Alternate layer LB films

Alternate-layer LB films (Y-type, ABAB) of long-chain amines and fatty acids maybe used for pyrroelectric appHcations (Fig. 5). Stearylamine, C gH2yNH2, and a series of straight-chain fatty acids, yield a thick film (several hundreds of monolayers) which gave a pyrroelectric coefficient of - 0.05 nC/(cm -K) (72). A coefficient of 0.3 nC/(cm -K) for an 11-monolayer sample of CO-tricosenoic acid and docosylamine C22H4 NH2 has been reported (73). 

Molecularly layered systems with built-in control of defects would therefore be highly interesting as an alternative to LB-films and we have developed 2 different approaches that are based on the self-organization of liquid crystals and on layer-by-layer adsorption from solution. Advantages and drawbacks of both techniques are briefly discussed below. 

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. 

A unique but widely studied polymeric LB system are the polyglutamates or hairy rod polymers. These polymers have a hydrophilic rod of helical polyglutamate with hydrophobic alkyl side chains. Their rigidity and amphiphilic-ity imparts order (lyotropic and thermotropic) in LB films and they take on a F-type stmcture such as that illustrated in Fig. XV-16 [182]. These LB films are useful for waveguides, photoresists, and chemical sensors. LB films of these polymers are very thermally stable, as was indicated by the lack of interdiffusion up to 414 K shown by neutron reflectivity of alternating hydrogenated and deuterated layers [183]. AFM measurements have shown that these films take on different stmctures if directly deposited onto silicon or onto LB films of cadmium arachidate [184]. 

In Fig. 15 are shown UV-visible absorption spectra of type a and type b of 15 bilayers consisting of an alternate A-S-D triad-CaT (1 5) and a pure CaT mono-layer and type a of 15 bilayers containing A-S dyads in place of the triads. Hie LB films of type a were deposited on quartz plates, but the LB film of type b on a chemically modified quartz plate by octadecyltrichlorosilane. An absorption peak around 450 nm can be assigned to the acylated perylene moieties for the dyad and... 

The reason for Nafion LB-film fabrication was the wish to obtain the highly ordered systems from perfluorinated ion exchange polymer with multilayered structure, where the ionic layers (conductors) would alternate with fluorocarbon polymer layers (insulators), and to investigate the properties of such films.74 This polymer contains a hydrophobic fluorocarbon polymeric chain and hydrophilic ionic groups, so it is sufficiently amphiphilic it has a comblike structure that makes it a suitable polymer for LB-film deposition. 

Stable Z-type LB films have been prepared, however, by the alternative deposition of two different monolayers (A and B) in a head-to-head ABAB arrangement [142]. Two different monolayers can be layered a number of different ways (AABBAA, AABAAB, ABBABB, etc.) and there are a myriad of ways to arrange three or more different monolayers in LB films. These types of LB films have no plane of symmetry (i.e. they are non-centrosymmetric) and manifest non-linear optical behavior [108,143]. Schematics of some of the different types of LB films are illustrated in Fig. 11. 

LB films prepared from alternating layers of long-chain TCNQ and long-chain TTF and transferred to glass substrates Absorption spectra and conductivity measurements... 

Fig. 123a. Simplified structure of the polyion complex LB films showing ionically bound mono-layers of conjugated polymer sandwiched between stearylamine spacer groups with interdigitated hydrocarbon tails, b Simplified organization of the heterostructure LB films showing alternating layers of 1 1 PTAA-StNH2 and SPAn-StNH2 [767]...

In this chapter an attempt has been made to discuss ordered structures made using the LB technique and employing relatively simple molecules all of one kind. In the next chapter, films made from preformed polymers, from polymerisable small molecules and from alternating layers of two distinct kinds of molecule will be discussed. Once again the emphasis will be on structure and the characterisation of order. The discussion of lipids and lipid-like materials is deferred until Chapter 8 which will discuss biomembranes and bioactive molecules. At this stage the reader may well ask What of the vast number of other simple materials from which LB films have been made ... 

In this chapter we turn to the study of LB films formed from polymers and LB films consisting of alternate layers of two different amphiphiles. In principle, of course, it would be possible to superimpose successive layers of three or more distinct amphiphiles but little has been done in this direction. However, a few examples of more complex alternating structures will be considered. 

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