pubs.acs.org/Langmuir © 2010 American Chemical Society
Synthesis of Narrowly Size-Distributed Metal Salt/Poly(HEMA) Hybrid Particles in Inverse Miniemulsion: Versatility and Mechanism Zhihai Cao,† Zhuo Wang,† Christine Herrmann,† Katharina Landfester,†,‡ and Ulrich Ziener*,† †
Institute of Organic Chemistry III - Macromolecular Chemistry and Organic Materials, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany, and ‡Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany Received September 3, 2010. Revised Manuscript Received October 16, 2010
Hybrid particles containing different hydrophilic metal salts such as tetrafluoroborates of iron(II), cobalt(II), nickel(II), copper(II), and zinc(II), and nitrates of cobalt(II), nickel(II), copper(II), zinc(II), and iron(III), and cobalt(II) chloride were synthesized via inverse miniemulsion polymerization of 2-hydroxyethyl methacrylate (HEMA). All salts delivered narrowly size-distributed hybrid particles with the exception of iron(III), where only the nitrate salt could be successfully employed. The size and size distribution of the hybrid particles were characterized by dynamic light scattering (DLS) and transmission electron microscopy (TEM). The particle morphology and the distribution of salt in the dried particles were observed by TEM. The influences of the type of metal salts and salt content on the particle size distribution were extensively investigated.
Introduction Polymeric nanoparticles containing solid or liquid, inorganic or organic, hydrophobic or hydrophilic compounds are of great interest for many applications.1 It is promising to encapsulate metal compounds (including metal or metal oxide nanoparticles, metal salts, and so on) in the core or immobilize them on the surface of polymer particles with respect to their potential applications in the fields of semiconductors, optics, catalysis, electronics, nanolithography, and so forth.2-7 Miniemulsion polymerization has been proven to be well-suited to prepare metal-containing nanoparticles due to its specific droplet nucleation mechanism.1,8,9 The metals or metal compounds can be homogeneously distributed in the polymer matrix or encapsulated as aggregates (metal or metal oxide nanoparticles). It is often difficult to directly encapsulate metal or metal oxide nanoparticles, and an additional surface modification of the nanoparticles is required to improve the colloidal stability against coalescence and the affinity between polymers and metal particles prior to polymerization.10,11 The encapsulation of metal salts via miniemulsion polymerization can be performed more easily than the encapsulation of metal or metal oxide particles because they can (molecularly) dissolve directly in the disperse phase. Highly uniform particles containing hydrophobic metal complexes via direct miniemulsion or emulsion *To whom correspondence should be addressed. E-mail: ulrich.ziener@ uni-ulm.de.
(1) Landfester, K. Angew. Chem., Int. Ed. 2009, 48, 4488–4507. (2) Mei, Y.; Sharma, G.; Lu, Y.; Ballauff, M. Langmuir 2005, 21, 12229–12234. (3) Suzuki, D.; Kawaguchi, H. Langmuir 2005, 21, 8175–8179. (4) Chatterjee, J.; Bettge, M.; Haik, Y.; Chen, C. J. J. Magn. Magn. Mater. 2005, 293, 303–309. (5) Manzke, A; Pfahler, C; Dubbers, O.; Plettl, A.; Ziemann, P.; Crespy, D.; Schreiber, E.; Ziener, U.; Landfester, K. Adv. Mater. 2007, 19, 1337–1341. (6) Schreiber, E.; Ziener, U.; Manzke, A.; Plettl, A.; Ziemann, P.; Landfester, K. Chem. Mater. 2009, 21, 1750–1760. (7) Cao, Z.; Wang, Z.; Herrmann, C.; Ziener, U.; Landfester, K. Langmuir 2010, 26, 7054–7061. (8) Asua, J. M. Prog. Polym. Sci. 2002, 27, 1283–1346. (9) Schork, F. J.; Luo, Y.; Smulders, W.; Russum, J. P.; Butte, A.; Fontenot, K. Adv. Polym. Sci. 2005, 175, 129–256. (10) Liu, P. Colloids Surf., A 2006, 291, 155–161. :: (11) Demir, M. M.; Koynov, K.; Akbey, U.; Bubeck, C.; Park, I.; Lieberwirth, I.; Wegner, G. Macromolecules 2007, 40, 1089–1100.
