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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Unique Hydrophobization and Hybridization via Direct Phase Transfer of ZrO2 Nanoparticles from Water to Toluene Producing Highly Transparent Polystyrene and Poly(methyl methacrylate) Hybrid Bulk Materials Kazushi Enomoto,† Yusuke Ichijo,† Masahiko Nakano,‡ Moriya Kikuchi,‡ Atsushi Narumi,§ Shin Horiuchi,∥ and Seigou Kawaguchi*,§ †

Department of Polymer Science and Engineering, Graduate School of Science and Engineering, ‡Faculty of Engineering, and Department of Organic Materials Science, Graduate School of Organic Materials Science, Yamagata University, 4-3-16, Jonan, Yonezawa 992-8510, Japan ∥ Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan §

S Supporting Information *

ABSTRACT: A versatile and promising organic−inorganic hybridization method is proposed for the fabrication of highly transparent polystyrene (PSt) and poly(methyl methacrylate) (PMMA) hybrid bulk materials containing highly crystalline ZrO2 nanoparticles (NPs) having a number-average diameter of 3.11 nm. The two key technologies that have been developed are the surface treatment, hydrophobization, and functionalization of ZrO2 NPs originally dispersed in water and their nanodispersion into a polymer continuous phase without any coagulation and/or agglomeration. A unique and fascinating surface treatment method is demonstrated in which the hydrophobization and phase transfer of ZrO2 NPs from water to toluene is simultaneously achieved. Transparent surface-modified ZrO2 nanodispersions in toluene are obtained by a gentle solvent exchange from a ternary solvent mixture composed of water, toluene, and methanol. The addition of carboxylic acids having more than four carbon atoms as the surface treatment agent enables this hydrophobization. The carboxylic acid-modified ZrO2 NPs prepared by the method are thoroughly characterized by small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), scanning TEM, TGA, and NMR, and IR spectroscopies. The surface-treated ZrO2 NP powders after drying possess the ability of redissolution or nanodispersion in several organic solvents and vinyl monomers. Further, optically transparent PSt and PMMA hybrid bulk materials with thicknesses of 10 mm are fabricated by the copolymerization of styrene (St) or methyl methacrylate (MMA) as a representative vinylic monomer with methacrylate-functionalized ZrO2 NPs as the multivinyl cross-linking agent. The optical properties of hybrid materials having a higher refractive index than that of the original homopolymers are measured and compared with the theories based on Fresnel refraction, Lorentz−Lorenz effective medium expansion, and Lambert−Beer and Rayleigh scattering equations. The present method provides promising candidates for different transparent hybrid materials consisting of inorganic and organic materials.

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INTRODUCTION Applications of organic materials are continuously increasing due to their advantages such as flexibility, miniaturization, lightweight, low cost, and easy mold fabrication. However, soft materials continue to trail behind inorganic materials from the point of views of thermal and chemical stabilities and physical © XXXX American Chemical Society

and electronic properties. Nevertheless, in optical applications, inorganic glasses have recently been replaced by a transparent Received: October 5, 2017 Revised: November 17, 2017

A

DOI: 10.1021/acs.macromol.7b02155 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules optical organic polymer. Organic polymers, however, have the disadvantage of low refractive indices because of the limited elements constituting the organic compounds. The improvement and/or control of optical properties such as refractive index and chromatic aberration have been one of the most attractive, continuing, interesting, and important issues in organic optical polymers in the past two decades.1−3 There have so far been two methods to improve the optical properties. First is a synthetic approach in which the elements with high atomic refractions, such as sulfur and halogens except for fluorine, aromatic groups with high molecular refractions, and/ or cyclohexane groups with small molar volumes, are introduced into a polymer chain. Many advanced sulfur-containing optical polymers with high refractive indices have so far been prepared.3 The second method is based on an organic−inorganic hybridization approach in which inorganic nanoparticles (NPs) with high refractive indices are dispersed into a polymer matrix, as described in this paper. Organic−inorganic hybrid materials are a highly functional advanced material in which organic and inorganic substances are mixed at a molecular or nanoscale level. Nowadays, synthetic progress in inorganic chemistry allows the production of myriad inorganic materials including NPs smaller than 10 nm,4−6 some of which are commercially available. Many transparent organic− inorganic nanohybrid thin films have been fabricated by taking advantage of interactions such as covalent bonding, hydrogen bonding, electrostatic interaction, and π−π interaction and by in situ sol−gel and polymerization reactions.7−15 The hybrids exhibit excellent thermal and mechanical properties, controllable electric and optical properties, and gas permeability.16 However, most of the studies were carried out for a silica-containing hybrid. When the filler size is much smaller than the visible wavelength, its light scattering effects (S) in hybrid materials may be discussed using the Rayleigh scattering theory.17,18 In case of spherical particle fillers, S is given by the following equation: ⎡ 4ϕpπ 4D3nm 4 ⎢ ⎢ S= λ4 ⎢ ⎢⎣

np

2

( ) ( ) nm np

nm

2

⎤2 − 1⎥ ⎥ + 2 ⎥⎥ ⎦

The transmissivity of hybrid materials can be estimated by eqs 1−3, and the results depend on the particle size, refractive index difference between the filler and matrix, and sample thickness. For example, when zirconia (ZrO2) NPs (np = 2.15,20 D = 10 nm) are nanodispersed in a poly(methyl methacrylate) (PMMA, nm = 1.49) matrix, the transmissivity (T) of the hybrid material (thickness (t) is 10 mm and volume fraction (ϕp) is 0.2) decreases to less than 10% at a λ of 589 nm. In contrast, when ZrO2 NPs with D = 4 nm are nanodispersed at the same t and ϕp, the T value is maintained at 79%. In these calculations, we assumed the nanodispersion of ZrO2 NPs in the polymer matrix, although the nanodispersion of inorganic particles with 4 nm diameter or less has not entirely been accomplished to date.21−26 Hence, an innovative and promising surface treatment and nanodispersion technique is required for the fabrication of highly transparent nanohybrid materials with high refractive indices. Titania (TiO2) and ZrO2 are representative inorganic NPs with high refractive indices. TiO2 is widely applied in paints, inks, milled papers, and cosmetics as a white pigment due to its high refractive indices, which are 2.72 (rutile type), 2.52 (anatase type), and 2.63 (brockite type).20 The dispersion of nanosized TiO2 into a polymer matrix may increase its refractive index, although photocatalytic action and coloration of TiO2 due to a high adsorption below 413.5 nm must be noted. In contrast, ZrO2 has a high refractive index (n = 2.1−2.2),20 excellent thermal and mechanical properties, chemical inertness, and potential for transparency in the near-ultraviolet range because of the absorption edge at 248 nm. Nanohybrid optical materials using ZrO2 NPs would be expected to have a high transparency in a wide range of the visible spectrum. Several works on the hybrid materials of ZrO2 NPs with polymers have been reported over the past decade.27−36 However, the reports for the fabrication of transparent hybrid bulk materials are very limited. Peng et al.37 reported 1 mm thick hybrid materials composed of ZrO2 NPs treated with silane coupling agent and epoxy resin. The refractive index was increased from 1.51 to 1.65 at 600 nm. Yong et al.38 reported the preparation of 2 mm thick hybrid materials composed of silicone resin and ZrO2 NPs by in situ sol−gel and polymerization reactions. The refractive index was increased from 1.578 to 1.583 at 633 nm. Quite recently, Liu et al.39 reported the easy syntheses of ZrO2 and TiO2 NPs using the hydrogen-bonding-assisted amidization reaction and the hybridization of the bifunctional methacrylatemodified ZrO2 with a mixture of methylstyrene and divinylbenzene. The contents of ZrO2 NPs in 1 mm thick material were increased up to 21.5 vol % to increase the refractive index to 1.652 at a λ of 589 nm, where the increment was 0.097. Additionally, the application of organic−inorganic hybrid materials in the area of photonic crystals and quantum dots has recently been reported.40−42 Most recently, Hühn et al.43 reported detailed preparation procedures for several types of inorganic NPs in organic solvent or aqueous solution, their surface functionalization protocol between hydrophilic and hydrophobic media using ligand exchange or polymer coating technique, and purification and characterization techniques of the obtained NP dispersions. Many types of inorganic NPs are recently commercially available as aqueous dispersions. However, they are unstable or highly sensitive to environment due to their high surface area or strong interaction among them. As shown in Scheme 1 as phase II, once the inorganic NP dispersions dry, it is almost impossible to untangle the coagulated NPs to their original

