Synthesis and nanostructure of strong polyelectrolyte brushes in amphiphilic diblock copolymer monolayers on a water surface

We synthesized an ionic amphiphilic diblock copolymer, poly(hydrogenated isoprene)-b-poly(styrenesulfonic acid) (PIp-h2-b-PSS), by living anionic polymerization, and the nanostructure of its monolayer spread on a water surface was directly investigated by the in situ X-ray reflectivity technique. The monolayer of the diblock copolymer on a water surface had a smooth hydrophobic PIp-h2 layer on water and a "carpet"/polymer brush double layer in a hydrophilic sodium polystyrene sulfonate (PSSNa) layer under the water. The surface pressure dependence and PSSNa chain length dependence of the PIp-h2 layer thickness and the brush nanostructure were quantitatively studied. The effect of salt concentration in the subphase was also investigated in aqueous solutions containing 0-2 M NaCl. The salt effect on monolayer structure occurred at around 0.2 M. The thickness of the PSS brush layer decreased at salt concentrations above 0.2 M, while no structural change was observed below 0.2 M. This critical salt concentration is thought to be related to the balance of ionic concentrations inside the brush and in bulk solution.     

Critical brush density for the transition between carpet-only and carpet/brush double-layered structures

The critical brush density, where transition from carpet-only structure to carpet/brush double-layered structure occurs, was estimated for a weakly ionic amphiphilic diblock copolymer, (diethylsilacyclobutane)34-b-(methacrylic acid)50, monolayer on water by an in situ X-ray reflectivity technique. The brush density in the monolayer was controlled from 0.11 to 0.60 brush chain/nm2 by changing surface pressure and mixing a poly(diethylsilacyclobutane) homopolymer separately synthesized. Only a carpet layer was formed at a low brush density condition, but a carpet and brush double layer was found for a higher brush density of more than 0.48/nm2. This brush density, which is fairly high, would be valuable for discussing the polyelectrolyte brush nanostructure.     

Salt effect on the nanostructure of strong polyelectrolyte brushes in amphiphilic diblock copolymer monolayers on the water surface

The nanostructure of a spread monolayer of diblock copolymers of poly(hydrogenated isoprene) and poly(styrenesulfonate) at the air/water interface were studied by in situ X-ray reflectivity as a function of the brush density and salt concentration. When the monolayer was compressed beyond the "critical brush density", its nanostructure changed from a flat, adsorbed "carpet" layer to a "carpet + brush" structure. The critical brush density was found to be about 0.12 nm-2, independent the brush length and salt concentration under a low-salt condition. The brush formation behavior was considered to be controlled by an electrostatic interaction between polyelectrolyte chains rather than by a steric hindrance. This might be because the distance between the chains at the critical point is rather long and also because of the effect of the salt on the critical brush density. The critical brush density increased at higher added salt concentration beyond 1 M. As a result, we found a new structure transition behavior of the polymer brushes between carpet-only and carpet + brush structures, which was induced by salt addition. Finally, we succeeded in the controlled release of salt ions from the salted brush layer by changing the brush density by compression of the monolayer.     

Nanostructure of a "carpet"-like dense layer/polyelectrolyte brush layer in a block copolymer monolayer at the air-water interface

The "carpet"/brush double layer structure in the polyelectrolyte layer in the amphiphilic diblock copolymer monolayer at the air-water interface was quantitatively studied by in situ neutron reflectometry in addition to X-ray reflectivity measurements. As a result of the higher contrast between polyelectrolyte [poly(methacrylic acid)] and solvent (D(2)O) for the neutron, the brush structure could be estimated more accurately as a function of surface pressure, that is, brush density. The thickness of the carpet layer, which is thought to be formed to reduce the interfacial free energy between water and the hydrophobic layer, was almost constant at 10-20 A at any surface pressure studied. Growth was clearly observed in the whole brush length with increasing surface pressure, and it was estimated to be almost 60% of the full-stretch length of the ionic polymer chain. Furthermore, by the comparison of density profiles by neutron and X-ray reflectometry, an anomalous hydration was suggested.     

