Direct preparation of core cross-linked micelles in a nonselective solvent

This is the first light scattering study demonstrating that the size of micelles, the aggregation number, and the mobility of the core blocks of the micelles could be controlled by the length of the cross-linker in the micellar cores. The core cross-linked micelles were prepared using a poly[(4-pyridinemethoxy-methyl)styrene]-block-polystyrene (PPySt-b-PSt) diblock copolymer and perfluoroalkyl dicarboxylic acid. The PPySt-b-PSt copolymer formed the micelles in THF, a nonselective solvent, in the presence of the perfluoroalkyl dicarboxylic acid. The light scattering studies demonstrated that the micellar size and aggregation number were dependent on the chain length of the perfluoroalkyl dicarboxylic acid. Perfluoroazelaic acid produced micelles with a larger hydrodynamic radius and higher aggregation number than tetrafluorosuccinic acid. The micellization proceeded through the formation of the pyridinium carboxylate and the cross-linkage between the PPySt blocks via the dicarboxylic acid. The core cross-linked micelles were thermally stable and maintained its structure with changes in the temperature. A ¹H NMR analysis revealed that the micelles prepared by perfluoroazelaic acid had more mobility of the core blocks than those by tetrafluorosuccinic acid.     

Micelle formation of a diblock copolymer having pyridine as pendant groups by carboxylic acids in nonselective solvents

The micelle formation of a poly(4-pyridinemethoxymethylstyrene)-block-polystyrene diblock copolymer (PPySt-b-PSt) was investigated in nonselective solvents using bifunctional and trifunctional carboxylic acids. The copolymer showed no self-assembly in 1,4-dioxane and tetrahydrofuran (THF) because the PPySt and PSt blocks were solvophilic to the solvents. Dynamic light scattering studies demonstrated that the copolymer formed micelles in the nonselective solvents in the presence of bifunctional carboxylic acids. Oxalic acid, maleic acid, citric acid, and phospholic acid promoted the micellization, while malonic acid, succinic acid, and glutalic acid had no effect on the micellization. The micellar size, aggregation number, and critical micelle concentration were dependent not only on the acid strength but also on the type of acid and the functionality. The micellization was also affected by the solvent quality. The micellization proceeded more effectively in 1,4-dioxane than in THF. It was found that the micellization occurred by hydrogen bonding between the pyridine moiety and the carboxylic acid and by the interaction among the carboxylic acids. This is because the copolymer needed over an equivalent of the acid to the PySt unit to complete the micellization. Furthermore, monofunctional carboxylic acid such as trichloroacetic acid and trifluoroacetic acid promoted the micellization, although dichloroacetic acid had no effect on the micellization.     

Thermodynamics and kinetics on micelle formation of a nonamphiphilic poly(vinylphenol)-<Emphasis Type="Italic">block</Emphasis>-polystyrene by α,ω-diamine

A poly(vinylphenol)-block-polystyrene diblock copolymer (PVPh-b-PSt) forms micelles in the presence of 1,4-butanediamine (BDA) in 1,4-dioxane, a nonselective solvent. The micellization proceeds through the formation of hydrogen bond cross-linking between the PVPh blocks via BDA, and the dissociation and reconstruction of the micelles is reversibly controlled by temperature. We explored the thermodynamics and kinetics on the micellization of the nonamphiphilic PVPh-b-PSt copolymer by BDA. Light scattering studies demonstrated that an equilibrium existed between the micelles and the unimers. The equilibrium constants were determined for the dissociation and the reconstruction of the micelles on the basis of variation in the aggregation number of the micelles. The equilibrium constant of the dissociation showed a good agreement with the reciprocal of the equilibrium constant of the reconstruction. Based on the equilibrium constants, the standard Gibbs energy, enthalpy, and entropy of the dissociation and reconstruction were estimated. The standard enthalpy was Δ H° = 30–40 kJ mol⁻¹ for the dissociation. The enthalpy of the reconstruction was obtained as a negative value, however, there was a negligible difference in the absolute values of Δ H° between the dissociation and the reconstruction. The rate constant of the micellization was ca. 10² times larger than the back reaction, and increased with a decrease in the temperature.     

