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Synthesis of Iron Oxide Rods Coated with Polymer Brushes and Control of Their Assembly in Thin Films Kohji Ohno Langmuir, Just Accepted Manuscript • DOI: 10.1021/la504429c • Publication Date (Web): 27 Dec 2014 Downloaded from http://pubs.acs.org on January 7, 2015

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Synthesis of Iron Oxide Rods Coated with Polymer Brushes and Control of Their Assembly in Thin Films

Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:

Langmuir la-2014-04429c.R1 Article 26-Dec-2014 Huang, Yun; Kyoto University, Institute for Chemical Research Ishige, Ryohei; Tokyo Institute of Technology, Department of Chemistry and Materials Science Tsujii, Yoshinobu; Kyoto University, Institute for Chemical Research Ohno, Kohji; Kyoto University, Institute for Chemical Research

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Synthesis of Iron Oxide Rods Coated with Polymer Brushes and Control of Their Assembly in Thin Films

6 Yun Huang,1 Ryohei Ishige,1 Yoshinobu Tsujii1,2 and Kohji Ohno1,*

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1

Institute for Chemical Research, Kyoto University, Uji, Kyoto, 611–0011, Japan and 2 JST, CREST, Tokyo, 102–0076, Japan

* To whom correspondence should be addressed. E-mail: [email protected]

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KEYWORDS: Living radical polymerization, polymer brush, hybrid particle, self-assembly

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ABSTRACT: We investigated the surface-initiated atom transfer radical polymerization (SI-

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ATRP) of methyl methacrylate (MMA) using monodisperse rod-type particles of iron oxide, β-

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FeOOH. The slow hydrolysis of iron(III) chloride yielded monodisperse β-FeOOH rods with an

5

average length-to-width ratio, L/W, of 6 (L = 210 nm and W = 35 nm on average). The surface of

6

the β-FeOOH rods were modified with a triethoxysilane derivative as an ATRP-initiating site,

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namely, (2-bromo-2-methyl)propionyloxypropyl triethoxysilane. The SI-ATRP of MMA,

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mediated by a copper complex, was performed using the initiator-coated β-FeOOH rods in the

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presence of a “sacrificial” free initiator. Well-defined poly(methyl methacrylate) (PMMA)

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brushes with molecular weights of up to 700,000 could be grafted on the β-FeOOH rods with a

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surface density as high as 0.3 chains/nm2. The resultant polymer-brush-afforded hybrid rods

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exhibited high dispersibility in various solvents for PMMA without forming aggregates. Thin

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films were prepared by dip-coating from a suspension of the hybrid rods and the rods were

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oriented in a specific direction in the films. The arrangement of the rods could be controlled by

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varying the chain length of the polymer brush and the withdrawal speed during the dip-coating

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process.

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INTRODUCTION

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Particle assemblies consisting of one-, two-, and three-dimensional ordered arrays have

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attracted considerable attentions owing to their potential for use in various applications,

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including in optical and electronic devices and biological and chemical sensors.1-11 Various types

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of particles having different sizes, shapes, and components have been employed for constructing

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these ordered assemblies.12-21 In addition, numerous techniques have been used for modifying the

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surfaces of the constituent particles to ensure that the assemblies can be constructed

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effectively.22,23 Among the various surface-modification methods developed so far, surface-

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initiated living radical polymerization (SI-LRP) is one of the most powerful ones, because of the

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inherent robustness and versatility of LRP techniques. We had developed a method for

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modifying silica particles by surface-initiated atom transfer radical polymerization (ATRP) and

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had synthesized hybrid nanoparticles consisting of a monodisperse silica particle as the core and

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a well-defined, concentrated polymer brush; the diameters of the particles had ranged from the

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nanometer scale to the micrometer scale.24,25 The fact that these hybrid particles were perfectly

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dispersible allowed us to fabricate two- and three-dimensional ordered arrays of the particles at

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air-water interfaces and in suspensions, respectively.24-29 In this study, in order to expand the

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versatility of the chemistry of surface-grafting polymers via LRP, we investigated the feasibility

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of using SI-ATRP in the case of rod-type particles.

