Polymers in the rubber state

To understand the behaviour of these materials, it is helpful to refer again to the modulus-temperature diagram for polymeric materials. Fig. 1.4, which shows the dependence of Young s Modulus, E, on temperature. 

As we have seen, most non-crystalline polymers can obtain a rubbery state, as shown in Fig. 1.4. At low temperatures, the polymer is glassy, with a high modulus (10 NM ). As the temperature is raised the polymer passes through the glass transition region (Tg), where it becomes visco elastic, with modulus very rate and temperature dependent. Beyond Tg, the polymer becomes rubbery, but not all polymers in the rubbery state show useful elastomeric properties most thermoplastics will flow irreversibly on loading - i.e. they are more visco than elastic. 

Some polymers, however, exhibit typically elastic, or rubbery, behaviour in the rubbery region these are the elastomers. They include conventional natural and synthetic rubbers, polyurethane elastomers, thermoplastic rubbers and plasticized PVC. Elastomers are characterized by highly elastic properties they can be strained, often to several hundred per cent, and 

In the case of semi-crystalline polymers, such as polypropylene, the rubbery state above Tg entails tough, horny properties, which derive from the crystalline structure. In highly crystalline polymers, such as acetal, this effect is further enhanced the region above Tg (-80°C for acetal) is far from rubbery, and the crystalline structure imparts hard, impact-resistant properties. 

An interesting application of this will be found in Chapter 3, where the plasticizer is used to control the Tg of PVC used in an injection-moulded product. 

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Deformation of a polymer in the rubber state—of importance in vacuum forming, pressure forming and warm forging techniques. 

POLYMER SCIENCE ASPECTS POLYMERS IN THE RUBBER STATE... 

We use variants of profile extrusion to produce tubing -with diameters of less then 1 mm and pipes with diameters exceeding 1 m, Wall thicknesses can vary from a few tens of micrometers up to several centimeters. Extruded window and door frames are more complex than pipes. Such profiles are largely hollow with internal ribs and fins that reinforce and divide the interior into two or more channels. We use solid rubber profiles in applications such as door seals and windshield wipers. We can produce foamed extrudates by incorporating a blowing agent, such as butane or carbon dioxide, into the polymer in the molten state. As the polymer exits the die, its internal pressure drops and the dissolved gas expands to form bubbles within the product. Examples of foamed extrudates include pipe insulation and automobile door gaskets. 

As already stated, mastication reactions are not limited to elastomers but can be extended to all polymers in the viscoelastic state. It is thus interesting to note that before the fundamental study of Watson and coworkers on cold rubber... 

While in the temperature range called the rubbery plateau, the soft polymer responds instantaneously and reversibly to applied stress and tends to be Hookean. In the rubber state, the polymer approaches Hooke s law for... 

While both solution and solid-state NMR has been routinely applied to polymers for many years, there have been a few recent applications of HRMAS to polymer systems, analyzing polymerization mechanisms and characterizing the resulting polymers in the swollen state. The vulcanization of butadiene rubber by cyclic disulfides was shown to follow two different mechanisms with two different classes of sulfur compounds - cross-linking progressed... 

In 4.3 we have already seen that polymers, in the rubber or fluid condition, crystallize much more rapidly when their chains are oriented. Therefore a stretched rubber, if stereospecific in its molecular structure, is able to crystallize at a temperature considerably above its equilibrium thermodynamic melting point. Also a thermoplast such as polyethylene, when in the molten state or in solution, can crystallize spontaneously when the chains are being orientated in elongational flow. The latter case is utilized when polyethylene is spun from a diluted solution (gel spinning process), resulting in fibres of super-high strength and stiffness ( Dyneema fibres). 

