pubs.acs.org/Langmuir © 2009 American Chemical Society
Surfactant (TTAB) Role in the Preparation of 2,7-Poly (9,9-dialkylfluorene-co-fluorenone) Nanoparticles by Miniemulsion P. Sarrazin,† D. Chaussy,† L. Vurth,‡ O. Stephan,‡ and D. Beneventi†,* †
LGP2, UMR 5518 CNRS - Grenoble-INP - CTP, 461 rue de la Papeterie, DU, BP. 65, 38402 Saint Martin d’H eres, France, and ‡LSP, 140 avenue de la Physique, BP. 87, 38402 Saint Martin d’H eres, France Received January 21, 2009. Revised Manuscript Received March 9, 2009
The role of tetradecyltrimethylammonium bromide (TTAB) and its partition between water, chloroform, and the chloroform/water interface during the miniemulsification of a photoluminescent polymer was investigated by indirect interfacial tension/elasticity measurements. Dynamic interfacial tension and elasticity measurements showed the presence of a gas-liquid phase transition at the chloroform/water interface and the formation of a rigid interface, which was supposed to promote emulsion stability. The parameters of the adsorption isotherms and the TTAB partition coefficient were obtained from surface tension isotherms. Dynamic surface tension measurements performed after TTAB water/chloroform extraction were used to compute TTAB partition between water, chloroform, and the chloroform/water interface. Model calculations allowed identifying (for the tested conditions) the minimum size of emulsion droplets before the onset of instability and the segregation of a sizable amount of TTAB in the final polymer nanoparticles, which induced a shift in the 2,7-poly(9,9-dialkylfluorene-co-fluorenone) (PF) photoluminescence emission band. The size of the emulsion droplets of the final polymer particles and the amount of segregated TTAB were in good agreement with the corresponding experimental values.
Introduction In the past few years, the electrical and optical properties of semiconducting polymers have been extensively investigated for both fundamental photophysics and applications in optoelectronics devices such as organic light-emitting diodes (OLEDs),1 organic thin film transistors,2 field effect transistors,3 solid-state lasers,4 photovoltaic cells,5 biological labels,6 and functional fibers-based materials.7-10 The use of organic polymers for electronic functions is mainly motivated by their low-end applications, where the low production and processing costs rather than advanced performance are the driving force. Another advantage of these materials is their excellent solution processability. Especially, organic conjugated polymers display a major processing advantage, since these materials are soluble in common organic solvents as a result of functionalization with a side chain. For this reason, polymer solutions in organic solvents can be easily processed by spin coating,1,11 drop casting,12 screen-printing,13 *Corresponding author. E-mail:
[email protected]. Tel: +33 4 76 82 69 54. Fax: +33 4 76 82 69 33. (1) 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 (London) 1990, 347, 539. (2) Torsi, L.; Cioffi, N.; Di Franco, C.; Sabbatini, L.; Zambonin, P. G.; BleveZacheo, T. Solid-State Electron. 2001, 45, 1479. (3) Xiao, D.; Xi, L.; Yang, W.; Fu, H.; Shuai, Z.; Fang, Y.; Yao, J. J. Am. Chem. Soc. 2003, 125, 6740. (4) McGehee, M. D.; Heeger, A. J. Adv. Mater. 2000, 12, 1655. (5) Hagfeldt, A.; Gratzel, M. Chem. Rev. 1995, 95, 49. (6) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013. (7) Hamedi, M.; Forchheimer, R.; Inganas, O. Nat. Mater. 2007, 6, 357. (8) Wistrand, I.; Lingstrom, R.; Wagberg, L. Eur. Polym. J. 2007, 43, 4075. (9) Huang, J.; Ichinose, I.; Kunitake, T. Chem. Commun. (Cambridge, U.K.) 2005, 1717. (10) Johnston, J. H.; Moraes, J.; Borrmann, T. Synth. Met. 2005, 153, 65. (11) Braun, D.; Heeger, A. J. Appl. Phys. Lett. 1991, 58, 1982. (12) Tang, C. W.; Vanslyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (13) Shaheen, S. E.; Radspinner, R.; Peyghambarian, N.; Jabbour, G. E. Appl. Phys. Lett. 2001, 79, 2996. (14) Hebner, T. R.; Wu, C. C.; Marcy, D.; Lu, M. H.; Sturm, J. C. Appl. Phys. Lett. 1998, 72, 519.
