Conditional repair by locally switching the thermal healing capability of dynamic covalent polymers with light

Abstract

Healable materials could play an important role in reducing the environmental footprint of our modern technological society through extending the life cycles of consumer products and constructions. However, as most healing processes are carried out by heat alone, the ability to heal damage generally kills the parent material's thermal and mechanical properties. Here we present a dynamic covalent polymer network whose thermal healing ability can be switched ‘on' and ‘off' on demand by light, thereby providing local control over repair while retaining the advantageous macroscopic properties of static polymer networks. We employ a photoswitchable furan-based crosslinker, which reacts with short and mobile maleimide-substituted poly(lauryl methacrylate) chains forming strong covalent bonds while simultaneously allowing the reversible, spatiotemporally resolved control over thermally induced de- and re-crosslinking. We reason that our system can be adapted to more complex materials and has the potential to impact applications in responsive coatings, photolithography and microfabrication.

Lowering the Healing Temperature of Photoswitchable Dynamic Covalent Polymer Networks

“…Three types of statistical copolymers were synthesized by atom transfer radical polymerization (ATRP) (see the Supporting Information). Whereas P1 exhibits the same composition as in our previous work (P(LMA‐co‐MIMA1)), the maleimide methacrylate MIMA2 in P2 (P(LMA‐co‐MIMA2)) contains a hexamethylene spacer unit to provide enhanced flexibility. In contrast to P1 and P2 , P3 is made from the more polar maleimide‐functionalized methacrylate MIMA3 bearing two ethylene glycol spacer units and the more polar macromonomer poly(ethylene glycol) methacrylate (PEGMA 300 ).…”

“…Thin films of a polymer network X@P were generally prepared by mixing X and P in such a ratio to assure the presence of 0.90 ± 0.05 equivalents of furyl groups per maleimide unit (see ref. for details on polymer film preparation) and therefore comparable likelihood of Diels–Alder adduct formation . The thermal healing process was qualitatively investigated by comparing optical micrographs of scratched thin films of X@P .…”

“…Herein, we describe a new set of bis‐furyl‐substituted diarylethene‐type crosslinkers ( X 2,2 and X 2,6 , Figure a,b) in combination with previously reported copolymer P(LMA‐co‐MIMA1) ( P1 ) to form dynamic covalent polymer networks capable of conditional healing (Figure c) . By optimizing the structure of the crosslinker we are able to decrease the temperature necessary for the retro Diels–Alder decrosslinking reaction to occur as compared to our initially described tetrafuryl‐substituted crosslinker X 4 (Figure b).…”

“…b) Chemical structures of crosslinkers and polymers utilized in this work to form various thermally reversible polymer networks. c) Chemical basis of our concept of conditional repair to locally switch the thermal healing capability of dynamic covalent polymers with light …”

“…Whereas short, side‐chained polymers such as poly(methyl methacrylate) with a high glass transition temperature preclude homogeneous film formation properties and thus the flow of the material, the introduction of dodecyl side chains in the polymer backbone such as in PLMA results in T g < −60 °C . On the contrary, the implementation of comonomers for introducing functionalities in the side chains increases the T g . Hence, enhancing the flexibility of the linker units is one obvious possibility for lowering the T g and thus improving the flow of the material .…”