Thermally self-healing polymers through a reversible Diels-Alder crosslinking process

In the present work of thesis, a new smart material was developed: its “smart” behaviour consists in the ability of activating a self-healing mechanism when subjected to a proper thermal stimulus. Thanks to its self-mending properties, this new material can potentially find applications in the field of smart coatings and adhesives. The new material synthesis process was analysed, followed by its characterization. Specifically, physicochemical, mechanical and thermal properties were studied. Moreover, a deep attention was given to the healing mechanism, by determining the thermal cycle required for healing activation, healing efficiency and recovery of mechanical properties. The self-healing material developed is based on a three-dimensional epoxy network composed by thermally reversible covalent bonds, obtained via Diels-Alder cycloaddition (DA). Basic constituents of the system are furan and maleimide compounds, which react via DA reaction to form a reversible network system. Firstly, the functionalization of epoxy resins with furfurylammine was carried out in order to obtain multi-functional furan compounds. Then, DA reaction of multi-functional furans with bismaleimides was exploited to form a thermo-sensitive polymer, totally constituted by reversible covalent bonds. Indeed, at the retro-DA temperature, DA adducts disconnect, recreating initial furan and maleimide monomers. Upon cooling, monomeric units reconnect in a totally reversible process, which can be used to self-heal damaged part of the polymeric material. Furthermore, compared to other self-healing mechanism, this mending process works without needing any other compounds such as catalysers or additional monomers and can be repeated multiple times. In the present work of thesis, two commercial epoxy compounds were used as basic precursors to be functionalized with furans: bi-functional Bisphenol A diglycidyl ether (DGEBA) and tri-functional N,N-Diglycidyl-4- glycidyloxyaniline (DGGO). Different formulations of furan functionalized epoxy resins were reacted with bismaleimides in order to obtain highly crosslinked thermally reversible network with good mechanical and adhesive properties. To verify the synthesis process efficiency, the polymer crosslinking degree was established using FTIR and UV spectroscopic analyses, whereas thermal reversibility via retro-DA reaction was demonstrated by means of DSC, DMA and solubility tests. In a later stage of the work, mechanical properties of DA smart materials were determined through tensile and adhesion tests. Moreover, tensile testing was also used to quantitatively calculate the healing efficiency of our self-mending formulations. The healing efficiency was also verified by means of visual analyses on microscopic and macroscopic scales. Finally, gloss measurements were conducted in order to evaluate the aesthetical recovery. It was demonstrated that when treated with a proper thermal cycle, the DA smart material synthesized is capable of efficiently mending complex scratch patterns, even multiple times and partially recovering mechanical properties with a 40% healing efficiency.