Crosslinked elastomers behavior

The disperse crosslinked elastomer phase shows behavior similar to that of a polymeric filler. Viscosity is highly dependent on shear rate [7], but hardly dependent on temperature [7]. The flow exponent is between 3 and 5. 

Hyperelastic models are often used to represent the behavior of crosslinked elastomers, where the viscoelastic response can sometimes be neglected compared with the nonlinear elastic response. Because UHMWPE behaves differently than do elastomers, there are only a few specific cases when a hyperelastic representation is appropriate for UHMWPE simulations. One such case is when the loading is purely monotonic and at one single loading rate. Under these conditions it is not possible to distinguish between nonlinear elastic and viscoplastic behavior, and a hyperelastic representation might be considered. Note that if a hyperelastic model is used in an attempt to capture the... 

Block copolymers display the unique behavior of being able to function as conventionally crosslinked elastomers over a certain temperature range, but they soften reversibly at high temperatures. As such, they can be processed as a thermoplastic. These polymers are macromolecules comprised of chemically dissimilar, terminally connected segments. According to the components that make up the segments, the thermoplastic rubbers can be classified according to Table 1. 

The viscoelastic properties of a series of fully cured epoxy resins with different crosslink densities and their trends have been discussed in some details in the previous section. Qualitatively these properties are shared by all crosslinked elastomers, although quantitatively they depend on molecular architecture and the chemical type of the network polymer and the crosslinking agent. Hence it is instructive to show the viscoelastic behavior of other model network systems and compare them. 

In the rubbery plateau, a new impediment to movement must be overcome entanglements along the polymer chain. In discussing the effects of entanglements in Chap. 2, we compared them to crosslinks. Is it any surprise, then, that rubbery behavior similar to that shown by cross-linked elastomers characterizes this region ... 

Recent work has focused on a variety of thermoplastic elastomers and modified thermoplastic polyimides based on the aminopropyl end functionality present in suitably equilibrated polydimethylsiloxanes. Characteristic of these are the urea linked materials described in references 22-25. The chemistry is summarized in Scheme 7. A characteristic stress-strain curve and dynamic mechanical behavior for the urea linked systems in provided in Figures 3 and 4. It was of interest to note that the ultimate properties of the soluble, processible, urea linked copolymers were equivalent to some of the best silica reinforced, chemically crosslinked, silicone rubber... 

The characteristic property of elastomers is their rubber-elastic behavior. Their softening temperature lies below room temperature. In the unvulcanized state, i.e. without crosslinking of the molecular chains, elastomers are plastic and thermo-formable, but in the vulcanized state—within a certain temperature range — they deform elastically. Vulcanization converts natural rubber into the elastic state. A large number of synthetic rubber types and elastomers are known and available on the market. They have a number of specially improved properties over crude rubber, some of them having substantially improved elasticity, heat, low-temperature, weathering and oxidation resistance, wear resistance, resistance to different chemicals, oils etc. 

Hydrogenation of double bonds is practiced with some elastomers such as SBR and NBR to increase high temperature and oxidative resistance [Hsieh, 1998 Hsieh and Quirk, 1996, Wrana et al., 2001], Only partial hydrogenation is performed since some double bonds are required to achieve the subsequent crosslinking required to achieve elastic behavior. 

Elastomers exhibit this behavior due to their unique, crosslinked structure (cf. Section 1.3.2.2). It has been found that as the temperatme of an elastomer increases, so does the elastic modulus. The elastic modulus is simply a measme of the resistance to the uncoiling of randomly oriented chains in an elastomer sample under stress. Application of a stress eventually tends to untangle the chains and align them in the direction of the stress, but an increase in temperatme will increase the thermal motion of the chains and make it harder to induce orientation. This leads to a higher elastic modulus. Under a constant force, some chain orientation will take place, but an increase in temperatme will stimulate a reversion to a randomly coiled conformation and the elastomer will contract. 

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-... 

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