Surface Dilatational Moduli of Poly(vinyl acetate) (PVAc) and PVAc

ARTICLE pubs.acs.org/Langmuir

Surface Dilatational Moduli of Poly(vinyl acetate) (PVAc) and PVAc-Poly(n-hexyl isocyanate) (PHIC) Blend Films at the Air
Water Interface Takako Morioka* and Masami Kawaguchi Division of Chemistry for Materials, Graduate School of Engineering, Mie University, 1577 Kurimamachiya, Tsu, Mie 514-8507, Japan ABSTRACT: Surface dilatational moduli of poly(vinyl acetate) (PVAc) film and blend films of PVAc and poly(n-hexyl isocyanate) (PHIC) were measured at the air
water interface. PVAc formed a film that was looser and also more stable against strain than the PHIC film. The apparent surface dilatational modulus and surface pressure of the blend films were superimposed on the lower concentration of PVAc, irrespective of the composition of PVAc. However, the additivity rule was not applicable to the apparent surface dilatational modulus and surface pressure. The scaling exponents of the apparent surface dilatational modulus against the added surface concentration decreased with an increase in the proportion of PVAc, suggesting that blend films gradually change from glass material to expanded films.

’ INTRODUCTION Surface rheological measurements, in certain cases, provide a powerful method for determining the stability of emulsions and foams. Correlation between the stability and the surface dilatational modulus of emulsions and foams has been illustrated using a pendant drop method,1
5 and various methods including the capillary method,6,7 capillary wave method,8
12 and interfacial shear rheometry2,13
15 have been applied to the measurement of surface rheological properties. The use of a Langmuir trough, on which two barriers are symmetrically equipped, is widely accepted as an easy-to-use method for measuring the surface dilatational modulus. In this technique, a strain is applied by moving the two barriers sinusoidally, the surface pressure response is measured, and the surface dilatational modulus is then calculated.16 This method has been used because the surface dilatational modulus and the stability of emulsions and foams are closely related.17,18 By employing this technique, many studies on the surface dilatational modulus of water-soluble polymers17,19,20 and proteins have been undertaken.18,21,22 Monolayers formed by water-insoluble polymers have also been extensively studied.23 The surface pressure isotherm characteristics of hydrophilic water-insoluble polymers are markedly different from those of hydrophobic polymers. A hydrophilic polymer forms an expanded film at the air
water interface, whereas a hydrophobic polymer forms a condensed film at the interface. Numerous data from surface rheological measurements of water-insoluble polymers have been amassed, and the more recent studies have been summarized by Monroy et al.24,25 These authors have studied various homopolymers by means of step-compression, oscillatory barrier, electrocapillary wave, and surface light scattering experiments. Because the miscibility of blend films can be inferred from the characteristics of the surface pressure isotherms, many studies have been performed on the blend films of insoluble polymers using this technique.26
34 r 2011 American Chemical Society

However, surface rheological measurements of the blend films of insoluble polymers have only been carried out for poly(vinyl acetate)/poly(dimethylsiloxane) using the quasi-elastic light scattering technique35 and for poly(vinyl acetate)/poly(4-hydroxy styrene) using step-compression, oscillatory barrier, electrocapillary wave, and surface light scattering experiments, especially for obtaining a wide range of frequencies;36 there are no reports on the measurement of the surface dilatational modulus of the blend film of insoluble polymers using only the oscillatory barrier method over a wide surface concentration range. Poly(vinyl acetate) (PVAc) is an archetypal flexible polymer that has been widely used as a stable expanded film.37 Poly(n-hexyl isocyanate) (PHIC) is known as a typical semiflexible polymer, and the solution properties38
40 as well as the solid state properties of PHIC have been reported.41 The stability of PHIC as a monolayer has been clarified by Kawaguchi et al.42
46 PHIC monolayers behave as typical condensed films. AFM measurements also revealed that PHIC formed bundles at the air
water interface and that the bundles could interpenetrate when the contour length of PHIC was sufficiently longer than the persistence length of PHIC.47 This interpenetration of bundles determines the film properties of PHIC. A film of PHIC is hard and brittle and can be classified as a two-dimensional glass material. The blend film of PVAc and PHIC has been investigated by surface pressure and fluorescence microscopy, and the compatibility of the blend film at the air
water interface has been confirmed.45 Because the compatibility of this system has already been carefully investigated, we focused on the surface dilatational modulus of the blend film of PVAc and PHIC. Prior to this, we compared the characteristics of the surface Received: December 24, 2010 Revised: June 8, 2011 Published: June 08, 2011 8672

