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On the preparation of Langmuir-Blodgett films of pyrrole derivatives
:'~ '
~~ "
MATERIALS
" ~ "
ELSEVIER
SCIENCE
&
ENGINEERING Materials Science and Engineering C 5 (1998) 223-226
12
On the preparation of Langmuir-Blodgett films of pyrrole derivatives A. Bonfiglio a.., M.T. Parodi b, B. Bianco b, G. Ruggeri ~ "Dipartimento di lngegneria Elettrica ed Elettronica, Universitc~ di Cagliari, Piazza d'Armi, 09123 Cagliari, Italia t, Domrtimento di ln,~egneria Biofisica ed Elettronica, Universit~l di Genova, Via Opera Pia, 1la, 16145 Genova, Italia • Dipat'timemo di Chimica e Chimica b~dustriale, Via Risorgimemo, 35, 56100 Pisa, Italia
Received 4 December 1996
Abstract
In this paper, we present a study of the preparation of Langmuir-Blodgett films of polymers obtained on a trough from mixtures made by pyn'ole derivatives. In particular, the comparison between films obtained by different molar ratios of 3-hexadecyl-pyrrole to pure pyIIole is shown, in terms of the kinetics of the process and of the compressibility of the products obtained. Depending on the molar ratio in the composition of the mixtures, differences have been found in the structure of the co-polymer obtained and in the behaviour of the resulting film. © 1998 Elsevier Science S.A. Keywords: Conductive polymers; Polymerization on trough; Langmuir-Blodgett films; Molecular electronics
1. I n t r o d u c t i o n
AlkyI pyrroIes are pyrrole derivatives with the property of forming Langmuir-Blodgett (LB) films [ 1 - 5 ] thanks to their lateraI alkyl chain. In a recent paper [6], we reported the study of the on-trough polymerization (chemically obtained) of two different alkyl-pyrroles, namely 3-hexadecyl-pyrrole (3HP) and 3-decyl-pyrrole (3DP). The study has demonstrated that a longer lateral alkyI chain improves the stability and the overall quality of the films obtained, even if electrical measurements [7] have shown that the conductivity of the films so obtained is very poor. The need for producing good quality LB films and, at the same time, conductive molecular assemblies, has suggested us to mix a high quality film-forming material (3HP) together with a high conductivity one (pure pyrrole [8] ), with the aim of obtaining a co-polymer with both qualities, namely conductivity and film-forming ability. Encouraged by our previous experience and by some other results reported in the Iiterature [ 9, I 0], we decided to try the polymerization reaction directly on the trough with the aim of obtaining a controlled twodimensional structure with optimal claaracteristics of order and conductivity. The aim of this work is the investigation of the behaviour of films made by 3HP-pyrrole mixtures in variable ratios, * Corresponding author. Tel: + 39-70-6755872; Fax: + 39-70-6755900; E-mail: annalisa@ DIEE.UNICA.IT 0928-4931/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved PIIS0928-493 1( 9 7 ) 0 0 0 4 7 - 7
with particular attention to the kinetics of the polymerization and to the compressibility of the film obtained.
2. Materials and methods
2.1. The synthesis 3-hexadecyI-pyrrole (3HP) (C2oHsvN, M.W. 291) was synthesized by tosylpyrrole acylation with the corresponding carboxylic acid derivatives and successive reduction of the obtained ketones with LiA1H4 as reported by Ruhe et al. [ 11 ]. The monomer is partially or totally soluble in chloroform, methylene chloride and tetrahydrofurane depending on molecular weight values. For what concerns pure pyrrole, the commercial product (Fluka) has been purified by distillation under reduced pressure and an inert atmosphere. At room temperature the substance is a liquid. The structures of both molecules are shown in Fig. I.
