Chemical Guiding of Magnetic Nanoparticles in Dispersed Media

Article pubs.acs.org/Langmuir

Chemical Guiding of Magnetic Nanoparticles in Dispersed Media Containing Poly-(methylmethacrylate-co-vinylpyrrolidone) Myriam G. Tardajos,† Inmaculada Aranaz,† Filiz Sayar,‡ Carlos Elvira,† Helmut Reinecke,† Erhan Piskin,‡ and Alberto Gallardo*,† †

Instituto de Ciencia y Tecnología de Polímeros, ICTP (CSIC), Juan de la Cierva 3, 28006 Madrid, Spain Hacettepe University, Department of Chemical Engineering, 06800 Beytepe, Ankara, Turkey

‡

S Supporting Information *

ABSTRACT: The differential reactivity of methylmethacrylate (MMA) and vinylpyrrolidone (VP) in free radical copolymerization, with stirring in methanol, renders an emulsified two phase system. The dispersed and continuous liquid phases contain copolymers rich in MMA and VP, respectively. When Fe3O4 magnetic nanoparticles (mNPs) stabilized with tetramethylammonium hydroxide are added to this emulsion, the mNPs are located in the continuous phase. Very small chemical changes in the methacrylic or vinylic chains are able to guide the mNP toward the interface or to the inside of the dispersed phase since quite a selective functionalization of each phase may be achieved separately. Thus, a small addition of methacrylic acid as comonomer (0.5% molar) guides all of the mNPs to the interface while a 0.5% molar of sulfopropyl methacrylate induces the migration of all mNPs to the dispersed phase. When 0.5% molar of a VP derivative bearing sulfonate functionality is added, the mNPs are found both in the interface and in the continuous phase. The addition of water allows solid MMA-based microspheres to be obtained incorporating the mNPs selectively either at the surface or in the core.

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INTRODUCTION We have recently shown1 that the simple conventional free radical copolymerization of methyl methacrylate (MMA) and vinylpyrrolidone (VP) in methanol with stirring yields, at high conversion, a high compositional heterogeneity which displays unusual phase separation behavior with partition of the copolymer chains between the two phases as a function of their composition. The dispersed phase is rich in copolymers with a high content of hydrophobic MMA units while the continuous phase is rich in copolymers with a high content of hydrophilic VP units. This phase separation occurs due to the upper critical solution temperature (UCST) phenomenon of the copolymers rich in MMA since pure PMMA exhibits a UCST of around 87 °C in methanol.2 Above the UCST, the polymer is soluble in methanol but below the UCST, the polymer−solvent interactions do not favor solubility, and a polymer-rich phase is formed, i.e., phase separation occurs. Figure 1 (left) shows a typical emulsion obtained for an equimolar copolymerization of MMA and VP. MMA and VP are monomeric precursors of typical solid cores and polymeric surfactants, respectively.3−5 Actually, the emulsion can be easily hardened by adding water leading to a hydrophobic MMA-based solid particle stabilized by some amphiphilic chains rich in VP (see Figure 1, right). The copolymerization of MMA and VP is self-stabilizing. Furthermore, the particles exhibit a surface nanostructuration.1 © 2012 American Chemical Society

This partition of polymer chains as a function of their composition may allow for quite selective and differentiated chemical functionalization by adding a small amount of functionalized monomer with a homologous polymerizable group, i.e., methacrylates or/and VP-derivatives. As recently described, the use of homologous comonomers is appropriate to functionalize these two types of chains since VP and VP derivatives are nonactivated monomers; during the copolymerization radical growing chains are much more reactive toward methacrylates.6 Our hypothesis is that the dispersed and the continuous phase of the parent emulsion in methanol may be functionalized quite selectively just by adding a third comonomer to the copolymerization. This third comonomer should be added in a small ratio to have a low influence on the phase separation phenomenon. This differentiated functionalization may be translated to the different components of the solid microspheres in water, i.e., a partial functionalization of VP may imply a functionalization of the polymeric surfactant and therefore a surface-focused methodology while a partial functionalization of the methacrylate part would mean a functionalization of the core component. A schematic representation is shown in Figure 2. Received: December 13, 2011 Revised: March 1, 2012 Published: March 8, 2012 5555

