Molar mass distribution
From Biocrawler, the free encyclopedia.
The Molar mass distribution in a polymer describes the relationship between a polymer fraction and the molar mass of that polymer fraction. In linear polymers the individual polymer chains rarely have the exact same degree of polymerization and there is always a distribution around an average value.
Different average values can be defined depending on the statistical method that is applied. The weighted mean can be taken with the weight fraction, the mole fraction or the volume fraction:
- Weight average molar mass or Mw
- Number average molar mass or Mn
- Volume average molar mass or Mv
- Z average molar mass or Mz
These different definitions have true physical meaning because different techniques in physical polymer chemistry often measure just one of them. For instance osmometry measures number average molar mass and small angle neutron scattering measures weight average molar mass.Mv and Mz are obtained from respectively viscosimetry and sedimentation analysis. In a typical distribution curve the average values are related to each other as follows Mn < Mv < Mw < Mz. Polydispersity of a sample is defined as Mw divided by Mn and gives an indication just how narrow a distribution is.
The molar mass distribution of a polymer sample depends on factors such as chemical kinetics and work-up procedure. Ideal step-growth polymerization gives a polymer with polydispersity of 2. ideal living polymerization results in a polydispersity of 1. By dissolving a polymer a insoluble high molar mass fraction may be filtered off resulting in an large reduction in Mw and a small increase in Mn thus reducing polydispersity.
Molecular weight distribution is often found in the literature but this phrase is technically incorrect.
