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Structural estimation of particle arrays at air-water interface based on silica particles with well-defined and highly grafted poly(methyl methacrylate).


Monodisperse ultrafine colloidal particles have recently received great attention concerning applications of composite materials (1-4). Surface modifications of inorganic particles have been widely studied (5-19). Surface graft polymerization is one of the most effective and versatile methods used in making many of these modifications. Recently, living polymerization techniques were successfully applied to surface-initiated graft polymerization to obtain well-defined polymer-grafted colloidal particles (13-19). Such polymer-modified particles have great potential for well-organized structures, i.e., two- or three-dimensional particle array substrates (19-24). In recent years, several groups have looked into the synthesis of a particle monolayer using the Langmuir-Blodgett (LB) technique or self-assembly of polymer modified particles (24-27).

Recently, a particle monolayer was fabricated at the air-water interface using a silica particle with well-defined grafted polymer prepared by surface-initiated atom transfer radical polymerization (ATRP) (27). The interparticle distance was controllable by the graft density and length of the polymer chain. However, for the polymer-grafted particle with low graft density, the monolayer structure on substrate contained aggregation and voids.

In this regard, activators regenerated by electron transfer (ARGET) for ATRP (ARGET ATRP) were applied to form polymer chains with a high molecular weight and high graft density on the silica surface. Matyjaszewski's group reported that possible side reactions in ATRP not only decrease the functionality of the polymer chains, but also limit the molecular weight attainable for some monomers (28), (29). In ARGET ATRP, lowering the concentration of catalyst involved in the polymerization minimizes the side reactions. This technique was also successfully applied to densely grafted polymer brushes from a silicon wafer in the presence of air (30).

In this work, the formation of silica particles coated with high graft density polymer by surface-initiated ARGET ATRP (Scheme 1) was attempted, with the aim of fabricating ordered and controllable assemblies of particles. In a previous study (27), low dense polymer-grafted silica with a graft density of 0.004-0.04 [nm.sup.-2] was used. In this study, the particle array structure was examined in the particle layer formed by highly dense polymer-grafted silica compared with those in the previous paper (27). Additionally, the multilayer fabrication of these particles using the LB technique was examined, and their structure was confirmed.




Monodisperse colloidal silica with aqueous dispersion, containing 15 wt% [SiO.sub.2] with a 151 nm diameter, was kindly offered by the Catalyst & Chemicals Ind., Japan. Methyl methacrylate (MMA) was purchased from the Wako Pure Chemical Ind., Japan and was distilled under reduced pressure prior to use. Tris[2-(dimethyl-amino)ethylJamine ([Me.sub.6]TREN) was synthesized according to a previously reported procedure (31). N,N,N',N',N" -pentamcthyldiethylene-triamine (PMDETA), tin(II) 2-ethyl-hexanoate [[Sn(EH).sub.2], and copper(II) chloride (all from Aldrich) were used as received. ATRP initiator, 2-bromo-N-(3-(triethoxysilyl)propyl)-2-methylpropanamide (BTPA) and free initiator, 2-bromo-N-butyl-2-methylpropanamide were synthesized according to a previous work (27). The highest grade solvents that are commercially available were used for this work.

Immobilization of BTPA on Silica Surface

Water was exchanged with 2-propanol as the dispersion medium of colloidal silica by successive centrifugal separation prior to synthesize of the initiator-coated [SiO.sub.2] nanoparticles (SP-BTPA). BTPA (1.1 g) and a 5 wt% [NH.sub.3] aqueous solution (6 mL), as an alkaline catalyst (19), were added to a silica-propanolic solution (60 mL), containing 3 g [SiO.sub.2] with a particle size of 151 nm. The mixture was refluxed for 24 h at 50[degrees]C and then cooled to room temperature. The initiator-attached silica particles (SP-BTPA) were separated from the mixture by centrifugal washing eight times with ethanol. The amount of attached initiator on the [SiO.sub.2] nanoparticle was obtained by TGA, and the range was 13.6-14.9 mg * [g.sup.-1] [SiO.sub.2], which corresponded to a grafting density of 2.2-2.4 BTPA molecules per [nm.sup.2].

