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Controlled spatial dispersion of CdSe tetrapod nanocrystals with amphiphilic block copolymer particles
Polymer 99 (2016) 399e403
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Short communication
Controlled spatial dispersion of CdSe tetrapod nanocrystals with amphiphilic block copolymer particles Seok Kyoo Seo, Jeewoo Lim, Hyemin Lee 1, Hyeonjun Heo, Kookheon Char* The National CRI Center for Intelligent Hybrids, The WCU Program of Chemical Convergence for Energy & Environment, School of Chemical & Biological Engineering, Seoul National University, Seoul 08826, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 June 2016 Received in revised form 12 July 2016 Accepted 16 July 2016 Available online 18 July 2016
We report, for the first time, a nanoscale control of the spatial distribution of semiconducting tetrapod (TP) nanocrystals within block copolymer particles. Polystyrene (PS) block copolymer (BCP)/CdSe TP nanocrystal hybrid particles were prepared by the nanoprecipitation of TP/BCP mixtures into methanol. The BCPs consisted of short, polar terminal block bearing methyl disulfide anchoring moiety which serves both to bind to CdSe TP surfaces as well as to drive self-assembly during nanoprecipitation into polar solvent systems. The resulting BCP/TP hybrid particles showed various spatial distribution and the number density of TPs with respect to individual polymer particles depending on the degree of polymerization of the block copolymer as well as the nature of the solvent in which nanoprecipitation was done. © 2016 Published by Elsevier Ltd.
Keywords: CdSe tetrapods Block copolymers Organic-inorganic hybrids
1. Introduction Semiconducting inorganic nanocrystals have received profound attention in the past decades for their interesting chemical and physical properties which could be utilized in a wide range of applications [1]. Recent synthetic advances have allowed for the preparation of well-defined semiconducting inorganic nanocrystals beyond those with simple spherical morphology (i.e., dots), and reliable methods for the preparation of higher order structures, such as nanorods and tetrapods (TPs), are now available [2]. TP nanocrystals, by the virtue of their geometric constraints that prevent them from forming flat structures on surfaces and interfaces, are of distinct interest [3]. Thus far, studies on the properties and applications of inorganic TP nanocrystals have largely been limited to colloidal dispersions in organic solvents [4], bare TPs on solid substrates [5], and simple blends between TPs containing small-molecule ligands and functional polymers [3,6]. Recently, we have reported the preparation of polymer-grafted CdSe TPs by appending end-functionalized polymeric brush ligands to the TP surface and demonstrated that this leads to significant enhancement in the homogeneity of TP dispersion in
* Corresponding author. E-mail address: [email protected] (K. Char). 1 Current address: LG Chemical, Research Park, South Korea. http://dx.doi.org/10.1016/j.polymer.2016.07.041 0032-3861/© 2016 Published by Elsevier Ltd.
polymeric matrices, even for the TPs with longer arm lengths [7]. While the study provides a strategy for forming well-dispersed hybrids between polymers and TPs, methods for fine-tuning the dispersion behavior are still desired. Extensive studies have been conducted on the dispersion behavior of various inorganic nanocrystals in a wide range of matrix materials [8]. Early studies involved the modification of the nanocrystals with polymeric ligands of judiciously chosen lengths to induce homogeneous dispersion of nanocrystals into desired soft matrices. Recently, Green et al. demonstrated “anisotropic dispersion” of polystyrene-modified gold nanoparticles within polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) block copolymer (BCP) nanoparticle/polystyrene blends and showed that different spatial distributions of gold nanoparticles could be obtained by varying nanoparticle size, concentration, the length of the polystyrene ligand, and the degree of polymerization of the matrix [9]. Given this and other studies on the controlled spatial dispersion of inorganic nanospheres and nanorods, it was envisioned that the identification of the handle for controlling the spatial distribution of semiconducting TP nanocrystals would provide an extremely useful tool for fine-tuning the properties and performance of TP/ polymer hybrids films and devices. We herein report, for the first time, a simple method which allows for the control of spatial distribution of CdSe TP nanocrystals within amphiphilic block copolymer particles. By changing formulation parameters such as the molecular weight of BCP
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S.K. Seo et al. / Polymer 99 (2016) 399e403
ligands and the composition of the non-solvent used for the nanoprecipitation, various spatial distributions were obtained, allowing for the localization of TPs either within the interior of the BCP particle matrix or exclusively on the BCP particle/non-solvent interface. Furthermore, conditions for achieving finely dispersed individual TPs are also outlined.
2. Experimental 2.1. Materials and methods PFPA and CMD were synthesized as described previously [10]. All other chemicals were commercially available and used as received unless otherwise stated. All air- and moisture-sensitive procedures were conducted under nitrogen atmosphere using standard Schlenk techniques. NMR spectra were obtained from Bruker Ascend TM-400 MHz spectrometer, and all proton chemical shifts were referenced to residual proton resonance of CDCl3 (7.26 ppm). Transmission electron microscopy (TEM) images were obtained on a Carl Zeiss LIBRA 120 microscope using 120 kV acceleration voltage. Samples were cast on carbon-coated copper grids from methanol suspensions. Spectral grade solvents were used unless otherwise noted. Tetrahydrofuran was distilled over sodium/benzophenone and degassed by bubbling argon through directly prior to use.