dx.doi.org/10.1021/la204900c | Langmuir 2012, 28, 5555−5561

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The mNPs used in this work are stabilized with tetramethylammonium hydroxide, i.e., they are magnetite (Fe3O4) nanoparticles (mNPs) with a cationic surface that may interact with complementary anionic or anionizable groups such as sulfonate or carboxylate. Thus, different monomers bearing sulfonic or carboxylic groups have been included in the copolymerization in low amounts such as 0.5% molar. The amount of added modifier of 0.5% molar was chosen as it is small enough to have a weak influence on the UCST phenomenon, but it is “high enough” to have chemical significance.

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EXPERIMENTAL DETAILS

Chemicals. Methylmethacrylate (MMA), methacrylic acid (MetCarb), and vinylpyrrolidone (VP) from Aldrich were distilled at reduced pressure and kept at −20 °C (MMA and Met-Carb) and 4 °C (VP). Sodium sulfopropyl methacrylate (Met-Sulf, Sigma) was used as received. Azo-bis-isobutyronitrile (AIBN, Aldrich) was recrystallized from ethanol. Sulfopropyl-vinylpyrrolidone monomer was prepared as described previously.19 Other reagents and solvents were used as received. Magnetite (Fe3O4) Nanoparticles Synthesis. The details of production and characterization of the magnetite (Fe3O4) nanoparticles (mNPs) were given elsewhere,16,20 and are briefly described here. A 120 mL aqueous solution of Fe2+ and Fe3+ salts (total 1.25 M) and 120 mL of 5 M NaOH solution were added into a reactor containing 160 mL of distilled water at 80 °C under N2 atmosphere by vigorous mixing. A black precipitate was formed at the early phase, and the medium was continuously stirred for 2 h at a 3000 rpm stirring rate and 80 °C. Magnetite particles were stabilized by slow addition of 10 mL of 25% (w/w) TMAOH. Superparamagnetic particles with an average size of 50 nm were obtained. The remanence of the particles was equal to zero, and the coercivity was almost negligible in the absence of an external magnetic field. The saturation magnetization was 6.6 emu/g at a magnetic field of 1.24 T. The nanoparticles were transferred from distilled water to an equal volume of methanol by dialysis using membranes with a cutoff of 3500. This solution in methanol was used for the study. Copolymerization. Copolymerizations were carried out under magnetic stirring by standard free radical reactions in methanol at 60 °C for 48 h using AIBN as the initiator. The reactions were performed in the absence of oxygen by bubbling nitrogen for 15 min before sealing the system. The total monomer and initiator concentrations were 1 and 0.015 mol/L, respectively. The initial feed molar fractions are indicated in Table 1.

Figure 1. (Upper) Schemes representing the type of chains and its location in the dispersed systems. (Bottom left) Optical image of a real emulsion obtained after the radical copolymerization of MMA and VP in methanol. (Bottom right) SEM image of solid microspheres obtained from the addition of water to the previous emulsion.

Figure 2. Schemes representing the hypothesis of selective functionalization of chains and phases.

In this work, we have tested our hypothesis of selective functionalization of phases by incorporating the magnetic nanoparticles (mNPs) into the parent emulsion. The interactions of nanoparticles with dispersed systems as liquid/ liquid emulsions or solid/liquid suspensions have interest from both fundamental and applied points of view, since novel materials may be prepared by profiting from these interactions.7,8 Colloidal systems may stabilize liquid/liquid emulsions (the so-called pickering emulsions)9 and selfassembly at interfaces,10 etc. These nanoparticles/interface interactions have been reported to be strongly dependent on the chemical characteristics of the interface, which may be modulated for instance by controlling the nature of the surfactant,11 by changing the pH or ionic strength of the media (in the case of water/oil emulsions),12,13 or by profiting from the Coulombic interactions of selected polyelectrolytes when added to polystyrene latexes.14 In the case of magnetic nanoparticles, the preparation of different composite materials with the nanoparticles located at the surface or in the core of solid polymeric entities has been reported during recent years: On one hand, polymer particles covered with magnetic materials may be of interest as active beads in different fields.15 On the other hand, the encapsulation of magnetic nanoparticles in polymeric matrixes modulates toxicity, stability, and compatibility.16,17 As magnetic nanoparticles have been proposed as nanoheaters for biomedical applications, their encapsulation in polymer beads may be used in medicine to combine this hyperthermia effect with other complementary purposes in drug delivery.18

