In-situ observing the UV curing process of self-wrinkled polymer films and enhancing the illuminance of LED chips

The micro-wrinkles on polymer film surfaces endow the films with unique optical properties . This study utilized a stereomicroscope for in-situ observation of the curing process of UV-curable self-wrinkled polyurethane acrylate films, investigating the critical time and microstructures during curing under varying UV intensities (5.6, 8.0, and 11.4 mW/cm 2 ), photoinitiator concentrations ( C PI , 1.6 wt% and 2.0 wt%), and film thicknesses ( h film , 60 and 120 μm). The acrylate conversion, surface gloss, arithmetic mean roughness (Ra), and micro-wrinkles' dimensions were compared across different curing parameters. Results demonstrated that the acrylate conversion increased with higher UV intensity, and continued to rise even after the surface micro-wrinkles stabilized. The UV intensity had the greatest influence on the critical time T 1 (from initial UV exposure to appearance of microstructures), with higher UV intensity reducing T 1 . UV intensity, C PI , and h film significantly impacted the critical time T 2 (from microstructure appearance to stable wrinkle formation), with T 2 decreasing as these three parameters increased. The sample F 2.0 – 120−H exhibited the shortest T 1 and T 2 , recorded at 90 s and 82.3 s, respectively. The amplitude ( A ), wavelength ( λ ), and A/λ values of the micro-wrinkles increased with h film but decreased with higher C PI and UV intensity. Moreover, a higher A/λ value correlates with an increased Ra and decreased gloss values. However, incomplete curing of some films can lead to relaxation and affect their gloss and Ra values. Finally, cured films demonstrated high transmittance and haze, with the illuminance of integrated LED chips covered by these films increasing by up to 27.5 %, indicating a promising application potential for self-wrinkled films in the optical field . • Prepare self-wrinkled film using only a medium-pressure mercury lamp in ambient air • Conduct in-situ observation of the wrinkling process using a stereomicroscope • Illuminance of LED chips covered with self-wrinkled film can increase by up to 27.5 %