Answer to Question 1
Crosslinks prevent the elastomer from creeping at high temperature. Also, crosslinks give some limitations to the range of elasticity for the polymer. These effects arise from the limiting effects of the crosslinks. The crosslinks bind the molecules together and do not permit unlimited stretching. Also, during creep, the crosslinks act as limiting forces to the continuation of creeping motion. If the crosslink density were increased, the amount of stretching would be lower, the strength higher, the hardness and the amount of creep would be lower.
Answer to Question 2
As with most plastic materials, when the temperature of an elastomer is lowered below its Tg, the polymer will embrittle. This occurs because the long-range flexibility of the polymer is decreased and the intermolecular forces increase as the polymer motion is reduced with the reduction in temperature. There fore, below Tg, elastomers are not elastomeric but are, rather, hard and brittle. The lower temperature range for elastomers is, therefore, the Tg.
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