Strategies to hydrophilize silicones via spontaneous adsorption of poly(vinyl alcohol) from aqueous solution

Abstract

It is challenging to achieve long-lasting hydrophilicity by surface modification of polydimethylsiloxane (PDMS), principally due to the hydrophobic recovery that occurs. This involves the migration of low molecular weight species from the bulk to the surface and is driven by the reduction of interfacial free energy. In this study, spontaneous adsorption of poly(vinyl alcohol) was carried out on Sylgard PDMS films and their modified derivatives. PDMSox1s, PDMSox60s, and PDMSox60s+2k were prepared by 1-s oxygen plasma, 60-s oxygen plasma, and 60-s oxygen plasma followed by covalent attachment of linear PDMS of 2 kDa molecular weight on PDMS films, respectively. Surface morphology was characterized by optical and atomic force microscopy and hydrophilicity was monitored by dynamic water contact angle measurements. It was found that negligible PVOH adsorption takes place on PDMSox60s due to the lack of hydrophobic driving force and that extensive PVOH thin film dewetting on PDMS and PDMSox1s results in insignificant improvement in hydrophilicity. However, a continuous PVOH thin film albeit with some small holes was obtained on PDMSox60s+2k. PDMSox60s+2k-PVOH exhibits advancing and receding contact angles of 80–90°/16 ± 2°, which are significantly lower than 123 ± 5°/97 ± 2° on unmodified PDMS. A range of static contact angles were also measured, some of which are lower than those reported in the literature. The PDMSox60s+2k-PVOH system demonstrates superior long-term and hydrolytic stability, which are attributed to the removal of the driving force for hydrophobic recovery by inserting a hydrophobic PDMS layer between a hydrophilic, plasma-oxidized, PDMS bulk and a hydrophilic PVOH exterior. This is a new concept in addressing hydrophobic recovery of hydrophilized silicones. The spontaneous nature of the adsorption process and the crystallinity of the PVOH barrier layer are the other advantages demonstrated in this study.