Synthetic Metals 111–112 Ž2000. 619–621 www.elsevier.comrlocatersynmet
Optical characterization of polymethylmethacrylate–poly ž3-alkylthiophene/ Langmuir–Blodgett films A. Bolognesi a,) , C. Botta a , G. Bajo a , A. Relini b, R. Rolandi b b
a Istituto di Chimica delle Macromolecole, CNR, Via E. Bassini 15, 20133 Milan, Italy Istituto Nazionale Fisica della Materia and Dip. Fisica UniÕersita` di GenoÕa, Via Dodecaneso 16, 16146 GenoÕa, Italy
Abstract We have synthesized a highly regioregular polyŽ3-alkylthiophene. containing an oxygen atom in the side chain, polyŽ3-pentylmethoxythiophene. ŽP5OMe.. We have studied the formation of Langmuir–Blodgett ŽLB. multilayer structures prepared by mixing P5OMe with highly isotactic polymethylmethacrylate ŽPMMA., and we have characterized the structure obtained by means of UV–Visible absorption, photoluminescence measurements and AFM. A certain degree of orientation of the macromolecules in the dipping direction was observed. q 2000 Elsevier Science S.A. All rights reserved. Keywords: PolyŽ3-alkylthiophene.; Polymethylmethacrylate; Langmuir–Blodgett films
1. Introduction Most of the recent developments in the field of polymeric electronic devices are due both to the preparation of thin films with a controlled supramolecular architecture w1x and to the synthesis of suitable polymeric structures w2x. The best supramolecular structure is not necessarily a highly ordered architecture, but it has to be optimized according to the peculiar application. In this context, the preparation and the study of new polymeric thin films provides new materials with improved properties in view of assembling new devices. In this work, we present the UV–Visible absorption characterization and an AFM preliminary investigation of thin film prepared by means of the Langmuir–Blodgett ŽLB. technique of polymethylmethacrylate ŽPMMA. and polyŽ3-pentylmethoxythiophene. ŽP5OMe, Fig. 1.. The use of the LB technique is particularly suitable because it
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allows a control of the thickness at the molecular level and in some cases, at least with some stiff macromolecules, it is possible w3x to induce macromolecules orientation during the transfer process.
2. Experimental Highly isotactic PMMA was prepared according to Ref. w4x. M w was 450.000 with M w rMn s 1.5. P5OMe synthesis has been reported in Ref. w5x: M w was 8.100. LB multilayers were prepared with a Lauda Filmwaage, computer controlled and following the procedure described in Ref. w6x. The mixture PMMArP5OMe was obtained by dissolving 1.5 mg of both PMMA and P5OMe in 6 ml of CHCl 3 . Transfer was performed at 12 mNrm. AFM imaging was performed in air with a Dimension 3000 microscope ŽDigital Instruments., equipped with a ‘‘G’’ scanning head, which has a maximum scan area of 70 = 70 mm2 . Single beam uncoated silicon cantilevers were used Ž125 mm length, type TESP, Digital Instruments.. Images were acquired in tapping mode. Typical scan areas ranged from 10 mm to 500 nm. Optimal imaging conditions were achieved at drive frequencies
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 3 9 2 - 6
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around 280 kHz, drive amplitude 300 mV and scan rate of 1 Hz.
3. Results and discussion We have recently reported a new synthetic approach to produce highly regioregular polyŽ3-alkylthiophenes. ŽPATs. w5x. With this method, a regioregularity as high as 98–100% can be obtained as detected by the 1 H-NMR spectrum. In this work, we focus our attention to one of the polymers synthesized with this method, P5OMe ŽFig. 1.. This polymer is different from other PATs reported in previous works w3,7x does not give oriented LB films. In fact, we have shown that PATs containing long side chains with polar groups may form LB structures oriented along the dipping direction and the degree of orientation seems to depend on the length of the side chains and on the nature of the polar groups. We have tried to induce orientation in this polymer during the transfer process by spreading on the water surface a mixed solution formed by P5OMe and PMMA. In fact, highly isotactic PMMA, with high molecular weight, narrow distributed, has been described in literature w6x to give oriented LB multilayer structures. The process of orientation has been described in detail by Brinkhuis and Schouten w6x: it has been reported that after a crystallization at a surface pressure of about 12 mNrm on the water surface, orientation of the macromolecules in the dipping direction occurs during the transfer process. In Fig. 2, the absorption spectrum of five layers of P5OMe is reported and compared to the spectrum of P5OMerPMMA LB film Žeight layers. obtained by spreading the P5OMerPMMA solution. The absorbance of P50Me in each layer is equivalent for the two structures by taking into account that, in the mixed structure, only 35% P50Me is present. As can be seen, the two spectra are slightly different: the vibronic structure which is present in the pure LB film at 600 nm disappears in the mixture structure indicating a different degree of aggregation of P5OMe. Fig. 3 reports the polarized UV–Visible absorption spectrum of P5OMerPMMA multilayered structure, obtained with light polarized parallel and orthogonal to the dipping direction. As can be seen, a partial orientation of the P5OMe backbones along the dipping direction is obtained, the dichroic ratio being about two. This value shows that
Fig. 1. Structure of P5OMe.
