Weak covalent crosslinks in thermoset materials for increased toughness

A method of forming an orthodontic appliance, comprising:

• providing a printable resin, the printable resin comprising:
• a weak crosslinking unit comprising a first reactive end group at a first end and a second reactive end group at a second end, the weak crosslinking unit comprising a weak crosslinking bond positioned between the first end and the second end;
• a monomer comprising a polymerizable group and a pendant group comprising a third reactive end group, the third reactive end group being capable of reacting with the first reactive end group and the second reactive end group;
• and
• an initiator,
• wherein the weak crosslinking bond has a bond dissociation energy from 50 kJ/mol to 325 kJ/mol, and
• curing the printable resin with a light source, wherein curing polymerizes the monomer to form a polymer matrix comprising a plurality of polymer chains, wherein the weak crosslinking unit is unreacted and dispersed within the polymer matrix;
• and
• heating the polymer matrix to cause the third reactive end group to react with the first and second reactive end groups to form a polymeric material, wherein the polymeric material comprises:
• a first polymer chain;
• a second polymer chain;
• and
• the weak crosslinking unit attached at a first end of the weak crosslinking unit to the first polymer chain and attached at a second end of the weak crosslinking unit to the second polymer chain.

An orthodontic appliance comprising a polymeric material comprising:

• a first polymer chain;
• a second polymer chain;
• and
• an activatable unit attached at a first end to the first polymer chain and attached at a second end to the second polymer chain,
• wherein the activatable unit converts to a weak crosslinking unit upon activation, the weak crosslinking unit being attached at a first end to the first polymer chain and attached at a second end to the second polymer chain, the weak crosslinking unit comprising a weak crosslinking bond positioned between the first end and the second end, the weak crosslinking bond having a bond dissociation energy from 50 kJ/mol to 325 kJ/mol.