Article pubs.acs.org/Langmuir
Fabrication of Anisotropic Porous Silica Monoliths by Means of Magnetically Controlled Phase Separation in Sol−Gel Processes Marco Furlan and Marco Lattuada*,‡ Institute for Chemical and Bioengineering, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland S Supporting Information *
ABSTRACT: Sol−gel accompanied by phase separation is an established method for the preparation of porous silica monoliths with well-defined macroporosity, which find numerous applications. In this work, we demonstrate how the addition of (superpara)magnetic nanocolloids as templates to a system undergoing a sol−gel transition with phase separation leads to the creation of monoliths with a strongly anisotropic structure. It is known that magnetic nanocolloids respond to the application of an external magnetic field by selfassembling into columnar structures. The application of a magnetic field during the chemically driven spinodal decomposition induced by the sol−gel transition allows one to break the symmetry of the system and promote the growth of elongated needlelike silica domains incorporating the magnetic nanocolloids, aligned in the direction of the field. It is found that this microstructure imparts a strong mechanical anisotropy to the materials, with a ratio between the Young’s modulus values measured in a direction parallel and perpendicular to the one of the field as high as 150, and an overall smaller average macropores size as compared to isotropic monoliths. The microstructure and properties of the porous monoliths can be controlled by changing both the system composition and the strength of the applied magnetic field. Our monoliths represent the first example of materials prepared by magnetically controlling a phase transition occurring via spinodal decomposition.
■
pores.19−21 This is of crucial importance in the preparation of membranes, in chromatography, and for the production of polymeric scaffolds for tissue engineering applications. The formation of bicontinuous structures is typically achieved by choosing conditions such that the phase separation occurs via spinodal decomposition, which is at some point arrested, leading to an interconnected macroporous structure.9,11,22,23 Sol−gel preparation of silica monoliths via phase separation has been heavily investigated in the last couple of decades, especially for the preparation of materials used in chromatographic applications.9 Several investigations have demonstrated that, in the case of silica, the addition to the initial solution of polymers, such as poly(ethylene glycol) (PEG) or poly(acrylic acid) (PAA) can induce a phase separation via spinodal decomposition as the sol−gel reaction proceeds. The relative amount between polymer and silica precursor is the major player in controlling the microstructure of the final material.23 In this work, we present a novel method that permits a control of the microstructure of silica monoliths obtained by sol−gel process with phase separation by means of an external magnetic field. This is achieved by introducing superparamagnetic nanocolloids as smart templates and exploiting their unique self-assembly properties in the presence of a field to prepare a new class of highly macroporous materials with
INTRODUCTION The quest for new strategies aiming at the preparation of macroporous materials with controlled properties has been ongoing for many decades. Macroporous materials find applications in a variety of fields, such as catalysis,1,2 chromatography,3−5 as membranes,6,7 and as scaffolds in tissue engineering.8 The vast majority of these materials consists of either ceramics, and in particular metal oxides, or polymers. To meet the requirements demanded by the applications, a broad palette of production methods has been developed. These include sol−gel processes, which have been exploited for the preparation of macroporous ceramics,9−13 phase separation in polymer solutions,7,6 polymerization methods in the presence of suitable porogens,14 foaming processes,8,15 lithographic techniques, and more recently self-assembly of nanobuilding blocks.3,4,16,17 Of particular interest are preparation methods based on phase separation, such as sol−gel processes and phase separation in polymer solutions.6,7,9−12,18 In both cases, the preparation starts with a homogeneous solution, and a phase separation is induced either by a chemical reaction or by a temperature quench, leading to a separation into two phases, where at least one of them is rich in solid content (polymer or ceramic material). The considerable advantages of these techniques are the mild operating conditions, and the possibility, by tuning the thermodynamics and the kinetics of the process, to obtain bicontinuous structures with controlled macropores size distribution and well interconnected © 2012 American Chemical Society
Received: June 13, 2012 Revised: July 31, 2012 Published: July 31, 2012 12655
dx.doi.org/10.1021/la302407j | Langmuir 2012, 28, 12655−12662
Langmuir
Article
Figure 1. Schematics of the magnetically templated sol−gel process. The process starts with the synthesis of magnetite nanocrystals via coprecipitation from iron salts in the presence of ricinoleic acid as capping agent. Magnetite nanocrystals are then encapsulated into polymer nanocolloids via miniemulsion free-radical polymerization. Afterward, the sacrificial nanocolloids are dispersed in a silica precursor solution and aligned by the application of magnetic field. Silica starts nucleating and depositing on top of the nanocolloids. Finally, the obtained monolith is calcined in order to remove the sacrificial nanocolloids leading to an anisotropic silica monolith.