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Lekkerkerker et al. employed an amine-functionalized polyisobutylene to sterically

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stabilize boehmite rod-like particles, and reported that the resulting hybrid rods formed a liquid

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crystal in toluene.30 Ji et al. reported the fabrication of gold nanorods grafted with a poly(N-

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isopropylacrylamide) brush using SI-ATRP. They envisaged that these nanorod hybrids would

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find use in smart in vivo drug delivery systems because the hybrids were sensitive to both

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temperature and near-infrared radiation.31 Boyes et al. reported the surface modification of gold

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nanorods via the postpolymerization immobilization of polymers prepared by reversible-

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fragmentation chain transfer (RAFT) polymerization. The polymers had a thiol-group at the

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chain end; the thiol-group was produced by using a reducing agent to convert the end group of

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the RAFT chain-transfer agent on the polymer just after the completion of the polymerization

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process.32 Zentel et al. employed poly(methyl methacrylate-b-dopamineacrylamide), which was

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also synthesized by RAFT, to modify the surfaces of TiO2 rods by exploiting the anchoring

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ability of the dopamine units. The surface-functionalized hybrid rods formed a liquid crystal in a

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solvent.33 They also fabricated a thin film of these hybrid rods in a poly(ethylene glycol) matrix

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through the dip-coating method and confirmed that the film exhibited birefringence under

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polarized optical microscopy.34

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Of the various rod-type particles synthesized so far, we chose β-FeOOH rods to use as the

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substrate for SI-ATRP process in this study, because the method for synthesizing β-FeOOH rods

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with a narrow size distributed is well established, and metal oxide surfaces are suitable for

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introducing initiation sites for polymerization. Zocher observed an iridescent layer at the bottom

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of a flask containing a suspension of β-FeOOH rod; this suggested the rods had formed some

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type of an ordered structure.35 Maeda et al. synthesized β-FeOOH rods with aspect ratios of 3.6–

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7.0, and were able to fabricate highly oriented structures of the rods in aqueous suspensions.36

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The ability of β-FeOOH rods to form ordered structures is particularly attractive as it allows for

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the fabrication of well-organized assemblies of polymer-brush-afforded hybrid particles.

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In this paper, we report the surface modification of β-FeOOH rods by the grafting of a

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high-density poly(methyl methacrylate) (PMMA) brush using SI-ATRP. Special attention was

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paid throughout all the processes, from initiator fixation process to polymerization process, to

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ensure that the rod-like particles did not form aggregates. We prepared thin films of the polymer-

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brush-afforded β-FeOOH rods using the dip-coating method. Finally, an oriented structure of the

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hybrid rods in the thin film was also fabricated.

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EXPERIMENTAL SECTION

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Materials. Ethyl 2-bromoisobutyrate (2-(EiB)Br, 98%) were obtained from Tokyo

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Chemical Industry Co., Ltd., Tokyo, Japan. 4,4’-Dinonyl-2,2’-bipyridine (dNbipy, 97%),

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copper(I) chloride (Cu(I)Cl, 99.9%), and iron(Ш) chloride hexahydrate (FeCl3·6H2O, ≥ 99.0%)

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were purchased from Wako Pure Chemical Industries, Ltd., Osaka, Japan. Methyl methacrylate

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(MMA, 99%) was obtained from Nacalai Tesque Inc., Osaka, Japan, and purified by flash

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chromatography over activated neutral alumina. An ATRP-initiator-holding silane coupling

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agent, (2-bromo-2-methyl)propionyloxypropyl triethoxysilane (BPE), was synthesized by

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following a previous report.25 Water was purified by a Milli-Q system (Nihon Millipore Ltd.,

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Tokyo, Japan) to a specific resistivity of ca. 18MΩ-cm. β-FeOOH rods were synthesized from

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aqueous FeCl3 solution (40 mM, 20 L) by slow hydrolysis for six month at room temperature,

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according to the method of Zocher and of Watson et al.35,37 The mean length (L) and width (W)

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of the resultant β-FeOOH rods were 210 and 35 nm, respectively, as measured by transmission

18

electron microscopy (TEM). All other reagents were used as received from commercial sources.

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Measurements. Gel permeation chromatographic (GPC) analysis was carried out at 40 °C

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on a Shodex GPC-101 high-speed liquid chromatography system (Showa Denko K.K., Tokyo,

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Japan) equipped with a guard column (Shodex GPC KF-G), two 30 cm mixed columns (Shodex

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GPC KF-806L, exclusion limit = 2 × 107), and a differential refractometer (Shodex RI-101).

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Tetrahydrofuran (THF) was used as the eluent at a flow rate of 0.8 mL/min. Poly(methyl

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methacrylate) (PMMA) standards were used to calibrate the GPC system. 1H (300MHz) spectra

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were obtained on a JEOL/AL300 spectrometer (JEOL, Tokyo, Japan). Thermal gravimetric

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analyses (TGA) were performed on a Shimadzu TGA-50 (Shimadzu, Kyoto, Japan) under a

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nitrogen atmosphere. Polarized optical microscopy (POM) observations were carried out by

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optical microscope BX51 (OLYMPUS, Tokyo, Japan). Transmission electron microscopy

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(TEM) observation was made on a JEOL transmission electron microscope (JEM-2100) operated

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at 200 kV. Field-emission scanning electron microscope (FE-SEM) observation was performed

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using a scanning electronic microscope JSM-6700F (JEOL) at an accelerating voltage of 1.5 kV.

