Comparison of the adsorption of cationic diblock copolymer micelles from aqueous solution onto mica and silica

The similarities and differences in the adsorption behavior of diblock poly(2-(dimethylamino)ethyl methacrylate)-b-poly(2-(diethylamino)ethyl methacrylate) (XqPDMA-PDEA, where X refers to a mean degree of quaternization of the PDMA of either 0, 10, 50, or 100 mol%) copolymers at the mica/ and silica/aqueous solution interfaces have been investigated. These diblock copolymers form core-shell micelles with the PDEA chains located in the cores and the more hydrophilic PDMA chains forming the cationic micelle coronas at pH 9. These micelles adsorb strongly onto both mica and silica due to electrostatic interactions. In situ atomic force microscopy (AFM) has demonstrated that the mean spacing and the dimension of the adsorbed micelles depend on both the substrate and the mean degree of quaternization of the PDMA blocks. In particular, the morphology of the adsorbed nonquaternized 0qPDMA-PDEA copolymer micelles is clearly influenced by the substrate type: these micelles form a disordered layer on silica, while much more close-packed, highly ordered layers are obtained on mica. The key reasons for this difference are suggested to be the ease of lateral rearrangement for the copolymer micelles attached to the solid substrates and the relative rates of relaxation of the coronal PDMA chains.     

Synthesis and characterization of shell cross-linked micelles with hydroxy-functional coronas: a pragmatic alternative to dendrimers?

Shell cross-linked (SCL) micelles with hydroxy-functional coronas have been constructed in aqueous solution by exploiting the micellar self-assembly behavior of a new thermoresponsive ABC triblock copolymer. This copolymer was prepared via atom transfer radical polymerization in a convenient one-pot synthesis and comprised a thermoresponsive core-forming poly(propylene oxide) (PPO) block, a cross-linkable central poly(2-(dimethylamino)ethyl methacrylate) (DMA) block, and a hydroxy-functional outer block based on poly(glycerol monomethacrylate) (GMA). DMF GPC analysis confirmed a unimodal molecular weight distribution for the PPO-PDMA-PGMA triblock copolymer precursor, with an M(n) of 12 100 and a polydispersity of approximately 1.26. This copolymer dissolved molecularly in aqueous solution at 5 degrees C but formed micelles with hydroxy-functional coronas above a critical micelle temperature of around 12 degrees C, which corresponded closely to the cloud point of the PPO macroinitiator. Cross-linking of the DMA residues using 1,2-bis(2-iodoethoxy)ethane produced SCL micelles that remained intact at 5 degrees C, i.e., below the cloud point of the core-forming PPO block. Dynamic light scattering studies confirmed that the SCL micelle diameter could be varied depending on the temperature employed for cross-linking: smaller, more compact SCL micelles were formed at higher temperatures, as expected. Since cross-linking involved quaternization of the DMA residues, the SCL micelles acquired cationic surface charge as judged by aqueous electrophoresis studies. These cationic SCL micelles were adsorbed onto near-monodisperse anionic silica sols, which were used as a model colloidal substrate. Thermogravimetric analyses indicated a SCL micelle mass loading of 2.5-4.4%, depending on the silica sol diameter and the initial micelle concentration. Aqueous electrophoresis measurements confirmed that surface charge reversal occurred after adsorption of the SCL micelles, and scanning electron microscopy studies revealed a uniform coating of SCL micelles on the silica particles.     

Characterization of layer-by-layer self-assembled multilayer films of diblock copolymer micelles

The in situ layer-by-layer (LbL) self-assembly of low Tg diblock copolymer micelles onto a flat silica substrate is reported. The copolymers used here were a cationic poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate) (50qPDMA-PDEA; 50q refers to a mean degree of quaternization of 50 mol % for the PDMA block) and zwitterionic poly(methacrylic acid)-block-poly(2-(diethylamino)ethyl methacrylate) (PMAA-PDEA), which has anionic character at pH 9. Alternate deposition of micelles formed by these two copolymers onto a silica substrate at pH 9 was examined. The in situ LbL buildup of the copolymer micelle films was monitored using zeta potential measurements, optical reflectometry, and a quartz crystal microbalance with dissipation monitoring (QCM-D). For a six layer deposition, complete charge reversal was observed after the addition of each layer. The OR data indicated clearly an increase in adsorbed mass with each additional micelle layer and suggest that some interdiffusion of copolymer chains between layers and/or an increase in the film roughness, and hence in the effective surface area of the micellar multilayers, must take place as the film is built up. QCM-D data indicated that the self-assembled micellar multilayers on a flat silica substrate undergo structural changes over a prolonged period. This is attributed to longer-term interdiffusion of the copolymer chains between the outer two layers after the initial adsorption of each layer is complete. The QCM-D data further suggest that the outer adsorbed layers adopt a progressively more extended conformation, particularly for the higher numbered layers. The morphology of each successive layer was characterized using in situ soft-contact atomic force microscopy, and micelle-like surface aggregates are clearly observed within each layer of the complex film, suggesting the persistence of aggregate structures throughout the multilayer structure.     

