Viscosity coefficient polydispersity

The extremely simple expressions for intrinsic viscosity at zero gradient and sedimentation coefficient containing in the denominator the square root of molecular weight agree very well with experimental data of theta solutions in the whole range from fully drained to impermeable coil. They turned out to be very good approximations of the more complicated expressions derived by Kirkwood-Riseman and Debye-Bueche ° and hence were used for the calculation of intrinsic viscosity of polydisperse samples. 

Another uncertainty arises from the influence of polydispersity. Intrinsic viscosity data were mostly obtained from fractions but the second virial coefficient data were chosen from unfractionated samples. The resulting error is probably not large since A2 depends only slightly on the width of the distribution [183, 184]. 

Third, a serious need exists for a data base containing transport properties of complex fluids, analogous to thermodynamic data for nonideal molecular systems. Most measurements of viscosities, pressure drops, etc. have little value beyond the specific conditions of the experiment because of inadequate characterization at the microscopic level. In fact, for many polydisperse or multicomponent systems sufficient characterization is not presently possible. Hence, the effort probably should begin with model materials, akin to the measurement of viscometric functions [27] and diffusion coefficients [28] for polymers of precisely tailored molecular structure. Then correlations between the transport and thermodynamic properties and key microstructural parameters, e.g., size, shape, concentration, and characteristics of interactions, could be developed through enlightened dimensional analysis or asymptotic solutions. These data would facilitate systematic... 

Experimentally, melts of low polydispersity that do not overtly spurt, slip, or succumb to other material instabilities will typically show steeply decreasing values of the viscosity and first normal stress coefficient in the shear-thinning region. Menezes and Graessley (1980) reported that t] oc y ° y i- at large y. These dependencies... 

This has an important implication. Measuring an exponent in a scattering experiment does not necessarily imply that one gets the fractal dimension directly. First one has to check the polydispersity by independent measurements, either with viscosity, or with second virial coefficient experiments. The latter may be calculated following the same steps as above and taking into account the interactions between the centers of masses of different polymers. 

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