The optical spectroscopy of poly(p-phenylene vinylene)/polyvinyl alcohol blends: from aggregates to isolated chromophores

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1 Synthetic Metals 142 (2004) The optical spectroscopy of poly(p-phenylene vinylene)/polyvinyl alcohol blends: from aggregates to isolated chromophores Thomas G. Bjorklund, Sang-Hyun Lim, Christopher J. Bardeen Department of Chemistry, University of Illinois, 600 S. Mathews Ave., Urbana, IL 61801, USA Received 29 April 2003; received in revised form 12 August 2003; accepted 2 September 2003 Abstract We report the effects of blending a non-conjugated polymer, polyvinyl alcohol (PVA), with the precursor polymer of poly(p-phenylene vinylene) (PPV). After thermal conversion, the PVA/PPV blends show markedly different absorption and luminescence spectra than neat PPV, even in blends containing only 1 wt.% PVA. These changes coincide with the disappearance of the nanocrystalline domains present in neat PPV. The temperature dependence of the fluorescence decay is not as sensitive to PVA concentration, but also disappears for blends containing more than 60% PVA. The extreme sensitivity of PPV s optical spectroscopy to small amounts of amorphous blends suggests that the ordered domains play an important role in determining its spectroscopic properties Elsevier B.V. All rights reserved. Keywords: Polymer; Polyvinyl alcohol; Poly(p-phenylene vinylene) 1. Introduction The ability to process electroluminescent conjugated polymers at low cost has allowed them to emerge as promising alternatives to inorganic materials in light-emitting diodes [1]. An improved understanding of the excited state structure and dynamics in conjugated polymers should aid in the design of more efficient materials for such applications. The prevailing view is that these materials can be thought of as collections of chain segments with varying conjugation lengths, whose proximity to each other does not affect their fundamental electronic structure, but whose intramolecular exciton states interact via weak, Forster-type interactions. Poly(p-phenylene vinylene) (PPV) and its derivatives are one of the most studied classes of conjugated polymers, and for derivatives like poly[2-methoxy-5-(2 -ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV), this picture seems to be accurate, since the absorption and emission spectra are largely unchanged whether the polymer is in a dilute blend, solution, or in a neat film. For unsubstituted PPV, however, the lack of sidechains allows for much closer packing of the conjugated chromophores, and it is an open question as to Corresponding author. address: bardeen@scs.uiuc.edu (C.J. Bardeen). whether this close-packing leads to the qualitative differences in the optical properties of PPV and MEH-PPV. Unfortunately, PPV chains cannot be isolated in solution or dilute blends due its insolubility, and thus studying the transition from non-interacting to interacting PPV chains is problematic. In this paper, we prepare a series of PPV samples blended with varying amounts of polyvinyl alcohol (PVA) in order to examine the role of intermolecular contacts in this polymer. By systematically changing the fraction of PVA in the PPV, we can tune both its absorption and emission over a limited range, and go from a strongly temperature-dependent fluorescence lifetime to a temperature-independent lifetime. Even at low levels of PVA doping, on the order of 1 wt.%, we see dramatic changes in its steady-state spectral properties and the temperature dependence of the fluorescence decay. At such low PVA concentrations, PPV does not undergo significant chemical alteration, and X-ray diffraction data shows that the main effect of blending PVA with PPV is to disrupt the formation of crystalline domains in the polymer film. The sensitivity of PPV s optical spectra to slight changes in its nanoscale morphology suggests that it is necessary to take intermolecular interactions into account in order to fully understand its electronic structure. These results are also of interest from a practical standpoint of tuning the properties of a conjugated polymer by very light doping of an inert polymer /$ see front matter 2003 Elsevier B.V. All rights reserved. doi: /j.synthmet

