Molecular engineering of pyroelectric polysiloxane Langmuir-Blodgett superlattices: synthesis, film preparation and pyroelectric properties

Molecular engineering of pyroelectric polysiloxane Langmuir-Blodgett superlattices: synthesis, film preparation and pyroelectric properties

T. Richardson*, W. H. Abd. Majid and R. Capan Department of Physics and Centre for Molecular Materials, University of Sheffield, Hounsfield Road, Sheffield S3 7RH, UK

and D. Lacey and S. Holder School of Chemistry, University of Hull, Cottingham Road, Hull HU6 7RX, UK (Received 25 April 1994; revised 27 June 1994)

A wide range of linear and cyclic polysiloxanes substituted with side chains containing carboxylic acid groups have been synthesized and characterized in terms of their Langmuir/Langmuir Blodgett (LB) film properties and their pyroelectric activity. The effects on these properties of varying the degree of side- group substitution, the length of aliphatic side groups, the incorporation of polar aromatic side groups and the deposition conditions utilized during the preparation of multilayer assemblies have been investigated. These materials form stable Langmuir layers at the air water interface which can be transferred onto substrates such as glass and aluminium-coated glass. The alternate layer LB deposition technique, in which each polysiloxane layer is co-deposited in an alternating stacking sequence with monolayers of a monomeric aliphatic amine compound, has been used to fabricate macroscopically polar films which display a temperature-dependent electric polarization, the 'pyroelectric effect'. Data are presented here for both linear and cyclic substituted polymer backbones showing that both systems provide useful insight into the pyroelectric behaviour of organic materials. Trends in the relationships between the pyroelectric activity and (1) the chemical structure of the materials and (2) the structure of the acid/amine superlattice have been identified and indicate that the optimum pyroelectric coefficient is observed for a linear copolysiloxane compound substituted with a polar aromatic pendant side group. Indeed, the pyroelectric coefficient measured for this material is ~10/tCm -2 K -~ which is currently the highest value reported for an LB assembly to our knowledge.

(Keywords: pyroelectricity; polysiloxane; Langmuir-Biodgett film)

I N T R O D U C T I O N

Pyroelectricity is the manifestation of the temperature dependence of the spontaneous polarization in the solid (or liquid crystalline) state. Altering the temperature of a pyroelectric material, sandwiched between two metal electrodes connected together via a sensitive ammeter or electrometer, results in the modification of its electrical polarization and thus an electrical current flows as long as the polarization continues to change. The sign of this current depends upon on whether the material is heated or cooled. Therefore pyroelectric materials are useful detectors capable of sensing temperature changes. The phenomenon of pyroelectricity j has been observed in a wide range of materials such as barium titanate 2, triglycine sulfate 3, poly(vinylidene difluoride) 4 and recently in various

* To whom correspondence should be addressed

Langmui~Blodget t (LB) materials 5. For most potentially pyroelectric solids, it is necessary to process the material after deposition or growth in order to achieve the non-centrosymmetry required to induce pyroelectric activity. For example, thin films of poly(vinylidene difluoride) must be electrically poled 6 after deposition in order to align the electric dipoles responsible for the pyroelectric effect. The activity of such a material is normally zero without such proces- sing.

Pyroelectric LB films have attracted much attention during the last decade because the LB deposition process itself can facilitate the preparation of electrically polar assemblies, thus removing the reliance on post-deposition poling treatments. The main virtue of the LB deposition technique is its utility in assembling sequentially stacked monolayers such that individual molecular dipoles within each monolayer can align in a preferred direction. The specific process involved is

0968-5677/94/01/0039 15 ~,~ 1994 Butterworth-Heinemann Ltd SUPRAMOLECULAR SCIENCE Volume 1 Number 1 1994 39

Pyroelectric polysiloxane LB superlattices: T. Richardson et al.

often referred to as the alternate layer LB technique7; the resulting multilayer films have an A B A B A . . . molecular arrangement in which two monolayers A and B are alternately co-deposited. Thus if the molecular dipoles associated with monolayers A and B are not equal, a net polarization results.

Several categories of LB film-forming materials have been studied over the last decade in order to determine their pyroelectric properties 8 ~0. One of the most interesting has been the acid-amine system 1l in which monolayers of long chain carboxylic acid molecules are co-deposited with monolayers of long chain alkyl- amines. The acid-amine interactions at each bilayer interface have been shown to lead to temperature- dependent macroscopic polarization, i.e. pyroelectricity 12. However, the chemical design of these molecules offers relatively little scope for modification. For instance, the effects of shortening the molecule or reducing the molecular surface packing density cannot easily be examined with this purely aliphatic monomer system since a minimum alkyl chain length of-C15H31 is required to maintain insolubility in water and thus to form a stable Langmuir layer on a water subphase.

Therefore, in this research programme an alternative approach has been taken in order to retain acid-amine interactions within alternate layer LB films, whilst allowing exciting modifications to the chemical structure to be made in order to examine their effect on the film-forming and pyroelectric properties. Several different polysiloxane backbones have been selected onto which a range of aliphatic and aromatic pendant side chains containing carboxylic acid headgroups have been grafted in order to obtain several families of polysiloxanes suitable for use with the LB deposition technique 13. These compounds have then each been alternately deposited with a monomeric alkylamine [eicosylamine: CH3(CH2)19NH2] to form polar structures possessing planes of acid-amine pairs at each bilayer interface in addition to other interesting molecular features which will be discussed later.

This paper describes the synthesis, film preparation and pyroelectric performance of alternate layer assemblies containing a wide range of novel linear and cyclic homo- and random copolysiloxanes.

E X P E R I M E N T A L

Physical measurements

Structures of the intermediates, monomers and polymers were confirmed by one or more of the follow- ing techniques: 1H and 13C n.m.r. (Jeol JNM-GX 270MHz spectrometer), i.r. spectroscopy (Perkin- Elmer 783 grating spectrophotometer) and mass spectrometry (Finnigan-MAT 1020G/MS spectrometer). Glass transition temperatures and melting points of the cyclic and linear polysiloxanes were measured using a Perkin-Elmer DSC-7C with thermal analysis data station and cooling accessory. The instru-

mental accuracy of the differential scanning calorimeter was calibrated against an indium standard. The purity of the intermediates and monomers were checked by n.m.r, spectroscopy, t.l.c. (single spot purity) and reverse phase chromatography (5mm pore size, 25 × 0.46cm, C18 Microsorb Dynamax column, eluting with acetonitrile or methanol). The polydisper- sity (Mw/Mn) and the degree of polymerization of the polymers were determined by g.p.c, using a PL gel column (5 mm, 30 × 0.75 cm, mixed C column) as the stationary phase and tetrahydrofuran (THF) as the mobile phase. The column was calibrated using polystyrene standards (Mp = 1000-450 000).

