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In-situ assembly of diblock copolymers onto submicron-sized particles for preparation of core-shell and ellipsoidal particles
Colloids and Surfaces A: Physicochem. Eng. Aspects 512 (2017) 80–86
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
In-situ assembly of diblock copolymers onto submicron-sized particles for preparation of core-shell and ellipsoidal particles Kosuke Hamada, Michinari Kohri ∗ , Tatsuo Taniguchi, Keiki Kishikawa Division of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• In situ assembly of amphiphilic diblock copolymers in the presence of silica particles was investigated. • Core-shell particles with tailored shell thickness were observed. • Ellipsoidal particles containing two core particles were prepared.
a r t i c l e
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Article history: Received 28 July 2016 Received in revised form 13 October 2016 Accepted 14 October 2016 Available online 17 October 2016 Keywords: Diblock copolymers Core-shell particles Ellipsoidal particles Self-assembly
a b s t r a c t We describe a facile and simple method to prepare colloidal architectures by the in situ assembly of amphiphilic diblock copolymers onto submicron-sized core particles. Polystyrene-b-poly(acrylic acid) diblock copolymers (PStm PAAn ) were directly assembled onto hydrophobic silica (SiO2 ) core particles, forming spherical core-shell particles. By varying the surface hydrophobicity of the core particles and the compositions of the diblock copolymers, core-shell particles with tailored shell thickness were observed. Furthermore, we demonstrated the preparation of ellipsoidal particles, which containing two core particles, based on this strategy. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Amphiphilic diblock copolymers self-assemble into nanostructures, e.g., micelles, cylinders, and vesicles, which have attracted
∗ Corresponding author. E-mail address: [email protected] (M. Kohri). http://dx.doi.org/10.1016/j.colsurfa.2016.10.035 0927-7757/© 2016 Elsevier B.V. All rights reserved.
considerable attention due to the possibility of numerous applications, including drug delivery systems, nano reactors, and diagnostic imaging [1–4]. Moreover, hybrid materials composed of diblock copolymers and particles are of interest in several fields, including biotechnology, pharmaceuticals, and synthetic chemistry [5–9]. A wide variety of nanoparticles has been incorporated into various polymer assemblies. For example, Taton et al. reported the preparation of gold nanoparticle-loaded diblock copolymer
K. Hamada et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 512 (2017) 80–86
micelles, which were shown to have polymer shell layers of approximately 15 nm in thickness on the hydrophobic gold nanoparticle cores of approximately 12 nm and 31 nm in diameter [5,6]. Park et al. demonstrated the control of the self-assembly structures of amphiphilic diblock copolymers in the presence of magnetic nanoparticles [7] or gold nanoparticles [8], which showed that the arrangement of nanoparticles and the polymer morphology could be controlled by changing the initial solvent composition, polymer chain lengths, and amount of nanoparticles. Eisenberg et al. also reported the preparation of gold nanoparticle aggregates with morphological control [9]. Nano-sized particles are incorporated into diblock copolymer assemblies such as micelles, cylinders, and vesicles. In these cases, it is required that the surface of nano-sized particles was usually pre-modified by diblock copolymers [5–9]. While most of the aforementioned research has focused on the use of nano-sized particles that were pre-coated by amphiphilic diblock copolymers, there are a few report that demonstrate block copolymer assembly using submicron-sized particle as a template [10,11]. The difference of submicron-sized particles and nano-sized particles for diblock copolymer assembly is the formation mechanism of assembled samples. For submicron-sized particles, diblock copolymer are assembled and aggregated onto the particles surface. Thus, assembled samples using submicron-sized particles and block polymer are easily obtained. Additionally, since block copolymers are simply assembled onto surface, variety of block copolymers, including functional polymers, will be applied to this method. It remains a challenge to develop an effective strategy for the direct preparation of colloidal architectures using submicronsized particles. Non-spherical particles, e.g., ellipsoidal, Janus, raspberry-like and dumbbell-like particles, have become attractive materials for basic research and industrial applications [12–18]. Although the seeded polymerization technique is effective for the preparation of non-spherical particles [19–22], it requires strict control of numerous parameters, such as the monomer composition, central seed particle size, polymerization time, reaction temperature, and density of the polymerizable groups on the seed surface, making it more difficult to optimize the reaction [23]. Therefore, it would be of great interest to develop a facile method for preparing non-spherical particles as well as spherical particles. Here, we report a facile and simple procedure for the preparation of colloidal architectures by the in situ assembly of amphiphilic diblock copolymers onto submicron-sized core particles (Fig. 1). By tuning the surface hydrophobicity of silica (SiO2 ) core particles and the compositions of polystyrene-b-poly(acrylic acid) diblock copolymers (PStm PAAn ), both spherical core-shell particles and non-spherical ellipsoidal particles were observed. A mechanism for the formation of the colloidal architectures is also proposed.
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washes with THF. St was dried over calcium hydride and distilled under reduced pressure. All other chemicals and solvents were of reagent grade and were used as received. 2.2. Measurements Scanning electron microscopy (SEM) micrographs were obtained using a JSM-6510A scanning electron microscope (JEOL). Transmission electron microscopy (TEM) micrographs were obtained using an H-7650 transmission electron microscope (Hitachi). FT-IR spectra were measured with an FTIR-420 spectrophotometer (JASCO). Thermogravimetric analysis (TGA) was performed in air at 10 ◦ C min−1 with a TG8120 thermogravimetry/differential thermal analyzer (Rigaku). The hydrodynamic diameter (Dh) of the particles in water was measured by dynamic light scattering using an ELSZ-1000ZS (Otsuka Electronics). UV–vis spectra were obtained using a U-3010 spectrophotometer (Hitachi). Static contact angle measurements were performed using a P200A (Meiwafosis). 2.3. Preparation of hydrophobic SiO2 core particles SiO2 particles (2.0 g) were dispersed in THF (50 mL) and stirred for 30 min under reflux. OTMS (3.6, 4.1, or 7.1 g, 9.6, 11, or 19 mmol) was then slowly added to the dispersion. After stirring for 24 h under reflux, the particles were separated and purified repeatedly by centrifugation (10,000 rpm, 10 min) before redispersion in DMF. 2.4. Synthesis of amphiphilic diblock copolymers (PStm PAAn ) Amphiphilic diblock copolymers of different compositions were prepared by sequential telluride-mediated polymerization (TERP) processes as described in a previous publication [24]. For example, for the synthesis of PSt41 PAA14 , St (4.0 g, 38 mmol), ACHN (94 mg, 0.38 mmol), and BTEE (176 L, 0.76 mmol) in PGMEA (4.1 mL) were added to a screw tube. The polymerization was conducted at 90 ◦ C for 14 h under nitrogen atmosphere, allowing monomer conversion to reach approximately 100%. Subsequently, tBA (second monomer; 2.0 g, 16 mmol) and AIBN (63 mg, 0.39 mmol) in PGMEA (7.2 mL) were added to the screw tube and were left to react for 20 h at 70 ◦ C. The polymers were purified by reprecipitation from THF into a large excess of methanol to form PSt41 PtBA14 . Excess TFA (5 eq. to tBA units) was added to the block copolymer obtained (2 g) in DCM (50 mL) and was stirred at room temperature under a nitrogen atmosphere. After 12 h, the solvents and regents were removed by evaporation, and the polymers were purified by reprecipitation from THF into a large excess of methanol, giving rise to PSt41 PAA14 diblock copolymers. 2.5. In situ assembly of diblock copolymers onto core particles
2. Experimental 2.1. Materials Styrene (St), tetrahydrofuran (THF), and methanol were obtained from Kanto Chemical. 2,2 -Azobisisobutylonitrile (AIBN), trifluoroacetic acid (TFA), and octadecyltrimethoxysilane (OTMS) were purchased from Tokyo Chemical Industry. 1,1 -Azobis(cyclohexane-1-carbonitrile) (ACHN), tert-butyl acrylate (tBA), dichloromethane (DCM), N, N-dimethylformamide (DMF), propylene glycol monomethyl ether acetate (PGMEA), and hydrofluoric acid (HF) were obtained from Wako Pure Chemical. Ethyl-2-butyltellanyl-2-methyl-propionate (BTEE) was graciously supplied by Otsuka Chemical and was used as received. SiO2 particles (MP-1040) were supplied by Nissan Chemical and were purified by stirring in concentrated nitric acid with subsequent
To PStm PAAn (2.8 mg, 0.1 wt%) and SiO2 core particles (5.7 mg, 0.2 wt%) in DMF (3 mL) was added deionized water (0.6 mL) in a dropwise manner (0.1 mL/min) using syringe pump (size of droplets: ca. 5 mm) at room temperature. The ratio of diblock copolymers to core particles was fixed to 1/2. The mixture was then stirred continuously for 1 h at 90 ◦ C to anneal polymers onto particles surface. (Stirring speed: 500-2 000 rpm. In these range, the difference was not observed because of high affinity of water and DMF.) Subsequently, the obtained particles were separated and purified repeatedly by centrifugation (13,500 rpm, 10 min) and were redispersed in water. Hollow particles were prepared as follows: SiO2 (1.5)@PSt41 PAA14 core-shell particles (10 mg) were stirred in excess hydrogen fluoride (HF) to remove the SiO2 core components. The sample was separated and purified repeatedly by
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Fig. 1. Schematic representation of spherical core-shell particles and non-spherical ellipsoidal particles by the in situ assembly of amphiphilic diblock copolymers onto submicron-sized core particles.
