Anisotropic elastomers

Keywords Anisotropic elastomers Ferrogels Magnetic composites ... 

In order to prepare anisotropic elastomers, the reaction mixture in the mould was placed between the poles of a large electromagnet (JM-PE-IJEOL, Japan) (Fig. 4). 

Brodowsky HM, Boehnke UC, Kremer F, Gebhard E, Zentel R. 1999. Mechanical deformation behavior in highly anisotropic elastomers made from ferroelectric liquid crystalline polymers. Langmuir 15 274 278. 

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

The wide choice available in plastics makes it necessary to select not only between TPs, TSs, reinforced plastics (RPs), and elastomers, but also between individual materials within each family of plastic types (Chapters 6 and 7). This selection requires having data suitable for making comparisons which, apart from the availability of data, depends on defining and recognizing the relevant plastics behavior characteristics. There can be, for instance, isotropic (homogeneous) plastics and plastics that can have different directional properties that run from the isotropic to anisotropic. Here, as an example, certain... 

Uniaxial deformations give prolate (needle-shaped) ellipsoids, and biaxial deformations give oblate (disc-shaped) ellipsoids [220,221], Prolate particles can be thought of as a conceptual bridge between the roughly spherical particles used to reinforce elastomers and the long fibers frequently used for this purpose in thermoplastics and thermosets. Similarly, oblate particles can be considered as analogues of the much-studied clay platelets used to reinforce a variety of materials [70-73], but with dimensions that are controllable. In the case of non-spherical particles, their orientations are also of considerable importance. One interest here is the anisotropic reinforcements such particles provide, and there have been simulations to better understand the mechanical properties of such composites [86,222],... 

When an expl is pressed in a die by means of a ram, the friction of the walls tends to cause press and density gradients. In addition, the one-dimensional compression can result in an anisotropic structure and produce pellets with residual strains. Where dimensional stability, uniformity and high density are essential to performance, hydrostatic pressing and isostatic pressing have been used. In both of these processes, the expl is compressed by the action of a fluid, from which it is separated by a rubber (or other elastomer) film... 

A simple model of an elastomer network is depicted in Fig. 7.1.8. The segmental motion of inter-cross-link chains is fast but anisotropic at temperatures of 100-150 K above the glass transition temperature The end-to-end vector R of such a chain reorients on a much slower timescale because it appears fixed between seemingly static cross-link points. As a result of the fast but anisotropic motion, the dipolar interaction between spins along the cross-link chains is not averaged to zero, and a residual dipolar coupling remains [Cohl, Gotl, Litl]. 

Synthesis of elastomers under uniform magnetic field can be used to prepare anisotropic samples. 

In Sect. 3.2 the preparation of magnetic polymer composites under uniform magnetic field has been described. The resulting composites show anisotropic behavior. The anisotropy manifests itself in the direction-dependent elastic modulus. Figure 13 shows carbonyl iron-loaded mPDMS elastomers. All three samples contain the same amount of filler particles, but the spatial distribution of the filler is different, as shown in the figure. 

When a sample of elastomer is stretched, it becomes anisotropic in that the network chains tend to orient themselves more in the direction of stretch than in the lateral directions. The more ordered chains favor the formation of crystallites. These crystallites will tie together a number of neighboring network chains, thereby exerting an additional crosslinking effect. This increase in the degree of crosslinking will in turn cause a rise in the elastic stress. The reason that these crystallites in fact act as crosslinks is attributable... 

Linear viscoelasticity is valid only imder conditions where structural changes in the material do not induce strain-dependent modulus. This condition is fulfilled by amorphous polymers. On the other hand, the structural changes associated with the orientation of crystalline polymers and elastomers produce anisotropic mechanical properties. Such polymers, therefore, exhibit nonlinear viscoelastic behavior. 

Spillmann CM, Ratna BR, Naciri J (2007) Anisotropic actuation in electroclinic liquid crystal elastomers. Appl Phys Lett 90 021911... 

In the preceding chapters the synthesis properties of linear liquid crystalline polymers are described, where different approaches exist to obtain the liquid crystalline state rod-like or disc-like mesogenic units are either incorporated in the polymer backbone or are attached as side groups to the monomer units of the main chain. Following conventional synthetic techniques these linear polymers can be converted to polymer networks. Compared to low molar mass liquid crystals and linear liquid crystalline polymers, these liquid crystalline elastomers exhibit exceptional new physical and material properties due to the combination and interaction of polymer network elasticity with the anisotropic liquid crystalline state. 

In this chapter we review the physical properties of liquid crystalline elastomers using publications until 1996. For a review of the physical properties of strongly cross-linked liquid crystalline polymers, which give rise to anisotropic nonliquid crystal line materials (duromers or anisotropic solids) we refer to the recent review by Hikmet and Lub [1]. 

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