Solution Of Amphiphilic Block Copolymer Research Articles

Computational Reverse-Engineering Analysis of Scattering Experiments (CREASE) on Amphiphilic Block Polymer Solutions: Cylindrical and Fibrillar Assembly

In this paper, we extend a recently developed computational reverse-engineering analysis for scattering experiments (CREASE) approach to analyze cylindrical, fibrillar, and elliptical cylindrical structures assembled in amphiphilic block copolymer solutions. With CREASE, one can deduce information about assembled structures characterized via small-angle scattering without having to rely on fits using conventional analytical scattering models that may be approximate or inapplicable for the system at hand. With scattering intensity profiles and information about the polymer solution (e.g., molecular weight, composition, and sequence) provided to CREASE as an input, the output from CREASE includes the shape, morphology, dimensions, and molecular packing in the assembled polymer structures within the solution. CREASE is comprised of two steps: the first step involves a genetic algorithm (GA) to determine the shape and dimensions of the domains in the assembled structure and the second step uses molecular simulations to reconstruct chain conformations and monomer-level arrangements within the assembled structure. This paper builds on our recent work that was focused on spherical assembled structures and extends it to nonspherical assembled structures like cylinders, fibrils, and cylinders with elliptical cross sections. We validate the GA step within CREASE by taking input scattering intensity profiles from a variety of assembled shapes with known shapes and dimensions and by producing outputs that match those known shapes and target dimensions. To demonstrate its use in real situations where microscopy may hint at the potential shapes without the user knowing the dimensions with certainty, we apply CREASE with different potential assembled shapes and compare the structural dimensions of the results.

Temperature-Induced Re-Entrant Morphological Transitions in Block-Copolymer Micelles.

Using a combination of a mean-field theoretical method and the numerical Scheutjens-Fleer self-consistent field approach, we predict that it is possible to have re-entrant morphological transitions in nanostructures of diblock copolymers upon variation in temperature-mediated solubility of the associating blocks. This peculiar effect is explained by the different rates in variation of the density of the collapsed core domains and the corresponding interfacial energy as a function of the temperature. The theoretical findings are supported by existing experimental observations of reversed sequences of the morphological transitions occurring upon temperature variation in solutions of amphiphilic block copolymers.

PRISM Theory Study of Amphiphilic Block Copolymer Solutions with Varying Copolymer Sequence and Composition

We present a comparison of Polymer Reference Interaction Site Model (PRISM) theory and molecular dynamics (MD) simulations for studying amphiphilic block copolymers in solution. We use a generic coarse-grained model to represent amphiphilic A–B block copolymers in implicit solvent with the solvophobicity of the B segments captured using effective B–B pairwise attraction modeled using the Lennard-Jones potential. We study the assembly of the amphiphilic A–B block copolymer as a function of solvophobicity for varying copolymer sequences (diblock and triblock) and composition (solvophobic-rich or solvophilic-rich). The PRISM theory equation along with the atomic Percus–Yevick closure is solved to obtain the intermolecular pair correlations in real space, g(r), and structure factors in Fourier space, S(k), for block copolymer solutions at increasing values of solvophobicity. The real-space intermolecular pair correlation functions and structure factors from PRISM theory and from MD simulations are compared di...

Competition between the micellization and the liquid–liquid phase separation in amphiphilic block copolymer solutions

We have formulated the mixing Gibbs energy density for the micelle phase of an amphiphilic block copolymer solution using mean-field lattice theory. By comparing it with the mixing Gibbs energy density for the homogeneous phase, we have observed the transition between the micellization and the liquid–liquid phase separation. The micellization is preferred over the liquid–liquid phase separation at a higher amphiphilicity and a higher degree of polymerization of the block copolymer chain as well as at a higher incompatibility between the two block chains.

