Suppression of the Skin-Core Structure of Poly(vinyl alcohol) Films by Adding Urea/Diethanolamine to Improve the Mechanical and Optical Properties

The microstructure of poly(vinyl alcohol) films formed during the solution drying process significantly influences their tensile ductility and optical transmittance. However, the “skin-core” structure of the dried PVA film is very obvious since the PVA solution dries extremely fast at a relatively low initial concentration (C₀). Herein, we propose a strategy to reduce the evaporation rate through the incorporation of plasticizer-type additives. For this purpose, urea and diethanolamine (DEA) were selected as the compound additives. On the one hand, the solubility of urea in water was improved by DEA because DEA is a good solvent for urea. On the other hand, more intermolecular hydrogen bonds were formed between the primary amine groups on urea and the hydroxyl (−OH) groups on PVA. The formation of PVA–plasticizer hydrogen bonding networks significantly prolonged the deceleration drying period and slowed down the drying process of the solution. In addition, the reduction in the content of single hydrogen-bonded water also favored the acquisition of a homogeneous structure. When the content of the urea/DEA compound additives was over 10%, the “skin-core” structure was suppressed. The structural changes in films have had an impact on both mechanical and optical performances. The fracture strain of the modified PVA film reaches 448.2%, and the average light transmittance in the visible range reaches 97.8% when the content of the compound additive is 20% of the PVA mass. These values are much higher than those of the control film, which are 9.2 and 92.2%, respectively. This study enhances comprehension of the film-forming mechanism and structural changes in PVA/H₂O/plasticizer systems, offering theoretical insights for improved industrial processing.