Structure and ordering kinetics of micelles in triblock copolymer solutions in selective solvents

We have used small-angle x-ray scattering (SAXS), and small-angle neutron scattering (SANS) to study the micelle structure of a polystyrene-block-poly(ethene-co-butene)-block-polystyrene triblock copolymer in dilute - semidilute solutions in solvents selective for either the outer styrene block (dioxane) or for the middle block (heptane or tetradecane). Measurements of equilibrium structure factors showed that micelles were formed in both types of selective solvents. In the case of dioxane, the micelles are isolated whereas in the case of heptane or tetradecane, a bridged micellar structure may be formed at higher copolymer concentrations. In both cases we observed an ordered cubic structure of insoluble domains (micellar cores) at high concentrations (> 8 %). The micellar scattering function was fit to the Percus-Yevick interacting hard-sphere model. The temperature dependence of the core radius, the hard-sphere interaction radius and the volume fraction of hard spheres were obtained. We also used synchrotron-based time-resolved SAXS to examine the kinetics of ordering of the micelles on a cubic lattice for many different temperature jumps into the ordered cubic phase starting from the disordered micellar fluid phase in different solvents at different concentrations. The time evolution of the structure changes was determined by fitting the data with Gaussians to describe the structure factor of the ordered Bragg peaks and the Percus-Yevick structure factor was used to describe the micellar fluid. The time dependence of the peak intensities and widths as well as of the micellar parameters will be presented. The results showing the kinetics of the transformation from the fluid to the ordered phase were analyzed using the Mehl-Johnson-Avrami theory of nucleation.     

Chain diffusion in the melt of an asymmetric diblock copolymer in the disordered and ordered state

 The self-diffusion in a polystyrene-b-polyisoprene diblock copolymer with a strongly asymmetric composition was investigated with dependence on temperature by pulsed field gradient (PFG) NMR. The diblock shows with decreasing temperature a disorder-to-order-transition at T _ODT=393 K from a micellar liquid-like to a bcc ordered state which was recently measured by SAXS [M. Schwab and B. Stühn, Phys. Rev. Lett. (1996) 76: 924]. Two diffusivities were observed, one of the free diblock chains and one of the diblock chains fixed in micelles. The volume fraction of free chains decreases with decreasing temperature. The diffusivity of the free chains must be related to chain stretching. The experiments show that within the time of the NMR experiment (300 ms) there is no exchange between the diblock chains in the free state and those fixed in the core of the micelles. Received: 25 March 1997 Accepted: 25 April 1997     

Anomalous rheology in a nanostructured diblock copolymer/hydrocarbon system and its kinetic origin

Nanostructured squalane solutions (5–20 wt %) of a diblock copolymer, poly(styrene‐b‐hydrogenated isoprene), were prepared by a cosolvent‐casting method. The as‐cast solutions behaved as viscous liquids with terminal flow behavior at room temperature. Upon heating, the solutions gelled, and they did not return to their starting liquidlike state upon cooling. Small‐angle X‐ray scattering (SAXS) revealed a random array of spherical micelles in the as‐cast solutions, which were hypothesized to be in a nonequilibrium state. This abnormal solidification with increasing temperature was correlated with the formation of body‐centered‐cubic (BCC) structures. Isothermal SAXS and rheology measurements also indicated that the rate of formation of BCC structures in the as‐cast solutions increased with temperature. A diffusion‐controlled nucleation‐and‐growth mechanism was proposed for the ordering process in the as‐cast polystyrene‐b‐hydrogenated polyisoprene/squalane solutions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1496–1505, 2004     

Water-induced micellar structures of block copoly(oxyethylene-oxypropylene-oxyethylene) in o-xylene

We previously reported the water-induced micelle formation of copoly(oxyethylene-oxy-propylene-oxyethylene), Pluronic L64, in o-xylene. The micellar properties could be controlled by varying the water to EO ratio (Z) in micelles. in micelles. In this paper, laser light scattering, transient electric birefringence (TEB), and synchrotron small-angle x-ray scattering (SAXS) were used to study the micellar structure at different Z values. Both TEB and SAXS results further confirmed the micellar shape transition from that of a sphere to a nonspherical shape. A comparison between TEB and dynamic light-scattering results as well as the SAXS experiments showed an ellipsoidal shape for micelles when Z > 1.3 with the oblate being the more reasonable form for fitting all the experimental parameters. The degree of asymmetry appeared to be not high. © 1993 John Wiley & Sons, Inc.     