Effect of salt concentration on the nanostructure of weak polyacid brush in the amphiphilic polymer monolayer at the air/water interface

The effect of salt concentration on the nanostructure of a spread monolayer of ionic amphiphilic diblock copolymer, (diethylsilacyclobutane)m-b-(methacrylic acid)n, at the air-water interface was directly investigated by in situ X-ray reflectivity and neutron reflectivity techniques. Previously, we had found that a poly(methacrylic acid) (PMAA) hydrophilic layer under the water was not in the form of a simple polyelectrolyte brush but consisted of a dense carpet upper layer and a diffuse brush lower layer when the hydrophilic chain was long enough. Here we observed this double layer formation in the monolayer in aqueous NaCl solution at a constant surface pressure. The effect of salt added to the subphase differed with the salt concentrations, that is, below or above 0.1 M. In the presence of NaCl up to 0.1 M, both the hydrophobic layer and brush layer thicknesses decreased. On the other hand, both of them increased in the presence of NaCl above 0.1 M. Also, the carpet layer thickness was almost constant independent of the salt concentration. In addition, the brush top roughness showed a maximum in the presence of 0.1 M NaCl. The increase of the charge number on the PMAA chain and the screening effect of the Coulomb interaction by added salt ions were considered to be responsible for these phenomena.     

Hydrophilic chain length dependence of the ionic amphiphilic polymer monolayer structure at the air/water interface

Detailed analysis of an interesting poly(methacrylic acid) (PMAA) brush structure in water of a diblock copolymer [(Et(2)SB(m)-b-(MMA)(n), where Et(2)SB is diethylsilacyclobutane] monolayer reported previously was performed by X-ray and neutron reflectometry and indicated that the hydrophilic layer formed with a relatively long PMAA chain is not a simple layer but is divided into two layers, that is, a "carpet"-like dense PMAA layer near the hydrophobic layer and a polyelectrolyte brush layer. The hydrophilic chain length dependence of the diblock copolymer monolayer was analyzed using m:n = 30:x polymer samples, especially of the PMAA double layer structure. With the increase in PMAA chain length in polymer samples, a carpet layer is mainly formed up to n approximately 50. With further increase in the PMAA chain length beyond n approximately 50, a well-defined brush layer appears. On the other hand, the variation in hydrophobic layer thickness with methacrylic acid unit is minimum at the critical PMAA length, that is, n approximately 50 under a constant surface pressure condition. It is strongly suggested that the two discrete layers contribute differently to surface pressure. Furthermore, from the comprehensive viewpoint, the major factor for brush formation was clarified not to be the absolute length of the PMAA chain, but the ratio of PEt(2)SB and PMAA chain length is an important factor.     

Sphere-to-rod transition of non-surface-active amphiphilic diblock copolymer micelles: a small-angle neutron scattering study

Micellization behavior of amphiphilic diblock copolymers with strong acid groups, poly(hydrogenated isoprene)-block-poly(styrenesulfonate), was investigated by small-angle neutron scattering (SANS). We have reported previously (Kaewsaiha, P.; Matsumoto, K.; Matsuoka, H. Langmuir 2005, 21, 9938) that this strongly ionic amphiphilic diblock copolymer shows almost no surface activity but forms micelles in water. In this study, the size, shape, and internal structures of the micelles formed by these unique copolymers in aqueous solution were duly investigated. The SANS data were well described by the theoretical form factor of a core-shell model and the Pedersen core-corona model. The micellar shape strongly depends on the hydrophobic chain length of the block copolymer. The polymer with the shortest hydrophobic chain was suggested to form spherical micelles, whereas the scattering curves of the longer hydrophobic chain polymers showed a q-1 dependence, reflecting the formation of rodlike micelles. Furthermore, the addition of salt at high concentration also induced the sphere-to-rod transition in micellar shape as a result of the shielding effect of electrostatic repulsion. The corona thickness was almost constant up to the critical salt concentration (around 0.2 M) and then decreased with further increases in salt concentration, which is in qualitatively agreement with existing theories. The spherical/rodlike micelle ratio was also constant up to the critical salt concentration and then decreased. The micelle size and shape of this unique polymer could be described by the common concept of the packing parameter, but the anomalously stable nature of the micelle (up to 1 M NaCl) is a special characteristic.     

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.     

