ARTICLE pubs.acs.org/Langmuir
Surfactant-Free Miniemulsion Polymerization as a Simple Synthetic Route to a Successful Encapsulation of Magnetite Nanoparticles Jose Ramos† and Jacqueline Forcada*,‡ †
Grupo de Física de Fluidos y Biocoloides, Departamento de Física Aplicada, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain ‡ Institute for Polymer Materials POLYMAT and Grupo de Ingeniería Química, Facultad de Ciencias Químicas, Universidad del País Vasco/EHU, Apdo. 1072, 20080 Donostia-San Sebastian, Spain ABSTRACT: Due to the existing interest in new hybrid particles in the colloidal range based on both magnetic and polymeric materials for applications in biotechnological fields, this work is focused on the preparation of magnetic polymer nanoparticles (MPNPs) by a single-step miniemulsion process developed to achieve better control of the morphology of the magnetic nanocomposite particles. MPNPs are prepared by surfactant-free miniemulsion polymerization using styrene (St) as a monomer, hexadecane (HD) as a hydrophobe, and potassium persulfate (KPS) as an initiator in the presence of oleic acid (OA)-modified magnetite nanoparticles. The effect of the type of cross-linker used [divinylbenzene (DVB) and bis[2-(methacryloyloxy)ethyl] phosphate (BMEP)] together with the effect of the amount of an aid stabilizer (dextran) on size, particle size distribution (PSD), and morphology of the hybrid nanoparticles synthesized is analyzed in detail. The mixture of different surface modifiers produces hybrid nanocolloids with various morphologies: from a typical coreshell composed by a magnetite core surrounded by a polymer shell to a homogeneously distributed morphology where the magnetite is uniformly distributed throughout the entire nanocomposite.
’ INTRODUCTION Polymer-coated magnetic nanoparticles are extensively used in pharmacy, biology, and medicine to transport biological compounds. Biomolecules can be separated quickly from a complex medium by fixing them to magnetic particles. Since magnetic supports can be separated from solutions containing other species such as suspended solids, cell fragments, and contaminants, the magnetic affinity separation is useful for crude samples. Direct and indirect methods of separation are frequently used in biological diagnosis, such as for detecting diseases with magnetic particles and for specific interactions between biological molecules (e.g., antibody/antigen or nucleic acids). In addition, magnetic nanoparticles can be used not only in vitro for diagnosis applications but also for in vivo therapeutic applications such as magnetic resonance imaging, targeted drug delivery, and hyperthermia. The magnetic latex must have the following characteristics, regardless of the application in the biomedical field: no sedimentation, small and uniform size and size distribution, high and uniform magnetic content, superparamagnetic behavior, no toxicity, and no iron leaking. In addition, for the magnetic nanoparticles to be biocompatible, the nanoparticles must be coated with biocompatible polymers in order to prevent the formation of large aggregates, geometry changes, and biodegradation when exposed to a biological system.1,2 r 2011 American Chemical Society
Miniemulsion polymerization offers the possibility for the encapsulation of different materials, ranging from liquid to solid, from organic to inorganic, and from molecularly dissolved to aggregated species into polymeric nanoparticles and nanocapsules.310 Particularly, the encapsulation of inorganic nanoparticles by polymer shells is of high interest because these hybrid nanoparticles exhibit enhanced, even novel properties (e.g., mechanical, chemical, electrical, rheological, magnetic, and optical) and offer very interesting actual and potential applications in different fields.1117 Most of the works found in literature devoted to encapsulate iron oxide nanoparticles by miniemulsion polymerization proposed a single-step miniemulsion process1844 consisting of four stages: (i) hydrophobization of the hydrophilic inorganic nanoparticles; (ii) dispersion and stabilization of the hydrophobized inorganic nanoparticles in monomer phase; (iii) miniemulsification of the lipophilic dispersion in water; and (iv) polymerization of droplets.4 The successful incorporation of hydrophilic iron oxide nanoparticles into hydrophobic polymer particles by miniemulsion polymerization relies on their surface modification to make iron oxide/polymer compatible. With this purpose, Received: March 1, 2011 Revised: April 18, 2011 Published: April 28, 2011 7222
