Polymer Molecular Weight Measurements, Explained by Basketball

By Jim Rancourt, founder and CEO of Polymer Solutions

  • In the NBA, there are 15 players to a team rosterbasketball
  • The average height is 6’7″
  • If two teams have an average height of 6’7″ are they the same? -Not necessarily
  • If one team has two players that are 7′ tall, does that make a difference on the court? -Absolutely

Molecular Weight and Basketball Teams

NCAA March Madness has gotten me thinking about one my favorite analogies for explaining the various ways to describe molecular weight: basketball teams. Molecular weight is a critical characteristic of plastic materials. In the most basic terms, molecular weight is the size of the molecule. The molecular weight of a plastic material has a direct bearing on how that material will perform throughout its life cycle and also on the ways a plastic material might fail. For example, when a product is sterilized, the molecular weight may be significantly decreased.  Processing that is not optimized can cause molecular weight deterioration due to the thermal and hydrolytic degradation that can occur during molding operations. There are many techniques used to measure molecular weight, with some providing more detailed measurements than others. The correct analytical approach must be selected with full awareness of the meaning of the molecular weight measurement that will result.  Cue the basketball analogy …

Are Teams With the Same Average Height the Same?

In the NBA there are 15 players to a team roster, with an average height of 6 feet, 7 inches.  If there are two teams that both have an average player height of 6 feet, 7 inches, are they the same?  Not necessarily.  One team could have an entire roster of players each measuring 6 feet, 7 inches tall, whereas another team might have a few extremely tall players and a few shorter players that result in the average.  Would it make a difference in the basketball game, or perhaps the Final Four, if one team has two players that are 7 feet tall?  Absolutely!  The same is true for molecular weight. The most basic molecular weight measurement is the number average molecular weight, Mn. This measurement is simply an average of the size of all molecules. This measurement is a great starting point for understanding the performance of a plastic material, perhaps to quickly rule-in or rule-out if the molecular weight might be the reason for a plastic failure issue. The analytical technique used to acquire this measurement is Dilute Solution Viscosity (DSV).  It is a relatively inexpensive analytical test and can be performed quickly, with a small amount of sample.  The result of this test will yield a single numerical value that is related to the molecular weight of the portion of the sample tested. The result can then be compared to control samples or known Mn measuresments of the material. If there is no known or expected outcome then it will be necessary to dig deeper, with a more rigorous analytical technique. By using Gel Permeation Chromatography (GPC), often referred to Size Exclusion Chromatography (SEC), you can determine if the plastic “team” has all 6-foot-7 players or if in fact there are a few 7-footers on the roster.  This technique will drill down to the molecular level and provide a distribution of the exact weight of individual molecules, rather than a single value. The resulting measurements provide the weight average molecular weight, Mw (an also Mn).  This measurement determines if high molecular weight or low molecular weight components skew the overall distribution and can also reveal if the polymer is a blend of several polymer molecular weight distributions.

Image by Polymer Solutions, Inc. and dekanaryas/123RF