2.2. Synthesis of CdSe tetrapods CdSe tetrapod nanocrystals were synthesized by continuous precursor injection (CPI) [2a]. Cadmium oleate (Cd(OA)2) solution was prepared by placing cadmium oxide (CdO, 10 mmol), oleic acid (OA, 7.8 ml), 1-octadecene (ODE, 6.2 ml), and n-trioctylphosphine (TOP, 1 ml) in a 3-neck round-bottom flask equipped with a condenser and heating the mixture to 280 C under nitrogen for 20 min. The solution was subsequently cooled down to 50 C, at which point it became transparent. Cetyltrimethylammonium bromide (CTAB, 0.14 mmol) was then added in one portion under nitrogen. Injection solution was prepared by adding selenium (Se, 12 mmol) and TOP (6 ml) to a separate 2-neck round-bottom flask equipped with a reflux condenser and stirring at 200 C until the mixture became transparent. After the solution was cooled to room temperature, 5 ml was withdrawn and added to the Cd(OA)2 solution. The mixture was then stirred for 5 min. A seed solution was prepared by placing Se (1 mmol) and ODE (10 ml) in a separate 3-neck round-bottom flask and heating to 300 C under nitrogen. When the solution become transparent, 2 mmol (CdO: OA: ODE ¼ 5 mmol: 5 ml: 5 ml) of Cd(OA)2 solution (4 ml) was injected into the solution quickly. The temperature of the mixture was held at 270 C for 15 min, after which ODE (6 ml) was added. The reaction mixture was then cooled down to room temperature. For the synthesis of CdSe tetrapod nanocrystals, the seed solution (5 ml), OA(2.25 ml), TOP(1.5 ml), ODE (21.25 ml) and CTAB (0.21 mmol) were placed in a 3-neck round-bottom flask coupled with a reflux condenser and heated to 260 C. And then, the injection solution (20 ml) was added to the seed solution with an injection rate of 0.4 ml/min for 50 min. In order to purify the products, the products were precipitated from chloroform/acetone. The precipitate was re-dispersed using chloroform or toluene and precipitation/re-dispersion procedure was repeated until the products were purified sufficiently.
2.3. Synthesis of PS-b- pPFPA Purified styrene monomer (PS50: 4.2 g, 40 mmol, PS300: 8.4 g, 80 mmol), BDTB (24.5 mg, 0.1 mmol) [11], and recrystallized AIBN (0.33 mg, 0.002 mmol) were mixed together in a Schlenk flask under nitrogen. After degassing through three freeze-pump-thaw cycles, the Schlenk flask was placed at an oil bath pre-heated to 100 C. After stirring at the temperature, the polystyrene macroinitiator was purified by precipitation in methanol three times and subsequently dried in a vacuum oven at 60 C for 24 h. Molecular weight of the polymers were controlled by varying reaction time and molar CTA/monomer/initiator ratio (PS50 microinitiator: CTA/ monomer/initiator ¼ 1:400:0.02 mol/mol, reaction time ¼ 2 h, conversion ¼ 0.125, yield ¼ 12%; PS300 microinitiator: CTA/monomer/initiator ¼ 1:800:0.02 mol/mol, reaction time ¼ 24 h, conversion ¼ 0.375, yield ¼ 35%) and were measured by gel permeation chromatography (THF). For the synthesis of the block copolymer, the macroinitiator, PFPA (20 eq. with respect to the macroinitiator), and AIBN were dissolved in THF (0.5e0.7 M PFPA). After degassing through three freeze-pump-thaw cycles, the solution was stirred under nitrogen at 70 C for 24 h. After the polymerization, the resulting PS-b-poly(PFPA) block co polymers were precipitated in methanol and were dried in a vacuum oven at 60 C for 24 h (19F NMR (300 MHz, CDCl3): d [ppm]: 164.27 (m), 159.90 (m), 154.48 (m)). 2.4. Synthesis of PS-b- PCMD PS-b-pPFPA (4 mmol PFPA) and CMD (0.86 mL, 8 mmol) were dissolved in THF and stirred at room temperature for 12 h. After the purification by precipitation in hexane three times, the desired polymer was obtained. 19F NMR (300 MHz, CDCl3): no signal. FT-IR: 1780 cm1 (C]O of ester) and 1515 cm1 (C]C of PFP) for PS-bPPFPA, 1674 cm1 (C]O of amide) for PS-b-PCMD. 2.5. Nanoprecipitation Nanoprecipitation of the block copolymers were performed by adding the PS-b-PCMD solution (dissolved in THF) to methanol dropwise under various polymer/THF/methanol ratios. For the incorporation of surface-modified CdSe tetrapod nanocrystals to the nanoprecipitation, CdSe tetrapod nanocrystals (1 mg) and PS-bPCMD block copolymers (5 mg) were dissolved in THF and stirred for 12 h and the solution was added dropwise to methanol. The morphology of colloidal polymer particles and the spatial distribution of tetrapod nanocrystals were investigated by TEM analysis. TEM samples were prepared by dropping the colloidal solution onto a TEM grid followed by the evaporation of solvent. 3. Results and discussion The CdSe TPs were obtained using the previously reported continuous precursor injection (CPI) method [2a]. The arm length and diameter were 50 nm and 6 nm, respectively. The polystyrene (PS) ligands were prepared by first synthesizing polystyrene-bpoly(pentafluorophenyl acrylate) (PS-b-PFPA) through sequential reversible addition-fragmentation chain-transfer (RAFT) polymerization followed by the removal of the chain transfer agent and the substitution of pentafluorophenyl units with cysteamine methyl disulfides to give PS-b-PCMD (Fig. 1). Two different PSn-b-PCMDm BCP ligands, with n/m values of 300:5 and 50:5, were prepared. Interestingly, the addition of a PS-b-PCMD solution in THF (5 mL, 1 mg/mL) into methanol (45 mL) resulted in well-defined spherical polymer particles, which were assumed to consist of PS-b-PCMD chains with polar CMD blocks forming the corona (Fig. 2).