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Langmuir inorganic nanoparticles have to be hydrophobized. In addition, apart from the enhancement of their surface hydrophobicity, in some cases different reactive groups are introduced for polymerization with the hydrophobic monomer. Like this, different surface modifiers have been used to obtain stable and hydrophobic magnetic nanoparticles, with oleic acid (OA) being the most common one.1835 The carboxylate head of OA is able to anchor on the surface of iron oxide nanoparticles, while its hydrophobic tail ensures steric stabilization as well as compatibility with the solvent. Oleoyl sarcosine acid,33,36 bis(2-ethylhexyl) sulfosuccinate (AOT),37 3-(trimethoxysilyl)propylmethacrylate (TPM),33,38 sorbitan oleate (Span-80),33,34,39 alkylolammonium salts of low molecular weight polycarboxylic acid polymer (Disperbik-106, Disperbik-108, and Disperbik-111),29,40 penta(propylene glycol) methacrylate phosphate (Sipomer PAM200),41 lauric acid,42,43 stearic acid,33 and 12-hexanoyloxy-9-octadecenoic acid have also been used.44 However, most of them caused (i) inhomogeneous distribution of the iron oxide nanoparticles inside and among the particles and/or (ii) pure polymer nanoparticles and/or (iii) bare (free) magnetite nanoparticles or magnetic aggregates in the aqueous phase and/or (iv) broad particle size distributions (PSDs), and/or (v) limited loading of the particles with magnetic material. In order to increase the magnetic content in the polymer nanoparticles, new procedures based on a double-step miniemulsion process are reported.9,4550 On the one hand, Landfester and co-workers4547 encapsulated high amounts of magnetite particles into polystyrene particles by a three-step preparation route including two miniemulsion processes. In the first step, OA coated magnetic nanoparticles in octane were prepared. In the second step, a dispersion of magnetite in octane was miniemulsified in water by using sodium dodecyl sulfate (SDS) as surfactant. After evaporation of the octane, the magnetite aggregates were mixed with a styrene (St) miniemulsion, and in the third step of the synthesis an ad-miniemulsification process was used to obtain final and full encapsulation. On the other hand, magnetite/polystyrene latexes with narrow size distribution and high magnetite content were also prepared by a hybrid miniemulsion polymerization process containing binary droplets.9,4850 First of all, magnetite nanoparticles modified with oleic acid were synthesized and dispersed in octane. This ferrofluid was added to an aqueous solution with SDS as surfactant and treated ultrasonically to obtain a miniemulsion composed of magnetite nanoparticle aggregations of droplets with a diameter of 100200 nm (Mag-droplets). Another miniemulsion made of St monomer droplets with a diameter of 34 μm (St-droplets) was prepared and mixed with Magdroplets to obtain a double-miniemulsion system, which contained microsized St droplets and nanosized magnetite aggregation droplets. With extremely low surfactant concentration, the nucleated loci were selectively controlled in the Magdroplets, as the result of smaller droplet size and larger surface ratio. With these double-step miniemulsion processes, from 40 to 86 wt % of magnetite can be encapsulated inside polymer nanoparticles. However, from an industrial point of view, these processes are very tedious and difficult to scale up. Thus, singlestep miniemulsion processes must be developed to achieve better control of the morphology of magnetic polymer nanoparticles (MPNPs). Recently, a successful single-step miniemulsion process was used by van Berkel et al.51 to coencapsulate gold and manganese ferrite nanoparticles homogeneously inside polymer particles.
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The key parameter was the right choice of the surface modifiers, which were thiol-terminated and carboxylic-acid-terminated polystyrene ligands prepared by reversible additionfragmentation (RAFT) polymerization and nitroxide-mediated polymerization (NMP), respectively. In addition, in our previous work, a synergistic effect was observed using OA together with TPM in the encapsulation of silica nanoparticles by miniemulsion polymerization of styrene.52 Therefore, the use of an adequate mixture of surface modifiers is fundamental for the successful encapsulation of inorganic nanoparticles by a single-step miniemulsion polymerization process. The preparation of MPNPs with the desired size, monodispersity, and morphology together with a high magnetite loading inside the nanocomposite particles by a single-step miniemulsion polymerization process has not been reported yet. However, the robustness and simplicity of this process makes it very suitable to be developed for obtaining well-designed magnetic latexes. In this work, having in mind the idea of obtaining a successful encapsulation of magnetite nanoparticles together with the use of a single-step polymerization, the effect of the surfactant concentration, from conventional to surfactant-free miniemulsion polymerization of styrene, on the encapsulation of magnetite nanoparticles is presented. Then, with the objective of obtaining homogeneously distributed magnetic polymer nanoparticles inside the final composite nanoparticles, the effect of the hydrophobic/hydrophilic character of the cross-linker (DVB and BMEP) used together with the effect of the amount of an aid stabilizer (dextran) will be analyzed in detail. Special attention is paid to size, PSDs, and morphology of the hybrid nanoparticles synthesized.
