François Tournilhac

Self-healing and thermoreversible rubber from supramolecular assembly

Rubbers exhibit enormous extensibility up to several hundred per cent, compared with a few per cent for ordinary solids, and have the ability to recover their original shape and dimensions on release of stress. Rubber elasticity is a property of macromolecules that are either covalently cross-linked or connected in a network by physical associations such as small glassy or crystalline domains, ionic aggregates or multiple hydrogen bonds. Covalent cross-links or strong physical associations prevent flow and creep. Here we design and synthesize molecules that associate together to form both chains and cross-links via hydrogen bonds. The system shows recoverable extensibility up to several hundred per cent and little creep under load. In striking contrast to conventional cross-linked or thermoreversible rubbers made of macromolecules, these systems, when broken or cut, can be simply repaired by bringing together fractured surfaces to self-heal at room temperature. Repaired samples recuperate their enormous extensibility. The process of breaking and healing can be repeated many times. These materials can be easily processed, re-used and recycled. Their unique self-repairing properties, the simplicity of their synthesis, their availability from renewable resources and the low cost of raw ingredients (fatty acids and urea) bode well for future applications.

Silica-Like Malleable Materials from Permanent Organic Networks

A polymer shows thermoset-like stability while displaying melt processability like that of a thermopolymer. Permanently cross-linked materials have outstanding mechanical properties and solvent resistance, but they cannot be processed and reshaped once synthesized. Non–cross-linked polymers and those with reversible cross-links are processable, but they are soluble. We designed epoxy networks that can rearrange their topology by exchange reactions without depolymerization and showed that they are insoluble and processable. Unlike organic compounds and polymers whose viscosity varies abruptly near the glass transition, these networks show Arrhenius-like gradual viscosity variations like those of vitreous silica. Like silica, the materials can be wrought and welded to make complex objects by local heating without the use of molds. The concept of a glass made by reversible topology freezing in epoxy networks can be readily scaled up for applications and generalized to other chemistries.

Metal-Catalyzed Transesterification for Healing and Assembling of Thermosets

Catalytic control of bond exchange reactions enables healing of cross-linked polymer materials under a wide range of conditions. The healing capability at high temperatures is demonstrated for epoxy-acid and epoxy-anhydride thermoset networks in the presence of transesterification catalysts. At lower temperatures, the exchange reactions are very sluggish, and the materials have properties of classical epoxy thermosets. Studies of model molecules confirmed that the healing kinetics is controlled by the transesterification reaction rate. The possibility of varying the catalyst concentration brings control and flexibility of welding and assembling of epoxy thermosets that do not exist for thermoplastics.

Activation and deactivation of self-healing in supramolecular rubbers

A remarkable self-healing property has been achieved recently with rubbers formed by a supramolecular network of oligomers. Here we explore this property through a tack-like experiment where two parts of supramolecular rubber are simply brought into contact and then taken apart. These experiments reveal that the self-adhesive strength of rubber surfaces is significantly enhanced by fracture or other damaging processes. The mechanical energy required to separate two fracture surfaces that were brought back into contact is about one order of magnitude larger than that for surfaces close to thermodynamic equilibrium. Moreover, we find that fracture faces stored apart at room temperature still self-heal after 12 h but that this self-healing can be fully deactivated within a couple of hours by annealing around 90 °C. More generally, these results provide useful quantitative data to investigate the intensity and kinetics of self-healing in these soft rubbers.

Demixing in simple fluids induced by electric field gradients

Phase separation in liquid mixtures is mainly controlled by temperature and pressure, but can also be influenced by gravitational, magnetic or electric fields. However, the weak coupling between such fields and concentration fluctuations limits this effect to extreme conditions. For example, mixing induced by uniform electric fields is detectable only at temperatures that are within a few hundredths of degree or less of the phase transition temperature of the system being studied. Here we predict and demonstrate that electric fields can control the phase separation behaviour of mixtures of simple liquids under more practical conditions, provided that the fields are non-uniform. By applying a voltage of 100 V across unevenly spaced electrodes about 50 µm apart, we can reversibly induce the demixing of paraffin and silicone oil at 1 K above the phase transition temperature of the mixture; when the field gradients are turned off, the mixture becomes homogeneous again. This direct control over phase separation behaviour depends on field intensity, with the electrode geometry determining the length-scale of the effect. We expect that this phenomenon will find a number of nanotechnological applications, particularly as it benefits from field gradients near small conducting objects.

Polyphilic molecules: Synthesis and mesomorphic properties of a four-block molecule

Abstract A four-block mesogen composed of three chemically different moieties is synthesized. Its mesomorphic properties are characterized by optical microscopy, differential thermal analysis and X-ray diffraction at small angles, demonstrating a smectic C structure between 92°C and 111°C. The notion of a polyphile is defined and the expected peculiarities of such compounds are emphasized.

Structural changes in block copolymers: coupling of electric field and mobile ions.

We argue that the presence of dissociated ions in block copolymers under electric fields can induce strong morphological changes and even lead to phase transitions. We investigate, in particular, diblock copolymers in the body centered cubic (bcc) phase. In pure dielectric materials (no free charges), a dielectric breakdown is expected to occur for large enough electric fields, preempting any structural phase transition. On the other hand, dissociated ions are predicted to induce a phase transition to a hexagonal array of cylinders, at fields of about 10 V/microm or even lower. The strength of this mechanism can be tuned by controlling the amount of free ions present.

Piezoelectric and pyroelectric properties of new polysiloxane smectic C∗ elastomers

Abstract We investigated the pyroelectric and piezoelectric effects of new smectic C ∗ liquid crystalline elastomers. The materials are made up of a flexible polysiloxane backbone bearing chiral mesogenic side groups and are cross-linked via alkylene chains. The polar structure of the mesophase is evidenced by the observation of piezoelectric and pyroelectric effects. Integration of the pyroelectric coefficient over the whole smectic C ∗ stability range allows the determination of the spontaneous polarization ( P S = 3–6 nC/cm 2 ). Due to the rubber elasticity, these materials exhibit a high piezoelectric response. The piezosignal at room temperature does not vanish after annealing at temperatures even much higher than the Curie point. The piezoelectric coefficient d 31 ranges from 0.5 pC/N for non-poled samples to 2.5 pC/N after application of a poling field.

Supramolecular Thermoplastic with 0.5 Pa·s Melt Viscosity

Design of materials with polymer-like properties at service temperature but able to flow like simple liquids when heated remains one of the important challenges of supramolecular chemistry. Combining these antagonistic properties is highly desirable to provide durability, processability, and recyclability of materials. Here, we explore a new strategy based on polycondensation reactions to design supramolecular polymer materials with stress at break above 10 MPa and melt viscosity lower than 1 Pa·s. We report the synthesis and rheological and mechanical properties (uniaxial tensile tests) of supramolecular polymers based on a multiblock polyamide architecture. The flexibility of polycondensation reactions made it possible to control the molecular size distribution, the strength of hydrogen bonds, and the crystallization of middle and end groups and to achieve targeted properties.

Structural properties of polyphilic mesogens. Toward longitudinal ferroelectricity

Abstract The segregation between unlike chemical species can be used to induce a polar symmetry. We synthesised a polyphilic molecule for this purpose. A steric model of its SC phase is proposed.