The Influence of the Furan and Maleimide Stoichiometry on the Thermoreversible Diels–Alder Network Polymerization

Abstract

In recent work, the thermoreversible Diels–Alder reaction between furan and maleimide functional groups has been studied extensively in the context of self-healing elastomers and thermosets. To elaborate the influence of the stoichiometric ratio between the maleimide and furan reactive groups on the thermomechanical properties and viscoelastic behavior of formed reversible covalent polymer networks, a series of Diels–Alder-based networks with different stoichiometric ratios was synthesized. Differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and dynamic rheology measurements were performed on the reversible polymer networks, to relate the reversible network structure to the material properties and reactivity. Such knowledge allows the design and optimization of the thermomechanical behavior of the reversible networks for intended applications. Lowering the maleimide-to-furan ratio creates a deficit of maleimide functional groups, resulting in a decrease in the crosslink density of the system, and a consequent decrease in the glass transition temperature, Young’s modulus, and gel transition temperature. The excess of unreacted furan in the system results in faster reaction and healing kinetics and a shift of the reaction equilibrium.

Structure–Property Relationships of Self-Healing Polymer Networks Based on Reversible Diels–Alder Chemistry

“…In previous work, it was reported how a decreasing molar ratio of the starting monomers (e.g., decreasing both r and [DA] eq,25°C ) leads to a decrease in both the equilibrium conversion and the rate at which it was attained during synthesis and healing. 28 In this work, special attention was given to keeping the DA concentration at 25 °C constant to study the effect of the stoichiometric ratio alone on the reaction kinetics. This is illustrated by simulating the formation of DA networks DPBM-FT5000-r0.83 and DPBM-FT3000-r0.52: Figure 15A shows the increase in conversion x = [DA]/[M 0 ] at 25 °C, starting from an initial state with conversion equal to zero.…”

“…Alternatively, the stoichiometric ratio between the reactive maleimide and furan functional groups has been exploited by the authors to greatly improve the healing rate, increasing both the reaction kinetics and the network mobility. 28 A low maleimide-to-furan ratio, achieved through introducing an excess of furan reactive groups in the network, leads to an increase in reaction kinetics of the DA reaction. 32 The faster reaction kinetics allow healing under ambient conditions, as demonstrated for a pneumatic soft robotic gripper, 33 manufactured from a DA elastomeric network with a low stoichiometric ratio.…”

Designing End‐of‐Life Recyclable Polymers via Diels–Alder Chemistry: A Review on the Kinetics of Reversible Reactions

“…[50] In the theory of network formation, the Flory-Stockmayer model is typically introduced (Equation ( 8)). [45,63,64] This model relates functionality of all reactants and the addition ratio of furan and maleimide with a gel conversion, i.e. the conversion at which all molecules have become part of one polymer network.…”

“…[41,44] In the case that one reactant is present in excess, it acts as functional solvent or plasticizer which can take part in the reaction, although this will limit the strength of the plastic. [45]…”

Laser sintering of self-healable and recyclable thermoset networks

“…The synthesis procedure was followed as previously described [43,44]. First, D400 was furanfunctionalized with a stoichiometric amount of FGE via irreversible epoxy-amine reaction at 60 ˚C for seven days.…”