2.2. On-trough polymerization The goal of this experiment was to record, under a fixed value of the surface pressure (rr*), all the barrier movements in order to detect the variations of the area occupied by the film caused by the polymerization. The experiments were performed with a KSV trough driven by a 386-SX personal
224
A. Bonfiglio et al. / Materials Science and Engineering C 5 (]998) 223-226
3. R e s u l t s a n d d i s c u s s i o n
CIt3
/
(Icm)a5 Previous to the examination of the data recorded during the experiment, it is necessary to give some information about the qualitative appearance of the films obtained as it may help in the interpretation of the data collected. Firstly, after polymerization, the color of the film changes from transparent to black. Secondly, decreasing the molar ratio between 3HP and pyrrole beside 1:1000, the film on the surface begins to visibly wrinkle under the pressure caused by the moving barrier and the extent of the wrinkling is larger near the barrier itself. In other words, the pressure of the batTier does not uniformly propagate through the film. The film is very rigid and the following attempts of deposition on glass substrates gave poor results as it was never possible to deposit more than one layer, either by vertical or horizontal dipping.
O
N
N
1
I
n
Pyrrole 3-hexadecyl-pyrrole Fig. 1. Structure of the moleculesemployed in the experiments. computer, at 20°C. The subphase employed for the polymerization reaction was a 50 mM FeC13 (Fluka Chemic) aqueous (purified with a Millipore system) solution. All experiments were carried out in a class 100 clean room. For what concerns experiments made with pure 3HP molecules, the 3HP monomer was first solved in chloroform ( 1 mg ml - ~) and then 30 ~xl of the solution obtained were spread on the surface of the subphase. The values recorded were: barrier position, barrier speed average, surface pressure (7r) and temperature. The typical value for ¢r* was 20 mN m - 1 (decided on the basis of an higher polymerization efficiency, as found in our previous experiments [ 6 ] ) . Control experiments were made with a pure water subphase. At the end of the observations (about 30 rain) the film was re-expanded and an isotherm of surface pressure versus area per molecule was recorded. For films made by co-polymers, the same procedure was followed except that the substance spread on the subphase was formed by mixing a 3HP solution (30 gl, 1 mg m l - ~, molar concentration 3.43X10 -3) and a pure pyrrole one (from 30 to 1500 /xl, 23 mg ml -~, molar concentration 3.43 × 1 0 - i ) . Large molar excesses of pyrrole were used in the mixtures due to the known high solubility of this material in water.
3.]. K i n e t i c s
During the kinetic measurements, the barrier moves depending on the area requested to assemble the molecules at the fixed value of the surface pressure, ( 7r* = 20 mN m - 1). The most relevant parameters recorded are: the average barrier speed, the surface pressure, the monolayer area. They are all in function of time in a typical ldnetics graphical representation (Figs. 2 and 3). First, we consider in Fig. 2 the kinetics recorded during the on-trough polymerization of 3HP. In Fig. 3 we present the curves obtained by repeating the experiment in the same conditions but with mixtures of 3HP-pyrrole with different molar ratios. Two curves are the fundamental parameters to be observed: the barrier speed average and the area occupied by the monolayer on the subphase. The barrier speed average is a value that accounts for the dynamic aspects of the polymerization. In fact, its value is
3HP
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--A-- Banier SpeedA ~ --x-- Sudace Fh'easu'e --¢-- Mor'~ayer Area
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Fig. 2. Kinetics of 3HP on a FeCI350 mM subphase in correspondenceof ~r* = 20 mN m- J.
225
A. Bonfiglio et al. / Materials Science and Engineering C 5 (1998) 223-226
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Fig. 3. Kinetics of the mixture 3HP-pyrroleon a FeCI350 mM subphase in correspondenceof ~-*= 20 nun m-~: (a) ratio, 1:100; (b) ratio, 1:500; (c) ratio, 1:1000; (d) ratio, 1:5000. given by the average barrier displacement in the time unit. As the barrier moves in order to maintain the surface pressure externally imposed, if something occurs that modifies the surface tension of the subphase (in this case the change in the monolayer organization due to polymerization), one can observe and monitor the corresponding variation of the bartier speed average. We characterized the polymerization of 3HP and 3DP with this method [6]. Our aim in the present work was to record the same data in the case ofa 3HP-pyrrole mixture ( Fig. 3) and to compare them with the case of a pure 3HP film (Fig. 2). A first interesting point is the time employed to reach the final surface pressure (20 mN m - ] ) ; it gives a quantitative measure of the rapidity of the reaction, but, at the same time, is influenced by the degree of coordination of the molecules inside the tilm. In the case of pure 3HP, the value is reached in 7 min. This time dramatically decreases in the case of the mixtures (ratio I:100, 4.17 rain; ratio i:500, 4 min). This indicates that the reaction is much more rapid when the amount of pyrrole is increased. In other words, the presence of pure pyrrole accelerates the reaction. Surprisingly, when the ratio is lower than 1:500, the time necessary to reach the fixed value of pressure begins to increase again. This can be explained with the wrinkling of the film, if one considers that the packing of the monolayer ( mainly driven by the coordination between alkyl chains and consequently strictly dependent on their distance) and therefore the surface tension and its dynamics are strongly influenced by this phenomenon. The increase of the time necessary to reach the final ,n-value is thus to ascribe mainly to a difficult
