ARTICLE pubs.acs.org/Langmuir
Macroporous Polymers Obtained in Highly Concentrated Emulsions Stabilized Solely with Magnetic Nanoparticles Alejandro Vílchez,*,†,‡ Carlos Rodríguez-Abreu,§ Jordi Esquena,† Angelika Menner,‡ and Alexander Bismarck*,‡ †
Institute for Advanced Chemistry of Catalonia, Consejo Superior de Investigaciones Científicas (IQAC-CSIC), CIBER de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Jordi Girona 18-26, 08034 Barcelona, Spain ‡ Department of Chemical Engineering, Polymer & Composite Engineering (PaCE) Group, Imperial College London, South Kensington Campus, London SW7 2AZ, U.K. § International Iberian Nanotechnology Laboratory (INL), Av. Mestre Jose Veiga, 4715-310 Braga, Portugal
bS Supporting Information ABSTRACT: Magnetic macroporous polymers have been successfully prepared using Pickering high internal phase ratio emulsions (HIPEs) as templates. To stabilize the HIPEs, two types of oleic acid-modified iron oxide nanoparticles (NPs) were used as emulsifiers. The results revealed that partially hydrophobic NPs could stabilize W/O HIPEs with an internal phase above 90%. Depending upon the oleic acid content, the nanoparticles showed either an arrangement at the oilwater interface or a partial dispersion into the oil phase. Such different abilities to migrate to the interface had significant effects on the maximum internal phase fraction achievable and the droplet size distribution of the emulsions. Highly macroporous composite polymers were obtained by polymerization in the external phase of these emulsions. The density, porosity, pore morphology and magnetic properties were characterized as a function of the oleic acid content, concentration of NPs, and internal phase volume of the initial HIPEs. SEM imaging indicated that a close-cell structure was obtained. Furthermore, the composite materials showed superparamagnetic behavior and a relatively high magnetic moment.
’ INTRODUCTION Polymeric nanocomposites typically consist of a polymeric matrix possessing embedded particles with at least one characteristic length in the nanometer range. By combining both components in a single material, any additional property coming from the inorganic part can be directly imparted to the polymer. One of the advantages of such materials is the large nanoparticle matrix interface. Because of the commercial interest in these materials, much research has been carried out in recent years.1 It is well established that optical, mechanical, thermal, and chemical properties can be enhanced by using nanoparticles (NPs). Such advanced functional materials have applications as hydrogen storage systems, electrical conductors, or optical devices.2 Generally speaking, NPpolymer nanocomposites can be obtained in two different ways.3 In the in situ approach, the NPs are synthesized using a monomer as the dispersion medium. However, in the ex situ technique the NPs are first synthesized and then embedded or incorporated into the medium before polymerization or cross-linking is carried out. Both techniques mentioned above have been applied either in the bulk,4,5 in an emulsion,6 or in highly concentrated emulsions7 (as well as in gel systems8,9) in order to obtain nonporous (e.g., ultrathin films), particulate (e.g., latex), or macroporous nanocomposites, respectively. r 2011 American Chemical Society
Highly concentrated emulsions, also called high internal phase emulsions (HIPEs), are characterized by possessing a volume fraction of the disperse phase that exceeds 0.74, which corresponds to the critical value for the most compact packing of monodisperse spherical droplets.10,11 HIPEs can be used as templates for the preparation of macroporous materials, also called polyHIPEs,12 via the polymerization of the external (continuous) emulsion phase.13,14 The first patent on the subject was registered by Unilever.15 Over the past two decades, the research has been mainly focused on controlling the open-cell structure of polyHIPEs. The external phases of typical emulsion templates were in many cases formed by mixtures of styrene cross-linked with divinylbenzene.16 The polymerized materials have typically densities of as low as 0.02 g/cm3, porosities of up to 95%, and pore sizes ranging from 1 to 50 μm. More recently, several reports have proposed the integration of metal nanostructures (such as gold17 or palladium NPs18) in already formed polyHIPEs. Such materials were used as catalyst supports. However, their synthesis processes consisted of a two-step method. An alternative approach based on a single-step method Received: August 19, 2011 Published: September 09, 2011 13342
dx.doi.org/10.1021/la2032576 | Langmuir 2011, 27, 13342–13352
Langmuir
ARTICLE
Table 1. Composition of HIPEs and Physical Properties of Poly-Pickering HIPEs after the Polymerization of the Continuous Phase of the HIPEsa sample
Φ (vol )b
Cp (w/w %)c
Fb (g/cm3)d
P (%)e
average diameter (μm)f
average diameter (μm)g
PNP1
75
0.5
1.09 ( 0.01
79.5 ( 0.7
21 ( 12
436 ( 108
PNP2 PNP3
75 75
1.5 3.0
1.09 ( 0.01 1.13 ( 0.01
78.3 ( 0.4 80.1 ( 0.4
22 ( 16 18 ( 13
154 ( 38 124 ( 38
CNP4
75
3.0
1.12 ( 0.01
81.1 ( 1.0
37 ( 22
237 ( 92
CNP5
85
3.0
1.13 ( 0.01
88.7 ( 1.2
32 ( 22
310 ( 83
CNP6
92.5
3.0
1.13 ( 0.01
94.0 ( 1.3h
27 ( 15
669 ( 189
CNP7
85
5.0
1.13 ( 0.01
87.5 ( 0.6
30 ( 19
287 ( 75
PNP8
85
5.0
1.13 ( 0.01
88.2 ( 1.0
24 ( 15
378 ( 98
a
PNP and CNP indicate the use of synthesized and commercial magnetite nanoparticles, respectively. b Internal phase volume. c Nanoparticle weight percentage with respect to the total monomer weight. d Bulk or skeletal density. e Porosity. f Average pore diameter corresponding to the smaller size populations. g Average pore diameter corresponding to the larger size populations. h Estimated by mass/volume measurements.
