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

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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.

Introduction

Silicones are widely used in science and technology due to their unique characteristics, such as elasticity, gas permeability, thermal stability, hydrophobicity, and reactivity [[1], [2], [3]]. Polydimethylsiloxane (PDMS) is the most common type of silicone. The hydrophobicity, however, causes issues in applications that require adhesion and wetting. Significant efforts ranging from plasma treatment to wet chemical approaches have been made to explore methods to hydrophilize PDMS [4,5]. Oxygen plasma treatment is the most common method to hydrophilize PDMS substrates and makes use of a gaseous mixture of high energy species, including electrons, ions, radicals, and excited species to oxidize surface methyl groups [6,7]. Plasma treatment results in the formation of a silica-like surface layer, SiOx [[8], [9], [10], [11], [12]].

Hydrophilized PDMS surfaces have been observed to recover their hydrophobicity rapidly, especially within the first few hours after exposure to air [[13], [14], [15]]. Hydrophobic recovery is spontaneous and is driven by the reduction of the high interfacial energy between the hydrophilic surface and air. Owen and others [11,16] attributed the recovery to a number of factors including reorientation of surface hydrophilic groups into the bulk [12,17,18], condensation of surface silanol groups [12], migration of low molecular weight (LMW) species from the bulk to the surface [9,11,17,19,20], in-situ generated surface cracks facilitating diffusion of LMW species [10,21], and in-situ created LMW species at the surface [11]. The diffusion of LMW species to the surface has been implicated as the major mechanism for the recovery [17]. Solvent extraction to remove free polymeric/oligomeric species prior to surface hydrophilization has been shown to reduce hydrophobic recovery [[22], [23], [24]]. However, this requires a large amount of organic solvent and is labor and energy intensive. We recently reported extraction of LMW species under reduced pressure for thin films and under ambient pressure for thick films prior to oxygen plasma treatment [25]. The method is effective at producing hydrophilized silicone samples that are stable during the monitoring period of 30 days. Despite efforts by us and others to remove existing LMW species, PDMS can undergo equilibration to create LMW species at moderate temperature [[26], [27], [28]]. Therefore, it is inevitable for the newly generated LMW species to migrate to the treated PDMS surface and reduce its hydrophilicity over time.

A potentially effective strategy to retard the migration of existing and newly generated LMW species is to attach a hydrophilic polymer layer to PDMS substrates. This can be accomplished by surface oxidation and grafting of hydrophilic polymers [[29], [30], [31], [32], [33], [34]]. Physisorption of polymers is an attractive alternative due to its spontaneity. More than a decade ago, poly(vinyl alcohol) (PVOH) adsorption from aqueous solution was established as a general method to hydrophilize hydrophobic substrates [[35], [36], [37], [38]]. PVOH is different from other water-soluble synthetic polymers in that it is atactic yet crystalline. The spontaneous PVOH adsorption was attributed to hydrophobic interactions and the subsequent crystallization of PVOH polymer chains at the interface [37]. Earlier attempts at adsorbing PVOH to unmodified PDMS substrates did not yield noticeable change in wettability [[39], [40], [41], [42]]. However, we recently reported significant enhancement in hydrophilicity via PVOH adsorption on silicon-wafer supported covalently attached linearPDMS of 2 kDa molecular weight (Si-linearPDMS2k) [43]. The substrate is completely covered by a continuous PVOH film of ∼3 nm in thickness. As the PDMS molecular weight increases, the adsorbed PVOH thin films dewet on thicker, more liquid-like PDMS layers upon exposure to air. 1-s oxygen plasma treatment of Si-linearPDMS substrates of higher molecular weights generated enough pinning sites to yield more continuous PVOH films [43]. Free LMW species are absent in the attached linearPDMS layers, therefore, hydrophobic recovery was not a concern. In many applications, however, thick/bulk PDMS substrates require hydrophilization. It is thus important to develop robust methods to address hydrophobic recovery caused by LMW species present in thick PDMS substrates. Trantidou et al. recently reported that stable PVOH films were obtained after depositing 1% PVOH solution onto oxygen plasma treated PDMS substrates and heating the samples to 110 °C for 15 min [44]. Static contact angles of 37 ± 19° and 50–55° were obtained on samples immediately and 30 days after deposition of 99% hydrolyzed PVOH, respectively, indicating some hydrophobic recovery. On the other hand, deposition of 87% hydrolyzed PVOH resulted in stable static contact angles ∼25° over the 30-day period [44]. The extent of hydrophilization is difficult to evaluate since dynamic contact angles were not reported.

In this study, spontaneous PVOH adsorption to unmodified and modified Sylgard PDMS films was carried out to determine the conditions under which long-lasting, hydrophilic PDMS can be accomplished. The emphasis was placed on the layered construction of the coating materials to remove the driving force for hydrophobic recovery without requiring the removal of LMW species.

Section snippets

Materials

Silicon wafers (100 orientation, P/B doped, resistivity 1–10 Ω-cm, thickness 475–575 μm) were purchased from International Wafer Service. Poly(vinyl alcohol) (PVOH: 89–98 kDa and 99 + % hydrolyzed) was purchased from Sigma–Aldrich. Sylgard-184 elastomer kit was purchased from Dow Corning. Polydimethylsiloxane trimethylsiloxy terminated (linearPDMS2k: M.W. = 2 kDa) was purchased from Gelest. HPLC-grade organic solvents were obtained from Pharmco. Oxygen gas (99.999%) was purchased from Middlesex

PDMS substrates

Sylgard-184, a commercial PDMS-based elastomer, was chosen because it is a widely used material. Here PDMS refers to crosslinked Sylgard PDMS and linearPDMS refers to uncrosslinked linear PDMS. Spin coating was used to prepare smooth PDMS samples of a few microns in thickness so that surface topography can be characterized by AFM and so that samples were thick enough to study the effect of LMW species on hydrophobic recovery. In addition to PDMS control, three modified PDMS substrates – PDMSox1s

Conclusions

PVOH adsorption was carried out on unmodified and modified Sylgard PDMS films to enhance wetting. On PDMS control and 1-s oxygen plasma oxidized PDMS, adsorption resulted in extensive PVOH dewetting hence insignificant improvement in hydrophilicity. On 60-s oxygen plasma treated PDMS, negligible adsorption took place due to the lack of hydrophobic driving force. On the other hand, covalent attachment of linear 2 kDa PDMS polymer to 60-s oxygen plasma treated PDMS resulted in a system that

Acknowledgements

Financial support was provided by the National Science Foundation (DMR-1404668) and Mount Holyoke College. The authors are grateful to Dr. Alexander Ribbe for his assistance with the AFM work.

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