Characterization of Langmuir-Blodgett films of parent polyaniline

ELSEVIER

Thin Solid Films 284-285

(1996)

Characterization of Langmuir-Blodgett A. Rid,

177-180

films of parent polyaniline

Jr. a, L.H.C. Mattoso b7c,G.D. Telles a, P.S.P. Herrmann b*c,L.A. Colnago b,c, N.A. Parizotto d, V. Baranauskas e, R.M. Faria ‘, O.N. Oliveira, Jr. a aInstitute de Fisica de S&J Curlos, USP CP 369, 13560-970 SZo Carlo& SP, Brazil ’ CNPDWEMBRAPA, Scfo Carlos CP 741, 13560-970 SCroCarlos. SP, Brazil ’ Institute de Quimica de .%io Carlos, DFQhXSP (Brazil), Go Carlos, Brazil d Depto de Fisioterapiu e T.O., (IFSCar. Srio Carlos. Brazil e Faculdade de Eng. Elhtrica, UNICAMP, Campinas. Brazil

Abstract Conducting Langmuir-Blodgett (LB) films have been fabricated from parent polyaniline (PAni) which was doped with functionalized acids. In order to optimize experimental conditions for the formation of stable Langmuir monolayers and their subsequent transfer onto solid substrates, PAni was dissolved in ten different combinations of chloroform solutions. Use was made of camphor sulfonic acid, dodecyl benzene sulfonic acid, and toluenesulfonic acid, and of the solvents N-methyl pyrrolidine and m-cresol as processing agents. Because acidic subphases have been employed, as-deposited LB films were already doped, which was confirmed by the appearance of a polaronic band in the UV-Vis absorption spectra. The absorbance peak increases with the number of deposited layers indicating that a suitable multilayer buildup is accomplished. When analysed by atomic force microscopy, PAni LB films show a fibrillar structure with the fibril width ranging from = 60 to 160 nm. Keywords: Polyaniline; Langmuir-Blodgett films; Doping; Microscopy

1. Introduction The use of functionalized acids for the processing of parent polyaniline (PAni) demonstrated by Heeger and coworkers [ 1] has made it possible to spread Langmuir monolayers of this polymer on acidic subphases [ 21. Despite the success obtained also in the transfer of PAni Langmuir-Blodgett (LB) multilayers onto glass substrates, it became clear that film quality depended strongly on the experimental conditions adopted for processing the polymer and also for spreading the monolayer. We have therefore systematically investigated the Langmuir monolayers of PAni+ functionalized acid + processing aids, spread from ten different types of chloroform solution. When optimized conditions are employed, good-quality Z-type LB films are fabricated which were then characterized by UV-Vis spectroscopy, atomic force microscopy and electrical conductivity measurements. 2. Experimental 2.1. Processing

of PAni

Parent polyaniline was chemically synthesized with excess aniline (monomer:oxidant ratio of 4: 1) according to the pro0040-6090/96/15.00 0 1996 Elsevier Science S.A. All rights reserved SSDIOO40-6090(95)08300-6

cedure described elsewhere [ 31. Ammonium peroxydisulfate was employed as the oxidizing agent in aqueous 1.O M HCl at about 0 “C. The polymer had a molecular weight, M,,,, of approximately 55 000 g mol-’ and was rendered soluble by being doped with functionalized protonic acids, namely camphorsulfonic (CSA) , toluenesulfonic (TSA) , and dodecyl benzenesulfonic (DBSA) acids, or by using N-methyl pyrrolidine (Nh4P) and m-cresol. CSA and DBSA were purchased from Aldrich (USA) while TSA and NMP were acquired from Merck. m-Cresol was purchased from Farmitalia Carlo Erba SpA (ITA). Ten different combinations of chloroform (from Merck) solutions were employed using these three acids and the solvents NMP and m-cresol as processing aids. The solutions were the following:

1. PAni + CSA + chloroform.

2. 3. 4. 5. 6. 7. 8.

