Non-surface activity and micellization behavior of cationic amphiphilic block copolymer synthesized by reversible addition-fragmentation chain transfer process

Interfacial Chemistry of Conical Fullerene Amphiphiles in Water

Amphiphiles are used for a variety of applications in our daily life and in industrial processes. They typically possess hydrophobic and hydrophilic moieties within the molecule, thereby performing a myriad of functions through the formation of two- and three-dimensional assemblies in water, such as Gibbs monolayers and micelles. However, these functions are often inseparable because they emerge from the same structural feature of the molecule, and are difficult to control because the structural diversity is limited to either long-chain hydrocarbons bearing a polar end group(s) or polymers bearing polar groups exposed to the exterior surface. In this Account, we describe the chemistry of a new class of amphiphiles, conical fullerene amphiphiles (CFAs), utilizing a superhydrophobic [60]fullerene group as a nonpolar apex with added structural features to make it soluble in water. By selective functionalization of only one side of the fullerene molecule, the CFA molecules spontaneously assemble in water through strong hydrophobic interactions among the fullerene apexes and exhibit unusual supramolecular and interfacial behavior. They form unilamellar micelles and vesicles at a critical aggregation concentration as low as micromolar, not showing any air-water and oil-water interfacial activity. The strong preference for self-assembly in water over monolayer formation at an air-water interface makes CFAs unique among conventional nonpolymeric surfactants. The CFA assemblies are often so mechanically robust that they can be transferred to the surface of a solid substrate and analyzed by high-resolution microscopy. Because of this rigid conical structure of a few nanometers in size, CFA molecules aggregate readily in water to form a hierarchical assembly with biomolecules and nanomaterials while maintaining the structural integrity of the CFA aggregate to form multicomponent agglomerates of controllable structural features. For instance, tissue-selective in vivo transport of DNA and siRNA has been achieved. Hybridization of a CFA vesicle with a transition metal catalyst enables the construction of a structurally defined nanospace and an interface for precise control of the nanoscale morphology of polymers. Solubilization of hydrophobic nanocarbons and nanoparticles is also achieved through hemimicelle formation on solid surfaces. The examples reported here illustrate the potential of the conical fullerene motif for the design of amphiphiles as well as supramolecular structures at molecular and tens of nanometers scale.

Toward Achieving Highly Ordered Fluorinated Surfaces of Spin-Coated Polymer Thin Films by Optimizing the Air/Liquid Interfacial Structure of the Casting Solutions

Thin polymer films with well-assembled fluorinated groups on their surfaces are not easily achieved via spin-coating film-fabrication methods because the solution solidifies very rapidly during spin-coating, which hinders the fluorinated moieties from segregating and organizing on the film surface. In this contribution, we have proposed a comprehensive strategy toward achieving well-ordered fluorinated thin films surfaces by optimizing the molecular organization at air/liquid interface of the film-formation solutions. To validate such a route, poly(methyl methacrylate) (PMMA) end-capped with several 2-perfluorooctylethyl methacrylate (FMA) units was employed as the model polymer for investigations. The air/solution interfacial structures were optimized by systematically changing the polymer chain structures and properties of the casting solvents. It was found that the polymers that form loosely associated aggregates (e.g., FMA₁- ec-PMMA₆₅- ec-FMA₁) and a solvent with better solubility to FMA while having not too low surface tension (i.e., toluene) can combine to produce solutions with well-assembled FMA at the interfaces. By spin-coating the solutions with well-organized interfaces, an ultrathin film with perfluorinated groups that were highly oriented toward the film surface was readily achieved, exhibiting surface energies as low as 7.2 mJ/m², which is among the lowest reported so far for the spin-coated thin films, and a very high F/C ratio (i.e., 0.98).

Surface Active to Non-Surface Active Transition and Micellization Behaviour of Zwitterionic Amphiphilic Diblock Copolymers: Hydrophobicity and Salt Dependency

