Elastomers double networking

Santangelo, P. G. Roland, C. M., Role of Strain Crystallization in the Fatigue Resistance of Double Network Elastomers. Rubber Chem. Technol. 2003,76,892-898. 

Santangelo PG, Roland CM (1994) The mechanical behavior of double network elastomers. Rubber Chem Technol 67(2) 359-365... 

In general, research on classes of materials is connected with that on materials with specific properties but includes somewhat more general research on composites, polyurethanes, epoxies, fluoropolymers, ferroelectric liquid crystals (especially those with fast switching times), polymer-polymer miscibility, double network elastomers, crystallization in polymers, polymeric Langmuir-Blodgett and other multilayer films, and polymer-stabilized synthetic membranes. 

There are some ongoing general studies of surface modification of elastomers and studies on the synthesis and characterization of double-network elastomers. Research was done on the suppression of crystallization in blended natural rubber and neoprene. Other studies of crystallization, for example, on positron emission tomography (PET), have also been done. 

FIGURE 15 Natural rubber sample on left is double network and on right is a conventional NR elastomer (a, b). Both are in a state of mechanical equilibrium (stress = 0). Nevertheless, as seen in (c), the double network transmits light through crossed polarizers, due to its inherent orientation. This is a violation of the stress optical rule, similar to that observed during creep recovery of uncrosslinked rubber. 

Networks can also be formed from chains in the deformed state. In this approach a first network is generally introduced in the undeformed state, the resulting elastomer is elongated, and a second network is introduced in the stretched state. Release of the stress permits the network to retract, but the second network of this double-network" structure prevents retraction down to the original dimensions (figure 7.4). The most interesting feature of the retracted network is the fact that it is anisotropic in structure and properties. 

In some cases, double networks have shown increases in orientability and strain-induced crystallization, as well as improved fatigue resis-tance. ° In fact, some results show that there maybe less of a compromise between failure properties in general and the modulus, which may be due in part to the decreased hysteresis observed for some of these elastomers. There have even been reports of improved thermal stabil-ity, although it is hard to visualize how this would occur. Finally, electrical resistivity is more sensitive to strain in carbon-black reinforced double networks. Better molecular understanding of these observations is being sought with, for example, extensive studies of residual strains and birefringence. ... 

Dry RubbGr. Because of its enhanced crystallizability, guayule rubber can exhibit superior failure properties in unfilled rubber compositions. As a double network, an elastomer cured a second time while in a deformed state guayule rubber exhibits substantially better fatigue resistance than deproteinized Hevea rubber (118). When compounded with carbon black, guayule rubber and Hevea rubber behave similarly (119). In tread and wire skim stocks, the compoimding and performance behavior of guayule rubber was comparable to that of Hevea rubber (Tables 7, 8) (120). 

Figure 11.9 Mapping of bond cleavage in self-reporting chemiluminescent elastomers that are toughened ty sacrificial bonds, (a) Bis(adamantyl)-1,2-dioxetane breaks under a mechanical force, resulting in chemiluminescence. (b) Intensity-coloured images of polymer networks during crack propagation of notched samples and schematic depiction of bond breaking around the crack tip. SN, DN, TN label elastomers of different molecular architecture ( single network , double network and triple network ). The dashed line indicates the perimeter of the sample. Vertical lines are artefacts of the detector.

Effect of Double Networking on Non-Linear Viscoelasticity of Elastomers... 

Reichert WF, Goritz D, Duschl EJ (1993) The double network, a model describing filled elastomers. Polymer 34(6) 1216-1221... 

The presence of filler in the rubber as well as the increase of the surface ability of the Aerosil surface causes an increase in the modulus. The temperature dependence of the modulus is often used to analyze the network density in cured elastomers. According to the simple statistical theory of rubber elasticity, the modulus should increase twice for the double increase of the absolute temperature [35]. This behavior is observed for a cured xmfilled sample as shown in Fig. 15. However, for rubber filled with hydrophilic and hydrophobic Aerosil, the modulus increases by a factor of 1.3 and 1.6, respectively, as a function of temperature in the range of 225-450 K. It appears that less mobile chain units in the adsorption layer do not contribute directly to the rubber modulus, since the fraction of this layer is only a few percent [7, 8, 12, 21]. Since the influence of the secondary structure of fillers and filler-filler interaction is of importance only at moderate strain [43, 47], it is assumed that the change of the modulus with temperature is mainly caused by the properties of the elastomer matrix and the adsorption layer which cause the filler particles to share deformation. Therefore, the moderate decrease of the rubber modulus with increasing temperature, as compared to the value expected from the statistical theory, can be explained by the following reasons a decrease of the density of adsorption junctions as well as their strength, and a decrease of the ability of filler particles to share deformation due to a decrease of elastomer-filler interactions. 

For a polymer to be useful as an elastomer, it must be possible to introduce cross-links in such a way as to bond a macroscopic sample into a continuous network. Generally, this requires the presence of double bonds or chemically functional groups along the chain. 

No such relationship was observed in the case of polyethylene mixtures with butyl rubber (Fig. 6). We came to the conclusion that the activity of polyacrylonitrile as a filler can be connected with EDA interactions between electrons of double bonds and -C E N groups. No such complex type has so far been detected in polymer mixtures. In the given instance an EDA complex could appear only at the interphase boundary and its concentration would be quite low. However, certain symptoms of its existence have been observed. PAN added in the amount of 30 phr raised the cis-1,4-poly-butadiene Tg towards higher temperature region by 4-15 K. This was observed by means of thermomechanical analysis under dynamic as well as under static conditions (Figs. 7 and 8). The presence of the immobilized layer of PB on the PAN domains was also established in studies carried out by the method of pulse NMR. In the mixtures of PB with PAN there appeared additional compliances of the relaxation time T2 "spin-spin" (Fig. 9), as well as relaxation time T "spin-network" (Fig. 10). This indicates that part of the elastomer has... 

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