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polymerization have been reported.5,6 These metal-containing hybrid particles have been successfully applied in the field of nanolithography as etching masks to prepare ordered arrays of nanopillars or nanoholes. However, further application of these metal-containing hybrid particles has been restricted by the low loading capacities of metal complexes in direct systems due to the poor solubility of most of the hydrophobic metal complexes in the disperse phase composed of the hydrophobic monomer and the (ultra)hydrophobe. The disperse phase of inverse miniemulsion consists of a polar mixture. Compared to direct systems, it is possible to load more metal salts via inverse miniemulsion polymerization because of the high solubility of hydrophilic salts in the polar mixture. The inverse miniemulsion polymerization has been extensively used to synthesize various hydrophilic (co)polymers.12-16 Recently, narrowly size-distributed hybrid particles containing a large amount of cobalt tetrafluoroborate (Co(BF4)2) were successfully prepared via inverse miniemulsion polymerization of 2-hydroxyethyl methacrylate (HEMA).7 In that paper, the influence of the type and amount of cosolvent, apolar continuous phase, amount of cobalt salt, and molecular weight of the polymeric surfactant on the particle size distribution and colloidal stability was investigated. Narrowly size-distributed Zn(NO3)2/ polyacrylamide particles were reported by Kobitskaya et al.17 The formation of the complex of acrylamide and Zn(NO3)2 was believed to favor the narrowing of the particle size distribution of Zn(NO3)2-containing polyacrylamide particles. More recently, narrowly size-distributed thermosensitive poly(N-isopropylacrylamide) nanocapsules were synthesized by using Co(BF4)2 as the lipophobe.18 (12) Landfester, K.; Willert, M.; Antonietti, M. Macromolecules 2000, 33, 2370– 2376. (13) Oh, J. K.; Tang, C.; Gao, H.; Tsarevsky, N. V.; Matyjaszewski, K. J. Am. Chem. Soc. 2006, 128, 5578–5584. (14) Oh, J. K.; Perineau, F.; Matyjaszewski, K. Macromolecules 2006, 39, 8003– 8010. (15) Blagodatskikh, I.; Tikhonov, V.; Ivanova, E.; Landfester, K.; Khokhlov, A. Macromol. Rapid Commun. 2006, 27, 1900–1905. (16) Crespy, D.; Landfester, K. Polymer 2009, 50, 1616–1620. (17) Kobitskaya, E.; Ekinci, D.; Manzke, A.; Plettl, A.; Ziemann, P.; Ziener, U.; Landfester, K. Macromolecules 2010, 43, 3294–3305. (18) Cao, Z.; Ziener, U.; Landfester, K. Macromolecules 2010, 43, 6353–6360.
Published on Web 11/08/2010
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Article
In the present paper, narrowly size-distributed particles containing a variety of metal salts including tetrafluoroborates, nitrates, and chlorides were successfully prepared via inverse miniemulsion polymerization of HEMA. The influence of the type and content of metal salts on the particle size distribution was thoroughly investigated in a wide range of salt types and contents.
Table 1. Particle Size (PS) and PDIs of Narrowly Size-Distributed Hybrid Particles Containing Different Salts salts
PS (nm) PDI
Fe(BF4)2
Co(BF4)2
Ni(BF4)2
Cu(BF4)2
Zn(BF4)2
128 0.019
129 0.014
126 0.014
121 0.017
125 0.012
Experimental Section Materials. The monomer, 2-hydroxyethyl methacrylate (HEMA, Aldrich, 97.0%), was purified by passing through a column filled with alumina and stored in the refrigerator before use. The block copolymer surfactant, poly(ethylene-co-butylene)b-poly(ethylene oxide) (P(E/B)-PEO), was synthesized according to the literature.19,20 The number average molecular weight of P(E/B)-PEO used in this report was 8700 g mol-1 determined by GPC calibrated against P(E/B)-PEO samples which were characterized by NMR. The initiator, R,R0 -azoisobutyronitrile (AIBN, Merck, 98.0%), the apolar solvent, cyclohexane (C6H12, VWR Prolabo, 99.5%), the cosolvent, methanol (Merck, 99.9%), and the metal salts, cobalt(II) tetrafluoroborate hexahydrate (Co(BF4)2 3 6H2O, Aldrich, 99.0%), iron(II) tetrafluoroborate hexahydrate (Fe(BF4)2 3 6H2O, Aldrich, 97.0%), nickel(II) tetrafluoroborate hexahydrate (Ni(BF4)2 3 6H2O, Strem Chemicals, 99.0%), copper(II) tetrafluoroborate hexahydrate (Cu(BF4)2 3 6H2O, Strem Chemicals, 99.0%), zinc(II) tetrafluoroborate hydrate (Zn(BF4)2 3 nH2O, n = 6-7, Aldrich, 99.0%), cobalt(II) nitrate hexahydrate (Co(NO3)2 3 6H2O, Merck, 99.0%), nickel(II) nitrate hexahydrate (Ni(NO3)2 3 6H2O, Strem Chemicals, 99.0%), copper(II) nitrate hemipentahydrate (Cu(NO3)2 3 2.5H2O, Aldrich, 98%), zinc(II) nitrate hexahydrate (Zn(NO3)2 3 6H2O, Fluka, 99.0%), iron(III) nitrate nonahydrate (Fe(NO3)3 3 9H2O, Acros Organics, 99.0%), and cobalt(II) chloride hexahydrate (CoCl2 3 6H2O, Acros, for analysis) were used as received.