(1)

where ϕp is the volume fraction of fillers and D is their diameter, nm and np are the refractive indices of the matrix and particle, respectively, and λ is the wavelength of light. This equation has been exclusively applied for the estimation of transmissivity of hybrid materials. On the other hand, the effect of Fresnel refraction (F) from the interface between the air and material on the transmissivity has to be considered as well. When a vertical incident light passes through a flat hybrid thick material, the intensity of the incident light is decreased by F, S, and F in the order from the front to the back of the material. Therefore, the actual transmissivity (T) of hybrid materials is expressed by ⎧ 4n ⎫2 ⎬ exp( −St ) T=⎨ ⎩ (n + 1)2 ⎭

(2)

where t is the sample thickness and n is the refractive index of the hybrid material, which is calculated by the Lorentz−Lorenz medium theory expressed by19 np2 − 1 n 2−1 n2 − 1 = ϕ + ϕm m2 p 2 2 n +2 np + 2 nm + 2

(3) B


Article

Macromolecules

4-phenoxybenzoic acid (>98.0%, p-PNBA) were purchased from Tokyo Chemical Industry Co., Ltd., Tokyo, Japan. Methanol (>99.5%), acetone (>99.0%), chloroform (>99.0%), 2-propanol (>99.7%), isobutylaldehyde (>97.0%), 2-(dimethylamino)ethyl methacrylate (>99.0%), acetic acid (>99.7%), propionic acid (>98.0%), hexadecanoic acid (>95.0%, HDA), octadecanoic acid (>95.0%, ODA), cis-9-octadecenoic acid (>65.0%, c-ODA), and dodecylphosphonic acid (>95.0%) were purchased from Wako Pure Chemical Industries, Ltd., Osaka, Japan. 3-Mercaptopropyltrimethoxysilane (>98.0%) was purchased from Dow Corning Toray Co., Ltd., Osaka, Japan, and 1-hydroxycyclohexyl phenyl ketone (>99.0%, IRUGACURE 184) was purchased from Sigma-Aldrich Co., LLC., Darmstadt, Germany. All the chemicals mentioned above were used as received. 3-Methacryloxypropyltrimethoxysilane (Dow Corning Toray Co., Ltd., Osaka, Japan), acrylic acid (Kanto Chemical Co., Tokyo, Japan, >97.0%), methacrylic acid (MA, Wako Pure Chemical Industries, Ltd., Osaka, Japan, >99.0%), and methyl methacrylate (MMA, Wako Pure Chemical Industries, Ltd., Osaka, Japan, >98.0%) were used after distillation under reduced pressure with CaH2. Styrene (St, Wako Pure Chemical Industries, Osaka, Japan, >99.0%) was purified by washing with a sodium hydroxide solution and saturated NaCl solution, then dried with calcium chloride, and distilled over CaH2 under reduced pressure. In Situ Hydrophobization of ZrO2 NPs via Phase Transfer from Water to Toluene. The typical surface modification procedure is described as follows. HA as a surface modifier (0.17 g, the weight fraction of the surface modifier in the feed, WF,M = 23 wt %) was added into a round-bottom flask fitted with a magnetic stir bar and diluted with a solvent mixture of methanol (15.0 mL) and toluene (0.5 mL). The aqueous dispersion of ZrO2 NPs (5.0 g, 0.57 g of ZrO2 NPs) was slowly added to the surface modifier solution under stirring. The slightly turbid dispersion was stirred for 2 h at room temperature and gently concentrated to about 5 mL with a rotary evaporator. Solvent mixtures of different volume ratios of methanol to toluene (changing from 30 to 0%) were added to the dispersion until it became transparent. Thereafter, the dispersion was again concentrated to about 5 mL. The procedure was repeated 5−6 times to remove water by azeotropic distillation, which eventually yielded a transparent toluene dispersion of ZrO2 NPs. The toluene dispersion was further evaporated and dried in vacuum at room temperature until it reached a constant weight. The white solid substance was obtained with a yield of 0.68 g and used for the solubility test and hybridization. The surface-modified ZrO2 NPs were further purified three times with acetone to remove free surface modifier. Structural characterizations of the purified ZrO2 NPs were conducted by Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), 1H NMR spectroscopy, and solidstate 13C CP/MAS NMR spectroscopy. Hybridization of ZrO2 NP with PSt or PMMA. A series of nanohybrid materials composed of ZrO2 NPs and PSt (ZrO2-PSt) were prepared by a typical catalyst-free thermal bulk (co)polymerization method. The ZrO2 NPs surface-modified using both HA and MA (HA/MA-ZrO2, 0.5 g) were dissolved (nanodispersed) into St (1.2 g, 11.5 mmol) and added to a test tube with an inner diameter of 10 mm. The transparent dispersions were bubbled with nitrogen for 15 min in an ice bath to remove oxygen. The thermal polymerization of St was conducted in an oil bath at 120 °C for 48 h. The insoluble fraction, i.e., the gel fraction (fgel), of ZrO2-PSt hybrids was determined as follows. A piece of sample (wB in weight) was washed with toluene using the Soxhlet apparatus for 24 h to remove un-cross-linked PSt and ZrO2 NPs. The gel fraction (fgel = wA/wB × 100) was then estimated by a gravimetric method using the weight ratios of ZrO2-PSt hybrids before and after the extraction with toluene, that is, wB and wA, respectively. ZrO2-PMMA hybrids were prepared by photopolymerization with 1-hydroxycyclohexyl phenyl ketone (IRUGACURE 184) as a photoinitiator at room temperature. The HA/MA-ZrO2 NPs (0.13 g) and photoinitiator (9 mg, 45.1 μmol) were dissolved into the MMA monomer (0.30 g, 3.0 mmol) to obtain a homogeneous dispersion. After deoxygenation by nitrogen bubbling in an ice bath for 15 min, the dispersion was injected into the cell of a Teflon spacer (1.0 mm thickness) sandwiched by two glass slides. The photopolymerization

Scheme 1. Surface Treatment Method Developed in This Study

sizes even in water. In addition, it is extremely difficult to nanodisperse them into organic media or polymer matrices without any aggregation. NPs are easily aggregated by solvent exchange from water to organic media (phase III), and this process is irreversible in most instances. In addition, since most of the organic polymers are generally immiscible in water but miscible in organic media, they have no compatibility with waterdispersible inorganic NPs. The introduction of an appropriate surface modifier onto the surface of NPs can be considered a possible way to nanodisperse them into different types of monomers to obtain organic−inorganic hybridization. Therefore, one of the most important approaches is to establish a surface modification technique to render the NPs nanodispersible in organic solvents and to ensure reversibility between the IV phase and V phase. To address this crucial issue, our basic idea for hybridization is concerned with strategies for surface treatment, hydrophobization, and functionalization of ZrO2 NPs originally dispersed in water and their nanodispersion into a polymer continuous phase without any coagulation and/or agglomeration. On the basis of this concept, we have developed a unique, versatile, fascinating, and promising surface modification method (gradual and gentle solventexchange method) for aqueous ZrO2 NP dispersion. The present paper reports in detail the surface modification method of the originally water-dispersible ZrO2 NPs, in which the hydrophobization and phase transfer of ZrO2 NPs from water to toluene are simultaneously conducted. In addition, detailed characterizations of the surface-treated ZrO2 NPs and the hybrids as well as the optical properties are reported.

