Molecular weight distribution

Molecular weight distribution describes how polymer-chain masses vary within a sample, from smaller chains to larger ones. It is commonly summarized by molecular-weight averages and dispersity.

Number-average molecular weight: The total mass of polymer molecules divided by the number of molecules in a sample. It weights every chain equally, making small chains count as much as large ones.
Melt viscosity: A measure of a polymer melt's resistance to flow under applied stress. High-molecular-weight chains can disproportionately increase resistance to flow during processing.
Polymer: A substance composed of molecules built from many covalently linked repeating units or related structural units. Its chains are the individual molecules whose masses form the distribution.
Z-average molecular weight: A molecular-weight average that weights polymer chains by the cube of their mass. It emphasizes the largest chains more strongly than the weight-average value.
Weight-average molecular weight: A molecular-weight average that weights each polymer chain in proportion to its mass. Its stronger emphasis on large chains reveals a different aspect of the distribution than the number average.
Polymer rheology: The study of how polymer materials deform and flow under forces. Distribution breadth helps explain why polymers with similar average mass can flow differently.
Molar mass: The mass of one mole of a substance, commonly expressed in grams per mole. Polymer molecular weights are reported as molar masses, though the sample contains many chain sizes.
Peak molecular weight: The molecular weight at the maximum of a measured polymer distribution curve. A peak locates the most prominent region but does not summarize the distribution's full breadth.
Dispersity: The ratio of weight-average to number-average molecular weight, indicating the breadth of a polymer's molecular-weight distribution. This ratio compresses the difference between two distribution-sensitive averages into one breadth measure.
Polymer processing: The methods used to shape polymer materials, including extrusion, molding, and fiber spinning. Manufacturing conditions must accommodate the chain populations that govern flow and relaxation.

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