Patterning of Polymers on a Substrate via Ink-Jet Printing of a Coordination Polymerization Catalyst**
By Johannes Huber, Abderramane Amgoune, and Stefan Mecking*
The generation of laterally structured patterns of conju gated, conductive polymers on flat substrates is a key step in the development of new technologies for the production of, for example, displays or low cost flexible circuits.[1–3]Printing and related procedures, in which the pattern of the individual pieces produced can also differ from one another, are patterning techniques suited for mass production and they are versatile with regard to the substrate.[4]However, when using polymers they are often hampered by limited solubility and the high viscosity of polymer solutions.[4b] This applies particularly to conjugated polymers, patterning of which, however, is of particular interest owing to their conductive and electro optical properties.[2c]
The direct patterning on a surface of a small amount of an appropriate compound, which promotes polymerization of subsequently added monomer, is an attractive approach to the aforementioned issue in principle.[5–7]Patterning of a transi tion metal coordination polymerization catalyst would be particularly attractive, as in catalytic polymerization one active site may form more than one polymer chain, and catalytic polymerization enables microstructure control.[8]
We report herein on the patterning of polymers via ink jet printing of a coordination polymerization catalyst. We chose polyacetylene as an example, as it is the prototype of a conjugated, conductive polymer and at the same time it is unprocessable from the neat bulk polymer owing to its insolubility and intractability.[9]
A prerequisite for this approach are suitable polymerization catalysts that tolerate the polar functional groups ubiquitous on most substrates. For example, paper or glass surfaces contain OH groups and adsorbed water. In general, late transition metal catalysts[10]are more tolerant towards polar groups than the more oxophilic early transition metal catalysts, as illustrated by their capability to co polymerize olefins with polar substituted monomers and to polymerize in aqueous emulsion.[11,12]
Catalysts based on cationic palladium complexes of bulky substituted trialkyl diphosphines are very active for the polymerization of acetylene.[13] The high stability of this catalyst system towards air and water[14]led us to investigate the approach of patterning via catalyst printing. A methanol solution of the in situ catalyst Pd(OAc)2/1,3 bis(ditert butylphosphino)propane was printed on paper or transparency sheets by employing a standard ink jet printer (see Experi mental for details). The printed substrates were subsequently exposed to an acetylene atmosphere at ambient pressure for a few hours. The catalyst retains its activity on these polar substrates, as evidenced by formation of a dark red polyacetylene pattern (Fig. 1). The catalyst productivity was estimated by weighing a defined area of paper before and after printing of the catalyst, and also after polymerization of acetylene. The productivity amounts to approximately 400 mol monomer converted per mol of metal present (see Experi mental).
The polymerization is thought to proceed as a chain growth polymerization in concentrated solution ([Pd]1 M). Excess phosphine or methanol may function as a solvent. This working hypothesis is based on the observation that excess phosphine or added methanol (a few drops added to the vessel with the patterned substrate during acetylene polymerization) accel erates the polymerization reaction. Note that phosphine and methanol are also active reagents in the formation of active species, which adds complexity to this issue.[15]
As expected for polyacetylene, the polymer formed is entirely insoluble in organic solvents. IR spectra of the polymer covered areas feature the characteristic signals of polyacetylene, with approximately equal amounts ofcis and trans repeat units.[16] The thickness of the polymer layer depends on the polymerization time. Samples from 1 to 4 days of polymerization were prepared with an average thickness of 9 to 30mm (see Supporting Information, Table S1). The roughness of the polymer surface was revealed by atomic force microscopy (AFM) to be typically0.1mm (Fig. 2).
‘‘Doping’’ with iodine afforded conductive circuit paths with a specific conductivity ranging from 6 to 70 S cm 1approxi mately, depending on the polymerization time (1 to 4 days).
Noteworthy, a conducting circuit path is already formed after only several hours of polymerization. Reported conductivities of bulk polyacetylene vary considerably, and are influenced by stretching induced orientation and the doping procedure.
Values range from 102 to 105S cm 1.[9,17] Polyacetylene is generally considered unstable in air.[9b–d] Nonetheless, [*] J. Huber, Dr. A. Amgoune, Prof. S. Mecking
Chair of Chemical Materials Science
Department of Chemistry University of Konstanz Universita¨tsstr. 10, 78457 Konstanz (Germany) E mail: stefan.mecking@uni konstanz.de
[**] A. A. thanks the Alexander von Humboldt Foundation for a research fellowship. S. M. is indebted to the Fonds der Chemischen Industrie.
We thank Christine Beierlein (Freiburg) for IR analyses. Supporting Information is available online from Wiley InterScience or from the authors.
First publ. in: Advanced Materials 20 (2008), pp. 1978-1981
Konstanzer Online-Publikations-System (KOPS) URL: http://www.ub.uni-konstanz.de/kops/volltexte/2008/6292/
URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-62924
conductivity was retained without any precautions for the exclusion of air. After 10 days, half of the original conductivity was still observed.
