Vitrimers are a class of plastics, which are derived from the thermosets and are very similar to them. Vitrimers are consisting 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.
Vitrimers are strong glass formers.
Polymer networks other than epoxy resins based on diglycidyl ether of bisphenol A have been utilized in vitrimer technology, such as polylactide, epoxidized soybean oil with citric acid, and polybutadiene.
Background and significance
Thermoplastics are on the one hand easy to process but on the other hand also easy corrodible by chemicals and mechanical stress. The opposite is true for 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 therefore in the desired shape, which is time-consuming, restricts the shape and is responsible for their high price.
If there was a way to create reversible covalent bonds, this would combine good processability, reparability and high performance. There have been already a number of 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 Customize complex objects. Vitrimers could be therefore a new and promising class of materials with numerous applications.
glass and glassformer
If the "melt" on an (organic) amorphous polymer is cooled down, it solidifies at the glass transition temperature Tv . Upon cooling, the hardness of the polymer increases in the neighborhood of Tv by several orders of magnitude. This hardening does not follow the Arrhenius equation, but the Williams-Landel-Ferry equation. Organic polymers are therefore 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 Tv and follows the Arrhenius law . Only through this gradual change in the viscosity glassblowing is possible. If one would try to shape an organic polymer in the same manner as a glass, it would at first be firmly and completely liquefy near Tv already at slightly higher temperatures. For a theoretical glassblowing of organic polymers the temperature must be controlled very precise.
Until today there were not known any organic strong glass formers. Strong glass formers can be shaped in the same way as glass (silicon dioxide) can be. With the vitrimers is now for the first time such a 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. 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 network density.
There is a number of applications imaginable on this basis. A surfboard from vitrimers could be brought into a new shape, scratches in a hood could be "cured" and cross-linked plastic or rubber items could be welded.
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