Hydrogen-Bonding-Induced Complexation of Polydimethylsiloxane

Aug 19, 2010 - Department of Chemistry, The Chinese University of Hong Kong, Shatin, Hong Kong. Langmuir , 2010 ... Yuhao Wang , Svetlana A. Sukhishvi...
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Hydrogen-Bonding-Induced Complexation of Polydimethylsiloxane-graft-poly(ethylene oxide) and Poly(acrylic acid)-block-polyacrylonitrile Micelles in Water Aijuan Hu,† Yushuang Cui,† Xiaoling Wei,‡ Zaijun Lu,*,† and To Ngai*,‡ †

School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China, and ‡ Department of Chemistry, The Chinese University of Hong Kong, Shatin, Hong Kong Received June 22, 2010. Revised Manuscript Received July 28, 2010

Polydimethylsiloxane-graft-poly(ethylene oxide) (PDMS-g-PEO) copolymers form micelles in water with PDMS as the core and PEO as the corona. The introduction of poly(acrylic acid)-block-polyacrylonitrile (PAA-b-PAN) block copolymers in water leads to the formation of micellar complexes due to the hydrogen bonding between carboxyl groups and ether oxygens among the PAA and PEO chains in the corona of the micelles. The effects of pH, molar ratios (r) of PAA/PEO, and the standing time on the directly mixing these two micelles in water have been investigated using laser light scattering (LLS) and transmission electron microscopy (TEM). Our results showed that the complexation between PAA and PEO in the corona was greatly enhanced at a pH below 3.5. For a fixed pH value, the interactions between these two micelles in water were governed by the value of r. At r < ∼0.6, mixing the two micelles in water resulted in a large floccule because the smaller PAA-b-PAN micelles act as physical cross-links, which are absorbed onto one PDMSg-PEO micelle and simultaneously bonded to PEO chains on the other micelles, forming bridges and causing flocculation. At ∼0.6 < r < ∼1.2, the mixing led to stable micellar complexes with a layer of PAA-b-PAN micelles absorbed onto the initial PDMS-g-PEO micelles. At r > ∼1.2, the resultant micellar complexes first remained stable, but they precipitated from solution after a long time standing.

Introduction The self-assembly of amphiphilic block copolymers in a selective solvent has stimulated a great deal of interest in physical chemistry and polymer science over the past 30 years.1 Usually, the micellization of diblock copolymers in selective solvents gives rise to core-shell micelles with the insoluble block as the core and the soluble block as the shell. The size and structure of micelles are affected by the chemical affinity of each block to the solvent, the chemical miscibility between the blocks, the molar mass of each block, and the ratio between the blocks.2 In addition to micelles consisting of a single kind of diblock copolymer, directly mixing homopolymer/block copolymer or two mutually interacting block copolymers in block-selective solvents can lead to micellar complexes with different shapes ranging from spheres2,3 to cylinders,4-6 *Corresponding authors. E-mail: [email protected] (Z.L.); tongai@cuhk. edu.hk (T.N.). (1) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: New York, 1998. (2) F€orster, S.; Antonietti, M. Adv. Mater. 1998, 10(3), 195–217. (3) Tuzar, Z.; Kratochvil, P. In Surface and Colloid Science; Matijevic, E., Ed.; Plenum Press: New York, 1993; Vol. 15, Chapter 1, pp 1-83. (4) Canham, P. A.; Lally, T. P.; Price, C.; Stubbersfield, R. B. J. Chem. Soc. 1980, 76(9), 1857–1867. (5) Zhang, L.; Eisenberg, A. Science 1995, 268(5218), 1728–1731. (6) Ding, J.; Liu, G.; Yang, M. Polymer 1997, 38(21), 5497–5501. (7) Yu, K.; Eisenberg, A. Macromolecules 1998, 31(11), 3509–3518. (8) Raez, J.; Barjovanu, R.; Massey, J. A.; Winnik, M. A.; Manners, I. Angew. Chem. 2000, 39(21), 3862–3865. (9) Frankowski, D. J.; Raez, J.; Manners, I.; Winnik, M. A.; Khan, S. A.; Spontak, R. J. Langmuir 2004, 20(21), 9304–9314. (10) Checot, F.; Lecommandoux, S.; Gnanou, Y.; Klok, H. A. Angew. Chem., Int. Ed. 2002, 41(8), 1339–1343. (11) Jenekhe, S. A.; Chen, X. L. Science 1999, 283(5400), 372–375. (12) Ding, J.; Liu, G. Macromolecules 1997, 30(3), 655–657. (13) Opsteen, J. A.; Cornelissen, J. J. L. M.; Hest, J. C. M. v. Pure Appl. Chem. 2004, 76(7-8), 1309–1319. (14) Pochan, D. J.; Chen, Z.; Cui, H.; Hales, K.; Qi, K.; Wooley, K. L. Science 2004, 306(5693), 94–97.