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dilatational behavior of PVAc with those of PHIC because the surface dilatational modulus properties for PHIC have already been investigated.46

’ EXPERIMENTAL SECTION Materials. PVAc was purchased from PolyScience (U.S.A.). It was diluted with ethanol, purified through dropwise precipitation in a large amount of water, and dried under vacuum. The PVAc sample was separated into 13 fractions in an acetone
hexane mixture. One fractionated sample was chosen, and its weight average molecular weight was determined to be 70  103 g mol
1 by GPC in tetrahydrofuran at 25 °C. PHIC was synthesized by polymerizing hexyl isocyanate in dimethylformamide with NaCN dispersed in dimethylformamide as the initiator.38,42 The resulting PHIC sample was diluted with toluene, purified through dropwise precipitation in a large amount of methanol, and dried under vacuum. The PHIC sample was separated into eleven fractions in a toluene
methanol mixture. One fractionated sample was chosen; the weight average molecular weight was determined to be 245  103 g mol
1 using intrinsic viscosity measurements in toluene, at 25 °C. From the corresponding molecular weight, 245  103 g mol
1, the contour length was calculated to be 336 nm, which is significantly larger than the 43 nm persistence length of PHIC.48 PVAc and PVAc-PHIC blend solutions were prepareed in chloroform (Nacalai Tesque, spectrum grade). The concentration of the respective spreading solutions was 0.5 mg mL
1. Surface Pressure Measurements. Surface pressure was recorded at 25.0 ( 0.1 °C using a KSV 2000 (KSV Instruments Ltd., Finland) Langmuir
Blodgett system for all measurements. The surface pressure isotherm was measured by the successive addition method for the trough area of 190 cm2. A time interval of 15
120 min was allowed to elapse for both solvent evaporation and surface pressure relaxation. Reproducibility was confirmed from a minimum of two measurements, and the error was less than 0.5 mN m
1. Surface dilatational measurements were also performed using the KSV 2000 system. Two barriers were mounted on both ends of the trough in this system in order to study the oscillating barriers. The oscillatory barrier method was described in detail in a previous paper.46 The apparent surface dilatational modulus |E*|app is defined by eq 1 jEjapp ¼ Δπ=2u0

ð1Þ

where Δπ is the difference between the maximum and the minimum surface pressures against an imposed strain u0. The imposed strain u0 is defined by eq 2 u ¼ u0 sin ωt

ð2Þ

where u is the sinusoidal oscillatory strain imposed by two barrier oscillation, ω is a given frequency, and t is time. The linear response of the surface pressure against the imposed strain is confirmed by the shape of the Lissajous orbit of the former versus the latter. An elliptical shape of the Lissajous orbit confirms a linear response of the surface pressure to the imposed strain, and the elastic and viscous components of the surface dilatational modulus can be obtained. In this study, the imposed strain was 1
20% and almost all of the surface pressure responses were nonlinear; consequently, only the apparent surface dilatational modulus was used throughout this study. The trough area was 190 cm2, and the frequency was fixed at 20 mHz.