Intrinsic parameters for the structure control of nonionic reverse micelles in styrene: SAXS and rheometry studies

Shape, size, and internal structure of nonionic reverse micelle in styrene depending on surfactant chain length, concentration, temperature, and water addition have been investigated using a small-angle X-ray scattering (SAXS) technique. The generalized indirect Fourier transformation (GIFT) method has been employed to deduce real-space structural information. The consistency of the GIFT method has been tested by the geometrical model fittings, and the micellar aggregation number (N(agg)) has been determined. It was found that diglycerol monocaprate (C(10)G(2)), diglycerol monolaurate (C(12)G(2)), and diglycerol monomyristate (C(14)G(2)), spontaneously self-assemble into reverse micelles in organic solvent styrene under ambient conditions. The micellar size and the N(agg) decrease with an increase in surfactant chain length, a scenario that could be understood from the modification of the critical packing parameter (cpp). A clear picture of one-dimensional (1-D) micellar growth was observed with an increase in surfactant weight fraction (W(s)) in the C(10)G(2) system, which eventually formed rodlike micelles at W(s) ≥ 15%. On the other hand, micelles shrunk favoring a rod-to-sphere type transition upon heating. Reverse micelles swelled with water, forming a water pool at the micellar core; the size of water-incorporated reverse micelles was much bigger than that of the empty micelles. Model fittings showed that water addition not only increase the micellar size but also increase the N(agg). Zero-shear viscosity was found to decrease with surfactant chain but increase with W(s), supporting the results derived from SAXS.     

Structure and rheology of reverse micelles in dipentaerythrityl tri-(12-hydroxystearate)/oil systems

We report the formation of reverse rod-like micelles and their rheological properties in novel nonionic surfactant, dipentaerythrityl tri-(12-hydroxystearate) (designated as WO-6)/oil systems without external water addition. Small-angle X-ray scattering (SAXS) was used to investigate the structure of the micelles and their flow properties were studied by rheological measurements. We found that WO-6 spontaneously self-assembles into reverse micelles in a variety of organic solvents at ambient conditions, their structure depending on solvent molecular architecture, surfactant concentration, and temperature. Rod-like micelles with a maximum length of ca. 12 nm and a cross section diameter of ca. 2 nm were observed in cyclohexane. When cyclohexane was replaced with a linear chain octane, the length and the cross section diameter were simultaneously increased. With a further increase of hydrocarbon chain length of solvent oils from octane to hexadecane, the rod-like micelles grew axially, keeping the cross section diameter (ca. 3 nm) virtually constant. Increasing surfactant concentration also favored one-dimensional micellar growth. On the other hand, micelles shrunk with the rise of temperature, which is similar to a rod-to-sphere transition, and is essentially the opposite temperature dependence to that often observed in aqueous micellar systems. A structural picture drawn by SAXS is well supported by rheology; the relative (zero-shear) viscosity of the WO-6/oil systems was found to be markedly greater than that expected for a dispersion of spherical particles due to the elongated micellar structure, despite quantitative inconsistency with semi-empirically predicted values for rigid rod-like particles.     

Formation of block copolymer micelles in solutions of a linear homopolymer in a good solvent

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 polystyrene-block-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.     

Static and dynamic structures of spherical nonionic surfactant micelles during the disorder-order transition

We have investigated the static and dynamic structures of nonionic surfactant micelles, a C(12)E(8)/water binary system, during the disorder-order transition using small angle x-ray scattering, static light scattering, and dynamic light scattering techniques. In the disordered phase, the micelles have spherical shape and intermicellar interactions are governed by the hard core and weak long ranged attractive potentials. With increase of the micellar concentration, the disordered micelles transform to the three characteristic ordered micellar phases, a hexagonally close packed lattice, a body centered cubic lattice, and an A15 lattice having area-minimizing structure. The stability of these phases is well explained by balance of a close packing rule and a minimal-area rule proposed by Ziherl and Kamien [Phys. Rev. Lett. 85, 3528 (2000)]. The role of hydrodynamic interactions in surfactant micellar solutions was compared with that in hard sphere colloidal particle suspensions.     