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tubes,7-9 vesicles,5,6,10-13 and donuts12,14,15 and thus has recently been a subject of interest. The micellar complexes formation can be mediated by different interactions such as interpolymer hydrogen bonding,16,17 electrostatic interactions,18 and metal-ligand coordinative bonds.19 Among these, the hydrogen-bonding-induced micellar complexes has attracted the most attention and extensively studied.16,20-30 For example, Jiang and co-workers20,21 prepared spherical micellar aggregates by simply mixing two polymers of carboxyl-terminated polyimide and poly(4-vinylpyridine) in their common solvent of chloroform due to hydrogen bonding between poly(4-vinylpyridine) and the carboxyl group of the polyimide. (15) Zhu, J.; Liao, Y.; Jiang, W. Langmuir 2004, 20(9), 3809–3812. (16) Hu, J.; Liu, G. Macromolecules 2005, 38(19), 8058–8065. (17) Li, G.; Shi, L.; Ma, R.; An, Y.; Huang, N. Angew. Chem., Int. Ed. 2006, 45(30), 4959–4962. (18) Fukushima, S.; Miyata, K.; Nishiyama, N.; Kanayama, N.; Yamasaki, Y.; Kataoka, K. J. Am. Chem. Soc. 2005, 127(9), 2810–2811. (19) Gohy, J. F.; Hofmeier, H.; Alexeev, A.; Schubert, U. S. Macromol. Chem. Phys. 2003, 204(12), 1524–1530. (20) Duan, H.; Chen, D.; Jiang, M.; Gan, W.; Li, S.; Wang, M.; Gong, J. J. Am. Chem. Soc. 2001, 123(48), 12097–12098. (21) Chen, D.; Jiang, M. Acc. Chem. Res. 2005, 38(6), 494–502. (22) Zhang, W.; Shi, L.; Gao, L.; An, Y.; Li, G.; Wu, K.; Liu, Z. Macromolecules 2005, 38(3), 899–903. (23) Lefevre, N.; Fustin, C. A.; Varshney, S. K.; Gohy, J. F. Polymer 2007, 48(8), 2306–2311. (24) Luo, S.; Liu, S.; Xu, J.; Liu, H.; Zhu, Z.; Jiang, M.; Wu, C. Macromolecules 2006, 39(13), 4517–4525. (25) Gao, W. P.; Bai, Y.; Chen, E. Q.; Li, Z. C.; Han, B. Y.; Yang, W. T.; Zhou, Q. F. Macromolecules 2006, 39(14), 4894–4898. (26) Bai, Y.; Gao, W. P.; Yan, J. J.; Ma, Y. G.; Liang, D. H.; Li, Z. C.; Han, B. Y.; Yang, W. T.; Chen, E. Q. Polymer 2008, 49(8), 2099–2106. (27) Zhang, W.; Shi, L.; Miao, Z. J.; Wu, K.; An, Y. Macromol. Chem. Phys. 2005, 206(23), 2354–2361. (28) Gohy, J. F.; Khousakoun, E.; Willet, N.; Varshney, S. K.; Jer^ome, R. Macromol. Rapid Commun. 2004, 25(17), 1536–1539. (29) Yan, X.; Liu, G.; Hu, J.; Willson, C. G. Macromolecules 2006, 39(5), 1906– 1912. (30) Xiong, D.; Shi, L.; Jiang, X.; An, Y.; Chen, X.; L€u, J. Macromol. Rapid Commun. 2007, 28(2), 194–199.

Published on Web 08/19/2010

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On the other hand, Yoshida et al. showed that complexation occurred when mixing 1,4-butanediamine and poly(vinylphenol)b-polystyrene (PVPh-b-PS) in 1,4-dioxane. This is because of the hydrogen-bonding cross-linking between the PVPh block and 1,4butanediamine.31 Recently, Kuo and co-workers have exploited the use of different solvents to modulate the morphology of the resulting micellar complexes due to the hydrogen bonding. Vesicles or patched spherical structures were obtained respectively in tetrahydrofuran (THF) and in dimethylformamide (DMF) when mixing poly(methyl methacrylate)-b-poly(4-vinylpyridine) (PMMAb-P4VP) with poly(styrene)-b-poly(vinylphenol) (PS-b-PVPh) at the stoichiometric molar ratio.32 Other morphologies were also obtained from mixtures between two block copolymers. For instance, Lu et al. reported the preparation of wormlike micellar complexes by the self-assembly of polye(ethylene glycol)-b-poly(acrylic acid) (PEG-b-PAA) and poly(N-isopropylacrylamide)-bpoly(4-vinylpyridine) (PNIPAM-b-P4VP) in ethanol, in which the length of the wormlike complexes could be tuned by varying the weight ratio of the two block copolymers.33 It should be noted that the above listed examples mostly dealt with the hydrogenbonding interactions that took place inside the core. Formation of hydrogen bonding between two copolymers can also take place in the corona, leading to another type of three-layered structures called core-shell-corona micelles or “onion-type” micelles. In this approach, conventional micelles are formed in the first step from self-assembly of an AB block copolymer, and an additional layer was added in the second step by using a CD block copolymer. For example, Zhang et al.27,34 showed that the micellization of the block copolymer poly(styrene-b-acrylic acid) (PS-b-PAA) in ethanol gave rise to spherical core-shell micelles with the PS blocks as the core and the PAA blocks as the shell. Upon adding poly(ethylene glycol)-b-poly(4-vinylpyridine) (PEG-b-P4VP) into the core-shell micellar solution, the P4VP block of the PEG-bP4VP chain penetrates into the PAA shell and was absorbed into the core-shell micelle to yield core-shell-corona micelle with PS block as the core, bonded PAA/P4VP blocks as the shell, and PEG block as the corona. A similar approach that led to coreshell-corona micelles has been reported by Shi et al., where PS-bPAA micelles in ethanol were decorated with PNIPAM-b-P4VP block copolymer.35 However, despite different combinations of block copolymers have been explored and large diversity of the micellar complexes that have been produced, little is known about their exact formation mechanism, and the influence of the interplay between competing parameters such as insolubilization of the complexes, conformation constraints, solvophobic-solvophilic balance, and so on. Moreover, directly mixing two mutually interacting micelles spanning distinct sizes in a common solvent has been scarcely investigated. In the present study, we have first synthesized polydimethylsiloxane-graft-poly(ethylene oxide) (PDMS-gPEO) graft copolymer and poly(acrylic acid)-block-polyacrylonitrile (PAA-b-PAN) block copolymer based on the hydrosilylation reaction and atom transfer radical polymerization (ATRP), respectively. The micellization of comb-type PDMS-g-PEO graft copolymer in water gives rise to large and loose micelle-like core(31) Yoshida, E.; Kunugi, S. Macromolecules 2002, 35(17), 6665–6669. (32) Kuo, S. W.; Tung, P. H.; Lai, C. L.; Jeong, K. U.; Chang, F. C. Macromol. Rapid Commun. 2008, 29(3), 229–233. (33) Xiong, D. a.; Shi, L.; Jiang, X.; An, Y.; Chen, X.; L€u, J. Macromol. Rapid Commun. 2007, 28(2), 194–199. (34) Zhang, W.; Shi, L.; Gao, L.; An, Y.; Wu, K. Macromol. Rapid Commun. 2005, 26(16), 1341–1345. (35) Xiong, D. a.; He, Z.; An, Y.; Li, Z.; Wang, H.; Chen, X.; Shi, L. Polymer 2008, 49(10), 2548–2552.