Another vivid example of the exceptional role of network topology is the unexpectedly high deformation abUity of hypercrosslinked polystyrenes under loading, which is usuaUy characteristic of conventional slightly cross-linked networks or linear polymers in the rubber elasticity state. Hypercrosslinked polymers, however, differ from the latter in that they retain their mobUity even at very low temperatures. In fact, hypercrosslinked materials do not exhibit typical features of polymeric glasses, nor are they typical elastomers. Their physical state thus cannot be described in terms of generaUy accepted notions. More likely, the hypercrosslinked networks demonstrate distinctly different, unique deformation and relaxation properties. 

These techniques rely upon high shear to cause bond scissions. Ruptured bonds result in formations of free-radical and ionic species. When this application of shear is carried out in the presence of monomers, block copolymers can form. This approach is exploited fairly extensively. Such cleavages of macromolecules can take place during cold mastication, milling, and extrusion of the polymers in the viscoelastic state. Both homolytic and heterolytic scissions are possible. The first yields free-radical and the second ionic species. Heterolytic scissions require more energy but should not be written off as completely unlikely." Early work was done with natural rubber. It swells when exposed to many monomers and forms a viscoelastic mass. When this swollen mass is subjected to shear and mechanical scission, the resultant radicals initiate polymerizations. The mastication reaction was shown to be accompanied by formation of homopolymers. Later, the technique was applied to many different polymers with many different monomers. ... 

In the following, we expect an Arrhenius-like temperature behavior for highly filled rubbers that is typically fotmd for polymers in the glassy state. Therefore, we measure—far above the polymer bulk glass transition temperature—the modulus G for small deformation amplitudes (0.2% in our case). This is depicted schematically in Fig. 36.10. One obtains a straight line of slope E /R by plotting log G (T) (or in the tensile mode log E (T)) vs. 1/T well above the bulk... 

A thermoplastic vulcanizate (TPV), represented by the lower right quadrant of Fig. 4.38, is a TPE produced by dynamic vulcanization, the process vulcanizing a vulcani-zable elastomer during its intimate mixing with a thermoplastic polymer in the molten state. A TPV comprises finely divided particles of highly cross-linked rubber in a continuous matrix of rigid thermoplastic. 

Finite strain elasticity the behaviour of polymers in the rubber-like state... 

FIG. 14-2. Dependence of diffusion coefficient of n-hexadecane on the degree of cross-linking in three polymers in the rubberiike state at 25°C. (PB) 1,4-Polybutadiene, cis trans vinyl = 40 53 7 (NR) natural rubber (pips denote four different initiai molecular weights before cross-linking, from 2.3 to 7.7 X 10 ) (SBR) styrene-butadiene random copolymer, 23.5% styrene. Abscissa is moles effective network strands per cubic centimeter estimated from swelling measurements. (Chen. )... 

Figure 1.1 describes the general regions of viscoelastic behavior for amorphous polymers where mechanochemistry may be conducted. The tensile and shear moduli for crystalline and amorphous polymers in the solid state are generally in the range of 10 dyn/cm. For the rubbery state, the value is about 10 dyn/cm, and it varies with the density of entanglements and chemical cross-links. The modulus can be calculated from the theory of rubber elasticity, and the short-time viscoelastic properties in the rubbery state are not unlike those of a common rubber band. 

As already stated, mastication reactions are not limited to elastomers but can be extended to all polymers in the viscoelastic state. It is thus interesting to note that before the fundamental study of Watson and co-workers on cold rubber mastication, Reid [110] had already found that when vinyl polymers and monomers are subjected to mechanical deformation, both degradation and polymerization occur simultaneously. If, however, the polymer is too soluble in the monomer, this method cannot be applied since the rubbery state, necessary to have effective shear, cannot be achieved. 

It is descriptive here to quote from Aklonis and McKnight, (1983). It is impossible to describe quantitatively the time ranges that give each type of behavior, since the temperature variable causes all these ranges to be relative. Accordingly. .. a plastic (a polymer in the glassy state) would have a modulus of a rubber on a time scale of perhaps a thousand years while a rubber might behave like a plastic on a nanosecond time scale. ... 

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