Oxidation-induced micellization of a diblock copolymer containing stable nitroxyl radicals

Oxidation-induced micellization was attained for a diblock copolymer containing 2,2,6,6-tetramethylpyperidine-1-oxyl (TEMPO). Poly(4-vinylbenzyloxy-TEMPO)-block-polystyrene (PVTEMPO-b-PSt) showed no self-assembly in carbon tetrachloride, a nonselective solvent. Dynamic light scattering demonstrated that the copolymer self-assembled into micelles of 49.5-nm hydrodynamic diameter when chlorine gas was added to the copolymer solution. The UV and electron spin resonance (ESR) analyses verified that as TEMPO was oxidized into the one-electron oxidant, that is, oxoaminium chloride (OAC) by the chlorine, the nonamphiphilic block copolymer became amphiphilic in nature, and thus, the polymers underwent micellization. An investigation of the relation between the micellization and the oxidation degree of the TEMPO into the OAC revealed that the micellization was induced by only 16% of the OAC. It was confirmed that the POAC-b-PSt micelles were spherical in shape by transmission electron microscopy observation. The micelles served as a two-electron oxidizing agent for benzyl alcohol to quantitatively give benzaldehyde. The micellar structure was maintained after the oxidation of benzyl alcohol without any dissociation into unimers because the OAC was converted into an insoluble hydroxylamine–hydrochloride salt. On the other hand, the micelles reacted with N,N,N′,N′-tetramethyl-1,4-phenylenediamine (TMPD) to produce Wurster’s blue chloride by a one-electron transfer from TMPD to the OAC, converting themselves into PVTEMPO-b-PSt unimers.     

Micelle formation induced by photo-Claisen rearrangement of poly(4-allyloxystyrene)-block-polystyrene

A novel micelle formation induced by the photo-Claisen rearrangement was attained using a poly(4-allyloxystyrene)-block-polystyrene (PASt-b-PSt) diblock copolymer. The photoreaction was performed in cyclohexane at room temperature without a catalyst. The conversion of the 4-allyloxystyrene units reached 90% by irradiation for 24 h. The photo-Claisen rearrangement of PASt-b-PSt into poly(3-allyl-4-hydroxystyrene)-block-PSt quantitatively proceeded up to a 20% conversion; however, the elimination of the allyl groups competitively occurred over the 20% conversion. The degrees of the photorearrangement and elimination showed good agreement in their material balance throughout the course of the reaction. Both of the photorearrangement and elimination finally reached ca. 50% degrees over 60% conversion. The light-scattering studies demonstrated that the PASt-b-PSt copolymer with a 36-nm hydrodynamic diameter as unimers formed micelles with a 98-nm diameter by irradiation.     

Preparation of micelles with azo dye and UV absorbent at their cores or coronas using non-amphiphilic block copolymers

Micelles with azo dye and UV absorbent at their cores or coronas were prepared from non-amphiphilic random diblock copolymers by α,ω-diamine. Poly[4-(phenylazophenoxymethyl)styrene-ran-4-(2-hydroxybenzophenoxymethyl)styrene-ran-vinylphenol]-block-polystyrene (P(AS-r-HBS-r-VPh)-b-PSt) and poly(vinylphenol)-block-poly[4-(phenylazophenoxymethyl)styrene-ran-4-(2-hydroxybenzophenoxymethyl)styrene-ran-styrene] (PVPh-b-P(AS-r-HBS-r-St)) diblock copolymers were prepared by living radical polymerization mediated by 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl. The former copolymer had a molecular weight of Mn[P(AS-r-HBS-r-VPh)-b-PSt] = 10,000-b-250,000 by ¹H NMR and a molar ratio of AS:HBS:VPh = 0.01:0.01:0.98, while the latter had a molecular weight of Mn[PVPh-b-P(AS-r-HBS-r-St)] = 10,000-b-111,000 and a molar ratio of AS:HBS:St = 0.02:0.03:0.95. The copolymers showed no self-assembly in 1,4-dioxane because this solvent was non-selective to the copolymers. Dynamic light scattering demonstrated that the copolymers formed micelles in the solvent in the presence of α,ω-diamine. The hydrodynamic radii of the micelles slightly increased with the copolymer concentration decrease, while the aggregation numbers were almost independent of the copolymer concentration. It was found that P(AS-r-HBS-r-VPh)-b-PSt formed smaller micelles with a lower aggregation number than PVPh-b-P(AS-r-HBS-r-St) because of the steric hindrance of the AS and HBS units present at the micellar coronas.     