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Article Scheme 1. Synthesis of PAA20-b-PAN75 Block Copolymer by ATRP

shell structure with the PDMS as the core and the PEO as the corona, while PAA-b-PAN block copolymer forms small micelles with the PAN as the core and the PAA as the corona. It is expected that directly mixing the large PDMS-g-PEO micelles with small PAA-b-PAN micelles in water should be able to form micellar complexes via the hydrogen-bonding interactions between PEO and PAA in the corona. A combination of laser light scattering (LLS) and transmission electron microscopy (TEM) was used to characterize the final micellar complexes. To achieve better insight into the hydrogen-bonding-induced complexation of these two micelles, the effects of pH, PEO/PAA molar ratios (r), and standing time have been investigated.

Experimental Section Materials. Acrylonitrile (AN) and tert-butyl acrylate (tBA) were distilled under vacuum. Copper(I) bromide (CuBr) and copper(I) chloride (CuCl) were purified by stirring in acetic acid, washing with ethanol and acetone, and then drying. Ethylene carbonate (EC) was recrystallized in anhydrous ethanol. Toluene was purified by distillation over phosphorus pentoxide (P2O5). Methyl 2-bromopropionate (MBP), allyl poly(ethylene oxide) (Mn ∼ 600 g/mol), tetrahydrofuran (THF), octamethylcyclotetrasiloxane (D4), tetramethylcyclotetrasiloxane (DH 4 ), hexamethyldisiloxane (MM), and other regents were used as received. Synthesis of PAA20-b-PAN75 Block Copolymer. The poly(acrylic acid)-block-polyacrylonitrile block copolymer, PAA20-bPAN75, was synthesized based on the atom transfer radical polymerization (ATRP) method.36 The synthetic route of PAA20b-PAN75 is outlined in Scheme 1. In the first step, to a 50 mL ampule flask, 1.6 mL of tBA, 0.23 mL of PMDETA, 0.24 mL of MBP, and 0.1521 g of CuBr were added. The reaction mixture was subjected to three freeze-pump-thaw (FPT) cycles and followed by backfilling with argon. The reaction was carried on at 60 C for 4 h. After completed the reaction, the mixture was diluted by THF and passed through an alumina column to remove the catalyst. 1H NMR (D3CCl): δ 1.12 (-CH(CH3)COOCH3), 1.39 ((CH3)3C-), 1.83 (CH2 of PtBA backbone), 2.23 (CH of PtBA backbone), 3.62 ((-CH(CH3)COOCH3). Mn = 2600 g/mol (1H NMR), Mw/Mn = 1.13 (GPC). In the second step, to a 50 mL ampule flask, 1.60 g of PtBA macroinitiator, 4 mL of AN, 0.0326 g of CuCl, 0.1014 g of Bpy, and 6 mL of EC were charged. After three FPT cycles, the flask was placed in a water bath at 70 C for 24 h to get the desired PtBA-b-PAN block copolymer. 1H NMR (DMSO-d6): δ 1.39 (shoulder (CH3)3C- of PtBA block), 2.06 (CH2 and CH of PtBA block and CH2 of PAN block), 3.15 (CH of PAN block). Mn = 6600 g/mol (1H NMR), Mw/Mn = 1.16 (GPC). The hydrolysis of 1.60 g of PtBA-b-PAN by 4.5 mL of trifluoroacetic acid (TFA) in 16.0 g of EC at 65 C, PAA20-b-PAN75 was finally obtained with a yield of 98.6%. 1H NMR (DMSO-d6): δ2.06 (CH2 and CH of PtBA block and CH2 of PAN block), 3.15 (CH of PAN block), 1.39 ((CH3)3C-), 12.24 (-COOH). (36) Tang, C.; Qi, K.; Wooley, K. L.; Matyjaszewski, K.; Kowalewski, T. Angew. Chem. 2004, 116(21), 2843–2847.

