Transformations of Poly(methoxy hexa(ethylene glycol) methacrylate

Department of Chemistry and Department of Biology, Indiana University, Bloomington, Indiana 47405, Department of Materials Science and Technology and ...
0 downloads 0 Views 667KB Size
Download PDF
Langmuir 2005, 21, 9747-9755

9747

Transformations of Poly(methoxy hexa(ethylene glycol) methacrylate)-b-(2-(diethylamino)ethyl methacrylate) Block Copolymer Micelles upon Metalation Lyudmila M. Bronstein,*,† Maria Vamvakaki,*,§,⊥ Maxim Kostylev,†,# Vasilios Katsamanis,|,⊥ Barry Stein,‡ and Spiros H. Anastasiadis|,⊥ Department of Chemistry and Department of Biology, Indiana University, Bloomington, Indiana 47405, Department of Materials Science and Technology and Department of Physics, University of Crete, 710 03 Heraklion, Crete, Greece, and Institute of Electronic Structure and Laser, Foundation for Research and Technology - Hellas, P.O. Box 1527, 711 10 Heraklion, Crete, Greece Received April 27, 2005. In Final Form: July 18, 2005 Micelle transformations upon metalation (i.e., incorporation of metal compounds and metal nanoparticle formation) in poly(methoxy hexa(ethylene glycol) methacrylate)-block-poly((2-(diethylamino)ethyl methacrylate)), PHEGMA-b-PDEAEMA, solutions have been studied using transmission electron microscopy (TEM) and photon correlation spectroscopy (PCS). Three different methods for the formation of metalated micelles are compared: (A) dissolution of the block copolymers in pure water followed by incorporation of platinic acid (H2PtCl6‚6H2O), (B) micellization in acidic molecular solutions of block copolymers induced by interaction of the protonated amino groups with the PtCl62- ions, and (C) incorporation of metal species in pH-induced micelles. The latter method leads to well-defined metalated micelles of 22-25 nm diameter containing nanoparticles with diameters of 1.3-1.5 nm. No nanoparticle aggregation is observed. Good agreement is obtained for the sizes of the platinic acid-containing micelles assessed by TEM and PCS.

Introduction Metal nanoparticles display unique desirable properties and characteristics in comparison to bulk materials and, consequently, have applications in many areas of modern technology. In particular, the enormous surface-to-volume ratio of nanoscale particles (about 100-3000 m2/cm3) is a promising feature for their use as catalysts. However, the large surface area carries significant energetic costs, and thus nanoparticles typically require effective stabilization that reduces the interfacial energy and prevents their agglomeration.1,2 In the past decade, the use of nanostructured polymeric matrixes for nanoparticle synthesis and stabilization has received considerable attention, and a number of polymeric materials have been shown to provide stabilization and control of particle growth.3-12 * To whom correspondence should be addressed. E-mail: [email protected]. E-mail: [email protected]. † Department of Chemistry, Indiana University. ‡ Department of Biology, Indiana University. § Department of Materials Science and Technology, University of Crete. | Department of Physics, University of Crete. ⊥ Institute of Electronic Structure and Laser. # Present address: Cornell University, Department of Chemistry and Chemical Biology, Baker Laboratory, Ithaca, New York 14853. (1) Schmid, G. P., Ed. Clusters and Colloids: From Theory to Applications; VCH: Weinheim, Germany, 1994. (2) Foester, S.; Antonietti, M. Adv. Mater. 1998, 10, 195-217. (3) Antonietti, M.; Wenz, E.; Bronstein, L.; Seregina, M. Adv. Mater. 1995, 7, 1000-1005. (4) Moffitt, M.; Vali, H.; Eisenberg, A. Chem. Mater. 1998, 10, 10211028. (5) Zhao, M.; Crooks, R. M. Adv. Mater. 1999, 11, 217-220. (6) Sidorov, S. N.; Volkov, I. V.; Davankov, V. A.; Tsyurupa, M. P.; Valetsky, P. M.; Bronstein, L. M.; Karlinsey, R.; Zwanziger, J. W.; Matveeva, V. G.; Sulman, E. M.; Lakina, N. V.; Wilder, E. A.; Spontak, R. J. J. Am. Chem. Soc. 2001, 123, 10502-10510. (7) Cuenya, B. R.; Baeck, S. H.; Jaramillo, T. F.; McFarland, E. W. J. Am. Chem. Soc. 2003, 125, 12928-12934.

Among the nanoparticle syntheses that have emerged, one promising method involves nanoparticle formation within the core of amphiphilic block copolymer micelles.3,4,13-20 Similar to surfactants, amphiphilic block copolymers tend to self-assemble in selective solvents and most commonly form spherical micelles with a solventinsoluble core.21,22 However, a range of other morphologies including, but not limited to, vesicles, rods, and needles are possible when manipulating the copolymer characteristicsscopolymer molecular structure, net co(8) Korchev, A. S.; Bozack, M. J.; Slaten, B. L.; Mills, G. J. Am. Chem. Soc. 2004, 126, 10-11. (9) Underhill, R. S.; Liu, G. Chem. Mater. 2000, 12, 2082-2091. (10) Liu, S.; Weaver, J. V. M.; Save, M.; Armes, S. P. Langmuir 2002, 18, 8350-8357. (11) Yan, X.; Liu, F.; Li, Z.; Liu, G. Macromolecules 2001, 34, 91129116. (12) Frankamp, B. L.; Uzun, O.; Ilhan, F.; Boal, A. K.; Rotello, V. M. J. Am. Chem. Soc. 2002, 124, 892-893. (13) Boutonnet, M.; Kizling, J.; Stenius, P.; Maire, G. Colloids Surf. 1982, 5, 209-225. (14) Bronstein, L. M.; Sidorov, S. N.; Gourkova, A. Y.; Valetsky, P. M.; Hartmann, J.; Breulmann, M.; Coelfen, H.; Antonietti, M. Inorg. Chim. Acta 1998, 280, 348-354. (15) Sidorov, S. N.; Bronstein, L. M.; Valetsky, P. M.; Hartmann, J.; Colfen, H.; Schnablegger, H.; Antonietti, M. J. Colloid Interface Sci. 1999, 212, 197-211. (16) Burke, S.; Eisenberg, A. High Perform. Polym. 2000, 12, 535542. (17) Seregina, M. V.; Bronstein, L. M.; Platonova, O. A.; Chernyshov, D. M.; Valetsky, P. M.; Hartmann, J.; Wenz, E.; Antonietti, M. Chem. Mater. 1997, 9, 923-931. (18) Platonova, O. A.; Bronstein, L. M.; Solodovnikov, S. P.; Yanovskaya, I. M.; Obolonkova, E. S.; Valetsky, P. M.; Wenz, E.; Antonietti, M. Colloid Polym. Sci. 1997, 275, 426-431. (19) Mayer, A. B. R.; Mark, J. E. Colloid Polym. Sci. 1997, 275, 333340. (20) Bronstein, L. M.; Sidorov, S. N.; Valetsky, P. M.; Hartmann, J.; Coelfen, H.; Antonietti, M. Langmuir 1999, 15, 6256-6262. (21) Hamley, I. U. The Physics of Block Copolymers; Oxford University Press: Oxford, U.K., 1998. (22) Hadjichristidis, N.; Pispas, S.; Floudas, G. A. Block Copolymers: Synthetic Strategies, Physical Properties, and Applications; WileyInterscience: Hoboken, NJ, 2003.

