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

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.