’ EXPERIMENTAL SECTION Materials. Styrene (St) monomer was purified by washing with a 10 wt % sodium hydroxide aqueous solution and stored at 18 °C until use. Divinylbenzene (DVB, Aldrich), bis[2-(methacryloyloxy)ethyl] phosphate (BMEP, Aldrich), hexadecane (HD, Sigma-Aldrich), potassium peroxodisulfate (KPS, Fluka), sodium dodecyl sulfate (SDS, Sigma-Aldrich), dextran 40 (Sigma), iron(III) chloride hexahydrate · · (FeCl36H2O, Sigma), iron(II) chloride tetrahydrate (FeCl24H2O, Aldrich), oleic acid (OA, Aldrich), 25% ammonium hydroxide solution (NH4OH, Fluka), 1 N hydrochloric acid (HCl, Panreac), and sodium hydroxide (NaOH, Panreac) were used without further purification. Double deionized (DDI) water was used throughout the work. Synthesis of Magnetite Nanoparticles. Oleic-acid-coated magnetite particles with the average size being about 10 nm were prepared by stoichiometric chemical coprecipitation of ferrous and ferric salts, as reported previously.26 Amounts of 54.04 g of FeCl3 3 6H2O and 19.88 g of FeCl2 3 4H2O (Fe3þ/Fe2þ = 2/1 mol/mol) were dissolved in 1300 mL of DDI water, and then 20 g of OA was dissolved in 120 mL of acetone and added under stirring. After 30 min, 150 mL of a 25% NH4OH solution was added, over a period of 1015 min, to the mixture under a nitrogen atmosphere and at room temperature. The resulting suspension was stirred for 1 h and then heated at 85 °C for 1 h. Then, the water-based Fe3O4 ferrofluid dispersion obtained was cooled to 70 °C, and 1 N HCl was added slowly to adjust the pH to 2 to ensure that oleate was converted to OA. The liquid was decanted; the black residue was washed several times with DDI water until the pH of the washing water was 7. The precipitate was then dried in an oven at 60 °C. The content of OA and magnetite was analyzed by thermogravimetric analysis (TGA) as being 50 wt % of OA and 50 wt % of Fe3O4. The dried OA-coated Fe3O4 particles were dispersed in St by slow stirring at room 7223
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temperature. In this way, an oil-based Fe3O4 ferrofluid (Fe3O4St) containing 40% of OA-coated Fe3O4 particles was obtained.
particles of magnetic latexes, and analyzed using the software Bolero (AQ Systems). Number- (dn), weight- (dw), and volume- (dv) average diameters as well as polydispersity indices (PDIs) were calculated from the PSDs.53 In addition, the fraction of MPNPs was determined as the number of MPNPs with respect to the total number of latex nanoparticles observed in TEM micrographs obviating the presence of bare magnetite nanoparticles due to their small size (about 10 nm). The final magnetic latexes were cleaned by the serum replacement method in order to determine accurately the surface charge density. The surface charge density (σ, μC/cm2) of the magnetic latexes was determined by conductometric titration, taking into account the volume and concentration of the titration agent (NaOH) used, and the surface area of the latex particles.54
Synthesis of Magnetic Polymer Nanoparticles (MPNPs). Based on the recipes given in Table 1, SDS (from 0 to 0.16 g), KPS (0.08 g), BMEP (from 0 to 0.16 g), and dextran (from 0 to 0.8 g) were dissolved in DDI water (72 g), and DVB (from 0 to 0.08 g) was dissolved in the mixture of HD (0.32 g) and Fe3O4St (8 g). Then, the two solutions were mixed together under stirring provided by a magnetic bar stirrer for 10 min and miniemulsified (model 450 sonifier, Branson) in an icecooled bath. All the miniemulsions were prepared under the same optimized conditions (output control, 8; duty cycle, 80%; sonication time, 10 min). Styrene was stable to a possible redox reaction caused by iron oxide, and no induced polymerization