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Fig. 1. a) Chemical structure and functional moieties of PS-b-CMD block copolymers used in the present study. Polar amide groups in the CMD block and non-polar polystyrene blocks facilitate the amphiphilic properties in selective solvents. b) TEM image of CdSe tetrapod used in this study.
Fig. 2. The PS50-b-PCMD5 block copolymers form colloidal polymer particles (b) during the typical precipitation process in a non-solvent for PS block (a) while forming chain precipitates with PS homopolymer alone, as shown in (c). The polar moieties in the PS-b-PCMD block copolymers stabilize non-polar polystyrene block chains in a polar solvent, as schematically depicted in (d).
With a simple method for well-defined PS-based BCP particle preparation at hand, similar nanoprecipitation procedures from THF solutions containing mixtures of CdSe TPs and PS-b-PCMD
ligands were conducted (Fig. 3). To prepare the mixed solutions, TP solutions in THF (1 mL, 1 mg/mL) were added to PS-b-PCMD solutions in THF (4 mL, 1.25 mg/mL). The relative amount of BCP ligands
Fig. 3. Incorporation of tetrapod (TP) nanocrystals within polymer particles. After the surface modification of TPs, the excess block polymers form colloidal particles with the surface-modified TPs in a polar solvent.
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with respect to TPs represents a large excess. The mixture was stirred at room temperature to induce the surface modification of TPs by the BCP ligands and subsequently poured into excessive methanol (45 mL), to prepare TP/BCP hybrid particles. The analysis of the resulting hybrid nanostructures is shown in Fig. 4. When a 5 mL THF solution containing CdSe TPs and PS50-bPCMD5 was added into 45 mL of methanol, a light-brown and nearly transparent solution was obtained without visible precipitate formation. The solution obtained, as verified with the TEM image, was composed of well-dispersed CdSe TPs individually “packaged” in polymer matrices (Fig. 4a), with one TP per polymer package. Interestingly, when PS300-b-PCMD5 was used instead of the shorter PS50-b-PCMD5, the resulting solutions contained polymer/TP hybrid particles in which TPs were localized exclusively on the surfaces of polymer particles with relatively low number densities of TPs (three to four TPs per particle, Fig. 4c). When the polarity of nanoprecipitation medium was decreased by changing the THF/methanol ratio to 10:40 (v/v), the samples with the shorter PS50-b-PCMD5 ligands showed polymer nanoparticles where the TPs were localized exclusively in the interior of polymer particles (Fig. 4b). When the longer PS300-b-PCMD5 was used, polymer particles with similar size were obtained with TPs distributed dominantly on the surfaces (Fig. 4d). The size of polymer particles and the number of TPs associated with a single particle increases with decreasing polarity of the nanoprecipitation medium. With the shorter PS50-b-PCMA5 ligands, TPs were incorporated at the interior of polymer particles whereas for the longer PS300-b-PCMA5 ligands, exactly opposite spatial distribution was obtained. It is presumed that the differences in the spatial distribution of TPs in relation to the molecular weight of BCP ligands are caused by the degree of interdigitation between the brushes attached to the TP surfaces and the unattached polymers. If there is already significant interpenetration between the polymer brushes attached to TP surfaces and unattached excess polymers (i.e., wet brushes), which could be thought of as the matrix polymer, in THF prior to nanoprecipitation, it would translate into significant coprecipitation upon nanoprecipitation. This would lead to the localization of TPs within extended matrices of excess polymers. On the contrary,
if the polymer chains attached to the TP surface and those constituting the matrix are unable to form enough interdigitation (i.e., dry brushes) prior to nanoprecipitation, they will separate upon contact with non-solvent (methanol), giving individually separated particles with TPs distributed on the surfaces of polymer particles. It is well known that polymer brushes with higher molecular weight provide better steric stabilization in core-shell colloids compared to lower molecular weight counterparts, and similar effect could be attributed to the observed spatial distribution induced by PS300-b-PCMD5 and PS50-b-PCMD5 BCP ligands. The possibility of the longer brushes, PS300-b-PCMD5, leading to inefficient surface modification (thus leading to simple blends rather than surface-bound hybrids) could be eliminated since the TPs treated with PS300-b-PCMD5 in THF were observed to be welldispersed