Polymer Virtually crosslinked elastomer

If the diol chain extender is used in exact molar proportion to the unreacted isocyanate, then a linear polyurethane elastomer is obtained. The resulting thermoplastic elastomer may be extruded or injection-moulded, and the properties arise from the ability of the hard and soft segments to form semi-crystalline domains that act as virtual crosslinks in the polymer and give it elastomeric properties, as shown in Figure 1.17. 

Liquid polymer While virtually all elastomers can be prepared in sufficiendy low molecular weights to exhibit fluid behavior, only those liquid polymers that can be converted to an elastomeric solid will be reviewed. It might appear that this could be accomplished by crosslinking, but this is not the case. While some crosslinking is desirable, the primary reaction required is one of chain extension that is the end-to-end attachments of the low molecular weight chains via reactive terminal groups. 

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