Polymer fractionation

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Polymers (often also called plastics ) are chain-like molecules that are built up either from the same repeating unit ( homopolymers ) or from different building blocks ( copolymers ). With a few exceptions (example: proteins ) they consist of a mixture of molecules with different chain lengths ( molar masses ). For polymers, averaged values ​​are therefore given as molecular mass . The number-average (Mn), weight-average (Mw) and viscosity -average molar mass (Mη) are common. Polydispersity (PDI) is a measure of the breadth of the molar mass distribution . Under polymer fractionation means the targeted modification of the molecular weight distribution.

Reasons for polymer fractionation

Molecular weight distributions of a polymer and two fractions obtained from it

The molar mass of polymers has a decisive influence on the properties of polymers and therefore determines their possible applications. Among other things, the flow properties , solubility , mechanical properties, and also the service life are largely determined by the molar mass of the polymer. For high-performance polymers, i.e. plastics that have to meet special requirements, not only the mean molar mass, but also the overall molar mass distribution is often decisive. Often the low and / or high molecular weight components interfere. Examples of such high-performance polymers are novolak as photoresist, hydroxyethyl starch (HES) as blood plasma expander , cellulose acetate as filter material and hyaluronic acid for medical / cosmetic applications. The initial distribution can be changed in a targeted manner through polymer fractionation. The figure shows the molar mass distributions of the starting material and two fractions obtained from it.

Analytical methods

Polymer fractionation for analytical purposes (scale: a few mg) is carried out using 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 molar mass distribution or - in the case of the FFF - to determine the particle size distribution of nanoparticles.

Preparative methods

Preparative polymer fractionation is based in most cases on chromatographic separation methods (e.g. preparative GPC, P-TREF and Baker-Williams fractionation ) and is therefore limited to the production of a few grams. Continuous Spin Fractionation is suitable for scales from several grams to kg or tons . The book Polymer Fractionation by F. Francuskiewicz provides an overview of preparative polymer fractionation .

literature

  • MJR Cantow: Polymer Fractionation. Academic Press, New York (1967)
  • LH Tung: Fractionation of Synthetic Polymers. Marcel Dekker, New York (1977), ISBN 0-8247-6547-8
  • F. Francuskiewicz: Polymer Fractionation. Springer, Berlin (1994), ISBN 978-0-387-57539-1
  • R. Koningsveld, LD Kleintjens, H. Geerissen, P. Schützeichel, BA Wolf: Fractionation. In: Comprehensive Polymer Science Volume 1 , Pergamon Press 1989, pp. 293-312, ISBN 0-08-036205-2
  • J. Eckelt, T. Haase, S. Loske, BA Wolf: Large scale fractionation of macromolecules. Macromolecular Materials and Engineering. 289, 2004, pp. 393-399.

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