Rubber elasticity three-dimensional network

The elasticity of a polymer is its ability to return to its original shape after being stretched. Natural rubber has low elasticity and is easily softened by hearing. Flowever, the vulcanization of rubber increases its elasticity. In vulcanization, rubber is heated with sulfur. The sulfur atoms form cross-links between the poly-isoprene chains and produce a three-dimensional network of atoms (Fig. 19.17). Because the chains are covalently linked together, vulcanized rubber does not soften as much as natural rubber when the temperature is raised. Vulcanized rubber is also much more resistant to deformation when stretched, because the cross-... 

Rubber materials are soft, elastic solids, made of mobile, flexible polymer chains (with a glass transition temperature (Tg) typically lower than 0 °C) which are linked together to form a three-dimensional network. They are characterised by a low, frequency independent elastic modulus (of the order 105 to 106 Pa) and usually by a large maximum reversible deformation (up to a few hundred per cent). Rubber elasticity is based on the properties of crosslinked polymer chains at large spatial scales, the presence of crosslinks ensures the reversibility of the deformation, while at short scales, mobile polymer chains behave as molecular, entropic springs. 

A three-dimensional network polymer, such as vulcanized rubber, does not dissolve in any solvent. It may nevertheless absorb a large quantity of a suitable liquid with which it is placed in contact and undergo swelling. The swollen gel is essentially a solution of solvent in polymer, although unlike an ordinary polymer solution it is an elastic rather than a viscous one. 

In summary, polymeric materials exhibit rubber elasticity if they satisfy three requirements (a) the polymer must be composed of long-chain molecules, (b) the secondary bond forces between molecules must be weak, and (c) there must be some occasional interlocking of the molecules along the chain lengths to form three-dimensional networks. Should the interlocking arrangements be absent, then elastomers lack memory, or have a fading memory and are not capable of completely reversible elastic deformations. 

From the reaction of such polyiners with silicone crosslinkers, which need to have at least three reactive groups, three-dimensional networks with rubber-like elasticity are obtained. The chemical nature of the silicone crosslinker to be used depends on the crosslinking system, further details of which are given in the next Chapter. 

Vulcanization effectively fixes the ends of flexible polymer chains to form three-dimensional networks. By this way, the stress relaxation of stretched polymer chains can be avoided, and the high entropy elasticity of the rubber can be produced. Moreover, the fixed chain ends also increase both melting points and glass transition temperatures of short flexible chains. 

The monomers of both artificial and natural rubber must be polymerized into a three-dimensional network to obtain the finished product. Different processes are used, although they are all quite similar. The process of polymerization (called vulcanization) involves the reaction between rubber isomers and sulfur to produce a polymer with enhanced elasticity and reduced plasticity (Table 4). The reaction between the monomers and sulfur is enhanced by the addition of accelerators and activators. Other chemicals that can be added to both natural and synthetic rubber include retardants, anti-oxidants, curing agents, reinforcers, fillers, ultraviolet inhibitors, softeners/extenders, stabilizers, blowing agents and colorants. Readers are referred to Rubber World Magazine s Blue Book (1997), which extensively surveys the various additives to both natural and synthetic rubbers. Those additives... 

Rubbers and gels are three-dimensional networks composed of mutually cross-linked polymers. They behave like solids, but they still have high internal degrees of freedom that are free from constraints of external force the random coils connecting the cross-links are free in thermal Brownian motion. The characteristic elasticity of polymeric materials appears from the conformational entropy of these random coils. In this chapter, we study the structures and mechanical properties of rubbers on the basis of the statistical-mechanical models of polymer networks. 

The theory of rubber elasticity is largely based on thermodynamic considerations. It will be briefly discussed as an example of how thermodynamics can be applied in polymer science. Eor more detailed information the reader is referred to the various textbooks [10-13]. It is assumed that there is a three-dimensional network of chains, that the chain units are flexible and that individual chain segments rotate freely, that no volume change occurs upon deformation, and that the process is reversible (i.e., true elastic behavior). Another usual assumption is that the internal energy U of the system does not change with deformation. Eor this system the first law of thermodynamics can be written as ... 

