Hybrid Nanoparticles with Hyperbranched Polymer Shells via Self

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Hybrid Nanoparticles with Hyperbranched Polymer Shells via Self-Condensing Atom Transfer Radical Polymerization from Silica Surfaces Hideharu Mori, Delphine Chan Seng, Mingfu Zhang, and Axel H. E. Mu¨ller* Lehrstuhl fu¨ r Makromolekulare Chemie II, and Bayreuther Zentrum fu¨ r Kolloide und Grenzfla¨ chen, Universita¨ t Bayreuth, D-95440 Bayreuth, Germany Received November 2, 2001. In Final Form: February 7, 2002 We present the synthesis of hyperbranched polymer-silica hybrid nanoparticles by self-condensing vinyl polymerization (SCVP) via atom transfer radical polymerization (ATRP) from silica surfaces. ATRP initiators were covalently linked to the surface of silica particles, followed by SCVP of an initiatormonomer (“inimer”) which has both a polymerizable acrylic group and an initiating group in the same molecule. Well-defined polymer chains were grown from the surface to yield hybrid nanoparticles comprised of silica cores and hyperbranched polymer shells having multifunctional bromoester end groups, as confirmed by elemental analyses and Fourier transform infrared measurements. Characterization of soluble polymers obtained in solution by gel permeation chromatography (GPC), GPC/viscosity, and NMR suggests the formation of highly branched polymers. Correlation of molecular parameters of the soluble polymers with the polymers grafted on the surface is discussed in view of theoretical considerations. Hydrolysis of the ester functionality of branched poly(tert-butyl acrylate), which was obtained by copolymerization of the inimer and tert-butyl acrylate, created branched poly(acrylic acid)-silica hybrid nanoparticles. The hybrid nanoparticles were characterized using transmission electron microscopy, field emission scanning electron microscopy, scanning force microscopy, and dynamic light scattering.

Introduction The well-controlled synthesis of nanocomposite materials consisting of a polymer with incorporated inorganic nanoparticles is an area of increasing research activity. The organic polymer shell determines the chemical properties of such materials and their interaction with the environment, whereas their physical properties are governed by both the size and shape of the inorganic core and the surrounding organic layer. Depending upon their composition and ordering, these nanoparticles exhibit novel size-dependent magnetic, optical, and materials properties that could find applications in a wide array of technologies, such as diffractive optics, electro-optical devices, information storage, and composites. On the other hand, silica-polymer composites often have improved mechanical and thermal properties relative to the unfilled polymer. Owing to such wide scientific and industrial interest, several processes have been developed to prepare polymer-silica particle nanocomposites.1-11 A promising approach to achieve organic shells is to interact the * To whom correspondence should be addressed. E-mail: [email protected]. Tel: 0921-55-3399. Fax: 0921-553393. (1) Suzuki, K.; Siddiqui, S.; Chappell, C.; Siddiqui, J. A.; Ottenbrite, R. M. Polym. Adv. Technol. 2000, 11, 92. (2) Rong, M. Z.; Zhang, M. Q.; Zheng, Y. X.; Zeng, H. M.; Walter, R.; Friedrich, K. J. Mater. Sci. Lett. 2000, 19, 1159. (3) Caruso, F.; Mo¨hwald, H. Langmuir 1999, 15, 8276. (4) Kaddami, H.; Gerard, J. F.; Hajji, P.; Pascault, J. P. J. Appl. Polym. Sci. 1999, 73, 2701. (5) Espiard, P.; Guyot, A. Polymer 1995, 36, 4391. (6) Barthet, C.; Hickey, A. J.; Cairns, D. B.; Armes, S. P. Adv. Mater. (Weinheim, Ger.) 1999, 11, 408. (7) Bourgeat-Lami, E.; Lang, J. J. Colloid Interface Sci. 1998, 197, 293. (8) Jethmalani, J. M.; Ford, W. T. Chem. Mater. 1996, 8, 2138. (9) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 602. (10) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 592. (11) Sondi, I.; Fedynyshyn, T. H.; Sinta, R.; Matijevic, E. Langmuir 2000, 16, 9031.

inorganic cores with initiators and to polymerize the latter in situ.9,10 In recent years, much attention has been paid to the use of controlled/“living” polymerizations from a flat surface,12 because this allows better control over the target molecular weight and molecular weight distribution of the target polymer. By using the techniques, a high grafting density and a controlled film thickness can be obtained, such as brushes consisting of end-grafted, strictly linear chains of the same length with the chains forced to stretch away from the flat surface. Several research groups have recently reported the application of controlled/ living polymerization systems on the synthesis of organic/ inorganic hybrids involving gold13,14 and silica15-19 nanoparticles. Notably, von Werne and Patten described the synthesis of well-defined polystyrene-nanoparticle hybrids by using spherical silica nanoparticles modified with 4-chloromethylphenyl type initiators for atom transfer radical polymerization (ATRP).16 They demonstrated that the structure of the resulting composites can be manipulated by using controlled/“living” radical polymerization through changes in the polymer’s grafting density, composition, structure, and molar mass. Bo¨ttcher et al. used a 2-chloro-2-phenylacetate type initiator attached to silica particles for ATRP of styrene.15 It was shown that after polymerization of a first generation of grafts, chain ends of the grafts are still active, and a second monomer feed (12) Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25, 677. (13) Watson, K. J.; Zhu, J.; Nguyen, S. T.; Mirkin, C. A. J. Am. Chem. Soc. 1999, 121, 462. (14) Jordan, R.; West, N.; Ulman, A.; Chou, Y.-M.; Nuyken, O. Macromolecules 2001, 34, 1606. (15) Bo¨ttcher, H.; Hallensleben, M. L.; Nuss, S.; Wurm, H. Polym. Bull. (Berlin) 2000, 44, 223. (16) von Werne, T.; Patten, T. E. J. Am. Chem. Soc. 1999, 121, 7409. (17) Perruchot, C.; Khan, M. A.; Kamitsi, A.; Armes, S. P.; von Werne, T.; Patten, T. E. Langmuir 2001, 17, 4479. (18) von Werne, T.; Patten, T. E. J. Am. Chem. Soc. 2001, 123, 7497. (19) Pyun, J.; Matyjaszewski, K.; Kowalewski, T.; Savin, D.; Patterson, G.; Kickelbick, G.; Huesing, N. J. Am. Chem. Soc. 2001, 123, 9445.

10.1021/la011630x CCC: $22.00 © 2002 American Chemical Society Published on Web 04/03/2002

Nanoparticles with Hyperbranched Polymer Shells

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Scheme 1. SCVP of an AB* Inimer (BPEA) from a Functionalized Silica Particle (B*f)a

a

Capital letters indicate vinyl groups (A) and active centers (A*, B*), and lowercase letters stand for reacted ones (a, b).