PAni PAni PAni PAni PAni PAni PAni

+ DBSA -+ chloroform. + TSA -+ chloroform. + CSA + chloroform. + DBSA f chloroform. + NMP -+ chloroform ( 1:9; 1: 1) . + m-cresol --) chloroform ( 1:9; 1: 1) . + CSA + m-cresol -+ chloroform ( 1:9).

178

A. Riul, Jr. et al. /Thin Solid Films 284-285 (I 996) I77-180

9. PAni +m-cresol +chloroform + CSA ( 1:9). 10. CSA +m-cresol + PAni + chloroform ( 1:9). The order of the sum indicates the order in which each product was added to the solution. The arrow indicates the product which was added in the last step of preparation. The proportions ( 1:1) and ( 1:9) correspond to the volume ratio employed for m-cresol or NMP and chloroform in the order indicated. The quantities of acid employed were sufficient for obtaining 50% of doping of PAni. Optimized values for concentration of the chloroform solutions lie between 0.05 and 0.25 mg ml- ‘. The solutions including functionalized acids and NMP were placed in an ultrasound bath and then filtered through filter paper. Because of this filtering process, it was necessary to estimate the final concentration which was achieved by comparison with a calibration curve prepared using the UV-Vis absorption spectra of a series of solutions. 2.2. Fabrication of langmuir monolayer-sand LBfilms Langmuir monolayers were spread from the ten chloroform solutions on the surface of pH = 2 acidic subphases prepared with either HCI, TSA or trifluoroacetic acid (TFA). The volume of solution spread varied from 300 to 1200 ~1depending on the type of solution. The speed of barrier compression was 30 mm min-‘. The monolayer surface pressure was measured with a Wilhelmy plate while the surface potential was monitored with a Kelvin probe. Langmuir-Blodgett (LB) films were deposited onto hydrophilic glass or IT0 (indium tin oxide-coated glass) substrates using the vertical dipping method with a dipping speed of OS-5 mm min- ‘. The surface pressure was kept at 18 mN m- ’ for films from PAni + CSA and at 12-15 mN m- ’ for films containing mcresol. The Langmuir-Blodgett work was carried out using a KSV 5000 Langmuir trough, mounted on an anti-vibration table in a Class 10000 clean room. The UV-Vis spectroscopy of deposited LB films was carried out in a double-beam, Hitachi U2000 spectrophotometer; surface potential measurements were obtained with a Trek probe Model 320. Conductivity measurements were carried out with the van der Pauw four-probe technique. Atomic force microscopy (AFM) images were obtained by the contact/ repulsive mode using a spring constant of 0.58 N m-‘, a scanning frequency of 4.5 Hz and a force variation between 7 nN and 14 nM in a Digital AFM-Nanoscope II instrument. The tip used was made of Si3Ni4.

3. Results and discussion 3.1. Formation

of stable Langmuir monolayer-s

Fig. 1 shows that reasonably steep pressure-areaisotherms can be obtained from PAni solutions. Hysteresis in a compression*xpansion cycle was small unless monolayer collapse was reached. Collapse was denoted by a change in the slope with which the surface pressure increases upon com-

40.

b

P

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a

sUJ20. !! n loQ 20

_I’!‘! 0

20

40

I

Area per monomer (A*) Fig. 1. Surface pressure-area curves from PAni + CSA; (b) CSA + m-cresol + PAni.

