Micelle Volume Research Articles

Soybean oil: a potentially new intravascular perfusate

Given that micelles of lipids are colloids, the hypothesis was generated that the rapid administration of large volumes of soybean oil micelles would be an effective perfusion fluid. We also hypothesized that oxygen loading would be enhanced due to the greater solubility of oxygen in lipids compared to water. A 100% lethal mouse model of blood loss was used to compare the ability of soybean oil micelles to that of Ringer's lactate, blood and other fluids, with respect to raising and maintaining the blood pressure for one hour. Oxygen on- and off-loading of various concentrations of soybean oil micelles was determined using mass spectroscopy. Nitric oxide uptake by micelles was also determined in a similar fashion. A 20% soybean oil emulsion was superior to Ringer's lactate in raising and maintaining blood pressure. A 20% soybean oil emulsion with 5% albumin added was superior to shed blood as well as solutions comprised of 5% albumin added to either normal saline or Ringer's lactate. There was a linear relationship between oxygen content and micelle concentration between 10% and 30%. Off-loading of oxygen from the micelles was nearly as fast as off-loading from water. Nitric oxide also loaded preferentially onto soybean oil micelles. (1) Soybean oil emulsions were superior to other fluids in restoring and maintaining the blood pressure; (2) oxygen-carrying ability of soybean oil micelles exceeds that of water and follows Henry's law between 10% and 30% w/v oil content; (3) nitric oxide was carried by the micelles; (4) animals receiving soybean oil micelles did not exhibit fat embolization; (5) colloids comprised of soybean oil-containing micelles may be used to replace blood loss and may be used to deliver oxygen and other potentially therapeutic gases such as nitric oxide to tissues.

Mass transport and electrolyte accessibility through hexagonally ordered channels of self-assembled mesoporous carbons

Ordered mesoporous carbons have attracted great interest for their potential use to electrodes for electric double layer capacitors. However, it is crucial to develop an electrode such that the active sites within the mesoporous carbons are accessible to electrolyte solution species via facile mass transport through well-defined pores. Here, we investigate the electrolyte accessibility and the effective mass transport for the ordered mesoporous carbons possessing p6mm symmetry, whereas having different mesochannel length, formed by soft-templating self-assembly. Mesoporous carbons were prepared from a thermosetting phenolic resin and a thermally decomposable copolymer template in ethanol/water solutions of different molar ratios. The general evolution of the mesophase follows the trends that are expected based on packing parameters due to swelling of the hydrophobic volume of the copolymer micelles. However, changes in mesochannel length and the degree of long-range order are found to depend on ethanol/water ratio. Electrochemical impedance spectroscopy was used to assess the transport properties. The impedance data clearly demonstrated that the degree of long-range order and channel length can be a dominant factor that determines the transport and the degree to which the equilibrium adsorption sites are accessible. The lengthy mesochannels in well-ordered mesoporous carbon serve as ion-highways and allow for very fast mass transport into the micropores of the channel frameworks.

Surface Tension, Density and Viscosity Studies on the Associative Behaviour of Oxyethylene-Oxybutylene Diblock Copolymers in Water at Different Temperatures

The associative properties of methoxy (CH3O-) ended E58B11 and hydroxyl (-OH) ended E56B19 oxyethylene-oxybuty- lene diblock copolymers in aqueous solution at different temperature are reported in this paper. For both copolymers, E represents an oxyethylene (-[CH2CH2O]-) unit and B an oxybutylene (-[CH(C2H5)CH2O]-) unit while the subscripts denote the number average block length. Surface tension measurements were used to find out surface excess concentrations (Γm), area per molecule (αs1 ) at air/water interface and Gibbs free energy for adsorption (△G0ads ) for the pre-micellar region at four temperatures. Likewise thermodynamic parameters of micellization such as, critical micelle concentrations (CMC), enthalpy of micellization (△H0mic ), standard free energy of micellization (△G0mic) and entropy of micellization (△S0mic ), were also obtained using surface tension measurements. Solution densities were used to deter-mine the partial specific volume of micelle ( Vmic) and micellar density (ρmic). Dilute solution viscosities have been used to estimate the intrinsic viscosities [η], solute-solvent interaction parameter (KH) and hydration value of micelle (Wh) at various temperatures. The effect of temperature on the micelle properties is also discussed.

