The diamide–imide
equilibrium was successfully exploited
for the synthesis of dynamic covalent polymer networks in which a
dissociative bond exchange mechanism leads to high processibility
at temperatures above ≈110 °C. Dynamic covalent networks
bridge the gap between thermosets and thermoplastic polymers. At the
operating temperature, when the network is fixed, dynamic covalent
networks are elastic solids, while at high temperatures, chemical
exchange reactions turn the network into a processible viscoelastic
material. Upon heating a dissociative network, the viscosity may also
decrease due to a shift of the chemical equilibrium; in such materials,
the balance between processibility and excessive flow is important.
In this study, a network is prepared that upon heating to above ≈110
°C dissociates to a significant extent due to a shift in the
amide–imide equilibrium of a bisimide, pyromellitic diimide,
in combination with poly(tetrahydrofuran) diamines. At room temperature,
the resulting materials are elastic rubbers with a tensile modulus
of 2–10 MPa, and they become predominantly viscous above a
temperature of approximately 110 °C, which is dependent on the
stoichiometry of the components. The diamide–imide equilibrium
was studied in model reactions with NMR, and variable temperature
infrared (IR) spectroscopy was used to investigate the temperature
dependence of the equilibrium in the network. The temperature-dependent
mechanical properties of the networks were found to be fully reversible,
and the material could be reprocessed several times without loss of
properties such as modulus or strain at break. The high processibility
of these networks at elevated temperatures creates opportunities in
additive manufacturing applications such as selective laser sintering.