Simple and Improved Approaches to Long-Lasting, Hydrophilic Silicones Derived from Commercially Available Precursors

Abstract

Three types of commercially derived methylsilicone materials, Sylgard-184, Q(V)Q(H) (an MQ-based silicone containing no dimethylsiloxane, D units), and D(V)D(H) (a D-based silicone with no additives), were judiciously chosen to study the conditions under which long-lasting hydrophilicity after oxygen plasma treatment can be obtained. A 30 s plasma treatment time under controlled conditions was found to be optimal in terms of achieving the lowest initial advancing and receding contact angles of θ(A)/θ(R) = 10°/5° with undetectable surface damage. Vacuum treatment, a necessary step prior to plasma ignition that has been overlooked in previous studies, as well as room temperature curing were explored as means to remove low molecular weight species. For thin films (a few micrometers), 40 min vacuum treatment was sufficient to achieve low dynamic contact angles of θ(A)/θ(R) = 51-56°/38-43° on all three types of silicones measured more than 30 days after the plasma treatments. These values indicate superior hydrophilicity relative to what has been reported. The small and slow rise in contact angle over time is likely caused by the intrinsic nature of the silicone materials, i.e., surface reorientation of hydrophilic functional groups to the bulk and condensation of surface silanol groups, and is thus unavoidable. For thick films (∼1 mm), room temperature curing in addition to vacuum treatment was required to reduce hydrophobic recovery and to achieve long-lasting hydrophilicity. The final contact angles for thick samples were slightly higher than the corresponding thin film samples due to the greater "reservoir" depth and migration length for mobile species. In particular, Sylgard exhibited inferior performance among the thick samples, and we attribute this to the additives in its commercial formulation. Furthermore, unlike polydimethylsiloxane-based silicones, Q(V)Q(H) does not contain equilibration products of the Dn-type; its thin films perform as well as those of Sylgard and D(V)D(H). Silicones without D units are promising materials with intrinsically low hydrophobic recovery characteristics and long-lasting hydrophilicity after oxygen plasma treatment.

Long-term hydrophilization of polydimethylsiloxane (PDMS) for capillary filling microfluidic chips

“…In general, three mechanisms have been proposed to contribute to hydrophobic recovery of PDMS: Bulk to surface migration of the low molecular weight species (LMW) (Toth et al 1994;Hillborg et al 2000Hillborg et al , 2004, surface re-structuring phenomena including re-orientation of hydroxyl groups toward bulk due to energetically unfavorable arrangement and condensation of silanol groups on the surface (Morra et al 1990;Bacharouche et al 2013), and generation of LMW species within the surface during the plasma treatment (Toth et al 1994;Kim et al 1999Kim et al , 2001a. Process parameters which significantly influence the contribution of such mechanisms include pre-polymer composition, curing temperature and time, sample thickness, vacuum treatment, plasma treatment parameters, post-plasma modifications and sample storage conditions (Nguyen 2014). Numerous efforts have been conducted on reducing, if not completely eliminating, the recovery over time by focusing on one of those process parameters.…”

Preparation of free‐standing silicone particles in aqueous heterogeneous system

“…Silicone is generally prepared through hydrolytic condensation of chlorosilane or alkoxysilane or through a hydrosilylation reaction between hydromethylsiloxane and methyl vinyl silicone using platinum as the catalyst in an organic medium . The hydrosilylation reaction is the most used method for commercial purposes, for example, in the silicone elastomer kit Sylgard 184 (Dow Corning) . Sylgard 184 contains two types of liquids, allowing the degree of cross‐linking to be easily modified.…”

“…Moreover, this process proceeds in alcohol medium (not aqueous medium). Although there are several patents prepared cross‐linked silicone particles using aforementioned hydrosilylation reaction, the metal catalysts remain in the products …”

Hydrophilic Surface Modification of PDMS Microchannel for O/W and W/O/W Emulsions

“…Photoinitiated grafting [25][26][27] strategies increase operational costs and become cumbersome in bonded microchannels due to non-uniformity of coatings. Nguyen et al [28] successfully achieved the extended hydrophilicity of PDMS using extended curing time and temperature, and exposure to vacuum prior to oxygen plasma treatment. Kovach and his co-workers [29] achieved long term hydrophilicity and hemocompatibility of PDMS microchannels using a plasma-assisted polyethylene glycol (PEG) grafting.…”