Article pubs.acs.org/Macromolecules
Polymer Janus Nanoparticles with Two Spatially Segregated Functionalizations Markus Urban,† Birger Freisinger,† Omayma Ghazy,‡ Roland Staff,† Katharina Landfester,† Daniel Crespy,† and Anna Musyanovych*,† †
Max Planck Institute for Polymer Research, Ackermannweg 10, Mainz 55128, Germany Institute of Organic Chemistry, Macromolecular Chemistry and Organic Materials, University of Ulm, Albert-Einstein-Allee 11, 89069 Ulm, Germany
‡
S Supporting Information *
ABSTRACT: Janus nanoparticles with a poly(L-lactide) face and a polystyrene-based face functionalized with amine or carboxylic acid groups were synthesized via two different approaches. In the first approach, the poly(styrene-co-methacrylic acid) or poly(styrene-co-2-aminoethyl methacrylate) copolymers were generated in situ in miniemulsion droplets before phase separation between the copolymers and the poly(L-lactide) occurred. In the second approach, the copolymers were prepared before the emulsification step. A solution containing the poly(L-lactide) and one of the copolymers was then emulsified, and the solvent was subsequently removed to induce a phase separation between the polymers, yielding a Janus morphology. The density of functional groups (amine or carboxylic acid) could be varied between 0 and 5 groups per nm2. Finally, we demonstrated that one face of the Janus nanoparticle could be selectively employed for a chemical reaction. Indeed, silver nanoparticles could be nucleated selectively on the poly(L-lactide) face.
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INTRODUCTION The design of composite particles containing at least two different polymer phases is attractive for advanced applications. Composite submicron particles have been extensively used to tailor the catalytic, electrical, optical, magnetic, mechanical, or thermal properties of materials.1−5 The term Janus particle was first mentioned by Casagrande and Veyssie in 19886 to describe spherical glass particles with one apolar and the other polar hemispheres. Janus particles contain two polymer phases that are segregated as two faces in the same particle. Several excellent reviews have been published in recent years, highlighting the main preparation strategies of Janus particles, their unique properties, and important applications.7−15 Because of the anisotropy in the properties of Janus particles and their geometry, they were employed for display technology,16 as building blocks for complex structures,17,18 and as solid surfactants for emulsions.19,20 Janus particles can be designed by microfluidics,12,16,21−23 self-assembly of block copolymers,19 Pickering emulsion routes,17,24 biphasic electrified jetting,25,26 seeded or swelling-diffusion emulsion polymerization,27−29 the temporary masking of one particle side followed by the modification of the unprotected side,30−33 and through internal phase separation inside the droplets induced by the evaporation of a solvent.34−44 The latter method has the advantage that it can be applied to many types of organic and inorganic materials, resulting in hybrid particles with controlled size and morphology.45 Furthermore, when it is combined with the miniemulsion process to create the droplets, © XXXX American Chemical Society
the evaporation can be performed in the quasi-absence of coalescence between the droplets,46 and nanoparticles42−44,47,48 or nanocapsules49−51 can be obtained. For the formation of polymer nanoparticles, the process involves emulsification of a polymer solution; i.e., droplets of the polymer solution are dispersed in another immiscible liquid phase. When the solvent is evaporated after the emulsification, the polymer precipitates and can form nanoparticles in the presence of a stabilizer. We reported previously the preparation of poly(L-lactide) (PLLA) particles displaying fluorescent and magnetic properties.47,48 In the current work, our aim was to create Janus nanoparticles with one biodegradable face and a nonbiodegradable face with a surface that can be functionalized in a controlled way, enabling differential surface modification. PLLA is a (bio)degradable polymer and was chosen as a “sacrificial phase”, thus providing the possibility to remove one part in a controlled way resulting in an asymmetric colloid. As another face, copolymers of styrene with few amounts of other functional comonomers were selected because of their immiscibility with PLLA and the hydrolytic stability of polystyrene.
