Polydispersity Index Calculator

Polydispersity Index (PDI) is a measure of the breadth of molecular weight distribution in a polymer sample. Polymers, unlike small molecules, consist of chains with varying lengths, and PDI quantifies how uniform or diverse these chain lengths are within a sample. It is defined as the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn), giving a dimensionless number that ranges from 1.0 (perfectly uniform) upward.

A PDI of exactly 1.0 indicates a monodisperse polymer, where every chain has identical molecular weight. This theoretical ideal is approached but never perfectly achieved in practice, even in living polymerization techniques. Real polymers have PDI values greater than 1.0, reflecting the statistical nature of chain growth. The broader the molecular weight distribution, the higher the PDI value. For example, a polymer with PDI of 2.0 has twice as much spread in chain lengths as a polymer with PDI of 1.5.

PDI is one of the most important characterization parameters in polymer science because molecular weight distribution directly affects material properties. Polymers with narrow distributions (low PDI) tend to have more predictable and uniform mechanical, thermal, and processing properties. Broad distributions can be advantageous in some applications, such as improving processability in melt processing operations.

The polydispersity index is calculated from two fundamental molecular weight averages that are typically determined by techniques such as gel permeation chromatography (GPC), also known as size exclusion chromatography (SEC), or by membrane osmometry and light scattering methods.

The number-average molecular weight (Mn) weights each chain equally regardless of size, while the weight-average molecular weight (Mw) gives more weight to larger chains. The ratio Mw/Mn therefore always equals or exceeds 1.0, with the value increasing as the distribution broadens.

PDI values can be classified into distinct ranges that correlate with polymerization methods and material properties:

This calculator determines PDI from two molecular weight measurements, typically obtained from gel permeation chromatography or other analytical techniques:

1. Enter Mw: Input the weight-average molecular weight in g/mol. This value is biased toward heavier chains in the distribution.
2. Enter Mn: Input the number-average molecular weight in g/mol. This value is biased toward lighter chains in the distribution.
3. View Results: The calculator computes PDI = Mw/Mn and classifies the distribution breadth based on established polymer science ranges.

The result includes both the numerical PDI value and a qualitative classification (monodisperse, very narrow, narrow, moderate, or broad) that indicates the likely polymerization method used to produce the sample.

PDI measurement is critical in quality control for polymer manufacturing. Pharmaceutical companies producing drug delivery nanoparticles require narrow PDI (typically below 0.2 for the polydispersity index as sometimes reported in nanoparticle sizing) to ensure consistent drug loading and release profiles. In the coatings industry, PDI affects film formation, viscosity, and final surface properties.

In biomedical applications, the PDI of biodegradable polymers like PLGA (poly(lactic-co-glycolic acid)) directly influences degradation rate and drug release kinetics. Narrow PDI polymers degrade more predictably, which is essential for controlled-release drug formulations. Food-grade polymers and thickeners also have specification ranges for PDI to ensure consistent texture, viscosity, and stability in consumer products.