Polymer fractionation
Polymers (often also called resins ) are chain-like molecules, which are either of the same repeating unit ( homopolymers ) or made of different components (copolymers ) are constructed. Apart from a few exceptions (for example, proteins) they consist of a mixture of molecules with different chain lengths ( molecular weights ). Therefore, averaged values are given as molecular mass for polymers. In use, the number average (Mn), the weight average (Mw) and the viscosity- average molar mass ( Mη ). A measure of the width of the molecular weight distribution is the polydispersity index (PDI). Under polymer fractionation is defined as the deliberate alteration of the molecular weight distribution.
Reasons for polymer fractionation
The molar mass of polymers has a significant influence on the properties of polymers and therefore determines their application possibilities. Among other things, the flow properties, solubility, mechanical properties, and also the service life can be largely determined by the molar mass of the polymer. For high-performance polymers, ie plastics that must meet specific requirements, not only the average molar mass, but also the entire molecular weight distribution is often critical. Often interfere with the low and / or high molecular weight components. Examples of such high-performance polymers are as novolak photoresist, hydroxyethyl starch (HES ) as a blood plasma expander, cellulose acetate filter material and hyaluronic acid for medical / cosmetic applications. Through a polymer fractionation, the output distribution can specifically change. The figure shows the molecular weight distributions of the starting material and of two fractions obtained therefrom.
Analytical Methods
The polymer fractionation for analytical purposes (scale: Some mg ) is carried out, inter alia, by means of the gel permeation chromatography (GPC), matrix assisted laser desorption / ionization - time of flight (MALDI -TOF) or field-flow fractionation ( FFF). These methods are used to determine the molecular weight distribution or - in the case of FFF - for determining the particle size distribution of nanoparticles.
Preparative methods
Preparative polymer fractionation is based in most cases on chromatographic separation methods (eg, preparative GPC, P- TREF and Baker Williams fractionation), and is therefore limited to the production of a few grams. For scales of several grams to kilograms or tonnes, the Continuous Spin Fractionation is suitable. An overview of preparative polymer fractionation offers to book Polymer Fractionation of F. Francuskiewicz.