Elucidating and Mitigating Instabilities of Poly(vinyl alcohol) Thin Films in Aqueous Environments.

In this study, 88% and 99% hydrolyzed poly(vinyl alcohol) (PVOH88%H and PVOH99%H, respectively) polymers were statically adsorbed and spin coated from an aqueous solution onto high molecular weight (HMW) polydimethylsiloxane (PDMS) substrates. The resulting PVOH thin films are unstable and rupture into fractal structures in a diffusion-limited aggregation fashion upon drying. The dynamics of these fractal thin films upon immersion in water and upon exposure to a single water droplet were closely examined. A newly developed "landmarking and overlaying" method was used to quantify the extent of polymer rearrangement under these conditions. Overall, both types of PVOH films exhibit instability in aqueous environments; however, PVOH88%H has faster desorption-readsorption kinetics at the substrate-solution and substrate-solution-air interfaces, resulting in more significant rearrangements upon water exposure. Ex situ cross-linking reactions using succinyl chloride in the vapor phase were carried out on the PVOH fractal thin films. Under the optimal reaction conditions, the PVOH fractal structures were entirely preserved upon water exposure. In situ cross-linking reactions using glutaraldehyde were performed on the PVOH thin films in contact with solution. Microscopic dewetting of PVOH on HMW PDMS and nanoscopic dewetting of PVOH on intermediate MW PDMS were eradicated. The in situ cross-linking results provide convincing evidence that PVOH dewetting takes place during the drying process and can be mitigated.