10.1021/la0511337 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/26/2005

9748

Langmuir, Vol. 21, No. 21, 2005

polymer length, and block length ratio.16,23-26 Characterization of the block copolymer structures in solution is necessary because their morphology may affect the properties of these materials and their hybrids with metallic nanoparticles; many studies have shown that small differences in the size and morphology of the copolymer structures often have considerable effects on the nanoparticle properties.7,27-29 Therefore, precise control of the copolymer and nanoparticle characteristics is essential for the successful applications of these materials. Earlier micellization and nanoparticle synthesis studies examined the effectiveness of various copolymer systems as nanoparticle matrixes in organic solvents.3,13,16,18 The behavior of poly(styrene)-block-poly(2-vinylpyridine) micelles containing gold nanoparticles in toluene solutions was studied using static and dynamic light scattering.30 More recently, many studies have focused on the use of water as the medium for these syntheses.9,14,15,20,31 Water has significant environmental and cost advantages with respect to organic solvents and is an optimum medium for catalytic reactions of hydrophilic substrates. Furthermore, stable polymer nanostructures in aqueous solutions have potential applications in drug delivery.32 However, the metal precursor ions do not enter spontaneously in nonfunctionalized hydrophobic micelle cores (such as polystyrene) in aqueous media. In such cases, particles form in the corona of the micelles and are thus weakly stabilized.19 This problem is overcome with the incorporation in the core-forming block of specific functional groups that readily form complexes with the metal precursor ions but do not necessarily lead to solubility in water. The presence of such groups facilitates precursor ion transport from the solution into the micelle core, yielding ionic or highly polar microdomains that effectively restrict and stabilize the nanoparticle growth.4,20 With the basic methodology established, it is important to design specific metal-polymer nanoscale systems that can be applied in modern research and industry. We report here on the micellization of block copolymer33 poly(methoxy hexa(ethylene glycol) methacrylate)-block-poly((2-(diethylamino)ethyl methacrylate)), PHEGMA-b-PDEAEMA, in water upon metalation. We believe that this system may be of significant value in a number of reactions that employ noble-metal catalysts in aqueous media. PHEGMA-b-PDEAEMA copolymers readily form stable micelles in water above pH 7 (the effective pKa of the PDEAEMA block), and they dissolve molecularly at low pH because of the protonation of the tertiary amine groups.33 The diethylamine groups present in the hydrophobic PDEAEMA block can form strong complexes with platinum compounds and thus enhance the transport of the metal anion into the micelle core. In addition, protonation of the nitrogen at acidic pH further accelerates (23) Luo, L.; Eisenberg, A. J. Am. Chem. Soc. 2001, 123, 1012-1013. (24) Burke, S. E.; Eisenberg, A. Polymer 2001, 42, 9111-9120. (25) Cao, L.; Manners, I.; Winnik, M. A. Macromolecules 2002, 35, 8258-8260. (26) Raez, J.; Tomba, J. P.; Manners, I.; Winnik, M. A. J. Am. Chem. Soc. 2003, 125, 9546-9547. (27) Cherstiouk, O. V.; Simonov, P. A.; Savinova, E. R. Electrochim. Acta 2003, 48, 3851-3860. (28) Campbell, C. T.; Parker, S. C.; Starr, D. E. Science 2002, 298, 811-814. (29) Coq, B.; Figueras, F. In Catalysis and Electrocatalysis at Nanoparticle Surfaces; Wieckowski, A., Savinova, E. R., Vayenas, C. G., Eds.; Marcel Dekker: New York, 2003; pp 847-875. (30) Moessmer, S.; Spatz, J. P.; Moeller, M.; Aberle, T.; Schmidt, J.; Burchard, W. Macromolecules 2000, 33, 4791-4798. (31) Li, Z.; Liu, G. Langmuir 2003, 19, 10480-10486. (32) Yesair, D. W. U.S. Patent 8,300,294, 1983. (33) Bailey, L.; Vamvakaki, M.; Billingham, N. C.; Armes, S. P. Polym. Prepr. 1999, 40, 263-264.

Bronstein et al. Scheme 1. Structure of Poly(methoxy hexa(ethylene glycol) methacrylate)-block-poly(2(diethylamino)ethyl methacrylate) (PHEGMA-b-PDEAEMA)

Table 1. Molecular Characteristics of the PHEGMA-b-PDEAEMA Block Copolymers

block copolymer

Mn by SECa

Mw/Mn by SECa

HEGMA/ DEAEMA mole ratio

PHEGMA30-b-PDEAEMA30 PHEGMA50-b-PDEAEMA20 PHEGMA50-b-PDEAEMA30 PHEGMA50-b-PDEAEMA34 PHEGMA50-b-PDEAEMA40 PHEGMA50-b-PDEAEMA50

9300 10 000 11 400 11 700 12 300 13 500

1.14 1.14 1.16 1.15 1.13 1.14

1.0 2.5 1.7 1.5 1.3 1.0

a

Using PMMA standards.