Effect of temperature and loading on the structure of β-casein/ibuprofen assemblies

β-Casein is a 24kDa amphiphilic and unstructured protein that self-assembles into small core–shell micelles at a wide range of concentrations, pH values and temperatures. We recently developed the micelles as nanocarriers for oral delivery of hydrophobic drugs. In this paper we examined the effect of the hydrophobic non-steroidal anti-inflammatory drug (NSAID) ibuprofen on the micellar structure, as a function of temperature and loading. Using cryo-transmission electron microscopy (cryo-TEM) we find two routes of organization – mixed micellization and co-assembly (aggregation). The time-dependent events that characterize the second routes has been examined in detail. At 25°C we find coexistence of small assemblies and larger aggregates of irregular (but defined) structures that contain the drug. Increasing the drug loading increases the relative number of the larger aggregates and their dimensions, leading eventually to the formation of long then branched structures, like in amphiphilic block copolymer solutions. Similar trends were identified for changes in the temperature. Combined, our results suggest that ibuprofen acts as a co-surfactant that possibly is localizes to the interface rather than being encapsulated in the micellar core as other NSAID hydrophobic drugs.

Robust coaxial capillary microfluidic device for the high throughput formation of polymersomes

A microfluidic device incorporating three coaxial capillaries set in a single block of ultra-high molecular weight polyethylene (UHMWPE) for the rapid formation of monodisperse double emulsions and polymersomes is described. The device utilizes easily interchangeable, coaxial capillaries whose geometry is maintained by a single UHMWPE block. Water-in-oil droplet formation using the device is characterized by measuring the diameter of the drops produced as a function of varying Reynolds, Weber, and capillary numbers. These parameters are used to characterize the dripping to jetting transition across a range of channel sizes. Double emulsions incorporating solutions of amphiphilic block copolymers are then processed to form large numbers of monodisperse polymersomes in an effective and controlled manner.

Hybrid Lattice Boltzmann/Dynamic Self-Consistent Field Simulations of Microphase Separation and Vesicle Formation in Block Copolymer Systems

We present a hybrid numerical method to introduce hydrodynamics in dynamic self-consistent field (SCF) studies of inhomogeneous polymer systems. It solves a set of coupled dynamical equations: The Navier–Stokes equations for the fluid flow, and SCF-based convection-diffusion equations for the evolution of the local monomer compositions. The Navier–Stokes equations are simulated by the lattice Boltzmann method and the dynamic self-consistent field equations are solved by a finite difference scheme. Two applications are presented: First, we study microphase separation in symmetric and asymmetric block copolymer melts with various values of shear and bulk viscosities, comparing the results to those obtained with purely diffusive dynamics. Second, we investigate the effect of hydrodynamics on vesicle formation in amphiphilic block copolymer solutions. In agreement with previous studies, hydrodynamic interactions are found to have little effect on the microphase separation at early times, but they substantially accelerate the process of structure formation at later times. Furthermore, they also contribute to selecting the pathway of vesicle formation, favoring spherical intermediates over aspherical (disklike) ones.

Thermodynamically stable, size selective solubilization of carbon nanotubes in aqueous solutions of amphiphilic block copolymers

Two molecular modes of amphiphilic block copolymer-carbon nanotube interactions have been identified in the literature, one involving the adsorption of individual block copolymer molecules on the carbon nanotubes and the other involving the adsorption of multimolecular, spherical micelles. In both cases, the nature of stability imparted to the dispersion of carbon nanotubes in the aqueous medium is kinetic, controlled by the steric barrier imposed by the adsorbed individual block copolymer molecules or the adsorbed micelles. In this study, we propose another mode of molecular interaction, wherein the block copolymer molecules self-assemble around the nanotube, generating aggregates in which the nanotubes are solubilized. In this case, the resulting system is a thermodynamically stable nanocolloidal solution, similar to aqueous surfactant solutions, in contrast to the kinetically stabilized nanotube dispersions. To examine whether such solubilization of nanotubes is possible, we have constructed a simple phenomenological theory for the free energy change associated with solubilization and have performed illustrative numerical simulations based on the theory. The calculated results for the commercially available symmetric PEO-PPO-PEO triblock copolymers and for the PEO-PPO diblock copolymers having the same composition and molecular weight as the triblocks show that indeed the block copolymer molecules are capable of solubilizing the carbon nanotubes. While the block copolymers whose natural curvature is cylindrical are the best candidates to solubilize the nanotubes, other block copolymers whose natural curvature is spherical or lamellar, are also found capable of solubilizing the nanotubes. Most interestingly, the solubilization is found to be size specific suggesting that this can be developed into a practical method to fractionate carbon nanotubes by their diameter. These results are applicable to both single-walled and multiwalled carbon nanotubes and the general conclusions are valid also for other diblock and triblock copolymers.

