Interfacial micellar structures from novel amphiphilic star polymers

An amphiphilic heteroarm star polymer containing 12 alternating hydrophobic/hydrophilic arms of polystyrene (PS) and poly(acrylic acid) (PAA) connected to a well-defined rigid aromatic core was studied at the air-water and the air-solid interfaces. At the air-water interface, the molecules spontaneously form pancakelike micellar aggregates which measure up to several microns in diameter and 5 nm in thickness. Upon reduction of the surface area per molecule to 7 nm2, the two-dimensional micelles merged into a dense monolayer. We suggest that confined phase separation of dissimilar polymer arms occurred upon their segregation on the opposite sides of the rigid disklike aromatic core, forcing the rigid cores to adopt a face-on orientation with respect to the interface. Upon transfer onto solid supports the PS chains face the air-film interface making it completely hydrophobic, and the PAA chains were found to collapse and form a thin flattened underlayer. This study points toward new strategies to create large 2D microstructures with facial amphiphilicity and suggests a profound influence of star molecular architecture on the self-assembly of amphiphiles at the air-water interface.     

Porous polystyrene microspheres having dimpled surface structures prepared within micellar assemblies of amphiphilic silica particles in water

Production of porous polystyrene microspheres having dimpled surface structures was demonstrated using amphiphilic and hydrophobic silica particles as structure-directing agents.     

Synthesis,surface accumulation,and micellar properties of amphiphilic block copolymers

Amphiphilic block copolymers, i.e., poly(methyl methacrylate)-b-poly(2-dimethylethylammoniumethyl methacrylate), were synthesized by the reaction between two prepolymers. Carboxyl-terminated poly(methyl methacrylate) and hydroxyl-terminated poly(2-dimethylaminoethyl methacrylate) were prepared by radical polymerization of the corresponding monomers in the presence of thioglycolic acid and 2-mercaptoethanol as a chain transfer agent, respectively. Two condensation methods, i.e., DCC and the acid chloride method, were used for the reactions of these prepolymers. The subsequent quarternization produced the amphiphilic block copolymers. Surface property of poly(methyl methacrylate) films containing this amphiphilic block copolymer was examined by measuring contact angles for water. The addition of only 0.5 wt% of the block copolymer was sufficient to make poly(methyl methacrylate) surfaces hydrophilic. The block copolymer formed a polymeric micelle in acetone–water mixed solvent.     

Controlled switchable surface

The macroscopic properties of a surface can be intelligently controlled by alternating the states of the modified molecules, such as polymers, metallic oxide, or self-assembled monolayers (SAMs). This article reviews various approaches to create a switchable surface and different types of external stimuli used to switch the surface properties. This area is of potential benefit for biomaterials, biosensors, information storage, microfluidic systems, adhesive materials, nanolithography, and so on.     

Adhesion between chemically heterogeneous switchable polymeric brushes and an elastomeric adhesive

We investigated the adhesive properties of binary heterogeneous polymer brushes made from end-functionalized polystyrene (PS) and poly(2-vinylpyridine) (P2VP) chains. The molecular organization of the mixed brush could be varied reversibly by exposure to selective solvents for PS (toluene) and for P2VP (acidic water). This exposure results in reversible switching of adhesive and wetting properties. The manner in which the adhesion switching occurs can be tuned by the composition of mixed brushes. However, the outer surface composition could be enriched more effectively in PS after the toluene treatment than in P2VP after the acidic water treatment. As a result, the mixed brush compositions that showed the largest difference in properties between an exposure to toluene and an exposure to water were the P2VP-rich compositions. Adhesive properties, tested against a soft hydrophobic pressure-sensitive adhesive (PSA) using a probe test, always showed smaller differences between solvent treatments than wetting properties with water, suggesting a much higher sensitivity of the hydrophobic/hydrophilic brushes to polar molecules than to nonpolar molecules.     

