Specular and Diffuse Reflectance of Phase‐Separated Polymer Blend Films

• Published in: Macromolecular rapid communications. Vol. 38; no. 12
• Main Authors: Nallapaneni, Asritha, Shawkey, Matthew D., Karim, Alamgir
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 01.06.2017
• Subjects: Angle of reflection; Annealing; Coating; Coatings; Devices; diffuse reflectance; Displays; Incident light; Light; Light emitting diodes; Liquid crystal displays; Luminous intensity; optics; Phase separation; Polymer blends; Polymer films; Polymers - chemistry; Reflectance; Reflection; Reflectors; Silver; surface roughening; Surface roughness; Temperature; Temperature effects; Thin films; Tuning; Ultraviolet radiation; phase separation; diffuse reflectance; optics; polymer blends; surface roughening
• ISSN: 1022-1336; 1521-3927
• DOI: 10.1002/marc.201600803

Diffuse reflectors have various applications in devices ranging from liquid crystal displays to light emitting diodes, to coatings. Herein, specular and diffuse reflectance from controlled phase separation of polymer blend films, a well‐known self‐organization process, are studied. Temperature‐induced spinodal phase separation of polymer blend films in which one of the components is selectively extracted is shown to exhibit enhanced surface roughness as compared to unextracted films, leading to a notable increase of diffuse reflectance. Diffuse reflectance of UV–visible light from such selectively leached phase‐separated blend films is determined by a synergy of varying lateral scale of phase separation (≈200 nm to 1 μm) and blend film surface roughness (0–40 nm). These critical parameters are controlled by tuning annealing time (0.5–3 h) and temperature (140, 150, 160 °C) of phase separation. Angle‐resolved diffuse reflection studies show that the surface‐roughened polymer films exhibit diffuse reflectance up to 40° from normal incident light in contrast to optically uniform as‐cast films that exhibit largely specular reflectance. Furthermore, the intensity of the diffusively reflected light can be enhanced (300–700 nm) or reduced (220–300 nm) significantly by coating the leached phase‐separated films with a thin silver over layer. Diffuse reflectors are used in liquid crystal displays, light emitting diodes, and solar panels. This study demonstrates the specular and diffuse reflectance of thermally phase‐separated polymer blend (polystyrene/polymethylmethacrylate(PMMA)) films in which polystyrene is selectively removed, leaving behind surface‐roughened PMMA films (SRPF). The resulting SRPF exhibits diffuse reflectance from nanosurface‐roughened films, which, in turn, depend on scale of phase separation.