A Study of Liquid Crystalline Elastomers as Piezoelectric Devices
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Although PVDF exhibits a large value of polarisation it has relatively poor ordering, compared to FLCPs which can exhibit a high degree of self-ordering. Hence, while the piezoelectric activity of some FLCP systems is similar in magnitude to PVDF, others may have the potential to surpass it. Further, their self ordering nature means that FLCPs do not require large thermal and electric poling fields to impose order unlike the case of PVDF, facilitating processing and fabrication. In general, organic systems are considerably less active than electro-ceramics such as lead zirconic titanate (PZT) [7,8], due to the molecular scale dipoles in the organics compared to the atomic scale dipoles in the ceramics. However, in certain applications, the lower modulus and ease of production offered by polymer systems offsets this reduction in sensitivity. An advantage which FLCPs have over ceramics and PVDF is their relatively low modulus which will produce a larger piezoelectric response. The most widely recognised measure of electromechanical activity in piezoelectrics is the d coefficient, defined as the ratio of dielectric displacement to applied mechanical stress. It is clear to see that this coefficient is a good indication of an electromechanical materials efficacy when used as a sensor. Due to the anisotropic nature of most piezoelectrics, the d term is described with reference to an orthogonal crystallographic system, via two subscripts defining, in this case, the direction of stress and resultant charge generated. They are referenced to the direction of net polarisation, designated as the z-axis or 3 direction. Hence the coefficient d33 refers to a force applied in and a charge generated in a plane perpendicular to the polarisation direction (z axis). It is also informative to note that the d coefficient can be approximated as illustrated in equation 1. d
Polarisation Modulus
Since the modulus of FLCPs can be considerably lower than the corresponding value for PVDF, they clearly offer the possibility of increased piezoelectric sensitivity. To achieve a high order parameter, a highly ordered, planar smectic C* phase is required. When a planar monodomain smectic C* phase is produced, the dipoles on the mesogenic chains will be unidirectional, thus maximising the magnitude of the spontaneous polarisation. Any subsequent perturbation to this idealised system will alter the Ps and therefore produce an electrical response. In order to achieve this it is usual to cool an appropriate liquid crystal from the isotropic state, via the nematic and smectic A states to the smectic C*. This phase sequence gives a more gradual increase in order parameter and hence leads to improved alignment and larger domain sizes. The helical superstructure can be unwound using modest surface; mechanical or electrical forces. In order to realise all of the above parameters a mixture of liquid crystal monomers is typically used. This provides for a system with a chiral dipole, the desired phase sequence and a highly ordered smectic C* phase over the
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