physical state of cis-1,4-polyisoprene

[jeenojose]

Dear all

Does the cis-1,4-polyisoprene, exist as a solid above its melting point? (in eye-vision). If so, please tell me the mechanism which causes the material to take the solid state. Also, is there any possibility that the material structurally remains to be amorphous below glass-transition temperature?

Thank you
Jeeno

 

[stancom]

Cis 1,4 polyisoprene (as in natural rubber), free of crystallinity, reamains a solid because its combination of a very high molecular weight (it can be several million) and accompanying chain entanglements. The crystal melting point increases with the temperature at which crystallization occurs but it never exceeds ca 30 C (excepting, of course,the strain induced crystallization that enhances strength properties).

The glass transition is essentially an instantaneous event. If the polymer is free of crystallinity as it normally will be in the processed or vulcanized state then you would expect an amorphous state to be retained below the transition, just as you might expect any existing crystallinity to be 'frozen in'. The level of crystallization always falls well short of 100% even if the polymer seeems to be virtually rock-hard and its influence on the glass transition is likely to be very small. Even so, in several standards we advise users to remove crystallization by heating at 70 C for a couple of hours before any low temperature testing.