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.     

Nucleo-copolymers: oligonucleotide-based amphiphilic diblock copolymers

For the first time, poly(butadiene) has been covalently linked to an oligonucleotide sequence and the resulting nucleo-copolymer exhibits amphiphilic properties in dilute aqueous solution, self-assembling into nanometer-sized vesicular structures.     

Synthesis and micellization of thermoresponsive galactose‐based diblock copolymers

Thermoresponsive double hydrophilic diblock copolymers poly(2‐(2′‐methoxyethoxy)ethyl methacrylate‐co‐oligo(ethylene glycol) methacrylate)‐b‐poly(6‐O‐methacryloyl‐D ‐galactopyranose) (P(MEO₂MA‐co‐OEGMA)‐b‐PMAGP) with various compositions and molecular weights were obtained by deprotection of amphiphilic diblock copolymers P(MEO₂MA‐co‐OEGMA)‐b‐poly(6‐O‐methacryloyl‐1,2:3,4‐di‐O‐isopropylidene‐D ‐galactopyranose) (P(MEO₂MA‐co‐OEGMA)‐b‐PMAlpGP), which were prepared via reversible addition‐fragmentation chain transfer (RAFT) polymerization using P(MEO₂MA‐co‐OEGMA) as macro‐RAFT agent. Dynamic light scattering and UV–vis studies showed that the micelles self‐assembled from P(MEO₂MA‐co‐OEGMA)‐b‐PMAlpGP were thermoresponsive. A hydrophobic dye Nile Red could be encapsulated by block copolymers P(MEO₂MA‐co‐OEGMA)‐b‐PMAGP upon micellization and released upon dissociation of the formed micelles under different temperatures. The galactose functional groups in the PMAGP block have specific interaction with HepG2 cells, and P(MEO₂MA‐co‐OEGMA)‐b‐PMAGP has potential applications in hepatoma‐targeting drug delivery and biodetection. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Stimulus-responsive self-assembly: reversible, redox-controlled micellization of polyferrocenylsilane diblock copolymers

In depth studies of the use of electron transfer reactions as a means to control the self-assembly of diblock copolymers with an electroactive metalloblock are reported. Specifically, the redox-triggered self-assembly of a series of polystyrene-block-polyferrocenylsilane (PS-b-PFS) diblock copolymers in dichloromethane solution is described. In the case of the amorphous polystyrene(n)-b-poly(ferrocenylphenylmethylsilane)(m) diblock copolymers (PS(n)-b-PFMPS(m): n = 548, m = 73; n = 71, m = 165; where n and m are the number-averaged degrees of polymerization), spherical micelles with an oxidized PFS core and a PS corona were formed upon oxidation of more than 50% of the ferrocenyl units by [N(C(6)H(4)Br-4)(3)][SbX(6)] (X = Cl, F). Analogous block copolymers containing a poly(ferrocenylethylmethylsilane) (PFEMS) metalloblock, which has a lower glass transition temperature, behaved similarly. However, in contrast, on replacement of the amorphous metallopolymer blocks by semicrystalline poly(ferrocenyldimethylsilane) (PFDMS) segments, a change in the observed morphology was detected with the formation of ribbon-like micelles upon oxidation of PS(535)-b-PFDMS(103) above the same threshold value. Again the coronas consisted of fully solvated PS and the core consisted of partially to fully oxidized PFS associated with the counteranions. When oxidation was performed with [N(C(6)H(4)Br-4)(3)][SbF(6)], reduction of the cores of the spherical or ribbon-like micelles with [Co(η-C(5)Me(5))(2)] enabled full recovery of the neutral chains and no significant chain scission was detected.     

