Elasticity of Rubber - Assignment Example

Elasti of Rubber Introduction and background A natural rubber is attained by means of coagulating latex from a type of tree known as Hevea Brasiliensis. It is predominantly composed of cis-1, 4-polyisoprene. Solidified natural rubber discerned in 1924 in Germany dates back to about fifty million years ago. Columbus found out for the duration of his second expedition to America concerning a game that was played by the Haiti nationals, in which balls of elastic tree-resins were used. The word rubber is derived from the ability of this material to remove (rub off) marks from paper, which was investigated by Priestley Joseph in the year 1770. Rubber substances are not confined to natural rubber, however. They take account of a great variety of synthetic polymers of comparable properties. An elastomer is considered as a polymer that shows evidence of rubber elastic properties, i.e. materials that can be possibly stretched to a number of times its original length without breaking and which, upon release of the stress, instantaneously returns to their original lengths. A rubber is more or less an elastic material, since its deformation is instantaneous and it further shows almost no slither (Bjork, 1988). The distinctive nature of rubbers was ascertained by John Gough in 1305, and described his findings and experiments as shown in this subsequent paragraph. A person has to clasp one end of the slip of a rubber between the thumb and forefinger of each hand; get the central point of the piece into slight contact with the lips; further lengthen the slip swiftly; and you will instantly make out a feeling of warmth in that section of the mouth that is in contact with stretched rubber. For this resin evidently grows warmer the further it is lengthened; and the edges of the lips possess a higher degree of sensibility, which facilitates them to discover these changes with greater facility than other parts of the body. The augment in temperatures, which is recognized in the event of extending any pieces of Caoutchouc, may be obliterated in on the spot, by allowing the slip to contract again; which it will do quickly by desirable quality of its own spring, as soft as the stretching forces cease to act as soon as it has been fully exerted. Gough made these comments regarding his second experiment: In any case one end of a slip of Caoutchouc is fastened to a rod of wood or metal, and some weight is fixed (added) at the other extremity/end; the thong will be seen to become longer with cold and shorter with heat (Mark, 1984). To make certain this concept, it is necessary for a self experimentation. One will only need a strip of rubber, a weight and a hair-dryer. Gough presented no better explanations to the unexpected findings, such as that the expected stiffness increments with rising temperatures and that heat is progressive throughout stretching duration. It took approximately fifty years prior to the formulation of thermodynamics of the rubber elasticity. Rubbers exhibit predominantly entropy-driven elasticity through measurements of force and specimen length at varied temperature levels. Thermo-elastic effects of rubber shows that stretched rubber samples which are subjected to constant uniaxial load contract reversibly on heating, and the same sample can give out heat reversibly if stretched. These two observable concepts are true and consistent with the view that the entropy of the rubber decreased on stretching. Molecular picture of the entropic forces is dated back to the theoretical work of 1930’s, when it was suggested that the covalently bonded polymer chains had been oriented during extension (Gumbrell, Rivlin, &, Mullins, 1953, p. 1495). Methods and procedures This study involved an investigation in the thermo-elastic behaviors and thermodynamics, with regards to the energetic and entropic elastic forces. At minimum strains, characteristically less than ? = L L 0 < 1.1 (where L and L0 are the dimensions of the unstressed and stressed specimens, respectively), the stress at constant strains diminish with respect to increasing temperatures, whereas at ? values greater than 1.1, the stress increases with increasing temperatures. This adjusts from negative through to positive temperatures coefficient is known as thermo elastic inversion. However, Joule observed this effect much earlier in 1859. The reasons for the negative coefficients at minimum sprains are the upbeat thermal growth and that the curvatures are achieved at invariable lengths. An increase in temperature causes thermal expansion, which is a rise in L0 and also a corresponding length extension in the perpendicular directions, and as a result a reduction in the accurate ? at invariable L. These effects would not be noted in case L0 was measured at every temperature change and in case the curves were taken at constant ?, linking to L0 at the actual experimental temperatures. Positive temperature coefficients are representative of entropy-driven elasticity. The reversible temperatures rise that transpires whenever any rubber bands are deformed can be detected by an individual’s lips, for example. It is basically due to the fact that the interior energy continues to be reasonably unaffected on deformation, i.e. dQ=-dW (when dE=0). In case work is carried out on the systems, it is then likely that heat will be produced hence leading to the rise in temperatures. The temperature increase under conditions of adiabatic can be substantial. Natural rubbers elongated to ?