Lowering the Healing Temperature of Photoswitchable Dynamic Covalent Polymer Networks

Abstract

of the Diels-Alder reaction [17] of a tetra-furylsubstituted molecular crosslinker and the maleimide side chains of a linear poly(lauryl methacrylate) (PLMA) backbone. This combination allowed for formation of a robust yet highly reversible network. Thermally induced healing, however, requires heating above 120 °C-a main drawback of Diels-Alder type decrosslinking-and thus running the risk of degradation of the material at the expense of thermal integrity. To overcome this problem, decreasing the activation barrier for the retro Diels-Alder decrosslinking reaction and thus lowering the healing temperature are highly desirable.Advances reported in the literature have shown that the structure of the crosslinker, including the valency, i.e., number of crosslinking functionality per crosslinker, strongly influences the flexibility of polymer networks, and thus the mechanical properties and healing ability of a polymer film. [18][19][20][21] In our previous work, [13] we chose a tetrafunctional motif X 4 ( Figure 1a,b) inspired by Wudl and co-workers [10] to warrant sufficient crosslinking. Moreover, healing temperatures of polymer networks are remarkably connected to the glass transition temperature (T g ) of the decrosslinked, linear polymethacrylate. Thus, a low T g of the starting material is the most important precondition for the mechanical properties of the resulting polymer film. During the healing process, the flow and flexibility of the linear polymer necessary for a smooth and homogeneous damage repair of a surface coating is tremendously enhanced in case of polymers with low T g . 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. [22][23][24] On the contrary, the implementation of comonomers for introducing functionalities in the side chains increases the T g . [13,24,25] 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. [24,[26][27][28] Furthermore, it is well-known for small molecular systems that polar surroundings lower the barrier for the retro Diels-Alder reaction. [29][30][31] Therefore, the introduction of polar comonomers, for example methacrylates bearing (oligo)ethylene glycol side chains, could in principle also influence the healing temperature. [24,26,32,33] Herein, we describe a new set of bis-furyl-substituted diarylethene-type crosslinkers (X 2,2 and X 2,6 , Figure 1a,b) in combination with previously reported copolymer P(LMA-co-MIMA1) Smart MaterialsTo reduce the environmental footprint of the modern society, it is desirable to elongate the lifetime of consumer products, for example by implementing healable coatings and protective layers. However, since most healing processes carried out by heat or light suffer from material degradation, improving t...

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