ion transport into the micelle core via ionic attraction between the positively charged amino group and the platinum anion. We used a number of relatively short copolymers (60-100 monomer units) with variable lengths of both blocks to determine the effect of the overall chain length and block length ratio on the micellar behavior of Pt-containing block copolymers. Experimental Section Materials. Diblock copolymers comprising hydrophilic methoxy hexa(ethylene glycol) methacrylate (HEGMA) and pHresponsive 2-(diethylamino)ethyl methacrylate (DEAEMA) units, PHEGMA-b-PDEAEMA (Scheme 1), were prepared by grouptransfer polymerization (GTP) chemistry in THF using glass reactors under dry nitrogen and standard Schlenk techniques as described elsewhere.33 The molecular weights and molecular weight distributions of the PHEGMA-b-PDEAEMA copolymers were determined by SEC using poly(methyl methacrylate) standards, whereas 1H NMR spectroscopy was used to assess their composition. SEC analysis of the diblock copolymers confirmed their narrow molecular weight distributions (Mw/Mn ) 1.13-1.16) and the good molecular weight control achieved. The relative copolymer compositions obtained by 1H NMR were in good agreement with the theoretical values. The molecular characteristics of the copolymers are shown in Table 1. A series of block copolymers were prepared in which the length of the hydrophilic PHEGMA block was kept constant at 50 units and the length of the pH-sensitive PDEAEMA block was varied systematically between 20 and 50 monomer units. Two symmetric block copolymers with overall degrees of polymerization of 60 and 100 were also synthesized. The platinum compounds, platinic acid (H2PtCl6 × 6H2O) and potassium hexachloroplatinate (K2PtCl6), were obtained from Reakhim Russia and Aldrich, respectively, and were used as received. Sodium borohydride (NaBH4) was also purchased from Aldrich and was used as a 12 w/v % solution in deionized water. Metalation of PHEGMA-b-PDEAEMA Micelles. Metalcontaining polymer samples were prepared by the addition of solid H2PtCl6‚6H2O in a 0.33 wt % polymer solution under vigorous stirring. H2PtCl6‚6H2O was added to the copolymer solutions in a 3:1 molar ratio of N/Pt. Three different procedures were used for the introduction of the metal compounds within the polymeric matrixes, and their effects on both the micelle characteristics and the size of the metal nanoparticles were investigated. In method A, the PHEGMA-b-PDEAEMA diblock copolymers were directly added to distilled water to obtain a 0.33 wt %

Transformations of Micelles upon Metalation polymer solution and were stirred overnight. Next, the metal precursor (H2PtCl6‚6H2O) was added to the copolymer solutions in a 3:1 molar ratio of N/Pt, and the mixtures were stirred overnight to allow for equilibration of metal incorporation. Excess ionic species were then removed by 3-fold ultrafiltration using an Amicon stirred ultrafiltration cell by Millipore with regenerated cellulose ultrafiltration membranes, NMWL: 3000. Note that in the early experiments this last procedure was not applied (Results and Discussion section). Next, metal reduction was carried out under vigorous stirring using a 3-fold excess of NaBH4 in deionized water, and the samples were allowed to stir overnight. Upon addition of the reducing agent, the solutions instantaneously turned black, and H2 gas rapidly evolved. In method B, a molecular copolymer solution was obtained by the addition of the copolymer to deionized water at pH 2 (adjusted with 12 M HCl) followed by overnight stirring to ensure complete polymer dissolution. Then the metal precursor was added, and the solution was stirred overnight to ensure equilibration in metal incorporation. The metal compound incorporation was followed by 3-fold ultrafiltration and metal reduction similar to that in method A. Finally, the pH was adjusted to 10 by the addition of 0.1 M NaOH (to facilitate comparison with method C below). In method C, the diblock copolymers were also added to deionized water adjusted to pH 2 and stirred overnight. Then the pH was raised from 2 to 10 using 0.1 M NaOH, and the solution was stirred overnight before the addition of the metal precursor to allow for the equilibrium formation of polymer micelles. Next, the metal precursor, H2PtCl6‚6H2O or K2PtCl6, was added to the micelle solution and was allowed to stir for 1 or 1-5 days, respectively, to ensure equilibration of metal incorporation. Finally, ultrafiltration and metal reduction were carried out similarly to that in method A. Characterization. Dynamic Light Scattering (DLS)-Photon Correlation Spectroscopy (PCS). The autocorrelation function of the polarized light scattering intensity GVV(q, t) ) 〈I(q, t) I(q, 0)〉/〈I(q, 0)〉2} was measured at different scattering angles, θ, using an ALV spectrophotometer and an ALV-5000 full digital correlator over the time range of 10-7-103 s; I(q, 0) is the mean scattering intensity. Generally, both the incident and the scattered beam were polarized perpendicular to the scattering plane (VV geometry). An Adlas diode-pumped Nd:YAG laser was used as the light source with wavelength of λ ) 532 nm and single mode intensity of ∼100 mW. The magnitude of the scattering wavevector is q ) (4πn/λ) sin(θ/2), where n is the refractive index of the medium. Under homodyne conditions, GVV(q, t) is related to the desired scattering field autocorrelation function C(q, t) by C(q, t) ) {[GVV(q, t) - 1]/f*}1/2, where f* is an experimental factor calculated by means of a standard. The experimental correlation functions C(q, t) are analyzed by performing the inverse Laplace transform (ILT) using the routine CONTIN assuming a superposition of exponentials for the distribution of relaxation times L(ln τ) (i.e., ∞ C(q, t) ) ∫-∞ L(ln τ) exp[-t/τ] d(ln τ)). The rate Γ of each process is calculated as the inverse of its mean relaxation time, whereas their dynamic intensities are calculated from the integrals under the peaks of L(ln τ) multiplied by I(q, 0). In the case of a diffusive process, its diffusion coefficient D is obtained from the slope of Γ versus q2 by Γ ) Dq2. The latter is related to the hydrodynamic radius Rh of the diffusing moiety by the Stokes-Einstein equation Rh ) kBT/(6πηD), where η is the viscosity of the solvent, kB is the Boltzmann constant, and T is the temperature of the sample (it is assumed that D at the low concentrations used corresponds to its limit for concentration c f 0). All measurements were performed at 20 °C. It is noted that all of the solutions investigated in the present article were stable for periods of more than a month where no indications of sedimentation or phase separation were observed independently of the presence or absence of the metal compound or the metal nanoparticles. Transmission Electron Microscopy (TEM). The block copolymers after addition of the metal precursor and the subsequent metal reduction were analyzed using transmission electron microscopy (TEM). A JEOL JEM1010 instrument at an electron accelerating voltage of 60 kV was employed for the measurements. Each sample was diluted to 0.75 mg/mL in deionized water. A drop of the diluted sample was then placed on a carbon-coated copper grid situated on absorbent paper and

Langmuir, Vol. 21, No. 21, 2005 9749 allowed to dry in the open air for 2 days. Particle and micelle sizes were determined using Scion Image software in conjunction with Adobe Photoshop 6.0.

