Adsorption of poly(vinyl alcohol) onto polydimethylsiloxane substrates: formation of continuous films, honeycomb structures, and fractal morphologies
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While polydimethylsiloxane (PDMS) has many uses in analytical, biomedical, and electronic systems, its hydrophobicity hinders some of its applications by causing incompatibility with aqueous media. Specifically, this can cause great difficulties to the polymer’s applications in bioengineering, as most biological systems are water-based. In this research, surface hydrophillization of PDMS was carried out by adsorption of poly(vinyl alcohol) (PVOH, 99% hydrolyzed, M.W. = 89-98 kDa) from aqueous solution. PDMS of different molecular weights, from 2 kDa to 116 kDa, were covalently attached to silicon wafer substrates. All the substrates were characterized using ellipsometry, contact angle goniometry, and atomic force microscopy before and after each step. Adsorbed PVOH thin films were only continuous on PDMS layers of 2 kDa and showed “dewetted” morphologies, such as honeycomb structures and fractal features, as the underlying PDMS molecular weight increases and decreases. The instability of the adsorbed PVOH thin films is likely caused by surface chemical and/or physical “defects”. PVOH morphologies on various PDMS substrates are determined by the density and extent of defects in different PDMS substrates, and/or the molecular weights (flexibility) of PDMS chains. Lower molecular weight PDMS are most easily hydrophilized by adsorbed PVOH as indicated by the low receding contact angle values. Higher molecular weight PDMS have incomplete coverage of PVOH, giving rise to high advancing contact angle values. In-situ imaging confirms that various PVOH morphologies are formed upon exposure to air, not in solution, and that PVOH desorption is minimal. The unique structural features of adsorbed PVOH thin films are likely the direct result of PVOH crystallization upon dehydration and are dependent on surface defects.