■
unique anisotropic structure and mechanical properties. Superparamagnetic nanocolloids have been widely investigated for their applications in electronics, bioseparations,24 catalysis,25 drug delivery, hyperthermia, and magnetic resonance imaging.26−29 It is well-established that superparamagnetic nanocolloids experience dipolar interactions in the presence of an external magnetic field that leads to their spontaneous selfassembly into one-dimensional chain-like structures.17,30−36 This behavior has been exploited to prepare magnetic responsive chains and rods, both rigid and flexible.33,35,36 The preparation of 3D anisotropic structures, achieved by inducing colloidal gelation of charge-stabilized polymer−magnetite nanoparticles in the presence of a magnetic field, and fixing of the final structure by means of a postpolymerization reaction, has also been reported.17 While the use of templating agents such as surfactants and block copolymers in sol−gel processes to fine-tune the properties of silica monoliths is a common practice,37 the use of colloids as templating agents has been only investigated in a few cases.38,39 To the best of our knowledge, the use of responsive nanoparticles as templates of silica materials has never been explored. We have prepared sterically stabilized magnetic nanocolloids via miniemulsion polymerization of styrene in the presence of hydrophobic magnetite nanocrystals. These nanocolloids were then used as templates for the preparation of porous silica monoliths via sol−gel method, and partially removed afterward through calcination. With the help of an external magnetic field applied during the sol−gel process, the spatial arrangement of the magnetic nanocolloids could be controlled. In this manner, the morphology of the bicontinuous phase formed as a result of the spinodal decomposition could be manipulated, leading to a strongly anisotropic microstructure with unique mechanical properties. In order to characterize the final monoliths, scanning electron microscopy, compressive stress measurements, and mercury porosimetry analysis were performed. We demonstrated that the microstructure of the final materials, their macroporosity, and their mechanical properties can be controlled by tuning the intensity of the applied magnetic field, the amount of sacrificial templates used, and the overall composition of the system.
EXPERIMENTAL SECTION
Materials and Instruments. 2,2′-Azobisisobutyronitrile purum ≥98% (AIBN), divinylbenzene technical grade, poly(ethylene glycol) (PEG) 10 kDa, and diethyl ether puriss were obtained from Fluka. Ricinoleic acid technical grade >80%, iron(II) chloride tetrahydrate Reagent-Plus 99% (FeCl2·4H2O), and acetone spectrophotometric ≥99.5% were obtained from Sigma-Aldrich. Iron(III) chloride hexahydrate extra pure 99+% (FeCl3·6H2O), and n-hexadecane 99% were obtained from Acros Organics. Styrene general purpose grade was obtained from Fisher Scientific. Tetramethoxysilane 98% (TMOS) was obtained from ABCR. Acetic acid glacial was obtained from Carlo Erba reagents. Ammonia solution 25% for analysis was obtained from Merck. Ethanol absolute analytical grade was obtained from Scharlau. Pluronic F-68 was obtained from BioChemica. If not specified, the chemicals were used as obtained. Dynamic light scattering (DLS) measurements have been performed using a Zeta Sizer Nano ZS (Malvern Instuments, UK). TEM pictures were recorded by a FEI Morgagni 268, while SEM pictures were recorded by a Zeiss Gemini 1530 FEG. Compressive stress measurements were conducted with an Instron Bluehill 5900 testing setup. Synthesis of Ricinoleic Coated Fe3O4. Oil-soluble magnetite nanocrystals were produced via a modification40 of the coprecipitation method developed by Massart.41 In a typical reaction, 3.90 g of FeCl2·4H2O and 10.71 g of FeCl3·6H2O were mixed in 180 mL of H2O. Once the salts were completely dissolved, a mixture composed of 8.56 g of ricinoleic acid and 4.8 g of acetone was added. Then, the solution was heated up to 80 °C, followed by the addition of 27 mL of NH3. The reaction was carried out for 30 min, then the solution was cooled down to room temperature. The product was then precipitated in acetone, washed 3 times with water and acetone, and then dried for 12 h. The product was subsequently redispersed in diethyl ether, filtered magnetically, and dried in a rota-vapor at 40 °C and 700 mbar. Synthesis of Polymer−Magnetite Nanocomposite Particles. Three grams of the synthesized magnetite nanocrystals were dispersed in 5.4 g of styrene, 0.6 g divinyl benzene, 0.125 g hexadecane, and 0.06 g AIBN. The obtained oil phase was then mixed with a solution composed of 48 g water and 0.60 g Pluronics F68. The obtained mixture was sonicated for 30 min at 70% amplitude with a duty cycle of 0.5 s. The miniemulsion was then transferred to a three-neck bottle, flushed with N2 for 5 min, and then heated up to 70 °C for 5 h, at which point almost complete conversion of the monomer was reached. The mean size of the produced particles was 137 nm. Synthesis of Nonmagnetic Polymer Nanoparticles. The synthesis of the nonmagnetic nanoparticles followed the same recipe of the synthesis of polymer−magnetite nanocomposite particles 12656
dx.doi.org/10.1021/la302407j | Langmuir 2012, 28, 12655−12662
Langmuir
Article
Figure 2. (a,b) SEM pictures of samples with a mass fraction of MP equal to 4.8 wt % and a MP/PEG ratio of 1:1, prepared in the absence (a) and in the presence (b) of a uniform magnetic field. (c,d) SEM pictures of samples with a mass fraction of MP equal to 0.5 wt % and a MP/PEG ratio of 1:9, prepared in the absence (c) and in the presence (d) of a uniform magnetic field. (e) Low-magnification SEM picture of a sample with a mass fraction of MP equal to 6.9 wt % and a MP/PEG ratio of 2.4:1, prepared in the presence of a uniform magnetic field. (f) High-magnification SEM picture of a fractured needle-like structure from a sample with a mass fraction of MP equal to 0.5 wt % and a MP/PEG ratio of 1:9, prepared in the presence of a uniform magnetic field. The magnetic field strength used in (b), (d), (e), and (f) was 1 T.