Dynamics of amphiphilic diblock copolymers at the air-water interface

Two polyisoprene-polyethyleneoxide diblock copolymers with different block length ratios adsorbed to the water surface were investigated by multiple angle of incidence ellipsometry, evanescent wave light scattering, and surface tension experiments. In a semidilute interfacial regime, the transition from a two-dimensional to a "mushroom" regime, in which polymer chains form loops and tails in the subphase, was discussed. A diffusion mechanism parallel to the interface was probed by evanescent wave dynamic light scattering. At intermediate concentrations, the interfacial diffusion coefficient D(∥) scales with the surface concentration Γ, as D(∥) ~ Γ(0.77) in agreement with the scaling observed for polymer solutions in a semidilute regime. At relatively high concentrations a decreasing of D(∥) is discussed in terms of increasing friction due to interactions between polyisoprene chains.     

Synthesis and chiral micellization behavior of optically active amphiphilic diblock copolymer bearing quinine pendants

A novel optically active amphiphilic diblock copolymer bearing quinine pendants poly(ethylene oxide)‐b‐poly(glycidyl triazolyl‐L ‐quinine) (MPEO‐b‐PGTQ) was synthesized by “click” reaction of alkyne‐modified diblock copolymer poly(ethylene oxide)‐b‐poly(glycidyl propargyl ether) (MPEO‐b‐PGPE) and 9‐N₃‐quinine. The structure and composition of copolymers were characterized by gel permeation chromatography, ¹H nuclear magnetic resonance spectroscopy (¹H NMR), elemental analysis and optical rotation measurements, which showed that the synthetic route could provide the copolymer with well‐defined composition and with similar optical activity compared to its parent quinine. The micellization behavior of this chiral copolymer was investigated in different solvent systems. The results from fluorescence spectroscopy, UV spectroscopy, dynamic light scattering, transmission electron microscopy, ¹H NMR and circular dichroism (CD) spectroscopy indicated that the MPEO‐b‐PGTQ could form regular chiral spherical micelles in H₂O and Tetrahydrofuran‐H₂O (10:90, V/V) systems, and the state of aggregated chiral micelles depended on the nature of the medium. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3640–3650, 2009     

Pressure-induced reentrant micellization of amphiphilic block copolymers in dilute aqueous solutions

The pressure-induced structural changes of a block copolymer, poly(2-ethoxyethoxyethyl vinyl ether)-block-poly(2-hydroxyethyl vinyl ether) (pEOEOVE-b-pHOVE) in aqueous solutions, were studied by means of small-angle neutron scattering (SANS) and dynamic light scattering (DLS) from atmospheric pressure up to 400 MPa. pEOEOVE-b-pHOVE formed a spherical micellar structure above 40 degrees C due to poor solubility of pEOEOVE. Micellization phase diagram was determined by DLS, and a covex-upward pressure-temperature (P-T) phase diagram was obtained having a peak around (P,T)=(150 MPa,48 degrees C). The SANS curves at 50 degrees C were analyzed as a function of P. The micellar core size decreased by pressurizing at low P's (P相似文献   

Reversible inside-out micellization of pH-responsive and water-soluble vesicles based on polypeptide diblock copolymers

The synthesis of a new zwitterionic diblock copolymer poly(l-glutamic acid)-b-poly(l-lysine) (PGA-b-PLys) is described, and its self-assembly into schizophrenic vesicles that can reversibly be produced in moderate acidic or basic aqueous solutions is reported. These pH-sensitive nanoparticles are expected to be very promising candidates in macromolecular nanobiotechnologies.     

侧基带有L-苯丙氨酸的手性两亲嵌段共聚物的合成及其自组装行为

N3-苯丙氨酸与嵌段共聚物聚乙二醇-b-聚炔丙基缩水甘油(MPEO-b-PGPE)发生"click"反应,合成了具有光学活性的两亲嵌段共聚物聚乙二醇-B-聚L-苯丙氨酸三唑基缩水甘油(MPEO-b-PGTP),用1H-NMR和元素分析对其结构和组成进行表征.并对其自组装行为进行研究,滴体积法测定MPEO-b-PGTP溶...     