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Fixation of ATRP-Initiator BPE on β-FeOOH Rods. A mixture of ammonia solution

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(28% NH3 aqueous solution, 6 g) and ethanol (33 g) was added dropwise into the suspension of

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β-FeOOH rods (5 g) in ethanol (400 g) under magnetically stirring, and the system was stirred

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for 1 h at room temperature. BPE (2.5 g) dissolved in ethanol (67 g) was added dropwise into the

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system, and the reaction mixture was continuously stirred for 5 days at room temperature. The

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modified β-FeOOH rods were collected by centrifugation and subsequently washed three times

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by consecutive centrifugation and redispersion in ethanol. Finally, the suspension of initiator-

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coated β-FeOOH rods in ethanol was solvent-exchanged to

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redispersion/centrifugation to obtain an anisole suspension to stock.

anisole by repeated

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Surface-Initiated ATRP on β-FeOOH Rods. Just before polymerization, the initiator-

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coated β-FeOOH rods in anisole were solvent-exchanged to MMA. A mixture of initiator-coated

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β-FeOOH rods in MMA containing a prescribed concentration of 2-(EiB)Br and dNbipy was

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quickly added to the Pyrex glass tube charged with a predetermined amount of Cu(I)Cl (solid).

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Oxygen in the solution was removed by three freeze-pump-thaw cycles and the tube was sealed

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off under vacuum. The polymerization was carried out in a shaking oil bath (TAITEC Corp.,

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Saitama, Japan, Personal H-10) thermostated at 60 °C and, after a prescribed time t, quenched to

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room temperature. An aliquot of the solution was taken out for NMR measurement to estimate

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monomer conversion and for GPC measurement to determine molecular weight and its

5

distribution. The rest of the reaction mixture was diluted by acetone and centrifuged to collect

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PMMA-grafted β-FeOOH rods (PMMA-FeRs). The cycle of centrifugation and redispersion in

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organic solvents of acetone/THF/toluene was sequentially carried out, and each cycle was

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repeated three times to obtain PMMA-FeRs perfectly free of the unbound (free) polymer. The

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purified PMMA-FeRs were treated with HF to cleave the graft polymer from the β-FeOOH rod

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surface as reported previously, and the molecular weight of graft polymer was determined by

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GPC.24

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In a typical run, the SI-ATRP of MMA was carried out in bulk state at 60 °C for 12 h with

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the starting materials of MMA (9.8 g, 98 mmol), 2-(EiB)Br (9.6 mg, 0.049 mmol), Cu(I)Cl (19

14

mg, 0.20 mmol), dNbipy (166 mg, 0.40 mmol), and initiator-coated β-FeOOH rods (50 mg). This

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gave a monomer conversion of 79 %, a free polymer with Mn = 165,000 and Mw/Mn = 1.27, and a

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graft polymer with Mn = 203,000 and Mw/Mn = 1.15, where Mn and Mw are the number- and

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weight-average molecular weights, respectively, and Mw/Mn is the polydispersity index. The

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polymer-grafted β-FeOOH rods were purified by repeated cycles of centrifugation (12,000 rpm

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for 20 min) and redispersion in acetone (3 × 200 mL), toluene (3 × 200 mL) and in THF (3 × 200

20

mL). The PMMA-FeRs were dispersed in toluene to obtain a toluene suspension to stock in

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refrigerator.

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Dip-Coating with PMMA-FeRs. A glass slide (size: 28 × 10 mm, thickness: 0.8–1.0 mm)

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was washed twice with water and once with ethanol using a bath sonicator. It was then rinsed

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with ethanol and dried at 60 °C.38 A programmable dip-coater (SDI Company Ltd., Kyoto, Japan,

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Nano Dip Coater ND-0407-S1) was used to fabricate thin films of the PMMA-FeRs. The cleaned

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slide was immersed in a suspension of the PMMA-FeRs (6 wt%) in toluene (1 mL) contained in

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a Pyrex cell (size: 22 × 15 × 4 mm) and then withdrawn from the suspension at a controlled

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speed of 0.1–10 µm/s. The entire setup was placed on a vibration isolator and covered with a

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windshield box. Thicknesses of films were measured with differential interference contrast

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microscope (DICM NT9100M-T09, Vecco Instrument. Ltd., USA).

