Measurement of molecular weight distribution

Gel Permeation Chromatography (CPC) is often the source of molecular wei t averages used in polymerization kinetic modelling Q.,2). Kinetic models also r uire measurement of molecular weight distribution, conversion to polymer, composition of monomers in a copolymerization rea tion mixture, copolymer composition distribution, and sequence length distribution. The GPC chromatogram often reflects these properties (3,. ... 

In Vienna, Mark published a number of fundamental papers. Their topics include polymerization mechanism (46, 47, 48), thermal polymerization (49, 50), polymerization kinetics (51), the effect of oxygen on polymerization (52), and measurement of molecular weight distribution (53). Guth and Mark expanded their modeling of extended and balled thread molecules to include rubber. The result of their studies was a series of very important papers in which the thermal effect on expansion and relaxation of rubber is explained (54, 55, 56). 

The polydispersity of dendritic molecules, expressed in the form of their poly-dispersity index (PDI), is directly related to their structural perfection. The PDI is a measure of molecular weight distribution. 

Some information can be obtained about the rate coefficients of ion-pair dissociation from measurements of molecular weight distribution. If the lifetime in any of the ionic states is short compared with its rate of reaction with monomer, the molecular weight distribution will be the simple narrow Poisson type expected for rapid initiation, propagation via a single species and no termination. A slower ion-pair dissociation process... 

SEC-viscometry combined with universal calibration can provide measurements of molecular weight distribution even when it is not possible to use a concen-... 

If the Mark-Houwink coefficients for the polymer-solvent system are known, then the IVD measured by SEC-viscornetry can be converted into the molecular weight distribution using the Mark-Houwink relation. This should give greater precision in the measurement of molecular weight distribution than SEC-vi scorn etry with universal calibration, because the IVD measurement is much less sensitive to experimental conditions than a calibration curve. 

Table 1 Measurement of Molecular Weight Distribution by SEC-Viscometry Selected Applications...

Considerable effort is being exerted currently to determine the compositional and molecular weight distributions of copolymers. The basic idea behind these techniques is fractionation of the copolymer by composition and subsequent measurement of molecular weight distributions of chemically uniform fractions by SEC. The problems with SEC characterizations that have been mentioned are circumvented if the samples are indeed compositionally uniform. 

Since the intrinsic viscosity measurements have been the cornerstone of industrial characterization measurements, it is expected that the measurement of this parameter and the other associated measurements, such as intrinsic viscosity distribution by GPC, will become more important from a quality control standpoint than the traditional measurement of molecular weight distribution, which has not previously been related to intrinsic viscosity. Of particular importance for quality control are the claims made for the technique, namely that IVD measurement is much less sensitive to column deterioration, flow rate variations and column loading. 

Methods are also available for rapidly estimating polymer molecular weight distribution. These methods include swelling, in which the amount of polymer precipitated vs nonsolvent added gives a cumulative distribution curve (i.e., higher molecular weights first precipitate out with nonsolvent addition) and tur-bidimetric titration, which uses turbidity induced by nonsolvent addition as a measure of molecular weight distribution. These methods are described in detail in Ref. 28. 

Qian K, Edwards KE, Siskin M, Olmstead WN, Mennito AS, Dechert GJ, Hoosain NE. Desorption and ionization of heavy petroleum molecules and measurement of molecular weight distributions. Energy Fuels. 2007 21 1042-7. 

Measurement of Molecular Weight Distributions. Energy Fuels, Vol.21, No.2, pp. 1042-1047... 

Tier 3 involves the measurement of molecular weight distribution. SDS-PAGE (sodium dodecyl sulfate polyacrylamide gel electrophoresis) or size exclusion chromatography (SEC) provides information of molecular weight distribution of protein composition on and estimated concentration of proteins >2.5 kDa. Ideally the lower limit detection of the method should be 1 ppm or less based on standard proteins. If the nonfood allergen or... 

In prachce, molecular weight is measured by solu-hon viscomefry. Dissolving a precise amount of resin in a solvent and measuring the change in viscosity of the solution versus pure solvent is correlated with resin molecular weight. While soluhon viscomefry does not give a direct measure of molecular weight distribution, the MJM of PVC is very nearly two in all cases as a result of the polymerizahon and chain transfer mechanism. 

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