Preparation of Inverse Miniemulsion and Polymerization. The surfactant P(E/B)-PEO (3 wt % with respect to the polar (disperse) solution) was dissolved in 12.5 g of C6H12 under magnetic stirring. A respective amount of metal salt was first dissolved in 0.5 g of methanol and then mixed with 1.5 g of HEMA to form a transparent solution. The polar solution was mixed with the surfactant solution. After 15 min pre-emulsification under strong magnetic stirring, the mixture was treated with 120 s ultrasound with a Branson 450W digital sonifier at 90% amplitude in an ice bath to prepare a miniemulsion. The initial miniemulsion was introduced to the reactor and purged with argon for 3 min under magnetic stirring. The argon protected reaction mixture was placed in a preheated oil bath (65 °C) and stirred for 3 h. The amount of metal salt varied in the range of 0.5-8.6 mol % relative to the molar amount of HEMA.
Characterization.
Dynamic Light Scattering (DLS).
The particle size and size distribution (as polydispersity index (PDI) and intensity particle size distribution) were measured by DLS (Nano-Zetasizer, Malvern Instruments) at 20 °C under the scattering angle of 173° at 633 nm wavelength. The dispersions were diluted with HD in a glass cuvette before the measurement. Particle sizes and PDIs are given as the average of three measurements. The PDI is a measure of the particle size distribution, and the PDI is a dimensionless number that describes the heterogeneity of the sample; it can range from 0 (monodisperse) to 1 (polydisperse). Transmission Electron Microscopy (TEM). TEM measurements were performed on a Philips EM 400 microscope. An amount of 1.5 μL of dispersion was diluted with 3 mL of C6H12, and then 1.5 μL of the diluted sample was placed on a 400-mesh carbon-coated copper grid and dried at 40 °C for (19) Schlaad, H.; Kukula, H.; Runloff, J.; Below, I. Macromolecules 2001, 34, 4302–4304. (20) Thomas, A.; Schlaad, H.; Smarsly, B.; Antonietti, M. Langmuir 2003, 19, 4455–4459.
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salts Fe(NO3)3 Co(NO3)2 Ni(NO3)2 Cu(NO3)2 Zn(NO3)2 CoCl2 PS (nm) 191 PDI 0.058
105 0.018
107 0.028
127 0.025
104 0.025
114 0.018
at least 4 days. To enhance the contrast, an additional carboncoating was performed before the measurement. Conductivity. Conductivity measurements were performed on an Inolab pH/Cond Level 1 pH conductivity meter with a Tetracon 325 conductivity cell. Fourier transform infrared (FTIR) Measurement. FTIR spectra were recorded on a Bruker IFS113v FTIR spectrometer (Rheinstetten/Karlsruhe). Thermogravimetric Analysis (TGA). The composite particles were collected by centrifuging the dispersion at 20 000 rpm for 1 h. The residual amounts of the Zn(NO3)2-containing composite particles were measured by TGA on a Mettler-Toledo TG/SDTA 851e instrument by heating from 25 to 1100 °C at the rate of 10 °C min-1 in an oxygen flow. The solid residue is ZnO.17
Results and Discussion Synthesis of Narrowly Size-Distributed Particles Containing Different Salts. In inverse miniemulsion, hydrophilic salts were extensively used as lipophobe to improve the droplet stability. So far, sodium salts were prevailingly used in inverse miniemulsion systems. However, compared to sodium salts, the transition metal salts have more advantages in further applications. Recently, it has been reported that narrowly size-distributed hybrid particles containing metal salts can be used in the field of nanolithography to prepare nanopillars or nanoholes.5,6,17 Compared to the abundance of metal salts/polymer hybrid particles synthesized by direct systems, the hybrid particles synthesized by inverse miniemulsion are relatively rare.7,16-18 In the present contribution, six transition metal cations (Fe2þ, Fe3þ, Co2þ, Ni2þ, Cu2þ, and Zn2þ) and three anions (BF4-, NO3-, and Cl-) were used to widen and better understand the scope of the concept we have presented in our previous publication to synthesize narrowly size-distributed particles by inverse miniemulsion of HEMA.7 Accordingly to the previous paper,7 one of the optimum combinations, namely, C6H12/methanol/P(E/B)-PEO (MW 8700 g mol-1), was used in the present contribution. Strictly speaking, the cosolvent is composed of methanol and water introduced by the crystal water of the salt. The PDIs obtained from DLS and listed in Table 1 were between 0.01 and 0.03 for all metal salt/ poly(HEMA) particles with the exception of the particles containing Fe(NO3)3 (PDI = 0.058). It should be pointed out that the technique of dynamic light scattering is challenging for multimodal distributions. Thus, TEM was employed for further (visual) characterization of particle size and distribution. All TEM images in Figure 1 and Supporting Information Figure S1 show the formation of narrowly size-distributed particles containing different metals with the exception of Fe(NO3)3. The narrow size distribution is underlined by the appearance of ordered hexagonal arrays of particles. Because of the instability of poly(HEMA) under the electron beam, some defects such as hollow structures appear in some TEM images. Most of the salts DOI: 10.1021/la103540d
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Figure 1. Representive TEM images of narrowly size-distributed hybrid particles containing (a) 2.3 mol % Fe(BF4)2, (b) 5.4 mol % Co(NO3)2, (c) 6.5 mol % CoCl2, and (d) 4.3 mol % Fe(NO3)3 synthesized by inverse miniemulsion polymerization of HEMA.
