Effect of Salt on Self-Assembly in Charged Block Copolymer Micelles

Microphase separation in polyelectrolytic diblock copolymer melt: Weak segregation limit

The authors present a generalized theory of microphase separation for charged-neutral diblock copolymer melt. The stability limit of the disordered phase for salt-free melt has been calculated using random phase approximation (RPA) and self-consistent-field theory (SCFT). Explicit analytical free energy expressions for different classical ordered microstructures (lamellar, cylinder, and sphere) are presented. The authors demonstrate that the chemical mismatch required for the onset of microphase separation (chi*N) in charged-neutral diblock melt is higher and the period of ordered microstructures is lower than those for the corresponding neutral-neutral diblock system. Theoretical predictions on the period of ordered structures in terms of Coulomb electrostatic interaction strength, chain length, block length, and chemical mismatch between blocks are presented. SCFT has been used to go beyond the stability limit, where electrostatic potential and charge distribution are calculated self-consistently. Stability limits calculated using RPA are in perfect agreement with the corresponding SCFT calculations. Limiting laws for the stability limit and the period of ordered structures are presented and comparisons are made with an earlier theory. Also, transition boundaries between different morphologies have been investigated.

Structure formation in films of weakly charged block polyelectrolyte solutions

A mean-field dynamic density functional theory is used to describe a phase diagram of concentrated solutions of weakly charged flexible block polyelectrolytes in a film. Electrostatics is taken into account by applying the local electroneutrality constraint (the Donnan membrane equilibrium approach). In the Donnan limit it is assumed that a salt added to the solution perfectly screens long-range electrostatic interactions. The phase diagram of a solution of a triblock polyelectrolyte in a film as a function of the solvent concentration and the charge of the polyelectrolyte (solvophilic) block is calculated for a given film thickness. The phase behavior of the block polyelectrolyte film arises from the interplay between surface-induced alignment and the electrostatically-driven structure formation. The observed mesoscopic structures (lamellar, perforated lamellar, cylindrical, micellar, and mixed phases) are oriented parallel to the surfaces for the considered case of morphologies unfrustrated by the film thickness. Structures with connections between parallel layers (bicontinuous, etc.) are not formed. As a result of surface-induced ordering, the region of ordered phases in a film is wider than in bulk and the phase boundary between ordered and disordered phases is more diffuse. As in the case of unconfined block polyelectrolyte solution, the solution in a film does not follow the lyotropic sequence of phases of such a block copolymer upon increase in the charge of the polyelectrolyte block. Upon changing the charge of the solvophilic copolymer block, transformations of copolymer morphology take place via change in curvature of polymeric domains. Due to confinement of a polyelectrolyte film, no swelling of solvophilic domains is observed.

Polyelectrolyte stars in planar confinement

We employ monomer-resolved molecular dynamics simulations and theoretical considerations to analyze the conformations of multiarm polyelectrolyte stars close to planar, uncharged walls. We identify three mechanisms that contribute to the emergence of a repulsive star-wall force, namely, the confinement of the counterions that are trapped in the star interior, the increase in electrostatic energy due to confinement as well as a novel mechanism arising from the compression of the stiff polyelectrolyte rods approaching the wall. The latter is not present in the case of interaction between two polyelectrolyte stars and is a direct consequence of the impenetrable character of the planar wall.

Study on the Synthesis of Poly(diglycidyl maleate-co-stearyl methacrylate) and Morphology Conversion of Their Self-Assembly Systems

A novel polymer of poly(diglycidyl maleate-co-stearyl methacrylate) (P(DGMA-co-SMA)) was synthesized by reaction between poly(maleic anhydride-co-stearyl methacrylate) (P(MA-co-SMA)) and epichlorohydrin. The self-assembly behavior of the resultant copolymer was investigated. It was found that the spheral aggregates could converse to nanorods after being aged for 2.5 days and nanolines composed of the nanorods were obtained after being aged for an additional 5.5 days. The mechanism of their self-assembly behavior and morphology conversion of self-assembly systems is discussed.

Sodium chloride-induced self-assembly of microfibers from nanofiber components

Self-assembly of a nucleotide bolaamphiphile, terminated with two nucleotide moieties including thymine, deoxyribose, and phosphodiester at both ends of the C20 oligomethylene chain, in sodium chloride aqueous solutions has been studied by SEM, LSM, FE-SEM, micro-FTIR, and zeta-potential measurement. The self-assembly behavior of the nucleotide bolaamphiphile was strongly dependent on the concentration of sodium chloride added. The nucleotide bolaamphiphile was found to hierarchically self-assemble to form micrometer-sized fibers (microfibers) consisting of bundles of entangled nanometer-sized fibers (nanofibers) under certain conditions of sodium chloride concentration (approximately 44 mM). The zeta-potential measurement suggested that the surface-charge tuning of the nanofibers induce the hierarchical self-assembly of the microfibers.