All the Petrarch polysiloxanes detailed in Table 1 were purchased from Fluorochem Ltd (Old Glossop, UK). The term polysiloxane is used in a completely general way and refers to any of the above polymeric materials. The prefixed PS denotes a linear homopolysiloxane with PS 100 being a linear homopolysiloxane and PS 50, PS 30 and PS 15 being linear copolysiloxanes with increasing amounts of (CH3)2SiO along the polymer backbone. The cyclic polysiloxane is designated according to the number of SiO units in the cyclic backbone.

From the data given in Table 2 it can be seen that there is good agreement, except for the data given for PS 15, between 29Si n.m.r., i H n.m.r, and the information supplied by Fluorochem for both the number of SiO units in the polymer backbone and for the ratio of (H)SiO(CH3) units (X) to (CH3)2SiO units (Y). With regard to the data on the number of SiO units in the polymer backbone for PS 15, there is good agreement between the information supplied by Fluorochem and 1H n.m.r, but the 298i n.m.r, gives this value at ~60% of that obtained by 1H n.m.r.

In this and previous articles 14 18 on our work in this area of LB thin films we have taken the IH n.m.r, data to be an accurate indication for both the number of SiO units in the polymer backbone and for the ratio X/Y. Thus the linear polysiloxane backbones cited in this paper will have the characteristics shown in Table 3.

Materials

The synthetic route used to prepare the substituted linear homo- and copolysiloxanes 4a--e and 5a-e and the cyclic polysiloxanes 6a and 6b incorporating the

Table 1 Polymer backbones and abbreviations used in the text

Polymer backbone Abbreviation

Poly(hydrogenmethylsiloxane)

50-55% Hydrogenmethylsiloxane: 45 50% dimethylsiloxane copolymer

30-35% Hydrogenmethylsiloxane: 65 70% dimethylsiloxane copolymer

15-18% Hydrogenmethylsiloxane: 82-85% dimethylsiloxane copolymer

Tetrahydrogenmethylcyclotetrasiloxane

PS 100

PS 50

PS 30

PS 15

Si4

40 SUPRAMOLECULAR SCIENCE Volume 1 Number 1 1994


Table 2 Structural data for the linear homo- and copolysiloxanes

,c.,,,sio /s iot . \ H ,Ix \ C H 3 )

Si(CH03

Code no.

Fluorochem 29Si n.m.r. IH n.m.r.

no.

Information from Fluorochem

X Y X Y X Y M W (%) (%)

No. of SiO units in polymer backbone

From M W 298i n.m.r. IH n.m.r. in column 9

PS 100 PS 120 33 ± 2 0 PS 50 PS 122.5 8 8 7 ± 1 6 ± 1 50-55 45-50 PS 30 PS 123 9 21 10 ~ I 17 ± 1 3(~35 65-70 PS15 PS123.5 2 16 6 ± 1 2 3 ± 1 15 18 82-85

2270 37 33 • 2 900-1000 14 16 13 ± 2

2000-2100 29 30 27 ± 2 2000-2500 31 18 29 ~ 2

benzyl esters of ~o-alkanoic acid as the side chain is shown in Scheme 1. The synthetic route to the corresponding substituted linear homo- and copolysiloxanes 13a and b and 14a--e and the cyclic polysiloxanes 15a-c incorporating, as the side chain, the benzyl esters of to-(4-alkoxyphenyl)alkanoic acid is shown in Scheme 2. The removal of the benzyl group and the structure of the linear homo- and copolysiloxanes and the cyclic polysiloxanes incorporating the co-alkanoic and the ~o-(4-alkoxyphenyl)alkanoic acid moieties used in this study are shown in Scheme 3.

Undec-10-enoic, pent-4-enoic acids (7a and b) and the acid chloride of undec-10-enoic acid were all purchased from Aldrich. The preparation of heptadec- 16-enoic acid (2) was made according to the method described by Barraud et al. 19. The esterification of compounds 2 and 7a and b using benzyl alcohol was carried out according to the method described by Hassner and Alexanian 2°. The procedure used for the hydrosilylation of the benzyl esters to give the substituted linear and cyclic polysiloxanes 4a-c, 5a-e and 6a and b and their subsequent purification is described by us in a previous paper j6.

4-(4-Methoxybenzoyl)butanoic acid (8) was prepared by the Friedel-Crafts acylation of an±sole (Aldrich) using 4-(chloroformyl)butanoic acid (Aldrich) in a similar way to that outlined in reference 21. Reduction of compound 8 was carried out with 2 molar equiva- lents of triethylsilane in trifluoroacetic acid to give 5-(4- methoxyphenyl)pentanoic a c i d (9) 22. Finally, the methyl group was removed by treatment with a large excess of hydrobromic acid in glacial acetic acid to give 5-(4- hydroxyphenyl)pentanoic acid (10) 23. 5-(4-Hydroxyphe- nyl)pentanoic and 3-(4-hydroxyphenyl)propanoic acids were converted to their corresponding ~o-(4-alkenoxy-

Table 3 Polymer characteristics

Code no. X Y No. of SiO units in polymer backbone

PS 100 33 ± 2 0 33 4- 2 PS50 7 ± 1 6 + 1 1 3 + 2 PS30 1 0 ± l 1 7 + 1 2 7 i 2 PS 15 6 ± 1 2 3 ± 1 2 9 ± 2

phenyl)alkanoic acids 11, 16a and 16b by using a variation on the Williamson synthesis 24. The preparation of the benzyl esters 12a-e and the subsequent hydrosilylation reaction to give the linear and cyclic polysiloxanes 13a and b, 14a-c and 15a-c was similar to the procedures outlined for the preparation of compounds 2 and 7a and b and the polysiloxanes 4a--e, 5a-e and 6a and b given in Scheme 1.

The removal of the benzyl group and the purification of the linear and cyclic polysiloxanes is described by us in a previous paper 16. Structures and identification numbers of the polymers are shown in Figure 1.

