Micelle Association Numbers Research Articles

Manipulating the structures and photoresponsive properties of an azobenzene-containing triblock copolymer by changing solvency and adding salts

We examined the structure–property relationship of a triblock copolymer comprising azobenzene sequences, namely, poly(6-[4-(4′-ethoxyphenylazo)phenoxy]hexyl methacrylate)-block-poly(ethylene glycol)-block-poly(6-[4-(4′-ethoxyphenylazo)phenoxy]hexyl methacrylate) (PMMAzo-b-PEG-b-PMMAzo), in mixed dimethylformamide (DMF)/H2O solvents, with the effects of solvency and salts being the main focus. When H2O (a PEG-selective solvent) was added to DMF (a neutral solvent), the poor compatibility between H2O and PMMAzo molecules led to the formation of micelles comprising PMMAzo and PEG in the core and corona, respectively. Increases in the H2O content in the mixed solvent decreased the solvency, which caused increases in the micelle association number and hydrophobic core radius. The azobenzene moieties confined in the micellar core inhibited their photoisomerization, which caused decreases in the photoisomerization kinetics and cis isomer content. The addition of KCl to the PMMAzo-b-PEG-b-PMMAzo solution increased the hydrophobicity of PEG chains and the core radius and decreased the corona thickness. Moreover, the KSCN addition to the aforementioned solution increased the hydrophilicity of PMMAzo segments, which caused the core radius and corona thickness of micelles to decrease and increase, respectively. Salt addition disturbed the compact packing of mesogens and reduced the mesogen association. Consequently, with an increase in the salt concentration, the content of H-type mesogen aggregates increased, whereas that of nonassociated mesogens decreased.

Micellization and Gelation of Mixed Copolymers P123 and F127 in Aqueous Solution

The micellization in dilute aqueous solution of Pluronic copolymers P123 (E21P67E21) and F127 (E98P67E98) and mixtures of the two was investigated using static and dynamic light scattering. Gelation of concentrated solutions of the two copolymers and their mixtures was studied using tube inversion and oscillatory rheometry. The two copolymers comicellized to give micelles with narrow size distributions. Clouding temperatures and critical micelle temperatures decreased as the proportion of P123 in the mixture was increased. Micelle association numbers of the mixed micelles lay between the values found for micelles of P123 and F127 alone, whereas micelle radii passed through maximum values in the range 0-50 wt % P123. As judged by the ratio of the thermodynamic to the hydrodynamic radius, the micelle interaction potential changes gradually from soft to hard as the proportion of P123 in the mixture is increased. Regions of cubic and hexagonal (birefringent) gel were defined for concentrated solutions. The high-temperature boundary of the 30 wt % cubic gel decreased monotonically from 90 to 43 degrees C as the proportion of P123 in the mixture was increased from 0 to 100 wt %, whereas the low-temperature boundary was essentially constant at 15 +/- 3 degrees C. Increasing the proportion of P123 in the mixture at 25 degrees C increased the concentration at which the cubic gel was first formed and decreased the concentration at which the hexagonal gel was first formed.

Conjugates of poly( dl-lactic acid) with ethylenediamino or diethylenetriamino bridged bis( β-cyclodextrin)s and their nanoparticles as protein delivery systems

Some biodegradable amphiphilic copolymers were synthesized by conjugating poly( dl-lactic acid) (PLA) onto ethylenediamino or diethylenetriamino bridged bis( β-cyclodextrin)s (bis-CDs). Double emulsion (DE) and nanoprecipitation (NP) methods were used to fabricate the nanoparticles of these copolymers entrapping bovine serum albumin (BSA) as a model protein. Effects of the experimental parameters, such as copolymer composition, BSA concentration, copolymer concentration and poly(vinyl alcohol) concentration, on particular size and encapsulation efficiency (EE) were investigated. Their EE to BSA could reach 83.5% at an optimized condition owing to the cooperative binding effect of the CD moiety with BSA. The core-corona structure of copolymer micelles fabricated from the nanoprecipitation was studied on the basis of 1H NMR and other measurements at various temperatures. The results showed that the core-corona structure kept stable below 50 °C (lower than T g). And increase of the micelle association number occurred above the T g because the size of the NPs became larger and proton signals of the liquid-like PLA cores could be observed in 1H NMR in D 2O at 60 °C. The release profiles of NPs showed a burst effect followed by a continuous release. Sodium dodecyl sulfate polyacrylamide gel electrophoresis, circular dichroic and fluorescence spectra were further used to identify the stability of BSA released from the NPs. The nanoparticles from the conjugates have a promising potential in nasal delivery system.