Reversible pH-triggered encapsulation and release of pyrene by adsorbed block copolymer micelles

The well-established ability of copolymer micelles to encapsulate and release hydrophobic molecules has been investigated following their adsorption onto silica particles. Here, a pH-responsive copolymer, poly(2-(dimethylamino)ethyl methacrylate)- b-poly(2-(diethylamino)ethyl methacrylate) (PDMA(106)- b-PDEA(25)), has been used to study the formation and dissociation of adsorbed micelles through pH variation. This copolymer behaves as free unimers in aqueous solutions below pH 8 and forms micelles 29 nm in hydrodynamic diameter above this pH. Encapsulation and release of a model hydrophobic compound (pyrene) by in situ adjustment of the solution pH has been compared for both free and adsorbed micelles using fluorescence spectrophotometry, epifluorescence microscopy, and zeta potential measurements. At basic pH values, pyrene is solubilized within the cores of micelles adsorbed on silica particles: addition of acid leads to micelle dissociation and release of the pyrene into the bulk aqueous solution. Micelle adsorption does not appear to hinder the extent of pyrene uptake/release. Moreover, this pH-responsive behavior is both reversible and reproducible over multiple pH cycles.     

pH-responsive diblock copolymer micelles at the silica/aqueous solution interface: Adsorption kinetics and equilibrium studies

The adsorption behavior of two examples of a weakly basic diblock copolymer, poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate) (PDMA-PDEA), at the silica/aqueous solution interface has been investigated using a quartz crystal microbalance with dissipation monitoring and an optical reflectometer. Dynamic and static light scattering measurements have also been carried out to assess aqueous solution properties of such pH-responsive copolymers. In alkaline solution, core-shell micelles are formed above the critical micelle concentration (cmc) by both copolymers, whereas the chains are molecularly dissolved (as unimers) at all concentrations in acidic solution. As a result, the adsorption behavior of PDMA-PDEA diblock copolymers on silica is strongly dependent on both the copolymer concentration and the solution pH. Below the cmc at pH 9, the cationic PDMA-PDEA copolymers adsorb as unimers and the conformation of the adsorbed polymer is essentially flat. At concentrations just above the cmc, the initial adsorption of copolymer onto the silica is dominated by the unimers due to their faster diffusion compared to the much larger micelles. Rearrangement of the adsorbed unimers and/or their subsequent displacement by micelles from solution is then observed during an equilibration period, and the final adsorbed mass is greater than that observed below the cmc. At concentrations well above the cmc, the much higher proportion of micelles in solution facilitates more effective competition for the surface at all stages of the adsorption process and no replacement of initially adsorbed unimers by micelles is evident. However, the adsorbed layer undergoes gradual rearrangement after initial adsorption. This relaxation is believed to result from a combination of further copolymer adsorption and swelling of the adsorbed layer.     

Synthesis and characterisation of new shell cross-linked micelles with amine-functional coronas