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Article Scheme 2. Synthesis of PDMS240-g-(PEO13)15 Graft Copolymer by Hydrosilylation Reaction

Synthesis of Graft Copolymer PDMS240-g-(PEO13)15. PDMS240-g-(PEO13)15 was prepared according to the well-documented hydrosilylation reaction.37,38 The synthetic route is summarized in Scheme 2. To a 150 mL three-neck flask with a reflux condenser and stirrer bar, 50.52 g of D4, 2.69 g of DH 4 , 0.46 g of MM, and 0.5 mL of concentrated sulfuric acid were added. The mixture was stirred at room temperature for 20 h. After that, 60 mL of toluene was added and subsequently followed by washing the toluene phase with water until the mixture became neutral. Evaporation of the solvent resulted in a hydrogen-containing polydimethylsiloxane (H-PDMS). 1H NMR (CCl4 benzene as the internal standard): δ 0.07 (-Si-CH3), 4.64 (-Si-H), 7.26 (H of benzene). Mn = 18 800 g/mol (1H NMR), Mw/Mn = 1.45 (GPC). wH% = (area of 4.64 ppm/area of 7.26 ppm)  (mb/ms)  7.743% = 0.0796% (where wH is the mass fraction of active hydrogen in H-PDMS, mb and ms are the mass of standard benzene and H-PDMS separately, 7.743% is the mass fraction of hydrogen in benzene). The grafted copolymer PDMS240-g-(PEO13)15 was synthesized by hydrosilylation of allyl poly(ethylene oxide) with H-PDMS. Typically, to a 100 mL three-neck flask equipped with a reflux condenser, 4.03 g of allyl polyether, 0.4 mL of isopropanol solution of chloroplatinic acid (1.67 wt %), and 8 mL of toluene were added. The solution was stirred and heated to 100 C under a nitrogen atmosphere. After addition of 8.01 g of H-PDMS within 0.5 h to the solution, the mixture was reacted at 120 C for 7 h. The crude product was fractionally distilled under reduced pressure to get the desired graft copolymer, PDMS240-g-(PEO13)15. 1H NMR (DCCl3): δ 0.07 (-Si-CH3), 0.47 (-Si-CH2CH2CH2O-), 1.61 (-Si-CH2CH2CH2O-), 3.65 (CH2CH2O- of PEO graft). Mn = 27 800 g/mol (1H NMR), Mn/Mw = 1.67 (GPC). The graft ratio is quantitative. Preparation of Micelles in Water. PAA20-b-PAN75 micelles were prepared by introducing 40.0 mL of water into the same volume of PAA20-b-PAN75 solution in DMF (1.0 mg/mL) under stirring at a constant flow rate of 15.0 mL/min by a SP100i syringe pump. DMF was then removed by dialyzing the mixture against water for 3 days. Similarly, PDMS240-g-(PEO13)15 micelles were prepared by introducing 40.0 mL of water into 10.0 mL of PDMS240-g-(PEO13)15 (10.0 mg) solution in THF using a syringe pump at the same flow rate. After that, THF solvent was removed by evaporation under reduced pressure. These two different kinds of micelles were further mixed together at the stoichiometric molar ratios in order to form the micellar aggrgates via the hydrogen-bonding complexation between the PAA and PEO chains in the corona. In this experiment, the concentration of the PDMS240-g-(PEO13)15 micelles was fixed as 1  10-5 g/mL, while the concentration of PAA20-b-PAN75 micelles was varied from 1  10-6 to 3  10-5 g/mL.

Physical Measurements. Gel Permeation Chromatography (GPC). GPC measurements were carried out on a Waters 515 liquid chromatograph equipped with two Waters Styragel columns and 2414 differential refractive index detector by using DMF as the eluant. The flow rate was 0.6 mL/min, and the temperature was 40 C. Monodispersed polystyrene (PS) samples (37) Feng, S.; Cui, M. React. Funct. Polym. 2000, 45(2), 79–83. (38) Kanner, B.; Reid, W. G.; Petersen, I. H. Ind. Eng. Chem. Res. Dev. 2002, 6(2), 88–92.