Synthesis and nanostructure of strong polyelectrolyte brushes in amphiphilic diblock copolymer monolayers on a water surface

We synthesized an ionic amphiphilic diblock copolymer, poly(hydrogenated isoprene)-b-poly(styrenesulfonic acid) (PIp-h2-b-PSS), by living anionic polymerization, and the nanostructure of its monolayer spread on a water surface was directly investigated by the in situ X-ray reflectivity technique. The monolayer of the diblock copolymer on a water surface had a smooth hydrophobic PIp-h2 layer on water and a "carpet"/polymer brush double layer in a hydrophilic sodium polystyrene sulfonate (PSSNa) layer under the water. The surface pressure dependence and PSSNa chain length dependence of the PIp-h2 layer thickness and the brush nanostructure were quantitatively studied. The effect of salt concentration in the subphase was also investigated in aqueous solutions containing 0-2 M NaCl. The salt effect on monolayer structure occurred at around 0.2 M. The thickness of the PSS brush layer decreased at salt concentrations above 0.2 M, while no structural change was observed below 0.2 M. This critical salt concentration is thought to be related to the balance of ionic concentrations inside the brush and in bulk solution.     

Architecture and solution properties of amphiphilic polymer brushes with peripheral charged ions

Functionalized poly(ethylene oxide) (PEO) macromonomers (alpha -tertiary amino and omega -methacryloyl groups) were prepared by ring-opening polymerization of live PEO anions end-capped with styrene oxide using 2-[2-(N ,N -dimethylamino)-ethoxy]ethanol potassium alkoxides as an initiator with methacryloyl chloride. PEO brushes were synthesized by free-radical homopolymerization of such PEO macromonomers. These brushes were converted into peripherally charged brushes by quaternization. We studied the solution properties of both types of brushes from the viewpoint of charge effect. It was found from dynamic light scattering (DLS) that the polymer brushes formed a single macromolecule in solution due to crowding of side chains. It was speculated from angular dependence measurements that the polymer brushes with large aspect ratios took a geometrical anisotropic conformation such as a cylinder. In methanol with a low dielectric constant, radius of gyration (R(G)), and cross-sectional radius of gyration (R(G,C)) of the polymer brushes with charged side chains were smaller than those of the polymer brushes without charges. In a solvent with a low dielectric constant such as methanol, ionic groups do not dissociate and condense. On the other hand, these physical values in an aqueous solution were somewhat larger than those of the polymer brushes without charges. In water with a high dielectric constant, peripherally charged brushes were strongly stabilized by electric double layers.     

Synthesis of macrocyclic molecular brushes with amphiphilic block copolymers as side chains

Macrocyclic molecular brushes c‐PHEMA‐g‐(PS‐b‐PEO) consisting of macrocyclic poly(2‐hydroxylethyl methacrylate) (c‐PHEMA) as backbone and polystyrene‐b‐poly(ethylene oxide) (PS‐b‐PEO) amphiphilic block copolymers as side chains were synthesized by the combination of atom transfer radical polymerization (ATRP), click chemistry, and single‐electron transfer nitroxide radical coupling (SET‐NRC). First, a linear α‐alkyne‐ω‐azido heterodifunctional PHEMA (l‐HC?C‐PHEMA‐N₃) was prepared by ATRP of HEMA using 3‐(trimethylsilyl)propargyl 2‐bromoisobutyrate as initiator, and then chlorine end groups were transformed to ? N₃ group by nucleophilic substitution reaction in DMF in the presence of an excess of NaN₃. The 3‐trimethylsilyl groups could be removed in the presence of tetrabutylammonium fluoride, and the product was cyclized by “click” chemistry in high dilution conditions. The hydroxyl groups on c‐PHEMA were transferred into bromine groups by esterification with 2‐bromoisobutyryl bromide and then initiate the ATRP of styrene. The formed macrocyclic molecular brushes c‐PHEMA‐g‐PS were coupled with the TEMPO‐PEO to afford the target macrocyclic molecular brushes c‐PHEMA‐g‐(PS‐b‐PEO) by SET‐NRC, and the efficiency is as high as 80～85%. All of the intermediates and final product were characterized with ¹H NMR, Fourier transform infrared (FTIR), and gel permeation chromatography in details © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Optically switchable liquid crystal photonic structures

Photo-optic materials offer the possibility of light controlled photonic devices, intelligent and environmentally adaptive optical materials. One strategy for creating these materials is the combination of structure formation through holographic photopolymerization and the variable optical properties of liquid crystals. Holographically patterned, polymer stabilized liquid crystals (HPSLCs) have proven to be useful optical materials. By incorporating photo-optic, azobenzene-derived liquid crystal blends into such material systems, we have generated practical photoresponsive optical materials.     