■
described in the previous section. The only difference is the absence of magnetite nanocrystals in the oil phase. The mean size of the produced particles was 130 nm. Synthesis of Silica Monoliths. The synthesis of the silica monoliths is based on a modification of a recipe described elsewhere.5 In a typical procedure, 0.34 g of PEG 10 kDa were dissolved in 5.465 mL of 0.01 M acetic acid, the obtained solution was cooled down to 0 °C, and then 1 mL of TMOS was added. The obtained mixture was stirred for 30 min at 0 °C, and then 0.235 mL of the above synthesized latex were added. The solution was stirred for 2 min more before pouring in two molds, one of which was put in an oven at 40 °C, while the second one was placed in a heat jacket inside a magnetic field of 1 T at 40 °C. Both samples were cured for 12 h. The obtained monoliths were first immersed in a solution of water−ethanol 1:1 by volume for 24 h and then in pure ethanol for additional 48 h. In order to remove all the water, the ethanol was replaced with fresh every 24 h. The monoliths were then dried in an oven at 50 °C for 3 days and finally calcined at 600 °C for 3 h. During this work, different parameters were varied; see Supporting Information Table S1. For the experiments with larger amounts of latex, the concentration of acetic acid was adjusted by the addition of a suitable amount of 1.25 M acetic acid solution.
RESULTS AND DISCUSSION
The production of silica macroporous monoliths via sol−gel process has been well-investigated in the literature.5,9−11 The most commonly investigated systems start from a homogeneous solution of silica precursor and a polymer acting as a porogen. The silica precursor (usually TMOS) is first hydrolyzed and subsequently polymerized. This polycondensation acts as a chemical cooling, which induces a phase separation in the system, usually occurring through a spinodal decomposition. The spinodal decomposition leads to the formation of a bicontinuous structure, with one phase rich in silica that becomes coarser, hardens, and eventually forms the skeleton of the monolith. In this work, we introduced a substantial modification to the above-described process. We accomplish this by adding from the beginning magnetic nanocolloids to the system, which act as templating agents, and we take advantage of their capability to align into string-like structures in the presence of a magnetic 12657
dx.doi.org/10.1021/la302407j | Langmuir 2012, 28, 12655−12662
Langmuir
Article
field. The application of a magnetic field during the chemically induced spinodal decomposition offers us the unique opportunity to control the morphology of the bicontinuous phase. A schematic of our process, which consists of two main steps, is shown in Figure 1. The first step is the production of magnetic nanocolloids, which are used in the second step, the sol−gel process, as a templates. Among the variety of available superparamagnetic nanoparticles, we utilized magnetite nanocrystals synthesized by coprecipitation method41 and coated with a long-chain fatty acid (ricinoleic acid) in order to render them oil-soluble. These nanocrystals were then dispersed in a monomer phase (styrene), miniemulsified in water in the presence of Pluronics F-68 as an emulsifier, leading to droplets that were eventually polymerized via free radical polymerization into polystyrene nanocolloids with a hydrodynamic diameter of ∼137 nm and magnetite content of 33 wt %. This preparation strategy fits the purpose of our work for a number of reasons. First of all, large quantities of magnetic nanocolloids with a narrow size distribution can be easily prepared at relatively high particle volume fractions (up to ∼15 wt %). The steric stabilizer enhances the stability of the nanocolloids under the conditions chosen for the sol−gel process, and also provides the nanocolloids with surface properties that would render them similar to the polymer (poly(ethylene glycol), (PEG)) that was added during the sol−gel process as porogen in many of the experiments. A last considerable advantage of our nanocolloids is their morphology, which can be seen in Figure S1 (Supporting Information). In fact, magnetite nanocrystals coated with a fatty acid are well-soluble in styrene (up to 40 wt %), but are incompatible with polystyrene. Therefore, upon polymerization magnetite nanocrystals tend to aggregate and accumulate on the surface of the nanocolloids because of the Pickering effect.42 Since silica has a strong tendency to nucleate on magnetite,43 these large magnetite patches ensure that the nanocolloids get trapped in the silica matrix. As shown in a previous work, vibration sample magnetometer (VSM) measurements show that the nanocolloids are superparamagnetic.17 The second step is the crucial part of our process. The magnetic nanoparticles are dispersed in a solution composed of water, acetic acid, and a silane precursor (TMOS), previously kept well-mixed for 30 min in an ice bath in order to produce a homogeneous solution. The dispersion is then transferred to a mold, which is typically placed between the poles of an electromagnet that generate a homogeneous magnetic field. In order to gain a better understanding of our process, we first performed experiments in the absence of a porogen, i.e., in the absence of a spinodal decomposition. The sol−gel process begins with the progressive condensation of TMOS, after which silica starts nucleating. The structure of the material obtained at the end of the process is shown in SI Figure S2a for the case of no application of a magnetic field. The material structure strongly resembles a colloidal gel, with silica almost completely covering the nanocolloids and linking them together. This suggests that silica has a strong propensity to nucleate right on the surface of the nanocolloids due to the strong affinity for magnetite, eventually covering them. Calcination of the structure to remove all the polymer leaves a silica and magnetite skeleton shown in SI Figure S2b with an almost identical structure to the one shown in SI Figure S2a. In the presence of a magnetic field, the nanoparticles align themselves almost instantaneously in the field direction, leading to the