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EXPERIMENTAL SECTION

Materials. An aqueous transparent dispersion of ZrO2 NPs (n = 2.1−2.220) with an average diameter of D ≈ 3.1 nm including a small amount of acetic acid (solid content of 11.4 wt %, pH = 3.04) was purchased from Sumitomo Osaka Cement Co., Ltd., Tokyo, Japan. Toluene (>99.0%), n-hexane (>95.0%), tetrahydrofuran (>99.0%, THF), methyl ethyl ketone (>99.0%, MEK), dichloromethane (>99.0%), acetonitrile (>99.0%), N,N-dimethylformamide (>99.0%, DMF), dodecylbenzenesulfonic acid (>90.0%), butyric acid (>99.0%, BA), isobutyric acid (>98.0%), hexanoic acid (>94.0%, HA), octanoic acid (>98.0%, OA), dodecanoic acid (>97.0%, DA), and calcium hydride (>95.0%, CaH2 ) were purchased from Kanto Chemical Co., Tokyo, Japan. 1-Butanol, 1-hexanol (>99.0%), n-hexylamine (>96.0%), 4-methylbenzoic acid (>98.0%, p-MBA), and C


Article

Macromolecules was carried out using an Ushio Optical ModuleX SX-U1251HQ (Ushio Inc., Tokyo, Japan) with an extra-high-pressure mercury lamp (40 mW/cm2) for 30 min. Measurements. Spectroscopic grade solventshexane, toluene, THF, and chloroformwere purchased from Kanto Chemical Co., Tokyo, Japan, and used as received. The dynamic light scattering (DLS) experiment was carried out using a DLS-7000 analyzer equipped with a photon correlator, GC-1000 (Otsuka Electronics Co., Ltd., Osaka, Japan), and a 10 mW He−Ne laser (wavelength, λ = 632.8 nm) at a scattering angle of 90° and temperature of 25 °C. The dispersions of ZrO2 NPs with weight concentration above 10 mg cm−3 were filtered through a hydrophilized PTFE filter with a pore size of 0.45 μm and then directly poured into a cylindrical quartz cell with an inner diameter of 9 mm. Evaluation of the relaxation spectrum A(τ) by CONTIN analysis and determination of the hydrodynamic diameter (DH) have been described elsewhere.44 Further, small-angle X-ray scattering (SAXS) measurements were conducted at 25 °C using a NANO-Viewer (Rigaku Co., Tokyo, Japan) as the X-ray source. The wavelength of the X-rays was 0.154 nm, and the sample-todetector distance was 650 mm. The scattering vector, q, defined as 4π sin(θ)/λ, with 2θ being the scattering angle, was determined from the Bragg reflection of powdered lead stearate. The scattering intensity, I(q), was detected using a high-speed 2D X-ray detector (PILATUS 100 K; DECTRIS Ltd., Baden, Switzerland) with a detector size of 487 × 195 pixels, covering the range of the scattering vectors from 0.1 to 1.0 nm−1. The excess scattering intensities, ΔI(q), were obtained as the difference between the I(q) values of the solvent and solution at the same q, taking into account the X-ray transmittance. Fourier transform infrared spectroscopy (FT-IR) was performed with a FT-720 spectrometer (HORIBA Ltd., Kyoto, Japan) using the KBr method. The resolution was 4 cm−1, and the accumulation time was 64. 1H, 13 C, and solid state 13C CP/MAS NMR spectra were recorded using a JNM-ECX400 spectrometer (JEOL Ltd., Tokyo, Japan). Thermal gravimetric analysis (TGA) was performed using a TGA4000 thermogravimetric analyzer (PerkinElmer, Ltd., Waltham, MA). Samples of approximately 3 mg were heated to 800 °C at a heating rate of 10 °C min−1 under a nitrogen flow of 20 mL min−1. To characterize the crystal structure of ZrO2 NPs, X-ray diffraction (XRD) measurements were performed using an Ultima IV diffractometer (Rigaku Co., Tokyo, Japan) at an incident X-ray wavelength of 0.154 nm. The optical properties of the hybrid materials were measured by using a multiwavelength Abbe refractometer (DR-M4, ATAGO Co., Ltd., Tokyo, Japan) at 25 ± 1 °C. An optically polished sample was placed on the prism, and its refractive index was determined from the total refraction at the interface between the sample and prism, where a mixture of sulfur and methylene iodide was used as the contact liquid (RE-1199, nD = 1.78). Interference filters (486 nm for nF, 589 nm for nD, and 656 nm for nC) were used for the determination of refractive index at each wavelength, and Abbe’s number (νD) was evaluated using the following equation:

n −1 νD = D nF − nC

cured with an SLS-150T light source (Sumita Optical Glass, Co., Saitama, Japan). The prepared sample was cut using an Ultramicrotome (Leica Microsystems, Co., Germany) equipped with a trimming diamond knife, Cryotrim 45 (DiATOME Co., Switzerland), into a trapezial shape. Finally, the sample was sliced by a diamond knife, Ultra (DiATOME Co., Switzerland), to 20 nm thickness, and the ultrathin section was placed on a Cu grid, Maxtaform grid HF36 400 mesh (NISSHIN EM Co., Japan), for TEM observation. The densities of the aqueous dispersion of ZrO2 NPs (ρdispersion) were measured using a DMA 4500 densimeter (Anton Paar GmbH, Austria) at 25 °C to determine the density of ZrO2 NPs, ρZrO2. The master dispersion of ZrO2 NPs was filtered through a cellulose acetate filter with a pore size of 0.45 μm. Different concentrations of aqueous dispersions of ZrO2 NPs were prepared by a gravimetric method. The weight fraction of the master dispersion of ZrO2 NPs (WZrO2) was determined by freeze-drying, followed by drying under reduced pressure at 40 °C until a constant weight was achieved. The net ZrO2 content was determined from the weight loss due to volatile components (H2O and acetic acid), which was determined using TGA until a constant weight was achieved at 250 °C. Assuming the ZrO2 NPs to be a rigid body, the density of the aqueous dispersion, ρdispersion, is expressed by

⎛ ρ ⎞ ρdispersion = ρs + ⎜⎜1 − s ⎟⎟C ZrO2 ρZrO ⎠ ⎝ 2

(5)

where ρs and CZrO2 are the density of water (0.997 05 g cm and the ZrO2 NP concentration (g cm−3), respectively.

−3

at 25 °C)

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RESULTS AND DISCUSSION Characterizations of ZrO2 NPs. A TEM photograph of ZrO2 NPs used in this study is presented in Figure 1, together

Figure 1. TEM and high-resolution TEM (HRTEM) (inset) photographs of ZrO2 NPs used in this study.