LANGMUIR FILM PROPERTIES OF CYCLIC AND LINEAR HOMO- AND COPOLYSILOXANES

Langmuir films of all materials were prepared using solutions of concentration around 0.2-0.3mgml ~ in which the solvent was either chloroform or a 1:1 mixture of chloroform and ethyl acetate (Tables 4 and 5). These solutions were spread dropwise onto the cleaned water surface (Elga UHP) of a constant perimeter single compartment Langmuir trough, allowing several minutes after spreading for solvent evaporation. The area in which the Langmuir layer was confined was reduced linearly at the rate of 0.7% s -1 until the minimum area was reached. The surface pressure was monitored using the well-known Wilhelmy plate method.

All of the polysiloxanes shown in Figure 1 contain pendant side groups which are terminated with polar carboxylic acid moieties which interact strongly with the water subphase. It is expected therefore that the polysiloxanes will be orientated with the acid groups on the water and the pendant rods directed approximately normal to the plane of the subphase surface. The linear polymers would appear as upright combs meandering across the Langmuir trough with their teeth aligned orthogonally to the water surface and the polysiloxane backbone aligned along the direction parallel to the water surface plane. The cyclic polysiloxanes are expected to behave similarly with the pendant side groups orthogonal to the water surface and the

SUPRAMOLECULAR SCIENCE Volume 1 Number 1 1994 41

Pyroelectric polysiloxane LB superlattices: T. Richardson et a I.

CH2=CH(CH2)8COCI

1.1

CH2=CH (CH2)8CO(CH2)sCO2H (1)

1.2

CH2=CH(CH2)14CO2H (2)

~ 1.3

1.3 CH2=CH (CH2)n.2CO2CH2C6H5 (3a-c)

1.4

CH2=CH(CH2)n.2CO2H

1 1 (CH3 k [ GH3 k / O H 4

s i o - - - - j - . . . . " 1 - \ 6HUy \(TH2)n / 33 .-I: 2

/ CO2CH2C6H 5 CO2CH2CeH5 CO2CH2CeH 5

(4a-¢) (5a-e) (6a,b)

(7a,b)

3a, n=4; 3b, n=10; 3c, n=16.

4a, n=4; 4b, n=10; 4c, n=16.

5a, n=4, x=7, y=6; 5b, n=10, x=7, y=6; 5c, n=10, x=10, y=17; 5d, n=10, x=6, y--23; 5e, n=16, x=7, y=6.

6a, n=10; 6b, n=16.

7a, n=4; 7b, n=10.

Scheme 1

1.1 (i) 1-morpholino-l-cyclohexene, triethylamine, chloroform; (ii) dil. HCI; (ii) KOH, H20; (iv) dil. HCI 1.2 hydrazine hydrate, KOH, diethylene glycol 1.3 N,N-dicyclohexylcarbodiimide, pyrrolidinopyridine, benzyl alcohol, dichloromethane 1.4 Spiers catalyst, appropriate polysiloxane backbone, toluene

siloxane ring orientated parallel to the surface. Surface pressure-area isotherms have been recorded for the materials shown in Figure 1. A typical example of an isotherm of a linear and cyclic polysiloxane is shown in Figure 2. The surface pressure rises gradually as the Langmuir layer is compressed unlike the classical three- phase behaviour of monomeric aliphatic acids. The area axes of these plots are given as the area per siloxane unit.

Table 4 shows the area per siloxane residue data for the cyclic polysiloxanes so far investigated. Two sets of area data are shown. The fourth column gives the areas measured at the surface pressure indicated (shown in

square parentheses) and the fifth column displays the values found by extrapolating the most steeply rising part of the surface pressure-area isotherm to zero surface pressure; the ratio between the two values gives an indication of the compressibility of the Langmuir layer. First, it is clear that the extrapolated area values lie in the range 0.33~).44 nm 2 except for cyclic polymer 22c which surprisingly shows a very large value at 0.78 nm 2. Molecular modelling using CPK space-filled models has suggested that the theoretical area per siloxane unit lies in the range 0.2-0.25 nm 2. A comparison of the cyclic and linear polysiloxanes by reference to Table 5 shows that the area per cyclic siloxane residue is

42 SUPRAMOLECULAR SCIENCE Vo lume 1 Number 1 1994


CHsOa 2.1_ II

CH,O--Q- C(CH&CO,H 2.2

(8)

HO--Q- O-MJQH

J (10)

2.4

CH2=CH(CH2)e0 + (CH2),C02H

(11)

J 2.5

2.5 cH~=CH(CH~),~O+-- (CH,&,,CO$H2CsH5 -

I

(12a-c)

W+- (CH2)4C0#

(9)

2.3

HO--Q- W,),CW

J 2.4

CH2=CH(CH2)~_20+(CH2)zC0$-i

Wa,b)

I 2.6

I I I {(F&2 {(F&---- +: jy @

0Ph(CH2)&02CH2CsH5 0Ph(CH2),,,C02CH2CeH5 0Ph(CH2),C02CH2C,HS

(13a,b) (14a-c) (15a-c)

12a, n=4, m=2; 12b, n=8, m=2; 12~ n&3, m=4.

13a, n=4, m=2; 13b, n=a, m=4.

14a, n=4, m=2, x=7, y=6; 14b, n=4, m=2, x=10, y=17; 14c, n&3, m=2, x=7, y=6_

15a, n=4, m=2; 15b, n=a, m=2; 15c, n=a, m=4.

16a, n=4; 16b, n=6

2.1 (i) CI.0C(CHz)&02H, AK&, 1,1,2,2Mrachloroethane; (ii) dil. HCI 2.2 2.3

(i) CFsCQH, (CHsCH2)sSiH; (ii) dil. NaOH; (iii) dil. HCI

2.4 (i) CHsCOzH, HBr; (ii) dil. NaOH; (iii) dil. HCI (i) appropriate bromoalkene, KOH, methanol, H20; (ii) dil. HCI

2.5 NJ-dicyclohexylcarbodiimide, pyrrolinopyridine, benzyl alcohol, CH$I, 2.6 Spiers catalyst, appropriate polysiloxane backbone, toluene

Scheme 2

always larger than the area per linear siloxane residue. This is explained by the fact that the pendant side groups on the cyclic polymers cannot pack together as efficiently as those on the corresponding linear backbone and also because the linear polymer molecules themselves are able to pack tightly like logs on a river whereas cyclic ring polymers pack less efficiently. As expected the aromatic-substituted cyclic polysiloxanes occupy a larger cross-sectional area than the aliphatic polymers owing to steric hindrance associated with the

phenoxy rings. It is important to note that these cyclic polysiloxanes exist in different isomeric forms25. No

attempt has been made by the authors yet to separate these forms although this will be necessary to facilitate a thorough interpretation of the area data described here.

