Entanglements in rubbers

Langley, N.R. and Polmanteer, K.E., Role of chain entanglements in rubber elasticity. Polym. Prep. Am. Chem. Soc. Div. Polym. Chem., 13(1), 235-240 (1972). 

Amorphous stereotactic polymers can crystallise, in which condition neighbouring chains are parallel. Because of the unavoidable chain entanglement in the amorphous state, only modest alignment of amorphous polymer chains is usually feasible, and moreover complete crystallisation is impossible under most circumstances, and thus many polymers are semi-crystalline. It is this feature, semicrystallinity, which distinguished polymers most sharply from other kinds of materials. Crystallisation can be from solution or from the melt, to form spherulites, or alternatively (as in a rubber or in high-strength fibres) it can be induced by mechanical means. This last is another crucial difference between polymers and other materials. Unit cells in crystals are much smaller than polymer chain lengths, which leads to a unique structural feature which is further discussed below. 

The role of chain entangling in cross-linked elastomers is an old issue which has not yet been settled. The success of Flory s new rubber elasticity theory 0-5) in describing some of the departures from the simple Gaussian theory has acted as a strong catalyst for new work in this area. 

Unfortunately, the method is based on a fairly large nunber of assumptions. If we want to relate GN to the pseudo-equilibrium rubber plateau modulus, G , and to the effect of chain entangling in ordinary networks produced by cross-linking in the unstrained state, the following assumptions are required ... 

Langley,N.R., Polmanteer,K.E. The relation of elastic modulus to crosslink and entanglement concentrations in rubber networks. J. Polymer Sci. Polymer Phys. Ed. 12,1023-1034 (1974). 

Several approaches to the description of molecular entanglements in polymers are available at present. A brief outline will be given here. The best known is the version of the binary hook [9,10] with some network features. At temperatures (T) exceeding the temperature of glass transition (Tg) for the polymer, the network density V(,i, is usually determined in the framework of the rubber-like elasticity, while for Tentanglement network is proven both theoretically and... 

The factors inhibiting macro-flow in vulcanised rubber are the chemical cross links due to vulcanisation (and probably also molecular entanglements), considerably assisted by the phenomenon of crystallisation. In unvulcanised raw rubber macro-flow may occur to a certain extend. In cellulose crystalline junction points of a high degree of stability are responsible for the exclusion of macro-flow. In rubber the chains are very flexible and consist of a large number of statistical chain sections in cellulose the chains are stiffer and consist of a small number of chain elements. In the former case the intermolecular forces are weak, in the latter case they are strong. 

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