1. What is Number Average Molecular Weight?

The number average molecular weight (M_n) is a way to characterize the molecular weight distribution of a polymer sample. It represents the total weight of all molecules divided by the total number of molecules in the sample.

2. How Does the Calculator Work?

The calculator uses the number average molecular weight equation:

Where:

• \( M_n \) — Number average molecular weight (g/mol)
• \( M_i \) — Molecular weight of the i-th species (g/mol)
• \( N_i \) — Number of molecules of the i-th species (unitless)

Explanation: The equation sums the product of each molecular weight and its count, then divides by the total number of molecules.

3. Importance of Mn Calculation

Details: M_n is particularly sensitive to the presence of low molecular weight species in the distribution. It's crucial for understanding polymer properties like viscosity, mechanical strength, and thermal behavior.

4. Using the Calculator

Tips: Enter pairs of molecular weights (in g/mol) and their corresponding counts. You can add as many pairs as needed. All molecular weights must be positive numbers, and counts must be positive integers.

5. Frequently Asked Questions (FAQ)

Q1: What's the difference between Mn and Mw?
A: M_n (number average) gives equal weight to each molecule, while M_w (weight average) gives more weight to heavier molecules. M_w is always ≥ M_n.

Q2: How is Mn measured experimentally?
A: Common methods include end-group analysis, osmometry, and size-exclusion chromatography with appropriate detection.

Q3: What does a high PDI (Mw/Mn) indicate?
A: A high polydispersity index (PDI) indicates a broad molecular weight distribution, while PDI=1 means all molecules are identical.

Q4: Why is Mn important in polymer science?
A: M_n determines many polymer properties like tensile strength, melt viscosity, and glass transition temperature.

Q5: Can this calculator be used for oligomers?
A: Yes, it works for any molecular weight distribution, from small molecules to high polymers.