are encapsulated successfully by the present technique based on the estimated amount of residue by TGA. For example, the encapsulation efficiencies in the systems with 2.1 and 8.6 mol % Zn(NO3)2 are 77 and 94%, respectively. Distribution of Salts in the Dried Particulate Poly(HEMA) Matrix. All hydrophilic salts used in this paper can easily dissolve in the disperse phase to form a transparent solution, and therefore, the salt is expected to homogenously distribute in the droplets. The final latex particle (after polymerization) consists of a concentrated solution of poly(HEMA) in the mixture of methanol and crystal water. Most probably, the salts can still homogenously distribute in the whole latex particles in the wet state (the presence of methanol and crystal water). In the dried state (after the removal of cosolvent in the particles and apolar solvent in the continuous phase), the distribution of salts (ions) in the poly(HEMA) matrix depends on the competition of the ion/poly(HEMA) and ion/ion interaction. The cobalt ion/HEMA and cobalt ion/poly(HEMA) interaction has been proven in our previous paper by FTIR spectroscopy and differential scanning calorimetry (DSC).7 In order to further confirm this interaction, FTIR spectroscopy was performed on poly(HEMA) with different Co(NO3)2 contents and the mixtures of HEMA and methanol containing different salts (see Supporting Information Figure S2). The spectra in Supporting Information Figure S2a show that the characteristic peak for the CdO stretching vibration in poly(HEMA) shifts from 1730 cm-1 for pure poly(HEMA) to 1726 cm-1 for poly(HEMA) containing 4.3 mol % Co(NO3)2. These results confirm the existence of an interaction between Co(NO3)2 and poly(HEMA). Compared to the mixture of methanol and HEMA, the band position belonging to the hydrogen bonded CdO groups is shifted by 4 cm-1 to lower wavenumbers due to the introduction of metal salt (Supporting Information Figure S2b). These results are in agreement with our previous results in ref 7 confirming the existence of interaction between metal salt and HEMA. It should be mentioned that the type of salt (different anions) had no effect on the position of the wavenumbers. The distribution of salts in the dried poly(HEMA) 18010 DOI: 10.1021/la103540d
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Figure 2. TEM images of narrowly size-distributed hybrid particles containing (a) 2.3 mol % Fe(BF4)2, (b) 6.5 mol % Cu(NO3)2, and (c) 6.5 mol % CoCl2 synthesized by inverse miniemulsion polymerization of HEMA.
matrix can be used to evaluate the interaction between the cation and anion. We assume that all employed metal BF4- salts distributed homogeneously in the dried poly(HEMA), as no particulate contrast was observed in the TEM images with high magnification (see Figures 2a and Supporting Information S3a-d). Contrarily, for the metal nitrates, some dark spots, presumably salt nanocrystals, were clearly observed in the TEM image of the particles containing Cu(NO3)2, (see Figure 2b), while Co(NO3)2, Ni(NO3)2, and Zn(NO3)2 could homogeneously distribute in the poly(HEMA) matrix (Supporting Information Figure S3e-g). The aggregates of salts were also observed in the product containing CoCl2 (Figure 2c). Concluded from these results, the distribution of salts in the poly(HEMA) matrix does not depend only on the type of cation but also on the anion. The salt’s ability to homogeneously distribute in the poly(HEMA) matrix decreased in the order of BF4- > NO3- > Cl-, obeying the reverse Hofmeister series (SO42- > CH3COO- > Cl- > NO 3 - > BF 4 -). 21,22 In fact, 4.3 mol % CoSO 4 and Co(CH3COO)2 cannot even completely dissolve in the mixture of methanol, crystal water, and HEMA, in agreement with the aforementioned finding. From these results, it can be deduced that the interaction between a given cation and different anions increased in the order of BF4- < NO3- < Cl-. Among the cations, copper(II) showed a stronger interaction with nitrate groups, compared to Co2þ, Ni2þ, and Zn2þ. Comparing the distribution of the different copper salts with the different cobalt salts in the poly(HEMA) matrix reveals that the type of anion has a more pronounced influence on the distribution (or the interaction between the cation and anion) than the type of cation, consistent with the fact that the Hofmeister effects of salts are mainly determined by the properties of the anion.23 Effects of the Type and Content of Salt on the Particle Size and Size Distribution. Droplet Stability. The droplets (21) Koga, Y.; Westh, P.; Davies, J. V.; Miki, K.; Nishikawa, K.; Katayanagi, H. J. Phys. Chem. A 2004, 108, 8533–8541. (22) Zhang, Y.; Cremer, P. S. Curr. Opin. Chem. Biol. 2006, 10, 658–663. (23) Lopez-Leon, T.; Santander-Ortega, M. J.; Ortega-Vinuesa, J. L.; Bastos-Gonzalez, D. J. Phys. Chem. C 2008, 112, 16060–16069.