Effect of NaCl and peptide concentration on the self-assembly of an ionic-complementary peptide EAK16-II

Previous work has examined the effects of such factors as pH and peptide concentration on the self-assembly of ionic-complementary peptides. This work focused on the effect of sodium chloride on the molecular self-assembly of an ionic-complementary peptide EAK16-II (AEAEAKAKAEAEAKAK). Surface tensions and dimensions of the self-assembled nanostructures were determined for a wide range of peptide and sodium chloride concentrations using axisymmetric drop shape analysis-profile (ADSA-P) and atomic force microscopy (AFM), respectively. The critical aggregation concentration, or critical self-assembly concentration (CSAC), of EAK16-II was not significantly affected by the presence of NaCl. However, the analysis of size variations in self-assembled nanostructures in response to changes in NaCl concentration indicated that the presence of NaCl does influence the resulting dimensions of the peptide nanostructures when the peptide concentration is below its CSAC. A critical NaCl concentration was identified at approximately 20mM, below which the equivalent radius of the peptide fibrils increased with increasing salt concentration, and above which the opposite response was observed. This critical NaCl concentration was further confirmed in the surface tension measurements, where the equilibrium surface tension and induction time of the peptide at low concentrations (<CSAC) decreased with increasing NaCl concentration up to approximately 20mM and a further increase caused the opposite trend.

Non-surface activity and micellization of ionic amphiphilic diblock copolymers in water. Hydrophobic chain length dependence and salt effect on surface activity and the critical micelle concentration

We reported previously (Macromolecules 2003, 36, 5321; Langmuir, 2004, 20, 7412) that amphiphilic diblock copolymers having polyelectrolytes as a hydrophilic segment show almost no surface activity but form micelles in water. In this study, to further investigate this curious and novel phenomenon in surface and interface science, we synthesized another water-soluble ionic amphiphilic diblock copolymer poly(hydrogenated isoprene)-b-sodium poly(styrenesulfonate) PIp-h2-b-PSSNa by living anionic polymerization. Several diblock copolymers with different hydrophobic chain lengths were synthesized and the adsorption behavior at the air/water interface was investigated using surface tension measurement and X-ray reflectivity. A dye-solubilization experiment was carried out to detect the micelle formation. We found that the polymers used in this study also formed micelles above a certain polymer concentration (cmc) without adsorption at the air-water interface under a no-salt condition. Hence, we further confirmed that this phenomenon is universal for amphiphilic ionic block copolymer although it is hard to believe from current surface and interface science. For polymers with long hydrophobic chains (more than three times in length to hydrophilic chain), and at a high salt concentration, a slight adsorption of polymer was observed at the air-water interface. Long hydrophobic chain polymers showed behavior "normal" for low molecular weight ionic surfactants with increasing salt concentration. Hence, the origin of this curious phenomenon might be the macroionic nature of the hydrophilic part. Dynamic light scattering analysis revealed that the hydrodynamic radius of the block copolymer micelle was not largely affected by the addition of salt. The hydrophobic chain length-cmc relationship was found to be unusual; some kind of transition point was found. Furthermore, very interestingly, the cmc of the block copolymer did not decrease with the increase in salt concentration, which is in clear contrast to the fact that cmc of usual ionic small surfactants decreases with increasing salt concentration (Corrin-Harkins law). These behaviors are thought to be the special, but universal, characteristics of ionic amphiphilic diblock copolymers, and the key factor is thought to be a balance between the repulsive force from the water surface by the image charge effect and the hydrophobic adsorption.

Reentrant morphological transitions in copolymer micelles with pH-sensitive corona

In contrast to self-assembled aggregates of conventional ionic (including polymeric) surfactants the equilibrium micelles of diblock copolymer with a pH-sensitive polyelectrolyte block can exhibit two inverse sequences of morphological transitions triggered by an increase in solution salinity. The direct sequence of the sphere-cylinder-lamella transitions is similar to that for the copolymer with a strongly dissociating ionic block and occurs at a high salt concentration in solution. The abnormal reversed sequence of the lamella-cylinder-sphere transitions is predicted to occur at relatively low ionic strength in solution. The origin of the reentrant transitions is coupling between aggregation and ionization in copolymer micelles.

Oil-in-water microemulsions stabilized by charged diblock copolymers

We present here oil-in-water microemulsions stabilized by charged diblock copolymers alone, along with their structural characterization by small-angle neutron scattering measurements. They consist of swollen spherical micelles containing small amounts of oil in their core, which is surrounded by a corona of stretched polyelectrolyte chains. Structural changes, including core size variations, are evidenced when using a cosurfactant, or upon addition of salt, through a contraction of the charged corona. Attempts to relate the micellar structure to the individual copolymer characteristics are also presented, and show that the size of the hydrophobic block mainly determines that of the micelles.