2 196 T.G. Bjorklund et al. / Synthetic Metals 142 (2004) Experimental The PPV is prepared from a precursor polymer in a previously described synthesis [2 4]. The precursor polymer is kept in aqueous solution after synthesis at a concentration of 8 mg/ml. An aqueous PVA (Aldrich, M w = 124, ,000, 99+% hydrolyzed) solution, also of 8 mg/ml, is used to make the blends. Combining the appropriate proportions of the two solutions made solutions of 0, 0.5, 5, and 50 wt.% PVA in PPV precursor. The solutions were then spin cast onto 0.5 mm thick sapphire windows at 600 rpm for 30 min. The films were then loaded into a tube furnace for thermal conversion to PPV. The films were converted at 190 C for 3 h with a ramp time of 30 min under a vacuum of <6 Pa After the conversion the weight percent of PVA in PPV, assuming complete conversion, is 0, 1, 10, and 69%. The samples were then transferred inside of an argon filled glove bag to a compressed helium cryostat and placed under vacuum. PVA was chosen as the best polymer to blend with PPV for three reasons. First, both PPV precursor and PVA are both soluble in water which facilitates blending. Other polymers soluble in organic solvents required blending multiple solvents together such as methylene chloride and methanol. Second, high molecular weight PVA can withstand temperatures up to 200 C, which is required because of the thermal conversion process. While other polymers soluble in organic solvents can withstand higher temperatures, other water-soluble polymers such as polyethylene glycol cannot withstand the temperatures needed to convert the precursor polymer to PPV. Third, PVA/PPV blends display no visible phase separation under examination with a conventional microscope, meaning that the blend is homogeneous on a length scale of 500 nm or less. All other polymers tried in this experiment showed visible phase separation under the same conditions. Absorption spectra of the samples were collected at room temperature with an Ocean Optics S2000 spectrometer. Fluorescence lifetimes are measured after excitation by a 150 fs, 400 nm pulse generated by a 40 khz amplified Ti:sapphire laser system. The front surface of the sample is excited at an angle of about 20. The fluorescence is collected normal to the film surface on the same side and focused onto a m spectrometer with a 150 groove/mm grating (SpectraPro 150). The dispersed spectrum is then incident on a streak camera (Hamamatsu Streak Scope C4334). The streak camera measures both the fluorescence spectrum and the fluorescence decay of all the samples. The time resolution of the measurement is limited by electronic jitter to 50 ps in the time window used for collection of the data. X-ray diffraction data for neat PPV and the 2% PVA/PPV blend were taken on a Bruker AXS P4 Single Crystal X-ray Diffractometer with a Bruker AXS Rotating Anode Monochromator as the Cu K radiation source and a Bruker AXS Hi-Star area detector. The neat PPV film and the 2% PVA/PPV films were spin cast onto borosilicate glass coverslips and converted as described above. The films were then removed from the substrates by floating them on top of dilute hydrofluoric acid solution until the coverslip detaches and falls away. After removal from the substrate the films are transferred to a container of deionized water for 15 min to rinse away any acid residue. There is no sign that PVA is lost during this step, either from changes in the spectroscopy or in the physical appearance of the films. The films are then transferred to a Teflon covered substrate to dry, peeled off and put into an X-ray capillary for measurement. The diffraction profiles were collected over the 2θ range of 0 60 with a collection time of 10 min for the neat PPV film and 120 min for the 2% PVA/PPV. 3. Results In order to investigate the structural alterations brought on by PVA doping, we examine the infrared spectra and X-ray diffraction of the blends. Fig. 1a shows the infrared absorption of the 1% PVA/PPV blend, which can be compared to the absorption of neat PVA before and after heating (Fig. 1b) and to neat PPV (Fig. 1c). When PVA is added to PPV, we see the appearance of intrinsic PVA peaks superimposed on top of the usual PPV peaks [5,6]. The PVA peaks [7] include the CH 2 stretch at 1450 cm 1 and the C H stretch at 2900 cm 1, both of which overlap the PPV peaks at those frequencies. In addition, there is a large, broad peak Fig. 1. Infrared absorption spectra of: (a) a 1% (w/w) PVA in PPV film; (b) a neat PVA before and after heating at 190 C; (c) a neat PPV film.