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EXPERIMENTAL SECTION
Materials. Methacrylic acid (MAA, Merck, 99%) and styrene (Merck, 99%) were distilled under reduced pressure and stored at −20 Received: July 2, 2014 Revised: October 3, 2014
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dx.doi.org/10.1021/ma5013545 | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
Article
°C before use. (1-Hexadecyl)trimethylammonium chloride (CTMACl, Alfa Aesar, 95%), 2-aminoethyl methacrylate hydrochloride (AEMA, Aldrich, 90%), 2,2′-azobis(2-methylbutyronitrile) (V59) and 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile) (V70, Wako Chemicals), BiomerL9000 (PLLA, Biomer, Germany, Mn ∼ 66 500 g mol−1, Mw ∼ 145 000 g mol−1 determined by GPC in chloroform), chloroform (Fisher Scientific, 99.98%), n-hexadecane (Merck, 99%), silver acetate (AgAc, Fluka, 99%), urotropine (Aldrich, 99%), poly(Nvinylpyrrolidone) (Aldrich, Mw ∼ 10 000 g mol−1), sodium n-dodecyl sulfate SDS (Alfa Aesar, 99%), and Lutensol AT50, which is a poly(ethylene oxide) hexadecyl ether with an EO block length of about 50 units (BASF), were used without further purification. Demineralized water was used throughout the work. Synthesis of P(S-co-MAA) in Solution. Different amounts of MAA and styrene (total amount of monomers = 1 g) were dissolved in chloroform (5 g) containing V70 (25 mg) (see Table S1). The free radical copolymerization was carried out at 45 °C for 20 h. After copolymerization, the reaction mixture was cooled down, and the polymer was collected after freeze-drying the mixture. Synthesis of P(S-co-AEMA) in Miniemulsion. Hexadecane (250 mg) and V59 (100 mg) were dissolved in styrene and mixed with a solution consisting of water (24 g), AEMA, and CTMA-Cl (125 mg) (see Table S2). After stirring for 1 h, the miniemulsion was prepared by ultrasonication for 120 s at 90% amplitude using a Branson sonifier W450 Digital, 1/2 in. tip under ice cooling. The polymerization was carried out at 72 °C for 20 h. After polymerization the particles were purified from CTMA-Cl and AEMA oligomers by repetitive centrifugation/redispersion in demineralized water for 100 min at 14 000 rpm and freeze-dried. In Situ Generation of the P(S-co-MAA) in the Miniemulsion Droplets for the Formation of Janus Particles. PLLA (300 mg) was dissolved in chloroform (10 g) and mixed with different amounts of MAA, styrene (total amount of monomers = 1 g), and V70 (see Table S1). Then, the obtained organic phase was added into the aqueous continuous phase consisting of water (24 g) and SDS (72 mg). After stirring for 1 h, the miniemulsion was prepared by ultrasonication for 180 s (30 s pulse, 10 s pause) at 70% amplitude using a Branson sonifier W450 Digital, 1/2 in. tip under ice cooling in order to prevent the evaporation of chloroform. The miniemulsion was transferred into a round-bottom flask with a wide neck, closed with a glass stopper, and heated at 45 °C for 20 h to copolymerize MAA and styrene. Afterward, the glass stopper was removed, and the chloroform was evaporated for 24 h at 40 °C. Formation of Janus Particles from Presynthesized PLLA and P(S-co-MAA). 300 mg of the freeze-dried P(S-co-MAA) and 300 mg PLLA were dissolved in 10 g of chloroform and mixed for 1 h with the aqueous solution consisting of water (24 g) and SDS (72 mg). The miniemulsion was prepared by ultrasonication for 180 s (30 s pulse, 10 s pause) at 70% amplitude using a Branson sonifier W450 Digital, 1/2 in. tip under ice cooling. The miniemulsion was transferred into a round-bottom flask with a wide neck and heated to 40 °C for 24 h to evaporate chloroform. In Situ Generation of the P(S-co-AEMA) in the Miniemulsion Droplets for the Formation of Janus Particles. PLLA (300 mg) and V70 (25 mg) were dissolved in chloroform (10 g) and mixed with different amounts of styrene (see Table S2). Then, the obtained organic phase was added into the aqueous continuous phase consisting of water (24 g), CTMA-Cl (125 mg), and different amounts of AEMA. After stirring for 1 h, the formation of miniemulsion, polymerization, and chloroform evaporation were performed as for the P(S-co-MAA) copolymers. Formation of Janus Particles from Presynthesized PLLA and P(S-co-AEMA). The freeze-dried P(S-co-AEMA) (300 mg) and PLLA (300 mg) were dissolved in chloroform (10 g) and mixed for 1 h with the aqueous solution consisting of water (24 g) and CTMA-Cl (125 mg). The miniemulsion was prepared by ultrasonication for 180 s (30 s pulse, 10 s pause) at 70% amplitude using a Branson sonifier W450 Digital, 1/2 in. tip under ice cooling. Afterward, the miniemulsion was transferred into a round-bottom flask with a wide neck and heated to 40 °C for 24 h to evaporate chloroform.
