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Combining reversible and permanent bonds in interpenetrating and partially reversible polymer networks for toughening and healing of thermosets

Cuvellier A.Combining reversible and permanent bonds in interpenetrating and partially reversible polymer networks for toughening and healing of thermosets,Vrije Universiteit Brussel, Brussels, Belgium,2021 (January).

Material scientists are constantly pushing boundaries, improving the performance of materials to make them lighter, stronger, cheaper, longer-lasting,.... Inspired by the ability of living systems to autonomously repair damage, various healing strategies have been developed to improve the reliability and lifetime of materials and avoid overdimensioning. For polymers the most promising strategy is the intrinsic approach, where the healing capability is inherent to the polymer chemistry. This can be achieved by using reversible chemistries. Amongst these the thermoreversible Diels-Alder cycloaddition between furan and maleimide functional groups proved to be an excellent candidate to create stimuli-responsive self-healing materials.
Due to their reversible character these covalent bonds are weaker than irreversible bonds and thus less energy is needed to break them. Consequently, not only can this reversible chemistry be used for healing, it is expected to be able to result in tougher materials when correctly designed. The idea is that by combining these reversible bonds with permanent bonds the reversible bonds will preferentially break dissipating large quantities of energy, while the permanent bonds keep the broken parts together, postponing failure to higher elongations. Upon removal of the force, due to entropy elasticity, the material will partially recover its shape and the broken reversible bonds will be able to reform, leading to the recovery of mechanical properties.
In this work, a reversible covalent Diels-Alder thermoset was combined with an irreversible elastomeric epoxy-amine network to obtain tough and healable stiff elastomers and thermosets (GPa level). Two different network architectures were studied. (i) Interpenetrating polymer networks (IPN) with each type of bond present in a separate network prepared through a sequential approach and a one-pot synthesis. (ii) Partially reversible polymer networks (PRPN) where both types of bonds are combined in a single network through a one-pot synthesis. Different thermal properties where obtained, with two glass transitions for the IPNs indicating a certain degree of phase separation, whereas the PRPNs only display one broad glass transition and thus mixing on the molecular level. By using a one-pot synthesis approach the fraction reversible bonds was varied, resulting in networks with different mechanical properties varying from 40 MPa to 1.5 GPa. For the sequential IPN a detrimental effect was observed on the toughness when both types of bonds were combined, but excellent results were obtained for the one-pot IPNs and PRPNs with increases in toughness ranging from a factor 3.5 to 16 compared to the reversible reference network. The energy dissipation, stress relaxation and creep behaviour due to the presence of these reversible bonds was also studied. Preliminary healing tests were performed to prove the reformation of broken reversible bonds. The PRPNs outperformed the IPNs with respect to toughening, however, better healing efficiencies were achieved with the IPNs.
In addition a detailed kinetic and thermodynamic study was performed in the absence of solvents. Calorimetry was used to investigate the influence of the stereochemistry of the furan-maleimide Diels-Alder reaction on the kinetics of the reversible network formation. Two stereoisomers are formed, with the endo isomer forming kinetically faster and undergoing cycloreversion at lower temperatures than the more thermodynamically stable exo isomer. Rate constants and activation energies for the forward and retro Dials-Alder reaction of both stereoisomers were derived. This was done for Diels-Alder systems using amorphous and crystalline maleimides.

BibTeX

@phdthesis{,
    author      = "Cuvellier, A.",
    title       = "Combining reversible and permanent bonds in interpenetrating and partially reversible polymer networks for toughening and healing of thermosets",
    institution = "Vrije Universiteit Brussel",
    year        = "2021",
    address     = ", Brussels, Belgium",
    month       = "January",
    doi         = "",
    note        = "",
}