was observed due to the low temperature used throughout all the miniemulsification process. The conventional and emulsifier-free miniemulsion polymerizations were carried in 0.10 L glass bottles immersed in a thermostatic bath. The bottles were tumbled end over end at 49 rpm. Once all the reagents were introduced and miniemulsified into the bottles, as was described before, the bottles were purged with nitrogen for 10 min and then introduced into the thermostatic bath at 70 °C for 24 h. Magnetite and Latex Characterizations. Transmission electron microscopy (TEM, TECNAI G2 20 TWIN 200 kV LaB6) was used to observe and analyze the PSDs and the microscopic morphology of magnetic polymer nanoparticles. PSDs were determined from TEM microphotographs on representative samples of more than 1000
’ RESULTS AND DISCUSSION Effect of the Surfactant Concentration. During the preparation process of MPNPs, the eventual formation of pure polymer nanoparticles is not desirable and must be avoided during the polymerization process. To minimize the number of these pure polymer nanoparticles, the amount of SDS used in the recipe was optimized. The amount of SDS (wt % with respect to the Fe3O4St mixture) used together with the number- (dn) and weight- (dw) average diameters, the PDIs, and the fractions of MPNPs obtained are shown in Table 2. As can be seen, reducing the concentration of SDS from 2 to 0 wt % (runs 14), larger particle diameters, lower PDI values, and higher fractions of MPNPs are obtained. In a previous work, the same effect of SDS on magnetite encapsulation was found.26 In that case, the optimum percentage of SDS was 23 wt %, but it was impossible to avoid the formation of pure polymer nanoparticles due to homogeneous and micellar nucleations. However, in our second work,27 a surfactant-free miniemulsion polymerization using 20 wt % sodium p-styrenesulfonate (NaSS) as ionic comonomer was successfully used in the encapsulation of magnetite nanoparticles, and neither bare magnetite nanoparticles nor pure polymer nanoparticles were observed. The absence of pure polymer nanoparticles formed during the polymerization process indicated that monomer droplet nucleation was achieved entirely. Micellar nucleation was avoided completely in the absence of surfactant, and homogeneous nucleation was also prevented.
Table 1. Recipes Used for the Miniemulsion Polymerizations of Styrene in the Presence of OA-Coated Fe3O4 Nanoparticles organic phasea
wt %c
OA-coated Fe3O4
40
HD
4
DVB
01 b
wt %c
water phase SDS
02
KPS
1
BMEP
02
Dextran
010
Fe3O4St: 8 g. b DDI water: 72 g. c With respect to the Fe3O4St mixture. Reaction conditions: T = 70 °C; rpm = 49; reaction time = 24 h. a
Table 2. Miniemulsion Polymerizations of Styrene in the Presence of OA-Coated Fe3O4 Nanoparticles: Effects of SDS, DVB, BMEP, and Dextran on Particle Size, Polydispersity Index, and Fraction of MPNPs surfactant run
SDS (wt %)
1 2
2 1.2
3
0.6
cross-linker DVB (wt %)
BMEP (wt %)
dextran (wt %)
4 5
1
6
dw (nm)
PDI
fraction of MPNPsa
43.7 50.8
54.1 62.4
1.239 1.229
0.49 0.79
60.4
71.5
1.184
0.81
163.0
178.9
1.098
0.92
344.4
366.0
1.063
1.00
97.1
106.3
1.095
0.97 0.95
7
5
134.4
147.0
1.094
8
10
127.6
131.9
1.034
0.96
5 10
97.4 89.0
110.3 99.3
1.132 1.116
0.98 0.95
9 10
a
2
dn (nm)
1 1
11
2
5
78.6
129.0
1.641
0.96
12
2
10
76.2
98.2
1.290
0.92
Calculated as the number of MPNPs with respect to the total number of latex particles observed in TEM micrographs. 7224
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Figure 1. Effect of the SDS surfactant concentration on MPNPs formation observed by TEM: (a) 2 wt % (run 1), (b) 1.2 wt % (run 2), (c) 0.6 wt % (run 3), and (d) without surfactant (run 4).