We have synthesized a range of zwitterionic amphiphilic diblock copolymers with the same hydrophilic block (carboxybetaine) but with different hydrophobic blocks (n-butylmethacrylate (n-BMA) or 2-ethylhexylacrylate (EHA)) by the reversible addition⁻fragmentation chain transfer (RAFT) polymerization method. Herein, we systematically examined the role of hydrophobicity and salt concentration dependency of surface activity and micellization behaviour of block copolymer. Transition from surface active to non-surface active occurred with increasing hydrophobicity of the hydrophobic block of block copolymer (i.e., replacing P(n-BMA) by PEHA). Foam formation of block copolymer slightly decreased with the similar variation of the hydrophobic block of block copolymer. Block copolymer with higher hydrophobicity preferred micelle formation rather than adsorption at the air⁻water interface. Dynamic light scattering studies showed that block copolymer having P(n-BMA) produced near-monodisperse micelles, whereas block copolymer composed of PEHA produced polydisperse micelles. Zimm plot results revealed that the value of the second virial coefficient (A₂) changed from positive to negative when the hydrophobic block of block copolymer was changed from P(n-BMA) to PEHA. This indicates that the solubility of block copolymer having P(n-BMA) in water may be higher than that of block copolymer having PEHA in water. Unlike ionic amphiphilic block copolymer micelles, the micellar shape of zwitterionic amphiphilic block copolymer micelles is not affected by addition of salt, with a value of packing parameters of block copolymer micelles of less than 0.3.

Aqueous solution behaviour of novel water-soluble amphiphilic copolymers with elevated hydrophobic unit content

The synthesis and aqueous solution behaviour of water-soluble copolymers poly(OEGMA-stat-styrene)-b-PDMAPMA with a relatively high content of styrene units were explored.

General considerations regarding the in vitro and in vivo properties of block copolymer micelle products and their evaluation

Block copolymer micelles are nanoparticles formed from block copolymers that comprise a hydrophilic polymer such as poly(ethylene glycol) and a poorly soluble polymer such as poly(amino acids). The design of block copolymer micelles is intended to regulate the in vivo pharmacokinetics, stability, and distribution profiles of an entrapped or block copolymer-linked active substance. Several block copolymer micelle products are currently undergoing clinical development; however, a major challenge in the development and evaluation of such products is identification of the physicochemical properties that affect the properties of the drug product in vivo. Here we review the overall in vitro and in vivo characteristics of block copolymer micelle products with a focus on the products currently under clinical investigation. We present examples of methods suitable for the evaluation of the physicochemical properties, non-clinical pharmacokinetics, and safety of block copolymer micelle products.

Nanostructure and salt effect of zwitterionic carboxybetaine brush at the air/water interface

Zwitterionic amphiphilic diblock copolymer, poly(ethylhexyl acrylate)-b-poly(carboxybetaine) (PEHA-b-PGLBT), was synthesized by the reversible addition-fragmentation chain transfer (RAFT) method with precise control of block length and polydispersity. The polymers thus obtained were spread onto the water surface to form a polymer monolayer. The fundamental property and nanostructure of the block copolymer monolayer were systematically studied by the surface pressure-molecular area (π-A) isotherm, Brewster angle microscopy (BAM), and X-ray reflectivity (XR) techniques. The π values of the monolayer increased by compression in relatively larger A regions. After showing a large plateau region by compression, the π value sharply increased at very small A regions, suggesting the formation of poly(GLBT) brush formation just beneath the water surface. The domain structure of μm size was observed by BAM in the plateau region. XR profiles for the monolayer at higher surface pressure regions clearly showed the PGLBT brush formation in addition to PGLBT carpet layer formation under the hydrophobic PEHA layer on the water surface, as was observed for both anionic and cationic brush layer in the water surface monolayer studied previously. The critical brush density, where the PGLBT brush is formed, was estimated to be about 0.30 chains/nm(2) for the (EHA)45-b-(GLBT)60 monolayer, which is relatively large compared to other ionic brushes. This observation is consistent with the fact that the origin of brush formation is mainly steric hindrance between brush chains. The brush thickness increased by compression and also by salt addition, unlike the normal ionic brush (anionic and cationic), whose thickness tended to decrease, i.e., shrink, by salt addition. This might be a character unique to the zwitterionic brush, and its origin is thought to be transition to an ionic nature from the almost nonionic inner salt caused by salt addition since both the cation and anion of the GLBT chain obtained counterions by the addition of salt. This stretching nature of the PGLBT brush depends on the ion species of the salt added, and it followed the Hofmeister series, i.e., more stretching in the order of Li(+) > Na(+) > K(+). However, it was rather insensitive to the anion species (Cl(-), Br(-), SCN(-)), which suggests that the carboxylic anion has a more dominant effect than the quaternized cation in GLBT although the former is a weak acid and the latter is believed to be a strong base.