The resolution of the procedure was probed by printing the catalyst solution in the form of thin lines on paper as well as
transparency sheets (Fig. 3). The resolution observed is similar to the resolution observed with the original ink; it is limited by the printer employed to ca. 0.25 mm when printing on transparency sheets. To confirm the complete separation of these individual lines, the polymer pattern was doped with iodine and the electrical resistance within the pattern was measured. The average resistance of a circuit path (Fig. 3, left) is 12 kV on 1 cm length, while the resistance between two different lines exceeds 20 MV. This confirms the reliability of the overall patterning process.
The preparation of multilayered structures was studied briefly. Two crossed polyacetylene paths separated by an insulating resin were prepared layer by layer by the catalyst printing technique. The conductivity was measured through and between the two (doped) layers. No short circuit between the layers was observed, while a similar conductivity was observed for the two layers (see Support ing Information for experimental details).
As a demonstration of the reliable conductivity of the circuit paths obtained, an electronic calculator (See Supporting Information) was constructed in which the keypad circuit paths were prepared via the catalyst printing technique. A 44 key pad (Fig. 4) was printed on plain paper and on a transparency sheet with the catalyst solution, followed by sub sequent surface polymerization of acet ylene and ‘doping’ with iodine. The keypad functioned perfectly over several weeks, in air. Remarkably, such flexible key pads can be rolled up without changing their conductivity (Fig. 4).
In conclusion, we have demonstrated the preparation of lateral structures of an insoluble, intractable polymer via ink jet printing of a coordination polymerization catalyst. Flexible substrates could be employed. Polyacetylene paths gener ated in this manner are suited as conductive circuit paths. We are cur rently studying the structuring of other difficult to process polymers via catalyst printing.
Experimental
General Conditions and Materials: Methanol (p.A. grade) supplied by Fluka, Pd(OAc)2 donated by Umicore, and methane sulfonic acid supplied by Acros were used as received. 1,3 bis(ditertbutyl) phosphinopropane was prepared according to [19]. Acetylene of 2.0 grade (99% purity) supplied by Sauerstoffwerk Friedrichshafen GmbH was used without further purification. AFM images were recorded on a JPK NanoWizard instrument in the intermittent contact mode using a silicon tip with a force constant of 40 N m 1and a resonant frequency of Figure 1. Patterns prepared by catalyst ink jet printing on glossy paper
(left) and on an overhead transparency sheet (right).
Figure 2. Topographic AFM image (left) and cross sectional topography trace (right) of a polymer surface (on ink jet transparency sheet as a substrate; polymerization time 4 days; average film thickness ca. 10mm).
Figure 3. Optical microscopy images of polymer lines prepared via catalyst printing. Lines patterned on a transparency sheet (left) [18], and on high gloss paper (right).
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about 300 kHz. Optical microscopy images were recorded on a Leica DM4000M microscope. Fourier transform IR analyses were carried out at the Institute of Macromolecular Chemistry in Freiburg employing attenuated total reflection (ATR). The circuit layouts were designed using Cadsoft Eagle 4.01 software.
Preparation of the Catalyst solution and Printing Process: Under a nitrogen atmosphere, a solution of 1,3 bis(ditertbutyl)phosphin opropane (598 mg, 1.8 mmol) in methanol (8 mL) was added to Pd(OAc)2(135.9 mg, 0.6 mmol) and the mixture was stirred for 30 min.
All further manipulations were carried out in air. The solution was filtered through a syringe filter (0.24mm) and methane sulfonic acid (140 mg) was added to the filtrate. A clear dark yellow solution was obtained. This catalyst solution, with a concentration of 75mmol Pd mL 1, was directly used for the printing experiments. All printing experiments were carried out using a modified HP Deskjet 720 desktop printer. The ink tank was replaced by a Teflon tube dipping into a vial containing the catalyst solution.
General Polymerization Procedure: Once printed on the substrate, the catalyst pattern was placed in a Schlenk tube under an argon flow, and the argon atmosphere was replaced by an acetylene atmosphere (ambient pressure). Exposure times were typically several hours, at room temperature.
Doping Process: The polyacetylene pattern obtained was doped by covering the substrate with coarse grained iodine for 10 to 30 min, depending on the thickness of the film. The excess iodine was removed after the doping procedure.
Estimation of the Conductivity: The polyacetylene film resistance was measured with a commercial multimeter on rectangular pattern films with defined dimensions (12.5 cm2). The conductivity was then estimated using the measured values of the resistance and the thickness.
Estimation of the Productivity: The catalyst consumption of the printer was determined by weighing the ink supply vessel before and after the printing process. After printing of the catalyst uniformly on a large area of an overhead sheet suited for ink jet printing (e.g., of A4 size, 2921 cm2) and surface polymerization of acetylene, a square piece of the coated sheet of 55 cm2size was cut out and weighed. The
weight was compared with that of the neat ink jet sheet of exactly the same dimensions.
From this data (corrected for the calculated weight of the Pd(OAc)2 applied, which resembles the non volatile portion of the catalyst), and the catalyst consumption per surface during printing, the catalyst produc tivity was determined.
Received: October 31, 2007 Published online: April 21, 2008
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Figure 4. 44 calculator keypad generated via catalyst ink jet printing on paper (top) and ink jet transparency sheet (bottom).
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