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, dibloc copolymer, with a molar fraction 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 cS. We present the resulting morphological phase diagram {pH, cS}, 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 liquidlike shortrange 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, pHdependent 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

Water-dispersed lamellar phases of symmetric poly(styrene)-block-poly(acrylic acid) diblock copolymers: Model systems for flat dense polyelectrolyte brushes

We investigate the static properties of a water-dispersed lamellar (L) phase formed in the melt state with a nearly symmetric poly(styrene)-block-poly(acrylic acid) (PS-b-PAA) diblock copolymer. The PAA brush is considered as a model flat polyelectrolyte (PE) brush of controlled surface density. Thanks to small-angle X-ray scattering, its behavior in water is studied as a function of (i) its ionization, through the pH of the dispersions which is increased by an addition of a known amount of a base, i.e. sodium hydroxyde NaOH, and (ii) in the presence of a monovalent salt, i.e. sodium chloride NaCl, of concentration C S . At low pH, we find that the brush effectively behaves as a neutral brush. At high pH, the brush is in the so-called “osmotic regime”, in which all sodium counterions are trapped within the brush volume and stretch the chains via an osmotic effect. The properties of such a brush in the presence of a monovalent salt, confirm this result, showing a C S -1/3 dependence in the brush height L O , in agreement with mean-field predictions. The L O -C S profiles at different ionizations give access to the actual brush internal charge fraction f. The results are found to be in very good quantitative agreement with experimental measures found in the literature, and can be completely and quantitatively described by Oosawa’s approach to counterion condensation in a semi-dilute to concentrated solution of charged, rod-like chains

Pearling instabilities in water-dispersed copolymer cylinders with charged brushes

We investigate the structural behavior of a poly(styrene)-block-poly(acrylic acid) diblock copolymer which forms hexagonally-packed PS cylinders (C-phase) in the melt state. The water dispersion of this structure provides hairy cylinders which comprise a PAA swollen cylindrical brush with a height h tunable via its degree of ionization and the ionic strength in the solution, and a water-free, PS cylindrical core of constant radius R C. Such system constitutes an “out-of-equilibrium” frustrated model system: the selective swelling of the PAA brush results in a frustration of the interface curvature, which the ratio h/R C allows to quantify. Upon heating at a temperature higher than the glass transition temperature of the PS core, the glassiness of the core is relieved and the mechanical constraints arising from the selective swelling of the structure can be relaxed: the cylinders undergo a cylinder-to-sphere transition upon annealing at high temperature, when above a frustration threshold h/R C ≈ 1.8. Thanks to a careful mapping of the transition diagram, an undulating cylindrical morphology (UC) is identified between unchanged cylinders ( h/R C $\leqslant$ 1.8) and spheres ( h/R C ⩾ 2.0), which appears to result from a Rayleigh-like pearling instability of the copolymer cylinders

Magneto-optical study of the orientation confinement of particles in ferrolyotropic systems

Ferrosmectics and ferrohexagonals are magnetic liquid crystals which consist of a lamellar and a hexagonal phase in which magnetic nanoparticles are incorporated. The magnetic field variation and the relaxation of the linear optical birefringence of ferrosmectics and ferrohexagonals and its time relaxation are measured in different geometries and compared with that of a ferrofluid sample. We interpret our data by the existence of a mean orientation of the magnetic particles in the lyotropic structure, which appears to be non-random in zero field. We conclude that the magnetic moments of the particles are preferentially aligned in the plane of the ferrosmectic layers and along the axis of the ferrohexagonal cylinders, respectively. To account for this preferred alignment, we propose that some of the particles in the ferrosmectic are adsorbed on the surfactant layers with their moment aligned in the lamellar plane, while the orientation of the moments in the ferrohexagonal is restricted to a cone around the cylinder axis

Optical properties of self-assembled plasmonic hyperbolic metasurfaces and metamaterials extracted by (Mueller matrix) spectroscopic ellipsometry

Hyperbolic metamaterials use the concept of controlling the propagative modes through the engineering of the dispersion relation, and are considered highly promising to reach different meta-properties. Spectroscopic Mueller Matrix Ellipsometry with variable angle of incidence and full azimuthal rotation of the sample is a powerful optical technique to characterize both anisotropic and bi-anisotropic materials. We here discuss the experimentally extracted uniaxial and biaxial optical properties of two self-assembled plasmonic systems that appear to have the appropriate meta-dispersion relations. The metasurface was produced by oblique incidence angle ion beam sputtering of glass followed by shadow deposition of Au [1]. The second bulk metamaterial was a block-copolymer based self-assembled hyperbolic metamaterial of nanocomposites based on metal nanoparticles embedded in a self-assembled anisotropic polymer host, presenting a strong spectrally selective optical anisotropy [2]. The extracted effective dielectric functions and the resulting dispersion relations are presented

Freeze-Fracture TEM Imaging of Robust Order in Swollen Phases of Amphiphilic Diblock Copolymers

We report on the structures exhibited by two different diblock poly(styrene)-b-poly(acrylic acid) (PS-b-PAA) copolymers in water, a selective solvent. Using a combination of X-ray scattering and freeze fracture-transmissionelectron microscopy (FF-TEM), we show that these structures can be widely swollen while retaining their initial morphology and a high degree of long-range order. The analysis of the FF-TEM pictures also evidences the presence of water crystallites of regular size and shape within the confined water domains. We relate the growth of these crystallites to the high local ionic strength of the water swelling the PAA brushes. Moreover, the confinement of the crystallites growth shows that the swollen phases have a very robust structure, potentially useful for confining colloidal particle