Synthesis of PMMA-Grafted Silica Particle by ARGET ATRP (Table 1, entry 3)
TABLE 1. Experimemtal conditions and results for ARGET ATRP of MMA from
silica nanoparticles
[[Sn[(EH).sub.2]].sub.o] = 5000:1:0.5:0.5; T = 60[degrees]C, in anisole
(0.75 volume equiv. vs. monomer)}.

Entry  [M]/[I]  Cu (a) (ppm)  Time (min)  Conv. (%)

1       5000        10            570        28
2       5000       100            330        33
3 (d)   5000       100           1680        32

Entry  [M.sub.n,theo.sup.b]     [[M.sub.w]/        [[M.sub.w]/
                             [M.sub.n.sup.c]]   [M.sub.n.sup.c]]

1           140,000          222,000 (228,000)     1.51 (1.54)
2           165,000          183,000(169,000)      1.35 (1.46)
3 (d)       160,000          152,000 (154,000)     1.29 (1.31)

(a) Corresponds to molar ratio vs. monomer.
(b) [M.sub.n,theo] = [[M.sub.o]/[[I].sub.o] X conversion X [M.sub.MMA].
(c) The values in parentheses denote molecular weight and
polydispersity index of free polymer, respectively.
(d) [[MMA].sub.o]/[[I].sub.o]/[[PMDETA].sub.o]/[[Sn[(EH).sub.2]].sub.o]
= 5000:1:0.5:0.5; T = 60[degrees]C, in anisole (0.75 volume equiv. vs.

Just before polymerization, SP-BTPA suspensed in ethanol was solvent-exchanged to anisole to prevent precipitation of the silica particle. Freshly distilled MMA (12 mL, 0.112 mol) and anisole (7 mL) were added to an open Schlenk flask. Next, a solution of [CuCl.sub.2] (1.48 mg, 1.11 X [10.sup.-2] mmol) and PMDETA (2.34 [micro]L, 1.11 X [10.sup.-2] mmol) in anisole (1 mL) was added. The resulting mixture was stirred for 10 min, and then a solution of Sn[(EH).sub.2] (3.56 [micro]L, 1.11 X [10.sup.-2] mmol), free initiator (3.6 mg, 1.6 X [10.sup.-2] mmol) and SP-BTPA (90 mg, 0.64 X [10.sup.-2] mmol) in anisole (1 mL) were added. The initial sample was taken, and the sealed flask was placed in an oil bath set at 60[degrees]C. Samples were taken at timed intervals and analyzed by GC and GPC. The polymerization was stopped by opening the flask and exposing the catalyst to air. The PMMA-grafted silica nanoparticles ([SiO.sub.2]-PMMA) were washed with THF by several dispersion-centrifugation cycles to remove any free PMMA produced from the free initiators. [SiO.sub.2]-PMMA was isolated by precipitation in an EDTA solution, washed several times with pure water and pure methanol, respectively, to remove the Cu catalyst.

Cleavage of the Graft Polymer from the Silica Nanoparticles

PMMA chains of [SiO.sub.2]-PMMA were cleaved as follows. The [SiO.sub.2]-PMMA sample was vigorously stirred in a poly(ethylene) bottle containing 5 mL of toluene, 5 mL of a 10 wt% aqueous HF solution and 5 mg of benzyltriethylammonium bromide, used as a phase transfer catalyst. After 24 h, the aqueous layer was removed, and the organic layer containing the cleaved polymer was evaporated to remove toluene, and then was measured using GPC.

Layering of [SiO.sub.2]-PMMA Particles by LB Technique

A diluted suspension (~2 X [10.sup.-4] g * [mL.sup.-1]) of [SiO.sub.2]-PMMA particles in chloroform was very carefully spread using a microsyringe on pure water surface of a Langmuir-Blodgett (LB) trough (150 X 155 [mm.sup.2]). The representative spreading amount was about 500 [micro]L. After the chloroform evaporated, the water surface was compressed and expanded several times repeatedly by moving the barriers at a constant speed of 0.025 mm * [s.sup.-1] to measure the surface pressure-area ([pi]-A) isotherm at 25[degrees]C. The particle monolayer was deposited on a cleaned glass substrate using the LB method with a speed of ~10 mm * [min.sup.-1] and a surface pressure of 20 mN * [m.sup.-1], which was chosen for this transfer. Above this value, the stability of monolayer decreased quickly. Then, the substrate was vertically inclined until the suspension was dry. The particle double- (transfer) layer was prepared by sequentially depositing a defined number of particle layers.


