Jongmin Lim

The solution behavior of the double polybetaine block copolymer poly(2-((2-(methacryloyloxy)ethyl)dimethylammonio)acetate)-block-poly(3-((2-(methacryloyloxy)¬ethyl)dimethylammonio)propane-1-sulfonate (PGLBT-b-PSPE) in sodium halide aqueous solutions was investigated. In the presence of salt ions, the unimer-to-micelle transition of PGLBT-b-PSPE that originated by Coulombic attraction between PSPE motifs was suppressed and shifted to much lower temperatures. The transition was more hindered by increases in the salt concentration due to additional counterion binding on the ionized site of PGLBT-b-PSPE chains which screens the dipole-dipole attractions. The specific ion effect was investigated on four different halides, Cl-, Br-, I- and F-. Cl- and two chaotropes (Br- and I-) apparently prevented micelle formation, and the hindering effectiveness on the PSPE pairing followed the general Hofmeister series of anions: I- > Br- > Cl-. More chaotropic anions strongly maintained the polymer chains in a fully hydrated state when the same amount of salts was incorporated. However, F-, which is classified as kosmotrope, only did small contribution on lowering the transition point and led to abrupt transition without showing a gradual phase change prior to the transition. The variations of hydrodynamic radius and zeta potentials of unimers and micelles gave hints of the solvation state of salt-incorporated PGLBT-b-PSPEs in each state. These results suggest that chaotropes tend to exist in the vicinity of the diblock polybetaine chain surface thus prominently influenced the thermoresponsive solution behavior, whereas kosmotropes prefer water molecules and cause minor changes in the PGLBT-b-PSPE aqueous solution.

We investigated the pH-responsive behavior of carboxybetaine-sulfobetaine diblock copolymer (PGLBT-b-PSPE) in aqueous solution through transmittance and dynamic laser light scattering while varying temperature. Alongside the temperature-responsive PSPE block which induces self-assembly of polymer micelles under critical temperatures, temperature-independent PGLBT motifs having protonation sites involved in different phase behaviors with pH change. The original upper critical solution temperature (UCST) type alteration of the block copolymer was common in both acidic and alkaline conditions with several differences. In acidic condition where the pH is lower than the pKa of PGLBT-b-PSPE, the transmittance of polymer solutions more abruptly dropped and became cloudy at higher temperatures compared to the case of salt-free solution. There were two simultaneous diffusive modes in the turbid state solutions equivalent to unimers or micelles and large aggregates over a few hundred nanometers, and those large aggregates did not disappear after the occurrence of the micellar form unlike neutral and basic solutions. The temperature-responsive behavior hardly changed in the alkaline solutions but the critical temperature significantly decreased. The surface charge of unimer and micelle state determined by zeta potential measurement varied from neutral or negative to positive with proton addition, and further positively increased under the micelle formation temperature, which indicates pH-responsive charge variations of PGLBT moiety on the polymer micelle surface. In spite of the positively charged outer surface of self-assembled polymer objects, two fast and slow diffusive modes representing micelles and large clusters are repeatedly observed in acidic solutions and some extent of size-grown particles eventually precipitated.

The block copolymer poly(2-((2-(methacryloyloxy)ethyl)dimethylammonio)¬acetate)-b-poly(3-(N-(2-metharyloylethyl)-N,N-dimethylammonio)propanesulfonate) (PGLBT-b-PSPE) was synthesized by reversible addition-fragmentation chain transfer (RAFT) technique under precise control. The PGLBT-b-PSPE block copolymers showed upper critical solution temperature (UCST) behavior originated from PSPE moieties. Unlike PSPE homopolymers, the transmittance change with temperature was gradual and unexpected retardation or slight changes in a reverse direction was found at the intermediate stage. Light scattering and 1H NMR studies proved that the block copolymers formed spherical micelles that were composed of a PSPE core and PGLBT shell around room temperature and lower temperatures, and slowly disassociated with temperature increase. During the transition, fast (small particle) and slow (large particle) diffusive modes were detected by dynamic light scattering (DLS), which implied that the unimers were escaping from the self-assembled structure and swollen micelles, respectively. At sufficiently high temperatures where the solutions became almost transparent, the slow mode eventually disappeared and only the fast mode remained. In addition, once the polymeric particles are formed, the size did not varied much with additional cooling. The transition point and the pattern of transmittance alteration were dependent on the degree of polymerization and the [PGLBT]:[PSPE] ratios; more PGLBT made the block copolymer less responsive to temperature and led the cloud point to lower degrees. However, random copolymers PGLBT-r-PSPE did not show any temperature-responsivity, and even small amount of GLBTs (10%) distributed in a PSPE chain significantly suppressed the transition.