Undesired films with brilliant whiteness produced from a transparent amorphous polymer solution during room temperature evaporation or water washing

During the use, storage, or cleaning of polyetherimide (PEI) solutions, undesired white films often appear. In this study, equivalent experiments of PEI solutions involving room temperature evaporation and solvent exchange with water are conducted to simulate their air exposure and water washing processes, respectively. Specifically, we focus on investigating the phenomenon and mechanism of white appearance in their films from room temperature evaporation with a cellular‐like porous structure and solvent exchange films that exhibit a finger‐like porous structure. Results obtained from FE‐SEM, UV–Vis spectroscopy, and Datacolor reflectance spectrophotometer indicate that both types of porous structures exhibit long‐range disorderliness, leading to multiple scattering of incident light and high reflectance across the visible light spectrum resulting in a white appearance. Furthermore, due to different structures caused by phase separation, mechanical properties of solvent exchange films are significantly inferior compared with those of films from room temperature evaporation. Finally, we verify the universality of these mechanisms for white film formation in PEI through experiments using PS and PMMA. This study provides insights into efficient utilization of polymer solutions and offers new strategies for biomimetic preparation of white films. Highlights White cellular‐like porous films were prepared by solvent evaporation method. White finger‐like porous films were produced through solvent exchange by water. Multiple scattering caused by above porous structures led to white appearance. Finger‐like porous films with nanosphere as the basic scatter unit had a luster. Universality of both white formation mechanisms was verified by PMMA and PS. This work was aim to elucidate the mechanism behind the formation of white artificial structures that appear in daily use of transparent amorphous polymer solutions.