Polystyrene-block-polyglycidol Micelles Cross ... - ACS Publications

Polystyrene-block-polyglycidol Micelles Cross-Linked with Titanium Tetraisopropoxide. Laser Light and Small-Angle X-ray Scattering Studies on Their Fo...
1 downloads 0 Views 1MB Size
Download PDF
pubs.acs.org/Langmuir © 2010 American Chemical Society

Polystyrene-block-polyglycidol Micelles Cross-Linked with Titanium Tetraisopropoxide. Laser Light and Small-Angle X-ray Scattering Studies on Their Formation in Solution Melanie Siebert,†,§ Artur Henke,†,§ Thomas Eckert,‡ Walter Richtering,‡ Helmut Keul,*,† and Martin M€oller*,† †

DWI and Institute of Technical and Macromolecular Chemistry, RWTH Aachen University, Pauwelsstrasse 8, 52056 Aachen, Germany, and ‡Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, 52056 Aachen, Germany. §Both authors contributed equally to this work Received July 12, 2010. Revised Manuscript Received September 24, 2010

Hybrid micelles from polystyrene-block-polyglycidol (PS-b-PG) copolymers with chemically cross-linked cores by titanium tetraisopropoxide (Ti(OC3H7)4) were prepared in toluene solution. Additionally, micellization of PS-b-PG copolymers with different mass fractions of polyglycidol (xPG), was studied by static and dynamic light scattering as well as small-angle X-ray scattering. It was observed that copolymers with xPG smaller than 0.5 self-assembled in toluene into spherical core-shell micelles with hydrodynamic radii Rh between 12 and 23 nm. On the other hand, copolymers with larger PG content formed particles with Rh = 50-70 nm and aggregation numbers of several thousands. The presence of these aggregates in solution was attributed to the nonequilibrated form of block copolymers upon dissolving, most probably due to hydrogen bonding. In the following, spherical PS-b-PG micelles were loaded in toluene with hydrochloric acid and titanium tetraisopropoxide. Confined hydrolysis of Ti(OC3H7)4 induced by HCl in the micellar core was confirmed by small-angle X-ray scattering experiments. The subsequent condensation of the precursor with hydroxyl groups of polyglycidol chains led to covalently stabilized hybrid organic-inorganic particles. The presence of cross-linked PS-b-PG micelles was proven in two ways. First, micelles with “frozen” core showed stable hydrodynamic size in time upon dilution below critical micellization concentration while non-cross-linked PS-b-PG micelles underwent disintegration under the same conditions within several hours. Second, light scattering experiments revealed the presence of stable, swollen particles in N,N-dimethylformamide, which is a good solvent for both blocks.

1. Introduction Amphiphilic block copolymers are an intriguing class of soft matter. Their unique behavior in solution or in bulk results from the presence of unlike types of monomers within the same polymer chain.1-6 As a consequence, copolymers can self-assemble in thermodynamically poor solvents for one of the blocks into spherical, rodlike, or vesicular particles.1,2 In the melt, on the other hand, incompatibility between blocks induces arrangement of the copolymer chains into lamellar or cylindrical types of structures, among others.5 The intensive studies on amphiphilic block copolymers within the last 40 years brought many potential applications of these systems, especially in the fields of nanotechnology and biomedicine.7,8 For example, there is a growing demand for new and versatile strategies for fabrication of nanostructured materials with potential application in microelectronics, sensors, catalysis, *Correspondence authors. H.K., [email protected]; M.M., moeller@ dwi.rwth-aachen.de. (1) Block Copolymers in Solution: Fundamentals and Applications; Hamley, I., Ed.; John Wiley & Sons: London, England, 2009. (2) Reiss, G. Prog. Polym. Sci. 2003, 28, 1107. (3) Hamley, I. W. Prog. Polym. Sci. 2009, 34, 1161. (4) Gohy, J.-F. Adv. Polym. Sci. 2005, 190, 65. (5) Abetz, V.; Simon, P. F. W. Adv. Polym. Sci. 2005, 189, 125. (6) Cohen Stuart, M. A. Colloid Polym. Sci. 2008, 286, 855. (7) Self-Assembly and Nanotechnology; Lee, S. Y., Ed.; John Wiley & Sons: New York, NY, U.S., 2008. (8) Bae, Y.; Katoka, K. Adv. Drug Deliv. Rew. 2009, 61, 768. (9) Nanostructured Materials and Nanotechnology; Nalwa, H. S, Ed.; Academic Press: New York, NY, U.S., 2002. (10) Springer Handbook of Nanotechnology; Bhushan, B., Ed.; Springer-Verlag: Berlin, Germany, 2004.

Langmuir 2010, 26(22), 16791–16800

or optical fields.9-11 In this particular case block copolymers show promising properties for controlled preparation of metallic or semiconductor nanoparticles.7,12,13 This follows from the fact that the growth of those nanoparticles can be limited in size to nanoscopic dimensions.7 There are four main procedures used to fabricate nanostructured materials by means of self-assembly strategy.14 Precursor and block copolymers can be co-selfassembled or associated through cooperative processes. Furthermore, the metal precursor either can be chemically linked to the copolymer chains (inorganic-organic hybrid) or can be immobilized within already formed micelles.7,14 The latter method offers a fast and reliable route for preparation and controlled ordering of the particles by using block copolymers based on polystyrene and poly(2-vinylpyridine), poly(ethylene oxide), or poly(methacrylic acid).14-16 When dissolved in nonpolar solvents, for example, toluene, these copolymers undergo inverse micellization with the hydrophilic block forming the core. In the following, the inner micellar compartment can be selectively loaded with metal salts/alkoxides and thus act as a nanoreactor for a reduction/ hydrolysis process of the precursor.17 It was shown that the formed (11) Haberkorn, N.; Lechmann, M. C.; Sohn, B. H.; Char, K.; Gutmann, J. S.; Theato, P. Macromol. Rapid Commun. 2009, 30, 1146. (12) Bang, J.; Jeong, U.; Ryu, D. Y.; Russell, T. P.; Hawker, J. Adv. Mater. 2009, 21, 4769. (13) Segalman, R. A. Mater. Sci. Eng. R 2005, 48, 191. (14) Roescher, A.; M€oller, M. Adv. Mater. 1995, 7, 151. (15) Spatz, J.; Roescher, A.; M€oller, M. Adv. Mater. 1996, 8, 337. (16) Glass, R.; M€oller, M.; Spatz, J. Nanotechnology 2003, 14, 1153. (17) K€astle, G.; Boyen, H.-G.; Weigl, F.; Lengl, G.; Herzog, T.; Ziemann, P.; Riethm€uller, S.; Mayer, O.; Hartmann, C.; Spatz, P. J.; M€oller, M.; Ozawa, M.; Banhart, F.; Garnier, M. G.; Oelhafen, P. Adv. Funct. Mater. 2003, 13, 853.

