Preparation and Morphology of Hybrids Composed of a Block

Oct 1, 2012 - A number-average molecular weight (Mn) of a macro-. CTA of PS was determined with 1H .... wPS:wP4VP:wh‑CdSe a. ϕPS:ϕP4VP:ϕh‑CdSe ...
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Preparation and Morphology of Hybrids Composed of a Block Copolymer and Semiconductor Nanoparticles via Hydrogen Bonding Atsushi Noro,* Kota Higuchi, Yoshio Sageshima, and Yushu Matsushita* Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan S Supporting Information *

ABSTRACT: We report a systematic study on preparation and morphological observation of hybrids composed of a block copolymer and hydroxy-capped semiconductor nanoparticles via hydrogen bonding. Three polystyrene-b-poly(4-vinylpyridine) (PS−P4VP) block copolymers with exactly the same PS chain length but with different P4VP chain length were synthesized via reversible addition−fragmentation chain transfer polymerization. To prepare hybrids, each PS−P4VP was mixed with hydroxy-capped nanoparticles made of cadmium selenide (h-CdSe), by varying a weight ratio of PS−P4VP:hCdSe. Morphology of three series of hybrids was observed by both transmission electron microscopy and small-angle X-ray scattering. Hybrids composed of h-CdSe and PS−P4VP bearing a long P4VP block represents uniform morphology of a single nanophase-separated structure, where domain spacing expansion and morphology transition induced by addition of h-CdSe were observed. On the other hand, nonuniform morphology, i.e., macrophase separation accompanied by overflow of h-CdSe from nanophase-separated domains, was observed in hybrids containing PS−P4VP bearing a short P4VP block. These results are attributed to hydrogen-bonding formation and the stoichiometric balance of functional groups.

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INTRODUCTION Well-ordered periodic nanostructures have been attaining much attention due to their high potential for applications to achieve high functional nanostructured materials.1−5 Nanophaseseparated structures6 of block copolymers are one of their good candidates;7,8 therefore, they have been studied extensively for more than four decades.9−15 For example, selective incorporation of inorganic nanoparticles into a specific location of nanophase-separated structures enables to provide the structures with prominent features such as optic, electronic, and magnetic properties, which leads to generation of remarkable photonic and microelectronic devices.16−27 One of the pioneering experimental studies on block copolymer/inorganic nanoparticle hybrids was carried out by Russell and Emrick et al., where they studied spin-coated thin films of hybrids composed of a polystyrene-b-poly(2-vinylpyridine) (PS−P2VP) block copolymer and surfactant-capped nanoparticles.28,29 Kramer et al. have succeeded in controlling the distribution of nanoparticles in nanophase-separated structures of PS−P2VP block copolymers by using PS or P2VP homopolymers as ligands.30−37 This approach was interesting in terms of reducing an enthalphic repulsive interaction between nanoparticles and block copolymers. Very recently, Watkins et al.38,39 demonstrated high loading capability and selective incorporation of nanoparticles into a specific constituent block, where a volume fraction of blocks for hydrogen bonding is larger than 0.5. Kramer and Hawker et al.40,41 reported localization and uniform distribution of © 2012 American Chemical Society

nanoparticles in the domain in response to the domain size of block copolymers, where they used multiple hydrogenbonding donors, poly(stynerene-r-4-vinylphenol), as ligands to enhance the solubility of nanoparticles in a nonpolar solvent such as dichloromethane. In addition to these good approaches, we and others have also reported hybrids composed of a block copolymer and metal salts42−49 (but not nanoparticles) via noncovalent bonding lately. We demonstrated that solvent casting with a nonpolar solvent for hybrid preparation easily led to formation of aggregates or precipitates because of strong interactions between components and that slow solvent casting with polar solvents was favorable to produce macroscopically homogeneous hybrids of block copolymer/metal salt.49 Here in this study, we report preparation and morphological observation of macroscopically homogeneous hybrids composed of a block copolymer and inorganic nanoparticles via hydrogen bonding (Figure 1). A systematic study was carried out by varying a molecular weight of one block and a weight ratio of a block copolymer and nanoparticles. Polystyrene-bpoly(4-vinylpyridine) (PS−P4VP) was used as a block copolymer for hydrogen bonding formation, where a volume fraction of P4VP for hydrogen bonding is smaller than 0.5. Because cadmium selenide nanoparticles were common Received: August 7, 2012 Revised: September 14, 2012 Published: October 1, 2012 8013