Carpetlike dense‐layer formation in a polyelectrolyte brush at the air/water interface

A carpetlike dense‐layer formation between a hydrophobic layer and a polyelectrolyte brush layer has been found in the monolayers of an ionic amphiphilic diblock copolymer, poly(1,1‐diethylsilacyclobutane)_m‐block‐poly(methacrylic acid)_n, on a water surface by an X‐ray reflectivity technique. By detailed analysis, we have found that the hydrophilic layer under the water is not a simple layer but is divided into two layers, that is, a carpetlike dense methacrylic acid (MAA) layer near the hydrophobic layer and a polyelectrolyte brush layer. We have also confirmed that a well‐established polyelectrolyte brush is formed only for the m:n = 43:81 polymer monolayer: For m:n = 40:10 and m:n = 45:60 polymer monolayers, only a dense MAA layer is formed. This dense‐layer formation should be the origin of the interesting hydrophobic‐layer thickness variation previously reported; The hydrophobic‐layer thickness takes a minimum as a function of the hydrophilic chain length at any surface pressure studied. An overview of the data for three samples with different chain lengths (m:n = 40:10, 45:60, or 43:81) has shown that the thickness of this dense layer is 10–20 Å and is independent of the surface pressure and polymerization degree of poly(methacrylic acid) (PMAA) in the range studied. This dense‐layer formation is explained by the reasonable speculation that contact with PMAA is thermodynamically more stable than direct contact with water for the diethylsilacyclobutane (Et₂SB) layer on water. In this sense, the dense layer acts like a carpet for the hydrophobic Et₂SB layer, and a 10–20‐Å thickness could be a critical value for the carpet. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1921–1928, 2003     

pH-responsive non-surface-active/surface-active transition of weakly ionic amphiphilic diblock copolymers

The weakly ionic amphiphilic diblock copolymer polystyrene-b-poly(acrylic acid) was synthesized by nitroxy radical-mediated living radical polymerization with precise control of block length, block ratio, and polydispersity. Systematical surface tension experiments and foam formation observations revealed that this polymer was non-surface active under neutral and alkaline (pH 10) conditions, while it was surface active under an acidic condition (pH 3). This result supports our proposed origin of non-surface activity; the image charge repulsion at the air/water interface is essential in addition to very stable micelle formation in the bulk solution. At a higher pH (pH 12), the polymer showed slight surface activity since the added NaOH played a role as an added salt. The critical micelle concentration (cmc) was estimated by static light scattering. Cmc increased with increasing added salt (NaCl) concentration as was observed for other strongly ionic non-surface-active polymers. Hence, this trend is characteristic for non-surface-active polymers. The pH dependence of cmc was minimum at pH 8–10. Since the acrylic acid block is fully ionized under this condition, the strong image charge repulsion at this condition accelerated micelle formation at a low polymer concentration, which consequently decreased cmc. Micelles in bulk solution were confirmed by dynamic light scattering, and the salt concentration and pH dependencies of the hydrodynamic radius of the micelles were also estimated. The pH-responsive non-surface-active/surface-active transition observed in this study strongly supports the fact that the image charge repulsion is an essential factor for non-surface activity in addition to stable micelle formation in solution.     

Unusual salt stability in highly charged diblock co-polypeptide hydrogels

The stability and properties of dilute solution hydrogels, synthesized by transition metal mediated polymerization of amino acid N-carboxyanhydrides (NCAs), have been studied in deionized (DI) water as well as various ionic media. These hydrogels are diblock amphiphilic copolymers of hydrophilic, charged segments of poly(l-lysine HBr) or poly(l-glutamic acid sodium salt), and helical, hydrophobic segments of poly(l-leucine). While many of these samples are able to form strong gels in deionized water at polymer concentrations as low as 0.25 wt %, stability in salt or buffer solutions was found to be only achieved at moderately higher polymer concentrations ( approximately 3.0 wt %). We have adjusted relative copolymer compositions and molecular weights to optimize hydrogel strength and polymer solubility in salt concentrations up to 0.5 M NaCl, as well as in cell growth media and aqueous buffers of varying pH. These materials are unique since they do not collapse in high ionic strength media, even though gel formation is contingent upon the presence of highly charged polyelectrolyte segments. The remarkable properties of these hydrogels make them excellent candidates for use as scaffolds in biomedical applications, such as tissue regeneration.     

