In the last lesson on ceramics, we saw that one way to classify ceramics is by their uses (refractories, glass, clay products, abrasives, etc.). Other possible classification categories might include crystal structure and whether they are crystalline or non-crystalline. For polymers, one useful classification is whether they are thermoplastic or thermosetting polymers. As you read in the last reading assignment, thermoplastics soften when heated and harden when cooled. This is totally reversible and repeatable. Most linear polymers and branched structure polymers with flexible chains are thermoplastics. This is in contrast to thermosetting polymers, which do not soften when heated due to strong covalent crosslinks. Thermoset polymers are generally harder and stronger than thermoplastics and have better dimensional stability.
For more information about thermoplastic (here referred to as thermo-softening) and thermosetting polymers watch this video (4:40):
The term polymer is used to describe a macromolecule made of many monomers or repeating units. The properties of these polymers all depend on a variety of factors. The monomer unit, the linkages between each monomer, and the intermolecular and intramolecular forces that exist between polymers. In this lesson, we will learn about two classes of polymers thermo-softening polymers and thermosetting polymers. We will also learn about their properties and how these properties arise. The term plastics is used to describe a wide range of polymers made of monomers all derived from the products obtained from the fractional distillation of crude oil. You may be familiar with polyethylene, polypropylene, and even polyvinylchloride. You can learn about the structure of these polymers, how they are made, and their real-life applications from other videos on our channel. Here we will focus on how these polymers respond to heat and why they respond the way they do.
Polyethylene, polypropylene, and polyvinyl chloride our thermo-softening polymers. This means that they soften when heated. When soft and in liquid form they can be molded into many different shapes. These plastics are used to make many everyday items such as window and door frames, pipes, wiring insulation, and waterproof clothing items just to name a few. This is made possible because polymers are not linked together. We can think of it like a bowl of noodles. Although the noodles are coiled and tangle with one another they are not linked. Like the noodles, these polymers can slide over one another making these items made from them soft and flexible. In fact, these polymers can only interact by weak intermolecular forces and can, therefore, be separated rather easily when heated giving them relatively low melting points. Some other thermo-softening polymers include polystyrene and polytetrafluoroethylene. Thermosetting polymers, on the other hand, do not soften when heated. Unlike thermo-softening polymers these thermosetting polymers are cross-linked to one another can you think about how this might affect the properties of these polymers? Pause, think, and continue when ready.
The presence of crosslinks hardens the overall structure. A good example of a thermosetting polymer is vulcanized rubber. Rubber tapped from para rubber trees is a polymer of isoprene monomers. It is a runny liquid that can be processed to make latex gloves, erasers, and party balloons. It can also be used to make car and bicycle tires though it has to be vulcanized first. For the vulcanization process, sulfur is added so the disulfide bridges link the polymers together. The presence of these cross-linkages greatly increases its strength and therefore does not soften easily when heated. Let's think about it. No matter how fast you ride your bike the tires do not change shape. Some other examples of thermosetting polymers include a substance used to make old TV sets and certain types of strong glue.
In summary, thermo-softening plastics are soft and melt when heated, whereas thermosetting plastics are hard and do not soften or change their shape when heated.
Now that we have discussed thermoplastic and thermosetting polymers let us review the different basic structures that polymers form and how that structure can determine whether the polymers are thermoplastic or thermosetting.