(4)

with a high-resolution (HRTEM) image as an inset, which shows single nanosized and monocrystalline ZrO2 NPs. The size distribution of ZrO2 NPs was determined by counting the size of 1000 NPs and is presented in Figure 2. Further, the number (Dn), volume (Dv), weight (Dw), z (Dz), and z + 3 (Dz+3) average diameters, standard deviation (σ), and coefficient of variation (Cv) were determined and are listed in Table 1. The XRD pattern of freeze-dried ZrO2 NPs is presented in Figure S1. The peak positions and intensity profile of ZrO2 NPs are in good agreement with those for fluorite-type ZrO2.46 The SAXS profile of a dilute aqueous solution (dispersion) of ZrO2 NPs (WZrO2 = 0.1 wt %) at 25 °C is presented in Figure 3, in which the βΔI(q)/ΔI(0) value vs q is shown as β as a shift coefficient of the longitudinal axis and as ΔI(0) since ΔI(q) at q = 0. The experimental scattering function of ΔI(q)/ΔI(0)

The surface-modified ZrO2 NPs were visualized using a scanning transmission electron microscope (STEM). The dispersion of purified HA-ZrO2 NPs in hexane (0.05 w/w) was dropped on a silicon grid (SiMPore Ultra SMTEM) and vacuum-dried for several hours. Thereafter, energy-dispersive X-ray spectrometry (EDX) measurements were performed using a STEM equipped with four SDD detectors (TECNAI OSIRIS, FEI Co., USA) with an accelerating voltage of 200 kV and acquisition time of 5 min. The elemental ratios of C and Zr in selected regions were determined from the collected energy spectra using the Cliff−Lorimer method.45 Morphologies of the hybrid materials were evaluated from the TEM images of an ultrathin section. TEM observation was conducted with a field emission transmission electron microscope (JEM-2100, JEOL Ltd., Tokyo, Japan) under 200 kV irradiation voltages. The ultrathin sections were prepared by the following procedure. A piece of hybrid material was embedded in light-curable resin, ARONIX (Toagosei, Ltd., Japan), and D


Article

Macromolecules

Pz(qDn), for the Schulz-distributed polydispersed spheres is expressed by47 576ω! (ω + 1)6 ⎧ 1 1⎛ω + 2⎞ 2 ⎟(qD ) ⎨ + ⎜ n 6 8⎝ ω + 1⎠ (qDn) (ω + 1)! ⎩ 2 ⎡ ⎛ qD ⎞⎤⎫ + ⎢G(qDn)(ω+ 1)/2 Q ⎜ n ⎟⎥⎬ ⎝ 2 ⎠⎦⎭ ⎣

Pz(qDn) =

where the functions G(qDn), Q(qDn/2), and F(qDn) are given by

Figure 2. Size distribution of ZrO2 NPs estimated by measuring the size of 103 NPs. Red curve shows the Schulz distribution function calculated by eq 6 with parameters Dn = 3.11 nm and ω = 15.

G(qDn) =

Table 1. Characteristics of ZrO2 NPs Used in This Study, Determined from TEM and SAXS Measurements method a

TEM SAXS

Dn (nm)

Dv (nm)

Dw (nm)

Dz (nm)

Dz+3 (nm)

b

c

b

b

b

3.11 3.11

3.30

3.30 3.30g

3.50 3.50g

4.06 4.08g

σ (nm) d

0.78 0.78f

Cve (%)

× sin[(ω + 2)F(qDn)] +

25.2 25.0

Determined by counting 103 ZrO2 NPs. bnth-order average diameter calculated by Dn = ∑NiDin/∑NiDin−1, where n is 1 for Dn, 2 for Dw, 3 for Dz, and 6 for Dz+3. cDv = {∑NiDi3/∑Ni}1/3. dσ = {∑(Di − Dn)2/ ∑Ni}1/2. eCv = (σ/Dn) × 100. fσ = Dn(1/(ω + 1))1/2. gDw, Dz, and Dz+3 were estimated from Dn value using the relation Dj = [(ω + n)/ (ω + 1)]Dn, where n = 2 for j = w, n = 3 for j = z, and n = 6 for j = z + 3.

⎡ (ω + 1)D ⎤ Dω exp⎢ − ⎥ Dn ⎣ ⎦

(qDn)2 ⎛ ω + 2 ⎞ ⎜ ⎟G(qD ) n 2 ⎝ ω + 1⎠

× cos[(ω + 3)F(qDn)]

(9)

⎛ qDn ⎞ ⎟ F(qDn) = arctan⎜ ⎝ ω + 1⎠

(10)

The black line for ZrO2 NPs in water in Figure 3 is the theoretical curve calculated from eqs 7−10 using two parameters, Dn = 3.11 nm and ω = 15, explaining the experimental SAXS profile in the region of q < 0.4 nm−1. However, the experimental data show a significant downward deviation from the theoretical curve with increasing q. This deviation possibly arose from the assumption that ZrO2 NPs are rigid spheres with smooth surfaces. The surfaces of crystalline inorganic NPs must have surface roughnesses due to the lattice imperfection at the surface, as will be discussed below. The influence of interface roughness on the form factor may be introduced by multiplying the Gaussian factor, exp(−δ2q2/2), with Pz(qDn), where δ is the thickness of the average interface roughness.48 The red line for ZrO2 NPs in water in Figure 3 is the theoretical curve calculated with the parameters Dn = 3.11 nm, ω = 15, and δ = 0.9 nm and almost perfectly describes the experimental scattering function of ZrO2 NPs in the region of q < 2 nm−1. The Dw, Dz, and Dz+3 values estimated by SAXS measurements are also listed in Table 1. The density of ZrO2 NPs, ρZrO2, determined at 25 °C using eq 5 from the slope in the plot of ρdispersion versus CZrO2 was 5.2 ± 0.1 g cm−3, as shown in Figure S2. For ZrO2 NPs, this value seems to be significantly smaller than the theoretical value of 6.05 g cm−3 for monocrystalline cubic ZrO2.46 A feasible explanation is the increase in surface area and lattice defects at the surface with decrease in the size of ZrO2 NPs. A simple model calculation based on the assumption of diamond shape-growth of NP as a function of size is presented in the Supporting Information. The ratio of Zr atoms existing at the surface to the total Zr atoms steeply increases, and the density, in turn, steeply decreases with decreasing size, especially for a size less than 10 nm, as shown in Figure S3b. This experimental result is a first example for the determination of the density of NPs in water, which is considerably lower than that of bulk. The hydrodynamic diameter (DH) of ZrO2 NPs in water at 25 °C was determined as 7 nm from DLS measurements.

versus q in a diluted dispersion shows an intramolecular scattering factor of ZrO2 NP, P(q). It may be interesting to compare it with the theoretical one for scattering bodies composed of polydispersed spheres. The Schulz distribution is considered the size distribution of the NPs. The Schulz distribution function, fω(D), of the particles with numberaverage diameter, Dn, and width of the distribution, ω (ω = 0, 1, 2, ...), is given by47 1⎞ ⎟ ⎠

(8)

and

Figure 3. SAXS profiles of pure ZrO2 NPs in water (WZrO2 = 0.1 wt %) and HA-ZrO2 in hexane (WHA‑ZrO2 = 0.1 wt %) at 25 °C. The black line represents the theoretical values for Schulz-distributed spherical particles with Dn = 3.11 nm and ω = 15. The red lines are the theoretical values for the Schulz-distributed spherical particles (Dn = 3.11 nm and ω = 15) with a surface roughness thickness of δ = 0.9 nm.

1 ⎛ω + ⎜ ω! ⎝ Dn

(ω + 1)2 (ω + 1)2 + (qDn)2

⎛ qD ⎞ qD 1 Q ⎜ n ⎟ = − cos[(ω + 1)F(qDn)] − n G(qDn)1/2 ⎝ 2 ⎠ 2 2

a

fω (D) =

(7)

ω+1

(6)

The ω value is estimated to be 15 from the relation σ = Dn/ (ω + 1)1/2, in which σ is the standard deviation (σ = 0.78 nm) and Dn = 3.11 nm, which were determined from TEM measurements (Table 1). The experimental size distribution is described by the Schulz distribution function, as indicated by the red line in Figure 2. The z-average scattering function, E


