Supramolecular polymer hydrogels induced by host-guest interactions with di-[cyclobis(paraquat-p-phenylene)] cross-linkers: from molecular complexation to viscoelastic properties

Supramolecular polymer networks have been designed on the basis of a -electron donor/acceptor complex: naphthalene (N)/cyclobis(paraquat-p-phenylene) (CBPQT4+=B). For this purpose, a copolymer of N,N-dimethylacrylamide P(DMA-N1), lightly decorated with 1 mol% of naphthalene pendant groups, has been studied in semi-dilute un-entangled solution in the presence of di-CBPQT4+ (BB) crosslinker type molecules. While calorimetric experiments demonstrate the quantitative binding between N and B groups up to 60 °C, the introduction of BB crosslinkers into the polymer solution gives rise to gel formation above the overlap concentration. From a comprehensive investigation of viscoelastic properties, performed at different concentrations, host/guest stoichiometric ratios and temperatures, the supramolecular hydrogels are shown to follow a Maxwellian behavior with a strong correlation of the plateau modulus and the relaxation time with the effective amount of interchain cross-linkers and their dissociation dynamics, respectively. The calculation of the dissociation rate constant of the supramolecular complex, by extrapolation of the relaxation time of the network back to the beginning of the gel regime, is discussed in the framework of theoretical and experimental works on associating polymers

Structure investigation of nanohybrid PDMA/silica hydrogels at rest and under uniaxial deformation

International audienceNano-hybrid hydrogels were prepared by cross-linking polymerization of N,N-dimethylacrylamide (DMA) within a dispersion of silica nano-particles. Working at constant polymer/water ratio, the mechanical properties of hydrogels can be finely tuned by changing either the level of covalent cross-linker and/or the amount of particles that act as physical cross-linkers through specific adsorption of PDMA chains. Whatever is the cross-linking ratio (from 0 to 1 mol%), the introduction of silica nano-particles dramatically improves the mechanical behavior of hydrogels with a concomitant increase of stiffness and nominal strain at failure. The physical interactions being reversible in nature, the dynamics of the adsorption/desorption process of PDMA chains directly controls the time-dependence of the mechanical properties. Small angle neutron scattering experiments, performed in contrast matching conditions, show that silica particles, which repel themselves at short range, remain randomly dispersed during the formation of the PDMA network. Although PDMA chains readily interact with silica particles, no significant variation of the polymer concentration was observed in the vicinity of silica surfaces. Together with the time dependence of physical interactions pointed out by mechanical analyses, this result is attributed to the moderate adsorption energy of PDMA chains with silica surfaces at pH 9. From 2D SANS experiments, it was shown that strain rapidly gives rise to a non affine deformation of the hybrid network with shearing due to the transverse compression of the particles. After loading at intermediate deformation, the particles recover their initial distribution due to the covalent network that is not damaged in these conditions. That is no longer true at high deformation where residual anisotropy is observed

Micelle formation, gelation and phase separation of amphiphilic multiblock copolymers

The phase behaviour of amphiphilic multiblock copolymers with a large number of blocks in semidilute solutions is studied by lattice Monte Carlo simulations. The influence on the resulting structures of the concentration, the solvent quality and the ratio of hydrophobic to hydrophilic monomers in the chains has been assessed explicitely. Several distinct regimes are put in evidence. For poorly substituted (mainly hydrophilic) copolymers formation of micelles is observed, either isolated or connected by the hydrophilic moieties, depending on concentration and chain length. For more highly substituted chains larger tubular hydrophobic structures appear which, at higher concentration, join to form extended hydrophobic cores. For both substitution ratios gelation is observed, but with a very different gel network structure. For the poorly substituted chains the gel consists of micelles cross-linked by hydrophilic blocks whereas for the highly substituted copolymers the extended hydrophobic cores form the gelling network. The interplay between gelation and phase separation clearly appears in the phase diagram. In particular, for poorly substituted copolymers and in a narrow concentration range, we observe a sol-gel transition followed by an inverse gel-sol transition when increasing the interaction energy. The simulation results are discussed in the context of the experimentally observed phase properties of methylcellulose, a hydrophobically substituted polysaccharide.Comment: 14 pages, 14 figures; Soft Matter (2011

pH/Temperature control of interpolymer complexation between poly(acrylic acid) and weak polybases in aqueous solutions

International audienc

Hybrid Complex Coacervate

Underwater adhesion represents a huge technological challenge as the presence of water compromises the performance of most commercially available adhesives. Inspired by natural organisms, we have designed an adhesive based on complex coacervation, a liquid-liquid phase separation phenomenon. A complex coacervate adhesive is formed by mixing oppositely charged polyelectrolytes bearing pendant thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) chains. The material fully sets underwater due to a change in the environmental conditions, namely temperature and ionic strength. In this work, we incorporate silica nanoparticles forming a hybrid complex coacervate and investigate the resulting mechanical properties. An enhancement of the mechanical properties is observed below the PNIPAM lower critical solution temperature (LCST): this is due to the formation of PNIPAM-silica junctions, which, after setting, contribute to a moderate increase in the moduli and in the adhesive properties only when applying an ionic strength gradient. By contrast, when raising the temperature above the LCST, the mechanical properties are dominated by the association of PNIPAM chains and the nanofiller incorporation leads to an increased heterogeneity with the formation of fracture planes at the interface between areas of different concentrations of nanoparticles, promoting earlier failure of the network-an unexpected and noteworthy consequence of this hybrid system.</p