Cy3 in AOT Reverse Micelles II. Probing Intermicellar Interactions Using Fluorescence Correlation Spectroscopy

Cyanine-3 (Cy3) fluorescent dye molecules confined in sodium di-2-ethylhexyl sulfosuccinate (AOT) reverse micelles were examined using dynamic light scattering and fluorescence correlation spectroscopy to probe the kinetics of Cy3 dye and reverse micelle aggregation. This study explored a range of reverse micelle sizes, defined as w(0) = [H(2)O]/[AOT], in which the occupation number ranged from one Cy3 molecule per ∼10(5) to ∼10(6) reverse micelles. These measurements reveal that in the smallest reverse micelle, w(0) = 1, the Cy3 molecules aggregate to form H-aggregate dimers, and the Cy3 dimerization is accompanied by the formation of a transient dimer between reverse micelles. Transient reverse micelle dimer particles are only observed in the small fraction of Cy3-labeled reverse micelles probed by fluorescence correlation spectroscopy and are not observed in the bulk solution probed by dynamic light scattering. Furthermore, fluorescence correlation spectroscopy makes it possible to probe the size and shape of these dimers, revealing prolate ellipsoid-shaped particles with twice the volume and surface area of a single reverse micelle.

Coagulation properties of ultrafiltered milk retentates measured using rheology and diffusing wave spectroscopy

The effect of concentration of milk by ultrafiltration on renneting has been widely studied as it is of great interest in dairy technology. Although a number of reports are available on the texture and microstructure of the milk gels formed at various concentrations, very little is understood on the effect of concentration on the stages preceding aggregation, or how concentration may affect the interactions between micelles. This study aims to investigate the renneting behavior of milk concentrated by ultrafiltration (without diafiltration) to 3× and 5× (v/v) and compare it to that of skim milk. The scattering properties of the casein micelles under quiescent conditions suggest that they deviate from hard-sphere behavior at 5× concentration (micelle volume fraction, ϕ = 0.5). The release of the caseinomacropeptide during renneting was not significantly different amongst the three different casein concentrations tested (1×, 3×, and 5×). No significant differences were also noted in the rennet coagulation time as detected by both diffusing wave spectroscopy and rheology. Concentrated milk samples formed significantly (p-value < 0.05) stiffer gels than regular milk due to an increased number of bonds in the network. The level of milk concentration also accelerated a change in the spatial distribution and rate of change of turbidity of the micelles because of a decrease in the overall inter-particle distance and increased collision frequencies. This in situ investigation of concentrated milk samples suggested that the changes in rennet coagulation with concentration are merely a cause of crowding effects.

A New Family of Nonionic Dendritic Amphiphiles Displaying Unexpected Packing Parameters in Micellar Assemblies

In this paper we report on the synthesis of a new family of nonionic dendritic amphiphiles that self-assemble into defined supramolecular aggregates. Our approach is based on a modular architecture consisting of different generations of hydrophilic polyglycerol dendrons [G1-G3] connected to hydrophobic C(11) or C(16) alkyl chains via mono- or biaromatic spacers, respectively. All amphiphiles complex hydrophobic compounds as demonstrated by solubilization of Nile Red or pyrene. The structure of the supramolecular assemblies as well as the aggregation numbers are strongly influenced by the type of the dendritic headgroup. While the [G1] amphiphiles form different structures such as ringlike and fiberlike micelles, the [G2] and [G3] derivatives aggregate toward spherical micelles of low polydispersity clearly proven by transmission electron microscopy (TEM) measurements. In the case of the biaromatic [G2] derivative, the structural persistence of the micelles allowed a three-dimensional structure determination from the TEM data and confirmed the aggregation number obtained by static light scattering (SLS) measurements. On the basis of these data, molecular packing geometries indicate a drastic mass deficit of alkyl chains in the hydrophobic core volume of spherical micelles. It is noteworthy that these highly defined micelles contain as little as 15 molecules and possess up to 74% empty space. This behavior is unexpected as it is very different from classical detergent micelles such as sodium dodecyl sulfate (SDS), where the hydrophobic core volume is completely filled by alkyl chains.

