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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
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
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
Functional principle
Glass and glass former
If the melt of an (organic) amorphous polymer is cooled down, it solidifies at the glass-transition temperature Tg. On cooling, the hardness of the polymer increases in the neighborhood of Tg by several
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
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.[2] 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.[3] 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.