Self-association of block copoly(oxyalkylene)s in aqueous solution. Effects of composition, block length and block architecture

The article deals with the association behaviour in dilute aqueous solution of block copoly(oxyalkylene)s in which hydrophilic poly(ethylene oxide) is combined with hydrophobic poly(propylene oxide), poly(1,2-butylene oxide) or poly(styrene oxide). Polymers with three simple architectures are considered, i.e. copolymers of type EmAn, EmAnEm and AnEmAn, where E denotes an oxyethylene unit, A denotes a hydrophobic oxyalkylene unit, and the subscripts m and n denote number-average block lengths in repeat units. The aim is to examine how composition, block length and block architecture govern two fundamental properties, critical micelle concentration (cmc) and micelle association number (N), for systems which are in dynamic equilibrium. Copolymers with properties known to be greatly affected by heterogeneity in composition are excluded from consideration. A uniform pattern of behaviour emerges when log(cmc) is plotted against reduced hydrophobic block length (x), consistent with the micellisation equilibrium changing from one between unimers and multimolecular micelles at low values of x, to one between unimolecular micelles and multimolecular micelles at high values of x. Support for this model is provided by the enthalpy of micellisation, values of which fall effectively to zero as x is increased. Values of the micelle association number are used to define a critical hydrophobic block length for micellisation (n(cr)) for each class of diblock copolymers, values of which apply equally well to the half-length of the central block of corresponding EmAnEm triblock copolymers. Given these values, and irrespective of block architecture, the overall scaling law for the weight-average association number of the micelles is shown to be Nw = n'(1.07)m(-0.63) where m is the length (or half-length) of the hydrophilic block, and n' is the effective length of the hydrophobic block, equal to its length (or half-length) minus the critical length, i.e. n' = n-n(cr).

Small-Angle Neutron Scattering Characterization of Micelles Formed by Poly(dimethylsiloxane)-graft-polyether Copolymers in Mixed Polar Solvents

The self-assembled structure of an amphiphilic poly(dimethylsiloxane)-graft-polyether copolymer has been investigated in binary solvents consisting of water and one of the following polar organic solvents: 2-propanol, ethanol, formamide, or glycerol. Micelle structural parameters were obtained by fitting the core−corona form factor and hard-sphere interaction structure factor to small-angle neutron scattering (SANS) data obtained at contrast conditions where the solvents were fully deuterated and the copolymer was not. The siloxane copolymer forms in water at 20 °C micelles that consist of a “dry” core with a radius of about 6 nm and a solvated (about 40% polymer) corona with a thickness of 2 nm; the relatively thin corona may be due to the comb-type architecture of the siloxane copolymer. The micelle radii, micelle core radii, micelle association numbers, and polymer volume fractions in the micelle core and corona decreased with increasing cosolvent/water ratio in the mixed solvents containing 2-propanol, ethanol, or formamide. On the contrary, the addition of glycerol led to an increase of the micelle association number. The above findings are discussed in terms of the relative location of the polar organic solvents in the interior or exterior of the siloxane copolymer micelles. In particular, the micelle association numbers observed for different water−cosolvent mixtures can be correlated to the octanol−water partition coefficients of the respective cosolvents. A transition from spherical to ellipsoid micelles was observed upon an increase in the temperature: this is attributed to the desolvation of the polyether chains. This structural transition correlates with the solvent quality in the mixed solvents: ethanol and 2-propanol, which solvate the interior of the micelles, tending to keep the micelles spherical, while increased glycerol content in the mixed solvent leads to the formation of ellipsoids at a fixed temperature.

Mixed micellisation of oxyethylene–oxybutylene diblock and triblock copolymers in water studied by light scattering

The mixed micellisation of copolymers E60B12 and E58B20E58 in water was investigated using static and dynamic light scattering (SLS and DLS). As expected, considering the similarity of their E block lengths, the radii of the micelles were constant irrespective of the proportions of the two copolymers in solution. Micelle association numbers, which differed for the two copolymers when micellised singly, depended on the proportions of the two copolymers in solution. This dependence was rationalised by taking account of the triblock copolymer looping in the micelle. The mixed micelles were found to have a harder intermicellar interaction potential than those formed from the single copolymers.

Aqueous Solutions and Gels of Diblock Copolymers of 1,2-Butylene Oxide and Ethylene Oxide Studied by Light Scattering and Rheology

Copolymers B20E430, B20E510, and B20E610 (B = oxybutylene repeat unit, E = oxyethylene repeat unit, subscripts indicate chain length in repeat units) were synthesized and characterized by gel permeation chromatography (for distribution width) and 13C NMR spectroscopy (for absolute molar mass and composition). Dynamic and static light scattering were used to determine micellar properties in dilute aqueous solution: e.g., micelle association numbers and radii. A tube-inversion method was used to define the mobile−immobile (hard gel) phase boundary. For copolymer B20E610, immobile gels form at concentrations as low as 2.9 wt %. Rheological measurements of dynamic modulus and yield stress served to characterize the gel properties and to confirm the phase boundaries. The results are combined with those from previous work on related block copolymers to obtain scaling relationships for the dependence of micellar and gel properties on E-block length, thus enabling prediction of the requirements for and properties of very dilute aqueous gels. The validity of treating the micelles as hard spheres is discussed.

