Temperature-Responsive Behavior of Double Hydrophilic Carboxy-Sulfobetaine Block Copolymers and Their Self-Assemblies in Water

The Effect of Block Ratio and Structure on the Thermosensitivity of Double and Triple Betaine Block Copolymers

AB-type and BAB-type betaine block copolymers composed of a carboxybetaine methacrylate and a sulfobetaine methacrylate, PGLBT-b-PSPE and PSPE-b-PGLBT-b-PSPE, respectively, were synthesized by one-pot RAFT polymerization. By optimizing the concentration of the monomer, initiator, and chain transfer agent, block extension with precise ratio control was enabled and a full conversion (~99%) of betaine monomers was achieved at each step. Two sets (total degree of polymerization: ~300 and ~600) of diblock copolymers having four different PGLBT:PSPE ratios were prepared to compare the influence of block ratio and molecular weight on the temperature-responsive behavior in aqueous solution. A turbidimetry and dynamic light scattering study revealed a shift to higher temperatures of the cloud point and micelle formation by increasing the ratio of PSPE, which exhibit upper critical solution temperature (UCST) behavior. PSPE-dominant diblocks created spherical micelles stabilized by PGLBT motifs, and the transition behavior diminished by decreasing the PSPE ratio. No particular change was found in the diblocks that had an identical AB ratio. This trend reappeared in the other set whose entire molecular weight approximately doubled, and each transition point was not recognizably impacted by the total molecular weight. For triblocks, the PSPE double ends provided a higher probability of interchain attractions and resulted in a more turbid solution at higher temperatures, compared to the diblocks which had similar block ratios and molecular weights. The intermediates assumed as network-like soft aggregates eventually rearranged to monodisperse flowerlike micelles. It is expected that the method for obtaining well-defined betaine block copolymers, as well as the relationship of the block ratio and the chain conformation to the temperature-responsive behavior, will be helpful for designing betaine-based polymeric applications.

Synthesis of zwitterionic polyelectrolyte nanogels via electrostatic-templated polymerization

Zwitterionic polyelectrolyte nanogels are prospective nanocarriers due to their soft loading pocket and regulated charges. We here report a facile strategy, namely, electrostatic-templated polymerization (ETP) for synthesizing zwitterionic nanogels with controlled size and properties. Specifically, with anionic-neutral diblock polymers as the template, zwitterionic monomers such as carboxybetaine methacrylate (CBMA) or carboxybetaine acrylamide (CBAA) are polymerized together with a cross-linker at pH 2 where the monomers exhibit only positive charge due to the protonation of the carboxyl group. The obtained polyelectrolyte complex micelles dissociate upon introducing a concentrated salt. The subsequent separation yields the released template and zwitterionic nanogels with regulated size and swelling ability, achieved by tuning the salt concentration and cross-linker fraction during polymerization. The obtained PCBMA nanogels exhibit charges depending on the pH, which enables not only the selective loading of different dye molecules, but also encapsulation and intracellular delivery of cytochrome c protein. Our study develops a facile and robust way for fabricating zwitterionic nanogels and validates their potential applications as promising nanocarriers for load and delivery of functional charged cargos.

Thermo-Responsive Polyion Complex of Polysulfobetaine and a Cationic Surfactant in Water

Poly(4-((3-methacrylamidopropyl)dimethylammonium)butane-1-sulfonate) (PSBP) was prepared via controlled radical polymerization. PSBP showed upper critical solution temperature (UCST) behavior in aqueous solutions, which could be controlled by adjusting the polymer and NaCl concentrations. Owing to its pendant sulfonate anions, PSBP exhibited a negative zeta potential of −7.99 mV and formed a water-soluble ion complex with the cationic surfactant cetyltrimethylammonium bromide (CTAB) via attractive electrostatic interaction. A neutral PSBP/CTAB complex was formed under equimolar concentrations of the pendant sulfonate group in PSBP and the quaternary ammonium group in CTAB. Transmittance electron microscopic images revealed the spherical shape of the complex. The stoichiometrically neutral-charge PSBP/CTAB complex exhibited UCST behavior in aqueous solutions. Similar to PSBP, the phase transition temperature of the PSBP/CTAB complex could be tuned by modifying the polymer and NaCl concentrations. In 0.1 M aqueous solution, the PSBP/CTAB complex showed UCST behavior at a low complex concentration of 0.084 g/L, whereas PSBP did not exhibit UCST behavior at concentrations below 1.0 g/L. This observation suggests that the interaction between PSBP and CTAB in the complex was stronger than the interpolymer interaction of PSBP.

