Photoinduced reaction and molecular orientation of poly(4‑methoxycinnamoyl norbornene) studied by real‑time FT-IR and PM-VLD spectroscopy

Photoinduced reaction and molecular orientation of poly(4-methoxycinnamoyl norbornene) were investigated using real-time Fourier transform infrared (FT-IR) spectroscopy and polarisation modulation vibrational linear dichroism (PM-VLD). The [2 + 2]-cycloaddition reaction of cinnamate groups was directly monitored via characteristic infrared bands, enabling real-time evaluation of reaction kinetics under linearly polarised UV irradiation. PM-VLD analysis showed that photoinduced molecular orientation developed preferentially perpendicular to the polarisation direction of incident UV light, and quantitative order parameters were determined. The dichroism reached a maximum at an early stage of irradiation and decreased upon prolonged exposure despite continued photoconversion. Liquid crystal alignment was evaluated by coating a nematic reactive mesogen onto photoaligned polymer layers, revealing a continuous decrease in optical brightness with increasing UV dose, indicating a discrepancy between bulk-averaged molecular orientation and surface-dominated alignment behaviour. The effect of a photosensitiser (Michler’s ketone) demonstrated enhanced photoreactivity accompanied by reduced orientational selectivity. Thermal stability tests further showed that the rigid polynorbornene backbone significantly improves alignment stability compared to poly(vinyl cinnamate). These results highlight the need to balance photoreactivity, molecular orientation, and backbone rigidity for effective photoalignment applications.