centrifugation (10 000 rpm for 5 min) and redispersion, and the solvent was exchanged with deionized water.
3. Results and discussion 3.1. Synthesis of desired core particles and amphiphilic diblock copolymers Three SiO2 core particles with a different hydrophobicity were prepared by silane coupling reagent (OTMS) treatment. The surface densities of the OTMS for the three SiO2 core particles, determined from the weight loss calculated using TGA, were approximately 1.5, 0.73, and 0.21 molecule/nm2 , respectively. The grafting density of silane coupling reagents depends on their molecular area [25]. For example, whereas the grafting density of ATRP initiator-bearing silane coupling reagents onto SiO2 particles is approximately 2–5 group/nm2 [26], the surface grafting density obtained with bulky RAFT CTA initiator was 0.15–0.54 group/nm2 [27]. This phenomenon is due to the rigidity of RAFT reagents. Because OTMS have long alkyl chains, it is reasonable that we obtain OTMS-bearing SiO2 particles with grafting densities of 0.21–1.5 molecule/nm2 . The wettability of particles obtained was measured by a static contact angle measurement [28,29]. The contact angles for water were 117◦ , 64◦ , and 20◦ on the glass plate surface, which were coated by SiO2 (1.5), SiO2 (0.73), and SiO2 (0.21) particles, respectively, indicating that the hydrophobicity of the SiO2 core particles could be controlled by adjusting the OTMS concentration (Fig. 2). For the synthesis of PSt homopolymers, telluride-mediated polymerization (TERP) was chosen, which is a living radical polymerization method ensuring relatively narrow molecular weight distributions [30–32]. With this polymerization system, PSt homopolymers were successfully prepared with the desired chain lengths (See Table S1). PStm PtBAn diblock copolymers were also prepared by sequential TERP method. GPC traces of the polymers obtained exhibited symmetric distributions (See Fig. S1). Table 1 summarizes the monomer conversions and molecular weights of the PStm PtBAn diblock copolymers. The Mn values of the PStm PtBAn diblock copolymers determined by GPC were similar to the theoretical values, indicating the successful preparation of diblock copolymers. After the formation of the PStm PtBAn diblock copolymers, the tert-butyl groups of the PtBA block were deprotected
Fig. 2. Plots of the contract angles for water on glass plates coated with SiO2 (1.5), SiO2 (0.73), and SiO2 (0.21) particles as a function of surface density of silane coupling reagents. The insets show a drop of water.
with TFA to produce PStm PAAn diblock copolymers [33,34]. The deprotection of the PtBA block was investigated by infrared (IR) measurement. The appearance of the peaks at 2580 cm−1 (O H stretching vibration), 1720 cm−1 (C O stretching vibration) and 1250 cm−1 (C O (ester) stretching vibration), which are presented in Fig. S2, confirmed the deprotection of the tert-butyl groups. The disappearance of the protons due to the tert-butyl group at 1.44 and 1.56 ppm in the 1 H NMR spectrum also indicated the complete deprotection of the PtBA blocks (see Fig. S3). These results indicated that the target diblock copolymers with controlled compositions were efficiently synthesized by sequential TERP using BTEE as a chain transfer agent. To investigate the self-assembly of PSt41 PAA14 , PSt86 PAA13 , and PSt156 PAA10 diblock copolymers, turbidity measurements were performed by adding water to DMF solutions of diblock copolymers. As shown in Fig. 3, the transmittances of the solutions were gradually decreased with increasing water amount from 7 to 10 wt% and reached a plateau at a water amount of 21 wt%, indicating water addition-induced self-assembly.
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Table 1 Molecular weights of the PStm PtBAn diblock copolymers. Polymera
Conversion [%]b
Mn , theory c
Mn , GPC d
Mw /Mn d
PSt41 PtBA14 PSt86 PtBA13 PSt156 PtBA10
97 92 64
8000 13000 22700
6400 10900 18100
1.39 1.45 1.34
Reaction time: 20 h. a PtBA length was determined by GPC measurements. b Determined by 1 H NMR spectra. c Calculated by ([tBA]/[PSt-Te]) × Mt BA × conversion + MPSt − Te . d Measured by GPC using PSt standards (Mp = 2630–96400).
3.2. In situ assembly of diblock copolymers onto core particles
Fig. 3. Transmittance of the solutions of PStm PAAn diblock copolymers in water: () PSt41 PAA14 , (䊏) PSt86 PAA13 , and (䊉) PSt156 PAA10 diblock copolymers. Transmittance was recorded at = 500 nm.
Fig. 4a shows a typical TEM image of the obtained particles. Because of the self-assembly behavior of PSt41 PAA14 diblock copolymers, in situ assembly of diblock copolymers onto SiO2 (1.5) core particles was induced by adding water into the mixture of the core particles and diblock copolymers in DMF, successfully producing SiO2 (1.5)@PSt41 PAA14 core-shell particles, having approximately 12 nm polymer-shell layers (Fig. 4a). By removing silica components from the obtained particles treated with HF, robust diblock copolymer capsules were formed, indicating the presence of diblock copolymer-shell layers (Fig. 4b). Fig. 4c shows the IR spectra of bare SiO2 (1.5) core particles, PSt41 PAA14 diblock copolymers, and SiO2 (1.5)@PSt41 PAA14 core-shell particles. The characteristic signals at 3020, 1500, 760, and 700 cm−1 correspond to the C H stretching mode, aromatic C C stretching mode, C H out-of-plane bending mode, and aromatic C C out-of-plane bending mode of the PSt moieties, respectively. The intensity of the typical SiO2 signals remained constant. Signals due to PAA moieties were not observed because the PAA layer
Fig. 4. TEM images of (a) SiO2 (1.5)@PSt41 PAA14 core-shell particles and (b) hollow particles. (c) FT-IR spectra of SiO2 (1.5) core particles, PSt41 PAA14 diblock copolymers and SiO2 (1.5)@PSt41 PAA14 particles. (d) Size distribution of bare SiO2 particles (red) and SiO2 (1.5)@PSt41 PAA14 core-shell particles (blue) measured by DLS in water. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. The effects of the hydrophobicity of the core particles on (a) the shell thickness and (b) the diblock copolymer amount.