packing of the film due to a poor coordination between the alkyl chains (now farther than with higher molar ratios) rather than to differences in the dynamics of the reaction. The value of the area occupied by the monolayer at the end of the polymerization reaction gives a quantitative, though indirect indication of the amount of material laying at the surface of the subphase. In particular, as the amount of 3HP is the same in all the experiments shown, it allows one to deduce how many pure pyrrole units belong to the final polymeric films. In the case of pure 3HP, the final value of the area is about I53 cm 2. It increases to 293 if the molar ratio is I:100, and to 441 with 1:500. As it is reasonable to think that the planar size of the single rings should be the same for 3HP and for pure pyrrole (the lateral chains do not affect the planar extension of the molecules as they are exposed externally when the pressure is high enough to cause molecular packing), one can derive the total number of pyrrole rings at the surface by dividing the total monolayer area by the area of a single pyrrole ring, as deduced by the case of the pure 3HP film. From this case, we derived the area of the single ring to be about 25 ~ 2 a t ~-= 20 mN m - 1. Therefore, in the case of a 1:100 molar ratio, the number of rings at the surface is 11.8 × 10 ~6, while in the case 1:500 it is 17.7 × 1016. Apart from the consideration that the large majority of the pure pyrrole molecules dissolves in the subphase before polymerization, the values derived indicate that the average ratio between 3HP and pyrrole molecules at the surface is about 1:1 in the first case and 1:2 in the second case. When the molar ratio is lower than 1:500, the overall area of the surface
226
A. Bo~figlio ez aI./ Materials Science and Engineering C 5 (t998) 223-226
layer increases but, at the same time, the film wrinkles and therefore the surface area measurement (i.e. the space between the barrier and the trough board) no longer corresponds to the real value of the monolayer area and it is not possible to deduce the ratio between 3HP and pyrrole at the surface. Anyway, the fact that this value has increased is confirmed by the wrinkling, as it can be explained only by admitting that, on average, there are more pyrrole rings (not bonded to the surface) between two contiguous surface active units.
explained with a wrinkling of the film. The change of slope around 15 mN m - ~is slightly visible in the i: I000 curve and it completely disappears in the 1:5000 one. Wrinkling does not allow us to have an idea of the average distance between alkyl chains and therefore to establish the degree of coordination between the chains. For these reasons, we do not consider the curves recorded in the cases I:1000 and 1:5000 to be very informative.
4. Conclusions
3.2. isodTerms The phenomena deduced from the kinetics are substantially confirmed in the isotherms. In Fig. 4 one can observe the isotherms surface pressure vs. monolayer area recorded in the case of a pure 3HP film and with various ratios between 3HP and pyrrole. As one can easily notice, the shape of the curve does not change when the molar ratio is 1:100 (except for a shift towards right of the curve due to a larger amount of molecules at the surface) while it changes quite dramatically when the molar ratio is 1:500. First there is a bigger shift towards the right of the curve, secondly there is a noticeable variation in the slope of the curve around 15 mN m that is less evident in the previous curves. From the kinetics data we derived that the average molar ratio between 3HP and pyrrole at the surface is l:1 in the case of the 1:100 mixture and 1:2 in the case of the i:500 mixture. It is straightforward to think that the coordination between the alkyl chains (that drives the packing in LB films) is weakly affected when the average distance between nearest chains is only doubled ( 1:100) while it is strongly modified when it becomes three times the original value (1:500). Therefore, the film which has alkyl units less close to each other needs to be compressed longer to rearrange the molecular disposition. Surprisingly, the shift towards the right is reduced in the cases of 1:1000 and 1:5000 molar ratios but again it can be
We have performed the analysis of the polymerization at the air-water interface of a Langmuir trough on mixtures made by 3HP and pure pyrroles in various molar ratios. Our experiments have shown that the presence of pure pyrrole accelerates the reaction. By varying the molar ratios between 3HP and pyrrole, we have observed that the average composition of the polymeric chains varies, in the sense that the number of pyrrole rings belonging to the chain increases with decreasing molar ratios. Beside the possible variations in terms of the conductivity of the chains, to be investigated, a straight consequence is a variation in the packing of the monolayer, observed in the isotherms, due to the fact that the alkyl chains, known to be the responsible of the molecular arrangement in LangmuirBlodgett films, become less densely packed when the molar ratio of pure pyrrole increases with respect to 3HP. Such a variation is reflected also in the appearance of the fihns, highly wrinkled at low molar ratios of 3HP.
