Electromechanical properties

The properties described in the previous section apply to networks exhibiting helical structures. This type of mesophase is obtained by introducing side mesogenic substituents with chiral end groups on the backbone [94, 138-141, 145-147], or by doping an achiral network with a chiral molecule [144,145]. As a result of the interaction between orientational (liquid-crystalline) and translational (network) degrees of freedom, these noncentrosymmetric systems exhibit rich electromechanical effects, such as piezoelectricity. 

Theoretical investigations by Brand [ 135] and Brand and Pleiner [136] predicted that a monodomain liquid-crystalline elastomer exhibiting a cholesteric or a chiral smectic C phase should display piezoelectric properties due to a modification of the pitch of the helix under strain. So, a piezoelectric voltage should be observed across the sample when a mechanical field is applied parallel to the helicoidal axis. In this description, the crosslinking density is supposed to be weak enough to allow the motion of the director, and deformations of the sample (compression, elongation, etc.) are assumed to be much smaller than those that should lead to a suppression of the helix. The possibility of a piezoelectric effect do not only concern cholesteric and chiral smectic C phases, but was also theoretically outlined for more exotic chiral layered systems such as chiral smectic A mesophases [137]. 

Preceding the reports on elastomers, piezoelectricity in chiral smectic C phases of low-molar weight molecules or of polymers has usually been observed. The special property is that the system possesses macroscopic electrical polarization without an external field, so it is classified as ferroelectric. 

Experimentally, piezoelectricity in cholesteric and chiral smectic C phases was reported for liquid-crystalline networks [140-147]. Multidomain lightly cross-linked systems were synthesized, then the orientation is obtained by mechanical strain [140] or by poling [147]. In other samples this orientation is performed prior to the crosslinking process [144, 146]. Macro-scopically oriented samples were subjected to either a static or a periodically varying strain. Open circuit voltages across the samples were measured that are linear functions of the applied strain [140-142, 144, 145], 

IV Behavior and Properties of Side Group Thermotropic Liquid Crystal Polymers 

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With the advent of these compounds in the 1960s, the hitherto more conventional insulating materials, such as phenol formaldehyde (popularly known as Bakelite) and wood (veneered impregnated) have been almost replaced by them. These compounds offer better electromechanical properties than conventional materials. Below we describe the basic mix and properties of these two basic compounds, for a brief reference. 

This is also known as Bulk Moulding Compound (BMC). It is blended through a mix of unsaturated polyester resin, crosslinking monomer, catalyst, mineral fillers and short-length fibrous reinforcement materials such as chopped glass fibre, usually in lengths of 6-25 mm. They are all mixed in different proportions to obtain the required electromechanical properties. The mix is processed and cured for a specific time, under a prescribed pressure and temperature, to obtain the DMC. 

Fig. 110. Temperature dependence of some electromechanical properties of RbsNb3OFis crystals. Reproduced from [443], A. I. Agulyansky, J. Ravez, R Von DerMuhll, A. Simon, Ferroelectrics 158 (1994) 139, Copyright 1994, with permission of Taylor Francis, Inc., http //www.routledge-ny.com.

Cairns, D. K. Crawford, G. P. 2005. Electromechanical properties of transparent conducting substrates for flexible electronic displays. Proc. IEEE. 93 1451-1458. 

Chubb, J.M., Bennett, J.L., Akera, T. and Brody, T.M. (1978) Effects of praziquantel, a new anthelmintic, on electromechanical properties of isolated rat atria. Journal of Pharmacology and Experimental Therapeutics 207, 284-293. 

M616ard, Philippe, Electromechanical Properties of Model Membranes and Giant Vesicle Deformations, 6, 185 see also Bivas, Isak, 6, 207. 

Grodzinsky, A.J., Lipshitz, H. and Glimcher, MJ. (1978) Electromechanical properties of articular cartilage during compression and stress-relaxation. Nature 275, 448 150... 

Lee, R.C., Frank, E.H., Grodzinsky, A.J. and Roylance, D.K. (1981) Oscillatory com-pressional behavior of articular cartilage and its associated electromechanical properties. ASME Journal of Biomechanical Engineering 103, 280-292... 

Far more frequently, however, measurements are made with a relatively thin film (often < 5 /rm) on a comparatively massive substrate (often > 400 //,m in thickness). In principle, the piezoelectric and electromechanical properties of the films can be extracted by fitting conventional resonance measurements. In practice, multiple resonances due to the film/substrate system are measured it is then necessary to deconvolute the data in order to extract information on the films itself. In practice, this is difficult to do unambiguously [21,22],... 

