Exploring formation rationale of skin-core heterogeneity during PVA solutions evaporation by laser-induced fluorescence analysis

Abstract

The structural heterogeneity is mainly characteristic as skin-core manner during the forming film of polymer solutions volatilization. Due to absence of convenient, efficient and high-sensitivity inspection method, it is still a great challenge to manifest the formation and evolution rules of skin-core heterogeneity. Herein, the real-time monitoring of the drying of poly (vinyl alcohol) (PVA) solutions with different initial concentrations (C0s) was performed by a combination of fluorescence staining and laser-induced fluorescence (LIF) analysis techniques. Thanks to the spatial resolution capability of LIF, the development trend of structural heterogeneity along the thickness direction can be acquired exactly, and the thickness of skin layer determined by the LIF method is in accordance with the result from electron microscopy characterization. Importantly, as increasing solution concentration during evaporation process, there exists a threshold of concentration ～25% for the appearance of skin-core divergence. Meanwhile, although the formation of skin-core structure upon a higher C0 is more readily than a lower one, the increasing of C0 benefits to depress structural heterogeneity. These findings are of significance for completing the scenario of the formation procedure of skin-core structure via solution evaporation, which plays a crucial role on dominating the ultimate performances in polymeric-film materials.