Polymer micelles with hydrophobic core and ionic amphiphilic corona. 2. Starlike distribution of charged and nonpolar blocks in corona

Mixed polymer micelles with hydrophobic polystyrene (PS) core and ionic amphiphilic poly(4-vinylpyridine)/poly(N-ethyl-4-vinylpyridinium bromide) corona (P4VP/PEVP) spontaneously self-assembled from mixtures of PS-b-PEVP and PS-b-P4VP macromolecules in dimethylformamide/methanol/water selective solvent. The fraction of PEVP units in corona was β = [PEVP]/([PEVP] + [P4VP]) = 0.05-1.0. Micelles were transferred into pure water via dialysis technique and pH was adjusted to 9, where P4VP blocks are insoluble. Structural characteristics of micelles as a function of corona composition β were investigated. Methods of dynamic and static light scattering, electrophoretic mobility measurements, sedimentation velocity, transmission electron microscopy, and UV spectrophotometry were applied. Spherical morphology with core (PS)-shell (P4VP)-corona (PEVP) organization was postulated. Micelles demonstrated a remarkable inflection in structural characteristics near β ～ 0.5-0.7. Above this region, aggregation number (m), core and corona radii of mixed micelles coincided with those of individual PS-b-PEVP micelles. When β decreased below 0.5, dramatic growth of aggregation number was observed, accompanied by growth in micelle size and stretching PEVP chains. At β below 0.2, dispersions of mixed micelles were unstable and easily precipitated upon addition of NaCl. Scaling relationships between micelle characteristics and β were obtained via minimization the micelle free energy, taking into account electrostatic, osmotic, volume, and surface contributions. Theoretical estimations predicted dramatic influence of β on aggregation number, m ～ β(-3). This result is in general agreement with experimental data and confirms the correctness of the core-shell-corona model. The inflection in micelle characteristics entails drastic changes in micelle dispersion stability in the presence of oppositely charged polymeric (sodium polymethacrylate) or amphiphilic (sodium dodecyl sulfate) complexing agents.     

Pyranine-induced micellization of poly(ethylene glycol)-block-poly(4-vinylpyridine) and pH-triggered release of pyranine from the complex micelles

The pyranine-induced micellization of poly(ethylene glycol)-block-poly(4-vinylpyridine) (PEG114-b-P4VP61) in aqueous solutions and pH-triggered release of pyranine from the complex micelles were studied by dynamic and static light scattering, transmission electron microscopy, 1H NMR spectroscopy, and UV-vis spectroscopy. At pH 2, the ionized pyranine can ionically cross-link the protonated P4VP block and result in well-defined spherical complex micelles with a P4VP/pyranine core surrounded by a PEG corona. The ratio of pyranine to pyridyl units can influence the structure and the properties of the resultant complex micelles. The complex micelles are stable upon dilution and heating but are sensitive to pH changes. pH-triggered release of the incorporated pyranine from the complex micelles demonstrates that the release behavior is pH-tunable and displays good controlled-release characteristics at pH approximately 4.     

Hollow capsules prepared from all block copolymer micelle multilayers

We introduce a novel and versatile approach for preparing hollow multilayer capsules containing functional hydrophobic components. Protonated polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) and anionic polystyrene-block-poly(acrylic acid) (PS-b-PAA) block copolymer micelles (BCM) were used as building blocks for the layer-by-layer assembly of BCM multilayer films onto polystyrene (PS) colloids. After removing the PS colloids, the stabilities of the formed BCM hollow capsules were found to be strongly dependent on the charge density of the hydrophilic corona segments (i.e., P4VP and PAA block segments) as well as the relative molecular weight ratio of hydrophobic core (i.e., PS segments) blocks and hydrophilic corona shells. Furthermore, in the case of incorporating hydrophobic fluorescent dyes into the PS core blocks of micelles, the hairy/hairy BCM multilayers showed well-defined fluorescent images after colloidal template removal process. These phenomena are mainly caused by the relatively high degree of electrostatic interdigitation between the protonated and anionic corona block shells.     

