Block Copolymer Micelles in Aqueous Media (original) (raw)
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Polystyrene-poly(methacrylic acid) block copolymer micelles
Macromolecules, 1994
Block copolymer micelles formed by diblock and triblock copolymers of styrene and methacrylic acid were characterized in solution in a mixed solvent with 80 vol % of dioxane and 20 vol % of water. Methods of static light scattering, quasielastic light scattering, differential refractometry, viscometry, sedimentation velocity, and densitometry were used. No unattached unimer molecules were observed. Three independent methods were employed for obtaining micellar weights. They agreed well with each other. No anomalous behavior was observed by any method. The micellar solutions were shown to contain almost exclusively single micelles; only a few samples (those producing the largest micelles) contained micellar clusters. The micelles behaved hydrodynamically and thermodynamically as impermeable spheres. The structure of the micellar shell was characterized in some detail. Relations between the aggregation number and the hydrodynamic radius of the micelles on the one hand and the sizes of the styrene and methacrylic acid blocks on the other were presented as scaling type phenomenological equations.
Collection of Czechoslovak Chemical Communications, 2002
The micellization behavior of a hydrophobically modified polystyrene-block-poly(methacrylic acid) diblock copolymer, PS-N-PMA-A, tagged with naphthalene between blocks and with anthracene at the end of the PMA block, was studied in 1,4-dioxane-methanol mixtures by light scattering and fluorescence techniques. The behavior of a single-tagged sample, PS-N-PMA, and low-molar-mass analogues was studied for comparison. Methanol-rich mixtures with 1,4-dioxane are strong selective precipitants for PS. Multimolecular micelles with compact PS cores and PMA shells may be prepared indirectly by dialysis from 1,4-dioxane-rich mixtures, or by a slow titration of copolymer solutions in 1,4-dioxane-rich solvents with methanol under vigorous stirring. In tagged micelles, the naphthalene tag is trapped in nonpolar and fairly viscous core/shell interfacial region. In hydrophobically modified PS-N-PMA-A micelles, the hydrophobic anthracene at the ends of PMA blocks tends to avoid the bulk polar solvent and buries in the shell. The distribution of anthracene tags in the shell is a result of the enthalpy-to-entropy interplay. The measurements of direct nonradiative excitation energy transfer were performed to estimate the distribution of anthracene-tagged PMA ends in the shell. The experimental fluorometric data show that anthracene tags penetrate into the inner shell in methanol-rich solvents. Monte Carlo simulations were performed on model systems to get reference data for analysis of time-resolved fluorescence decay curves. A comparison of experimental and simulated decays indicates that hydrophobic traps return significantly deep into the shell (although not as deep as in + The study is a part of the long-term Research Plan of the School of Science No. MSM 113100001. aqueous media). The combined light scattering, fluorometric and computer simulation study shows that the conformational behavior of shell-forming PMA blocks in non-aqueous media is less affected by the presence of nonpolar traps than that in aqueous media.
Polymer, 1996
Solutions of polystyrene-b-poly(ethylene/propylene) block copolymer and homopolystyrene in toluene (a good solvent of both copolymer blocks) were studied by dynamic light scattering and viscometry. The presence of homopolystyrene in the solution induces the formation of micelles with a core which consists of poly(ethylene/propylene) blocks. The dynamic light scattering measurements showed that these micelles have a narrow size distribution. The influence of the polystyrene concentration on the structural parameters of the micelles was analysed. The hydrodynamic radius and molar mass remained constant in the polystyrene concentration range studied. However, the diffusion of the micelles was very influenced by the increment of the solution viscosity caused by the polystyrene chains.
Macromolecules, 2007
We report the structure and dynamics of micelles of the amphiphilic diblock copolymers poly(nbutyl acrylate)-block-poly(acrylic acid) (PnBA-PAA). These self-assembled nanostructures consist of a liquid hydrophobic core and a pH-and ionic strength-sensitive hydrophilic corona. In the first part of this series, 1 we reported the synthesis and micellization of these block copolymers in aqueous media without the need of any cosolvent. Here we present a detailed study on the structural and dynamic properties of these micelles in aqueous solutions under various conditions using static and dynamic light scattering (SLS, DLS), small-angle neutron scattering (SANS), and cryogenic transmission electron microscopy (cryo-TEM). The block copolymers spontaneously dissolve in water, forming rather monodisperse micelles. Although the corona thickness depends on external stimuli, such as pH and salinity, the micelles do not significantly change their shape or aggregation number upon modifications of these parameters, in spite of the liquidlike nature of the hydrophobic block at room temperature. Moreover, the structure of the formed micelles depends on the preparation conditions: aggregates of micelles are initially formed when the polymers are dissolved in saline aqueous solutions even at pH 6.5, which disintegrate within weeks, resulting in isolated micelles with significantly larger size compared to micelles at the same ionic strength but initially prepared in the absence of added salt. The results are explained in terms of a kinetic control of the micellization process, which is dynamic in terms of unimer exchange but slow on the experimental time scale in adapting to external stimuli.
