PKG-323 Lecture 3.4: Molecular Weight and MW Distribution

Why are polymers described using average molecular weights instead of a single absolute value?

Polymerization produces macromolecules of varying lengths and molecular masses.

What is the correct order of average molecular weights in polymers?

Mz > Mw > Mv > Mn

What is the number average molecular weight (Mn)?

The ordinary average molecular weight equal to total mass divided by total number of molecules.

What is the formula for number average molecular weight (Mn)?

Mn = (ΣNiMi) / (ΣNi)

What is the weight average molecular weight (Mw)?

An average that accounts for the weight fraction of each molecular size and is always greater than Mn.

What is the formula for weight average molecular weight (Mw)?

Mw = (ΣNiMi²) / (ΣNiMi)

What is the viscosity average molecular weight (Mv)?

The average molecular weight most commonly used in industry because it is easy to measure.

How is viscosity average molecular weight determined?

By dissolving the polymer in a solvent and measuring viscosity.

What is the Z-average molecular weight (Mz)?

An average molecular weight that emphasizes large molecules more than Mw.

How is Z-average molecular weight determined?

By size exclusion chromatography.

What is polydispersity (Ð)?

A measure of the molecular size distribution within a polymer sample.

What is the formula for polydispersity?

Ð = Mw / Mn

What does a polydispersity value of 1 mean?

All molecules are the same size, which does not occur in synthetic polymers.

What are typical polydispersity values for commercial polymers?

Ð is greater than or equal to 1 and most are greater than 2.

What defines a narrow molecular weight distribution?

Ð < 6 with molecules of roughly the same length.

What defines a broad molecular weight distribution?

Molecules have significantly varying lengths.

How does increasing molecular weight affect polymer properties?

It increases strength and resistance to flow (viscosity).

What is the effect of a narrow molecular weight distribution?

Increased strength but increased resistance to flow.

What is the effect of a broad molecular weight distribution?

Lower viscosity, higher melt flow, and easier processing.

Which molecular weight average best correlates with mechanical properties?

Weight average molecular weight (Mw).

What is gel permeation chromatography (GPC)?

A method that separates polymer chains by size using porous beads.

What is the principle behind GPC?

Large molecules elute first while small molecules elute later.

What information does GPC provide?

Mn, Mw, and the full molecular weight distribution (Ð).

What is chemical analysis for molecular weight determination?

Titration of polymer end groups.

What is a limitation of chemical analysis methods?

They require suitable solvents and polymers with specific functional groups.

What is the dilute solution viscosity method?

A method that determines molecular weight by measuring solution viscosity.

What is the relationship between viscosity and molecular weight?

Viscosity increases as molecular weight increases when total mass is constant.