Published on Web 10/13/2010

DOI: 10.1021/la102780y

16791

Article

Siebert et al.

micelles are kinetically frozen in the presence of a precursor due to specific physical interaction between both components.18 Due to strong coordinative bonding between metal salt and 2-vinylpyridine units, the spherical shape of polystyreneb-poly(2-vinyl pyridine) micelles loaded with HAuCl4 can be preserved after deposition on the surface, even at copolymer concentrations where lamellar types of ordering should prevail. As a consequence, loaded PS-b-P2VP micelles can arrange into hexagonal pattern after spin-or dip coating on the substrate. This specific copolymer-metal precursor interaction can be regulated by the chemical structure of the copolymer, the length of the hydrophilic block, and the amount of loaded precursor.18 By using sequential anionic polymerization with a sec-butyllithium/phosphazene base initiating system, we prepared recently well-defined polystyrene-block-polyglycidol copolymers.19 Polyglycidol is a water-soluble, biocompatible polyether with a hydroxyl group in each repeating unit.20 Thus, it receives growing attention as a functional alternative to poly(ethylene oxide).20 For instance, in analogy to polystyrene-b-poly(ethylene oxide) copolymer, PS-b-PG micelles can serve as matrices for a sol-gel transition of metal alkoxides. Furthermore, the polyglycidol block can undergo covalent cross-linking with metal alkoxides via condensation, giving systems with new properties. Till now, such materials have not been studied in detail.21,22 Increased stability of micelles can offer new advantages over classical micellar systems that are prone toward dissolution upon experiencing concentration or temperature fluctuations.23 For example, Huo et al. showed that micellar structures of pluronic F127 block copolymer can be locked upon hydrolysis of tetraethoxysilane around a core. This led to hybrid nanoparticles with high stability and potential application as carriers of pharmaceuticals or imaging agents.22 Even though the synthesis of copolymers with polystyrene and polyglycidol blocks has been reported,24,25 its micellization was not studied before. Here, we present static and dynamic light scattering as well as small-angle X-ray scattering results on polystyrene-b-polyglycidol copolymer micelles in toluene. The micellization process of PS-b-PG copolymers was investigated as a function of the mass fraction of the polyglycidol block in the copolymer. In the following, the micellar cores were cross-linked by titanium tetraisopropoxide via hydrolysis and subsequent condensation with hydroxyl groups of the polyglycidol segments.

2. Experimental Part 2.1. Materials. Dry toluene (g99.9%), N,N-dimethylformamide (DMF g99.9%) and acidic ion-exchange resin Amberlyst 15 (dry, moisture e1.5%) were purchased from Aldrich and used as received. HCl (37%, aqueous solution) and titanium(IV) tetraisopropoxide (Ti(OC3H7)4, purum) were obtained from Fluka and used without further treatment. Syringe filters with a PTFE membrane and pore size of 1 μm were obtained from Whatman. 2.2. Polymer Synthesis. The detailed description can be found elsewhere.19 All reagents used for anionic polymerization (18) Spatz, J. P.; Sheiko, S.; M€oller, M. Macromolecules 1996, 29, 3220. (19) Siebert, M.; Keul, H.; M€oller, M. Des. Monomers. Polym., DOI:10.1163/ 138577210X530657. (20) Keul, H.; M€oller, M. J. Polym. Sci., Polym. Chem. 2009, 47, 3209. (21) Steunou, N.; F€orster, S.; Florian, P.; Sanchez, C.; Antonietti, M. J. Mater. Chem. 2002, 12, 3426. (22) Huo, Q.; Liu, J.; Wang, L.-Q.; Jiang, Y.; Lambert, T. N.; Fang, E. J. Am. Chem. Soc. 2006, 128, 6447. (23) Read, E. S.; Armes, S. P. Chem. Commun. 2007, 3021. (24) Toy, A. A.; Reinicke, S.; M€uller, A. H. H.; Schmalz, H. Macromolecules 2007, 40, 5231. (25) Barriau, E.; Marcos, A. G.; Kautz, H.; Frey, H. Macromol. Rapid Commun. 2005, 26, 862.

16792 DOI: 10.1021/la102780y

were purified according to well established procedures. The synthesis of polystyrene-block-polyglycidol block copolymers was realized in a two-step procedure. (i) Styrene and ethoxyethyl glycidyl ether (EEGE) were polymerized by sequential anionic polymerization in benzene by using a sec-butyllithium/phosphazene base (t-BuP4) initiating system, and (ii) the complete deprotection of PEEGE block occurred with an acidic ion-exchange resin in THF. Successful synthesis of the corresponding PS-b-PG block copolymers was confirmed by NMR in DMSO (99.8%, Deutero GmbH). The characteristics of the copolymer samples used in this study are given in Table 1. The synthesis of polystyrene-block-poly(ethylene oxide) copolymer is described in detail elsewhere.26 The sample with 100 polystyrene and 270 ethylene oxide units (Mn=22800, xPEO=0.53) was used.