Association Properties of Triblock Copolymers in Aqueous Solution: Copolymers of Ethylene Oxide and 1,2-Butylene Oxide with Long E-blocks

Four triblock copolymers, EmB20Em, with m in the range 58−260, were prepared by sequential anionic polymerization of 1,2-butylene oxide followed by ethylene oxide. E denotes an oxyethylene repeat unit and B an oxybutylene repeat unit. The copolymers were characterized by gel permeation chromatography (for distribution width) and 13C NMR spectroscopy (for absolute molar mass and confirmation of triblock architecture). Static and dynamic light scattering were used to study the micellization and micelle properties of the copolymers in dilute aqueous solution, particularly the mass-average association number and thermodynamic and hydrodynamic radii. At a given temperature, the micelle association number decreased as the E-block length was increased while the radii increased. The gelation of relatively concentrated aqueous solutions of the copolymers was also investigated by a tube inversion method. The minimum concentration for gelation decreased in the range 9−13.3 wt % as the E-block length was increased. C...

Small-Angle Neutron Scattering Investigation of the Temperature-Dependent Aggregation Behavior of the Block Copolymer Pluronic L64 in Aqueous Solution

Small-angle neutron scattering (SANS) was used to investigate the temperature dependence of the structure of micelles formed by the poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) Pluronic L64 amphiphilic block copolymer in aqueous (D2O) solution. Different models for the form factor and the structure factor have been considered for fitting the SANS scattering patterns. By comparing the various models, we found that in the intermediate block copolymer concentration range, e.g., 2.5 wt % Pluronic L64, the micelles are well separated while the intermicellar interaction remains strong. Thus the more appropriate model was that of core−corona form factor and hard sphere structure factor, which allowed the determination of the core, micelle, and hard sphere interaction radii, the micelle association number, and the solvent volume fraction in the micelle core and corona. Upon an increase of temperature (in the range 35−55 °C) the micelle radius increased by ≈10%, accompanied by the loss of water in the micelle core. These trends coincide with the second relaxation process (attributed to the micelle core dehydration) observed in recent kinetics studies (Kositza, et al. Macromolecules 1999, 32, 5539). The SANS data also indicated a stronger increase of the micelle association number occurring above 47 °C, reflecting a strong increase of the intermicellar interactions. These results support the attribution of the third relaxation process observed in the aforementioned kinetic studies to micelle clustering.

Effects of block architecture and composition on the association properties of poly(oxyalkylene) copolymers in aqueous solution

A block copolymer of hydrophilic poly(ethylene oxide) and a hydrophobic poly(alkylene oxide) can associate in dilute aqueous solution to form micelles. The results of recent investigations of the micellisation behaviour and micelle properties of such copolymers are described. Copolymers of ethylene oxide with propylene oxide, 1,2-butylene oxide or styrene oxide are considered, including aspects of their preparation. Experimental methods for determination of critical conditions for micellisation, micelle association number and spherical-micelle radius are summarised. Effects of temperature, composition, block length and block architecture (diblock, triblock and cyclic-diblock) are described and, where possible, related to the predictions of theory. Brief consideration is given to the dynamics of micelle formation/dissociation, to cylindrical micelles, and to effects of added salts.

Polyoxyalkylene Block Copolymers in Formamide−Water Mixed Solvents: Micelle Formation and Structure Studied by Small-Angle Neutron Scattering

We investigated the solution properties of an amphiphilic poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO−PPO−PEO) block copolymer (Pluronic P105: EO37PO58EO37) in mixed solvents consisting of water and formamide (a nonaqueous polar solvent). The critical micellization concentration and temperature, the thermodynamic parameters of micellization, and the micelle structural parameters were obtained from small-angle neutron scattering (SANS) as a function of the formamide−water ratio, solution temperature, and block copolymer concentration. PEO−PPO−PEO block copolymers self-assemble in formamide−water mixed solvents with increasing temperature, indicating an endothermic micellization. Upon an increase of the formamide−water ratio, the enthalpy and entropy of micellization become smaller than the respective quantities in water. The micelle core and corona radii, the hard-sphere interaction distance of the micelles, the micelle association number, as well as the polymer volume fraction in...