Table 5 gives the area per molecule data for the linear polysiloxanes studied during this work. The areas evaluated at 25 mN rn-’ (unless indicated) lie in the range 0.14-0.21 nm2 except polymer 18e which shows a value of 0.27nm2 and the PS15 and PS30 polymers (He and d)



Scheme 3

C02C%!-Y45 C02C%C6H5

(4a-c) (5a-e)

C02CbC6H5

030)

C02H

(17a-c)

(20a,W

COPH CO,H

(18a-e) Wa,W

p-&---@y @

OPh(CH,),CO,H 0Ph(CH2),C02H

(21a-c) (22a-c)

{+$3 f 2 {(PA---- +i$y 0Ph(CH2),C02CH2C6H5 0Ph(CH2),C0,CH2C6HS

(13a,W (14a-c)

3.1 5% PdlC, ethanol and/or ethyl acetate, HQ)

0Ph(CH2),,,C0fiH2C6HS

(15a-c)

which yield very small area values. These small values

suggest that either (1) the Langmuir films of these materials are imperfect and contain regions in which the film is

several molecules thick or (2) the polysiloxane backbone has bunched or coiled such that the side groups have

become more closely packed. To a first approximation, the flexibility of the polysiloxane backbones would be expected to increase as the degree of side-group substitution decreased; bunching or coiling may therefore be observed where the backbones are only lightly substituted. The area loss of all of the Langmuir films formed was found to be ~0.5% h-’ below their collapse pressure. Further details of the structure of the surface pressure- area isotherms can be found in the literature26.

FABRICATION OF POLAR LB FILMS

CONTAINING POLYSILOXANES

The alternate layer deposition technique was used to prepare non-centrosymmetric, polar multilayer LB films in which each polysiloxane was co-deposited with

monomeric eicosylamine [CHs(CH2)i9NH2]. A two- compartment Langmuir trough was used for this purpose2’. The subphase was ultra-pure water at pH 5.9 (stabilized in air ambient) and 20°C. The eicosylamine was deposited at 22.5 mNm_’ in all cases, the deposition surface pressure of each polysiloxane layer being shown in Tables 4 and 5 (column 6). The substrates used consisted of Super Premium



Linear-Aliphatic Polysiloxanes

C02H C02H

(17a-c) (18a-e)

17a, n=4; 17b, n=lO; 17c, n=16.

18a, n=4, x=7, y=6; 18b, n=lO, x=7, y=6; 18c, n=lO, x=10, y=17; 18d. n=lO, x=6, y=23; 188, n=l6, x=7, y=6.

Cyclic-Aliphatic Polysiloxanes

C02H

(19a,b)

19a. n=lO; 19b, n=16.

Linear-Aromatic Polysiloxanes

OPh(CHZ),,,C02H

(20a,b) 0Ph(CH2),C02H

(21 a-c)

20a, n=4. m-2; 20b, n=8. m=4.

2la. n=4, m=2, x=7, y=6; 21b, n=4, m=2, x=10, ~~17; 21c, n=8, m=2, x=7, y=6.

Cyclic-Aromatic Polysiloxanes

0Ph(CH2),C02H

(22a-c)

Figure 1 Structure of the linear and cyclic polymers

22a, n=4, m=2; 22b, n=8, m=2; 22c, n=6, m=4.

Table 4 Area per siloxane data and deposition surface pressures for the cyclic polysiloxanes

Compound Siloxane backbone

Pendant side group Area/siloxane unit at [nmNm-‘1

(nm*)

Area/siloxane Deposition unit (extrapolated surface to A = 0) pressure

(nm*) (mNm_‘)

19a Si4 -(CHz),,COzH 0.23 [20.0] 0.37 22.5 19b Si4 -(CH2)&02H 0.18 [17.0] 0.33 15.0 22a Si4 -(CH&,0Ph(CH2)2C02H 0.30 [20.0] 0.44 20.0 22b Si4 -(CH&OPh(CH&COzH 0.33 [20.0] 0.42 20.0 22c Si4 -(CH&0Ph(CH&C02H 0.54 [17.0] 0.78 15.0



Table 5 Area per siloxane data and deposition surface pressures for the linear polysiloxanes

Compound Siloxane Pendant side group Area/siloxane backbone unit at [ n m N m i]

(nm 2)

Area/siloxane unit (extrapolated to n -- 0) (nm 2)

Deposition surface pressure (mNm I)

17b PS 100 -(CH2)IoCO2H 0.21 [25.0] 18a PS 50 -(CH2)4CO2H 0.18 [25.0] 18b PS 50 -(CH2)toCO2H 0.18 [25.0] lge PS 50 -(CH2)I6CO2H 0.27 [20.0] 18c PS 30 -(CH2hoCO2H 0.05 [20.0] 18d PS 15 -(CH2)toCO2H 0.04 [25.0] 20a PS 100 -(CH2)4OPh(CH2)2CO2H 0.14 [25.0] 21a PS 50 -(CH2)4OPh(CH2)2CO2H 0.18 [25.0] 21c PS 50 -(CH2)8OPh(CH2)2CO2H 0.15 [25.0] 21b PS 30 -(CH2)4OPh(CH2)2CO2H -- "

0.23 25.0 0.24 25.0 0.23 25.0 0.32 20.0 0.09 20.0 0.07 25.0 0.24 25.0 0.25 25.0 0.20 25.0

"Difficulty in dissolving

Microscope (Merck Ltd, Dorset, UK) glass slides coated with a 50nm layer of ultra-pure aluminium. Details of the device geometry are described in the next section. The hydrophilic substrate was initially positioned underneath the water surface. It was then withdrawn through the compressed polysiloxane (mono)layer (A) at a speed of ~ 1 0 m m m i n i. The

a)

3~

,'-" 30

z0

10'

~ 5 U3

2

0.1 0.2 0.3 o:, o:5 o:~ 03

Area per siloxane unit (nm 2)

0.8

b)

2o

tJ 5

U3 0

Figure 2

22b

o'~ o., 0.6 o.s i ,'.~ i. , ~16 l.~

Area per siloxane unit (nm 2)

Surface pressure-area isotherms of (a) linear copolysiloxane 21a and (b) cyclic polysiloxane 22b

monolayer-coated substrate was subsequently inserted through the compressed eicosylamine monolayer (B) at a speed of 60mmmin -1. This alternate layer process was repeated until the desired number of polysiloxane/ amine bilayers had been deposited. Finally, the substrate was withdrawn through the polysiloxane layer to yield an A B A B A . . . type polar LB film containing an odd number of layers. Transfer ratios for both the polysiloxane and the eicosylamine monolayers were close to unity (>0.95).