In the present work, the miniemulsion polymerization carried out without surfactant (run 4) hardly showed pure polymer nanoparticles because the absence of SDS favored only droplet nucleation. This can be confirmed with the lower PDI obtained with respect to the miniemulsion polymerizations carried out in the presence of SDS (runs 13). The effect of the surfactant concentration on the morphology of the different magnetic latexes obtained is shown in Figure 1. The distribution of magnetite inside the nanocomposite particles is more homogeneous in the polymerization carried out in the absence of surfactant (run 4, Figure 1.d) than that in the presence of SDS (runs 13, Figure 1ac). In addition, the reduction of the pure polymer nanoparticles formation can be clearly observed from 2 wt % SDS (Figure 1a) to 0 wt % SDS (Figure 1d). Therefore, surfactant-free miniemulsion polymerization is a more convenient process to synthesize MPNPs having the magnetite more homogeneously distributed inside the nanocomposite particles and narrower PSDs, and avoiding pure polymer nanoparticle formation, compared to conventional miniemulsion polymerization. Effect of the Type of Cross-Linker. It is well-known that cross-linkers play an important role in miniemulsion polymerization. DVB is one of the most common hydrophobic crosslinkers used in styrene polymerizations. In a kinetic study on a St
emulsion polymerization using DVB as cross-linker,55 it was found that when increasing the DVB content the polymerization rate of the whole reaction was accelerated, because of its high reactivity. In addition, DVB as comonomer was more prone to form the polymer matrix, especially during the earlier period of the polymerization process. Recently, Xu et al.9 carried out a morphological manipulation of MPNPs by means of a kinetic control of a miniemulsion polymerization process using DVB as cross-linker. To obtain MPNPs with different morphologies, the DVB content in the monomer mixture was adjusted during the preparation of the monomer miniemulsion, concluding that DVB helps at the early stages of the reaction both the formation of a polymer matrix and increasing the polymerization rate. In this work, with the aim of improving the homogeneity of the distribution of magnetite inside MPNPs, different amounts of DVB were added in the surfactant-free miniemulsion polymerization of Fe3O4St ferrofluid. The best result is obtained by using 1 wt % (with respect to the Fe3O4St mixture) DVB (run 5, Table 2). As can be seen, a larger particle diameter and a lower PDI value compared to those of the polymerization carried out without DVB (run 4) are obtained. In Figure 2a and b, different morphologies of MPNPs can be observed when the reaction is carried out without cross-linker or by using DVB in the polymerizations. As can be seen in Figure 2a (run 4), without 7225
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Figure 2. Effect of the type of cross-linker on MPNP formation observed by TEM: (a) without cross-linker (run 4); with 0.5 mmol of cross-linker and (b) 1 wt % DVB (run 5), and (c) 2 wt % BMEP (run 6).
cross-linker, the homogeneous distribution of magnetite inside the core of MPNPs is not as well achieved as compared to that of the particles shown in Figure 2b (run 5). Here, magnetite nanoparticles are homogeneously distributed in the cores of the MPNPs and a thick polymer shell surrounded them. In addition, using DVB, no pure polymer nanoparticles are observed and only MPNPs are produced as observed by means of the fraction of MPNPs obtained. The faster polymerization rate of the system when DVB is used together with the higher hydrophobicity of DVB with respect to that of St retards more effectively the diffusion of monomer out of the miniemulsion droplets (Ostwald ripening). In addition, the lack of mobility of polymer chains retains magnetite nanoparticles inside the core of MPNPs. The same coreshell morphology was reported in the batch emulsion polymerization of styrene in the presence of swollen ferrofluid droplets as seed, when DVB was used as crosslinker.56,57 Without DVB, they obtained a “Janus-like” morphology, but the increase in the amount of DVB favored the spreading of the polymer around the magnetic emulsion. The homogeneous coreshell morphology was reached when at least 40 wt % DVB was used. In order to analyze the effect of the type in terms of hydrophilicity/hydrophobicity of the cross-linker used, a more hydrophilic one (BMEP) is used instead of DVB. It is well-known that phosphate ions form bidentate complexes with adjacent sites on the iron oxide surface.58 The reason for using BMEP is that this comonomer acts as a surface modifier of magnetite and as a hydrophilic cross-linker of polymer chains at the same time. The concentration of BMEP used is the same than that used with DVB (0.5 mmol) and the results are shown in Table 2 (run 6). As can be seen, smaller particle diameters are obtained compared to those obtained in polymerizations carried out without cross-linker (run 4) and using the same concentration of DVB (run 5). This is due to the ionic character provided by the phosphate group of BMEP. The reduction in size of the particles with the amount of an ionic comonomer added was previously observed by using NaSS.27 PDI and amount of pure polymer nanoparticles are higher than those obtained in run 5, where DVB is used as cross-linker, and the values are very close to those obtained in run 4 without crosslinker. The morphology of the MPNPs prepared with BMEP as cross-linker can be observed in Figure 2c. As can be seen, the magnetite nanoparticles are distributed inside all nanocomposite particles instead of forming a coreshell structure as in the previous
