Permanently cross-linked materials have outstanding mechanical properties and solvent resistance, but they cannot be processed and reshaped once synthesized. Non-cross-linked polymers and those with reversible cross-links are processable, but they are soluble. We designed epoxy networks that can rearrange their topology by exchange reactions without depolymerization and showed that they are insoluble and processable. Unlike organic compounds and polymers whose viscosity varies abruptly near the glass transition, these networks show Arrhenius-like gradual viscosity variations like those of vitreous silica. Like silica, the materials can be wrought and welded to make complex objects by local heating without the use of molds. The concept of a glass made by reversible topology freezing in epoxy networks can be readily scaled up for applications and generalized to other chemistries.
Vitrimers, strong organic glass formers, are covalent
networks
that are able to change their topology through thermoactivated bond
exchange reactions. At high temperatures, vitrimers can flow and behave
like viscoelastic liquids. At low temperatures, exchange reactions
are very long and vitrimers behave like classical thermosets. The
transition from the liquid to the solid is reversible and is, in fact,
a glass transition. By changing the content and nature of the catalyst,
we can tune the transesterification reaction rate and show that the
vitrimer glass transition temperature and the broadness of the transition
can be controlled at will in epoxy-based vitrimers. This opens new
possibilities in practical applications of thermosets such as healing
or convenient processability in a wide temperature range.
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