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Addition to Wikipedia Project Proposal #1: Vitrimers

Instructions: The original Wiki article "Vitrimers" is shown below, with my addition in BOLD.

Vitrimers are a class of plastics, which are derived from thermosetting polymers (thermosets) and thermoplastics, and have the advantages of both of these materials. 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. 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.

Besides epoxy resins based on diglycidyl ether of bisphenol A, also other polymer networks have been used for vitrimer technology, such as polylactic acid (polylactide), polyhydroxyurethanes, epoxidized soybean oil with citric acid and polybutadiene.

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.

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.

The first Vitrimer was created by the French researcher Ludwik Leibler, head of laboratory at CNRS, France's national research institute. In 2011, Leibler and co-workers developed silica-like networks using well-established the transesterification reaction of epoxy and fatty dicarboxylic or tricarboxylic acids. The synthesized networks have both hydroxyl and ester groups, which undergo exchange reactions (transesterifications) at high temperatures, resulting in the ability of stress relaxation and malleability of the material. On the other hand, the exchange reactions are suppressed to a great extent when the networks are cooled down, leading to a behavior like a soft solid. This whole process is based only on exchange reactions, which is the main difference from that of thermalplastics.

Functional principle

Glass and glass former

If the melt of an (organic) amorphous polymer is cooled down, it solidifies at the glass-transition temperature T_g. On cooling, the hardness of the polymer increases in the neighborhood of T_g 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_g 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_g 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, which can behave like viscoelastic fluid at high temperatures. Unlike classical polymer melts, whose flow properties are largely dependent on friction between monomers, vitrimers become the viscoelastic fluid because of exchange reactions at high temperatures as well as monomer friction. These two processes have different activation energies, resulting in a wide range of viscosity variation. Moreover, because the exchange reactions follow Arrhenius' Law, the change of viscosity of vitrimers also follows an Arrhenius relationship with the increase of temperature, differing greatly from conventional organic polymers.

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, which means that neither the decomposition of polymer linkages nor the decrease of integrity of polymers happens when transesterification reactions take place. 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.

The concentration of ester groups in vitrimers is shown to have a huge influence on the rate of transesterification reactions. In the work done by Hillmyer, et. al., about polyactide vitrimers, they demonstrated that the more ester groups present in the polymer, the faster the rates of relaxation will be, leading to better self-healing performance. Polyactide vitrimers which are synthesized by cross linking reactions of hydroxylterminated 4-arm star-shaped poly((±)-lactide) (HTSPLA) and methylenediphenyl diisocyanate (MDI) with the presence of cross-linking and transesterification catalyst stannous (II) octoate [Sn(Oct)₂], have many more ester groups than all previous vitrimers; therefore, this material has a significantly high stress relaxing rate compared to other polyester based vitrimer systems.

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 curedand cross-linked plastic or synthetic rubber items could be welded. Vitrimers which are prepared from metathesis of dioxaborolanes with different polymers that are commercially available, can have both good processibility and outstanding performance such as mechanical, thermal, and chemical resistance. The polymers that can be utilized in such methodology range from poly(methylmethacrylate), polystyrene, to polyethylene with high density and cross-linked robust structures, which makes this preparative method of vitrimers able to be applied to a wide range of industries.