can be initiated to further chain growth. Synthesis of hybrid nanoparticles possessing well-defined tethered block copolymers by using ATRP has been also reported by Pyun et al.19 Herein, we report the synthesis of hyperbranched polymer-silica hybrid nanoparticles by modifying the surface of silica nanoparticles with initiators for ATRP and by using the functionalized particles for self-condensing vinyl polymerization (SCVP). In contrast to previous reports, we have focused on the synthesis of core-shell nanostructures via the covalent attachment of hyperbranched polymers to inorganic nanoparticles. Highly branched polymers play an increasingly important role in interface and surface sciences, since their distinctive chemical and physical properties can be used advantageously as functional surfaces and as interfacial materials. Due to the highly compact and globular shape as well as the monodispersity, for example, dendrimers attached to flat surfaces20-23 are useful for many applications, such as data storage or nanolithography systems. The surface chemistry and interfacial properties of hyperbranched polymers have also become a field of growing interest.24-31 A highly branched poly(acrylic acid) film attached to a flat gold surface has been successfully applied for a number of technical applications including corrosion (20) Hierlemann, A.; Campbell, J. K.; Baker, L. A.; Crooks, R. M.; Ricco, A. J. J. Am. Chem. Soc. 1998, 120, 5323. (21) Li, J.; Piehler, L. T.; Qin, D.; Baker, J. R., Jr.; Tomalia, D. A.; Meier, D. J. Langmuir 2000, 16, 5613. (22) Tully, D. C.; Trimble, A. R.; Fre´chet, J. M. J.; Wilder, K.; Quate, C. F. Chem. Mater. 1999, 11, 2892. (23) Tully, D. C.; Fre´chet, J. M. J. Chem. Commun. (Cambridge, U.K.) 2001, 1229. (24) Bergbreiter, D. E.; Tao, G.; Franchina, J. G.; Sussman, L. Macromolecules 2001, 34, 3018. (25) Bergbreiter, D. E.; Tao, G. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 3944. (26) Fujiki, K.; Sakamoto, M.; Sato, T.; Tsubokawa, N. J. Macromol. Sci., Pure Appl. Chem. 2000, A37, 357. (27) Shirai, Y.; Shirai, K.; Tsubokawa, N. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 2157. (28) Hayashi, S.; Fujiki, K.; Tsubokawa, N. React. Funct. Polym. 2000, 46, 193. (29) Mackay, M. E.; Carmezini, G.; Sauer, B. B.; Kampert, W. Langmuir 2001, 17, 1708. (30) Beyerlein, D.; Belge, G.; Eichhorn, K.-J.; Gauglitz, G.; Grundke, K.; Voit, B. Macromol. Symp. 2001, 164, 117. (31) Voit, B. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 2505.

inhibition, chemical sensing, cellular engineering, and micrometer-scale patterning, due to an extremely high density of functional groups at the surface.32 Recently, there was an interesting report describing the use of spherical substrates for the synthesis of hyperbranched polymers. Bharathi and Moore reported a new synthetic procedure in which slow addition of AB2 monomer takes place on an insoluble solid support, providing hyperbranched polymers with low polydispersity and controlled molecular weight.33,34 In a previous paper,35 we demonstrated a novel synthetic approach to prepare hyperbranched polymers grafted from a planar surface in which a silicon wafer grafted with an R-bromoester type initiator layer was used for SCVP via ATRP. This reaction is based on an initiator-monomer (“inimer”) of the general structure AB*, where the double bond is designated A and B* is a group capable of initiating the polymerization of vinyl groups. In this way, we were able to create novel surface architectures, in which the characteristic nanoprotrusions with different densities and sizes are composed of hyperbranched polymers tethered directly to the flat surface. In the present work, the silica particles densely covered with a monolayer of a 2-bromoisobutyryl fragment (B*) were used as a core molecule of the structure B*f (representing a polyfunctional initiator), instead of a planar surface, as can be seen in Scheme 1. Because both the AB* inimer and the functionalized silica particles have groups capable of initiating the polymerization, the chain growth can be started from both B* in the initiators immobilized on the (32) (a) Zhou, Y.; Bruening, M. L.; Bergbreiter, D. E.; Crooks, R. M.; Wells, M. J. Am. Chem. Soc. 1996, 118, 3773. (b) Bruening, M. L.; Zhou, Y.; Aguilar, G.; Agee, R.; Bergbreiter, D. E.; Crooks, R. M. Langmuir 1997, 13, 770. (c) Peez, R. F.; Dermody, D. L.; Franchina, J. G.; Jones, S. J.; Bruening, M. L.; Bergbreiter, D. E.; Crooks, R. M. Langmuir 1998, 14, 4232. (d) Ghosh, P.; Amirpour, M. L.; Lackowski, W. M.; Pishko, M. V.; Crook, R. M. Angew. Chem., Int. Ed. Engl. 1999, 38, 1592. (e) Franchina, J. G.; Lackowski, W. M.; Dermody, D. L.; Crooks, R. M.; Bergbreiter, D. E.; Sirkar, K.; Russell, R. J.; Pishko, M. V. Anal. Chem. 1999, 71, 3133. (f) Zhou, M.; Bruening, M. L.; Zhou, Y.; Bergbreiter, D. E.; Crooks, R. M. Isr. J. Chem. 1997, 37, 277. (g) Aoki, A.; Ghosh, P.; Crooks, R. M. Langmuir 1999, 15, 7418. (33) Bharathi, P.; Moore, J. S. J. Am. Chem. Soc. 1997, 119, 3391. (34) Bharathi, P.; Moore, J. S. Macromolecules 2000, 33, 3212. (35) Mori, H.; Bo¨ker, A.; Krausch, G.; Mu¨ller, A. H. E. Macromolecules 2001, 34, 6871.

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silica particles and a B* group in the inimer. Both of the activated B* groups can add to the double bond, A, to form the ungrafted or grafted dimer with a new propagating center, A*. Further addition of AB* inimer or dimer to A* and B* centers results in hyperbranched polymers. Welldefined hyperbranched polymers grafted on the silica particles should contain a high density of bromoester end groups at the outermost surface, because a bromine atom acting as a branching point migrates to an end group of the branched chain formed by the next monomer addition, and repetition of this step would bring a bromine atom to the outer surface. Such surface multifunctionality is ideally independent of the surface curvature of the core particle and the layer thickness of the polymer shell, which could not be achieved by linear polymers. The hybrid nanoparticles comprised of silica cores and hyperbranched polymer shells should have interesting properties for many applications, after suitable modification of the end groups. Introduction of functional groups, such as carboxylic acid groups, also can be also attained by copolymerization of the AB* inimer and a protected monomer (tert-butyl acrylate), followed by hydrolysis of tert-butyl groups. These polymers grafted on the nanoparticles can be designed to have a fairly open structure, allowing the functional materials, such as metal ions, to penetrate the film more easily than in conventional linear polymer layers. Thus, a well-controlled synthesis for these materials can lead to the creation of novel core-shell hybrids that are controllable on the nanoscopic scale and have chemically sensitive interfaces. Experimental Section Materials and Monomers. CuBr (95%, Aldrich) was purified by stirring overnight in acetic acid. After filtration, it was washed with ethanol and diethyl ether and then dried. N,N,N′,N′′,N′′Pentamethyldiethylenetriamine (PMDETA, 99%, Aldrich) and ethyl 2-bromo-2-isobutyrate (EBIB, 98%, Aldrich) were distilled and degassed. tert-Butyl acrylate (tBuA, BASF AG) was fractionated from CaH2, stirred over CaH2, and distilled and degassed in high vacuum. Other reagents were commercially obtained and used without further purification. Synthesis of an acrylic AB* inimer, 2-(2-bromopropionyloxy)ethyl acrylate (BPEA), was conducted by the reaction of an R-bromoacid halide with 2-hydroxyethyl acrylate in the presence of pyridine as reported previously.35,36 The inimer was degassed by three freeze-thaw cycles. The hydrophilic silica gel (Aerosil 200, Degussa) used in this study had a specific surface area of 200 ( 25 m2/g (determined by the Brunauer-Emmett-Teller method) and a mean particle diameter of 16 nm. It was dehydrated prior to reaction by heating at 120 °C in vacuo for 24 h. Synthesis of the r-Bromoester Type Initiator on Silica Particles. The preparation of the R-bromoester initiator attached to silica particles was conducted by the reaction of the crude trichlorosilyl derivative with silica particles in a manner similar to that reported previously (Scheme 2).35,37 The trichlorosilyl R-bromoester, which has reactive species capable of bonding to the silica surface and a latent R-bromoester, was prepared by the reaction of hex-5-enol with 2-bromo-isobutyryl bromide, followed by the hydrosilylation reaction with trichlorosilane. To 1.0 g of Aerosil 200 dispersed in 40 mL of dry toluene, 4 mL of toluene solution of the crude trichlorosilyl derivative (ca. 0.24 mmol) was added dropwise under nitrogen. Then, triethylamine (1 mL, 7.2 mmol) was added dropwise to the mixture, and the reaction mixture was stirred at room temperature overnight. The mixture was filtered and rinsed repeatedly with toluene, methanol, and dichloromethane. The white product obtained was then dried in vacuo at room temperature overnight and stored at room temperature in a drybox. (36) Matyjaszewski, K.; Gaynor, S. G.; Kulfan, A.; Podwika, M. Macromolecules 1997, 30, 5192. (37) Husseman, M.; Malmstro¨m, E. E.; McNamara, M.; Mate, M.; Mecerreyes, D.; Benoit, D. G.; Hedrick, J. L.; Mansky, P.; Huang, E.; Russell, T. P.; Hawker, C. J. Macromolecules 1999, 32, 1424.