Table 1. Labels and Feed Molar Compositions of the Copolymerizations Studied in This Work reaction

fMMA

fVP

M-V M-VS MS-V MC-V

0.50 0.50 0.495 0.495

0.50 0.495 0.50 0.50

fVP-Sulf

fMet-Sulf

fMet-Carb

0.005 0.005 0.005

Liquid/Liquid Emulsions and Solid Microspheres. To the corresponding liquid/liquid emulsion obtained after copolymerization in methanol, an aliquot of the mNPs solution was added in a volume ratio of 2:1. Emulsion droplets were hardened under stirring in a shaker by slowly dropping water into the emulsion giving rise to the formation of solid particles. The solutions were centrifuged at 3000 rpm for 30 s (microfuge B, Beckman). The supernatant was removed, and the centrifugate was resuspended in pure water and kept in solution until needed for further characterization. Characterization. Liquid/liquid emulsions were assessed using optical microscopy (eclipse E400, Nikon). Optical images have been included in the Supporting Information. Particle morphology and size 5556


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were studied under a scanning electron microscope (SEM) (XL30, Philips) operating at 25 kV. One droplet (15−20 μL) of the corresponding suspension was placed on a glass surface (14 mm diameter) and allowed to dry; after which, the sample was covered with gold using a sputter coating device (Polaran SC7640, Thermo VG Scientific). Energy-dispersive X-ray spectroscopy (EDX) analysis was performed using EDAX equipment with super ultra tiny window (sutw) and an active area of 10 mm2. The C/Fe ratios were obtained by averaging three measurements. The UCST was determined by measuring the optical transmittance of the solution of the copolymers in MeOH/H2O at 600 nm as a function of temperature. The mixtures were prepared adding the proper amount of water to the initial emulsions in methanol. The analysis was made using a Cary 3 BIOVarian UV−visible spectrophotometer. Temperature was reduced from 60 to 10 °C at a rate of 1 °C min−1. The UCST was defined as the temperature at the inflection point of the absorbance versus temperature curve.

below this limiting value (which corresponds to a conversion of around 0.66 as well) will be part of the dispersed or continuous phase, respectively. Doing a simple mass balance, the average MMA molar fraction in the continuous phase, for a theoretical 100% conversion, is 0.075. Figure 3 shows that the compositional heterogeneity is not “symmetric”, i.e., the chains richer in MMA incorporate approximately 20% molar of VP units while the chains richer in VP are almost pure PVP. In other words, the use of methacrylates to functionalize the MMA-rich chains compared to the use of VP-derivatives to functionalize the VP-rich chains leads to a higher selectivity since the methacrylates do not incorporate into the chains richer in VP due to the MMA consumption. Considering that 0.5% molar of generic functionalized methacrylate or VP derivatives (Met-F or VP-F) are incorporated and assuming that there is a complete homology in reactivities (Met-F has the same reactivity as MMA and VP-F has the same reactivity as VP), the MMA or VP molar fractions are proportional to those of Met-F or VP-F. This assumption has been made on the basis of the high differential reactivity between VP derivatives (a low activated monomer) and the methacrylic family,21−23 which makes the reactivity difference between monomers within the same family comparatively low and of little significance. Therefore and assuming 100% conversion, a “selectivity factor” SF for both phases may be calculated, which we define as the ratio of the concentrations of the functionality in the homologous and nonhomologous phase:

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RESULTS AND DISCUSSION Theoretical Description of the System. Methacrylates like MMA exhibit a much higher reactivity in free radical copolymerization than VP,21−23 and as a consequence, the simultaneous free radical polymerization of both monomers yields materials with high compositional heterogeneity, which in this case leads to the formation of two phases with very different compositions. MMA is preferentially consumed, and copolymer chains rich in MMA are formed during the first stages of the reaction due to its inherent higher reactivity. After most of the MMA has been consumed, chains rich in VP are produced. Figure 3 shows the theoretical instantaneous and