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RESULTS AND DISCUSSION

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Synthesis of ATRP-Initiator-Coated β-FeOOH Rods. Using a previously reported

11

method, we prepared narrowly size-distributed β-FeOOH rods from a dilute aqueous solution of

12

FeCl3; the long- and short-axis lengths of the rods were 210 and 35 nm, respectively. The

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reaction conditions for introducing the ATRP-initiating sites on the surfaces of the β-FeOOH

14

rods were similar to the ones we had used previously for modifying the surfaces of silica

15

particles. The starting materials for the reaction were the β-FeOOH rods (5 g), ethanol (500 g), a

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28% aqueous NH3 solution (24 g), and BPE (2.5 g). However, this attempt of surface

17

modification was unsuccessful and resulted in visually noticeable aggregates of the β-FeOOH

18

rods soon after the addition of the aqueous NH3 solution. Even though most of these aggregates

19

disappeared as the reaction proceeded, an extremely tiny amount of the aggregates did remain in

20

the system. This point will be discussed later in the paper.

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To overcome this issue, we deduced the concentration of the NH3 solution to a quarter of

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the previous value. It was expected that the use of a low-concentration NH3 solution would

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decrease the ionic strength of the system and consequently retard particle aggregation, resulting

2

in perfectly dispersible ATRP-initiator-coated β-FeOOH rods. A low-concentration NH3 solution

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also decreases the reaction rate; however, prolonging the reaction time to 5 days ensured that a

4

sufficient number of initiation sites were introduced on the β-FeOOH rods. An elemental

5

analysis of the initiator-coated β-FeOOH rods indicated a bromine content of 0.85%, which,

6

along with the known density (1.98 g/cm3) and the surface area of the spheroidal particles,

7

suggested a surface density of approximately 0.9 initiator-molecules/nm2. The resulting

8

suspension of the initiator-coated β-FeOOH rods in anisole could be stably stored in a

9

refrigerator for at least 6 months.

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Surface-Initiated ATRP of MMA using β-FeOOH Rods. The initiator-coated β-FeOOH

11

rods were subsequently employed for the copper-mediated ATRP of MMA in bulk (Scheme 1).

Length=210 nm Aspect ratio=6

2-(EiB)Br MMA CuCl/dNbipy CuCl/dNbipy

BPE NH4OH cat. in EtOH

60 oC

O

CH3

EtO

= EtO

Si

CH2

EtO

BPE

12

O

3

C

C

O

CH3

C

Br H 3C

CH3

O

C

CH2

Br CH3

2-(EiB)Br (free initiator)

Scheme 1. Schematic representation of the synthesis of polymer-coated β-FeOOH rods by surface-initiated atom transfer radical polymerization.

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1.6 1.2 0.8 0.4 0.0 0

2

4

6

8

10

12

t/h

Mw / Mn

1.6 1.4 1.2 1.0

200

Mn / 1000

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ln( [M]0 / [M] )

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Mn,calcd

100

50

0

0

20

40

60

80

100

Conversion / %

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Figure 1. (a) Plot of ln([M]0/[M]) vs reaction time t and (b) the number-average molecular weights (Mn) and polydispersity indices (Mw/Mn) of the graft (○) and free (●) polymers as functions of monomer conversion for the polymerization of methyl methacrylate (MMA) in bulk at 60 °C using the initiator-coated β-FeOOH rods (0.5 wt%): [MMA]0/[ethyl 2bromoisobutylate]0/[Cu(I)Cl]0/[4,4’-dinonyl-2,2’-bipyridine]0 = 2200/1/4/8.

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During the polymerization process, attention was paid to the following two points, given

2

by previous experience with SI-ATRP. First, we ensured that the initiator-coated β-FeOOH rods

3

were not dried before the polymerization process. This resulted in them being homogeneously

4

dispersible in the polymerization medium.24 Second, the “sacrificial” or free initiator, 2-(EiB)Br,

5

was added to the polymerization mixture. The role of the free initiator is to accumulate an

6

appropriate amount of the Cu(II) species via the termination of the polymer radicals early during

7

the polymerization process and thus to control the process through the so-called persistent radical

8

effect.39-44 Figure 1a shows the first-order kinetic plot of the monomer concentration for the

9

polymerization of MMA in bulk with 2-(EiB)Br in the presence of the initiator-coated β-FeOOH

10

rods (see Supporting Information). The linear relation of the plot during the early stage of the

11

polymerization indicates that the concentration of the propagating radical species remained

12

constant (Figure 1a). However, the plot exhibited a slightly concave shape with an increase in the

13

polymerization time. This may be attributed to a large increase in the system viscosity owing to

14

higher monomer conversion and resultantly increased molecular weight of the produced

15

polymer, namely, a gel effect. The β-FeOOH rods purified after the polymerization process were