Mori et al. Scheme 2. Synthetic Route for the r-Bromoester Type Initiator Grafted on a Silica Particle

Elemental analysis yielded the following: C, 1.87; H, 0.32; Br, 1.02; residue (SiO2), 96.26. The graft densities of the initiator molecules were determined from the elemental analysis using the bromine contents or the content of residual SiO2 remaining after combustion. Elemental analysis also indicated that the starting silica nanoparticle contained 0.48% carbon, 0.13% hydrogen, and undetectable Br. The weight fraction of the initiator molecule was determined by the ratio of the fraction of the respective element (C or Br) in the sample to that in the initiator (silane compound without Cl). The surface coverage was calculated from the weight fraction and the specific surface area of the bare substrate (200 m2/g). Polymerization. All polymerizations were carried out in a round-bottom flask sealed with a plastic cap under nitrogen. A representative example is as follows: The functionalized silica particle (0.122 g) was added to a round-bottom flask containing degassed BPEA (2.178 g, 8.68 mmol) and CuBr(I) (0.0100 g, 0.0697 mmol). As soon as PMDETA (0.0121 g, 0.0698 mmol) was added, the system became green, indicating the start of the polymerization. The heterogeneous mixture was stirred at 30 °C and became viscous gradually. After 2 h, the polymerization was stopped by cooling. Conversion of the double bonds detected by 1H NMR of the ungrafted material was >99%. The reaction mixture was dissolved in tetrahydrofuran (THF), and the polymer-silica hybrid particles were separated from the mixture by filtration using a extraction thimble (Schleicher & Schuell GmbH, 603 grade, made of cellulose fibers) and rinsed with THF. Afterward, any absorbed polymer formed was removed from the silica particles by Soxhlet extraction in THF for 24 h. The residue was dried under vacuum to give polymer-silica particles (0.515 g, corresponding to 3.2 g polymer/g silica). The weight fraction of the grafted polymer was determined either by the difference in weight between the functionalized silica particles in feed and the recovered polymer-silica particles after purification by Soxhlet extraction or by the ratio of the measured carbon content in the remaining products to the calculated one in the polymer. The surface coverage, Γ (mg/m2), was calculated from the weight fraction of the grafted polymer and the specific surface area of the bare substrate (200 m2/g), and the grafting density, Σ (chains/ nm2), of polymer layers was calculated by the following equation:

Σ ) Γ × 10-21 NA/Mn(absolute) where NA is Avogadro’s number and Mn is the absolute numberaverage molecular weight estimated by gel permeation chromatography (GPC)/viscosity of the soluble polymers. After the polymerization, characterization of the soluble polymers was conducted by conventional GPC, GPC/viscosity, and 1H NMR measurements. The soluble polymer had Mn ) 30 800 and Mw/Mn ) 2.42 (as determined by GPC/viscosity using universal calibration), compared to Mn ) 4600 and Mw/Mn ) 2.32 (as determined by GPC using linear polystyrene standards). The determinations of the proportion of B* in the ungrafted polymers, the reactivity ratio of A* and B* groups, r ) kA/kB, and the degree of branching (DB) were performed by evaluation of 1H NMR spectra according to a method reported previously.35,36,38,39 Under (38) Matyjaszewski, K.; Gaynor, S. G.; Mu¨ller, A. H. E. Macromolecules 1997, 30, 7034. (39) Yan, D.; Mu¨ller, A. H. E.; Matyjaszewski, K. Macromolecules 1997, 30, 7024.

Nanoparticles with Hyperbranched Polymer Shells the condition described above, b (fraction of reacted B* units) ) 0.38 was observed, corresponding to r ) kA/kB ) 6.3, and DB ) 0.44. Similarly, an acrylic AB* inimer, BPEA, was copolymerized with tBuA in the presence of the functionalized silica particles. BPEA (0.500 g, 1.99 mmol) was added to a round-bottom flask containing the functionalized silica particles (0.110 g), CuBr(I) (0.0172 g, 0.120 mmol), PMDETA (0.0308 g, 0.178 mmol), and tBuA (1.56 g, 12.2 mmol). The flask was placed in an oil bath at 60 °C for 2 h. Conversion of the double bonds detected by 1H NMR of the ungrafted material was >95%. After the purification of the insoluble fraction by Soxhlet extraction in THF for 24 h, the residue was dried under vacuum to give a polymer-silica hybrid particle (0.250 g). The soluble polymer had Mn ) 16 800 and Mw/Mn ) 4.23 (as determined by GPC/viscosity) and Mn ) 8100 and Mw/Mn ) 4.36 (as determined by GPC using linear PtBuA standards). The ungrafted polymers obtained by the copolymerizations were removed in a manner similar to that obtained by SCVP of BPEA. Hydrolysis of Branched Poly(tert-butyl acrylate) Grafted on Silica Particles. The branched PtBuA-silica hybrid nanoparticles (0.154 g, 0.65 mmol ester) were dissolved in dichloromethane (3 mL), and an about 7-fold molar excess of trifluoroacetic acid (0.5 g, 4.4 mmol, with respect to the ester groups) was added. The heterogeneous mixture was stirred at room temperature for 24 h. The nanoparticles were separated by filtration or decantation, washed with dichloromethane repeatedly, and thoroughly dried under vacuum at 50 °C. Characterization. After preparation, the polymers only absorbed to the surface were washed with THF and subsequently passed through a neutral alumina column to remove the catalyst residues. The remaining solution was characterized by GPC using THF as the eluent at a flow rate of 1.0 mL/min at room temperature. Two GPC systems were used in order to get apparent molecular weights as well as absolute ones. For GPC system I, the column set was as follows: 5 µm PSS SDV gel, 102, 103, 104, 105 Å, 30 cm each. The detectors were a Waters 410 differential refractometer and a Waters photodiode array detector operated at 254 nm. Narrow PS standards (PSS, Mainz) were used for the calibration of column set I. The calibration curve of PtBuA was also used for the polymers obtained by homo- and copolymerizations of tBuA. For GPC system II, the column set was as follows: 5µ PSS SDV gel, 103 Å, 105 Å and 106 Å, 30 cm each. The detectors were a Shodex RI-71 refractive index detector, a Jasco uvidec-100-III UV detector (λ ) 254 nm), and a Viscotek viscosity detector H 502B. Absolute molecular weights of soluble polymers were determined by universal calibration40 using the viscosity module of the PSS-WinGPC scientific V 6.1 software package. 1H NMR spectra were recorded in CDCl3 with a Bruker AC-250. Fourier transform infrared (FT-IR) spectra were recorded on a Bruker Equinox 55 spectrometer. The samples containing silica particles were prepared as KBr pellets, while the analysis of viscous polymers was performed with NaCl plates. The elemental analyses were performed by Ilse Beetz Mikroanalytisches Laboratorium (Kulmbach). Morphology and average particle sizes of the polymer-silica hybrid particles were characterized using transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), scanning force microscopy (SFM), and dynamic light scattering (DLS). Bright field TEM was performed using a Zeiss electron microscope (CEM 902) operated at 80 kV. The samples for TEM observation were prepared by mounting a drop of a diluted THF suspension (1 mg/100 mL) on carbon-coated Cu grids and allowing the samples to dry in air. SFM height and phase images were taken on a Digital Instruments Dimension 3100 microscope operated in Tapping Mode (free amplitude of the cantilever ≈ 30 nm, set point ratio ≈ 0.98). The samples were prepared on polished silicon wafers by dip-coating from 1 mg/ 100 mL suspensions in THF. The same samples prepared on silicon wafers were directly characterized by FE-SEM. FE-SEM was performed using a LEO 1530 Gemini microscope (acceleration voltage, 1.0 kV). DLS was performed at room temperature on an ALV DLS/SLS-SP 5022F compact goniometer system with (40) Benoıˆt, H.; Grubisic, Z.; Rempp, P.; Decker, D.; Zilliox, J. G. J. Chem. Phys. 1966, 63, 1507.

