Natural rubber deformation

Originally, vulcanization implied heating natural rubber with sulfur, but the term is now also employed for curing polymers. When sulfur is employed, sulfide and disulfide cross-links form between polymer chains. This provides sufficient rigidity to prevent plastic flow. Plastic flow is a process in which coiled polymers slip past each other under an external deforming force when the force is released, the polymer chains do not completely return to their original positions. 

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. 

Natural rubber is known to be more elastic (deformable) than gutta-percha. Is there any obvious difference in the structures in the two strands which might lead to a difference in the properties of the real polymers ... 

Can you provide a qualitative explanation of why natural rubber becomes more and more hard (less deformable) as the percentage of incorporated sulfur is increased ... 

The standard polymers used for rubber linings consist of materials that are cross-linkable macromolecules which, on mixing with suitable reactants that form strong chemical bonds, change from a soft deformable substance into an elastic material. These polymers include natural rubber and its corresponding synthetic, c/s-polyisoprene, styrene-butadiene rubber, polychloroprene, butyl rubber, halogenated butyl rubbers, acrylonitrile-... 

The elasticity of a polymer is its ability to return to its original shape after being stretched. Natural rubber has low elasticity and is easily softened by hearing. Flowever, the vulcanization of rubber increases its elasticity. In vulcanization, rubber is heated with sulfur. The sulfur atoms form cross-links between the poly-isoprene chains and produce a three-dimensional network of atoms (Fig. 19.17). Because the chains are covalently linked together, vulcanized rubber does not soften as much as natural rubber when the temperature is raised. Vulcanized rubber is also much more resistant to deformation when stretched, because the cross-... 

NONAFFINE DEFORMATION OF ELONGATED NATURAL RUBBER 21.3.1 Brief Introduction... 

Id. The Ideal Rubber.—The data available at present as summarized above show convincingly that for natural rubber (dE/dL)T,v is equal to zero within experimental error up to extensions where crystalhzation sets in (see Sec. le). The experiments of Meyer and van der Wyk on rubber in shear indicate that this coefficient does not exceed a few percent of the stress even at very small deformations. This implies not only that the energy of intermolecular interaction (van der Waals interaction) is affected negligibly by deformation at constant volume—which is hardly surprising inasmuch as the average intermolecular distance must remain unchanged—but also that con-... 

Easily deformable gelatinous particles which comprise part of the yellow fraction of fresh natural rubber latex. Lutoids break down on the addition of ammonia and pass into the latex serum. 

The characteristic property of elastomers is their rubber-elastic behavior. Their softening temperature lies below room temperature. In the unvulcanized state, i.e. without crosslinking of the molecular chains, elastomers are plastic and thermo-formable, but in the vulcanized state—within a certain temperature range — they deform elastically. Vulcanization converts natural rubber into the elastic state. A large number of synthetic rubber types and elastomers are known and available on the market. They have a number of specially improved properties over crude rubber, some of them having substantially improved elasticity, heat, low-temperature, weathering and oxidation resistance, wear resistance, resistance to different chemicals, oils etc. 

For the shortness let us confine to the numerical analysis of the isothermal and adiabatic deformation of natural rubber, which at comparatively low chains cross-linking can be described as a melt. 

Temperature change at adiabatic deformation of natural rubber was calculated accordingly to eq. (43) which under assumption cv = cv°N, where cv° = cp° is molar heat of the isoprene carries to... 

Figure 2. Temperature increasing at adiabatic deformation of natural rubber at its stretching (y z > l) and...

Figure 3. Dependence of the Young s module on the multiplicity of linear deformation Az at stretching and squeezing of natural rubber along z axis. Calculation has been done in accordance with the eq. (78) (see the explanations in text). 

Becker 2S) measured 10-min isochronal values of dW/dl) for natural rubber vul-canizate the results are shown in Figs. 14 and 15 for dW/dli and dWjdI2, respectively. His data are almost similar in the form of dW/dli and dW/bI2 to those in Fig. 12 and 13, but less sensitive to deformation except in the region of very small deformation. 

Fig. 26 A. dW/bli measured by Becker for natural rubber vulcanizate at small deformations and 10 min. [Reproduced from Becker, C. W. J. Polymer Sci., Part C, 16, 2893 (1967), a part of Fig. 4 and a part of Fig. 5.]...

Rubber - An elastomer, generally implying natural rubber, but used loosely to mean any elastomer, vulcanized and unvulcanized. By definition, a material that is capable of recovering from large deformations quickly and forcibly. 

Gutta-percha, the trans 1,4-isomer of natural rubber, is hard and brittle at room temperature. The reason for the difference in properties between the cis and trans isomers readily can be seen by inspecting molecular models. The chains with trans double bonds are able to lie along side of each other, forming a semicrystalline array, as shown in Figure 13-2. This ordered arrangement cannot be deformed easily, hence the material is hard and brittle. However, when the double bonds are cis, steric hindrance prevents the chains from assuming a similar ordered structure and the bulk of the material exists in a... 

Fig. 1 a,b. Strain amplitude dependence of the complex dynamic modulus E E l i E" in the uniaxial compression mode for natural rubber samples filled with 50 phr carbon black of different grades a storage modulus E b loss modulus E". The N numbers denote various commercial blacks, EB denotes non-commercial experimental blacks. The different blacks vary in specific surface and structure. The strain sweeps were performed with a dynamical testing device EPLEXOR at temperature T = 25 °C, frequency f = 1 Hz, and static pre-deformation of -10 %. The x-axis is the double strain amplitude 2eo... 

Production of mechano-radical in Nylons by either larger deformations or fractures were extensively investigated by DeVries and his collaborators (17,18,53), Peterlin and his coworkers (54, 55) and Becht and Fischer (22). ESR studies on mechano-radicals of other pol3rmers, including polystyrene (49, 56, 57), pol)ndnylacetate (49,57—59) poly o-methylstyrene (11), polyisobutyrene (11), Natural Rubber (22), other elasttxners (60-62), polysaccharides (6ii), and cellulose (63, 64). 

An elastomer is a polymer that can be deformed to a great extent and still recover its original form when the deforming stress is removed. The term rubber was introduced by Joseph Priestley, who observed that such materials can be used to rub out pencil marks. Natural rubber is a polymer of isoprene (2-methylbutadiene). The isoprene molecule contains two double bonds of which polymerization removes only one natural rubber is therefore unsaturated, containing one double bond per isoprene unit. In polymeric isoprene, the geometry at each double bond... 

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