A small-angle X-ray scattering study of the structure of lysozyme-sodium dodecyl sulfate complexes

The structure of lysozyme-sodium dodecyl sulfate (SDS) complexes in solution is studied using small-angle X-ray scattering (SAXS). The SAXS data cannot be explained by the necklace and bead model for unfolded polypeptide chain interspersed with surfactant micelles. For the protein and surfactant concentrations used in the study, there is only marginal growth of SDS micelles as they complex with the protein. Being a small and rather rigid protein, lysozyme can penetrate the micellar core which is occupied by flexible and disordered paraffin chains and also the shell occupied by the hydrated head groups. A partially embedded swollen micellar model seems appropriate and describes well the scattering data. The SAXS intensity profiles are analyzed by considering the change in the electron scattering length density of the micellar core and shell due to complexation with protein and treating the intermicellar interaction using rescaled mean spherical approximation (RMSA) for charged spheres.     

Thin film order of symmetric diblock copolymers

The ordering in thin films of symmetric diblock copolymers of polystyrene and poly(methyl methacrylate) has been investigated by neutron reflectivity as a function of film thickness and temperature. The order-disorder transition in the thin films was found to lose its first order character in that the transition occurs in a continuous manner without the correlation length becoming infinite. In addition, a transition from a partially to fully ordered state was observed which was fully reversible. This transition depended in a power law manner on the film thickness and extrapolates to the bulk order-disorder transition temperature for thick films.     

Microrheology of wormlike micellar fluids from the diffusion of colloidal probes

The microrheology of cationic micellar solutions has been investigated as a function of added organic salts using quasielastic light scattering (QELS). Two organic salts, sodium p-toluene sulfonate and sodium salicylate, were used to induce microstructural changes in cetyl trimethylammonium bromide (CTAB) micelles. The mean-squared displacement (MSD) of polystyrene probe particles embedded in CTAB micellar solutions was monitored by QELS in the single-scattering regime. Through the use of the generalized Stokes-Einstein relationship, the frequency-dependent complex shear moduli of each fluid were estimated from the Laplace transform of the corresponding MSD. The salt-induced transition from nearly spherical to elongated wormlike micelles and consequent changes in fluid response from viscous to viscoelastic are clearly captured by microrheology.     

Structure and interactions of block copolymer micelles of Brij 700 studied by combining small-angle X-ray and neutron scattering

Spherical micelles of the diblock copolymer/surfactant Brij 700 (C(18)EO(100)) in water (D(2)O) solution have been investigated by small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS). SAXS and SANS experiments are combined to obtain complementary information from the two different contrast conditions of the two techniques. Solutions in a concentration range from 0.25 to 10 wt % and at temperatures from 10 to 80 degrees C have been investigated. The data have been analyzed on absolute scale using a model based on Monte Carlo simulations, where the micelles have a spherical homogeneous core with a graded interface surrounded by a corona of self-avoiding, semiflexible interacting chains. SANS and SAXS data were fitted simultaneously, which allows one to obtain extensive quantitative information on the structure and profile of the core and corona, the chain interactions, and the concentration effects. The model describes the scattering data very well, when part of the EO chains are taken as a "background"contribution belonging to the solvent. The effect of this becomes non-negligible at polymer concentrations as low as 2 wt %, where overlap of the micellar coronas sets in. The results from the analysis on the micellar structure, interchain interactions, and structure factor effects are all consistent with a decrease in solvent quality of water for the PEO block as the theta temperature of PEO is approached.     

Upper critical solution temperature switchable micelles based on polystyrene‐block‐poly(methyl acrylate) block copolymers

The solubility behavior of well‐defined poly(methyl acrylate) homopolymers as well as polystyrene‐block‐poly (methyl acrylate) block copolymers is discussed in this contribution. A solubility screening in ethanol–water solvent mixtures was performed in a high‐throughput manner using parallel turbidimetry revealing upper critical solution temperature behavior for poly(methyl acrylate). Moreover, the self‐assembly behavior of the block copolymers into micellar structures was investigated by dynamic light scattering (DLS), transmission electron microscopy (TEM), and cryo‐TEM revealing upper critical solution temperature switchability of the micelles, which was evaluated by DLS at different temperatures. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Local dynamics of micelles of new long-chain surfactants in aqueous media

The molecular dynamics, organization, and phase state of aqueous solutions of new long-chain cationic surfactants with saturated hydrocarbon radicals (from C₁₆ to C₂₂) containing one or two hydroxyl groups in their polar heads are studied by the spin-probe EPR spectroscopy. In the region of micellar solutions, local mobility of surfactant molecules slightly changes with an increase in the length of hydrocarbon radical, whereas the order parameter of micelles increases notably. The addition of two hydroxyl groups to the polar part of long-chain (C ₂₂) surfactant molecule considerably decreases local mobility and increases the ordering of micellar system compared to the micelles of analogous surfactant with one hydroxyl group. Phase transition from micellar to a solid state is observed in this system with a decrease in temperature. The addition of KCl to aqueous surfactant solution lowers the local mobility, increases the order parameter of micelles, and can cause changes in the phase state of a system. In the presence of salt, the correlation time of probe rotation and its order parameter depend on surfactant concentration. Apparently, this is explained by changes in the shape of micelles upon variations in surfactant concentration.     