within PS matrices whereas simple blends (without surface modification) led to massive phase segregation [7]. -These microscale hybrid colloidal particles which incorporate CdSe TP nanocrystals either in the interior or at the surface of polymer particles in a controlled manner constitute highly useful building blocks for the assembly of distinctive macroscale polymer/ TP phase-separated structures. We expect that hybrid colloidal particles bearing TPs on the surface can be used to construct cocontinuous channels for charge transfer in which the volume fraction and connectivity among TPs can be controlled by polymer particle size and the relative content of TP nanocrystals. Hybrid colloidal particles harboring TPs in the interior are expected to give homogeneously dispersed structures in the macroscale. However, unlike the dispersion structure of surface-modified TP within bulk polymer matrices, the number density of TPs can be controlled by varying the preparation parameters. Specific applications of these nanoscale hybrid particles are currently under progress. 4. Conclusion We have demonstrated that the spatial distribution of CdSe TP nanocrystals within PS-based block copolymer particles can be easily controlled by varying the molecular weight of block copolymer ligands and the solvent conditions used for the particle preparation. When shorter polymer ligands were used, TPs were
Fig. 4. TP-incorporated polymer particles. TPs are confined inside the particles with PS50-b-PCMD5 (a & b) while TPs are located at the surfaces of polymer particles in the case with PS300-b-PCMD5 (c & d). The size of polymer particles and the number of TPs in a particle are controlled by the volume ratio of THF to methanol (a & c: 5 ml-THF/45 ml-MeOH; b & d: 10 ml-THF/40 ml-MeOH). Scale bar ¼ 200 nm.
S.K. Seo et al. / Polymer 99 (2016) 399e403
observed to be distributed exclusively in the interior of polymer particles, with more polar nanoprecipitation conditions leading to more finely divided polymer-TP hybrid particles, as verified with TEM imaging. When the higher molecular weight polymer ligands were employed, the TPs were obtained exclusively at the surface of polymer particles, an observation which is attributed to the poor interdigitation between longer polymer chains prior to nanoprecipitation. When less polar mixed solvent conditions were applied for particle preparation, the number density of TPs per polymer particle was increased. Using these two distinct handles for controlling the spatial distribution of TPs in TP/polymer particle hybrids, various well-defined building blocks for the macroscale assembly of electronic materials could be envisioned.
[3]
[4]
[5] [6]
Acknowledgments This work was supported by the NRF of Korea through the National Creative Research Initiative Center for Intelligent Hybrids (Grant No. 2010-0018290). The authors would also like to acknowledge the World Class University Program of the NRF of Korea funded by the Ministry of Education, Science, and Technology (Grant No. R31-10013). KC also acknowledges the Gutenberg Research College (GRC) Fellowship.
[7]
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[9]
[10]
References [1] L. Zhao, Z.Q. Lin, Crafting semiconductor organic-inorganic nanocomposites via placing conjugated polymers in intimate contact with nanocrystals for hybrid solar cells, Adv. Mater. 24 (32) (2012) 4353e4368. [2] (a) J. Lim, W.K. Bae, K.U. Park, L. zur Borg, R. Zentel, S. Lee, K. Char, Controlled synthesis of CdSe tetrapods with high morphological uniformity by the persistent kinetic growth and the halide-mediated phase transformation, Chem. Mater. 25 (8) (2013) 1443e1449; (b) L. Manna, D.J. Milliron, A. Meisel, E.C. Scher, A.P. Alivisatos, Controlled
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growth of tetrapod-branched inorganic nanocrystals, Nat. Mater. 2 (6) (2003) 382e385; (c) L. Manna, E.C. Scher, A.P. Alivisatos, Synthesis of soluble and processable rod-, arrow-, teardrop-, and tetrapod-shaped CdSe nanocrystals, J. Am. Chem. Soc. 122 (51) (2000) 12700e12706; (d) Z.A. Peng, X.G. Peng, Nearly monodisperse and shape-controlled CdSe nanocrystals via alternative routes: nucleation and growth, J. Am. Chem. Soc. 124 (13) (2002) 3343e3353. B.Q. Sun, E. Marx, N.C. Greenham, Photovoltaic devices using blends of branched CdSe nanoparticles and conjugated polymers, Nano Lett. 3 (7) (2003) 961e963. Y. Sung, J. Lim, J.H. Koh, L.J. Hill, B.K. Min, J. Pyun, K. Char, Uniform decoration of Pt nanoparticles on well-defined CdSe tetrapods and the effect of their Pt cluster size on photocatalytic H-2 generation, CrystEngComm 17 (44) (2015) 8423e8427. Y. Cui, U. Banin, M.T. Bjork, A.P. Alivisatos, Electrical transport through a single nanoscale semiconductor branch point, Nano Lett. 5 (7) (2005) 1519e1523. J. Lim, D. Lee, M. Park, J. Song, S. Lee, M.S. Kang, C. Lee, K. Char, Modular fabrication of hybrid bulk heterojunction solar cells based on breakwater-like CdSe tetrapod nanocrystal network infused with P3HT, J. Phys. Chem. C 118 (8) (2014) 3942e3952. J. Lim, L. zur Borg, S. Dolezel, F. Schmid, K. Char, R. Zentel, Strategy for good dispersion of well-defined tetrapods in semiconducting polymer matrices, Macromol. Rapid Commun. 