Polymers in Modern Life

WHAT ARE POLYMERS?

Polymers are natural or man-made molecules, frequently called macro-molecules. They are composed of smaller units, monomers, which have reacted together to give a long chain, rather like a string of beads. In the simplest polymers, the monomers are identical and the polymer is named by prefixing 'poly' to the name of the monomer from which it is derived. Thus the polymer from ethylene is poly(ethylene), although in common usage the brackets are omitted. The monomers that constitute a polymer may be the same in which case they are called homopolymers or they may contain more than one monomer in which case they are copolymers. Additional monomers in a polymer may be randomly copolymerised to give random copolymers or may be polymerised in alternating blocks of identical monomers forming block copolymers.

-M-M-M-M- -M-N-N-M-N-M- -M-M-M-N-N-N-
Homopolymer Random copolymer Block copolymer

Some polymers contain chemical bonds or cross-links between the long chains. Cross-links may be introduced during the synthesis of the polymer as is the case in the thermosetting polymers, which include the well known phenol-formaldehyde resins, but they may also be introduced into an existing polymer by a chemical reaction. This method of making cross- linked polymers is used in the process of rubber vulcanisation or sulfur cross-linking which was one of the earliest chemical reactions carried out on a naturally occurring polymer [cis-poly(isoprene)] obtained from the latex of the tree Hevea braziliensis. Many other polymeric materials are found in living organisms. The most common are the polysaccharides, which include starch, the food store of seeds, and cellulose, the structural material of plants. A second important group are the polymerisation products of amino acids, the polypeptides, of which the proteins are widely distributed in living organisms. In the polypeptides the sequence of monomer units is much more complex than in the case of man-made polymers and the order in which the monomers are put together changes their nature and biological function.

-A1-A2-A3- where A1, A2, etc.
A4A5- are different amino acids
Polypeptide

Thus muscle, collagen (in bone), keratin (in hair, nails and beaks) and albumin are all copolymers of very similar amino acids but have quite different physical properties. In deoxyribonucleic acid (DNA), the genetic template, the sequence of monomers is precise and variations are the cause of genetic mutations. Although the polypeptides are of ultimate importance in life processes they are not important in the context of materials and will not be considered further in this book. However, they have had a significant impact on modern polymer science since the synthesis of the first man-made poly amide fibre, Nylon, by Carothers was modelled on the structure of a silk, a naturally occurring polypeptide.

NATURAL POLYMERS

Cellulose

The most easily recognised natural polymer is cellulose, the most abundant organic polymer on earth. It consists of glucose units and is the major component of wood although it is also found in the stems and leaves of many plants. Cotton is a particularly pure form of cellulose.

In woody materials, the long crystalline fibrils of cellulose are bound into a composite structure by lignin, a macromolecule based on polyphenols. Lignin, which is present to the extent of 25–30% in most woods, is a cross-linked polymer rather similar to man-made phenol–formaldehyde resins and may be looked upon as a 'glue' which gives wood its permanent form (Figure 1.1).

The overall effect is a very strong material that can bear enormous tensions and bending stresses without breaking. However, as every cricketer knows, wood can break under violent impact. This is because it is weakest along the direction of the fibres and the lignin which is a weaker polymer delaminates (separates) between the cellulose fibres. It is an interesting tribute to the evolution of natural materials that since the discovery of the synthetic polymers, man has employed the same principle of orientated fibre (glass or carbon) reinforcement in the manufacture of polymer composites with a strength (in the fibre direction) similar to that of steel.

Pure cellulose biodegrades relatively rapidly in the natural environment. Nature's abundant cellulosic litter in the form of leaves, grass, plant stems, etc. is bioassimilated in one season to give useful biomass. Branches and tree trunks take much longer to biodegrade and it is not always appreciated that, in some parts of the world, tree trunks and branches on the sea-shore present a much more significant litter problem than most of the commodity plastics. It may take decades and in some cases centuries for some fallen trees to disintegrate and biodegrade in the natural environment whereas a polyethylene container in the same situation would disappear due to photo-biodegradation in as many years. The situation may be quite different inside a forest environment, which is much more conducive to biological attack. Both cellulose and lignin biodegrade; the former much more rapidly than the latter by enzyme-catalysed hydrolytic depolymerisation of cellulose to its constituent sugars, which are assimilated by the cell. This process begins with the attachment of microflora to the hydrophilic (water-loving) surface of the tree trunk and the death of the microorganisms in t urn provides humus for the growth of seedlings which eventually cannibalise the dead trunk. Polyethylene which is a hydrophobic polymer (that is it repels water) cannot undergo biodegradation unless it is modified by abiotic peroxidation. Sunlight catalyses this process and the rate-controlling process in the natural environment is photooxidation. This will be discussed in more detail in Chapter 5.

Wood-pulp cellulose is the basis of paper manufacture. The process involves separating the cellulose fibre from the resinous component of wood by treatment with alkali and carbon disulfide, an environmentally polluting process (Chapters 2, 5). Papermaking has been strongly criticised in recent years by ecologists due to the rapid depletion of the forests and this has resulted in an increase in recycling of used paper.

Cellulose fibres are crystalline and very strong materials when they are dry. However, they are hydrophilic and in the presence of moisture they absorb water, becoming permeable by microorganisms. For this reason paper became much less important as a food packaging material when the cheap hydrophobic synthetic polymers emerged in the second half of the 20th century.

The hydrophilic nature of cellulose is due to the high concentration of hydroxyl groups in the molecule. In the absence of water this gives rise to the association of the long molecules by hydrogen bonding. These 'cross-links' are weak compared to the valency bonds that hold together...