Polymer Micelle Formation without Gibbs Monolayer Formation:  Synthesis and Characteristic Behavior of an Amphiphilic Diblock Copolymer Having Strong Acid Groups

Abstract

A novel amphiphilic diblock copolymer having sulfonic acid groups in its hydrophilic chain with poly(diethylsilacyclobutane) as a hydrophobic segment was synthesized by living anionic polymerization followed by treatment with 1,3-propane sultone. The polymer backbone with a degree of polymerization of 18 (hydrophobic):37 (hydrophilic) was synthesized, and three kinds of diblock copolymers with different degrees of sulfonation (R ) 0, 0.50, 0.81) were prepared as sodium salts. The surface tension of aqueous solutions of highly sulfonated polymers as including the R ) 0 (all carboxylic acid) polymer did not decrease with polymer concentration. The solutions did not show foam formation even after vigorous shaking, and almost no adsorption of polymer molecules was observed at the air/water interface by X-ray reflectivity measurement. However, micelle formation with a radius of 140-200 Å was confirmed by dynamic light scattering and small-angle X-ray scattering experiments. Hence, this polymer showed "micelle formation" without "Gibbs monolayer formation" in water. The surface tension decrease in 0.5 M NaCl aqueous solution was also small, but adsorption of polymer was observed by X-ray reflectivity in addition to formation of foam by shaking. The strong ionic character of this novel class of amphiphilic polymer is thought to be the origin of this curious behavior.

pH-responsive non-surface-active/surface-active transition of weakly ionic amphiphilic diblock copolymers

“…The surface tension data is somewhat scattered, but the small "drop" of surface tension near cmc might have a physical meaning. We observed similar behavior, i.e., small surface tension decrease near cmc, for other non-surface active polymers, and named it "M-Point" [1]. We could not understand its meaning and interpretation at that stage, but this point was repeatedly observed, and we are convinced that this is not an experimental error.…”

“…However, after 0.1M NaCl addition, the solution showed very good foam formation, which means that the polymer became surface active. This salt effect was observed for all non-surface active polymers ever studied [1][2][3][4][5][6][7][8][9] and means that the non-surface activity is electrostatic in origin, probably due to the image charge repulsion at the air/water interface. As shown in Figure 4 Since pH 12 is an overneutralized condition, the excess NaOH acts as a salt, and behavior similar to that in Fig.3 was observed.…”

“…Various ionic amphiphilic diblock copolymers including strongly anionic [1][2][3][4][5][6], weakly anionic [7], and also cationic [8,9] polymers show a non-surface active nature. We have systematically investigated this unique, somewhat out of common sense of surface and interface science, property, and proposed that the essential origin of non-surface activity is strong image charge repulsion [10][11][12][13][14] at the air/water interface [2,3,5,6,9].…”

Solubility behavior of amphiphilic sulfonated copolymers based on styrene–stearyl methacrylate and styrene–stearyl cinnamate

“…Amphiphilic copolymers containing ionic groups have been divided in two classes according to their dissociation behavior: weak acid/base and strong acid/base. The synthesis of polyelectrolytes containing in their structure strong acid groups has not been extensively reported, and this could be related to the difficulty in synthesizing them due to the large differences between (1) the hydrophilicity and hydrophobicity of the segments and (2) the monomer solubility . Chemical modification of the polymer chain is an alternative to obtain copolymers containing strong acid groups, for example, by inserting sulfonic groups following homogeneous or heterogeneous methods .…”

Why Ionic Amphiphilic “Block” Copolymer Can Be Non-surface Active? Comparison of Homopolymer, Block and Random Copolymers of Poly(styrenesulfonate)

“…DLS instrument was Photal SLS-7000DL equipped with GC-1000 Correlator (Otsuka Electric, Osaka). The decay rate Γ was evaluated by fitting of the time correlation function of scattered field, g (1) (q,¸), and the translational diffusion coeffecient D was evaluated from the slope of Γ vs. q 2 plot. The hydrodynamic radius R h was calculated from D values by appling StokesEinstein equation.…”