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Macromolecules

as surface modifiers with the weight fraction in feed, WF,M, ranging from 9 to 95 wt %. The results are summarized in Table 2. When silane coupling agents, alcohols, aldehydes, and amines were used as the surface modifier, heterogeneous and/ or turbid toluene dispersions were obtained at all the WF,M values, as shown in Figure S4. In remarkable contrast, when acids such as dodecylphosphonic acid (WF,M = 50 wt %), dodecylbenzenesulfonic acid (WF,M = 17 wt %), and dodecanoic acids (DA) were used, transparent toluene solutions (nanodispersions) were obtained from the original aqueous ZrO2 dispersion. These results imply that the modifiers having both an acid group and alkyl chain are suitable for the present hydrophobization procedure. Thus, we examined in detail with a series of carboxylic acid derivatives with different numbers of carbon. When acetic, propionic, and acrylic acids (WF,M < 95 wt %) were used, in which the number of carbon is less than 4, heterogeneous toluene dispersions were produced. However, when carboxylic acids with more than four carbons (WF,M ≥ 23 wt %) were used, transparent toluene nanodispersions were obtained, as shown in Figure S5, irrespective of the presence of branching and double bond and/or aliphatic and aromatic compounds. The use of carboxylic acid having a longer alkyl chain allows one to produce a transparent toluene nanodispersion in less quantity. DLS measurements were performed for the resultant homogeneous toluene dispersions, which indicated that the ZrO2 NPs form stable nanodispersions in toluene with DH ranging from 8 to 14 nm. The toluene dispersions were very stable and did not produce any aggregation for several months. The obtained toluene solutions (nanodispersions) were further dried under vacuum to produce solid white powders of surface-modified ZrO2 NPs, and their resolubility (re-nanodispersibility) tests were conducted. It is well-known that dried NPs are very difficult to re-nanodisperse

Figure 4. Relaxation spectra A(τ) plotted against DH of HA-ZrO2 NPs renanodispersed into organic solvents and aqueous dispersion of original ZrO2 NPs.

The relaxation spectrum is presented in Figure 4, which shows a size distribution broader than that estimated by TEM. Moreover, the DH value is considerably larger than the Dz+3 values determined by TEM and SAXS. The difference between the values possibly originated from the surface roughness, strong interactions between ZrO2 NPs, dynamic weak aggregation, and hydrated water molecules. Hydrophobization of ZrO2 NPs via Phase Transfer from Water to Toluene. Several types of surface treatment agents were examined to determine the chemical nature suitable for a hydrophobization agent of ZrO2 NPs originally nanodispersed in water via phase transfer from water to toluene without any strong agglomeration. Our idea is based on a gentle, tender, and ZrO2-NP-friendly environment change during hydrophobization because NPs have an inherent tendency to aggregate and reduce the surface area under environmental stress. Silane coupling agents, amines, alcohols, aldehyde, phosphoric acid, sulfonic acid, and carboxylic acids were examined

Table 2. Hydrophobization Results of Aqueous ZrO2 NPs Using Different Weight Fractions (WF,M) of Various Surface Modifiersa WF,M (wt %) functional group silane alcohol

aldehyde amine phosphoric acid sulfonic acid carboxylic acid

a

modifier

carbon no.

9

17

23

33

50

95

(3-mercaptopropyl)trimethoxysilane 3-methacryloxypropyltrimethoxysilane 2-propanol 1-butanol 1-hexanol isobutylaldehyde n-hexylamine 2-(dimethylamino)ethyl methacrylate dodecylphosphonic acid dodecylbenzenesulfonic acid acetic acid propionic acid acrylic acid butyric acid (BA) isobutyric acid methacrylic acid (MA) hexanoic acid (HA) octanoic acid (OA) dodecanoic acid (DA) hexadecanoic acid (HDA) octadecanoic acid (ODA) cis-9-octadecenoic acid (cODA)

6 10 3 4 6 5 6 8 12 18 2 3 3 4 4 4 6 8 12 16 18 18

× × × × × × × × × × × × × × × × × × × × × ×

× × × × × × × × × ○ × × × × × × ○ ○ ○ ○ ○ ○

× × × × × × × × × ○ × × × ○ ○ ○ ○ ○ ○ ○ ○ ○

× × × × × × × × × ○ × × × ○ ○ ○ ○ ○ ○ ○ ○ ○

× × × × × × × × ○ ○ × × × ○ ○ ○ ○ ○ ○ ○ ○ ○

× ○ × × × × × × ○ ○ × ○ × ○ ○ ○ ○ ○ ○ ○ ○ ○

Symbols: (○) transparent ZrO2 NPs toluene dispersion; (×) turbid toluene dispersion. F


Article

Macromolecules Table 3. Results of Re-Nanodispersibilities of Surface-Modified ZrO2 NPs in Diverse Solventsa

a

solvent

SPb (MPa)1/2

pure ZrO2

BAc

MAc

HAd

OAd

DAd

HDAd

ODAd

c-ODAc

p-MBAc

p-PNBAc

n-hexane MMA toluene THF styrene chloroform MEK dichloromethane acetone DMF 2-propanol acetonitrile methanol water

14.9 18.0 18.2 18.6 19.0 19.0 19.0 19.8 20.3 21.7 23.5 24.3 29.7 47.9

× × × × × × × × × × × × × ×

□ □ ○ ○ ○ ○ □ ○ □ × □ × × ×

× × ○ ○ □ ○ □ □ □ × × × × ×

○ □ ○ ○ ○ ○ □ ○ □ × × □ × ×

□ ○ ○ ○ ○ ○ × ○ × × × × × ×

□ □ ○ ○ ○ ○ × ○ × × × × × ×

□ ○ ○ ○ ○ ○ × ○ × × × × × ×

□ ○ ○ ○ ○ ○ × ○ × × × × × ×

○ × ○ ○ ○ ○ × ○ × × × × × ×

× ○ ○ ○ ○ ○ ○ ○ × ○ × □ × ×

× ○ ○ ○ ○ ○ □ ○ × ○ × × × ×

Symbols: (○) nanodispersion; (□) nanodispersion but partially suspension; (×) no dispersion. bSolubility parameter. cWF,M = 23 wt %. WF,M = 17 wt %.

d

demonstrate advantages of the present hydrophobization process. In the first experiment, HA (WF,M = 23 wt %) was directly added to the original ZrO2 NP water dispersion, intensively stirred for 2 h at room temperature, and then freezedried to produce a solid powder. In the second experiment, toluene and HA were added to the water dispersion, and the reaction mixture was refluxed for 1 day followed by evaporation. The ZrO2 NP powders surface-modified by these methods did not fully renanodisperse into any solvent. Characterization of Surface-Modified ZrO2 NPs. The 1 H NMR spectra of HA-ZrO2 NPs and HA in CDCl3 are presented in Figure S6. Considerably broad peaks due to the protons of HA are observed in the spectra of HA-ZrO2 NPs. In addition, no peak is observed in the range from 10 to 12 ppm due to the acid proton of HA. The methylene protons (b) right next to the α-carboxyl group produced much broader peaks than the distant ω-methyl protons (e) did, suggesting that the carboxyl group as a head is physically adsorbed and/or chemically bound to the surface of the ZrO2 NPs and strongly restricted the motion. The FT-IR spectra of calcined ZrO2 (top), HA-ZrO2 (WF,M = 17 wt %) (middle), and HA (bottom) are shown in Figure 5. The peak ascribed to the carboxyl group (μCO) of HA at 1710 cm−1 completely disappears, and peaks due to carboxylate, asymmetric stretching vibrations (μCOO−asym), and symmetric vibrations (μCOO−sym) newly appear at 1554 and 1463 cm−1, respectively. Two interesting observations to be noted are the differences between the extinction coefficients and the wavenumbers of μCOO−asym and μCOO−sym. The extinction coefficient of the asymmetric stretching vibration in a typical carboxylate anion is usually much higher than that of the symmetric one.49 As shown in Figure 5, however, these values in HA-ZrO2 are almost similar, implying that the carboxylate is not the carboxylate anion. Another point is the differences between their wavenumbers. A typical carboxylate anion has a difference of approximately 150 cm−1. The fact that the difference is 91 cm−1 in HA-ZrO2 implies that the binding fashion of HA molecule to the surface of the ZrO2 NPs is of bidentate type,50 which is shown as insets in Figure 5 and Figure S6. To directly confirm the existence of HA at the surfaces of ZrO2 NPs, STEM observation was performed. Figure 6a shows an image of a high-angle annular dark field (HAADF) STEM image in which a NP with a diameter of 4 nm is observed. Figures 6b and 6c show the EDX elemental maps of Zr and C

