Unraveling the structure-property relationships of thermoreversible diels-alder-based networks as self-healing polymers

The reversibility of the Diels-Alder reaction, its sufficiently fast reaction kinetics,
catalyst-free nature, and the high equilibrium conversion at room temperature make it a
suitable and attractive candidate to create intrinsic self-healing polymer networks. The
Diels-Alder-based reversible polymer networks have the potential to prolong their
lifespan by restoring their structure after mechanical damage and can be reprocessed and
recycled thermally. The properties of Diels-Alder-based polymer networks, such as the
thermophysical properties, mechanical behavior, and healing ability, can be extensively
tuned by varying the network design parameters, like the molecular weight and
functionality of the multifunctional diene (e.g., furan) and dienophile (e.g., maleimide)
monomers. This work greatly expands the fundamental understanding of other network
design parameters on the thermophysical, viscoelastic and self-healing properties of
Diels-Alder-based polymer networks and the resulting freedom to design these properties
for a wide range of applications.
In this work, Reversible polymer networks were made with different maleimide-to-furan
ratios to systematically study the influence of the stoichiometry between functional
groups on properties of the formed polymer network such as glass transition temperature,
Young’s modulus, overall stress-strain behavior, fracture properties, gel transition
temperature, reaction kinetics and equilibrium thermodynamics, and self-healing
behavior (efficiency, time and temperature). When the ratio of maleimide-to-furan ratio
is lowered, there's a shortage of maleimide groups, leading to reduced crosslinking and
subsequently lower glass transition temperature, Young's modulus, and gel transition
temperature. Excess of unreacted furan speeds up reactions and healing kinetics. This
study showed that starting from just two monomers bismaleimide (DPBM) and furan-
functionalized Jeffamines (F400) various polymer networks can be produced. These
networks exhibit mechanical properties ranging from rigid thermosets to flexible
elastomers that can self-heal at room temperature.
This study highlights the capability of Diels-Alder polymers to undergo efficient room-
temperature healing, fully restoring mechanical properties within a few hours. Immediate
partial recovery is observed shortly after fractured surfaces are rejoined. This rapid
healing is achieved by using an off-stoichiometric maleimide-to-furan ratio in the
polymer network. The research explores six Diels-Alder polymers, extensively
investigating the impact of crosslink density on self-healing, thermal, and mechanical
behavior. Various crosslink densities are attained by adjusting monomer molecular weight
or employing the off-stoichiometric ratio. While decreasing crosslink density via a lower
maleimide-to-furan ratio accelerates healing significantly, it slightly reduces mechanical
performance. Conversely, reducing crosslink density by using longer monomers achieves
faster healing to a lesser extent but higher mechanical performance.
Mixing high and low molar mass furan-functionalized monomers introduces a new
dimension for modifying the properties of reversible polymer network blends. These
blends contain different proportions of elastomeric and thermoset phases, with varying
crosslink densities and glass transition temperatures. The resulting phase-separated
blends combine the advantages of the individual networks and mitigate the limitations of
pure elastomers and thermosets. Increasing the content of the thermoset phase results in
a stiffer network with higher modulus, stress, and lower strain at break. Conversely,
adding 10-20% by weight of the elastomer phase to thermosets increases both stress and
strain at the break without major changes in the modulus, leading to a toughened
thermoset due to stress dissipation in the blended network. Achieving compatibility
between these components leads to properties that cannot be achieved solely by altering
monomer properties or adjusting the stoichiometric ratio between maleimide and furan.
The study highlights the effects of phase-separated blends comprising elastomeric and
thermoset regions.
The influence of thermal reprocessing on Diels-Alder (DA) polymer networks was
thoroughly investigated through extensive thermal analysis techniques over five
consecutive cycles. After the initial reprocessing cycle, changes in mechanical properties
were significant due to solvent evaporation and increased crosslink density from
improved DPBM miscibility, resulting in higher Young's modulus and fracture stress, and
reduced fracture strain. However, subsequent cycles had less effect on mechanical
properties. DSC measurements showed that the glass transition temperature and
dissociation peaks of DA adducts remained consistent and aligned with the pristine
network, confirming the potential for repeated thermal reprocessing without losing
properties.