Fluorescence Study of Aggregation in Water of PEO-Grafted

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Fluorescence Study of Aggregation in Water of PEO-Grafted Polydiphenylamine Fengjun Hua and Eli Ruckenstein* Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, New York 14260 Received November 11, 2003. In Final Form: March 10, 2004 Fluorescence spectra of water-soluble conducting poly(ethylene oxide)-grafted polydiphenylamines (PDPAg-PEOs with the Mns of PEO 350, 750, and 2000) in water were determined and interpreted. The emissions of the PDPA-g-PEOs occurred in the range from 360 to 700 nm and were dependent on their concentrations, PEO chain length, extent of oxidation, pH, and temperature. An optimum concentration, above which the fluorescence intensity decreased dramatically because of quenching, was observed. This quenching was a result of the aggregation of PDPA-g-PEO macromolecules in water. The doped PDPA-g-PEO molecules provided a lower optimum concentration than the corresponding reduced-state macromolecules. A rodshaped microstructure of nanoscale size was generated through the aggregation of the PDPA-g-PEO macromolecules. This microstructure was also confirmed by dynamic light scattering, atomic force microscopy, and surface elemental analysis.

Introduction The π-conjugated polymers became well-known in the past decades because of their useful electronic, photonic, and electroluminescence properties.1-6 As one of the most important π-conjugated polymers, polyaniline was studied extensively and used to prepare ultrathin films, nanotubes, and thin shell-hollow capsules.7-11 However, the stiffness of the PANI backbone and the hydrogen-bonding interactions between the amine moieties of adjacent chains are responsible for its poor processibility and solubility in organic solvents. To improve the solubility of PANI in organic compounds, many approaches have been employed, involving the copolymerization of aniline with its derivatives,12,13 suitable substitutions on the emeraldine base at the N sites,14,15 and the use of various organic acid dopants.16,17 The first observation of fluorescence from a quinine solution in sunlight was reported by Herschel in 1845. The fluorescence could be generated after the fluorophore * To whom correspondence should be addressed. Tel: (716) 645-2911,ext.2214.Fax: (716)645-3822.E-mail: FEAELIRU@acsu. buffalo.edu. (1) Park, Y. W.; Heeger, A. J.; Druy, M. A.; MacDiarmid, J. J. Chem. Phys. 1980, 73, 946. (2) Cao, Y.; Andreatta, A.; Heeger, A. J.; Smith, P. Polymer 1989, 30, 2305. (3) Yue, J.; Wang, Z.; Cromack, K. R.; Epstein, A. J.; MacDiarmid, A. G. J. Am. Chem. Soc. 1991, 113, 2665. (4) Fou, A. C.; Rubner, M. F. Macromolecules 1995, 28, 7115. (5) Antoun, S.; Karasz, F. E.; Lenz, R. W. J. Polym. Sci., Part A: Polym. Chem. 1988, 26, 1800. (6) Eckjardht, H.; Shacklette, L. W.; Jen, K. Y.; Elsenbaumer, R. L. J. Chem. Phys. 1989, 91, 1310. (7) Cheung, J. H.; Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2712. (8) Huang, K.; Wan, M. X. Chem. Mater. 2002, 14, 3486. (9) Mezzenga, R.; Ruokolainen, J.; Fredrickson, G. H.; Kramer, E. J.; Moses, D.; Heeger, A. J.; Ikkala, O. Science 2003, 299, 1872. (10) Shi, X. Y.; Briseno, A. L.; Sanedrin, R. J.; Zhou, M. F. Macromolecules 2003, 36, 4093. (11) Wan, M. X.; Wei, Z. X.; Zhang, Z. M.; Zhang, L. J.; Huang, K.; Yang, Y. S. Synth. Met. 2003, 135/136, 175. (12) Kinlen, P. J.; Liu, J.; Ding, Y.; Graham, C. R.; Remsen, E. E. Macromolecules 1998, 31, 1735. (13) Roy, B. C.; Gupta, M. D.; Bhoumik, L.; Ray, J. K. Synth. Met. 2002, 130, 27. (14) Planes, G. A.; Morales, G. M.; Miras, M. C.; Barbero, C. Synth. Met. 1998, 97, 223. (15) DeArmitt, C.; Armes, S.; Winter, J.; Uribe, F. A.; Gottesfeld S.; Mombourquette, C. Polymer 1993, 34, 158.