Results and Discussion The effect of metalation (i.e., metal ion incorporation and metal nanoparticle formation) on solution behavior and the micellization process of pH-responsive PHEGMAb-PDEAEMA diblock copolymers was investigated using TEM and PCS. The influence of copolymer characteristics such as molecular weight and composition on the size of the metal-containing micellar structures was also studied. In our earlier study, we reported the pH-dependent micellization of these diblock copolymers in the absence of the platinum precursors.34 Herein we compare three different methodologies (Experimental Section) of metal incorporation for the two symmetric diblock copolymers PHEGMA30-b-PDEAEMA30 and PHEGMA50-b-PDEAEMA50 and investigate a broader block copolymer composition range for method B (see below). Method A. In this method (Scheme 2a), the PHEGMAb-PDEAEMA diblock copolymers are added to pure water at pH ∼7.5 without any previous adjustment of the solution pH and are stirred for 24 h. The incorporation of platinic acid (H2PtCl6‚6H2O) into this solution drops the pH to ∼6.5 and causes partial protonation of the tertiary amine groups and their electrostatic interaction with the PtCl62ions. Finally, metal reduction using NaBH4 results in metal nanoparticle formation and causes an increase in the solution pH to 8.0. It should be noted that the copolymer is not very soluble in water at pH 7.5 where it is added initially. This results in the formation of block copolymer aggregates (discussed below), which are schematically shown as large spherical micelles in Scheme 2a. The other inadequacy of this method is the formation of nanoparticles in the exterior of the micellar core. This is due to some precursor ions that remain in the solution outside the micelles after the H2PtCl6 addition, which, when the reducing agent is added, leads to particle nucleation outside the micelles. The TEM micrograph in Figure 1 illustrates this phenomenon for PHEGMA30-bPDEAEMA30 diblock copolymer. Metal nanoparticles formed in these solutions are relatively large (up to 5.5 nm), and many are located outside the micelles (dark spherical shapes in the background). To overcome this problem, we introduced a 3-fold ultrafiltration step 24 h after the addition of the Pt precursor that removes all of the ions remaining outside of the micelles. Removal of the excess ions considerably improves the results because upon reduction no nanoparticles form outside the micelles. According to elemental analysis data, the Pt content in a solid polymer sample is now 9.65 wt %, providing a degree of platinic acid incorporation of about 99%. The TEM images of PHEGMA50-b-PDEAEMA50 micelles after incorporation of platinic acid and after nanoparticle formation are shown in Figure 2. The TEM results on the diameters of the metalated micellar structures (as well as on the diameters of the synthesized nanoparticles) are shown in Table 2 and will be discussed together with the PCS data. Figure 3 shows the intensity autocorrelation functions for the various stages during the metalation of a PHEGMA50-b-PDEAEMA50 diblock copolymer solution using method A at scattering angle θ ) 90°. The distributions of relaxation times multiplied by the total scattering (34) Vamvakaki, M.; Papoutsakis, L.; Katsamanis, V.; Afchoudia, T.; Fragouli, P. G.; Iatrou, I.; Hadjichristidis, N.; Armes, S. P.; Sidorov, S.; Zhirov, D.; Zhirov, V.; Kostylev, M.; Bronstein, L. M.; Anastasiadis, S. H. Faraday Discuss. 2004, 128, 129-147.

9750

Langmuir, Vol. 21, No. 21, 2005

Bronstein et al.

Scheme 2. Schematic Representation of the Synthetic Procedures Followed for the Metal Nanoparticle Formation Using Methods A, B and Ca

a The hydrophobic micelle cores comprising neutral DEAEMA units are shown in black; the protonated amine-hexachloroplatinate anion containing cores, in grey; and the metal nanoparticle-containing hydrophobic cores, in black with white spots (nanoparticles).

intensities are shown in the insets and can be directly compared. After reduction with NaBH4, the solution turns black because of metal nanoparticle formation. Thus, a 3-fold dilution of the sample is required prior to DLS analysis because of the high absorbance of the dark-colored metal species of the 0.33 wt % copolymer solution. This concentration was used for all reduced samples discussed below. In pure water (Figure 3a), one relaxation process with constant and very high intensity and a q2-dependent relaxation rate is observed in the distribution of relaxation times, shown in the inset. The diffusivity of this process (D ) 8.57 × 10-8 cm2/s) corresponds to a hydrodynamic radius of Rh ) 25 ( 0.5 nm that is attributed to the formation of block copolymer aggregates. This aggregation is due to the poor solubility of the deprotonated hydrophobic PDEAEMA block at high pH that results in the observed nonequilibrium frozen aggregate structures of high intensity.35-37

When the platinic acid is introduced into the copolymer solution, one process with lower intensity and diffusivity D ) 1.19 × 10-8 cm2/s that corresponds to a smaller hydrodynamic radius of Rh ) 18 ( 0.5 nm dominates the correlation function, shown in Figure 3b. We attribute the decrease in the hydrodynamic radius to the protonation of the tertiary amine groups by the platinic acid, which results in the partial decomposition of the larger aggregates discussed above and the detachment of some free polymer chains from the aggregate structure. However, these free polymer chains are not observed by PCS because of the very strong intensities of the process corresponding to the aggregates, which dominates over the significantly lower scattering intensity from the unimer chains. The autocorrelation function of a 0.1 wt % metal nanoparticle-containing copolymer solution, shown in Figure 3c, depicts two diffusive processes. Diffusion coefficients D1 ) 1.68 × 10-7 cm2/s and D2 ) 2.10 × 10-8 correspond to hydrodynamic radii of Rh,1 ) 13.1 ( 0.5 nm and Rh,2 ) 105 ( 3 nm. The first is related to the metal nanoparticle-containing polymer micelles, whereas the second is related to the few micellar aggregates formed by the clustering of some micelles; this aggregation may be understood to be due to short-range attraction between the micellar coronas, which can dominate at high pH.38 The low intensities of the peak related to the slow process signify a very small number of aggregates. It is surprising that small, well-defined micelles are formed after metal reduction, despite the large polymer aggregates obtained in the two preceding steps. Because reduction with NaBH4 leads to decomposition of the “cross links” due to the PtCl62anions, we believe that this allows the rearrangement and equilibration of the micelles. The metalation of the second symmetric diblock copolymer of lower overall molecular weight, PHEGMA30b-PDEAEMA30, in pure water was also investigated. The

(35) Selb, L.; Gallot, Y. In Developments in Block Copolymers; Goodman, I., Ed.; Elsevier Applied Science Publishers: New York, 1985; Vol. 2.