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Figure 2. GPC chromatograms of PS and three PS−P4VP synthesized via RAFT polymerization. Chromatograms are displayed as follows: left (blue) for PS−P4VP(41K), second left (green) for PS− P4VP(22K), second right (red) for PS−P4VP(4K), and right (black) for PS.

weight by polystyrene standards was basically used to estimate the PDI of each sample. A number-average molecular weight (Mn) of a macroCTA of PS was determined with 1H NMR (Varian). A volume fraction of a PS block (ϕPS) of PS−P4VP was also determined on the basis of 1 H NMR analysis. Mn, PDI, and ϕPS of each polymer sample are listed in Table 1. Three PS−P4VP block copolymers were coded as PS− P4VP(X), where X represents the molecular weight of P4VP in PS− P4VP. Although the PDI of PS−P4VP(41K) is a bit broader than the other two, there is no need to care about effects of polydispersity on single morphology formation according to the previous literature.57−60 Synthesis of 2-Mercaptoethanol-Capped Cadmium Selenide (CdSe) Nanoparticles. A Cd(ClO4)2·nH2O solution at pH 12.5 under the presence of 2-mercaptoethanol was mixed with a freshly prepared, oxygen-free, aqueous solution of sodium hydrogen selenide to produce hydroxy-capped CdSe nanoparticles with ligands of 2mercaptoethanol, it being abbreviated as h-CdSe hereafter for simplicity. Purification of h-CdSe nanoparticles was carried out by precipitating the nanoparticles into isoproparnol, followed by centrifugation to separate a precipitate of nanoparticles and the supernatant. The precipitate of nanoparticles was dispersed in DMF to prepare a homogeneous 3 wt % solution. Details in the synthesis were described elsewhere.61 The average size of the core of a nanoparticle determined by TEM was 6.9 nm. The weight fraction of organic components of ligands was also determined ∼16 wt % by thermogravimetric analysis. (See also Figures S2 and S3 on characterization of the nanoparticles.) Preparation of Hybrids. Figure 1 is an illustration of a PS− P4VP/h-CdSe hybrid at the molecular level. Since a nanoparticle is coated with 2-mercaptoethanol via metal-to-ligand coordination between thiol ends and nanoparticle surfaces, there are many hydroxy groups on nanoparticles. These hydroxy groups are utilized for hydrogen-bonding formation with pyridine groups in PS−P4VP, which should induce selective incorporation of h-CdSe into a P4VP phase. For uniform preparation, a slow solvent casting procedure with a polar solvent was used.62−66 First, a 4 wt % PS−P4VP solution in DMF was prepared, and then it was mixed with a solution of h-CdSe. The mixture solution was transferred to a Teflon Petri dish, and DMF in the mixture solution was evaporated slowly on a hot plate at 50 °C for 72 h. The obtained solvent-cast film was thermally annealed at 150 °C for 72 h in vacuo. To investigate effects of the amount of h-CdSe mixed with block copolymers, a weight fraction of h-CdSe in PS− P4VP/h-CdSe hybrids was varied systematically from 0 to 0.3. These hybrids were coded as X-Y, where X represents the molecular weight of P4VP in PS−P4VP while Y means the weight fraction of h-CdSe in PS−P4VP/h-CdSe hybrids. Table 2 summarizes codes and characteristics of all hybrids prepared in this study. Spectroscopy. Fourier transform infrared (FT-IR) spectroscopy was conducted to identify the hydrogen-bonding interaction in hybrids between hydroxy groups on h-CdSe and pyridine groups in PS−P4VP. The instrument used was FT-IR 6100 (Jasco. Japan). Each