The directed self-assembly of amphiphilic diblock copolymers in selective solvents

The directed self-assembly of diblock copolymers in solvents is studied systematically using a simulated annealing method. Effects of the shape, scale, and adsorption capacity of the induced surface on the morphology of the aggregates are examined. A variety of morphologies are predicted. By increasing the scale of induced surface, the micellar shape transforms from cylinder to sheet with a tail and finally to thin sheet without tail. The shape of induced surface determines the sheet’s shape, such as rounded and square. Configurations of hydrophobic blocks and interfacial energies are investigated by calculating the mean square end-to-end distances and the contact numbers between hydrophobic monomer and other species, respectively.     

pH-responsive non-surface-active/surface-active transition of weakly ionic amphiphilic diblock copolymers

The weakly ionic amphiphilic diblock copolymer polystyrene-b-poly(acrylic acid) was synthesized by nitroxy radical-mediated living radical polymerization with precise control of block length, block ratio, and polydispersity. Systematical surface tension experiments and foam formation observations revealed that this polymer was non-surface active under neutral and alkaline (pH 10) conditions, while it was surface active under an acidic condition (pH 3). This result supports our proposed origin of non-surface activity; the image charge repulsion at the air/water interface is essential in addition to very stable micelle formation in the bulk solution. At a higher pH (pH 12), the polymer showed slight surface activity since the added NaOH played a role as an added salt. The critical micelle concentration (cmc) was estimated by static light scattering. Cmc increased with increasing added salt (NaCl) concentration as was observed for other strongly ionic non-surface-active polymers. Hence, this trend is characteristic for non-surface-active polymers. The pH dependence of cmc was minimum at pH 8–10. Since the acrylic acid block is fully ionized under this condition, the strong image charge repulsion at this condition accelerated micelle formation at a low polymer concentration, which consequently decreased cmc. Micelles in bulk solution were confirmed by dynamic light scattering, and the salt concentration and pH dependencies of the hydrodynamic radius of the micelles were also estimated. The pH-responsive non-surface-active/surface-active transition observed in this study strongly supports the fact that the image charge repulsion is an essential factor for non-surface activity in addition to stable micelle formation in solution.     

Synthesis and micellization of amphiphilic poly(ethylene oxide)‐block‐poly(methylidene malonate 2.1.2.) diblock copolymers

Poly(ethylene oxide)‐block‐poly‐(methylidene malonate 2.1.2.) diblock copolymers were prepared by anionic polymerization. They were characterized by SEC, ¹H NMR spectroscopy and DSC. Their micellization behavior was examined in aqueous medium. The micelle size, as determined by dynamic light scattering, could be correlated to the degree of polymerization of each sequence in agreement with existing theories.     

New amphiphilic diblock copolymers: surfactant properties and solubilization in their micelles

Several series of amphiphilic diblock copolymers are investigated as macrosurfactants in comparison to reference low-molar-mass and polymeric surfactants. The various copolymers share poly(butyl acrylate) as a common hydrophobic block but are distinguished by six different hydrophilic blocks (one anionic, one cationic, and four nonionic hydrophilic blocks) with various compositions. Dynamic light scattering experiments indicate the presence of micelles over the whole concentration range from 10(-4) to 10 g x L(-1). Accordingly, the critical micellization concentrations are very low. Still, the surface tension of aqueous solutions of block copolymers decreases slowly but continuously with increasing concentration, without exhibiting a plateau. The longer the hydrophobic block, the shorter the hydrophilic block, and the less hydrophilic the monomer of the hydrophilic block is, the lower the surface tension is. However, the effects are small, and the copolymers reduce the surface tension much less than standard low-molar-mass surfactants. Also, the copolymers foam much less and even act as anti-foaming agents in classical foaming systems composed of standard surfactants. The copolymers stabilize O/W emulsions made of methyl palmitate as equally well as standard surfactants but are less efficient for O/W emulsions made of tributyrine. However, the copolymer micelles exhibit a high solubilization power for hydrophobic dyes, probably at their core-corona interface, in dependence on the initial geometry of the micelles and the composition of the block copolymers. Whereas micelles of copolymers with strongly hydrophilic blocks are stable upon solubilization, solubilization-induced micellar growth is observed for copolymers with moderately hydrophilic blocks.     

