5. Glass, rubber and melt phase

We distinguish three different phases in the polymer: the glass phase, the rubber phase and the melt phase. On top of this the polymer can be partially crystalline and partially amorphous.

Glass phase

At low enough temperatures the rotation time of the Kuhn segments is far more than 1 second. The macromolecules are rigid now. The polymer is difficult to deform: any load will hardly change the shape of the polymer molecules. The chain segments can only bend a little bit. An applied force will only result in a small deformation of the plastic. When a polymer is in this condition then it is said to be in the glass phase.

Many plastic products like pipes, toys, cutlery, etcetera, need to be able to take up some load when used. That means that these products are made of plastics that are in the glass phase at room temperature.

Plastic pipes made from PVC in the glass phase

Although it may take a long time, rotation of the Kuhn segments is still possible: we only have to wait long enough. Suppose that on average it takes 1 million seconds for a Kuhn segment to rotate. If we would now apply a force on the plastic for a time longer than 1 million seconds, then the Kuhn segments do have sufficient time to rotate and the plastic will start to deform. This deformation of course happens very slowly and it is called creep of the polymer.

Rubber phase

With increasing temperatures the rotation time of the Kuhn segments will decrease. The temperature at which the rotation time is 1 second is the glass-rubber transition temperature. Above this temperature the rotation time is less than 1 second. In this condition the polymer molecules will easily deform under an applied load due to the rotating Kuhn segments. The polymer is now in the rubber phase. It is a flexible material.

Plastics that are in the rubber phase at room temperature is used for flexible products like automotive tires and rubber bands.

Elastic band made from a polymer in the rubber phase

In the rubber phase reptation of the macromolecules can be neglected. It would simply take a too long time before that happens. For reptation over some distance many millions of Kuhn segment rotations are needed.

Melt phase

Since the rotation time of the Kuhn segments keeps on decreasing with increasing temperature the time for the molecules to reptate into a new position will decrease. At temperatures where the reptation time is 1 second or less the macromolecules easily move into new positions. The polymer will forget its shape after a short time. It has changed from a rubber into a liquid.

The melt phase is typically used to give the polymer its final shape for use. For example, pipes are made by extrusion of polymer melt, plastic toys are made by injection moulding and bottles are made by blow moulding the melt.

The properties of the polymer now have changed from that of a rubber into that of a melt. The melt phase is defined as the temperature region where the reptation time is less than 1 s. The macromolecules can reptate on a human time scale, causing stresses to be released in less than a second. The stress relaxation times have reduced to less than a second.

Amorphous and crystalline polymers

Due to their nature polymer molecules like to have a random shape. Combined with other macromolecules they form a disordered structure (amorphous). However, some polymers are able to arrange their macromolecules with some regularity with neighbouring macromolecules. The regular structures now formed are called crystalline regions.

In a crystalline region the macromolecules are more closely packed than in an amorphous region. The free volume is less and this causes the macromolecules to be strongly hindered in their movements. They are less mobile in a crystalline region. When macromolecules are arranged in a crystalline structure they form a rigid and strong plastic, even when the amorphous part of the polymer would be in the rubber phase.

Regular crystalline regions in an amorphous matrix

Polymers are usually classified into amorphous polymers and crystalline polymers. Amorphous polymers have a fraction of crystalline material that is usually less than 10 %. Crystalline polymers have a crystalline fraction that is larger than 50 %. Typical examples of amorphous polymers are polycarbonate (PC), polystyrene (PS) and polyvinylchloride (PVC). Typical examples of crystalline polymers are polyethylene (PE) and polyethylene terephthalate (PET).

At a high enough temperature the crystalline region will melt. This is the crystalline melting point. The crystalline melting point is always higher than the glass-rubber transition temperature.

Summary

  • Dependant on the temperature polymers are in the glass phase, the rubber phase or the melt phase.
  • In the glass phase the Kuhn segments have a rotation time of (much) more than 1 second. The plastic is rigid on a human time scale.
  • A force applied for a long time is still able to deform the polymer in the glass phase. The time should be longer than the time that the Kuhn segments need to rotate. This slow deformation is called creep.
  • In the rubber phase the Kuhn segments rotate in a time much less than 1 second and the reptation time of the macromolecules is much higher than 1 s. The plastic is flexible.
  • In the melt phase the reptation time of the macromolecules is less than 1 second. In this condition the plastic can be shaped into products by means of extrusion, injection moulding or blow moulding.
  • The polymer molecules usually form a disordered structure. This is called amorphous.
  • Polymers have a limited possibility to arrange their molecules in a densely packed regular structure. This is called crystalline.
  • The crystalline regions form rigid regions in the amorphous phase.
  • A polymer is called crystalline when the fraction of crystalline material is more than 50 %.

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