(36) Zhang, L.; Shen, H.; Eisenberg, A. Macromolecules 1997, 30, 1001-1011. (37) Zhang, L.; Eisenberg, A. Macromolecules 1999, 32, 2239-2249. (38) Nyrkova, I. A.; Semenov, A. N. Faraday Discuss. 2005, 128, 113.

Figure 1. TEM image of the Pt nanoparticles formed in PHEGMA30-b-PDEAEMA30 micellar solution following the reduction of H2PtCl6 by NaBH4 without previous ultrafiltration.

Transformations of Micelles upon Metalation

Langmuir, Vol. 21, No. 21, 2005 9751

Figure 2. TEM images of PHEGMA50-b-PDEAEMA50 micelles after incorporation of platinic acid (a) and metal nanoparticles formed after reduction (b). Table 2. Radii of the Copolymer-H2PtCl6 Complexes and Metal Nanoparticle Diameters

c

copolymer

methoda

Rh (nm)b

Rt (nm)c

dn (nm)d

H30-D30 H50-D50 H30-D30 H50-D50 H30-D30 H50-D50

A A B B C C

10.9 18.0 11.0 12.2 9.9 9.9

8.5 ( 3.7 17.8 ( 7.9 7.4 ( 2.0 9.7 ( 2.0 7.2 ( 2.0 9.2 ( 2.4

1.6 ( 0.5 1.4 ( 0.4 1.3 ( 0.3 1.3 ( 0.3 1.3 ( 0.3 1.3 ( 0.4

a See Experimental Section. b Hydrodynamic radius by DLS. Micelle radius from TEM. d Pt nanoparticle diameter from TEM.

Figure 4. Intensity autocorrelation functions for the metalation of a PHEGMA30-b-PDEAEMA30 diblock copolymer solution using method A at scattering angle θ ) 90°. (a) Copolymer dissolved in water at pH 7.5 and 0.33 wt % (squares), (b) in the presence of H2PtCl6‚6H2O and c ) 0.33 wt % (circles), and (c) after metal reduction using NaBH4 at pH 9 and c ) 0.1 wt % (triangles). (Inset) distribution of relaxation times multiplied by the total scattering intensity (normalized to that of toluene).

Figure 3. Intensity autocorrelation functions for the metalation of a PHEGMA50-b-PDEAEMA50 diblock copolymer solution using method A at scattering angle θ ) 90°. (a) Copolymer added to water at pH 7.7 and c ) 0.33 wt % (squares), (b) in the presence of H2PtCl6‚6H2O at pH 6.5 and c ) 0.33 wt % (circles), and (c) after metal reduction using NaBH4 at pH 8.0 and c ) 0.1 wt % (triangles). (Inset) distribution of relaxation times multiplied by the total scattering intensity (normalized to that of toluene).

smaller molecular weight copolymer was expected to be more soluble in water, and thus it should lead to less aggregation. Figure 4 shows the intensity autocorrelation functions of a PHEGMA30-b-PDEAEMA30 diblock copolymer solution before and after metal incorporation at scattering angle θ ) 90°. In pure water (Figure 4a), two relaxation processes are consistently observed in the distributions of relaxation times with diffusivities D1 ) 2.02 × 10-7 cm2/s and D2 ) 2.24 × 10-8 cm2/s, which correspond to hydrodynamic radii of Rh,1 ) 10.9 ( 0.5 nm and Rh,2 ) 99 ( 3 nm, respectively. The first is attributed

to polymer micelles, whereas the second is attributed to large micellar aggregates. The formation of regular micelles of small size in pure water for PHEGMA30-bPDEAEMA30 is in contrast to the larger nonequilibrium aggregates obtained for the PHEGMA50-b-PDEAEMA50 diblock copolymer in the same medium. This is indeed due to the lower overall molecular weight of the diblock copolymer and especially the shorter hydrophobic block, which facilitates the rearrangement of the copolymer chains in the aqueous medium and the formation of equilibrium micelles even by direct dissolution in water.39,40 When H2PtCl6 is added to the micellar solution of PHEGMA30-b-PDEAEMA30, three processes are observed in the distribution of relaxation times (Figure 4b). The three diffusion coefficients (D1 ) 1.74 × 10-6 cm2/s, D2 ) 2.03 × 10-7 cm2/s, and D3 ) 3.11 × 10-8 cm2/s) correspond to hydrodynamic radii of Rh,1 ) 1.3 ( 0.2 nm, Rh,2 ) 10.9 ( 0.5 nm, and Rh,3 ) 71 ( 3 nm. The first process is related to free polymer chains, the second to polymer micelles, (39) Chu, B. Langmuir 1995, 11, 414-421. (40) Tuzar, Z. In Solvents and Self-Organization of Polymers; Webber, S. E. M., P., Tuzar, Z., Eds.; Kluwer Academic Publishers: Dordtrecht, The Netherlands, 1996; pp 1, 309.