Phase behavior and microstructures of nonionic fluorocarbon surfactant in aqueous systems

The phase behavior and self-assembled structures of perfluoroalkyl sulfonamide ethoxylate, C8F17SO2N(C3H7)(CH2CH2O)20H (abbreviated as C8F 17EO20), a nonionic fluorocarbon surfactant in an aqueous system, has been investigated by the small-angle X-ray scattering (SAXS) technique. The C8F17EO20 forms micelles and different liquid crystal phases depending on the temperature and composition. The fluorocarbon micellar structure induced by temperature or composition change and added fluorocarbon cosurfactant has been systematically studied. The SAXS data were analyzed by the indirect Fourier transformation (IFT) and the generalized indirect Fourier transformation (GIFT) depending on the volume fraction of the surfactant and complemented by plausible model calculations. The C8F17EO20 forms spherical type micelles above critical micelle concentration (cmc) in the dilute region. The micelle tends to grow with temperature; however, the growth is not significant on changing temperature from 15-75 degrees C, which is attributed to the higher clouding temperature of the surfactant (>100 degrees C). On the other hand, the micellar structure (shape and size) is apparently unaffected by composition (1-25 wt %) at 25 degrees C. Nevertheless, addition of fluorocarbon cosurfactant of structure C8F17SO2N(C3H7)(CH2CH2O)H (abbreviated as C8F17EO1) to the semidilute solution of C8F17EO20 (25 wt %) favors micellar growth, which finally leads to the formation of viscoelastic wormlike micelles, as confirmed by rheometry and supported by SAXS. The onset sphere-to-wormlike transition in the structure of micelles in the C8F17EO20/water/C8F17EO1 system is due to the fact that the C8F17EO1 tends to go to the surfactant palisade layer so that the critical packing parameter increases due to a decrease in the effective cross-sectional area of the headgroup. As a result, spherical micelles grow into a cylinder, which after a certain concentration entangle to form a rigid network structure of wormlike micelles.     

Micelle structure in isotropic aqueous colloids of a poly(oxyethylene) amphiphile C12E8

Micelle structure in aqueous colloids in the isotropic liquid phase (L₁) of a nonionic amphipile (n-dodecyl octa(oxyethylene glycol) monoether (C₁₂E₈) has been investigated as a function of concentration and temperature using light scattering (LS), viscometry, NMR, and small-angle X-ray scattering (SAXS).The spherical micelles, having a radius of 28–31 Å, remain in a wide concentration range from very dilute to ca. 42 wt %. The micelle size increases sligthly with increasing temperature in the range of 25–60 °C. In the concentrated colloids, the spherical micelles are likely to be arranged in a certain ordered structure. Even at such a high concentration as 30 wt %, the isotropic colloid shows Newtonian flow. This suggests that interaction between micelles in the ordered structure is very weak and the structure is very fragile. Moreover, coexistence of the isotropic phase and the ordered structure in L₁ phase is discussed.     

Interaction of meso-tetrakis (4-sulfonatophenyl) porphyrin with cationic CTAC micelles investigated by small angle X-ray scattering (SAXS) and electron paramagnetic resonance (EPR)

Small angle X-ray scattering (SAXS) and electron paramagnetic resonance (EPR) have been used to investigate the interaction of the water-soluble meso-tetrakis (4-sulfonatophenyl) porphyrin (TPPS(4)) with cationic cethyltrimethylammonium chloride (CTAC) micelles. To evaluate if the porphyrin protonation state affects its interaction with the micelle, both SAXS and EPR measurements were performed at pH 4.0 and 9.0. The best-fit SAXS curves were obtained assuming for CTAC micelle a prolate ellipsoidal shape in the absence and upon incorporation of 2-10 mM TPPS(4). SAXS results show that the presence of porphyrin impacts on micellar hydrophobic core, leading to a micellar reassembling into smaller micelles. Lineshapes of EPR spectra of 5- and 16-doxyl stearic acids (5- and 16-DSA, respectively) bound to 100 mM CTAC micelles exhibited slight changes as a function of porphyrin concentration. Spectral simulations revealed an increase of mobility restriction for both spin probes, especially at higher porphyrin concentration, where a small reduction of environment polarity was also observed for 16-DSA. The spin labels monitored only slight differences between pH 4.0 and 9.0, in agreement with the SAXS results.     

