation around the valence bond, however, not only extend but also stiffen the molecule. The completely stiff coils - an idealization that never occurs but is very suitable for computing purposes - are oriented in the flow without any alteration of their shapes and shape distribution. The intrinsic viscosity thereby decreases with increasing orientation. The effect is the more accessible the longer the molecule, the deviations from the initial viscosity at rest being proportional to the square of the gradient and the fourth power of the degree of polymerization. The real molecules are neither quite stiff nor completely soft. Thus their intrinsic viscosity ought to lie between the two limiting cases, the decrease with the gradient being smaller than for the stiff coils. Taking into account the hydrodynamic interaction of the chain elements, the gradient dependence cannot be computed precisely. Qualitative considerations yield an increased viscosity contribution with the stretching of the coils, since they become more permeable the more they are extended. Thus the intrinsic viscosity of quite soft molecules should increase with the gradient. In completely stiff coils, however, the partial solvent immobilization does not change the gradient dependence, on the whole, since the shapes and the shape distribution are unaltered by the flow, although the appearing of the effect is displaced to higher gradients, and the absolute value of the initial intrinsic viscosity is very markedly reduced. Experiments are too rare and not sufficiently precise to allow any reliable test of the theory. In most cases, the concentrations and the gradients are too high for an unambiguous extrapolation to infinite dilution and zero gradient viscosity. But they nevertheless seem to exclude the case of quite soft molecules and are not in contradiction with the general features of the theory.
Additional Material:
5 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/pol.1952.120080607
