Rubber, natural structure

Gutta-percha. The name, derived from Malayan ge-tah pertcha=latex of the percha tree, for a natural rubber (structure, see there) from the gutta-percha trees Palaquium gutta and P. oblongifolia, Sapotaceae) with properties similar to those of balata. In Sumatra, Java, and south east India, the rapidly coagulating latex of incised trees is collected, rapidly kneaded, and marketed as raw G. Pure G. is the all-trans-isorntr of polyisoprene, related to balata molecular mass ca. 100000. In contrast to the cis-isomeric natural rubber, G. is hard and less elastic but not brittle, it softens at 25-30°C, becomes plastic at 60 °C, and melts at >100°C with decomposition and formation of a sticky mass. For uses, see literature. 

Oxygenated Functions. Oxygenated functions on carbon black surface were observed in the early 1950s [70] and completely characterized by H. P. Boehm in the 1960s [71]. At this time, interaction between carbon black and natural rubber was considered the consequence of chemical reactions between the carbon black surface s acidic groups and basic moieties present in the natural rubber structure [71a]. 

There are two possible structures for poly-l,4,polyisoprene. Natural rubber structure is of the cis form. The tram forms (the structure of guta perch or balata gum) have higher melting point and higher glass transition temperatures (see below). 

Furthermore, the C=C bonds in the natural rubber structure might induce poor thermal and oxidative resistance in the natural rubber blends. Thus, Thawornwisit and coworkersproposed the preparation of hydrogenated natural rubber, which is one of the chemical modifications available to improve the oxidation and thermal resistance of diene-based natural rubber before blending with poly(methyl methacrylate-co-styrene). The poly(methyl methacrylate-co-styrene) was resistant to the outdoor environment and had excellent optical properties with a high refractive index, but it was extremely brittle and had low impact strength. Hydrogenated natural rubber could, however, be used as an impact modifier, as well as to improve its thermal and oxidative resistance for these acrylic plastics. 

R 620 T. Kameda and T. Asakura, Study of Solid C NMR on Natural Rubber Structure and Mobility under Tension , Kobunshi Kako, 2004,53,102... 

A typical cyclized natural rubber structure containing a tricyclic and a bicyclic ring is shown below. 

A detailed discussion of the history structure and applications of natural rubber appears in the May 1990 issue of the Journal of Chemical Education... 

Polymers of chloroprene (structure [XII]) are called neoprene and copolymers of butadiene and styrene are called SBR, an acronym for styrene-butadiene rubber. Both are used for many of the same applications as natural rubber. Chloroprene displays the same assortment of possible isomers as isoprene the extra combinations afforded by copolymer composition and structure in SBR offsets the fact that structures [XIIll and [XIV] are identical for butadiene. 

Natural rubber is composed of polymerized isoprene units. When rubber is under tension, ozone attacks the carbon-carbon double bond, breaking the bond. The broken bond leaves adjacent C = C bonds under additional stress, eventually breaking and placing shll more stress on surrounding C = C bonds. This "domino" effect can be discerned from the structural formulas in Fig. 9-4. The number of cracks and the depth of the cracks in rubber under tension are related to ambient concentrations of ozone. 

In addition to plastics materials, many fibres, surface coatings and rubbers are also basically high polymers, whilst in nature itself there is an abundance of polymeric material. Proteins, cellulose, starch, lignin and natural rubber are high polymers. The detailed structures of these materials are complex and highly sophisticated in comparison the synthetic polymers produced by man are crude in the quality of their molecular architecture. 

If a rubbery polymer of regular structure (e.g. natural rubber) is stretched, the chain segments will be aligned and crystallisation is induced by orientation. This crystallisation causes a pronounced stiffening in natural rubber on extension. The crystalline structures are metastable and on retraction of the sample they disappear. 

Several other elastic materials may be made by copolymerising one of the above monomers with lesser amounts of one or more monomers. Notable amongst these are SBR, a copolymer of butadiene and styrene, and nitrile rubber (NBR), a copolymer of butadiene and acrylonitrile. The natural rubber molecule is structurally a c/i -1,4-polyisoprene so that it is convenient to consider natural rubber in this chapter. Some idea of the relative importance of these materials may be gauged from the data in Table 11.14. 

The close structural similarities between polychloroprene and the natural rubber molecule will be noted. However, whilst the methyl group activates the double bond in the polyisoprene molecule the chlorine atom has the opposite effect in polychloroprene. Thus the polymer is less liable to oxygen and ozone attack. At the same time the a-methylene groups are also deactivated so that accelerated sulphur vulcanisation is not a feasible proposition and alternative curing systems, often involving the pendant vinyl groups arising from 1,2-polymerisation modes, are necessary. 

It was found that the amount of chlorine that could be removed (84-87%) was in close agreement to that predicted by Flory on statistical grounds for structure Figure 12.10(a). It is of interest to note that similar statistical calculations are of relevance in the cyclisation of natural rubber and in the formation of the poly(vinyl acetals) and ketals from poly(vinyl alcohol). Since the classical work of Marvel it has been shown by diverse techniques that head-to-tail structures are almost invariably formed in addition polymerisations. 

Block copolymer chemistry and architecture is well described in polymer textbooks and monographs [40]. The block copolymers of PSA interest consist of anionically polymerized styrene-isoprene or styrene-butadiene diblocks usually terminating with a second styrene block to form an SIS or SBS triblock, or terminating at a central nucleus to form a radial or star polymer (SI) . Representative structures are shown in Fig. 5. For most PSA formulations the softer SIS is preferred over SBS. In many respects, SIS may be treated as a thermoplastic, thermoprocessible natural rubber with a somewhat higher modulus due to filler effect of the polystyrene fraction. Two longer reviews [41,42] of styrenic block copolymer PSAs have been published. 

Both side groups and carbon-carbon double bonds can be incorporated into the polymer structure to produce highly resilient rubbers. Two typical examples are polyisoprene and polychloroprene rubbers. On the other hand, the incorporation of polar side groups into the rubber structure imparts a dipolar nature which provides oil resistance to these rubbers. Oil resistance is not found in rubber containing only carbon and hydrogen atoms (e.g. natural rubber). Increasing the number of polar substituents in the rubber usually increases density, reduces gas permeability, increases oil resistance and gives poorer low-temperature properties. 

Pressure-sensitive adhesives (PSAs) based on acrylic, natural rubber and silicone are employed primarily for ease of application. To name Just a few applications, PSAs bond decals to surfaces, interior decorative surfaces to interior panels, interior trim pieces in place directly or hook and loop tape for the same purpose, structural shims in place during manufacturing and acoustic (sound deadening) materials to body skin interior surfaces. Tape products with pressure-sensitive adhesive on one or both surfaces are used for such functions as cargo compartment sealing, as a fluid barrier to prevent spills and leaks in the lavatories and... 

FIGURE 8.16 The structure of isoprene (2-methyl-l,3-butadiene) and the structure of head-to-tail and tail-to-tail linkages. Isoprene itself can be formed by distillation of natural rubber, a linear head-to-tail polymer of isoprene units. 

Examine structures of the different forms of rubber provided. Which is natural rubber and which is guttapercha How many monomers are in each strand ... 

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 ... 

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