3 T.G. Bjorklund et al. / Synthetic Metals 142 (2004) at around 3400 cm 1 which corresponds to the OH stretch of the PVA. The peak at 1100 cm 1 in the spectrum is partially due to the C O H stretch of PVA alone, but is also the expected frequency of the C O C asymmetric stretching mode which has been observed previously in such blends [8]. Such an ether linkage is expected if the OH group of the PVA attacks the methylene carbon of the PPV precursor, with the tetrahydrothiophene acting as a leaving group, leaving an ether cross-link in place of a double bond. A model calculation of the spectrum that involved adding together the weighted spectra of neat PPV and neat PVA after heating showed good agreement with the blend spectrum even at 1100 cm 1, without adding additional C O C amplitude. Simply heating the PVA by itself does not lead to the appearance of any new large peaks, as shown in Fig. 1b. The yield of the ether forming reaction must be less than 100%, as evidenced by the lack of significant ether vibrational peaks and the existence of the large free OH peak at 3400 cm 1. Comparison of the X-ray diffraction for neat PPV and a 2% PVA/PPV blend (Fig. 2) shows that the addition of the PVA reduces the crystallinity of the PPV polymer. The spectral behavior of the 2% blend is similar to that of the 1% blend. Not only does the large peak at 20 broaden considerably, the diffraction peak, which is related to the long-range order along an axis perpendicular to the polymer chains [9] completely disappears. The disappearance of this peak, along with the broadening of the (1 1 0) peak, indicates that what crystalline regions remain must have much smaller domain sizes than in the neat PPV film, where a Scherrer analysis of the X-ray diffraction peak width yields domain sizes of 5 10 nm [10]. Similar changes in crystallinity and crystallite size have also been observed in neat PPV films prepared using different precursor solvents, and in that case as well the changes in the X-ray diffraction were correlated with a blue-shift in the absorption spectrum [2]. Finally, the diffraction of the 2% PVA/PPV blend is qualitatively similar to what is observed in MEH-PPV, which also has no observable peaks due to interchain ordering [11]. The room temperature absorption and fluorescence spectra of PPV and the 1, 10, and 69% PVA/PPV blends are shown in Fig. 3a d. The neat PPV film absorption spectrum has some vibronic structure with peaks at 458 nm and 488 nm. As PVA is added to the PPV a blue-shift is observed in the absorption spectrum accompanied by a loss of the vibronic structure. Even just 1 wt.% of PVA in PPV causes a large shift of 30 nm to the blue of neat PPV. For PVA concentrations greater than 10% the absorption peak stops shifting and remains stationary at 410 nm. The shapeless absorption of the PVA/PPV blends is reminiscent of the amorphous MEH-PPV, although MEH-PPV s absorption spectrum is considerably redshifted due to its electron-donating alkoxy sidegroups. The origin of the apparent structure in the 69% absorption spectrum in Fig. 3d is unknown, but is partially due to noise in the measurement due to the low sample Fig. 2. X-ray diffraction profiles for (a) a 2% (w/w) PVA in PPV film and (b) a neat PPV film. Fig. 3. Room temperature normalized absorption ( ) and fluorescence ( ) spectra of (a) neat PPV, (b) 1% PVA/PPV, (c) 10% PVA/PPV, and (d) 69% PVA/PPV.

4 198 T.G. Bjorklund et al. / Synthetic Metals 142 (2004) Fig. 4. Fluorescence decays at the temperatures of 15 ( ), 100 ( ), 200 ( ) and 290 K ( ) for (a) neat PPV, (b) 1% (w/w) PVA in PPV, (c) 10% (w/w) PVA in PPV, and (d) 69% (w/w) PVA in PPV films. absorption. PPV s fluorescence spectrum is also quite sensitive to the amount of blended PVA. In the room temperature fluorescence, a blue-shift of about 20 nm is observed from neat PPV to 69% PVA/PPV. The shape of the spectrum changes as well. From Fig. 3, the ratio of the high energy 0 0 peak at 520 nm to the 0 1 peak at 555 nm changes from 0.5 at 290 K in neat PPV to 1.5 at 290 K in a 1% PVA/PPV blend. No loss of vibronic structure is observed in the fluorescence spectra, however, just a redistribution of intensity. The addition of the PVA to the PPV films also affects the fluorescence lifetime. Fig. 4a d shows the temperature dependence of the fluorescence decays for neat PPV and 1, 10, and 69% PVA/PPV samples. In all the samples, the fluorescence decays are non-exponential, typical of what is observed in conjugated polymers. We concentrate on the first nanosecond of the fluorescence decay. Neat PPV (Fig. 4a) has a very strong temperature dependence of its fluorescence decay with 1/e times ranging from 250 to 750 ps after lowering the temperature from 290 to 15 K [10]. Upon addition of PVA to PPV the temperature dependence of the PPV becomes weaker and the 1/e times for the 1% PVA in PPV blend (Fig. 4b) range from 500 to 750 ps when lowering the temperature from 290 K down to 15 K. This trend continues through the 10% PVA in PPV (Fig. 4c) until at 69% PVA in PPV (Fig. 4d) virtually no temperature dependence of the fluorescence decays remains and the 1/e time converges to a value of 500 ps. Perhaps surprisingly, this temperature-independent lifetime is shorter than that of neat PPV at the lowest temperatures. 