So far, the discussion has been confined to polymers with only a single type of repeat unit, but in reality, a large and growing number of commercial polymers are actually composed of different types of unit attached together by chemical covalent bonds. They are known as copolymers, and can comprise just two different units (binary copolymers) or three (ternary), and so on. It is one of the common strategies used by molecular engineers to manipulate the properties of polymers to gain just the right combination of properties for a specific application.
One of the best known examples involves polystyrene. In its homopolymer form, it is a rigid, transparent thermoplastic which is also very brittle. It thus finds little application for stressed applications in its original state. It also shows a glass transition temperature of about 97 °C, so is useless for containers which could hold boiling water (like very hot coffee). The glass transition temperature (Tg) is the temperature at which an amorphous thermoplastic becomes flexible and rubbery (Box 4). This problem can be solved by copolymerizing styrene with acrylonitrile to produce SAN polymer, where the styrene and acrylonitrile units alternate along the backbone chain of the material (Figure 22). SAN is commonly used for transparent drinks containers since the acrylonitrile units raise the Tg to about 107 °C.
The problem of brittleness can be solved in a quite different way. Paradoxically, if rubber (polybutadiene) chains are grafted onto the main backbone polystyrene chain, the graft copolymer so formed (Figure 22) is much tougher owing to molecular segregation of the rubber chains into tiny particles. Although they reduce the stiffness of the copolymer compared with the parent PS, the particles act as nuclei for minute crazes. Such crazes are so plentiful when the solid is stressed, that a great deal of energy is absorbed and so the bulk material appears ductile and tough. The material is HIPS or high-impact polystyrene. The benefits of both high Tg and toughness are achieved with ABS, a terpolymer of the three component repeat units, with butadiene present to about 25 weight % (Figure 22). The strategy of adding rubber particles to toughen a brittle polymer is known as rubber-toughening.
Because the different repeat units added to the original polymer are always covalently bonded, they are in effect locked into the structure, so in theory at least, the composition is infinitely variable. This is quite unlike metal alloys or mixed glasses, where the composition is only possible between certain, fixed limits. Thus mild steel is an alloy with 0.1–0.4% carbon and small deviations above or below cause large changes in properties. So what happens if the second rubbery component in a copolymer is increased? Not surprisingly, the properties change from those of a plastic to that of a reinforced rubber. In fact, copolymerization of butadiene and styrene was employed at a very early stage in the development of synthetic rubber during the last World War particularly. It was found that the stiffness of polybutadiene rubber could be improved by copolymerization with about 24 weight% styrene, to give a random copolymer of the two units, known as SBR (Figure 22).
Box 4 Thermal transitions in polymers
The two most important thermal transitions exhibited by polymers are the glass transition temperature, Tg and the crystalline melting temperature, Tm. The glass point is the temperature at which amorphous polymer becomes elastomeric and flexible as the temperature is raised. The crystalline melting point is that point when the crystalline component loses coherence and long-range order. Since most polymers are rarely completely crystalline owing to chain entanglements, most crystalline polymers will show both a Tg and a Tm. This is illustrated below by the thermogram for polyethylene terephthalate) or PET (Figure 23), where the Tg is shown at about 84 °C by the inflection in the curve. The material used was sampled from a soft drinks bottle.
The large dip at about 248 °C represents the melting point. The thermogram was obtained using a technique known as differential scanning calorimetry or DSC for short. It is a very accurate way of evaluating the thermal properties of polymers, the apparatus needing only a few milligrams of the polymer for analysis. The temperature of the sample is raised in a controlled and regular way (horizontal scale of Figure 23). The device measures heat flow into or from the sample, as shown by the vertical scale in Figure 23.
What is the mole fraction of styrene in SBR rubber? (The mole fraction is based on the relative number of each unit present, rather than weight:
weight fraction = w1/(w1 + w2)
and, n = w/M R
where n is the number of moles, and w the weight of any component.)
For 100 g of SBR, there will be a mass of 24 g of styrene units and 75 g of butadiene present in the copolymer. Hence the number of moles of each will be:
nS = 25/ = 25/104 = 0.24
nB = 75/ = 75/54 = 1.39
Thus the mole fraction of styrene is:
0.24/(0.24 + 1.39) = 0.24/1.63 = 0.147 or 14.7%
Alternatively, it was found in the 1960s that another way of putting the units together was possible. If, rather than using a mixture of monomers, they were added sequentially, then a block copolymer resulted (Figure 24). The properties are different again to those of the random copolymer because each type of chain segregates together to form minute domains, as shown by the microstructure of Figure 24. Such materials retain thermoplastic behaviour yet behave as crosslinked rubbers, and the SBS block copolymer was the first commercial thermoplastic elastomer or TPE. Although most polymer chains are incompatible with one another, there are some exceptions to the rule. One in particular has gained commercial success, and is a blend of polystyrene and poly(phenylene oxide) (PPO). The Tg is increased as is the toughness of the resulting physical mixture of different chains, and the polymer mixture is known by the trade name Noryl. It is used widely for enclosure of consumer products.
Self assessment question 3
uPVC for window frames is too brittle to be acceptable in a product destined to last more than 50 years without damage. The plastic currently used for such products is a copolymer where poly (ethyl aery late) rubber of repeat unit structure
is grafted onto PVC backbone chains. What will be the microstructure of the material, why should it be a more acceptable material for window frames, and what composition should the copolymer possess?
The structure of a grafted polymer would comprise domains of the rubber phase embedded in a matrix of PVC. The domains would be very small, and toughen the plastic matrix by creating tiny crazes when or if stressed. It would be tough and resist the formation of brittle cracks. This is an essential requirement for a window frame, which must sit in a building for 50 years or more. The worst stresses might occur through faulty fitment, distortion in the opening from ground movement or settlement. They might be expected to be worst at corners in the frame, where there are sharp changes in shape. Such corners are weak points in the structure owing to the need for welding of the material here. The optimum composition would be about 25 wt% rubber to give the best toughening effect.