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Hu et al. were used as standards. THF was used as the eluant with a flow rate of 1.0 mL/min for the sample of PDMS-g-PEO. Nuclear Magnetic Resonance Spectroscopy (1H NMR). 1H NMR spectra were recorded on a Bruker AV400 spectrometer using tetramethylsilane (TMS) as the internal standard. For H-PDMS, 1H NMR was measured by taking benzene as classic inner standard matter to calculate the mass fraction of active hydrogen in tetrachloromethane (CCl4) (see Supporting Information for the figures). Infrared Spectra (IR). IR spectra were recorded at room temperature on a Nicolet 20SX FTIR spectrometer using polymer films that had been cast onto KBr pellets from water solution and then vacuum-dried. All IR spectra were obtained within the spectral range 4000-400 cm-1. Fluorimetric Measurements. Fluorimetric measurements were carried on a PerkinElmer LS 55 fluorescence spectrometer with the excitation wavelength at 335 nm. Pyrene was used as the fluorescence probe with the concentration of 2  10-7 g/mL. The critical micelle concentration (cmc) of PAA20-b-PAN75 and PDMS240-g-(PEO13)15 copolymers were found to be 3.38  10-7 and 1.06  10-8 g/mL, respectively. Laser Light Scattering (LLS). LLS measurements were conducted on an ALV/DLS/SLS-5022F spectrometer with a multi-τ digital time correlation (ALV5000) and a cylindrical 22 mW UNIPHASE He-Ne laser (λ0 = 632 nm) as the light source. In static LLS,39 the weight-average molar mass (Mw), the root-meansquare radius of gyration ÆRg2æz1/2 (or written as ÆRgæ), and the second virial coefficient A2 were obtained from the angular dependence of the absolute excess time-average scattering intensity or Rayleigh ratio Rvv(q). In dynamic LLS,40 the intensityintensity time correlation function G(2)(t,q) was measured to determine the line-width distribution G(Γ). For a pure diffusive relaxation, Γ is related to the translational diffusion coefficient (D) of the scattering object in dilute solution or dispersion by Γ = Dq2 at C f 0 and q f 0. On the other hand, G(Γ) can be converted into hydrodynamic radius (Rh) from the Stokes-Einstein equation: Rh = kBT/(6πηD), where kB, T, and η are the Boltzmann constant, the absolute temperature, and the solvent viscosity, respectively. Hydrodynamic radius distribution f (Rh) was calculated from the Laplace inversion of a corresponding measured G(2)(t,q) using the CONTIN program. All dynamic LLS experiments were conducted at a scattering angle (θ) of 30. All the micellar solutions were kept at 25 C during the measurements and standing. The samples were dust-free by passing through either 0.8 μm Millipore filter or 0.45 μm Millipore filter before the measurements. The refractive index increment (dn/dC) values of the micelles were measured by using a precise differential refractometer.41 Transmission Electron Microscopy (TEM). The morphologies of the micellar complexes were observed on a JEM-100X II transmission electron microscope. To prepare TEM samples, a drop of the dilute aqueous solution was deposited onto a carboncoated copper mesh grid. The samples were then directly dried in air by the evaporation or in some case by freeze-drying. Some samples were also stained with RuO4 for 40 min to enhance the electron density contrast under the imaging.

Results and Discussion Micellization of Block Copolymers. Figures 1 and 2 show the typical hydrodynamic radius distributions f (Rh) and TEM images of PAA20-b-PAN75 and PDMS240-g-(PEO13)15 micelles in water, respectively. Both copolymers form micelles in aqueous solutions and show only one relatively narrow distribution. In Figure 1, the peak located at 20 nm is due to the micellization of (39) Chu, B. Laser Light Scattering, 2nd ed.; Academic Press: New York, 1991. (40) Berne, B. J.; Pecora, R. Dynamic Light Scattering; Plenum Press: New York, 1976. (41) Wu, C.; Xia, K. Q. Rev. Sci. Instrum. 1993, 65(3), 587–590.

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Article Table 1. LLS and TEM Characterizations of PAA20-b-PAN75 and PDMS240-g-(PEO13)15 Micelles in Watera

micelle sample

dn/dC/(L/mg)

ÆRhæ/nm

ÆRgæ/nm

ÆRgæ/ÆRhæ

0.138 20 ( 1 16.2 0.79 PAA20-b-PAN75 0.271 140 ( 2 118.5 0.85 PDMS240-g-(PEO13)15 a ÆRæ is the average radius of micelles measured by TEM. ÆFæ is defined as Mw,agg/(4πÆRhæ3NA/3).

Figure 1. Hydrodynamic radius distributions f (Rh) of PAA20-bPAN75 and PDMS240-g-(PEO13)15 micelles in water at the concentration of 1  10-5 g/mL. The scattering angle is at 30.

Figure 2. TEM images of PAA 20 -b-PAN 75 and PDMS 240 -g(PEO 13 )15 micelles in water: (a) PAA 20 -b-PAN 75 micelles; (b) PDMS240-g-(PEO13)15 micelles, where the inset is the image of the graft copolymer after the freeze-drying.