Interaction between block copolymer micelles and azobenzene-containing surfactants: from coassembly in water to layer-by-layer assembly at the interface

In this paper, we describe the use of block copolymer micelles to incorporate Azo-AOT, an azobenzene-containing amphiphile having a structure suitable for reverse micelle formation and the fabrication of polyelectrolyte/micelle multilayer films. Interestingly, it is found that the PS21-PAA157 micelles can incorporate more Azo-AOT molecules than the PS115-PAA15 micelles, which is different from the case of incorporation of noncharged hydrophobic molecules. Moreover, Azo-AOT incorporated into the PS21-PAA157 micelles undergoes a faster photoisomerization than in the PS115-PAA15 micelles, which seems to be related to different aggregation states of Azo-AOT in the two micelles. From the data of UV-vis spectra, we can infer that Azo-AOT adopts a reverse micelle-like aggregation state in the PS115-PAA15 micelles and disperses in the interface between the core and corona of PS21-PAA157 micelles. These polyelectrolyte/micelle films incorporating functional amphiphiles have great potential in the field of functional thin films.     

Interactions in binary mixed systems involving a sugar-based surfactant and different n-alkyltrimethylammonium bromides

In this paper, mixtures of sugar-based decanoyl-N-methylglucamide with three different n-alkyltrimethylammonium bromides (n=12 (DTAB), 14 (TTAB), and 16 (CTAB)) have been studied using conductance and fluorescence spectroscopic techniques. The critical micelle concentration values of pure and mixed systems were determined by both the conductance and the pyrene 1:3 ratio methods. The experimental results were interpreted using thermodynamic mixing approaches based on the pseudophase separation model. These analyses allowed us to determine the interaction parameters and the composition of the mixed micelles through the whole composition range. Since all the ionic surfactants used in this study have the same headgroup, the differences observed between the three mixed systems were attributed to the lengths of their hydrocarbon chains. It was established that, besides interactions of electrostatic character, additional short-range interactions must be considered. By using the static quenching method, the mean micellar aggregation numbers of mixed micelles were obtained. In the cases of the mixed systems with DTAB and TTAB it was observed that the aggregation number is initially reduced with the participation of the ionic component, remaining almost constant and close to the aggregation number of the pure ionic micelle. However, in the systems involving CTAB it is observed that the size of micelles initially increases and then decreases slightly for mixtures with a high content of the ionic component. The hydrophobic index pyrene 1:3 ratio was used to examine possible changes in the micellar micropolarity; however, no definitive conclusions could be derived from these experiments. In order to study the evolution of the local viscosity of the mixed micelles upon addition of the ionic surfactant, fluorescence polarization measurements were carried out with two different probes, fluorescein and coumarin 6. It was found that the participation of the ionic component in the mixed micelle induces the formation of less ordered structure than that of pure nonionic micelles. An attempt was made to correlate these effects with the interaction parameters obtained from the theoretical mixing model and, consequently, with the alkyl chain length of the ionic components.     

Solvation dynamics in ionic liquid swollen p123 triblock copolymer micelle: a femtosecond excitation wavelength dependence study

Femtosecond solvation dynamics of coumarin 480 (C480) in a mixed micelle is reported. The mixed micelle consists of a triblock copolymer (PEO)20-(PPO) 70-(PEO)20 (Pluronic P123) and an ionic liquid (IL), 1-pentyl-3-methylimidazolium tetrafluoroborate ([pmim][BF4]). At a low concentration (0.3 M), the sparingly water soluble IL ([pmim][BF4]) penetrates the hydrophobic PPO core of the P123 micelles. Thus emission maximum of C480 in the core (accessed at lambdaex=375 nm) in 0.3 M IL is red-shifted by 8 nm from that in its absence and the red edge excitation shift (REES) is large (19+/-1 nm). At a high concentration (0.9 M), the ionic liquid [pmim][BF4] invades both the core and corona region and the mixed micelle exhibits very small REES (3+/-1 nm). Anisotropy decay and solvation dynamics in different regions of the mixed micelle are studied by variation of excitation wavelength (lambda ex). In P123 micelle, the average rotational time () is 2800 ps in the core (at lambdaex=375 nm) and 1350 ps in the corona region (at lambdaex=435 nm). In 0.3 M [pmim][BF4], tau rot at the core of the mixed micelle decreases to 1950 ps while that in the corona remains unaffected. In 0.9 M IL, both the core and corona (lambda ex=375 and 435 nm) exhibit similar and short approximately 600 ps. In 0.3 M IL, solvation dynamics in the core region (lambdaex=375 nm) of P123 micelle is about 2 times faster than in its absence. In 0.3 M IL, solvation dynamics in the corona region (lambdaex=435 nm) is approximately 100 times faster than that in the core. In 0.9 M IL, the solvation dynamics in the core and in the corona is, respectively, approximately 9 times and 4 times faster than that in 0.3 M IL.     