Langmuir, Vol. 18, No. 9, 2002 3685 a He-Ne laser (λ0 ) 632.8 nm) using an ALV 5000/E correlator. Prior to the light scattering measurements, THF suspensions (2 mg/mL for the functionalized silica particle, 1 mg/mL for the polymer-silica particles) were filtered using Millipore Teflon filters with a pore size of 3.0 µm into a dust-free cylindrical cuvette. The CONTIN analysis of the autocorrelation functions was employed. By fitting an integral type model function to the correlation function using a constrained regularization method, one can get the intensity weighted decay time distribution function. Then, under the assumption that the scattering particles behave as hard spheres in dilute solution and within the Rayleigh-Debye theory the particle radius distribution function is calculated from the decay time distribution function using the Stokes-Einstein equation. After measurements at four or five different angles (30, 60, 90, 120, and 150°), the hydrodynamic radius of the species was obtained by extrapolation to q2 f 0.

Results and Discussion Preparation of the r-Bromoester Type Initiator on Silica Particles. In this study, the R-bromoester group was selected as a group capable of initiating from both the functionalized silica surface and the AB* inimer. The formation of a 2-bromoisobutyryl fragment (B*) layer on the silica surface was conducted by the reaction of the crude trichlorosilyl compound with silica particles. Subsequently, the sample was washed repeatedly in order to remove free attachable initiator from the monolayer. Elemental analysis indicated that the modified nanoparticles contained, on average, 1.02% bromine, which corresponds to a weight fraction of the initiator molecule of 3.54%. This is comparable with the values determined using carbon content (3.21%) or the SiO2 residue remaining after combustion (3.74%). From this value, it can be derived that about 0.13 mmol initiator/g of silica particles and 0.4 initiator molecules/nm2 are immobilized on the silica particles, corresponding to about 320 initiator molecules per silica particle. The number of initiator molecules per area obtained in this study is comparable to the values (0.8-1.6 initiator molecules/nm2) reported in the case of azo type initiators grafted on silica surfaces (surface area ) 285 m2/g) obtained under similar reaction conditions.10 SCVP from the Functionalized Silica Particles. Bulk polymerization of an acrylic AB* inimer, BPEA, with the functionalized silica particles was carried out in the presence of CuBr(I) and PMDETA at the ratio of [BPEA]0/ [CuBr]0/[PMDETA]0 ) 125:1:1. The reaction was performed at a weight ratio of BPEA to functionalized silica particles ≈ 18, which translates into a molar ratio of [BPEA]0/[grafted initiators]0 ≈ ca. 550. The ratio corresponds to the ratio of number of B* groups in solution to that grafted on the surface. Under that condition, full conversion was reached after 2 h at 30 °C. The weight fraction of the surface-attached poly(BPEA) chains obtained after Soxhlet extraction was ca. 3.2 g polymer/g silica. As can be seen in Table 1, the value decreases with decreasing [BPEA]0/[CuBr]0 ratio, which is the same tendency observed in SCVP of BPEA on a flat silicon surface.35 The surface coverage calculated by the elemental analysis and specific surface area of the bare silica substrate (200 m2/g) is 9.9-21 mg/m2. These values are several times higher than that obtained on the flat silicon surface under similar conditions,35 which may be due to the differences in the surface area and surface curvature of the substrates. Surface crowding should be significant in the chain growth of hyperbranched polymers from the flat surface, while higher surface curvature of the spherical nanoparticle can help to reduce the steric crowding during the polymerization. The grafting densities estimated from the surface coverage are approximately 0.25-0.4 chains/

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Table 1. Polymerization of BPEA with the Functionalized Silica Particles entrya

[BPEA]0/[catalyst]0

time (h)

g polymer/g SiO2

fraction of inimer grafted on SiO2 (%)

carbon contentb (%)

surface coveragec (mg/m2)

grafting densityd (chains/nm2)

1 2 3e

125 50 50

2 0.5 24

3.2 2.1 1.1

18 12 6

32.9 28.0 15.2

21.3 9.9 2.7

0.41 0.25 0.19

a Polymerization with CuBr(I)/N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA) at 30 °C. The BPEA/SiO ratio in the feed is 2 18 wt %. Conversion of the double bonds as determined by 1H NMR was >95% in all cases. b Determined by elemental analysis. c Calculated from carbon content and surface area of silica (200 m2/g). d Grafting density ) surface coverage × NA (Avogadro’s number)/Mn(absolute).e Ethyl acetate (50 vol % to BPEA) was used as a solvent.

Figure 1. FT-IR spectra of unmodified silica particles (a); poly(BPEA) grafted on silica particles (see Table 1), surface coverage ) 2.7 mg/m2 (b), 9.9 mg/m2 (c), and 21.3 mg/m2 (d); and poly(BPEA) obtained in solution (e).

nm2, suggesting that more than half of the potential initiators act as initiating groups for SCVP. Both ungrafted and grafted polymers are produced in this system, since chain growth can be started from both the B* initiators immobilized on the silica particles and from B* groups in the inimers. Calculation from the weight fraction of the grafted polymers shows that about 10-20% of BPEA polymerizes from the surface of the silica nanoparticles under the conditions used in this study, and the values are apparently higher than that on the flat silicon surface. Remaining BPEA acts as an AB* monomer for normal SCVP to produce ungrafted polymer, which was used for the characterization of molecular parameters. The polymerization of BPEA with the functionalized silica particles was also conducted in solution, aiming to control both the grafting density and the morphology of the resulting hyperbranched polymer-silica hybrid nanoparticles. When the polymerization was conducted in ethyl acetate (50 vol % to BPEA), the fluid reaction mixture was kept without solidification even after the conversion of the AB* inimer reached more than 95%. However, usage of ethyl acetate as a polymerization solvent led to a decrease in the weight fraction of the grafted polymer and the surface coverage. As described later, the characterization of the soluble polymer indicates that the molecular weights and the degree of branching are lower than those obtained in the bulk polymerization. The successful preparation of hyperbranched polymersilica hybrid nanoparticles was confirmed by FT-IR measurements and elemental analyses. The FT-IR spectra of the hybrid particles obtained by SCVP from the functionalized silica particles are depicted in Figure 1 and are characterized by the C-H stretching vibration between