was too thin. The core-shell particles, however, enhanced their dispersion stability in water, and the -potential was negative (–42 mV), indicating the presence of PAA moieties in the outermost layer of the core-shell particles. The volume-average diameter of the SiO2 (1.5)@PSt41 PAA14 core-shell particles in water, measured by DLS, was approximately 177 nm, which was larger than the 107 nm of bare SiO2 particles (Fig. 4d). The effect of the surface hydrophobicity of the core particles on the in situ assembly of the diblock copolymers was investigated. When SiO2 (0.21) and SiO2 (0.73) particles were used as the core particles, as well as SiO2 (1.5) particles, spherical core-shell particles were also obtained (see Fig. S4 upper row). As shown in Fig. 5a, shell thicknesses, measured by TEM photographs, were increased from 4 nm to 10 nm with the hydrophobicity of the core particles (SiO2 (0.21) and SiO2 (0.73)). Further increasing the hydrophobicity of the core particles (SiO2 (1.5)) did not affect the shell thickness. The same trend was observed for the polymer contents measured by TGA (Fig. 5b). The effect of the diblock copolymer compositions on the in situ assembly behavior was investigated. With PSt86 PAA13 diblock copolymer, the TEM images showed the presence of spherical coreshell particles, regardless of the core particles (see Fig. S4 lower row). As shown in Fig. 5a,b, the shell thickness and polymer content increased with the hydrophobicity of the core particles, and reached a plateau, which is in agreement with the results using PSt41 PAA14 diblock copolymers (vide supra).
Fig. 6. TEM images of (a) SiO2 (0.73)@PSt156 PAA10 and (b) SiO2 (1.5)@PSt156 PAA10 particles. Insets show the number of SiO2 core particles (n) in the PStm PAAn diblock copolymers.
The shell thickness of SiO2 @PSt156 PAA10 core-shell particles also increased with increasing core particle hydrophobicity (Fig. 5a). The contents of PSt156 PAA10 diblock copolymers were dramatically increased compared with the other polymers (Fig. 5b). When PSt156 PAA10 diblock copolymers were assembled onto SiO2 (0.21) particles, large aggregates were observed in addition to the core-shell particles in the TEM image (data not shown). The SEM image of SiO2 (0.21)@PSt156 PAA10 also showed the presence of approximately 281 nm of aggregates (See Fig. S5a). To investigate the formation of large aggregates, samples were prepared in the absence of SiO2 (0.21) core particles. The addition of water to DMF solutions of PSt156 PAA10 diblock copolymers gave particles with a diameter of approximately 345 nm (See Fig. S5b). Again, PSt156 PAA10 diblock copolymer aggregates were easily formed in the addition of water. Thus, the large aggregates were due to the self-assembly samples of PSt156 PAA10 diblock copolymers, and the
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Fig. 7. Proposed mechanism of the formation of irregular-shaped particles.
presence of these aggregates would cause increased polymer contents. The SEM images of SiO2 (0.73)@PSt156 PAA10 and SiO2 (1.5)@PSt156 PAA10 particles showed the presence of nonspherical particles as well as spherical particles (See Fig. S6). As shown in Fig. 6a,b, TEM observations clearly indicated that the non-spherical particles contained two or more SiO2 core particles. In each sample, 100 particles were measured to evaluate the number of SiO2 core particles (n) in the PSt156 PAA10 polymers. From the TEM image, the SiO2 (0.73)@PSt156 PAA10 particles were spherical and ellipsoidal particles containing one and two SiO2 (0.73) core particles, and the percentages of samples (n = 1,2) were 56% and 40% (Fig. 6a inset). Although more experiments were needed to produce ellipsoidal particles selectively, relatively uniform particles were obtained. On the other hand, when the SiO2 (1.5) particles, which have high-hydrophobicity, were used as the core particle, the number of SiO2 (1.5) particles in the samples was not quite uniform (Fig. 6b inset). These morphological changes may be attributed to the following possible causes. The first cause is the core particles aggregating due to the hydrophobicity of the core particles. Fig. S7 shows the aggregation behavior of the core SiO2 particles in the absence of diblock copolymers. After adding water to DMF dispersions of SiO2 core particles, the transparencies at 500 nm gradually decreased with increasing water contents. The transparency of SiO2 (1.5) particles rapidly decreased, strongly suggesting the formation of core particles aggregates. Thus, PSt156 PAA10 diblock copolymers and core particles would be competitively assembled, giving rise to the irregular-shaped particles (Fig. 7). The other is fusion of some particles [35]. As the system experiences the solvent shifting from DMF to water, hydrophobic PS156 -PAA10 polymers will show high affinity with the highly hydrophobized SiO2 (1.5) core particles, producing irregular-shaped particles that was fused by involving other particles. From previous reports, non-spherical particles were separated using centrifugation [36]. Thus, we carried out the separation of ellipsoidal particles and core-shell particles by centrifugation. However, it was hard to separate these two particles because of their similar properties. Particle classifier using microchannel device will be useful to separate two particles. More experiments are needed to establish particle separation process. It is early to draw conclusions with the results presented here, and more experiments are needed to clarify detailed mechanisms of irregular-shaped particles formation. In principle, by controlling both the hydrophobicity of the core particles and the compositions of the diblock copolymers for the in situ assembly method, we were able to prepare ellipsoidal particles as well as spherical core-shell particles. Detailed studies will be needed for the precise control of the amount of non-spherical particles, and these studies are currently underway in our laboratory.
4. Conclusions In conclusion, we demonstrated the facile and simple fabrication of colloidal architectures via the in situ assembly of PStm PAAn diblock copolymers onto SiO2 core particles. Spherical core-shell particles with the desired shell thicknesses were prepared by varying the surface hydrophobicity of the core particles and the compositions of the diblock copolymers. When PSt156 PAA10 was used, binary in situ assembly of diblock copolymers and core particles occurred, forming ellipsoidal particles which containing two core particles. The results presented here are a promising step in the development of functional polymer particles with tunable morphologies. Additional studies include the development of applications of this methodology using other functional diblock copolymers. The results of these investigations will be described in future reports. Acknowledgements This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (No. 23750119). We would like to thank Dr. Masaki Ishihara of Otsuka Chemical for the TERP method. We gratefully acknowledge Otsuka Chemical and Nissan Chemical Industries for providing reagents. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa.2016.10. 035. References [1] Y. Mai, A. Eisenberg, Self-assembly of block copolymers, Chem. Soc. Rev. 41 (2012) 5969–5985. [2] N.S. Cameron, M.K. Corbierre, A. Eisenberg, Asymmetric amphiphilic block copolymers in solution: a morphological wonderland, Can. J. Chem. 77 (1999) 1311–1326. [3] B. Nandan, A. Horechyy, Hairy core–shell polymer nano-objects from self-assembled block copolymer structures, ACS Appl. Mater. Interfaces 7 (2015) 12539–12558. [4] M.J. Wang, H. Wang, A.C. Chen, C. Chen, Y. Liu, Morphological control of anisotropic self-assemblies from alternating poly(p-dioxanone)-poly(ethylene glycol) multiblock copolymer depending on the combination effect of crystallization and micellization, Langmuir 31 (2015) 6971–6980. [5] Y. Kang, T.A. Taton, Core/shell gold nanoparticles by self-assembly and crosslinking of micellar, block-copolymer shells, Angew. Chem. Int. Ed. 44 (2005) 409–412. [6] B.S. Kim, J.M. Qiu, J.P. Wang, T.A. Taton, Magnetomicelles: composite nanostructures from magnetic nanoparticles and cross-linked amphiphilic block copolymers, Nano Lett. 5 (2005) 1987–1991. [7] R.J. Hickey, A.S. Haynes, J.M. Kikkawa, S.J. Park, Controlling the self-assembly structure of magnetic nanoparticles and amphiphilic block-copolymers: from micelles to vesicles, J. Am. Chem. Soc. 133 (2011) 1517–1525.