SFMet‐F =

SFVP‐F =

[fMMA‐cum ]dispersed phase [fMMA‐cum ]continuous phase [fVP‐cum ]continuous phase [fVP‐cum ]dispersed phase

=

=

0.72 = 9.6 0.075

1 − 0.075 = 3.3 1 − 0.72

Using the data from Figure 3, the selectivity is 3 times higher in the case of using Met-F compared to the use of VP-F. Using a Met-F comonomer, more than 95% of the functionality will be located in the dispersed phase while in the case of VP-F, 76% of the functionalized units will be located in the continuous phase for a reaction with a 100% conversion. These calculations show the theoretical feasibility of the initial hypothesis: both phases can be functionalized with certain selectivity, with selectivity higher in the case of the dispersed phase rich in methacrylate units. The following paragraphs describe the usefulness of these theoretical considerations. Location of Magnetic Nanoparticles (mNPs) in Liquid/ Liquid Emulsions. Interactions of several dispersed systems based on poly(MMA-co-VP) copolymers with magnetic nanoparticles (mNPs) stabilized with tetramethylammonium hydroxide have been studied. The mNPs have a cationic surface that may interact with complementary anionic groups such as sulfonate units. Therefore, the influence of the incorporation of a very small amount (0.5% molar) of the sulfonate functionality by using the monomers indicated in the Figure 4 has been studied. Methacrylic acid was also studied for comparison reasons. The sulfonate group is a permanent anion that may strongly interact with the cationic surface of mNPs. The carboxylic unit is a weak acid that will be partially ionized in water. Four different copolymerizations, as shown in Table 1, have been

Figure 3. Theoretical instantaneous (solid line) and cumulative (dashed line) molar fraction of MMA in the copolymer chains versus conversion for an equimolar copolymerization in a closed vessel. The dotted line at an MMA molar fraction of 0.42 tentatively indicates the instantaneous composition limit between the populations of the two phases.

cumulative molar fraction of MMA in the copolymer chains versus conversion for an equimolar copolymerization in a closed vessel. This graph has been obtained using Copol software.24 The dotted line at a MMA molar fraction of 0.42 indicates a tentative limit of the instantaneous composition of the polymer chain distribution between the two phases, according to our previous studies.1 This value was inferred from the NMR analysis of the dispersed phase, which gave an average MMA molar fraction of 0.72.1 This average MMA molar fraction must correspond to the cumulative data at the limiting point in Figure 3 (0.42). Individual chains with compositions above or 5557


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In the MC-V emulsion, the mNPs seem to balance the interactions between the two phases. They interact with the COOH groups of the MMA-rich chains in the dispersed phase, but they are not located inside as in the case of the MS-V emulsion. The interaction here is not as strong as with the permanent anion SO3−; the affinity to the continuous phase probably causes the particle to be guided to the interface to fulfill both affinities. This behavior may be seen as an example of nanoparticle self-assembly at a liquid/liquid interface, a wellknown phenomenon for different colloidal systems in oil/water or liquid/liquid interfaces.13 As has been mentioned before, the polar or charge characteristics of the surface may modulate the interactions of the nanoparticles with the liquid/liquid interfaces.14−16 In this context, the presence of carboxylic groups in the dispersed phase in the MC-V system allows for the selective interactions of the mNPs at the interface. This balance of interactions that causes the mNPs to be located at the interface in the MC-V system is very sensitive to the modifier load. Because of the high selectivity mentioned before, the amount of COOH groups in the continuous phase is much smaller than in the dispersed phase (around 10 times smaller). However, the absolute amount of COOH in this “preferred” continuous phase increases with increasing MetCarb molar fraction, which influences the aforementioned balance of interactions. Actually, the mNPs gradually move from the interface to the continuous phase as it is shown in Figure S4 in the Supporting Information. In this figure, the interactions of mNPs with a MC-V emulsion with a 2% molar of modifier are shown. The mNPs are agglomerated in the continuous phase and partially interact with the interface. Preparation of Solid Microspheres Loaded with mNPs. The emulsions M-VS and MC-V can be easily hardened at room temperature rendering solid microspheres by adding water dropwise. Figures 5 and 6 show SEM micrographs of the