formation of chains of particles. These chains are progressively covered and cross-linked together by silica, creating a network of aligned chains as can be observed in SI Figure S3. One should also note that chains are not perfectly straight, due to the competition between dipolar interactions that induce chaining and diffusion, which tends to drive the assembly process toward a diffusion-limited path and still plays an important role for nanocolloids in the size range of 100−200 nm. It is worth noting that when the same process is applied to nanocolloids prepared following an almost identical recipe, but without magnetite nanocrystals, the obtained structures are completely different, as SI Figure S4 shows. The monoliths do not resemble colloidal gels, but rather show a silica matrix with polymer nanoparticles agglomerated together and not welldispersed inside it. The difference in the observed structure is clearly imputable to the lack of magnetite on the surface of the nanocolloids, which promotes nucleation and growth of a silica layer. The most interesting structures are, however, observed when PEG, often used in the preparation of silica monoliths as a porogen, has been added to control the structure of silica monoliths. Some examples are shown in Figure 2. PEG promotes spinodal decomposition in the system, which leads to the formation of a percolated structure made of smooth micrometer-sized silica beads, and almost equally sized macropores (see Figure S5 in Supporting Information). By carefully tuning the amount of solvent and PEG used, the size and the volume of beads and of the macropores can be tuned.11 We first tested the effect of PEG addition to our system containing polymer−magnetic nanoparticles. As previously discussed, silica nucleates on the magnetic nanocolloids, due to the strong affinity between magnetite and silica. This leads to the accumulation of the nanocolloids in the silica-rich phase, where they become incorporated in the percolated matrix leading to a beads-like structure similar to the one obtained in the absence of the nanocolloids, as Figure 2a,c shows. The application of a magnetic field leads to the formation of dramatically different structures, which first manifest themselves in the macroscopic appearance of the materials, as SI Figure S6 shows. While samples prepared in the absence of a magnetic field have a featureless appearance, those prepared in the presence of a field have a fibrous-like structure, loosely reminiscent of that of wood. Microscopically, the magnetic nanocolloids are still coated by silica and accumulate in the silica-rich regions. However, due to their dipolar interactions, they align themselves in the direction of the magnetic field altering the morphology of the silica-rich domains. The bicontinuous domains obtained as a result of the spinodal decomposition evolve into very long needle-like silica domains all oriented in the direction of the applied field, with approximately the same diameter as the large spherical silica beads formed in the absence of a field, as Figure 2b,d shows. SEM pictures do not permit a precise estimate of the lengths of these needles, which appear to be hundreds of micrometers long, and are densely packed together, as Figure 2e reveals. We can therefore claim that, in this case, our process consists of a magnetically driven control of the spinodal decomposition process normally occurring in the presence of a porogen. The amount of PEG relative to that of nanocolloids affects the size of the obtained beads and needle-like structures. As can be observed by comparing Figure 2a,b with Figure 2c,d, both the particle and needle sizes increase with increase of the ratio 12658
dx.doi.org/10.1021/la302407j | Langmuir 2012, 28, 12655−12662
Langmuir
Article
of PEG to nanocolloids. This effect has been quantified by measuring the average diameter of both particles and needles prepared in the presence and in the absence of a magnetic field, respectively, from SEM pictures. The results are reported in SI Figure S7, where it can be seen that the diameter of the needles and particles, plotted versus the ratio of PEG amount to nanocolloids amount, increases with increasing the percentage of PEG. This increase is more pronounced when the percentage of PEG is higher than 90%. These results suggest that the role of magnetic fields is only to change the shape and spatial arrangement of the silica domains, but not their diameter. One remarkable feature of the monoliths prepared by the proposed method is that very low amounts of nanocolloids are sufficient to induce a drastic change in the microstructure of the monoliths upon application of a