86
K. Hamada et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 512 (2017) 80–86
[8] Q. Luo, R.J. Hickey, S.J. Park, Controlling the location of nanoparticles in colloidal assemblies of amphiphilic polymers by tuning nanoparticle surface chemistry, ACS Macro Lett. 2 (2013) 107–111. [9] Y. Mai, L. Xiao, A. Eisenberg, Morphological control in aggregates of amphiphilic cylindrical metal–polymer brushes, Macromolecules 46 (2013) 3183–3189. [10] C.M. Grozea, M. Rabnawaz, G. Liu, G. Zhang, Coating of silica particles by fluorinated diblock copolymers and use of the resultant silica for superamphiphobic surfaces, Polymer 64 (2015) 153–162. [11] C. Schatz, Two non-covalent methods to decorate nanoparticles with block copolymers, Macromol. Chem. Phys. 215 (2014) 945–957. [12] S. Wang, F. Ma, H. Zhao, N. Wu, Bulk synthesis of metal–organic hybrid dimers and their propulsion under electric fields, ACS Appl. Mater. Interfaces 6 (2014) 4560–4569. [13] J. Liu, G. Liu, M. Zhang, P. Sun, H. Zhao, Synthesis and self-assembly of amphiphilic janus laponite disks, Macromolecules 46 (2013) 5974–5984. [14] C. Herrmann, M.B. Bannwarth, K. Landfester, D. Crespy, Re-dispersible anisotropic and structured nanoparticles: formation and their subsequent shape change, Macromol. Chem. Phys. 213 (2012) 829–838. [15] J. Yoon, A. Kota, S. Bhaskar, A. Tuteja, J. Lahann, Amphiphilic colloidal surfactants based on electrohydrodynamic co-jetting, ACS Appl. Mater. Interfaces 5 (2013) 11281–11287. [16] C. Kaewsaneha, P. Tangboriboonrat, D. Polpanich, M. Eissa, A. Elaissari, Janus colloidal particles: preparation, properties, and biomedical applications, ACS Appl. Mater. Interfaces 5 (2013) 1857–1869. [17] M. Kohri, Y. Nannichi, H. Kohma, D. Abe, T. Kojima, T. Taniguchi, K. Kishikawa, Size control of polydopamine nodules formed on polystyrene particles during dopamine polymerization with carboxylic acid-containing compounds for the fabrication of raspberry-like particles, Colloids Surf. A 449 (2014) 114–120. [18] D. Nagao, K. Goto, H. Ishii, M. Konno, Preparation of asymmetrically nanoparticle-supported, monodisperse composite dumbbells by protruding a smooth polymer bulge from rugged spheres, Langmuir 27 (2011) 13303–13307. [19] S. Xu, W.F. Ma, L.J. You, J. Li, J.M. Guo, J.J. Hu, C.C. Wang, Toward designer magnetite/polystyrene colloidal composite microspheres with controllable nanostructures and desirable surface functionalities, Langmuir 28 (2012) 3271–3278. [20] E.B. Mock, H.D. Bruyn, B.S. Hawkett, R.G. Gilbert, C.F. Zukoski, Synthesis of anisotropic nanoparticles by seeded emulsion polymerization, Langmuir 22 (2006) 4037–4043. [21] F. Ito, G. Ma, M. Nagai, S. Omi, Study of particle growth by seeded emulsion polymerization accompanied by electrostatic coagulation, Colloids Surf. A 201 (2002) 131–142. [22] M. Okubo, T. Fujibayashi, M. Yamada, H. Minami, Micron-sized monodisperse, snowman/confetti-shaped polymer particles by seeded dispersion polymerization, Colloid. Polym. Sci. 28 (2005) 1041–1045.
[23] S. Sacanna, D.J. Pine, Shape-anisotropic colloids: building blocks for complex assemblies, Curr. Opin. Colloid Interface Sci. 16 (2011) 96–105. [24] Y. Nakamura, Y. Kitada, Y. Kobayashi, B. Ray, S. Yamago, Quantitative analysis of the effect of azo initiators on the structure of ␣-polymer chain ends in degenerative chain-transfer-mediated living radical polymerization reactions, Macromolecules 44 (2011) 8388–8397. [25] J. Chen, M. Liu, C. Chen, H. Gong, C. Gao, Synthesis and characterization of silica nanoparticles with well-defined thermoresponsive PNIPAM via a combination of RAFT and click chemistry, ACS Appl, Mater. Interfaces 3 (2011) 3215–3223. [26] T.V. Werne, T.E. Patten, Atom transfer radical polymerization from nanoparticles: a tool for the preparation of well-defined hybrid nanostructures and for understanding the chemistry of controlled/living radical polymerizations from surfaces, J. Am. Chem. Soc. 123 (2001) 7497–7505. [27] C. Li, J. Han, C.Y. Ryu, B.C. Benicewicz, A versatile method to prepare RAFT agent anchored substrates and the preparation of PMMA grafted nanoparticles, Macromolecules 39 (2006) 3175–3183. [28] E. Öztürk, E. Turan, T. Caykara, Formation of poly(octadecyl acrylate) brushes on a silicon wafer surface, Polym. Int. 61 (2012) 586. [29] N. Durand, B. Boutevin, G. Silly, B. Ameduri, B. Boutevin, Grafting from polymerization of vinylidene fluoride (VDF) from silica to achieve original silica–PVDF core–shells, Macromolecules 44 (2011) 8487–8493. [30] S. Yamago, K. Iida, J. Yoshida, Organotellurium compounds as novel initiators for controlled/living radical polymerizations. Synthesis of functionalized polystyrenes and end-group modifications, J. Am. Chem. Soc. 124 (2002) 2874–2875. [31] S. Yamago, Precision polymer synthesis by degenerative transfer controlled/living radical polymerization using organotellurium organostibine, and organobismuthine chain-transfer agents, Chem. Rev. 109 (2009) 5051–5068. [32] S. Yamago, Y. Nakamura, Recent progress in the use of photoirradiation in living radical polymerization, Polymer 54 (2013) 981–994. [33] R. Francis, B. Lepoittevin, D. Taton, Y. Gnanou, Toward an easy access to asymmetric stars and miktoarm stars by atom transfer radical polymerization, Macromolecules 35 (2002) 9001–9008. [34] H.S. Sundaram, D. Raghavachari, Controlled radical polymerization of tert-butyl acrylate at ambient temperature: effect of initiator structure and synthesis of amphiphilic block copolymers, J. Polym. Sci. A: Polym. Chem. 50 (2012) 996–1007. [35] D.J. Kraft, J. Hilhorst, M.A.P. Heinen, M.J. Hoogenraad, B. Luigjes, W.K. Kegel, Patchy polymer colloids with tunable anisotropy dimensions, J. Phys. Chem. B 115 (2011) 7175–7181. [36] P.M. Johnson, C.M. van Kats, A. van Blaaderen, Synthesis of colloidal silica dumbbells, Langmuir 21 (2005) 11510–11517.
