pi / c = RT/Mn + BC
This situation can be realized if the molecular weight isn't too high, and the solvent isn't too good (B isn't too large). Data for polystyrene in a good solvent, toluene, are depicted in PC Fig. 10.3.
The influence of the molecular weight on this type of plot is well-illustrated by study of samples of polystyrene with molecular weights of 30,900 to 612,000 in toluene, F Fig. 116, reproduced from Krigbaum, W. R.; Flory, P. J. "Statistical Mechanics of Dilute Polymer Solutions. IV. Variation of the Osmotic Second Virial Coefficient with Molecular Weight" J. Am. Chem. Soc. 1953, 75, 1775-1794). The second virial coefficient is positive in all cases, but it becomes smaller as Mn increases. However, this decrease in B is not as rapid as the decrease in Mn itself. Therefore the error that would be produced in the calculation of Mn from the measurement at a single concentration, say 0.5 g/100 cc, becomes larger as the molecular weight increases.
The measurement of the second virial coefficient at various temperatures provides a convenient method for determining the Theta temperature in a particular solvent, as illustrated, F Fig. 119, for polyisobutylene in benzene. The Theta temperature is the temperature at which B changes sign.
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