Simple synthesis of zwitterionic diblock copolymers for the application on metal nanoparticles preparation

AB diblock copolymers of poly(2-(dimethylamino)ethyl metharylate-block-potassiurn acrylate) were prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization. The structure of the block polymer was determined by the nuclear magnetic resonance (NMR) spectroscopy and the gel permeation chromatography. Moreover, it has also been shown that the diblock copolymers exhibit aggregate as function of the pH according to the result of ¹H-NMR spectroscopy, FT-IR absorption spectra, UV-vis transmittance spectroscopy, transmission electron microscopy and ultrasonic particle size analyzer. The result was attributed that such AB diblock copolymers were tailored to undergo pH-induced self-assembly. Furthermore, the aggregate can be as template of metal nanoparticles preparation, and the sizes of the aggregate, in turn, strongly control nanoparticle sizes.     

Adsorption characteristics of zwitterionic diblock copolymers at the silica/aqueous solution interface

The adsorption of a zwitterionic diblock copolymer, poly(2-(diethylamino)ethyl methacrylate)-block-poly(methacrylic acid) (PDEA59-PMAA50), at the silica/aqueous solution interface has been characterised as a function of pH. In acidic solution, this copolymer forms core-shell micelles with the neutral PMAA chains being located in the hydrophobic cores and the protonated PDEA chains forming the cationic micelle coronas. In alkaline solution, the copolymer forms the analogous inverted micelles with anionic PMAA coronas and hydrophobic PDEA cores. The morphology of the adsorbed layer was observed in situ using soft-contact atomic force microscopy (AFM): this technique suggests the formation of a thin adsorbed layer at pH 4 due to the adsorption of individual copolymer chains (unimers) rather than micelle aggregates. This is supported by the remarkably low dissipation values and the relatively low degrees of hydration for the adsorbed layers, as estimated using a combination of quartz crystal microbalance with dissipation monitoring (QCM-D) and optical reflectometry (OR). In alkaline solution, analysis of the adsorption data suggests a conformation for the adsorbed copolymers where one block projects normal to the solid/liquid interface; this layer consists of a hydrophobic PDEA anchor block adsorbed on the silica surface and an anionic PMAA buoy block extending into the solution phase. Tapping mode AFM studies were also carried out on the silica surfaces after removal from the copolymer solutions and subsequent drying. Interestingly, in these cases micelle-like surface aggregates were observed from both acidic and alkaline solutions. The lateral dimension of the aggregates seen is consistent with the corresponding hydrodynamic diameter of the copolymer micelles in bulk solution. The combination of the in situ and ex situ AFM data provides evidence that, for this copolymer, micelle aggregates are only seen in the ex situ dry state as a result of the substrate withdrawal and drying process. It remains unclear whether these aggregates are caused by micelle deposition at the surface during the substrate withdrawal from the solution or as a result of unimer rearrangements at the drying front as the liquid recedes from the surface.     

Synthesis and "schizophrenic" micellization of double hydrophilic AB4 miktoarm star and AB diblock copolymers: structure and kinetics of micellization