the block copolymer PAA20-b-PAN75 in water with PAN block as the core and PAA block as the shell. However, the self-assembly of PDMS240-g-(PEO13)15 in water leads to a much larger micellelike core-shell structure (ÆRhæ ∼ 140 nm) with PDMS as the core and grafted PEO chains as the shell. Other parameters characterized using LLS and TEM are summarized in Table 1. It is well-known that the ratio ÆRgæ/ÆRhæ can reflect the structure of a polymer chain or a particle. For uniform nondraining sphere, hyperbranched cluster, and random coil, ÆRgæ/ÆRhæ are ∼0.774, 1.0-1.2, and 1.5-1.8, respectively.42,43 The ÆRgæ/ÆRhæ ratios of PAA20-b-PAN75 and PDMS240-g-(PEO13)15 micelles in water are 0.79 and 0.85, respectively, thus suggesting that the formed micelles are spherical, which have been further confirmed by the TEM images as shown in Figure 2. For PAA20-b-PAN75 micelles, the radius obtained from TEM is 18 ( 2 nm, which is slightly smaller than those measured by dynamic LLS (20 ( 1 nm). This is reasonable because dynamic LLS measured the size of the micelles in a solution state while TEM determined the size in a dry state. However, for PDMS240-g-(PEO13)15 micelles, the size determined from TEM (ÆRæ ∼ 70 ( 5 nm) is significantly smaller than that measured by dynamic LLS (ÆRhæ ∼ 140 ( 2 nm). The likely reason is that the formed PDMS240-g-(PEO13)15 micelles have a very (42) Brown, W. Light Scattering: Principles and Development; Clarendon Press: Oxford, 1996. (43) Douglas, J. F.; Roovers, J.; Freed, K. F. Macromolecules 1990, 23(18), 4168–4180.

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Mw/(g/mol)

ÆFæ/(g/cm3)

ÆRæ/nm

8.67  106 1.01  108

3.98  10-1 1.46  10-2

18 ( 2 70 ( 5

Figure 3. pH dependence of ÆRhæ of PDMS240-g-(PEO13)15/ PAA20-b-PAN75 mixed micelles in water with the standing time of 1 day. The molar ratio (r) of PAA/PEO is fixed as ∼0.61, and the scattering angle is 30.

loose structure in water. This is possible because short PEO chains are grafted on the periphery of the backbone which may not effectively shield the solvophobic PDMS completely to form regular core-shell micelles. Conversely, some of PEO grafts would be embedded inside the core when the PDMS backbones undergo inter- and intrachain aggregation.44 The PEO grafts in the core can be swollen by water, leading to the formation of a less compacted micellar complex. It is expected that during sample preparation for TEM measurements the size of PDMS240g-(PEO13)15 micelles will significantly decrease owing to the water evaporation. However, it is interesting to note that directly freezedrying PDMS240-g-(PEO13)15 can keep the integrity of the resulting structure with a radius around ∼130 nm, as shown in the inset of Figure 2b. The average chain density (ÆFæ) of PDMS240-g-(PEO13)15 micelles in water is 1.46  10-2 g/cm3. This provides the further evidence that PDMS240-g-(PEO13)15 micelles have a loose structure. Influence of pH Value. The hydrogen-bonding complexation between PEO and PAA chains in aqueous solutions has been extensively studied. It was suggested that such a hydrogen bond is formed between ether and carboxylic acid in aqueous solutions. However, it is worth pointing out that PAA is a weak polyelectrolyte (pKa ∼ 5.6);45 the ionization of carboxylic group in PAA backbone and the interpolymer hydrogen-bonding formation between PEO and PAA are thus pH-dependent.46 In such a way, we first investigated the influencing of pH value on the PDMS240-g-(PEO13)15 micelles when mixing with the PAA20-bPAN75 micelles at a fixed molar ratio (r = 0.61) of PAA/PEO. Figure 3 shows that in the pH range of 3.8-5.6 there is no significant different in the ÆRhæ of the mixed micelles from pure PDMS240-g-(PEO13)15 micelles in water; i.e., the hydrogen-bonding complexation between the PEO and PAA in the corona is inconspicuous, or at least the interaction is not strong enough to form a stable micellar complex. Presumably, within such pH ranges, carboxylic groups in PAA backbone are partially ionized, (44) Huo, H.; Ngai, T.; Goh, S. H. Langmuir 2007, 23(24), 12067–12070. (45) Niwa, M.; Hayashi, T.; Higashi, N. Langmuir 1990, 6(1), 263–268. (46) Yang, S.; Zhang, Y.; Zhang, X.; Xu, J. Soft Matter 2007, 3, 463–469.

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Figure 6. TEM images of PDMS240-g-(PEO13)15/PAA20-b-PAN75 Figure 4. FTIR spectra of (a) PAA20-b-PAN75 block copolymer and (b) PDMS240-g-(PEO13)15/PAA20-b-PAN75 mixed micelles in water with r = 1.52 and pH = 3.5.

Figure 5. Hydrodynamic radius distributions f (Rh) of PDMS240g-(PEO13)15/PAA20-b-PAN75 mixed micelles with different molar ratios (r) of PAA/PEO after mixing for 2 days at 25 C, where the solution pH was kept at 3.5. The scattering angle is 30.

