Covalent adaptable network
When a stimulus (for example heat, light, pH, ...) is applied to the material, these dynamic bonds become active and can be broken or exchanged with other pending functional groups, allowing the polymer network to change its topology.
At low temperatures, the molecular motion of the polymer chains is limited due to chain-entanglements, resulting in a hard and glassy material.
In the framework of sustainability, the combination of the mechanical properties of thermosets with the reprocessability of thermoplastics through the introduction of dynamic bonds has been the topic of numerous research studies.
The thermoreversible nature of the physical cross-links results in polymer materials with improved mechanical properties without losing reprocessability.
The properties of these physical networks are highly dependent on the used backbone and type of non-covalent interactions, but typically they are brittle at low temperature and become elastic or rubbery above Tg.
[7] Recent advancements in the field of CANs have shown their potential to someday replace conventional non-recyclable thermosetting materials.
[13] Upon application of a stimulus, the equilibrium shifts to the dissociated state, resulting in a temporarily decreased cross-link density in the network.
When a sufficient amount of dynamic bonds dissociate due to the equilibrium being shifted below the gel point, the material will suffer a loss of dimensional stability and show a sudden and drastic viscosity decrease.
The decreased viscosity caused by rapid dynamic bond exchanges enables stress relaxation and network topology rearrangements in these materials.
Dynamic exchange reactions are also often activated by direct infrared heating with the assistance of photothermal fillers (e.g. carbon black, graphene, and gold nanoparticles).
Some examples of this include the systems based on photoreversible cycloaddition that require ultraviolet (UV) irradiation, as well as photo-triggered radical reshufflings of sulfur-based dynamic covalent bonds.
[21] Thermosets are currently in high demand for high-performance composites that are heavily needed in lightweight engineering and ultrahigh-performance mechanical parts.
Applications include: packaging, remediation, energy storage, electromagnetic absorption, sensing and actuation, transportation and safety, defense systems, thermal flow control, information industry, catalysts, cosmetics, sports, etc.
For example, the addition of a resistive heater for electrothermal conversion (e.g. single walled carbon nanotubes) can allow for an on-demand mechanical property switch via an electric current.
[23] Alternatively, by adding a filler like graphene oxide, light irradiation can be used for an induced photo-thermal effect allowing for switching of the mechanical properties as a response to light-irradiation.
Currently, plastics are the most common raw material used for 3D printing due to their wide availability, diversity and light weight.
[12] The mechanism behind this reaction can be attributed to the cleavage of disulfide linkages (RS−SR) into thiyl radicals (2 RS•), resulting in reprocessability and self-healing characteristics for the bulk material.
[13][32] Typically, recyclability is restricted to thermoplastic materials, as said materials consist of polymer chains which are not bonded to each other at the molecular level; as a result, they can be melted down and reformed (as the addition of thermal energy allows the chains to untangle, move past each other, and adopt new configurations), but this comes at the expense of their physical robustness.
