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 the repeating units in a polymer but because of the very large number of hydroxyl groups in cellulose, the molecules are held firmly together when dry to give the strong crystalline structure referred to above. Hydrophilic solvents can 'break' hydrogen bonds, resulting in swelling of the fibres. By using appropriate solvents, cellulose can be reformed into fibres (rayon) and films (cellophane) which retain the essential structure (and biodegradability) of cellulose. Replacement of the hydroxyl hydrogen in cellulose by hydrophobic groups such as methyl, ethyl or acetyl decreases its hydrophilicity. It can then be plasticised to give a thermoplastic material that can be processed to films. However, cellulose derivatives are more susceptible to water swelling than the hydrocarbon polymers, which are discussed below and they have been largely replaced by synthetic polymers in packaging applications.

Starch

Starch is the main energy storage system in plants and is closely related chemically to glycogen, the energy storage 'fuel' in animals. Starch consists of two polysaccharide components: amylose, a long unbran-ched chain of D-glucose units, and amylopectin, which is made from the same monomer units but cross-linked to give a much higher number of units in each molecule. It is, like wood, a composite structure in which the amylopectin provides the 'glue' that holds the starch granule together. Starch itself, unlike wood or paper, has no application as an industrial material but it will be seen in a later chapter that corn or potato starches have considerable potential as biodegradable thermoplastic polymers if appropriately plasticised. However, starch is much more readily hy-drolysed to glucose in the presence of microorganisms than cellulose and the main problem is to retain its integrity during use.

Wool, Hair and Silk

Like feathers and hair, wool consists mainly of keratin. This polypeptide contains L-cystine which is the oxidised (disulfide) form of L-cysteine. The former acts as a cross-link which determines the crimped nature of wool. In human hair this process is, of course, carried out reversibly in 'permanent waving':

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Silk, by contrast, consists of fibroin which does not contain cystine and is a straight fibre.

Natural Rubber [cis-poly(isoprene)]

The development of natural rubber as the first major industrial polymer was a very significant achievement of the 19th century. Rubber latex, as it comes from the tree, is a very unpromising material. It very rapidly loses its most useful mechanical property, namely elasticity, owing to attack by oxygen in the atmosphere (Chapter 3) which also transforms it into a very biodegradable material. Equally important to the development of rubber as an industrial product was the discovery of 'vulcanisation' (Vulcan, the God of fire) by Charles Goodyear in the USA and Thomas Hancock in England in 1839. Sulfur vulcanisation, or 'curing' as it is still called by rubber technologists, transformed the essentially thermoplastic polymer, which had poor dimensional stability, to a strong cross-linked matrix. Vulcanised rubber was found by Hancock to be a suitable material for bicycle tyres and in the 20th century it was developed for motor car tyres. A comparison of the environmental durability of an early bicycle tyre with that of a modern automobile tyre would be instructive but unfortunately none of the original rubbers now exist since they have all degraded to resinous nonresilient oxidation products. A modern tyre, unlike the early bicycle tyres, has to withstand the stresses and strains of high speed motorway travel (fatigue) at temperatures up to 100 °C and this is achieved by a complex combination of antioxidants which also give it protection against environmental pollutants such as ozone. Evidence for the environmental stability of modern tyre rubbers can be seen in many farmyards where discarded tyres are used as weights to hold down plastic covers. Some of these have been there for decades and show little sign of deterioration or biodegradation. This has nothing to do with the inherent biostability of the rubber molecule since we know that rubber latex peroxidises and biodegrades very rapidly when exposed to the outdoor environment. The durability of automotive tyres is rather a function of the exceptional effectiveness of the synthetic antioxidants that have been developed to protect the rubber from peroxidation during use. As will be seen later, the control of peroxidation is one of the mechanisms that is currently used in the development of polymers that can be programmed to biodegrade in a controlled way in the environment.