leading to weaker cooperative hydrogen bonding between PEO and PAA chains in the micelles. Besides, it needs to bear in mind that in LLS the scattering intensity is proportional to the square of the apparent molar mass so that LLS is very sensitive to largesized objects (aggregates) particularly at smaller scattering angle (θ = 30). The almost constant ÆRhæ located at 140 nm actually represents only the PDMS240-g-(PEO13)15 micelles in water even thought the smaller PAA20-b-PAN75 micelles are coexisting in the mixed solution. When the pH is decreased below a value of 3.5, ÆRhæ increases and the size distribution becomes broader. We conjecture that the micellar complexes are formed due to the hydrogen-bonding interactions between PEO and PAA chains among the two micelles. FTIR spectra shown in Figure 4 further confirm the formation of hydrogen bonding between PAA and PEO because in the PDMS240-g-(PEO13)15/PAA20-b-PAN75 mixed solution the band centered at 3542 cm-1 corresponding to the hydroxyl group shifts to 3123 cm-1. Meanwhile, the band corresponding to carboxyl group also shows a red shift from 1723 to 1736 cm-1. Further acidification triggers fast macroscopic phase separation. This is because the carboxylic groups in PAA are fully protonated so that stronger cooperative hydrogen bonding between PEO/PAA are expected, leading to the phase separation. Therefore, in the following studies, the pH value in the PDMS240-g-(PEO13)15 and PAN75-b-PAA20 mixed micelles was fixed at 3.5. 14506 DOI: 10.1021/la102539v

complex aggregates with different unit molar ratios (r) of PAA/ PEO after mixing for 21 days: (a) r = 0.06 (unstained), the inset is the partial “zoomed-in” image; (b) r = 0.91 (freeze-dried and stained with RhO4).

Effects of Molar Ratios (r) of PAA/PEO. Figure 5 shows the effects of molar ratios (r) of PAA/PEO on hydrodynamic radius distribution f (Rh) of PDMS240-g-(PEO13)15/PAA20-bPAN75 mixed micelles in water after mixing for 2 days. The initial PDMS240-g-(PEO13)15 micelles in water are relative monodispere and the ÆRhæ is ∼140 nm. Obviously, the size of the micelles significantly increases, and the size distribution becomes broader after adding a small amount of PAA20-b-PAN75 micelles (r = 0.06). The corresponding TEM image (Figure 6a) clearly reveals that mixing these two micelles in water at such a low r value leads to large agglomerates. It suggests that the smaller PAA-b-PAN micelles act as physical cross-links, which not only are absorbed onto the initial PDMS240-g-(PEO13)15 micelles through the hydrogen bonding between the PAA and PEO chains in the corona but also are contiguous with PEO chains on other micelles, serving to bind them together and causing the flocculation. It is interesting to note that the broad peak disappears with increasing the concentration of PAA20-b-PAN75 micelles. Instead, there exists a bimodal peak. At r = 0.91 and 1.52, the peaks located at 20 nm are attributed to free PAA20-b-PAN75 micelles, while the peaks located at 160 nm can be related to PDMS240-g-(PEO13)15/ PAA20-b-PAN75 micellar complexes. It suggests that when a sufficient amount of PAA20-b-PAN75 micelles is introduced, they can be absorbed surrounding the PDMS240-g-(PEO13)15 micelle shells through the cooperative hydrogen bonding complexation between PAA and PEO chains, resulting in large micellar complexes. The TEM image in Figure 6b shows that after freezedrying the PDMS240-g-(PEO13)15/PAA20-b-PAN75 complex aggregates at r = 0.91 has the radius around 150 nm, which is very close to those measured by DLS. Therefore, it should be safe to assume that the final micellar complexes consist with a layer of PAA-b-PAN micelles absorbed onto the initial PDMS-g-PEO micelles. Meanwhile, there also exist some smaller particles, corresponding to the free PAA20-b-PAN75 micelles in water. Kinetic Process of Complexation. Figure 7 shows that at low PAA20-b-PAN75 micelle concentration (r = 0.06) ÆRhæ of the mixed micelles increases gradually from 138 to 618 nm over time due to the formation of large agglomerates which can be referred to our above discussions. The corresponding IT/I0 (Figure 8) increases first but decreases after standing for 4 days. The decrease of IT/I0 is assigned to the initial formation of nonequilibrium aggregates and subsequent structural rearrangements into the final loosely packed agglomerates. At intermediate PAA20-b-PAN75 micelle concentration, r = 0.91, ÆRhæ increases from 137 to 165 nm in 1 day and exists without any visible changes. Note that the corresponding IT/I0 exhibits similar behavior, suggesting that the final micellar complexes consisting with a layer of PAA20-b-PAN75 Langmuir 2010, 26(18), 14502–14508

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Figure 7. Standing time dependence of ÆRhæ of PDMS240-g(PEO13)15/PAA20-b-PAN75 mixed micelles with different molar ratios (r) of PAA/PEO in water at the solution pH of 3.5. The scattering angle is 30. The error bars have been set 3 standard deviations for the measurements, where error bars are not visible indicates that the error bars are less than the size of symbols. Figure 9. Change of ÆRhæ of PDMS240-g-(PEO13)15/PAA20-bPAN75 micellar complexes in water relative to different molar ratios (r) of PAA/PEO along with the standing time of 15 min (0), 2 days (4), 9 days (O), 16 days (]), and 19 days (3). The insets are the schemes of complex aggregates.