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Article Table 2. Effect of the Time of Addition on the Particle Sizes and PDIs Co(BF4)2
Co(NO3)2
CoCl2
delay time (min)
particle size (nm)
PDI
delay time (min)
particle size (nm)
PDI
delay time (min)
particle size (nm)
PDI
0 15 30 60
130 125 126 127
0.014 0.009 0.020 0.016
0 15 30 60
130 124 130 134
0.014 0.007 0.015 0.023
0 15 30 60
114 120 126 129
0.018 0.032 0.022 0.008
in the initial miniemulsion can be “frozen” by the polymerization. The evolution of initial droplets before they are completely “frozen” may affect the final particle size distribution. As the polymerization rate and induction time could vary with the different types of salt or different salt contents, it is necessary to know how stable the droplets are before they are nucleated. Due to the difficulty to measure accurately droplet size and size distribution, the retention time of the droplets was varied by varying the waiting time of addition of initiator. The relationships between the addition time and the final particle size were used to evaluate the droplet stability, as reported in the literature.24 Three typical recipes containing Co(BF4)2 (2.1 mol % to HEMA), Co(NO3)2 (4.3 mol %), and CoCl2 (6.5 mol %), from which narrowly size-distributed particles were obtained, were adopted. In order to simulate the real situation of droplets before polymerization, we put the initial miniemulsions in the oil bath at the reaction temperature instead of storing them at room temperature. As shown in Table 2, the effects of the delay times on the particle size and PDIs were hardly observed in the systems containing Co(BF4)2 and Co(NO3)2. The slight increase of the size of the droplet containing CoCl2 with the increase of the addition time could be regarded as a hint that molecular diffusion could be slightly less suppressed in these systems than in the systems containing Co(BF4)2 and Co(NO3)2. Recalling the conclusion in the former section that the interaction between the cation and anion increased in the order of BF4- < NO3- < Cl-, it is reasonable to suppose that the dissociation ability of cobalt salts in the mixture of methanol, crystal water, and HEMA decreases in the order of Co(BF4)2 > Co(NO3)2 > CoCl2. The increase of the ion concentration in the droplets favors the suppression of the molecular diffusion. Therefore, the droplets containing Co(BF4)2 and Co(NO3)2 showed a better droplet stability compared to the droplets containing CoCl2 (see below). Particle Size and Size Distribution. In direct and inverse miniemulsion systems, a suitable amount of hydrophobe (direct) or lipophobe (inverse) must be added to improve the droplet stability by establishing an osmotic pressure in the droplets to counteract the Laplace pressure.1 Besides the establishment of an osmotic pressure in the droplets, the hydrophilic salts in inverse miniemulsion can influence the interfacial properties of the disperse and continuous phases and interact with the other compounds in the disperse phase including monomer, surfactant, and so on.7,16,17 In addition, the dissociation of salts depending on the nature of salt and the organic mixture also plays an important role for the particle properties and colloidal stability. Therefore, it is necessary to extensively investigate the dependence of particle size and size distribution on the type and content of salt. The dependencies of particle size and size distribution of hybrid particles containing BF4- salts on the type of cations and salt content are shown in Figure 3 and Supporting Information Figure S4. For a specific metal salt (for example, Fe(BF4)2; see Figure 3), the particle size and size distribution strongly depend on the salt content. In the range of 0.5-4.2 mol % to HEMA, with the increase of (24) Landfester, K.; Bechthold, N.; Tiarks, F.; Antonietti, M. Macromolecules 1999, 32, 5222–5228.
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Figure 3. Dependence of the particle sizes and PDIs of the hybrid particles synthesized by inverse miniemulsion polymerization of HEMA on the salt contents of Fe(BF4)2.
Figure 4. Relationship between the conductivity and salt contents of the solutions composed of different tetrafluoroborate salts, HEMA, and methanol.
Fe(BF4)2 content, the particle size increases from about 80 nm to a maximum (about 130 nm) at 2.1 mol % and then decreases to a constant value at about 105 nm above 3.2 mol %, while the PDIs undergo a reverse trend. With the increase of Fe(BF4)2 content, the PDI decreases to a minimum (about 0.01) at 2.1 mol % and then increases to about 0.07 at 4.3 mol %. The overall trends of the particle sizes and PDIs with the increase of salt content are very similar for Co(BF4)2, Ni(BF4)2, Cu(BF4)2, and Zn(BF4)2 (see Supporting Information Figure S4). The particles with the narrowest particle size distribution and largest particle size could be synthesized at 2.1 mol % of metal BF4- salts regardless of the type of cation as shown in Figure 1a and Supporting Information Figure S1a-d. The miniemulsions were not very stable with 0.5 mol % BF4salt indicated by the fact that after the reaction mixture was introduced into the reactor, some visible mixture of oil and hydrophilic compounds resided on the wall of the previous container. The residual mixture was presumably composed of large droplets or macroscopically separated oil/water mixture due DOI: 10.1021/la103540d
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Figure 6. Relationship between the conductivity and salt contents of the solutions composed of different metal salts, HEMA, and methanol.
