Poly(vinyl alcohol) Film Stability: Investigation of Film Formation and Dewetting to Access Continuous and Discontinuous Poly(vinyl alcohol) Thin Films

Abstract

Polymer thin films find extensive use in a diverse range of applications, spanning everyday uses such as anti-stick coatings on cookware to high performance implementation such as self-healing coatings. Polymer thin films are often used as a means of surface modification, wherein the interfacial properties of a technologically useful material are altered. Morphology (physical structure) of films plays a significant role in the modification of surface properties. Film morphologies are commonly categorized as continuous or discontinuous. Continuous film morphologies enable complete masking of bulk material properties. For instance, continuous polymer films have been employed to increase biocompatibility of biomedical implant materials. Alternatively, discontinuous polymer thin films have been leveraged for applications requiring topographical patterning or roughness, such as microlens array fabrication.

Film morphology is generally dictated by film stability; unstable films dewet (rupture) leading to discontinuous morphologies. For semicrystalline polymer films, such dewetting can be facilitated by polymer aggregation and crystallization. Together, dewetting and subsequent polymer aggregation govern film morphology. Studying dewetting and aggregation processes enables purposeful experimental design to prepare films with desired morphologies. This work seeks to probe film stability and morphology of poly(vinyl alcohol) (PVOH) thin films on polydimethylsiloxane (PDMS) substrates, enabling the preparation of both continuous and discontinuous PVOH films by modifying film stability.

The preparation of continuous PVOH films offers a route to hydrophilize hydrophobic PDMS which is useful for reducing biofouling in microfluidics applications. The complementary fabrication of discontinuous PVOH films was also explored because such films are useful for applications requiring topographically rough or patterned surfaces. PVOH film stability was first characterized for films prepared through adsorptive processes (static adsorption and spin coating). Crosslinking was then used to modify film stability, allowing access to both continuous and discontinuous morphologies.

PVOH crystallinity was found to influence both film morphology and stability. Effects of film crystallinity were probed by comparing the behavior of 88% hydrolyzed PVOH (PVOH88%H) with 99% hydrolyzed PVOH (PVOH99%H). PVOH88%H films exhibited fractal morphologies (typical of semicrystalline polymer films) with droplet-like dewetting patterns (typical of amorphous polymer films). PVOH99%H films exhibited fractal morphologies (typical of semicrystalline polymer films) but no features associated with amorphous polymer films. These film morphologies suggest that PVOH99%H has greater semicrystalline character than PVOH88%H, which is expected given the greater ability of PVOH99%H to hydrogen bond. The greater crystallinity of PVOH99%H films imparted PVOH99%H films with greater stability against heat exposure (thermal annealing) and water exposure (solvent annealing) relative to PVOH88%H films. This was documented by decreased polymer rearrangement observed in PVOH99%H films upon thermal and solvent annealing as compared to PVOH88%H films.

Glutaraldehyde crosslinking was employed to stabilize PVOH films. Stabilization of PVOH films via crosslinking enabled the preparation of films with both discontinuous and continuous morphologies that exhibited less polymer rearrangement upon solvent annealing. Crosslinking was performed either after adsorption of PVOH to PDMS from aqueous solution but before film drying (in-situ crosslinking) or after film drying (ex-situ crosslinking). In-situ crosslinking stabilized films against dewetting, yielding microscopically continuous films. These films were sufficiently stabilized to prevent significant morphology changes during solvent annealing. Ex-situ crosslinking enabled the preparation of discontinuous films that similarly did not undergo significant morphology changes during solvent annealing. In-situ crosslinking was successfully performed for PVOH88%H and PVOH99%H while ex-situ crosslinking was only successfully performed for PVOH88%H.