Physical deposition behavior of charged amphiphilic diblock copolymers: Effect of charge distribution and electric field

Coarse-grained molecular dynamics simulations are used to investigate physical deposition behavior of charged amphiphlic diblock copolymers. The effects of solvent selectivity, charge distribution in amphiphlic diblock copolymers, and electric field strength on deposition conformations are studied qualitatively. Flat amphiphilic bilayers, which consist of hydrophilic monolayer and hydrophobic brush, are formed by physical deposition of charged amphiphlic diblock copolymers in nonselective solvents. For physically deposited amphiphlic diblock copolymers in selective solvents, amphiphilic bilayers consist of disc-shaped hydrophilic monolayers and hydrophobic nanospheres are found. This study sheds light on the formation of various amphiphlic diblock copolymer deposition conformations in different solvents and interaction mechanism of different components. Furthermore, the evolution of physical deposition process of charged amphiphlic diblock copolymers layer offers new insight to the controlling of amphiphilic bilayer thickness, hydrophobic nanosphere size, and interface property of depositional amphiphlic diblock copolymers.     

Adsorption of the protein bovine serum albumin in a planar poly(acrylic acid) brush layer as measured by optical reflectometry

The adsorption of bovine serum albumin (BSA) in a planar poly(acrylic acid) (PAA) brush layer has been studied by fixed-angle optical reflectometry. The influence of polymer length, grafting density, and salt concentration is studied as a function of pH. The results are compared with predictions of an analytical polyelectrolyte brush model, which incorporates charge regulation and excluded volume interactions. A maximum in adsorption is found near the point of zero charge (pzc) of the protein. At the maximum, BSA accumulates in a PAA brush to at least 30 vol %. Substantial adsorption continues above the pzc, that is, in the pH range where a net negatively charged protein adsorbs into a negatively charged brush layer, up to a critical pH value. This critical pH value decreases with increasing ionic strength. The adsorbed amount increases strongly with both increasing PAA chain length and increasing grafting density. Experimental data compare well with the analytical model without having to include a nonhomogeneous charge distribution on the protein surface. Instead, charge regulation, which implies that the protein adjusts its charge due to the negative electrostatic potential in the brush, plays an important role in the interpretation of the adsorbed amounts. Together with nonelectrostatic interactions, it explains the significant protein adsorption above the pzc.     

High‐Density Liquid‐Crystalline Polymer Brushes Formed by Surface Segregation and Self‐Assembly

High‐density polymer brushes on substrates exhibit unique properties and functions stemming from the extended conformations due to the surface constraint. To date, such chain organizations have been mostly attained by synthetic strategies of surface‐initiated living polymerization. We show herein a new method to prepare a high‐density polymer brush architecture using surface segregation and self‐assembly of diblock copolymers containing a side‐chain liquid‐crystalline polymer (SCLCP). The surface segregation is attained from a film of an amorphous base polymer (polystyrene, PS) containing a minor amount of a SCLCP‐PS diblock copolymer upon annealing above the glass‐transition temperature. The polystyrene portion of the diblock copolymer can work as a laterally mobile anchor for the favorable self‐assembly on the polystyrene base film.     

Electrolyte-induced collapse of a polyelectrolyte brush

We have investigated the electrolyte-induced collapse of a polyelectrolyte brush covalently attached to a planar solid surface. Positively charged poly-4-vinyl [N-methyl-pyridinium] (MePVP) brushes were prepared in situ at the surface by free radical chain polymerization using a surface-immobilized initiator monolayer ("grafting from" technique) and 4-vinylpyridine as the monomer, followed by a polymer-analogous quaternization reaction. The height of the brushes was measured as a function of the external salt concentration via multiple-angle null ellipsometry. As predicted by mean-field theory, the height of the MePVP brushes remains unaffected by the addition of low amounts of external salt. At higher salt concentrations the brush height decreases. The extent to which the brush shrinks strongly depends on the nature of the salt present in the environment. MePVP brushes collapse to almost the dry layer thickness upon the addition of potassium iodide to a contacting aqueous medium. In contrast, the collapse of MePVP brushes having bromide or chloride counterions is much less pronounced. These brushes remain in a highly swollen state even after large amounts of salt have been added to the solution.     