35 (19) (2014) 1685e1691. F. Mathias, A. Fokina, K. Landfester, W. Tremel, F. Schmid, K. Char, R. Zentel, Morphology control in biphasic hybrid systems of semiconducting materials, Macromol. Rapid Commun. 36 (11) (2015) 959e983. X.C. Chen, P.F. Green, Structure of thin film polymer/nanoparticle systems: polystyrene (PS) coated-Au nanoparticle/tetramethyl bisphenol-A polycarbonate mixtures (TMPC), Soft Matter 7 (3) (2011) 1192e1198. (a) M.D. Conner, V. Janout, I. Kudelka, P. Dedek, J.Y. Zhu, S.L. Regen, Perforated monolayers - fabrication of calix[6]arene-based composite membranes that function as molecular-sieves, Langmuir 9 (9) (1993) 2389e2397; (b) M. Eberhardt, R. Mruk, R. Zentel, P. Theato, Synthesis of pentafluorophenyl(meth)acrylate polymers: new precursor polymers for the synthesis of multifunctional materials, Eur. Polym. J. 41 (7) (2005) 1569e1575. Y.K. Chong, J. Krstina, T.P.T. Le, G. Moad, A. Postma, E. Rizzardo, S.H. Thang, Thiocarbonylthio compounds [S¼C(Ph)S-R] in free radical polymerization with reversible addition-fragmentation chain transfer (RAFT polymerization). Role of the free-radical leaving group (R), Macromolecules 36 (7) (2003) 2256e2272.
Contents lists available at ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Short communication
Controlled spatial dispersion of CdSe tetrapod nanocrystals with amphiphilic block copolymer particles Seok Kyoo Seo, Jeewoo Lim, Hyemin Lee 1, Hyeonjun Heo, Kookheon Char* The National CRI Center for Intelligent Hybrids, The WCU Program of Chemical Convergence for Energy & Environment, School of Chemical & Biological Engineering, Seoul National University, Seoul 08826, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 June 2016 Received in revised form 12 July 2016 Accepted 16 July 2016 Available online 18 July 2016
We report, for the first time, a nanoscale control of the spatial distribution of semiconducting tetrapod (TP) nanocrystals within block copolymer particles. Polystyrene (PS) block copolymer (BCP)/CdSe TP nanocrystal hybrid particles were prepared by the nanoprecipitation of TP/BCP mixtures into methanol. The BCPs consisted of short, polar terminal block bearing methyl disulfide anchoring moiety which serves both to bind to CdSe TP surfaces as well as to drive self-assembly during nanoprecipitation into polar solvent systems. The resulting BCP/TP hybrid particles showed various spatial distribution and the number density of TPs with respect to individual polymer particles depending on the degree of polymerization of the block copolymer as well as the nature of the solvent in which nanoprecipitation was done. © 2016 Published by Elsevier Ltd.
Keywords: CdSe tetrapods Block copolymers Organic-inorganic hybrids
1. Introduction Semiconducting inorganic nanocrystals have received profound attention in the past decades for their interesting chemical and physical properties which could be utilized in a wide range of applications [1]. Recent synthetic advances have allowed for the preparation of well-defined semiconducting inorganic nanocrystals beyond those with simple spherical morphology (i.e., dots), and reliable methods for the preparation of higher order structures, such as nanorods and tetrapods (TPs), are now available [2]. TP nanocrystals, by the virtue of their geometric constraints that prevent them from forming flat structures on surfaces and interfaces, are of distinct interest [3]. Thus far, studies on the properties and applications of inorganic TP nanocrystals have largely been limited to colloidal dispersions in organic solvents [4], bare TPs on solid substrates [5], and simple blends between TPs containing small-molecule ligands and functional polymers [3,6]. Recently, we have reported the preparation of polymer-grafted CdSe TPs by appending end-functionalized polymeric brush ligands to the TP surface and demonstrated that this leads to significant enhancement in the homogeneity of TP dispersion in
* Corresponding author. E-mail address: [email protected] (K. Char). 1 Current address: LG Chemical, Research Park, South Korea. http://dx.doi.org/10.1016/j.polymer.2016.07.041 0032-3861/© 2016 Published by Elsevier Ltd.
