Several important terms and concepts must be understood in order to discuss fully the synthesis, characterisation, structure and properties of polymers
Apolymeris a substance composed of molecules which have long sequences of one or more species of atoms or group of atoms linked to each other by primary, usually covalent bonds. The emphasis upon substance in this definition is to highlight that although the words polymer and macromolecule are used interchangeably, the latter strictly defines the molecules of which the former is composed.
Macromolecules are formed by linking together monomer molecules through chemical reactions, the process by which this is achieved being known as polymerisation. For example, polymerisation of ethylene creates polyethylene, a typical sample of which may contain molecules with 50,000 carbon atoms linked together in a chain. It is this long chain nature which sets polymers apart from other materials and gives rise to their characterisationtic properties.
The definition of macromolecules presented up to this point implies that they have a linear skeletal structure which may be presented by a chain with two ends. Whilst this is true for many macromolecues, there are also many with non-linear skeletal structures:
Branched polymers have side chains, or branches, of significant length which are bonded to the main chain at branch points (also known as junction points ), and are characterised in terms of the number and size of the branches. Network polymers have three dimentional structures in which each chain is connected to all others by sequences of junction points and other chains. Such polymers are said to be cross linked and are characterised by their crosslink density, or degree of crosslinking, which is related directly to the number of junction points per unit volume.
Non linear polymers may be formed by polymerisation, or can be prepared by linking together pre-existing chains. Variations in skeletal structure give rise to major differences in Properties.
The formal definition of a hompolymer is a polyemer derived from one species of monomer. However, the word homopolymer often is used more broadly to describe polymers whose structure can be represented by multiple repetitions of a single type of repeat unit which may contain one ore more species of monomer unit. The latter is sometime referred to as a structural unit.
The chemical structure of a polymer usually is represented by that of the repeat unit enclosed by brackets. Thus the hypothetical homopolymer where n is the number of repeat units linked together to form the macromolecule.
Basic naming nomenclature places the prefix 'poly' before the name of the monomer, the monomer's name being contained within the parentheses unless it is a simple single word. In structure based nomenclature the prefix poly is followed in parentheses by word which describe the chemical structure of the repeat unit.
A copolymer is a polymer derived from more than one species of monomer. However, in accordance with use of the word homopolymer, it is common practise to use a structure based definition. Thus the word copolymer is more commonly used to describe polymers whose molecules contain two or more different types of repeat unit.
There are several catagories of copolymer, each being characterised by a particular form of arrangement of repeat units along the polymer chain. For simplicity, the representation of these categeories will be illistrated by copolymers containing only two different types of repeat unit
Statistical copolymers are copolymers in which the sequential distribution of the repeat units obeys known statistical laws (e.g)Markovian). Random copolymers are a special type of statistical copolymer in which the distribution of repeat units is truly random eg:
Alternating copolymers have only two different types of repeat unit and these are arranged alternately along the polymer chain eg:
Statistical, random and alternating copolymers generally have properties which are intermediate to those of the corresponding homopolymers. Thus by preparing such copolymers it is possible to combine the desirable properties of the homopolymers into a single material. This is not normally possible by blending because many homopolymers are immiscible with each other.
Block copolymers are linear copolymers in which the repeat units exist only in a long sequence, or blocks, of the same type. eg:
Graft copolymers are branched polymers in which the branches have a different chemical structure to that of the main chain. In there simplest form they consist of a main homopolymer chain with branches of a different homopolymer eg:
In distinct contrast to the types of copolymers described earlier, block and graft copolymers usually show property characteristics of each of the constituent homopolymer. They also have some unique properties arising from the chemical linkage(s) between the homopolymer sequences preventing them from acting entirely independently of each other.
The current principle of nomenclature for copolymers are indicated in the table below where A and B represent source or structure based names for these repeat units. Thus a statistical copolymer of ethylene and propyleneis named poly(ethylene-stat-propylene), in certain cases, additional square brackets are required. for example an alternating copolymer of styrene and maliec anhydride is named poly[styrene-alt-(maleic anhydride)].
|Type of copolymer||Example of nomenclature|
The most common way of classifying polymers is
where they are first split into three groups, thermoplastic, elastomersand thermosets. Thermoplastics are then further seperated into those which are crystalline and those which are amorphous. This method of classification has an advantage in comparison to others since it is based essentially upon the underlying molecular structure of the polymers.
Thermoplastics, often referred to just as plastics, are linear or branched polymers which can be melted upon the application of heat. They can be moulded (and remoulded) into virtually any shape using processing techniques such as injection moulding and extrusion, and now constitute by far the largest proportion of the polymers used in industry. Generally, thermoplastics do not crystallise easily upon cooling to the solid state because this requires considerable ordering of the highly coiled and entangled macromolecules present in the liquid state. Those which do crystallize invariably do not form perfectly crystalline materials but instead are semi-crystalline with both crystalline and amorphous regions. The crystalline phases of such polymers are characterised by their melting temperature (Tm). Many thermoplastics are, however, completely amorphous and incapable of crystallisation, even upon annealing. Amorphous polymers (and amorphous phase of semi-crystalline polymers) are characterised by their glass transition temperature (Tg), the temperature at which they transform abruptly from the glassy state (hard) to the rubbery state (soft). This transition corresponds to the onset of chain motion; below Tg the polymer chains are unable to move and are 'frozen' in position. Both Tm and Tg increase with increaseing chain stiffness and increasing forces of intermoleculer attraction.
Elastomers are crosslinked rubbery polymers (ie rubbery networks) that can be stretched easily to high extensions (eg 3 - 10 times their original extension) and which rapidly recover their original dimensions when the applied stress is released. This extremely important and useful property is a reflection of their molecular structure in which the network is of low crosslink density. The rubbery polymer chains become extended upon deformation but are prevented from permanent flow by the crosslinks, and driven by entropy, spring back to their original positions on removal of stress. The word rubber, often used in place of elastomer, preferably should be used for describing rubbery polymers which are not crosslinked.
Thermosets normally are rigid materials and are network polymers in which chain motion is greatly restricted by a high degree of crosslinking. As for elastomers, they are intractable once formed and degrade rather than melt upon the application of heat.
Trevor Clowes document last modified 23 May 2003 University of sheffield, England