5.5.1 Non-uniform mixtures Moulded rubbers and plastics are compounds of a polymer matrix and a variety of additives. The mixing history of the material before and during the moulding process can have a critical influence upon the final product properties. If mixing is done badly then the microstructure of the moulding can be non-uniform. Lack of uniformity can cause variations of strength and other physical properties within the moulding. The degree of dispersion or distribution of relatively minor quantities of addit
5.5 Orientation in polymers Viscoelasticity, like thermodynamics, is concerned with the correlation of controllable variables and bulk, macroscopic phenomena. But one unique feature of polymeric materials is that the molecular unit, the polymer chain, can be highly anisotropic, i.e. the chain can be fully extended, or curled up in an amorphous equilibrium state without any net orientation. In fact, unoriented polymer is rarely encountered in manufactured products because of the different ways it is processed to shape. B
5.4 Dynamic mechanical properties Viscoelasticity is not experienced just under quasi-static conditions, i.e. when the imposed stresses and strains are constant or change only slowly. Polymers, and particularly rubbers, are often deliberately selected for products which are to be subjected to dynamic mechanical loading. Tyres are an obvious example where the unique high strain elasticity and energy absorbing qualities of rubbers make them the natural choice of material. Stress analysis involves the use of the frequency-depend
5.3.2 Effects of structure on viscoelasticity If a single measurement of ER(t) is taken at an arbitrary but fixed interval of time, say 10 seconds, then it will vary with temperature in a way rather similar to the viscoelastic master curve. Such a curve for atactic polystyrene is shown in Figure 48, where the various zones of behaviour are identified. The effect of lightly crosslinking the material is to eliminate flow of any kind, extending
5.3.1 Time-temperature superposition For amorphous polymers above their Tgs, there is a convenient approximation which makes experiments easier. It is known as time-temperature superposition, and it relates time to temperature for viscoelastic materials. A sequence of measurements of ER
(t) is performed at different temperatures at a fixed initial strain. The time scale might be limited between several seconds and say 100 hours. The curves obtained on uncrosslinked polyisobutylen
5.3 Viscoelasticity and master curves An immediate consequence of the viscoelasticity of polymers is that their deformations under stress are time dependent. If the imposed mechanical stress is held constant then the resultant strain will increase with time, i.e. the polymer creeps. If a constant deformation is imposed then the induced stress will relax with time (stress relaxation). Figure 46 shows the creep strain response to a constant stress followed
5.2.2 Viscous behaviour Viscous flow is not recoverable. When the stress is removed from a viscous fluid the strain remains. Hence the work energy is not returned to the forcing agency and has to be otherwise dissipated. Figure 45 illustrates this schematically by showing the strain response in such a viscous material when a simple stress history has been imposed upon it. 5.2.1 Elastic and viscoelastic behaviour When an elastic (not elastomeric, or long range elastic) material is stressed, there is an immediate and corresponding strain response. Figure 43 illustrates this by showing schematically the strain response to a particular stress history. Note that when the stress is removed the strain also returns to zero. So in a perfectly elastic material all the deformation is returned to the forcing agency. If this energy had 5.2 Viscoelasticity of polymers The simplest models for the deformation behaviour of an ideal material are those of Hookean linear elasticity in the solid state, and Newtonian linear viscosity in the liquid state. The end point of elastic deformation is either fracture or plastic flow, with the latter taking place at a constant yield stress in the ideal case. Whilst the behaviour of many real materials does approximate to these idealised models, that of polymers deviates markedly from them. In particular, their solid state 5.1 The behaviour of polymers The manufacture of polymer products is controlled by two often conflicting demands: the quality of the finished article in terms of its response to its environment and the ease or difficulty of processing it to shape. Both factors are controlled by what is termed viscoelasticity, namely, the behaviour of the polymer in response to applied stress or strain, and temperature. It is important to appreciate the duality in terms of the elastic and viscous responses of polymer solids and poly 4.6.2 Material costs in manufacturing For high added-value products like boats and cars, material costs form a relatively small proportion of total costs. For directly manufactured products, however, which are sold without much assembly or finishing, material costs do form a relatively large proportion of the total production cost. This applies particularly to polymeric containers for foods and drinks but not, for example, to containers for more sophisticated products like electronic or electrical goods. What is much more importa 4.6.1 Prices of polymers Prices of bulk and speciality polymers (Table 9) broadly reflect the degree of chemical processing and treatment needed to make them. Thus the polyolefins, which are directly polymerized from cracker streams, are generally the cheapest followed by vinyl derivatives of ethylene like PS and PVC. Derived polymers which require more complex treatment, such as ABS, PET and polyester thermosets are generally more expensive by f 4.5.1 The copolymer equation It can be shown that the rate of change of monomer concentration in any copolymerization is given by the equation where [M1] and [M2] are the concentrations of monomers 1 and 2 at any instant and r1
and r2., are reactivity ratios. The reactivity ratios represent the rate at which one type of gr 4.5 Copolymerization The alloying of metals to improve their properties is widespread and although many polymers used today are relatively pure (e.g. polystyrene, nylon), an increasing number are mixtures of two or more polymers. As with metals, one reason for doing this is to increase the range of properties. The major practical problem, however, is that homopolymers blend together with difficulty and even where blends are possible, as in some thermoplastics, phase separation can occur readily. This proble 4.4 Step growth polymerization 4.3.5 Co-ordination polymerization While most free radical and ionic polymerizations are carried out homogeneously, there is another important class of reaction which is often performed with solid catalysts. These reactions, discovered in the mid-fifties, have revolutionized polymer manufacture by permitting much less severe polymerization conditions than with other systems and by allowing a greater degree of control of polymer structure. Ziegler-Natta catalysts, as they are called, will convert vinyl and diene monomers 4.3.4 Ionic polymerization Free radicals are indiscriminate in the compounds they attack, and their non-selective nature in polymerization reactions leads to problems such as chain branching and transfer which affect the structure of the polymer produced. Anionic polymerization overcomes many of these problems. A typical commercial (but also see Box 8) anionic reaction is the polymerization of styrene using butyllithium, C4H9 4.3.3 Termination and transfer There are basically three ways in which chains terminate. The first is known as coupling and occurs when two free radicals join together. This can be represented by the general equation Such a mechanism significantly increases molecular mass, if it results in two polymer chains joining. This is the main mechanism which terminates the po 4.3.2 Propagation Once a small number of chains have been started, propagation involves successive addition of monomer units to achieve chain growth. At each step the free radical is regenerated as it reacts with the double bond. So in the case of styrene the propagation step is
The free radical can also add on in a different way to produce
4.3.1 Initiation Initiation is the mechanism which starts the polymerization process. Vinyl monomers are quite easily polymerized by a variety of activating methods. Styrene, for example, can be converted to solid polymer simply by heating, and ultraviolet light can have exactly the same effect. Usually, however, an activating agent is used. This is an unstable chemical which produces active species that attack the monomer. A good example is benzoyl peroxide which splits up when heated:
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