Figure 4. Structures of the comonomers added in a 0.5% molar to equimolar copolymerizations of MMA and VP in methanol.

comparatively studied. M-V corresponds to an unfunctionalized reference copolymer, and the subscripts of M or V indicate the type of functionalization in the homologous polymerizable group (S for sulfonic and C for carboxylic groups) added in a 0.5% molar solution. When mNPs are incorporated into the control emulsion, MV, they are located, partially agglomerated, in the continuous phase (see the Supporting Information, Figure S1). No mNPs were found in the dispersed phase or in the interface, indicating that the mNPs exhibit a higher affinity to the hydrophilic VPrich chain environment. If sulfonate functionality is introduced via the VP derivative (VP-Sulf), the mNPs are located preferentially in both the interface and in the continuous medium (see the Supporting Information, Figure S2), which is in agreement with the initial hypothesis: the use of VP derivatives predominantly functionalizes the VP-rich chains located in the continuous phase. Also, the location of mNPs at the interface is quite interesting because this indicates that the VP-rich chains are effectively interacting and stabilizing the MMA-rich droplets. As indicated in the Introduction, MMA and VP are monomeric precursors of typical solid core and polymeric surfactants. Thus, amphiphilic PVP, which is soluble in organic and polar media including water, is used as a surfactant, for example, in the dispersion polymerization of MMA or styrene to stabilize the PMMA or PS particles.3−5 On this basis, we propose that the chains rich in VP located at the continuous phase are able to stabilize, in polar media such as methanol/water, the dispersed entities rich in the more hydrophobic MMA. The guiding of magnetic particles to the interface or the surface when functionalizing the PVP chains is in agreement with that because it confirms a partial location of those chains in the interface or on the surface. The optical micrographs of Figure S3 (in the Supporting Information) show the interactions that take place when the methacrylate chains are the carriers of the functionalities. The use of Met-Sulf causes all of the mNPs to be located in the inside of the dispersed phase (see the Supporting Information, Figure S3, upper), while the use of Met-Carb gives rise to a very peculiar result: most of the mNPs were located at the interface (see the Supporting Information, Figure S3, bottom). Both results again confirm the initial hypothesis: both phases can be functionalized selectively. Moreover, the fact that most of the mNPs were found at the interface or in the dispersed phase is in agreement with the higher selectivity predicted for the functionalization of the methacrylate system compared to the derivatization of VP. In the MS-V case, the MMA-rich chains present in the dispersed phase bear sulfonate groups that strongly interact with the cationic surface of the mNPs.

Figure 5. SEM micrograph of the microspheres obtained by adding water to the emulsified system M-VS + mNPs. The C/Fe ratios of representative areas have been included. A region with mNPs aggregates is also indicated in the graph.

microspheres obtained from M-VS and MC-V respectively. Microspheres from these systems have mNPs randomly located at the surface as indicated by the EDX analysis. This is clearly related to the parent emulsions where the mNPs were mainly located at the interfaces (see the Supporting Information, Figures S2 and S3, upper). Furthermore, aggregates of mNPs outside the microspheres are observed in the micrograph of MVS but not in the case of MC-V. This is again in agreement 5558


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for those water/methanol ratios are lower than the UCST in pure methanol, and a minimum UCST exists for a particular volume ratio. In Figure 8, the UCST values of the systems

Figure 6. SEM micrograph of the microspheres obtained by adding water to the emulsified system MC-V + mNPs. The C/Fe ratios of representative areas have been included.

with the initial study of the parent emulsions as well as with the initial hypothesis: in the M-VS system, the functionalized VPrich chains are mainly present in the continuous phase and at the interface (stabilizing the “hydrophobic” droplets) while in the MC-V case the functionalization is quite selective toward the dispersed phase. In any case, it seems that, once the solid microspheres have been formed, the mNPs stay on the surface guided by the sulfonate or carboxylic groups. M-VS and MC-V are dispersed systems during the whole process of water addition at room temperature. The droplets of the parent liquid/liquid emulsion in methanol become the final solid microspheres in water. The droplets “nucleate” the spherical microspheres. The MS-V emulsion, however, cannot be hardened with water at room temperature because the phase separation disappears when a critical methanol/water composition is reached. At high water/methanol ratios, a “precipitation-like” process takes place and irregularly shaped precipitates are obtained instead of microspheres (see Figure 7).