Probing solubilization sites in block copolymer micelles using fluorescence quenching

The solubilization sites provided by micelles formed by a diblock copolymer with one neutral hydrophobic block, polystyrene, and one charged hydrophilic block, poly(acrylic acid) or poly(methacrylic acid), have been studied by fluorescence quenching of pyrene by polar and nonpolar quenchers. Pyrene solubilized into these micelles is distributed between the inner corona and the micelle core. The fraction of pyrene residing in the inner corona is almost unity for star micelles, where the corona-forming blocks are larger than the core-forming blocks, and around 0.5 for crew-cut micelles where the opposite situation prevails. The kinetics of the quenching process depends on the pyrene location, i.e. is static in the micelle core, and largely dynamic in the inner corona at low quencher concentration. The rate constants for fluorescence quenching by nitromethane are shown to increase with increasing pH.     

Double‐hydrophilic block copolymer for encapsulation and two‐way pH change‐induced release of metalloporphyrins

A double hydrophilic block copolymer composed of poly(acrylic acid) (PAA) and poly(4‐vinyl pyridine) (P4VP) was obtained through hydrolysis of diblock copolymer of poly(tert‐butyl acrylate) (PtBA) and P4VP synthesized using atom transfer radical polymerization. Water‐soluble micelles with PAA core and P4VP corona were observed at low (acidic) pH, while micelles with P4VP core and PAA corona were formed at high (basic) pH. Two metalloporphyrins, zinc tetraphenylporphyrin (ZnTPP) and cobalt tetraphenylporphyrin (CoTPP), were used as model compounds to investigate the encapsulation of hydrophobic molecules by both types of micelles. UV–vis spectroscopic measurements indicate that micelles with P4VP core are able to entrap more ZnTPP and CoTPP as a result of the axial coordination between the transition metals and the pyridine groups. The study found that metalloporphyrins encapsulated by the micelles with PAA core could be released on pH increase, while those entrapped by the micelles with P4VP core could be released on pH decrease. This behavior originates from the two‐way pH change‐induced disruption of PAA‐b‐P4VP micelles. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1734–1744, 2006     

Reorganization of hydrogen-bonded block copolymer complexes

Poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) copolymers and poly(acrylic acid) (PAA) have been mixed in organic solvents. Complexation via hydrogen bonding occurs between the P4VP and PAA blocks. Those insoluble complexes aggregate to form the core of micelles surrounded by a corona of PS chains. Reorganization of these structures occurs upon addition of acidic or basic water, which results in the breaking of the hydrogen bonds between the P4VP and PAA blocks. After transfer of the initial complexes in acidic water, micelles consisting of a PS core and a protonated P4VP corona are observed. In basic water, well-defined nanoparticles formed by the PS-b-P4VP copolymers are obtained. It is demonstrated that these nanoparticles are stabilized by the negatively charged PAA chains. Finally, thermally induced disintegration of the micelles is investigated in organic solvents.     

