Effects of Plasticizers on Structural Transitions and Heterogeneity of Poly(vinyl alcohol) Films as Studied via AIE Materials

Solution casting is a common processing method in industrial production. The phase-structure transitions driven by solvent evaporation directly impact the quality of the final film. However, due to the presence of solvents and other additives in the solution, it is difficult to detect the condensed matter structure during the drying process. This work studies structural transitions and heterogeneity of a poly(vinyl alcohol) (PVA) film during water evaporation using glycerol as a plasticizer. Aggregation-induced emission (AIE) molecules are utilized to indicate the change in PVA chain dynamics during evaporation. The gelation, crystallization, and glass transition can be clearly detected via emission intensity change of the AIE molecules during water evaporation. It is found that the critical concentrations triggering structural transitions increase with the glycerol content, attributed to the release of PVA segmental mobility by the plasticizer. Furthermore, structural uniformity in the thickness direction is realized while the PVA film was loaded with 20% glycerol. It is revealed that glycerol diminishing the heterogeneity is due to the dual effect of hydrophilic glycerol reducing the evaporation rate and enhancing diffusion. Finally, the mechanism of the plasticizer in delaying structural transitions and ameliorating the heterogeneity in PVA films is discussed, which could provide theoretical guidance for preparation of polymer films via solution casting.