Effect of Salt Concentration on the Nanostructure of Weak Polyacid Brush in the Amphiphilic Polymer Monolayer at the Air/Water Interface

Abstract

The effect of salt concentration on the nanostructure of a spread monolayer of ionic amphiphilic diblock copolymer, (diethylsilacyclobutane)m-b-(methacrylic acid)n, at the air-water interface was directly investigated by in situ X-ray reflectivity and neutron reflectivity techniques. Previously, we had found that a poly(methacrylic acid) (PMAA) hydrophilic layer under the water was not in the form of a simple polyelectrolyte brush but consisted of a dense carpet upper layer and a diffuse brush lower layer when the hydrophilic chain was long enough. Here we observed this double layer formation in the monolayer in aqueous NaCl solution at a constant surface pressure. The effect of salt added to the subphase differed with the salt concentrations, that is, below or above 0.1 M. In the presence of NaCl up to 0.1 M, both the hydrophobic layer and brush layer thicknesses decreased. On the other hand, both of them increased in the presence of NaCl above 0.1 M. Also, the carpet layer thickness was almost constant independent of the salt concentration. In addition, the brush top roughness showed a maximum in the presence of 0.1 M NaCl. The increase of the charge number on the PMAA chain and the screening effect of the Coulomb interaction by added salt ions were considered to be responsible for these phenomena.

Evaluation of Small Ion Distribution in the Polyelectrolyte Brush at the Air/Water Interface by Neutron Reflectometry

“…The effect of NaCl addition to water subphase for poly(Et 2 SB)-b-poly(MAA) monolayer system was duly investigated by XR and NR in our previous study. [8] The result was in principle in agreement qualitatively with previous theoretical studies. [20][21][22][23] NaCl concentration was changed from OM up to 1M, and what was interesting was the observation of expandingshrinking behavior of the monolayer with increasing salt concentration.…”

Evaluation of Small Ion Distribution in the Polyelectrolyte Brush at the Air/Water Interface by Neutron Reflectometry

“…The effect of NaCl addition to water subphase for poly(Et 2 SB)-b-poly(MAA) monolayer system was duly investigated by XR and NR in our previous study. [8] The result was in principle in agreement qualitatively with previous theoretical studies. [20][21][22][23] NaCl concentration was changed from OM up to 1M, and what was interesting was the observation of expandingshrinking behavior of the monolayer with increasing salt concentration.…”

Nanostructure of a Poly(acrylic acid) Brush and Its Transition in the Amphiphilic Diblock Copolymer Monolayer on the Water Surface

“…Zhulina et al predicted this phenomenon theoretically . We observed the same trend for the PMAA brush previously and for the PAA brush in this study. Currie et al reported a similar trend, and Kurihara et alalso observed an increase in repulsive force by salt addition via a surface force measurement (SFA).…”

“…This observation should mean that some transition occurs under the 0.10 M NaCl condition. Similar behavior was also observed for the monolayer having a PMAA brush . The results of detailed analysis and discussion will be given later with the results of XR analysis.…”

Critical Brush Density for the Transition between Carpet-Only and Carpet/Brush Double-Layered Structures. 2. Hydrophilic Chain Length Dependence

“…Each profile corresponds to a different f value, but the thickness of the hydrophobic layer was kept almost constant by tuning the A values (and hence the π values). As was observed previously, − , very clear Kiessig fringes were observed for all three polymers used in this study. Also, a small fringe at about q = 0.06 is observed in this study clearer than in a previous study on the critical brush density .…”

Evaluation of Small Ion Distribution in the Polyelectrolyte Brush at the Air/Water Interface by Neutron Reflectometry

“…The effect of NaCl addition to water subphase for poly(Et 2 SB)-b-poly(MAA) monolayer system was duly investigated by XR and NR in our previous study. [8] The result was in principle in agreement qualitatively with previous theoretical studies. [20][21][22][23] NaCl concentration was changed from OM up to 1M, and what was interesting was the observation of expandingshrinking behavior of the monolayer with increasing salt concentration.…”

Nanostructure of a Poly(acrylic acid) Brush and Its Transition in the Amphiphilic Diblock Copolymer Monolayer on the Water Surface

“…Zhulina et al predicted this phenomenon theoretically . We observed the same trend for the PMAA brush previously and for the PAA brush in this study. Currie et al reported a similar trend, and Kurihara et alalso observed an increase in repulsive force by salt addition via a surface force measurement (SFA).…”

“…This observation should mean that some transition occurs under the 0.10 M NaCl condition. Similar behavior was also observed for the monolayer having a PMAA brush . The results of detailed analysis and discussion will be given later with the results of XR analysis.…”

Critical Brush Density for the Transition between Carpet-Only and Carpet/Brush Double-Layered Structures. 2. Hydrophilic Chain Length Dependence

“…Each profile corresponds to a different f value, but the thickness of the hydrophobic layer was kept almost constant by tuning the A values (and hence the π values). As was observed previously, − , very clear Kiessig fringes were observed for all three polymers used in this study. Also, a small fringe at about q = 0.06 is observed in this study clearer than in a previous study on the critical brush density .…”

Evaluation of Small Ion Distribution in the Polyelectrolyte Brush at the Air/Water Interface by Neutron Reflectometry

“…The effect of NaCl addition to water subphase for poly(Et 2 SB)-b-poly(MAA) monolayer system was duly investigated by XR and NR in our previous study. [8] The result was in principle in agreement qualitatively with previous theoretical studies. [20][21][22][23] NaCl concentration was changed from OM up to 1M, and what was interesting was the observation of expandingshrinking behavior of the monolayer with increasing salt concentration.…”

Nanostructure of a Poly(acrylic acid) Brush and Its Transition in the Amphiphilic Diblock Copolymer Monolayer on the Water Surface

“…Zhulina et al predicted this phenomenon theoretically . We observed the same trend for the PMAA brush previously and for the PAA brush in this study. Currie et al reported a similar trend, and Kurihara et alalso observed an increase in repulsive force by salt addition via a surface force measurement (SFA).…”

“…This observation should mean that some transition occurs under the 0.10 M NaCl condition. Similar behavior was also observed for the monolayer having a PMAA brush . The results of detailed analysis and discussion will be given later with the results of XR analysis.…”

Critical Brush Density for the Transition between Carpet-Only and Carpet/Brush Double-Layered Structures. 2. Hydrophilic Chain Length Dependence

“…Each profile corresponds to a different f value, but the thickness of the hydrophobic layer was kept almost constant by tuning the A values (and hence the π values). As was observed previously, − , very clear Kiessig fringes were observed for all three polymers used in this study. Also, a small fringe at about q = 0.06 is observed in this study clearer than in a previous study on the critical brush density .…”