Ionic effects in collapse of polyelectrolyte brushes

We investigated the effect of counterion valence on the structure and swelling behavior of polyelectrolyte brushes using a nonlocal density functional theory that accounts for the excluded-volume effects of all ionic species and intrachain and electrostatic correlations. It was shown that charge correlation in the presence of multivalent counterions results in collapse of a polyelectrolyte brush at an intermediate polyion grafting density. At high grafting density, the brush reswells in a way similar to that in a monovalent ionic solution. In the presence of multivalent counterions, the nonmonotonic swelling of a polyelectrolyte brush in response to the increase of the grafting density can be attributed to a competition of the counterion-mediated electrostatic attraction between polyions with the excluded-volume effect of all ionic species. While a polyelectrolyte brush exhibits an "osmotic brush" regime at low salt concentration and a "salted brush" regime at high salt concentration regardless of the counterion valence, we found a smoother transition as the valence of the counterions increases. As observed in recent experiments, a quasi-power-law dependence of the brush thickness on the concentration ratio can be identified when the monovalent counterions are replaced with trivalent counterions at a fixed ionic strength.     

The structure and phase transitions in polymer blends,diblock copolymers and liquid crystalline polymers: The Landau-Ginzburg approach

The polymer systems are discussed in the framework of the Landau-Ginzburg model. The model is derived from the mesoscopic Edwards Hamiltonian via the conditional partition function. We discuss flexible, semiflexible and rigid polymers. The following systems are studied: polymer blends, flexible diblock and multi-block copolymer melts, random copolymer melts, ring polymers, rigid-flexible diblock copolymer melts, mixtures of copolymers and homopolymers and mixtures of liquid crystalline polymers. Three methods are used to study the systems: mean-field model, self consistent one-loop approximation and self consistent field theory. The following problems are studied and discussed: the phase diagrams, scattering intensities and correlation functions, single chain statistics and behavior of single chains close to critical points, fluctuations induced shift of phase boundaries. In particular we shall discuss shrinking of the polymer chains close to the critical point in polymer blends, size of the Ginzburg region in polymer blends and shift of the critical temperature. In the rigid-flexible diblock copolymers we shall discuss the density nematic order parameter correlation function. The correlation functions in this system are found to oscillate with the characteristic period equal to the length of the rigid part of the diblock copolymer. The density and nematic order parameter measured along the given direction are anticorrelated. In the flexible diblock copolymer system we shall discuss various phases including the double diamond and gyroid structures. The single chain statistics in the disordered phase of a flexible diblock copolymer system is shown to deviate from the Gaussian statistics due to fluctuations. In the one loop approximation one shows that the diblock copolymer chain is stretched in the point where two incompatible blocks meet but also that each block shrinks close to the microphase separation transition. The stretching outweights shrinking and the net result is the increase of the radius of gyration above the Gaussian value. Certain properties of homopolymer/copolymer systems are discussed. Diblock copolymers solubilize two incompatible homopolymers by forming a monolayer interface between them. The interface has a positive saddle splay modulus which means that the interfaces in the disordered phase should be characterized by a negative Gaussian curvature. We also show that in such a mixture the Lifshitz tricritical point is encountered. The properties of this unusual point are presented. The Lifshitz, equimaxima and disorder lines are shown to provide a useful tool for studying local ordering in polymer mixtures. In the liquid crystalline mixtures the isotropic nematic phase transition is discussed. We concentrate on static, equilibrium properties of the polymer systems.     

Micellization of non-surface-active diblock copolymers in water. Special characteristics of poly(styrene)-block-poly(styrenesulfonate)

Strongly ionized amphiphilic diblock copolymers of poly(styrene)-b-poly(styrenesulfonate) with various hydrophilic and hydrophobic chain lengths were synthesized by living radical polymerization, and their properties and self-assembling behavior were systematically investigated by surface tension measurement, foam formation, hydrophobic dye solubilization, X-ray reflectivity, dynamic light scattering, small-angle neutron scattering, small-angle X-ray scattering, and atomic force microscope techniques. These copolymer solutions in pure water did not show a decrease of surface tension with increasing polymer concentration. The solutions also did not show foam formation, and no adsorption at the air/water interface was confirmed by reflectivity experiments. However, in 0.5 M NaCl aq solutions polymer adsorption and foam formation were observed. The critical micelle concentration (cmc) was observed by the dye solubilization experiment in both the solutions with and without added salt, and by dynamic light scattering we confirmed the existence of polymer micelles in solution, even though there was no adsorption of polymer molecules at the water surface in the solution without salt. By the small-angle scattering technique, we confirmed that the micelles have a well-defined core-shell structure and their sizes were 100-150 A depending on the hydrophobic and hydrophilic chain length ratio. The micelle size and shape were unaffected by addition of up to 0.5 M salt. The absence of polymer adsorption at the water surface with micelle formation in a bulk solution, which is now known as a universal characteristic for strongly ionized amphiphilic block copolymers, was attributed to the image charge effect at the air/water interface due to the many charges on the hydrophilic segment.     