Underwater Adhesion of Multiresponsive Complex Coacervates

International audienceMany marine organisms have developed adhesives that are able to bond under water, overcoming the challenges associated with wet adhesion. A key element in the processing of several natural underwater glues is complex coacervation, a liquid–liquid phase separation driven by complexation of oppositely charged macromolecules. Inspired by these examples, the development of a fully synthetic complex coacervate‐based adhesive is reported with an in situ setting mechanism, which can be triggered by a change in temperature and/or a change in ionic strength. The adhesive consists of a matrix of oppositely charged polyelectrolytes that are modified with thermoresponsive poly(N‐isopropylacrylamide) (PNIPAM) grafts. The adhesive, which initially starts out as a fluid complex coacervate with limited adhesion at room temperature and high ionic strength, transitions into a viscoelastic solid upon an increase in temperature and/or a decrease in the salt concentration of the environment. Consequently, the thermoresponsive chains self‐associate into hydrophobic domains and/or the polyelectrolyte matrix contracts, without inducing any macroscopic shrinking. The presence of PNIPAM favors energy dissipation by softening the material and by allowing crack blunting. The high work of adhesion, the gelation kinetics, and the easy tunability of the system make it a potential candidate for soft tissue adhesion in physiological environments

Enhancement of the Adhesive Properties by Optimizing the Water Content in PNIPAM-Functionalized Complex Coacervates

Most commercially available soft tissue glues offer poor performance in the human body. We have developed an injectable adhesive whose setting mechanism is activated by a change in environmental factors, i.e., temperature and/or ionic strength. The material and setting process are inspired by the adhesive processing mechanism observed in natural maritime glues. Complex coacervation, a liquid-liquid phase separation between oppositely charged polyelectrolytes, is thought to play an important role in the processing. Complex coacervates are characterized by a high water content, which inevitably weakens the glue. Here, we aim to increase the adhesive performance by systematically tuning the water content. Among the several strategies here explored, the most effective one is the mechanical removal of water using an extruder, resulting in an increase of work of adhesion by 1 order of magnitude compared to the original formulation

Tuning the Interactions in Multiresponsive Complex Coacervate-Based Underwater Adhesives

In this work, we report the systematic investigation of a multiresponsive complex coacervate-based underwater adhesive, obtained by combining polyelectrolyte domains and thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) units. This material exhibits a transition from liquid to solid but, differently from most reactive glues, is completely held together by non-covalent interactions, i.e., electrostatic and hydrophobic. Because the solidification results in a kinetically trapped morphology, the final mechanical properties strongly depend on the preparation conditions and on the surrounding environment. A systematic study is performed to assess the effect of ionic strength and of PNIPAM content on the thermal, rheological and adhesive properties. This study enables the optimization of polymer composition and environmental conditions for this underwater adhesive system. The best performance with a work of adhesion of 6.5 J/m2 was found for the complex coacervates prepared at high ionic strength (0.75 M NaCl) and at an optimal PNIPAM content around 30% mol/mol. The high ionic strength enables injectability, while the hydrated PNIPAM domains provide additional dissipation, without softening the material so much that it becomes too weak to resist detaching stress. © 2019 by the authors. Licensee MDPI, Basel, Switzerland

Study of sodium dodecyl sulfate/poly(propylene oxide) methacrylate mixed micelles for the synthesis of thermo-associative polymers by micellar polymerization

cited By 23International audienceWater soluble associative polymers based on polyacrylamide (PAM) grafted with short poly(propylene oxide) (PPO) chains as pendant groups were synthesized. Two different methods were used (1) homogeneous (water + acetonitrile) (2) micellar copolymerization, in the presence of sodium dodecyl sulphate (SDS) to solubilize the hydrophobic comonomer, PPO methacrylate (PPOMA). As expected from the literature, random or blocky distribution of PPO along the PAM chain, respectively, is obtained by these two methods. This work focuses on the characterization of the micellar reactional medium in order to have a good estimate of the length of the PPO sequences along the main chain. An extensive study of the mixture SDS/PPOMA was performed, by using fluorimetry, light scattering and conductimetry, to characterize the initial mixed micelles (critical micellar concentration and aggregation number). It is shown that the total aggregation number of the micelles and that of SDS decrease when the concentration of PPOMA increases. In addition, preliminary turbidimetry and viscosimetry studies of the thermo-associative properties of the two polymers are given. © 2002 Society of Chemical Industry

Thermoresponsive Complex Coacervate-Based Underwater Adhesive

Sandcastle worms have developed protein-based adhesives, which they use to construct protective tubes from sand grains and shell bits. A key element in the adhesive delivery is the formation of a fluidic complex coacervate phase. After delivery, the adhesive transforms into a solid upon an external trigger. In this work, a fully synthetic in situ setting adhesive based on complex coacervation is reported by mimicking the main features of the sandcastle worm's glue. The adhesive consists of oppositely charged polyelectrolytes grafted with thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) chains and starts out as a fluid complex coacervate that can be injected at room temperature. Upon increasing the temperature above the lower critical solution temperature of PNIPAM, the complex coacervate transitions into a nonflowing hydrogel while preserving its volume—the water content in the material stays constant. The adhesive functions in the presence of water and bonds to different surfaces regardless of their charge. This type of adhesive avoids many of the problems of current underwater adhesives and may be useful to bond biological tissues.</p