Modulation of Double Zwitterionic Block Copolymer Aggregates by Zwitterion-Specific Interactions

Transformable double hydrophilic block copolymer assemblies are valid as a biocompatible smart macromolecular system. The molecular mechanisms in the spontaneous assembly of double zwitterionic diblock copolymers composed of a poly(carboxybetaine methacrylate) (PCB2) and a poly(sulfobetaine methacrylate) (PSB4) chains (PCB2-b-PSB4) were investigated by the modulation of the aggregates in response to nondetergent zwitterions. The PCB2-b-PSB4 diblock copolymers with a high degree of polymerization PSB4 block produced aggregates in salt-free water through "zwitterion-specific" interactions. The PCB2-b-PSB4 aggregates were dissociated by the addition of nondetergent sulfobetaine (SB4) and carboxybetaine (CB2) molecules, while the aggregates showed different aggregation modulation processes for SB4 and CB2. Zwitterions with different charged groups from SB4 and CB2, glycine and taurine, hardly disrupted the PCB2-b-PSB4 aggregates. The PCB2-b-PSB4 aggregate modulation efficiency of SBs associated with the intercharge hydrocarbon spacer length (CSL) rather than the symmetry with the SB in the PSB chain. These zwitterion-specific modulation behaviors were rationalized based on the nature of zwitterions including partial charge density, dipole moment, and hydrophobic interactions depending on the charged groups and CSL.

pH-Switchable Latexes Based on the Nonionic Amphiphilic Diblock Copolymer with a Chargeable End-Group on the Core-Forming Block

Reversible addition-fragmentation transfer (RAFT) dispersion polymerization of styrene was performed in an ethanol-water mixture using a Z-group carboxylated poly(N-acryloylmorpholine) (PNAM) macro-RAFT agent, and dialysis was performed against water to produce the PNAM_x-PS_y-COOH (PS = polystyrene) diblock copolymer latexes. This new formula is developed for the fabrication of pH-switchable copolymer latexes through an end-group response approach. The PNAM₄₄-PS₁₃₄-COOH latex is unstable at suitably low pH values (pH ≤ 4), and these aggregated spherical nanoparticles are redispersed successfully by adding base as determined by analysis of their dynamic light scattering (DLS) diameters and transmission electron microscopy (TEM) data. Negative zeta potential (-19.4 mV at 0.02% w/w) of the original latex indicated that carboxylic acid end-groups were anchored on the surface of the PS core via the polymerization-induced self-assembly (PISA) process and exposed to the solvent. Protonation of carboxylate groups reduces the degree of hydration of the PS core with a great impact on the free energy of the core/solvent interface, inducing the aggregation of PNAM₄₄-PS₁₃₄-COOH latex particles. A comparative experiment where the carboxylic acid end-group is designed on the PNAM stabilizer block proves that no pH-switchable behavior occurs in this case. Moreover, the vesicle-like nanoparticles composed of PNAM₄₄-PS₄₂₈-COOH copolymers have an apparently anionic character (zeta potential ≈ -33.5 mV at 0.02% w/w) and are still pH-switchable with a lower critical flocculation point (pH 2-3). More importantly, the latex composed of PNAM₁₁₈-PS₁₅₁-COOH diblock copolymers is insensitive to the solution pH.

Effects of Halide Anions on the Solution Behavior of Double Hydrophilic Carboxy-Sulfobetaine Block Copolymers

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 hindered more by increases in the salt concentration because of 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 a kosmotrope, only made a small contribution to 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 chaotropic halides tend to exist in the vicinity of the diblock polybetaine chain surface and thus prominently influenced the thermoresponsive solution behavior, whereas kosmotropic F^- prefers water molecules and causes minor changes in the PGLBT-b-PSPE aqueous solution.

Effects of pH on the Stimuli-Responsive Characteristics of Double Betaine Hydrophilic Block Copolymer PGLBT-b-PSPE

We investigated the pH-responsive behavior of the carboxybetaine-sulfobetaine diblock copolymer poly(2-(2-(methacryloyloxy)ethyl)dimethylammonio)acetate-block-3-((2-(methacryloyloxy)ethyl)dimethylammonio)propane-1-sulfonate (PGLBT-b-PSPE) in aqueous solution under varying temperatures. Alongside the temperature-responsive PSPE block which induces self-assembly of polymer micelles under the upper critical solution temperature, the PGLBT motifs having protonation sites caused additional changes in the phase behaviors. In acidic conditions where the pH is lower than the pK_a 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 solutions. There were two simultaneous diffusive modes in the turbid solutions equivalent to unimers or micelles and large aggregates over a few hundred nanometers. Unlike in neutral and basic conditions, those large aggregates did not disappear after the emergence of the polymer micelles. The trend of the temperature-responsive behavior hardly changed in the alkaline solutions; however, the critical temperature significantly decreased. The surface charge of the unimers and self-assembled objects determined by zeta potential measurement varied from neutral or negative to positive with proton addition and further positively increased below the micelle formation temperature. This indicates the cationization of PGLBT moieties and their arrangement in the outer layer of the polymer micelle surface. In spite of the positively charged outer surface, two fast and slow diffusive modes representing micelles and large clusters were repeatedly observed in acidic solutions, and to some extent, size-grown particles eventually precipitated.