=5 attains some temperature changes, which is 2-5 K higher than that before any deformations. Conversely, whenever external forces are withdrawn and the specimens allowed to return to their initial and unstrained states, equivalent temperature drop is experienced (Anthony, Caston, & Guth, 1942, p.826). Results and analysis It is fundamental to split the elastic forces into entropic and energetic involvements. Stresses acting on the rubber networks have the impacts of stretching out and orienting the chains existing between the crosslink connections. This will resultantly reduce the entropy of the chains and hence giving rise to entropic forces. The alterations in chain conformations are expected to change the intermolecular internal energy. The packing of the chains may also have impacts influences that may consequently affect the intermolecular-related internal energies. Both the inter- and intra-molecular potentials contribute to the energetic forces. The subsequent thermodynamic managements give rise to expressions distinguishing between the energetic and entropic contributions to the resilient forces. According to the first and second laws of thermodynamics, the internal energy transformation (dE) in a uniaxially strained system exchanging heat (dQ) and deformation and pressure volume work (dW) reversibly is given by: dE = T dS - p dV + f dL Where dS is the differential change in entropy, p dV represents Pressure Volume Work and, f dL represents Work done by the stretch From this point, it is evidenced that it is apposite to note that the applied forces are perceptibly vectors (designated f) although in this action is regarded as a scalar (represented by f; being the absolute values of the vectors). The Gibbs liberated force (G) is given by: G = E + pV – TS = H-TS Any complicated quandaries to eliminate the predominant effects of volume alterations on the interior energy can be solved easily through solutions provided by Lippmann, Gee and Elliot; who demonstrated that it is much possible to derive the changes in internal energy at constant volumes from stress strain measurements which are attained at steady pressure levels. The internal energy contributions to the elastic forces at constant volume are on the minimum at ? < 2.7. Mark (1984) and Treloar (1975) have gathered fe f statistics, where fe is determined as the force elements involving the changes in inner energy at unvarying volume and f being the total force for natural rubber: fe f =0.18±0.03 (? ?2). Mark, in his explanations, bring to a closes from the already gathered data that fe f is cannot be influenced by dilutions, such as swelling of the network polymer in low molar mass solvents. Thus, fe f is controlled by the intramolecular energetic, i.e. the energy differences between dissimilar conformational circumstances of both negative and positive values of fe f (Mark, 1993). Discussion and conclusions Polyethylene shows a negative fe f value (-0.42). During stretching of cross linked or molten polyethylene noticeably large entropy forces build up and the internal energy at determined and constant volumes reduces due to the fact that a number of gauche conformers are transferred into trans-states. The energetic force must then be negative in such a case. Other polymers such as natural rubber and poly (dimethyl siloxane) exhibit positive fe f values, i.e. the extended conformation are of higher energy than the unstrained structure (Gaylord, & Higgs, 1990, p.90). The low-energy conformation of poly (dimethyl siloxane) is all-trans, but this gives the chain a non-extended circular form due to the difference in bond angles for O-Si-O and Si-O-Si. {(fe/ f)v=constant} =T{[d(1n(r^2)v0)/dT]/dT} The temperature coefficient of the measurement of the tranquil polymer molecules, {d(1n(r^2)v0)/dT} found from stress-strain statistics for a assortment of crosslinked polymers is in accordance with estimates obtained from viscometry of the chain dimensions in theta solvents at dissimilar temperature levels. Polyethylene confirms negative fe f and {d(1n(r^2)v0)/dT}. The trans-content in polyethylene becomes increasingly lower with increasing temperatures and hence the sizes of the random coils reduce with increasing temperatures. Works Cited Anthony, P. C., Caston, R. H. & Guth, E. J. Phys. Chem.1942, 46, 826. Bjork, F. Ph.D. Thesis: Dynamic stress relaxations of rubber substances, Polymers Technology Department, Royal Institute of Technology, Stockholm, Sweden, 1988. Gaylord, R.J. & Higgs, P.G. Polymer, 1990, 31, 70. Gumbrell, S. M., Rivlin, R. S. &, Mullins, L. Trans. Faraday Soc. 1953, 49, 1495 Mark, J.E. Rubber elastic states, in Physical Property of Polymers, Mark, J. E. (ed.), American Chemical Society, Washington, DC, 1984 Mark, J.E. The rubber elastic state, in Physical Properties of Polymers, 1993 Appendix I Polymer Diluent v2^a ( fe f )^b Polyethylene Polyethylene Polyethylene None n-C30H62 n-C32H64 1.00 0.50 0.30 -0.42 -0.64 -0.50 Natural rubber Natural rubber Natural rubber None n-C16H34 Decalin 1.00 0.34-0.98 0.20 0.17 0.18 0.14 Poly(dimethylsiloxane) Trans(1, 4 polyisoprene) Trans(1, 4 polyisoprene) None None decalin 1.00 1.00 0.18 0.25 -0.10 -0.20 Read More