Wesslau molecular weight distribution

Crosslinking and main-chain scission in Marlex-50 has been reinvestigated recently by Dole and co-workers [272] using the Charlesby— Pinner function modified for a Wesslau molecular weight distribution (Table 18). Evidence of increase of Gc L with dose has been obtained and related to vinylene decay. GCL and Gcs at zero dose are almost independent of temperature but at 27 Mrad, GCL increases with temperature. 

A computer simulation of size-exclusion chromatography-viscom-etry-light scattering is described. Data for polymers with a Flory-Schulz molecular weight distribution (MWD) are simulated, and the features of the different detector signals are related to the molecular weight and polydispersity of the distribution. The results are compared with previously reported simulated results using a Wesslau MWD. 

Good description of the distribution was obtained with 2-5 components. The Wesslau model requires the smallest number of components to define a molecular weight distribution for HDPE. 

In order to model polymer transport phenomena of this type, where polydispersity effects are important, it is not adequate to consider the polymer as a single component of concentration, c, as has been done so far in this chapter. The polymer itself is made up of many components which are different only in their size (although the Mark-Houwink parameters that apply for the polymer will be esentially the same for each of the polymer subcomponents). Thus it is necessary to use a multicomponent representation of the polymer molecular weight distribution in order to model the polymer behaviour adequately in such experiments. Brown and Sorbie (1989) have adopted this approach in order to model the Chauveteau-Lecourtier results quantitatively. They used a multicomponent representation of the MWD based on a Wesslau distribution function (Rodriguez, 1983, p. 134) with 26 discrete fractions being used to represent the xanthan. For this case, a set of convection-dispersion equations including dispersion and surface exclusion... 

Figure 1. Wesslau and Flory-Schulz differential weight-fraction MWDs on a logarithmic scale, where W is the weight fraction and M is the molecular weight. Both distributions are for M = 10,000 gjmol and M /M = 2.0.

Figure 8-5. Integral mass distribution for a Wesslau distribution. The viscosity-average degree of polymerization (X,), and the mass fraction were measured. The viscosity-average degree of polymerization of the original material with the already known exponent the median value Xjvf, and equation (8-23) and (8-24) were used to calculate the number- and weight-average molecular weights.

Compare the cumulative distribution of Problem 6.14 with the Wesslau model on logarithmic probability paper using the product ax as the measure of molecular weight. 

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