was brought to an excited state, and the excess vibrational energy was lost rapidly to the solvent. The emission occurred at a longer wavelength (Stokes shift) when the fluorophore returned to the ground state. The fluorescence spectrum was typically independent of the excitation wavelength and the solvent relaxation occurred rapidly in about 10-10s.18 Fluorescence is typically produced by aromatic molecules and conjugated oligomers.19 The π-conjugated poly(p-phenylene vinylene) (PPV) has generated a lot of interest because it is used in the fabrication of PPV-based electroluminescence devices.19,20 Recently, Lee reported that a self-assembled multilayer thin film of water-soluble conjugated aromatic polyimine could provide fluorescence.21 Takahashi reported about the fluorescent behavior of a lyotropic liquid-crystalline poly(potassium oxy (x-sulfonate-1,4-phenylene)oxy (x-nitroterephthaloyl)).22 In this case, the fluorescence had its origin in the complexes formed between the potassium sulfonate-substituted aromatic moiety and the nitrosubstituted aromatic moiety and also in the dimer of the potassium sulfonate-substituted moiety. The fluorescence behavior was related to the phase change in the lyotropic liquid-crystalline system. In a previous paper, we reported the preparation of a water-soluble conductive polydiphenylamine (PDPA) with poly(ethylene oxide) (PEO) chains grafted to the nitrogens of the PDPA backbone.23 The PDPA-g-PEO with a combshaped architecture exhibited high solubility in organic compounds, such as chloroform and tetrahydrofuran, and also in water. Three PDPA-g-PEOs with different PEO (16) Rethi, M.; Ponrathnam, S.; Rajan, C. R. Macromol. Rapid. Commun. 1998, 19, 119. (17) Yin, W.; Ruckenstein, E. Synth. Met. 2000, 108 (1), 39. (18) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd ed.; Kluwer Academic/Plenum Publishers: New York, 2000. (19) Haugland, R. P. Handbook of Fluorescence Probes and Research Chemicals; Molecular Probes, Inc.: Eugene, Oregon, 1996. (20) Feast, W. J.; Tsibouklis, J.; Pouwer, K. L.; Groenendaal, L.; Meijer, E. W. Polymer 1996, 37, 5017. (21) Lee, T. S.; Kim, J.; Kumar, J.; Tripathy, S. Macromol. Chem. Phys. 1998, 199, 1445. (22) Takahashi, H.; Horie, K.; Yamashita, T.; Machida, S.; Hannah, D. T. B.; Sherrington, D. C. Macromol. Chem. Phy. 1996, 197, 2703. (23) Hua, F.; Ruckenstain, E. Macromolecules 2003, 36, 9971.

10.1021/la036124h CCC: $27.50 © 2004 American Chemical Society Published on Web 04/16/2004

