Electrostatic forces opposing rotation

Non-bonding interactions, associated with steric hindrance, are not the only type of force that can create an energy barrier to rotation. Also of significance are 

The temperature of the glass to rubber transition depends on the energy barriers opposing internal rotation. It is now a simple matter to determine how these depend on chemical composition. 

In the same way, we can compare chains of similar steric volume but different polarity. Consider non-polar polypropylene and polar poly(vinyl chloride) (PVC). 

The examples of polypropylene and of poly(vinyl chloride) require some further comment. After all, much of the poly(vinyl chloride) that we use at room temperature is flexible and leather-like , while commercial polypropylene is stiff. These observations are directly contrary to the glass transition temperatures listed above. 

As wiU be discussed in Chapter 6, commercial polypropylene is isotactic and crystalline at room temperature. Thus the modulus is that of a partially crystalline 

---

The mechanism of plasticisation of PVC is interesting. We have seen that the high glass transition is a result of electrostatic forces between carbon-chlorine dipoles opposing chain internal rotations. Consequently, anything that wiU screen or diminish the interdipole forces, and indeed the interchain separation, wiU thereby lower the transition temperature.This is exactly what the plasticiser molecules do. 

It is an intriguing fact that nucleons in nuclei, electrons in atoms, as well as large cosmic objects such as solar systems and ev galaxies, are more dominated by rotational than by linear motion, although in our daily life the latter seems to be a more common phenomenon. In rotation there is a balance between two forces the centrifugal force of inertia, which tries to move a body away from a center point, and an attractive force (gravitational, electrostatic, etc.), which opposes the separation. 

---