Elastomer networks, swelling

Another way to deform an elastomer network is to put it in contact with a solvent. In this case molecules of solvent are absorbed in the network, giving rise to a phenomenon known as swelling. Swelling of a network by the... 

An important methodology established early on is an equilibrium volume swelling approach and application of the Flory-Rehner equation [134, 135]. While this is satisfactory for simple one component elastomer networks it is not particularly convenient experimentally and is of dubious value for multi-component elastomers. 

A cross-linked elastomer cannot dissolve in a solvent. Dispersion is resisted because the cross-links restrict the movement and complete separation of the chains, but the elastomer does swell when the solvent molecules diffuse into the network and cause the chains to expand. This expansion is counteracted by the traidency of the chains to coil up and, eventually, an equihbiium degree of swelling is established that depends on the solvent and the cross-hnk density i.e., the higher the cross-Unk density, the lower the swelling. 

Both Mg and the interaction parameter Xi can be calculated from measurements of network swelling when the elastomer is brought in contact with a solvent. We can assume that the total free-energy change will then be composed of... 

The change of viscoelastic behavior of ciosslinked elastomers on swelling depends on polymer physical network. This, in turn, depends on the concentration and nature of the plasticizer in the material expressed by parameter m describing the ability of liquid to affect physical network of elastomer. 

With decreasing ability of PEU and PBU to be deformed because of increase in its chemical network density, curves of dependences of 8 and plasticizer volume fraction, < ), become concave. The critical strain is heavily reduced already by a small content of a polar plasticizer. The data demonstrate a strong influence of chemical network on the critical strain of elastomer on swelling. 

The density of cross-links in the elastomer network was determined by the method of equilibrium swelling according to standard PN-74/C-04236. The samples of the composite material were subjected to equilibrium swelling in toluene for 48 h at room temperature. Then, the swollen samples were weighed on a torsion balance, next dried at a temperature of 60 to a constant weight and after 48 h they were reweighed. Based on the Flory-Rehner s equation [20] the crosslink density was determined, assuming the elastomer-solvent interactions parameter as x = 0.501 + 0.273 Vr, where Vr is the volume fraction of the elastomer in the swollen gel. 

Only when chemical bonds between neighboring molecules are introduced is a raw elastomer converted into a rubber vulcanizate, which is essentially a three-dimensional network structure (see Figure 5.3). The process is referred to as vulcanizahon or curing, or more accurately, as cross-linking. A cross-linked elastomer, or rubber vulcanizate, is capable of large reversible deformations within a broad temperature range and does not dissolve, but only swells in solvents and other liquids. 

As pointed out earlier, an elastomer cross-linked above its gel point will not dissolve in a solvent, but will absorb it and swell. The swelling will continue until the forces of swelling balance the retroactive forces of the extended chains of the network. The cross-link density can then be calculated from the degree of swelling using the Flory-Rehner equation ... 

The structure of an elastomer comprises a network of chains, meaning that there are gaps between adjacent chains. Indeed the elasticity of rubber relies on substantial thermal motion of the chains, which would not be possible if the chains were closely packed. The free volume available in the rubber means that some liquids can enter the rubber and cause swelling - sometimes very large amounts of swelling. For example the ability of oil to swell natural rubber is well known. 

Many studies of vulcanized elastomer blends have revealed discontinuities in physical property trends attributable to poor interfacial bonding. Recently Rehner and Wei (5) have observed discontinuities in the swelling of blended crosslinked networks swollen in a common solvent. This departure from an averaged swelling behavior, based upon compositional ratios and the swelling behavior of the two homophases, re-... 

Table I. Swelling of Various Elastomer Crosslinked Networks in Differential Solvents at — 25°C...

Mechanical properties of crosslinked elastomers are influenced not only by the volume-average crosslink density but also by network heterogeneity. The influence of structural defects (such as residual sol, dangling chains, chain loops and the heterogeneity of the junction distribution) on the viscoelastic properties and the equilibrium swelling data is still under discussion. Local methods which probe molecular properties are very suitable for the determination of the degree of network heterogeneity [11]. 

Elastomers. In the Investigation of the effects of methanol/ gasoline blends on elastomers [2] we established that the ultimate stress and the ultimate elongation In swelled elastomers are linearly related to the volume of rubber Vj. In the swelled network, I.e.,... 

Linear Regression Analysis of Tensile Strength (o.) and Ultimate Elongation (Ej ) Versus Volume of Elastomer In Ethanol Swelled Network... 

Linear polymers are usually characterized in solution. However, if enough crosslinking has occurred, such as in the vulcanization of elastomers, the polymer will swell but cannot dissolve in typical solvents. Then, it is impossible to derive molar mass (molecular weight) information from solution behavior. However, the average molar mass of the chains in the polymer network can be determined from swelling (1,2). 

If the elastomer is swollen to below the equilibrium swelling so that no deswelling will occur upon deformation, the statistical expression for the shear modulus [equation (6-59)] can be readily modified. We define Vr as the ratio of the unswollen volume to the swollen one, which is identical to the volume fraction of polymer in the mixture. The number of network chains per unit volume then becomes NQVr and the mean square end-to-end distance of the network chain is now r02 / Vr2/3. Equation (6-59) then reads... 

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