up to the original primary particle size. Indeed, freeze-dried original ZrO2 NPs are not re-nanodispersible in any solvent, as shown in Table 3. Interestingly, the surface-modified ZrO2 NPs with carboxylic acids containing more than four carbon atoms spontaneously re-nanodispersed (redissolved) into not only toluene but also various organic solvents such as THF, chloroform, and dichloromethane, and vinyl monomers such as MMA and St. DLS and SAXS measurements were performed to confirm the renanodispersion. Figure 4 displays the curves of A(τ) against DH for the re-nanodispersion of hexanoic acid-modified ZrO2 NPs (HA-ZrO2) in different organic solvents such as hexane, toluene, THF, and chloroform. All the curves exhibit a unimodal distribution with an average DH value of 7−9 nm. The size is in good agreement not only with that of the original ZrO2 NPs in water (7 nm) within experimental error but also with the values deduced from the core−shell model consisting of 3.11 nm core surrounded with alkyl chains. The SAXS profile of HA-ZrO2 NPs in hexane is also presented in Figure 3. The experimental form factor in hexane is quantitatively described in terms of the theoretical curve (red line) with the same parameters as those for water: Dn = 3.11 nm, ω = 15, and δ = 0.9 nm. These results strongly support the spontaneous renanodispersion (redissolution) of ZrO2 NP powders. It may be fair to discuss several important features of the present hydrophobization process via phase transfer from water to toluene. Since water is originally immiscible with toluene, the direct phase transfer of ZrO2 NPs from water to toluene does not take place even in the presence of an appropriate carboxylic acid. Moreover, the direct addition of carboxylic acids to the aqueous ZrO2 NP dispersion induced significant agglomeration. The tertiary compatibilizer solvent between water and toluene, i.e., methanol, plays an important role in this process. The interface between water and toluene disappears with the addition of excess amount of methanol. Further, ethanol and 2-propanol were examined as a compatibilizer in this process, which successfully yielded transparent toluene dispersions. Another noteworthy feature is the gentle and gradual solventexchange via azeotropic distillation of a mixture of water, methanol, and toluene. This process prevents the intense coagulation among ZrO2 NPs during hydrophobization and gradually alters the originally water-dispersible NPs to toluenedispersible ones. Two control experiments were conducted to G


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Macromolecules

WF,M = 23 wt %, while the mole fraction of MA in the feed, xF,MA, was increased from 0 to 1.0, as listed in Table 4. Table 4. Re-Nanodispersibility Test and Characterization of Purified HA/MA-ZrO2 NPs renanodispersibilitya 1

Figure 5. FT-IR spectra of calcined ZrO2 NPs, HA-ZrO2 NPs, and HA. The schematic cross-sectional view of the binding fashion between carboxylic acid and the surface of ZrO2 NP is presented in the inset.

A (nm2 molecule−1)c

xHA

run

xF,HA

St

MMA

H NMR

1-1 1-2 1-3 1-4 1-5 1-6

1 0.75 0.53 0.33 0.16 0

○ ○ ○ ○ ○ ○

□ □ ○ ○ × ×

0.77 0.50 0.56 0.40

13 C CP/ MAS

0.75 0.53 0.44 0.28

WMb (wt %)

H NMR

13 C CP/ MAS

16.3 16.2 11.4 13.1 10.3 8.9

0.31 0.29 0.41 0.36 0.45 0.46

0.31 0.29 0.41 0.34 0.43 0.46

1

a Symbols: (○) nanodispersion; (□) nanodispersion but partially suspension; (×) aggregation. bDetermined from the weight loss of purified HA/MA-ZrO2 NPs. cThe area occupied by a modifier on ZrO2 NP surface is calculated from eq 11.

The surface-modified ZrO2 NPs were purified three times by washing with acetone to remove the unbounded (free) modifiers. The net quantity (WM) of surface modifier exactly bound to the ZrO2 NP surface was then determined by TGA measurements and is listed in Table 4. The WM values are significantly lower than the WF,M (23 wt %) in the feed and decreases as xF,MA increases. This is most likely because some parts of carboxylic acids are not exactly bound to the ZrO2 NP surface and/or evaporated during the hydrophobization process. The boiling points of MA and HA are 161 and 205 °C, respectively. The FT-IR spectra of HA/MA-ZrO2 NPs showed peaks corresponding to MA and HA and revealed that the acid groups were bound in the bidentate form (Figure S7), similar to the HA-ZrO2 system (Figure 5). The 1H NMR spectrum of the HA/MA-ZrO2 system (xF,HA = 0.53) is shown in Figure S8. The broad signals due to the vinyl protons of MA reasonably appeared at 5.4 and 6.0 ppm together with the protons ascribed to HA at 0.9−2.5 ppm. The mole fraction of HA, xHA, was determined from the proton signal ratio of the vinyl protons of MA (a) and the methyl protons (f) of HA (xHA = 2f/(3a + 2f)) and are listed in Table 4. This method seems appropriate at first glance. However, the method may also provide an inaccurate value because of the difference in the restricted mobility of the species bound to the ZrO2 NPs. The extent of perturbation in motion may decrease with increase in distance from the NP surface. Thus, the vinyl protons (a) of MA are more restricted than the ω-methyl protons (f) of HA, leading to the overestimation of HA fraction. Figure 7 shows the 13C CP/MAS NMR spectra of HA/MA-ZrO2 NPs. Relatively sharp peaks due to the carboxylate group separately appeared at 174 ppm (d) for MA and at 181 ppm for HA (e). The mole fractions, xHA, were estimated using the respective peak intensity ratios (xHA = e/(d + e)) and are listed in Table 4. Figure 8 shows a plot of xF,HA against xHA, which is determined by 1H NMR in CDCl3 (squares) and 13C CP/MAS NMR (circles). The difference between the xHA values of the two methods became remarkable in the region of xF,HA < 0.5, most likely due to the higher vapor pressure of MA than that of HA at the same temperature and pressure during the hydrophobization process.

Figure 6. (a) High-angle annular dark field (HAADF) STEM image of HA-ZrO2 NPs on silicon grid. (b−d) EDX elemental maps of Zr, C, and Zr + C. (e) EDX spectra from the regions enclosed in circles 1 and 2 in (d).

elements, in which the atoms are indicated in red and green, respectively. Figure 6d shows the merged image of Figures 6b and 6c, indicating that carbon atoms unquestionably exist around the Zr atoms. Figure 6e shows the EDX spectra of the regions enclosed in circles 1 and 2 in Figure 6d. In region 1, the intensities of the characteristic X-ray due to Zr and C atoms are clearly observed, while in region 2, they are not observed within the experimental error, implying that ZrO2 NP is covered with HA. The XRD pattern of dodecanoic acid (DA)-modified ZrO2 NPs (DA-ZrO2) is presented in Figure S1 and is almost the same as that of pure ZrO2 NPs, showing that the crystal structure of original ZrO2 NPs did not change during the surface modification process. Characterization of Surface Comodified ZrO2 NPs Using MA and HA. ZrO2 NPs with (co)polymerizable methacrylate functional groups on their surfaces were prepared by comodification of various mole ratios of MA to HA as surface modifiers. These NPs were used for hybridization to clarify the influencing mechanism of (co)polymerizable functional group on hybridization, as will be discussed later. The total amount of surface modifiers was fixed in the feed at H


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Macromolecules

thermal polymerization at 120 °C. Before polymerization, the reaction mixture was completely transparent, whereas after polymerization, phase separation occurred, as shown in Figure 9.

Figure 7. 13C CP/MAS NMR spectra of purified HA/MA-ZrO2 NPs with various xF,HA.