Colloidlike behaviour of a polystyrene-b-poly(ethylene-co-propylene) diblock copolymer dissolved in n-decane investigated by pulsed field gradient nuclear magnetic resonance

The self-diffusion of a polystyrene-b-poly(ethylene-co-propylene) diblock copolymer dissolved in a preferential solvent for the aliphatic block, n-decane, was investigated by pulsed field gradient NMR. The diblock copolymer forms micelles in solution, the structure of the solid polymer being preserved in the native solution because the polystyrene is in the glassy state. The equilibrium state is attained upon heating which again freezes in upon cooling to room temperature. The hydrodynamic radius of the micelles decreases by about 50% during this heating–cooling process. The concentration dependence of the self-diffusivity shows typical colloidlike behaviour, and it can be described by a Vogel–Fulcher–Tammann-like equation. No indications of crystallization at higher concentrations are observed in the micellar solution because the micellar sizes are slightly polydisperse. The self-diffusivity was measured up to the glasslike state, where in-cage- diffusion and dynamic heterogeneities could be detected. Received: 14 April 1999 Accepted in revised form: 14 June 1999     

Effect of added poly(oxyethylene)dodecyl ether on the phase and rheological behavior of wormlike micelles in aqueous SDS solutions

Upon the addition of a short EO chain nonionic surfactant, poly(oxyethylene) dodecyl ether (C12EOn), to dilute micellar solution of sodium dodecyl sulfate (SDS) above a particular concentration, a sharp increase in viscosity occurs and a highly viscoelastic micellar solution is formed. The oscillatory-shear rheological behavior of the viscoselastic solutions can be described by the Maxwell model at low shear frequency and combined Maxwell-Rouse model at high shear frequency. This property is typical of wormlike micelles entangled to form a transient network. It is found that when C12EO4 in the mixed system is replaced by C12EO3 the micellar growth occurs more effectively. However, with the further decrease in EO chain length, phase separation occurs before a viscoelastic solution is formed. As a result, the maximum zero-shear viscosity is observed at an appropriate mixing fraction of surfactant in the SDS-C12EO3 system. We also investigated the micellar growth in the mixed surfactant systems by means of small-angle X-ray scattering (SAXS). It was found from the SAXS data that the one-dimensional growth of micelles was obtained in all the SDS-C12EOn (n=0-4) aqueous solutions. In a short EO chain C12EOn system, the micelles grow faster at a low mixing fraction of nonionic surfactant.     

Supramolecular self-assembly of multiblock copolymers in aqueous solution

A unique pH-dependent phase behavior from a copolymer micellar solution to a collapsed hydrogel with micelles ordered in a hexagonal phase was observed. Small-angle neutron scattering (SANS) was used to follow the pH-dependent structural evolution of micelles formed in a solution of a pentablock copolymer consisting of poly((diethylaminoethyl methacrylate)-b-(ethylene oxide)-b-(propylene oxide)-b-(ethylene oxide)-b-(diethylaminoethyl methacrylate)) (PDEAEM25-b-PEO100-b-PPO65-b-PEO100-b-PDEAEM25). Between pH 3.0 and pH 7.4, we observed the presence of charged spherical micelles. Increasing the pH of the micelle solution above pH 7.4 resulted in increasing the size of the micelles due to the increasing hydrophobicity of the PDEAEM blocks above their pKa of 7.6. The increase in size of the spherical micelles resulted in a transition to a cylindrical micelle morphology in the pH range 8.1-10.5, and at pH >11, the copolymer solution undergoes macroscopic phase separation. Indeed, the phase separated copolymer sediments and coalesces into a hydrogel structure that consists of 25-35 wt % water. Small-angle X-ray scattering (SAXS) clearly indicated that the hydrogel has a hexagonal ordered phase. Interestingly, the process is reversible, as lowering of the pH below 7.0 leads to rapid dissolution of the solid into homogeneous solution. We believe that the hexagonal structure in the hydrogel is a result of the organization of the cylindrical micelles due to the increased hydrophobic interactions between the micelles at 70 degrees C and pH 11. Thus we have developed a pH-/temperature-dependent, reversible hierarchically self-assembling block copolymer system with structures spanning nano- to microscale dimensions.