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In-situ assembly of diblock copolymers onto submicron-sized particles for preparation of core-shell and ellipsoidal particles Kosuke Hamada, Michinari Kohri ∗ , Tatsuo Taniguchi, Keiki Kishikawa Division of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• In situ assembly of amphiphilic diblock copolymers in the presence of silica particles was investigated. • Core-shell particles with tailored shell thickness were observed. • Ellipsoidal particles containing two core particles were prepared.
a r t i c l e
i n f o
Article history: Received 28 July 2016 Received in revised form 13 October 2016 Accepted 14 October 2016 Available online 17 October 2016 Keywords: Diblock copolymers Core-shell particles Ellipsoidal particles Self-assembly
a b s t r a c t We describe a facile and simple method to prepare colloidal architectures by the in situ assembly of amphiphilic diblock copolymers onto submicron-sized core particles. Polystyrene-b-poly(acrylic acid) diblock copolymers (PStm PAAn ) were directly assembled onto hydrophobic silica (SiO2 ) core particles, forming spherical core-shell particles. By varying the surface hydrophobicity of the core particles and the compositions of the diblock copolymers, core-shell particles with tailored shell thickness were observed. Furthermore, we demonstrated the preparation of ellipsoidal particles, which containing two core particles, based on this strategy. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Amphiphilic diblock copolymers self-assemble into nanostructures, e.g., micelles, cylinders, and vesicles, which have attracted
∗ Corresponding author. E-mail address: [email protected] (M. Kohri). http://dx.doi.org/10.1016/j.colsurfa.2016.10.035 0927-7757/© 2016 Elsevier B.V. All rights reserved.
considerable attention due to the possibility of numerous applications, including drug delivery systems, nano reactors, and diagnostic imaging [1–4]. Moreover, hybrid materials composed of diblock copolymers and particles are of interest in several fields, including biotechnology, pharmaceuticals, and synthetic chemistry [5–9]. A wide variety of nanoparticles has been incorporated into various polymer assemblies. For example, Taton et al. reported the preparation of gold nanoparticle-loaded diblock copolymer
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micelles, which were shown to have polymer shell layers of approximately 15 nm in thickness on the hydrophobic gold nanoparticle cores of approximately 12 nm and 31 nm in diameter [5,6]. Park et al. demonstrated the control of the self-assembly structures of amphiphilic diblock copolymers in the presence of magnetic nanoparticles [7] or gold nanoparticles [8], which showed that the arrangement of nanoparticles and the polymer morphology could be controlled by changing the initial solvent composition, polymer chain lengths, and amount of nanoparticles. Eisenberg et al. also reported the preparation of gold nanoparticle aggregates with morphological control [9]. Nano-sized particles are incorporated into diblock copolymer assemblies such as micelles, cylinders, and vesicles. In these cases, it is required that the surface of nano-sized particles was usually pre-modified by diblock copolymers [5–9]. While most of the aforementioned research has focused on the use of nano-sized particles that were pre-coated by amphiphilic diblock copolymers, there are a few report that demonstrate block copolymer assembly using submicron-sized particle as a template [10,11]. The difference of submicron-sized particles and nano-sized particles for diblock copolymer assembly is the formation mechanism of assembled samples. For submicron-sized particles, diblock copolymer are assembled and aggregated onto the particles surface. Thus, assembled samples using submicron-sized particles and block polymer are easily obtained. Additionally, since block copolymers are simply assembled onto surface, variety of block copolymers, including functional polymers, will be applied to this method. It remains a challenge to develop an effective strategy for the direct preparation of colloidal architectures using submicronsized particles. Non-spherical particles, e.g., ellipsoidal, Janus, raspberry-like and dumbbell-like particles, have become attractive materials for basic research and industrial applications [12–18]. Although the seeded polymerization technique is effective for the preparation of non-spherical particles [19–22], it requires strict control of numerous parameters, such as the monomer composition, central seed particle size, polymerization time, reaction temperature, and density of the polymerizable groups on the seed surface, making it more difficult to optimize the reaction [23]. Therefore, it would be of great interest to develop a facile method for preparing non-spherical particles as well as spherical particles. Here, we report a facile and simple procedure for the preparation of colloidal architectures by the in situ assembly of amphiphilic diblock copolymers onto submicron-sized core particles (Fig. 1). By tuning the surface hydrophobicity of silica (SiO2 ) core particles and the compositions of polystyrene-b-poly(acrylic acid) diblock copolymers (PStm PAAn ), both spherical core-shell particles and non-spherical ellipsoidal particles were observed. A mechanism for the formation of the colloidal architectures is also proposed.
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washes with THF. St was dried over calcium hydride and distilled under reduced pressure. All other chemicals and solvents were of reagent grade and were used as received. 2.2. Measurements Scanning electron microscopy (SEM) micrographs were obtained using a JSM-6510A scanning electron microscope (JEOL). Transmission electron microscopy (TEM) micrographs were obtained using an H-7650 transmission electron microscope (Hitachi). FT-IR spectra were measured with an FTIR-420 spectrophotometer (JASCO). Thermogravimetric analysis (TGA) was performed in air at 10 ◦ C min−1 with a TG8120 thermogravimetry/differential thermal analyzer (Rigaku). The hydrodynamic diameter (Dh) of the particles in water was measured by dynamic light scattering using an ELSZ-1000ZS (Otsuka Electronics). UV–vis spectra were obtained using a U-3010 spectrophotometer (Hitachi). Static contact angle measurements were performed using a P200A (Meiwafosis). 2.3. Preparation of hydrophobic SiO2 core particles SiO2 particles (2.0 g) were dispersed in THF (50 mL) and stirred for 30 min under reflux. OTMS (3.6, 4.1, or 7.1 g, 9.6, 11, or 19 mmol) was then slowly added to the dispersion. After stirring for 24 h under reflux, the particles were separated and purified repeatedly by centrifugation (10,000 rpm, 10 min) before redispersion in DMF. 2.4. Synthesis of amphiphilic diblock copolymers (PStm PAAn ) Amphiphilic diblock copolymers of different compositions were prepared by sequential telluride-mediated polymerization (TERP) processes as described in a previous publication [24]. For example, for the synthesis of PSt41 PAA14 , St (4.0 g, 38 mmol), ACHN (94 mg, 0.38 mmol), and BTEE (176 L, 0.76 mmol) in PGMEA (4.1 mL) were added to a screw tube. The polymerization was conducted at 90 ◦ C for 14 h under nitrogen atmosphere, allowing monomer conversion to reach approximately 100%. Subsequently, tBA (second monomer; 2.0 g, 16 mmol) and AIBN (63 mg, 0.39 mmol) in PGMEA (7.2 mL) were added to the screw tube and were left to react for 20 h at 70 ◦ C. The polymers were purified by reprecipitation from THF into a large excess of methanol to form PSt41 PtBA14 . Excess TFA (5 eq. to tBA units) was added to the block copolymer obtained (2 g) in DCM (50 mL) and was stirred at room temperature under a nitrogen atmosphere. After 12 h, the solvents and regents were removed by evaporation, and the polymers were purified by reprecipitation from THF into a large excess of methanol, giving rise to PSt41 PAA14 diblock copolymers. 2.5. In situ assembly of diblock copolymers onto core particles
2. Experimental 2.1. Materials Styrene (St), tetrahydrofuran (THF), and methanol were obtained from Kanto Chemical. 2,2 -Azobisisobutylonitrile (AIBN), trifluoroacetic acid (TFA), and octadecyltrimethoxysilane (OTMS) were purchased from Tokyo Chemical Industry. 1,1 -Azobis(cyclohexane-1-carbonitrile) (ACHN), tert-butyl acrylate (tBA), dichloromethane (DCM), N, N-dimethylformamide (DMF), propylene glycol monomethyl ether acetate (PGMEA), and hydrofluoric acid (HF) were obtained from Wako Pure Chemical. Ethyl-2-butyltellanyl-2-methyl-propionate (BTEE) was graciously supplied by Otsuka Chemical and was used as received. SiO2 particles (MP-1040) were supplied by Nissan Chemical and were purified by stirring in concentrated nitric acid with subsequent
To PStm PAAn (2.8 mg, 0.1 wt%) and SiO2 core particles (5.7 mg, 0.2 wt%) in DMF (3 mL) was added deionized water (0.6 mL) in a dropwise manner (0.1 mL/min) using syringe pump (size of droplets: ca. 5 mm) at room temperature. The ratio of diblock copolymers to core particles was fixed to 1/2. The mixture was then stirred continuously for 1 h at 90 ◦ C to anneal polymers onto particles surface. (Stirring speed: 500-2 000 rpm. In these range, the difference was not observed because of high affinity of water and DMF.) Subsequently, the obtained particles were separated and purified repeatedly by centrifugation (13,500 rpm, 10 min) and were redispersed in water. Hollow particles were prepared as follows: SiO2 (1.5)@PSt41 PAA14 core-shell particles (10 mg) were stirred in excess hydrogen fluoride (HF) to remove the SiO2 core components. The sample was separated and purified repeatedly by
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Fig. 1. Schematic representation of spherical core-shell particles and non-spherical ellipsoidal particles by the in situ assembly of amphiphilic diblock copolymers onto submicron-sized core particles.