Figure 9. Optical images of ZrO2-PSt hybrid materials with WF,HA/MA‑ZrO2 of 30 wt % and various xF,HA.

This is because of entropy loss due to the linking of styrene (ΔS = −105 J K−1 mol−1 for styrene52) before and after polymerization. The size of ZrO2 NPs used was 3.11 nm, which is comparable to the diameter of the unperturbed radius of gyration of the PSt chains having a molecular weight of 1.43 × 104 g mol−1.53 The Gibbs free energy of the hybrid materials, ΔGmix, may be treated in the framework of the Flory− Huggins−Scott lattice theory.54 ΔGmix for the mixture consisting of polymer 1, with degree of polymerization P1, and ZrO2 NPs (with PZrO2) is given by

1

Figure 8. Plot of xHA against xF,HA determined by H NMR in CDCl3 (squares) and 13C CP/MAS NMR (circles). Average surface coverage, Aav (diamonds), is plotted against xF,HA.

The area occupied by a modifier on a ZrO2 NP with a smooth surface (A) is expressed by51 A av =

⎞ 6M ⎛ 100 − 1⎟ ⎜ ρZrO DNA ⎝ WM ⎠ 2

(11)

ϕZrO ϕ ΔGmix 2 = 1 ln ϕ1 + ln ϕZrO + ϕ1ϕZrO χ1,ZrO 2 2 2 RT P1 PZrO2

where M and NA denote the average molecular weight of HA (116.2 g mol −1 ) and MA (86.1 g mol −1 ) at x HA and the Avogadro’s number, respectively. The Aav values (0.29−0.46 nm2 molecule−1) of HA/MA-ZrO2 are calculated from eq 11 and summarized in Table 4. Although the Aav (red diamond) values are somewhat scattered, they decrease with increasing xHA because the cross-sectional area of linear HA is slightly smaller than that of MA, as shown in Figure 7. The Aav value determined is comparable to that (0.20 nm2) of densely packed aliphatic carboxylic acid on a plane substrate such as an LB membrane. The relatively low Aav value most likely results from the surface curvature of NP spheres and/or the assumption of smooth surface. The number of MA per ZrO2 NP is estimated to be 160, 210, 290, 300, and 400 molecules for xF,HA = 0.75, 0.53, 0.33, 0.16, and 0, respectively. Thus, the HA/MA-ZrO2 NPs are considered a multivinyl monomer with a huge range of vinyl groups at the surface of ZrO2 NP core. Preparation of PSt and PMMA Hybrid Bulk Materials. Since HA/MA-ZrO2 NPs are re-nanodispersible into styrene monomer as listed in Table 4, the hybridization was directly conducted by the polymerization of styrene as a continuous phase. First, the hybridization of HA-ZrO2 (xF,HA = 1) with styrene (WF,HA‑ZrO2 = 30 wt %) was examined by catalyst-free

(12)

where R is the gas constant, T is the Kelvin temperature, ϕ1 and ϕZrO2 are the volume fractions of the polymer and ZrO2 NPs, and χ1,ZrO2 is the interaction parameter between the polymer and ZrO2 NPs. Before polymerization, P1= 1; thus, the entropy term remains positive enough to yield a transparent dispersion. However, after polymerization, P1 increases, resulting in significant decrease in the entropy term. As a result, ΔGmix is nearly determined by the enthalpy term, value of χ1,ZrO2. Since HA-ZrO2 NPs are at least dissolved into the styrene monomer, the χ1,ZrO2 value is thought to be small but definitely positive. This makes ΔGmix positive, thus inducing phase separation during polymerization. To prevent phase separation and agglomeration of ZrO2 NPs during polymerization, the effect of polymerizable functional groups was examined. The hybridization was carried out using HA/MA-ZrO2 NPs with xF,HA ranging from 0.63 to 0 and a constant WF,HA‑ZrO2 of 30 wt %. Figure 9 shows the optical photographs of the obtained hybrid materials of PSt and ZrO2 NPs (ZrO2-PSt). When HA/MAI


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Macromolecules ZrO2 NPs with xF,HA = 0.63 were used, the phase separation was dramatically suppressed; however, agglomerations of ZrO2 NPs occurred, yielding a turbid material. In remarkable contrast, when HA/MA-ZrO2 NPs with xF,HA ranging from 0.53 to 0.08 were used, optically transparent hybrid bulk materials were obtained. At xF,HA = 0, the material became turbid again, most likely due to the increase in the χ1,ZrO2 value. The gel fraction of hybrid materials, fgel, was determined using a Soxhlet apparatus with toluene as a solvent for 24 h. The fgel of ZrO2-PSt hybrid material containing HA-ZrO2 NPs (xF,HA = 1) was 26% owing to the recovery of the agglomerated ZrO2 NPs. The fgel value was approximately 100% for the hybrids of ZrO2 NPs with xF,HA other than 1. This result implies that the network and grafting structure in the hybrid materials consisting of HA/MA-ZrO2 NPs as a multi-cross-linking agent play a dominant role in the suppression of aggregation of ZrO2 NPs during polymerization.

Figure 11. Optical images of (a) PSt-based and (b) PMMA-based hybrid materials containing WF,HA/MA‑ZrO2 ≥ 30 wt % using HA/MAZrO2 NPs with WM of 19.3 wt % and xF,HA of 0.43.

consistent with those (46.9 and 65.1 wt %) calculated from the values of WF,HA/MA‑ZrO2 and WM. Highly transparent 1 mm thick hybrid materials composed of ZrO2 NPs and PMMA (ZrO2-PMMA) were also prepared by photopolymerization, as shown in Figure 11b. Optical Properties of Hybrid Materials. The experimental data of WF,ZrO2, volume fraction (ϕF,ZrO2), refractive index (nD), and Abbe’s number (νD) of ZrO2-PSt and ZrO2PMMA hybrid materials are summarized in Table 5. These materials were prepared using HA/MA-ZrO2 NPs with WM of 19.3 wt % and xF,HA of 0.43. The ϕF,ZrO2 of 27.6% for PSt and 28.7% for PMMA hybrids are the highest loadings reported in the literature so far.37,39 The increments in the refractive indices of hybrid materials are estimated to be 0.084 for PSt and 0.134 for PMMA as compared to that of PSt and PMMA. Figure 12 shows the plots of nD versus WF,ZrO2 for hybrid materials of ZrO2-PSt (squares) and ZrO2-PMMA (circles), in which HA/MA-ZrO2 NPs with xF,HA = 0.43 and WHA/MA = 19.3 wt % were used, respectively. The refractive index significantly increased with increasing WF,ZrO2. Further, the dependence of nD on WF,ZrO2 is compared with the Lorentz−Lorenz effective medium expansion theory, expressed as19

Figure 10. TEM image of a 20 nm thick specimen of ZrO2-PSt hybrid material with WF,HA/MA‑ZrO2 of 30 wt % and xF,HA of 0.16.