PYROELECTRIC DEVICE S TRU CTU RE AND PYROELECTRIC ASSESSMENT

In order to measure the pyroelectric activity of the alternate layer LB films, it is necessary to fabricate a capacitor-type device in which the LB multilayer forms the dielectric material. The geometry of the pyroelectric test sample is shown in Figure 3. Its simple structure consists of a glass substrate (as described above) on which a 50nm layer of aluminium has been thermally evaporated (Edwards 306A coating unit). The aluminium coating has been patterned using straight- forward shadow-masking techniques such that a thin strip of glass remains uncoated along the long axis of the microscope slide. The alternate layer LB film is then deposited over the entire surface of this substrate except for a region blanked over using a thin tape. The upper electrodes consist of 50nm thick aluminium strips thermally evaporated onto the LB layer. This process must be performed with care in order not to damage the organic film. Evaporation at a rate of <0.1 nms J for the first 5nm followed by a gradual increase in the rate to <0 .5nms -t ensures that high quality electrodes are produced.

The test device is placed in an evacuated pyroelectric test chamber described in detail elsewhere 28. The princi- ple of the measurement of the pyroelectric coefficient arises from the simple theory associated with the phenomenon. The change in polarization which occurs when the temperature of a pyroelectric material is altered results in an electrical current if the two electro-


Pyroelectric polysiloxane LB superlattices: 7". Richardson et al.

• # A1/Au base electrode

I"~'~'~'~$"* " ' ~ ~ ~ i " :?'" '~z,.:.:.'+':'."-~." ", • ,'~." :~, -~.~ "-,~." ".'--'.--:

[:"~: , ] G l a s s s u b s t r a t e

LB super la t t ice - 40 n m

il "

Figure 3 Pyroe lec t r i c tes t dev ice g e o m e t r y

des of the pyroelectric cell are short-circuited. This current is given by Ip where:

I v = FA (dT/dt) (1)

where F is the pyroelectric coefficient (the rate of change of polarization with respect to temperature dP/dT), A is the area of overlap of the two electrodes and dT/dt is the rate of change of temperature with respect to time. This equation suggests a simple yet elegant method for the measurement of the pyroelectric coefficient. This consists of heating and cooling the pyroelectric cell in a triangular wave fashion whose amplitude is restricted to ~I°C and whose frequency is ~0.02-0. ! Hz. Linear heating and cooling ramps can be achieved with the use of current-driven Peltier thermo- electric pumps. If the dielectric material exhibits pyroelectricity, then the expected current response would resemble the derivative of the imposed temperature profile and therefore would appear as a square- wave signal. Indeed, such square-wave current responses are observed in practice as shown schemati- cally in Figure 4a. Only the peak-to-peak current value, Ip_p and the total rate of change of temperature (i.e. heating and cooling rates are both used) are required for calculating the pyroelectric coefficient provided the

electrode overlap area is known. In practice, a range of heating and cooling rates are often imposed upon the cell and a plot of Ip_p against (dT/dt)total is drawn, where (dT/dt)total is given by:

(dT/dt)total ~- I(dT/dt)heating[ + I(dT/dt)coolingl (2)

Such a plot is shown in Figure 4b indicating the expected linear relationship between Ip_p and (dT/dt)total. The gradient of this line yields the product AF and thus the pyroelectric coefficient can be evaluated. All measurements are performed in an evacuated (at least 1.33Pa), earthed chamber. The contacts between the current-measuring device (a Keithley 617 electrometer) and the pyroelectric cell electrodes are made using small globules of silver- loaded paint. The samples are kept under vacuum with short-circuited electrodes for at least 1 h before testing begins. The pyroelectric current profiles are recorded using either the analogue output of the electrometer and a suitable chart-recorder or via an interface to a personal computer. Each pyroelectric measurement consists of performing ~10 full temperature cycles (heating and cooling) about a mean temperature. This mean temperature can be modified by adjusting the Peltier heater drive current appropriately. Thus the


Pyroelectric polysiloxane LB superlattices: T. Richardson et a l.

a)

i - - - - I

L__ I

V a c u u m chamber

I

i

K

A A / / V V

T u n e

b)

8 <

"~ 7

~ 5 6 U

"~ 4

o 3 K

o_.., 0

0

" " " I - " " I " " - I " " - I - - I " [ ] " " "

J

!

. . . ~ t . . . . , . . . . i . . . . ! . . , . I . . . . ~ . . . . . . I

0.01 0.02 0.03 0.04 0.05 0.06 0.07

Rate of change of temperature, dT/dt (K s l )

Figure 4 (a) Schematic diagram of the measurement configuration and the ideal temperature and pyroelectric current profiles and (b) real data of the peak peak pyroelectric current plotted versus the rate of change of temperature

temperature dependence of the pyroelectric coefficient itself can be evaluated over the desired temperature range. In these experiments a range of 15~0°C has been adopted.

PYROELECTRIC BEHAVIOUR OF POLYSILOXANE/EISCOSYLAMINE LB FILMS: OPTIMIZATION USING M O L E C U L A R E N G I N E E R I N G

The pyroelectric activity of a large number of alternate layer samples of both cyclic and linear polysiloxanes has been determined during this research programme. In this section the iterative molecular engineering steps in the optimization of the pyroelectric performance of these materials are described. First, the pyroelectric properties of cyclic polysiloxanes are detailed followed by those of their linear relatives. Finally, a comparison is made between the two types of polymer backbones.

Pyroelectricity in LB films containing cyclic polysiloxanes

The pyroelectric coefficient of 11 layer alternate layer LB films of each cyclic polysiloxane co-deposited with eicosylamine was measured over a range of temperature. For purposes of comparison, their pyroelectric coefficients at room temperature (21°C) are shown on the histogram in Figure 5a. There are several interesting features of these data. It is clear that the film containing the shorter side chain aliphatic-substituted cyclic polymer 19a displays a larger coefficient than that incorporating the -(CH2) I6CO2H substituted cyclic polymer 19b. This is partially a result of the increase in the density of acid-amine pairs associated with the former case. Such acid amine pairs are known to give rise to a temperature-dependent dipole moment as a result of proton transfer between the two moieties. Decreasing the volume in which a constant number of acid-amine pairs provide such dipoles results in an increase in the polarization (the total dipole moment

48 S U P R A M O L E C U L A R SCIENCE V o l u m e 1 N u m b e r 1 1994


a)

~.--'1"4 I 1.2

1

= 0.8

0.6

0 4 u 8 0.2

a, 0

i~iiiil;~i:iiiilili:iiiiiiiiiiiiii!ii~i!i!