Fluorescence Spectra of PDPA-g-PEOs

chain lengths [PDPA-g-PEO-350, PDPA-g-PEO-750, and PDPA-g-PEO-2000, prepared using PEO-350 (Mn ) 350), -750 (Mn ) 750), and -2000 (Mn ) 2000), respectively] were synthesized by substituting the tosyloyl end group of tosylate PEO with an amine moiety of the PDPA. The number-average molecular weights of the three copolymers, 35 600, 64 000, and 176 500, respectively, were determined on the basis of a polystyrene standard curve of a gel permeation chromatograph (GPC, Waters). We found that these polymers in water could emit fluorescence by themselves after being excited at their maximum absorption. The room-temperature conductivities of the compressed pellets of HCl-doped PDPA-g-PEO-350, PDPAg-PEO-750, and PDPA-g-PEO-2000 were determined using the conventional four-point method and found to be 4.2 × 10-2, 7.0 × 10-3, and 5.6 × 10-4 S/cm, respectively. These disks were nonfluorescent. In the present paper, we determined the fluorescence emission spectra of three PDPA-g-PEOs and of the corresponding HCl-doped grafted macromolecules in water and investigated the fluorescence dependence on the polymer concentration, the PEO chain lengh, pH, temperature, extent of oxidation, and the concentration of an oppositely charged polyanionic additive. The goal of this investigation was to relate the fluorescence emission to the polymer aggregation. The aggregation decreased the fluorescence intensity due to quenching. The wellorganized microstructures generated via aggregation were examined by dynamic light scattering (DLS), atomic force microscopy (AFM), and surface elemental analysis. A possible microstructure is suggested. Experimental Section The synthesis of PEO-grafted polymers and their characterization were reported in a previous paper.23 The grafting was carried out through a “graft onto” process, the tosylate-functionalized PEO chains being reacted with the amine moieties of the PDPA backbone. PDPA was prepared via a HCl-mediated polymerization of the DPA monomer, using (NH4)2S2O8 as initiator. The doped PDPA was neutralized with a 1.0 N ammonium aqueous solution, and this was followed by reduction with hydrazine. To avoid side reactions, such as cross-linking, a high excess of PEO-Tos compared to the secondary amine moieties of PDPA ([tosylate]/[NH] ) 3/1) was employed to ensure the generation of a fully grafted PDPA. The tosylate PEO was prepared via the esterification of the HO-ended PEO with tosylol chloride. The resulting PDPA-g-PEOs were oxidized with compressed air in the presence of a 3.0 N HCl aqueous solution. The FTIR spectrum of the neutralized PDPA-g-PEO exhibited bands at 1109, 1595, and 1494 cm-1, which can be assigned to the C-O-C stretching vibration of the oxyethylenes of the PEO side chains, the quinoid rings, and the phenyl groups of the PDPA backbone, respectively. The 1H NMR spectrum in deuterated chloroform provided multisignals in the range 6.8-7.8 ppm and signals at 3.60, 3.37, and 3.50 ppm, which can be assigned to the aromatic hydrogens of the PDPA backbone, the oxyethylene hydrogens of the ethylene oxides (EO), the end of the oxymethylene hydrogens of the PEO side chains, and the methylene hydrogens connected with the nitrogens, respectively. The 13C NMR spectrum exhibited peaks at 69, 58,61, 130, 142, and 148 ppm, which can be assigned to the oxyethylene carbons of the EO units, the end oxymethylene carbons of the PEO chains, the N-C connection between PEO and PDPA, the double-substituted phenyl ring carbon of PDPA, and the two aromatic carbons near the oxyalkyl substitution at nitrogen, respectively. Fluorescence measurements were carried out using a SLMAMINCO Model 8100 spectrofluometer. Dried polymer samples were dissolved in water for selected concentrations. To eliminate the dust, the sample solution was introduced with a syringe in a filter equipped with a Millipore membrane of 0.45 µm pore size. The filtered liquid was located in a 2 mL cell and the latter introduced in the chamber of the equipment and subjected to UV excitation of 310 or 320 nm wavelength. The emission spectra

