High-Birefringence Photoalignable Polymeric Films via In Situ Mesogenic Modification

We report a strategy for tuning birefringence in photoalignable liquid-crystalline copolymer (PLCP) films through in situ modification of mesogenic side groups. A PLCP containing 4-formylphenyl benzoate (PA) and cinnamic acid (CA) units was reacted with five phenylamine derivatives after axis-selective photoreactions induced by linearly polarized light. The condensation sequence and phenylamine structure strongly affected molecular reorientation and birefringence development. Microwave-assisted processing enabled rapid condensation and simultaneous molecular reorganization, yielding a maximum birefringence of Δn = 0.47 at 527 nm for 2-aminofluorene (AF)-modified films. Thick (∼2000 nm) AF-modified PLCP films exhibited high transparency and retardation values up to 625 nm and were further applied to highly efficient near-infrared polarization gratings operating at 1550 nm with diffraction efficiencies of 86-87%. These results demonstrate that in situ mesogenic modification provides an effective route to highly birefringent PLCP films for photonic applications.