Model Natural rubber

The chemistry of the accelerated vulcanization of BR, SBR, and EPDM appears to have much in common with the vulcanization of natural rubber. Before the formation of cross-links, the rubber is first sulfurated by accelerator-derived polysulfldes (Ac-S -Ac) to give macromolecular, polysulfidic intermediates (rubber-Sx-Ac), which then form crosslinks (rubber-S -rubber). As in the case of natural rubber, the average length of a crosslink (its sulfidic rank, the value of x in the cross-link, rubber-Sx-rubber) increases with the ratio of sulfur concentration to accelerator concentration (S/Ac) used in the compounded rubber mix. However, in the case of BR or SBR, the cross-link sulfidic rank is not nearly as sensitive to S/Ac as it is in the case of natural rubber. Model compound studies of the vulcanization of EPDM (e.g., wherein ethylidenenorbomane was used as a model for EPDM) indicate that the polysulfidic rank of the EPDM cross-links probably responds to changes in S/Ac in a natural rubber-hke fashion. 

Antioxidants may be assessed in a variety of ways. For screening and for fundamental studies the induction period and rate of oxidation of petroleum fractions with and without antioxidants present provide useful model systems. Since the effect of oxidation differs from polymer to polymer it is important to evaluate the efficacy of the antioxidant with respect to some property seriously affected by oxidation. Thus for polyethylene it is common to study changes in flow properties and in power factor in polypropylene, flow properties and tendency to embrittlement in natural rubber vulcanisates, changes in tensile strength and tear strength. 

The results of searches of these two chemical classes matched by polymer are presented in Table II. Three candidate polymers (natural rubber, neoprene, and nitrile) were found with 15 models giving acceptable test data. 

For a moderately crossllnked network, equation (13) predicts a declining stress with lamellae formation from the amorphous melt. A stress Increase can be achieved with this model only by reorientation of the chain axis to the directions perpendicular (or nearly so) to the stress direction. If then this model is suitable for lightly crystalline materials, its behavior is in good accord with the observations of Luch and Yeh (6) on stretched natural rubber networks. They reported simultaneous lamellae formation and declining network stress. 

Meade, B.J. andWoolhiser, M., Murine models for natural rubber latex allergy assessment, Methods, 27, 63, 2002. 

The cDNAs of the cA-prenyltransferase of H. brasiliensis was successfully identified and expressed in E. coli. The in vitro polymerization of IPP after initiation with FPP using the expressed c/x-prenyltransferase resulted in low degrees of polymerization [267, 268]. After addition of rubber particles to this polymerization, the molecular weight increased tremendously [269], It can be concluded that the rubber particles are essential for rubber biosynthesis. Katarina Cornish established a detailed structural model of the in vivo synthesis of natural rubber in the rubber particle monolayer membrane and partially explained this behavior (see Fig. 12) [251],... 

If material is neo-Hookean, its Mooney-Rivlin plot ought to give a horizontal line and hence yield C2 = 0. Thus one is tempted to consider that nonzero C2 must be associated in one way or another with the deviation of a given material from the idealized network model, and it is understandable why so many rubber scientists have concerned themselves with evaluating the C2 term from the Mooney-Rivlin plot of uniaxial extension data. However, the point is that a linear Mooney-Rivlin plot, if found experimentally, does not always warrant that its intercept and slope may be equated to 2(9879/,) and 2(91V/9/2), respectively. This fact is illustrated below with actual data on natural rubber (NR) and styrene-butadiene copolymer rubber (SBR). 

The new absorptions in the spectra of crosslinked rubber are assigned on the basis of 13C solution NMR chemical shifts for a variety of model compounds, such as pentenes and mono-, di- and tri-sulfidic compounds, by using the 13C chemical shift substituent effect. From the calculated values for particular structural units, the experimental spectra of a sulfur vulcanized natural rubber 194,195,106), natural rubber cured by accelerated sulfur vulcanization 197 y-irradiation crosslinked natural rubber198 and peroxide crosslinked natural rubber and cis-polybutadiene 193 1991 are assigned. 

It is theorized that between the complex network structure of the unaccelerated system and the simpler network structure of the accelerated system, structures made up of the two models represent natural-rubber vnlcani7ares made at various times and temperatures of cures, with different reactant concentrations, and showing the effects of other variants. 

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... 