Figure 1. Schematic illustration of preparation of a PS−P4VP block copolymer and hydroxy-capped CdSe nanoparticles via hydrogen bonding.

semiconductor nanoparticles,50,51 mercaptoethanol-capped cadmium selenide was used as nanoparticles coated with hydroxy groups for hybrid preparation. Hydrogen-bonding formation was investigated by FT-IR measurements while transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) were also conducted for morphological observation.

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EXPERIMENTAL SECTION

Materials. 4-Vinylpyridine (4VP) was purchased from Aldrich. Aluminum selenide (99.9%) and cadmium perchlorate n-hydrate (Cd(ClO4)2·nH2O) were purchased from Mitsuwa Chemicals Co., Ltd., for preparation of semiconductor nanoparticles, while other reagents including 2-mercaptoethanol were purchased from Kishida Regents Chemicals Co., Ltd. Styrene and 4VP for polymerization were purified by an aluminum oxide column before use. N,N-Dimethylformamide (DMF) as a casting solvent was used after removing water with molecular sieves. The others were used as received. Synthesis and Characterization of Block Copolymers. Three polystyrene-b-poly(4-vinylpyridine) (PS−P4VP) block copolymers were synthesized via reversible addition−fragmentation chain transfer (RAFT) polymerization.52−54 Styrene was polymerized in bulk at 130 °C for 9.5 h, by using a monofunctional chain transfer agent (CTA), S1-dodecyl-S′(α,α′-dimethyl-α″-acetic acid) trithiocarbonate.55 After deactivation of radicals by lowering a temperature with liquid nitrogen, the crude product was purified with reprecipitation into methanol three times to obtain a macromolecular CTA (macro-CTA) of PS. Then, 4VP was polymerized in bulk at 70 °C for several tens of minutes under the presence of 2,2′-azobis(isobutyronitrile) (AIBN) and a macro-CTA of PS.56 Polymerization of 4VP from a macro-CTA was terminated by deactivating radicals with liquid nitrogen. After purification of crude products by precipitating into hexane and rinsing with methanol, PS−P4VP block copolymers were dried in vacuo. See also details of synthesis in ref 49. To determine a polydispersity index (PDI) of each polymer, GPC was performed by using three TSK-GEL G4000HHR columns combined with a DP-8020 dual pump and a UV detector (Figure 2), where the wavelength was set at 220 nm. The eluent was DMF, and the flow rate was 1 mL/min. A calibration curve of molecular 8014

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Table 1. Molecular Characteristics of a PS Macro-CTA and PS−P4VP Block Copolymers code

Mna

ϕPSb

PDIc

Mn(P4VP)d

morphologye

D/nmf

PS PS−P4VP(41K) PS−P4VP(22K) PS−P4VP(4K)

39 000 80 000 61 000 43 000

1.00 0.52 0.66 0.92

1.10 1.43 1.22 1.17

41 000 22 000 4 000

L C S

66 60 22

Number-average molecular weight calculated by using Mn(PS) (= 39 000), ϕPS of PS−P4VP, and bulk densities of component polymers, i.e., 1.05 for PS and 1.17 for P4VP. bVolume fraction of PS determined by 1H NMR. Bulk densities of component polymers were used for calculation. c Polydispersity index determined by GPC. Calibration was done with polystyrene standards. dNumber-average molecular weight of P4VP blocks. e Morphology in a bulk state determined by both TEM and SAXS. L: lamellar structure; C: cylindrical structure; S: spherical structure. fDistance between domains for each morphology at room temperature determined on the basis of SAXS data. a