The essential concept involved in the statistical theory of rubber elasticity is that a macroscopic deformation of the whole sample leads to a microscopic deformation of individual polymer chains. The microscopic model of an ideal rubber consists of a three-dimensional network with junction points of known functionality greater than 2. An ideal rubber consists of fully covalent junctions between polymer chains. At short times, high-molecular-weight polymer liquids behave like rubber, but the length of the chains needed to describe the observed elastic behavior is independent of molecular weight and is much shorter than the whole chain. The concept of intrinsic entanglements in uncrosslinked polymer liquids is now well established, but the nature of these restrictions to flow is still unresolved. The following discussion focuses on ideal covalent networks. 

The requirement for ease of processing demands that the rubber behave as a plastic material. While the effect of heating will aid in converting the tough elastic polymer into a plastic, the process is far from complete. Many grades of polymer are available in a pre-crosslinked form to assist in this respect. These polymers have usually been crosslinked prior to coagulation of the emulsion from which they were derived. The three-dimensional network helps to reduce the elastic component of the compound but at the expense, usually, of mechanical strength. Reclaimed rubbers form a supply of polymer with similar properties because of the residual crosslink structure from the parent material. 

Vulcanization is the process by which the linear rubber molecules are linked to form a three dimensional network comprising crosslinks formed by one or more sulfur atoms this is a result of heating the liquid rubber with sulfur. Crosslinking increases the elasticity and the strength of rubber about ten-fold, but the amount of erosslinking must be controlled in order to prevent the formation of brittle and inelastic rubber. The properties of rubber that are improved by vulcanization are tensile strength, elasticity, hardness, tear strength, abrasion resistance, and resistance to chemicals. The traditional... 

The structural formula of natural rubber (NR) is shown in Scheme 12.1. NR is sticky and non-elastic by nature. Crosslinking is a reaction of polymers to form a three-dimensional network. Crosslinking of rubber is called vulcanization. The crosslinking of NR makes it elastic. When NR is irradiated by high-energy radiation, hydrogen atoms of the trunk chain, mainly of methylene groups oc to double bonds, are ejected and radical sites are formed (Scheme 12.2), and these radical sites are combined into C-C crosslinks. ... 

As indicated earlier, the essential structural feature of a rubber vulcanizate is its flexible three-dimensional network. It is this arrangement which leads to the characteristic elastic behaviour. Comparative values for some properties of typical vulcanizates of common elastomers are given in Table 18.1. When natural rubber is stretched, crystallization of the highly regular chains occurs and the material shows a high tensile strength. The addition of fillers such as carbon black results in some increase in strength but the effect is not so marked as with elastomers for which stress-induced crystallization is not possible. 

Despite these differences in chain flexibility, molecular weight, and functionality, elastomers have the formation of a three-dimensional network in the final processing step in common with the thermoset resins such as the phenolic, epoxy, and acrylate resins. Crosslinking of a rubber renders it insoluble and, in addition, yields a material which has strongly enhanced physical properties, such as tensile properties (modulus, tensile strength, and elongation at break) and elastic recovery (under both compression and tension). 

Viscoelastic Response far above the Glass Temperature Tg The Fluid State. From Figure 10 or Figure 12 one can see the fluid state response of the polymer. This is the portion of the curve at long times or high temperatures from the rubbery plateau to the end of relaxation where the polymer would take the shape of whatever container held it, ie, it is a liquid. There are several fundamental aspects to polymer behavior in this region. On the rubbery plateau, the polymer chains behave as if they were part of a three-dimensional network and their response can be described from modern rubber elasticity theories. This behavior is beyond the scope of the current review and the reader is referred to... 

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. 

Equation (2.53) is stating that the network modulus is the product of the thermal energy and the number of springs trapped by the entanglements. This is the result that is predicted for covalently crosslinked elastomers from the theory of rubber elasticity that will be discussed in a little more detail below. However, what we should focus on here is that there is a range of frequencies over which a polymer melt behaves as a crosslinked three-dimensional mesh. At low frequencies entanglements... 

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