Rubber elastomeric networks

Linear Elastic and Rubber Elastic Behavior. Although stiffening is quite noticeable in the glassy regime of the amorphous phase, the most spectacular effect is seen in the rubber elastic regime phase, as already evoked in the case of reinforcement by cellulose whiskers (2). The PA6-clay hybrids example presented in Table 3 is quite representative of the situation encoimtered with semi crystalline thermoplastics, but elastomeric networks benefit as well of clay layer dispersion with a two- to threefold increase in modulus for polyurethane or epoxy networks... 

The Theory of Kuhn and Grun. The theory of birefringence of deformed elastomeric networks was developed by Kuhn and Griin and by Treloar on the basis of the same procedure as that used for the development of the classical theories of rubber-like elasticity (48,49). The pioneering theory of Kuhn and Griin is based on the affine network model that is, upon the application of a macroscopic deformation the components of the end-to-end vector for each network chain are assumed to change in the same ratio as that of the corresponding dimensions of the macroscopic sample. 

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. 

Recently, new models were proposed which can indeed be used to characterize the structure of real elastomeric networks in view of their mechanical properties 6, 7 However several parameters are necessary to describe the relationship between molecular and macroscopic deformations and therefore stress-strain measurements are generally not sufficient to conclude without any ambiguity on the validity of these elaborated theories- Another possible test consists in measuring molecular orientation in stretched rubbery networks- With this in view, the photoelastic properties of rubbers have been widely investigated However birefringence data... 

Vulcanisation of the covers is done in steam autoclaves, where the rubber polymer is crossHnked to the elastomeric network The cover is finished by mechanical tooling, drilling and grinding to the required geometrical dimensions. 

B. Innovative Elastomeric Networks Based on Functionalized Ethylene-Propylene Rubbers and Hydroxyl Terminated Polybutadiene... 

New elastomeric networks based on saturated ethylene-propylene rubbers grafted with succinic anhydride groups (EPR-g-SA) crosslinked with a hydroxyl-terminated polybutadiene (HTPB) are hereafter described. Infrared techniques are employed to follow the kinetics of the monoesterification reaction and to assess its potential thermoreversibility, either on the macromolecular system (EPR-g-SA-I-HTPB), or on a model system, formed by EPR-g-SA and a low molecular weight diol, namely 1,9-nonandiol. 

One of the simplest ways to introduce the crosslinks required for rubber-like elasticity is to carry out a copolymerization in which one of the comonomers has a functionality of three or higher [9,21]. This method, however, has been used primarily to prepare materials so heavily crosslinked that they are in the category of hard thermosets rather than elastomeric networks, as it has already been mentioned [11]. The more common techniques include vulcanization (addition of sulfur atoms to unsaturated sites), peroxide thermolysis (covalent bonding through free-radical generation), end linking of functionally terminated chains (isocyanates to hydroxyl-terminated polyethers, organosilicates to hydroxyl-terminated polysiloxanes, and silanes to vinyl-terminated polysiloxanes) [18],... 

Mark JE. Bimodal elastomeric networks. In Mark JE, Lai J, editors. Elastomers and rubber elasticity. Washington, DC American Chemical Society 1982. p. 349-66. 

The mechanical properties of elastomeric networks are described within the theory of rubber elasticity, which accounts for the behavior of a network—in fact, its elastic modulus—as a function of its molecular parameters (number of elastic... 

As illustrated in Figure 2, elastomeric networks consist of chains joined by multifunctional junctions. As early as 1934, it was suggested by Guth and Mark and by Kuhn that the elastic retractive force exhibited by rubber upon deformation arises from the entropy decrease associated with the diminished number of conformations available to deformed polymer chains. It is, therefore, of primary interest to study the statistics of a polymer chain and to establish the elastic equation of state for a single chain. 

It is somewhat difficult conceptually to explain the recoverable high elasticity of these materials in terms of flexible polymer chains cross-linked into an open network structure as commonly envisaged for conventionally vulcanised rubbers. It is probably better to consider the deformation behaviour on a macro, rather than molecular, scale. One such model would envisage a three-dimensional mesh of polypropylene with elastomeric domains embedded within. On application of a stress both the open network of the hard phase and the elastomeric domains will be capable of deformation. On release of the stress, the cross-linked rubbery domains will try to recover their original shape and hence result in recovery from deformation of the blended object. 

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