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and inkjet printing 14,15 to functionalize a wide range of suitable substrates. Recently, the preparation of nanoparticles in aqueous dispersions has attracted considerable attention in organic optoelectronics because of their cost effectiveness16,17 and the possibility to obtain stable dispersions that can be used for inkjet printing of electroactive devices18,19 or natural fibers treatment.20-22 Organic semiconducting polymer nanoparticles can be prepared by miniemulsion polymerization, where the monomer is polymerized to form polymer particles without changing the droplets identity.23 Other possibilities are the formation of artificial latexes by reprecipitation of hydrophobic polymers in water24,25 or solubilization of a preformed polymer in organic/ aqueous mixtures and its miniemulsification in water to give stable emulsions with droplets size in the 50-500 nm range.26 After evaporation of the solvent, a polymer dispersion is obtained in less restrictive conditions, avoiding the presence of residual polymerization byproducts. Miniemulsions are stable emulsions consisting of stable droplets created by shearing a system containing an organic solvent, water, a surfactant, and a highly (15) Hoth, C. N.; Choulis, S. A.; Schilinsky, P.; Brabec, C. J. Adv. Mater. 2007, 19, 3973. (16) An, B.-K.; Kwon, S.-K.; Jung, S.-D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410. (17) Fu, H.-B.; Yao, J.-N. J. Am. Chem. Soc. 2001, 123, 1434. :: (18) Mannerbro, R.; Ranlof, M.; Robinson, N.; Forchheimer, R. Synth. Met. 2008, 158, 556. (19) Mauthner, G.; Landfester, K.; Kock, A.; Bruckl, H.; Kast, M.; Stepper, C.; List, E. J. W. Organic Electronics 2008, 9, 164. (20) Agarwal, M.; Lvov, Y.; Varahramyan, K. Nanotechnology 2006, 17, 5319–5325. (21) Peng, C. Q.; Thio, Y. S.; Gerhardt, R. A. Nanotechnology 2008, 19, 10. (22) Sarrazin, P.; Valecce, L.; Beneventi, D.; Chaussy, D.; Vurth, L.; Stephan, O. Adv. Mater. 2007, 19, 3291. (23) Stejskal, J. J. Polym. Mater. 2001, 18, 225. (24) Kong, F.; Wu, X. L.; Huang, G. S.; Yuan, R. K.; Chu, P. K. Thin Solid Films 2008, 516, 6287. (25) Zhao, L.; Lei, Z. X.; Li, X. R.; Li, S. B.; Xu, J.; Peng, B.; Huang, W. Chem. Phys. Lett. 2006, 420, 480. :: (26) Landfester, K.; Montenegro, U.; Scherf, R.; Guntner, U.; Asawapirom, S.; Patil, D.; Neher, T. Adv. Mater. 2002, 14, 651.
Published on Web 04/06/2009
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hydrophobic compound.27,28 The miniemulsion could be destabilized by either Ostwald ripening, flocculation, or coalescence. Suppression of these processes is required for the formation of a stable miniemulsion. Coalescence and flocculation are controlled by the effective use of a surfactant, while Ostwald ripening can be efficiently suppressed by the addition of a hydrophobic agent (ultrahydrophobe) to the dispersed phase. The ultrahydrophobe promotes the formation of an osmotic pressure inside the droplets, counteracting the Laplace pressure and avoiding diffusion from one droplet to the surrounding aqueous medium.29 A fairly recent investigation has shown that 2,7-poly(9,9dialkylfluorene-co-fluorenone) (denoted PF), can be synthesized by nickel(0) coupling (using the Yamamoto route) of the corresponding dibromo-monomers30 to obtain a stable and efficient yellowish photoluminescence (PL).31 PF has demonstrated a high capacity to be miniemulsified with a cationic surfactant such as tetradecyltrimethylammonium bromide (TTAB).22 Surfactant is known to avoid particle coalescence by adsorbing at the surface of the emulsion droplets. It has been observed that the droplet size strongly depends on the surfactant adsorption density: higher surfactant adsorption gives more stable and smaller emulsion droplets.27 The size of the droplet can be adjusted by the appropriate selection of surfactant type and concentration. However, some aspects regarding the surfactant role during the miniemulsification process, such as surfactant partition between water and solvent and at the water/solvent interface, remain not fully understood. The purpose of this investigation is to understand the behavior of the cationic surfactant (TTAB) during the miniemulsification of a PFpreformed photoluminescent polymer.
