Phase Behavior of Polyelectrolyte Block Copolymers in Mixed Solvents

We have studied the phase behavior of the poly(n-butyl acrylate)-b-poly(acrylic acid) block copolymer in a mixture of two miscible solvents, water and tetrahydrofuran (THF). The techniques used to examine the different polymers, structures and phases formed in mixed solvents were static and dynamic light scattering, small-angle neutron scattering, nuclear magnetic resonance and fluorescence microscopy. By lowering the water/THF mixing ratio X, the sequence unimers, micron-sized droplets, polymeric micelles was observed. The transition between unimers and the micron-sized droplets occurred at X = 0.75, whereas the microstructuration into core-shell polymeric micelles was effective below X = 0.4. At intermediate mixing ratios, a coexistence between the micron-sized droplets and the polymeric micelles was observed. Combining the different aforementioned techniques, it was concluded that the droplet dispersion resulted from a solvent partitioning that was induced by the hydrophobic blocks. Comparison of poly(n-butyl acrylate) homopolymers and poly(n-butyl acrylate)-b-poly(acrylic acid) block copolymers suggested that the droplets were rich in THF and concentrated in copolymers and that they were stabilized by the hydrophilic poly(acrylic acid) moieties.Comment: 11 pages, 12 figures, to appear in Macromolecule

Polymer-Nanoparticle Complexes : from Dilute Solution to Solid State

We report on the formation and the structural properties of supermicellar aggregates also called electrostatic complexes, made from mineral nanoparticles and polyelectrolyte-neutral block copolymers in aqueous solutions. The mineral particles put under scrutiny are ultra-fine and positively charged yttrium hydroxyacetate nanoparticles. Combining light, neutron and x-ray scattering experiments, we have characterized the sizes and the aggregation numbers of the organic-inorganic complexes. We have found that the hybrid aggregates have typical sizes in the range 100 nm and exhibit a remarkable colloidal stability with respect to ionic strength and concentration variations. Solid films with thicknesses up to several hundreds of micrometers were cast from solutions, resulting in a bulk polymer matrix in which nanoparticle clusters are dispersed and immobilized. It was found in addition that the structure of the complexes remains practically unchanged during film casting.Comment: 18 pages, 11 figures, 2 table

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

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

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

How a coating is hydrated ahead of the advancing contact line of a volatile solvent droplet

The wettability of coatings is very sensitive to the amount of solvent they may contain. When a droplet of volatile solvent, such as water, is deposited on a substrate, its vapor may quickly condensate just ahead of the contact line. We give an estimation of the extent of solvent uptake by a coating of variable thickness e , in front of an advancing contact line of given velocity U . Depending on the values of e and U , we observe three regimes: at low velocity and for a thin coating, the coating adsorbs a fraction of solvent that can quickly equilibrate across its entire thickness, so that it mainly appears solvophilic, while this is not the case for a thick coating. For high velocities, regardless the coating thickness, the coating ahead of the contact line does not have enough time to adsorb a significant amount of solvent, so that it mainly appears solvophobic. All these phenomena appear to be controlled by a molecular cut-off length

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