Shell cross-linked (SCL) micelles with amine-functional coronas have been constructed in aqueous solution by exploiting the micellar self-assembly of new thermo-responsive ABC triblock copolymers. These copolymers were prepared via atom transfer radical polymerisation (ATRP) in convenient one-pot syntheses and comprised a thermo-responsive core-forming poly(propylene oxide) [PPO] block, a cross-linkable central poly(glycerol monomethacrylate) [GMA] block and an amine-functional outer block based on either poly(2-(dimethylamino)ethyl methacrylate) [DMA] or poly([2-(methacryloyloxy)ethyl]trimethyl ammonium chloride) [QDMA]. DMF GPC analysis indicated an M_n of 17,700 and an M_w/M_n of 1.46 for the PPO-PGMA-PDMA triblock copolymer. The DMA residues of the PPO-PGMA-PDMA triblock copolymer were reacted with methyl iodide to prepare copolymers with differing degrees of quaternisation. Each triblock copolymer dissolved molecularly in aqueous solution at 5 °C and formed micelles with amine-functional coronas above a critical micelle temperature (CMT) of around 12 °C, which corresponded closely to the cloud point of the PPO macro-initiator. Cross-linking of the GMA residues in the inner shell using divinyl sulfone produced SCL micelles that remained intact at 5 °C, i.e. below the cloud point of the core-forming PPO block. Aqueous electrophoresis studies confirmed that these SCL micelles had considerable cationic surface charge, as expected. The cationic SCL micelles were adsorbed onto a near-monodisperse anionic silica sol, which was used as a model colloidal substrate. Thermogravimetric analyses indicated SCL micelle mass loadings of 6.1-15.5 wt.%, depending on the initial micelle concentration. Aqueous electrophoresis studies confirmed that surface charge reversal occurred on adsorption of the SCL micelles and scanning electron microscopy studies revealed the presence of SCL micelles on the silica particles.     

Micellar aggregation in blends of linear and cyclic poly(styrene-b-isoprene) diblock copolymers

The morphology of micelles formed from blends of linear and cyclic poly(styrene-b-isoprene) (PS-b-PI) block copolymers has been investigated in solution using dynamic light scattering (DLS) and in thin solid deposits by atomic force microscopy (AFM) and transmission electron microscopy under cryogenic conditions (cryo-TEM). Micelles of the pure cyclic PS(290)-b-PI(110) copolymers are wormlike cylindrical objects built by unidirectional aggregation of 33 nm wide sunflower micelles, while the linear block copolymer having the same volume fraction and molar mass forms spherical micelles 40 nm in diameter. The DLS, AFM, and cryo-TEM results consistently show that the addition of the linear copolymer (even for amounts as low as 5% w/w) to the cyclic copolymer rather favors the formation of spherical micelles at the expense of the cylindrical aggregates. Those results clearly show that the linear block copolymer chains can be used to stabilize the thermodynamically unstable elementary sunflower micelle. The thermal stability of the micelles (from the pure copolymers and from the blends) has been examined in solid deposits with in situ AFM measurements. Coalescence starts at about 70 degrees C, and the surface roughness shows a two-step decrease toward a fully homogeneous and flat structure.     

Incorporation of block copolymer micelles into multilayer films for use as nanodelivery systems

This work demonstrates the potential application of stimulus responsive block copolymer micelles as triggerable delivery systems for use within multilayer films. Cationic, pH-responsive micelles of poly[2-(dimethylamino)ethyl methacrylate-block-poly(2-(diethylamino)ethyl methacrylate)] (PDMA-PDEA) were deposited on anionic polystyrene latex particles. The charge reversal of the surface and the amount of adsorbed polymer were monitored by zeta potential measurements and colloidal titrations, respectively. Prior to adsorption, the PDMA-PDEA micelles were loaded with a hydrophobic dye, and UV-vis spectroscopy was used to determine the amount of dye encapsulated within a monolayer of micelles. It was found that subtle chemical modification of the PDMA-PDEA diblock copolymer via permanent quaternization of the PDEA block results in micelles with tunable loading capacities. Multilayers of cationic micelles of partially quaternized PDMA-PDEA and anionic polyelectrolyte (poly(sodium 4-styrene sulfonate)) were deposited on the surface of polystyrene latex particles by sequential adsorption. UV-vis analysis of the dye present within the multilayer after the addition of each layer demonstrates that the micelles are sufficiently robust to retain encapsulated dye after multiple adsorption/washing cycles and can thus create a film that can be increasingly loaded with dye as more micelle layers are adsorbed. Multiple washing cycles were performed on micellar monolayers of PDMA-PDEA to demonstrate how such systems can be used to bring about triggerable release of actives. When performing several consecutive washing steps at pH 9.3, the micelle structure of the PDMA-PDEA micelles in the monolayer is retained, resulting in only a small reduction in the amount of encapsulated dye. In contrast, washing at pH 4, the structure of the micelle layers is severely disrupted, resulting in a fast release of the encapsulated dye into the bulk. Finally, if a sufficient number of micelle/homopolyelectrolyte layers are adsorbed, it is possible to selectively dissolve the latex template, resulting in hollow capsules.     