Materials and Methods Chemicals. 2,7-dibromofluorene, 2,7-dibromofluorenone, sodium hydride, tetradecyl bromide, bipyridine, and cyclooctadiene were provided by Aldrich. TTAB was supplied by Fluka, and bis(1,5-cyclooactadiene) nickel(0) was obtained from Strem Chemicals. Toluene, methanol, acetone, HCl, CHCl3, NaCl, NaOH, and N,N-dimethylformamide were of analytical grade. Synthesis of the Photoluminescent Polymer. The monomer was synthesized starting from 10 mmol of 2,7-dibromofluorene dissolved in 50 mL of N,N-dimethylformamide. A 20 mmol portion of sodium hydride was added to the mixture at room temperature. After 4 h stirring, 30 mmol of tetradecyl bromide were slowly added, and the mixture was stirred for an additional period of 12 h. Then, the 2,7-dibromo-9,9-di-n-tetradecylfluorene was purified by column chromatography using pentane as eluant. The copolymer was synthesized by mixing 2,7-dibromo9,9-di-n-tetradecylfluorene (1.8 mmol) and 2,7-dibromofluorenone (0.2 mmol), Bipyridine (2 mmol), cyclooctadiene (2.5 mmol) and bis(1,5-cyclooctadiene)nickel(0) (2 mmol). Toluene (16 mL) and N,N-dimethylformamide (6 mL) were added via syringe, and the mixture was stirred at 80 C for 3 days. After cooling, the polymer (PF) was precipitated by pouring the solution in a methanolacetone-concentrated HCl mixture. The solid was collected by filtration and purified by subsequent precipitation in a methanol mixture. (27) Landfester, K.; Bechthold, N.; Tiarks, F.; Antonietti, M. Macromolecules 1999, 32, 5222. (28) Schork, F. J.; Poehlein, G. W.; Wang, S.; Reimers, J.; Rodrigues, J.; Samer, C. Colloids Surf., A 1999, 153, 39. (29) Landfester, K. Annu. Rev. Mater. Res. 2006, 36, 231. (30) Yamamoto, T.; Morita, A.; Miyazaki, Y.; Maruyama, T.; Wakayama, H.; Zhou, Z.; Nakamura, Y.; Kanbara, T.; Sasaki, S.; Kubota, K. Macromolecules. 1992, 25, 1214. (31) Panozzo, S.; Vial, J. C.; Kervella, Y.; Stephan, O. J. Appl. Phys. 2002, 92, 3495.
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Preparation of the Aqueous Polymer Dispersion. The polymer (10 mg) was dissolved in 1 mL CHCl3 (CF) and added to 5 mL of an aqueous TTAB (17 mg) solution. After 15 min of stirring for pre-emulsification, the miniemulsion was prepared by ultrasonicating the mixture for 15 min (150 W). The sample was then stirred in a thermostatic bath (50 C) for 30 min to evaporate the CHCl3 and subsequently filtrated on a 1.2 μm membrane. PL measurements of the native polymer dissolved in CF and of the particle dispersion before and after CF evaporation were performed using the 365 nm line of a filtered mercury arc lamp for the excitation. PL spectra were recorded with a 10 cm monochromator followed by an AsGa photomultiplier tube (1500 V). Zeta potential and the average particle size of photoluminescent colloidal particles were measured by electrophoresis and dynamic light scattering (DLS; Malvern, Zetasizer nanoZS) in a 8.5 10-3 mol L-1 TTAB solution to avoid aggregation. Miniemulsion Surface and Interfacial Tension. Air/water and CF/water dynamic interfacial tensions were measured using an axisymmetric drop shape analysis tensiometer32 (ADSA, ITConcept) and the rising bubble and pendant drop techniques, respectively. CF droplets with volume ranging from 4 to 7 μL were formed in 7 mL TTAB solutions with concentration ranging between 2 10-5 and 1.1 10-3 mol L-1. After 3 h of relaxation, when the interfacial tension was near equilibrium, the elasticity of both air/water and CF/water interface was measured applying a sinusoidal variation of the bubble/drop surface area at frequencies ranging between 0.01 and 0.5 Hz. The surface tension of water-TTAB solutions was also measured using the maximum bubble pressure (MBP) technique with a bubble lifetime triggered to 1 s (Sita T60) in order to obtain a one-to-one correlation between surface tension and TTAB concentration also above the critical micelle concentration (cmc). Surfactant Partition in Miniemulsion. The TTAB partition between water and chloroform during emulsification was investigated using a modified indirect technique based on surface tension measurement.33 TTAB was extracted from 25 mL surfactant aqueous solutions with concentration ranging between 5.3 10-4 and 8.5 10-3 mol L-1, with 25 mL CF. After 15 min of ultrasonication and 2 days of storage at rest to reach partition equilibrium, 15 mL of the organic phase was sampled and evaporated at room temperature. The dry TTAB deposit was then redissolved in 20 mL of deionized water, and surface tension was measured by MBP (1 s). TTAB concentration estimated using the surface tension versus TTAB concentration curve (1 s) was used to calculate the CF TTAB concentration. The amount of residual surfactant in PF particles after CF evaporation was evaluated by elemental analysis of Br (ICP) on pure TTAB, PF, and room-temperature dried dispersion samples, as well as on PF suspensions (after filtration on a 0.45 μm membrane and washing three times with deionized water to remove surfactant molecules). Thermal analysis of the pristine polymer (PF), the roomtemperature dried dispersion, and blends of TTAB-PF were carried out with a TA Instruments DSC Q100 between -80 and 300 C (5 C/min) after quenching in liquid nitrogen. The shape of PF particles dried on a silica substrate were examined by atomic force microscopy (AFM, Topometrix) in air at 23 C. The AFM was operated in tapping mode using a conical-shaped Si tip (20 nm) with resonance frequency of 200 Hz.