Table 2. Codes and Characteristics of PS−P4VP/h-CdSe Hybrids code

wPS:wP4VP:wh‑CdSea

ϕPS:ϕP4VP:ϕh‑CdSeb

wP4VP/wh‑CdSec

npyridine/nOHd

morphologye

D/nmf

41K-0.05 41K-0.10 41K-0.20 41K-0.30 22K-0.05 22K-0.10 22K-0.20 22k-0.30 4K-0.05 4K-0.10 4K-0.20 4K-0.30

0.46:0.49:0.05 0.44:0.46:0.10 0.39:0.41:0.20 0.34:0.36:0.30 0.60:0.35:0.05 0.57:0.33:0.10 0.51:0.29:0.20 0.44:0.26:0.30 0.87:0.08:0.05 0.82:0.08:0.10 0.73:0.07:0.20 0.64:0.06:0.30

0.51:0.47:0.02 0.50:0.47:0.03 0.48:0.45:0.07 0.45:0.43:0.12 0.65:0.33:0.02 0.64:0.33:0.03 0.61:0.32:0.07 0.58:0.30:0.12 0.90:0.08:0.02 0.89:0.08:0.03 0.86:0.07:0.07 0.81:0.07:0.12

9.8 4.6 2.1 1.2 7.0 3.3 1.5 0.87 1.6 0.8 0.35 0.2

42.9 20.3 9.0 5.3 30.7 14.6 6.5 3.8 7.2 3.4 1.5 0.9

L L L L C L L L S M(S/C) M(S/C) M(S/C)

56 69 70 77 59 54 58 60 25 20/36 27/46 27/53

a Weight fraction ratio of PS:P4VP:h-CdSe, which was calculated by using a weight fraction of PS−P4VP in the hybrid, ϕPS of neat PS−P4VP, and bulk densities of component polymers. bVolume fraction ratio of PS:P4VP:h-CdSe, which was calculated on the basis of the assumption that the density of ligands on the surface of an h-CdSe core is the same as the bulk density of 2-mercaptoethanol (1.11 g/cm3) . Note that the bulk density of CdSe is 5.81 g/cm3 and that the core diameter of h-CdSe is 6.9 nm (determined by TEM). cRatio value wP4VP to wh‑CdSe. dStoichiometric ratio value of pyridine groups to hydroxy groups, which was calculated on the basis of the assumption that the density of ligands on the surface of an h-CdSe core is the same as the bulk density of 2-mercaptoethanol (1.11 g/cm3). eMorphology in a bulk state determined by TEM and SAXS. L: lamellar structure; C: cylindrical structure; S: spherical structure; M: macrophase separation. fDistance between domains for each morphology at room temperature determined on the basis of SAXS data.

arbitrary units. There is an absorption peak at 3430 cm−1 on a spectrum of h-CdSe, indicating the presence of hydroxy groups originated from 2-mercaptoethanol as ligands on nanoparticles, which is also supported by the spectrum of neat 2mercaptoethanol. There is also an absorption peak at 3420 cm−1 on a neat PS−P4VP(41K) spectrum, but it suggests an O−H stretching vibration mode from carboxy ends of PS− P4VP(41K) associated with a RAFT agent residue.55 A new absorption peak at around 3290 cm−1 is seen in a spectra of 41K-0.05, where 3290 cm−1 is the lower wavenumber than the original peak position of hydroxy groups (3430 cm−1), indicating hydrogen bonding is formulated between pyridine groups on PS−P4VP and hydroxy groups on h-CdSe. As the amount of h-CdSe in hybrids increases, this absorption peak shifted to lower wavenumber, which also indicates the presence of hydrogen bonds regardless of the amount of h-CdSe amount added. FT-IR spectra of the 970−1015 cm−1 region in Figure 3b also support hydrogen bonding between pyridine groups on PS−P4VP and hydroxy groups on h-CdSe, where the absorbance at 993 cm−1 (indicating free pyridine67) decreases while the absorbance at 1003 cm−1 (indicating hydrogenbonded pyridine) increases by the addition of h-CdSe. Figure 4a compares DSC thermograms of neat PS− P4VP(41K) and 41K-Y hybrids, whereas Figure 4b presents the corresponding derivative curves. Neat PS−P4VP(41K) possesses two peaks at 100 and 146 °C on the derivative curve in Figure 4b, representing Tgs of PS and P4VP components,