Langmuir, Vol. 20, No. 10, 2004 3955 in the range 360-700 nm were scanned with a photomultiplier tube as detector. The emission spectra at high temperatures were determined by heating the sample at a rate of 2 °C/min. The data were collected after a selected temperature was kept for 10 min.24 The extinction coefficients of the reduced PDPA-g-PEO-2000 (excitation wavelength 310 nm) and HCl-doped PDPA-g-PEO2000 (excitation wavelength 320 nm) solutions were 0.119 and 0.214 L cm-1 g-1 at pH ) 2, 0.124 and 0.197 L cm-1 g-1 at pH ) 4, 0.118 and 0.187 L cm-1 g-1 at pH ) 7, 0.124 and 0.174 L cm-1 g-1 at pH ) 10, and 0.118 and 0.169 L cm-1 g-1 at pH ) 12, respectively. They remained constant in the concentration range investigated (below 1.0 g/L). To examine the fluorescence behavior in the presence of an anionic polyelectrolyte additive, polystyrene sulfonic acid (PSSA), which for doped polymers was expected to increase, via electrostatic cross-linking, the aggregation, was first prepared from the corresponding polystyrene sulfonic sodium (PSS sodium), purchased from Aldrich (Mn ) 10 000, PDI ) 1.2). The PSS sodium was dissolved in water and passed through a H+-type ionexchange resin (IR 1200 H resin from Aldrich) column, and the resulting polystyrene sulfonic acid (PSSA) solution was concentrated under vaccum at 60 °C. The water was removed under vacuum at room temperature for at least 2 days, and a transparent PSSA solid was thus obtained. The proton (1H) and carbon (13C) NMR, UV-vis absorption, and FTIR measurements were carried out on 400 MHz INOVA400, Thermo Spectronic Genesys-6, and Perkin-Elmer-FTIR 1760 equipment, respectively. Atomic force microscopy (AFM, Quesant Scan Atomic Co.) was used to investigate the conformation of the macromolecular aggregates. The aqueous polymeric solutions were spin-coated onto freshly cleaned mica plates at 1000 rpm and subsequently examined under AFM in the tapping mode. The elemental analysis of the polymer samples was carried out using a Perkin-Elmer Model 2400 C, H, N analyser. The chlorine and sulfur contents were determined by the oxygen flask method. The surface elemental analysis was carried out on a VG ESCA/SIMSLAB MK II using a Mg KR radiation source (1253.6 eV). Each polymer sample was dissolved in water at a concentration of 0.5 g/L and spin-coated on freshly cleaned mica. The coated mica was dried under high vacuum at room temperature for at least 7 days before the ESCA examination. The dynamic light scattering (DLS) experiments were performed using an ALV 5000-correlator and krypton ion laser with a wavelength of λ ) 647.1 nm. The sample solutions were filtered through Millipore Teflon membranes of 0.45 µm pore size into 10-mL cells that were sealed with Teflon stoppers before any measurement. The autocorrelation function of the scattered light intensity was analyzed using the CONTIN method developed by Provencher.25

Results and Discussion NMR Indirect Evidence for Aggregation. As pointed out in a previous paper,23 the prepared PDPA-g-PEOs were in reduced state and soluble in chloroform. In the 1H NMR spectra of the three PDPA-g-PEOs in deuterated chloroform (20 g/L), the integral ratios of the signals due to the aromatic hydrogens of the PDPA backbone and the signal due to the oxymethylenes of the PEO chains were near the expected values. However, the 1H NMR spectra of the three PDPA-g-PEOs in deuterated water with the same concentration of 20 g/L as in deuterated chlorofrom indicated that, whereas the signal intensity around 3.60 ppm due to the oxymethylenes of the EO units decreased in the expected sequence, the signals of the aromatic hydrogens in these polymers were almost undetectable (see Figure 1a). The 13C NMR spectrum of a 60 g/L PDPA-gPEO-2000 sample in deuterated water exhibited, after running for 12 h, weak signals above 130 ppm due to the aromatic carbons (Figure 1b). Similar results were ob(24) Bright, F. V. Fluorescence Spectroscopy in Instrumental Analysis; McCullough, T. O.; Barker, B. J., Eds.; Xavier University Press: Cincinnati, OH, 1992. (25) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 213.

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Figure 2. UV-vis spectra and the corresponding emission spectra of PDPA-g-PEO-2000 and HCl-doped PDPA-g-PEO2000 in water. (a) UV-vis spectrum of PDPA-g-PEO-2000; (a′) emission spectrum of PDPA-g-PEO-2000 (excitation wavelength 310 nm); (b) UV-vis spectrum of HCl-doped PDPA-g-PEO2000; (b′) emission spectrum of HCl-doped PDPA-g-PEO-2000 (excitation wavelength 320 nm). Scheme 1. Chemical Structures of HCl-Doped PDPA-g-PEOs