2.3. Preparation of Polystyrene-block-polyglycidol and Polystyrene-block-poly(ethylene oxide) Solutions. Copoly-

mer stock solutions (10 g dm-3) were obtained by direct dissolution of the block copolymers in toluene. For copolymers with polyglycidol mass fractions xPG up to 0.5, several hours were required for complete dissolution, while for copolymers with higher mass fractions of polyglycidol, homogeneous solutions were obtained after several days of stirring (eventually additional ultrasound treatment was applied). PS-b-PEO solutions were obtained by stirring the copolymer at 80 °C for several hours. Samples with the desired concentrations were prepared by dilution of the stock solution. 2.4. Loading with Titanium Alkoxide. PS-b-PG micellar solutions in toluene (5 or 10 g dm-3) were treated with concentrated HCl (loading of 0.5 or 1.0 equiv of HCl/glycidol unit). After being stirred for at least 24 h at room temperature, the HCl-loaded micelles were treated with the titanium precursor (titanium isopropoxide, Ti(OC3H7)4, 0.5 or 1.0 equiv of Ti(OC3H7)4/glycidol unit). The solutions were stirred for additional 24-48 h to ensure complete micelle loading.

2.5. Static and Dynamic Light Scattering Measurements. In static light scattering (SLS), the intensity of light scattered by polymer solution is expressed through so-called Rayleigh ratio Rθ. For dilute polymer solution, the latter parameter is related to the weight-average molecular weight Mw, radius of gyration Rg, and the second virial coefficient A2 according to formula 1:27   Kc 1 1 2 2 ¼ 1 þ Rg q þ 2 A2 c ð1Þ Rθ Mw 3 where c is the polymer concentration, K is so-called optical constant defined as 4πn2(dn/dc)2/(NAλ04) with n, (dn/dc), NA, and λ0 being the refractive index of the solvent, the refractive index increment of the polymer solution, the Avoradro constant, and the wavelength of the laser light in vacuum, respectively. Additionally, q is the length of the scattering vector, defined as (4πn/λ0) sin(θ/2) with θ being the scattering angle. In micellar solutions, however, the molecular weight of the formed particles changes strongly with copolymer concentration, especially close to the micellization threshold. In such a case, a classical dilution approach and extrapolation of the results to c f 0 can lead to erroneous results. Instead, the Rayleigh ratio can be measured at finite polymer concentration c (smaller than critical overlapping concentration c*).27 The relationship between the excess Rayleigh ratio at a given polymer concentration c and weight-average molecular weight can be expressed as Kc/Rθ ≈ 1/Mw,app(1 þ c/c*).27 In the present work, this latter approach was used. All experiments were done at 25 ( 0.1 °C. SLS measurements were carried out on the modified Sofica goniometer (SLS-Systemtechnik, Denzlingen) combined with a helium-neon laser (26) Albrecht, K. Ph.D. Thesis, RWTH Aachen, 2007. (27) LaRue, I.; Adam, M.; Zhulina, E. B.; Rubinstein, M.; Pitsikalis, M.; Hadjichristidis, N.; Ivanov, D. A.; Gearba, R. I.; Anokhin, D. V.; Sheiko, S. S. Macromolecules 2008, 41, 6555.

Langmuir 2010, 26(22), 16791–16800

Siebert et al.

Article Table 1. Characteristics of Copolymer Samples Used in the Experiments mass fraction

Pn copolymer

a

Mn PS-b-PG, Da

PS40-b-PG210 19800 28800 PS120-b-PG220 23100 PS120-b-PG140 22100 PS160-b-PG70 a Values obtained on the basis of 1H NMR.

PS block

PG block

xPS

xPG

(dn/dc), mL/g

40 120 120 160

210 220 140 70

0.22 0.43 0.55 0.76

0.78 0.57 0.45 0.24

0.0131 0.0173 0.0321 0.0580

Table 2. Critical Micellization Concentrations, Hydrodynamic Radii, Rh, Radii of Gyration, Rg, Rg/Rh Ratios, and Aggregation Numbers (Measured at 1 g dm-3) for Investigated Copolymer Samples copolymer

cmc (g dm-3)

Rh (nm)

Rg,app (nm)

Rg/Rh

Nagg,appe (at 1 g dm-3)

0.80 12.5a 10.0c 0.8 35 PS160-b-PG70 PS120-b-PG140 0.25 22.5a 18.0c 0.8 490 b d 0.02 51.5 63.0 1.2 15100 PS120-b-PG220 0.05 43.5b 49.0d 1.1 8700 PS40-b-PG210 a Rh values obtained by extrapolation to infinite dilution and zero scattering angle. b Apparent Rh values obtained at 1 g dm-3 after extrapolation to zero scattering angle. c Apparent Rg values obtained with the SAXS method at a copolymer concentration of 10 g dm-3. d Apparent Rg values obtained with the static laser light scattering method at a copolymer concentration of 1 g dm-3. e Values determined at λ0 = 633 nm.