PAni

monolayers:

(a)

pression, in a similar fashion observed for other polymeric materials [ 41. The area per monomer (9 1 g mol - ‘) varied between 4 and 9 A2 for most monolayers, in agreement with data in the literature for similar PAni films [ 5-71. The only exception is the PAni +m-cresol +CSA case for which a much larger area was observed, probably due to the effect of m-cresol which makes the polymer chains more extended, consistent with the so-called secondary doping effect [ 8] ‘. It is known that PAni undergoes a confonnational change from coil-like to rod-like chains due to the appropriate combination of dopant (CSA) and solvent (m-cresol) . It must be stressed that this phenomenon is accompanied by an increase in conductivity in free-standing films, but such an increase was only noticed when the deposited LB films were exposed to mcresol for long periods, as discussed in the next sub-section. High collapse pressures were obtained for the majority of the solutions. Monolayer stability was also excellent for some of the solutions, with only very small decreases in area being observed over a long time period when the surface pressure is kept constant. When ease of processibility is taken into account, the combinations 8,9 and 10 are considered the best solutions. They were therefore employed in the fabrication of the LB films to be reported here. Surface potential isotherms were also obtained in order to verify whether strong modifications to molecular packing occur during monolayer compression, The main features of the data shown in Fig. 2 are the constant, almost zero value for A V at large areas per monomer and the absence of any major changes in A V when the monolayer reaches the condensed state. The increase in A Vat a given critical area ( 10.9 A2 for the monolayer in Fig. 2) has been observed for a number of aliphatic, simple compounds such as fatty acids, fatty alcohols and phospholipids and is attributed to the structuring of the monolayer [ 91. The critical area is always larger than the area at which the pressure starts to rise. Even though ’ There has been some debate over the name secondary doping. The name has been challenged especially because after this doping process the material does not return to its original situation, i.e. no complete dedoping can be achieved. Some authors take the view that the phenomenon should be termed as an order-inducing effect.

A. Rid,

Jr. et cd. /Thill

Solid Fi1tn.s 284-285

(1996)

179

177-180

mately 760 nm in the UV-Vis

-O.’ 0'--50 Area per monomer

(A’,

Fig. 2. Surface potential curve from a (PAni + CSA) monolayer.

molecules are much more complex, such a structuring is also manifest in the A V-A curves for PAni monolayers. The lack of major changes in A V in the condensed state, or even after monolayer collapse, indicates that no drastic molecular rearrangement takes place upon monolayer compression. The highest value for A V, for closely packed monolayers, lied between 0.35 and 0.40 V for all monolayers investigated. polymer

3.2. Fabric&w

md cluvacteriz,atior~ of PAni LBjiltns

The as-deposited LB films are already doped, as demonstrated by the appearance of a polaronic peak at approxi0.25,

(4

29

0.20

absorption

spectra shown in

Fig. 3(a). The peak absorbance increases with the number of deposited layers (Fig. 3(b)) indicating that multilayer buildup is being accomplished. In-plane conductivity values of ca. 10-j S cm-- ’ were obtained for most films. Also, no significant anisotropy is observed with regards to the in-plant direction of the measurement in the four-probe van der Pauw technique. The lack of anisotropy is to be expected since there should be no preferential direction of deposition of polymer molecules on the substrate, and this appears to be confirmed by data for other conducting LB films [ IO]. In just one case we did measure a higher conductivity. This occurred for a sample that was exposed to m-cresol vapor for 30 days, which caused the conductivity to increase to IO- ’ S cm ~ ’ in an anisotropic fashion. The increase may be associated with the secondary doping caused by m-cresol, while the anisotropy could be due to the preferential orientation of the molecules. This is in fact consistent with the order-inducing effect of m-cresol. When compared with published conductivity data for similar PAni LB films. these measured values lie between those quoted by Cheung and Rubner [ 51 (up to IO-- ’ S cm ’ ) and byPunkkaeta1. [7] (lo-‘Scm-‘). Secondary doping owing to exposure to m-cresol appears to be consistent with some UV-Vis absorption data. When

I o.25)(b) I

/\

.

Wavelength (nm) Fig. 3. (a) uv-vis

Number of layers

spectra of (PAni + CSA) LB films. The numbers indicate the number of deposited layers. (b) Peak absorbance

(760 nm) versus number

of deposited layers.

0.08,

(b) 0.08

0.06-

.