Fig. 2. UV–Visible absorption spectra of P5OMe LB films: dotted line, pure P5OMe; Full line, P5OMerPMMA matrix.
PMMA matrix induces a certain degree of orientation on the PAT system. Moreover, the chains are preferentially oriented in the dipping direction which, according to Brinkhuis and Schouten w6x, is also the preferential orientation of the PMMA matrix. It is also interesting to note that the same degree of orientation which is reached by PMMA is obtained for P5OMe. In fact, PMMA alone was reported to have a dichroic ratio of about 2.5 Žafter annealing at 1208C. as detected by FTIR measurements w6x which is near to the value found in our mixed P5OMerPMMA multilayers with a different spectroscopical technique. A preliminary AFM investigation was performed on the LB multilayered structure formed by the PMMArP5OMe mixture. In Fig. 4, a typical image is reported. The film is formed by grains elongated in the dipping direction; in this direction their size ranges from 50 to 100 nm. Each grain is structured in parallel bands approximately 20 nm wide, always perpendicular to the elongation direction. A further inspection of the bands shows that they are formed by small grains whose diameter is approximately 20 nm. The film roughness is around 3 nm. Occasionally, holes were present in the surface, allowing the measurement of the layer thickness, which turned out to be approximately 24 nm.
Fig. 3. UV–Vis absorption spectra of P5OMerPMMA LB films in polarized light Žparallel and perpendicular indicate the orientation of the polarizer with respect to the dipping direction..
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4. Conclusions Mixed layers of highly regioregular polyŽ3-alkylthiophene. with highly isotactic PMMA have been prepared by means of the LB technique. The PMMA matrix has been found to induce a certain degree of orientation on the P5OMe polymer chains in the dipping direction. Preliminary data from AFM measurements indicate that the two polymers do not segregate in different domains: this is in agreement with the UV–Visible absorption spectrum of the mixed structure showing no vibronic structure and with its PL spectrum, which has the same shape and same position as the P5OMe PL solution. Acknowledgements Fig. 4. Tapping mode AFM image of a P5OMerPMMA Ž1r1 by weight. film formed by eight layers.
Surface topography measurements do not allow to distinguish the two polymeric structures, PMMA and P5OMe. Phase imaging in tapping mode does not show any evidence of phase separation between the two components. We have no definitive explanation yet of this phenomenon: it seems that the two polymeric chains are not well separated in domains, but rather they form a solid solution. This hypothesis is in agreement with the lack of vibronic structure in the UV–Visible spectrum and with the PL spectrum of the mixed LB multilayers which is equivalent to the PL of the toluene P5OMe solution. This possibility needs more experimental work to be completely verified.
This work was supported by the Progetto Finalizzato Materiali Speciali per Tecnologie Avanzate II, Project DEMO. References w1x P.K.H. Ho, M. Gaanstrom, ¨ R.H. Friend, N.C. Greenham, Adv. Mater. 10 Ž1998. 769. w2x A. Kraft, A.C. Grimsdale, A.B. Holmes, Angew. Chem. Int. Ed. 37 Ž1998. 403. w3x A. Bolognesi, F. Bertini, G. Bajo, A. Provasoli, D. Villa, O. Ahumada, Thin Solid Films 289 Ž1996. 129. w4x K. Hatada, K. Ute, K. Tanaka, T. Kitayama, Y. Okamoto, Polymer J. 17 Ž1985. 977. w5x A. Bolognesi, W. Porzio, G. Bajo, G. Zannoni, L. Fannig, Acta Polymerica 50 Ž1999. 151. w6x R.H. Brinkhuis, A.J. Schouten, Macromolecules 24 Ž1991. 1496. w7x A. Bolognesi, G. Bajo, D. Comoretto, P. Elmino, S. Luzzati, Thin Solid Films 299 Ž1997. 169.