’ RESULTS AND DISCUSSIONS Figure 1a shows the surface pressure isotherms for PVAc, PHIC, and PVAc-PHIC blend films as a function of the surface concentration of PVAc (ΓPVAc) for PVAc and PVAC-PHIC

Figure 1. Surface pressure isotherms for PVAc, PHIC, and PVAc-PHIC blend films as a function of ΓPVAc for PVAc and PVAc-PHIC blend films and as a function of ΓPHIC for PHIC film. (a) PVAc (9), 4/1 PVAcPHIC (b), 1/1 PVAc-PHIC (2), 1/2 PVAc-PHIC (1), 1/9 PVAcPHIC (), and PHIC ((). (b) Surface pressure at the plateau as a function of XPVAc.

blend films and as a function of the surface concentration of PHIC (ΓPHIC) for the PHIC film. The isotherms of all of the samples show an increase in surface pressure as the surface concentration increases and eventually saturates as it reaches a plateau. At the plateau, the films are likely to form multilayers. Below ΓPVAc = 0.2 mg m
2, the surface pressure isotherms for all of the PVAc-PHIC blend films are well superimposed over those for PVAc. This implies that the surface pressure of PVAc, dominates the surface pressure properties of the blend films until the surface pressure reaches the plateau. Figure 1b shows the surface pressure at the plateau as a function of the molar composition of PVAc (XPVAc). The surface pressure at the plateau increases with an increase in XPVAc, suggesting that PVAc and PHIC are compatible at the air
water interface as opposed to the occurrence of the preferential adsorption of PVAc at the air
water interface. This result is consistent with a previous study in which Kawaguchi and Suzuki reported that PVAc and PHIC were compatible at the air
water interface.45 For the PVAc film, the slope of the double-logarithmic plot of surface pressure versus surface concentration was 2.8; this slope indicates that the air
water interface presents a good solvent condition for PVAc. This result is consistent with that of the previous study.37 The overlapping surface concentration (Γ*) can be estimated by extrapolating the straight-line portion of the surface pressure isotherm to zero surface pressure. The dilute regime (Γ < Γ*) and the semidilute regime (Γ > Γ*) can then be determined. The semidilute regimes for PVAc and PHIC are 0.5
0.9 and 0.96
1.02 mg m
2,46 respectively. A plateau region is defined as the concentrated regime in which the film is likely to form a multilayer, as mentioned above. In a previous paper, we reported 8673

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Figure 2. Apparent surface dilatational modulus of PVAc in a semidilute regime of 0.8 mg m
2.

Figure 4. Surface dilatational modulus of PVAc, PHIC and PVACPHIC blend films as functions of (a) Γ and (b) ΓPVAc (ΓPHIC for PHIC film); symbols: PVAc (9), 4/1 PVAc-PHIC (b), 1/1 PVAc-PHIC (2), 1/2 PVAc-PHIC (1), 1/9 PVAc-PHIC (), and PHIC ((). Figure 3. Static compression modulus and apparent surface dilatational modulus of PVAc. The solid line shows the static compression modulus, and the solid square shows the apparent surface dilatational modulus.

decrease in the minimum value of the surface pressure response during several barrier oscillatory cycles. The static compression modulus (Es) can be estimated from eq 3

that the apparent surface dilatational modulus of PHIC in the semidilute regime was more strain dependent than the elastic and viscous components of the surface dilatational modulus in the dilute regime. Therefore, it is meaningful to compare the surface dilatational modulus of PVAc with that of PHIC in the semidilute regime. Figure 2 shows the apparent surface dilatational modulus (|E*|app) of PVAc in the semidilute regime of 0.8 mg m
2 as a function of imposed strain, u0. The apparent surface dilatational modulus of PVAc gradually increases with the strain until the imposed strain increases by 5
10% and reachs a plateau at ∼25 mN m
1. This behavior is significantly different from that of PHIC.46 The apparent surface dilatational modulus of PHIC rapidly increases with the strain until the strain increases by 3%, attains the maximum value of ∼30 mN m
1, and decreases drastically after the point at which the strain had increased by 3%. Moreover, negative hysteresis was observed for the Lissajous orbit of surface pressure versus the sinusoidal strain for PHIC.46 Thus, PVAc should form a looser film at the air
water interface as compared to PHIC because PVAc attains a maximum value at a greater strain as compared to PHIC. Moreover, the apparent surface dilatational modulus of PVAc became constant, whereas that of PHIC decreased with an increase in strain. This result suggests that PVAc forms a tougher film than does PHIC and that the former is more stable at the air
water interface than the latter. However, in the concentrated regime (ΓPVAc > ∼1 mg m
2), PVAc film easily leaks out of the edge of the trough during the surface dilatational modulus measurement, whereas such a leak was not observed for the PHIC film for any of the examined concentrations.46 This leakage resulted in a

