Vitrimers

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Vitrimers ( English Vitrimers ) are a class of plastic that is derived from classic thermosets and has strong similarities with them. They are made up of covalent networks that can change their topology through thermally activated bond exchange reactions. Vitrimers are powerful glass formers. At high temperatures they flow and behave like a viscoelastic liquid. At low temperatures, the exchange reactions are immeasurably slow (“frozen”) and the vitrimers behave like classic thermosets .

Their behavior opens up new possibilities in the application of thermosets such as self-healing or easy processing in a wide temperature range.

Background and meaning

While thermoplastics are easy to process, but also susceptible to chemicals and relatively little mechanically resilient, the opposite applies to thermosets. Thermoplastics consist of covalently bonded molecular chains that are connected to one another by weak interactions (e.g. Van der Waals forces ). As a result, they can be easily processed by melting (or in some cases also from the solution ), but are also susceptible to suitable solvents and creep under constant stress. Thermoplastics can be reversibly deformed above their glass transition temperature or above the crystallite melting temperature and processed by extrusion , injection molding and welding. Thermosetting plastics, on the other hand, consist of molecular chains that are linked together by covalent bonds to form a permanent network. As a result, they have outstanding mechanical properties and thermal and chemical resistance. This makes them an indispensable part of load-bearing components in the automotive and aircraft industries. Due to their irreversible linkage via covalent bonds, they can no longer be deformed as soon as the polymerization is complete. They must therefore be polymerized in the desired shape, which is time-consuming, restricts the shape and makes them very expensive.

If there was a way to make covalent bonds reversible, this would combine good processability, reparability and high performance. Numerous strategies have already been tried out to make such plastics possible. Vitrimers are a successful solution to these attempts, they combine the desired properties of both classes: They show the mechanical and thermal properties of thermosets and at the same time can be deformed under the influence of heat. Vitrimers can be welded like (silicon) glasses or metals . Welding by simple heating enables the production of complex objects. Vitrimers could therefore be a new and promising class of materials with numerous application possibilities.

Function and principle

Glasses and glass formers

When the “melt” of an (organic) amorphous polymer cools down, it solidifies at the glass transition point T v . On cooling, the hardness of the polymer in the vicinity of this point increases by several orders of magnitude . It does not follow the Arrhenius equation , but the Williams-Landel-Ferry equation . Organic polymers are therefore referred to as "fragile glass formers" (from the English fragile = weak). Silicon glass (e.g. window glass), on the other hand, is called a strong glass former. Its viscosity changes only very slowly in the vicinity of the glass transition point T v and follows Arrhenius' law. Glass bubbles are only possible through this gradual change in viscosity . If one were to try to shape an organic polymer such as glass, it would initially be solid and, in the vicinity of T v , would completely liquefy at a slightly higher temperature, e.g. B. drip down. The temperature would therefore have to be controlled very precisely and in a complex manner in order to “blow glass from organic polymers”.

Until now, no organic materials were known to be powerful glass formers. Strong glass formers, like glass ( silicon dioxide ), can be shaped into any shape in the same way as glass. With the vitrimers, such a material is now available for the first time.

Mode of action: transesterification and temperature influence

The working group around Ludwik Leibler demonstrated the functional principle of the vitrimers using the example of epoxy duromers. Epoxy thermosets can be represented as vitrimers if transesterification reactions can be introduced and controlled. In the system examined, carboxylic acids or carboxylic acid anhydrides must be used as hardeners . A change in the topology is possible through transesterification reactions. These transesterification reactions have no influence on the number of links or the (average) functionality of the polymer. At high temperatures the polymer can flow like a viscoelastic liquid. As the temperature is lowered, the transesterification reactions slow down until they eventually "freeze" (become immeasurably slow). Below the (temperature) point at which this is the case ( topology freezing transition or vitrification , T v ), vitrimers behave like normal, permanently linked thermosets. The vitrimers shown by way of example had a modulus of elasticity of 1 MPa to 100 MPa below T v , depending on the network density .

Applications

Numerous applications are conceivable on this basis. A surfboard made of vitrimers could be given a new shape, scratches in a bonnet could be “healed” and cross-linked plastic or rubber items could be welded together.

Web links

Individual evidence

  1. ↑ Mathieu Capelot, Miriam M. ceasing, François Tournilhac, Ludwik Leibler: Catalytic Control of the Vitrimer Glass Transition . In: ACS Macro Letters . 2012, doi : 10.1021 / mz300239f .
  2. ↑ Damien Montarnal, Mathieu Capelot, François Tournilhac, Ludwik Leibler: silica-Like Malleable material from permanent Organic Networks . In: Science . tape 334 , November 2011, doi : 10.1126 / science.1212648 .
  3. ↑ a b Mathieu Capelot, Damien Montarnal, François Tournilhac, Ludwik Leibler: Metal-catalyzed transesterification for healing and assembling of thermosets . In: J. Am. Chem. Soc . tape 134 , no. 18 , 2012, p. 7664-7667 , doi : 10.1021 / ja302894k .