Results and Discussion Interfacial Tension of TTAB-Water-CF Systems. The cmc’s graphically determined from air/water and CF/water :: (32) Mobius, D.; Miller, R. Drops and Bubbles in Interfacial Research; Elsevier: Amsterdam, 1998. (33) Ravera, F.; Ferrari, M.; Liggieri, L.; Miller, R.; Passerone, A. Langmuir 1997, 13, 4817.
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Table 1. Experimental Parameters of the LS Isotherm Obtained for TTAB Adsorption at the Air/Water and CF/Water Interfacesa interface air/water CF/water
Γ¥ (mol m-2) -6
2.7 10 2.0 10-6
aL (mol L-1)
Α (A˚2)
-6
6.0 10 9.5 10-6
55 83
Dlteff (m2 s-1) -12
0.3-1.7 10 0.2-1.4 10-12
2 -1 Dst eff (m s ) -12
6 10
γmin (mN m-1)
cmc (mol L-1) -4
9 10 1 10-3
37 1.9
Γ¥/aL (m) 7.5 10-4 2.1 10-4
a Diffusion coefficients were obtained with long adsorption time approximations for TTAB concentrations ranging between 1 10-5 and 1 10-4 mol L-1.
interfacial tension plots exhibit similar values (Table 1), thereby indicating that TTAB depletion from water due to surfactant partition between water and the CF droplet during ADSA measurements was negligible.34 The near-equilibrium interfacial tension data shown in Figure 1 were interpreted using the Langmuir-Szyszkowski (LS) equations: Γ ¼ Γ¥
c aL þ c
c γ ¼ γ0 -RTΓ¥ ln 1 þ aL
ð1Þ
ð2Þ
where Γ is the surface excess at the c equilibrium concentration, Γ¥ is the surface excess at the saturation of the interface, aL is the concentration corresponding to half of the maximum surface excess, and γ0 is the interfacial tension in the absence of surfactants. Adsorption of TTAB at the air/water interface was in good agreement with surface tension isotherms of cationic homologues.35-37 Surfactant adsorption at the CF/ water interface displayed a slightly different behavior. Adsorption parameters obtained with eqs 1 and 2 (Table 1) indicate that TTAB adsorption at the air/water interface begins at lower concentration when compared to that at the CF/water interface. In addition, it is noticed that TTAB molecules are more densely packed at the air/water interface when compared to the CF/water interface. At saturation, the surface area of adsorbed TTAB molecules increased from 55 to 83 A˚2. This increase may be explained by the high solubility of alkyl trimethylammonium cations in CF38 in conjunction with the dispersive interactions and hydrogen bonding of TTAB by CF.39 The dynamic interfacial tension of the CF/water interface shown in Figure 2b had a monotone character, indicating that the diffusive flux from the rich surfactant phase (water) to the interface was larger than the flux to CF.40 Moreover, the small volume ratio between CF and the TTAB solution used during both interfacial tension measurements and miniemulsification (0.14 and 0.2, respectively) allowed neglecting the effect of surfactant depletion on the interfacial crowding.41 The effective diffusion coefficient of TTAB to the air/water and CF/water interface was then obtained from data given in Table 1 and interpolation of dynamic interfacial tension data shown in (34) Yeung, A.; Dabros, T.; Masliyah, J. J. Colloid Interface Sci. 1998, 208, 241. (35) Rosen, M. J. Surfactants and Interfacial Phenomena; Wiley Interscience: New York, 1975. (36) Szymczyk, K.; Zdziennicka, A.; Janczuk, B.; Wojcik, W. Colloids Surf., A 2005, 264, 147. (37) Beneventi, D.; Carre, B.; Gandini, A. Colloids Surf., A 2001, 189, 65. (38) Reck, R. A.; Harwood, H. J.; Ralston, A. W. J. Org. Chem. 1947, 12, 517. (39) Okazaki, M.; Hara, I. J. Phys. Chem. 1976, 80, 64. (40) Liggieri, L.; Ravera, F.; Ferrari, M.; Passerone, A.; Miller, R. J. Colloid Interface Sci. 1997, 186, 46. (41) Ravera, F.; Ferrari, M.; Liggieri, L. Adv. Colloid Interface Sci. 2000, 88, 129.