16

treated with HF to cleave the siloxane linkages at the grafting point and hence to recover the

17

graft polymer free from the core rod. The cleaved polymer was subjected to GPC-based analysis

18

(see Supporting Information). Figure 1b shows the evolution of the number-average molecular

19

weight, Mn, and the polydispersity index, Mw/Mn, of the cleaved polymers, as well as those of the

20

free polymer produced simultaneously from the free initiator, 2-(EiB)Br. The solid line shows

21

the theoretical value of Mn (Mn,theo = MMMA × C × {[MMA]0/([2-(EiB)Br]0 + Ifix}), where MMMA

22

is the molecular weight of MMA, C is the monomer conversion, [MMA]0 and [2-(EiB)Br]0 are

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the concentrations of MMA and 2-(EiB)Br in the feed, respectively, and Ifix is the number of

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initiation sites available on the surface of all β-FeOOH rods. The Ifix value was calculated from

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the number of initiation site on each the β-FeOOH rod and the amount of β-FeOOH rods. It can

3

be seen that the Mn values of the graft and free polymers were nearly the same, with both

4

increasing in proportion to monomer conversion; however, both the Mn values were slightly

5

higher than the Mn,theo value. The reason for this is not clear. The Mw/Mn ratio remained lower

6

than 1.2 for most samples. All these results indicate that the polymerization of MMA initiated on

7

the surfaces of the β-FeOOH rods proceeded in a living fashion, resulting in the β-FeOOH rods

8

coated with a shell of well-defined PMMA (PMMA-FeRs).

-2

0.5

Graft Density / chains nm

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0.4 0.3 0.2 0.1 0.0

0

2

4

6

8

10

12

t/h

9

Figure 2. Time dependence of the graft density of poly(methyl methacrylate) grown on the surfaces of β-FeOOH rods.

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Determination of Graft Density of PMMA-FeRs. TGA was performed on the PMMA-

11

FeRs to determine the mass (m) of the surface-grafting PMMA per one β-FeOOH rod (see

12

Supporting Information). For the calculation to the density of the β-FeOOH rod determined as

13

follows. The PMMA-FeRs (Mn of the graft polymer = 197,000) were dispersed in solvent

14

mixtures of various densities, and then subjected to the centrifugation at 1000 rpm for 10 min.

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The average overall density of PMMA-FeRs was estimated to be approximately 1.3 g/mL,

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because the PMMA-FeRs did not settle in the solvent mixtures with densities greater than 1.3

2

g/mL. Using the known density of PMMA (1.2 g/mL) and the TGA result (PMMA content of 87

3

wt%), the density of the β-FeOOH rod was calculated to be 1.98 g/mL. The graft density (σ) was

4

then calculated using Eqs. (1), (2) and (3):24,45

5

σ=

6

S=

(m / M n ) Av S

π 2

W 2 (1 +

e2 = 1 − 7

L sin −1 e) We

W2 L2

(1)

(2)

(3)

8

where Av is Avogadro’s number, and S is the surface area of one β-FeOOH rod. To estimate the

9

S value, the rod was reasonably assumed to be a prolate spheroid with a mean length (L) of 210

10

nm, a mean width (W) of 35 nm, and eccentricity, e as was revealed by TEM observation (see

11

below). Figure 2 shows that the graft density of the samples obtained at different polymerization

12

time. From the kinetic consideration, i.e., sufficiently fast initiation even on solid surfaces, and

13

its experimental verification on, e.g., a silicon wafer and a silica particle, the graft density should

14

be nearly constant under this polymerization condition but seemingly increased slightly in Figure

15

2. In this case, the weight fraction of PMMA and hence the graft density was possibly under

16

estimated especially for the samples with relatively low amount of graft polymer, which may

17

come from the estimation error of the core weight by TGA. From this consideration, we finally

18

concluded that the graft density was approximately 0.3 chains/nm2; this value is somewhat

19

smaller than those obtained in the case of silica particles. However, more importantly, the graft

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density was high enough to suggest that the layer of the polymer grafts was in the concentrated-

2

brush regime.

(a)

(b)

500 nm

3

500 nm

Figure 3. Transmission electron microscopy images of films of β-FeOOH rods end-grafted with poly(methyl methacrylate) brushes (PMMA-FeRs): (a) PMMA-FeRs fabricated using initiator-coated β-FeOOH rods prepared with high-concentration ammonia solutions. (b) PMMA-FeRs fabricated using initiator-coated β-FeOOH rods prepared with lowconcentration ammonia solutions (optimized condition).

