Synthesis and characterization of a poly(dimethylsiloxane)-poly (ethylene oxide) block copolymer for fabrication of amphiphilic surfaces on microfluidic devices

A poly(dimethylsiloxane)-poly(ethylene oxide) (PDMS-PEO) vinyl terminated block copolymer has been synthesized via a simple hydrosilylation reaction between hydride-terminated PDMS and PEO divinyl ether. This prepolymer can be subsequently cross-linked into an elastomer in a second hydrosilylation reaction involving a methylhydrosiloxane-dimethylsiloxane copolymer, forming a material suitable for the purposes of fabricating microfluidic devices. The presence of the PEO block in the prepolymer chain results in a much more hydrophilic material following cross-linking. The surface water contact angle of the PDMS-PEO material is 65° ± 3 (n = 6), as opposed to approximately 110° for native PDMS. Droplets of water straddled by air within molded channels of the PDMS-PEO are concave in shape with contact angles where the fluid meets the side walls of 32° ± 4 (n = 8), while droplets in PDMS microchannels are more convex with contact angles of 95° ± 6 (n = 6). The length of the PDMS-PEO prepolymer chain and the multifunctional hydride cross-linker chains appear to dictate the durability of the elastomeric material. Young's modulus measurements yielded values of 0.94 ± 0.08,2.6 ± 0.8, and 1.91 ± 0.06 MPa for a [5% vinyl excess prepolymer and 10-fold excess of cross-linker], [10% vinyl excess prepolymer and 5-fold excess of cross-linker], and 10:1 PDMS, respectively, confirming that the elasticity of the cross-linked PDMS-PEO is similar to that of PDMS (Sylgard 184:10:1 mixture of elastomeric base to elastomer curing agent). The PDMS-PEO material still possesses enough PDMS character to allow molded channel architectures to be sealed between two pieces of the block copolymer by conformai contact. As a result of the more hydrophilic nature of the material, the channels of devices fabricated from this polymer are self-filling when using aqueous buffers, making it more user-friendly than PDMS for applications calling for background electrolytes void of organic modifiers. Different compositions of PDMS-PEO devices feature different electroosmotic flow values with the 5% vinyl excess prepolymer EOF values of 2.5 ± 0.7 × 10^-4 and 5.7 ± 0.8 × 10^-4 cm²/(V s) at pHs 6 and 9, respectively, and 1.2 ± 0.3 × 10^-4 and 2.5 ± 0.3 × 10^-4 cm²/(V s) for the 10% vinyl excess prepolymer device at pHs 6 and 9, respectively.