Molecular weight determination membrane osmometry

Membrane osmometry is one of two osmometry techniques that are used to determine molecular weight. The other is vapor-pressure osmometry. The latter requires calibration using samples of known molecular weight, while membrane osmometry is an absolute technique. Only membrane osmometry is described here. The osmotic pressure of a polymer solution is directly related to the number-average molecular weight of the polymer and is useful when Af is less than about 500,000. The basic principle is that if a polymer solution and pure solvent are placed on opposite sides of a semi-permeable membrane, i.e., one that allows solvent to pass but not polymer, there will be a tendency for solvent to flow into the solution, where its chemical potential is lower. If the pressure of the solution is raised above that in the solvent, the chemical potential will be balanced, and the flow will stop when the pressure difference reaches the osmotic pressure, n. The thermodynamic expression required to determine the molecular weight is the van t Hoff equation ... 

Attempts were made to determine number average molecular weights (Mn) by osmometry (Mechrolab Model 502, high speed membrane osmometer, 1 to 10 g/1 toluene solution at 37 °C), however, in many instances irreproducible data were obtained, probably due to the diffusion of low molecular weight polymer through the membrane. This technique was abandoned in favor of gel permeation chromatography (GPC). 

It is possible to add a second, molecular weight-sensitive detector to an SEC system to provide a direct means of absolute molecular weight calibration without the need to resort to external standards. These detectors represent refinements in classic techniques, such as light-scattering photometry, capillary viscometry (for intrinsic viscosity), and membrane osmometry for on-line molecular weight determination. Yau recently published a review of this subject with comparisons of the properties and benefits of the principal detectors currently in use (22). The present discussion is restricted to lightscattering and viscometry detectors because Yau s osmometry detector is not yet commercially available. The reader is referred to Chapter 4 for a comprehensive discussion of molecular weight-sensitive detectors. 

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. 

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