Self-organization in water of well-defined amphiphilic poly(vinyl acetate)-b-poly(vinyl alcohol) diblock copolymers

Self-organization of well-defined poly(vinyl acetate)-b-poly(vinyl alcohol) (PVAc-b-PVA) amphiphilic diblock copolymers with different hydrophilic block sizes was reported.

Sonication-induced formation of size-controlled self-assemblies of amphiphilic Janus-type polymers as optical tumor-imaging agents

In this study, amphiphilic Janus-type polymers were synthesized via ring-opening metathesis polymerization (ROMP), multiple vicinal diol formation, and grafting of poly(ethylene glycol) monomethyl ether (mPEG). These amphiphilic polymers formed self-assemblies, which were a mixture of micelles and multimicellar aggregates, in water. By choosing suitable Janus-type polymers and irradiating an aqueous solution of polymers using a sonicator, either small micelles or large multimicellar aggregates were obtained selectively. Hydrophobic substituents controlled the aggregation-disaggregation behavior, leading to the formation of metastable self-assemblies by sonication. The formation of self-assemblies with a uniform size was affected by ultrasonic frequency, rather than power. In vivo optical tumor imaging revealed that the large-size multimicellar aggregates persisting for a long time in blood circulation slowly accumulated in tumor tissues. In contrast, the tumor site was rapidly, clearly visualized using the small-size micelles.

Photoresponsive block copolymer: synthesis, characterization, and surface activity control

Amphiphilic block copolymers bearing chromophores are used to achieve photoresponses upon exposure to suitable light, which alter molecular properties, but the photostimulus surface activity control of amphiphilic block copolymers remains to be elucidated. In this work, a series of novel amphiphilic block copolymers consisting of a carboxymethyl betaine monomer (called GLBT) and 4-ethoxy-4'-methacrylamide (EMAAB) with different block ratios have been synthesized using a reversible addition-fragmentation chain-transfer (RAFT) polymerization process. Copolymers were observed to be self-assembled in the aqueous solution above a critical micelle concentration, which was determined by static light scattering measurements and formed vesicles of 120-170 nm in diameter, at different pH values. Copolymers were found to be surface-active at pH 7 but exhibited non-surface activity at acidic and alkaline pH values. After being irradiated with 360 nm UV light, copolymers showed a significant photoresponse both at the surface and in bulk solution as a result of the photoinduced isomerization of azochromophores. The surface property of copolymers was significantly affected by UV irradiation at pH 7, and block copolymers became non-surface-active. The bulk properties changed considerably upon UV exposure where polymer vesicles transformed to micelles as a result of the polarity difference between two azo isomers (cis and trans isomers). All of these transitions were found to be reversible. A new method to control the surface active/nonactive and vesicle/micelle transitions by light and pH has been established by introducing an azobenzene chromophore and GLBT into amphiphlic diblock copolymers.

Chain length dependence of non-surface activity and micellization behavior of cationic amphiphilic diblock copolymers

The cationic and anionic amphiphilic diblock copolymers with a critical chain length and block ratio do not adsorb at the air/water interface but form micelles in solution, which is a phenomenon called "non-surface activity". This is primarily due to the high charge density of the block copolymer, which creates a strong image charge effect at the air/water interface preventing adsorption. Very stable micelle formation in bulk solution could also play an important role in the non-surface activity. To further confirm these unique properties, we studied the adsorption and micellization behavior of cationic amphiphilic diblock copolymers of poly(n-butyl acrylate)-b-poly(3-(methacryloyloxy)ethyl)trimethylammonium chloride) (PBA-b-PDMC) with different molecular weights of hydrophobic blocks but with the same ionic block length. These block copolymers were successfully prepared via consecutive reversible addition-fragmentation chain transfer (RAFT) polymerization. The block copolymer with the shortest hydrophobic block length was surface-active; the solution showed surface tension reduction and foam formation. However, above the critical block ratio, the surface tension of the solution did not decrease with increasing polymer concentration, and there was no foam formation, indicating lack of surface activity. After addition of 0.1 M NaCl, stable foam formation and slight reduction of surface tension were observed, which is reminiscent of the electrostatic nature of the non-surface activity. Fluorescence and dynamic and static light scattering measurements showed that the copolymer with the shortest hydrophobic block did not form micelles, while the block copolymers formed spherical micelles having radii of 25-30 nm. These observations indicate that micelle formation is also important for non-surface activity. Upon addition of NaCl, cmc did not decrease but rather increased as observed for non-surface-active block copolymers previously studied. The micelles formed were very stable, and their size decreased by only ∼5 nm after addition of 0.1 M NaCl.