Molecular Weights
Synthetic polymers have a distribution of molecular weights.
- Relative distributions can be measured by size exclusion chromatography (SEC), also called gel permeation chromatography (GPC).
- Absolute distributions can be measured by mass spectrometry, provided the molecular weights are not too high.
- The distributions can be characterized by different types of average molecular weights (such as Mn, Mw, Mz) and ratios derived therefrom. The most commonly used ratio is the polydispersity index, Mw/Mn,
Some methods that determine absolute molecular weights are
- Mn: end group analysis, colligative properties (vapor pressure lowering, ebulliometry, cryoscopy, osmometry)
- Mw: Light scattering, neutron scattering, ultracentrifugation
- Mz: Ultracentrifugation
Molecular weights are also measured by relative methods, which require calibration with known samples.
Examples are viscosity and size exclusion chromatography.
Return to the index
June 24, 1999
Wayne L. Mattice: wlm@polymer.uakron.edu