has been developed by Menner et al. and described in a sequence of articles. In the first stage, they started adding SiO2 nanoparticles,19 carbon nanotubes, and TiO2 NPs20 to the external phase of W/O HIPEs, giving rise to polyHIPEs with enhanced mechanical properties. Subsequently, they took advantage of the ability of the nanoparticles to stabilize emulsions (typically denoted as Pickering emulsions21). Pickering HIPEs with high internal phase volume contents were stabilized using small amounts of oleic acid surface-modified SiO2 (HIPEs with up to 92% internal phase)22 and TiO223,24 nanoparticles. After the polymerization of the template, poly-Pickering-HIPE nanocomposites with high porosities, containing nanoparticles embedded in the walls, were obtained. These macroporous polymers had a typical close-cell structure. Such close-cell polymer foams are commonly used as thermal insulators.25 It is well known from the literature that particles, by analogy to surfactants, can adsorb strongly at oilwater interfaces, acting as an efficient barrier against droplet coalescence.26 However, to obtain stable emulsions some requirements are necessary: interalia, the contact angle of the nanoparticles at the oilwater interface, must be near 90°.27 Consequently, the nanoparticles should be partially wetted by both oil and water phases. The phase that preferentially wets the particles will be the external phase.28 The stability of Pickering emulsions depends on various factors such the nanoparticle concentration,29 their size, and their wettability.26 Although a large variety of inorganic nanoparticles have been used to stabilize emulsions,27,30,31 only a few (SiO222,32 and TiO224 NPs) have been used to stabilize Pickering HIPEs, which then have been used as templates for the preparation of polyPickering-HIPE nanocomposites. Other types of nanoparticles and aspects such as their size and hydrophobicity have not been studied in depth in such systems. Even though numerous investigations have been incorporated into the design of magnetic polymeric particles,6,33 little work has been done concerning magnetic macroporous polymeric materials.79 Here, we report the stabilization of Pickering W/O HIPEs with iron oxide NPs and the use of these HIPEs as templates for the preparation of macroporous polymer foams with a magnetic response. We undertook a more systematic study of the applicability of the oleic acid surface-modified NPs initiated by Menner et al.23 Two types of NPs with different sizes and different oleic acid contents have been chosen. The ability of both NPs to act as efficient emulsifiers has been compared. The results have revealed that partially hydrophobic magnetic NPs can stabilize HIPEs with an internal phase content
of up to 92.5%. Consequently, highly porous polymer foams have been obtained. The density, porosity, cell morphology, nanoparticle arrangement, and magnetic properties were characterized as a function of the nature of NPs, the oleic acid content and concentration of NPs, and the internal phase volume of the emulsions used to prepare the poly-Pickering HIPEs.
’ MATERIALS AND METHODS Materials. Styrene (g99%), oleic acid (g90%), iron(III) chloride hexahydrate (FeCl3 3 6H2O, g 98%), and anhydrous iron(II) chloride (FeCl2, g 99%) were purchased from Sigma-Aldrich. Oil-soluble initiator α,α0 -azoisobutyronitrile (AIBN, g 96%), cross-linker divinylbenzene (technical grade, 50%), and ammonium hydroxide (32 wt % NH3) were purchased from Merck. Styrene and divinylbenzene were purified before use by passing through neutral chromatographic aluminum oxide in order to remove polymerization inhibitors. The rest of the chemicals were used as received. In all experiments, Milli-Q water was used. One of the two types of iron oxide nanoparticles, used in this work, was acquired from Sigma-Aldrich (nanopowder,