Figure 5. Dependence of the particle sizes and PDIs of the hybrid particles synthesized by inverse miniemulsion polymerization of HEMA on the salt contents of (a) Co(NO3)2, (b) Cu(NO3)2, and (c) Fe(NO3)3.
to the poor suppression of molecular diffusion. Surprisingly, the particles in the resulting dispersion showed a narrow particle size distribution indicated by low PDIs (0.025-0.045) and TEM images in Supporting Information Figure S5. In addition, the particle size (about 80 nm) was hardly influenced by the type of cation in the systems with 0.5 mol % BF4- salts. We assume that those systems are rather dominated by an emulsion type polymerization and the particles are most probably formed by micellar or homogeneous nucleation instead of droplet nucleation predominant in miniemulsion. The miniemulsions with more than 1 mol % BF4- salts were visually stable, supporting that the droplet stability is enhanced with the increase of salt content due to the establishment of a more effective osmotic pressure in the droplets. 18012 DOI: 10.1021/la103540d
The particle size reduced to about 105 nm when the salt contents were above 3.2 mol % for all the BF4- salts. According to the results of conductivity in Figure 4, the concentration of ion species in the droplets increased with the increase of salt content. In other words, the osmotic pressure established in the droplets was expected to increase with the increase of salt content to counteract a higher Laplace pressure, leading to the decrease of particle size. However, the particle size distribution became broader as indicated by the PDIs in Figure 3 and Supporting Information Figure S5 and TEM images in Supporting Information Figure S6 compared to the particles containing 2.1 mol % salt regardless of the type of salts. The reason for broadening of the particle size distribution is not yet fully understood, but the discussion of this issue is given in the following part of this paper combining the results from the nitrate salts. The nitrates of Co (II), Ni(II), and Zn (II) showed a very similar dependence of the particle size and PDIs on the salt contents as the corresponding tetrafluoroborates (see Figure 5a and Supporting Information Figure S7). The particle size increased with increasing salt content to a maximum and then decreased to a constant value. A low PDI was observed in the system with a low salt content (about 1.1 mol % for the metal nitrates), which then increased a little with the increase of salt content to 2.2 mol %. The PDIs decreased with the further increase of salt content to a minimum, and then increased again with the increase of salt contents. For the initial miniemulsions containing metal nitrates, the residuals were observed in the systems with 1.1 mol % salts, higher than those of BF4- salts (0.5 mol %). The fact that more nitrate salts were required to prepare a stable miniemulsion could be regarded as a sign that the ability of nitrate to establish an osmotic pressure is lower than that of BF4- salts. We assume that the different dissociation abilities of nitrates and tetrafluoroborates may be responsible for the different osmotic pressures established in the droplets with the same content of NO3- and BF4- salts. Although the conductivity of electrolyte cannot be directly correlated with the concentration of different ions in solution, the large differences of conductivities between the NO3and BF4- salts at the same molar content seen in Figure 6 could be indirect evidence that BF4- salts more easily dissociate in the mixture of HEMA, methanol, and crystal water than the nitrate salts. As discussed in the former section, the interaction between the cations and NO3- group is stronger than that between the cations and BF4- group. This finding also supports the fact that the dissociation ability of nitrates is weaker than that of tetrafluoroborate if the cation is the same. The optimum salt content for preparing narrowly size-distributed hybrid particles containing Langmuir 2010, 26(23), 18008–18015
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Figure 7. TEM images of hybrid particles synthesized by inverse miniemulsion polymerization of HEMA with different Co(NO3)2 contents: (a) 1.1 mol %, (b) 2.2 mol %, (c) 3.2 mol %, (d) 4.3 mol %, (e) 5.4 mol %, (f) 6.5 mol %, and (g) 8.6 mol %.