Nanowear studies in reversibly switchable polystyrene-poly(acrylic acid) mixed brushes

Wear studies were performed on polystyrene (PS)-poly(acrylic acid) (PAA) mixed polymer brushes and corresponding monobrushes in a dried state. The aim was to study the wear mechanism in polymer brush surfaces as well as to investigate the effect of switching of PS + PAA binary brush surfaces (on treatment with the selective solvents for the PS and PAA) on the wear process. Wear experiments were carried out using atomic force microscopy (AFM) under a controlled environment. The wear experiments were performed as a function of scan number using a sharp silicon nitride tip to induce the wear on the sample surfaces. The wear mechanism on different brush surfaces was influenced by molecular entanglement as well as adhesion and friction on the sample surface. The wear process on the PS monobrush surface treated with toluene took place via formation of the ripples. On the other hand, a typical wear mode observed on the PAA monobrushes was removal of the polymeric material from the surface. For the mixed brush surface treated with toluene (selective solvent for PS) where PS chains dominated the top of the sample surface, the typical wear mode observed was ripple formation similar to that observed for the PS monobrushes. However, when a mixed brush was treated with ethanol and pH 10 water so that PAA chains dominated the top layer, wear occurred via removal of material. The amount of wear on the surfaces increased with the number of scans. Furthermore, the load and scan velocity dependence of the wear process was also investigated. Wear on polymer brush surfaces increased on increasing the load and/or decreasing the scan speed. The present study shows that wear can be controlled/tuned using mixed responsive brushes.     

Salt effect on the nanostructure of strong polyelectrolyte brushes in amphiphilic diblock copolymer monolayers on the water surface

The nanostructure of a spread monolayer of diblock copolymers of poly(hydrogenated isoprene) and poly(styrenesulfonate) at the air/water interface were studied by in situ X-ray reflectivity as a function of the brush density and salt concentration. When the monolayer was compressed beyond the "critical brush density", its nanostructure changed from a flat, adsorbed "carpet" layer to a "carpet + brush" structure. The critical brush density was found to be about 0.12 nm-2, independent the brush length and salt concentration under a low-salt condition. The brush formation behavior was considered to be controlled by an electrostatic interaction between polyelectrolyte chains rather than by a steric hindrance. This might be because the distance between the chains at the critical point is rather long and also because of the effect of the salt on the critical brush density. The critical brush density increased at higher added salt concentration beyond 1 M. As a result, we found a new structure transition behavior of the polymer brushes between carpet-only and carpet + brush structures, which was induced by salt addition. Finally, we succeeded in the controlled release of salt ions from the salted brush layer by changing the brush density by compression of the monolayer.     

Photoreversibly switchable superhydrophobic surface with erasable and rewritable pattern

We report the facile fabrication of a functional nanoporous multilayer film with wettability that is reversibly tunable between superhydrophobicity and superhydrophilicity with UV/visible irradiation. Our approach controls surface roughness with an electrostatic self-assembly process and makes use of the photoresponsive molecular switching of fluorinated azobenzene molecules. Selective UV irradiation onto the nanostructured substrate was used to realize substrates with erasable and rewritable patterns of extreme wetting properties. Our findings will open up new avenues for external stimuli-responsive smart surfaces.     

Effects of chain architectures on the surface structures of conjugated rod-coil block copolymer brushes

Rod-coil block copolymers are of unique and interesting characteristics since their physical properties can be reversibly tuned in response to the external stimuli, such as change in solvent quality. In this study, dissipative particle dynamics is used to investigate the surface structures of rod-coil polymer brushes tethered onto a surface. When immersed in the selective solvent for the coil blocks, rod blocks tend to form aggregates. Our results show that linear and Y-shaped polymer brushes exhibit similar aggregative behavior. However, some of the surface structures can be acquired within experimentally attainable surface grafting density only for Y-shaped polymer brushes. On the other hand, comblike polymer brushes are found to possess more diverse aggregative manners than linear brushes. Surface structures with aggregates taking the forms of cones, cylinders, or layers of spheres are found. By controlling the aggregative structures, it is possible for us to adjust the physical properties, such as optical function, of the material.     