Table 3. Surface Charge Densities of Some of the MPNPs σstrong (μC/cm2)
σweak (μC/cm2)
run 4
3.9 ( 0.5
59 ( 1
run 5
4.2 ( 0.7
run 6
3.0 ( 0.1
43 ( 3
run 10
3.6 ( 0.4
51 ( 2
run 12
2.8 ( 0.8
62 ( 1
cases: run 4 (Figure 2a) without cross-linker and run 5 (Figure 2b) with DVB. This is due to the better compatibility between polymer and magnetite imparted by BMEP. However, in run 6, the homogeneity in the distribution of the magnetite nanoparticles in each nanocomposite particle is worse than that in the case of using DVB as cross-linker (run 5), and close to that observed without using cross-linker (run 4). This is probably due to a higher mobility of the polymer chains inside the nanocomposite particles caused by the lower cross-linking density achieved by using BMEP cross-linker. In our previous work,27 the mobility or diffusion inside MPNPs of OA molecules, which are covering magnetite nanoparticles, was reported by quantifying the presence of carboxylic groups provided by OA onto the surface of MPNPs observed through conductometric titration. In Table 3, surface charge densities of the different MPNPs prepared are shown. They are determined by taking into account the two contributions to the total surface charge density: the first one (σstrong) due to the strong acid groups provided by the protonated sulfate groups of the initiator (KPS), and the second one (σweak) corresponding to the weak acid groups consisting of the carboxylic groups of OA and the protonated phosphate groups, if any, of BMEP. As can be seen, both σstrong and σweak are similar in all the polymerizations carried out. Regarding σweak, the surfactant-free miniemulsion polymerization carried out without cross-linker (run 4) shows 59 μC/cm2 of charge due to the carboxylic groups provided by OA. Therefore, the excess of OA nonbound to magnetite could diffuse from the core to the surface of the magnetic latex. In the case of using DVB as cross-linker (run 5) no weak surface charge is detected on the corresponding MPNPs surface, thus the crosslinked chains hinder the diffusion of OA molecules. These results differ from those obtained by Braconnot et al.57 in which no differences in surface charge densities of magnetic latexes were obtained when DVB was used in the formulation. In addition, 7226
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Figure 3. Effect of dextran as an aid stabilizer on MPNPs formation observed by TEM: (a) 5 wt % (run 7), (b) 10 wt % (run 8), (c) 5 wt % with 1 wt % DVB (run 9), (d) 10 wt % with 1 wt % DVB (run 10), (e) 5 wt % with 2 wt % BMEP (run 11), and (f) 10 wt % with 2 wt % BMEP (run 12).
they obtained higher surface charge densities (>150 μC/cm2) than those attributed to the interfacial carboxylic group of OA, observing that hydroxyl groups of the iron oxide nanoparticles were also titrated.58 These differences are due to the different polymerization process used to obtain MPNPs. Braconnot et al.57 used an emulsion process in which St and DVB diffused
from droplets to MPNPs, whereas in the present work a miniemulsion process in which St and DVB together with magnetite form a stable droplet is used. In this way, the mobility of the chains is more restricted. On the other hand, in the case of using BMEP as cross-linker (run 6), 43 μC/cm2 of charge coming from weak acid groups is detected through conductometric titrations. These weak 7227
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Langmuir groups are carboxylic groups of OA and phosphate groups of BMEP and are impossible to separate from each other. The presence of weak acid groups on the surface of MPNPs when BMEP was used indicates that the cross-linked structure formed inside each nanocomposite particle is more homogeneous than that formed by using DVB, in which the core of the nanocomposite particle is highly cross-linked. Effect of the Use of Dextran. One of the most common polymer stabilizers used to coat magnetic iron oxide nanoparticles is dextran. Dextran is a polysaccharide polymer composed exclusively of R-D-glucopyranosyl units with varying degrees of chain length and branching. Dextran has been used often as a polymer coating mostly because of its biocompatibility.59 In this work, different amounts (5 and 10 wt % with respect to the Fe3O4St ferrofluid) of dextran with a molecular weight of 40 kDa are added with the objective of improving the homogeneity inside the magnetic nanocomposites. Table 2 shows the number- and weight-average diameters, PDIs, and fractions of MPNPs for the surfactant-free