Polymer

Polymer

A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

s are chainlike molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s that are made of the same repetition unit. Aside from few exceptions (e.g. protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s) polymers consist of molecules with different chain lengths. Therefore average values are given for the molecular weight

Molar mass

Molar mass, symbol M, is a physical property of a given substance , namely its mass per amount of substance. The base SI unit for mass is the kilogram and that for amount of substance is the mole. Thus, the derived unit for molar mass is kg/mol...

 like the number average, the weight average or the viscosity average molar mass. A measure for the width of the molecular weight distribution

Molar mass distribution

In linear polymers the individual polymer chains rarely have exactly the same degree of polymerization and molar mass, and there is always a distribution around an average value. The molar mass distribution in a polymer describes the relationship between the number of moles of each polymer species...

 is the polydispersity index

Polydispersity index

In physical and organic chemistry, the polydispersity index , is a measure of the distribution of molecular mass in a given polymer sample. The PDI calculated is the weight average molecular weight divided by the number average molecular weight. It indicates the distribution of individual...

. The targeted manipulation of the molecular weight distribution of a polymer by removing short and/or long chain material is called polymer fractionation.

Reasons for polymer fractionation

The molecular weight of polymers has a large influence on their properties and therefore determines the applications. Among others the flow behavior, the solubility

Solubility

Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...

, the mechanical properties but also the lifetime are influenced by the molecular weight. For high duty polymers – polymers that have to fulfill elevated demands – not only the molecular weight but also the molecular weight distribution is important. This especially holds true if low and/or high molecular material disturbs a given task.

Analytical Methods

Polymers can be fractionated on an analytical scale by size exclusion chromatography

Size exclusion chromatography

Size-exclusion chromatography is a chromatographic method in which molecules in solution are separated by their size, and in some cases molecular weight . It is usually applied to large molecules or macromolecular complexes such as proteins and industrial polymers...

 (SEC), Matrix-assisted laser desorption/ionization

Matrix-assisted laser desorption/ionization

Matrix-assisted laser desorption/ionization is a soft ionization technique used in mass spectrometry, allowing the analysis of biomolecules and large organic molecules , which tend to be fragile and fragment when ionized by more conventional ionization methods...

 (MALDI) or Field Flow Fractionation

Field Flow Fractionation

Field-flow fractionation, abbreviated FFF, is a separation technique where a field is applied to a fluid suspension or solution pumped through a long and narrow channel, perpendicular to the direction of flow, in order to cause separation of the particles present in the fluid, dependent on their...

 (FFF). These methods are used to determine the molecular weight distribution.

Preparative Methods

In most cases the fractionation of polymers on a preparative scale is based on chromatographic methods (e.g. preparative SEC or Baker-Williams fractionation). Therefore the production is normally limited to few grams only. For large scales of several grams up to kg or even tons the “Continuous Spin Fractionation” can be used. F. Francuskiewicz gives an overview about preparative polymer fractionation.

Literature

• M.J.R Cantow Polymer Fractionation Academic Press, New York (1967)
• L.H. Tung Fractionation of Synthetic Polymers Marcel Dekker, New York (1977)
• F. Francuskiewicz Polymer Fractionation Springer, Berlin (1994)
• R. Koningsveld, L.D. Kleintjens, H. Geerissen, P. Schützeichel, B.A. Wolf „Fractionation“ in: Comprehensive Polymer Science Volume 1 Pergamon Press, Oxford (1989) 293-312

External links

• http://www.wee-solve.de - WEE-Solve GmbH: Service provider for polymer fractionation