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Figure 1. Interfacial tension of air/water and CF/water interfaces at increasing concentration of TTAB. Air/water and CF/water are the near-equilibrium surface tensions. Air/water-1s is the dynamic surface tension as determined by MBP measurement with a bubble lifetime of 1 s.
Figure 2 using the Joos relations,42,43 known to describe diffusion-controlled adsorption close to equilibrium: RTΓ2 γ -γe ¼ 2c 3
sffiffiffiffiffiffiffiffiffiffi π Dlteff t
ð3Þ
where Dlteff is the effective diffusion coefficient for a long adsorption time, t is the adsorption time, c is the surfactant bulk concentration, and γe is the equilibrium interfacial tension. Dlteff values (Table 1) were 1 order of magnitude lower than those of ionic and nonionic surfactants with similar molar weight44,45 and of the TTAB diffusivity (Dst eff) obtained by fitting MBP data (Figure 1) with the short time approximation of the Ward and Tordai equation46 and eqs 1 and 2, 2c γðtÞ ¼ γ0 -RTΓ¥ ln 1 þ Γ¥
rffiffiffiffiffiffiffiffiffiffi! Dsteff t π
ð4Þ
namely, 6 10-12 m2 s-1. This mismatch was attributed to the different state of adsorbed TTAB molecules. At the onset of adsorption, a gas-like phase forms at the air/water interface. On this basis, dynamic surface tension can be correlated to surfactant diffusion from the bulk solution to the surface using eq 4. Whereas, after a long adsorption time (when t f¥), a transition from a gas to a twodimensional (2D) condensed phase may occur. We are going on (42) (43) (44) (45) (46)
Van den Bogaert, P.; Joos, P. J. Phys. Chem. 1979, 83, 2244. Li, J.; Fainerman, V. B.; Miller, R. Langmuir 1996, 12, 5138. Eastoe, J.; Dalton, J. S. Adv. Colloid Interface Sci. 2000, 85, 103. Theander, K.; Pugh, R. J. J. Colloid Interface Sci. 2001, 239, 209. Ward, A. F.; Tordai, L. J. Phys. Chem. 1946, 14, 453.
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Figure 3. Influence of TTAB concentration and oscillating frequency on the surface elasticity of the air/water (a) and CF/water interface (b) (full lines represent theoretical values). Figure 2. Air/water (a) and CF/water (b) interfacial tension at different TTAB concentrations plotted as a function of t-0.5.
the assumption that it is reflected by the presence of an inflection √ point in γ(t) curves.47-49 Surface tension versus 1/ t plots obtained for the air/water and CF/water interfaces (Figure 2) displayed an abrupt slope variation at ∼0.1 s-0.5 instead of a expected linear profile,45 indicating the formation of TTAB aggregates at both the air/water and CF/water interfaces. Low Dlteff values were therefore associated with a progressive gasliquid phase transition of adsorbed molecules with an ensuing decrease in the interface tension relaxation kinetics.47,50 Interfacial Elasticity of TTAB-Water-CF Systems. For all tested oscillation frequencies, Figure 3 shows that the elasticity of both air/water and CF/water interfaces progressively increased with TTAB concentration to reach a maximum (47) Fainerman, V. B.; Vollhardt, D.; Melzer, V. J. Chem. Phys. 1997, 107, 243. (48) Vollhardt, D.; Melzer, V.; Fainerman, V. Thin Solid Films 1998, 327-329, 842. (49) Tomassone, M. S.; Couzis, A.; Maldarelli, C. M.; Banavar, J. R.; Koplik, J. J. Chem. Phys. 2001, 115, 8634. (50) Melzer, V.; Vollhardt, D.; Weidemann, G.; Brezesinski, G.; Wagner, R.; :: Mohwald, H. Phys. Rev. E 1998, 57, 901.