19a 19b 22a 22b 22e

b)

1.8

1.6 • ..,,..,.o 22a

r) v ~ 1 . 4

~ " 1 . 2

.~ l

0.8

0.6

0.4 ~ 19a

0.2 ~ . , . 1 . I

0 10 20 30 4 0 50

Temperature, T (*C)

Figure 5 (a) Comparison of the pyroelectric coefficients of cyclic polysiloxanes 19a, 19b, 22a, 22b and 22c at 21 C and (b) the temperature dependence of the pyroelectric coefficient for polymers 19a and 22a

per unit volume). Since this polarization is temperature dependent, the pyroelectric coefficient rises. However, this increase cannot be explained purely by volume considerations since the nominal percentage volume increase is ~20% whereas the increase in the pyroelectric coefficient is nearly 150%. Clearly, there must be a second pyroelectric mechanism which has become more active in the short-chain case. Further research is currently underway to attempt to identify this source of pyroelectricity. The increase in the density of acid- amine pairs arises directly from the incorporation of the cyclic siloxane ring in this system. A fatty acid monomer of this chain length [-(CH2h0CO2H] would not be stable as a monolayer at the air-water interface.

The pyroelectric behaviour of the alternate layer films containing the aromatic-substituted polysiloxanes are also very interesting. First, it is evident that a reduction in the length of the alkyl chain separating the siloxane ring from the phenoxy group from -(CH2)8- (22b) to -(CH2)4- (22a) results in a rise in the pyroelectric coefficient by a factor of ~13. The reason for this dramatic increase is not yet understood, but as in the previous case, clearly cannot be explained only in terms of the increase in volume. A comparison of cyclic polymers 22b and 22e shows that increasing the length of the alkyl spacer group between the benzene ring and

the carboxylic acid headgroup causes an enhancement of the pyroelectric activity by a factor of ~7. In this case, the density of acid-amine pairs has actually decreased yet the pyroelectric coefficient has increased. This strongly suggests that the second mechanism responsible for pyroelectric activity in these materials is indeed the dominant one.

Finally, it is informative to compare the activity of the films containing cyclic polymers 19a and 22a. The pendant side-chain length is very similar in both molecules although their dipolar nature is very different. The inclusion of the phenoxy moiety has resulted in an increase in activity by a factor of ~3. This result shows that the strategy of incorporating polar units into the side chain of cyclic polysiloxanes has succeeded in enhancing their pyroelectric activity. Figure 5b depicts the temperature dependence of the pyroelectric coefficient associated with LB films of these two compounds. The coefficient of the film containing the aliphatic pendant side groups is virtually temperature-independent over this range, yet the film containing the aromatic side groups is strongly dependent on temperature.

Pyroelectricity & LB fihns containing linear homo- and copolysiloxanes

The data described in the above section in particular have provided encouragement to investigate the temperature-dependent pyroelectric behaviour of alternate layer films containing linear polysiloxanes in order to determine whether the same trends are observed in such systems. This work has been divided into three main parts on investigations into the effect on the pyroelectric activity of modifying (1) the degree of aliphatic pendant side-chain substitution utilizing homo- and copolysiloxane backbones, (2) the length of the side groups grafted onto the backbone in the purely aliphatic systems and (3) the inclusion of phenoxy groups within the side groups in order to increase the polarization in this direction.

Homo- and copolysiloxane backbones incorporating aliphatic side chains, Four different kinds of polysiloxane backbone have been used facilitating the synthesis of linear polymers 17b, 18b, 18e and 18d. These materials all contain pendant -(CH2)10CO2H side chains but the degree of substitution (the number of side chains per unit length) varies according to the fraction of silicon-hydrogen bonds along the backbone. Alternate layer films (25 layers) of three of these materials were produced using eicosylamine as the co-deposited layer. Linear polymer 18d was found not to transfer onto solid substrates from the air-water interface. Figure 6a shows the pyroelectric coefficients at 21°C; it is clear that the highest activity (1.1/tCm ZK-]) is observed for the PS 50 copolymer, 18b. At first inspection this may appear surprising since the density of acid-amine interactions is a maximum in the alternate layer film incorporating the homopolysi-


Pyroelectric polysiloxane LB superlattices: 7-. Richardson e ta I.

-'~ 1.2

: : L

0.8 g

0.6

§ 0.4

0.2

0 17b 18b 18e

b) 2.1

, - - - ,

1.9 E r.) 1.7

1.5

"B 1.3

u

0.9

o ~>, 0.7

0.5 16 32

i i i I I I I "

18b

o 1 8 c

. . I . . I . . . I . . . I . . . 1 . . . I . . . I . . .

18 20 22 24 26 28 30

Temperature, T ('C)

F i g u r e 6 (a) Comparison of the pyroelectric coefficients of linear polysiloxanes 17b, 18b and 18c at 21°C and (b) their temperature dependence

loxane 17b. However, it has been suggested by several researchers29 3~ that a dipolar tilting mechanism, in which dipoles aligned roughly normal to the substrate plane tilt when the temperature changes, presents a second source of pyroelectricity in addition to the temperature-dependent acid-amine dipole. It is proposed that in this case the free volume created due to the unsubstituted silicon sites provides encouragement for this tilting or reorientational process. The PS 30 copolymer, 1Be, yields an intermediate activity suggesting that there may be a trade-off between the pyroelectric contributions arising from both proton transfer associated with the acid-amine interactions and reorientation of the dipoles through a tilting process. Figure 6b gives the temperature dependence of the pyroelectric coefficient in these three LB films showing that the above trend exists over the entire range 17-30°C.

Effect of adjusting the length of the aliphatic side chain. Linear polymers 18a, 18b and 18e provide a series in which the length of the substituted aliphatic acid side chain has been modified whilst the degree of substitution remains unaltered. An investigation of the pyroelectric behaviour of polysiloxane/eicosylamine alternate layer films of these polymers was performed

in order to establish the effect of reducing the density of acid-amine pairs. The PS 50 backbone was used owing to its success in the previous investigation. Figure 7a shows the pyroelectric coefficients at 29°C. First, the -(CHz)16CO2H substituted copolysiloxane 18e yields only a very small pyroelectric activity. At 21°C there is little difference in activity of the -(CH2)10COzH and -(CH2)4CO2H substituted polymers 18b and 18a, respectively. However, as shown in Figure 7a, at 29°C this difference is large, the shorter chain derivative showing a 50% higher pyroelectric response. Reference to the section 'Pyroelectricity in LB films containing cyclic polysiloxanes' indicates that a similar trend was observed for the case of the cyclic aliphatic acid substituted polymers 19a and 19b. Figure 71) presents the pyroelectric data for these three LB films as a function of temperature showing that the difference in activity of polymers 18a and 18b progressively increases with increasing temperature. Interestingly, in both cases the pyroelectric activity increases with increasing temperature but the temperature dependence is greater for polymer 18a.