9752

Langmuir, Vol. 21, No. 21, 2005

and the third to micellar aggregates. It appears that the significant drop in the solution pH by the addition of platinic acid causes partial protonation of the DEAEMA units and leads to increased molecular solubility and thus to an equilibrium between free chains and metalated micelles. When the addition of excess NaBH4 for the metal reduction brings the pH to 9, both micelles and micellar aggregates containing metal nanoparticles (D1 ) 2.21 × 10-7 cm2/s, Rh,1 ) 10.0 ( 0.5 nm and D2 ) 3.15 × 10-8 cm2/s, Rh,2 ) 70 ( 3 nm, respectively) are observed (Figure 4c). No free chains are observed because of the high solution pH33 and probably because of micelle reorganization that takes place during nanoparticle formation. Obviously, the nanoparticles are accommodated within the micelle voids because no significant change in the micelle diameter is observed. Table 2 presents the diameters of block copolymer micelles containing PtCl62- ions both from PCS and TEM data. It is worth noting that the Pt species incorporated inside the micelles provide the contrast necessary for TEM visualization of the micelles. At the same time, when the nanoparticles are formed within the micelles, the contrast between the nanoparticles and the micelle material is so high that the micelles become nearly invisible. The exceptions are the micelles formed by arene-containing blocks (polystyrene, poly(4-vinylpyridine), etc.).41,42 In this work, no estimation of micelle sizes after nanoparticle formation was possible by TEM. The data presented in Table 2 show good agreement between the sizes of the micelles containing platinic acid assessed by TEM and PCS. However, the sizes obtained from TEM are always somehow smaller than those obtained by PCS. It should be noted that PCS estimates a micelle size in solution whereas TEM deals with a dried sample. Besides, the platinum species should be located within the PDEAEMA micelle core or at the core-shell interface, and thus the diameter assessed by TEM may represent the micelle core diameter rather than the overall micelle size. Mean Pt particle diameters measure 1.6 and 1.4 nm for PHEGMA30b-PDEAEMA30 and PHEGMA50-b-PDEAEMA50, respectively (the standard deviations of the Pt nanoparticle sizes are also presented in Table 2). Method B. In this method, the diblock copolymer is introduced to water adjusted to pH 2. Addition of the copolymer causes an increase in the solution pH to 2.4 due to the basic nature of the PDEAEMA block. At this pH, the PDEAEMA block is fully protonated, and thus the copolymer exists as single chains (unimers) in the aqueous medium (molecular solution). This procedure circumvents the problem of irregular polymer aggregation observed in method A, as observed by PCS below. Micellization in a molecular copolymer solution can be induced either by complex formation between the PDEAEMA block and the metal precursor20 or by a pH increase above the effective pKa of the PDEAEMA block (i.e., deprotonation of the tertiary amine groups). In method B, we used metal complexation to induce micellization, whereas in method C we describe the pH-induced micellization followed by the interaction with the metal compound. In method B, H2PtCl6‚6H2O was added to the solution, causing the pH to decrease to 2.2, and this was followed by metal reduction using NaBH4 to form the metal (41) Seregina, M. V.; Bronstein, L. M.; Platonova, O. A.; Chernyshov, D. M.; Valetsky, P. M.; Hartmann, J.; Wenz, E.; Antonietti, M. Chem. Mater. 1997, 9, 923-931. (42) Bronstein, L. M.; Chernyshov, D. M.; Volkov, I. O.; Ezernitskaya, M. G.; Valetsky, P. M.; Matveeva, V. G.; Sulman, E. M. J. Catal. 2000, 196, 302-314.

Bronstein et al.

Figure 5. Intensity autocorrelation functions for the metalation of a PHEGMA50-b-PDEAEMA50 diblock copolymer solution using method B at scattering angle θ ) 90°. (a) Copolymer dissolved in water at pH 2.4 and 0.33 wt % (squares), (b) in the presence of H2PtCl6‚6H2O at pH 2.2 and c ) 0.33 wt % (circles), and (c) after reduction using NaBH4 at pH 10 and c ) 0.1 wt % (triangles). (Inset) distribution of relaxation times multiplied by the total scattering intensity (normalized to that of toluene).

nanoparticles and adjustment of the pH to 10 using NaOH. Figure 5 shows the intensity autocorrelation functions at scattering angle θ ) 90° for a PHEGMA50-b-PDEAEMA50 diblock copolymer solution at all stages of metalation with method B. At low pH before the incorporation of the metal compound (Figure 5a), two relaxation processes with low intensities are consistently observed in the distribution of relaxation times. The fast process corresponds to the diffusion of single polymer chains with diffusion coefficient D1 ) 7.18 × 10-7 cm2/s that corresponds to a hydrodynamic radius of Rh,1 ) 3.1 ( 0.2 nm, whereas the slower process with diffusivity D2 ) 2.67 × 10-8 cm2/s corresponds to Rh,2 ) 83 ( 3 nm and is related to polymer aggregates, as extensively discussed in our recent paper.34 Upon addition of the platinic acid (Figure 5b), one single process dominates the correlation function with very strong intensity and slower dynamics; its diffusivity of D ) 1.81 × 10-7cm2/s corresponds to a hydrodynamic radius of Rh ) 12.2 nm. This process is attributed to the formation of micelles via a metal-induced micellization process that is primarily due to electrostatic interactions of the protonated PDEAEMA units and the PtCl62- ions. Replacement of chlorine atoms in PtCl62- by nonprotonated PDEAEMA amino groups may occur as well but is much slower than the electrostatic interaction of the oppositely charged ions. Such ion-induced aggregation phenomena have been previously reported by us20 and other authors.43 In the latter case, multivalent cations are added to the solution of an oppositely charged block copolymer and lead to large, rather ill-defined structures because of the formation of nonequilibrium structures when metal cations are involved. However, the ion-induced micellelike structures observed in the present study have sizes that are comparable to those of the well-defined micelles formed at high pH in the absence of metal ions (Table 3), suggesting the formation of stable equilibrium structures. We believe that because metal anions are bulkier than cations and a negative charge is spread between six chlorine atoms, the electrostatic interaction between (43) Li, Y.; Gong, Y.-K.; Nakashima, K.; Murata, Y. Langmuir 2002, 18, 6727-6729.

