Effect of Molecular Weight
on Polymer Properties

Almost all properties of a polymer depend on the molecular weight and its distribution, that is, low and high molecular weight polymers will have vastly different mechanical and thermo-physical properties. For example, polymeric materials consisting of a rather small number of repeat units (often called oligomers1) will be rather soft or even liquid and will possess little or no strength whereas polymeric materials of high molecular weight will be solid, and have much improved mechanical properties. One usually observes a steep rise in the mechanical and visco-elastic properties with increasing molecular weight until a certain molecular weight is reached, beyond which the properties are nearly independent of the chain length, that is, all properties will eventually reach an asymptotic value. It was found, that the molecular weight dependence of many properties can be described by an expression of the form

Examples of properties that are very sensitive to the molecular weight are softening and melting point, solution and melt viscosity, solubility, mechanical strength and moduli. Not only the average molecular weight but also its distribution has a noticeable effect on the physical, mechanical, and processing properties. In most cases, a narrow molecular weight distribution will yield better mechanical and processing properties than a broad distribution because the low molecular weight portion will act as a plasticizer and soften the polymeric material, whereas an exceedingly high molecular weight portion will make processing of the polymer resin very difficult due to its disproportionately high contribution to the melt viscosity.

Notes
  1. Oligomers have properties in between monomers and polymers. Since they are made up of a relative small number of repeat units, reomoval of one unit has a noticeable effect on their properties which is not the case with polymers. Typical examples of oligomers include liquid paraffin, curable resins such as epoxides and phenolics, as well as many plasticizers, tackifiers and lubricants. 

  • Summary/h2>

    Molecular Weight

    The two most fundamental properties of a polymer are its chemical structure and its molecular weight distribution.

  • These two characteristics determine directly all cohesive forces, mechanical properties (moduli and strength), transition temperatures (melting and softening point), viscosity, and solubility and miscibility.

  • In a more indirect way, they also control the morphology (degree of crystallinity and packing density), molecular mobility, and various relaxation phenomena, i.e. the visco-elastic response behavior of the polymer.

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