Elastomer Behaviour

At temperatures above their melting points, semi-crystalline polymers resemble amorphous polymers. We find the same states as described for crosslinked amorphous materials, provided that there are covalent bonds between the chains. 

From the above, we know the conditions a polymer must satisfy, if it is to 

Furthermore, crosslinking removes any possibility of reusing the polymer, which will neither dissolve, nor flow. They are thus impossible to recycle. Another means of crosslinking, called physical crosslinking, was therefore developed to avoid some of these problems. The result is thermoplastic elastomers, which are more and more often found in automobile parts, sports shoes, and many other applications. 

Physical crosslinking can also be achieved by incorporating crystallisable blocks into the chain. An example from French industry is PEBAX. 

Pure elastic behaviour is characterised by the fact that the force-strain curve remains the same during loading and unloading. A return to zero deformation upon unloading is not sufficient. 

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ROSLANIEC z and RATUSZYNSKA H, Characteristics of ester-ether elastomer behaviour in contact with food , Polimery Tworzywa Wielkoczasteczkowe, 1990 35(11/12) 450-453. 

Figure 11.6 Modelling of elastomer behaviour contributing to the peeling of an onion ...

Figure Bl.19.35. Experimental nanoindentation eurves obtained with the AFM showing the loading and unloading behaviour of (a) an elastomer and highly oriented pyrolytie graphite and (b) a gold foil. (Taken from [183]. figure 4.)...

An eiegant theoreticai treatise of poiymer physics which conveys an intuitive understanding of the behaviour of macromoiecuies. Charrier J-M 1990 Polymeric Materials and Processing Plastics, Elastomers and Composites (Munich Hanser)... 

The influence of the gel content in polyisoprene-tackifier blends on creep resistance and peel behaviour have been recently studied [62]. The gel content was achieved by cross-linking the adhesives with electron beam irradiation. The molecular weight of the soluble fraction in the blend was always dominated by that of the initial elastomer. Creep resistance was achieved either through molecular weight increases or gel content increases. However, the peel strength is strongly... 

However, not all properties are improved by filler. One notable feature of the mechanical behaviour of filled elastomers is the phenomenon of stresssoftening. This manifests itself as a loss of stiffness when the composite material is stretched and then unloaded. In a regime of repeated loading and unloading, it is found that part of the second stress-strain curve falls below the original curve (see Figure 7.13). This is the direct opposite of what happens to metals, and the underlying reasons for it are not yet fully understood. 

Veenstra H., Hoogvfiet R.M., Norder B., De B., and Abe P. Microphase separation and rheology of a semicrystalUne poly(ether-ester) multiblock copolymer, J. Polym. Sci. B. Polym Phys., 36, 1795, 1998. Garbrieelse W., SoUman M., and Dijkstra K., Microstmcture and phase behaviour of block copolyfether ester) thermoplastic elastomers. Macromolecules, 34, 1685, 2001. 

Kumar G., Neelakantan N.R., and Subramanian N., Mechanical behaviour of polyacetal and thermoplastic polyurethane elastomer toughened polyacetal, Polym. Plastics TechnoL Eng., 32, 33, 1993. Newmann W. et al.. Preprints, 4th Rubber Technology Conference, London, May 22-25, 1962. Farrissey W.J. and Shah T.M., Handbook of Thermoplastic Elastomers (Walker B.M. and Rader C.P., eds.). Van Nostrand Reinhold, New York, 1988. 

Fakirov S, Fakirov C, Fischer EW, and Stamm M. Deformation behaviour of poly(ether ester) thermoplastic elastomers as revealed by SAXS. Polymer, 1991, 32, 1173-1180. 

See, J.L. Leblanc, Insight into elastomer—filler interactions and their role in the processing behaviour of mbber compounds, Prog. Rubber Plast. TechnoL, 10/2, 110-129, 1994, for a pictorial representation of such a morphology. 

Also termed glass temperature or Tg. The temperature at which the stiffness of an elastomer subjected to low temperatures changes most rapidly. If the glass temperature is close to the operational temperature the material will be leathery in its behaviour rather than rubber-like. Approximate glass transition temperatures for different polymers are NR -70 °C SBR -52 °C HR -75 °C PCP -40 °C and silicone rubber -85 °C. 

Thermoplastic elastomers are materials which exhibit elastomeric behaviour at room temperature, but which can be processed as thermoplastics. Before one can understand the performance of these materials an understanding of how they can give their unique properties of elasticity and thermoplasticity is required this is best done by considering the styrene-butadiene-styrene (SBS) thermoplastic elastomers. 

Before briefly discussing each type it is necessary to consider the performance of thermoplastic elastomers, and the problem of defining service temperature limits for them. The structural features that convey the ability to be processed as a thermoplastic are also a limiting factor in their use. Since it is the pseudocrosslinks that allow these materials to develop elastomeric behaviour, any factor which interferes with the integrity of the pseudocrosslinks will weaken the material, and allow excessive creep or stress relaxation to occur under the sustained application of stress and strain. Temperature is obviously one such factor. 

The thicker lines represent the sequences of hard urethane segments, and the clusters of these effectively act as crosslinks, making the material act as a conventional elastomer. When the temperature is raised high enough, the clusters disassociate and the material can be made to flow when subsequently cooled, the clusters can reform and the material again exhibits elastomeric properties. Thus these materials show elastomeric behaviour at room temperature, but can be processed as thermoplastics. Hence the name of the material class - thermoplastic elastomers. 

Flexible PVC is appreciated for the versatility of its characteristics according to the formulation. Significant quantities of fillers and plasticizers are used to optimize some of the characteristics such as behaviour at low temperatures, fire resistance, flexibility and hardness in the elastomer range, low price, electric insulation, easy welding and joining, possibility of transparency, food contact, fireproofing. 

Alloying with elastomers improves impact behaviour and processing. 

Summary of some general assessments concerning chemical behaviour of PPS at room temperature, which are not necessarily representative of PPS/elastomer alloys or of all grades of PPS. These general indications should be verified by consultation with the producer of the selected alloys and by tests under operating conditions. 

SBS, as well as all elastomers rich in double bonds, are not suited to exposure to light, UV or ozone, whereas SEES show good behaviour. 

TENSILE DEFORMATION BEHAVIOUR OF THE POLYMER PHASE OF FLEXIBLE POLYURETHANE FOAMS AND POLYURETHANE ELASTOMERS... 

Atomic force microscopy and attenuated total reflection infrared spectroscopy were used to study the changes occurring in the micromorphology of a single strut of flexible polyurethane foam. A mathematical model of the deformation and orientation in the rubbery phase, but which takes account of the harder domains, is presented which may be successfully used to predict the shapes of the stress-strain curves for solid polyurethane elastomers with different hard phase contents. It may also be used for low density polyethylene at different temperatures. Yield and rubber crosslink density are given as explanations of departure from ideal elastic behaviour. 17 refs. 

In recent years, the behaviour of liquid crystalline polymers including elastomers has been a subject of considerable interest 104,105). It is known that small molecule liquid crystals turn into a macroscopic ordered state by external electric or magnetic fields. A similar behaviour seems to occur for liquid-crystalline polymer networks under mechanical stress or strain. 

Elastomer blends consisting of two immiscible components are heterogeneous rubberlike materials both components of which are in the rubbery state. Such blends consist usually of either a matrix and a discrete phase or two interpenetrating continuous phases (interpenetrating networks). At homogeneous deformations of such blends, the contribution of either component to the thermomechanical behaviour of the material is determined by the content of the component and the individual characteristics of its chains. 

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