Controlled supramolecular assembly of micelle-like gold nanoparticles in PS-b-P2VP diblock copolymers via hydrogen bonding

We report a facile strategy to synthesize amphiphilic gold (Au) nanoparticles functionalized with a multilayer, micelle-like structure consisting of a Au core, an inner hydroxylated polyisoprene (PIOH) layer, and an outer polystyrene shell (PS). Careful control of enthalpic interactions via a systematic variation of structural parameters, such as number of hydroxyl groups per ligand (N(OH)) and styrene repeating units (N(PS)) as well as areal chain density of ligands on the Au-core surface (Σ), enables precise control of the spatial distribution of these nanoparticles. This control was demonstrated in a lamellae-forming poly(styrene-b-2-vinylpyridine) (PS-b-P2VP) diblock copolymer matrix, where the favorable hydrogen-bonding interaction between hydroxyl groups in the PIOH inner shell and P2VP chains in the PS-b-P2VP diblock copolymer matrix, driving the nanoparticles to be segregated in P2VP domains, could be counter balanced by the enthalphic penalty of mixing of the PS outer brush with the P2VP domains. By varying N(OH), N(PS), and Σ, the nanoparticles could be positioned in the PS or P2VP domains or at the PS/P2VP interface. In addition, the effect of additives interfering with the hydrogen-bond formation between hydroxyl groups on Au nanoparticles and P2VP chains in a diblock copolymer matrix was investigated, and an interesting pea-pod-like segregation of Au nanoparticles in PS domains was observed.     

Double-polyelectrolyte, like-charged amphiphilic diblock copolymers: swollen structures and pH- and salt-dependent lyotropic behavior

We consider a symmetrical poly(styrene- stat-(acrylic acid))- block-poly(acrylic acid), i.e., PSAA- b-PAA, diblock copolymer, with a molar fraction phi AA = 0.42 of acrylic acid, in the more hydrophobic PSAA statistical first block. We investigate its structural behavior at constant concentration in water using small-angle neutron scattering (SANS) by varying (i) the ionization of its acrylic acid motives via the pH by adding NaOH and (ii) the ionic strength of the solution by increasing the NaCl salt concentration c S. We present the resulting morphological phase diagram {pH, c S}, in which we identified two different lamellar phases presenting a smectic long-range order at small-to-intermediate ionizations and a spherical phase with a liquid-like short-range order at larger ionization. In the low-ionization regime, the first lamellar phase comprises a water-free PSAA lamellar core surrounded by a dense poly(acrylic acid) brush swollen with water. Its mostly hydrophobic core still being glassy, this phase is unable to reorganize and is frozen in. A detailed analysis of the SANS data shows the osmotic nature of the polyelectrolyte brush, in which the Na+ counterions are confined so that local electroneutrality is satisfied. Above the pH at which the PSAA statistical block starts ionizing, the PSAA lamellar core melts. The second lamellar phase identified then comprises a PSAA core thinner than that of the frozen-in previous phase, implying a significant increase of the core/water interface and a decrease of the brush surface density. The transition from the first lamellar phase to the second one can be quantitatively shown to result from the balance between the two contributions: (i) the extra interfacial cost between the thinner core and water and (ii) the associated gain in entropy of mixing for the counterions confined inside the brush. At even higher ionization, the diblocks finally form spherical objects with a very small, pH-dependent aggregation number and reach an apparent onset of self-association. When the highest ionization investigated is reached, the cores of these final spherical core-shell objects are found to contain a significant amount of water. We thereby demonstrate that at constant concentration, pH, and ionic strength both trigger a transition from frozen to molten hydrophobic phases as well as unexpected morphological transitions.