Reversible control of self-assembly of a diblock copolymer supporting Wittig reagent

The reversible control of self-assembly of a diblock copolymer supporting Wittig reagent was attained by changing the volume ratio of the mixed solvent. Poly(4-vinylbenzyltriphenylphosphonium chloride)-block-polystyrene (PPCl-b-PSt) self-assembled into micelles with the PPCl block cores in C₆H₆. The micelles were completely dissociated into unimers by the addition of CH₃CN at a 5/5 volume ratio of C₆H₆/CH₃CN. As a result of further increasing the CH₃CN, the reversed micelles with the PPCl block shells were produced. The copolymer served as the Wittig reagent for 9-anthracenecarboxaldehyde to produce a block copolymer with the pendent anthracene. The resulting copolymer also provided micelles in C₆H₆.     

Micelle formation of a nonamphiphilic poly(vinylphenol)-<Emphasis Type="Italic">block</Emphasis>-polystyrene diblock copolymer in ethyl acetate

The micelle formation of a poly(vinylphenol)-block-polystyrene diblock copolymer was studied in ethyl acetate, a nonselective solvent using α,ω-diamine. The copolymer formed micelles in ethyl acetate in the presence of a small amount of the α,ω-diamine. Light scattering studies demonstrated that the micellization was dependent on the grade, the bulkiness, and the conformation of the diamines. The copolymer needed more diamine with the increasing grade of the diamine, due to a decrease in the basicity of the diamine. The bulkiness of the diamines also reduced the efficiency of the micellization by hindering the formation of the hydrogen bond cross-linking. Similarly, the conformation of the diamine affected the micellization, since the conformation determined the intramolecular spatial distance between the animo groups. Trans-1,2-cyclohexanediamine was more effective than the cis-isomer to produce the micelles. Furthermore, (1S,2S)-(+)-1,2-cyclohexanediamine, one of the mirror image isomers composing the trans-isomer, was more effective in producing the micelles than the trans-isomer. The interaction between the mirror image isomers also obstructed the micellization. The micellization, coupled with the thermoresponsivity of the micelles, were influenced by the solvent quality. The dissociation of the micelles into unimers was suppressed in ethyl acetate, while the reconstruction was promoted, in comparison with those in 1,4-dioxane and THF.     

Reduction-induced micellization of a diblock copolymer containing stable nitroxyl radicals

A novel micelle formation induced by reduction was attained using a diblock copolymer supporting 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO). Poly(4-vinylbenzyloxy-TEMPO)-block-polystyrene (PVTEMPO-b-PSt) showed ultraviolet (UV) absorption at 467 nm as λ _max based on the TEMPO radicals. As the phenylhydrazine was added to the copolymer solution in benzene, the UV absorbance decreased. The decrease in the absorbance suggested that the TEMPO radicals were reduced to the colorless hydroxylamine by phenylhydrazine. The PVTEMPO-b-PSt copolymer showed no self-assembly in benzene due to the nonselective solvent. A light scattering study demonstrated that the scattering intensity of the copolymer increased with a decrease in the UV absorbance. The hydrodynamic diameter of the copolymer rapidly increased with the addition of phenylhydrazine and became almost steady over the molar ratio of phenylhydrazine to the VTEMPO unit of 0.2. It was found that the hydroxylamine in the micelles reverted to the TEMPO radicals by oxidation with oxygen.     