Cholic acid micelles—controlling the size of the aqueous cavity by PEGylation

Data show that cholic acid (CA) micelles are less densely packed and much smaller than micelles formed by typical surfactants, suggesting that CA derivatives can be used to synthesize drug nanocarriers. Presumably, the formation of internal cavities is favored by the facial characteristics of the CA molecule, i.e. the convex molecular structure that is hydrophobic on one side and hydrophilic on the other. Here, we present a thermodynamical approach to quantify the effect of facial characteristics on forces governing the self-assembling process of CA molecules. We show that facial characteristics favor the entrapment of water molecules at interfaces upon CA aggregation, which weakens the attraction between CA hydrophobic moieties. Our computer simulations suggest that these effects contribute significantly to the tendency of CA molecules to form small "hollow-core" micelles. The attachment of polyethylene glycol (PEG) molecular chains to CA increases the repulsive forces in the system, reducing even further the micelle size. We use the present molecular model and experimental critical micelle concentration (cmc) data for CA-PEG systems to predict the change of the micelle size and cavity volume with the increase of the PEG chain length (x). Our computations indicate that the CA-PEG micelles are good candidates for drug delivery. The structural stability of CA-PEG micelles was further assessed by molecular dynamics simulations. We also tested the drug loading efficiency of this system and found an average of 0.5 mg paclitaxel load per 20 mg of CA-PEG polymer. The present study helps to identify critical parameters that control structural properties of the CA based nanocarriers and suggests practical means to optimize the ratio between micelle size and volume of the internal cavity.

Slow dynamics of wormlike micelles

We present an extensive rheology study of the wormlike micelle system lecithin-water-cyclohexane. In this system the micelles are really wormlike, meaning that there are no signs of micellar branching, as it has previously been demonstrated by NMR self-diffusion experiments (R. Angelico, U. Olsson, G. Palazzo and A. Ceglie, Phys. Rev. Lett., 1998, 81, 2823). Wormlike micelles break and recombine, processes that are important for the stress relaxation. When branching is highly unfavorable, micelle recombination reactions only involve micelle ends, the concentration of which are very low when the micelles are very long. Hence, the break and recombination kinetics is very slow for true wormlike micelles. In the present system, the stress relaxation times are of the order of an hour. This is about three to four orders of magnitude longer than what commonly is observed in systems claimed to contain wormlike micelles. We conclude that systems with true wormlike micelles are very rare. An exponential stress relaxation is observed except at lower concentrations, where the micellar breaking time appears to exceed the reptation time. Because of the slow dynamics, the linear elastic modulus can be obtained from small rapid shear deformations, for which the system obeys Hooke's law. Larger deformations result in a fracture of the micellar network at a critical strain γ* ∼ ϕ−1, where ϕ is the micelle volume fraction. For γ < γ* we may still obtain fracture, although with a lag time, τ*, that decreases with increasing γ, and vanishes when γ = γ*. Extrapolating τ* to zero deformation we obtain the estimate ≈250 s at the highest concentration ϕ = 0.29. We interpret this time as the equilibrium micellar breaking time at rest, a quantity not previously measured.

Micelle Morphology and Mechanical Response of Triblock Gels

The effect of polymer concentration on mechanical response and micelle morphology of ABA and AB copolymers in B-selective solvents has been systematically studied. Micelle morphology was determined using a combination of small-angle X-ray scattering, shear, and birefringence while mechanical response at low and high strains was determined using indentation techniques. Self-consistent field theory calculations were used to determine micelle volume fraction profiles and to construct an equilibrium phase map. The transition from spherical to cylindrical micelles increases the triblock gel modulus and energy dissipation. Combining knowledge of gel relaxation time, which determines the rate at which the gel can equilibrate its micelle structure, with the equilibrium phase map allows estimation of the experimental temperatures and time scales over which kinetic trapping will arrest micelle structure evolution. Kinetic trapping enables cylindrical morphologies to be obtained at significantly lower polymer fractions than is possible in equilibrated systems.

Expanding mesoporosity of triblock-copolymer-templated silica under weak synthesis acidity

With initial aging at low temperature for enough time, silicas with large mesoporosity were synthesized using triblock copolymer as template agent under weak acidities. SBA-15 with periodic mesostructure and short mesochannels could be synthesized at pH 2.5–3.0 within weak acidity range, and the surface areas, pore diameters and pore volumes reached up to ca. 1000 m 2/g, 8.8 nm and 2.0 cm 3/g, respectively, which were significantly higher than those of the conventional SBA-15 synthesized under strong acidities. Mesoporous silica with wormhole structure and abundant textural porosity was formed at pH ∼ 3.5. The increased hydrophobic volume of the copolymer micelles at elevated pH values was responsible for the enlargement of mesoporosity in the products. The materials synthesized under weak acidities showed lower hexagonal ordering in comparison to the general SBA-15 synthesized under strong acidities because the decreased hydronium ion concentration induced relatively weaker assembly forces during the synthesis. Nonetheless, the short mesochannels and large pore diameter in the products might be beneficial to some applications in which fast diffusion of molecules is required.