Es ¼
Að∂π=∂AÞT

ð3Þ

The Es value of PVAc was estimated from the surface pressure isotherm in Figure 1a. Figure 3 shows the Es value of PVAc and the apparent surface dilatational modulus of PVAc when the imposed strain was 10%. The Es value and the apparent surface dilatational modulus were in good agreement within the experimental range. The smaller value of the apparent surface dilatational modulus observed in the smaller strain range might be related to a looser structure of PVAc at the air
water interface. The Es value at 0.8 mg m
2 was ∼32 mN m
1; this value is slightly larger than the values of ∼27 mN m
1 and ∼24 mN m
2 reported in studies by Kawaguchi et. al and by Monroy et. Al, respectively.49,50 Furthermore, Kawaguchi et. al studied the surface light scattering at the same surface concentration, and the resulting value was in good agreement with the calculated Es value in that study. Figure 4, panels a and b, shows the apparent surface dilatational moduli of PVAc, PHIC, and PVAc-PHIC blend films as functions of the added surface concentration, Γ and ΓPVAc (ΓPHIC for PHIC film), respectively. The imposed strain was fixed at 10% for the PVAc, 4/1 PVAc-PHIC, and 1/1 PVAcPHIC films because negative hysteresis was not observed for the corresponding films. On the other hand, because the 1/2 PVAcPHIC, 1/9 PVAc-PHIC, and PHIC films showed negative hysteresis at higher values of Γ, the maximum values of the apparent surface dilatational modulus obtained at 10% or at less than 10% strain were plotted. Regardless of the value of XPVAc, the apparent surface dilatational modulus of the blend films increases logarithmically with an increase in Γ in the semidilute 8674

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Figure 5. Scaling exponent of the apparent surface dilatational modulus as a function of XPVAc.

regime. In the concentrated regime, the apparent surface dilatational modulus did not exhibit the smaller value, whereas the static compression modulus went to zero (see Figure 1a). In Figure 4b, the apparent surface dilatational moduli of PVAc and PVAc-PHIC blend films are superimposed upon ΓPVAc. However, a deviation between the curves occurs at a smaller ΓPVAc value of 0.1 mg m
2. Figure 5 shows the scaling exponent of the apparent surface dilatational modulus against Γ as a function of XPVAc. The scaling exponent is defined by eq 4 jEjapp µ Γn

Figure 6. Apparent surface dilatational modulus as a function of XPVAc at ΓPVAc values of (a) 0.3 and (b) 0.8 mg m
2.

ð4Þ

where n is the scaling exponent. Therefore, the scaling exponent can be estimated from the slope of the plots in Figure 4a. The scaling exponent decreases with an increase in XPVAc, and the scaling exponent of PHIC (i.e., 4.6) was determined in our previous study.46 Here, PVAc formed an expanded film and PHIC formed a two-dimensional glass material, as mentioned above. Thus, the blend films gradually changed from a glass-like material to expanded films as the value of XPVAc increased. The scaling exponent (n) of PVAc was in good agreement with the value of 2.4 obtained for the scaling exponent of the elastic component of the surface dilatational modulus (E0 ) from a previous study,51 and it was slightly smaller than 2.8.52 Here, the surface pressure response against imposed strain was linear for the PVAc film. In addition, the apparent surface dilatational modulus and E0 had the almost the same value in the PVAc film system. Figure 6, panels a and b, shows the apparent surface dilatational modulus as a function of XPVAc at ΓPVAc values of 0.3 and 0.8 mg m
2, respectively. The respective surface concentrations correspond to the dilute regime and the semidilute regime for a PVAc film. The surface dilatational moduli decrease rapidly with an increase in XPVAc, irrespective of the value of ΓPVAc. This result indicates that the surface dilatational modulus is not additive for this system. Figure 7 shows the apparent surface dilatational modulus of the 1/1 PVAc-PHIC blend film at an added surface concentration of 1.8 mg m
2, where both PVAc and PHIC are in the semidilute regime. The respective surface concentrations of PVAc and PHIC are 0.8 and 1.0 mg m
2. The added values of the apparent surface dilatational moduli of PVAc at 0.8 mg m
2 surface concentration and of PHIC at 1.0 mg m
2 surface concentration are also plotted in Figure 7 as a function of imposed strain. If a PVAc-PHIC blend film is ideally miscible,