Density functional simulations for self-assembling micellar structures in amphiphilic block copolymer solutions

Density functional simulations are performed to study various self-assembling micellar structures in amphiphilic block copolymer solutions. It was shown that onion structures (stacked multilayer structures) are formed when the hydrophilic interaction is weak. Simulation results imply that the onion structures are different from monolayer vesicle structures which is formed when the hydrophilic interaction is rather strong. The difference and similarity between the vesicle structures and onion structures, or between their kinetic formation pathways are discussed.

Spontaneous Formation of Complex Micelles from a Homogeneous Solution

We present an extensive computer simulation study of structure formation in amphiphilic block copolymer solutions after a quench from a homogeneous state. By using a mesoscopic field-based simulation method, we are able to access time scales in the range of a second. A "phase diagram" of final structures is mapped out as a function of the concentration and solvent philicity of the copolymers. A rich spectrum of structures is observed, ranging from spherical and rodlike micelles and vesicles to toroidal and net-cage micelles. The dynamical pathways leading to these structures are analyzed in detail, and possible ways to control the structures are discussed briefly.

Calculation of the Micellar Structure of Polymer Surfactant on the Basis of the Density Functional Theory

Amphiphilic block copolymer solutions form various micellar structures including micelles and vesicles. We applied the density functional theory for block copolymers which we have proposed to amphiphilic block copolymer systems. The 3 dimensional simulation for AB diblock copolymer solutions and AB diblock copolymer / A homopolymer blends has been done and it is shown that the spherical micelles, cylindrical micelles and spherical vesicles are formed. It is also shown that the phase diagram for AB diblock copolymer / A homopolymer blends qualitatively agrees with the phase diagram obtained by the experiment.

Mechanism of Gold Metal Ion Reduction, Nanoparticle Growth and Size Control in Aqueous Amphiphilic Block Copolymer Solutions at Ambient Conditions

Spontaneous formation and efficient stabilization of gold nanoparticles with an average diameter of 7 approximately 20 nm from hydrogen tetrachloroaureate(III) hydrate (HAuCl4.3H2O) were achieved in air-saturated aqueous poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymer solutions at ambient temperature in the absence of any other reducing agent. The particle formation mechanism is considered here on the basis of the block copolymer concentration dependence of absorption spectra, the time dependence (kinetics) of AuCl4- reduction, and the block copolymer concentration dependence of particle size. The effects of block copolymer characteristics such as molecular weight (MW), PEO block length, PPO block length, and critical micelle concentration (cmc) are explored by examining several PEO-PPO-PEO block copolymers. Our observations suggest that the formation of gold nanoparticles from AuCl4- comprises three main steps: (1) reduction of metal ions by block copolymer in solution, (2) absorption of block copolymer on gold clusters and reduction of metal ions on the surface of these gold clusters, and (3) growth of metal particles stabilized by block copolymers. While both PEO and PPO blocks contribute to the AuCl4- reduction (step 1), the PEO contribution appears to be dominant. In step 2, the adsorption of block copolymers on the surface of gold clusters takes place because of the amphiphilic character of the block copolymer (hydrophobicity of PPO). The much higher efficiency of particle formation attained in the PEO-PPO-PEO block copolymer systems as compared to PEO homopolymer systems can be attributed to the adsorption and growth processes (steps 2 and 3) facilitated by the block copolymers. The size of the gold nanoparticles produced is dictated by the above mechanism; the size increases with increasing reaction activity induced by the block copolymer overall molecular weight and is limited by adsorption due to the amphiphilic character of the block copolymers.