generating green light with a wavelength of λ0 = 543 nm or ALV setup with CGS-3 goniometer, pseudo cross-correlation detector and helium-neon laser with λ0 = 633 nm. The time-averaged intensity of the laser light scattered by PS-b-PG solutions was measured in the angular range between 25° and 145° with a step of 5° and chosen copolymer concentrations (typically between 1.0 and 10 g dm-3). In the following, the apparent weight-average molecular weight Mw,app and the apparent z-average radius of gyration Rg,app were calculated at a given concentration from the square root value of the Kc/Rθ. Mw,app is correlated with the apparent aggregation numbers of the micellar aggregates through the Nagg,app=Mw,app/Mn,copolymer relationship. The Nagg,app values for PS-b-PG copolymers at 1 g dm-3 are given in Table 2. For PS120-b-PG140 and PS120-b-PG140 copolymers, the radius of gyration of the micelles was smaller than the resolution of used static light scattering setup and the small-angle X-ray scattering method was used to estimate Rg,app (for details see below). For PS40-bPG210 and PS120-b-PG240 copolymers the apparent radii of gyration obtained at 1 g dm-3 are summarized in Table 2 (see also section 3.1.2). Dynamic light scattering (DLS) experiments were performed in a self-beating mode on an ALV setup consisting of an ALV-SP8 goniometer, an ALV-SIPC photomultiplier, a multiple τ digital real-time ALV-7004 correlator, and a solid state laser (Koheras) with a wavelength of 473 nm. Experimentally measured autocorrelation functions (ACF) of intensity fluctuations g(2)(t) were analyzed with CONTIN algorithm, giving the distribution of the relaxation time τ. The time of a measurement at a single angle was set to 180 or 300 s and in some case more than 600 s, depending on data statistics. At a given copolymer concentration and scattering angle, the relaxation time is proportional to the apparent translation diffusion coefficient Dapp(q,c) according to τ = (Dapp(q,c)q2)-1. The apparent diffusion coefficient was measured in the angular range between 30° and 120° with a step of 5° (in some cases 10°) and extrapolated to zero scattering angle (q f 0). In the following, Dapp(c,qf0) was measured at different copolymer concentrations above the critical micellization onset. The corresponding hydrodynamic radius was calculated using the StokesEinstein relationship and values obtained for PS120-b-PG140 and PS120-b-PG140 from double extrapolation are summarized in Table 2. For PS40-b-PG210 and PS120-b-PG240, the apparent Rh’s given in Table 2 are measured at 1 g dm-3 (see also section 3.1.2). The time-averaged scattered intensity changes used for the determination of the critical micellization concentrations (cmc) of the PS-b-PG copolymers in toluene were measured at 90° using the ALV setup described above. Langmuir 2010, 26(22), 16791–16800

In all experiments, solutions were first measured without filtration and if necessary they were purified using PTFE syringe filters with 1 μm pore size. The refractive index increment (dn/dc) of PS-b-PG toluene solutions was measured with an Optilab DSP differential refractometer (Wyatt) operating at 690 nm. Prior to the measurements, solutions were filtered through PTFE syringe filters with 1 μm pore size.

2.6. Small-Angle X-ray Scattering Measurements (SAXS).

All measurements were done at 25 °C, using a S-MAX3000 pinhole camera and MicroMax-002þ microfocus X-ray generator from Rigaku. The wavelength of the generated radiation was 1.54 A˚, and the range of the scattering vector from 0.004 to 0.16 A˚ was covered. PS120-b-PG140 and PS120-b-PG140 copolymers with a concentration of 10 g dm-3 were filtered through 1 μm PTFE filters directly into capillary tubes that were sealed before measurements. The obtained scattering curves were transformed via indirect Fourier transformation (ITF) into a pair-distance distribution functions.28 2.7. Thermal Gravimetric Analysis (TGA). Measurements were carried out on Netzsch Proteus TG 209C. Nonfiltered and filtered toluene copolymer samples loaded with HCl and Ti(OC3H7)4 were placed on the aluminum pans and solvent was evaporated. Samples were heated from 30 to 500 °C with a heating rate of 10 °C min-1. After cooling to room temperature, the mass loss between samples was measured.

3. Results and Discussion In this work well-defined polystyrene-block-polyglycidol copolymers were used to prepare micelles with cross-linked cores using titanium tetraisopropoxide. Before cross-linking the micelles in toluene, the association process of synthesized copolymers in toluene was studied by laser light and small-angle X-ray scattering techniques to get the basic macromolecular characteristics of the micelles formed. 3. 1. Micellization of PS-b-PG Copolymers in Toluene. 3.1.1. Determination of PS-b-PG Critical Micellization Concentration. PS-b-PG copolymers self-assemble in toluene above critical micellization concentration (cmc), forming micelles with a polyglycidol core and a polystyrene corona. The onset of the micellization process was determined for all copolymers by monitoring the changes of the scattered light intensity (28) Vestergaard, B.; Hansen, S. J. Appl. Crystallogr. 2006, 39, 797.

DOI: 10.1021/la102780y

16793

Article

Figure 1. Laser light intensity scattered by PS-b-PG solutions at different copolymer concentrations. The inset shows magnification of the same plot with micellization onset marked for each copolymer with an arrow. Measurements are performed at a scattering angle of 90°.

Figure 2. Normalized intensity autocorrelation functions g(2)(t) measured at a scattering angle of 90° for PS160-b-PG70 copolymer in toluene at different concentrations. The critical micellization concentration for this sample is calculated on the basis of data from Figure 1 and is equal to 0.8 g dm-3.

at an angle of 90° as a function of copolymer concentration (Figure 1). The points from which the intensity started to increase were ascribed as micellization onsets (arrows in the inset of Figure 1). As typically reported,29,30 the calculated cmc values decrease in exponential manner with increasing length of nonsoluble constituent, namely PG block (Table 2). The formation of micelles was followed simultaneously by dynamic light scattering. The normalized autocorrelation functions (ACF) of intensity fluctuations g(2)(t) obtained at a scattering angle of 90° are shown in Figure 2 for PS160-b-PG70 copolymer in toluene as an example. It can be seen that below cmc the intensity autocorrelation functions for nonaggregated copolymer chains (in this particular case at 0.5 g dm-3) decayed within several nanoseconds, which (29) Yang, Y.-W.; Deng, J.-N.; Yu, G.-E.; Zhou, Z.-K.; Attwood, D.; Booth, C. Langmuir 1995, 11, 4703. (30) Crothers, M.; Attwood, D.; Collett, J. H.; Yang, Z.; Booth, C.; Taboada, P.; Mosquera, V.; Ricardo, N. M. P. S.; Martini, L. G. A. Langmuir 2002, 18, 8685.

