WHAT IS A POLYMER?
A monomer is a molecule typically made from between 4 and 10 atoms. They are constructed so that they will be relatively reactive with a desire to grab hold of the monomers around them in the process mentioned above called polymerisation. Polymers and polymerisation processes are very diverse and there are a variety of ways to classify them. As an example, proteins in nature are formed by a condensation polymerisation process. Amino acids react together forming DNA from which all protein is derived. Water is released as a by product.
Sometimes polymer systems are not simply a straight chain of similar repeating units. They can consist of a variety of different chain lengths and can also have branches.
In thermosets, they can be completely cross linked:
It is also possible to polymerise two or more different monomers in a process called copolymerisation.
The more complex polymers will have higher relaxation temperatures, softening points and melting temperatures. Properties such as tensile strengths will also be higher as the inter chain and intermolecular interactions increase.
Identification and characterisation of polymers is not particularly easy. This makes it very difficult to check polymers in a factory environment for example for a regular quality. Often minor quality variations can be dealt with by adjusting the process conditions but this has a danger of allowing a broader range of quality of the finished products. To-date, there is a great deal of ‘trial and error’ in the polymer converting industry.
Most polymers are organic with carbon bonds as their backbone but there are examples of silicon backboned polymers. Silicone polymers do not have carbon as part of the backbone structure. Many silanes are known which are analogous to the hydrocarbons with Si-Si bonds. These compounds are not very stable and hence not very useful.
Silicones on the other hand have an alternating -Si-O- type structure. This basic structural unit is found in many rocks and minerals in nature including common sand.
Types of common synthetic polymer:
Polymers can generally be grouped as Thermoplastics (amorphous or semi crystalline) or Thermosets
- Thermoplastic - can be easily moulded
- Amorphous eg polystyrene, Perspex
- Semi crystalline eg polyethylene, polypropylene, nylon
An amorphous polymer has no order to the arrangement of the molecules in the solid state. They can be compared to cooked spaghetti.
A semi crystalline polymer will have regions of amorphicity (randomly ordered molecules) and regions of distinct order.
- Thermoset – one off cure
- Epoxy resins, eg Araldite
- Cyanoacrylates, eg Superglue
There are some key properties and general statements that can be made about synthetic polymers;
- Generally, amorphous polymers have a glass transition (Tg), a softening point but no melt temperature (Tm)
They are usually clear and can be brittle in their ‘glassy’ state.
Tend not to be too flexible.
- Generally, a semi crystalline polymer will have a weaker glass transition, a softening point and a clear melt temperature
Are very often opaque and fairly resistant to shattering.
Tend to have a ‘waxy’ feel and can be fairly flexible.
- Generally, thermosets have a glass transition, no melt temperature but a degradation at higher temperatures
They are often mechanically very strong and can bond with other materials very easily as they polymerise (cross link). Are easily incorporated into composite structures and are often used to form these with glass, carbon fibres etc.. In non crosslinked form, are often used as adhesive systems.
There are linear polymers, eg olefins and polymers with an aromatic (ring structure) ‘backbone’
Side group arrangements:
Polymers with molecules in which substituent groups of atoms are arranged at random above and below the backbone chain of atoms, when the latter are arranged in the same plane. The opposite of stereospecific polymers.
Polymers whose monomer units are oriented alternately dextro and levo or a polymer structure in which monomer units attached to the polymer backbone alternate in a-b-a-b fashion on one side of the backbone and, if present on the other side, are arranged in a b-a-b-a fashion
Pertaining to a type of polymeric molecular structure containing a sequence of regularly spaced, asymmetric atoms arranged in like configuration in a polymer chain. The term isotactic is sometimes used to denote a polymer structure in which monomer units attached to a polymer backbone are identical on one side but alternated on the other side of the backbone. Materials containing isotactic molecules may exist in highly crystalline form because of the high degree of order that may be imparted to such structures.