Polyether nanoparticles from covalently crosslinked copolymer micelles

Double hydrophilic block copolymers poly(ethylene oxide)-b-polyglycidol were synthesized using living anionic polymerization. The polyglycidol blocks were made hydrophobic by the esterification of a part of hydroxyl groups with cinnamic acid, thus simultaneously attaching UV-sensitive double bonds to the polymer backbone. The block copolymers were found to spontaneously associate in aqueous solution forming well-defined micelles, where the corona of the micelles was formed of EO units and the cores consisted of hydrophobic glycidyl cinnanamate units. The critical micelle concentration was determined by light-scattering measurements and fluorescence spectroscopy. Stabilization of micelles was obtained by covalently crosslinking the cores of polyether micelles formed from amphiphilic block copolymers of the type poly(ethylene oxide)-b-poly(glycidol-co-glycidyl cinnamate) (denoted EO(113)-b-(Gl(33)-co-GlCA(33-x))). To obtain stable nanoparticles double bonds of cinnamate units contained in core were crosslinked under UV irradiation. The kinetics of the stabilization process was investigated using SEC-MALLS and UV spectroscopy. The parameters of the micelles and nanogels were calculated from the light-scattering data.     

Complex micelles formed from two diblock copolymers for applications in controlled drug release

Double‐responsive core‐shell‐corona complex micelles for applications in drug release were formed from self‐assembly of two diblock copolymers PtBA‐b‐ PNIPAM and PtBA‐b‐P4VP. The two diblock copolymers coaggregated into core‐shell complex micelles in acidic water with the hydrophobic PtBA blocks as the common core and soluble PNIPAM/P4VP blocks as the mixed shell. Increasing temperature or pH value, the micelles converted into core‐shell‐corona micelles because of the collapse of PNIPAM or P4VP blocks as the inner shell and soluble P4VP or PNIPAM chains stretching outside as the outer corona. The anti‐inflammation drug naproxen (NAP) was loaded as the model drug in micelles in acidic water and released because of the ionization of NAP in alkaline solutions. Compared with pure core‐shell micelles, release of NAP from core‐shell‐corona complex micelles avoided the burst diffusion and the release rate is more easily controlled by tuning the composition of the mixtures or by adjusting the pH of the medium. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1804–1810, 2009     

Sphere-to-rod transition of non-surface-active amphiphilic diblock copolymer micelles: a small-angle neutron scattering study

Micellization behavior of amphiphilic diblock copolymers with strong acid groups, poly(hydrogenated isoprene)-block-poly(styrenesulfonate), was investigated by small-angle neutron scattering (SANS). We have reported previously (Kaewsaiha, P.; Matsumoto, K.; Matsuoka, H. Langmuir 2005, 21, 9938) that this strongly ionic amphiphilic diblock copolymer shows almost no surface activity but forms micelles in water. In this study, the size, shape, and internal structures of the micelles formed by these unique copolymers in aqueous solution were duly investigated. The SANS data were well described by the theoretical form factor of a core-shell model and the Pedersen core-corona model. The micellar shape strongly depends on the hydrophobic chain length of the block copolymer. The polymer with the shortest hydrophobic chain was suggested to form spherical micelles, whereas the scattering curves of the longer hydrophobic chain polymers showed a q-1 dependence, reflecting the formation of rodlike micelles. Furthermore, the addition of salt at high concentration also induced the sphere-to-rod transition in micellar shape as a result of the shielding effect of electrostatic repulsion. The corona thickness was almost constant up to the critical salt concentration (around 0.2 M) and then decreased with further increases in salt concentration, which is in qualitatively agreement with existing theories. The spherical/rodlike micelle ratio was also constant up to the critical salt concentration and then decreased. The micelle size and shape of this unique polymer could be described by the common concept of the packing parameter, but the anomalously stable nature of the micelle (up to 1 M NaCl) is a special characteristic.     