magnetic field. SI Figure S8 shows SEM pictures of monoliths prepared in the presence of an external magnetic field, containing respectively 0.2 wt %, 0.1 wt %, and 0.05 wt %. It is found that 0.1 wt % of particles is sufficient to induce a very clear formation of needle-like structures, while 0.05 wt % of magnetic nanocolloids is not sufficient to alter the entire structure of the monolith. Not surprisingly, the change in microstructure of the materials induced by the application of a magnetic field consequently alters their pore size distribution. Figure 3a shows the pore size distributions, measured by mercury porosimetry, of monoliths prepared with porogen, of some pairs of samples with different compositions, where one was prepared in the presence and one in the absence of a magnetic field. As a general trend, the application of a magnetic field at a given composition influences the macroporosity of the material and leads to a slightly smaller pore size, and to less defined and typically broader pore size distribution. This is a consequence of the needle-like structures being more tightly packed together than a random bead configuration, thus reducing the pore space among them. As the amount of PEG with respect to the nanocolloids increases, the average pore size increases from around 300 nm for a sample containing 50 wt % of PEG to several micrometers for a sample containing 90 and 95 wt %. This is expected, since an increase in PEG amount leads to an increase in the average bead (or needle) size, and this correspondingly affects the size of pores. A similar effect of the magnetic field is observed in the case of samples prepared in the absence of PEG, as shown in SI Figure S9. The pore size of the monoliths is affected not only by the composition, but also by the strength of the magnetic field applied during the sol−gel process. This can be observed from Figure 3b, showing how the pore size distribution is shifted toward larger size values as the intensity of the applied magnetic field is reduced. Between a monolith prepared in the presence of a field of 0.02 T and one prepared at 0.005 T, there is little difference, meaning that the structures between the two experiments are very similar to one another. This is confirmed by SEM pictures, as can be seen in SI Figure S10, where there are only a few small needle-like structures, aligned inside a random matrix of beads for the experiment at 0.02 T (see SI Figure S10 (a)), whereas for the experiment at 0.005 T (see SI Figure S10 (b)) only a random structure is visible. The most dramatic effect of the application of a magnetic field during sol−gel is in the mechanical properties of the final monoliths, which are used here as a quantitative measure of anisotropy. The mechanical strength of the samples has been quantified by performing compressive stress measurements and
Figure 3. Pore size distribution for different samples. (a) Samples prepared both in the absence (NMF) and in the presence (MF) of a magnetic field, containing different magnetic nanoparticles to PEG (MP:PEG) ratios, as reported in the legend. A magnetic field strength of 1 T was applied during sample preparation. (b) Samples prepared both in the absence and in the presence of a magnetic field, using different magnetic field strengths, as reported in the legend. The composition of the samples was kept constant: 0.5 wt % MP, 4.8 wt % PEG, and 14.2 wt % TMOS.
extracting the Young’s moduli. The parallelepiped-shaped samples have been measured in two directions, which in the case of samples prepared in the presence of an external magnetic field correspond to the directions parallel and perpendicular to the field, respectively. The mechanical stress measurements were performed after the calcination step so that the polymer was completely removed and did not influence the properties of the final silica monoliths. To better compare the different monoliths, we normalized the elastic modulus by multiplying it by the measured cross-sectional area of the sample and dividing the result by its effective area. The effective area was calculated by dividing the weight of the monolith by the product of the density of silica by the measured sample height. In this way, the elastic modulus was normalized to that of an equivalent density silica monolith. This was performed because porous materials with different compositions tend to shrink to different extents, making a direct comparison difficult without the aforementioned normalization. SI Figure S11 shows a typical example of compressive stress versus strain curves. A remarkable general feature of the monoliths prepared in the presence of a magnetic field is the enormous mechanical anisotropy that the magnetic field is capable of inducing. As shown in Figure 4a, the difference between the Young’s moduli 12659
dx.doi.org/10.1021/la302407j | Langmuir 2012, 28, 12655−12662
Langmuir
Article
mediate between those of samples prepared in the presence of a field and those measured perpendicular to the direction of the field and parallel to it. These effects can be explained by the needle-like clusters observed in samples prepared in a magnetic field, which are all aligned in the field direction imparting a tremendous strength to the structure. In the perpendicular direction instead, the mechanical properties are inferior to those of isotropic samples because the needles are loosely bound together laterally by only a few links.