Micelles and gels of oxyethylene–oxybutylene diblock copolymers in aqueous solution: The effect of oxyethylene-block length

Block copolymer E90B10 (E=oxyethylene, B=oxybutylene) was synthesised and characterised by gel permeation chromatography and 13C NMR spectroscopy. Dynamic light scattering (DLS) and static light scattering (SLS) were used to characterise the micelles in solution (both in water and in aqueous 0.2 mol dm-3 K2SO4), yielding the micellar association numbers, the hydrodynamic and thermodynamic radii, and related expansion factors. Micellar parameters were also obtained by small-angle neutron scattering (SANS) for solutions of a similar copolymer, E86B10, in water, i.e., the association number, the hard-sphere radius, the micelle volume fraction and the corresponding expansion factors. A comparison of the appropriate quantities showed good agreement between the two techniques. SANS gave additional information e.g., volume fraction profiles for the micelles and volume fraction of water in the micelle core. Moderately concentrated solutions of copolymer E90B10 were studied in the gel state by small-angle X-ray scattering (SAXS) in tandem with rheology (oscillatory shear). Values for the dynamic elastic modulus (G′) of the gels significantly exceeded 104 Pa across the range of temperature (25–80°C) and frequency (0.1–100 rad s-1) explored. The SAXS patterns for the orientated gels showed them to have a body-centred-cubic structure, as expected for packed, spherical micelles. This structure persisted over a wide concentration range, e.g., until crystallisation of the E blocks occurred at high concentration (70 wt.% copolymer). By combining the present and published results, a comparison was made of the micelle and gel properties of copolymers with the same B-block length but different E-block lengths, i.e., E90B10, E40B10 and E18B10. As would be expected, as the E-block length was increased, the micelle association numbers decreased while the micelle radii and expansion factors increased. The critical gel concentration (cgc) also decreased as the E-block length was increased. Moreover, the cgc correlated quantitatively with the thermodynamic expansion factors obtained by SLS and SANS from the exclusion properties of the micelles.

Micellization of Graft Copolymers of Alkyl Methacrylates with α-Methyl-ω-hydroxypoly(oxyethylene) Methacrylates

The solution properties of graft copolymers having a poly(methyl methacrylate-co-lauryl methacrylate) backbone and poly(oxyethylene) branches of different compositions were studied by static and dynamic light scattering methods in ethanol–water mixed solvents. The graft copolymers formed multimolecular aggregates in mixed solvents with water content higher than or equal to 10 vol%. The difference in solubility of lauryl and poly(oxyethylene) side chains of copolymers seems to be a dominant factor in determining their association behavior. The micelle formation is preferred in such solvents (water content ≥30 vol%) where the difference in solubilities of LAM and PEO side chains on graft copolymers is higher than a critical one. In conditions where there is a lower difference in solubilities of the side chains (water content 10–20 vol%), the random association of copolymers dominates. Copolymers were fully soluble in ethanol with low water content (1 vol%). The highest association number of micelles and consequently the lowest polydispersity were obtained with copolymers having the longest PEO side chains (Mw= 5000 g mol−1).

Association and surface properties of diblock copolymers of ethylene oxide and DL‐lactide in aqueous solution

AbstractDiblock copolymers of ethylene oxide and DL‐lactide (EmLn copolymers) were prepared by sequential anionic polymerisation and characterised by gel permeation chromatography (GPC) and 13C NMR. Their association and surface properties in aqueous solution were studied by surface tension and dynamic and static light scattering. Comparison of the critical micelle concentrations (cmc) and micelle association numbers (Nw) of the EmLn copolymers with published values for oxyethylene/oxybutylene (EmBn) copolymers indicated a slightly lower hydrophobicity for a lactide unit: i.e. L ≃ 0.75B. The temperature dependence of the cmc of copolymer E42L12 was analysed to obtain a standard enthalpy of micellisation ΔmicH0 ≈ 40 kJ · mol−1.

Self-assembly of alcohol ethoxylate non-ionic surfactants in aqueous solution

M. Ameri, D. Attwood, J. H. Collett and C. Booth, J. Chem. Soc., Faraday Trans., 1997, 93, 2545 DOI: 10.1039/A701662J

Association of Diblock and Triblock Copolymers of Ethylene Oxide and Butylene Oxide in Aqueous Solution

The association properties of block copolymers of ethylene oxide (E) and 1,2-butylene oxide (B) manufactured by The Dow Chemical Company were studied by static and dynamic light scattering. The critical micelle concentrations and micelle association numbers so obtained were combined with literature values for related copolymers in order to discuss the effect ofblock architecture on the association properties of linear diblock and triblock copolymers. The measurements significantly increase the available data for E m B n E m triblock copolymers and provide new insight into effects caused by E-block distributions originating from the different rates of reaction of ethylene oxide with primary and secondary oxyanions during copolymerization.