Co(NO3)2, Ni(NO3)2, and Zn(NO3)2 also shifted to a higher value (4.3 mol %) compared to the BF4- salts (2.1 mol %). Instead of the narrow range of salt content (close to 2.1 mol %) of the BF4salts, the range of Co(NO3)2, Ni(NO3)2, and Zn(NO3)2 contents to access the narrowly size-distributed particles became relatively broader, lying in the range of 4.3-6.5 mol %. As an example, the TEM images of hybrid particles with different Co(NO3)2 contents are shown in Figure 7. Consistent with the PDIs obtained by DLS, small particles with a narrow size distribution were obtained in the system with 1.1 mol % Co(NO3)2 as shown in Figure 7a. Both small and large particles could be observed in the products with 2.2 and 3.2 mol % Co(NO3)2. According to these results, we assume the micellar or homogeneous nucleation in the system with 1.1 mol % Co(NO3)2 might be a relevant mechanism due to the instability of droplets with such a low Co(NO3)2 content. With the increase of Co(NO3)2 content, the droplet stability increased accordingly, and therefore, more and more particles were formed via droplet nucleation. However, the molecular diffusion could not be suppressed effectively in the system with a relatively low Co(NO3)2 content like 2.2 and 3.2 mol %. Consequently, some small particles were still produced by homogeneous nucleation. Small particles are hardly seen in the products with 4.3-8.6 mol % Co(NO3)2. We believe the droplet nucleation dominated in these systems due to the excellent suppression of molecular diffusion. Instead of the formation of a large amount of small particles to broaden the particle size distribution, a small amount of relatively larger particles appeared in the product with 6.5 and 8.6 mol % Co(NO3)2, leading to a broader particle size distribution. Similar results were observed in the systems with a high content of other metal salts including BF4- and NO3- salts (Supporting Information Figure S6). The absence of small particles in the systems containing a high salt content could be reasonably ascribed to the effective establishment of an osmotic pressure to counteract the Laplace pressure, by which the molecular diffusion among the particles and thus secondary (homogeneous) nucleation could be Langmuir 2010, 26(23), 18008–18015
suppressed effectively. As reported in our previous paper, in a specific apolar solvent, the particle size distribution strongly depends on the properties of surfactant and the disperse phase.7 It has been reported that the cloud point of PEO or its copolymers such as poly(ethylene oxide)-poly(propylene oxide) could be significantly influenced by the introduction of salts and depend on the type and concentration of the salts.25,26 This means that the properties of P(E/B)-PEO might change with the change of salt content. Moreover, according to the results in the literature, the influence of salt on the cloud points of compounds containing PEO chains will become more significant with the increase of salt content.25 Therefore, the failure to control the particle size distribution in the system with a high salt content might be due to the change of surfactant properties. The sizes and PDIs of the particles containing Cu(NO3)2 (see Figure 5b) underwent a very similar variation with increasing salt contents as the systems containing Co(NO3)2, Ni(NO3)2, and Zn(NO3)2. However, the requirement of salt content to obtain a stable miniemulsion increased to 3.2 mol % in the systems with Cu(NO3)2. In addition, the salt content required to prepare narrowly size distributed particles increased to 6.5 mol % Cu(NO3)2, higher than 4.3 mol % of the nitrates of Co2þ, Ni2þ, and Zn2þ. According to these results, we suppose that the dissociation ability of Cu(NO3)2 is relatively poorer than that of Co(NO3)2, Ni(NO3)2, and Zn(NO3)2. This assumption was supported by the formation of some salt nanocrystals of Cu(NO3)2 in the dried poly(HEMA) matrix due to the relatively strong interaction between Cu2þ and NO3- and the lower conductivity than Co(NO3)2, Ni(NO3)2, and Zn(NO3)2 at the same molar content (Supporting Information Figure S8). As shown in Figure 5c, the size of particles containing Fe(NO3)3 increased with increasing salt content in the range of 2.1-4.3 mol % and then decreased. Generally, the particle sizes were larger than those of the particles containing the same amount (25) Alexandridis, P.; Holzwarth, J. F. Langmuir 1997, 13, 6074–6082.
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Figure 8. (a) Particle sizes and PDIs and (b-h) TEM images of hybrid particles synthesized by inverse miniemulsion polymerization of HEMA with different CoCl2 contents: (b) 2.2 mol %, (c) 3.2 mol %, (d) 4.3 mol %, (e) 5.1 mol %, (f) 6.5 mol %, (g) 7.7 mol %, and (h) 8.6 mol %.
