In
physicalPhysical chemistry is the study of macroscopic, atomic, subatomic, and particulate phenomena in chemical systems in terms of physical laws and concepts...
and
organic chemistryOrganic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
, the
polydispersity index (
PDI), is a measure of the distribution of
molecular massThe molecular mass of a substance is the mass of one molecule of that substance, in unified atomic mass unit u...
in a given
polymerA polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
sample. The PDI calculated is the weight average molecular weight divided by the number average molecular weight. It indicates the distribution of individual
molecular massThe molecular mass of a substance is the mass of one molecule of that substance, in unified atomic mass unit u...
es in a batch of
polymerA polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
s. The PDI has a value equal to or greater than 1, but as the polymer chains approach uniform chain length, the PDI approaches unity (1). For some natural polymers PDI is almost taken as unity.
The PDI from polymerization is often denoted as:
Mn is more sensitive to molecules of low molecular mass, while Mw is more sensitive to molecules of high molecular mass.
A polymer material is denoted by the term
polydisperse if its chain lengths vary over a wide range of molecular masses. This is characteristic of man-made polymers.
http://www.chemicool.com/definition/polydisperse.html. Natural organic matter produced by the decomposition of plants and wood debris in soils (humic substances) also has a pronounced polydispersed character. It is the case of
humic acidHumic acid is a principal component of humic substances, which are the major organic constituents of soil , peat, coal, many upland streams, dystrophic lakes, and ocean water. It is produced by biodegradation of dead organic matter...
s and fulvic acids, natural
polyelectrolytePolyelectrolytes are polymers whose repeating units bear an electrolyte group. These groups will dissociate in aqueous solutions , making the polymers charged. Polyelectrolyte properties are thus similar to both electrolytes and polymers , and are sometimes called polysalts. Like salts, their...
substances having respectively higher and lower molecular weights.
Effect of Polymerization Mechanism on PDI
Typical PDI's vary based on the mechanism of polymerization and can be affected by a variety of reaction conditions. In synthetic polymers, it can vary greatly due to reactant ratio, how close the
polymerizationIn polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
went to completion, etc. For typical addition
polymerizationIn polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
, values of the PDI can range around 10 to 20. For typical step polymerization, most probable values of the PDI are around 2 —Carothers' equation limits PDI to values of 2 and below.
Living polymerizationIn polymer chemistry, living polymerization is a form of addition polymerization where the ability of a growing polymer chain to terminate has been removed. This can be accomplished in a variety of ways. Chain termination and chain transfer reactions are absent and the rate of chain initiation is...
, a special case of addition polymerization, leads to values very close to 1. Such is the case also in biological polymers, where the PDI can be very close or equal to 1, indicating only one length of polymer is present.
Methods to Determine the Polydispersity
- Gel permeation chromatography
Gel permeation chromatography is a type of size exclusion chromatography , that separates analytes on the basis of size. The technique is often used for the analysis of polymers. As a technique, SEC was first developed in 1955 by Lathe and Ruthven. The term gel permeation chromatography can be...
(also known as Size exclusion chromatographySize-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...
).
- Light scattering
Light scattering is a form of scattering in which light is the form of propagating energy which is scattered. Light scattering can be thought of as the deflection of a ray from a straight path, for example by irregularities in the propagation medium, particles, or in the interface between two media...
measurements
- Direct calculation from MALDI or from Electrospray
The name electrospray is used for a device that employs electricity to disperse a liquid or for the fine aerosol resulted in this process. The method is sometimes improperly called electrohydrodynamic atomization. High voltage is applied to a liquid supplied through an emitter...
mass spectrometry.