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Macromolecules 1998, 31, 3148-3150
Langmuir Film Polymerization To Form a Soluble Poly(phenylenevinylene) (PPV) Precursor Polymer
Scheme 1. Water Soluble Precursor Route to PPV
J. H. Batten and R. S. Duran* The Butler Polymer Research Laboratories, University of Florida, Gainesville, Florida 32611 Received October 29, 1997 Revised Manuscript Received March 9, 1998
Introduction. The synthesis of poly(phenylenevinylene) (PPV) via the water soluble polyelectrolyte precursor route (Scheme 1) was first introduced in 1968 by Wessling and Zimmerman.1 Since this time, there has been considerable interest in manipulating the properties of the polymer formed. This interest has led to the polymer’s recent application in LED technology.2 One method used to manipulate the polymer’s properties is to prepare Langmuir-Blodgett (LB) films of precursor polymer (B).3-5 These LB films are then converted to fully conjugated PPV (C) by thermal elimination of the sulfonium moiety. When PPV was generated by this technique, it was found that its mean conjugation length was extended, which indicates a more defect free polymer as compared with traditionally cast thin films. The extended conjugation has been attributed to the ability of the precursor polymer to orient itself more effectively in a Langmuir environment. Recently, a polymeric LED was prepared using LB multilayer films of PPV.6 The work reported herein expands upon this research by forming the precursor polymer in a Langmuir environment prior to deposition. In situ polymerization may further improve the quality of films produced by eliminating overlapped polymer chains, reducing chain coiling, and allowing better orientation of pendant side groups. This method has also proven to be an effective way to produce polymer with base sensitive pendant groups. As part of this work, an amphiphilic monomer has been synthesized and polymerized in a Langmuir environment. Results and Discussion. In order for a molecule to be surface active, it must have both hydrophilic and hydrophobic characteristics. By the addition of a hydrophobic group to A, these requirements for the monomer are met. For synthetic ease and the possibility of future cleavage of the hydrophobic chain, octadecyl 2,5-bis(tetrahydrothiopheniumylmethyl)benzoate dibromide (1) was synthesized as shown in Scheme 2. The first attempts to form Langmuir monolayers at the air-purified water interface at room temperature were unsuccessful; however, when the subphase temperature was cooled to 11 °C, stable monolayers formed. All of the subsequent work was done on cooled subphases. The Π-A isotherms of 1 (Figure 1) on water had an onset area at 58 Å2 molecule-1 and a collapse pressure of 18 mN/m. Hysteresis studies (Figure 1) revealed that the film could be compressed and expanded reversibly up to 10 mN/m. Isobaric creep data (film area vs time) showed that the film was stable (less than 10% area loss) for a minimum of 1 h at applied surface pressures up to 8 mN/m. At greater surface pressures, the film was unstable over time. When this polymerization is carried out in 3-dimensions, a stoichiometric amount of base is used so that
Scheme 2. Synthetic Route to 1: (i) 1-Octadecanol, H2SO4, Ethyl Ether (Yield 89%); (ii) NBS, Benzoyl Peroxide, CCl4 (80%); (iii) Tetrahydrothiophene, MeOH (68%)
the reaction does not proceed to the fully conjugated polymer; however, in Langmuir polymerizations a large excess of base is needed because of the small amount that is available at the surface. On the basis of the concentration of oxidant used in the polymerization of aniline type monomers in Langmuir monolayers,7 0.10 and 0.030 M NaOH subphases were studied. The isotherms of 1 on both basic subphases were similar in shape to those on water. However, the onset areas were greater at 67 Å2 molecule-1 and less reproducible ((5%). On expansion of the film, even from very low surface pressures (2 mN/m), hysteresis was present. Upon the second compression, the onset area was smaller and the hysteresis was less, which suggests that the initial
Figure 1. Isotherm (‚‚‚) and hysteresis (s) plots of 1 on purified water at 11 °C. Barrier speed ) 1.5 Å2 molecule-1 min-1.
S0024-9297(97)01597-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/08/1998
Macromolecules, Vol. 31, No. 9, 1998
Figure 2. Isobaric creep plots of 1 on water (- - -), 0.030 M NaOH (‚‚‚), and oxygen-reduced 0.030 M NaOH (s) at 11 °C and 8 mN/m.
change that occurred in the film had come to equilibrium. Hysteresis indicates a phase transition or a chemical reaction in the monolayer. Gel permeation chromatography (GPC) of the film provided evidence that dimers were formed, but there was no indication of polymeric material. Isobaric creep data showed that there was less than a 15% area decrease of the film at applied pressures up to 8 mN/m after 1 h. Previous reports have indicated that high molecular weight polymer only forms in the absence of oxygen.8 Therefore, the film was further studied on a degassed, 0.030 m NaOH subphase under an argon atmosphere. The isotherm and hysteresis plots were similar to those discussed in the previous paragraph, but the striking difference was observed in the isobaric creep measurements. After 1 h, the percent surface area decrease was between 25 and 30% at constant applied surface pressures between 4 and 8 mN/m. Figure 2 compares the isobaric creep plots of 1 on water, 0.03 M NaOH, and oxygen-reduced 0.30 M NaOH at a constant applied surface pressure of 8 mN/m. In the case of the oxygenreduced basic environment, the film area decreased most rapidly in the first 20 min of the reaction and, after 45 min, there was less area change. The relationship between monolayer area decrease and polymer formation has been documented and is attributed to the replacement of van der Waals radii with covalent bonds.9 In this case, area reduction can also be attributed to the elimination of tetrahydrothiophene and to the reduction of the ionic character of the film. After 1 h under polymerization conditions, the applied surface pressure was removed and the film was allowed to expand so that further studies of the monolayer could be done. Isotherms of the resulting films showed a 25% decrease of the onset (52 Å2 molecule-1) and little hysteresis even after compression to 20 mN/m, which is beyond the collapse of the monomer. Further evidence of film stability was that after 1 h, isobaric creep plots showed less than 10% area decrease at constant pressures up to 18 mN/m. The resulting films, which were soluble in THF and chloroform were collected for further evaluation by extracting the film from the subphase with chloroform or by aspirating the film and subphase through a glass frit, which collected the organic material.