2840 and 3040 cm-1 and by the strong CdO vibration at 1740 cm-1. These peaks are also found to be characteristic of the soluble hyperbranched polymer obtained in solution. As can be seen in Figure 1b-d, the intensities of these peaks increase with increasing surface coverage. On the other hand, the broad band around 1100 cm-1 assigned to the Si-O stretching vibration on silica is observed clearly. Hence, the spectra of the hybrid particles obtained by SCVP from the functionalized silica particles appear as the superposition of the spectra of the two components, silica and poly(BPEA), suggesting that the hyperbranched polymers are covering the silica surface. To obtain quantitative information about the chemical structure of the covalently attached polymer layers, the atomic compositions of several samples were investigated by elemental analysis. For example, the atomic composition of sample 2 (see Table 1) was as follows: C, 28.0; H, 3.5; Br, 18.8. The relative atomic composition of the organic part is in agreement with the value (C, 25.1; H, 2.9; Br, 20.8) calculated from the weight fraction of the grafted polymer determined by gravimetry. These experimental data suggest that SCVP of the AB* inimer from the surface of the silica nanoparticle yields hybrid particles comprised of a silica core and grafted hyperbranched polymer layers having multifunctional bromoester end groups. To prove that polymers simply absorbed to the silica surface can be readily removed by the extraction described above, a control experiment with a mixture of the functionalized silica particles and the ungrafted soluble poly(BPEA) formed in solution was conducted. The mixture was stirred in THF at room temperature for 1 h, and then the remaining particles were washed by Soxhlet extraction in THF for 24 h. As expected, only a tiny amount of polymer could be attached to the surfaces of the functionalized silica particles. From elemental analysis, the grafting density of the attached material was calculated to be 2 mg polymer/g SiO2, which is about 3 orders of magnitude lower than that of the materials prepared by SCVP from the functionalized silica particles. The negligible amount of absorbed polymer was also confirmed by FT-IR measurement of the remaining particles after Soxhlet extraction of the mixture. The result indicates that the remaining polymer is covalently linked to the silica particles and any loosely bound polymer chains are completely removed by the treatment described above. To understand the growth characteristics of the surfaceinitiated polymerization as well as the molecular parameters of the grafted polymers, it is necessary to cleave these chains from the surface at their points of attachment. However, attempts to achieve this were unsuccessful in the case of the hybrids prepared by the technique described here. For example, treatment of the hyperbranched polymer-silica hybrid particles in toluene suspension with aqueous HF (5%) and a phase transfer catalyst, which has been used to detach polystyrene or poly(methyl methacrylate) (PMMA) grafted to a silica surface,16,18 was found to only give only low molecular weight products. This may be due to chain scission reactions on ester


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Table 2. Characterization of Soluble Polymers Obtained by SCVP samplea

Mn,GPCb (Mw/Mn)

Mn,GPC-VISCO (Mw/Mn)

Rc

bd

re

DBf

1 2 3

4600 (2.32) 4600 (2.85) 2400 (2.12)

30800 (2.42) 23800 (3.01) 8500 (2.07)

0.38 0.36 0.34

0.38 0.34 0.28

6.3 8.4 14.6

0.44 0.43 0.26

a See Table 1. b Calibrated against polystyrene standards. Mark-Houwink exponent as determined by GPC/viscosity measurement. d Fraction of reacted B* units as determined by 1H NMR. e r ) k /k , as determined according to ref 38. f Degree of branching A B as determined according to ref 39. c

linkages of the poly(BPEA), which was also confirmed by NMR measurements of the soluble fraction and FT-IR measurement of the remaining insoluble part. Therefore, characterization of the ungrafted soluble polymers obtained in solution, combined with theoretical considerations, was conducted in order to get information on the molecular parameters of the hyperbranched polymers. The unattached polymer chains were separated from the covalently bound polymers by Soxhlet extraction with an appropriate solvent (THF) and used to estimate the molecular weight, the molecular weight distribution, and the degree of branching. These results are summarized in Table 2. The soluble polymers obtained in the bulk polymerization at a ratio of [BPEA]0/[CuBr]0 ) 50 and 125 showed apparent number-average molecular weight Mn ) 4600 and polydispersity index Mw/Mn ) 2.3 and 2.9, respectively (as determined by GPC using linear polystyrene standards). To obtain true molecular weights, a viscosity detector and universal calibration were used. The absolute molecular weights (Mn,GPC-VISCO ) 30 800 and 23 800, respectively) are much higher than the corresponding apparent one, indicating a lower hydrodynamic volume of the hyperbranched polymers. However, the polydispersity is not strongly affected. The [BPEA]0/ [CuBr]0 ratio has a slight effect only on the molecular weights and the polydispersity of the soluble polymers. Use of a polymerization solvent leads to the decrease in the molecular weights. In all cases, low values of the Mark-Houwink exponent (R ) 0.34-0.38) are observed, showing undoubtedly a densely packed three-dimensional structure resulting from the hyperbranched topology. The reactivity ratio, r ) kA/kB ) 6.3-8.3, and the degree of branching, DB ) 0.43-0.44, which can be calculated from evaluation of 1H NMR spectra of the soluble polymers obtained in the bulk polymerization system, also indicate the achievement of the highly branched structures. We have reported a series of theoretical studies of the SCVP,39,41-45 which give valuable information, such as the kinetics, the molecular weights, the molecular weight distribution, and the degree of branching. The calculated molecular weight distribution of polymers formed in SCVP without initiators (conventional SCVP in bulk or solution) is broader than that obtained from SCVP in the presence of f-functional initiators.42,45 The presence of core-forming molecules (a core molecule of the structure B*f, see Scheme 1) leads to a considerable narrowing of the polydispersity index, which decreases with increasing initiator func(41) Mu¨ller, A. H. E.; Yan, D.; Wulkow, M. Macromolecules 1997, 30, 7015. (42) Radke, W.; Litvinenko, G. I.; Mu¨ller, A. H. E. Macromolecules 1998, 31, 239. (43) Litvinenko, G. I.; Simon, P. F. W.; Mu¨ller, A. H. E. Macromolecules 1999, 32, 2410. (44) Simon, P. F. W.; Mu¨ller, A. H. E. Macromol. Theory Simul. 2000, 9, 621. (45) Yan, D.; Zhou, Z.; Mu¨ller, A. H. E. Macromolecules 1999, 32, 245.