Diblock Copolymer Micelles in Solvent Binary Mixtures. 1. Selective Solvent/Precipitant
Macromolecules, 1995
The association of a block copolymer polystyrene-b-poly(ethylene/propylene) in mixtures of 5-methyl-2-hexanone (a selective solvent of polystyrene block) and 2-pentanol (a precipitant of both copolymer blocks) has been studied by static and dynamic light scattering and viscometry. Micelle structural parameters have been analyzed as a function of the solvent composition. As the precipitant percentage increases in the solvent mixture, the molar mass increases, the apparent radius of gyration and the second virial coefficient decrease, and the hydrodynamic radius remains constant. Static light scattering was used to establish the dependence of the critical micelle temperature on concentration. Dynamic light scattering measurements have shown that the micelles have narrow size distributions and micellization can be thermodynamically treated as a closed association. The light scattering results were used to calculate the standard Gibbs energies, enthalpies, and entropies of micellization. As the 2-pentanol percentage increases, the standard Gibbs energy becomes more negative, whereas the standard enthalpy and entropy remain almost constant.
Diblock Copolymer Micelles in a Dilute Solution
Macromolecules, 2005
We performed theoretical and experimental investigations of dilute solutions of micelles of neutral amorphous diblock copolymers in selective solvents. The ranges of thermodynamic stability of spherical, cylindrical, and lamellar morphologies along with the equilibrium sizes and aggregation numbers of micelles are calculated and measured. For high molecular weight copolymers it is shown that the sphere-to-cylinder transition as well as precipitation of the micelles associated with cylinder-to-lamella transition occurs when aggregates have a crew-cut structure with the thickness of the corona smaller than the radius of the core. Similar to starlike micelles with corona larger than the core, the equilibrium parameters of crew-cut micelles are determined by the balance between the free energy of the corona and the surface energy of the core. The elastic free energy of the core remains small compared to the corona and surface free energies; however, it determines the transitions between different morphologies. The theoretical predictions including the existence of crew-cut spherical micelles, the range of stability of cylindrical micelles, and the significance of the contributions of the logarithmic corrections to scaling are in good agreement with experiments on polystyrene-polyisoprene block copolymer micelles in heptane, a selective solvent for the polyisoprene block.
Formation of block copolymer micelles in solutions of a linear homopolymer in a good solvent
Macromolecular Rapid Communications, 1995
The existence of micelles of polystyrene-block-poly(ethylene/propene) in solutions of polystyrene in toluene was investigated. Toluene is a good solvent of both copolymer blocks whereas polystyrene and poly(ethylene/propene) are immiscible polymers. The presence of homopolystyrene at high enough concentration can induce the micellization of polystyreneblock-poly(ethylene/propene) in solution of a good solvent such as toluene. The thermodynamics of this new micelle system at a given polystyrene concentration was studied. Light scattering measurements were carried out in order to determine the critical micelle temperature (CMT) of different micellar solutions. Standard Gibbs energy, enthalpy and entropy of micellization were estimated from CMT and concentration data. The numerical values found were less negative than those found for micelle systems consisting in a block copolymer dissolved in a single selective solvent.
Polymer, 1998
The micellization thermodynamics and micelle structural parameters for polystyrene-b-poly(ethylene/butylene)b-polystyrene copolymers (SEBS) in n-octane were studied. This solvent is selective for the middle poly(ethylene/butylene) block of the copolymer. The copolymer samples have similar chemical composition and different molar mass. Standard thermodynamic functions of micellization (ΔG°, ΔH° and ΔS°) were determined by light scattering. All the functions showed more negative values for the SEBS copolymer with a larger molar mass. The structural parameters of micelles formed by different triblock copolymers were determined from static light scattering and viscosity measurements. Micelle molar mass, association number and viscometric hydrodynamic radius increase with the length of the copolymer chain for a constant copolymer composition; however, the radius of gyration decreases. Finally, the results found for SEBS micelles in n-octane are compared to those found for SEBS micelles in 4-methyl-2-pentanone. The different behaviour found supports the idea that the micelles of a triblock copolymer in a selective so vent of the middle block will show a shell formed by loops of the middle block in order that the two outer blocks go into the micelle core and even some poorly solvated outer blocks could be extended out of the micelle.
Diblock Copolymer Micelles in Solvent Binary Mixtures. 2. Selective Solvent/Good Solvent
Macromolecules
Solutions of polystyrene-b-poly(ethylene/propylene) block copolymer in mixtures of 4-methyl-2-pentanone (a selective solvent of the polystyrene block) and 2-chlorobutane (a good solvent of both copolymer blocks) were studied by static and dynamic light scattering and viscometry, Micelle structural parameters and micellization thermodynamics have been analyzed as a function of the solvent composition. The micelle molar mass decreased as the 2-chlorobutane percentage in the binary mixture increased. However the apparent radius of gyration, the second virial coefficient, and the hydrodynamic radius hardly showed variations. Static light scattering was used to establish the dependencies of the critical micelle temperature on concentration. The standard Gibbs energies, enthalpies, and entropies of micellization have been calculated for the different solvent mixtures from these relationships, The three thermodynamic magnitudes became less negative as the good solvent percentage increased.