Influence of substrate hydrophobicity on the adsorption of an amphiphilic diblock copolymer

The adsorption of poly(tert-butylmethacrylate)-block-poly(2-(dimethylamino-ethyl) methacrylate) (PtBUMA-b-PDMAEMA) was studied by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) analysis performed on dried samples. The copolymer was dissolved in toluene at concentrations below (0.01 wt%) and above (0.05 and 1 wt%) the CMC; silicon (SiOH) and CH(3)-grafted silicon (SiCH(3)) were used as substrates. Whatever the concentration and the substrate, a layer of individual copolymer molecules, 1.5-3 nm thick, formed rapidly. The adsorbed amount was slightly higher and the resistance to AFM tip scraping was stronger on SiOH than on SiCH(3). This is attributed to hydrogen bonding between the PDMAEMA block and the OH groups of the silicon surface, leading to polarization of the adsorbed layer. Above the CMC, on SiOH, randomly scattered dot-like features (about 5 nm high) observed by AFM were attributed to individual micelles, which were not displaced by drying. On SiCH(3), the particles found on the top of the adsorbed layer were micelle aggregates, about 50 nm thick, the lateral size of which was strongly influenced by the rate of drying. This further difference between SiCH(3) and SiOH is tentatively attributed to the exposure of PDMAEMA by the adsorbed layer formed on SiCH(3), while only PtBUMA would be exposed by the layer adsorbed on SiOH. The red blood cell shape and the size of the micelles observed in single layers indicate that the PtBUMA corona was not made compact as a result of drying.     

Novel biomimetic surfactant: synthesis and micellar characteristics

Novel biomimetic surfactants based on cholesterol as the hydrophobic segment and poly[2-(methacryloyloxy)ethyl phosphorylcholine] (pMPC) as the hydrophilic segment were synthesized in the present study by atom transfer radical polymerization (ATRP) of 2-(methacryloyloxy)ethyl phosphorylcholine (MPC) using a cholesterol-based macroinitiator. The association behavior of cholesterol-block-poly[2-(methacryloyloxy)ethyl phosphorylcholine] (Chol-pMPCs) in aqueous solution was studied by (1)H NMR spectroscopy, fluorescence probe technique, and atomic force microscopy (AFM). The (1)H NMR spectrum of the polymer in CD(3)OD showed both the cholesterol group and the phosphorylcholine group while the cholesterol group did not appeared in the (1)H NMR spectrum of the polymer in D(2)O, which implied the formation of a micelle structure. Fluorescence excitation spectra of a pyrene probe solubilized in the aggregates of Chol-pMPCs suggested the presence of a critical micelle concentration (cmc) in water. The critical micelle concentrations of the polymers CMPC10, CMPC20 and CMPC40 were determined to be 7.27 x 10(-3), 13.47 x 10(-3), and 20.77 x 10(-3) mg . mL(-1), respectively. AFM images of the aggregates on mica suggested that the pMPC block formed the biocompatible micelle coronas and the cholesterol block formed the hydrophobic micelle cores. These new biomimetic diblock copolymers were evaluated as "stealthy" nanocapsules for the delivery of hydrophobic drugs. The anti-cancer drug adriamycin (ADR) was chosen as a hydrophobic drug to be incorporated into the inner core of the micelles and the morphology of the drug-loaded micelles were observed by AFM.     

Self-association of water-soluble fluorinated diblock copolymers in solutions

The self-association of the fluorinated diblock copolymer, poly(methacrylic acid)-block-poly(perfluorooctylethyl methacrylate) (PMAA-b-PFMA), in water has been investigated by light scattering, potentiometry, atomic force microscopy, and transmission electron microscopy. The size of the polymer micelles increases, as the degree of dissociation of the PMAA blocks increases. Since the charged PMAA block takes the stretched structure, PMAA-b-PFMA can easily form large micelles due to the low steric hindrance of PMAA blocks. Addition of NaCl shielded electrostatic repulsion in the PMAA chain and induced the formation of smaller micelles than water without NaCl did because of the bulky structure of the PMAA chain in the shell of the micelles. The micelle of PMAA-b-PFMA in ethanol is larger than that of poly(t-butyl methacrylate)-block-poly(perfluorooctylethyl methacrylate) (PtBMA-b-PFMA) in ethanol as a result of the higher steric hindrance of the PtBMA block. The dimensions of the core and shell of the micelles were estimated. The micelle of PMAA-b-PFMA in water possesses a rather thick shell and a large volume per molecule, consistent with the extended PMAA chain. On the other hand, the shell of the micelle in an ethanol solution of PtBMA-b-PFMA is thin but has a large surface area. Facts are consistent with the shrunk structure of the PtBMA block in poor solvent.     