tionality, f. Thus, the molecular weight and molecular weight distribution of the ungrafted polymer obtained in solution might be different from those of the grafted polymer produced by a surface-initiated SCVP. On the other hand, the effect of the f-functional initiators on the degree of branching was calculated to be negligible under batch conditions used here (inimers and initiators grafted on the surface are mixed instantaneously).42 This indicates that the degree of branching does not depend on whether polymer is formed in solution or on the surface of particles. Therefore, it is reasonable to suppose that SCVP of BPEA with functionalized silica particles provides surfacegrafted poly(BPEA) having a highly branched structure, even if the correlation of the molecular parameters of the soluble polymers with those of the polymers grafted on the surface is not confirmed experimentally. Copolymerization of Inimer with tert-Butyl Acrylate From the Functionalized Silica Particles. Since one of our targets is the preparation of a variety of branched polymer-silica hybrid particles, we also attempted to prepare surface-grafted branched polymers by selfcondensing vinyl copolymerization (SCVCP). In this study, tBuA was selected as a comonomer, because it is a protected precursor for poly(acrylic acid) (PAA) and has a variety of possible applications. For example, Sondi et al. reported encapsulation of nanosized silica by conventional radical polymerization of tBuA and the use of the system in the encapsulated inorganic resist technology.11 Poly(acrylic acid)-silica composites have been extensively investigated, owing to many scientific and industrial applications, such as biomaterial carriers,46,47 intelligent environment-responsive surfaces,1 thermal reinforcement,48,49 and optical chemical sensing.50 Further, it has been shown that SCVCP of an AB* inimer with a conventional vinyl monomer in the presence of the functionalized flat silicon surface leads to grafted branched polymers, allowing for the control of the film thickness, surface morphology, roughness, and chemical structure.35 Thus, SCVCP in the presence of the functionalized silica particles is a reliable method to prepare functional branched polymer-silica hybrid nanoparticles, in which one can tune the architecture, chemical and physical properties, and particle morphology by the choice of comonomers and their composition in the feed. The surface-initiated polymerization can be initiated in two ways (Scheme 3): (i) the addition of the active B* group of the functionalized silica particle to the vinyl group A of the AB* inimer forming a dimer with two active sites, A* and B*, and (ii) the addition of a B* group to the vinyl group of monomer M forming a dimer with one active site, M*. The formation of ungrafted dimers, which should have one double bond, takes place at the same time. Both the initiating B* group and the newly created propagating centers A* and M* can react with any vinyl group in the system. Thus, we have three different types of active centers, A*, B*, and M* on the silica surface, which can react with double bonds A (inimer and macromolecules; each macromolecule contains strictly one double bond) and M (monomer). Bulk copolymerization of BPEA with tBuA was conducted with CuBr/PMDETA in the presence of the functionalized silica particles. Homopolymerization of (46) Shimomura, M.; Kikuchi, H.; Matsumoto, H.; Yamauchi, T.; Miyauchi, S. Polym. J. (Tokyo) 1995, 27, 974. (47) Yoshinaga, K.; Kondo, K.; Kondo, A. Colloid Polym. Sci. 1997, 275, 220. (48) Chan, C. K.; Chu, I. M. Polymer 2001, 42, 6089. (49) Caykara, T.; Gu¨ven, O. J. Appl. Polym. Sci. 1998, 70, 891. (50) Shi, Y.; Seliskar, C. J. Chem. Mater. 1997, 9, 821.

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Scheme 3. Initial Steps in Copolymerization of an AB* Inimer with a Conventional Monomer (M) from a Silica Particlea

Figure 2. FT-IR spectra of (a) branched PtBuA (γ ) 1.1) obtained in solution, (b) branched PtBuA (γ ) 1.1) grafted on silica particles, (c) branched PtBuA (γ ) 6.1) grafted on silica particles, and (d) linear PtBuA grafted on silica particles.

a Capital letters indicate vinyl groups (A and M) and active centers (A*, B*, M*), and lowercase letters stand for reacted ones (a, b, m).

tBuA from the functionalized silica particles with EBIB as a controlling initiator (B*) was also conducted as a comparison. Table 3 summarizes characteristic features involving the surface coverage and the atomic compositions determined by elemental analysis of the resulting polymer-silica hybrid particles, as well as the molecular parameters of the soluble polymers. The conditions for the copolymerizations were adjusted to yield polymers quantitatively (the conversion determined by 1H NMR was >95% in both cases). When the copolymerization was carried out at a comonomer ratio of [tBuA]0/[BPEA]0 ) γ ) 1.1 or 6.1, the surface coverage was about 15-20 mg/ m2. These values are comparable to that of the grafted polymer obtained by SCVP of the AB* inimer. The relatively low apparent molecular weights of the resulting polymers may be due to the highly branched structure, as branched polymers have smaller hydrodynamic volumes than their linear analogues. The difference between the apparent molecular weights determined by GPC and the absolute one evaluated by GPC/viscosity also supports the highly branched structures. The BPEA composition

in the resulting copolymers determined by 1H NMR is in good agreement with the comonomer composition in the feed which corresponds to the γ value. The homopolymerization of tBuA from the functionalized silica particles resulted in lower surface coverage and gave soluble polymers with a number-average molecular weight close to the predicted value and a relatively broad polydispersity. Figure 2 shows the FT-IR spectra of branched (b,c) and linear (d) PtBuA’s grafted on silica particles and of soluble branched polymer (a). The absorption bands due to the carbonyl (1735 cm-1) and the C-H stretching vibrations (2800-3050 cm-1) are clearly visible in the grafted and ungrafted polymers involving higher BPEA content (γ ) 1.1). The spectra of the branched PtBuA having lower BPEA content (γ ) 6.1) and the linear PtBuA grafted on silica particles reveal the carbonyl peak at 1730 cm-1. The broad band around 1100 cm-1 resulting from Si-O stretching on silica is observed in all the grafted samples. FT-IR spectra of the products obtained by SCVCP from the functionalized silica particles show vibration bands corresponding to both the branched polymer and silica particles. Elemental analysis was used to determine the chemical composition of the products obtained by the copolymerization. The bromine contents of the branched polymer-silica hybrid nanoparticles (4.9% for entry 5 and 12.0% for entry 4) are dependent upon the comonomer composition in the feed and are apparently lower than that of the polymer obtained by SCVP (18.8% for entry 2). The value of the linear PtBuA-silica hybrid nanoparticles is comparable to that of the functionalized silica particles before the polymerization. These results are in qualitative

Table 3. Synthesis and Characterization of Highly Branched and Linear Poly(tert-butyl acrylate)s entrya

γb

carbon contentc (%)

Br contentc (%)

surface coveraged (mg/m2)

Mn,GPCe (Mw/Mn)

Mn,GPC-VISCO (Mw/Mn)

BPEA ratio in polymerf (mol %)

4g 5g 6h

1.1 6.1 ∞

37.6 46.5 6.7

12.0 4.9 0.76

19.8 15.5 0.40

4800 (4.29) 8100 (4.36) 19000 (1.60)

8300 (5.41) 16800 (4.23) 19800 (1.59)

45 14 0

a Bulk polymerization at 60 °C with CuBr/PMDETA at a ratio of ([tBuA] + [BPEA] )/[CuBr] ) 105-125. Conversion of double bonds 0 0 0 as determined by 1H NMR was >95% in all cases. b γ ) [tBuA]0/[BPEA]0. c Determined by elemental analysis. d Calculated from carbon content and surface area of silica (200 m2/g). e Calibrated against PtBuA standards. f Determined by 1H NMR. g Copolymerization of BPEA and tBuA for 2 h. h Homopolymerization of tBuA for 15 h in the presence of EBIB (equimolar to catalyst).


