In equation (8.2) m1, m2, m3, ... are the masses of each species, and mt is obtained by multiplying the molecular weight of each species by the number of molecules of that weight; that is, mt = niMi. Thus the molecular weight appears as the square in the numerator of equation (8.2); the weight average molecular weight is therefore biased towards larger molecules. Another consequence is that Mw > Mn; the average molecular weight of a polymer measured by light scattering must be greater than that obtained by osmotic pressure measurements if the polymer is polydisperse (that is, contains a range of molecular weights). The ratio Mw/Mn expresses the degree of poly-

dispersity. Table 8.2 (p. 281) shows actual values for the number average and weight average molecular weights for dextrans (micro-bial polysaccharides used as plasma expanders).

8.1.3 Polymer mixtures or blends

The different phase behaviours which are realised with representative molecular architectures are shown in Fig. 8.5 (a) to (c). Such mixtures produce solid polymeric structures such as films and membranes with distinctive morphologies.



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