Figure 8. UCST dependence from the MeOH volume fraction in water/methanol mixtures of the systems studied in this work. A total of 2 equiv of TBA (per sulfonate group) has been added in the MS-V + TBA system.

studied in this work and determined by turbidimetry are represented as a function of the methanol volume fraction. The cosolvency effect is evident for all systems. MS-V exhibits a comparatively broad interval of UCST below room temperature. This is probably related to the high hydrophilia of the sulfonate groups that are incorporated into the “hydrophobic” MMA-rich chains, as these are the chains responsible for this UCST transition behavior. The addition of water at room temperature to MS-V leads immediately to complete homogenization. Upon continuing to add water, at higher water/alcohol ratio (volume fraction of methanol around 0.75 according to Figure 8) the UCST exceeds room temperature again and a “precipitation-like” process takes place, leading to an irregularly shaped solid precipitate. The M-VS system also incorporates sulfonate groups, but these are located on the VP-rich chains that populate the continuous phase and they do not have such an effect on the UCST of the copolymers forming the dispersed phase. In this case, a phase separation takes place during all stages upon the addition of water at room temperature, until solid microspheres are formed. Therefore, the selective functionalization of the dispersed or continuous phases (MMA-rich or VP-rich chains, respectively) discussed above has a very different influence on the UCST dependence and on the hardening process at room temperature (as well as on the formation of spherical microspheres). MC-V, which like MS-V bears the functionalities in the methacrylate chains, shows a behavior very similar to that of M-VS but unlike that of MS-V. In this MC-V system, the UCST values are above room temperature for any methanol/water ratio, and the droplets may be hardened to solid microspheres. This may be explained in terms of the different nature of the functionalities of MC-V and MS-V: an uncharged or partially ionized carboxylic function and a permanent sulfonate anion, respectively. In the particularly interesting case of the MS-V emulsion in which the mNPs could be guided to the inside of the drops of the dispersed phase (see the Supporting Information, Figure S3, bottom), the dramatic decrease of the corresponding UCST upon the addition of water impedes the preparation, at room temperature, of solid polymeric microspheres loaded in the

Figure 7. SEM micrograph of the precipitate obtained by adding water to the emulsified system MS-V (without mNPs).

This phenomenon, the formation of a homogeneous solution when adding water, must be related to the well-known singular cosolvency behavior of PMMA in methanol/water mixtures (as well as in other mixtures of water with lower aliphatic alcohols).2,25 Although water is a nonsolvent for PMMA, enhanced solvation of the polymer chains may be found for certain methanol/water ratios.26 This means that UCST values 5559


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role of the VP-rich chains as stabilizers. Furthermore, the emulsions can be hardened by adding water dropwise resulting in microspheres. This allows us to prepare, in a very simple way, composite microspheres selectively locating the mNPs for different purposes. Further optimization of the process (i.e., size, load, surface density of groups...) as well as a study of the influence of different parameters on the magnetic performance will be addressed in the future. Particularly interesting will be a study into the influence of the modifier concentration as has been mentioned before for the MC-V system. The phase separation and the hardening process are very sensitive to the concentration of modifiers with methacrylic homology. Small changes have a strong influence on the UCST values, the cosolvency effect, the concentration in the dispersed phase, and the copolymer partition (and therefore on “the sensitivity factor” as defined in the manuscript).

core with mNPS. This decrease in the UCST however can be chemically modulated, and microspheres encapsulating mNPs in the core may be easily obtained. Assuming the sulfonate charge causes this deviation, we have complexed the extra anion with a “hydrophobic” cation, tert-butylammonium (TBA). As a consequence, the sulfonate anion is “blocked” and the UCST curve moves to higher temperatures as can be seen in Figure 8. Despite the complexation with TBA, the sulfonate groups still guide all the mNPs to the inside of the droplets of the dispersed phase in the emulsion in methanol (see the Supporting Information, Figure S5). The upward modulation of the UCST allows the preparation of spherical solid microspheres loaded with mNPs in the core (as confirmed by EDX) via the addition of water at room temperature (see Figure 9).

