Polymer Molecular Weight and Intrinsic Viscosity

The practical importance of intrinsic viscosity cannot be overstated. It is the most common measure of polymer molecular weight. The relationship between the intrinsic viscosity and the molecular weight can be determined as follows. Consider a polymer liquid consisting of a distribution of molecules, each with molecular weight 

Afi and weight fraction Wj. From the definition of the intrinsic viscosity, we can define an intrinsic viscosity [tilj for each component of the distribution and an average intrinsic viscosity, as follows  

Notice that approaches as a approaches 1.0 for values of a in the typical range 0.5 a 1, is intermediate between and M, but typically closer to M.  

Although (ro/M) is a function of temperature and solvent character, in many cases it is not very sensitive to either variable at temperatures near the theta temperature. The ratio of [q] to [q]0f therefore provides a good estimate of a, no matter what solvent was used to measure [q]0f. Taking values of K and a from Table 7.6, we find for polyisobutylene in cyclohexane at 30° (see Table 7.7)  

Therefore, for At = 10 g/mol, = 3.56, that is, the individual polymer molecules occupy almost 4 times the volume in cyclohexane than they do in any theta solvent. It was mentioned in Section 2.5 that polymers in contact with solvents of like solubility parameter 5 should show the greatest solvation effects, that is, swelling of networks or enhancement of viscosity of solutions. Now we can identify this solvating ability with a, the expansion factor. 

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