centrifugation (10 000 rpm for 5 min) and redispersion, and the solvent was exchanged with deionized water.
3. Results and discussion 3.1. Synthesis of desired core particles and amphiphilic diblock copolymers Three SiO2 core particles with a different hydrophobicity were prepared by silane coupling reagent (OTMS) treatment. The surface densities of the OTMS for the three SiO2 core particles, determined from the weight loss calculated using TGA, were approximately 1.5, 0.73, and 0.21 molecule/nm2 , respectively. The grafting density of silane coupling reagents depends on their molecular area [25]. For example, whereas the grafting density of ATRP initiator-bearing silane coupling reagents onto SiO2 particles is approximately 2–5 group/nm2 [26], the surface grafting density obtained with bulky RAFT CTA initiator was 0.15–0.54 group/nm2 [27]. This phenomenon is due to the rigidity of RAFT reagents. Because OTMS have long alkyl chains, it is reasonable that we obtain OTMS-bearing SiO2 particles with grafting densities of 0.21–1.5 molecule/nm2 . The wettability of particles obtained was measured by a static contact angle measurement [28,29]. The contact angles for water were 117◦ , 64◦ , and 20◦ on the glass plate surface, which were coated by SiO2 (1.5), SiO2 (0.73), and SiO2 (0.21) particles, respectively, indicating that the hydrophobicity of the SiO2 core particles could be controlled by adjusting the OTMS concentration (Fig. 2). For the synthesis of PSt homopolymers, telluride-mediated polymerization (TERP) was chosen, which is a living radical polymerization method ensuring relatively narrow molecular weight distributions [30–32]. With this polymerization system, PSt homopolymers were successfully prepared with the desired chain lengths (See Table S1). PStm PtBAn diblock copolymers were also prepared by sequential TERP method. GPC traces of the polymers obtained exhibited symmetric distributions (See Fig. S1). Table 1 summarizes the monomer conversions and molecular weights of the PStm PtBAn diblock copolymers. The Mn values of the PStm PtBAn diblock copolymers determined by GPC were similar to the theoretical values, indicating the successful preparation of diblock copolymers. After the formation of the PStm PtBAn diblock copolymers, the tert-butyl groups of the PtBA block were deprotected
Fig. 2. Plots of the contract angles for water on glass plates coated with SiO2 (1.5), SiO2 (0.73), and SiO2 (0.21) particles as a function of surface density of silane coupling reagents. The insets show a drop of water.
with TFA to produce PStm PAAn diblock copolymers [33,34]. The deprotection of the PtBA block was investigated by infrared (IR) measurement. The appearance of the peaks at 2580 cm−1 (O H stretching vibration), 1720 cm−1 (C O stretching vibration) and 1250 cm−1 (C O (ester) stretching vibration), which are presented in Fig. S2, confirmed the deprotection of the tert-butyl groups. The disappearance of the protons due to the tert-butyl group at 1.44 and 1.56 ppm in the 1 H NMR spectrum also indicated the complete deprotection of the PtBA blocks (see Fig. S3). These results indicated that the target diblock copolymers with controlled compositions were efficiently synthesized by sequential TERP using BTEE as a chain transfer agent. To investigate the self-assembly of PSt41 PAA14 , PSt86 PAA13 , and PSt156 PAA10 diblock copolymers, turbidity measurements were performed by adding water to DMF solutions of diblock copolymers. As shown in Fig. 3, the transmittances of the solutions were gradually decreased with increasing water amount from 7 to 10 wt% and reached a plateau at a water amount of 21 wt%, indicating water addition-induced self-assembly.
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Table 1 Molecular weights of the PStm PtBAn diblock copolymers. Polymera
Conversion [%]b
Mn , theory c
Mn , GPC d
Mw /Mn d
PSt41 PtBA14 PSt86 PtBA13 PSt156 PtBA10
97 92 64
8000 13000 22700
6400 10900 18100
1.39 1.45 1.34
Reaction time: 20 h. a PtBA length was determined by GPC measurements. b Determined by 1 H NMR spectra. c Calculated by ([tBA]/[PSt-Te]) × Mt BA × conversion + MPSt − Te . d Measured by GPC using PSt standards (Mp = 2630–96400).
3.2. In situ assembly of diblock copolymers onto core particles
Fig. 3. Transmittance of the solutions of PStm PAAn diblock copolymers in water: () PSt41 PAA14 , (䊏) PSt86 PAA13 , and (䊉) PSt156 PAA10 diblock copolymers. Transmittance was recorded at = 500 nm.
Fig. 4a shows a typical TEM image of the obtained particles. Because of the self-assembly behavior of PSt41 PAA14 diblock copolymers, in situ assembly of diblock copolymers onto SiO2 (1.5) core particles was induced by adding water into the mixture of the core particles and diblock copolymers in DMF, successfully producing SiO2 (1.5)@PSt41 PAA14 core-shell particles, having approximately 12 nm polymer-shell layers (Fig. 4a). By removing silica components from the obtained particles treated with HF, robust diblock copolymer capsules were formed, indicating the presence of diblock copolymer-shell layers (Fig. 4b). Fig. 4c shows the IR spectra of bare SiO2 (1.5) core particles, PSt41 PAA14 diblock copolymers, and SiO2 (1.5)@PSt41 PAA14 core-shell particles. The characteristic signals at 3020, 1500, 760, and 700 cm−1 correspond to the C H stretching mode, aromatic C C stretching mode, C H out-of-plane bending mode, and aromatic C C out-of-plane bending mode of the PSt moieties, respectively. The intensity of the typical SiO2 signals remained constant. Signals due to PAA moieties were not observed because the PAA layer
Fig. 4. TEM images of (a) SiO2 (1.5)@PSt41 PAA14 core-shell particles and (b) hollow particles. (c) FT-IR spectra of SiO2 (1.5) core particles, PSt41 PAA14 diblock copolymers and SiO2 (1.5)@PSt41 PAA14 particles. (d) Size distribution of bare SiO2 particles (red) and SiO2 (1.5)@PSt41 PAA14 core-shell particles (blue) measured by DLS in water. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. The effects of the hydrophobicity of the core particles on (a) the shell thickness and (b) the diblock copolymer amount.
