A particle at interface is a simplest system concerning interfaces and has wide variety of interests ranging from basic science to industrial process . Among the particulate fabrications, in particular, there has been a great interest in creating materials with anisotropic property. One is to fabricate materials with anisotropic shape such as disc or rod. Another is to make anisotropic nature with spherical shape, i.e., Janus particles  . In the latter category, preparation process is further categorized into two. One is to making use of association behavior like micelle or emulsion including non-equilibrium status     . The other is to utilizing the planner interface, for example, air/water interface or liquid/liquid interface    , which concerns the topic of this paper.
In this paper, we describe the fabrication of polymer-grafted particle monolayer with anisotropic features introduced at the air-aqueous medium interface. We have been studying polymer-grafted particle system at the air-water and oil-water interfaces -. We systematically investigated the effects of molecular weight, graft density, core particle, polymer species, and interface on the polymethacrylate-grafted particle monolayer structure by π-A isotherm measurement, SEM observation and X-ray reflectivity measurement. So far, we found that polymethacrylate-grafted particles forms monolayer at air-water interface and grafted polymethacrylate chains are fairly extended at the interfaces. On the basis of these previous findings, we verified to fabricate anisotropic polymer-grafted particle by utilizing potential reactivity at the air/water interface.
2. Experimental Method
2.1. Synthesis and Characterization of PtBMA-Grafted Polystyrene Latex
Polystyrene latex with chloromethyl groups was synthesized by soap-free emulsion polymerization as previously described . The averaged diameter of polystyrene latex was estimated by dynamic light scattering with DLS-8000 (Otsuka Electronics Co., Ltd., Hirakata, Japan). Polydispersity was confirmed to be within 1%. Graft polymerization of PMMA from polystyrene latex surface via copper-mediated ATRP was carried out as follows  : The polystyrene latex dispersion of DME was mixed with freshly distilled PtBMA, CuBr (I) and N,N,N',N",N"-pentamethyldiethylenetrimamime (PMDETA). The mixture was carefully bubbled and degassed by ultrasonification under argon atmosphere. Then free initiator, α-chloro-p-xylene, was added to the mixture. Typical feed is 40 mg of polystyrene latex, 6.6 ml of dry MMA, 7 mg of CuBr, and 32 mg of PMDETA with 6.4 mg of free initiator. The solution was degassed by four or five freeze-pump-thaw cycles and the ample was sealed off completely with argon purge. The polymerization was carried out at 80˚C for 4 days to obtain PtBMA-grafted polystyrene latex (PSL-PtBMA).
Molecular weight and polydispersity of free polymer were confirmed by GPC equipped with column oven (CTO-10A/10AC, Shimadzu, Kyoto, Japan) and refractive index detector (RI153, JASCO Corp., Tokyo, Japan). Obtained polymer-grafted particle was firstly characterized by dynamic light scattering (DLS). Time correlation function was fitted to single exponential function, and hydrodynamic diameter and high monodispersity was confirmed.
Estimation of graft density is performed by utilizing IR spectrum measurement as follows: Calibration curves for P(tBMA) and PSL was obtained respectively with 2-cyanoacetoamide as internal standard as same as previous study . The following peaks were picked up for calibration curves; 1475 cm−1 (PSL, benzene ring in-plane vibration), 1140 cm−1 (PtBMA, C-O stretching), and 2270 cm−1 (cyanoacetoamide, stretching). The linier relationships were obtained between molar ratio of [tBMA]/[2-cyanoacetoamide] (or [styrene]/[2-cyanoacetoamide]) and peak area ratio [C-O stretching]/[ stretching] (or [benzene ring in-plane vibration]/[ stretching]). By IR measurement of PSL-PtBMA as in Figure 1, the amount ratios of PtBMA to PSL were obtained for each sample. Then, the graft density of PtBMA was calculated with the two assumptions that 1) the molecular weight of grafted PtBMA is equal to Mn obtained by GPC, and 2) polystyrene latex was made of only polystyrene with density of 1.047.
2.2. Preparation of PMMA-Grafted Polystyrene Latex Monolayer and Measurement of π-A Isotherms
The preparation of particle monolayer was carried out as previously reported -. The chloroform solution of 2.5 × 10−4 g/ml PSL-PtBMA was spread by microsyringe on ultra-pure water surface and HCl solution (pH3), respectively in Langmuir-Blodgett (LB) trough (150 mm × 160 mm). After evaporation of solvent, the water surface was compressed and expanded by Teflon® barrier at 0.025 mm/sec to measure π-A isotherm at 25˚C by USI-3-00 (USI System, Fukuoka, Japan). At various surface pressures on a second compression, particle layer was deposited on glass plate by LB method at the speed of 10 mm/min. We observed the deposited particle layers by SEM (SM-7000FSK, JEOL Ltd., Tokyo, Japan). In order to estimate the basic property of a deposited layer on a substrate, contact angle measurement was measured by dropping 0.5 μL pure water on a substrate and observing the drop shape by a digital camera with a-close-up lens .
Figure 1. IR spectrum for PSL-PtBMA added with cyanoacetoamide with internal standard.
