Early Detection of Polymer Deformation via Poly(Urea-Formaldehyde) Microcapsules Encapsulated With Charge Transfer Precursors

Abstract

Herein, we present a self-reporting coating achieved through the simple integration of microcapsules containing charge transfer precursors into a polymer matrix. Utilizing 1,2,4,5-tetracyanobenzene (TCNB) as electron acceptor (A), carbazole (CARB) and dibenzothiophene (DBZP) as electron donors (D), two charge transfer complexes (CTCs) exhibiting red and green colors were successfully assembled. Poly(urea-formaldehyde) microcapsules encapsulating either donor or acceptor components were synthesized via in situ polymerization of an oil-in-water emulsion. A systematic investigation was conducted to assess their composition, morphology, thermal stability, and self-reporting ability. The microcapsules incorporated into the poly-(dimethylsiloxane) (PDMS) matrix ruptured upon mechanical failure of the polymer coating. This rupture facilitated the release of donor and acceptor components into the matrix, where they subsequently formed CTCs, resulting in a significant color transformation. To visually indicate crack penetration depth, we developed a multilayered polymer incorporating various types of microcapsules. When subjected to scratches of differing depths on the polymer matrix, different CTCs are activated that allow for visual detection of crack penetration depth based on their fluorescence color. Furthermore, this methodology can be integrated with hexamethylene-diisocyanate (HDI), a self-healing reagent, to develop a smart coating with autonomous self-healing and self-reporting functions. The dual-function coating is achieved by integrating the HDI and CTCs components into one microcapsule in the matrix, without any external intervention. The CTCs-based detection materials make this matrix a powerful tool for damage indication and seem to be also suitable for other polymers.