16794 DOI: 10.1021/la102780y

Siebert et al.

made impossible estimation of the hydrodynamic radius of the copolymers at this scattering angle. In fact, the shape of the ACF below the cmc was not so different from pure toluene in all investigated cases. To explain this phenomenon, one should take into account the values of the refractive increment indices determined for PS-b-PG copolymers in toluene. As presented in Table 1, they decreased from 0.058 to 0.0131 mL g-1 with increasing length of the polyglycidol block. Thus, the excess intensity scattered in solution by nonaggregated copolymers chains, which additionally possess relatively low molar masses, can be almost neglected. On the other hand, above critical micellization concentration, the intercepts and decay time of g(2)(t) increased significantly. Such a strengthening of a measured signal could originate only from scattering objects possessing higher molar mass and size than the native chains, i.e., from PS-b-PG micelles. 3.1.2. Characterization of PS-b-PG Micelles. The intrinsic size of PS-b-PG micelles with different mass fraction of polyglycidol block was determined from angular and concentration dependent laser light scattering measurements. First, the changes of the apparent hydrodynamic radii as a function of the scattering vector at copolymer concentration of 1 g dm-3 are presented in Figure 3A. With increasing length of the polyglycidol block and increasing molecular weight of the copolymer (compare mainly Mn of PS120b-PG220 and PS40-b-PG210), the radius of the particles increases. The size distribution of formed micelles was obtained by CONTIN and cumulant analysis (so-called polydispersity index PDI) of the autocorrelation functions. Monodisperse and polydisperse systems are characterized by PDI values below 0.1 and above 0.3, respectively.31 The distribution of the relaxation times at θ = 30° for PS-b-PG samples prepared from 1 g dm-3 copolymer solutions are shown in Figure 3B. It can be noticed that micelles show moderately polydisperse spectra, which is additionally confirmed by PDI values in the range 0.18-0.23. Additionally, in all cases, the distribution of these micellar aggregates is unimodal. The small peaks observed in some spectra at short relaxation times are due to the scattered points of the autocorrelation functions and in consequence generation of the artifacts by CONTIN analysis. A concentration dependency of the diffusion coefficient was also elaborated for all synthesized copolymers. The diffusion coefficient extrapolated to zero scattering angle (qf0) is correlated with the polymer concentration according to eq 2.32 Dapp ðc;q f 0Þ ¼ Dz ð1 þ kD cÞ

ð2Þ

where Dapp(c,q f 0) is the diffusion coefficient measured at concentration c, Dz is the z-average diffusion coefficient at infinite dilution, and kD is the so-called dynamic virial coefficient that depends on the static second virial coefficient A2 and concentration dependence of the hydrodynamic friction in a given solvent.32 The dynamic virial coefficient is then obtained from the slope of the Dapp(c,q f 0) = f(c) relationship. It can be seen that for the PS160-b-PG70 sample, the diffusion coefficient depends significantly on concentration and the corresponding kD is equal to 1.53 mL g-1. For PS120-b-PG140 micelles the kD value is only slightly positive, which indicates that hydrodynamic behavior of these micellar particles is close to hard sphere. The observed difference in hydrodynamic behavior might at least partially result from shorter PS and longer PG block lengths in the latter copolymer, which in consequence should influence the structure and solvation of the particle. On the other (31) Laser Light Scattering: Basic Principles and Practice; Chu, B., Ed.; Academic Press: New York, NY, U.S., 1991. (32) Burchard, W. Adv. Polym. Sci. 1999, 143, 114.

Langmuir 2010, 26(22), 16791–16800

Siebert et al.

Figure 3. (A) Angular dependence of the apparent hydrodynamic radius and (B) distributions of the relaxation time measured at a scattering angle of 30° for PS-b-PG copolymers. The total concentration of polymers is 1 g dm-3.

Figure 4. Concentration dependence of the diffusion coefficient for PS-b-PG copolymers in toluene.

hand, for copolymers with xPG >0.5, dynamic virial coefficients are between -0.2 and -0.3 mL g-1 (Figure 4). The high positive kD value indicates swelling of the micelles with decreasing polymer amount, which can be explained by the Langmuir 2010, 26(22), 16791–16800

Article

fact that at high concentrations, particles are partly squeezed by repulsive osmotic interactions between swelled coronas. Negative values of the dynamic virial coefficient indicate that attractive interaction between micelles is preferred over repulsive. As a consequence, for samples with kD < 0 the hydrodynamic radius as well as the radius of gyration decreased with copolymer concentration in toluene (data not shown) and in such a case extrapolation to infinite dilution was not performed. Instead, apparent values at 1 g dm-3 are given in Table 2 for comparison. For samples with the lowest PG mass fractions, the radius of gyration could not be determined with the used light scattering equipment, indicating a size below set-up resolution. Thus, Rg values were estimated by the small-angle X-ray scattering method. For PS160-b-PG70 and PS120-b-PG140 copolymers the radius of gyration obtained by SAXS was equal to 10 and 18 nm, respectively. By calculating the ratio between the radius of gyration and the hydrodynamic radius, one obtains additional information on the particle shape.31 The Rg/Rh ratio for micelles built by copolymers with xPG 0.5 were ca. 20 times higher than these measured for PS160-b-PG70 and PS120-b-PG140 micelles. One cannot exclude that obtained Nagg,app values are influenced by uncertainty in determined dn/dc values. However, such high aggregation numbers were already reported in the literature for other systems and were attributed to nonspecifically aggregated block copolymer chains rather than well-defined single micelles. For example, Tuzar et al. reported values of Nagg as large as 4000 and Rh = 52 nm in a case of polycaprolactone-b-polystyrene-bpolycaprolactone copolymers in a mixture of toluene and 2,2,3,3tetrafluoropropanol.35 Xu et al. observed that PS-b-PEO copolymers formed particles in water with the aggregation numbers between 6700 and 12 000 and radii in a range of 50-70 nm.36 According to the literature, formation of such anomalous particles in block copolymer solutions can have different origins. Zhou and Chu showed that commercial pluronic underwent abnormal association due to the presence of different impurities, among which PPO homopolymer seemed to play the most crucial role.37,38 Stejskal et al. discussed that block copolymers do not (33) Vagberg, L. J. M.; Cogan, K. A.; Gast, A. P. Macromolecules 1991, 24, 1670. (34) Boyko, V.; Richter, S.; Mende, M.; Schwarz, S.; Zschoche, S.; Arndt, K.-F. Macromol. Chem. Phys. 2007, 208, 710.  Lednicky, F. Macromol. Chem. 1988, (35) Tuzar, Z.; Stehlicek, J.; Konak, C.; 189, 221. (36) Xu, R.; Winnik, M. A.; Hallett, F. R.; Riess, G.; Croucher, M. D. Macromolecules 1991, 24, 87. (37) Zhou, Z.; Chu, B. Macromolecules 1987, 20, 3089.