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ASSOCIATED CONTENT

S Supporting Information *

Optical micrographs of emulsified systems. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge support from Grants MAT2010-20001 and MAT2008-2174. I.A. and M.G.T. acknowledge ́ the Consejo Superior de Investigaciones Cientificas (CSIC) and Ministerio de Ciencia y Educación, Spain, for the JAE-doc and FPI grants, respectively. We thank David Gomez and Mónica Nieto for the SEM analysis.

Figure 9. SEM micrograph of the microspheres obtained by adding water to the emulsified system M-VS + TBA + mNPs. The C/Fe ratios of representative areas have been included.

All the microspheres described in this work were obtained using a common methodology (and the same polymer concentration, 1 M), and they exhibited an average size in the range of 1−10 μm and a broad size distribution in agreement with the data reported previously.1 We know today (data to be published) that the size characteristics of the microspheres (and of the parent drops) are more related to the initial polymer concentration in methanol than to the characteristics of the VP-co-MMA chains. We are currently preparing submicrometer spheres with narrower size distributions from less concentrated solutions (0.25 M).

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REFERENCES

(1) Aranaz, I.; Reinecke, H.; Elvira, C.; Gallardo, A. Compositionallytunable surface nanostructuration of microspheres obtained from a self-stabilizing copolymerization of methylmethacrylate and vinylpyrrolidone. Polymer 2011, 52, 2991. (2) Cowie, J. M. G.; Garay, M. T.; McEwen, I. J. Polymer-cosolvent systems: synergism and antisynergism for solvent mixtures for poly(methyl methacrylate). Polym. Commun. 1986, 27, 122. (3) Ishii, N.; Inoue, K. Synthesis and regular reflection property of cocoon-like poly(methyl methacrylate) particles by seeded suspension polymerization. Polym. Bull. 2009, 63, 653. (4) Zhang, H. T.; Yuan, X. Y.; Huang, J. X. Study of kinetics and nucleation mechanism of dispersion copolymerization of methyl methacrylate and acrylic acid. React. Funct. Polym. 2004, 59, 23. (5) Karagoz, B.; Gunes, D.; Bicak, N. Preparation of crosslinked poly(2-bromoethyl methacrylate) microspheres and decoration of their surfaces with functional polymer brushes. Macromol. Chem. Phys. 2010, 211, 1999. (6) Tardajos, M. G.; Nash, M.; Rochev, Y.; Reinecke, H.; Elvira, C.; Gallardo, A. Homologous copolymerization route to functional and biocompatible poly-vinylpyrrolidone. Macromol. Chem. Phys. 2012, 213, 529. (7) Binks, B. P. Colloidal particles at liquid interfaces. Phys. Chem. Chem. Phys. 2007, 9, 6298. (8) Caruso, F.; Spasova, M.; Susha, A.; Giersig, M.; Caruso., R. A. Magnetic nanocomposite particles and hollow spheres constructed by a sequential layering approach. Chem. Mater. 2001, 13, 109. (9) Pickering, S. U. Emulsions. J. Chem. Soc. 1907, 91, 2001.

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CONCLUSION This work is devoted to the description of a new and simple procedure to chemically guide colloids in dispersed media containing heterogeneous copolymeric chains. It is shown here that methanolic emulsified systems containing heterogeneous poly(MMA-co-VP), having continuous and dispersed phases containing chains rich in VP and MMA, respectively, can be selectively functionalized in a simple one pot procedure by using small amounts of homologous functionalized monomers. This selectivity has been confirmed by monitoring the location of magnetic nanoparticles (mNPs) with cationic surfaces. Thus, the addition of a 0.5% molar of sulfopropylmethacrylate causes all of the mNPs to be located inside the droplets of the dispersed phase. The use of 0.5% molar methacrylic acid allows for the directional location of most mNPs at the interface. When 0.5% molar sulfopropyl-VP is used, the mNPs are located in both the continuous phase and the interface confirming the 5560


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Chemical Guiding of Magnetic Nanoparticles in Dispersed Media

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