(337f) Light Stimulus Responsive Solid to Gas Formation By Photochemistry on Different Polymer Surfaces.

Responsive materials that evolve gas after being exposed to external stimuli can have possible applications as active surfaces and for ultrasound imaging. 2-azidoanthracene can act as a photochemical source of N₂ gas when dissolved in an optically transparent polymer like poly(methyl methacrylate) (PMMA). Irradiation at 365 or 405 nm of a 150 μm thick polymer film submerged in water causes the rapid appearance of a surface layer of bubbles. For an azide concentration of 0.1 M and a light intensity of 140 mW/cm², the yield of gas bubbles is calculated to be approximately 40%. The dynamics of bubble growth depend on surface morphology, light intensity, and azide concentration. A combination of nanoscale surface roughness, high azide concentration, and high light intensity is required to attain the threshold N₂ gas density necessary for rapid, high-yield bubble formation. Other azide molecules have been evaluated as sources of N₂, with the finding that the rate of N₂ production is enhanced by the proximity of the azide group to the conjugated ring and the addition of extra azide groups. The N₂ bubbles adhere to the polymer surface and survive for days underwater. The ability to generate stable gas bubbles “on demand” using light permits the demonstration of photoinduced flotation and patterned bubble arrays. When generated on microparticles or free-standing polymer fibers, they can potentially be used to improve contrast in bio-imaging using ultrasound.