The synthesized ATRP initiator (BTPA) and free initiator were characterized by [.sup.1]H NMR spectroscopy on a JEOL JNM-A500. Monomer conversion was determined using HP 5890 series 2 gas chromatography equipped with a HP-101 column (25 m X 0.32 mm X 0.3 [micro]m) and FID detector. The initial temperature was 40[degrees]C, and the final temperature of 200[degrees]C was reached at a heating rate of 20[degrees]C * [min.sup.-1]. Number-averaged molecular weight ([M.sub.n]), weight-averaged molecular weight ([M.sub.w]), and molecular weight distribution (MWD) denoted by polydidpersity index (PDI, [[M.sub.w]/[M.sub.n]]) were determined by THF GPC that was calibrated with polystyrene standards and equipped with an Agilent 1100 pump, 2 Agilent Plgel Mixed-C columns, and an Agilent 1100 RI detector. The (average) size and distribution of the particles were measured in ethanol by a dynamic light scattering (DLS) method on an Ohtsuka DLS-700S. The LB preparation of the particle monolayer was performed on a USI-3-00 (USI system, Fukuoka, Japan). Scanning electron micrograph (SEM) observations were taken by a JEOL JSM-6320F. Thermo-gravimetric analyses (TGA) on the composites were performed on a Shimadzu TGA-50.


Surface-Initiated ARGET ATRP of MM A on [SiO.sub.2] Nanoparticle

Surface-initiated ARGET ATRP of MMA on [SiO.sub.2] nanoparticles mediated by copper (Scheme 1) was carried out in the presence of the free initiator to control not only the graft polymerization but also to easily measure the molecular weight (and its distribution) of the resulting grafted PMMA (18), (19), (32-35).

Table 1 shows the experimental conditions and properties of PMMA by surface-initiated ARGET ATRP at 60[degrees]C. High molecular weight polymers were obtained when MMA was polymerized with 10 and 100 ppm copper versus monomer using [Me.sub.6]TREN as a ligand (Table 1, entries 1-2). However, the grafted polymer had relatively large polydisperisity index. The higher activation rate of the Cu(I) complex with [Me.sub.6]TREN was probably the reason for this. According to Queffelec et al. (36), the Cu(I)/[Me.sub.6]TREN system yielded completely uncontrolled polymerization of MMA, whereas the Cu(I)/PMDETA system would have been suitable for the polymerization of MMA (37), (38). Therefore, in a subsequent experiment (Table 1, entry 3), MMA was polymerized using PMDETA as ligand. The polymerization was slower, but a polymer with a high molecular weight ([M.sub.n] = 152,000) and relatively low PDI (1.29) was obtained.

Figures 1 and 2 illustrate the kinetic plot and evolution of the molecular weight and polydispersity index of free polymers with conversion for entries 1-2 and entry 3 of Table 1, respectively. The polymerization of MMA in [Me.sub.6] TREN ligand system did not exhibit the characteristics of a well-controlled living polymerization. Although the kinetic plots are roughly linear, the number-average molecular weight ([M.sub.n]) versus monomer conversion plot showed deviations from the expected linear correlation, as well a large PDI (up to [[M.sub.w]/[M.sub.n]] = 1.54). Slow initiation was the reason for the difference between the obtained and expected values. A similar kinetic behavior caused by slow initiation was reported in the ATRP of MMA from silica colloids (33). Additionally, for lower catalyst concentrations (Fig. 1, entry 1(*)), the molecular weight decreased as polymerization proceeded. The decrease of [M.sub.n] and the large PDI were both consistent with the interparticle or intraparticle crosslinking/branching process (33).



Figure 2 shows that control of the molecular weight for ARGET ATRP of MMA improved in the PMDETA ligand system. The plot of entry 3 is nearly linear, indicating that the kinetics was first-order in the monomer and that the concentration of the propagating species was constant during the polymerization. The obtained molecular weights were found to be close to expected values, and the resulting polymer had a narrow molecular weight distribution. Additionally, the (average) molecular weight and PDI of cleaved graft polymer were nearly the same as those of the free polymer.

