Polymer molecular weight (MW) is the molecular mass of a polymer chain. While small molecules of the same elemental composition all exhibit the same molecular weight, polymer chains exhibit different characteristics. These chains are compounded from multiple small molecules that can alter the mechanical behavior of the polymer and skew its molecular weight. This makes determining a consistent molecular weight for synthetic polymer chains almost impossible. As such, a bell curve distribution is used to determine polymer molecular weights and find an average value for synthetic polymer chains.

Three distinct molecular weight averages are used to ascribe a numeric value for the molecular weight distribution of polymer chains:

  • The number average molecular weight (Mn), which describes the lowest portion of molecular weight of the sample;
  • The weight average molecular weight (Mw), which describes the average that is closest to the center of the bell curve;
  • The Z average molecular weight (Mz), which describes the highest portion of molecular weight of the sample.


The Importance of Polymer Molecular Weight

The physiochemical properties of a polymer are entirely dependent upon the molecular weight and its distribution throughout the polymer chain. Consistency is critical for numerous manufacturers in sectors as varied as the pharmaceutical, biotech, litigation, and petroleum industries. The properties of commercial polymers also depend upon the molecular weight of the chains used in their construction. Products such as car parts and food packaging depend upon ductile materials that exhibit good plastic deformation to withstand rugged daily use.

Polymeric materials are fundamentally varied. Those consisting of a low number of repeated units, thus low molecular weight, tend to exhibit soft or even fluidic properties with little mechanical strength. Conversely, larger chains with increased molecular weight display solid properties with vastly improved physical integrity over their shorter chain counterparts.

These properties can determine the softening and melting points of a polymeric product, its solution and melt viscosity, and its elasticity.


Determining Polymer Molecular Weight

Molecular weight is determined by multiplying the mass of each subsequent repeating unit in the polymer chain, before adding the mass of either end group. This provides a range of values that are used to establish the molecular weight distribution of the material. The average molecular weight is the most common value of merit for manufacturers, but the low and high portion molecular weights are also important for understanding the entire distribution curve and establishing the broader characteristics of the polymer.

The processing properties of polymeric materials are largely determined by their molecular weight distribution, with narrower distribution curves typically suggesting improved mechanical and processing properties compared to broader distributions. This is due to the plasticizing effects of the low portion molecular weight that softens the material, while the high portion disproportionately contributes to the melt viscosity.


Molecular Weight Distribution with RQM+ Lab Services

RQM+ Lab Services provides unparalleled expertise in the field of molecular weight analysis, using cutting-edge chromatographic techniques to determine polymer molecular weight distributions with outstanding degrees of accuracy. We use Gel Permeation Chromatography (GPC) to determine polymeric molecular weight distributions for a broad range of products.

If you would like any more information about GPC technology or analyzing polymer molecular weight, please do not hesitate to contact us.

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