Liquid Crystal Elastomers with Piezoelectric Properties
- PDF / 1,650,564 Bytes
- 3 Pages / 604.8 x 806.4 pts Page_size
- 56 Downloads / 281 Views
ure 1. (a) Sample of an LCE without meclianical déformation. U is translucent because of the non-uniform orientation of the director. (b) Déformation of an LCE leads to a macroscopically ordered System, which is transparent.
MHS BULLETIN/JANUARY1991
This response of the LC-phase structure to an applied mechanical field is similar to the effect of electric or magnetic fields on low molecular weight liquid crystals (LMLC), as illustrated in Figure 1. Figure la shows an undeformed LCE. Because of the non-uniform orientation of the director, the sample scatters light strongly so the elastomer is translucent like frosted glass. On the other hand, applying a mechanical field the director becomes uniformly aligned and the sample is transparent (Figure lb). Such a macroscopically ordered rubber exhibits optical properties very similar to single crystals. Thèse properties of LCEs offer new prospects for technical application, e.g., in nonlinear and integrated optics. In addition, LCEs exhibit physical properties that can only be observed due to the
shape retaining network structure, that prevents macroscopic flow as a response to a mechanical field. Discussion Recently, Brand 67 pointed out theoretically that chiral nematic (cholesteric) elastomers should show interesting piezoelectric properties. In a cholesteric elastomer there exists orientational long-range order of the long axis of the LC molécules, as exists in the well-known nematic phase. In addition, due to the présence of chiral molécules, a helicoidal superstructure perpendicular to the long axis of the molécules is observed. This is shown schematically in Figure 2. The helicoidal structure may be characterized by a pitch, p, where the molecular long axis has performed a rotation of 360°. Due to this helicoidal structure, the cholesteric phase exhibits exceptional optical properties. It shows reflection of circular polarized light at the w a v e l e n g t h \ R . The w a v e l e n g t h of reflection is related to the pitch by: \ R = np
(1)
where n is the average refractive index and p is the pitch of the helicoidal structure.
i I
COMPRESSION
1u Figure 2. Déformation of the helicoidal superstructure leads to a variation of the pitch (therefore also KR) and simultaneously to the development of a piezoelectric voltage-
29
Liquid Crystal Elastomers with Piezoelectric Properties
Since the cholesteric phase lacks inversion symmetry macroscopically (because of the helicoidal superstructure), Brand pointed out by symmetry arguments that piezoelectricity must be observed6: a compression or elongation of the helicoidal structure along its helical axis induces an electric field in the direction of the helical axis. In LMLCs such a mechanical déformation of the helicoidal structure cannot occur; the mechanical field simply causes flow. In LCEs however, the LC molécules are attached to the network, preventing macroscopic flow. Consequently, a mechanical déformation of the sample parallel to the helical axis should cause the same effect as a mechanical force applied
Data Loading...