Figure 1. (A) 1H NMR spectra of various graft copolymers with different PEO lengths in deuterated water: (a) PDPAg-PEO-350, 20 g/L (slightly soluble); (b) PDPA-g-PEO-750, 20 g/L (partially soluble); (c) PDPA-g-PEO-2000, 20 g/L; (d) PDPAg-PEO-2000, 60 g/L. (B) 13C spectrum of PDPA-g-PEO-2000, 60 g/L in deuterated water.

tained in the NMR spectra in deuterated water of HCldoped PDPA-g-PEOs. In conclusion, the hydrogens or the carbons of the phenyl rings could not be completely detected by NMR in aqueous solutions. These observations provide indirect evidence for the aggregation of the polymers. The phenyl rings could not be detected in water because most of them aggregated into large particles and therefore the peaks broaden dramatically. In contrast, in deuterated chloroform one could detect both the hydrogens of PEO and those of PDPA, because no aggregation or only very small aggregates were formed by the PDPAg-PEO macromolecules in chloroform. The fluorescence experiments presented below provide additional insight about the aggregation of the PDPA-g-PEO macromolecules. Fluorescence Emission of Reduced PDPA-g-PEOs and HCl-Doped PDPA-g-PEOs in Water. As expected, a typical UV-vis spectrum and the fluorescence emission spectrum of reduced PDPA-g-PEO-2000 in water display almost mirror images of each other. The benzenoid rings (N-substituted diphenylamine units) of the PDPA backbone exhibit a maximum absorbance at 310 nm in the UV-vis spectrum and could be excited at that wavelength to produce fluorophores, which emit at the longer wavelength of 405 nm (Stockes shift; see Figure 2a,a′).18,21,24 The emission spectrum of HCl-doped PDPA-g-PEO-2000 possesses a wide peak in the range of 360-700 nm (Figure 2b′). The vibronic features around 500 nm are apparent and arise because of a Woods anomaly in the emission grating monochromator.24 As shown in the UV-vis absorption spectrum (Figure 2b), two absorption bands at 320 and 630 nm are present, which can be assigned to a

π-π* transition of the benzenoid ring (N-substituted diphenylamine) and an exciton transition, respectively. After doping, the delocalization of charges along the PDPA backbone produces conjugated segments of different lengths: the N-substituted diphenylamine, the quinoid ring, the N,N′-diphenyl benzidine radical cation, and the N,N′-diphenyl benzidine dication (Scheme 1). The electrons distributed along the PDPA backbone are responsible for the red shift after excitation and the decrease of the quantum yield. Optimum Concentrations of Reduced PDPA-gPEO-2000 and HCl-Doped PDPA-g-PEO-2000 and the Fluorescence Quenching. The intensity of the fluorescence is plotted against wavelength for various polymer concentrations of the reduced PDPA-g-PEO-2000 and HCldoped PDPA-g-PEO-2000 in Figure 3a,b. Both families of curves reveal the existence of an optimum concentration, above which the fluorescence intensity decreases with increasing concentration. The optimum concentrations are 0.4 and 0.04 g/L for the reduced PDPA-g-PEO-2000 and HCl-doped PDPA-g-PEO-2000, respectively. Our experiments have shown that these optimum concentrations and fluorescence spectra did not change with the geometry

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Figure 3. Emission spectra of PDPA-g-PEO-2000 and HCl-doped PDPA-g-PEO-2000 (excitation wavelength 320 nm) for various concentrations.