■
CONCLUSION In this work, we have demonstrated how sol−gel transition of silica accompanied by phase separation can be substantially modified by the use of magnetic nanocolloids as templates. Our magnetic nanocolloids consist of polystyrene nanoparticles prepared by miniemulsion polymerization, in which magnetite nanocrystals are encapsulated. These nanocolloids can respond to the application of an external magnetic field by aligning into chain-like structures. When these nanocolloids are added to a solution containing a silica precursor undergoing a sol−gel transition with phase separation, the structure of the resulting porous silica monoliths can be controlled by applying a magnetic field. In the absence of a magnetic field, the structure of the monoliths is unaffected by the presence of the nanocolloids, which are incorporated in a silica skeleton made of interconnected beads. On the other hand, the application of magnetic field during sol−gel transition allows one to control the morphology of the bicontinuous structure formed by the spinodal decomposition, leading to the formation of materials with extremely long needle-like domains aligned in one direction. Remarkably, very small amounts of nanocolloids are required to induce drastic variations in the final structure, and to control various properties in the final material. In particular, the average pore size of the monoliths can be reducedand the size distribution broadenedby increasing the amount of nanocolloids and the intensity of the applied field. Similarly, the mechanical properties of these monoliths are enormously affected by the application of a magnetic field during the sol−gel transition, and a greater than 2 orders of magnitude mechanical anisotropy between directions parallel and perpendicular to the field has been achieved. This process not only is restricted to silica monoliths but also, by adjusting the synthesis conditions, could be generalized to produce a broad range of materials, both ceramic and polymeric, that can be synthesized via a sol−gel process.
Figure 4. Elastic modulus as a function of (a) the applied magnetic field strength during the synthesis. The composition of all samples is as follows: 0.5 wt % magnetic nanoparticles, 4.8 wt % PEG, and 14.2 wt % TMOS. ◊ refer to measurements performed in the direction of the applied magnetic field. ■ refer to measurements performed in a direction perpendicular to that of the applied magnetic field. Note that data corresponding to samples prepared in the absence of an external field have been reported for graphical clarity at a field intensity equal to the Earth's magnetic field value (i.e., 70 μT) and (b) magnetic nanoparticles weight fraction. ⧫ refer to values measured in the direction of the applied magnetic field, while ■ to values measured in a direction perpendicular to that of the applied magnetic field. Δ refer to measurements performed on monoliths produced in the absence of a magnetic field. The magnetic field strength applied during the synthesis was 1 T. All the measurements were carried on without the application of a magnetic field.
■
in the directions parallel and perpendicular to the field, applied during the reaction, is increasing with strength of the applied field, until a saturation for high fields value is reached. For high enough magnetic fields, the values of Young’s modulus in directions parallel to the field can be more than 2 orders of magnitude higher than the one perpendicular to the field, and can be 20 times higher than that of a material prepared in the absence of a field. The values of the modulus measured in the perpendicular direction instead decrease as the field increases. On the other hand, samples prepared in the absence of the field are isotropic, thus confirming that only the application of the field during the reaction is responsible for anisotropy. Figure 4b shows that an increase in magnetic nanoparticle concentration leads to an increase of the elastic modulus in the parallel direction, which remains approximately constant in the perpendicular direction. The values of the elastic modulus of monoliths produced outside the magnetic fields are inter-
ASSOCIATED CONTENT
S Supporting Information *
Table with a list of performed experiments, TEM picture of magnetite-polymer nanocomposites, SEM pictures of a sample produced without PEG, SEM picture of a sample produced with nonmagnetic colloids, SEM Pictures of a monolith produced without nanocolloids, photographs of the monoliths, chart reporting size vs PEG to MP ratio, SEM Pictures of monolith obtained with different MP concentration, chart reporting pore volume vs pore radius of monolith prepared without PEG, SEM pictures of samples prepared with different magnetic field intensity, and chart reporting compressive stress vs compressive strain for a monolith prepared both in the presence and in the absence of a magnetic field. This material is available free of charge via the Internet at http://pubs.acs.org. 12660