SYNTHETIC POLYMERS

Synthetic polymers are made by two main processes. The carbon-chain polymers result from the 'opening' of the double bond in the original olefin by addition polymerisation to give new carbon-carbon bonds:

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The hetero-chain polymers are mainly made by elimination of water between a carboxylic acid and an alcohol or an amine to give a polyester or a polyamide respectively. These are the condensation polymers:

[FORMULA NOT REPRODUCIBLE IN ASCII] (1.3)

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Similar products can be made without the elimination of water by 'ring-opening' polymerisation of monomeric cyclic monomers, the lactones and lactams. In this case the 'repeat unit' in the polymer is the same as that in the monomer:

[FORMULA NOT REPRODUCIBLE IN ASCII] (1.2)

Polyolefins

This is the most important group of carbon-chain polymers made by the general reaction (1.2) in which R is hydrogen (polyethylene) or an alkyl group (e.g. CH3 in polypropylene). In practice, polyethylene is not a single polymer. Three main sub-divisions are recognised.

Low Density Polyethylene

Low density polyethylene (LDPE) was the earliest polyolefin to be manufactured by the free radical polymerisation of ethylene. It is not 'pure' poly(methylene). The latter is a very linear polymer and can be synthesised in the laboratory by decomposition of diazomethane. As a result of its method of manufacture, LDPE contains alkyl groups of varying lengths pendant to the polymer chain and some carbon–carbon double bonds. These irregularities reduce its crystallinity compared with poly(methylene) so that it normally consists of 50–60% of crystal-line domains dispersed in amorphous polymer. The amorphous matrix is a very viscous liquid which acts like a rubber and gives the polymer its very high resistance to impact. LDPE is still widely used because of its exceptional toughness.

High Density Polyethylene

High density polyethylene (HDPE) is made by a different (ionic) process which gives higher relative molar masses (Mr) This, coupled with its lower chain-branching (it is sometimes called 'linear' polyethylene), increases the ratio of crystalline to amorphous polymer which makes it much stronger than LDPE. However, the reduction in energy-absorbing amorphous phase also reduces its toughness.

Linear Low Density Polyethylenes

A compromise between LDPE and HDPE has been achieved in the commercial development of linear low density polyethylenes (LLDPEs), which are a series of polymers in which ethylene is copolymerised with a small proportion of other alkylethylenes. This gives controlled branching without sacrificing Mr or toughness.

Polypropylene

Polypropylene (PP) is made by a similar ionic reaction to HDPE and LLDPE, using organometallic catalysts. Unlike the ethylene polymers it exists in stereoisomeric forms due to the asymmetry of the methyl substituted carbon atom. The common industrial material is the isotactic form which can be stretched to give highly crystalline films and fibres. It is less tough than the LDPE but this may be improved by copolymerisation with a small amount of ethylene.

The major application of the polyolefins is in packaging (films and bottles). HDPE is aesthetically as well as technically an alternative to paper since it has the 'crinkly' feel of paper but unlike paper it has high resistance to water and hence microorganisms. PP is also widely used as a hydrophobic outer wrapping in the packaging of many commodities (e.g. cigarettes) where it has largely replaced cellophane, and in bottles as a replacement for glass.

There is little doubt that the emergence of the polyolefins in the mid-years of the present century transformed food distribution because of their effectiveness as sterile barriers against microbial contaminants. These advantages over glass, coupled with higher impact resistance and lower cost, were immediately obvious to the consumer but more recently it has also been recognised that the use of plastics in the packaging of foodstuffs is much less energy intensive than the use of traditional packaging materials (Chapter 2).

An important use of polyethylene films is in 'shrink-wrap' packaging in which the films are given a 'built-in' stress which is released on heating with contraction of the film over goods to be packaged. This is a convenient and economic alternative to traditional 'boxing' in cardboard. A rather similar packaging application is 'stretch-wrap' polyethylene film for hay and silage. This mainly black wrapping film has replaced hay stacks in the countryside and is also a major source of plastics pollution in the countryside (Chapter 5).

Another important use for the crystalline polyolefins is in industrial and, to a lesser extent, domestic fibres. Polypropylene fibres have also largely replaced the naturally derived sisal in agriculture. This fibrous material physically resembles sisal but unlike the natural product it does not readily biodegrade during use or after discard. Agricultural twines are often made very cheaply by slitting uniaxial stretched polypropylene sheets in the direction of the applied stress. The non-biodegradability of discarded PP twines has been overcome in recent years by making the polymer photo-biodegradable (Chapter 5).