measurement was carried out under reduced pressure at room temperature. Thermal Analysis. Differential scanning calorimetry (DSC) was carried out to examine Tgs of neat PS−P4VP and hybrids. Approximately 10 mg of samples was sealed in aluminum pans for measurements. The instrument used was EXSTAR DSC 6100 (Seiko Instruments Inc.). An empty hermetic pan was set as a reference in every case. Nitrogen was flowed as a purge gas. The temperature ramp rate of 10 °C/min was used for all measurements. Morphological Observation. Transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) were carried out to observe morphologies of PS-P4VP/h-CdSe hybrids. Ultrathin sections with a thickness of ca. 50 nm were prepared from thermally annealed hybrid films by using a microtome, Leica Ultracut FCS. Ultrathin sections of neat PS-P4VP were exposed with an iodine vapor to stain the P4VP phase. An electron microscope of JEM-1400 was used at an acceleration voltage of 120 kV for morphological observation. SAXS was also carried out at room temperature in the beamlines 15A and 6A at the Photon Factory, KEK, Tsukuba, Japan. The X-ray wavelength was 0.15 nm, and the camera length was ca. 2.2 m. The incident beam was set parallel to the sample film surface.

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RESULTS Characterization of PS−P4VP/h-CdSe Hybrids. Figure 3a compares FT-IR spectra of the 3000−3800 cm−1 region for neat PS−P4VP(41K), 41K-Y hybrids, h-CdSe, and neat 2mercaptoethanol. Here, a 41K-Y series were used as a representative for hybrid characterization. A horizontal axis represents wavenumber, while a vertical axis is absorbance in 8015