pH-responsive behavior of selectively quaternized diblock copolymers adsorbed at the silica/aqueous solution interface

The desorption and subsequent pH-responsive behavior of selectively quaternized poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate) (PDMA-PDEA) films at the silica/aqueous solution interface has been characterized. The copolymer films were prepared at pH 9, where micelle-like surface aggregates are spontaneously formed on silica. The subsequent rinse with a copolymer-free electrolyte solution adjusted to pH 9 causes partial desorption of the weakly or non-quaternized copolymers, but negligible desorption for the highly quaternized copolymers. Further rinsing with a pH 4 electrolyte solution results in additional desorption and extension (swelling) of the remaining adsorbed copolymer film normal to the interface. This pH-responsive behavior is reversible for two pH cycles (9-4-9-4) as monitored by both quartz crystal microbalance with dissipation monitoring (QCM-D) and also zeta potential measurements. The magnitude of the pH-responsive behavior depends on the mean degree of quaternization of the PDMA block. Moreover, a combination of contact angle data, zeta potential measurements and in situ atomic force microscopy (AFM) studies indicates that the pH-responsive behavior is influenced not only by the number of cationic binding sites on the adsorbed copolymer chains but also by the adsorbed layer structure.     

Cross-linking of cationic block copolymer micelles by silica deposition

Diblock copolymer micelles comprising cationic poly(2-(dimethylamino)ethyl methacrylate) (PDMA) coronas and hydrophobic poly(2-(diisopropylamino)ethyl methacrylate) (PDPA) cores are used as nanosized templates for the deposition of silica from aqueous solution at pH 7.2 and 20 degrees C. Both noncross-linked and shell cross-linked (SCL) micelles can be coated with silica without loss of colloid stability. Under optimized conditions, the silica deposition is confined to the partially quaternized cationic PDMA chains, leading to hybrid copolymer-silica particles of around 35 nm diameter with well-defined core-shell morphologies. 1H NMR studies confirmed that the PDPA cores of these copolymer-silica particles became protonated at low pH and deprotonated at high pH, which suggests possible encapsulation and controlled release applications. Moreover, in situ silica deposition effectively stabilizes the PDPA-PDMA micelles, which remain intact on lowering the solution pH (whereas the original noncross-linked PDPA-PDMA micelles dissociate in acidic solution). This suggests a convenient route to silica-stabilized SCL micelles under mild conditions.     

高分子表面活性剂在固/液界面上的吸附形态

采用紫外光谱、XPS研究了羧甲基纤维素型高分子表面活性剂在硅胶 /水界面上的吸附形态 ,结果表明随着高分子表面活性剂溶液浓度增大 ,分子在硅胶表面的吸附由单层逐渐变为多层 ,生成半胶束结构 .     

In situ adsorption studies of a 14-amino acid leucine-lysine peptide onto hydrophobic polystyrene and hydrophilic silica surfaces using quartz crystal microbalance, atomic force microscopy, and sum frequency generation vibrational spectroscopy

The adsorption of a 14-amino acid amphiphilic peptide, LK14, which is composed of leucine (L, nonpolar) and lysine (K, charged), on hydrophobic polystyrene (PS) and hydrophilic silica (SiO2) was investigated in situ by quartz crystal microbalance (QCM), atomic force microscopy (AFM), and sum frequency generation (SFG) vibrational spectroscopy. The LK14 peptide, adsorbed from a pH 7.4 phosphate-buffered saline (PBS) solution, displayed very different coverage, surface roughness and friction, topography, and surface-induced orientation when adsorbed onto PS versus SiO2 surfaces. Real-time QCM adsorption data revealed that the peptide adsorbed onto hydrophobic PS through a fast (t < 2 min) process, while a much slower (t > 30 min) multistep adsorption and rearrangement occurred on the hydrophilic SiO2. AFM measurements showed different surface morphologies and friction coefficients for LK14 adsorbed on the two surfaces. Surface-specific SFG spectra indicate very different ordering of the adsorbed peptide on hydrophobic PS as compared to hydrophilic SiO2. At the LK14 solution/PS interface, CH resonances corresponding to the hydrophobic leucine side chains are evident. Conversely, only NH modes are observed at the peptide solution/SiO2 interface, indicating a different average molecular orientation on this hydrophilic surface. The surface-dependent difference in the molecular-scale peptide interaction at the solution/hydrophobic solid versus solution/hydrophilic solid interfaces (measured by SFG) is manifested as significantly different macromolecular-level adsorption properties on the two surfaces (determined via AFM and QCM experiments).     

