Denis Bendejacq

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.

Interactions between Sulfobetaine-Based Polyzwitterions and Polyelectrolytes

We investigate the interactions between sulfobetaine-based polyzwitterions and polyelectrolytes, either positive or negative ones, i.e., poly(DADMAC)s and poly(AA)s. Three different sulfobetaine motifs denoted SPE, SPP, and SHPP have been considered, presenting slight chemical changes either in the function carrying the zwitterionic group or in the zwitterionic motif itself. All three poly(sulfobetaine)s normally present critical temperatures (T(c)) above which they become fully soluble. The association with polyelectrolytes directly affects the critical temperature in a highly nonmonotonic fashion as the mixture composition is varied. Thanks to layer-by-layer deposition in a reflectometric cell, we demonstrate that a selective attraction exists between polyzwitterions and polyelectrolytes, from which an association follows at a nanoscopic scale as demonstrated by small-angle X-ray scattering and atomic force microscopy. The association of polyzwitterions with polyelectrolytes, however, is site-specific since it exists only between positive polyelectrolytes (i.e., polycations) and polyzwitterions based on SPE or SPP motifs. The range in which the association affects the critical temperature, T(c), is found to largely depend on the molecular weights of both zwitterionic and cationic species. As a result, the complexation and the creation of a hybrid object, referred to as a complex, also depend on the same parameters. By varying the latter from a few thousands to several millions, we define rules for the existence of this complex. In particular, a minimum polyzwitterion molecular weight is needed to observe alterations of the critical temperatures and closure of the complexation cone. Finally, within a Flory-like approach, we consider the polyzwitterion/polyelectrolyte complex as an effective statistical copolymer, whose composition comprises a fraction phi(A) of excess zwitterionic motifs as the majority species and a fraction 1 - phi(A) of complex motifs. We thereby reduce a polymer/polymer/solvent ternary system to a copolymer/solvent binary one, an assumption valid within the limit of small additions of cationic species. The approach predicts the reciprocal critical temperature 1/T(c) to be quadratic in phi(A), which agrees very well with all experimental results, even for a large mismatch between the molecular weights of both species, and regardless of the zwitterionic motif, SPE or SPP.

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

Abstract.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.

One and two dimensional self-assembly of comb-like amphiphilic copolyelectrolytes in aqueous solution

The spontaneous self-assembly of a novel comb-like amphiphilic copolyelectrolyte in water was studied by small angle neutron scattering and freeze fracture transmission electron microscopy. The copolymer was synthesized by randomly grafting pendant quaternized alkylamine moieties to a poly(styrene) chain. The effects of the grafting density and the length of the alkyls (C12–C18) on the structure were investigated systematically. The polymers were soluble over a broad range of grafting densities (35–80 mol%) where they assembled either into lamellae (35–55 mol%) consisting of a double layer of polymers or cylinders (65–80 mol%) formed by bundling of several chains. At higher concentrations, electrostatic repulsion drove smectic and hexagonal ordering of, respectively, the lamellae and the rods. TEM images showed micro-domains of stacked lamellae that were in some cases closed upon themselves to form onion-like particles. Exposure of hydrophobic patches at the surface due to defects of the self-assembly led to the formation of bonds between lamellae or cylinders. As a consequence visco-elastic solutions or hydrogels were formed at higher concentrations.

Facile formulation of high density well-ordered nanoparticle–copolymer nanocomposites

Robust water-swollen lamellar phases of an amphiphilic diblock copolymer were used to organize 7 nm large cerium oxide nanoparticles (NPs) from aqueous dispersions into well-ordered nanocomposites. The pH of the mixture controls the amount of nanocrystals successfully incorporated, found as high as 12 vol% of the total composite sample.

Wetting properties of charged and uncharged polymeric coatings—effect of the osmotic pressure at the contact line

We study the wetting behaviour of water droplets on three types of thin water-soluble polymeric coatings spin-coated on a silicon wafer—(i) a neutral polymer, (ii) a cationic polyelectrolyte, and (iii) a zwitterionic polymer. We investigate the dynamical spreading of drops as well as their drying on these surfaces. The influence of a salt—added either in the water droplets or inside the coating—is investigated to better differentiate the three types of polymer. We find that the salt tends to promote spreading on charged polymeric coatings when it is added inside the coating, yet not when added in the drops. Salts inside a coating simply increase its surface tension, resulting in lower contact angles during the spreading motion of a drop. On the other hand, when drying starts on the neutral polymer, the drop starts receding (i.e. its radius decreases) immediately after its spreading motion has stopped, whereas for both charged polymers, it remains pinned on the substrate until the very end of drying. When an increasing amount of salt is incorporated inside the two types of charged coatings, the contact line tends to unpin faster. As charges and salts obviously have a strong impact on the pinning/receding of the contact line, we hypothesize that a contribution of osmotic nature drives the motion of the contact line of drying water drops on polymeric coatings. There is a competition between (i) the evaporation from the edge which favours recession and (ii) a flux of water resulting from the osmotic pressure imposed by the higher concentration of species at the edge of the drop. The osmotic pressure being high for charged polymers, water evaporating on a charged coating keeps flowing from the inside of a drop to the outer edge, and compensates for the fast evaporation in the edge: macroscopically, this keeps the apparent contact line at the same position so that it appears pinned as more and more dissolved polymer molecules are dragged toward the edge and accumulate in a ring like deposit. When a salt is added, the osmotic drive weakens and charged polymers eventually dry like neutral ones.

How does water wet a hydrosoluble substrate

Thin layers made of water soluble polymers are good candidates to improve the wettability of surfaces as they are theoretically expected to be totally wettable. Yet, in this work we show that the wetting of a droplet of a volatile solvent, water, advancing on such films strongly depends on the degree of coating hydration beyond the contact line. The phenomenon originates not from the diffusion of water molecules through the coating, but from the evaporation from the advancing drop itself and re-condensation beyond the contact line. The faster a droplet is travelling on a coating, the less extensive the phenomenon of evaporation/recondensation beyond the contact line: macroscopically, the more hydrophobic the coating will appear. Expectedly, the contact angle θ strongly depends on the coating thickness e and the droplets advancing velocity U, two parameters that regulate the solvent uptake. More precisely, θ depends on product eU, which we predicted theoretically in a previous paper and verify here experimentally. The practical outcome of these findings is that hydrophilizing a substrate with a hydrophilic, fully water-soluble polymer will be more or less successful depending both on its thickness and on whether the substrate is static or moving during its use.

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-transmission electron 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 particles.

A novel elastomer/liquid crystal polymer nanocomposite created in situ from controlled radical graft-polymerization

We present the original design and properties of a novel, all-polymer hierarchical composite comprising an elastomer and a fluorinated liquid crystal polymer. The material follows from the phase separation between the two highly incompatible components, occuring in situ during the controlled radical graft-polymerization of the liquid crystal monomer onto functionalized elastomeric chains. We demonstrate that the composite remarkably combines the mechanical properties, static and dynamic, of both an elastomer and a semi-crystalline polymer. We thereby illustrate how the careful choice of a graft can provide a simple rubbery material with original and antonymic functionalities. Here, the temperature-sensitive liquid crystal polymeric domains act as hard particulate fillers under their fusion temperature and liquid softening inclusions above, with fast reversible transitions from one state to another. A variety of composites combining physico-chemical features difficult to marry otherwise may be designed following this simple method.