Formation of Silver Nanoparticles on the PLLA Face of PLLA/PS Janus Particles. PLLA/PS particles were first dialyzed against nonionic Lutensol AT50 solution. 4 g of particle dispersion was mixed with 12 g of Lutensol AT50 solution (0.25 wt %) and stirred for 5 h under ambient conditions. Afterward, particles were transferred in the dialyzing tube (MWCO 14 000 Da) and dialyzed overnight against demineralized water. For the formation of silver nanoparticles, 4 g of 0.1 wt % PS/PLLA particle dispersion was mixed with 7 mg of silver acetate and stirred gently for 24 h at 23 °C. After that, 30 mg of urotropine and 1 mg of poly(N-vinylpyrrolidone) were added to the dispersion and stirred for another 5 h at 80 °C. Characterization. After the formation of the particles by solvent evaporation, some water evaporated as well. The loss of water was compensated, and the dispersion was centrifuged at 2000 rpm for 20 min. Before characterization, the particles were purified by dialysis (MWCO 100 000 Da) to remove the excess of surfactant and the water-soluble oligomers until the conductivity reached a value below 9 μS cm−1. The size of the particles was characterized by dynamic light scattering using a Malvern Zeta Nanosizer with a measuring angle of 173°. The monomer conversion was determined gravimetrically. The molecular weight of the copolymer was measured by GPC in THF using polystyrene as standard. Copolymer composition was determined by potentiometric titration and/or 1H NMR spectroscopy. For the P(S-co-AEMA) copolymers, the integral of the amino group between 8.2 and 8.8 ppm was compared with the aromatic area of PS between 6.2 and 8.1 ppm. For the potentiometric titration the copolymer or the composite particles were dissolved in THF/water, and NaOH (0.1 M) was used for titration. 1H NMR of the composite particles was measured in DMSO-d6 with 300 MHz in heterophase. The morphology of the particles was studied using a Philips EM 400 or a Zeiss EM 902 transmission electron microscope, both working at 80 kV. The dispersions were diluted with water, dropped on a carboncoated 300 mesh copper grid, dried at ambient temperature, and coated with carbon afterward. The homogeneity of the composite particles was studied by preparative ultracentrifugation.52,53 Water and sucrose solutions with three different concentrations were layered to get a density gradient from 1.00 to 1.34 g cm−3 and centrifuged at 4 °C for 2 h at 41 000 rpm (288 000g) under vacuum using a Beckman L855 M ultracentrifuge with a SW 41 Ti rotor. The amount of functional groups per particle and per unit area was calculated from titration with polyelectrolytes. Carboxyl and amine groups were titrated using 0.001 M poly(diallyldimethylammonium chloride) (PDADMAC) or 0.001 M sodium polyethylene sulfonate (PES-Na), respectively. The particle dispersions were titrated at pH 2.5 and 9 (samples prepared with SDS) or at pH 2.5, 6, and 10 (samples prepared with CTMA-Cl).
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RESULTS AND DISCUSSION Formation of Janus Nanoparticles. Janus nanoparticles with one biodegaradable face (PLLA) and with one functional face (copolymer based on polystyrene) were prepared by the emulsion-solvent evaporation technique. The copolymer can be either prepared in situ in the miniemulsion (Figure S1a) or presynthesized before the emulsification process (Figure S1b). In the first approach, low molecular weight (