Study on the synthesis of spherulitic calcium carbonate composite in amphiphilic PS-b-PAA solution and its crystalline structure

Spherulitic calcium carbonate composite is synthesized in the solution of amphiphilic block copolymer PS-b-PAA. FE-SEM and XRD measurements show that the size of the particulates decrease with the augment of PS-b-PAA concentration, and the morphology as well as the crystalline structure are changed too. TG-DTA together with IR analysis is applied to investigate the thermal dynamic behavior of the composite. The results show that the composite is mainly composed of two phases, that is, the nanocrystalline calcium carbonate and the PS-b-PA–Ca composites. In DTA curves, PS phase decomposes first with a large heat release at the temperature range between 300 and 400 °C. However, the PAA chains have relatively higher thermal stability, probably due to the structural CaO bond, and decompose between 400 and 500 °C. The above studies on the morphology and crystal structure of the calcium carbonate and the thermal stability indicate that the micellization of amphiphilic block copolymers, the stretching of ionized PAA chains and the preferential induction of neighboring carboxyl group on the precipitation of calcium carbonate may cause the formation of the spherulitic composite.

The synthesis of spherical calcium carbonate composite in amphiphilic PS-b-PAA solution and its thermal dynamic characteristic

Spherical calcium carbonate composite is synthesized in the solution of amphiphilic block copolymer of polystyrene(PS) and poly(acrylic acid)(PAA). SEM and XRD measurements show that the diameter of the particulates decreases with the augment of the PS-b-PAA concentration, crystalline in the composite is calcite and its morphology as well as the structure is changed too. TG-DTA together with IR analysis is applied to investigating the thermal dynamic behavior of the composite. The results show that the composite is mainly composed of two phases, that is, the nano-crystalline calcium carbonate and the PS-b-PA-Ca composites. PS phase decomposes first with a large heat release at about 330 °C. However, the PAA chains have relatively high thermal stability, probably due to the structural Ca-O bond, and decomposes at above 400 °C. Matching opinions are used to explain the possible reasons for the regular as well as the particular characteristics of the composite corresponding to a certain copolymer concentration.

Characterization of aqueous micellar solutions of amphiphilic block copolymers of poly(acrylic acid) and polystyrene prepared via ATRP. Toward the control of the number of particles in emulsion polymerization

A series of diblock, triblock and star-block copolymers composed of polystyrene and poly(acrylic acid) were synthesized by ATRP. The structure of the copolymers, the size of the blocks and the composition were varied, keeping however a short polystyrene block and a poly(acrylic acid) content larger than 60 mol% to allow solubility in alkaline water. Their micellization was studied by static and dynamic light scattering and the influence of their structural characteristics on the aggregation number, N agg, was examined at low salt concentration and alkaline pH. It was shown that micelles were in thermodynamic equilibrium and that N agg followed the power law N agg∼ N A −0.9 N S 2 (with N A, the total number of acrylic acid units in the copolymer and N S, the total number of styrene units), that is characteristic of amphiphile micelles formed from strongly segregated block copolymers. Moreover, N agg was independent of salt concentration in the investigated range. The same copolymers were previously used as stabilizers in emulsion polymerization [Macromolecules 34 (2001) 4439]. The final latex particle concentration, N p, was compared with N m, the initial micelle concentration. This enabled us to conclude that among the block copolymers studied, those with high acid content behaved like low molar mass surfactants. In contrast, those with low acid content formed stable micelles that could be directly nucleated to create latex particles, allowing a good control over N p.

Temperature-Induced Solubilization of Hydrocarbons in Aqueous Block Copolymer Solutions

In this study, the use of aqueous solutions of amphiphilic block copolymers as an extractant for hydrocarbons has been examined. The solubilization of naphthalene in poly(ethylene oxide)−poly(propy...