of the fluorometer cells employed (we used different fluorometer cells provided by Starna Cells, Inc). The optimum concentration can be related to the beginning of the aggregation and occurs because the macromolecular chains quench one another. The HCl-doped PDPA solutions contains the chloride anion (Cl-), which can be a quencher.26,27 Indeed, by adding NaCl the quenching was increased. Because the absorbance was below 0.2, the inner filter effect had a minor effect on our experiments. The doped PDPA-g-PEO-2000 generated larger aggregates than the neutral one, probably because the hydrogen bonding induced was stronger than the electrostatic repulsion. To further examine the quenching effect of aggregation, an anionic polyelectrolyte, polystyrene sulfonic acid (PSSA), which was expected to promote the aggregation of HCl-doped copolymers, was added to both the reduced and HCl-doped PDPA-g-PEO-2000 aqueous solutions. The fluorescence intensity of the reduced PDPA-g-PEO-2000 solution (0.03 g/L) remained almost the same, even for an SSA/DPA mole ratio of unity. However, in the case of HCldoped PDPA-g-PEO-2000 solution (0.03 g/L), a de(26) Illsley, P. N.; Verkman, A. S. Biochemistry 1987, 26, 1215. (27) Martin, A.; Narayanaswamy, R. Sensors Actuators B 1997, 3839, 330.

crease of the fluorescence intensity occurred with increasing PSSA content in water, as shown in Figure 4. Being uncharged, the reduced polymer could not interact with the polyanions of the dissociated PSSA and the intensity of the fluorescence remained almost unchanged. In contrast, the positively charged doped polymer could interact with those anions, which became a kind of crosslinker, thus stimulating the aggregation and decreasing the fluorescence intensity. Dependence of the Fluorescence Emision Spectra on the Oxidation Degree, PEO Chain Length, pH, and Temperature. The reduced PDPA-g-PEOs can be directly transformed into HCl-doped PDPA-g-PEOs by bubbling air through a 3.0 N HCl aqueous solution containing the macromolecules. Figure 5 shows a red shift of the fluorescence emission dependent on the oxidation time. During oxidation, new conjugated chemical structures of the PDPA backbone were formed, such as the quinoid rings, the N,N′-diphenylbenzidine radical cations, and the N,N′-diphenylbenzidine dications (Scheme 1). These structures generated a fluorophore after excitation that was responsible for the shift to longer wavelengths (red shift). Because the protonation of PDPA-g-PEO changed the distribution of electrons along the PDPA backbone and

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Figure 4. Emission spectra of 0.03 g/L HCl-doped PDPA-gPEO-2000 (excitation wavelength 320 nm) in the presence of polystyrene sulfonic acid (PSSA) for various mole ratios of styrene sulfonic acid (SSA)/DPA: (a) PSSA ) 0, (b) SSA/DPA ) 0.3, (c) SSA/DPA ) 0.5.

Figure 5. Emission spectra of PDPA-g-PEO-2000 for various oxidation times (excitation wavelength 320 nm). Small samples were taken out periodically from a 0.1 g/L PDPA-g-PEO-2000, 3.0 N HCl aqueous solution and dried under vacuum at room temperature at least for 2 days.

this effect is responsible for the red shift, the fluorescence intensity and the area integral emission intensity have decreased with increasing extent of oxidation. The fluorescence emission spectra of HCl-doped PDPAg-PEOs for various PEO chain lengths are presented in Figure 6. For a polymer concentration of 0.03 g/L, both the intensity area integral and the red shift increased with increasing PEO chain length (see Figure 6a). The concentration of 0.03 g/L was near the optimum concentration of the HCl-doped PDPA-g-PEO-2000 (0.04 g/L), but much higher than the optimum concentrations of HCldoped PDPA-g-PEO-750 and -PEO-350 (0.01 and 0.003 g/L, respectively). This occurred because the macromolecules with longer PEO chains generated for steric reasons smaller aggregates and thus were subjected to lower quenching. For the very small polymer concentration of 0.001 g/L, much smaller than the optimum concentrations of the three polymers, the intensity area integral increased in the opposite direction, being higher for the shorter PEO chains. This occurred because the grafted macromolecules were much less, or even not at all, aggregated for shorter

Figure 6. Emision spectra of (a) HCl-doped PDPA-g-PEO350, (b) HCl-doped PDPA-g-PEO-750, and (c) HCl-doped PDPAg-PEO-2000 for two concentrations in water (excitation wavelength 320 nm).

