Nondestructive Photo-Cross-Linking of Microphase-Separated Diblock Polymers through Coumarin Dimerization

Photoinduced polyelectrolyte complexation for the formation of stable films with reversible crosslinking

Thin films formed by complexation of oppositely charged polyelectrolytes have significant potential in applications such as separation membranes, biocompatible or anticorrosion coatings, and drug delivery systems. While layer-by-layer deposition is a reliable method for producing conformal films, this multi-step process limits scalability. In this study, we functionalize polymers with photoactive protecting and crosslinking groups, allowing a one-step approach for preparing polyelectrolyte complex (PEC) films. To achieve this goal, we introduced o-nitrobenzyl and coumarin groups into a polyanion. The o-nitrobenzyl protecting groups can be selectively deprotected upon exposure to 365 nm light, revealing charged pendent groups that initiate polyelectrolyte complexation. Simultaneously, the coumarin units in the copolymers undergo dimerization, enhancing the solvochemical stability of the PEC films. Notably, short-wave UV irradiation (254 nm) enables retrocyclization of the coumarin dimers, returning the PEC film to its uncrosslinked state. These UV-driven deprotection, crosslinking, and de-crosslinking processes provide a versatile and tunable platform for fabricating reversibly crosslinked films. By integrating photoresponsive polymers and reversible covalent linkages, this approach offers new opportunities for designing PEC materials with tunable dynamic properties for advanced applications.

Photocrosslinkable Polymeric Bicontinuous Microemulsions

We present an approach to photocrosslink bicontinuous microemulsions derived from ternary blends of poly(methoxyethyl acrylate) (PM, Mn = 4200 g/mol), poly(hexyl methacrylate-co-coumarin methacrylate) (PHC, Mn = 6800 g/mol), and PM-b-PHC diblock polymer (Mn = 19,400 g/mol) in a phase-selective manner, enabling structural characterization at an unprecedented level of detail. This strategy utilizes the [2 + 2] photodimerization reaction of coumarin derivatives to covalently crosslink blends without the use of harsh reagents or disruptive thermal treatment, thus preserving the intricate network structure throughout curing. The resulting crosslinked bicontinuous microemulsions exhibited rubbery behavior at elevated temperatures, achieving an elastic shear modulus of nearly 1 MPa at 70 °C, owing to the presence of the three-dimensional co-continuous network morphology. The dimensional stabilization afforded by crosslinking further allowed the microstructure to be directly imaged by scanning electron microscopy and atomic force microscopy. Contrary to recent theoretical findings, the BμE appears in a wide temperature and compositional window, suggesting that it is a robust feature of these blends. As a proof of concept demonstrating both the utility of bicontinuous microemulsion-derived materials and versatility of this strategy toward broader applications in energy storage and transport, the uncrosslinked portion of a cured blend was extracted by washing and replaced with an ionic liquid; the resultant heterogeneous solid electrolyte exhibited a room-temperature conductivity of 2 mS/cm, approximately one-quarter that of the pure ionic liquid.