DOI: 10.1021/la102780y

16795

Article

Siebert et al.

Scheme 1. Reaction between Titanium Tetraisopropoxide and Hydroxyl Groups of Polyglycidol Block Catalyzed by Hydrochloric Acid

form equilibrated micelles in a solution if the particle core is in a glassy state or nonswollen or if there are specific interactions within a core.39 Moreover, frozen micellar structures can be observed if copolymers experience temporal structuring in a solid state (before solution preparation).39 Polyglycidol is a polymer with a glass transition temperature around -20 °C.40 As a consequence, one can exclude formation of a glassy core within the micelles. On the other hand, it can be assumed that pendant hydroxyl groups can form intermolecular hydrogen bonds. Recently, Rangelov et al. studied properties of high molecular weigh polyglycidol in water by static and dynamic light scattering methods.41 They claim that hydroxyl groups in the PG chain might form hydrogen bonds. Thus, we believe that association through hydrogen bonds is the most probable explanation for the formation of nonequilibrated PS-b-PG micelles by copolymers with high polyglycidol content where cooperative bonding of hydroxyl groups should be strong. This could also explain long times required for dissolution of these copolymers in toluene. It is obvious that this subject needs more detailed studies which are on the way. 3.2. Hybrid Micelles Based on PS-b-PG Copolymer and Titanium Tetraisopropoxide. 3.2.1. Influence of HCl(aq) on the Structure of PS-b-PG Micelles in Toluene. As copolymers with PG content higher than 0.5 formed nonequilibrated micelles in toluene and showed complex aggregation behavior, we decided to use PS120-b-PG140 (with xPG = 0.45) for the preparation of cross-linked particles. This sample was characterized by well-defined structure and hydrodynamic behavior also at higher concentrations, which is especially important from the application point of view. In the present work, self-assembled copolymer chains in toluene are first loaded with concentrated aqueous HCl (38) Zhou, Z.; Chu, B. Macromolecules 1988, 21, 2548.  Plestil, J.; Kratochvil, P. Polymer (39) Stejskal, J.; Sikora, D. H.; Konak, C.; 1992, 33, 3675. (40) Vandenberg, E. J. Coordination Polymerization in Polymer Science and Technology; Price, C. C.; Vandenberg, E. J., Eds.; Plenum: New York, NY, U.S., 1983; p11. (41) Rangelov, S.; Trzebicka, B.; Jamroz-Piegza, M.; Dworak, A. J. Phys. Chem. B 2007, 111, 11127.

16796 DOI: 10.1021/la102780y

solution, followed by the addition of titanium tetraisopropoxide. In such a case, the aqueous phase diffuses into the micellar core, forming a compartment where a sol-gel process occurs. The presence of the acid accelerates hydrolysis and condensation.42 As shown in Scheme 1, HCl activates the titanium alkoxide for condensation with the hydroxyl groups of polyglycidol, leading to a covalent linking of the organic and inorganic components. When PS120-b-PG140 solutions are loaded with aqueous HCl, solution turbidity appears. The higher the loading is, the stronger is the opacity of the samples. This can be explained by the fact that a not well-dispersed aqueous phase immediately forms big miniemulsion droplets stabilized by the copolymer chains. In this way, a certain fraction of primarily formed small spherical micelles is lost. For further studies, we chose a system with 0.5 or 1.0 equiv of HCl with respect to the glycidol unit as typically used in the previous reports.15 To reduce the turbidity, the solutions were filtrated before light scattering experiments through 1 μm PTFE filters. Thermogravimetrical measurements revealed that after filtration of loaded samples, around 10-15% of the initial polymer mass is lost (data not shown). In Figure 5A the distribution of the relaxation time of the autocorrelation function at a scattering angle of 90° for 0.5 equiv of HCl-loaded PS120-b-PG140 micelles is shown. The distribution is described by two relaxation processes. It is to be noted that both of them show pure diffusive character as the plot of Γ vs q2 passes the origin (inset in the Figure 5B). The spectrum is dominated by a peak at higher relaxation times (socalled slow mode) which corresponds to the diffusion of the particles with the apparent Rh = 100 nm. However, one can also see a pronounced peak at shorter relaxation time (so-called fast mode), which should correspond to the single micelles. To clearly assign both modes to certain types of structures formed in solution, angular dependent measurements were performed (Figure 5B). The fast mode does not show angular dependency (Figure 5B), giving an extrapolated Rh value around 20 nm. As a consequence, it corresponds to single micelles loaded with HCl. (42) Hench, L. L.; West, J. K. Chem. Rev. 1990, 90, 33.

Langmuir 2010, 26(22), 16791–16800

Siebert et al.

Article

Figure 6. Small angle X-ray scattering results for PS120-b-PG140 Figure 5. (A) Distribution of the hydrodynamic radius for PS120b-PG140 copolymer micelles in toluene without and with 0.5 equiv of HCl loading, at a scattering angle of 90°. (B) Angular dependency of the slow (squares) and the fast mode (circles) for PS120-bPG140 copolymer micelles in toluene loaded with 0.5 equiv of HCl. The inset of (B) shows an angular dependency of the relaxation rate of both modes. The initial concentration of the copolymer in solution is equal to 5 g dm-3. Solutions are filtered through 1 μm PTFE filter.