Conformatinal Dynamics Of Cytochrome c Encapsulated In AOT Reverse Micelles

Scalable interior volume of the reverse micelle provides a convenient way to investigate the impact of aqueous solvation and confinement on the structure, stability and dynamics of proteins and peptides. Here we report transient absorption study of CO rebinding to cytochrome c encapsulated in AOT reverse micelles. Encapsulation of cytochrome c is associated with the destabilization of the protein structure and concomitant dissociation of Met-80 from the heme iron as evident from the disappearance of the absorption band at 695 nm. Reduction of cytochrome c and subsequent exposure to CO result in the formation of CO bound protein with a Soret band located at 415 nm. Upon photodissociation, the ligand rebinding occurs as a two-step process with the first step having the lifetime of 7 μs and slower process with the lifetime of roughly 50 μs. No significant impact of the reverse micelle size (w = 10, 20 and 80) on the kinetics of CO rebinding to cytochrome c was observed. On the other hand, CO rebinding to the model compound, microperoxidase-11, is multi-phasic with 6 μs and 50 μs kinetics on the microsecond timescale and an additional kinetics on the millisecond timescale. These data show that the encapsulation of protein within the negatively charged reverse micelles results in the heterogenous population and/or distribution of protein within the reverse micelle.

On the concentration-induced growth of nonionic wormlike micelles

We present a comprehensive study of the concentration-induced growth of nonionic wormlike micelles in dilute solutions, below the overlap concentration, where we combine static and dynamic light scattering and NMR self-diffusion. The data are analyzed in detail in terms of the number-averaged contour length, assuming an exponential size distribution, as predicted by theory, and that the micellar flexibility can be described by the wormlike chain model with a certain persistence length. A very good agreement between the different experimental methods is obtained. The number-averaged contour length increases with increasing micelle volume fraction. The data are consistent with a power law, where the power law exponent is in the range 0.5-0.8, depending on the system. The result is in reasonable agreement with the theory of living polymers.

Solubilization kinetics for polycyclic aromatic hydrocarbons transferring from a non-aqueous phase liquid to non-ionic surfactant solutions

A modified rotating disk apparatus was used to investigate the mass transfer of two polycyclic aromatic hydrocarbon (PAH) compounds, naphthalene and phenanthrene from a synthesized non-aqueous phase liquid (NAPL) comprised of hexadecane and the 2 PAHs into different non-ionic surfactant solutions. Major factors influencing the rate of solubilization of PAHs from a NAPL in micelles of different non-ionic surfactants were determined. As the surfactant concentration increased, the mass transfer coefficients for both PAHs from the NAPL decreased. The maximum rates of solubilization of the PAHs however increase with surfactant dose. The rate of solubilization was found to be limited by rates of desorption of mixed micelles from the NAPL and their rate of diffusion into the bulk solution phase. The influence of the surfactant molecular structure on the kinetics of the solubilization process was investigated. The results suggested that the length of the alkyl portion of the non-ionic surfactant and the micelle volume influenced the solubilization kinetics. The results of the investigation improve our ability to provide a rational basis for selecting the optimum surfactant and dose to enhance the solubilization of PAHs from NAPLs.

Physicochemical studies on hydrophobic PEO‐PPO‐PEO triblock copolymer micelles in SDS micellar environment