4

TEM Observations of PMMA-FeRs. Monolayers of the PMMA-FeRs were prepared

5

using a previously reported procedure. A droplet of a suspension of the PMMA-FeRs in toluene

6

was deposited on the surface of pure water, resulting in the formation of a thin film at the air-

7

water interface after the toluene had evaporated.24 Figure 3a shows a TEM image of the

8

transferred film of the PMMA-FeRs (Mn of the graft polymer = 160,000), which were

9

synthesized from the initiator-coated β-FeOOH rods prepared using the high-concentration NH3

10

solution, as described above. The β-FeOOH core rods are visible as the dark rods. However, the

11

PMMA chains, which should have formed fringes around the cores of the β-FeOOH rods, could

12

barely be observed, owing to their significantly lower electron density. A few of the β-FeOOH

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rods formed doublets (or aggregates), which remained even after repeated ultrasonication. On the

14

other hand, Figure 3b shows a TEM image of a monolayer of the PMMA-FeRs (Mn of the graft

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polymer = 140,000), which were synthesized from the initiator-coated β-FeOOH rods prepared

2

using the low-concentration NH3 solution, as described above. The hybrid rods were perfectly

3

dispersed throughout the film, and no aggregates were noticed. The high dispersibility of the

Figure 4. Transmission electron microscopy images of films of β-FeOOH rods end-grafted with poly(methyl methacrylate) brushes (PMMA-FeRs). Number-average molecular weights of the PMMA grafts were (a) 58,000, (b) 88,000, (c) 140,000, and (d) 197,000. 4 5

PMMA-FeRs proves the importance of the concentration of the NH3 solution used in the

6

initiator-coating process. Figure 4 shows that a similarly good dispersibility was observed in the

7

monolayers of the PMMA-FeRs with graft chains of varied lengths. The interparticle distance

8

increased with an increase in the molecular weight of the graft polymer. In addition, the PMMA-

9

FeRs were preferentially oriented along a direction to some extent. This encouraged us to try to

10

fabricate oriented films of the PMMA-FeRs, as discussed below.

11

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Table 1. Characteristics of Various PMMA-FeRs

a

Sample Code

Mn(graft) a

Mw/Mn(graft) a

Graft density (chains/nm2)

R1

4,000

1.20

0.24

R2

16,000

1.08

0.23

R3

58,000

1.33

0.20

R4

197,000

1.17

0.36

R5

700,000

1.25

0.38

Number average molecular weight and polydispersity index of PMMA grafts.

3

4

Figure 5. Photographs of thin films prepared by dip-coating β-FeOOH rods end-grafted with poly(methyl methacrylate) brushes (PMMA-FeRs) on glass slides with a withdrawal speed of 2 µm/s. Number-average molecular weights of the PMMA grafts were (a) 4,000, R1, (b) 16,000, R2, (c) 58,000, R3, (d) 197,000, R4, and (e) 700,000, R5.

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Fabrication of Thin Films of PMMA-FeRs by Dip-Coating. PMMA-FeRs consisting of

2

PMMA grafts with different Mn values were deposited on glass slides by dip-coating; the

3

withdrawal speed was 2 µm/s. Table 1 shows the characteristics of the PMMA-FeRs used for the

4

dip-coating experiment. Figure 5 shows photographs of these obtained thin films of the PMMA-

5

FeRs. The β-FeOOH rods caused the films to be yellow. Further, with an increase in the Mn

6

value of the PMMA graft, that is, with a decrease in the volume fraction of the β-FeOOH core,

7

the films became more transparent. Especially, the samples, R1 and R2, were apparently opaque.

8

Figure 6 shows FE-SEM images of the surface of these thin films. A few cracks were observed

9

in the thin films made of samples, R1 and R2. This is one of the reasons why these PMMA-FeRs

10

did not form transparent (or smooth) films. Probably, less entanglement among shorter PMMA

11

chains densely grafted onto rods is responsible for the cracks. Meanwhile, the samples of the

12

higher Mn values, R3-R5, gave transparent films with smoother surfaces. Interestingly, the R3

13

sample was aligned parallel to the film surface and perpendicular to the withdrawal direction.

14

With an increase in the graft chain length, the degree of this in-plane orientation decreased. The

15

R5 sample was arranged in an almost isotropic manner even with a few of them being arranged

16

perpendicular to the film surface; the white dots presumably denote the pointed ends of the rods

17

(Figure 6e).

18

The observed dependence of the chain length on the orientational ordering of the PMMA-

19

FeRs can be understood on the basis of their overall structures. Figure 7 shows the structural

20

models supposed for the PMMA-FeRs with graft chains of various lengths in solvent for PMMA.