Figure 3. FT-IR spectra of (a) branched PtBuA (γ ) 6.1) obtained in solution, (b) soluble branched PAA (γ ) 6.1), (c) branched PAA (γ ) 6.1) grafted on silica particles, (d) branched PAA (γ ) 1.1) grafted on silica particles, and (e) a mixture of soluble branched PAA (γ ) 6.1) and unmodified silica particles.

agreement with the assumption: on the average, the hyperbranched polymers obtained by SCVP carry one bromoester function per monomer unit, while the functionality decreases with increasing γ. The significant difference in the bromine content among the hybrids grafted with the hyperbranched poly(BPEA), branched copolymer of BPEA and tBuA, and linear PtBuA suggests the feasibility of controlling the surface chemical functionalities on the outermost surface of the polymer-silica hybrid nanoparticles. The tert-butyl ester groups in the branched PtBuA grafted on silica particles were hydrolyzed using a 5-8 excess of trifluoroacetic acid at room temperature for 24 h. The hydrolysis of soluble polymers was also conducted as a comparison. Figure 3a,b shows the FT-IR spectra of the soluble branched PtBuA (γ ) 6.1) before and after hydrolysis. The spectrum of the branched PtBuA is characterized by the C-H stretching vibration between 2840 and 2940 cm-1 and by the strong CdO vibration at 1730 cm-1. After hydrolysis, the acid functionality is clearly visible as the broad absorption from 2400 to 3800 cm-1. The carbonyl stretch shifts slightly (1725 cm-1), corresponding to free carbonyl bond, and a shoulder on the lower wavenumbers (1640 cm-1) is observed, which is due to hydrogen-carboxy bonded carbonyl groups. The complete hydrolysis of tert-butyl ester groups in the soluble polymers was also confirmed by 1H NMR. The more detailed characterization of the soluble branched copolymers synthesized by SCVCP of BPEA and tBuA as well as the hydrolysis products will be reported in a subsequent publication. The typical broad band around 1100 cm-1 resulting from Si-O stretching on silica is also observed in all hybrids grafted with the branched copolymers, in addition to the acid functionality and the CdO vibration. In the spectrum of the simple mixture of the soluble branched PAA obtained in solution and unmodified silica particles, there is a broad peak between 2800 and 3800 cm-1, which is different from the results for other

Langmuir, Vol. 18, No. 9, 2002 3689

hydrolyzed samples of hybrids. The broad peak may correspond to the OH groups of bare silica particles, as can be seen in Figure 1a, suggesting that silica particles are not covered completely by the branched PAA in the sample prepared simply by mixing. These FT-IR analyses confirmed the presence of the acidic group in the polymer shell of the hybrid after the hydrolysis, and the resulting branched PAA layers exist predominantly surrounding silica particles. The hydrolysis was also confirmed by elemental analysis. The atomic compositions of the hydrolyzed branched copolymer-silica hybrid particles were as follows: C, 32.1; H, 4.2 (calcd: C, 34.5; H, 4.0, γ ) 6.1) and C, 29.5; H, 3.5 (calcd: C, 32.8; H, 4.0, γ ) 1.1), respectively. The existence of the carboxylic acids on the hybrid nanoparticles was also identified by titration. These results indicate that the copolymerization of BPEA with tBuA, followed by hydrolysis of linear segments, yields characteristic silica hybrid nanoparticles with the tethered branched PAA chains as a shell in a controlled fashion. Characterization of Branched Polymer-Silica Hybrid Nanoparticles. Ultrafine particles have lately attracted great attention for various advanced applications. Many efforts have been paid to control of aggregation of inorganic particles and to design of the shape and morphology. One promising way for preventing the aggregation of nanoparticles is to surround them with an appropriate layer, involving various organic molecules and polymers. However, it is normally difficult to prepare ultrafine polymer-modified particles, because uncontrolled aggregations may occur during the polymerization and purification process. On the basis of this consideration, preliminary characterization of the morphology for the branched polymer-silica hybrid nanoparticles was conducted by DLS, TEM, SFM, and FE-SEM. DLS studies were carried out on the selected branched polymer-silica hybrid particles in THF at the concentration of 1.0 mg/mL at room temperature. In all cases, DLS measurements were conducted at four or five different angles, and the hydrodynamic radius of the species was obtained by extrapolation to q2 f 0. The particle size distributions obtained by CONTIN analysis of the DLS data at a 30° observation angle are presented in Figure 4. In the case of sample 2 (Figure 4b), the branched polymer-silica hybrid particles show a unimodal distribution, independent of the scattering angle. After extrapolation to zero angle (q2 f 0), the average hydrodynamic radius (Rh) is 77 ( 1 nm. In the case of sample 5 (Figure 4c), however, the distribution is bimodal, even though the particles with bigger sizes are obviously dominant. The average Rh values of the two species are 25 ( 17 and 140 ( 14 nm, respectively, after extrapolation to q2 f 0. The relatively high range of the Rh for the smaller species may be caused by the uncertainty of the mathematical fitiing due to its small fraction. The functionalized silica particle also shows a bimodal distribution. As shown in Figure 4a, the area fraction with a small diameter is around 4.5% of the total area of this sample. The extrapolated average Rh values of the two peaks are 20 ( 2 and 85 ( 3 nm, respectively. Even the Rh of the smaller species is larger than that of the bare silica particle (average diameter ) 16 nm) used as a starting material, suggesting that the observed Rh values do not correspond to a single functionalized silica particle. The large value is comparable to that of the branched polymer-silica hybrid particles. This is an indication that the agglomerated structures of the branched polymer-silica hybrid particles are partially derived from that of the functionalized silica particles.

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Figure 5. Schematic representation of (a) a core-shell morphology of branched polymers grafted on a silica particle and comparison of (b) isolated and (c) aggregated forms of the branched polymer-silica particles.

Figure 4. Particle size distribution from DLS at a 30° observation angle: (a) the functionalized silica particle, (b) poly(BPEA)-silica hybrid particles obtained by SCVP at a ratio of [BPEA]0/[CuBr]0 ) 50, and (c) branched PtBuA-silica hybrid particles obtained by SCVCP (γ ) 6.1).

In the spherical core-shell model (Figure 5a), the core is formed by SiO2, while the shell is composed of the branched polymers. Assuming that a silica particle is covered homogeneously by the branched polymers without any aggregation, the shell thickness is given by hshell ) Rh - Rcore and is calculated from the following equation:

Vshell )

4π δ (Rh3 - Rcore3) ) 3 F

(1)

where Vshell is the volume of the polymer shell, F ) 1.1

g/cm3 is the mass density of PMMA, and δ is the weight of the grafted polymer per silica particle, which is calculated by dividing the observed weight ratio (g polymer/g silica) by the number of silica particles per 1 gram. The number of the silica particles can be estimated from the density (ca. 2 g/cm3) and the diameter (2Rcore ) 16 nm) of silica to be around 2.3 × 1017/g silica. From the equation and the experimental data, the shell thickness is estimated to be about 5 and 10 nm for sample 2 and sample 1, respectively. When the radius Rh is obtained from DLS, a large value is obtained: for example, hshell ) Rh (77 nm) - Rcore (8 nm) ) 69 nm for sample 2. The value is too high for a single branched polymer-silica hybrid particle, indicating the existence of some kind of aggregated forms, as shown in Figure 5c. Figure 6 shows representative TEM micrographs of the branched polymer-silica hybrid particles obtained from dilute THF suspension. The resulting particles are seen to form clusters of micrometer size (Figure 6a). Such aggregation morphologies were also observed in other branched polymer-silica hybrid particles. Similar agglomerated morphologies were reported in polystyrenesilica composites prepared by a noncontrolled radical polymerization from silica particles modified with azo type initiators.51 A possible reason is the fact that the primary particles themselves (Aerosil 300, Degussa, 2R ) 7 nm in their case; Aerosil 200, Degussa, 2R ) 16 nm in our case) form large aggregates of 0.1 to several micrometers due to van der Waals interparticle attraction and the aggregation is kept somehow during the preparation of the functionalized silica particles as well as the following polymerization and purification processes. Relatively small particles about 30-70 nm in size are also visualized in TEM (Figure 6b). Recently, von Werne and Patten16,18 have reported successful TEM observations of the unaggregated polystyrene-nanoparticle hybrids prepared by ATRP from spherical silica particles having a narrow size distribution (average diameters ) 70 and 300 nm, prepared using the Sto¨ber process52). They also demon(51) Ribbe, A.; Prucker, O.; Ru¨he, J. Polymer 1996, 37, 1087. (52) Philipse, A. P.; Vrij, A. J. Colloid Interface Sci. 1989, 128, 121.


