Molecular weight determination, polymers osmotic pressure method

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. 

Typical molecular weight g mol range of polymer Anionic polymerization, active center sodium thiolates determined by osmotic pressure method M 1-6 X 10 (13)... 

In addition to its major use in determining the number-average molecular weight (Ma) of polymers, osmometry has also been used to determine M of block copolymer micelles. The method involves determining the osmotic pressure (77) across a membrane that is permeable to solvent only. Because osmotic pressure is a colligative property, it depends on the number of particles, and hence yields Ma. It also depends on the interactions between particles, and thus... 

In lyophilic colloidal systems, e.g. in polymer solutions, the disperse phase concentration may be sufficiently high, and the osmotic pressure reaches values that can be reliably measured. In this case osmotic pressure measurements, along with the application of osmosis-related phenomena, such as cryoscopy and ebullioscopy, provide methods for the study of such systems. For example, these methods allow one to determine the molecular weights of polymers. 

The recent studies on light scattering in polymer solutions lead to principally the same results as the osmotic pressure method. Compare the chapter on molecular weight determinations, p. 146. 

Polystyrene can be prepared as follows A mixture of styrene, detergent (Na-dodecanoate), and water is agitated ultrasonically to produce a fine emulsion. On the addition of hydrogen peroxide (initiator), PS is obtained as a polymer, which can be extracted after filtration. The polymer molecular weight is determined by various methods (such as light scattering and osmotic pressure). 

Osmometry is an important method of determining the molecular weight of macromolecules such as polymers and proteins. The advantages of this method for macromolecules are that membranes permeable to the solvent but not to the macromolecule are easily found, and the osmotic pressure (or height of the solvent) is large and easily measured. " ... 

Methods of determining molecular weight include osmotic pressure, light scattering, gel permeation chromatography, dilute solution viscosity, vapor pressure, freezing temperatures, and others. Odian GC (2004) Principles of polymerization. John Wiley and Sons Inc., New York. Ehas HG (2003) An introduction to plastics. John Wiley and Sons, New York. Slade PE (2001) Polymer molecular weights, vol 4. Marcel Dekker, New York. 

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