Ionic amphiphilic diblock copolymers, such as polystyreneb-poly(styrenesulfonate), show non-surface activity when some requirements are satisfied. The main origin of this unique property has been thought to be an image charge repulsion at the air/water interface. In this study, surface activity/non-activity was examined for random copolymers in addition to homopolymers. Random copolymers of styrene and styrenesulfonate were found to be surface active, while the poly(styrenesulfonate) homopolymer was slightly surface active. This observation cannot be interpreted by image charge repulsion alone since these are all polyanions and the block copolymer is mostly hydrophobic. Stable polymer micelle formation in the block copolymer system is also an essential origin of non-surface activity.Ionic amphiphilic diblock copolymers, which consist of hydrophobic and ionic blocks, show non-surface activity under suitable conditions.19 Their aqueous solutions show no or little reduction of surface tension and foam formation, while a multimolecular micelle is formed in the bulk solution. This phenomenon is, in a sense, out of common sense of surface and interface science since the micelle has been believed to be formed by the "surfactant (surface active agent)" above the critical micelle concentration (cmc). We have systematically investigated this unique property for strongly 1,2,4,5 and weakly 3 anionic amphiphilic block copolymers and also cationic 6,7 block copolymers. As a result, we have proposed an image charge effect 1014 at the air/water interface as the first essential origin of this phenomenon. Since the hydrophilic segment is a polyelectrolyte, which has many charges, the block copolymer adsorbed at the air/water interface by a hydrophobic effect is destabilized by strong image charge repulsion from the water surface. When salt was added into the solution, the polymers changed to be surface active; the surface tension of the solution decreased and showed foam formation activity. From this result, it is obvious that the electrostatic effect plays an important role in the origin of non-surface activity.4 Also, when the hydrophobic block becomes long enough compared to the length of the hydrophilic, ionic block, the polymer shows surface activity. The hydrophobic adsorption effect might be stronger than the image charge repulsion. Also, the total length (molecular weight) should be long enough to be surface active. Chen et al. reported that the polymer with a block length of m:n = 30:30 was non-surface active while m:n = 15:15 polymer showed surface activity.9 In this sense, non-surface activity is some kind of polymer effect. Furthermore, non-surface activity was also observed and has been reported by other researchers for different systems. 1523On the other hand, most polyelectrolyte homopolymers are slightly surface active. 24,25 Sen et al. reported the reduction of surface tension of aqueous poly(styrenesulfonate) (PSS) solutions, 24 and Yim et al. detected the adsorbed poly(syrenesulfonate) potassium salt molecule...

Strong polyampholytes comprising cationic vinylbenzyl trimethylammonium chloride (VBTAC) bearing a pendant quaternary ammonium group and anionic sodium p-styrenesulfonate (NaSS) bearing a pendant sulfonate group were prepared via reversible addition-fragmentation chain-transfer polymerization. The resultant polymers are labelled P(VBTAC/NaSS)n, where n indicates the degree of polymerization (n = 20 or 97). The percentage VBTAC content in P(VBTAC/NaSS)n is always about 50 mol%, as revealed by 1H NMR measurements, meaning that P(VBTAC/NaSS)n is a close to stoichiometrically charge-neutralized polymer. Although P(VBTAC/NaSS)n cannot dissolve in pure water at room temperature, the addition of NaCl or heating solubilizes the polymers. Furthermore, P(VBTAC/NaSS)n exhibits upper critical solution temperature (UCST) behavior in aqueous NaCl solutions. The UCST is shifted to higher temperatures by increasing the polymer concentration and molecular weight, and by decreasing the NaCl concentration. The UCST behavior was measured ranging the polymer concentrations from 0.5 to 5.0 g/L.

Amphoteric statistical equivalent copolymers (P(2VP/NaSS) n ) composed of 2-vinylpyridine (2VP) and anionic sodium p -styrenesulfonate (NaSS) were prepared via reversible addition–fragmentation chain transfer polymerization. The degrees of polymerization ( n ) were 19 and 95. The monomer reactivity ratio, time conversion profile, and 1 H nuclear magnetic resonance diffusion-ordered spectra suggested that the copolymerization of 2VP and NaSS provided statistical or near to random copolymers. P(2VP/NaSS) n exhibited an upper critical solution temperature (UCST) in acidic aqueous solutions on the basis of the charge interactions between the protonated cationic 2VP and anionic NaSS units. With an increase in pH value, the interaction was weakened because of the deprotonation of the 2VP units, thus reducing the UCST. At high [NaCl], the electrostatic interactions among the polymers were weakened because of the screening effect, and again, the UCST was reduced. With an increase in polymer concentration, the intra- and interpolymer interactions increased because of some entanglement, and the UCST consequently increased. Electrostatic interactions among the polymer chains with high molecular weight occurred easier than those among the low-molecular-weight polymer chains, which increased the UCST. The UCST also increased when deuterium oxide was used instead of hydrogen oxide, which was due to the isotopic effect. Hence, the UCST of P(2VP/NaSS) n can be adjusted according to the desired application.

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