PEO chains, and as a result, the chromophores of the PDPA backbone had a higher exposure. The bulk elemental analysis has shown that all three HCl-doped PDPAg-PEOs possessed comparable doping extents, because they provided comparable Cl/N ratios. Indeed, these ratios for PDPA-g-PEO-350, PDPA-g-PEO-750, and PDPA-gPEO-2000 were 0.47, 0.43, and 0.41, respectively (see Table 1). It is worth noting that the fluorescence emission spectrum of HCl-doped PDPA-g-PEO-2000 was affected by the pH of the aqueous solution (see Figure 7). By increasing the pH from 10 to 12, the emission exhibited a mild blue shift, but the area integral intensities remained comparable. When the doped PDPA-g-PEO-2000 was dedoped with NH4OH, the total number of chromophores remained the same, because the N,N′-diphenylbenzidine radical cations and the N,N′-diphenylbenzidine dications were transformed into quinoid rings through dedoping. However, in the presence of HCl, additional fluorescence quenching was caused by the chloride quencher, which decreased the emission intensity. Blue shifts in fluorescence were observed when the pH was changed from 7 to 4 and from 7 to 2. The spectra exhibited at pH ) 2 and

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Table 1. Surface and Bulk Elemental Stoichiometries of PDPA-g-PEO-2000 and HCl-Doped PDPA-g-PEO-2000 atomic ratio polymer PDPA-g-PEO-2000 HCl-doped PDPA-g-PEO-2000a)

in the bulk (expected) C122.02H216.32O56.11N1.00Cl0.00S0.00 (C101.50H190.00O45.75N1.00Cl0.00S0.00) C118.25H210.26O52.715N1.00Cl0.41S0.00) (C101.50H190.00O45.75N1.00Cl1.00S0.00)

on the surfaceb

in the surface region after 20 nm sputtering

C152.31O81.28N1.00Cl0.00S0.00

C134.43O75.02N1.00Cl0.01S0.00

C178.54O97.20N1.00Cl0.10S0.00

C140.00O72.06N1.00Cl0.38S0.00

a The graft copolymers were oxidatively doped in air in the presence of 3 N HCl solutions for at least 72 h. b The polymer solution was prepared by dissolving a polymer sample in water at a concentration of 0.4 g/L, and by spin coating on freshly cleaned mica. The film was dried under high vacuum at room temperature for at least 7 days before the ESCA measurement.

Figure 7. pH dependence of emission spectrum of HCl-doped PDPA-g-PEO-2000 (excitation wavelength 320 nm).

Figure 9. (a) Hydrodynamic radius distribution function of PDPA-g-PEO-2000 as a function of concentration in water (scattered degree of 60°). (b) Average hydrodynamic radius of PDPA-g-PEO-2000 and HCl-doped PDPA-g-PEO-2000. Figure 8. Temperature dependence of the emission intensity of PDPA-g-PEO-2000 for a concentration of 0.4 g/L in water (excitation wavelength 310 nm).

4 a very narrow peak at 375 nm. This peak could be a result of the fluorescence caused by the single polymer chains, because as the pH was lowered, the increased number of positive charges on the backbone induced repulsion between chains, breaking up the aggregates so that single polymer chains could be observed. The dependence of the emission spectrum of the reduced PDPA-g-PEO-2000 on temperature is presented in Figure 8. The intensity slowly increased when the temperature was raised from 20 to 70 °C. The emission intensity first increased with increasing temperature because of lower aggregation. However, starting with 75 °C, the intensity decreased with increasing temperature. This decrease occurred because the PEO has lost its water of hydra-

tion,28,29 thus becoming insoluble in water. As a result, the grafted polymer precipitated from the aqueous solution and the fluorescence intensity decreased. Microstructure of PDPA-g-PEOs via Aggregation in Water. As already noted, the dependence of the fluorescence intensity on the concentration of the grafted copolymer revealed that the PDPA-g-PEO macromolecules aggregated in water. The dynamic light scattering (DLS)30,31 was employed by us to confirm the existence of such microstructures, and the results are presented in Figure 9. By changing the concentration between 0.0459 and 0.286 g/L, the average hydrodynamic radius (〈Rh〉) of the reduced PDPA-g-PEO-2000 in water increased from 45.5 to 195.2 nm, values which are much larger than those (28) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145. (29) Malmsten, M.; Lindman, B. Macromolecules 1993, 26, 1282.