Transformations of Micelles upon Metalation

Langmuir, Vol. 21, No. 21, 2005 9753

Table 3. Hydrodynamic Radii of All Species Obtained by PCS for the PHEGMA-b-PDEAEMA Diblock Copolymers Using Method B copolymers H30-D30 H50-D20 H50-D30 H50-D34 H50-D40 H50-D50

Rh,unimers Rh,aggregates Rh,micelles Rh,platinic Rh,reduced (nm)a (nm)a (nm)b (nm)c (nm)d 3.0 4.0 4.3 4.4 4.4 4.4

72 80 75 68 75 76

10.3 11.3 13.9 13.1 13.2 13.2

11.0 12.0 12.6 10.8 11.7 12.2

12.2 11.0 12.9 13.1 13.6 14.5

a Nonmetalated species. b Nonmetalated micelles formed at pH 10. c Metalated micelles in the presence of H2PtCl6. d Metal nanoparticle-containing micelles.

anions and protonated PDEAEMA units is weaker than that of cations,43 thus allowing micelle equilibration. It should be noted that such well-defined metal-induced micelles can, in principle, be utilized for the development of sensors. Finally, after metal reduction, both micelles and micellar aggregates (Rh,1 ) 14.5 ( 0.5 nm and Rh,2 ) 109 ( 3 nm, respectively) exist in the solution (Figure 5c). The effect of metal incorporation and metal nanoparticle formation as a function of copolymer composition and molecular weight for all PHEGMA-b-PDEAEMA copolymers (see Table 1) was investigated by PCS utilizing method B. The hydrodynamic sizes for all species (both metalated and nonmetalated) found for the copolymers used in this study are summarized in Table 3. At low pH, unimers and polymer aggregates of similar sizes for all copolymers are obtained, suggesting that there is no significant effect on the hydrodynamic size of the single chains for the range of copolymer molecular weights examined. Next, upon addition of the metal precursor at pH 2, metal-induced micellization of the copolymer leads to the formation of PHEGMA-b-PDEAEMA-H2PtCl6 micelles with sizes between 11.0 and 12.6 nm, which are only slightly smaller than those obtained at pH 10 in method C (Table 3) and with no obvious trend as a function of copolymer molecular weight and composition, at least within the range investigated in this study. Moreover, the sizes of these micelles obtained by PCS are in good agreement with the micelle characteristics discussed below by TEM. A very small number of micellar aggregates, which are not reported in Table 3, with sizes between 60 and 80 nm were also observed upon metal compound incorporation. Finally, reduction of the micelles containing PtCl62- ions using NaBH4 results in the formation of metal nanoparticle-containing micelles with hydrodynamic radii in the range of 11.0 to 14.5 nm and few micellar aggregates with Rh values between 73 and 109 nm (not reported in

Table 3). The metal nanoparticle-containing micelles exhibit only a slight increase in size with the copolymer molecular weight and composition for the copolymers examined in this study. However, it is noted that the sizes of both the micelles containing platinum ions and nanoparticles are very similar to those of the well-defined nonmetalated micellar structures formed at high pH. This suggests that the hydrodynamic micellar size at equilibrium is not significantly affected by the presence of the metal compound or nanoparticles. It is noteworthy that no unimers were observed when micelle formation was induced by the incorporation of platinic acid in the molecular solution at pH 2, whereas unimers did appear when H2PtCl6‚6H2O was added to the polymer solution at pH 7.5 (method A). A possible explanation for this phenomenon is the following. Incorporation of the PtCl62ions into the preformed micelles or micellar aggregates starts from the micelle core periphery. This also leads to the decrease in pH that may locally be substantial and may allow the dissolution of some unimers not connected with platinum anions. When the PtCl62- ions are incorporated into the molecular solution, all unimers interact with these ions simultaneously, thus no unimers are left in the solution. A TEM image of the PHEGMA50-b-PDEAEMA50 micelles formed after the incorporation of platinic acid using method B is shown in Figure 6a. One can see regular micelles with a mean radius of 9.7 nm (Table 2), corroborating the PCS results. The TEM image of the block copolymer after reduction reveals that Pt nanoparticles tend to aggregate (Figure 6b), whereas no aggregation was observed for nanoparticles obtained by method A (Figure 2b). We think that when micelles are formed directly by complexation at pH 2 they should be swollen, which facilitates the diffusion of Pt nanoparticles within the micelles and thus aggregation, although this does not result in a measurable increase in particle size (Table 2). Again (similarly to method A) for PHEGMA50-b-PDEAEMA50, the sizes of the H2PtCl6-containing micelles evaluated by TEM are somehow smaller than those estimated by PCS. Method C. In this method, the pH of the molecular block copolymer solution prepared at pH 2 was raised to 10. This causes deprotonation of the tertiary amine block, which becomes hydrophobic, and leads to the formation of well-defined micelles (see below). Next, H2PtCl6 is added to the micellar copolymer solution at pH 10, and the pH drops to ∼7.5. This leads again to partial protonation of the amino groups and the incorporation of the metal anions in the cores of the micelles due to electrostatic interactions.

Figure 6. TEM images of the PHEGMA50-b-PDEAEMA50 micelles after incorporation of platinic acid (a) and Pt nanoparticle formation (b) using method B.

9754

Langmuir, Vol. 21, No. 21, 2005

Bronstein et al.

Figure 7. TEM image of PHEGMA50-b-PDEAEMA50 micelles filled with H2PtCl6 using method C (a) and the histogram of the micelle size distribution (b). Table 4. Sizes of PHEGMA-b-PDEAEMA-H2PtCl6 Micelles and Pt Nanoparticles from TEM Using Method C block copolymer

Rt (nm)a

dn (nm)b

PHEGMA30-b-PDEAEMA30 PHEGMA50-b-PDEAEMA20 PHEGMA50-b-PDEAEMA30 PHEGMA50-b-PDEAEMA34 PHEGMA50-b-PDEAEMA40 PHEGMA50-b-PDEAEMA50

7.2 ( 2.0 8.6 ( 1.9 8.0 ( 1.5 9.0 ( 1.9 8.6 ( 2.0 9.2 ( 2.4

1.3 ( 0.3 1.2 ( 0.3 1.3 ( 0.4 1.3 ( 0.4 1.5 ( 0.4 1.3 ( 0.4

a Metalated micelles in the presence of H PtCl . b Pt nanoparticle 2 6 diameter.

