Ferro-, pyro- and piezoelectric behaviour

LB films are good candidate materials for exhibiting ferro-, pyro- and piezoelectric properties. Such behaviour requires a macroscopically polar structure normally inherent in acentric materials. Y-type LB films are unable to exhibit such properties, since these materials possess a centre of symmetry. LB films deposited by the X- or Z-type mode are therefore required, or combinations of different materials deposited alternately in the Y-type mode will also give AB-type multilayers having an overall macroscopic polarity. 

All ferroelectric materials are also piezo- and pyroelectric, although there are no known examples of ferroelectric LB films. However, bulk phase phthalocyanine (2) is known to undergo a ferroelectric transition at low temperature. By modifying the structure to achieve reversible spontaneous polarization upon application of an external electric field, it may be possible to produce ferroelectric LB films of such materials at more accessible temperatures. Applications for materials of this type include bistable switching devices and memory storage systems. 

The occurrence of piezoelectric behaviour in LB films has been known for some time [57,58], and a 30 X-type layer LB film of (37) was found to give opposite signs of the piezoelectric strain coefficients d i and d [59], the latter having a value of 1.5 pC which is approximately an order of magnitude lower than that of the well-documented polymer poly(vinylidene fluoride) (PVDF). Values for 31 of 0.023 and 0.170 pC N have also been obtained for alternate-layer structures of 22-tricosenoic acid with docosylamine, and a ruthenium complex with docosanoic acid respectively [60]. As the use of pyroelectric materials in detector applications requires that the materials possess only low levels of piezoelectricity (high levels introduce problems of microphony), this suggests that the former materials would be better suited for pyroelectric detector applications, while the latter system would be more appropriate for piezoelectric-based applications. 

There are no reports of ferro-, pyro- or piezoelectric properties being exhibited by LB polymers, although in principle there is no reason why such materials should not exhibit electroactive behaviour of this type. By incorporating suitable functional groups into polymerizable structures, it should be possible to produce useful LB pyro- and piezoelectric polymers 

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