Effect of polymer concentration on molecular alignment behavior during scanning wave photopolymerization

Molecularly aligned liquid-crystalline (LC) polymer films hold great promise for next-generation high-performance photonics, electronics, robotics, and medical devices. Photoalignment methods capable of achieving precise molecular alignment in a noncontact manner have been actively studied. Recently, we proposed the concept of using spatiotemporal photopolymerization to induce molecular diffusion and the resulting alignment, termed scanning wave photopolymerization (SWaP). The spatial gradient of the polymer concentration is the dominant factor in inducing the molecular diffusion and alignment of LCs. However, the effect of polymer concentration on molecular alignment behavior remains unclear. In this study, we performed SWaP at different exposure energies to modulate the polymer concentration during polymerization. We found that a certain polymer concentration was required to initiate the alignment. Furthermore, the phase diagram of the polymer/monomer mixtures and real-time observations during SWaP revealed that phase emergence and unidirectional molecular alignment occurred simultaneously when the polymer concentration exceeded 50%. Since SWaP achieves molecular alignment coincident with photopolymerization, it has the potential to revolutionize material fabrication by consolidating the multiple-step processes required to create functional materials in a single step. Schematic illustrations of the alignment behavior induced by SWaP. Photopolymerization was conducted with a scanned UV slit light. Uniaxial molecular alignment was induced when the polymer concentration in the exposure area was high, while it was random when the polymer concentration was low.