Figure 7 shows a TEM micrograph of PHEGMA50-bPDEAEMA50 micelles after metalation with H2PtCl6 and the histogram of the micelle sizes. The mean radius of the micelles for all block copolymers studied varies from 7.2 to 9.2 nm (Table 4), although as in method B no clear trend with increasing block copolymer net length is observed. Regular spherical micellar aggregates are also present in all samples. Finally, metal reduction is carried out using NaBH4, which causes an increase in the pH to 8.8 and results in the formation of the metal nanoparticles within the micelles. Pt nanoparticle sizes are independent of the block copolymer net length (Table 4). Figure 8 shows the intensity autocorrelation functions for the metalation of a PHEGMA50-b-PDEAEMA50 diblock copolymer solution. In good agreement with our earlier study on the micellization of PHEGMA-b-PDEAEMA diblock copolymers,34 at pH 10, before metal incorporation, the distribution of relaxation times, shown in the inset in Figure 8a, exhibits a dominant mode with strong and constant intensity and a q2-dependent rate (not shown) that corresponds to the diffusion of polymer micelles with a hydrodynamic radius of Rh,1 ) 12.2 ( 0.5 nm. A second slower diffusive process can hardly be seen (D2 ) 2.24 × 10-8 cm2/s, Rh,2 ) 99 ( 3 nm); it has significantly lower intensity, which is q-dependent (radius of gyration Rg ≈ 89 ( 3 nm) and is attributed to the formation of a small number of micellar aggregates. Upon addition of H2PtCl6, which causes the pH to decrease to ∼7.5, besides the diffusion of polymer micelles of slightly smaller size (Rh,2 ) 9.9 ( 0.5 nm), two other processes are also evident in the correlation function (Figure 8b). The fast process with diffusivity D1 ) 6.84 × 10-7 cm2/s and Rh,1 ) 3.2 ( 0.2 nm is attributed to free polymer chains, whereas the slow mode with D3 ) 3.12 × 10-8 cm2/s and Rh,3 ) 71 ( 2 nm is due to polymer aggregates. The decrease in the size of the micelles and the formation of free polymer chains upon incorporation

Figure 8. Intensity autocorrelation functions for the metalation of PHEGMA50-b-PDEAEMA50 diblock copolymer solutions using method C at scattering angle θ ) 90°. (a) Copolymer micelles at pH 10 and c ) 0.33 wt % (squares), (b) in the presence of H2PtCl6‚6H2O at pH 7.5 (circles), and (c) after reduction with NaBH4 at pH 8.4 and c ) 0.1 wt % (triangles). (Inset) distribution of relaxation times multiplied by the total scattering intensity (normalized to that of toluene).

of the metal compound is attributed to the partial protonation of the amine groups by H2PtCl6, which become hydrophilic and thus lead to the detachment of free polymer chains from the micellar structure (see discussion in method A). This partial decomposition of the micellar structures is further supported by the sharp decrease observed in the intensity of the process that corresponds to the micelles, which suggests a reduction in the number of micelles upon metal incorporation. The appearance of large aggregates is also in good agreement with previous work34 where the coexistence of some polymer aggregates of low intensity was observed in a unimer copolymer solution. Reduction with NaBH4 causes an increase in the solution pH to 8.8 (Scheme 2). The intensity autocorrelation function of a 0.1 wt % metal nanoparticle-containing copolymer solution is shown in Figure 8c for scattering angle θ ) 90°. The distribution of relaxation times shown in the inset depicts two diffusive processes (D1 ) 1.65 × 10-7 cm2/s and D2 ) 2.34 × 10-8 cm2/s) that correspond to hydrodynamic radii of Rh,1 ) 13.4 ( 0.5 nm and Rh,2 ) 94 ( 3 nm. The first corresponds to polymer micelles with

Transformations of Micelles upon Metalation

metal nanoparticles, whereas the second is attributed to a few micellar aggregates. Similar behavior was observed for the metalation of PHEGMA30-b-PDEAEMA30 using method C. Thus, before metal incorporation, at pH 10, micelles and micellar aggregates with Rh,1 ) 10.3 ( 0.5 nm and Rh,2 ) 91 ( 3 nm, respectively, are obtained. The addition of platinic acid causes a decrease in the solution pH to ∼7.5, the partial protonation of the DEAEMA units with the detachment of free polymer chains, and a decrease in the number of micelles discussed above for PHEGMA50-bPDEAEMA50. Three processes are consistently observed corresponding to Rh,1 ) 1.4 ( 0.2 nm, Rh,2 ) 9.9 ( 0.5 nm, and Rh,3 ) 68 ( 2 nm attributed to free polymer chains, polymer micelles, and polymer aggregates, respectively. Upon metal reduction, the solution pH increases to 8.8, and metal nanoparticle-containing micelles and micellar aggregates with Rh,1 ) 11.7 ( 0.5 nm and Rh,2 ) 63 ( 2 nm, respectively, are observed. The trend of a slight decrease in the micelle size after the incorporation of platinic acid followed by a size increase after metal reduction should be explained by the partial detachment of unimers at lower pH when H2PtCl6‚6H2O is added (see discussion for methods A and C), followed by reorganization of the micelles including all unimers from the solution. Conclusions We analyzed three different methods for the metalation of PHEGMA-b-PDEAEMA diblock copolymer systems. We have shown that the micellar characteristics of PHEGMAb-PDEAEMA depend only slightly on the net length of

Langmuir, Vol. 21, No. 21, 2005 9755

the block copolymer and on the synthetic procedures. Micelles of PHEGMA-b-PDEAEMA diblock copolymers in the range of 60 to 100 monomer units are usually welldefined spherical structures when synthesized under optimum conditions. Micelle formation upon direct dissolution of the copolymer in pure water is strongly dependent on the copolymer characteristics. Equilibrium micellar structures in the aqueous medium are obtained only for the lower molecular weight (60 monomer units) and lower hydrophobic content (30 units) copolymers. Metal compound incorporation in a molecular copolymer solution results in a complexation-induced micellization process and the formation of stable equilibrium copolymer structures in contrast to the large aggregates obtained in pure water. Finally, metal incorporation in a micellar copolymer solution results in the partial decomposition of the micellar structures followed by their reorganization upon metal reduction. The constant micellar size upon metal reduction suggests the formation of equilibrium micelles with the nanoparticles accommodated in the voids of the micellar cores. Acknowledgment. Part of this research was sponsored by NATO’s Scientific Affairs Division in the framework of the Science for Peace Programme (grant SfP974173), by the Greek General Secretariat of Research and Technology, and by the Russian Foundation for Basic Research (grant 04-03-32928). We thank Professor S. P. Armes for his help during the synthesis of the diblock copolymers. LA0511337