Natural rubber structure macroscopic structures

From a theoretical point of view, the equilibrium modulus very probably gives the best characterization of a cured rubber. This is due to the relationship between this macroscopic quantity and the molecular structure of the network. Therefore, the determination of the equilibrium modulus has been the subject of many investigations (e.g. 1-9). For just a few specific rubbers, the determination of the equilibrium modulus is relatively easy. The best example is provided by polydimethylsiloxane vulcanizates, which exhibit practically no prolonged relaxations (8, 9). However, the networks of most synthetic rubbers, including natural rubber, usually show very persistent relaxations which impede a close approach to the equilibrium condition (1-8). 

The greater stability of the crystalline state of networks formed from unoriented but crystalline chains compared with networks formed from amorphous polymers, can be explained in the same way as for networks formed from axially oriented natural rubber. Although prior to network formation the crystallites are randomly arranged relative to one another, portions of chains are still constrained to lie in parallel array. The cross-linking of the predominantly crystalline polymer cannot, therefore, involve the random selection of pairs of units. The units that can be paired are limited by the local chain orientation imposed by the crystalline structure. An increase in the isotropic melting temperature of such networks would therefore be expected. It can be concluded that orientation on a macroscopic scale is not required for partial order in the liquid state to develop. Concomitantly a decrease in the entropy of fusion will result, which reflects the increase in molecular order in the melt. This is an important concept that must be kept in mind when studying the properties of networks formed in this manner. This conclusion has important implications in studying the properties of networks formed from unoriented crystalline polymers. 

The statistical theory is remarkable in that it enables the macroscopic deformation behaviour of a rubber to be predicted from considerations of how the molecular structure responds to an applied strain. However, it is important to realize that it is only an approximation to the actual behaviour and has significant limitations. Perhaps the most obvious problem is with the assumption that end-to-end distances of the chains can be described by the Gaussian distribution. This problem has been highlighted earlier in connection with solution properties (Section 3.1) where it was shown that the distribution cannot be applied when the chains become extended. It can be overcome to a certain extent with the use of more sophisticated distribution functions, but the use of such functions is beyond the scope of this present discussion. Another problem concerns the value of N. This will be governed by the number of junction points in the polymer network which can be either chemical (cross-links) or physical (entanglements) in nature. The structure of the chain network in an elastomer has been discussed earlier (Section 4.4.5). There will be chain ends and loops which do not contribute to the strength of the network, but if their presence is ignored it follows that if all network chains are anchored at two cross-links then the density, p, of the polymer can be expressed as... 

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