On the other hand, for the slow mode the hydrodynamic radius strongly depends on the scattering angle and the extrapolated Rh value was equal to 136 nm. These big particles in a sample loaded with HCl(aq) resulted from water droplets in toluene solution stabilized by the PS120-b-PG140 copolymer. However, it should be emphasized that the scattered intensity is proportional to the size in the power of six and thus their number fraction after filtration is small. 3.2.2. Loading of Titanium Tetraisopropoxide into PS-bPG(HCl) Micelles in Toluene. As discussed above, aqueous solution of hydrochloric acid within the micelles catalyzes the reaction between the titanium alkoxide precursor and the glycidol units, leading to the formation of inorganic particles with sizes limited to the micellar core. To confirm constrained hydrolysis of titanium tetraisopropoxide within the micelles, small-angle X-ray scattering was used, as titanium possesses a very high contrast in comparison with organic polymeric material. In Figure 6A, the pair-distance distribution function p(r) for neat PS120-b-PG140 copolymer sample at concentration of 10 g dm-3 is shown and compared with samples of the same copolymer concentration but loaded with 1.0 equiv of HCl/titanium tetraisopropoxide. Langmuir 2010, 26(22), 16791–16800

copolymer micelles in toluene without and with 1.0 equiv of Ti(OC3H7)4 and HCl/Ti(OC3H7)4. (A) Pair-distance distribution function and (B) scattered X-ray intensities as a function of the scattering vector. In (B), the scattering curve for PS120-b-PG140 loaded with Ti(OC3H7)4 is shifted upward by a factor of 7 to prevent curve overlay. Inset: X-ray intensity scattered by Ti(OC3H7)4 in toluene. The initial concentration of copolymer in solution is equal to 10 g dm-3. Solutions are filtered through 1 μm PTFE filter.

For comparison purposes, a curve for micelles loaded only with 1.0 equiv of precursor is added to the plot. p(r) reveals maxima at 18, 20, and 22 nm for micelles without loading, loaded only with metal precursor and loaded with HCl/ Ti(OC3H7)4, respectively. The observed difference is a consequence of swelling of the PG core by the low-molecular-weight components. Additionally, the peak maximum of the pair-distance distribution function for loaded micelles increases. For sample containing HCl/Ti(OC3H7)4 the signal is 4 times higher than for neat micelles. This enhancement comes from Ti-based nanoparticles formed upon hydrolysis of the precursor in an acidic microenvironment inside the core. Additionally, a small increase of the p(r) for the particles loaded only with titanium alkoxide is observed. As hydrolysis of the alkoxides is very sensitive to even a small amount of H2O, the latter effect results from the hydrolysis of the precursor by traces of water always bound to hydrophilic polymers. It is also important to note that the maximum distance r of the pair-distance distribution function changed in the latter case only slightly, indicating that hydrolysis of titanium tetraisopropoxide occurs in the interior of the micelles or closely to the core-corona interface. On the other hand, for micelles loaded with HCl/Ti(OC3H7)4 one can see a pronounced DOI: 10.1021/la102780y

16797

Article

Siebert et al.

Figure 7. Normalized intensity autocorrelation functions g(2)(t) and the corresponding distributions of the hydrodynamic radii for PS120-bPG140 copolymer micelles in toluene at different concentrations. (A) Samples loaded with 0.5 equiv of HCl and (B) samples loaded with 0.5 equiv of HCl/Ti(OC3H7)4. The initial concentration of copolymer in solution is equal to 5 g dm-3. Solutions are filtered through 1 μm PTFE filter. Measurements are performed at a scattering angle of 90°.

shoulder in p(r). This phenomenon results from imposition of the pair-distance distribution functions from particles with two different sizes-one should remember that we observed a fraction of particles with Rh>100 nm in DLS measurements. The latter conclusion is supported by the results presented in Figure 6B. It can be seen that scattering curves obtained for unloaded PS-b-PG micelles and loaded with titanium alkoxide (shifted to avoid curve overlaying) are almost identical; thus they can be ascribed by the same form factor. On the other hand, the scattering curve for the sample containing both HCl and titanium precursor has the same slope in the q range between 0.01 and 0.1 A˚; however, it is shifted upward for the scattering vectors 16798 DOI: 10.1021/la102780y

below 0.01 A˚ in comparison with two other systems. It indicates a presence of the scattering objects with different length scales (bigger sizes), which after indirect Fourier transformation appear as a shoulder in the pair-distance distribution function. However, if lower scattering vectors were accessible, one should observe the evolution of the whole scattering curve of these aggregates and in consequence second peak in the p(r) should be observed.28 In the inset to Figure 6B, one can additionally see the intensity changes vs the scattering vector of a pure precursor in toluene. The obtained intensities are about 2-3 orders of magnitude smaller than for copolymer micelles without loading, and the experimental points are strongly scattered. The calculated radius Langmuir 2010, 26(22), 16791–16800

Siebert et al.

Figure 8. (A) Laser light intensity scattered by PS-b-PG copolymer micelles loaded with 0.5 equiv of HCl and Ti(OC3H7)4 as a function of time after dilution from 5 to 0.05 g dm-3. Intensities scattered by PS-b-PG solution are normalized against the intensity of toluene. (B) Normalized intensity autocorrelation functions g(2)(t) and normalized Rh distribution obtained for the same sample after 3 weeks from dilution. All measurements are performed at a scattering angle of 90°.

of gyration is equal to ca. 2 nm; however, this value is apparent due to high uncertainty of the experimental data. Nevertheless, on this basis we can state that titanium tetraisopropoxide was stable in the toluene used and noncontrolled hydrolysis of the alkoxide (for example, outside micelles) in the experiments with PS-b-PG copolymers can be ruled out. To verify the permanent linkage between the organic and inorganic components and thus formation of stable hybrid particles, the following experiments were performed. First, micellar solutions loaded with HCl and titanium tetraisopropoxide were diluted far below the critical micellization concentration. Second, the loaded micelles were isolated by freeze-drying and treated with DMF, which is a good solvent for both blocks. If covalent bonds are not formed within the core, micelles should disintegrate to unimers. In Figure 7, the normalized intensity autocorrelation functions g(2)(t) and the corresponding distributions of the hydrodynamic radii are shown for PS120-b-PG140 micelles with 0.5 equiv of HCl/ Ti(OC3H7)4 at different concentrations. For comparison, a block copolymer solution containing only HCl was diluted in the same way. Langmuir 2010, 26(22), 16791–16800

Article

Figure 9. (A) Normalized intensity autocorrelation functions g(2)(t) and (B) the corresponding distribution of the hydrodynamic radius measured at a scattering angle of 90° for PS-b-PG micelles loaded at c = 5 g dm-3 in benzene with 0.5 equiv of HCl and Ti(OC3H7)4 and subsequent freeze-drying and redissolution in DMF.

