1. What is Polydispersity Index?

Definition: Polydispersity Index (PDI) is a measure of the distribution of molecular mass in a given polymer sample.

Purpose: It indicates the breadth of the molecular weight distribution and is important for understanding polymer properties.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( PDI \) — Polydispersity Index (unitless)
• \( M_w \) — Weight-average molecular weight (kg/mol)
• \( M_n \) — Number-average molecular weight (kg/mol)

Explanation: The weight-average molecular weight is divided by the number-average molecular weight to determine how broad or narrow the molecular weight distribution is.

3. Importance of Polydispersity Index

Details: PDI values help predict polymer behavior:

• PDI = 1: Monodisperse (all chains same length)
• PDI > 1: Polydisperse (varying chain lengths)
• Typical step-growth polymers have PDI ≈ 2

4. Using the Calculator

Tips: Enter both molecular weight values in kg/mol. The values must be > 0, and M_w should typically be greater than M_n.

5. Frequently Asked Questions (FAQ)

Q1: What does a high PDI indicate?
A: A high PDI (>2) indicates a broad molecular weight distribution, which affects mechanical properties and processing.

Q2: What's a typical PDI for step-growth polymers?
A: Step-growth polymers typically have PDI around 2 due to their random nature of chain growth.

Q3: How is PDI different for chain-growth polymers?
A: Chain-growth polymers often have wider distributions with PDI values between 1.5-20 depending on the polymerization mechanism.

Q4: What analytical techniques measure M_w and M_n?
A: Techniques include gel permeation chromatography (GPC), light scattering (for M_w), and osmometry (for M_n).

Q5: Can PDI be less than 1?
A: No, since M_w is always ≥ M_n, PDI cannot be less than 1.