Chain length, elastomeric networks

With regard to elastomers of controlled structure, those having unusual distributions of network chain lengths have been of particular interest [88,89]. The most novel elastomer of this type consists of a binary combination of unusually short network chains (molecular weights of a few hundred) and the much longer chains typically associated with elastomeric behavior (molecular weights of ten or twenty thousand). Such a network is sketched in Figure 6. 

The tenet of classical rubber theory has been that the chains are in random networks and the networks comprise a Gaussian distribution of end-to-end chain lengths. However, the mechanisms and molecular bases for the elasticity of proteins are more complex than that of natural rubber. In biological systems elastomeric proteins consist of domains with blocks of repeated sequences that imply the formation of regular stmctures and domains where covalent or noncovalent cross-linking occurs. Although characterised elastomeric proteins differ considerably in their precise amino acid sequences they all contain elastomeric domains comprised of repeated sequences. It has also been suggested that several of these proteins contain p-tums as a structural motif (Tatham and Shewry 2000). 

Falender, J. R. Yeh, G. S. Y. Mark, J. E., The Effect of Chain Length Distribution on Elastomeric Properties. 2. Comparisons Among Networks of Varying Degrees of Randomness. Macromolecules 1979,12,1207-1209. 

Mark, J. E., Elastomeric Networks with Bimodal Chain-Length Distributions. Acc. Chem. Res. 1994,27, 271-278. 

Sharaf, M. A. Mark, J. E. Al-Ghazal, A. A.-R., Elastomeric Properties of Poly(dimethylsiloxane) Networks Having High-Functionality Crosslinks and Bimodal Chain-Length Distributions. J.Appl. Polym. Sci. Symp. 1994, 55, 139-152. 

Erman, B. Mark, J. E., Calculations on Trimodal Elastomeric Networks. Effects of Chain Length and Composition on Ultimate Properties. Macromol-ectdes 1998,31, 3099-3103. 

The theory of rubber elasticity (Section 9.7) assumes a monodisperse distribution of chain lengths. Earher, the weakest link theory of elastomer rupture postulated that a typical elastomeric network with a broad distribution of chain lengths would have the shortest chains break first, the cause of failure. This was attributed to the limited extensibility presumably associated with such chains, causing breakage at relatively small deformations. The flaw in the weakest link theory involves the implicit assumption that all parts of the network deform affinely (24), whereas chain deformation is markedly nonaffine see Section 9.10.6. Also, it is commonly observed that stress-strain experiments are nearly reversible right up to the point of rupture. 

Mark JE. Elastomeric networks with bimodal chain-length distributions. Acc Chem Res 1994 27 271-8. 

Sharaf MA, Mark JE, Al-Ghazal AA-R. Elastomeric properties of poly(dimethylsiloxane) networks having high-functionality crosslinks and bimodal chain-length distributions. J Appl Polym Sci Symp 1994 55 139-52. 

Erman B, Mark JE. Calculations on trimodal elastomeric networks. Effects of chain length and composition on ultimate properties. Macromolecules 1998 31 3099-103. 

Typical silicone elastomeric commercial products are complex, multimodal networks with a variety of chemically distinct side groups, crosslink sites, physical entanglements, free chain ends, end-to-end chain lengths, and filler phases that all influence/control the overall network properties (see multiple entries in this volume and Figure 11.1). 

Mark, J. E. Tang, M.-Y., Dependence of the Elastomeric Properties of Bimodal Networks on the Length and Amount of the Short Chains. J. Polym. Set,... 

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