4. Discussion Clearly, even a small (1 wt.%) fraction of PVA blended with PPV results in large changes in its photophysical properties. One concern is that the PVA may be reacting with the PPV during the conversion process since it is known that the precursor polymer can react with PVA s OH groups to form ether groups on the linkages between the phenyl rings of the polymer backbone. Although the IR spectra show no sign of large amounts of ether linkages, we can calculate a worst case scenario as well. A calculation of the relative number

5 T.G. Bjorklund et al. / Synthetic Metals 142 (2004) of hydroxyl groups to the number of vinyl linkages available in 1% PVA in PPV gives a ratio of 1 hydroxyl group per 42 vinyl groups. This means, at most, about 2 3% of the vinyl linkages in the blend could be reacted with the PVA. Assuming the effective chromophore size in PPV is 8 10 repeat units [12], this means only about 1 out of every 5 chromophores would have reacted. This fraction of reacted chromophores is not enough to account for the large changes observed in the spectra from neat PPV to the 1% PVA/PPV blend. Therefore, the redistribution of the fluorescence intensity observed from neat PPV to the 1% PVA/PPV blend is unlikely to have resulted from simple chemical interruption of conjugation. Also, if a chemical reaction were occurring in the blended films we would expect the absorption spectra of the blends to continue shifting to higher energy as the average effective chromophore length decreased. Instead, we see that the absorption stops shifting at a peak position of about 410 nm. Finally, the possibility that PVA somehow prevents the thermal conversion from occurring, leaving significant amounts of unreacted precursor, is also unlikely, since the absorptivity of a 1% PVA/PPV blend and that of a neat PPV film are the similar. If there were a substantial amount of unconverted precursor, we would expect substantially less visible absorption in the blends. In short, there is considerable evidence that the effects observed in this paper are not simply the result of interrupted conjugation. A clue to the origin of these changes can be obtained from our X-ray data, which shows that the amount of PPV contained in large (>5 nm) crystallites essentially vanishes when a small amount of PVA is added. The addition of PVA to PPV disrupts the crystalline order of the film, but there is not enough PVA to significantly isolate the polymer chains from each other. We can compare the number of non-conjugated carbon atoms to conjugated carbon atoms as a crude estimate for the amount of aliphatic filler in between conjugated segments. In MEH-PPV, where the alkoxy sidechains prevent the formation of crystalline domains, this non-conjugated to conjugated ratio is This ratio is similar to the 69% PVA/PPV blend, which has a non-conjugated to conjugated ratio of 1.3, while the 1% PVA/PPV sample has a ratio of only But even this small amount is sufficient to destroy crystalline order. This loss of order leads to a broadening of the absorption, while the shape of the PVA/PPV blend fluorescence becomes similar to that of MEH-PPV [10,13,14]. Qualitatively similar changes in the absorption and fluorescence spectra have also been observed by Friend and coworkers [15,16] in composites of PPV and silica nano-particles. The concentrations of our samples cover a similar range as the silica nano-composites, which had concentrations of 0, 15, 30, and 50 vol.%, but also allows us to examine very low concentrations like the 1% blend. In the PPV/silica nano-composites the changes observed in the steady-state fluorescence lineshape were attributed to a competition between two types of species in the film [15]. One species, identified as a singlet exciton localized on a single chain, was assumed to be favored in the amorphous regions of the film. It is this species that dominates the emission from the blends, and which most resembles MEH-PPV emission. The other species, favored in the more crystalline regions of the film, was assumed to be an aggregate species whose formation is thermally activated and which dominated the emission in neat PPV at room temperature. From our work, this aggregate species must be very sensitive to the degree of crystallinity, since even a small disruption of the order in a 1% PVA/PPV blend can remove it. The disruption of the sample s nanoscale morphology by PVA also modifies the temperature-dependent fluorescence decay data. The fluorescence lifetime of neat PPV is known to have a strong