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TEM Observations. TEM images of ultrathin sections of neat PS−P4VP and hybrids are shown in Figure 5, where all scale bars are 100 nm. Parts a, f, and k in Figure 5 are images of neat PS−P4VP(X) (X = 41K, 22K, and 4K). Iodine vapor staining was performed for neat PS−P4VP(X) before observation to enhance contrast between a PS phase and a P4VP phase. PS−P4VP(X) expresses different morphologies: lamellae for PS−P4VP(41K) (Figure 5a), cylinders for PS− P4VP(22K) (Figure 5f), and spheres for PS−P4VP(4K) (Figure 5k), depending on volume fractions. Images of hybrids are also displayed in Figure 5. Despite no treatment with any staining agents, images with high contrast were observed for all hybrids, where a darker (electron-rich) phase must be a hybrid composed of P4VP and h-CdSe, while a lighter phase is PS. It is very clear that all 41K-Y hybrids prepared in this study (Y ≤ 0.30) represent lamellar structures as well as neat PS−P4VP(41K) (Figure 5b−e). With regard to 22K-Y hybrids, a cylindrical structure was kept at Y = 0.05 as neat PS−P4VP(22K) (Figure 5g); however, morphology transition occurred at Y = 0.10, and lamellar structures were stable at Y ≥ 0.10. This is probably induced by the increase in a volume fraction of a P4VP/h-CdSe hybrid phase due to selective incorporation of h-CdSe into a P4VP phase via hydrogen bonding. 4K-Y hybrids also gave the same spherical structures at Y = 0.05 as neat PS−P4VP(4K). Though morphological transition from spherical to cylindrical structures was also recognized in this series (Figure 5m−o), spherical structures were also observed as shown in Figure 5p−r, which were accompanied by h-CdSe aggregation in Figure 5q and r, suggesting occurrence of macrophase separation. SAXS Measurements. Small-angle X-ray scattering was conducted to acquire quantitative data of morphologies in a wide range. Figure 6 displays X-ray scattering profiles of neat PS−P4VP and hybrids. Vertical axes denote logarithmic intensities in arbitrary units, and horizontal axes denote the scattering vector, q = (4π sin θ)/λ, where 2θ and λ represent the scattering angle and the wavelength of X-ray, respectively. In Figure 6a, a SAXS profile of neat PS−P4VP(41K) is shown at the bottom, where peaks appear at relative q values of 1, 3, and 5, indicating a lamellar structure. Weak intensities of peaks at relative q of 2, 4, and 6 suggest that composition of one phase in lamellae should be close to 0.5, which agrees with the volume fraction ratio of PS:P4VP of PS−P4VP(41K) as shown in Table 1. It should be noted that the peaks on a profile of a 41K-0.05 hybrid at the second bottom in Figure 6a shifted to higher q. Domain spacing of lamellar structures, D, can be easily estimated by the equation of D = 2π/q1, where q1 denotes the magnitude of the scattering vector at the first peak; therefore, D of a lamellar structure of 41K-0.05 was determined to be 56 nm, which is definitely a smaller domain spacing than that of neat PS−P4VP(41K), 66 nm (see also Tables 1 and 2). This initial shrinkage of domain spacing by the addition of h-CdSe is attributed to hybridization by hydrogen bonding, which was also seen in a different noncovalent bonded systems.49 As the amount of h-CdSe in hybrids increases, the peaks shifted to lower q, meaning that a domain size increased due to incorporation of h-CdSe. This domain size expansion might be induced by increase in an effective χ parameter value between PS and hybrid phases. Figure 6b represents SAXS results of neat PS−P4VP(22K) and the hybrids. Neat PS−P4VP(22K) at the bottom and 22K0.05 at the second bottom possess peaks at relative q of 1, 41/2, 71/2, 91/2, and 161/2, indicating cylindrical structures, which is

Figure 3. FT-IR spectra of neat PS−P4VP(41K), 41K-Y hybrids, hCdSe, and neat 2-mercaptoethanol. (a) A region between 3000 and 3800 cm−1. (b) A region between 970 and 1015 cm−1. A horizontal axis denotes wavenumber, while a vertical axis is absorbance in arbitrary units. The color for each spectra is as follows: black for neat PS−P4VP(41K), red for 41K-0.05, dark yellow for 41K-0.10, green for 41K-0.20, blue for 41K-0.30, purple for CdSe, and gray (dashed line) for neat 2-mercaptoethanol.

Figure 4. DSC thermograms of neat PS−P4VP(41K) and 41K-Y hybrids: (a) heat flow; (b) derivative curves. Thermograms are displayed in the order of magnitude of h-CdSe amount added from bottom to top, i.e., PS−P4VP(41K), 41K-0.05, 41K-0.10, 41K-0.20, and 41K-0.30.

respectively.68 As can be seen in Figures 4a and 4b, a higher Tg corresponding to a Tg of P4VP increases as 149, 151, 152, and 154 °C with increase in an amount of h-CdSe added, while a lower Tg corresponding to a Tg of PS remains almost constant. This strongly suggests slow molecular motion of P4VP due to selective incorporation of h-CdSe into a P4VP phase. These DSC results are consistent with those of FT-IR. 8016

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Figure 5. TEM images of neat PS−P4VP(X) and the hybrids of X-Y. P4VP blocks in neat PS−P4VP(X) were stained with an iodine vapor to enhance contrast between a PS phase and a P4VP phase, whereas all hybrids were not stained at all. All scale bars represent 100 nm.

nanophase-separated grains,57 suggesting macrophase separation, which agrees with TEM results.

