Weight Average MolecularWeight
المؤلف:
sami matar & Lewis. F. Hatch
المصدر:
Chemistry of PETROCHEMICAL PROCESSES
الجزء والصفحة:
p 319
20-9-2017
993
Weight Average MolecularWeight
The weight average molecular weight (Mw) is the sum of the products of the weight of each species present and its molecular weight, divided by the sum of all the species’ weights:
Wi = weight of each polymeric species
Mi = molecular weight of each polymeric species
Substituting NiMi = Wi, the weight average molecular weight can be defined as Larger, heavier molecules contribute more to Mw than to Mn. Light scattering techniques and ultracentrifugation can determine Mw.
The following simple example illustrates the difference between Mn and Mw: Suppose a sample has six macro-molecules. Three of them have a molecular weight = 1.0 × 104, two have a molecular weight = 2.0 × 104, and one has a molecular weight = 3.0 × 104:
In monodispersed systems Mn = Mw.
The difference in the value between Mn and Mw. indicates the polydispersity of the polymer system. The closer Mn is to Mw, the narrower the molecular weight spread. Molecular weight distribution curves for polydispersed systems can be obtained by plotting the degree of polymerization i versus either the number fraction, Ni, or the weight fraction, Wi.
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