21

Because the surface curvature of pointed ends of the core rod is greater than that of their lateral

22

surfaces and the polymer grafts is in the stretched state, the aspect ratio of the PMMA-FeRs

23

should be significantly affected by the length of the graft polymers. The shape anisotropy of the

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(a)

(b)

(c)

(d)

Page 18 of 32

(e)

1

Figure 6. Field emission scanning electron microscopy (FE-SEM) images of thin films prepared by dip-coating β-FeOOH rods end-grafted with poly(methyl methacrylate) brushes (PMMA-FeRs) on glass slides. Number-average molecular weights of the PMMA grafts were (a) 4,000, R1, (b) 16,000, R2, (c) 58,000, R3, (d) 197,000, R4, and (e) 700,000, R5. During the dip-coating process, the withdrawal speed was 2 µm/s, and the concentration of PMMA-FeRs in toluene was 6 wt%. Arrows show the withdrawal direction of the dipcoating process. The scale bars represent 1 µm.

2

PMMA-FeRs decreased with an increase in the length of graft polymer. It is likely that the shape

3

anisotropy reflects the orientational order of the R3 sample shown in Figure 6c. Owing to the

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same reason, the overall structure of the R5 rods was nearly spherical, resulting in some of them

2

being observed standing perpendicular to the film surface, as shown in Figure 6e.

3

Figure 7. Illustration showing the effect of the length of the polymer chain on the overall structure of β-FeOOH rods end-grafted with poly(methyl methacrylate) brushes.

4

To investigate the effect of the withdrawal speed during the dip-coating process on the

5

orientation of the PMMA-FeRs in the thin films, we dip-coated a suspension of the R4 rods in

6

toluene (6 wt%) at speeds of 0.1–10 µm/s. Figure 8 shows FE-SEM images of the surface of

7

resultant thin films. The thickness of films decreased with increasing withdrawal speed (see

8

Supporting Information). The direction of withdrawal was horizontal to the plane of the FE-SEM

9

images, as indicated by black arrows. Similar to that was noticed in Figure 6d, the R4 rods were

10

arranged almost randomly in the films prepared at withdrawal speeds greater than 2 µm/s.

11

However, at lower speeds, the rods aligned preferentially perpendicular to the withdrawal

12

direction. Further, and more importantly, a highly oriented order of the hybrid rods was observed

13

in the film fabricated at a speed of 0.1 µm/s, as shown in Figure 8f. Bottom-left insets in Figure 8

14

show the Fast Fourier transforms (FFT) of each FE-SEM image. The contrasts (intensity) of the

15

FFT images correspond to power spectra, which is equal to the scattering intensity. The FFT

16

images show similar geometry to the scattering patterns from the oriented nematic phase of low-

17

molecular-weight rigid molecules.46 The horizontal diffuse peak (bright region) appears in the

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direction perpendicular to the orientation direction (long-axis direction of rods) and any clear

2

diffraction spot does not appear. These features indicate only the orientational order without

3

long-range positional order, i.e. nematic feature rather than smectic one.47,48 In order to

4

quantitatively compare the degree of orientation, the order parameter was evaluated from the

5

azimuthal-intensity distribution for the diffuse peaks in the FFT pattern (see, the values of the

6

insets in Figure 8). Here, the value of the two-dimensionally orientational order parameter λ is

7

defined as Eqs. (4) and (5):49, 50, 51

8

λ = 2 cos 2 φ − 1

9

cos ϕ 2

∫ =

π

0

cos 2 ϕ I (q0 , ϕ ) dϕ

∫

π

0

I (q0 , ϕ ) dϕ

(4)

(5)

10

where φ is the azimuthal angle (orientation direction is defined as 0 degree) and I(q0, φ) is the

11

azimuthal-intensity distribution of the diffuse peak obtained at q0 (q0 is the radial distance from

12

the center of the FFT image to the peak position and corresponds to the inverse value of the

13

average lateral distance between the particles). Herein, λ = 1 corresponds to the perfectly

14

uniaxial orientation, and λ = 0 to the completely isotropic state in plane. The film of the R4 rods

15

dip-coated at the 0.1 µm/s shows λ = 0.58, which is a typical value for a nematic phase in liquid

16

crystals.52 With increasing speed, the λ values decreased. These results indicate that a lower

17

speed of dip-coating is one of the key issues to achieve a high orientational order of PMMA-

18

FeRs. Some groups reported the formation of films with oriented rod-type particles by drop-

19

casting their suspensions onto solid surfaces. In these systems, the key for orientation is the

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1

2

Langmuir

formation of liquid crystal phases at the droplet edge of the suspension where the particle

Figure 8. Field emission scanning electron microscopy (FE-SEM) images of thin films prepared by dip-coating β-FeOOH rods end-grafted with poly(methyl methacrylate) brushes (PMMA-FeRs, R4 rods) on glass slides. Number-average molecular weights of the PMMA grafts was 197,000. The withdrawal speeds during the dip-coating process were (a) 10, (b) 5, (c) 2, (d) 1, (e) 0.5, and (f) 0.1 µm/s. The arrows show the withdrawal direction of dipcoating process and the black scale bars indicate 1 µm. Bottom left insets show Fast Fourier Transform images from the FE-SEM images with a 8.5 × 8.5 µm2 region. The white scale bars represent 30 µm−1, and the values in the insets are the orientational order parameter of each film. The color scale bar for the FFT images is inset on the right side of (f).