miniemulsion polymerizations carried out with 5 wt % (run 7) and 10 wt % (run 8) of dextran. As can be seen, increasing the amount of the polymer stabilizer from 0 (run 4) to 10 wt %, smaller particle diameters, lower PDI values, and higher fractions of MPNPs are obtained, achieving the monodispersity in size (PDI < 1.05) according to Tsaur and Fitch60 in the case of using 10 wt % dextran (run 8). On the other hand, Figure 3 shows the effect of the aid stabilizer on the morphology of the MPNPs. As can be seen, slight differences are observed in the nanocomposites morphology using 5 wt % (run 7, Figure 3a) and 10 wt % dextran (run 8, Figure 3b) compared to the polymerization carried out without dextran (run 4, Figure 1.d). Therefore, the main improvement achieved by adding dextran is the narrowing of the PSD of the MPNPs. These results are in agreement with those obtained by Ladaviere et al.61 They reported the synthesis of polysaccharide-coated nanoparticles by direct emulsion polymerization of styrene in the presence of native dextran of different molecular weights and using KPS as initiator. Surprisingly, in spite of the lack of surface-active properties of native dextran, stable polystyrene latexes with a very low amount of coagulate were obtained. Particle size decreased with dextran concentration and molecular weight, obtaining highly monodisperse polystyrene nanoparticles at high dextran concentration. They studied the mechanism of chemical modification of dextran and found that in situ amphiphilic graft copolymers of dextran and polystyrene were formed which conferred a great stability to the nanoparticles. In the present work, the same kind of in situ dextran-based amphiphilic copolymers were formed, improving the stability, reducing the size, and lowering the PDI of the MPNPs. However, the distribution of magnetite inside nanocomposite did not improve because amphiphilic copolymer was only at the particle surface. Miniemulsion polymerizations with 5 and 10 wt % dextran are also carried out in the presence of DVB as hydrophobic crosslinker (runs 9 and 10, respectively), and in the presence of BMEP as hydrophilic cross-linker (runs 11 and 12, respectively). As can be seen in Table 2, in both cases, smaller diameters and lower PDI values are also obtained when the amount of dextran increases. However, PDI values are much higher than those obtained in the absence of dextran (runs 5 and 6). These results suggest that dextran plays an important role in particle nucleation and that droplet nucleation is not the only mechanism during particle formation. Regarding the fraction of MPNPs, only slight differences are observed, and in all cases fractions are higher than 0.9.
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Surface charge densities of the MPNPs prepared with 10 wt % dextran and DVB or BMEP as cross-linker (runs 10 and 12, respectively) are shown in Table 3. As in the previous cases, the contribution to the charge due to sulfate groups provided by the initiator (σstrong) is similar in all polymerizations; however, the contribution due to weak (carboxylic and phosphate, if any) acid groups (σweak) is higher than that of their homologous without dextran (runs 5 and 6). This implies that the mobility of the polymer chains and magnetite inside the nanocomposite particle, mainly in the core, is less reduced in the presence of dextran probably due to a better compatibility between the different environments inside the particles. The effect of dextran together with DVB or BMEP on the morphology of the MPNPs can also be observed in Figure 3. As in the case of using DVB alone (run 5, Figure 2b), MPNPs prepared in the presence of DVB and using 5 wt % (run 9, Figure 3c) and 10 wt % (run 10, Figure 3d) dextran are mainly constituted by a core of magnetite nanoparticles surrounded by a shell of polymer. However, using dextran, magnetite nanoparticles are much less agglomerated inside the nanocomposite particles. This corroborates the better compatibility between magnetite and polystyrene achieved by using dextran. On the other hand, magnetite nanoparticles inside the MPNPs synthesized in the presence of BMEP using 5 wt % (run 11, Figure 3e) and 10 wt % (run 12, Figure 3f) dextran are homogeneously distributed inside all nanocomposites. These results can be explained by taking into account the different amphiphilic copolymer formed with DVB or BMEP. Dextran-based amphiphilic graft copolymers are formed in the aqueous phase by reacting dextran with St and DVB or BEMP, and when they become enough hydrophobic and cross-linked, enter the droplets improving the compatibility and dispersibility of the nanocomposite. The diffusion of a dextran-based amphiphilic copolymer inside a nanocomposite depends on the type of crosslinker. On the one hand, using DVB, the dextran-based amphiphilic copolymer is much more hydrophobic, and thus, it can diffuse to the core of the particle favoring the dispersibility of the magnetite in the core. On the other hand, using BMEP, the dextran-based amphiphilic copolymer is more hydrophilic, favoring the homogeneity of the nanocomposite.
