Vitrimers

Vitrimers are a class of plastics, which are derived from thermosetting polymers (thermosets) and are very similar to them. Vitrimers consist of molecular, covalent networks, which can change their topology by thermally activated bond-exchange reactions. At high temperatures they can flow like viscoelastic liquids, at low temperatures the bond-exchange reactions are immeasurably slow (frozen) and the Vitrimers behave like classical thermosets at this point.

The first Vitrimer was created by the French researcher Ludwik Leibler, head of laboratory at CNRS, France's national research institute.^[1]

Vitrimers are strong glass formers.

Their behavior opens new possibilities in the application of thermosets like self-healing or simple processibility in a wide temperature range.^[2][3][4]

Polymer networks other than epoxy resins based on diglycidyl ether of bisphenol A have been used in vitrimer technology, such as polylactic acid (polylactide),^[3] polyhydroxyurethanes,^[4] epoxidized soybean oil with citric acid,^[5] and polybutadiene.^[6]

Background and significance

Thermoplastics are easy to process, but also easy to corrode by chemicals and mechanical stress. The reverse is true of thermosets. Thermoplastics are made of covalent bond molecule chains, which are connected by weak interactions (e.g., van der Waals forces).

Thus they can be easily processed by melting (or in some cases also from solution), but are also susceptible against appropriate solvents creep under constant load. Thermoplastics can be deformed reversibly above their glass-transition temperature or their crystalline melting point and be processed by extrusion, injection molding, and welding. Thermosets, however, are made of molecular chains, which are interconnected by covalent bonds to a stable network.

Thus, they have outstanding mechanical properties and thermal and chemical resistance. They are an indispensable part of structural components in automotive and aircraft industry. Due to their irreversible linking by covalent bonds molding is not possible anymore as soon as the polymerization is completed. They must be polymerized thus in the desired shape, which is time-consuming, restricts the shape and is responsible for their high price.^[7]

If there was a way to create reversible covalent bonds, this would combine high processability, repairability, and performance. There have been already many strategies tried that would allow such plastics. Vitrimers combine the desirable properties of both classes: they show the mechanical and thermal properties of thermosets and can be also molded under the influence of heat. Vitrimers can be welded like (silicon) glasses or metals. Welding by simple heating allows the creation of complex objects. Vitrimere like (silicon) glasses or metals are welded. A weld by simply heating allows customizing complex objects. Vitrimers could thus be a new and promising class of materials with many uses.^[8]

Functional principle

Glass and glass former

If the melt on an (organic) amorphous polymer is cooled down, it solidifies at the glass-transition temperature T_v. On cooling, the hardness of the polymer increases in the neighborhood of T_v by several orders of magnitude. This hardening follows the Williams-Landel-Ferry equation, not the Arrhenius equation. Organic polymers are thus called fragile glass formers. Silicon glass (e.g., window glass), is in contrast labelled as a strong glass former. Its viscosity changes only very slowly in the vicinity of the glass-transition point T_v and follows the Arrhenius law. Only via this gradual change in viscosity is glassblowing possible. If one would try to shape an organic polymer in the same manner as a glass, it would at first firmly and fully liquefy near T_v already at slightly higher temperatures. For a theoretical glassblowing of organic polymers, the temperature must be controlled very precisely.

Until 2010, no organic strong glass formers were known. Strong glass formers can be shaped in the same way as glass (silicon dioxide) can be. Vitrimers are the first such material discovered.

Effect of transesterification and temperature influence

The research group led by Ludwik Leibler demonstrated the operating principle of vitrimers at the example of epoxy thermosets. Epoxy thermosets can be represented as vitrimers, when transesterification reactions can be introduced and controlled. In the studied system as hardeners must be used carboxylic acids or carboxylic acid anhydrides.^[8] A topology change is possible by transesterification reactions. These transesterification reactions do not affect the number of links or the (average) functionality of the polymer. By that the polymer can flow like a viscoelastic liquid at high temperatures. When the temperature is lowered, the transesterification reactions are slowed down, until they finally freeze (be immeasurably slow). Below this point vitrimers behave like normal, classical thermosets. The shown case-study polymers did offered a elastic modulus of 1 MPa to 100 MPa, depending on the bonding network density.

Applications

There are many uses imaginable on this basis. A surfboard of vitrimers could be brought into a new shape, scratches on a car body could be cured and cross-linked plastic or synthetic rubber items could be welded.

External links

• ESPCI ParisTech

References