was too thin. The core-shell particles, however, enhanced their dispersion stability in water, and the -potential was negative (–42 mV), indicating the presence of PAA moieties in the outermost layer of the core-shell particles. The volume-average diameter of the SiO2 (1.5)@PSt41 PAA14 core-shell particles in water, measured by DLS, was approximately 177 nm, which was larger than the 107 nm of bare SiO2 particles (Fig. 4d). The effect of the surface hydrophobicity of the core particles on the in situ assembly of the diblock copolymers was investigated. When SiO2 (0.21) and SiO2 (0.73) particles were used as the core particles, as well as SiO2 (1.5) particles, spherical core-shell particles were also obtained (see Fig. S4 upper row). As shown in Fig. 5a, shell thicknesses, measured by TEM photographs, were increased from 4 nm to 10 nm with the hydrophobicity of the core particles (SiO2 (0.21) and SiO2 (0.73)). Further increasing the hydrophobicity of the core particles (SiO2 (1.5)) did not affect the shell thickness. The same trend was observed for the polymer contents measured by TGA (Fig. 5b). The effect of the diblock copolymer compositions on the in situ assembly behavior was investigated. With PSt86 PAA13 diblock copolymer, the TEM images showed the presence of spherical coreshell particles, regardless of the core particles (see Fig. S4 lower row). As shown in Fig. 5a,b, the shell thickness and polymer content increased with the hydrophobicity of the core particles, and reached a plateau, which is in agreement with the results using PSt41 PAA14 diblock copolymers (vide supra).
Fig. 6. TEM images of (a) SiO2 (0.73)@PSt156 PAA10 and (b) SiO2 (1.5)@PSt156 PAA10 particles. Insets show the number of SiO2 core particles (n) in the PStm PAAn diblock copolymers.
The shell thickness of SiO2 @PSt156 PAA10 core-shell particles also increased with increasing core particle hydrophobicity (Fig. 5a). The contents of PSt156 PAA10 diblock copolymers were dramatically increased compared with the other polymers (Fig. 5b). When PSt156 PAA10 diblock copolymers were assembled onto SiO2 (0.21) particles, large aggregates were observed in addition to the core-shell particles in the TEM image (data not shown). The SEM image of SiO2 (0.21)@PSt156 PAA10 also showed the presence of approximately 281 nm of aggregates (See Fig. S5a). To investigate the formation of large aggregates, samples were prepared in the absence of SiO2 (0.21) core particles. The addition of water to DMF solutions of PSt156 PAA10 diblock copolymers gave particles with a diameter of approximately 345 nm (See Fig. S5b). Again, PSt156 PAA10 diblock copolymer aggregates were easily formed in the addition of water. Thus, the large aggregates were due to the self-assembly samples of PSt156 PAA10 diblock copolymers, and the
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Fig. 7. Proposed mechanism of the formation of irregular-shaped particles.
presence of these aggregates would cause increased polymer contents. The SEM images of SiO2 (0.73)@PSt156 PAA10 and SiO2 (1.5)@PSt156 PAA10 particles showed the presence of nonspherical particles as well as spherical particles (See Fig. S6). As shown in Fig. 6a,b, TEM observations clearly indicated that the non-spherical particles contained two or more SiO2 core particles. In each sample, 100 particles were measured to evaluate the number of SiO2 core particles (n) in the PSt156 PAA10 polymers. From the TEM image, the SiO2 (0.73)@PSt156 PAA10 particles were spherical and ellipsoidal particles containing one and two SiO2 (0.73) core particles, and the percentages of samples (n = 1,2) were 56% and 40% (Fig. 6a inset). Although more experiments were needed to produce ellipsoidal particles selectively, relatively uniform particles were obtained. On the other hand, when the SiO2 (1.5) particles, which have high-hydrophobicity, were used as the core particle, the number of SiO2 (1.5) particles in the samples was not quite uniform (Fig. 6b inset). These morphological changes may be attributed to the following possible causes. The first cause is the core particles aggregating due to the hydrophobicity of the core particles. Fig. S7 shows the aggregation behavior of the core SiO2 particles in the absence of diblock copolymers. After adding water to DMF dispersions of SiO2 core particles, the transparencies at 500 nm gradually decreased with increasing water contents. The transparency of SiO2 (1.5) particles rapidly decreased, strongly suggesting the formation of core particles aggregates. Thus, PSt156 PAA10 diblock copolymers and core particles would be competitively assembled, giving rise to the irregular-shaped particles (Fig. 7). The other is fusion of some particles [35]. As the system experiences the solvent shifting from DMF to water, hydrophobic PS156 -PAA10 polymers will show high affinity with the highly hydrophobized SiO2 (1.5) core particles, producing irregular-shaped particles that was fused by involving other particles. From previous reports, non-spherical particles were separated using centrifugation [36]. Thus, we carried out the separation of ellipsoidal particles and core-shell particles by centrifugation. However, it was hard to separate these two particles because of their similar properties. Particle classifier using microchannel device will be useful to separate two particles. More experiments are needed to establish particle separation process. It is early to draw conclusions with the results presented here, and more experiments are needed to clarify detailed mechanisms of irregular-shaped particles formation. In principle, by controlling both the hydrophobicity of the core particles and the compositions of the diblock copolymers for the in situ assembly method, we were able to prepare ellipsoidal particles as well as spherical core-shell particles. Detailed studies will be needed for the precise control of the amount of non-spherical particles, and these studies are currently underway in our laboratory.
4. Conclusions In conclusion, we demonstrated the facile and simple fabrication of colloidal architectures via the in situ assembly of PStm PAAn diblock copolymers onto SiO2 core particles. Spherical core-shell particles with the desired shell thicknesses were prepared by varying the surface hydrophobicity of the core particles and the compositions of the diblock copolymers. When PSt156 PAA10 was used, binary in situ assembly of diblock copolymers and core particles occurred, forming ellipsoidal particles which containing two core particles. The results presented here are a promising step in the development of functional polymer particles with tunable morphologies. Additional studies include the development of applications of this methodology using other functional diblock copolymers. The results of these investigations will be described in future reports. Acknowledgements This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (No. 23750119). We would like to thank Dr. Masaki Ishihara of Otsuka Chemical for the TERP method. We gratefully acknowledge Otsuka Chemical and Nissan Chemical Industries for providing reagents. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa.2016.10. 035. References [1] Y. Mai, A. Eisenberg, Self-assembly of block copolymers, Chem. Soc. Rev. 41 (2012) 5969–5985. [2] N.S. Cameron, M.K. Corbierre, A. Eisenberg, Asymmetric amphiphilic block copolymers in solution: a morphological wonderland, Can. J. Chem. 77 (1999) 1311–1326. [3] B. Nandan, A. Horechyy, Hairy core–shell polymer nano-objects from self-assembled block copolymer structures, ACS Appl. Mater. Interfaces 7 (2015) 12539–12558. [4] M.J. Wang, H. Wang, A.C. Chen, C. Chen, Y. Liu, Morphological control of anisotropic self-assemblies from alternating poly(p-dioxanone)-poly(ethylene glycol) multiblock copolymer depending on the combination effect of crystallization and micellization, Langmuir 31 (2015) 6971–6980. [5] Y. Kang, T.A. Taton, Core/shell gold nanoparticles by self-assembly and crosslinking of micellar, block-copolymer shells, Angew. Chem. Int. Ed. 44 (2005) 409–412. [6] B.S. Kim, J.M. Qiu, J.P. Wang, T.A. Taton, Magnetomicelles: composite nanostructures from magnetic nanoparticles and cross-linked amphiphilic block copolymers, Nano Lett. 5 (2005) 1987–1991. [7] R.J. Hickey, A.S. Haynes, J.M. Kikkawa, S.J. Park, Controlling the self-assembly structure of magnetic nanoparticles and amphiphilic block-copolymers: from micelles to vesicles, J. Am. Chem. Soc. 133 (2011) 1517–1525.
