Normal molecular weight distribution

Figure 8-6. Calculated W/A , ratios as a function of the exponents t, for a Schuiz-Flory (—) or a generalized logarithmic normal (--) molecular-weight distribution for. in cadi case. w/M, 2. SI may be A , or (After H.-G. Elias, R. Bareiss and J. G.

If the average molecular weight of the total oligomer is experimentally available, its monomer dependence allows one to decide which of the two reaction schema (set a or set b) is applicable. In either case, a normal molecular weight distribution (Schulz-Flory distribution) results. This means that the weight fraction nip of oligomer of degree of... 

These normal stresses are more pronounced for polymers with a very broad molecular weight distribution. Viscosities and viscoelastic behavior decrease with increasing temperature. In some cases a marked viscosity decrease with time is observed in solutions stored at constant temperature and 2ero shear. The decrease may be due to changes in polymer conformation. The rheological behavior of pure polyacrylamides over wide concentration ranges has been reviewed (5). 

In SEC, universal calibration is often utilized to characterize a molecular weight distribution. For a universal calibration curve, one must determine the product of log(intrinsic viscosity molecular weight), or log([7j] M). The universal calibration method originally described by Benoit et al. (9) employs the hydro-dynamic radius or volume, the product of [tj] M as the separation parameter. The calibration curves for a variety of polymers will converge toward a single curve when plotted as log([7j] M) versus elution volume (VJ, rather than plotted the conventional way as log(M) versus V, (5). Universal calibration behavior is highly dependent on the absence of any secondary separation effects. Most failures of universal calibration are normally due to the absence of a pure size exclusion mechanism. 

The block copolymer produced by Bamford s metal carbonyl/halide-terminated polymers photoinitiating systems are, therefore, more versatile than those based on anionic polymerization, since a wide range of monomers may be incorporated into the block. Although the mean block length is controllable through the parameters that normally determine the mean kinetic chain length in a free radical polymerization, the molecular weight distributions are, of course, much broader than with ionic polymerization and the polymers are, therefore, less well defined,... 

An alternate approach was taken by a shaft seal manufacturer and a polymer producer working together [2]. The project involved making a series of butadiene-acrylonitrile polymers of known acrylonitrile contents with various molecular weights and molecular weight distributions then making seals under normal manufacturing conditions. There were four polymers evaluated ... 

Fig. 66a. GPC traces of a PS sample (initial M = 4.34 xlO6, Mw/M = 1.06), initial concentration = 200 ppm before and after 50 cycles of freeze-and-thaw degradation (the curves are normalized to the same height), b Molecular weight distribution of the virgin...

Our initial studies (23) were performed in toluene, and Table I shows the results from the polymerization of a number of representative monomers. The data reported in Table I are for direct addition of the monomer to the sodium dispersion. Inverse addition often leads to higher molecular weights, although the overall polymer yields are usually lower (15,23). The results in Table I show that, under these reaction conditions, a bimodal molecular molecular weight distribution is normally obtained. Furthermore, it is obvious that the crude polymer yields drop precipitously as the steric hindrance in the monomer increases. 

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