If there is one key property for polymers, it is molecular weight --- actually itÆs the presence of the very-high-molecular weight fraction. ItÆs these very long molecules which give the polymer its high elongation, toughness, impact strength, long-term creep resistance and resistance to stress cracking. In semi-crystalline polymers, these long-chain molecules get captured in one or more adjacent crystalline sections and act as links (tie molecules) between the crystallites. In amorphous polymers, the chains are so long that they get severely entangled with the other molecules. Unlike short chains that can easily slip through the entangled mass (a term we define as flow), the long chains are tied together such that pulling on one causes all the others to resist the movement. These long chains form the ômortar between the bricksö which gives the strength to the plastic part.
Of course, itÆs the high molecular weight fraction which is also responsible for high resin viscosity which shows up as low extrusion rates, short injection flow lengths, and poor knit-line strengths. Also, when you ôforceö the material to flow, these long molecules which are carrying a disproportionate amount of the load, tend to break, therein, slightly reducing the overall molecular weight average.
When there is a failure, one often wants to determine if the molecular weight of the material is the same as that which was specified, or has it been altered during processing or in use. After all, small reductions in average molecular weight can be responsible for significant reduction in tie-molecule density, i.e., performance.
There are a number of methods by which one can determine different average molecular weights --- melt rheology, solution viscosity, membrane osmometry, gel permeation chromatography (also called size exclusion chromatography), and light scattering techniques. Each has its own advantages as well as limitations, however, the one most widely used is also the simplest and often the most sensitive measurement --- melt flow rate.
As discussed earlier, a very small change in molecular weight can result in a very significant alteration of a materialÆs properties. Most of the other methods measure properties of the polymer which are directly proportional to the average molecular weight. Small changes in molecular weight therefore can be lost in the normal variability of the measurements. Melt flow rate, on the other hand, is exponentially related to the average molecular weight of the material:
MFR = Mw3.4
The power factor makes melt flow rate an extremely sensitive tool for measuring small changes in molecular weight.
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