AbstractThe shape, size, aggregation, hydration, and correlation times of water insoluble PEO‐PPO‐PEO triblock copolymer micelles with sodium dodecylsulfate (SDS) micelles were investigated using transport studies and dynamic light scattering technique. From the conductance of micellar solutions of the polymer in 25 mM SDS and 5 mM NaCl, the hydration of polymer micelles were determined using the principle of obstruction of electrolyte migration by the polymer. The asymmetry of the micellar particles of polymer and polymer‐SDS mixed micellar systems in 5 mM NaCl and their average axial ratios were calculated using intrinsic viscosity and hydration data obeying Simha–Einstein equation. Hydration number and micellar sizes were variable with temperature. The shape of the polymer micelles has been ellipsoidal rather than spherical. The micellar volume, hydrodynamic radius, radius of gyration, diffusional coefficients as well as translational, rotational and effective correlation times have been calculated from the absolute values of the axes. The partial molal volume of polymer micelles has also been determined and its comparison with the molar volume of pure polymer suggested a volume contraction due to immobilization of the water phase by the hydrophilic head groups of the polymer. The thermodynamic activation parameters for viscous flow favor a more ordered water structure around polymer micelles at higher temperatures. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2410–2420, 2007

The colloid structural forces as a tool for particle characterization and control of dispersion stability

Knowing the size and interactions of colloid particles, one can predict the stepwise thickness transitions and the contact angles of particle-containing liquid films. Here, we consider the inverse problem, viz. how to determine the particle properties by measurements with liquid films. We carried out experiments with films formed from aqueous solutions of two nonionic surfactants, Brij 35 and Tween 20, which contain spherical micelles of diameters in the range 7-9 nm. From the measured contact angles, we determined the micelle aggregation number and volume fraction. In addition, from the measured disjoining-pressure isotherms we determined the micelle diameter. In other words, the liquid-film measurements give information about the micelles, which is analogous to that obtainable by dynamic and static light scattering. Furthermore, we investigate the predictions of different quantitative criteria for stability-instability transitions, having in mind that the oscillatory forces exhibit both maxima, which play the role of barriers to coagulation, and minima that could produce flocculation or coalescence in colloidal dispersions (emulsions, foams, suspensions). The interplay of the oscillatory force with the van der Waals surface force is taken into account. Two different kinetic criteria are considered, which give similar and physically reasonable results about the stability-instability transitions. Diagrams are constructed, which show the values of the micelle volume fraction, for which the oscillatory barriers can prevent the particles from coming into close contact, or for which a strong flocculation in the depletion minimum or a weak flocculation in the first oscillatory minimum could be observed.

Micelles for the Self‐Assembly of “Off–On–Off” Fluorescent Sensors for pH Windows

A micellar approach is proposed to build a series of systems featuring an "off-on-off" fluorescent window response with changes in pH. The solubilizing properties of micelles are used to self-assemble, in water, plain pyrene with lipophilized pyridine and tertiary amine moieties. Since these components are contained in the small volume of the same micelle, pyrene fluorescence is influenced by the basic moieties: protonated pyridines and free tertiary amines behave as quenchers. Accordingly, fluorescence transitions from the "off" to the "on" state, and viceversa, take place when the pH crosses the pK(a) values of the amine and pyridine fragments. To obtain an "off-on-off" fluorescent response in this investigation we use either a set of dibasic lipophilic molecules (containing covalently linked pyridine and tertiary amine groups) or combinations of separate, lipophilic pyridines and tertiary amines. The use of combinations of dibasic and monobasic lipophilic molecules also gives a window-shaped fluorescence response with changes in pH: it is the highest pyridine pK(a) and the lowest tertiary amine pK(a) that determine the window limits. The pK(a) values of all the examined lipophilic molecules were determined in micelles, and compared with the values found for the same molecules in solvent mixtures in which they are molecularly dispersed. The effect of micellization is to significantly lower the observed protonation constants of the lipophilized species. Moreover, the more lipophilic a molecule is, the lower the observed logK value is. Accordingly, changing the substituents on the basic moieties or modifying their structure, tuning the lipophilicity of the mono- or dibases, and choosing among a large set of possible combination of lipophilized mono- and dibases have allowed us to tune, almost at will, both the width and the position along the pH axis of the obtained fluorescent window.

