Hydrophilicity-Hydrophobicity Transformation, Thermoresponsive Morphomechanics, and Crack Multifurcation Revealed by AIEgens in Mechanically Strong Hydrogels.

Biomimetic exploration of stimuli-responsive and crack-resistant hydrogels is of great academic and practical significance, although the rational design of tough hydrogels is limited by insufficient mechanism study due to the lack of imaging techniques to "see" hydrogels at mesoscale level. A series of composite hydrogels with compartmentalized thermal response is designed by incorporating aggregation- and polarity-sensitive fluorescent probes in a poly(N-isopropylacrylamide) (PNIPAM) network grafted with poly(N,N-dimethylacrylamide) side-chains. The fluorescence technique is explored as a powerful tool to directly visualize their hydrophilicity-hydrophobicity transformation and the composition-dependent microphase separation.

Molecular mechanism of abnormally large nonsoftening deformation in a tough hydrogel.

Tough soft materials usually show strain softening and inelastic deformation. Here, we study the molecular mechanism of abnormally large nonsoftening, quasi-linear but inelastic deformation in tough hydrogels made of hyperconnective physical network and linear polymers as molecular glues to the network. The interplay of hyperconnectivity of network and effective load transfer by molecular glues prevents stress concentration, which is revealed by an affine deformation of the network to the bulk deformation up to sample failure.

Thermally Triggered Injectable Underwater Adhesives.

A novel bioinspired underwater adhesive based on the injectable aqueous solution of a graft copolymer with a thermoresponsive backbone is reported, which turns into a sticky hydrogel just below body temperature. With this topology, the collapse of the backbones upon the thermal transition leads to the formation of a percolating network of strong hydrophobic domains. Similar to pressure-sensitive adhesives (PSAs), the hydrogel goes through fibrillation and extensive energy dissipation in large deformations, giving it an edge over conventional chemical hydrogels, which are typically elastic and inherently nonsticky.

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(-isopropylacrylamide) (PNIPAM) chains.

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(-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.

Dual Responsive Regulation of Host-Guest Complexation in Aqueous Media to Control Partial Release of the Host.

The regulation of the concentration of a wide range of small molecules is ubiquitous in biological systems because it enables them to adapt to the continuous changes in the environmental conditions. Herein, we report an aqueous synthetic system that provides an orchestrated, temperature and pH controlled regulation of the complexation between the cyclobis(paraquat-p-phenylene) host (BBox) and a 1,5-dialkyloxynaphthalene (DNP) guest attached to a well-defined dual responsive copolymer composed of N-isopropylacrylamide as thermoresponsive monomer and acrylic acid as pH-responsive monomer. Controlled, partial release of the BBox, enabling control over its concentration, is based on the tunable partial collapse of the copolymer.

Effect of responsive graft length on mechanical toughening and transparency in microphase-separated hydrogels.

Effective remote control of mechanical toughening can be achieved by using thermo-responsive grafts such as poly(N-isopropylacrylamide) (PNIPAm) in a hydrophilic covalently cross-linked polymer network. The weight ratio of PNIPAm grafts in the network may impart such a thermo-responsive mechanical reinforcement. Here, we show that the network topology - especially graft length - is likewise crucial.

Hydrophobic Hydrogels with Fruit-Like Structure and Functions.

Normally, a polymer network swells in a good solvent to form a gel but the gel shrinks in a poor solvent. Here, an abnormal phenomenon is reported: some hydrophobic gels significantly swell in water, reaching water content as high as 99.6 wt%.

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.

Cold and Hot Gelling of Alginate-graft-PNIPAM: a Schizophrenic Behavior Induced by Potassium Salts.

Recently, alginates (ALG) characterized by high mannuronic content (M blocks) have been shown to undergo a reversible sol/gel transition during cooling in the presence of potassium salts. Cold gelling takes place at low temperatures, just below 0 °C for a KCl concentration of 0.3 mol/kg, but the aggregation process can be easily shifted to higher temperatures by increasing the salt concentration.

Hydrogels with Dual Thermoresponsive Mechanical Performance.

Dual thermoresponsive chemical hydrogels, combining poly(N-isopropylacrylamide) side-chains within a poly(N-acryloylglycinamide) network, are designed following a simple and versatile procedure. These hydrogels exhibit two phase transitions both at low (upper critical solution temperature) and high (lower critical solution temperature) temperatures, thereby modifying their swelling, rheological, and mechanical properties. These novel thermo-schizophrenic hydrogels pave the way for the development of thermotoughening wet materials in a broad range of temperatures.

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 π-electron donor/acceptor complex: naphthalene (N)/cyclobis(paraquat-p-phenylene) (CBPQT = 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-CBPQT (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.

Recognition-Mediated Hydrogel Swelling Controlled by Interaction with a Negative Thermoresponsive LCST Polymer.