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at 6 10-5 mol L-1, which was followed by a drop, at high surfactant concentration, attributed to the dampening effect of TTAB diffusion at/from the interface. Within the range of tested concentrations, the CF/water interface displayed a lower elasticity than the air/water one, which was related to lower Gibbs elasticity: ε0 ¼
dγ d ln Γ
ð5Þ
and TTAB surface activity (Γ¥/aL) shown in Figure 3 and Table 1, respectively. Experimental data were compared with the interfacial elasticity calculated using TTAB adsorption parameters given in Table 1 and the Lucassen-Van Den Tempel (LVDT) equation,51 i.e., a purely diffusional model with no interfacial phase transition, ε0 jεj ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ 2ξ þ 2ξ2
ð6Þ
(51) Lucassen, J.; Van Den Tempel, M. Chem. Eng. Sci. 1972, 27, 1283.
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Figure 4. TTAB partitioning between water and CF below the cmc. Dotted lines represent experimental data interpolation with a polynomial function, kp(c).
where ξ2 is the ratio between the diffusion relaxation frequency, Dst dc 2 ω0 ¼ eff 2 dΓ
ð7Þ
and the surface oscillation frequency, ω. Figure 3 shows that the interfacial elasticity given by the LVDT model is strongly affected by both the TTAB concentration and the surface oscillation frequency, thereby reflecting the influence of diffusional relaxation on the interfacial elasticity. For all tested TTAB concentrations and oscillation frequencies, LVDT elasticity gives lower values than experimental ones. Moreover, at TTAB concentrations below 2-3 10-5 mol L-1, with negligible diffusional relaxation, also Gibbs elasticity was lower than the experimental data. This deviation from a purely diffusive model was interpreted as reflecting the formation of a rigid condensed phase at both the air/water and CF/water interfaces, which was also at the origin of the low effect of the oscillation frequency on interfacial elasticity when compared to the LVDT model prediction. The convergence of experimental data to model prediction at high TTAB concentrations was interpreted as reflecting the dominant effect of diffusional relaxation, which (at tested frequencies) dampened the effect of surface area oscillation on the interfacial tension with an ensuing drop in interfacial elasticity. Present results are in good agreement with previous findings showing the formation of a condensed surfactant phase at both the air/water and solvent/ water interfaces after a long adsorption time.52-54 Below the cmc, TTAB aggregation at the CF/water interface promotes the stability of CF emulsion droplets by increasing the rigidity of the CF/water interface and leading to a positively charged surfactant layer at the CF droplet surface. Furthermore, excessive surfactant concentration is expected to have a negative role on emulsion droplet stabilization. Figure 1 shows that, at the cmc, the CF/water interface reaches saturation, which corresponds to a plateau in the surface electrical charge and in the repulsion (52) Sztukowski, D. M.; Yarranton, H. W. Langmuir 2005, 21, 11651. (53) Santini, E.; Liggieri, L.; Sacca, L.; Clausse, D.; Ravera, F. Colloids Surf., A 2007, 309, 270. (54) Yarranton, H. W.; Sztukowski, D. M.; Urrutia, P. J. Colloid Interface Sci. 2007, 310, 246.
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Figure 5. Variation of the surface tension of the water-TTABCF system during ultrasonication. Arrow indicates the beginning of ultrasonication.
forces providing the electrostatic stabilization of CF droplets. According to the generally accepted role of surface elasticity on bubble-emulsion droplets stabilization,55 the drop in interfacial elasticity (Figure 3) attributed to the dominant effect of diffusional relaxation is therefore expected to promote droplet collision and coalescence. TTAB Partition during Miniemulsion Preparation. Figure 4 shows that, for all tested concentrations, TTAB partition in liquid phases was favorable to CF. Below the cmc, the partition coefficient kp ¼
c cCF
ð8Þ
where c and cCF are the surfactant equilibrium concentrations in water and in CF, respectively, had a constant value. Close to the cmc, when c increased from 6 to 9.8 10-4 mol L-1, kp decreased from 0.4 to 0.1. We think that this drop in kp can be attributed to TTAB separation in CF as solvated monomer. As a result, efficient micelle formation would occur only after CF saturation. Surface tension measurements recorded during the miniemulsification process (Figure 5) show a initial increase in surface tension from 36.8 to 41.6 mN m-1, beginning after mechanical pre-emulsification (first 20 min), followed by a plateau. As expected, it reflects the progressive TTAB adsorption on the surface of freshly formed CF droplets and partitioning between water and CF.27 It is noticed that, even after prolonged ultrasonication, surface tension remained constant indicating that, after 7 min of ultrasonication, TTAB adsorption and partitioning reached equilibrium. Thereafter, the partition coefficient kp, and parameters of LS equations obtained for TTAB adsorption at the CF/water interface were used to calculate TTAB distribution in water and CF and at the CF/water interface. The amount of surfactant adsorbed on chloroform droplets was calculated as Μads ¼ Γ 3 Stot
ð9Þ
(55) Valkovska, D.; Danov, K. D. J. Colloid Interface Sci. 2001, 241, 400.