Theoretical and experimental studies on the surface structures of conjugated rod-coil block copolymer brushes

A combined theoretical and experimental investigation of conjugated rod-coil block copolymer brushes is reported. The theoretical study for the surface structures of rod-coil block copolymer brushes was established based on the simulation method of dissipative particle dynamics. The effects of solvent stimuli, grafting density, and rod-coil block ratio of the polymer brushes on the surface structures were examined. The rod blocks of polymer brushes were found to be well-dispersed on the surface in their good solvents. On the other hand, aggregative domains of the rod blocks were formed in their poor solvents with the conformations of isolated islands or worm-like structures depending on the grafting density of the polymer brushes. The aggregative domains tend to stay on top of the coil blocks for small rod-to-coil block ratio. However, the submergence of the aggregative domains into the coil blocks is thermodynamically preferred for large enough rod-to-coil block ratio. New multifunctional amphiphilic rod-coil block copolymers, poly-[2,7-(9,9-di-n-hexylfluorene)]-block-poly-[poly(ethylene glycol) methyl ether methacrylate]-block-poly-[3(tripropoxysilyl)propyl methacrylate] (PF-b-PPEGMA-b-PPOPS), with two different block ratios were synthesized and used to prepare the corresponding polymer brushes via the grafting- method. The effects of stimuli factors on the surface structures characterized by the atomic force microscopy images were consistent with the theoretical results. Furthermore, the photophysical properties of PF-b-PPEGMA-b-PPOPS brushes were significantly varied by the solvent stimuli. The emission peaks originated from the aggregation and/or excimer formation of PF blocks were observed after methanol treatment. The photoluminescence intensity and its efficiency were well correlated to the surface structure and the methanol content in mixed solvents. Our study demonstrates how the surface structures and photophysical properties of rod-coil block copolymer brushes response to environmental stimuli.     

Controlled protein assembly on a switchable surface

The strategy presented in this work supplies a general method of controlling protein assembly on a switchable low-density SAM, which may open a new way to design functional biocomposite films for biosensors or protein chips.     

Homeotropic and planar structures in liquid-crystalline polymer brushes

The ordering in polymer brushes formed by macromolecules with mesogenic groups in the main chain is investigated. The numerical method of self-consistent field approximation was used. The existence of two different liquid crystalline nematic states is shown: homeotropic (HLC) and planar (PLC) states. The free energy of the HLC state is always less than that of the PLC state. However, with the increase of energy of anisotropic interactions, (with decrease in temperature) our numerical procedure leads us to either one or another state depending on the grafting density. The results obtained show that both brush surfaces, play an essential role in establishing the concrete LC state structure. The grafting surface and the external surface force the planar order.     

Monte Carlo simulation for the micellar behavior of amphiphilic comb-like copolymers

Micellar behaviors in 2D and 3D lattice models for amphiphilic comb-like copolymers in water phase and in water/oil mixtures were simulated. A dynamical algorithm together with chain reptation movements was used in the simulation. Three-dimension displaying program was pro-grammed and free energy was estimated by Monte Carlo technigue. The results demonstrate that reduced interaction energy influences morphological structures of micelle and emulsion stems greatly; 3D simulation showing can display more direct images of morphological structures; the amphiphilic comb-like polymers with a hydrophobic main chain and hydrophilic side chains have lower energy in water than in oil.     

Ferrocenyl amphiphilic Janus dendrimers as redox‐responsive micellar carriers

Amphiphilic Janus dendrimers have attracted increasing attention due to their asymmetric structures and various functional properties compared to the conventional symmetric macromolecules. Herein, a novel ferrocenyl‐terminated amphiphilic Janus dendrimer containing nine hydrophilic triethylene glycol branches was synthesized by two synthetic routes, namely the typical chemo selective coupling method and the mixed modular approach. Chemical redox triggers, namely Fe₂(SO₄)₃ as oxidant and ascorbic acid as reductant, could regulate the self‐assembly behavior of the Janus dendrimer in water through the redox‐switching between ferrocene and ferricinium cations, and the change of micelles formed were investigated and confirmed through scanning electron microscopy and dynamic light scattering. The cargo‐loading property of the micelles self‐assembled by the Janus dendrimer was further proved by the successful fabrication of Rhodamine B (RhB)‐loaded micelles, and the oxidation‐triggered release behavior of the encapsulated RhB could be mediated by changing the concentration of oxidants. This work provides an effective approach to prepare ferrocenyl‐terminated amphiphilic Janus dendrimers and the self‐assembled micelles might be used as a promising molecular carrier in areas such as drug delivery and catalysis.