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Figure 10. AFM images of PDPA-g-PEO-2000 and HCl-doped PDPA-g-PEO-2000 for various concentrations in water: (a) PDPAg-PEO-2000, 0.4 g/L; (b)HCl-doped PDPA-g-PEO-2000, 0.4 g/L; (a′) PDPA-g-PEO-2000, 0.01 g/L; (b′) HCl-doped PDPA-g-PEO-2000, 0.01 g/L.

of the single macromolecules. It is obvious that the macromolecules aggregated with the hydrophilic PEO side chains exposed to water and the PDPA hydrophobic backbones hidden inside. For the HCl-doped PDPA-g-PEO2000, a similar result was found, but the 〈Rh〉 curve was above that of the reduced PDPA-g-PEO-2000 (Figure 9b). The doped PDPA-g-PEO-2000 macromolecules possess higher conjugation along the PDPA backbone and hence smaller flexibility, and this increased the size of the hydrodynamic radius of their aggregates. The atomic force microscopy (AFM) provided the images from Figure 10. For the reduced PDPA-g-PEO-2000 at a concentration of 0.5 g/L, which is above its optimum concentration (0.4 g/L) (Figure 3), the particles have an average size of over 150 nm. For a concentration of 0.01 g/L, the particles became smaller in size, below 60 nm. For the HCl-doped PDPA-g-PEO-2000 at a concentration of 0.5 g/L, which is above the optimum concentration (0.04 g/L), the average

particle size was above 200 nm. At the higher dilution of 0.01 g/L, rod-shaped particles with a smaller diameter of 23 nm and a length near 80 nm were formed. The microstructure formation is sketched in Scheme 2. Table 1 lists the results of the surface elemental analysis for reduced PDPA-g-PEO-2000 and HCl-doped PDPA-gPEO-2000 films, which were prepared via spin-coating on mica plates with water as solvent and a concentration of 0.5 g/L. For the reduced PDPA-g-PEO-2000, the C/N and O/N ratios on the surface and after 20 nm sputtering were higher than those in the bulk polymer. On the surface, the C content was somewhat higher than expected because of the contamination of the surface, which occurs in any XPS analysis.32 For HCl-doped PDPA-g-PEO-2000, the Cl content after sputtering 20 nm was near that in the bulk. These results suggest a microstructure with the grafted PEO chains as a shell exposed to water.

(30) Yang, Y. L.; Xie, Z. W.; Wu, C. Macromolecules 2002, 35, 3426. (31) Chu, B. Laser Light Scattering, 2nd ed.; Academic Press: New York, 1991.

(32) Kumar, S. N.; Gaillard, F.; Bouyssoux, G.; Sartre, A. Synth. Met. 1990, 36, 111.

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Scheme 2. A Possible Aggregate of HCl-Doped PEO-Grafted PDPA in Water

Conclusions In conclusion, fluorescence spectra of a series of synthesized water-soluble PDPA-g-PEOs copolymers were determined. The reduced PDPA-g-PEOs and HCl-doped PDPA-g-PEOs produced fluoroscence in dilute water solutions. The emission can be related to their concentrations, the PEO side chain length, the extent of oxidation, pH, and temperature. An optimum concentration for each polymer was determined because of the quenching induced particularly by the aggregation. The HCl-doped PDPAg-PEO possessed a lower optimum concentration than that

of the corresponding reduced PDPA-g-PEO. A microstructure could be identified and the dynamic light scattering and atomic force microscopy measurements confirmed the presence of rod-shaped nanoscale aggregates. Acknowledgment. We are thankful to Drs. F. Bright and J. R. Errington for allowing us to use their equipment and for helpful comments. Dr. R. Kita from Max-Planck Institute in Mainz, Germany, carried out the dynamic light scattering measurements. LA036124H