Characteristics of [SiO.sub.2]-PMMA Particles

The characteristics of [SiO.sub.2]-PMMA prepared by ARGET ATRP are summarized in Table 2. From the values of [M.sub.n] and the amount of grafted PMMA, the graft densities of about 0.93, 0.80, and 0.28 [nm.sup.-2] were calculated for each of the three samples, respectively. These values were remarkably higher than those previously attained on a silica surface (27). For a high graft density, the particle monolayer at the air--water interface was expected to form more easily due to the large number of hydrophilic carboxyl groups. In practice, a well-ordered particle monolayer was obtained at the air--water interface.
TABLE 2. Characteristics of [SiO.sub.2]-PMMA prepared by ARGET ATRP.

Sample  [M.sub.[n.graft]]  [M.sub.w]/[M.sub.n]  Attached PMMA (b)
code    (a)                (a)                  (mg * [g.sup.-1]

S1         222.000               1.51                6167
S2         183.000               1.35                4413
S3         152.000               1.29                1260

Sample code  Graft density (c) (N * [nm.sup.-2])  [D.sub.h] (d) (nm)

S1                     0.93                               904
S2                     0.80                               811
S3                     0.28                               565

(a) Determined by GPC measurement with polystyrene standard.
(b) Determined by weight decrease from 100 to 800[degrees]C on TGA.
(c) Calculated from [M.sub.n] and attached PMMA.
(d) Hydrodynamic diameter measured by dynamic light scattering. Core
[SiO.sub.2] particle diameter is 151 nm.

The hydrodynamic diameter and size distribution of [SiO.sub.2]-PMMA were obtained by fitting the time correlation function [g.sup.(1)](q, [tau]) to a single exponential. Figure 3 illustrates a representative time correlation function and [q.sup.2]-[GAMMA] plot for the S3 sample of Table 2. The diameter was found to be ca. 565 nm. The corresponding size distribution is also shown as an inset in Fig. 3a. However, the observed particle diameter was approximately three times greater than the calculated diameter obtained from the compact core-shell model (24) using a [SiO.sub.2] core and PMMA shell. This indicates that the polymer chains adopted a stretched conformation from the surface, which is characteristic of a polymer brush. This phenomenon could be interpreted as being a consequence of the strong anisotropic interactions among the polymer chains arising from the high surface density (25), (33). The other [SiO.sub.2]-PMMA particle samples (S1, S2) were nearly monodisperse and had a fully expanded chain conformation.


Preparation of [SiO.sub.2]-PMMA Particle Array and Measurement of ([pi]-A) Isotherm

Figure 4 depicts the surface pressure-area ([pi]-A) isotherms for three different [SiO.sub.2]-PMMA particles spread onto the air-water interface. The shape of the curves indicates that all of the [SiO.sub.2]-PMMA particles formed a stable monolayer at the air-water interface and showed some interesting characteristics. The most notable point is that the measured occupied area was much larger than the one expected from the hydrodynamic diameter of the polymer-modified particle. The onset area for the increasing surface pressure in the particle array system is generally several times larger than the area corresponding to the particle diameter (39). Thus, this characteristic [pi]-A isotherm suggests that PMMA chains were fully expanded at the air-water interface.


Another characteristic of the [pi]-A isotherms is that all of the isotherms showed a plateau-like region around 15 mN * [m.sup.-1], which was probably due to the structural change that was necessary to offset the increase in the surface pressure (27). An X-ray reflectivity study about the structural change in the plateau-like region was reported in a previous work (40).