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Figure 7. Particle radius of the miniemulsified photoluminescent polymer before (CF) and after (PF) CF evaporation.
Figure 6. Influence of the size of CF droplets after emulsification on the state of the aqueous phase and of the water/CF interface. (a) TTAB residual concentration in water and corresponding surface tension. (b) Interfacial tension and TTAB excess at the CF/water interface.
where Stot is the cumulative area of the CF/water interface, Stot ¼ 4πr2 3
VCF 3VCF ¼ 3 4=3πr r
ð10Þ
VCF is the volume of chloroform emulsified in water (1 mL in this study) and r the radius of CF emulsion droplets. By coupling eqs 1, 9, and 10 we obtain Μads ¼ Γ¥ 3
c 3VCF 3 aL þ c r
ð11Þ
The amount of surfactant dissolved in chloroform was obtained from the experimental CF/water partition coefficient kp ΜdisCF ¼
c VCF kp 3
ð12Þ
The sum of TTAB partitioned in the organic phase, in water, MdisH2O = c 3 VH2O, and adsorbed at the CF/water interface gave the total amount of TTAB in the system: Mtot ¼ Μads þ ΜdisCF þ ΜdisH2 O 6750 DOI: 10.1021/la900259x
ð13Þ
Equation 13 was used taking the radius of CF droplets as variable and the experimental VCF/VH2O ratio and initial TTAB concentration in water, namely, 0.2 and 9 10-3 mol L-1. Figure 6a shows that TTAB molecules progressively deplete from water as the size of CF droplets decreases. The inflection point in the TTAB concentration versus droplet radius curve shown in Figure 6a was attributed to the decrease in the interface excess concentration of TTAB at the CF/water interface. Indeed, with droplet radii above 120 nm, TTAB concentration in water is close to/above the cmc: the CF/water interface is saturated (Γ = Γ¥) and TTAB depletion from water is only due to the increase of the CF/water interface surface area (associated with a decrease in the CF droplet radius). Figure 6b shows that, with droplet radii below 120 nm, TTAB interface excess linearly decreases, thereby limiting the effect of CF droplet radius on TTAB depletion. Moreover, the sharp rise in interfacial tension, from 8 to 21 mN m-1 when droplet radius decreases from 120 to 100 nm, points out the onset of thermodynamic instability. A radius of 120 nm can be therefore considered as the lower limit for the formation of stable CF droplets under our tested conditions. The equilibrium surface tension of CF emulsions (Figure 5) and Figure 6a show that, after ultrasonication, the residual TTAB concentration in water is slightly below the cmc, i.e., 8.5 10-4 mol L-1, and that the TTAB partition balance predicts the formation CF droplets with ∼140 nm radius, close to the limit value of 120 nm. The radius predicted for CF droplets was in reasonable agreement with the size measured 10 min after ultrasonication (Figure 7), which displayed a slightly asymmetric monomodal distribution with a peak located at 180 nm. Furthermore, the measured size of the final PF nanoparticles shown in Figure 7 was in good agreement with the calculated ones for PF and CF volume ratios, ∼10-2, used during the emulsification process. The plot of TTAB partition (Figure 8) shows that, with CF droplets radii below 120 nm, nearly all TTAB molecules are adsorbed at the CF/water interface. Above this critical value, the TTAB fraction dissolved in water and CF progressively increases and the partition between the two phases is proportional to their volume ratio and to the partition coefficient kp. Taking into account that segregation of TTAB molecules occurs in the organic phase during the solvent evaporation Langmuir 2009, 25(12), 6745–6752
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Figure 8. Partition of TTAB in water and CF and at the CF/water interface plotted as a function of CF droplet size after emulsification. TTAB weight fraction is expressed with respect to dry solutes in CF (i.e., TTAB and PF). Table 2. Bromide and TTAB Contents sample
TTAB
PFFO
dried dispersion
washed and dried dispersion
experimental Br content (%) theoretical Br content (%) experimental TTAB content (%) theoretical TTAB content (%)
23.4
2.5
17.3
6.5
23.4
ND
16.6
ND
100
0
70
19
100
0
66
19-27a
Figure 9. PF PL spectra. Dashed line: PF solid state; dotted line: PF in chloroform; solid line: PF colloidal dispersion.
process (formation of solid PF nanoparticles), TTAB mass balance shows (Figure 8) that its fraction in the final solid particles rapidly increases for particle radii between 120 and Langmuir 2009, 25(12), 6745–6752
Figure 10. AFM imaging (a) and profile (b) of PF nanoparticles on a silicon wafer.
