Free volume and activation energy for movement in the glass

12 Free volume and activation energy for movement in the glass 

In the glass, there is insufficient free volume for the chains to be able to move. What then defines the critical size of the free volume that is required for backbone motion  

In order for some of the trans conformations to change into gauche and vice versa, but not to undergo translation, a finite space is required for the cooperative process. This is the free volume required for the backbone rotation that changes the glass into a rubber. 

A very clear indication of the importance of free volume is the way that the observed transition temperature changes with pressure. The application of pressure decreases the free volume and so requires further thermal expansion to enable the chain movement. This, of course, raises the observed transition temperature. This is important when one looks at polymers used in applications where the material is subjected to high pressure, such as seals in diving bells, hydraulic systems, etc. A polymer at atmospheric pressure may be above its transition temperature and so its elastomeric properties allow it to provide an effective seal. However, application of pressure, such as that experienced when a diving 

So now we can extend the answer to our question What do the molecules need Answer The molecules need energy, time and space.  

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