Acknowledgements This work was supported by Italian Progetto MURST 40% 'Microetettronica: Tecnologie, dispositivi e sensori' and by the EC-Network "Langmuir-Blodgett films for Molecular Electronics and Bioelectronics'.
Isotherms 4O --o---~----~----~--
E v
Mix 1:1000 Mix 1:500 Mix 1:100 Mix 1:5000 3HP
25
15
\
E
={
O9 5
1O0
2120
3C0
400
500
600
700
800
Monolayer Area (crr#) Fig. 4. Isotherms of 3HP and the other mixtures after polymerization on a FeCI3 50 mM subphase (T=20°C).
References [ 1] K. Hong, R.B. Rosner, M.F. Rubner, Chem. Mater. 2 (t990) 82-88. [2] R.S. Duran, H.C. Zhou, Polymer 33 (19) (1992) 4019-4023. [3] P. Quint, TI Bailey, W. Sigmund, M. Hara, W. Knoll, H. Sasabe, R.S. Duran, Polymer 35 ( I ) (I994) 281-282. [4] D.M. Collard, C.N. Sayre, Polymer 35 ( 1) (t994) I96-197. [5] D. Stanke, M.L. Hallensieben, L. Toppare, Synth. Met. 73 (1995) 267-272. [6] M.T. Parodi, A. Bonfiglio, B. Bianco, G. Ruggeri, F.Ciardelli, Thin Solid Films 295 (1997) 234-240. -[7] A: Bonfigtio, E. Di Zitti, M.T. ParodL B. Bianco, G. Ruggeri, Mat. Sc. Eng. C5 (1998) 251-254. [8] A.F. Diaz. K K Kanazawa, G.P. Gardini, J. Chem. Sot., Chem. Comrrmn. (1979) 635. [9] K. Hong, F. Rubner, Thin Solid Nims 179 (I989) 215-220. [ 10] K. Hong, R.B. Rosner, M.F. Rubner, Chem. Mater. 2 (1990) 82-88. [ 11 ] J. Ruhe, T.A. Ezquerra, G. Wegner, Makromol. Chem. Rapid Commun. 10 (1989) I03.
~~ "
MATERIALS
" ~ "
ELSEVIER
SCIENCE
&
ENGINEERING Materials Science and Engineering C 5 (1998) 223-226
12
On the preparation of Langmuir-Blodgett films of pyrrole derivatives A. Bonfiglio a.., M.T. Parodi b, B. Bianco b, G. Ruggeri ~ "Dipartimento di lngegneria Elettrica ed Elettronica, Universitc~ di Cagliari, Piazza d'Armi, 09123 Cagliari, Italia t, Domrtimento di ln,~egneria Biofisica ed Elettronica, Universit~l di Genova, Via Opera Pia, 1la, 16145 Genova, Italia • Dipat'timemo di Chimica e Chimica b~dustriale, Via Risorgimemo, 35, 56100 Pisa, Italia
Received 4 December 1996
Abstract
In this paper, we present a study of the preparation of Langmuir-Blodgett films of polymers obtained on a trough from mixtures made by pyn'ole derivatives. In particular, the comparison between films obtained by different molar ratios of 3-hexadecyl-pyrrole to pure pyIIole is shown, in terms of the kinetics of the process and of the compressibility of the products obtained. Depending on the molar ratio in the composition of the mixtures, differences have been found in the structure of the co-polymer obtained and in the behaviour of the resulting film. © 1998 Elsevier Science S.A. Keywords: Conductive polymers; Polymerization on trough; Langmuir-Blodgett films; Molecular electronics
1. I n t r o d u c t i o n
AlkyI pyrroIes are pyrrole derivatives with the property of forming Langmuir-Blodgett (LB) films [ 1 - 5 ] thanks to their lateraI alkyl chain. In a recent paper [6], we reported the study of the on-trough polymerization (chemically obtained) of two different alkyl-pyrroles, namely 3-hexadecyl-pyrrole (3HP) and 3-decyl-pyrrole (3DP). The study has demonstrated that a longer lateral alkyI chain improves the stability and the overall quality of the films obtained, even if electrical measurements [7] have shown that the conductivity of the films so obtained is very poor. The need for producing good quality LB films and, at the same time, conductive molecular assemblies, has suggested us to mix a high quality film-forming material (3HP) together with a high conductivity one (pure pyrrole [8] ), with the aim of obtaining a co-polymer with both qualities, namely conductivity and film-forming ability. Encouraged by our previous experience and by some other results reported in the Iiterature [ 9, I 0], we decided to try the polymerization reaction directly on the trough with the aim of obtaining a controlled twodimensional structure with optimal claaracteristics of order and conductivity. The aim of this work is the investigation of the behaviour of films made by 3HP-pyrrole mixtures in variable ratios, * Corresponding author. Tel: + 39-70-6755872; Fax: + 39-70-6755900; E-mail: annalisa@ DIEE.UNICA.IT 0928-4931/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved PIIS0928-493 1( 9 7 ) 0 0 0 4 7 - 7