All of the monolayers of [SiO.sub.2]-PMMA were successfully transferred onto a cleaned glass substrate by LB deposition at a constant surface pressure of 20 mN [m.sup.-1]. A partial high array structure was observed more in the transferred films, shown in Fig. 5, than in the low density polymer-grafted silica (27). Although many large voids were also observed, Fig. 5a shows the formation of a close packed structure in the mono-particle layer. The highly particle-arrayed structure was probably due to the strong steric repulsion between the grafted polymer chains. The reason for the small number of particles in Fig. 5a is still unclear. The particle layer formed at the air/water interface was transferred onto a glass substrate, and SEM images of the transferred particles were taken. Presumably, a strong hydrophobic interaction between the high molecular weight grafted polymer chains on silica might simultaneously work to reduce number of transferred particles during transcription of the particles from the air/water interface on the glass substrate. However, for the S2 and S3 samples (Fig. 5b and c), the particles were separately located, and the distance between each particle seemed to be almost uniform. The interparticle distance calculated from the [pi]-A isotherms turned out to be close to the distance obtained from the SEM image. The array quality was also estimated by laser confocal microscopic (LCM) images (Fig. 5d-f). The LCM images were in fair agreement with the SEM observations.


Particle layering for the S3 sample was performed by repeating the depositing and drying treatments aiming for 3D ordered structure. Figure 6 represents the cross-sectional SEM images of the particle monolayer, double-layer, and five-layer that were used to investigate the morphology of the multilayers prepared by LB deposition, where the five-layer was obtained from five successive transfer steps in the LB deposition experiment. Hemispherical [SiO.sub.2]-PMMA particles of the monolayer were observed on the smooth, flat surface. The height of the monolayer measured from the lowest surface of the matrix was about 170 nm, which was comparable with the size estimated by the Gaussian chain model. The height of the double-layer was almost the same as that of the monolayer, as shown in Fig. 6b. However, the five-layer height increased remarkably compared with the monolayer. Additionally, the particles were not spherical in shape, but instead, many of the particle aggregates formed a ridge line. The structural model schematic depicted in Fig. 7 was suggested by the observations made for the morphology of the particle multilayer. It was speculated from the SEM images that the PMMA chains strongly interacted with glass substrate due to their hydrophilicity, but PMMA chains in contact with air shrunk in order to lower their surface area. Thus, the hemispherical monolayer of particles was formed on the glass substrate (Fig. 7a). When the monolayer with a nonclose packed structure was subjected to repeated-transfer steps in a LB deposition experiment, the voids between particles filled in to form a denser structure (Fig. 7b). Consequently, further layering past the fifth transfer step may give a particle array in the form of ridge line, as illustrated in Fig. 7c.




Silica particles coated with well-defined, highly grafted PMMA were prepared by surface-initiated ARGET ATRP of MMA with an initiator-fixed silica particle under a limited amount of air. Polymerization of MMA from silica nanoparticles by the ARGET ATRP technique exhibited good molecular weight control with a relatively low PDI in a [CuCl.sub.2]/PMDETA system but not in a [CuCl.sub.2]/[Me.sub.6] TREN system. Thus, the PMDETA ligand was more suitable for the ARGET ATRP of MMA. PMMA chains exerted interparticle interactions over an extremely long range due to high surface density at the air-water interface. Consequently, [pi]-A isotherms and SEM observations confirmed that the silica particles with well-defined, highly dense PMMA chains formed a stable particle monolayer at the air-water interface. This method was also used to fabricate a particle multi-layer using repeated transfer steps in a LB deposition. The thickness of the double-layer was almost the same as that of monolayer because the particles of double-layer were probably located at vacant positions of the glass substrate. In the five-layer, the particle array formed a ridge line.


This work was supported for two years by a Pusan National University Research Grant. The authors thank the Kyushu Institute of Technology for Langmuir-Blodgett.


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Jung-Min Moon, (1) Bong-Soo Kim, (1) Hyun-Jong Paik, (1) Jang-Oo Lee, (1) Emiko Mouri, (2) Kohji Yoshinaga (2)

(1) Division of Chemical Engineering, Pusan National University, Geumjeonggu, Busan 609-735, Republic of Korea

(2) Department of Applied Chemistry, Kyushu Institute of Technology, Sensui, Tobata, Kitakyushu 804-8550, Japan

Correspondence to: Jang-Oo Lee; e-mail:

Published online in Wiley InterScience (

[C] 2009 Society of Plastics Engineers

DOI 10.1002/pen.21639
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Author:Moon, Jung-Min; Kim, Bong-Soo; Paik, Hyun-Jong; Lee, Jang-Oo; Mouri, Emiko; Yoshinaga, Kohji
Publication:Polymer Engineering and Science
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Geographic Code:9JAPA
Date:Jun 1, 2010
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