Figure 10 shows a TEM image of an ultrathin section (20 nm) of ZrO2-PSt hybrid material containing WF,HA/MA‑ZrO2 of 30 wt % with xF,HA of 0.16. The image shows that ZrO2 NPs are nanodispersed into the PSt matrix as a continuous phase. Here, we discuss the preparation of ZrO2-PSt hybrids containing a higher WF,HA/MA‑ZrO2 than 30 wt %. Figure 11a displays the optical images of ZrO2-PSt hybrids containing WF,HA/MA‑ZrO2 = 58 and 80 wt %. As can be seen, highly transparent hybrid materials are obtained. The WM of the modified ZrO2 NPs used was determined by TGA to be 19.3 wt %. The net ZrO2 content in the hybrid materials, as determined by TGA, was 48.7 wt % for WF,HA/MA‑ZrO2= 58 and 64.2 wt % for WF,HA/MA‑ZrO2 = 80 wt %. These values are almost

ϕF, i =

wi ρi n wi ∑i = 1 ρ i

=

nZrO2 2 − 1 n 2−1 n2 − 1 = ϕ + ϕHA HA 2 ZrO2 2 2 n +2 nZrO2 + 2 nHA + 2 + ϕMA

nMA 2 − 1 nMA 2 + 2

+ ϕm

nm 2 − 1 nm 2 + 2

(13)

where the subscripts “ZrO2”, “HA”, “MA”, and “m” in ϕ and n signify those of the ZrO2 NPs, HA, MA, and the matrix, respectively. The volume fraction of the species i in feed, ϕF,i, is expressed by wi ρi

wHA/MA ‐ ZrO2(100 − WM) ρZrO

+

wHA/MA ‐ ZrO2WMWF,HA ρHA

2

Here, wi is the weight of species i, wi is the wt % of species i, and WF,HA (0.5 w/w) is the weight fraction of HA in the surface

+

wHA/MA ‐ ZrO2WM(1 − WF,HA ) ρMA

+

100wm ρm

(14)

modifiers for HA/MA-ZrO2. The first, second, third, and fourth terms in the denominator in eq 14 are the feed volume of the J


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Macromolecules

Table 5. Volume Fraction (ϕF) of Each Component in the Hybrid Materials of PSt or PMMA and ZrO2, Refractive Index (nD), and Abbe’s Number (νD) ϕF (v/v)b run

matrix

2-1 2-2 2-3 2-4 3-1 3-2 3-3 3-4

PSt

WF,ZrO2 (wt %)

ZrO2

matrix

HA

MA

nD

νD

0 23.7 46.9 65.1 0 24.4 40.4 64.6

0 0.059 0.152 0.276 0 0.068 0.133 0.287

0 0.874 0.676 0.411 0 0.854 0.717 0.388

0 0.036 0.093 0.169 0 0.042 0.081 0.175

0 0.031 0.079 0.144 0 0.036 0.069 0.150

1.592 1.613 1.637 1.676 1.492 1.527 1.559 1.626

31.0 32.8 28.0 30.0 57.8 50.7 49.5 46.7

PMMA

a

HA/MA-ZrO2 NPs with WM of 19.3 wt % and xF,HA of 0.43 are used. bThe volume fraction of ZrO2 NPs in the hybrid materials is calculated from eq 14.

Figure 12. Dependence of nD on WF,zrO2 for ZrO2-PSt hybrids (squares) and ZrO2-PMMA hybrids (circles). HA/MA-ZrO2 NPs with xF,HA = 0.43 and WHA/MA = 19.3 wt % were used. The black and red lines represent the theoretical values calculated from the Lorentz− Lorenz theory using the values of ρ and nD before and after the polymerization of MA, respectively.

Figure 13. Dependence of νD on ϕF,ZrO2 for ZrO2-PSt hybrids (squares) and ZrO2-PMMA hybrids (circles). HA/MA-ZrO2 NPs with xF,HA = 0.43 and WHA/MA = 19.3 wt % were used. The black, red, and blue lines represent the theoretical values calculated from a combination of Lorentz−Lorenz theory and linear relationship between λ and n using the indicated νD,HA, respectively.

We next discuss the effect of refractive index of the surface modifier on the refractive index of the dispersions and hybrids. Figure S9 shows the comparison of refractive indices of styrene dispersions containing ZrO2 NPs surface treated with different surface modifiers (HA (nD = 1.419), p-PNBA (nD = 1.567), and p-MBA (nD = 1.503), see Table S2). It is evident that the refractive index increases with increase in the refractive index of surface modifier at the same ϕF,M, indicating the significance of refractive index of the modifiers used. The results of refractive index and Abbe’s number of the hybrids are shown in Table S3 and Figures S10 and S11, which reveal the non-negligible effect of the species of surface modifiers on the optical properties of the resulting hybrids. Finally, we discuss the luminous transmittance of the hybrid material with WZrO2 of 23.7 wt % (run 2-2 in Table 5). The value of transmittance at 10 mm thickness was calculated to be about 90% using eqs 1−3. This result is reasonable, judging from the appearance of the hybrid bulk rod.

corresponding components, respectively. The black lines in Figure 12 represent the theoretical values calculated from eqs 13 and 14 using the values of ρ and nD listed in Table S2. The experimental data show a significant upward deviation with increasing WZrO2. A feasible explanation for the deviation is the increment in the ρ and nD values of MA upon polymerization. Since the ρ and nD values of poly(methacrylic acid) (PMA) are not known, those of PMMA were used instead of PMA. The red lines calculated with these parameters reasonably explain the experimental data. Figure 13 shows the plots of νD value versus WF,ZrO2 for the ZrO2-PSt (squares) and ZrO2-PMMA (circles) hybrid materials. Although the νD value of ZrO2-PSt is almost constant at all the values of ϕF,ZrO2, the νD value of ZrO2-PMMA slightly decreased with increasing ϕF,ZrO2. Since the adsorption edge of ZrO2 is located at 248 nm, we assumed that the dependence of n on λ in the range from 486 to 656 nm has a linear relationship, which is given by the following expression: (n − 1)(589.3 − λ) n= D + nD 170.2νD

■

CONCLUSIONS This paper reports a unique and fascinating surface treatment and hybridization method for the production of highly transparent hybrid bulk materials. The method includes simultaneous hydrophobization and phase transfer of ZrO2 NPs from water to toluene via a gentle and gradual solvent exchange from a ternary solvent mixture composed of water, toluene, and methanol. Addition of organic acids with more than four carbon atoms as the surface treatment agent enables this hydro-

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The black, red, and blue lines represent the theoretical values calculated from eqs 13−15 using νD,HA of 30, 20, and 10, respectively. The experimental values of νD for ZrO2-PSt and ZrO2-PMMA are in relatively good agreement with those for the calculated lines when νD,HA/MA‑ZrO2 of 20 and 30 were used. K


Article

Macromolecules

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phobization. The carboxylic acid-modified ZrO2 NPs were thoroughly characterized by SAXS, DLS, scanning TEM, TGA, and NMR, and IR spectroscopies, which indicated that carboxylic acids are bonded on the surface of ZrO2 NPs in a bidentate fashion. The surface-treated ZrO2 NP powders after being completely dried possessed the ability of redissolution or nanodispersion in several organic solvents and vinyl monomers. Further, optically transparent PSt and PMMA hybrid bulk materials with thicknesses of 10 mm were fabricated by the copolymerization of St or MMA as a representative vinylic monomer with methacrylate-functionalized ZrO2 NPs as the multivinyl cross-linking agent. The optical properties of hybrid materials having higher refractive indices than an original homopolymer does are well described by the theory based on the Lorentz−Lorenz effective medium expansion. The maximum loading amount of the modified ZrO2 NPs is up to 80 wt %, and the refractive indices (nD) of the PSt-based and PMMA-based hybrid materials are 1.676 (Δn = 0.084) and 1.626 (Δn = 0.134), respectively. The present method provides a promising approach to controlling the refractive indices of different advanced optical polymers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02155. Figures S1−S11 and Tables S1−S3 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone +81-238-26-3182; Fax +81-238-26-3182; e-mail [email protected] (S.K.). ORCID

Seigou Kawaguchi: 0000-0002-5283-781X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas, “New Polymeric Materials Based on Element-Blocks (No. 2401)” (Proposal No. JP25102506 and JP15H00721), of the Ministry of Education, Culture, Sports, Science, and Technology, Japan. K. Enomoto appreciates the support by Grant-in-Aid from JSPS, Research Fellowship for Young Scientists (Proposal No. JP16J06696), and Innovative Flex Course for Frontier Organic Materials Systems (iFront) at the Graduate School of Yamagata University. We thank Editage for English language editing.

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REFERENCES

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