3. Results and Discussion
The characteristics of core PS particle and PSL-PtBMA are summarized in Table 1 and Table 2, respectively. Important parameters for PSL-PtBMA, molecular weight of grafted polymers, graft density and hydrodynamic size in the solution, are summarized in Table 2. Although we obtained PtBMA homopolymer with high monodisperse nature (Mw/Mn = 1.15 - 1.24) in the comparable system without PSL, PtBMA molecular weight with PSL showed wider molecular weight distribution. Since the hydrodynamic size of PSL used here is 118 nm, PtBMA brush layer with ca.10-nm thickness covers around PSL latex core. The graft density of 0.14 is relative high graft density -.
PSL-PtBMA of well-characterized sample is applied to monolayer formation at the air/water interface. Figure 2 shows the π-A isotherm of PSL-PtBMA at the air-water interface as curve 1. The π-A isotherm is reproducible and shows the increase of surface pressure at a very large occupied area. For example, expected occupied area with PSL particles with closest packing is indicated by straight line in Figure 1, which shows more than 1/10 smaller area. As also discussed in the
Table 1. Characteristics of PSL.
Table 2. Characteristics of PSL-PtBMA.
Figure 2. π-A isotherms for PSL-PtBMA on a pure water and on a HCl aqueous solution. Straight line indicates the occupied area of PSL core at closest packing.
former our papers, this indicates that polymer chains grafted from particles are much extended on the water surface  . In this data, the occupied area at 1 mN/m is corresponds to the interparticle distance of 388 nm assuming closest packing of particle at the air-water interface. On the other hand, calculated interparticle distance with stretched conformation of Mn = 76,000 is to be 386 nm. These two values are in agreement. We do know that the molecular weight used here is relative value and contains errors, however, the agreement of these values supports the extended fairly conformation of PtBMA on a watersurface similar to PMMA system .
SEM images of deposited monolayer by LB method at several constant surface pressuresare presented in Figures 3(a)-(c). These SEM images support the monolayer formation, not multilayer. We can see individual PSL core with separation of more than 150 nm.
In order to introduce “anisotropicity” in the particle monolayer by utilizing the nature of air-water interface, monolayer formation of PSL-tBMA is conducted on acidic aqueous solution of pH3. Partial proceeding of PtBMA hydrolysis in water subphase may occur because the hydrolysis of PtBMA into PMAA is irreversible reaction while PtBMA remains at the air side. However, the acidic condition here is milder than usual hydrolysis condition for PtBMA  . Thus, small fraction of tBMA monomer units would be hydrolyzed to give methacrylic acid units.
In Figure 2, π-A isotherm obtained on acidic subphase is also presented as curve 2. The π-A isotherm on acidic subphase shows clear shift to larger occupied area before plateau region. On the other hand, two π-A isotherms at different conditions match after plateau region. Based our previous data, below the plateau region the interaction between polymer chains grafted from particle is observed while the interaction between hard spheres is observed above the plateau region . Considering the interpretation of π-A isotherm, it is reasonable
Figure 3. SEM images of deposited PSL-PtBMA monolayer on glass substrates at constant surface pressures ((a), (d): 10 mN/m, (b), (e): 15 mN/m, (c) (f): 30 mN/m) from pure water (a)-(c) and HCl aq. solution (d)-(f). Schematic figures are also shown.
that the difference between the π-A isotherms are observed only below the plateau region. In addition to that, assuming the hydrolysis proceeds in some part, the interaction between PMAA (the resultant of hydrolysis of PtBMA) is enhanced little bet by the electrostatic interaction (although almost MAA is not dissociated in acidic pH range), which agrees with the tendency that surface pressure is increased at the larger occupied area.
SEM images of deposited monolayer by LB method at several constant surface pressures are presented in Figures 3(d)-(f). SEM images obtained at acidic condition shows more aggregated states than the ones at the air-water interface. The anisotropic nature of PSL-PtBMA at acidic condition would favors formation of large assembly of polymer-grafted PSL although we cannot ascertain whether the large aggregates are formed at the water surface or at the glass substrates.
We also estimate the surface property of deposited particle monolayer on substrate. Figure 4 shows variation of contact angle of water droplet on the particle monolayers with varying surface pressure. The values of contact angles are similar at the two different subphase conditions above 5 mN/m. This is rather reasonable that the outer surface would be PtBMA because particle monolayer is deposited as Y type. In the studied range, aggregation shown in Figure 3 is not so effective to the contact angels. By changing the substrate to more hydrophobic one, we could tune the contact angle by PSL-PtBMA from acidic subphase.
In the view of fabricating anisotropic particles monolayer, we investigated the behavior of PSL-P(t-butyl methacrylate) at the air/acidic water interface. PtBMA was polymerized from monodisperse polystyrene latex (PSL) surface by ATRP to givemonodisperse PtBMA-grafted PSL (PSL-PtBMA). PSL-PtBMA monolayer was characterized by π-A isotherms and SEM. PSL-PtBMA monolayer deposited from acidic condition shows more aggregated state on glass substrate rather than the ones deposited from pure water, however, the contact angle of the both
Figure 4. Contact angle of water droplets measured on PSL-PtBMA monolayer deposited on glass substrates at constant surface pressures with different subphase conditions.
deposited film shows similar values.
We are greatly thankful to Prof. Kohji Yoshinaga (Professor Emeritus of Kyushu Institute of Technology) for his advice in the experiments especially for the synthesis of the samples. This work was financially supported by a grant-in-aid (No. 19750099, 24850015) from JSPS.