dx.doi.org/10.1021/la302407j | Langmuir 2012, 28, 12655−12662
Langmuir
■
Article
and morphology of macroporous polymers obtained by bulk polymerization. Macromolecules 2001, 34, 658−661. (15) Blaker, J.; Maquet, V.; Jérôme, R.; Boccaccini, A.; Nazhat, S. Mechanical properties of highly porous PDLLA/Bioglass composite foams as scaffolds for bone tissue engineering. Acta Biomater. 2005, 1, 643−652. (16) Yao, X.; Yao, H.; Li, Y. Hierarchically aligned porous scaffold by ice-segregation-induced self-assembly and thermally triggered electrostatic self-assembly of oppositely charged thermosensitive microgels. J. Mater. Chem. 2009, 19, 6516−6520. (17) Furlan, M.; Brand, B.; Lattuada, M. Magnetic gelation: a new method for the preparation of polymeric anisotropic porous materials. Soft Matter 2010, 6, 5636−5644. (18) Amatani, T.; Nakanishi, K.; Hirao, K.; Kodaira, T. Monolithic Periodic Mesoporous Silica with Well-Defined Macropores. Chem. Mater. 2005, 17, 2114−2119. (19) Lee, B. P.; Douglas, J. F.; Glotzer, S. C. Filler-induced composition waves in phase-separating polymer blends. Phys. Rev. E 1999, 60, 5812−5822. (20) Jones, R. A. L.; Norton, L. J.; Kramer, E. J.; Bates, F. S.; Wiltzius, P. Surface-directed spinodal decomposition. Phys. Rev. Lett. 1991, 66, 1326−1329. (21) Vaynzof, Y.; Kabra, D.; Zhao, L. H.; Chua, L. L.; Steiner, U.; Friend, R. H. Surface-Directed Spinodal Decomposition in Poly[3hexylthiophene] and C(61)-Butyric Acid Methyl Ester Blends. ACS Nano 2011, 5, 329−336. (22) Kanamori, K.; Nakanishi, K.; Hanada, T. Spinodal decomposition in siloxane sol-gel systems in macroporous media. Soft Matter 2009, 5, 3106−3113. (23) Lofton, C. M.; Milz, C. B.; Huang, H.; Sigmund, W. M. Bicontinuous porosity in ceramics utilizing polymer spinodal phase separation. J. Eur. Ceram. Soc. 2005, 25, 883−889. (24) Ditsch, A.; Yin, J.; Laibinis, P. E.; Wang, D. I. C.; Hatton, T. A. Ion-exchange purification of proteins using magnetic nanoclusters. Biotechnol. Prog. 2006, 22, 1153−1162. (25) Lu, A. H.; Schmidt, W.; Matoussevitch, N.; Bonnemann, H.; Spliethoff, B.; Tesche, B.; Bill, E.; Kiefer, W.; Schuth, F. Nanoengineering of a magnetically separable hydrogenation catalyst. Angew. Chem., Int. Ed. 2004, 43, 4303−4306. (26) Neuberger, T.; Schopf, B.; Hofmann, H.; Hofmann, M.; von Rechenberg, B. Superparamagnetic nanoparticles for biomedical applications: Possibilities and limitations of a new drug delivery system. J. Magn. Magn. Mater. 2005, 293, 483−496. (27) Satarkar, N. S.; Hilt, J. Z. Magnetic hydrogel nanocomposites for remote controlled pulsatile drug release. J. Controlled Release 2008, 130, 246−251. (28) Asmatulu, R.; Zalich, M. A.; Claus, R. O.; Riffle, J. S. Synthesis, characterization and targeting of biodegradable magnetic nanocomposite particles by external magnetic fields. J. Magn. Magn. Mater. 2005, 292, 108−119. (29) Yang, J.; Park, S. B.; Yoon, H. G.; Huh, Y. M.; Haam, S. Preparation of poly epsilon-caprolactone nanoparticles containing magnetite for magnetic drug carrier. Int. J. Pharm. 2006, 324, 185−190. (30) Janjua, M.; Nudurupati, S.; Singh, P.; Aubry, N. Electric fieldinduced self-assembly of micro- and nanoparticles of various shapes at two-fluid interfaces. Electrophoresis 2011, 32, 518−526. (31) Grzelczak, M.; Vermant, J.; Furst, E. M.; Liz-Marzan, L. M. Directed Self-Assembly of Nanoparticles. ACS Nano 2010, 4, 3591− 3605. (32) Martin, J. E.; Gulley, G. Field-structured composites for efficient, directed heat transfer. J. Appl. Phys. 2009, 106 (33) Singh, H.; Laibinis, P. E.; Hatton, T. A. Rigid, superparamagnetic chains of permanently linked beads coated with magnetic nanoparticles. Synthesis and rotational dynamics under applied magnetic fields. Langmuir 2005, 21, 11500−11509. (34) Dreyfus, R.; Baudry, J.; Roper, M. L.; Fermigier, M.; Stone, H. A.; Bibette, J. Microscopic artificial swimmers. Nature 2005, 437, 862− 865.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address ‡
Adolphe Merkle Institute, University of Fribourg, route de l’ancienne Papeterie CP 209, CH-1723 Marly 1, Switzerland. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors acknowledge the support by the Electron Microscopy of ETH Zurich (EMEZ) and Frank Krumeich for the SEM pictures, the help of Dr. Josef Kaufmann for mercury porosimetry measurements, and the help of Dr. Kirill Feldman and Florian Hirt for the compression measurements. A special thanks goes to Professor Massimo Morbidelli for fruitful discussions and for his support. The financial support from the Swiss National Science Foundation (SNF), with grant number 200021-116687, is gratefully acknowledged.
