Dominique Hourdet

Synthesis of thermoassociative copolymers

Abstract The synthesis of thermoassociative water-soluble copolymers, with polyether as pendant chains, was developed by macromonomer copolymerization and grafting techniques. The discussion was specially focused on the coupling reactions, between ω-methyl α-aminopolyethers and carboxylic units borne on the backbone, which provide in quantitative yield well-defined grafted copolymer structures. According to the thermodynamical properties of poly(ethylene oxide) or poly(propylene oxide) side-chains (lower critical solution temperature), aqueous solutions of grafted copolymers exhibit a microphase separation of polyether grafts upon heating and generate, above C ∗ , thermothickening properties of the solutions. The general basis of this behaviour is discussed, and the roles played by structural and external parameters highlighted.

Reversible thermothickening of aqueous polymer solutions

A thermally induced associative polymer was developed by grafting lower critical solution temperature side chains onto a water-soluble backbone. Based upon the thermodynamic properties of the pendent chains, acting as reversible crosslinkers with varying temperature, such a system was prepared from a poly(ethylene oxide)-modified poly(acrylic acid) (PAA-g-PEO). In aqueous solution this polymer provides an enhancement of the viscosity on heating, the extent of which can be controlled by polymer concentration, grafting ratio and salt concentration.

Aqueous solution behavior of new thermoassociative polymers

A new kind of water-soluble polymer was obtained by grafting side chains, characterized by a phase separation on heating (Lower Critical Solution Temperature LCST), on a hydrosoluble backbone. For semidilute solutions, the side chains associate as the temperature exceeds a critical temperature (Tass), which is close to their LCST. Microdomains are formed which act like physical crosslinking units between the main chains, and an increase in the aqueous solution viscosity is observed. Systems based on 2-Acrylamido-2-methyl propane sulfonic acid (AMPS) backbone and polyethylene oxide (PEO) side chains were developed. Their rheological behavior in both dilute and semi-dilute states was studied by varying differents parameters such as polymer and salt concentrations, grafting ratio, etc. Fluorescence measurements indicate the formation of hydrophobic microdomains on heating, in agreement with the thickening properties of the solutions.

Reversible adhesion between a hydrogel and a polymer brush

We have developed a new experimental methodology to investigate the adhesive properties of hydrogels on solid surfaces under fully immersed conditions. The method, based on contact mechanics, provides time-resolved reproducible and quantitative data on the work of adhesion between a hydrogel at swelling equilibrium and a planar surface grafted with responsive brushes. We used poly(N,N-dimethylacrylamide) (PDMA) and polyacrylamide (PAM) as model gels and poly(acrylic acid) (PAA) as pH dependent polymer brush. The effect of pH, contact time and debonding velocity on adhesive interactions was specifically investigated. As expected from molecular interactions, we found that adhesion increased as the pH decreased and this was attributed to the formation of hydrogen bonds at the interface. Surprisingly, however, the buildup of adhesion increased slowly with the time of contact up to one hour and depended markedly on debonding velocity despite the very elastic nature of the hydrogels. Furthermore, the maximum pH where adhesion was observed was significantly higher for the couple PAM–PAA than for the couple PDMA–PAA, in contrast with the onset of molecular interactions in dilute solutions.

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.

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. The graft copolymers, CMG-g-PEPO and CMT-g-PEPO exhibited interesting thermo-associating behavior which was evidenced by the detailed rheological and fluorescence measurements. The visco-elastic properties (storage modulus, G; loss modulus, G) of the copolymer solutions were investigated using oscillatory shear experiments. The influence of salt and surfactant on the T(assoc) was also studied by rheology, where the phenomenon of Salting out and Salting in was observed for salt and surfactant, respectively, which can give an easy access to tunable properties of these copolymers. These thermo-associating polymers with biodegradable nature of CMG and CMT can have potential applications as smart injectables in controlled release technology and as thickeners in cosmetics and pharmaceutical formulations.

Recognition-mediated hydrogel swelling controlled by interaction with a negative thermoresponsive LCST polymer

Abstract 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 CBPQT4+, 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. The immersion of NaphtGel in a solution of poly(N‐isopropylacrylamide) with tetrathiafulvalene (TTF) end groups complexed with CBPQT4+ induced positive thermoresponsive behaviour. The LCST‐induced dethreading of the polymer‐based pseudorotaxane upon heating led to transfer of the CBPQT4+ host and a concomitant swelling of NaphtGel. Subsequent cooling led to reformation of the TTF‐based host–guest complexes in solution and contraction of the hydrogel.

Hydrophobically modified acrylamide-based hydrogels

The synthesis of acrylamide based hydrogels incorporating a small proportion of hydrophobic functional groups results in a hydrogel structure with improved mechanical properties. In particular increased mechanical toughness and tensile strength whilst maintaining a relatively high swelling ratio. A method of increasing toughness has been developed by introducing a number of long alkyl chain hydrophobic groups into the hydrophilic structure. These hydrophobic groups create domains of densely clustered polymer chains within the highly swollen hydrogel matrix. Single edged notch tensile tests demonstrate a significant increase in fracture energy when the hydrophobes are able to interact in aqueous solutions compared to gels swollen in organic solvents in which the hydrophobes act independently. Therefore it is speculated that these domains increase energy dissipation around the crack tip during fracture causing an increase in mechanical toughness.

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.