Poly(N-isopropylacrylamide) (PNIPAM)-based tetrafunctional atom transfer radical polymerization (ATRP) macroinitiator (1b) was synthesized via addition reaction of mono-amino-terminated PNIPAM (1a) with glycidol, followed by esterification with excess 2-bromoisobutyryl bromide. Well-defined double hydrophilic miktoarm AB4 star copolymer, PNIPAM-b-(PDEA)4, was then synthesized by polymerizing 2-(diethylamino)ethyl methacrylate (DEA) via ATRP in 2-propanol at 45 degrees C using 1b, where PDEA was poly(2-(diethylamino)ethyl methacrylate). For comparison, PNIPAM-b-PDEA linear diblock copolymer with comparable molecular weight and composition to that of PNIPAM-b-(PDEA)4 was prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization. The pH- and thermoresponsive "schizophrenic" micellization behavior of the obtained PNIPAM65-b-(PDEA63)4 miktoarm star and PNIPAM70-b-PDEA260 linear diblock copolymers were investigated by 1H NMR and laser light scattering (LLS). In acidic solution and elevated temperatures, PNIPAM-core micelles were formed; whereas at slightly alkaline conditions and room temperature, structurally inverted PDEA-core micelles were formed. The size of the PDEA-core micelles of PNIPAM65-b-(PDEA63)4 is much smaller than that of PNIPAM70-b-PDEA260. Furthermore, the pH-induced micellization kinetics of the AB4 miktoarm star and AB block copolymers were investigated by the stopped-flow light scattering technique upon a pH jump from 4 to 10. Typical kinetic traces for the micellization of both types of copolymers can be well fitted with double-exponential functions, yielding a fast (tau1) and a slow (tau2) relaxation processes. tau1 for both copolymers decreased with increasing polymer concentration. tau2 was independent of polymer concentration for PNIPAM65-b-(PDEA63)4, whereas it decreased with increasing polymer concentration for PNIPAM70-b-PDEA260. The chain architectural effects on the micellization properties and the underlying mechanisms were discussed in detail.     

Water-dispersible, uniform nanospheres by heating-enabled micellization of amphiphilic block copolymers in polar solvents

Uniform nanospheres with tunable size down to 30 nm were prepared simply by heating amphiphilic block copolymers in polar solvents. Unlike reverse micelles prepared in nonpolar, oily solvents, these nanospheres have a hydrophilic surface, giving them good dispersibility in water. Furthermore, they are present as individual, separated, rigid particles upon casting from the solution other than continuous thin films of merged micelles cast from micellar solution in nonpolar solvents. These nanospheres were generated by a heating-enabled micellization process in which the affinity between the solvent and the polymer chains as well as the segmental mobility of both hydrophilic and hydrophobic blocks was enhanced, triggering the micellization of the glassy copolymers in polar solvents. This heating-enabled micellization produces purely well-defined nanospheres without interference of other morphologies. The micelle sizes and corona thickness are tunable mainly by changing the lengths of the hydrophobic and hydrophilic blocks, respectively. The heating-enabled micellization route for the preparation of polymeric nanospheres is extremely simple, and is particularly advantageous in producing rigid, micellar nanospheres from block copolymers with long glassy, hydrophobic blocks which are otherwise difficult to prepare with high efficiency and purity. Furthermore, encapsulation of hydrophobic molecules (e.g., dyes) into micelle cores could be integrated into the heating-enabled micellization, leading to a simple and effective process for dye-labeled nanoparticles and drug carriers.     

pH/enzyme/light triple-responsive vesicles from lysine-based amphiphilic diblock copolymers

We report the synthesis of pH- and enzyme-responsive amphiphilic diblock copolymers through reversible addition-fragmentation chain transfer polymerization of a lysine-derived methacrylate monomer comprising p-nitrobenzyl carbamate (pNBC) functionality using a poly(ethylene glycol)-modified macro-chain transfer agent. Depending on the hydrophobic block length, the diblock copolymers self-assemble to form spherical micelles, wormlike micelles, and bilayered vesicles in the aqueous solution. The responsive behaviors of the polymeric vesicles to pH, enzyme, and light are investigated in detail. As the pH lowers to pH 5.0, the polymeric vesicles undergo a morphological transition from vesicles to spherical micelles. In the presence of nitroreductase and a cofactor NADH, the decomposition of pNBC releases the ε-NH₂ of the lysine moiety and hence induces the generation of the vesicles with crosslinked membranes at pH 7.4. Moreover, owing to the degradation of pNBC moiety under UV irradiation, the polymeric vesicles also demonstrate a photo-responsive feature. As the irradiation time prolongs, it is observed a light-triggered morphological transition from vesicles to wormlike micelles with network-like structures.