For people with little or no knowledge of chemistry.
Structure of a molecule
All matter (solids, liquids and gases) is ultimately composed of atoms. By 'ultimately' we mean that an atom cannot be broken down into further smaller bits of the same kind. There are more than 118 different kinds of atoms, each kind being known as an element. However, atoms are usually joined together with other atoms which can be of the same or of a different kind. Atoms combined with other atoms are called molecules, the linkages between the atoms being called bonds. Molecules containing different kinds of atoms are called compounds. The number and kind of each of the atoms in the molecule, their relative positions and the nature of the bonds joining them, make a unique compound. These factors constitute what is called the structure of a molecule.
Chemistry is conveniently divided into different areas. The area dealing with compounds of the element carbon is known as organic chemistry. In the field of textile organic chemistry, compounds comprising atoms of carbon with those of hydrogen, oxygen, nitrogen or chlorine or combinations of these, are the most common. Each element is represented by an atomic symbol; in the case of the above five elements the symbols are: C, H, O, N and Cl respectively. The molecular structure of an organic compound can be displayed using these symbols joined up by short lines or sometimes a double line (rarely, a triple line) representing the bonds. For example, a molecule of water would be shown as H–O–H. It is interesting that these elements have specific capabilities of joining up — that means the number of bonds they can make to other atoms. In the case of our five elements C, H, O, N and Cl these numbers are 4, 1, 2, 3 and 1 respectively. An atom's combining number is called its valency and it is inherent in the nature of the atom.
Although these valencies restrict atoms' combining power, organic compounds are unimaginably varied, as are their properties. For example, a particular compound might decompose spontaneously at room temperature, whereas another is stable but will decompose when heated. Decomposition means breaking up the original structure to form smaller and different molecules. However, specific desired changes in the molecular structure of an organic compound can be brought about by processes called chemical reactions. For example, a chemical reaction can be of the form Compound A mixed with Compound B reacts to form Compounds C and D. Sometimes minor amounts of other substances known as catalysts are added to start or to speed up reactions.
Some molecules are very large and contain groups of atoms which are repeated over and over again being held together by strong chemical bonds. These are known as polymers and the process by which they are made is called polymerisation.
Categorisation of chemical substances
Chemical substances are often categorised based on the nature of their principal structural features. Thus, we have alcohols, esters, amides, carboxylic acids, etc. These kinds of terms are useful in describing a compound with such a group in it. It is the reason there are so many fibres called polyesters and polyamides, the prefix ‘poly’ meaning that many of the indicated groups (ester or amide) are repeated in the molecule.
Synthetic polymers
Many polymers are synthetic, e.g., polyamides, polyesters etc., made from petroleum for example, and many others occur naturally, e.g., proteins, carbohydrates etc. However, several important polymers are prepared by converting a naturally occurring polymer into a fibre suitable for making textiles. Such fibres are often called semi-synthetic. Thus, a major component of wood is cellulose; a naturally occurring carbohydrate polymer which can be treated to convert it into viscose — a semi-synthetic fibre that is still a carbohydrate polymer but now in fibrous form suitable for textile use. Cotton and flax are also forms of cellulose, but they are suitable for textiles without the kind of treatment needed to make viscose. All chemical reactions in nature take place at ambient temperature and pressure and are mediated by enzymes. Enzymes are proteins with specific structures which act as catalysts for the whole gamut of natural processes, one of them being polymerisation. By contrast, the polymerisation reactions used to manufacture semi-synthetic or fully synthetic fibres, need varied conditions, temperatures, pressures etc., according to the process, and often require catalysts which at the present time are never proteins.
The polymer called cellulose is composed of repeating groups having the structure of glucose, which is itself composed of six carbon atoms, twelve hydrogen atoms and six oxygen atoms. Glucose has a length of 1 nanometre (10-9m). In the fibre there are several hundred of these glucose units, largely strung in a line. Each glucose unit has three places on it which can react with other substances. In practice the substance with which cellulose is most commonly reacted is acetic anhydride, and the product is a variety of cellulose acetate. As there are several hundred (x3) places where a cellulose molecule can react with acetic anhydride, a vast number of cellulose acetate varieties will be formed. The principal factors affecting this reaction are the quantity of acetic anhydride used in relation to the amount of cellulose, how long the reaction is allowed to take place and the temperature. Note that the reaction products are always going to be mixtures with different amounts of acetate except when conditions are such that all the glucose groups in all of the cellulose molecules are fully reacted. In that case the product will be cellulose triacetate. Both the fully and the partially reacted varieties are suitable for commercial use, the partially reacted ones being called cellulose diacetate. In practice, for convenience and for historical reasons, cellulose acetate is the name given to this partially reacted product.
Summary
Summarising, polymers are large molecules, comprising repeated groups of atoms all being held together by strong chemical bonds. All natural, semi and fully synthetic fibres are polymers.