Adsorption of poly(ethylene oxide)-b-poly(epsilon-caprolactone) copolymers at the silica-water interface

The adsorption of amphiphilic poly(ethylene oxide)-b-poly(epsilon-caprolactone) and poly(ethylene oxide)-b-poly(gamma-methyl-epsilon-caprolactone) copolymers in aqueous solution on silica and glass surfaces has been investigated by flow microcalorimetry, small-angle neutron scattering (SANS), surface forces, and complementary techniques. The studied copolymers consist of a poly(ethylene oxide) (PEO) block of M(n) = 5000 and a hydrophobic polyester block of poly(epsilon-caprolactone) (PCL) or poly(gamma-methyl-epsilon-caprolactone) (PMCL) of M(n) in the 950-2200 range. Compared to homoPEO, the adsorption of the copolymers is significantly increased by the connection of PEO to an aliphatic polyester block. According to calorimetric experiments, the copolymers interact with the surface mainly through the hydrophilic block. At low surface coverage, the PEO block interacts with the surface such that both PEO and PCL chains are exposed to the aqueous solution. At high surface coverage, a dense copolymer layer is observed with the PEO blocks oriented toward the solution. The structure of the copolymer layer has been analyzed by neutron scattering using the contrast matching technique and by tapping mode atomic force microscopy. The experimental observations agree with the coadsorption of micelles and free copolymer chains at the interface.     

Surface Reconstruction by a Coassembly Approach

Materials with switchable surfaces, capable of changing surface properties under external stimuli, are playing a pivotal role in many applications, such as tissue engineering, biosensors, and drug/protein delivery. In this research silica particles with patterned and switchable surfaces are fabricated. Surface micelles on silica particles are formed by coassembly of polymer brushes and “free” block copolymer chains in a selective solvent. The cores of the surface micelles are crosslinked by anthracene photodimerization. After quaternization of the coronae, amphiphilic surface micelles are prepared. The surface micelles are able to rearrange in different media. After treatment with an organic solvent, the surfaces of silica particles are occupied by hydrophobic polymer components; in aqueous solution, the positively charged polymer chains are on the surfaces. The switching of the surface micelles results in changes in surface composition and wetting behaviors.     

Mechanism of mesoporous silica formation. A time-resolved NMR and TEM study of silica-block copolymer aggregation

The dynamics of the synthesis of a mesoporous silica material SBA-15 is followed using time-resolved in situ 1H NMR and transmission electron microscopy (TEM). Block copolymer-silica particles of two-dimensional hexagonal symmetry evolve from an initially micellar solution. The synthesis was carried out with the block copolymer Pluronic P123 (EO20-PO70-EO20) at 35 degrees C and using tetramethyl orthosilicate as the silica precursor. By using TEM, we can image different stages during the evolution of the synthesis. Flocs of spherical micelles held together by the polymerizing silica are observed prior to precipitation. With time, the structure of these flocs evolves and the transition from spherical to cylindrical hexagonally packed micelles can be monitored. The signal from the methyl protons of the PO part was recorded with 1H NMR. One observes a continuous increase in the signal width but with distinct changes in the spectral characteristics occurring in narrow time intervals. The spectral changes can be attributed to structural changes of the self-assembled aggregates. The 1H NMR and TEM studies reveal the same mechanism of formation. It is concluded that the aggregation is caused by a micelle-micelle attraction induced by oligomeric/polymeric silica that adsorbs to the EO palisade layer of the micelles and has the ability to bridge to another micelle. This adsorption also favors the formation of cylindrical aggregates relative to spherical micelles. The sequence of NMR and TEM observations can then be interpreted as the following sequence of events: (i) silicate adsorption on globular micelles possibly accompanied with some aggregate growth, (ii) the association of these globular micelles into flocs, (iii) the precipitation of these flocs, and (iv) micelle-micelle coalescence generating (semi)infinite cylinders that form the two-dimensional hexagonal packing.     

