Elastomer crosslink density

It is observed that Slapp of nano-CaCOs is less than the commercial CaCOs filled SBR which is attributed to greater crosslinking of rubber, as the uniform dispersion of nano-CaCOs brings the chains closer and keeps them intact with nanoparticles. Swelling depends on elastomer crosslinking density and solvent used. Solvent penetration is more in commercial micron size CaCOa than the nano-CaCOs rubber composites. 

E. Case V Predicting Crosslink Density Changes in Silicone Elastomers Due to Aging... 

Figure 29 Characteristic variation of crosslink density of silicone elastomers subjected to aging.

Even with elastomers, it is often desirable to phase mix the material for certain combinations of physical properties. For example, elastomers with high hardness and flexural modulus are prepared from polyisocyanates, polyols, and polyamines of high functionality, creating a high crosslink density. Although polyol chains... 

Studies of crosslinked networks of LC polymers have been proposed ( ), and prepared without disruption of the mesophase (11-14). Crosslinked elastomers were shown to retain the mesophase up to certain crosslink densities (15,16). 

Atomic force microscopy and attenuated total reflection infrared spectroscopy were used to study the changes occurring in the micromorphology of a single strut of flexible polyurethane foam. A mathematical model of the deformation and orientation in the rubbery phase, but which takes account of the harder domains, is presented which may be successfully used to predict the shapes of the stress-strain curves for solid polyurethane elastomers with different hard phase contents. It may also be used for low density polyethylene at different temperatures. Yield and rubber crosslink density are given as explanations of departure from ideal elastic behaviour. 17 refs. 

The Poisson s ratio (A//Aw, where A/ is the change in length produced by a change of width, Aw) of an isotropic liquid is 0.5, and that of an elastic solid is about 0.2. The value of P for an elastomer, such as NR, is 0.5, and this value decreases as the elastomer is cured with increasing amounts of sulfur, i.e., as the crosslink density increases. Likewise, P for rigid PVC is about 0.3, and this value increases progressively as the plasticizer content is increased. 

The Tg of elastomers must be below the use temperature. The high degree of cold flow which is characteristic of polymers at temperatures above the Tg is reduced by the incorporation of a few crosslinks to produce a network polymer with a low crosslink density. 

The diffusion coefficient D is always related inversely to the crosslink density of vulcanized elastomers. When Z)is extrapolated to zero concentration of the diffusate, it is related to the weight of the principal section of the elastomer, i.e., the weight of the segments between crosslinks. 

The products with low crosslink density are elastomers, but infusible, hard plastics are obtained when these polymers are crosslinkcd with large amounts of sulfur (40%). 

Cyclopentene is readily available as a byproduct in the ethylene production. Norbornene 2-ethylhexyl carboxylate is obtained by the Diels-Alder reaction of 2-ethylhexyl acrylate with cyclopenta-diene (5). Norbornene isobornyl carboxylate, norbornene phen-oxyethyl carboxylate, and other related monomers are synthesized according to the same route. Polymers obtained from these esters exhibit excellent properties in terms of controlling the crosslinking density, the associated product modulus, and the glass transition temperature (Tg), thus allowing tailoring the properties of elastomers, plastics and composites. Other suitable monomers are summarized in Table 1.1 and sketched in Figure 1.2. 

The stress-optical coefficient, Ka, of an elastomer network is a constant, independent of extension ratio and crosslink density. It is directly proportional to the difference between the longitudinal and transverse polarizabilities of the statistical chain segment (fei — 2) ... 

At temperatures well below Tg, when entropic motions are frozen and only elastic bond deformations are possible, polymers exhibit a relatively high modulus, called the glassy modulus (Eg) which is on the order of 3 Gpa. As the temperature is increased through Tg the stiffness drops dramatically, by perhaps two orders of magnitude, to a value called rubbery modulus Er. In elastomers that have been permanently crosslinked by sulphur vulcanization or other means, the values of Er, is determined primarily by the crosslink density the kinetics theory of rubber elasticity gives the relation as... 

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]. 

Multidimensional NMR spectroscopy proves to be a powerful method to reveal structural and dynamical information at the molecular level in elastomers. Residual dipolar couplings can be measured site-selectively and correlated with the crosslink density and mechanical stress. The local segmental order and information on local molecular motions can be also obtained with newly developed 2D NMR methods. The information at the molecular level can be correlated with macroscopic properties of elastomers and provides the basis for a better design of material properties for specific applications. 

The degree of toughness is determined by the crosslink density of the matrix, the elastomer particle size and size distribution, the volume fraction of the elastomeric phase, and the degree of adhesion between the epoxy matrix and the particle. The formulating procedure was found to have as strong an effect on the fracture toughness as the materials themselves.16... 

The importance of crosslink density has already been encountered in terms of the vulcanisation (i.e. sulphur-crosslinking) of natural rubber. With low crosslink densities (i.e. low levels of sulphur) the product is a flexible elastomer, whereas it is a rigid material when the crosslink density is high. 

Elastomers are crosslinked rubbery polymers (i.e. rubbery networks) that can be stretched easily to high extensions (e.g. 3 to 10 their original dimensions) and which rapidly recover their original dimensions when the applied stress is released. This extremely important and useful property is a reflection of their molecular structure in which the network is of low crosslink density. The rubbery polymer chains become... 

Several features of this route to grafted PEO polymers are attractive. PEO is not very soluble in silicone, a property that can be exploited to direct reaction to interfaces. The ability to control degree of functionality at one or both ends of the PEO polymer permits independent control of crosslink density and total PEO content. Finally, any residual OH or allyl groups can be used for subsequent functionalization. We describe below strategies to graft in (co-cure) and graft to (post-cure) silicone elastomers with PEO. 

Epoxy networks may be expected to differ from typical elastomer networks as a consequence of their much higher crosslink density. However, the same microstructural features which influence the properties of elastomers also exist in epoxy networks. These include the number average molecular weight and distribution of network chains, the extent of chain branching, the concentration of trapped entanglements, and the soluble fraction (i.e., molecular species not attached to the network). These parameters are typically difficult to isolate and control in epoxy systems. Recently, however, the development of accurate network formation theories, and the use of unique systems, have resulted in the synthesis of epoxies with specifically controlled microstructures Structure-property studies on these materials are just starting to provide meaningful quantitative information, and some of these will be discussed in this chapter. 

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