’ CONCLUSIONS Surfactant-free miniemulsion polymerization is a more convenient process than the conventional one using SDS as surfactant to synthesize MPNPs with the magnetite more homogeneously distributed inside the nanocomposite particles, which also have narrower PSDs, and avoiding pure polymer nanoparticles formation. The use of DVB as a hydrophobic cross-linker in a surfactantfree miniemulsion polymerization produces magnetic composite particles with larger particle diameters together with a narrower PSD and without formation of pure polymer nanoparticles. The morphology of the MPNPs formed is coreshell-type with the agglomerated magnetite remaining in the core surrounded by a thick polymer shell. However, the use of BMEP as a hydrophilic cross-linker provides a lower particle diameter maintaining the PSD and the fraction of MPNPs similar to its homologous surfactant-free miniemulsion polymerization carried out without cross-linker. Additionally, magnetite nanoparticles were distributed inside all nanocomposites due to the better compatibility between polymer and magnetite imparted by BMEP. 7228
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Langmuir The use of dextran as an aid stabilizer is checked. Dextran decreased the size and narrowed the PSD of MPNPs, maintaining the morphology similar to that obtained with the surfactantfree miniemulsion polymerization carried out without using it. However, a synergistic effect on the morphology of MPNPs is observed when dextran together with a cross-linker is used in the polymerizations. MPNPs composed by a nonagglomerated magnetite core surrounded by a polymer shell and magnetite homogeneously distributed along the entire nanocomposite particle are obtained using DVB and BMEP, respectively. The results presented here indicate that MPNPs having a homogeneous distribution of magnetite inside can be synthesized using an adequate mixture of surface modifiers by means of a single-step miniemulsion polymerization process.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work has been supported by the Spanish Ministerio de Ciencia e Innovacion (MICINN)/Programa Nacional de Materiales (MAT2009-13,155-C04-01). The authors thank Ibai Aranburu for his valuable help in the synthesis of the magnetic latexes. J. Ramos acknowledges financial support by the MICINN: Subprograma Juan de la Cierva (JCI-2008-2217). ’ REFERENCES (1) Elaissari, A.; Veyret, R.; Mandrand, B.; Chatterjee, J. In Colloidal Biomolecules, Biomaterials, and Biomedical Applications; Elaissari, A., Ed.; Marcel Dekker: New York, 2004; Vol. 116, p 1. (2) Rahman, M. M.; Elaissari, A. Adv. Polym. Sci. 2010, 233, 237. (3) Landfester, K.; Weiss, C. K. Adv. Polym. Sci. 2010, 229, 1. (4) Forcada, J.; Ramos, J. In Miniemulsion Polymerization Technology; Mittal, V., Ed.; Wiley & Scrivener Publishing LLC: New Jersey and Massachusetts, 2010; p 71. (5) Bernardy, N.; Romio, A. P.; Barcelos, E. I.; Dal Pizzol, C.; Dora, C. L.; Lemos-Senna, E.; Araujo, P. H. H.; Sayer, C. J. Biomed. Nanotechnol. 2010, 6, 181. (6) Fuchs, A. V.; Will, G. D. Polymer 2010, 51, 2119. (7) Landfester, K.; Musyanovych, A.; Mail€ander, V. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 493. (8) van Berkel, K. Y.; Hawker, C. J. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 1594. (9) Xu, Y.; Xu, H.; Gu, H. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 2284. (10) Chen, R.; Chu, F.; Gauthier, L. C.; Chaduc, I.; Bourgeat-Lami, E.; Lansalot, M. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 2329. (11) Schottner, G. Chem. Mater. 2001, 13, 3422. (12) Caruso, F. Adv. Mater. 2001, 13, 11. (13) Liu, Y.; Miyoshi, H.; Nakamura, M. Int. J. Cancer 2007, 120, 2527. (14) Saunders, B. R.; Turner, M. L. Adv. Colloid Interface Sci. 2008, 138, 1. (15) Iskandar, F. Adv. Powder Technol. 2009, 20, 283. (16) Yuan, J.; Mueller, A. H. E. Polymer 2010, 51, 4015. (17) Caseri, W. J. Mater. Chem. 2010, 20, 5582. (18) Reimers, G. W.; Rholl, S. A.; Snyder, R. W.; Khalafalla, S. E. Mater. Sci. Eng. 1974, 15, 129. (19) Liu, X.; Guan, Y.; Ma, Z.; Liu, H. Langmuir 2004, 20, 10278. (20) Csetneli, I.; Faix, M. K.; Szilagyi, A.; Kovacs, A. L.; Nemeth, Z.; Zrinyi, M. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 4802.
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