Preparation of micelles with azobenzene at their coronas or cores from ‘nonamphiphilic’ diblock copolymers

Micelles with azobenzene at the coronas or the cores were prepared by the micellization of nonamphiphilic diblock copolymers through hydrogen bond cross-linking. We used 4-(phenylazophenoxymethyl)styrene (AS) as the azobenzene. A poly(vinylphenol)-block-poly(AS-co-styrene) diblock copolymer (PVPh-b-P(AS-co-St)) was prepared by combination of the nitroxide-mediated living radical polymerization and the hydrolysis. The copolymer contained ca. 1 mol% of the azobenzene units in the P(AS-co-St) blocks on the basis of ¹H NMR analysis. The PVPh-b-P(AS-co-St) copolymer showed no micellization in 1,4-dioxane, the nonselective solvent. Dynamic light scattering demonstrated that the copolymer formed micelles in the presence of 1,4-butanediamine (BDA) in this solvent. ¹H NMR analysis revealed that the azobenzene moieties were located at the coronas of the micelles, because the signals of the aromatic protons originating from the azobenzene had no changes in the shape and the intensity by the micellization. UV analysis supported the presence of the azobenzene at the micellar coronas. The size of the PVPh-b-P(AS-co-St) micelles was independent of the copolymer concentration. On the other hand, the aggregation number of the micelles was dependent not only on the copolymer concentration but also on the kind of the diamine. A poly(AS-co-vinylphenol)-block-polystyrene diblock copolymer (P(AS-co-VPh)-b-PSt) formed the micelles with the azobenzene at the cores of the micelles by BDA. UV analysis demonstrated that the azobenzene at the micellar cores still had the potential to function as photorefractive switching.     

Micelle formation induced by photolysis of a poly(<Emphasis Type="Italic">tert</Emphasis>-butoxystyrene)-<Emphasis Type="Italic">block</Emphasis>-polystyrene diblock copolymer

We found the novel photolysis-induced micellization of the poly(tert-butoxystyrene)-block-polystyrene diblock copolymer (PBSt-b-PSt). PBSt-b-PSt with a molecular weight of Mn(PBSt-b-PSt) = 15,000-b-97,000 showed no self-assembly in dichloromethane and existed as isolated copolymers with a hydrodynamic diameter of 16.6 nm. Dynamic light scattering demonstrated that the copolymer produced micelles with a 63.0 nm hydrodynamic diameter when the copolymer solution was irradiated with a high-pressure mercury lamp at room temperature in the presence of bis(alkylphenyl) iodonium hexafluorophosphate, a photoacid generator. The ¹H NMR analysis revealed that the micellization resulted from the photolysis of the PBSt blocks into insoluble poly(vinyl phenol) blocks based on the fact that the signal intensity of the tert-butyl protons decreased over time during the irradiation. It was found that the micellization rapidly proceeded as the degree of the photolysis reached over 50% and was completed at 90%.     

TEM observation of nonamphiphilic copolymer micelles

Light scattering and transmission electron microscope (TEM) measurements were preformed for micelles of a nonamphiphilic poly(vinylphenol)-block-polystyrene diblock copolymer (PVPh-b-PSt) to determine the shape of the micelles. The micelles were prepared by the self-assembly of the copolymer in 1,4-dioxane, a nonselective solvent, in the presence of 1,4-butanediamine. The logarithm of the normalized time correlation function of the scattered field, lnG₁(τ), linearly decayed versus the delay time, τ. The diffusion coefficient measured in the range of the scattering angles from 30° to 150° was almost independent of the square of the magnitude of the scattering vector. The linear decay of lnG₁(τ) vs τ and the angular-independence of the diffusion coefficient suggested that the monodisperse spherical micelles were formed by the micellization. The TEM observations confirmed the formation of uniform spheres.     