of the other nitrate salts. Although the PDIs show a minimum (0.058), narrowly size-distributed particles could not be prepared even in the system with the optimum salt content. It should be pointed out that the polymerization rates of the system containing Fe(NO3)3 were relatively lower than those of the systems containing any of the other metal salts. The low polymerization rate could be reasonably attributed to the relatively low redox potential between Fe2þ/Fe3þ. Actually, taking advantage of this property, the complexes of iron(III) salts such as FeCl3 have been used as catalyst in atom transfer radical polymerization (ATRP).27,28 However, the low polymerization rate was not expected to be the main reason for the broad particle size distribution, because intentionally delaying the start of the polymerization by late addition of the initiator did not have a significant influence on the particle size distribution (see above). The oxidation of the propagating radicals by Fe3þ can lead to termination and thus to the formation of oligomers or low molecular polymer chains. The polymer synthesized in the presence of Fe(NO3)3 dissolved readily in methanol in a few seconds to form a transparent solution proving the low molecular weight of the “polymers”, while the polymers synthesized in the presence of the other salts underwent a process from swelling to partially dissolving to form a turbid solution after 24 h magnetic stirring. It should be pointed out that the polymers containing Cu(NO3)2 could form a transparent solution, but the dissolution process took much longer compared to the polymer with Fe(NO3)3. The existence of low molecular weight polymer chains or oligomers would deteriorate the colloidal and droplet stability as reported in the literature on living radical polymerization in miniemulsion.29,30 We also observed
that more coagulum (9.0 wt % to HEMA) is produced in the systems with Fe(NO3)3 compared to the systems containing the other types of salt, for example, about 3 wt % in the systems containing Co(NO3)2. Therefore, it is reasonable to attribute the broad particle size distribution in the systems with Fe(NO3)3 to the existence of low molecular weight polymer chains or oligomers. Most probably, the low contrast of particles shown in the TEM image (Figure 1d) is also due to the low molecular weight of the polymers. In the system encapsulating CoCl2, the particle size increased again with increasing the salt content to a maximum (135 nm) at 5.1 mol % and then decreased. The PDIs decreased first to a minimum (0.018) at 6.5 mol % and then increased. Contrary to the low PDIs (0.024 and 0.04) at 4.3 and 5.1 mol %, some small particles were observed in the TEM images (Figure 8d and e), presumably formed by homogeneous nucleation. Combining the DLS and TEM results, narrowly size-distributed CoCl2 containing particles were successfully synthesized in the system with 6.5 mol % CoCl2 content. Compared to Co(BF4)2 (2.1 mol %) and Co(NO3)2 (4.3 mol %), the requirement of salt content to effectively suppress the molecular diffusion shifted to a higher value (6.5 mol %). This again could be ascribed to the relatively poor dissociation ability of CoCl2 in the mixture of HEMA, methanol, and crystal water compared to Co(BF4)2 and Co(NO3)2 supported by the relationship between the conductivity and the salt types (see Figure 6) and the formation of salt aggregate (see Figure 2c). Both the TEM and DLS results point out that the particle size distribution is broadened by a further increase of the CoCl2 content.
(26) Ataman, M. Colloid Polym. Sci. 1987, 265, 19–25. (27) Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101, 2921–2990. (28) Kamigaito, M.; Ando, T.; Sawamoto, M. Chem. Rev. 2001, 101, 3689–3745. (29) Luo, Y.; Tsavalas, J.; Schork, F. J. Macromolecules 2001, 34, 5501–5507. (30) de Brouwer, H.; Tsavalas, J. G.; Schork, F. J.; Monteiro, M. J. Macromolecules 2000, 33, 9239–9246.
Conclusion In conclusion, the fundamental concept for the preparation of hybrid metal salt containing poly(HEMA) particles via inverse miniemulsion could be successfully extended to a broad variety of
18014 DOI: 10.1021/la103540d
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different metal salts. In detail, narrowly size-distributed particles containing different hydrophilic metal salts including tetrafluoroborates of iron(II), cobalt(II), nickel(II), copper(II) and zinc(II) and nitrates of cobalt(II), nickel(II), copper(II) and zinc(II), and cobalt(II) chloride were successfully synthesized via inverse miniemulsion polymerization of 2-hydroxyethyl methacrylate (HEMA). The particle size distribution could not be controlled in the systems containing Fe(NO3)3 due to the existence of low molecular weight polymer chains and oligomers. All employed salts were homogeneously distributed in the dried poly(2-hydroxyethyl methacrylate) (poly(HEMA)) matrix with the exception of Cu(NO3)2 and CoCl2. Some salt nanocrystals of Cu(NO3)2 and CoCl2 were randomly distributed in the poly(HEMA) matrix as shown by TEM. This could be regarded as the sign of a stronger interaction between the cation and anion in these two salts, compared to the other salts. For all employed salts, the dependence of particle size and PDI on the salt content was very similar. The particle size increased with increasing salt content to a maximum and then decreased to about 100 nm. For a stable miniemulsion, the PDIs decreased with the increase of salt content to a minimum and then increased. The requirement of salt content to obtain a stable miniemulsion depends on the salts and varies between 1% and 3.2%. Similarly, the salt contents to access narrowly size distributed particles range between 2.1 and 6.5 mol % depending on the type of salt. These results could be perfectly correlated with the dissociation ability of the metal salt.
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The salt content required to form a stable miniemulsion and to synthesize narrowly size-distributed particles shifts to a higher value with the increase of the interaction between the cation and anion due to the decrease of the dissociation ability of the metal salt. The effectiveness of osmotic pressure in the droplets strongly depends on the ion concentration in the disperse phase. Thus, monomers with stronger interactions with metal ions than HEMA may promote the dissociation of salt and subsequently improve the control of size and size distribution. The reason for broadening of the particle size distribution in the system with a relatively high salt content is not yet fully understood and is under investigation. We suspect that the variation of the properties of the disperse phase and the interaction between the ions and surfactant may play an important role in governing the particle size distribution. Acknowledgment. We greatly thank G. Weber for the synthesis of P(E/B)-PEO and the TGA measurements and E. Kaltenecker for the FTIR investigations. The support by Deutsche Forschungsgemeinschaft (DFG) within the Cooperative Research Center SFB 569 is gratefully acknowledged. Supporting Information Available: Additional TEM images and graphs. This material is available free of charge via the Internet at http://pubs.acs.org.
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