polymeric matrices, even for the TPs with longer arm lengths [7]. While the study provides a strategy for forming well-dispersed hybrids between polymers and TPs, methods for fine-tuning the dispersion behavior are still desired. Extensive studies have been conducted on the dispersion behavior of various inorganic nanocrystals in a wide range of matrix materials [8]. Early studies involved the modification of the nanocrystals with polymeric ligands of judiciously chosen lengths to induce homogeneous dispersion of nanocrystals into desired soft matrices. Recently, Green et al. demonstrated “anisotropic dispersion” of polystyrene-modified gold nanoparticles within polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) block copolymer (BCP) nanoparticle/polystyrene blends and showed that different spatial distributions of gold nanoparticles could be obtained by varying nanoparticle size, concentration, the length of the polystyrene ligand, and the degree of polymerization of the matrix [9]. Given this and other studies on the controlled spatial dispersion of inorganic nanospheres and nanorods, it was envisioned that the identification of the handle for controlling the spatial distribution of semiconducting TP nanocrystals would provide an extremely useful tool for fine-tuning the properties and performance of TP/ polymer hybrids films and devices. We herein report, for the first time, a simple method which allows for the control of spatial distribution of CdSe TP nanocrystals within amphiphilic block copolymer particles. By changing formulation parameters such as the molecular weight of BCP
400
S.K. Seo et al. / Polymer 99 (2016) 399e403
ligands and the composition of the non-solvent used for the nanoprecipitation, various spatial distributions were obtained, allowing for the localization of TPs either within the interior of the BCP particle matrix or exclusively on the BCP particle/non-solvent interface. Furthermore, conditions for achieving finely dispersed individual TPs are also outlined.
2. Experimental 2.1. Materials and methods PFPA and CMD were synthesized as described previously [10]. All other chemicals were commercially available and used as received unless otherwise stated. All air- and moisture-sensitive procedures were conducted under nitrogen atmosphere using standard Schlenk techniques. NMR spectra were obtained from Bruker Ascend TM-400 MHz spectrometer, and all proton chemical shifts were referenced to residual proton resonance of CDCl3 (7.26 ppm). Transmission electron microscopy (TEM) images were obtained on a Carl Zeiss LIBRA 120 microscope using 120 kV acceleration voltage. Samples were cast on carbon-coated copper grids from methanol suspensions. Spectral grade solvents were used unless otherwise noted. Tetrahydrofuran was distilled over sodium/benzophenone and degassed by bubbling argon through directly prior to use.
2.2. Synthesis of CdSe tetrapods CdSe tetrapod nanocrystals were synthesized by continuous precursor injection (CPI) [2a]. Cadmium oleate (Cd(OA)2) solution was prepared by placing cadmium oxide (CdO, 10 mmol), oleic acid (OA, 7.8 ml), 1-octadecene (ODE, 6.2 ml), and n-trioctylphosphine (TOP, 1 ml) in a 3-neck round-bottom flask equipped with a condenser and heating the mixture to 280 C under nitrogen for 20 min. The solution was subsequently cooled down to 50 C, at which point it became transparent. Cetyltrimethylammonium bromide (CTAB, 0.14 mmol) was then added in one portion under nitrogen. Injection solution was prepared by adding selenium (Se, 12 mmol) and TOP (6 ml) to a separate 2-neck round-bottom flask equipped with a reflux condenser and stirring at 200 C until the mixture became transparent. After the solution was cooled to room temperature, 5 ml was withdrawn and added to the Cd(OA)2 solution. The mixture was then stirred for 5 min. A seed solution was prepared by placing Se (1 mmol) and ODE (10 ml) in a separate 3-neck round-bottom flask and heating to 300 C under nitrogen. When the solution become transparent, 2 mmol (CdO: OA: ODE ¼ 5 mmol: 5 ml: 5 ml) of Cd(OA)2 solution (4 ml) was injected into the solution quickly. The temperature of the mixture was held at 270 C for 15 min, after which ODE (6 ml) was added. The reaction mixture was then cooled down to room temperature. For the synthesis of CdSe tetrapod nanocrystals, the seed solution (5 ml), OA(2.25 ml), TOP(1.5 ml), ODE (21.25 ml) and CTAB (0.21 mmol) were placed in a 3-neck round-bottom flask coupled with a reflux condenser and heated to 260 C. And then, the injection solution (20 ml) was added to the seed solution with an injection rate of 0.4 ml/min for 50 min. In order to purify the products, the products were precipitated from chloroform/acetone. The precipitate was re-dispersed using chloroform or toluene and precipitation/re-dispersion procedure was repeated until the products were purified sufficiently.