Inclusion of aromatic side groups. Since the phenomenon of pyroelectricity relies upon the existence of polar entities whose polarization is temperature- dependent, the next step in the optimization of the

=-" 3.5

3

2.5 L-. ~ 2 0~

1.5

0.5

K" 0 18a 18b 18e

b)

3 E

L

o

,. 1

2"- 0

18

F i g u r e 7

i I i ] i

1 8 a

1 8 b

t [] ~ [3 i [] t o ~ 1 8 e . . . . . . . I

20 22 24 26 28 30 32

Temperature, T (°C)

(a) Comparison of the pyroelectric coefficients of linear polysiloxanes 18a, 18b and 1Be at 29°C and (b) their temperature dependence

50 SUPRAMOLECULAR SCIENCE Vo lume 1 Number 1 1994


effect in the polysiloxane/eicosylamine system involved exchanging the purely aliphatic acid side chains for more polar aromatic acid side chains. A phenoxy group was chosen as the first candidate for this purpose resulting in polymers 20a, 21a, 21b and 21e. Polymer 21b was found to be insoluble in a number of suitable spreading solvents. Figure 8a shows the pyroelectric coefficients at 21°C of the aromatic side- chain polymers 20a and 21a in comparison to polymers 17b and 18b which are both substituted with a -(CH2)10CO2 H aliphatic chain of similar length to the -(CH2)4OPh(CHz)zCO2H moiety. It is clear in both the aliphatic (17b, 18b) and aromatic (20a, 21a) cases that the films containing the PS 50 copolymer backbone yield the larger pyroelectric coefficient. The aromatic substituted PS 50 copolymer 21a yields the highest coefficient and its value is about three times larger than the corresponding aliphatic polymer 18b. However, the change is reversed in the case of the aliphatic (17b) and aromatic (20a) substituted homopolyrners in that exchanging the aliphatic side groups for aromatic pendants actually leads to a small reduction in the pyroelectric activity.

A dipolar tiliting/reorientational model fits well the above data if consideration is given to the free volume available for reorientation of the pendant side groups.

a)

-'-" 3

E 2.5 U

~. 2

• ~ 1..5

• ~ 0.5

0 17b 20a 18b 21a

b) _,.-., 5

4 ::L

s

• / 21a

In both the aliphatic and aromatic pairs of polymers (17b, 18b and 20a, 21a, respectively) there is less restric- tion to the movement of the side groups in the case of the PS 50 copolymer backbone. Consideration of the aliphatic and aromatic substituted homopolymers (17b, 20a) also results in the conclusion that there is more steric hindrance between adjacent aromatic side groups compared to adjacent aliphatic side chains. However, the polarity of the side chain also plays a major role in determining the pyroelectric activity within these films. Exchanging the aliphatic chains for the phenoxy-based aromatic groups grafted onto the copolymer backbone, in which each side group is relatively free to reorien- tate, results in the movement of a larger dipole and therefore a greater polarization change. Figure 8b presents the temperature-dependent pyroelectric coefficients of the materials described in this section.

18b

[] o O [] 2 a I ~ I [ . l I . I I

18 20 22 24 26 28 30


Thickness dependence of pyroelectric activity

The results described above show that the highest activity in this series of LB films is exhibited by the multilayer containing linear polymer 21a. Using this polymer, a detailed study of the dependence of the pyroelectric activity on the number of layers within the alternate layer LB film has been undertaken. In princi- ple, the pyroelectric coefficient should be thickness independent. However, relatively small secondary pyroelectric effects 32 show some thickness dependence and may therefore be important in interpreting data obtained from LB films of different numbers of layers.

Figure 9 depicts the thickness dependence of the pyroelectric coefficient for alternate layers containing the PS 50 copolymer films substituted with the -(CH2)IoCO2H aliphatic side chain (18b) and the -(CH2)4OPh(CH2)2CO2H aromatic side chain (21a). Both curves show that an optimum thickness is observed around 23 layers. A thickness dependence for the pyroelectric coefficient is well known for certain inorganic films 33 and it has also been observed for 22- tricosenoic acid/docosylamine LB films 34 and 5-(p- dodecyloxyphenyl)pyrazine-2-carboxylic acid (DOPC)/

10

E 8

~- 6

o

4 o

o o e~

0 0

16 32

Figure 8 (a) Comparison of the pyroelectric coefficients of linear polysiloxanes 17b, 20a, 18b and 21a at 21°C and (b) their temperature dependence

" i " i " ' i I

I

f

21a

/ ,, / '

N

f [] \

II 18b

10 15 20 25

Number of monolayers, N

%

I . .

3O 35

Figure 9 Dependence of the pyroelectric coefficient of linear polymers 18b and 21a on the total number of monolayers within the LB film at 28°C

S U P R A M O L E C U L A R SCIENCE V o l u m e 1 N u m b e r 1 1994 51

Pyroelectric polysiloxane LB superlattices: 7-. Richardson et al.

12

IO

8 . 2

U ~ 6

.~ 4 u

.~ 2 P .

o 15

I I I I I I I " "

21a

• , ] . . I . . . I . . I . . I I . . I .

17 19 21 23 25 27 29 31


Figure 10 Temperature dependence of the pyroelectric coefficient of linear polymer 21a

stearic acid multilayers 35. In the former case, the coefficient was measured to be 0 . 5 8 / ~ C m - 2 K -1 for an 11 layer film and 0 . 8 5 p C m -2 K -1 for a 99 layer film. This increase was at tr ibuted to the reduct ion o f the secondary pyroelectric effect due to thermal expansion of the substrate in compar i son to the pr imary pyroelectric effect due entirely to the LB film. In the second case, the activity was seen to fall by 50% when the thickness was increased by a factor o f 3. I.r. spectroscopy indicated that the decrease in the coefficient was accompanied by a decrease in the average tilt angle o f the hyd roca rbon chains o f the D O P C and stearic acid molecules with respect to the substrate plane. It was proposed that this resulted in a reduct ion in the normal (measured) componen t o f the macroscopic polarization which in turn provided a possible mechanism for the reduced pyroelectric activity.

