Elastomer network properties

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. 

A large number of macroscopic properties of elastomer networks are closely related to the density of network junctions and the extent of their fluctuations. Qualitatively, any increase of network density causes an increase in stress, whereas fluctuations of network junctions leads to a decreasing stress. It is generally believed that a formation of additional network junctions resulting fi-om the presence of filler particles in the elastomer matrix is one of the reasons for the improvement of mechanical properties of filled elastomers. However, the application of macroscopic techniques does not provide reliable results for the network structure in filled elastomers. Furthermore, a lack of information exists on the dynamic behavior of adsorption junctions. The present study fills the gap of knowledge in this area. 

The reinforcement of rubber by the presence of active fillers is a complex phenomenon that depends on the characteristics of the elastomer network and the properties of the fillers. The influential properties are the particle size, the morphology of particle aggregates, and the surface properties. The role of the geometrical characteristics of the tiller is well understood, whereas the significance of the surface properties is more difficult to analyze. This situation stems essentially from the lack of adequate methods to analyze the surface of such small particles and from the fact that fillers differ from each other and need to be considered individually. 

Chang, W. V., Bloch, R.,Tscho l, N. W. A New Measure of Strain to Describethe Mechanical Response of Elastomer Networks, in Chemistry and Properties of Crosslinked Polymers (S. S. Labana ed.). Academic Press, New York 1977... 

A new approach was developed to describe deformation behavior of crosslinked amorphous elastomers. - Stress relaxation arrd change of network properties during deformation of an elastomer in the whole range of strain up to the rapture of material is taken into accormt. 

Anisotropic elastomers, network, and gels with liquid crystal properties represent an area of research in the general domain of self-assembling materials... 

Brehmer M, Zentel R, Giesselmann F, Germer R, Zugenmaier P (1996) Coupling of liquid crystalline and polymer network properties in LC-elastomers. Liq Cryst 21(4) 589-596. doi 10.1080/02678299608032868... 

For example, hydrosilylation was employed for end linking poly-dimethylsiloxane networks to verify a statistical method of describing elastomer network structures and how they related to macroscopic properties where the required empirical information was the degree of conversion of the reactive groups (1). Subsequent work on the same system revealed, however, an extraneous side reaction which consumed - SiH without forming end links (,2). The side reaction was originally presumed... 

Brehmer M, Zentel R, Giepehnann F, Germer R, Zugenmaier P (1996) Coupling of liquid crystalline and polymer network properties in LC-elastomers. Liq Cryst 21(4) 589-596 Broer DJ, Finkehnaim H, Kondo K (1988) In-situ photopolarization of an oriaited liquid-crystalline acrylate. Makromol Chem 189 185-194 Broer DJ, Boven J, Mol GN, Challa G (1989a) In-situ photopolymerization of wiented liquid crystalline acrylates. Makromol Chem 190 2255-2268... 

Payne has proposed a classical splitting of all the effects involved in rubber reinforcement which can be further refined with respect to contemporary views (Figure 5.40). In addition to the intrinsic properties of the elastomer network, the CB particles bring first mere hydrodynamic effects, which are further enhanced by strong rubber-filler interactions, and interaggregate interactions which are weaker and depending on strain level. 

Polymers with the mechanical and chemical properties we have discussed in this section are called elastomers. In the next couple of sections we shall examine the thermodynamic basis for elasticity and then apply these ideas to cross-linked polymer networks. 

Properties such as low permanent set, low creep and low hysteresis are really measures of the efficiency of the heat fugitive network system. This is a complex function of the morphology. As a very general statement, the problem would seem to be less important with the harder grades of thermoplastic elastomer. 

Phthalazinone, 355 synthesis of, 356 Phthalic anhydride, 101 Phthalic anhydride-glycerol reaction, 19 Physical properties. See also Barrier properties Dielectric properties Mechanical properties Molecular weight Optical properties Structure-property relationships Thermal properties of aliphatic polyesters, 40-44 of aromatic-aliphatic polyesters, 44-47 of aromatic polyesters, 47-53 of aromatic polymers, 273-274 of epoxy-phenol networks, 413-416 molecular weight and, 3 of PBT, PEN, and PTT, 44-46 of polyester-ether thermoplastic elastomers, 54 of polyesters, 32-60 of polyimides, 273-287 of polymers, 3... 

Hydroxy-terminated PDMS, however, has disadvantages. The monofunctional ends limit the number of connections between the polymer (or oligomer) molecule and the glass network to two. This limitation raises the possibility that some PDMS molecules are not tied at both ends to the glass network if the polycondensation does not go to completion i.e. there may be "dangling" or loose PDMS chains in the final sol-gel material. This occurance of free ends would indeed be anticipated since the extent of reaction most likely is not 100%. Hence, the physical properties, specifically the mechanical behavior of the overall material, would be expected to suffer as a result of loose PDMS chains in the system. Disregarding this potential problem, the mechanical behavior of the sol-gel hybrids are, ultimately, influenced by the mechanical behavior of the modifying elastomer ... 

Because of the improvements in properties exhibited by elastomers having bimodal distributions [5], there have been attempts to prepare and characterize "trimodal" networks [56]. The calculations suggest that adding a small amount of very high molecular weight end-linkable polymer could further improve mechanical properties. 

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