2.3. Synthesis of PS-b- pPFPA Purified styrene monomer (PS50: 4.2 g, 40 mmol, PS300: 8.4 g, 80 mmol), BDTB (24.5 mg, 0.1 mmol) [11], and recrystallized AIBN (0.33 mg, 0.002 mmol) were mixed together in a Schlenk flask under nitrogen. After degassing through three freeze-pump-thaw cycles, the Schlenk flask was placed at an oil bath pre-heated to 100 C. After stirring at the temperature, the polystyrene macroinitiator was purified by precipitation in methanol three times and subsequently dried in a vacuum oven at 60 C for 24 h. Molecular weight of the polymers were controlled by varying reaction time and molar CTA/monomer/initiator ratio (PS50 microinitiator: CTA/ monomer/initiator ¼ 1:400:0.02 mol/mol, reaction time ¼ 2 h, conversion ¼ 0.125, yield ¼ 12%; PS300 microinitiator: CTA/monomer/initiator ¼ 1:800:0.02 mol/mol, reaction time ¼ 24 h, conversion ¼ 0.375, yield ¼ 35%) and were measured by gel permeation chromatography (THF). For the synthesis of the block copolymer, the macroinitiator, PFPA (20 eq. with respect to the macroinitiator), and AIBN were dissolved in THF (0.5e0.7 M PFPA). After degassing through three freeze-pump-thaw cycles, the solution was stirred under nitrogen at 70 C for 24 h. After the polymerization, the resulting PS-b-poly(PFPA) block co polymers were precipitated in methanol and were dried in a vacuum oven at 60 C for 24 h (19F NMR (300 MHz, CDCl3): d [ppm]: 164.27 (m), 159.90 (m), 154.48 (m)). 2.4. Synthesis of PS-b- PCMD PS-b-pPFPA (4 mmol PFPA) and CMD (0.86 mL, 8 mmol) were dissolved in THF and stirred at room temperature for 12 h. After the purification by precipitation in hexane three times, the desired polymer was obtained. 19F NMR (300 MHz, CDCl3): no signal. FT-IR: 1780 cm1 (C]O of ester) and 1515 cm1 (C]C of PFP) for PS-bPPFPA, 1674 cm1 (C]O of amide) for PS-b-PCMD. 2.5. Nanoprecipitation Nanoprecipitation of the block copolymers were performed by adding the PS-b-PCMD solution (dissolved in THF) to methanol dropwise under various polymer/THF/methanol ratios. For the incorporation of surface-modified CdSe tetrapod nanocrystals to the nanoprecipitation, CdSe tetrapod nanocrystals (1 mg) and PS-bPCMD block copolymers (5 mg) were dissolved in THF and stirred for 12 h and the solution was added dropwise to methanol. The morphology of colloidal polymer particles and the spatial distribution of tetrapod nanocrystals were investigated by TEM analysis. TEM samples were prepared by dropping the colloidal solution onto a TEM grid followed by the evaporation of solvent. 3. Results and discussion The CdSe TPs were obtained using the previously reported continuous precursor injection (CPI) method [2a]. The arm length and diameter were 50 nm and 6 nm, respectively. The polystyrene (PS) ligands were prepared by first synthesizing polystyrene-bpoly(pentafluorophenyl acrylate) (PS-b-PFPA) through sequential reversible addition-fragmentation chain-transfer (RAFT) polymerization followed by the removal of the chain transfer agent and the substitution of pentafluorophenyl units with cysteamine methyl disulfides to give PS-b-PCMD (Fig. 1). Two different PSn-b-PCMDm BCP ligands, with n/m values of 300:5 and 50:5, were prepared. Interestingly, the addition of a PS-b-PCMD solution in THF (5 mL, 1 mg/mL) into methanol (45 mL) resulted in well-defined spherical polymer particles, which were assumed to consist of PS-b-PCMD chains with polar CMD blocks forming the corona (Fig. 2).
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Fig. 1. a) Chemical structure and functional moieties of PS-b-CMD block copolymers used in the present study. Polar amide groups in the CMD block and non-polar polystyrene blocks facilitate the amphiphilic properties in selective solvents. b) TEM image of CdSe tetrapod used in this study.
Fig. 2. The PS50-b-PCMD5 block copolymers form colloidal polymer particles (b) during the typical precipitation process in a non-solvent for PS block (a) while forming chain precipitates with PS homopolymer alone, as shown in (c). The polar moieties in the PS-b-PCMD block copolymers stabilize non-polar polystyrene block chains in a polar solvent, as schematically depicted in (d).
With a simple method for well-defined PS-based BCP particle preparation at hand, similar nanoprecipitation procedures from THF solutions containing mixtures of CdSe TPs and PS-b-PCMD
ligands were conducted (Fig. 3). To prepare the mixed solutions, TP solutions in THF (1 mL, 1 mg/mL) were added to PS-b-PCMD solutions in THF (4 mL, 1.25 mg/mL). The relative amount of BCP ligands
Fig. 3. Incorporation of tetrapod (TP) nanocrystals within polymer particles. After the surface modification of TPs, the excess block polymers form colloidal particles with the surface-modified TPs in a polar solvent.
