AN INVESTIGATION OF THE STABILITY AND STRUCTURE OF POLY(VINYL ALCOHOL) THIN FILMS ON POLYDIMETHYLSILOXANE
Date
2023-07-14
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Abstract
The hydrophilization of polydimethylsiloxane (PDMS) using poly(vinyl alcohol) (PVOH) thin films has been studied to utilize hydrophilic materials in PDMS-based micro- and nanomanufactured devices more effectively. Previous studies have determined that PVOH spontaneously adsorbs from aqueous solution onto hydrophobic substrates due to the hydrophobic effect. Following spontaneous adsorption, PVOH thin films are stabilized via crystallization driven by hydrogen bonding.
PVOH thin films can also be prepared via adsorptive spin coating on flat substrates. We have found that the PVOH-PDMS system varies from the well-established Meyerhofer model, which predicts film thickness from spin rate, solution viscosity (polymer concentration), and other variables. We proposed a new model to describe film thickness (h) from spin coating, which includes polymer-substrate interactions (spontaneous adsorption) and polymer-polymer interactions (e.g., crystallization and the hydrophobic effect). We hypothesized that these two types of interactions form the h1 and h2 thicknesses of the multilayered spin-coated PVOH-PDMS thin films. However, recent evidence has shown that the structure of polymer thin films is even more complex and nuanced.
In this study, we continue to elucidate the complex structure and dynamics of PVOH-PDMS thin films via static adsorption, adsorptive spin coating, and water annealing experiments. We use PDMS polymers of various molecular weights (MWs) (PDMS340, MW = 340 Da; PDMS2k, MW = 2 kDa; PDMS9k, MW = 9.430 kDa; PDMS49k, MW = 49.350 kDa) since the PDMS polymers vary in hydrophobicity and mobility. We also study two different PVOH polymers (PVOH88%H, MW = 85-124 kDa, 87-89% hydrolyzed; PVOH99%H, MW = 89-98 kDa, 99+% hydrolyzed) since the PVOH polymers vary in hydrophobicity and crystallinity. We vary the time before water annealing (0 h vs. 24 h) to determine the stability of partially dry vs. completely dry films. We characterize film thickness, wettability, and morphology to identify changes in the PVOH thin film layers.
We can separate the spontaneously-adsorbed h1 layer into sublayers: a tightly-bound h1 layer and a loosely-bound h1 layer. The tightly-bound h1 layer is first formed when PVOH adsorbs onto the PDMS substrate and is resistant to desorption in water. We found evidence of this layer on almost all of the PVOH-PDMS systems in this study. The only exception was the PVOH99%H-PDMS340 system, in which the hydrophobicity of the substrate and the polymer were insufficient to drive spontaneous adsorption. The loosely-bound h1 layer is formed on top of the tightly-bound h1 layer and is desorbed during water annealing. We found evidence of this layer on PDMS9k substrates alone. While a hydrophobic substrate is generally sufficient to form a tightly-bound PVOH h1 layer, it is insufficient at forming a loosely-bound h1 layer.
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polymer thin films, static adsorption, spontaneous adsorption, adsorptive spin coating, polydimethylsiloxane, PDMS, poly(vinyl alcohol), PVOH, water annealing, film stability, h1, spin cast