Crystallizing elastomers

Neoprene AF ( 963). It is a polychloroprene modified with methacrylic acid. Although it is a slow-crystallizing elastomer, the cohesive strength develops very rapidly and it has improved creep resistance at high temperature compared with Neoprene AC or AD. The improved properties of Neoprene AF are derived from the interaction between the carboxyl functionality with the metal oxides added in the solvent-borne polychloroprene adhesives. 

E.H. Andrews, Crack propagation in a strain-crystallizing elastomer, J. Appl. Phys., 32, 542-548, 1961. 

Shahinpoor, M., Elastically-activated artificial muscles made with liquid crystal elastomers. Proceedings of SPIE 7th Annual International Symposium of Smart Structures and Materials, EAPAD Conf, 3987, pp. 187-192, 2000. 

M. Warner and E.M. Terentjev, Liquid Crystal Elastomers, Oxford University Press, New York, 2003. 

Knight, D. P., and Vollrath, F. (2002). Biological liquid crystal elastomers. Philos. Trans. R Soc. Lond. B Biol. Sci. 357, 155-163. 

Wang. Y. cl al. Twinning in MgSio3 Peruvskite." Science. 468 (April 27. t990 . Warner. M.. and E.M. Tcrcnijcv Ijquitl Crystal Elastomers. Oxford University Press. New Yoil. NY. 2003. 

The reversible shape change in molecular materials was found for the first time in 2001 by using azobenzene-containing liquid crystal elastomers [39]. Figure 23.6... 

Figure 23.6 Contraction fraction of azobenzene-containing liquid crystal elastomers, (Lo - f-O/f-o, at 298 K against the time upon irradiation with ultraviolet light and in the dark, to and L, represent lengths of the elastomers in the initial state and after the time, respectively.

Defining permanent memory of macroscopic global alignment in liquid crystal elastomers... 

ISO 8013 is confined to static strain conditions and can seriously underestimate the creep that occurs under dynamic loading. The creep rate in cycled rubber is higher than that predicted by a simple Boltzmann superposition, but linearity is still observed between creep and logarithmic time or the logarithm of the number of cycles, as long as a physical mechanism applies [39,40]. The increase is most striking with strain-crystallizing elastomers such as natural rubber. 

Electrostrictive polymers have a spontaneous electric polarization. Electrostriction results from the change in dipole density of the material. These polymers contain molecular or nanocrystaUine polarizations that align with an applied electric field. PVDF copolymers with nano-sized crystalline domains, electrostrictive graft copolymers, and liquid crystal elastomers fall under this category. 

Fig. 1.7 Actuation mechanism of a liquid crystal elastomer. The application of an electric field results in the realignment of intrinsically polarized liquid crystal mesogens. The mesogens are either grafted to elastomer chains or incorporated within them. The elastomer chains prevent the free flow of the mesogens and couple their motion to bulk stresses and strain...

Warner M, Terentjev M (2003) Liquid crystal elastomers. Oxford Science Publications, Oxford... 

Thomsen DL, Keller P, Naciri J, Pink R, Jeon H, Shenoy D, Ratna BR (2001) Liquid crystal elastomers with mechanical properties of a muscle. Macromolecules 34 5868... 

Li MH, Keller P (2006) Artificial muscles based on liquid crystal elastomers. Philos Trans R Soc A 364 2763... 

Shenoy DK, Thomsen DL, Srinivasan A, Keller P, Ratna BR (2002) Carbon coated liquid crystal elastomer film for artificial muscle applications. Sens Actuators A 96 184... 

Finkelmann H, Shahinpoor M (2002) Electrically controllable liquid crystal elastomer-graphite composite artificial muscles. Proc SPIE 4695 459... 

Chambers M, Einkelmann H, Remskar M, Sanchez-Ferrer A, Zalar B, Zumer S (2009) Liquid crystal elastomer-nanoparticle systems for actuation. J Mater Chem 19 1524... 

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

LSCE liquid single crystal elastomer MAD molding area diagram... 

The techniques used to obtain the untwisted SmC phase structure in low molar mass LCs and SCLCPs are limited to thin layers. In contrast to this, LC elastomers can be macroscopically uniformly oriented by mechanical deformations [230], and this orientation process is not limited to thin samples or suitable dielectric anisotropy of the material. Furthermore, for LC elastomers the oriented structure can be chemically locked in by crosslinking, resulting in the so-called liquid single crystal elastomers [231],... 

Static mechanical properties in the vicinity of the nematic-isotropic transition in liquid single crystal elastomers (LSCEs) have been investigated [10, 11]. In Fig. 5 the deformation L/Lq (mon) is plotted as a function of the reduced temperature red- Here Lo(mon) denotes the length of the LSCE at the phase transition temperature of the nematic-isotropic phase transition and... 

While there are not many NMR investigations of the isotropic-nematic transition in sidechain liquid crystal elastomers [20, 21], they turned out to be crucial in answering the question whether strained LCEs show a first order phase transition with a classical two phase region or whether there is a continuous change of the local degree of nematic order, since one is in a state beyond the critical point. As already mentioned earlier in this section this question could not be settled conclusively with the mechanical and optical investigations described above. 

Mechanical Properties of Monodomains Liquid Single Crystal Elastomers... 

Ultrasonic experiments using laser induced phonon spectroscopy have been performed in a nematic liquid single crystal elastomer [48]. The experiments reveal a dispersion step for the speed of sound and a strong anisotropy for the acoustic attenuation constant in the investigated frequency range (100 MHz -1 GHz). These results are consistent with a description of LCEs using macroscopic dynamics [54-56] and reflect a coupling between elastic effects and the nematic order parameter as analyzed in detail previously [48]. 

As shown in Figure 10.15, the tear strength of strain-crystallizing elastomers is greatly enhanced over the range of tear rates and temperatures at which... 

FIGURE 10.15 Fracture energy, G, for a strain-crystallizing elastomer, natural rubber, as a function of temperature, T, and rate of tearing, R. (From Greensmith and Thomas (1955).)... 

Although amorphous elastomers are found to tear steadily, at rates controlled by the available energy for fracture, G (as shown in Figures 10.13 and 10.14), strain-crystallizing elastomers do not tear continuously under small values of G, of less than about 10" J/m for natural rubber, for example (see Figure 10.15). Nevertheless, when small stresses are applied repeatedly, a crack will grow in... 

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