86
K. Hamada et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 512 (2017) 80–86
[8] Q. Luo, R.J. Hickey, S.J. Park, Controlling the location of nanoparticles in colloidal assemblies of amphiphilic polymers by tuning nanoparticle surface chemistry, ACS Macro Lett. 2 (2013) 107–111. [9] Y. Mai, L. Xiao, A. Eisenberg, Morphological control in aggregates of amphiphilic cylindrical metal–polymer brushes, Macromolecules 46 (2013) 3183–3189. [10] C.M. Grozea, M. Rabnawaz, G. Liu, G. Zhang, Coating of silica particles by fluorinated diblock copolymers and use of the resultant silica for superamphiphobic surfaces, Polymer 64 (2015) 153–162. [11] C. Schatz, Two non-covalent methods to decorate nanoparticles with block copolymers, Macromol. Chem. Phys. 215 (2014) 945–957. [12] S. Wang, F. Ma, H. Zhao, N. Wu, Bulk synthesis of metal–organic hybrid dimers and their propulsion under electric fields, ACS Appl. Mater. Interfaces 6 (2014) 4560–4569. [13] J. Liu, G. Liu, M. Zhang, P. Sun, H. Zhao, Synthesis and self-assembly of amphiphilic janus laponite disks, Macromolecules 46 (2013) 5974–5984. [14] C. Herrmann, M.B. Bannwarth, K. Landfester, D. Crespy, Re-dispersible anisotropic and structured nanoparticles: formation and their subsequent shape change, Macromol. Chem. Phys. 213 (2012) 829–838. [15] J. Yoon, A. Kota, S. Bhaskar, A. Tuteja, J. Lahann, Amphiphilic colloidal surfactants based on electrohydrodynamic co-jetting, ACS Appl. Mater. Interfaces 5 (2013) 11281–11287. [16] C. Kaewsaneha, P. Tangboriboonrat, D. Polpanich, M. Eissa, A. Elaissari, Janus colloidal particles: preparation, properties, and biomedical applications, ACS Appl. Mater. Interfaces 5 (2013) 1857–1869. [17] M. Kohri, Y. Nannichi, H. Kohma, D. Abe, T. Kojima, T. Taniguchi, K. Kishikawa, Size control of polydopamine nodules formed on polystyrene particles during dopamine polymerization with carboxylic acid-containing compounds for the fabrication of raspberry-like particles, Colloids Surf. A 449 (2014) 114–120. [18] D. Nagao, K. Goto, H. Ishii, M. Konno, Preparation of asymmetrically nanoparticle-supported, monodisperse composite dumbbells by protruding a smooth polymer bulge from rugged spheres, Langmuir 27 (2011) 13303–13307. [19] S. Xu, W.F. Ma, L.J. You, J. Li, J.M. Guo, J.J. Hu, C.C. Wang, Toward designer magnetite/polystyrene colloidal composite microspheres with controllable nanostructures and desirable surface functionalities, Langmuir 28 (2012) 3271–3278. [20] E.B. Mock, H.D. Bruyn, B.S. Hawkett, R.G. Gilbert, C.F. Zukoski, Synthesis of anisotropic nanoparticles by seeded emulsion polymerization, Langmuir 22 (2006) 4037–4043. [21] F. Ito, G. Ma, M. Nagai, S. Omi, Study of particle growth by seeded emulsion polymerization accompanied by electrostatic coagulation, Colloids Surf. A 201 (2002) 131–142. [22] M. Okubo, T. Fujibayashi, M. Yamada, H. Minami, Micron-sized monodisperse, snowman/confetti-shaped polymer particles by seeded dispersion polymerization, Colloid. Polym. Sci. 28 (2005) 1041–1045.
[23] S. Sacanna, D.J. Pine, Shape-anisotropic colloids: building blocks for complex assemblies, Curr. Opin. Colloid Interface Sci. 16 (2011) 96–105. [24] Y. Nakamura, Y. Kitada, Y. Kobayashi, B. Ray, S. Yamago, Quantitative analysis of the effect of azo initiators on the structure of ␣-polymer chain ends in degenerative chain-transfer-mediated living radical polymerization reactions, Macromolecules 44 (2011) 8388–8397. [25] J. Chen, M. Liu, C. Chen, H. Gong, C. Gao, Synthesis and characterization of silica nanoparticles with well-defined thermoresponsive PNIPAM via a combination of RAFT and click chemistry, ACS Appl, Mater. Interfaces 3 (2011) 3215–3223. [26] T.V. Werne, T.E. Patten, Atom transfer radical polymerization from nanoparticles: a tool for the preparation of well-defined hybrid nanostructures and for understanding the chemistry of controlled/living radical polymerizations from surfaces, J. Am. Chem. Soc. 123 (2001) 7497–7505. [27] C. Li, J. Han, C.Y. Ryu, B.C. Benicewicz, A versatile method to prepare RAFT agent anchored substrates and the preparation of PMMA grafted nanoparticles, Macromolecules 39 (2006) 3175–3183. [28] E. Öztürk, E. Turan, T. Caykara, Formation of poly(octadecyl acrylate) brushes on a silicon wafer surface, Polym. Int. 61 (2012) 586. [29] N. Durand, B. Boutevin, G. Silly, B. Ameduri, B. Boutevin, Grafting from polymerization of vinylidene fluoride (VDF) from silica to achieve original silica–PVDF core–shells, Macromolecules 44 (2011) 8487–8493. [30] S. Yamago, K. Iida, J. Yoshida, Organotellurium compounds as novel initiators for controlled/living radical polymerizations. Synthesis of functionalized polystyrenes and end-group modifications, J. Am. Chem. Soc. 124 (2002) 2874–2875. [31] S. Yamago, Precision polymer synthesis by degenerative transfer controlled/living radical polymerization using organotellurium organostibine, and organobismuthine chain-transfer agents, Chem. Rev. 109 (2009) 5051–5068. [32] S. Yamago, Y. Nakamura, Recent progress in the use of photoirradiation in living radical polymerization, Polymer 54 (2013) 981–994. [33] R. Francis, B. Lepoittevin, D. Taton, Y. Gnanou, Toward an easy access to asymmetric stars and miktoarm stars by atom transfer radical polymerization, Macromolecules 35 (2002) 9001–9008. [34] H.S. Sundaram, D. Raghavachari, Controlled radical polymerization of tert-butyl acrylate at ambient temperature: effect of initiator structure and synthesis of amphiphilic block copolymers, J. Polym. Sci. A: Polym. Chem. 50 (2012) 996–1007. [35] D.J. Kraft, J. Hilhorst, M.A.P. Heinen, M.J. Hoogenraad, B. Luigjes, W.K. Kegel, Patchy polymer colloids with tunable anisotropy dimensions, J. Phys. Chem. B 115 (2011) 7175–7181. [36] P.M. Johnson, C.M. van Kats, A. van Blaaderen, Synthesis of colloidal silica dumbbells, Langmuir 21 (2005) 11510–11517.