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with respect to TPs represents a large excess. The mixture was stirred at room temperature to induce the surface modification of TPs by the BCP ligands and subsequently poured into excessive methanol (45 mL), to prepare TP/BCP hybrid particles. The analysis of the resulting hybrid nanostructures is shown in Fig. 4. When a 5 mL THF solution containing CdSe TPs and PS50-bPCMD5 was added into 45 mL of methanol, a light-brown and nearly transparent solution was obtained without visible precipitate formation. The solution obtained, as verified with the TEM image, was composed of well-dispersed CdSe TPs individually “packaged” in polymer matrices (Fig. 4a), with one TP per polymer package. Interestingly, when PS300-b-PCMD5 was used instead of the shorter PS50-b-PCMD5, the resulting solutions contained polymer/TP hybrid particles in which TPs were localized exclusively on the surfaces of polymer particles with relatively low number densities of TPs (three to four TPs per particle, Fig. 4c). When the polarity of nanoprecipitation medium was decreased by changing the THF/methanol ratio to 10:40 (v/v), the samples with the shorter PS50-b-PCMD5 ligands showed polymer nanoparticles where the TPs were localized exclusively in the interior of polymer particles (Fig. 4b). When the longer PS300-b-PCMD5 was used, polymer particles with similar size were obtained with TPs distributed dominantly on the surfaces (Fig. 4d). The size of polymer particles and the number of TPs associated with a single particle increases with decreasing polarity of the nanoprecipitation medium. With the shorter PS50-b-PCMA5 ligands, TPs were incorporated at the interior of polymer particles whereas for the longer PS300-b-PCMA5 ligands, exactly opposite spatial distribution was obtained. It is presumed that the differences in the spatial distribution of TPs in relation to the molecular weight of BCP ligands are caused by the degree of interdigitation between the brushes attached to the TP surfaces and the unattached polymers. If there is already significant interpenetration between the polymer brushes attached to TP surfaces and unattached excess polymers (i.e., wet brushes), which could be thought of as the matrix polymer, in THF prior to nanoprecipitation, it would translate into significant coprecipitation upon nanoprecipitation. This would lead to the localization of TPs within extended matrices of excess polymers. On the contrary,
if the polymer chains attached to the TP surface and those constituting the matrix are unable to form enough interdigitation (i.e., dry brushes) prior to nanoprecipitation, they will separate upon contact with non-solvent (methanol), giving individually separated particles with TPs distributed on the surfaces of polymer particles. It is well known that polymer brushes with higher molecular weight provide better steric stabilization in core-shell colloids compared to lower molecular weight counterparts, and similar effect could be attributed to the observed spatial distribution induced by PS300-b-PCMD5 and PS50-b-PCMD5 BCP ligands. The possibility of the longer brushes, PS300-b-PCMD5, leading to inefficient surface modification (thus leading to simple blends rather than surface-bound hybrids) could be eliminated since the TPs treated with PS300-b-PCMD5 in THF were observed to be welldispersed within PS matrices whereas simple blends (without surface modification) led to massive phase segregation [7]. -These microscale hybrid colloidal particles which incorporate CdSe TP nanocrystals either in the interior or at the surface of polymer particles in a controlled manner constitute highly useful building blocks for the assembly of distinctive macroscale polymer/ TP phase-separated structures. We expect that hybrid colloidal particles bearing TPs on the surface can be used to construct cocontinuous channels for charge transfer in which the volume fraction and connectivity among TPs can be controlled by polymer particle size and the relative content of TP nanocrystals. Hybrid colloidal particles harboring TPs in the interior are expected to give homogeneously dispersed structures in the macroscale. However, unlike the dispersion structure of surface-modified TP within bulk polymer matrices, the number density of TPs can be controlled by varying the preparation parameters. Specific applications of these nanoscale hybrid particles are currently under progress. 4. Conclusion We have demonstrated that the spatial distribution of CdSe TP nanocrystals within PS-based block copolymer particles can be easily controlled by varying the molecular weight of block copolymer ligands and the solvent conditions used for the particle preparation. When shorter polymer ligands were used, TPs were
Fig. 4. TP-incorporated polymer particles. TPs are confined inside the particles with PS50-b-PCMD5 (a & b) while TPs are located at the surfaces of polymer particles in the case with PS300-b-PCMD5 (c & d). The size of polymer particles and the number of TPs in a particle are controlled by the volume ratio of THF to methanol (a & c: 5 ml-THF/45 ml-MeOH; b & d: 10 ml-THF/40 ml-MeOH). Scale bar ¼ 200 nm.
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observed to be distributed exclusively in the interior of polymer particles, with more polar nanoprecipitation conditions leading to more finely divided polymer-TP hybrid particles, as verified with TEM imaging. When the higher molecular weight polymer ligands were employed, the TPs were obtained exclusively at the surface of polymer particles, an observation which is attributed to the poor interdigitation between longer polymer chains prior to nanoprecipitation. When less polar mixed solvent conditions were applied for particle preparation, the number density of TPs per polymer particle was increased. Using these two distinct handles for controlling the spatial distribution of TPs in TP/polymer particle hybrids, various well-defined building blocks for the macroscale assembly of electronic materials could be envisioned.
[3]
[4]
[5] [6]
Acknowledgments This work was supported by the NRF of Korea through the National Creative Research Initiative Center for Intelligent Hybrids (Grant No. 2010-0018290). The authors would also like to acknowledge the World Class University Program of the NRF of Korea funded by the Ministry of Education, Science, and Technology (Grant No. R31-10013). KC also acknowledges the Gutenberg Research College (GRC) Fellowship.
[7]
[8]
[9]
[10]
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