Enhanced stability of ZnTPPS by polymeric gold nanoparticles in acidic aqueous solutions

Novel kind of core-shell corona complex micelles were prepared, which enhanced both the hydrolytic stability and the photostability of water-soluble zinc tetrakis(4-sulfonatophenyl) porphyrin (ZnTPPS) in acidic aqueous solutions. The core-shell gold nanoparticles (AuNPS) were synthesized by reducing HAuCl₄ and di-thioester terminated block copolymer, poly(Nisopropylacrylamide)-block-poly(4-vinylpyridine) (PNIPAM-b-P4VP). The complex micelles with gold core, P4VP/ZnTPPS shell and PNIPAM corona were formed by simple mixing of gold nanoparticles and ZnTPPS. The photochemical properties of the complex micelles were studied by UV–Visiblespectroscopy and fluorescence spectroscopy. The results showed trapping of ZnTPPS in the positively charged micellar shell that effectively prevented demetallation of the ZnTPPS that would occur in acidic aqueous solutions. Furthermore, with appropriate concentration of gold nanoparticles, ZnTPPS in the complex micelle had excellent photostability by suppression of generation of reactive oxygen species (ROS). The enhanced stability of ZnTPPS in acidic aqueous media could be extensively used for photocatalysis and in solar cells.     

Spectroscopic studies on the photostability and photoactivity of metallo-tetraphenylporphyrin in micelles

Inspired by natural photosynthesis, we employed polymeric micelles to enhance the water solubility and photostability of hydrophobic metallo-tetraphenylporphyrin (metallo-TPP) by complexation with poly(ethylene glycol)-block-poly(4-vinylpyridine) (PEG-b-P4VP) via axial coordination. The structure and photochemical properties of the complex micelles were characterized by UV-visible spectroscopy, fluorescence spectroscopy, and laser light scatting. The photostability and electron transfer ability of metalloporphyrins in the micelles were investigated under continuous irradiation. The results show that metallo-TPPs entrapped in the micellar cores possess higher photostability and better electron transfer ability. The hydrophobic metalloporphyrins remains active inside the micelles which is reminiscent of chlorophyll protein complex in photosynthesis. The use of micelle thus may provide a promising system on designing photocatalysts for targeting applications in solar energy conversion and photodynamic tumor therapy (PDT).     

Block copolymer micelle shuttles with tunable transfer temperatures between ionic liquids and aqueous solutions

Four poly((1,2-butadiene)- block-ethylene oxide) (PB-PEO) diblock copolymers were shown to self-assemble into micelles with PB cores and PEO coronas (including spheres, cylinders, and vesicles) in the ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]). All four systems exhibited the "micelle shuttle" (He, Y.; Lodge, T. P. J. Am. Chem. Soc. 2006, 128, 12666-12667), whereby PB-PEO micelles transferred, reversibly and with preservation of micelle structure, from an aqueous phase at room temperature to a hydrophobic ionic liquid at high temperature. The micelle size (both mean and distribution) depends on whether it was initially dissolved in water or in the ionic liquid, but the initial micelle structures in the ionic liquid were shown by dynamic light scattering to be preserved during the transfer and persist essentially unchanged for months in both the ionic liquid and water. The transfer was shown to be driven by the deteriorating solvent quality of water for PEO at high temperature, while the ionic liquid remains a good solvent. The transfer temperature could be tuned by adding ionic or nonionic additives to the aqueous phase to change the solvent quality of water for PEO, and by using ionic liquids with different polarity.     

Modulating the catalytic activity of Au/micelles by tunable hydrophilic channels

Polymeric micelles with a polystyrene core, poly(acrylic acid)/poly(4-vinyl pyridine) (PAA/P4VP) complex shell and poly(ethylene glycol) & poly(N-isopropylacrylamide) (PEG & PNIPAM) mixed corona were synthesized and used as the supporter for the gold nanoparticles (GNs). It was concluded from the result of ¹H NMR characterization that hydrophilic channels formed around PEG chains when PNIPAM collapsed above its lower critical solution temperature. The density of the channels in the corona can be tuned by changing the weight ratios of PEG chains to PNIPAM chains. The GNs were set in the PAA/P4VP complex layer and the catalytic activity of the GNs can be modulated by the channels. The catalytic activity increased with increasing the density of the channels in the corona. Meanwhile, the whole Au/micelle nanoparticles were stabilized by the extended PEG chains.     