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1

concentration gradually increases with the solvent evaporating off.53-60 However, this is

2

presumably not the case with the R4 sample, with a relatively low aspect ratio in a swollen state

3

of the PMMA graft. Therefore, the mechanism of orientation should be different but is still

4

unclear.

5

With further increasing the withdrawing speed of e.g., 10 µm/s (Figure 8a), the film shows

6

a relatively higher order parameter, and more interestingly, the orientation direction changed

7

more or less to the withdrawal direction. This may suggest that there exist two forces: one is the

8

force to orient the rod perpendicular to the dipping direction, and the other is the force of the

9

shear stress experienced by the withdrawing. Then, the orientation direction and also the

10

11

orientation order can be also affected by the speed.

Figure 9. Polarized optical microscopy images of a thin films prepared by dip-coating βFeOOH rods end-grafted with poly(methyl methacrylate) brushes (PMMA-FeRs, R4 rods) on glass slides. Number-average molecular weights of the PMMA grafts was 197,000. The withdrawal speed during the dip-coating process was 0.1 µm/s. The sample was observed between two-crossed polarizers in horizontal (polarizer indicated by P) and vertical directions (analyzer indicated by A) with a phase compensator (U-TP530 530 nm retardation plate: the optical axis is indicated with the white arrow). (a) and (b) show R4 rods aligned parallel and perpendicular to the direction of the compensator, respectively. The scale bars indicate 50 µm.

12

Macroscopic orientation of the thin film was observed by POM. Figure 9 shows a POM

13

image of the thin film formed by the R4 rods at a withdrawal speed of 0.1 µm/s. The observed

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birefringence strongly suggested the existence of an optically anisotropic structure in the film,

2

that is, the presence of an oriented structure of the hybrid rods, as was noticed in the FE-SEM

3

image in Figure 8f. It should be noted that the transparency of the film allowed for this type of

4

microscopy-based characterization. Any orientation defect (e.g. Schlieren texture) is not

5

observed in the POM image, which indicate that the highly oriented structure of R4 rods formed

6

in a large area, as shown in Figure 9. The POM picture reveals that highly oriented homogeneous

7

structure of the rods can be fabricate in macroscopic scale by the dip-coating method.

8

CONCLUSIONS

9

Highly dispersible ATRP-initiator-coated β-FeOOH rods were successfully synthesized by

10

optimizing the reaction condition for the fixation of the initiator-holding silane-coupling agent,

11

BPE, on their surfaces. The SI-ATRP of MMA with the initiator-coated β-FeOOH rods

12

proceeded in a living manner, producing PMMA-FeRs consisting of PMMA brushes with target

13

molecular weights of up to 700,000. Owing to the excellent dispersibility of the PMMA-FeRs,

14

thin films of the rods could be fabricated successfully by the dip-coating method. By tuning the

15

dip-coating conditions, we were able to control the degree of orientation of the PMMA-FeRs in

16

the films. This technique should be applicable to other particles as well, owing to the robustness

17

and versatility of the SI-ATRP process, and should lead to the fabrication of particle-assembled

18

systems with unique functionalities.

19 20

ASSOCOATED CONTENT

21

Supporting Information

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Page 24 of 32

H NMR spectra and GPC traces of SI-ATRP of MMA, TGA curves of PMMA-FeRs, and

2

thickness of films made by dip-coating. This material is available free of charge via the internet

3

at http://pubs.acs.org.

4 5

ACKNOWLEDGMENT

6

We sincerely thank Professor H. Yano (Research Institute for Sustainable Humanosphere,

7

Kyoto University) for allowing us to use the FE-SEM apparatus. We greatly acknowledge JNC

8

Corporation for their kind donation of tetraethoxysilane and triethoxysilane.

9 10

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For Table of Contents Use Only

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Synthesis of Iron Oxide Rods Coated with Polymer Brushes and Control of Their Assembly

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in Thin Films

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Yun Huang, Ryohei Ishige, Yoshinobu Tsujii and Kohji Ohno*

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