Membrane osmometry, determination weight

Of the preponderance of small ions, the colligative properties of polyelectrolytes in ionising solvents measure counterion activities rather than Molecular weight. In the presence of added salt, however, correct Molecular weights of polyelectrolytes can be measured by membrane osmometry, since the small ions can move across the membrane. The second virial coefficient differs from that previously defined, since it is determined by both ionic and non-ionic polymer-solvent interactions. 

Analytical procedures The molecular weights of the polyisobutylenes (Systematic name poly(l,l-dimethylethylene) and of the polynorbornadienes (Systematic name poly(3,5-tricyclo[2.2.1.02, b]heptylene) were determined by membrane osmometry in toluene solution and those of the polystyrenes were determined by vapour-pressure osmometry in chloroform. 

Cotmnercial cellulose triacetate samples were fractionated by both fractional precipitation and preparative gel permeation chromatography (GPC). The triacetate fractions were characterized by vlsco-metry, high speed membrane osmometry (HSMO) and GPC. A fair agreement has been found between the molecular weights of various triacetate fractions determined by the three procedures. 

Narrow Molecular Weight Triacetate Fractions. Narrow molecular weight cellulose triacetate fractions were obtained by both fractional precipitation and preparative GPC as described above. The number average molecular weight (1 ) of the various fractions and cuts was determined by high speed membrane osmometry. A linear dependence of GPC elution volume on log molecular weight for all cellulose triacetate fractions was found in both methylpyrroli-done and dichloromethane. 

For polydisperse polymer samples, measurements that lead directly to the determination of the molecular weight, such as light-scattering photometry and membrane osmometry, are referred to as absolute molecular weight methods. Techniques such as viscometry are not absolute molecular weight methods because they require calibration using an absolute molecular weight technique. 

Membrane Osmometry and Viscometry. Number-average molecular weights of PMMA were determined with a Mechrolab model 501 highspeed membrane osmometer in toluene at 60 °C except for samples 122-4 and 122-6 which were determined at 40°C. 

Molecular weights are determined by end-group analysis (Mn), membrane osmometry (Mn), viscometry (Mv), size exclusion chromatography (Mm), light scattering photometry, and sedimentation (Mw). Any molar mass computed by these methods must be evaluated critically, in view of a dependence on methodology. 

Berth, G., Dautzenberg, H., Lexow, D., and Rother, G. (1990). The determination of molecular weight distribution of pectins by calibrated GPC. Part I. Calibration by light scattering and membrane osmometry. Carbohydr. Polym. 12 39-59. 

Fig. 18. Plot of apparent number-average molecular weight M , as determined by membrane osmometry, for a cellulose diacetate fraction in tetrachloroethane (TCE) against the temperature of measurement401 O, M in TCE , M in THF...

Table 6. Number average molecularJ weight Mn of cellulose acetate fractions with < F)) = 2.92, determined by membrane osmometry in various solvents at 25 °C 39>...

The Mark-Houwink relation for polypropylene in o-dichlorobenzene at 130°C was calibrated as follows. A series of sharp fractions of the polymer was obtained by fractionation, and the molecular weight of each fraction was determined by membrane osmometry in toluene at 90"C. The samples were then dissolved in o-dichlorobenzene at I30°C and their intrinsic viscosities ([ ]) were measured. The resulting data fitted an expression of the form... 

Berth, G., Anger, H. and Lexow, D. 1980. The determination of the molecular weights of pectins by means of membrane osmometry in aqueous solution, Nahrung, 24(6) 529-534. 

Membrane osmometry was measured on a few samples to determine the number average molecular weight (Mn). Regenerated cellulose membranes were used, and were found to be highly swollen in NMP, leading to both very long equilibration times and excellent retention of low molecular weights. Table I below shows the results of these and the above measurements for the samples reported here. 

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