with particular attention to the kinetics of the polymerization and to the compressibility of the film obtained.
2. Materials and methods
2.1. The synthesis 3-hexadecyI-pyrrole (3HP) (C2oHsvN, M.W. 291) was synthesized by tosylpyrrole acylation with the corresponding carboxylic acid derivatives and successive reduction of the obtained ketones with LiA1H4 as reported by Ruhe et al. [ 11 ]. The monomer is partially or totally soluble in chloroform, methylene chloride and tetrahydrofurane depending on molecular weight values. For what concerns pure pyrrole, the commercial product (Fluka) has been purified by distillation under reduced pressure and an inert atmosphere. At room temperature the substance is a liquid. The structures of both molecules are shown in Fig. I.
2.2. On-trough polymerization The goal of this experiment was to record, under a fixed value of the surface pressure (rr*), all the barrier movements in order to detect the variations of the area occupied by the film caused by the polymerization. The experiments were performed with a KSV trough driven by a 386-SX personal
224
A. Bonfiglio et al. / Materials Science and Engineering C 5 (]998) 223-226
3. R e s u l t s a n d d i s c u s s i o n
CIt3
/
(Icm)a5 Previous to the examination of the data recorded during the experiment, it is necessary to give some information about the qualitative appearance of the films obtained as it may help in the interpretation of the data collected. Firstly, after polymerization, the color of the film changes from transparent to black. Secondly, decreasing the molar ratio between 3HP and pyrrole beside 1:1000, the film on the surface begins to visibly wrinkle under the pressure caused by the moving barrier and the extent of the wrinkling is larger near the barrier itself. In other words, the pressure of the batTier does not uniformly propagate through the film. The film is very rigid and the following attempts of deposition on glass substrates gave poor results as it was never possible to deposit more than one layer, either by vertical or horizontal dipping.
O
N
N
1
I
n
Pyrrole 3-hexadecyl-pyrrole Fig. 1. Structure of the moleculesemployed in the experiments. computer, at 20°C. The subphase employed for the polymerization reaction was a 50 mM FeC13 (Fluka Chemic) aqueous (purified with a Millipore system) solution. All experiments were carried out in a class 100 clean room. For what concerns experiments made with pure 3HP molecules, the 3HP monomer was first solved in chloroform ( 1 mg ml - ~) and then 30 ~xl of the solution obtained were spread on the surface of the subphase. The values recorded were: barrier position, barrier speed average, surface pressure (7r) and temperature. The typical value for ¢r* was 20 mN m - 1 (decided on the basis of an higher polymerization efficiency, as found in our previous experiments [ 6 ] ) . Control experiments were made with a pure water subphase. At the end of the observations (about 30 rain) the film was re-expanded and an isotherm of surface pressure versus area per molecule was recorded. For films made by co-polymers, the same procedure was followed except that the substance spread on the subphase was formed by mixing a 3HP solution (30 gl, 1 mg m l - ~, molar concentration 3.43X10 -3) and a pure pyrrole one (from 30 to 1500 /xl, 23 mg ml -~, molar concentration 3.43 × 1 0 - i ) . Large molar excesses of pyrrole were used in the mixtures due to the known high solubility of this material in water.