Communications to the Editor 3149
Figure 3. Absorption spectra of 1: monomer (s); after polymerization (- - -); after heat treatment (‚‚‚).
Figure 4. Fluorescence spectra of 1 after polymerization and heat treatment. Excitation spectra before (- - -) and after (s) heat treatment and emission spectra before (‚‚‚) and after heat treatment (-‚-). λem ) 420 nm, λex ) 380 nm, in chloroform.
The UV-vis spectra (Figure 3) of the monomer and films before and after heat treatment (4 h, 250 °C, under vacuum) showed that there was a red shift upon polymerization and a further shift after heating, which indicates that a partially eliminated precursor polymer was formed in the Langmuir monolayer and a more highly conjugated PPV was formed upon heat treatment. The spectrum of the heat-treated polymer lacks the λmax between 400 and 500 nm that is typically reported for PPV and its derivatives. However, the tailing peak shown in Figure 3 has been previously reported for PPV derivatives and was attributed to the steric and electronic effects of the substituents.10 The polymer formed in the monolayer was fluorescent with a λmax at 434 nm in the emission spectrum (Figure 4). Heating the sample did not significantly alter the fluorescence spectrum; however, the λmax of the excitation spectra shifted from 382 to 400 nm on heat treatment. FT-IR analysis of the films produced in this study indicated that the ester had not hydrolyzed to the carboxylate anion, which was evident by the large C-H stretch between 2960 and 2880 cm-1 and the absence of the broad -O-H stretch between 3300 and 2500
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cm-1. This effect was observed in earlier Langmuir studies done with ethyl palmitate, which showed there was no apparent ester hydrolysis when the films were compressed and the ester functional groups were orientated out of the basic subphase.11 GPC analysis showed a major peak that corresponded to a Mw of ca. 13 000 and a polydispersity of 2.3. The most probable reason for the relatively low Mw is the inability to rigorously exclude oxygen from the reaction environment. The GPC results were obtained using a PG gel linear mixed bead column with a chloroform mobile phase. The instrument was calibrated with polystyrene standards. In conclusion, these results indicate that 1 has been successfully polymerized to form a partially eliminated sulfonium salt precursor polymer of PPV in a Langmuir environment. It is also one of the few examples of a chemical (as opposed to photochemical) polymerization in a monolayer. Further, this is a unique method that allowed the preparation of an ester-substituted PPV, which would have probably undergone some degree of cleavage to form the carboxylate anion under normal polymerization conditions. This polymerization technique could be extended to form PPV with base sensitive functional groups. Acknowledgment. We are grateful to the NASA Headquarters Graduate Student Researchers Fellowship program for financial support of this work. We also
Macromolecules, Vol. 31, No. 9, 1998
thank Kevin Ley for the fluorescence data, KSV Instruments for technical support, and Microcal software. Supporting Information Available: 1H and 13C NMR, mass spec, and elemental analysis data for the compounds produced in Scheme 2 (1 page). Ordering and accessing information is given on any current masthead page.
References and Notes (1) Wessling, R. A.; Zimmerman, R. G. U.S. Patent 3 401 152, 1968. (2) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539. (3) Nishikata, Y.; Kakimoto, M.; Imai, Y. Thin Solid Films 1989, 179, 191. (4) Era, M.; Kamiyama, K.; Yoshiura, K.; Momii, T.; Murata, H.; Tokito, S.; Tsutsui, T.; Saito, S. Thin Solid Films 1989, 179, 1. (5) Wu, A.; Yokoyama, S.; Watanabe, S.; Kakimoto, M.; Imai, Y.; Araki T.; Iriyama, K. Thin Solid Films 1994, 244, 750. (6) Wu, A.; Jikei, M.; Kakimoto, M.; Imai, Y.; Ukishima, S.; Takahashi, Y. Chem. Lett., 1994, 2319. (7) Zhou, H.; Stern, R.; Batich, C.; Duran, R. S. Makromol. Chem. Rapid. Commun. 1990, 11, 409. (8) Denton, F. R.; Lahti, P. M.; Karasz, F. E. J. Polym, Sci., Part A: Polym. Chem. 1992, 30, 2223. (9) Tieke, B. In Polymerization in Organized Media; Paleos, C. M., Ed.; Gordon and Breach Science: Philadelphia, 1992; Chapter 2. (10) Jin, J.; Lee, Y. Macromolecules 1993, 26, 1805. (11) Davies, J. T.; Rideal, E. K. Interfacial Phenomena; Academic: New York, 1963; p 285.
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