Are polymers viscoelastic below the glass transition temperature
Properties of plastics:
The glass transition temperature
What does glass have to do with plastics is certainly the question that is on the tip of your tongue. But if you know that the glass we normally use is made of silicates (SiO42-) exists, and that this individual SiO4-Tetrahedra are linked together (see the drawing below), it becomes clear that in principle glass, just like plastics, is also a polymer.
But what kind of transition is it referred to in the term "glass transition temperature"?
Perhaps you have already noticed that plastics are more brittle and less elastic in winter (or in the freezer) than at warmer temperatures, and that they become more elastic and flexible when heated (for example, it helps to keep the lid under warm water for Tupperware jars) so that it fits more easily on the can). All of this has to do with the glass transition temperature, or Tg for short.
The glass transition temperature is the temperature at which polymers (but only completely or partially amorphous polymers) change from the liquid or rubber-elastic, flexible state to the glassy or hard-elastic, brittle state; it is therefore also called the "softening temperature". It is specific for each plastic, which means that plastics can be differentiated on the basis of their glass transition temperature.
Some plastics are used below their glass transition temperature, e.g. polystyrene, polymethyl methacrylate (this is made of e.g. Plexiglas) or polyethylene terephthalate, i.e. when they are hard and brittle, other plastics are used above their glass transition temperature when they are elastic and flexible, e.g. polyisoprene and polybutadiene (two types of synthetic rubber) or polyethene.
Glass transition and melting
Before we deal more with the glass transition temperature, the following important fact should be made clear: The transition from the glass state to the liquid state is not the same as melting. Admittedly, it looks absolutely the same for the non-chemist, the plastic becomes soft and fluid, but for the chemist there are two different processes. Melting takes place with substances (including plastics) that are crystalline: the supply of heat dissolves the ordered crystal lattice, resulting in a disordered liquid. In the case of substances whose molecules are already in a disordered state in the solid state, in so-called amorphous solids, the crystal lattice does not have to be dissolved by adding energy, the substance "only" becomes liquid.
The difference becomes clear when you look at the temperature profile when heating a crystal and an amorphous solid: When a crystal begins to melt, its temperature remains constant until it is completely melted, i.e. the heat supplied is used exclusively to destroy the crystal lattice , not to raise the temperature of the melting substance (this heat is called latent heat of fusion referred to, from the Latin latens = hidden). In the case of amorphous solids, on the other hand, all the heat supplied is used to increase the temperature of the substance; there is no crystal lattice that has to be destroyed.
This is illustrated in the diagrams below:
Most plastics consist of crystalline and amorphous areas, i.e. they have both a melting temperature (for the crystalline areas) and a glass transition temperature (for the amorphous areas).
A closer look at the glass transition
The following "formula" helps to understand all of this better:
This means: the warmer it is, the more and the faster particles such as our plastic molecules move, the colder it is, the less they move (from a physical point of view, heat is nothing more than kinetic energy, i.e. kinetic energy).
For an amorphous plastic this means: If it is cold, i.e. below the glass transition temperature in the glassy, hard-elastic state, the molecular chains hardly move (a little movement is always present as long as a substance is not at absolute zero of 0 K (-273 ° C)). If the plastic is now slowly heated, the chains move more and more, but still hold together until the point at which longer sections of the molecular chains can move freely with the glass transition temperature is reached and the plastic becomes soft, elastic and finally liquid.
This also explains the easier brittleness of plastics below the glass transition temperature: In the elastic state, it is no problem for the molecular chains to slide past each other in order to evade pressure or some other external force. However, if the molecules are in place - as in the hard-elastic glass state - there are two possibilities: Either they withstand the pressure and remain in the position they are in, or they are separated by the pressure and the plastic breaks apart.
What does the glass transition temperature depend on?
The glass transition temperature essentially depends on the structure of the polymers, whereby various factors can be distinguished:
- Main chain flexibility
This is one of the most important factors for the level of the glass transition temperature, and it is easy to understand why: A plastic has a low glass transition temperature if the molecular chains can move so well even at low temperatures that the plastic is flexible and elastic. The more flexible the chains are, the better it works.
Let's take a silicone as an example:
Here the main chain, which consists alternately of oxygen and silicon atoms, is so flexible that the glass transition temperature is -127 ° C, so polydimethylsiloxane is liquid at room temperature and is used, for example, as a thickener in some shampoos.
A plastic that does not have a transition to an elastic state, but instead decomposes when heated without becoming soft, is polyphenyl sulfone:
Here the chains are so stiff that the plastic never softens. Because this is extremely impractical for processing, one usually builds between the sulfone groups (-SO2-) other groups, e.g. ether bonds (-C-O-C-), which cause the plastic to have a glass transition temperature that is below the decomposition temperature.
- Barbed side chains
The side chains that are attached to the main chain also influence the glass transition temperature. This is easy to understand: if the chains are smooth and without side groups, they can slide past each other more easily and move better, so the glass transition temperature is low. If side chains are attached to the main chain, the individual molecular chains get caught more easily, and it is more difficult for the main chain to move if the weight of the side groups has to be moved, so the glass transition temperature is higher.
Of course, it depends on the type and number of side chains how great the influence on the glass transition temperature is.
- Side chains as spacers
However, side groups can also lower the glass transition temperature. It works as follows: the further apart the molecular chains are, the more empty space there is in the plastic and the easier it is for the chains to move, which means that the glass transition temperature is lowered. You can get more distance between the individual chains by adding plasticizers, these are small molecules that are stored between the chains and thus keep them at a distance, but the side chains of the plastic molecules can also have a similar effect.
An example of this are the various polymethacrylates shown below: The longer the side chains, the lower the glass transition temperature, and while polymethyl methacrylate (PMMA for short) is generally known as "Plexiglas" as a hard plastic, polybutyl methacrylate is already soft at room temperature.
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