Natural rubber structure bulk polymer

The existence of pure amorphous bulk polymers has been a controversial issue since the beginning of polymer science. Natural rubber yields an X-ray pattern tiiat contains only amorphous halos, typical of any liquid. Nevertheless, it was difficult for many scientists to believe that molecules with a polymeric chain structure could pack in a truly amorphous way. There are still papers that are submitted for publication that assert that amorphous rubbery polymers are actually composed primarily of microcrystalline domains. This issue has been clarified by the incisive theoretical and experimental work of Flory. It is now understood that there are polymers that exhibit liquid crystalline phases upon melting of the crystals. The nature of the noncrystalline state of pure bulk polymers depends on tiie detailed local structure of the chain and the ratio of die persistence lengtii of die chain to the diameter of the mer. Molecules that are conformationally dexible enough to have a small persistence length can exist in the amorphous liquid state. 

Many bulk polymers that are crystallized from a melt are semicrystalline and form a spherulite structure. As implied by the name, each spherulite may grow to be roughly spherical in shape one of them, as foimd in natural rubber, is shown in the transmission... 

Compared with natural rubber SBR compounds are lacking in green strength. Recent developments using labile crosslinking systems to reduce this deficiency are described. The general vulcanisation behaviour of SBR is reviewed and the network structures of the vulcanisates are compared with those of natural rubber. Whilst the chapter is primarily concerned with emulsion SBRs, which constitute the bulk of the market, the relative merits and demerits of the newer solution polymers are considered. 

This issue of specific surface area hints at how one might change the nature of reinforcement. In typical micro- and macrocomposites, the properties are dictated by the bulk properties of both the matrix and the flUer. This relationship between the properties of the composite and the properties of the filler is what leads to the stiffening and degraded elongation mentioned earlier. In the case of nanocomposites, the properties of the material are instead tied to the interface. Terms like bound polymer, bound rubber, and interphase have been used to describe the polymer at or near the interface, where significant deviations from bulk structure and properties are known to occur (Fig. 6.2). 

Our present theories of rubber elasticity are mostly of the kinetic theory type. In some of these theories the elasticity of a single chain is multiplied by the number of effective chains in the network to provide the total elasticity. In these theories no information concerning the statistical properties of the network structure can be inferred since either no structural aspects are evaluated or a simple structural nature is assumed. These structuril properties should be of great interest in furthering our understanding of polymers in bulk. Thus the strain dependence of X-ray scattering measurements from networks, which have heavy atoms at the cross-links and/or the chain ends, should provide some correlation between the structural and elastic properties of polymer networks. 

In other theories of rubber elasticity, the network structure is explicitly considered. However, the polymer on the surface is taken to be fixed (according to an affine deformation) upon deformation. - A truly statistical mechanical theory would also treat the surface statistically. More fundame ntally, however, in these theories the fixed point character of the surface i hen completely determines the behavior of the bulk material. This would appear to be nonsense in the thermodynamic limit of infinite volume, unless the fixed surface were of finite extent. In this case, the theory is no longer statistical in nature. 

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