Abstract
Two reversible polymer networks, based on Diels–Alder cycloadditions, are selected to discuss the opportunities of mobility-controlled self-healing in ambient conditions for which information is lacking in literature. The main methods for this study are (modulated temperature) differential scanning calorimetry, microcalorimetry, dynamic rheometry, dynamic mechanical analysis, and kinetic simulations. The reversible network 3M-3F630 is chosen to study the conceptual aspects of diffusion-controlled Diels–Alder reactions from 20 to 65 °C. Network formation by gelation is proven and above 30 °C gelled glasses are formed, while cure below 30 °C gives ungelled glasses. The slow progress of Diels–Alder reactions in mobility-restricted conditions is proven by the further increase of the system’s glass transition temperature by 24 °C beyond the cure temperature of 20 °C. These findings are employed in the reversible network 3M-F375PMA, which is UV-polymerized, starting from a Diels–Alder methacrylate pre-polymer. Self-healing of microcracks in diffusion-controlled conditions is demonstrated at 20 °C. De-gelation measurements show the structural integrity of both networks up to at least 150 °C. Moreover, mechanical robustness in 3M-F375PMA is maintained by the poly(methacrylate) chains to at least 120 °C. The self-healing capacity is simulated in an ambient temperature window between −40 and 85 °C, supporting its applicability as self-healing encapsulant in photovoltaics.
Highlights
Self-healing polymer networks have the ability to healdefects in order to maintain and restore functional properties [1,2,3,4,5,6]
The heat flow and heat capacity evolutions during reaction can be measured simultaneously, which give indications on the decrease in reaction rate during vitrification. This was proven in previous publications for irreversible thermosets [47,48,49] and recently reversible furan–maleimide Diels–Alder thermosets [38]
The results show that the mechanical properties of the damaged material are restored during the healing step at 20 ◦ C, and that a powder rectangular bar with acceptable mechanical properties, around 1.7 GPa at 20 ◦ C, is obtained
Summary
Self-healing polymer networks have the ability to heal (micro-)defects in order to maintain and restore functional properties [1,2,3,4,5,6]. The first stage of the healing process is the sealing step, in which the gap between crack surfaces is closed. This requires the creation of a sufficiently mobile phase, enabling close contact between both sides of the damaged site. The second stage is the healing step, during which the initial polymer properties are restored [4,7]. Self-healing polymers are designed based on (i) an extrinsic approach using pre-embedded healing agents. Polymers 2020, 12, x FOR PEER REVIEW [8,9,10] or microvascular systems [11,12,13,14]) or (ii) an intrinsic approach using reversible covalent [15–
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