Entanglements:

Polymers can be described as materials with the capacity to assume an enormous array of thermally equilibrated structural configurations.  Under the heading of polymers we can define a subclass of materials, plastics, which generally have molecular weights above 10 kDa and which are generally based on covalently bonded linear chains with simple chemical structures.  (That is, denatured proteins are excluded from plastics.)  For the most part plastics have moderate to low chain persistence (chain rigidity), so display sufficient flexibility that at some point in increasing molecular weight, different chains can physically interact with each other so that a mechanical stress can be transferred from one chain to another.  The structural conformation that leads to transfer of a mechanical stress from one chain to another is termed an entanglement, although the physical picture of two chains twisted or tangled together has significantly lost favor in the polymer physics community over the past 5 years.  Current thinking implies that an entanglement involves a certain chain packing density and probability of overlap.  For example is currently held that longer chain persistence leads to a greater number density of entanglements since rods are more likely to interact with each other compared to random coils.

If the quiescent (structurally undeformed) mechanical properties of a polymer melt are plotted as a function of molecular weight in a log/log plot we find an initial weak or linear dependence followed, at approximately 10 kDa) by a strong dependence on molecular weight.  The transition molecular weight is termed the entanglement molecular weight.  Polymers above the entanglement molecular weight are useful as plastics while those below the entanglement molecular weight display features of low molecular weight materials, generally brittle powders.  For example, candle wax is low molecular weight polyolefins of the same molecular structure as polyethylene.  We can compare the mechanical properties of a candle compared with a milk jug.  Both materials are hydrophobic and can be molded into a desirable shape, however, candle wax is useless as a bottle and can not be processed in a blow molding operation to form a milk jug because it displays no melt strength and no resistance to tearing.  Both of these properties are associated with the entanglement of polymer chains in polyethylene.  Polystyrene is a somewhat brittle and clear amorphous polymer used for CD cases and other packaging.  Polystyrene with molecular weights below 10 kDa has no industrial use and is a white powdery material that can not be molded into useful parts and also displays no melt strength for drawing.  Similar differences between low molecular weight polymers and plastics can be given for any polymeric material. 

Industrial polymers or plastics then can be defined as materials possessing chain structure with thermally equilibrated configurational freedom that display chain entanglements.  Elastomers and thermoset polymers are subclasses of polymers where most entanglements are replaced by chemical bonds that link different polymer chains in a permanent network structure.  These links between chains are termed crosslinks.  The difference between thermosets and elastomers is that use temperature for an elastomer or rubber (butyl rubber) is well above the solidification temperatures (glass and crystalline transitions temperatures) while for a thermoset (epoxy) the use temperature is well below the transition temperatures.  Generally, the polymer chains between crosslinks for an elastomer or thermoset is somewhat below or close to the entanglement molecular weight for the uncrosslinked polymer.  This is for several reasons.  Firstly, thermosets and elastomers are processed in the liquid state so that flow and mold filling ability are enhanced with lower molecular weight prepolymers and secondly lower molecular weights between entanglements lead to higher modulus and tougher rubbers and thermosets, as can be predicted by a state of thermo-configurational equilibrium (rubber elasticity theory).