Viscosity in dilute solutions

Chain-growth polymerizations are diffusion controlled in bulk polymerizations. This is expected to occur rapidly, even prior to network development in step-growth mechanisms. Traditionally, rate constants are expressed in terms of viscosity. In dilute solutions, viscosity is proportional to molecular weight to a power that lies between 0.6 and 0.8 (22). Melt viscosity is more complex (23) Below a critical value for the number of atoms per chain, viscosity correlates to the 1.75 power. Above this critical value, the power is nearly 3 4 for a number of thermoplastics at low shear rates. In thermosets, as the extent of conversion reaches gellation, the viscosity asymptotically increases. However, if network formation is restricted to tightly crosslinked, localized regions, viscosity may not be appreciably affected. In the current study, an exponential function of degree of polymerization was selected as a first estimate of the rate dependency on viscosity. 

Evans and Spurlin have studied the effect of carboxyl groups present in 0-ethylcellulose on its viscosity in dilute solution. Although free carboxyl groups do not affect viscosity, neutralization with metal ions increases the viscosity, especially in non-polar solvents. According to these workers, the effect of bound, metal ions is to produce large electrostatic forces of attraction between the chains to which they are attached. 

Let s write also the temperature dependencies of the coefficients of a frictional component of the viscosity in diluted solution... 

Simha, R The concentration dependence of viscosities in dilute solutions. Journal of Colloid Science 5(4), pp. 386-392 (1950). 

Their viscosities in dilute solution appear to be essentially low compared with those caused by rigid rods and the concentration interaction is reduced. This behavior may be attributed to the curling, the deforma-bility and to the changed orientation of such particles in the flow gradient. ... 

We can relate the coil volume Rp to the viscosity in dilute solutions the viscosity increase 5i/ due to a small concentration c in polymers has the form... 

An unfortunate usage of the term bulk viscosity is common among polymer chemists, in the sense of the ordinary shear viscosity of a polymer in bulk, as contrasted with its viscosity in dilute solution. In acoustics, bulk viscosity means the viscosity associated with a change in volume, and this definition fits in best with the nomenclature of viscoelasticity. In this book, the complex dynamic bulk viscosity refers to 77 = K /iu). 

Inherent viscosity In dilute solution viscosity measurements, inherent viscosity is the ratio of the natural logarithm of the relative viscosity to the concentration of the polymer in grams per 100 mL of solvent. 

Intrinsic viscosity In dilute solution viscosity measurements, intrinsic viscosity is the limit of the reduced and inherent viscosities as the concentration of the polymeric solute approaches zero and represents the capacity of the polymer to increase viscosity. 

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