Polymers are macromolecules consisting of repetition units, so-
Both biopolymers and their derivates and also special synthesized polymers fulfil high duties in medicine, cosmetics and industry. For these special applications it is often very harmful that the polymers consist of molecules with different molecular weight. For linear molecules this means that they have a broad molecular weight distribution. This characteristic can be a problem for a technical application and for the application as pharmaceutical products.
For special demands it is necessary to remove harmful material which cannot be avoided during the synthesis. This kind of separation of shorter or longer chains is called fractionation.
Unlikely low molecular weight components that consist of one kind of molecule only, polymers consist of a mixture of molecules with different molar masses. Therefore the molar masses of polymers are always gives as averages. There exist different ways of determine the average differing in the kind of weighting. Commonly used average molar masses are the number average molar mass Mn, the weight average molar mass Mw, and the z average molar mass Mz:
ni is the amount of molecules having the molar mass Mi.
The different averages can be determind with different methods (Mn by osmosis, Mw by light scattering, and Mz by ultracentrifugation). The most powerful methods to determine the molar masses are the Gel Permeation Chromatography (GPC, also Size Exclusion Chromatography, SEC) and MALDI-
The polydispersity index is a measure for the broadness of the molecular weight distribution. For polymers that consist of molecules with one kind of chain length only (e.g. proteins), the polydispersity index is 1. As the difference in chain length increase the polydispersity index is rising.
By removing short and/or long chains -
Methods that enable the production of larger amounts are based on liquid-