Characterizing the pH-responsive behavior of thin films of diblock copolymer micelles at the silica/aqueous solution interface

The pH-responsive behavior of cationic diblock poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate) copolymer micelles adsorbed at the silica/aqueous solution interface has been characterized. The micellar morphology of this copolymer, initially adsorbed at pH 9, can be dramatically altered by lowering the solution pH. The original micelle-like morphology of the adsorbed copolymer chains at pH 9 completely disappears as the pH is decreased to 4, and a brush-like layer structure is produced. This change results from protonation of the copolymer chains: the subsequent electrostatic repulsions within the film drive the copolymer chains to expand into the aqueous phase. Returning the solution pH from 4 to 9 causes this brush-like layer to collapse, with atomic force microscopy images suggesting degradation of the film. Hence, the pH-responsive behavior of the copolymer film exhibits irreversible morphological changes. Measurements of the adsorbed/desorbed amounts of the copolymer film were conducted using both a quartz crystal microbalance with dissipation monitoring (QCM-D) and optical reflectometry (OR). After an initial rinse at both pH values, the OR adsorbed mass becomes almost constant during subsequent pH cycling, whereas the corresponding QCM-D adsorbed mass changes significantly but reversibly in response to the solution pH. Since the QCM-D measures a bound mass that moves in tandem with the surface, the discrepancy with the OR data is due to changes in the amount of bound water in the copolymer film as a result of the pH-induced changes in surface morphology. The larger effective mass observed at pH 4 suggests that the brush-like layer contains much more entrapped water than the micellar films at pH 9. The pH dependence of the contact angle of the adsorbed film is consistent with the changes observed using the other techniques, regardless of whether the solution pH is altered in situ or the aqueous solution is completely replaced. In fact, comparison of these two approaches provides direct evidence of the exposure of adsorbed micelle core blocks to the solution during pH cycling and the concomitant impact upon all the other measurements.     

Non-surface activity and micellization of ionic amphiphilic diblock copolymers in water. Hydrophobic chain length dependence and salt effect on surface activity and the critical micelle concentration

We reported previously (Macromolecules 2003, 36, 5321; Langmuir, 2004, 20, 7412) that amphiphilic diblock copolymers having polyelectrolytes as a hydrophilic segment show almost no surface activity but form micelles in water. In this study, to further investigate this curious and novel phenomenon in surface and interface science, we synthesized another water-soluble ionic amphiphilic diblock copolymer poly(hydrogenated isoprene)-b-sodium poly(styrenesulfonate) PIp-h2-b-PSSNa by living anionic polymerization. Several diblock copolymers with different hydrophobic chain lengths were synthesized and the adsorption behavior at the air/water interface was investigated using surface tension measurement and X-ray reflectivity. A dye-solubilization experiment was carried out to detect the micelle formation. We found that the polymers used in this study also formed micelles above a certain polymer concentration (cmc) without adsorption at the air-water interface under a no-salt condition. Hence, we further confirmed that this phenomenon is universal for amphiphilic ionic block copolymer although it is hard to believe from current surface and interface science. For polymers with long hydrophobic chains (more than three times in length to hydrophilic chain), and at a high salt concentration, a slight adsorption of polymer was observed at the air-water interface. Long hydrophobic chain polymers showed behavior "normal" for low molecular weight ionic surfactants with increasing salt concentration. Hence, the origin of this curious phenomenon might be the macroionic nature of the hydrophilic part. Dynamic light scattering analysis revealed that the hydrodynamic radius of the block copolymer micelle was not largely affected by the addition of salt. The hydrophobic chain length-cmc relationship was found to be unusual; some kind of transition point was found. Furthermore, very interestingly, the cmc of the block copolymer did not decrease with the increase in salt concentration, which is in clear contrast to the fact that cmc of usual ionic small surfactants decreases with increasing salt concentration (Corrin-Harkins law). These behaviors are thought to be the special, but universal, characteristics of ionic amphiphilic diblock copolymers, and the key factor is thought to be a balance between the repulsive force from the water surface by the image charge effect and the hydrophobic adsorption.