Spherical polyelectrolyte block copolymer micelles: structural change in presence of monovalent salt

Spherical polyelectrolyte block copolymer micelles were investigated as a function of added NaCl salt concentration using Small-Angle Neutron Scattering (SANS) and Light Scattering (LS). The micelles are formed by the self-association of charged-neutral copolymers made of a long deuterated polyelectrolyte moiety (NaPSS(d))251 and a short hydrophobic moiety (PEP)52. In presence of salt, the core shape and the aggregation number of the micelles are not affected. The hydrodynamic radius of the micelle is found to be identical to the radius of the whole micelle deduced from neutron scattering and thus the hydrodynamic radius is a valid measure of the corona thickness. At the lowest salt concentrations investigated the thickness of the corona, R(s), remains essentially constant and a contraction is observed above an added-salt concentration c(s) of 2 x 10(-2) M where this crossover concentration corresponds to the average ionic strength of the free counterions in the corona. The contraction takes place while maintaining a rod-like behavior of the chains at short scale and obeys to: R(s) approximately c(s)(-0.18). The exponent 0.18 suggests an electrostatic persistence length proportional to the Debye screening length.

Structure and phase behavior of polyelectrolyte star solutions

Using the recently developed effective interaction potentials between polyelectrolyte stars, we examine the structure and phase behavior of solutions of the same. The effective interaction is ultrasoft and density dependent, owing to the integration of the counterionic degrees of freedom. The latter contribute extensive volume terms that must be taken into account in drawing the phase diagram of the system. The structural behavior of the uniform fluid is characterized by anomalous structure factors, akin to those found previously for solutions of uncharged star polymers. The phase diagram of the system is very rich, featuring a fluid phase at low arm numbers of the stars, two reentrant melting regions, as well as a variety of crystal structures with unusual symmetry. The physical origin of these features can be traced back to the ultrasoft nature of the effective interaction potential.

Structure of hydrophobically and hydrogen-bonded complexes between amphiphilic copolymer and polyacid in water

We observed by SANS and NMR the structure of intermolecular complexes formed through hydrogen bonding and hydrophobic interactions between a polyacid and a neutral copolymer surfactant (PEO-PPO-PEO). The polyacid is perdeuterated and the contrast variation method enables us to measure separately the structure factor of each component in the complex and the cross structure factor as a function of the pH and the temperature. The evolution of NMR spectra and relaxation times with pH and temperature give a complementary insight into the inner structure of the aggregates. The interaction between the aggregates and the aggregation number of polyacid chains are governed by electrostatic effects; they depend on the polyacid charge and are insensitive to temperature. The number of copolymer chains which results from the cooperative action of hydrogen bonding and hydrophobic interactions is sensitive to both pH and temperature. The complexation preserves the micellar core-corona structure of the copolymer and shrinks the polyacid chains which adopt a compact structure. The non-dissociated polyacid sequences are bound to the PPO part of the copolymer forming the hydrophobic core of the complex, whereas the ionized polyacid sequences form with the PEO a stabilizing hydrated corona.

Complex coacervation core micelles. Colloidal stability and aggregation mechanism

Complex coacervation core micelles were prepared with various polyelectrolytes and oppositely charged diblock copolymers. The diblock copolymers consist of a charged block and a water-soluble neutral block. Our experimental technique was dynamic light scattering in combination with titrations. At mixing ratios where the excess charge of the polyelectrolyte mixture is approximately zero, micelles may be formed. The colloidal stability of these micelles depends on the block lengths of the diblock copolymers and the molecular weight of the homopolymers. In addition, the chemical nature of the corona blocks and nature of the ionic groups of the polyelectrolytes also influence the stability and aggregation mechanism. A corona block that is three times longer than the core block is a prerequisite for stable micelles. If this ratio is further increased, the molecular weight of the homopolymers as well as the type of the ionic groups starts to play a major role. With very asymmetric block length ratios, no micelles are formed. In addition, if the neutral block is too short, the polymeric mixture forms a macroscopic precipitate. With a constant core block, the aggregation number decreases with increasing corona block length, as is predicted by scaling models for polymeric micelles with a neutral corona.

Microphase separation of weakly charged block polyelectrolyte solutions: Donnan theory for dynamic polymer morphologies

A mean-field dynamic density functional theory for the phase behavior of concentrated weakly charged block polyelectrolyte solutions is developed, using the Donnan membrane equilibrium approach to account for electrostatic interactions. In this limit all long-range electrostatic interactions are canceled and the net charge density in any region on a coarse-grained scale is zero. The phase diagram of a model triblock polyelectrolyte in solution as a function of the charge of the solvophilic block and the solvent concentration is established. Different mesoscopic structures (lamellar, bicontinuous, hexagonal, micellar, and dispersed coexisting phases) are formed depending on the copolymer charge asymmetry. It is found that upon changing the charge of the solvophilic copolymer block the polyelectrolyte solution does not follow the lyotropic sequence of phases of this polymer. Upon increase in the charge of the solvophilic blocks, changes in copolymer morphology take place by means of change in curvature of polymeric domains.