■
REFERENCES
(1) Sachse, A.; Galarneau, A.; Fajula, F.; Di Renzo, F.; Creux, P.; Coq, B. Functional silica monoliths with hierarchical uniform porosity as continuous flow catalytic reactors. Microporous Mesoporous Mater. 2011, 140, 58−68. (2) Studart, A. R.; Gonzenbach, U. T.; Tervoort, E.; Gauckler, L. J. Processing routes to macroporous ceramics: a review. J. Am. Ceram. Soc. 2006, 89, 1771−1789. (3) Marti, N.; Quattrini, F.; Butté, A.; Morbidelli, M. Production of polymeric materials with controlled pore structure: the ″Reactive gelation″ process. Macromol. Mater. Eng. 2005, 290, 221−229. (4) Bechtle, M.; Butté, A.; Storti, G.; Morbidelli, M. Preparation of macroporous methacrylate-based monoliths for chromatographic applications by the Reactive Gelation Process. J. Chromatogr., A 2010, 1217, 4675−4681. (5) Minakuchi, H.; Nakanishi, K.; Soga, N.; Ishizuka, N.; Tanaka, N. Octadecylsilylated Porous Silica Rods as Separation Media for Reversed-Phase Liquid Chromatography. Anal. Chem. 1996, 68, 3498−3501. (6) Tanaka, T.; Lloyd, D. R. Formation of poly(l-lactic acid) microfiltration membranes via thermally induced phase separation. J. Membr. Sci. 2004, 238, 65−73. (7) Tanaka, T.; Tsuchiya, T.; Takahashi, H.; Taniguchi, M.; Ohara, H.; Lloyd, D. R. Formation of Biodegradable Polyesters Membranes via Thermally Induced Phase Separation. J. Chem. Eng. Jpn. 2006, 39, 144−153. (8) Matuana, L. M.; Diaz, C. A. Study of Cell Nucleation in Microcellular Poly(lactic acid) Foamed with Supercritical CO2 through a Continuous-Extrusion Process. Ind. Eng. Chem. Res. 2010, 49, 2186−2193. (9) Nakanishi, K. Pore Structure Control of Silica Gels Based on Phase Separation. J. Porous Mater. 1997−06−01, 4, 67−112. (10) Nakanishi, K.; Kobayashi, Y.; Amatani, T.; Hirao, K.; Kodaira, T. Spontaneous Formation of Hierarchical Macro-Mesoporous EthaneSilica Monolith. Chem. Mater. 2004, 16, 3652−3658. (11) Nakanishi, K.; Tanaka, N. Sol-Gel with Phase Separation. Hierarchically Porous Materials Optimized for High-Performance Liquid Chromatography Separations. Acc. Chem. Res. 2007, 40, 863− 873. (12) Nakanishi, K.; Amatani, T.; Yano, S.; Kodaira, T. Multiscale Templating of Siloxane Gels via Polymerization-Induced Phase Separation. Chem. Mater. 2008, 20, 1108−1115. (13) Wen, J.; Wilkes, G. L. Organic/Inorganic Hybrid Network Materials by the Sol-Gel Approach. Chem. Mater. 1996, 8, 1667−1681. (14) Santora, B. P.; Gagne, M. R.; Moloy, K. G.; Radu, N. S. Porogen and cross-linking effects on the surface area, pore volume distribution, 12661
dx.doi.org/10.1021/la302407j | Langmuir 2012, 28, 12655−12662
Langmuir
Article
(35) Biswal, S. L.; Gast, A. P. Mechanics of semiflexible chains formed by poly(ethylene glycol)-linked paramagnetic particles. Phys. Rev. E 2003, 68 (36) Goubault, C.; Jop, P.; Fermigier, M.; Baudry, J.; Bertrand, E.; Bibette, J. Flexible magnetic filaments as micromechanical sensors. Phys. Rev. Lett. 2003, 91 (37) Mann, S.; Burkett, S. L.; Davis, S. A.; Fowler, C. E.; Mendelson, N. H.; Sims, S. D.; Walsh, D.; Whilton, N. T. Sol-Gel Synthesis of Organized Matter. Chem. Mater. 1997, 9, 2300−2310. (38) Antonietti, M.; Berton, B.; Goltner, C.; Hentze, H. Synthesis of mesoporous silica with large pores and bimodal pore size distribution by templating of polymer latices. Adv. Mater. 1998, 10, 154. (39) Velev, O.; Kaler, E. Structured porous materials via colloidal crystal templating: From inorganic oxides to metals. Adv. Mater. 2000, 12, 531−534. (40) Furlan, M.; Kluge, J.; Mazzotti, M.; Lattuada, M. Preparation of biocompatible magnetite-PLGA composite nanoparticles using supercritical fluid extraction of emulsions. J. Supercrit. Fluids 2010, 54, 348− 356. (41) Massart, R. Preparation of aqueous magnetic liquids in alkaline and acidic media. IEEE Trans. Magnetics 1981, 17, 1247−1248. (42) Sacanna, S.; Kegel, W. K.; Philipse, A. P. Thermodynamically stable pickering emulsions. Phys. Rev. Lett. 2007, 98, 158301. (43) Philipse, A.; Vananbruggen, M.; Pathmamanoharan, C. Magnetic Silica dispersions - preparation and stability of surfacemodified Silica particles with a magnetic core. Langmuir 1994, 10, 92− 99.
12662
dx.doi.org/10.1021/la302407j | Langmuir 2012, 28, 12655−12662