Most polymeric thermoresponsive hydrogels contract upon heating beyond the lower critical solution temperature (LCST) of the polymers used. Herein, we report a supramolecular hydrogel system that shows the opposite temperature dependence. When the non-thermosesponsive hydrogel NaphtGel, containing dialkoxynaphthalene guest molecules, becomes complexed with the tetra cationic macrocyclic host CBPQT , swelling occurred as a result of host-guest complex formation leading to charge repulsion between the host units, as well as an osmotic contribution of chloride counter-ions embedded in the network.

Thermoresponsive Toughening with Crack Bifurcation in Phase-Separated Hydrogels under Isochoric Conditions.

A novel mode of gel toughening displaying crack bifurcation is highlighted in phase-separated hydrogels. By exploring original covalent network topologies, phase-separated gels under isochoric conditions demonstrate advanced thermoresponsive mechanical properties: excellent fatigue resistance, self-healing, and remarkable fracture energies. Beyond the phase-transition temperature, the fracture proceeds by a systematic crack-bifurcation process, unreported so far in gels.

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

Nano-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.

Synthesis and characterization of PEPO grafted carboxymethyl guar and carboxymethyl tamarind as new thermo-associating polymers.

New thermo associating polymers were designed and synthesized by grafting amino terminated poly(ethylene oxide-co-propylene oxide) (PEPO) onto carboxymethyl guar (CMG) and carboxymethyl tamarind (CMT). The grafting was performed by coupling reaction between NH2 groups of PEPO and COOH groups of CMG and CMT using water-soluble EDC/NHS as coupling agents. The grafting efficiency and the temperature of thermo-association, T(assoc) in the copolymer were studied by NMR spectroscopy.

Probing pH-responsive interactions between polymer brushes and hydrogels by neutron reflectivity.

We investigated the effect of specific interactions on the structure of interfaces between a brush and a hydrogel on the polymer chain length scale. We used a model system for which the interactions between the brush and the gel are switchable. We synthesized weak polyelectrolyte brushes of poly(acrylic acid) and hydrogels of polyacrylamide and poly(N,N-dimethylacrylamide) which interact solely when the poly(acrylic acid) is mainly in its acidic form.

Nanoparticle solutions as adhesives for gels and biological tissues.

Adhesives are made of polymers because, unlike other materials, polymers ensure good contact between surfaces by covering asperities, and retard the fracture of adhesive joints by dissipating energy under stress. But using polymers to 'glue' together polymer gels is difficult, requiring chemical reactions, heating, pH changes, ultraviolet irradiation or an electric field. Here we show that strong, rapid adhesion between two hydrogels can be achieved at room temperature by spreading a droplet of a nanoparticle solution on one gel's surface and then bringing the other gel into contact with it.

Structure of surfaces and interfaces of poly(N,N-dimethylacrylamide) hydrogels.

We investigated the surface structure of hydrogels of poly(N,N-dimethylacrylamide) (PDMA) hydrogels synthesized and cross-linked simultaneously by redox free radical polymerization. We demonstrate the existence of a less cross-linked layer at the surface of the gel at least at two different length scales characterized by shear rheology and by neutron reflectivity, suggesting the existence of a gradient in cross-linking. The composition of the layer is shown to depend on the degree of hydrophobicity of the mold surface and is weaker for more hydrophobic molds.

Cyclodextrin polymer nanoassemblies: strategies for stability improvement.

The main goal of this work was to develop two strategies for stabilization of nanoassemblies made of β-cyclodextrin polymer and amphiphilic dextran associated through host-guest complexes. The first strategy was to coat the nanoassemblies with a dextran derivative bearing adamantyl anchoring groups and hydrophilic poly(ethylene oxide-co-propylene oxide) side chains to increase the steric repulsion between the nanoassemblies. The second strategy developed was to post-reticulate the nanoassemblies upon UV irradiation.

Strain induced clustering in polyelectrolyte hydrogels.

Systematic large strain compression measurements have been performed on polyelectrolyte hydrogels based on modified PAA crosslinked by bifunctional thiols. For compressive strains larger than a critical value depending on polymer concentration, we observed a significant hysteresis, strain-hardening and a stress plateau during unloading. This was attributed to strain-induced ionic clustering due to electrostatic interactions that can become attractive if chains are close enough to each other.

Responsive hybrid self-assemblies in aqueous media.

Responsive copolymers have been prepared by grafting onto a poly(acrylamide-co-sodium acrylate) backbone [PAM-co-PANa] poly(N-isopropylacrylamide) stickers [PNIPA] characterized by a lower critical solution temperature (LCST) in water. From adsorption isotherms and DSC studies performed on PNIPA/silica mixtures, it was shown that PNIPA chains irreversibly interact with silica particles and that at low coverage they partially lose their responsiveness with temperature. When PNIPA is grafted onto a PAM-co-PANa backbone, which has no specific attraction to silica surfaces (only electrostatic repulsions), their binding process remains very similar to the one analyzed with PNIPA chains alone.