Real-time molecular-level visualization of mass flow during patterned photopolymerization of liquid-crystalline monomers

Single-particle fluorescence imaging is used to monitor dynamic processes that occur during patterned photopolymerization of liquid-crystalline monomers. A spatial gradient of chemical potential can be created at the border of bright and dark regions by structured illumination in the photopolymerization process, leading to mutual diffusion of polymers and monomers. Analysis of the fluorescence from single quantum dots doped into the monomers at minute concentrations enables visualization of highly directional flow from the illuminated region where the photopolymerization proceeds toward a masked unpolymerized region. This directional mass flow causes flow-induced orientation of the polymers that is subsequently fixed by completion of the polymerization reaction, resulting in a mesoscopic aligned area of the polymer film. The light-induced processes that enable the alignment of long molecules have been described by scientists in Japan. The long chain of molecules that form a polymer is often tangled and disorganized. However, in so-called liquid-crystalline polymers, the molecular chains are straight. Aligning these chains with each other creates materials useful for photonic and biomedical applications. Current alignment techniques are slow and complex or require the addition of a dye, which alters the material’s properties. Martin Vacha and Atsushi Shishido from the Tokyo Institute of Technology (in Tokyo and Yokohama) and co-workers used single-particle fluorescence imaging to understand the molecular processes that occur during patterned photopolymerization, a novel method for aligning liquid-crystalline polymers which avoids the disadvantages above. They show that optical illumination induces chemical differences between the bright and shadowed regions, which helps alignment. Single-particle fluorescence imaging is used to monitor dynamic processes that occur during patterned photopolymerization of liquid-crystalline monomers. Spatial gradient of chemical potential created at the border of bright and dark regions by structured illumination leads to mutual diffusion of polymers and monomers. Fluorescence of single quantum dots doped into the monomers visualizes highly directional mass flow from the illuminated region where the photopolymerization proceeds toward a masked unpolymerized region. The flow-induced orientation of the polymers is subsequently fixed by completion of the polymerization reaction, resulting in a mesoscopic aligned area of the polymer film.