Molecular weight distribution characterization

Bueche (57) has estimated the appropriate moment of a particular molecular weight distribution, characterized by a long tail at high molecular weight, that should apply in this theory. For this distribution. 

For large molecules observed at high values of q, the average S(q) must be calculated. A synthetic polymer sample usually has a molecular weight distribution characterized by a continuous function. Let f(M)dM be the weight fraction of molecules with molecular weights between M and M + dM. Then the summation in Eq. (8.5.1) may be replaced by an integration over M... 

The phenomena we discuss, phase separation and osmotic pressure, are developed with particular attention to their applications in polymer characterization. Phase separation can be used to fractionate poly disperse polymer specimens into samples in which the molecular weight distribution is more narrow. Osmostic pressure experiments can be used to provide absolute values for the number average molecular weight of a polymer. Alternative methods for both fractionation and molecular weight determination exist, but the methods discussed in this chapter occupy a place of prominence among the alternatives, both historically and in contemporary practice. 

The molecular weight distribution of LLDPE resins is usually characterized in industry by the ratios of melt indexes measured in the same apparatus using different loads (2.16, 10.16, and 21.6 kg). The commonly used ratios are melt flow ratio, MFR) and I q/I2. Both of these ratios... 

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. 

In addition, there is the difficulty of controlling the grafted side chains in molecular weight distribution. In the context of this chapter, the authors hope to give a systematic, comprehensive, up-to-date review on the various methods of grafting, the methods used for characterization of grafted substrates, and their possible end use applications. 

If Mark-Houwink coefficients were supplied at setup time, the chromatogram may be converted into the differential molecular weight distribution of the specimen. Various averages characterizing this molecular weight distribution are then calculated. The molecular weight distribution may be written to a file. 

Then, the macromolecular characterization is necessary to obtain the molecular weight distribution of the polymeric material and the average molecular weights. For this purpose, the first important condition is to get a perfectly molecular soluble material which means to avoid aggregation and/or take off insoluble material. This point was previously discussed [12]. The polysaccharide must be isolated preferentially as a sodium salt form to be fully soluble in water or in presence of some NaCI used to screen electrostatic interactions. 

Although poly(organo)phosphazenes have been characterized using dilute solution techniques (4-10), attempts to characterize polydichlorophosphazene directly have been limited (11,12,13). The presence of gel and the fact that polydichlorophosphazene is moisture sensitive generally have precluded an accurate analysis of its molecular weight and molecular weight distribution... 

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