dependence on temperature (Fig. 4a) [17]. Examination of the first nanosecond of PPV fluorescence and anisotropy decays shows an Arrhenius behavior with an activation energy of about 170 cm 1 [10]. The origin of this decay is believed to be thermally activated energy transfer to quenching defects [18]. As PVA is added to the PPV the temperature dependence of the fluorescence lifetimes becomes smaller until at 69% PVA/PPV the lifetime of 500 ps becomes almost completely independent of temperature. Although the 1% PVA blend is sufficient to transform the steady-state optical spectra in Fig. 3, there is still a reasonably pronounced sensitivity to temperature in the fluorescence decay in Fig. 4b. This suggests that the energy transport process is not as dependent on the crystallinity, but instead depends on a quantity like the average interchain distance. By the time the fraction of PVA becomes greater than that of PPV (69%), the fluorescence decay looks like that of a single chromophore, with an exponential decay that is independent of temperature over the first decade or more. Interestingly, that decay is actually shorter (500 ps as opposed to 750 ps) than that of neat PPV at 4 K. Ideally, if the emission from 69% PVA/PPV blend and neat PPV at low temperature were both from the same type of isolated chromophore, then their lifetimes would be the same. This difference in lifetimes may be because the PVA chemically modifies the PPV segments, or it may be because the PPV emission is not truly isolated as a single chain exciton even at the lowest temperatures. For example, if the PPV exciton has some H-aggregate character, its decay time would be expected to be longer than that of an isolated chain [19,20]. 5. Conclusions In summary, we have observed that even a small amount of PVA can cause a transition from a bulk PPV film dominated by interchain species to a film behaving as a collection of isolated polymer segments. The disruption of crystalline order in PPV upon the addition of PVA, as evidenced by changes in the X-ray diffraction, leads to large changes in the spectroscopy. Even a 1 wt.% PVA/PPV blend tends toward behavior that is qualitatively similar to what is observed in MEH-PPV. By the time the proportion of PVA is about

6 200 T.G. Bjorklund et al. / Synthetic Metals 142 (2004) equal to that of PPV, the conjugated polymer is transformed from something that looks like an aggregate species to a dilute blend, where the fluorescence decay is independent of temperature. The fact that such small amounts of PVA are sufficient to affect the spectroscopy of PPV demonstrates that the role of crystallinity and the detailed intermolecular interactions are important in determining the properties of this material. Finally, it would be interesting to test the effects of blending of small amounts of PVA with PPV on the performance of light-emitting diodes as well [8], since such blending may break up the aggregates which are thought to quench luminescence without completely ruining the transport properties of the conjugated polymer. Acknowledgements CJB acknowledges support from a 3M Non-Tenured Faculty Award and NSF grants CHE and CHE References [1] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A.D. Santos, J.L. Bredas, M. Logdlund, W.R. Salaneck, Nature 397 (1999) 121. [2] T.G. Bjorklund, S.-H. Lim, C.J. Bardeen, Synth. Met. 126 (2002) 295. [3] S.-H. Lim, T.G. Bjorklund, C.J. Bardeen, Chem. Phys. Lett. 342 (2001) 555. [4] J.D. Stenger-Smith, R.W. Lenz, G. Wegner, Polymer 30 (1989) [5] D.D.C. Bradley, R.H. Friend, H. Lindenberger, S. Roth, Polymer 27 (1986) [6] M.M.d. Kok, A.J.J.M.v. Breemen, R.A.A. Carleer, P.J. Adriaensens, J.M. Gelan, D.J. Vanderzande, Acta Polym. 50 (1999) 28. [7] B.J. Holland, J.N. Hay, Polymer 42 (2001) [8] W.-P. Chang, W.-T. Whang, Polymer 37 (1996) [9] D. Chen, M.J. Winokur, M.A. Masse, F.E. Karasz, Polymer 33 (1992) [10] T.G. Bjorklund, S.-H. Lim, C.J. Bardeen, J. Phys. Chem. B 105 (2001) [11] S.-H. Chen, A.-C. Su, Y.-F. Huang, C.-H. Su, G.-Y. Peng, S.-A. Chen, Macromolecules 35 (2002) [12] H.S. Woo, O. Lhost, S.C. Graham, D.D.C. Bradley, R.H. Friend, C. Quattrocchi, J.L. Bredas, R. Schenk, K. Mullen, Synth. Met. 59 (1993) 13. [13] R. Jakubiak, C.J. Collison, W.C. Wan, L.J. Rothberg, B. Hsieh, J. Phys. Chem. A 103 (1999) [14] T. Nguyen, I.B. Martini, J. Liu, B.J. Schwartz, J. Phys. Chem. B 104 (2000) 237. [15] P.K.H. Ho, J.-S. Kim, N. Tessler, R.H. Friend, J. Chem. Phys. 115 (2001) [16] P.K.H. Ho, R.H. Friend, J. Chem. Phys. 116 (2002) [17] C.M. Heller, I.H. Campbell, B.K. Laurich, D.L. Smith, D.D.C. Bradley, P.L. Burn, J.P. Ferraris, K. Mullen, Phys. Rev. B 54 (1996) [18] L.J. Rothberg, M. Yan, F. Papadimitrakopoulos, M.E. Galvin, E.W. Kwock, T.M. Miller, Synth. Met. 80 (1996) 41. [19] J. Cornil, D.A.d. Santos, X. Crispin, R. Silbey, J.L. Bredas, J. Am. Chem. Soc. 120 (1998) [20] F.C. Spano, J. Chem. Phys. 118 (2003) 981.