200 nm to progressively converge to a value close to 50%. Elemental analysis (Table 2) performed on the PF dispersion without and with particles washing exhibits a decrease in the bromide content from 17.3 to 6.5% (TTAB content decreasing from 70 to 19%). It is noticed that TTAB weight fractions obtained by mass balance are in good agreement with experimental data, indicating that adsorbed TTAB can be efficiently removed from the particles by washing. Most of the TTAB dissolved in CF remains segregated in PF particles during solvent evaporation. Emulsion Characteristics. Previous studies have shown that PF has different PL features depending of its state.31 In a dilute chloroform solution, it exhibits a main PL band at around 450 nm (blue) originating from the fluorene segments, whereas, in the solid state, the main band is close to 600 nm and is attributed to a total exciton transfer from the alkyl-fluorene segments to fluorenone moieties, which develop π-stacking conformation. In our case, the aqueous dispersion of nanoparticles displayed a PL spectra similar to the pristine dry polymer (Figure 9), which means that all chloroform has been removed upon the evaporation process (thermal treatment of the emulsion). The comparison of solid PF and miniemulsified PF spectra shows that the presence of surfactant only slightly affects PL properties: a minor shift of the main band from 600 to 570 nm is observed, which may be associated with the deformation of the fluorenone π-stacking conformation. The obtained particles had a narrow monomodal size distribution (Figure 7) centered on a 28 nm radius and a positive DOI: 10.1021/la900259x
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zeta potential with a mean value close to + 80 mV. The thermal analysis of the synthesized copoly(fluorene-fluorenone) revealed the presence of a glass transition temperature (at -20 C) and a melting endothermal peak (at 50 C) corresponding to the liquid-crystalline (LC) mesophase of this kind of organic polymers.56 The comparison of differential scanning calorimetry (DSC) thermograms of the dried, dried and washed emulsion, and the PF in solid state did not reveal any remarkable effect of TTAB on the thermal properties of the miniemulsified PF. The glass transition temperature of PF was not influenced by the surfactant. Low thermal transition temperatures lead to flexible particles that can be rapidly modified by weak thermal treatments. This feature was confirmed by the AFM analysis (Figure 10) of the dried dispersion, which revealed a flattening of the hypothetical native spherical particles to a cylinder with a ∼250 nm diameter and ∼16 nm thickness for the largest ones. The equivalent sphere diameter was 116 nm, and it was consistent with the size of the largest PF particles detected by DLS (Figure 7), namely 100-120 nm.
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Conclusions
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The following general conclusions can be drawn from this investigation: - Interfacial tension and elasticity measurements showed the presence of a gas-liquid phase transition at both the air/water and CF/water interface, which was supposed to promote the stability of CF emulsion droplets during the miniemulsification process by lowering the effect of diffusional relaxation on the elasticity of the CF/water interface. (56) Grell, M.; Bradley, D. D. C.; Inbasekaran, M.; Woo, E. P. Adv. Mater. 1997, 9, 798.
6752 DOI: 10.1021/la900259x
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The influence of TTAB concentration on its partition between water and CF was evaluated by using MBP surface tension measurements. A drop in the partition coefficient was obtained close to the cmc, showing that, above the cmc, TTAB partition in CF is strongly favored. Parameters of interfacial tension isotherms and partition coefficients were used to establish a surfactant mass balance between water, CF, and the CF/water interface. The onset of emulsion instability due to surfactant depletion from the CF/water interface was determined as a function of the emulsion droplets radius, and the minimum droplet radius before the onset of instability was consistent with the size of CF droplets obtained by DLS. Surfactant mass balance and elemental analysis showed that a non-negligible amount of TTAB (19-27%) remains segregated in the final PF particles, inducing a shift in the main PF emission band, namely from 600 to 570 nm. The presence of TTAB in the PF particles did not affect the thermal properties of the polymer, which was characterized by a low glass transition temperature (-20 C) and which assumed a flat-disk shape (instead of spherical) when dried at room temperature.
Acknowledgment. The authors wish to thank Dr. Dubreuil (CERMAV, Grenoble) for his expertise in taking AFM measurements. Dynamic surface tension and interfacial elasticity measurements were performed at Centre Technique du Papier. This work was supported by a MEN grant.
Langmuir 2009, 25(12), 6745–6752