It can be seen that at c = 0.05 g dm-3 a scattering signal from micelles containing only HCl is no longer detected, as a consequence of dissociation below the cmc (0.25 g dm-3, Table 2). On the other hand, autocorrelation functions measured for the micelles with titanium precursor showed no change upon dilution. This indicates that the stability of these micelles in the presence of the precursor is much higher than that of the native PS-b-PG micelles. The corresponding hydrodynamic radii distributions were also evaluated and are given in Figure 7B. Here, one can clearly see that a decrease in concentration of the HCl-loaded micellar system induced disintegration. On the other hand, the distributions for micelles immobilized with HCl and Ti(OC3H7)4 are the same even below cmc. Slight broadening may come from swelling of the micelles below the cmc. However, as reported for coordinative interactions between polymer and metal salts, the stability of the micelles can be strongly enhanced and several weeks are required to see their full disintegration on the surface.18 For this reason, we checked the long-time stability of a 0.05 g dm-3 PS120-b-PG140 micellar solution containing HCl and titanium tetraisopropoxide. For this purpose the scattered light intensity was monitored up to 6 weeks from dilution (Figure 8). The laser light intensities scattered by PS120-b-PG140 solutions decreased around 25% up to 2 weeks from dilution, which led also DOI: 10.1021/la102780y

16799

Article

to a decrease of the intercept of the normalized intensity autocorrelation function (Figure 8B). Additionally, the distribution of the hydrodynamic radius recorded after 3 weeks from solution preparation revealed a maximum in Rh,app close to the single micellar particles, as shown in the inset of Figure 8B. Thus, the observed intensity drop is mainly due to dissolution of larger particles, which contributed more strongly to the scattered intensity; however, as already mentioned, their number fraction in the sample is low. This dissolution effect can arise from partial hydrolysis of the C-O-Ti bond in acidic aqueous environment present in the micellar core. To exclude all these factors, the stability of formed hybrid micelles was checked with another approach. The block copolymer without and with HCl and titanium tetraisopropoxide were first prepared in benzene and stirred for at least 24 h, followed by freeze-drying. Preparation of a sample in benzene instead of toluene had no influence on micellar size and its distribution. It was also found that redissolving of freeze-dried micelles in benzene resulted in the same particle size and slightly broader distribution (data not shown). However, during the freeze-drying step, water and HCl are removed from the internal micellar compartment and as a consequence hydrolysis of the C-O-Ti bond can be effectively limited. Loaded and unloaded micelles were redispersed in DMF and analyzed with light scattering. The intensity autocorrelation function for solution containing hybrid cross-linked micelles and the calculated distribution are shown in Figure 9A,B, respectively. Additionally, the ACF for block copolymer micelles loaded only with HCl is also shown for comparison purposes. First, it can be clearly seen that micelles with hydrochloric acid and titanium tetraisopropoxide were characterized by a normalized autocorrelation function with a high intercept while for block copolymer containing only HCl, the ACF was again similar to that for pure toluene. A high ACF intercept in DMF indicates undoubtedly that the stability of the hybrid PS-b-PG micelles could result only from permanent connection between polyglycidol units and hydrolyzed precursor within the core. The calcu(43) Gutierrez, J.; Tercjak, A.; Garcia, I.; Peponi, L.; Mondragon, I. Nanotechnology 2008, 19, 155607.

16800 DOI: 10.1021/la102780y

Siebert et al.

lated distribution of Rh,app shows a relatively broad peak with a maximum around 70 nm. This proves that the presence of a good solvent for both blocks leads to swelling of the micelles. It should be emphasized that no changes in the distribution or the value of scattered intensity were observed even after 3 weeks from preparation, thus hydrolysis of C-O-Ti bond did not take place. Finally, we compared the behaviors of PS-b-PG micelles and polystyrene-block-poly(ethylene oxide) copolymer under similar conditions. In the latter case, interactions between polymer and precursor are based on hydrogen-bonding only.43 The results for loaded and nonloaded PS-b-PEO micelles are given in the Supporting Information. The conclusion from performed experiments is that relatively weak hydrogen bonds formed between PEO units and hydrolyzing precursor are not enough to ensure integrity of the particles in solution below the cmc.

4. Conclusions Polystyrene-block-polyglycidol copolymers form inverse micelles in toluene. The characterization of particles formed by static and dynamic light scattering methods revealed that copolymers containing a mass fraction of polyglycidol block below 0.5 selfassemble into core-shell micelles while samples with higher PG content form nonspecifically aggregated particles. Their presence results most probably from multiple hydrogen bond formation between PG units that can strongly restrict their complete equilibration in solution. Loading of well-defined core-shell micelles with HCl and titanium tetraisopropoxide leads to hydrolysis of the titanium precursor within the micellar core. It was also proven that a sol-gel process leads to a permanent connection between glycidol units and the inorganic component via a condensation reaction. As a consequence, stable hybrid micelles are obtained, which in the following can be used for highly controlled nanostructuring of TiO2 on the surface and for the preparation of release systems. Supporting Information Available: Dynamic light scattering results for HCl loaded and HCl/Ti(OC3H7)4 loaded poly(ethylene oxide)-block-polystyrene copolymer micelles in toluene. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(22), 16791–16800