consistent with TEM results. On the other hand, hybrids of 22K-Y (Y = 0.10, 0.20, and 0.30) showed a series of characteristic peaks of lamellar structures, whose relative peak q values are 1, 2, (or 3,) and 4. The morphology transition from cylindrical to lamellar structures induced by h-CdSe incorporation is also consistent with TEM results, which was also seen in other hybrids composed of h-CdSe and PS−P4VP with Mn(P4VP) of 10K (see also Figure S5 in Supporting Information). It is apparent that peaks shift to a lower q region with increasing Y for 22K-Y hybrids, suggesting that incorporation of h-CdSe in 22K-Y hybrids induces expansion of lamellar domain spacing as well as 41K-Y hybrids (Table 2). Profiles of PS−P4VP(4K) and the hybrids are provided in Figure 6c. There are peaks at relative q of 1, 21/2, 31/2, and 41/2 on the bottom profile of neat PS−P4VP(4K), indicating a bcc spherical structure. Weak intensities at the peak positions of 21/2 and 41/2 are attributed to the form factor reflecting composition of spherical structures, which are mentioned by a detailed SAXS study on morphology of block copolymer/ homopolymer blends.69,70,71 A 4K-0.05 hybrid gave a similar profile to that of a neat sample. Further addition of h-CdSe into PS−P4VP(4K) seems to induce peak splitting; i.e., the first peak on a profile of 4K-0.05 at 0.28 nm−1 seems to be split into a small peak at the lower q and a large peak at the higher q on a profile of a hybrid: for example, peaks at 0.19 and 0.31 nm−1 for 4K-0.10. These imply coexistence of two or more regions of

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DISCUSSION Morphology of Hybrids Depending on the Chain Length of P4VP Blocks. As mentioned in the Introduction, there are several reports on morphologies of hybrids composed of a block copolymer and inorganic nanoparticles. Here, our study used a recently focused attractive interactions between hydrogen-bonding groups on nanoparticles and those on block copolymers, which were confirmed by FT-IR measurements as shown in Figure 3. Solvent casting with a polar solvent of DMF was also carried out for sufficient time, followed by annealing. These molecular design and careful solvent-casting procedure are probably the main reasons for selective incorporation of hCdSe into a P4VP phase as well as uniform and single morphology formation observed in 41K-Y and 22K-Y hybrids, where h-CdSe behaves like a filler or a swelling agent. The increase in domain spacing and morphology transition look quite natural, since these phenomena were also induced by the addition of homopolymers (or hydrogen-bonded homopolymers73,74) in block copolymer/homopolymer blends.69,70,71 To the contrary, 4K-Y hybrids with Y ≥ 0.10 brought macrophase separation with overflow of h-CdSe, i.e., nonuniform morphology formation. It is evident that the chain length of P4VP is the main difference among molecular characteristics 8017

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Figure 6. SAXS profiles of neat PS−P4VP(X) and the hybrids of X-Y: (a) neat PS−P4VP(41K) and 41K-Y hybrids; (b) neat PS−P4VP(22K) and 22K-Y hybrids; (c) neat PS−P4VP(4K) and 4K-Y hybrids.

hydrogen-bonding connection between PS−P4VP and h-CdSe should not work properly because of an inappropriate molar amount of pyridine groups for connecting. In fact, this inappropriate hydrogen-bonding connection between PS− P4VP and h-CdSe caused macrophase separation, where some h-CdSe, which was incorporated into a P4VP phase due to hydrogen bonding, induced morphological transition from spherical structure (Figure 5k,l) to cylindrical one (Figure 5m−o), while some excess h-CdSe became the aggregation, which was excluded from nanophase-separated domains (Figure 5q,r). This phenomenon was also seen in hybrids of h-CdSe and PS−P4VP bearing very short P4VP blocks (see also Figure S6). The macrophase separation induced by excess h-CdSe could also be associated with the discussion on entropic loss of molecular conformation at mixing of PS−P4VP(4K) and h-CdSe, which is explained by a relative size ratio of nanoparticle diameter to domain distance of a P4VP block.75−78