Zwiazki powierzchniowo czynne z grupy polioksyetylenowanych estrow kwasow tluszczowych. Cz. II. Podstawowe wielkosci lepkosciowe a rownowaga hydrofilowo-lipofilowa produktow oksyetylenowania estrow metylowych kwasow tluszczowych oleju rzepakowego

Aqueous solutions of homologous series of Rofam type surfactants (ZPC) based on rape oil fatty acids methyl esters, differing in oxyethylene segments content (n T E ), were the subjects of investigations. Content of oxyethylene segments n T E , determined by 1 H NMR, allowed to verify ZPC molecular weight and to calculate structural hydrophilic-lipophilic balance (HLB, by Davis or Griffin methods) as well as required solubility level of the balance (HLB R ) (Table 1). The following values were determined on the basis of viscometric measurements: limiting viscosity number [η], viscometric average molecular weight (Mη), specific volume (Ω) and hydrodynamic radius of micelle (Robs) (Table 2 and 3). Viscometric measurements let also calculate average number of water molecules hydrating the micelle built of Rofam homologous structures. Determination of critical micellar concentration (cmc) was the base for calculation of thermodynamic potential of micelle formation (ΔG 0 m ) and micellar level of hydrophilic-lipophilic balance (HLB c m c ) (Table 4, Fig. 1-3). The value of surfactant's lipophilic core upthrust over the phase boundary (A m , Fig. 4) let analyze the effect of oxyethylene segments' content in Rofam molecule structure on its potential solubilization properties. The results of investigations and the values calculated on this basis make clearer the mechanism of solubilization of lipophilic drugs what determines the pharmaceutical and biological availability of micellar adducts from the model forms of drugs.

Forced folding and structural analysis of metastable proteins.

A significant fraction of the proteins encoded by the human and other genomes appears to be significantly unfolded in vitro. This will undoubtedly hamper attempts to characterize their structure by classical crystallographic or solution NMR methods. Here we show that encapsulation of a metastable protein within the restricted volume a reverse micelle can be used to force fold the protein and allow its characterization by modern methods of NMR spectroscopy. This may have significant utility in the context of structural proteomics. In addition, variation of the inner volume of the reverse micelle can be used to probe the character of the manifold of unfolded states.

Effect of pH on the viscosity of heated reconstituted skim milk

Reconstituted skim milk was adjusted to pH values between 6.5 and 6.7 and heated (90°C) for up to 30 min. The change in the viscosity, as well as the casein micelle size and the level of whey protein association with the micelles, was monitored for each milk sample. The change in the viscosity was dependent on the pH of the milk at heating. At pH 6.5, the viscosity increased markedly during the early stages of heating, and plateaued on prolonged heating. The magnitude of the change in viscosity decreased as the pH of the milk was increased, so that only small changes in viscosity were observed on heating at pH 6.7. The change in viscosity was linearly correlated with the change in particle volume (calculated from the particle size). This indicates that the viscosity changes were related to a change in the volume fraction of the casein micelles as a consequence of heating. The level of denatured whey proteins associating with the micelles was also markedly dependent on the pH at heating. High levels associated at pH 6.5, and the level associated was lower as the pH was increased to pH 6.7. The changes in viscosity and casein micelle volume were correlated with the levels of denatured whey proteins that were associated with the casein micelles, although there was some deviation from linearity at low levels of association (<15%).

Thermosensitive sol–gel transition behaviors of poly(ethylene oxide)/aliphatic polyester/poly(ethylene oxide) aqueous solutions

AbstractThe thermoreversible gelation of Pluronic [poly(ethylene oxide) (PEO)–polypropylene oxide (PPO)–PEO] aqueous solutions originates from micelle formation and micelle volume changes due to PEO–water and PPO–water lower critical solution temperature behavior. The micelle volume fraction is known to dominate the sol–gel transition behavior of Pluronic aqueous solutions. Triblock copolymers of PEO and aliphatic polyesters, instead of PPO, were prepared by hexamethylene diisocyanate coupling and dicyclohexyl carbodiimide coupling. Through changes in the molecular weight and hydrophobicity of the polyester middle block, the hydrophobic–hydrophilic balance of each block was systematically controlled. The following aliphatic polyesters were used: poly(hexamethylene adipate) (PHA), poly(ethylene adipate) (PEA), and poly(ethylene succinate) (PESc). With the hydrophobicity and molecular weight of the middle block increasing, the critical micelle concentration at the same critical micelle temperature decreased, and the absolute value of the micellization free energy increased. The micelle size was rather insensitive to temperature but slightly decreased with increasing temperature. PEO–PHA–PEO and PEO–PEA–PEO triblock copolymers needed high polymer concentrations to form gels. This was ascribed to the tight aggregation of PHA and PEA chains in the micelle core due to strong hydrophobic interactions, which induced the contraction of the micelle core. However, because of the relatively hydrophilic core, a PEO–PESc–PEO aqueous solution showed gelation at a low polymer concentration. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 772–784, 2004