Rubbery

Solid, rubbery silicones likewise retain their plasticity at low temperatures and are resistant to many forms of chemical attack they are now incorporated in paints for resisting damp and for waterproofing. Silicones are also used in moulds to avoid sticking of the casting to the mould. 

When sulphur is melted viscosity changes occur as the temperature is raised. These changes are due to the formation of long-chain polymers (in very pure sulphur, chains containing about 100 (X)0 atoms may be formed). The polymeric nature of molten sulphur can be recognised if molten sulphur is poured in a thin stream into cold water, when a plastic rubbery mass known as plastic sulphur is obtained. This is only slightly soluble in carbon disulphide, but on standing it loses its plasticity and reverts to the soluble rhombic form. If certain substances, for example iodine or oxides of arsenic, are incorporated into the plastic sulphur, the rubbery character can be preserved. 

Rubbery materials are usually lightly cross-linked. Their properties depend on the mean distance between cross links and chain rigidity. Cross linking can be quantified by the use of functions derived from graph theory, such as the Rao or molar Hartmann functions. These can be incorporated into both group additivity and QSPR equations. 

They are used as high-temperature structural adhesives since they become rubbery rather than melt at about 300°C. 

At still longer times a more or less pronounced plateau is encountered. The value of the plateau modulus is on the order of 10 N m", comparable to the effect predicted for cross-linked elastomers in Sec. 3.4. This region is called the rubbery plateau and the sample appears elastic when observed in this time frame. 

At the transition between glassy and rubbery behavior, a distinct relaxation occurs. From one viewpoint, the molecules have enough time to jostle into more relaxed conformations from another, they have enough thermal energy to do so. 

In the rubbery plateau, a new impediment to movement must be overcome entanglements along the polymer chain. In discussing the effects of entanglements in Chap. 2, we compared them to crosslinks. Is it any surprise, then, that rubbery behavior similar to that shown by cross-linked elastomers characterizes this region ... 

At longer times an increase in compliance marks the relaxation of the glassy state to the rubbery state. Again, an increase of temperature through Tg would produce the same effect. 

The plateau compliance is characteristic of rubbery behavior where chain entanglements play the role of effective crosslinks. 

The upswing in compliance from the rubbery plateau marks the onset of viscous flow. In this final stage the slope of the lines (the broken lines in Fig. 3.12) is unity, which means that the compliance increases linearly with time. 

With T as the independent variable, the transition between glassy and rubbery behavior can be read directly at Tg. Note that Tg is about 100° lower for poly(methyl acrylate) than for poly(methyl methacrylate). 

The component with the lower viscosity tends to encapsulate the more viscous (or more elastic) component (207) during mixing, because this reduces the rate of energy dissipation. Thus the viscosities may be used to offset the effect of the proportions of the components to control which phase is continuous (2,209). Frequently, there is an intermediate situation where a cocontinuous or interpenetrating network of phases can be generated by careflil control of composition, microrheology, and processing conditions. Rubbery thermoplastic blends have been produced by this route (212). 

Chlorinated rubberis often used in combination with medium od drying type alkyds. The alkyd gives better toughness, flexibdity, adhesion, and durabdity, and the chlorinated mbber contributes to faster drying and better resistance to water and chemicals. The principal appHcations are highway traffic paint, concrete floor, and swimming pool paints. 

Many of the most floppy polymers have half-melted in this way at room temperature. The temperature at which this happens is called the glass temperature, Tq, for the polymer. Some polymers, which have no cross-links, melt completely at temperatures above T, becoming viscous liquids. Others, containing cross-links, become leathery (like PVC) or rubbery (as polystyrene butadiene does). Some typical values for Tg are polymethylmethacrylate (PMMA, or perspex), 100°C polystyrene (PS), 90°C polyethylene (low-density form), -20°C natural rubber, -40°C. To summarise, above Tc. the polymer is leathery, rubbery or molten below, it is a true solid with a modulus of at least 2GNm . This behaviour is shown in Fig. 6.2 which also shows how the stiffness of polymers increases as the covalent cross-link density increases, towards the value for diamond (which is simply a polymer with 100% of its bonds cross-linked. Fig. 4.7). Stiff polymers, then, are possible the stiffest now available have moduli comparable with that of aluminium. 

Polymers, too, creep - many of them do so at room temperature. As we said in Chapter 5, most common polymers are not crystalline, and have no well-defined melting point. For them, the important temperature is the glass temperature, Tq, at which the Van der Waals bonds solidify. Above this temperature, the polymer is in a leathery or rubbery state, and creeps rapidly under load. Below, it becomes hard (and... 

Now take another batch of sulphur flowers, but this time heat it well past its melting point. The liquid sulphur gets darker in colour and becomes more and more viscous. Just before the liquid becomes completely unpourable it is decanted into a dish of cold water, quenching it. When we test the properties of this quenched sulphur we find that we have produced a tough and rubbery substance. We have, in fact, produced an amorphous form of sulphur with radically altered properties. 

As the temperature is raised above T, one might expect that flow in the polymer should become easier and easier, until it becomes a rather sticky liquid. Linear polymers with fairly short chains (DP < 10 ) do just this. But polymers with longer chains (DP > 10 ) pass through a rubbery state. 

Fig. 23.7. A modulus diagram for PMMA. It shows the glassy regime, the gloss-rubber transition, the rubbery regime and the regime of viscous flow. The diagram is typical of linear-amorphous polymers.

When styrene and butadiene are polymerised, the result is a mixture of distinct molecules of polystyrene and of a rubbery copolymer of styrene and butadiene. On cooling, the rubbery copolymer precipitates out, much as CuAlj precipitated out of aluminium alloys, or FejC out of steels (Chapters 10 and 11). The resulting microstruc-... 

As the author pointed out in the first edition of this book, the likelihood of discovering new important general purpose materials was remote but special purpose materials could be expected to continue to be introduced. To date this prediction has proved correct and the 1960s saw the introduction of the polysulphones, the PPO-type materials, aromatic polyesters and polyamides, the ionomers and so on. In the 1970s the new plastics were even more specialised in their uses. On the other hand in the related fields of rubbers and fibres important new materials appeared, such as the aramid fibres and the various thermoplastic rubbers. Indeed the division between rubbers and plastics became more difficult to draw, with rubbery materials being handled on standard thermoplastics-processing equipment. 

Whether or not a polymer is rubbery or glass-like depends on the relative values of t and v. If t is much less than v, the orientation time, then in the time available little deformation occurs and the rubber behaves like a solid. This is the case in tests normally carried out with a material such as polystyrene at room temperature where the orientation time has a large value, much greater than the usual time scale of an experiment. On the other hand if t is much greater than there will be time for deformation and the material will be rubbery, as is normally the case with tests carried out on natural rubber at room temperature. It is, however, vital to note the dependence on the time scale of the experiment. Thus a material which shows rubbery behaviour in normal tensile tests could appear to be quite stiff if it were subjected to very high frequency vibrational stresses. 

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