polydispersitetsindeks

Polydispersity index, often abbreviated as PDI, is a measure used to quantify the distribution of molecular masses in a sample of polymers. It is a dimensionless number that indicates the breadth of the molecular weight distribution. A PDI value of 1 signifies that all polymer molecules in the sample have the same molecular weight, which is characteristic of a monodisperse sample. In reality, perfect monodispersity is rarely achieved.

For most synthetic polymers, the PDI is typically greater than 1. A higher PDI value indicates a broader distribution of molecular weights, meaning there is a wider range of polymer chain lengths present in the sample. Conversely, a lower PDI value suggests a narrower distribution, where the molecular weights are more uniform. The PDI is calculated as the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn). Mw takes into account the mass of each polymer chain, giving more weight to larger molecules, while Mn is simply the total mass divided by the total number of molecules. Therefore, PDI = Mw / Mn.

The polydispersity index is an important parameter in polymer science and engineering as it influences many material properties, including viscosity, mechanical strength, and solubility. For instance, polymers with a lower PDI often exhibit more predictable and reproducible properties. The method of polymerization and the conditions under which it is carried out can significantly affect the PDI of the resulting polymer. Different techniques are used to measure Mw and Mn, such as gel permeation chromatography (GPC), also known as size exclusion chromatography (SEC).