Selective fluorination of the surface of polymeric materials after stereolithography 3D printing
Megan A. Catterton, Alyssa N. Montalbine, and Rebecca R. Pompano
With the microfluidics community embracing 3D resin printing as a rapid fabrication method, controlling surface chemistry has emerged as a new challenge. Fluorination of 3D printed surfaces is highly desirable in many applications due to chemical inertness, low friction coefficients, anti-fouling properties and the potential for selective hydrophobic patterning. Despite sporadic reports, silanization methods have not been optimized for covalent bonding with polymeric resins. As a case study, we tested the silanization of a commercially available (meth)acrylate-based resin (BV-007A) with a fluoroalkyl trichlorosilane. Interestingly, plasma oxidation was unnecessary for silanization of this resin, and indeed was ineffective. Solvent-based deposition in a fluorinated oil (FC-40) generated significantly higher contact angles than deposition in ethanol or gas-phase deposition, yielding hydrophobic surfaces with contact angle > 110° under optimized conditions. Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectroscopy indicated that the increase in contact angle correlated with consumption of a carbonyl moiety, suggesting covalent bonding of the silane without plasma oxidation. Consistent with a covalent bond, the silanization was resistant to mechanical damage and hydrolysis in methanol, and was stable over long-term storage. When tested on a suite of photocrosslinkable resins, this silanization protocol generated highly hydrophobic surfaces (contact angle > 110°) on three resins and moderate hydrophobicity (90 – 100°) on the remainder. Selective patterning of hydrophobic regions in an open 3D-printed microchannel was possible in combination with simple masking techniques. Thus, this facile fluorination strategy is expected to be applicable for resin-printed materials in a variety of contexts including micropatterning and multiphase microfluidics.
Keywords: Two-phase microfluidics, Droplet microfluidics, low surface energy, Digital Light processing (DLP), stereolithography printing (SLA)