A polymeric material is build-up from many randomly coiled polymer molecules that are extensively intertwined with each other. At many positions the molecules are even entangled with each other, which results in a kind of network that connects all polymer molecules with each other. Because of these entanglements the interaction between the molecules is very strong. Each individual molecule will intersect with many other molecules. The polymer structure can best be visualized with, for example, a pile of worms or a pan with spaghetti.
Because of their unordered structure the molecules do not fit perfectly together. This creates some empty space between the molecules that is called free volume. This free volume takes up about 5 to 10 % of the total volume of a polymer structure. With increasing temperatures the free volume increases and with reducing temperatures the free volume reduces.
The level of free volume is very important for the mobility of the polymer molecules. At high temperatures, where the free volume is relatively large, the mobility of the molecules will be very high. They are able to move around in the polymer structure. It is like a room in which only a few people are present: they can wander around without seriously hindering their neighbours.
At low temperatures however the free volume is low. Now the molecules are so close together that they hinder each other in their movements. Just consider the same room but now densely packed with people: with enough people pressed in the room it may even be impossible for them to move their arms or legs.
- The polymer molecules form a disordered structure.
- The molecules are strongly intertwined with many neighbouring molecules. They form a network.
- In between the molecules some free volume is present.
- Usually the free volume is so low that the molecules hinder each other strongly in their movements. The mobility will be low.
- At increasing temperatures the free volume increases. Due to this the mobility of the polymer molecules increases too.