Natural rubber oligomers with (i-pinene terminal units have been synthesized by Gutierrez and Tlenkopatchev via metathesis depolymerization [151]. The bulk degradation of natural rubber in the presence of (i-pinene, as chain-transfer agent, and C4 (0.1 mol% related to polymers with Mn between 490 and 1,700 kDa) produced monoterpene-terminated oligomers in yields between 80% and 90%. The results were further confirmed with model CM reactions using (i-pinene and (Z)-3-methyl-2-pentene. 

The inner sides of the walls are coated with vulcanised natural rubber or P.V.C. In an electrolyzer of the described size the bottom measures 660 sq.dm and is not coated. It is electrically connected to the main cathode bus-bar, located on the insulators under the cell. Older models had walls and the bottom made of concrete and the current was led in by iron disks, inserted into... 

The above interpretations of the Mullins effect of stress softening ignore the important results of Haarwood et al. [73, 74], who showed that a plot of stress in second extension vs ratio between strain and pre-strain of natural rubber filled with a variety of carbon blacks yields a single master curve [60, 73]. This demonstrates that stress softening is related to hydrodynamic strain amplification due to the presence of the filler. Based on this observation a micro-mechanical model of stress softening has been developed by referring to hydrodynamic reinforcement of the rubber matrix by rigid filler... 

Fig. 12a,b. Fitting functions of storage and loss moduli of carbon black filled natural rubber according the L-N-B-model (from [90])... 

With the notable exceptions of natural rubber and gutta-percha, almost all naturally occurring polymers are optically active. Historically, interest in optically active synthetic polymers has focused on modeling natural polymers, interpreting the conformational properties of macromolecules in solution, and investi-... 

According to this mechanism, natural rubber chains are expected to have one dimethylallyl terminal unit and one isoprenyl pyrophosphate terminal unit the latter may give rise to a hydroxyl group by hydrolysis. From this point of view, acyclic terpenes in the generalized structure (II) may be appropriate models for the structural characterization of natural polyisoprenes by 13C NMR spectroscopy. 

Graham, who was one of the first to consider the permeabilities of natural rubber films to a wide range of gases, found responses such as that seen in Fig. 2a. The description he formulated in 1866 of the so-called "solution-diffusion" mechanism still prevails today (30). He postulated that a penetrant leaves the external phase in contact with the membrane by dissolving in the upstream face of the film and then undergoes molecular diffusion to the downstream face where it evaporates into the external phase again. Mathematically, one can state the solution-diffusion model in terms of permeability, solubility and diffusivity coefficients, as shown in Eq(2). 

The basic form of the model of P D used in this work, which will be described elsewhere in detail, [22] treats the diffusion of small amounts of "simple" spherical penetrants, such as gas molecules, in "smooth-chained" polymers, such as poly (ethylene terephthalate) (PET) and cis-polyisoprene (natural rubber). [12] Whenever necessary, generalized equations are being used, for example for simple nonspherical penetrants [13] and for polymers which possess closely spaced, bulky side groups such as poly(vinyl chloride) (PVC). [14]... 

The theory proposed for equilibrium swelling and diffusion is based on the assun tlon that the hydrophilic impurities are present in particulate form and are dispersed throughout the rubber. The precise nature of this impurity in natural rubber is not known so it was decided to make a model rubber by adding 0.17. of a hydrophilic Impurity (sodium chloride) to a solution polymerised synthetic rubber (cis-polyisoprene) Ich is chemically the same as natural rubber. Using this model rubber it is possible to check the theory more precisely since both the nature and concentration of the hydrophilic lgq>urlty in the model rubber are known. It is proposed that the water diffuses through the rubber and forms droplets of solution inside the rubber where there are particles of the hydrophilic impurity thereby causing a non-uniform distribution of water in the rubber. The... 

A model rubber was made from cis-polyisoprene, a synthetic rubber which is chemically similar to natural rubber, to which had been added a small amount of hydrophilic impurity. This was 0.17. of sodium chloride in most of the experiments but 1% of an animal protein (bovine albumen) was also used for one set of experiments and this gave similar results to those obtained when sodium chloride was used, demonstrating that the phenomenon is not a feature of one type of impurity only. Since the cis-polyisoprene used was solution polymerized it was relatively free from hydrophilic impurities before mixing. The desired amount of sodium chloride was dissolved in water to form a concentrated solution. This solution was added to the rubber on a heated mill, the water then evaporated producing a fine dispersion of... 

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