of hybrids; therefore, the short chain length of P4VP in PS− P4VP(4K) is probably the cause of nonuniform morphology formation. Although 4K-Y hybrids with Y ≥ 0.10 represent macrophase separation, it is natural that organic block copolymers and inorganic nanoparticles phase-separate macroscopically if we take into account of the large difference between those in their chemical nature, i.e., a repulsive force between those or enthalphic contributions on demixing. In our hybrids, however, both h-CdSe and PS−P4VP(X) have functional groups for hydrogen bonding. This attractive interaction induces uniform morphology formation in 41K-Y and 22K-Y hybrids because components were bound at the molecular level via hydrogen bonding, even if there generates a large repulsive force between components. But note that such connection by hydrogen bonding does not always hold; for instance, in the case that stoichiometry of functional groups for hydrogen bonding is out of balance. 4K-Y hybrids with Y ≥ 0.10 is probably the case because PS−P4VP(4K) has a small molar amount of pyridine groups for hydrogen bonding because of short P4VP blocks. Table 2 lists a stoichiometric number ratio of pyridine groups to hydroxy groups (npyridine/nOH) as well as a P4VP/h-CdSe weight ratio (wP4VP/wh‑CdSe), where the latter should be proportional to the former. (See also descriptions in the Supporting Information.) Although npyridine/nOH can include uncertainty because the estimation was based on the assumption that the density of ligands on the surface of an hCdSe core is the same as the bulk density of 2-mercaptoethanol (1.11 g/cm3), a stoichiometric threshold of pyridine vs alcohol in this study appears to be at around 3.5, below which

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SUMMARY In conclusion, we have implemented a systematic study on preparation and morphological observation of hybrids composed of a block copolymer and hydroxy-capped semiconductor nanoparticles via hydrogen bonding. Three PS− P4VP block copolymers were synthesized for hybrid preparation, whereas mercaptoethanol-capped CdSe nanoparticles (hCdSe) were used as a counterpart. Hydrogen bonding between hydroxy groups on h-CdSe and pyridine groups on P4VP blocks was confirmed by FT-IR measurements, and DSC analysis also found selective incorporation and homogeneous 8018

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dispersion of h-CdSe into a P4VP phase. Complementary morphology observations with TEM and SAXS revealed the following two results: formation of uniform morphology of a single nanophase separated structure in hybrids containing PS− P4VP with longer P4VP; nonuniform morphology formation, i.e., macrophase separation with splitting into two orderedstructures accompanied by overflow of h-CdSe in hybrids containing PS−P4VP with shorter P4VP. The former result includes domain spacing expansion and morphology transition, which is natural if h-CdSe behaves like a homopolymer in block copolymer/homopolymer blends. The latter result was probably induced by stoichiometric imbalance of functional groups for hydrogen bonding, which is originated from the short chain length of P4VP. All these results of this systematic study suggest beneficial guidelines for designing useful nanophase-separated hybrids with remarkably optical, electrical, and magnetic properties.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information for experiments; Figures S1−S6. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.M.); [email protected] (A.N.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Shigeo Arai at the Ecotopia Science Institute in Nagoya University for his help in TEM observations. The authors also thank Dr. Yuya Shinohara at the University of Tokyo and Mr. Tatsuo Hikage at High Intensity X-ray Diffraction Laboratory in Nagoya University for his assistance in SAXS measurements. The authors are grateful for financial support through the Grant-in-Aid (KAKENHI) projects (no. 22245038 (Y.M.), no. 23655123 (A.N.), and no. 24685035 (A.N.)) from JSPS. They also thank the Program for Leading Graduate Schools at Nagoya University entitled “Integrate Graduate Education and Research Program in Green Natural Sciences”. Use of synchrotron X-ray source was supported by Photon Factory, KEK in Tsukuba, Japan (no. 2010G59 and no. 2012G176 for A.N.)

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