Preparation of light-stable micelles with azo dyes from a nonamphiphilic random block copolymer

Light-stable micelles with azo dyes were prepared by micelle formation of a nonamphiphilic diblock copolymer containing azobenzene and UV absorbent at ca. 1 mol% as the unit ratios. The nonamphiphilic block copolymer consists of two different kinds of random copolymer blocks: poly[4-(phenylazophenoxymethyl)styrene-co-vinylphenol] (P(AS-co-VPh)) and poly[4-(2-hydroxybenzophenoxymethyl)styrene-co-styrene] (P(HBS-co-St)). This random block copolymer, P(AS-co-VPh)-b-P(HBS-co-St) formed the micelles in the presence of 1,4-butanediamine (BDA) through hydrogen bond cross-linking between the VPh units via BDA. The micelles had the azobenzene moieties at the cores and the UV absorbents at the coronas. The micelles showed a small color difference in color fading experiments, in comparison with the unimers and with micelles having no UV absorbent at the coronas. It is significant that the diblock copolymer forms the micelles and has the UV absorbents at the coronas to suppress the color fading. Furthermore, the chain length of ,-diamines had no effect on the hydrodynamic radius of the micelles, but affected the aggregation number and the cmc.     

Microstructure transition of hydrophilic modified ibuprofen and Pluronic copolymer F127 complexes

The effect of the aggregation state of Pluronic copolymer (PEO₁₀₀–PPO₆₅–PEO₁₀₀, F127) and the concentration of hydrophilic modified ibuprofen (Ibuprofen–PEG800, IP800) on the interaction between F127 and IP800 was systematically investigated by nuclear magnetic resonance, dynamic light scatter (DLS), surface tension, and freeze-fractured transmission electron microscopy. In the solution of F127 unimers (5 °C), F127 unimers tended to wrap around IP800 micelles, and the binding model of F127 unimers to IP800 micelles transferred from wrapping around to partly threading through with increasing IP800 concentration. The latter binding model was straightly confirmed by nuclear Overhauser enhancement spectroscopy. As the aggregation state of F127 is in the beginning of the micellization (20 °C), the addition of IP800 significantly promoted the micellization of F127 to form the F127/IP800 complex with F127 micelles as the skeleton called the F127–micelle complex. The sudden decrease of the size obtained from DLS stemmed from the disruption of the F127–micelle complex and accompanying rehydration of PPO which is weaker compared with refs. The amount of IP800 to disintegrate the F127–micelle complex increased in the F127–micelle-dominated solution (40 °C) compared to that at 20 °C.     

聚甲基丙烯酸甲酯接枝聚氧乙烯在甲苯中的聚集态结构

两亲接枝共聚物;胶束;聚甲基丙烯酸甲酯接枝聚氧乙烯在甲苯中的聚集态结构     

Photolysis-induced micellization of a poly(4-tert-butoxystyrene)-block-polystyrene diblock copolymer

A novel micellization induced by photolysis was attained using a poly(4-tert-butoxystyrene)-block-polystyrene diblock copolymer (PBSt-b-PSt). BSt-b-PSt showed no self-assembly in dichloromethane and existed as isolated copolymers. Dynamic light scattering demonstrated that the copolymer produced spherical micelles in dichloromethane by the irradiation with a high-pressure mercury lamp in the presence of photoacid generators, such as bis(alkylphenyl)iodonium hexafluorophosphate (BAI), diphenyliodonium hexafluorophosphate (DPI), and triphenylsulfonium triflate (TPS). The irradiation time to promote the micellization increased in the order of BAI < DPI < TPS, depending on the UV absorption intensity of the photoacid generators. The efficiency to promote the micellization was also dependent on the block length of the copolymer. Under an identical PBSt block length, the copolymer with the shorter PSt block length more easily formed micelles. The ¹H NMR analysis confirmed that the PBSt-b-PSt copolymer was converted into poly(4-vinyl phenol)-block-PSt, resulting in micelles by self-assembly.     

Synthesis and aggregation behaviour of amphiphilic block copolymers with random middle block

The effect of microstructure on the aggregation behaviour of symmetrical di- and triblock copolymers P(BMA)-b-P(MAA) and P(BMA)-b-P(BMA-co-MAA)-b-P(MAA) with a molecular weight of 40,000 g/mol was studied. The critical micelle concentration, hydrodynamic radius and morphology of the micelles were determined by fluorescence spectroscopy, dynamic light scattering and scanning force microscopy (SFM). Whereas no effect of the microstructure on the critical micelle concentration could be detected, the hydrodynamic radius decreased from di- to triblock copolymer from 53 to 36 nm. The decrease of about 32% corresponds to the length of the random middle block within the triblock copolymer so that the reduction in hydrodynamic radius was caused by a complete orientation of the random middle block at the core corona interface. Finally, the SFM investigation showed that dehydration of micelles on a substrate is accompanied by formation of a physisorbed monolayer with a thickness of 2 nm on which the micelles are deposited.     