Langmuir, Vol. 18, No. 9, 2002 3691

Figure 6. Representative ΤΕΜ images of the branched PtBuA-silica hybrid particles obtained by SCVCP (γ ) 1.1).

strated that the spacing of these particles within the array was dependent on the molecular weight of the grafted polymer. However, agglomeration took place at certain monomer conversions which was most likely assisted by chain entanglement of higher molar mass linear polymer chains. Well-defined growth of highly branched polymers from the surface of the silica nanoparticles should show different tendencies, because less chain entanglement is expected compared to linear ones. A branched polymer could bind covalently to only one individual particle even at the last stage of polymerization, as the ungrafted macromolecules have strictly one double bond in the chain and the grafted polymers should not contain any vinyl groups. This means that one individual particle might be linked with many branched polymers due to the high number of active centers; however, the branched polymers may not lead to any cross-linking between nanoparticles. Hence, the system is considered to help to prevent interparticle attraction, giving unaggregated core-shell nanostructures, as can be seen in Figure 5b. Occasionally, such isolated forms were observed in TEM, SFM, and FESEM measurements of the branched polymer-silica hybrid particles obtained in this study. Recently, SFM has been successfully applied to the characterization of a variety of hybrid nanoparticle films.7,19,53 Particularly, Pyun et al.19 demonstrated that the successful grafting of polymers to nanoparticles via ATRP and morphology changes of (sub)monolayers of particles were confirmed by SFM. Figure 7 shows tapping mode SFM images of the branched polymer-silica hybrid particles obtained from dilute THF suspension. An aggregated form of about 1.5 µm2 in size is seen (Figure 7a), which comprises about 20 particles. The phase images (53) Sato, H.; Ohtsu, T.; Komasawa, I. J. Colloid Interface Sci. 2000, 230, 200.

Figure 7. Representative SFM images of the branched PtBuAsilica hybrid particles obtained by SCVCP (γ ) 1.1): (a) phase image (50°); (b) higher magnification phase image taken from the area inside the box indicated in (a); (c) cross section taken at the position indicated by the dotted line in (b).

shown in regions of different phase contrast were tentatively identified as particle cores (bright spots) and branched polymer (matrix between the bright spots). Parts b and c of Figure 7 represent a magnification of representative isolated particles and the cross section of the dotted line in the magnification, respectively. Spherical particles of about 10 nm in height are also observed, which are comparable to the size of the bare silica particle (average diameter ) 16 nm) used as a starting material. These results suggest that the primary particles of the bare silica can be dispersed in the polymer matrix without aggregation, forming the micrometer-sized polymer-silica hybrid. Note that dark halos around bright particle cores were occasionally observed in the functionalized silica particles before the polymerization, suggesting that the thickness of the polymer shell could not be determined by SFM. The unaggregated smaller structures, as can be seen in Figure 7b, were predominantly observed in the SFM measurements, compared to those in TEM observation.

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Figure 8. Representative FE-SEM image of the functionalized silica particles.

This is mainly due to the different substrates and the different sample preparation methods for TEM and SFM in the present study. Several authors7,53 describing SFM imaging for ultrafine particles pointed out that the particles must be bound strongly to the substrate to prevent them from being swept out by the scan of the tip. In our case, the existence of organic layers on the silica particles may help to solve the problem. For the sample preparation of the heterogeneous materials for SFM measurements, solvent evaporation can induce mechanical forces that may lead to the destruction of large aggregates formed in solution. The visualization of relatively smaller particles in SFM measurements may come from the mechanical forces and the interaction of the branched polymer-silica hybrid nanoparticles with the substrate during the sample preparation process. To get more information on the particle size, the shape, and the aggregation states, the samples prepared on polished silicon wafers by dip-coating were directly investigated by FE-SEM. The FE-SEM micrograph of the functionalized silica particles shows that spherical particles exist on the silicon surface without aggregation, as can be seen in Figure 8. The diameter of the particles is in the range of 40-150 nm, which is comparable to that determined by DLS. The branched PtBuA-silica hybrid particles obtained by SCVCP at γ ) 6.1 showed two different types of images. Spherical particles of about 40160 nm in diameter are observed (Figure 9a), corresponding to isolated particles. Aggregated forms are also seen (Figure 9b), which comprise many small particles. These isolated and aggregated forms are considered to be derived from the larger and smaller species of the functionalized silica particles (Rh obtained from DLS ) 85 ( 3 and 20 ( 2 nm), respectively. Parts c and d of Figure 9 represent low-magnification images of the isolated and aggregated forms of the branched PtBuA-silica hybrid particles obtained by SCVCP at γ ) 1.1. Figure 9d shows a micrometer-sized aggregation form containing continuous layers of silica particles. The silica particles are arranged within a hybrid film on the silicon surface, and the morphology may be related to the size and composition of the grafted polymers in the hybrid. In the isolated form (Figure 9c), bright spherical spots and dark halos around cores are observed. The dark halos were not observed in the functionalized silica particles before the polymerization, suggesting that the isolated form may consist of one to four silica particles as a core and the grafted copolymer shell existing around the core. DLS, TEM, SFM, and FE-SEM measurements give valuable information on the size, the morphology, and the aggregation states of the branched polymer-silica hybrid particles obtained in this study. These results

Figure 9. Representative FE-SEM images of the branched PtBuA-silica hybrid particles obtained by SCVCP at γ ) 6.1 (a,b) and γ ) 1.1 (c,d).

consistently indicate that the hybrid nanoparticles comprising the silica core and the hyperbranched polymer shell exist as isolated and aggregated forms. The precise determination of the difference in the mean particle size between the functionalized silica particles and the branched polymer-silica hybrid particles was not successful, due to the relatively large distribution of the particle size and uncontrolled aggregation forms of commercial silica particles. Experiments aiming to control the size and distribution of the branched polymer-silica hybrid nanoparticles obtained by this method are now in progress and will be reported elsewhere. Conclusions Hyperbranched polymer-silica hybrid nanoparticles were synthesized by surface-initiated SCVP of an acrylic AB* inimer from silica nanoparticles functionalized with monolayers of ATRP initiators. Elemental analyses and FT-IR measurements of the resulting products indicate the formation of nanoparticles composed of an inorganic silica core and an outer layer of covalently attached hyperbranched polymer with a high density of bromoester end groups. The relatively high weight fraction and the surface coverage of the grafted polymers were observed for SCVP as well as for copolymerization with tert-butyl acrylate. Molecular parameters of the soluble polymers, such as molecular weight, polydispersity, and degree of branching, were characterized by GPC, GPC/viscosity, and NMR measurements, suggesting the formation of highly branched architectures. The functionality of the end groups on the surface and the chemical composition as well as the structure of the branched polymers grafted on the silica nanoparticles could be controlled by composition in the feed during the SCVCP. Novel core-shell nanomaterials, branched PAA-silica hybrid nanoparticles, were obtained after hydrolysis of linear segments of the branched PtBuA. The present work can be applied to a wide range of inorganic materials having different sizes


and shapes for surface-initiated SCVP and SCVCP to allow the preparation of new functional branched polymermetal hybrid nanoparticles. Acknowledgment. The authors thank C. Drummer (BIMF) and A. Go¨pfert for FE-SEM (SFB 481) and TEM

Langmuir, Vol. 18, No. 9, 2002 3693

measurements. D.C.S (University of Bordeaux I) acknowledges a grant from the Erasmus exchange program. This work was supported by the Deutsche Forschungsgemeinschaft (Schwerpunkt “Benetzung und Strukturbildung an Grenzfla¨chen” and SFB 481). LA011630X

Hybrid Nanoparticles with Hyperbranched Polymer Shells via Self

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