The op t imum thickness effect displayed by the polysiloxane/eicosylamine films would appear to be the first time such an effect has been observed in an LB film. It is possible that both the secondary pyroelectric effect (causing a gradual increase in measured coefficient) and the change in orientational architecture (resulting in a decrease in activity) may act together explaining the opt imum thickness as the t rade-off point associated with the two mechanisms. Fur ther research effort is currently being aimed at explaining this feature.

Finally, Figure 10 shows the temperature-dependent pyroelectric activity for the alternate layer film containing the aromatic-subst i tuted polymer 21a. The op t imum thickness sample o f 21 layers possesses a coefficient which rises to nearly 1 0 # C m - 2 K -1. This is the largest pyroelectric coefficient ever reported for an alternate layer LB film and is approximately one-third o f the value exhibited by commercial ly available poly(vinylidene difluoride) sensors.

C O N C L U S I O N S

It has been shown that the pyroelectric activity within a family o f cyclic and linear homo- and copolysiloxanes

(alternately deposited using the LB technique with monomer ic eicosylamine) has been iteratively optimized by tailoring both the chemical structure of the side-chain polysiloxanes and the film architecture to incorporate the molecular features which have been found to be responsible for high pyroelectric sensitivity. The coefficient o f nearly 10/~C m 2 K-J at 30°C is the highest value reported for an LB to our knowledge. This research p rog ramme has now been funded for a further 2 years in order that the next generation of polysiloxane compounds within this family can be designed and evaluated in terms of their pyroelectric performance.

A C K N O W L E D G E M E N T S

The SERC is gratefully acknowledged for the funding of a studentship (S.H.) and for agreeing to fund the next stage in this research programme. The Malaysian and Turkish governments are also thanked for student- ships (W.H.A.M. and R.C.).

R E F E R E N C E S

1 Lang, S. B. 'Sourcebook of Pyroelectricity', Gordon and Breach, New York, 1974

2 Lock, P. J. Appl. Phys. Lett. 1971, 19, 390 3 Fatuzzo, E. and Merz, W. J. 'Ferroelectrics', North-Holland,

Amsterdam, 1967 4 Fukuda, E. Ultrasonics 1968, 6, 229 5 Smith, G. W., Daniel, M. F., Barton, J. W. and Ratcliffe, N.

Thin Solid Films 1985, 132, 125 6 Wada, Y. and Hayakawa, R. Jpn J. Appl. Phys. 1976, 15, 2041 7 Holcroft, B., Petty, M. C., Roberts, G. G. and Russell, G. J.

Thin Solid Films 1985, 134, 83 8 Blinov, L. M., Davydova, N. N., Lazarev, V. V. and Yudin, S. G.

Soy. Phys. Solid State 1982, 24, 1523 9 Christie, P., Roberts, G. G. and Petty, M. C. Appl. Phys. Lett.

1986, 48, 1101 10 Richardson, T., Holder, S. and Lacey, D. J. Mater. Chem. 1992,

2(11), 1155 11 Christie, P., Roberts, G. G. and Petty, M. C. J. Phys. D 1986,

19, 1167 12 Jones, C. A., Petty, M. C. and Roberts, G. G. Thin Solid Films

1989, 160, 117 13 McArdle, C. B. (Ed.) 'Side Chain Liquid Crystal Polymers'

Chapman and Hall, New York, 1989 14 Richardson, T., Holder, S. and Lacey, D. Mol. Cryst. Liq.

Cryst. 1993, 236, 145 15 Richardson, T., Roberts, G. G., Holder, S. and Lacey, D. Thin

Solid Films 1992, 210]211,299 16 Richardson, T., Holder, S. and Lacey, D. J. Mater. Chem. 1992,

2(11), 1155 17 Majid, W. H. A., Richardson, T., Holder, S. and Lacey, D. Thin

Solid Films 1994, 243, 378 18 Richardson, T., Majid, W. H. A., Cochrane, E. C. A., Holder, S.

and Lacey, D. Thin Solid Films 1994, 242, 61 19 Barraud, A., Rosilio, C. and Ruaudel-Teixier, A. J. Coll.

Interface Sci. 1977, 62, 509 20 Hassner, A. and Alexanian, V. Tetrahed. Lett. 1978, 46, 4475 21 Vogel, A. I. 'Textbook of Practical Organic Chemistry', 4th

Edn, Longman, London, 1973, p. 269 22 West, C., Donelly, S. J. and Doyle, M. P. d. Org. Chem. 1973,

38, 2675 23 Doxsee, K. M., Feigel, M., Stewart, K. D., Canary, J. W.,

Knobler, C. B. and Cram, D. J. d. Am. Chem. Soc. 1987, 109, 3090

24 Gray, G. W., Jones, B. and Manson, F. d. Chem. Soc. 1957, 333

52 S U P R A M O L E C U L A R SCIENCE V o l u m e 1 N u m b e r 1 1994

Pyroelectric polysiloxane LB superlattices: T. Richardson e t a l.

25 Beevers, M. S. and Semlyen, J. A. Polymer 1971, 12, 378 26 Holder, S. J. PhD Thesis, University of Hull, 1994 27 Holcroft, B., Petty, M. C., Roberts, G. G. and Russell, G. J.

Thin Solid Films 1985, 134, 83 28 Poulter, M. W. PhD Thesis, University of Oxford, 1992 29 Tsibouklis, J,, Petty, M., Song, Y., Richardson, R., Yarwood, J.,

Petty, M. C. and Feast, W. J. J. Mater. Chem. 1991, 1,819 30 Madden, P., personal communication, 1990 31 Richardson, T., Majid, W. H. A., Capan, R., Lacey, D. and

Holder, S. 'lEE Colloquium on Molecular Electronics Devices', Digest No. 1994/084, lEE, London, 1994

32 Ashwell, G. J. (Ed.) 'Molecular Electronics', Research Studies Press, Taunton, 1992

33 Cady, W. G. 'Piezoelectricity', McGraw-Hill, New York, 1956 34 Christie, P., Roberts, G. G. and Petty, M. C. J. Phys. D 1986,

19, 1167 35 Kamata, T., Umemura, J., Takenaka, T. and Koizumi, N.

J. Phys. Chem. 1991, 95, 4092


Documents

Molecular engineering of pyroelectric polysiloxane Langmuir-Blodgett superlattices: synthesis, film preparation and pyroelectric properties