3.]. K i n e t i c s
During the kinetic measurements, the barrier moves depending on the area requested to assemble the molecules at the fixed value of the surface pressure, ( 7r* = 20 mN m - 1). The most relevant parameters recorded are: the average barrier speed, the surface pressure, the monolayer area. They are all in function of time in a typical ldnetics graphical representation (Figs. 2 and 3). First, we consider in Fig. 2 the kinetics recorded during the on-trough polymerization of 3HP. In Fig. 3 we present the curves obtained by repeating the experiment in the same conditions but with mixtures of 3HP-pyrrole with different molar ratios. Two curves are the fundamental parameters to be observed: the barrier speed average and the area occupied by the monolayer on the subphase. The barrier speed average is a value that accounts for the dynamic aspects of the polymerization. In fact, its value is
3HP
80Q.
--A-- Banier SpeedA ~ --x-- Sudace Fh'easu'e --¢-- Mor'~ayer Area
60,~
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609
1
40
30
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~2020Q
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9
0
5
10
15
20
25
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Fig. 2. Kinetics of 3HP on a FeCI350 mM subphase in correspondenceof ~r* = 20 mN m- J.
225
A. Bonfiglio et al. / Materials Science and Engineering C 5 (1998) 223-226
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Fig. 3. Kinetics of the mixture 3HP-pyrroleon a FeCI350 mM subphase in correspondenceof ~-*= 20 nun m-~: (a) ratio, 1:100; (b) ratio, 1:500; (c) ratio, 1:1000; (d) ratio, 1:5000. given by the average barrier displacement in the time unit. As the barrier moves in order to maintain the surface pressure externally imposed, if something occurs that modifies the surface tension of the subphase (in this case the change in the monolayer organization due to polymerization), one can observe and monitor the corresponding variation of the bartier speed average. We characterized the polymerization of 3HP and 3DP with this method [6]. Our aim in the present work was to record the same data in the case ofa 3HP-pyrrole mixture ( Fig. 3) and to compare them with the case of a pure 3HP film (Fig. 2). A first interesting point is the time employed to reach the final surface pressure (20 mN m - ] ) ; it gives a quantitative measure of the rapidity of the reaction, but, at the same time, is influenced by the degree of coordination of the molecules inside the tilm. In the case of pure 3HP, the value is reached in 7 min. This time dramatically decreases in the case of the mixtures (ratio I:100, 4.17 rain; ratio i:500, 4 min). This indicates that the reaction is much more rapid when the amount of pyrrole is increased. In other words, the presence of pure pyrrole accelerates the reaction. Surprisingly, when the ratio is lower than 1:500, the time necessary to reach the fixed value of pressure begins to increase again. This can be explained with the wrinkling of the film, if one considers that the packing of the monolayer ( mainly driven by the coordination between alkyl chains and consequently strictly dependent on their distance) and therefore the surface tension and its dynamics are strongly influenced by this phenomenon. The increase of the time necessary to reach the final ,n-value is thus to ascribe mainly to a difficult
packing of the film due to a poor coordination between the alkyl chains (now farther than with higher molar ratios) rather than to differences in the dynamics of the reaction. The value of the area occupied by the monolayer at the end of the polymerization reaction gives a quantitative, though indirect indication of the amount of material laying at the surface of the subphase. In particular, as the amount of 3HP is the same in all the experiments shown, it allows one to deduce how many pure pyrrole units belong to the final polymeric films. In the case of pure 3HP, the final value of the area is about I53 cm 2. It increases to 293 if the molar ratio is I:100, and to 441 with 1:500. As it is reasonable to think that the planar size of the single rings should be the same for 3HP and for pure pyrrole (the lateral chains do not affect the planar extension of the molecules as they are exposed externally when the pressure is high enough to cause molecular packing), one can derive the total number of pyrrole rings at the surface by dividing the total monolayer area by the area of a single pyrrole ring, as deduced by the case of the pure 3HP film. From this case, we derived the area of the single ring to be about 25 ~ 2 a t ~-= 20 mN m - 1. Therefore, in the case of a 1:100 molar ratio, the number of rings at the surface is 11.8 × 10 ~6, while in the case 1:500 it is 17.7 × 1016. Apart from the consideration that the large majority of the pure pyrrole molecules dissolves in the subphase before polymerization, the values derived indicate that the average ratio between 3HP and pyrrole molecules at the surface is about 1:1 in the first case and 1:2 in the second case. When the molar ratio is lower than 1:500, the overall area of the surface
226
A. Bo~figlio ez aI./ Materials Science and Engineering C 5 (t998) 223-226
layer increases but, at the same time, the film wrinkles and therefore the surface area measurement (i.e. the space between the barrier and the trough board) no longer corresponds to the real value of the monolayer area and it is not possible to deduce the ratio between 3HP and pyrrole at the surface. Anyway, the fact that this value has increased is confirmed by the wrinkling, as it can be explained only by admitting that, on average, there are more pyrrole rings (not bonded to the surface) between two contiguous surface active units.