Figure 7. Apparent surface dilatational modulus of the 1/1 PVAc-PHIC blend film at the surface concentration of 1.8 mg m
2 where both PVAc and PHIC are in the semidilute regime (0.8 mg m
2 for PVAc and 1.0 mg m
2 for PHIC) (9) and the added values of the apparent surface dilatational modulus of PVAc and PHIC at surface concentrations of 0.8 and 1.0 mg m
2, respectively (b).

the measured surface dilatational modulus may coincide with the calculated one. However, in the present case, the latter is found to be smaller than the former, and this implies that PVAc and PHIC are compatible but partly immiscible, as mentioned above. Moreover, the added values of the surface dilatational moduli of PVAc at 0.8 mg m
2 surface concentration and of PHIC at 1.0 mg m
2 surface concentration reached a maximum at 3% strain and they gradually decreased with increasing strain; this reflected that the PHIC film at 1.0 mg m
2 had a maximum surface dilatational modulus at 3% strain. On the other hand, the surface dilatational modulus of the 1/1 PVAc-PHIC blend film at an added surface concentration of 1.8 mg m
2 exhibited a maximum value at 3% strain and tended to a constant value as the strain increased, indicating that the 1/1 PVAc-PHIC blend film at 1.8 mg m
2 was stable against strain. 8675

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Figure 8. Apparent surface dilatational modulus of the 4/1 PVAc-PHIC blend film as a function of strain in the concentrated regime of ΓPVAc = 1.54 mg m
2.

Although PVAc films are more stable than PHIC films in the semidilute regime, PVAc films tend to leak out of the edge of the trough in the concentrated regime (ΓPVAc > ∼1 mg m
2), as was mentioned above. On the other hand, the PVAc-PHIC blend films do not leak out of the edge of the trough in the concentrated regime, even when the ratio of PHIC to PVAc is rather small, such as in the 4/1 PVAc-PHIC blend film. Figure 8 shows the apparent surface dilatational modulus of the 4/1 PVAc-PHIC blend film as a function of imposed strain in the concentrated regime of ΓPVAc = 1.54 mg m
2, where the surface concentration of PHIC is in the dilute regime at 0.57 mg m
2. The plot of the surface dilatational modulus shows a bump at a strain of 3% and then the value of the modulus becomes constant, suggesting that the 4/1 PVAc-PHIC film is stable at the air
water interface.

’ CONCLUSIONS First, the apparent surface dilatational modulus of a PVAc film was measured at the air
water interface and compared with that of a PHIC film. In the semidilute regime, PVAc formed a looser film that was more stable against strain than the PHIC film, but in the concentrated regime, the PVAc film easily leaked out of the edge of the trough. The apparent surface dilatational moduli of PVAc-PHIC blend films were subsequently measured, and it was found that the apparent surface dilatational modulus was superimposed on the low ΓPVAc regime. However, the additivity rule was not applicable to either the apparent surface dilatational modulus or the surface pressure of this system. In the semidilute regime, the power (scaling exponent) of the apparent surface dilatational modulus against added surface concentration decreased with an increase in the PVAc component of the blend film; this suggests that the blend films gradually change from a two-dimensional glass material to expanded films. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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