Double electron electron resonance as a method for characterization of micelles

Double electron electron resonance (DEER) is an experimental technique used to determine distance between electron spins. In this work, we show that it can be used to study the properties of micelles in solution, specifically their volume and the aggregation number. The feasibility of the method is tested on micelles of Pluronic block copolymers, PEO(x)-PPO(y)-PEO(x), built from chains of poly(ethylene oxide) (PEO), comprising the more hydrophilic corona, and a poly(propylene oxide) (PPO) block constituting the hydrophobic core. In this work, the dimensions of the hydrophobic core of micelles of Pluronic L64 (x = 13, y = 30), P123 (x = 20, y = 70), and F127 (x = 106, y = 70) and their aggregation number were studied. This was done using the spin-probe 4-hydroxy-tempo-benzoate (4HTB), which is hydrophobic and is localized in the hydrophobic core of the micelles and does not dissolve in aqueous solution. The measurements were carried out on frozen solutions, freeze quenched after equilibration at 50 degrees C. It was found that the hydrophobic core radii occupied by 4HTB in 7.5 wt % F127 and 6 wt % L64 are 4.0 +/- 0.05 and 3.8 +/- 0.1 nm, respectively, and the corresponding aggregation numbers are 57 +/- 2 and 206 +/- 14. The micelles of 6 wt % P123 were found to have a rod shape, and the addition of 4HTB at concentrations higher than 0.7 mM resulted in a phase transitioned to spherical micelles. Finally, this study also showed that the micelle structure is preserved upon rapid freezing.     

Study of the ordered structures of poly(styrene-b-vinyl4pyridine) in a solution state by using small-angle X-ray scattering and generalized indirect Fourier transform

The structures of the mesophases and their subunits (micelles) of poly(styrene-b-vinyl4pyridine) (PS-b-P4VP) in a toluene solution were studied by using small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and generalized indirect Fourier transform (GIFT) methods. The structures of PS-b-P4VP, such as the individual micelle, the face-centered cubic (fcc) and body-centered cubic (bcc), and the lamellar, were identified. The SAXS from the PS-b-P4VP solution showed a good contrast for the micelle, even in a low concentration of 0.25 wt %. As the concentration increases, the fcc and both fcc and bcc appear for the packing of the micelles of PS(3.3K)-b-P4VP(4.7K) and PS(12K)-b-P4VP(11.8K), respectively. The lamellar structure was also identified, with a further increase in the concentration for PS(3.3K)-b-P4VP(4.7K). These structures were also identified via TEM images.     

Self-assembly of charged amphiphilic diblock copolymers with insoluble blocks of decreasing hydrophobicity: from kinetically frozen colloids to macrosurfactants

We have investigated the self-assembly properties in aqueous solution of amphiphilic diblock copolymers with insoluble blocks of different hydrophobicity and demonstrated that the condition to obtain dynamic micelles is to design samples with insoluble blocks of low enough hydrophobicity. We focus here on results with new water-soluble amphiphilic diblock copolymers poly(diethyleneglycol ethylether acrylate)-b-poly(acrylic acid), or PDEGA-b-PAA. The physical characteristics of PDEGA-b-PAA micelles at high ionization have been determined by small angle neutron scattering (SANS). We show that PDEGA-b-PAA samples form micelles at thermodynamic equilibrium. The critical micelle concentrations (CMCs) decrease strongly with ionic strength and temperature due to a solvent quality decrease for, respectively, the corona and the core. This behavior of reversible aggregation is remarkable as compared to the behavior of kinetically frozen aggregation that has been widely observed with samples of similar architecture and different hydrophobic blocks, for example, poly(styrene)-b-poly(acrylic acid), PS-b-PAA, and poly(butyl acrylate)-b-poly(acrylic acid), PBA-b-PAA. We have measured the interfacial tension between water and the homopolymers PDEGA and PBA at, respectively, 3 and 20 mN/m at room temperature, which permits one to estimate the energy cost to extract a unimer from a micelle. The results are consistent with a micelle association that is fast for PDEGA-b-PAA and kinetically frozen PBA-b-PAA. Hence, PDEGA-b-PAA samples form a new system of synthetic charged macrosurfactant with unique properties of fast dynamic association, tunable charge, and water solubility even at temperatures and NaCl concentrations as high as 65 °C and 1 M.