explained with a wrinkling of the film. The change of slope around 15 mN m - ~is slightly visible in the i: I000 curve and it completely disappears in the 1:5000 one. Wrinkling does not allow us to have an idea of the average distance between alkyl chains and therefore to establish the degree of coordination between the chains. For these reasons, we do not consider the curves recorded in the cases I:1000 and 1:5000 to be very informative.
4. Conclusions
3.2. isodTerms The phenomena deduced from the kinetics are substantially confirmed in the isotherms. In Fig. 4 one can observe the isotherms surface pressure vs. monolayer area recorded in the case of a pure 3HP film and with various ratios between 3HP and pyrrole. As one can easily notice, the shape of the curve does not change when the molar ratio is 1:100 (except for a shift towards right of the curve due to a larger amount of molecules at the surface) while it changes quite dramatically when the molar ratio is 1:500. First there is a bigger shift towards the right of the curve, secondly there is a noticeable variation in the slope of the curve around 15 mN m that is less evident in the previous curves. From the kinetics data we derived that the average molar ratio between 3HP and pyrrole at the surface is l:1 in the case of the 1:100 mixture and 1:2 in the case of the i:500 mixture. It is straightforward to think that the coordination between the alkyl chains (that drives the packing in LB films) is weakly affected when the average distance between nearest chains is only doubled ( 1:100) while it is strongly modified when it becomes three times the original value (1:500). Therefore, the film which has alkyl units less close to each other needs to be compressed longer to rearrange the molecular disposition. Surprisingly, the shift towards the right is reduced in the cases of 1:1000 and 1:5000 molar ratios but again it can be
We have performed the analysis of the polymerization at the air-water interface of a Langmuir trough on mixtures made by 3HP and pure pyrroles in various molar ratios. Our experiments have shown that the presence of pure pyrrole accelerates the reaction. By varying the molar ratios between 3HP and pyrrole, we have observed that the average composition of the polymeric chains varies, in the sense that the number of pyrrole rings belonging to the chain increases with decreasing molar ratios. Beside the possible variations in terms of the conductivity of the chains, to be investigated, a straight consequence is a variation in the packing of the monolayer, observed in the isotherms, due to the fact that the alkyl chains, known to be the responsible of the molecular arrangement in LangmuirBlodgett films, become less densely packed when the molar ratio of pure pyrrole increases with respect to 3HP. Such a variation is reflected also in the appearance of the fihns, highly wrinkled at low molar ratios of 3HP.
Acknowledgements This work was supported by Italian Progetto MURST 40% 'Microetettronica: Tecnologie, dispositivi e sensori' and by the EC-Network "Langmuir-Blodgett films for Molecular Electronics and Bioelectronics'.
Isotherms 4O --o---~----~----~--
E v
Mix 1:1000 Mix 1:500 Mix 1:100 Mix 1:5000 3HP
25
15
\
E
={
O9 5
1O0
2120
3C0
400
500
600
700
800
Monolayer Area (crr#) Fig. 4. Isotherms of 3HP and the other mixtures after polymerization on a FeCI3 50 mM subphase (T=20°C).
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