Figure 8. Standing time dependence of relative scattering intensity (IT/I0) of PDMS240-g-(PEO13)15/PAA20-b-PAN75 mixed micelles with different molar ratios (r) of PAA/PEO in water at the solution pH of 3.5. The scattering angle is 30.

micelles surrounded the PDMS240-g-(PEO13)15 micelles are stable and reach the equilibrium within 1 day. At high PAA20-b-PAN75 micelle concentration, r = 1.52, the changes of ÆRhæ and corresponding IT/I0 are almost the same with r = 0.91 at the initial 10 days. Surprisingly, the size of the formed micellar complexes increases sharply hereafter and eventually leads to macroscopic phase separation. The decrease of corresponding IT/I0 observed at the final stage is in a good agreement with precipitation phenomena. Some studies have indicated that the solubility of PAA/PEO micellar complexes in aqueous solution is relatively poor after the hydrogen-bonding formation, which could be the reason leading to phase separation after a long-time standing.47-51 However, the kinetic process related to the micellar complexes coexisting with excess PAA20-b-PAN75 micelles in water so far is unclear. Further studies are needed as the understanding will enable us to control the final structure and properties of the micellar complexes. Figure 9 summarizes the relationship among the hydrodynamic radius ÆRhæ, molar ratios (r) PAA/PEO, and standing time of the (47) Smith, K. L.; Winslow, A. E.; Petersen, D. E. Ind. Eng. Chem. 1959, 51(11), 1361–1364. (48) Jr., F. E. B.; Lundberg, R. D.; Callard, R. W. J. Polym. Sci., Part A 1964, 2(2), 845–851. (49) Ikawa, T.; Abe, K.; Honda, K.; Tsuchida, E. J. Polym. Sci., Polym. Chem. Ed. 1975, 13(7), 1505–1514. (50) Khutoryanskiy, V. V.; Dubolazov, A. V.; Nurkeeva, Z. S.; Mun, G. A. Langmuir 2004, 20(9), 3785–3790. (51) Staikos, V. V. K. G. Hydrogen-Bonded Interpolymer Complexes: Formation, Structure and Applications; World Scientific Publishing Co.: Singapore, 2009; p 250.

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formed micellar complexes in water via hydrogen bonding. At the first 15 min mixing, there is no significant change in the ÆRhæ of the mixed micelles, indicating that the hydrogen-bonding complexation between the PEO and PAA in the corona is inconspicuous. After that, the complexation process can be divided into three parts. In region I (r < 0.61), ÆRhæ decreases when r increases, indicating that sufficient amount of PAA20-b-PAN75 micelles are needed to form the micellar complexes through the cooperative hydrogen-bonding complexation between PAA and PEO chains. However, at low value of r, the initial formed micellar complexes are in a nonequilibrium state, and therefore the size of the micellar complexes increases with the standing time. In region II (r = 0.61-1.21), ÆRhæ increases to 165 nm within 1 day and exists without any visible changes. The almost constant ÆRhæ with different r ratios and standing time suggests that the hydrogen-bonding complexation between PDMS240-g-(PEO13)15 and PAA20-bPAN75 micelles in water likely proceeds stoichiometrically. In region III (r > 1.21), the formed micellar complexes remain stable during the first 9 days, but ÆRhæ is unexpectedly increased after that, leading to final macroscopic phase separation. The observed increase of ÆRhæ in existing large amount of free PAA20-b-PAN75 micelles in water should be an interesting subject for the insight of the kinetic studies.

Conclusion In summary, block copolymers PAA20-b-PAN75 and grafted copolymers PDMS240-g-(PEO13)15 were successfully synthesized and self-assembled into micelles in aqueous solutions. The studies of the mixing of these two micelles in water reveal that the hydrogen-bonding-induced complexation is no obvious in the pH range of 3.8-5.6 due to the partial ionized PAA blocks which may hinder the formation of strong cooperative hydrogen bonds between PAA and PEO chains. When the solution pH is reduced below 3.5, larger micellar complexes are formed due to the absorption of a layer of smaller PAA20-b-PAN75 micelles to the initial PDMS240-g-(PEO13)15 micelles. The further acidification, however, triggers the phase separation. Moreover, our results also indicate that the dependence of the hydrogen-bonding-induced DOI: 10.1021/la102539v

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complexation between the two micelles in water on PAA/PEO molar ratios (r) and standing time can be divided into three regions. In region I (r < 0.61), PAA20-b-PAN75 micelles act as physically cross-links, and PDMS240-g-(PEO13)15 micelles are “glued” together to form loosely packed agglomerates. The initial formed micellar complexes are in a nonequilibrium state, and therefore the size of the micellar complexes increases with the standing time. In region II (r = 0.61-1.21), very stable spherical micellar complexes are obtained through the adsorption of a layer of PAA20-b-PAN75 micelles around the shell of PDMS240g-(PEO13)15 micelles; the hydrogen-bonding complexation between PDMS240-g-(PEO13)15 and PAA20-b-PAN75 micelles in water likely proceeds stoichiometrically. In region III (r > 1.21), the formed micellar complexes first remained stable, but they precipitated from solution after a long time standing.

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We conjecture that the coexisting of excess free PAA20-b-PAN75 micelles in micellar complexes may play a significant role. Acknowledgment. We thank Prof. Guangzhao Zhang and Dr. Dinghai Xie for help in laser light scattering measurements and discussions. The financial support of this work by the Natural Science Foundation of Shandong province (2008ZRB02432) and Hong Kong Special Administration Region (HKSAR) General Research Fund (CUHK402809, 2160387) is gratefully appreciated. Supporting Information Available: Figures showing 1H NMR spectra of PtBA, PtBA-b-PAN, PAA-b-PAN, H-PDMS, and PDMS-g-PEO. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(18), 14502–14508