The effect of long-wavelength ultraviolet photo-cross-linking on microphase-separated coumarin-containing block polymers was studied by photorheometry and small-angle X-ray scattering. This model system consisted of three photo-cross-linkable diblock polymers of poly(methoxyethyl acrylate)-b-poly(hexyl methacrylate-co-coumarin methacrylate) with different volume fractions of the cross-linkable coumarin-containing block, which microphase separated into lamellar and cylindrical morphologies. All polymers stiffened upon exposure to 365 nm light, with much greater relative increases in moduli recorded for lamellae-forming polymers (ca. 3200% increase) compared to the cylinder-forming polymer (ca. 550% increase). Disordering transitions that were evident in un-cross-linked samples were no longer observed after cross-linking in the ordered state, and domain sizes were found to remain stable to heating. The photo-cross-linking reaction only proceeded under active irradiation (i.e., cross-linking does not persist when the UV radiation is turned off), indicating a high degree of spatiotemporal control over curing in this system. Finally, at constant concentration of couamarin within the cross-linkable block, the cure rate was largely independent of polymer composition, suggesting a constant local concentration of coumarin moieties within the segregated cross-linkable domains. These findings establish a set of specific structure–property relationships governing the phase-selective photo-cross-linking of diblock polymers that can guide the design of robust nanostructured materials.
We present an approach to photocrosslink bicontinuous microemulsions derived from ternary blends of poly(methoxyethyl acrylate) (PM, M n = 4200 g/mol), poly(hexyl methacrylate-co-coumarin methacrylate) (PHC, M n = 6800 g/ mol), and PM-b-PHC diblock polymer (M n = 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.
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