Stacking-Order Effects on Directional Optical Properties of Al2O3/Poly(Methyl Methacrylate) Bilayer Scattering Films

Bilayer scattering films offer a practical route to independently controlling transmission and reflection in transparent optical systems, yet the separate roles of nanoparticle loading and stacking order remain underexplored. In this work, Al2O3/poly(methyl methacrylate) bilayer films with four concentration pairs (5/15, 15/5, 5/30, and 9/30 wt%) were fabricated by sequential spin-coating and characterized by wavelength-dependent transmittance, reflectance, integrating-sphere haze decomposition, and optical surface interferometry under both glass-side (G-incident) and scattering-layer-side (SL-incident) illumination. Two independent design parameters govern the optical response: (i) the maximum nanoparticle concentration sets the overall scattering regime, and (ii) the layer stacking order controls the partitioning between parallel transmittance (PT) and diffuse transmittance (DT). Bilayers with a maximum loading of 15 wt% maintained total transmittance (TT) ≈ 99% and moderate haze (25%–40%), while reciprocal 5/15 and 15/5 wt% pairs exhibited a ~15 percentage-point shift in PT despite identical total loading. Incorporation of a 30 wt% layer shifted films into a diffusion-dominant regime (DT > 68%, haze > 70%), and positioning this layer at the incident side under SL illumination suppressed visible reflectance below 1%. These results provide a practical composition–sequence design map for transparent optical coatings in display, privacy, and anti-glare applications.