Polydispersitytekijä

Polydispersitytekijä, often referred to as the polydispersity index (PDI), is a measure used in polymer science to describe the distribution of molecular weights in a polymer sample. A polymer sample is rarely composed of chains of identical length. Instead, it contains a range of chain lengths, and the polydispersitytekijä quantifies how broad this distribution is.

The PDI is calculated as the ratio of the weight-average molecular weight ($M_w$) to the number-average molecular weight ($M_n$). Therefore, PDI = $M_w$ / $M_n$. The number-average molecular weight is the sum of the molecular weights of all the polymer chains divided by the total number of polymer chains. The weight-average molecular weight takes into account the contribution of longer chains more heavily.

A monodisperse sample, meaning all polymer chains have the same molecular weight, would have a PDI of 1. In reality, all synthetic polymers are polydisperse, and their PDI values are typically greater than 1. A PDI value close to 1 indicates a narrow molecular weight distribution, which is often desirable for achieving specific material properties. Conversely, a higher PDI suggests a broader distribution of chain lengths, which can influence the polymer's physical and mechanical characteristics, such as viscosity, solubility, and mechanical strength. Understanding and controlling the polydispersitytekijä is crucial for tailoring polymer properties for various applications.