Inhibition of the Skin-Core Structure in Poly(Vinyl Alcohol) Films by Increasing the Diffusion-Induced Dominated Drying Process

As the core material to realize the polarization function, the homogeneity of poly­(vinyl alcohol) (PVA) film directly affects the polarization performance, transmittance, color tone, and other key optical indicators of the polarization film. However, due to the rapid volatilization of solvents in the actual production process, the phase transition occurs in a very short time, and there is not enough time to attain the equilibrium state, which will cause a gradient structure in the thickness direction. Herein, we proposed a strategy to suppress the skin-core structure by shifting the drying process of PVA solution from evaporation-induced dominance to diffusion-induced dominance to slow down the drying rate. For this purpose, PVA aqueous solutions with five different initial concentrations (C₀) from 10 wt % to 21.4 wt % were prepared, and the film formation process was monitored in situ. It was found that there existed a critical concentration; i.e., when the C₀ of the PVA aqueous solution exceeded this concentration (18 wt %), the dried film was homogeneous, and when the C₀ was lower than this value, the skin-core structure appeared. Moreover, by monitoring the drying process of PVA solutions with different C₀s in real-time, we observed that the drying rate of PVA solutions can be effectively slowed when the double hydrogen-bonded water and multiple hydrogen-bonded water were predominant as the critical concentration exceeded 18 wt %. Thus, the homogeneous PVA film is formed by retarding the evaporation and crystallization process. This study contributes to the structural development of PVA optical films with uniform structures and provides theoretical support for the design and fabrication of polarized films.