An electrochemical stimulus provides a two-fold advantage since it can stimulate molecular actuations through clean redox reactions and also detects simultaneously the resulting molecular movements by sensing the changes in the electromechanical properties of a bistable molecule. CV is often employed to both write and read the electromechanical movements in the redox-active bistable... 

The search for piezoceramics having optimum electromechanical properties has been extended to cover MPB compositions of PbTi03 with various relaxors. The search is assisted by using a relationship established between the Curie point of the MPB composition and the tolerance factor (see Section 2.7.3) for the relaxor component [10]. 

Table 6.3 Electromechanical properties of some relaxor-based single crystals (data taken from [11])...

Unmodified Pb(Zr, Ti)03 ceramics are seldom used. The dielectric and electromechanical properties are tailored to a particular device requirement by changing the Zr/Ti ratio and by addition of dopants or modifying agents. Thousands of these formulations have been documented, and some of the more important are summarized. 

Activation and conductivity at room temperature are problems that can be addressed by the incorporation of other electronic structures that increase carrier transport. Crystal morphology is an important parameter in the boron doping process to determine uncompensated acceptors (Aa-Ad) for different crystal facets as a function of doping concentration. The temperature coefficient of resistance for a CVD diamond film can be changed by boron doping. As conductivity depends on the crystal phase, the combined electromechanical properties can be exploited in sensor applications both for resistive temperature detectors and for pressure transdu-cers. " As electrical conductivity is related linearly with boron concentration, a better-controlled process may allow for the development of better semiconductor devices improving crystal quality and operating limits. ... 

Meleard, P., Gerbeaud, C., Pott, T., and Mitov, M.D. (2000) Electromechanical properties of model membranes and giant vesicle deformahon, in Giant Vesicles (eds P.L. Luisi and P. Walde), John Wiley Sons, Ltd, Chichester, pp. 185-205. 

One single property of filler - electric conductivity - affects many properties of the final products. These properties include electric insulation, conductivity, superconductivity, EMI shielding, ESD protection, dirt pickup, static decay, antistatic properties, electrocatafysis, ionic conductivity, photoconductivity, electromechanical properties, thermo-electric conductivity, electric heating, paintability, biocompati-bilify, etc. Possession of one of these properties in a polymer can make it useful in industiy and eveiyday use. Examples are given in Chapter 19. Here, the electrical... 

Ludwig J, Oliver D, Frank G, Klocker N, Gummer AW, et al. 2001. Reciprocal electromechanical properties of rat prestin The motor molecule from rat outer hair cells. Proc Natl Acad Sci USA 98 4178-4183. 

That fluid continues to flow after compression has ceased (l.e., during relaxation) is evidenced by its electromechanical properties (2 2). When the tissue is compressed, an electric potential difference occurs between the surface and the deepest regions. This potential difference was attributed to the flow of fluid and entrained counterions with respect to the net negatively charged solid matrix (2.2). That is, the observed potential is essentially a streaming current potential. The decay of this potential following compression was found to exactly parallel the load dissipation curves with time (22). 

Gao Q, Scheinbeim JI, Newman BA (2000) Dipolar intermolecular interactions, structural development, and electromechanical properties in ferroelectric polymer blends of nylon-11 and poly(vinylidene fluoride). Macromolecules 33 7564... 

Xu H, Cheng ZY, Olson D, Mai T, Zhang QM, Kavamos G (2001) Ferroelectric and electromechanical properties of poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoro-ethylene) terpolymer. Appl Phys Lett 78 2360... 

Carpi F, DeRossi D, Kombluh R, Pelrine R, Somer-Larsen P (2008) Dielectric elastomers as electromechanical transducers, fundamentals, materials, devices, models and applications of an emerging electroactive polymer technology. Elsevier Press, Amsterdam Chapter 7 Benslimane M, Kiil H-E, Tryson MJ (2010) Electromechanical properties of novel large strain PolyPower film and laminate components for DEAP actuator and sensor applications. Proc SPIE 7642 764231... 

Mohamed B, Kill H-E, Tryson MJ (2010) Electromechanical properties of novel large strain PolyPower film and laminate components for DEAP actuator and sensor applications. Proc SPIE 7642 764231... 

Alternating current measurements were used to investigate the electromechanical properties in combined photo-crosslinked cholesteric LCEs. In a compound showing both, a cholesteric and a chiral SmC phase, the electromechanical response in the cholesteric phase was considerably higher. 

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