Polymers

Polymers

Polymers usually possess a certain amount of crystallinity, and their tensile strength increase with molecular weight. Also, the greater the crystallinity, the greater is the tensile strength, the lower is the solubility and the higher is the m.p.

Polymers may be classified into three groups:

1. Natural, e.g., rubber, proteins, cellulose;
2. Semi-synthetic, e.g., nitrocellulose, cellulose acetate; and
3. Synthetic, e.g., nylon, Bakelite, Perspex.

Plastics form a group of high polymers which have a fair range of deformability and mouldability, particularly at high temperatures. In plastics the polymers formed do not all have the same molecular weight, and since the polymers are not amenable to the ordinary methods of separation, the molecular weight of a ‘polymer’ is the average molecular weight. Polymerization is carried out with the object of building up compounds with predicted properties, and since the properties of a plastic depend on the degree of polymerization it is necessary to stop polymerization when the desired average molecular weight is reached. This may be done by various means, e.g., variation of the concentration of the catalyst. The average molecular weight of plastics varies from about 20,000 (e.g., nylon) to several hundred thousand (e.g., polyvinyl chloride, 250,000).

Plastics are generally tough, resistant to the action of acids and alkalis, and not very much affected over a fair range of temperature. They can be moulded to any desired shape or form.

Plastics are of two main types, thermoplastic and thermosetting. Thermoplastics are linear polymers which are soluble in many organic solvents, and which soften on heating and become rigid on cooling. The process of heat-softening, moulding and cooling can be repeated as often as desired, and hardly affects the properties of the plastic. Typical thermoplastics are cellulose acetate, nitrocellulose and vinyl polymers such as polythene, Perspex, etc.

Thermosetting plastics are three-dinensional polymers which are insoluble in any kind of solvent, and which can be heat-treated only once before they set, i.e., their formation, after which heating results in chemical decomposition, and hence they cannot be ‘reworked’. Typical thermosetting plastics are phenol-formaldehyde, urea-formaldehyde, melamine-formaldehyde, silicones, etc.

In thermoplastics the chains are, more or less, free chemically, but are held together by van der Waals’ forces. It is possible, however, to link together these linear molecules (cf. the rungs of a ladder) and the cross-linking agent converts the thermoplastic into a thermosetting plastic, e.g., in the vulcanization of rubber the sulphur cross-links the long chains. Furthermore, such thermosetting plastics may be reconverted into thermoplastics by opening the cross-linked polymers, e.g., the reclaiming of rubber. Most thermosetting plastics may be regarded as cross-linked polymers.

Those plastics which do not soften very much with rise in temperature are made soft and readily workable by the addition of certain compounds known as plasticisers; e.g., polyvinyl chloride is extremely stiff and hard, but addition of tricresyl phosphate makes it soft and rubber-like.