I3

L!

em z 0.04

10.04 . m

2

n

0.02

9 0.02-

l

. 0.00 400

o.oo+ 600

800

Wavelength (nm) Fig. 4. (a) UV-vis spectra of (CSA + number of deposited layers

1000

0

10

20

30

Number of layers

m-cresol+ PAni) LB films. The numbers indicate the number of deposited layers. (b) Absorbance

(760 nm) versus

180

A. Riul, Jr. et (11./Thin Solid Films 28&285 (I 996) 177-l 80

200

Inm

0

0

200

400

600

LBFH Fig. 5. AFM image of LB films of PAni pre-doped with CSA. Scanned area 800 X 800 nm’.

LB films were deposited from Langmuir monolayers prepared with m-cresol the UV-Vis spectra take the form shown in Fig. 4(a). The polaronic band is now much broader than in Fig. 3, and presents a tail for increasing wavelengths which has been attributed to delocalization as in secondary doping for other PAni films [ 111. The absorbance (taken at 760 nm) again increased linearly, within experimental error, with the number of layers (Fig. 4(b) ) . Interestingly, upon inspecting the as-deposited films we failed to notice the strong odour characteristic of m-cresol which probably indicates that mcresol was not incorporated into the LB film. It may rather have been mixed with the aqueous subphase as the latter indeed presented the m-cresol odour. A fibrillar-like structure indicating some preferential orientation was observed in AFM studies, as shown in Fig. 5. Common fibril width and apparent heights could range from -60 to = 160 nm and from = 1 to = 10 nm, respectively, depending on the region of the sample analysed. Previous work [ 121 on scanning tunneling microscopy of parent polyaniline cast from NMP in the form of free-standing thick films also presented a fibrillar-like morphology with widths of the same order as ours (from 10 to 140 nm) . Theirporosity, however, was much greater than ours (valleys heights varying from 0.5 nm to 70 nm). These results seem to be consistent with a more compact and less porous morphology for the PAni films produced by the LB technique.

Acknowledgements The financial assistance from FAPESP, (Brazil) is gratefully acknowledged.

CNPq and Finep

References [ 11 Y. Cao. P. Smith and A.J. Heeger, Synrh. Met, 55-57 ( 1993) 3514. [2] A. Riul Jr.,L.H.C. Mattoso, S.V. Mel1o.G.D. Tellesand0.N. Oliveira, Jr., Synrh. Met.. 71 ( 199.5) 2067. 131 L.H.C. Mattoso. A.G. MacDiarmid and A.J. Epstein, Synrlr. Mer., 68 (1994) 1. [4] L.H.C. Mattoso, S.V. Mello, A. Rim, Jr., O.N. Oliveira, Jr.. and R.M. Faria, Thin Solid Films, 244 ( 1994) 7 14. [S] J.H. Cheung and M.F. Rubner, T/rin Solid Films, 244 ( 1994) 990. [6] M.E. Agbor, M.C. Petty, A.P. Monkman and M. Harris, Synth. Met.. 55-57 ( 1993) 3789. [7] E. Punkka, K. Laakso, H. Stubb, K. Levon and W-Y. Zheng, Thin Solid F&us, 515-520 ( 1994) 243. [8] A.G. MacDiarmid and A.J. Epstein, Syrtth. Mer., 65 ( 1994) 103. [9] H. Morgan, D.M. Taylor and O.N. Oliveira, Jr., Biochim. Biophy.~. Am. 1062 (1991) 149. [IO] J.H. Cheung, Ph.D. Thesis, Massachusetts Institute of Technology, EUA, 1993. [ 111 A.G. MacDiarmid and A.J. Epstein, Bruzilim Con/. on Polymem, Sso Paulo, Brazil, October 1993, Brazilian Polymer Association, SPo Carlos, Brazil, p. 544. [ 121 J.G. Mantovani, R.J. Warmack, B.K. Annis, A.G. MacDiarmid and E. Scherr, J. A&. Polym. Sci., 40 ( 1990) 1693.