Formation and microstructure transition of F127/TX-100 complex

Formation and structure transition of the complex composed of triblock copolymer F127 and nonionic surfactant TX-100 have been investigated by 1H NMR spectroscopy, dynamic light scattering (DLS), and isothermal titration calorimetry (ITC). Three TX-100 concentration regions are identified, within which TX-100/20 mg/mL F127 complex undergoes different temperature-induced structure transitions. In low concentration region (< 9.42 mM), F127 single molecular species (unimers) wrap around TX-100 micelles forming F127/TX-100 complex with TX-100 micelle as the skeleton at a lower temperature (5 degrees C), and the skeleton transfers to F127 micelle at higher temperature (40 degrees C); in intermediate TX-100 concentration region (9.42-94.85 mM), the skeleton of F127/TX-100 complex transfers from TX-100 micelle successively into F127 micelle and TX-100 micelle again upon heating. The interaction of F127 with TX-100 is saturated in high TX-100 concentration region (> 157.57 mM), and free TX-100 micelles coexist with larger clusters of F127/TX-100 complexes. In addition, TX-100-induced F127/TX-100 complex formation and structure transition are also investigated at constant temperatures. The results show that within 5-10 degrees C, F127 unimers mainly adsorb on the surface of TX-100 micelles just like normal water soluble polymers; in the temperature region of 15-25 degrees C, TX-100 micelles prompts F127 micelle formation. Within 30-40 degrees C, TX-100 inserts into F127 micelles leading to the breakdown of F127 aggregates at higher TX-100 concentrations, and the obtained unimers thread through TX-100 micelles forming complex with TX-100 micelle as skeleton.     

Synthesis of stable “gold nanoparticle–polymeric micelle” conjugates: A new class of star “molecular chimera” that self‐assemble into linear arrays of spherical micelles

Stable and aggregation‐free “gold nanoparticle–polymeric micelle” conjugates were prepared using a new and simple protocol enabled by the hydrogen bonding between surface‐capping ligands and polymeric micelles. Individual gold nanoparticles were initially capped using a phosphatidylthio–ethanol lipid and further conjugated with a star poly(styrene‐block‐glutamic acid) copolymer micelle using a one‐pot preparation method. The morphology and stability of these gold–polymer conjugates were characterized using transmission electron microscopy (TEM) and UV–vis spectroscopy. The self‐assembly of this class of polymer‐b‐polypeptide in aqueous an medium to form spherical micelles and further their intermicelle reorganization to form necklace‐like chains was also investigated. TEM and laser light scattering techniques were employed to study the morphology and size of these micelles. Polymeric micelles were formed with diameters in the range of 65–75 nm, and supermicellular patterns were observed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3570–3579, 2007     

Micellization-induced deprotonation of thermoresponsive surfactant CAE-85 — the telechelic carboxylic group derivative of Pluronic P85

Temperature-induced micellization of CAE-85, a carboxylic acid end-standing derivative of a triblock copolymer Pluronic P85, was studied by attenuated total reflectance Fourier-transform infrared spectroscopy (ATR FTIR) and density functional theory (DFT) model calculations. It was found that in polymer micelles carboxyl end groups dissociated and it was a two stage process. The first stage of deprotonation appeared with the onset of micellization and it was in agreement with predictions of existing models and theories for ionization processes in micellar corona. In micelles well above the critical micellization temperature, the degree of CAE-85 deprotonation increased further to values significantly higher compared to unimer solution. It is proposed that such deprotonation correlates with the formation of hydrogen bonded carboxyl end groups enabled by sufficient density of chains in the corona of developing micelles. It was demonstrated that the proton dissociation constant, pK_m, specific to the micellar form existed and was different from the proton dissociation constant of solution of unimers, pK_a.