Characterization of bonding between poly(dimethylsiloxane) and cyclic olefin copolymer using corona discharge induced grafting polymerization

Abstract

Thermoplastics have been increasingly used for fabricating microfluidic devices because of their low cost, mechanical/biocompatible attributes, and well-established manufacturing processes. However, there is sometimes a need to integrate such a device with components made from other materials such as polydimethylsiloxane (PDMS). Bonding thermoplastics with PDMS to produce hybrid devices is not straightforward. We have reported our method to modify the surface property of a cyclic olefin copolymer (COC) substrate by using corona discharge and grafting polymerization of 3-(trimethoxysilyl)propyl methacrylate; the modified surface enabled strong bonding of COC with PDMS. In this paper, we report our studies on the surface modification mechanism using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and contact angle measurement. Using this bonding method, we fabricated a three-layer (COC/PDMS/COC) hybrid device consisting of elastomer-based valve arrays. The microvalve operation was confirmed through the displacement of a dye solution in a fluidic channel when the elastomer membrane was pneumatically actuated. Valve-enabled microfluidic handling was demonstrated.

Figure 1

(a) Layout of the two-dimensional protein separation device containing a horizontal channel AB and vertical channels CD. The size of the device is 1” x 3”. (b) An exploded view of intersections with a smaller number of channels is shown for simplicity. Channels connecting to reservoir C are staggered with those connecting to reservoir D. Two individually addressable control channels (gray color) are located at each side of the AB channel and they function as a part of microvalves.

Figure 2

ATR-FTIR spectra of COC in the native form and those after various surface treatments. (a) Native COC; (b) COC surface activated by corona discharges; (c) COC surface activated and then treated with TMSPMA; and (d) COC surface activated, TMSPMA-treated, and exposed to the second corona discharges, followed by annealing.

Figure 3

(a) Chemical structure of 3-(trimethoxysilyl)propyl methacrylate (TMSPMA). (b) Chemical structure of (3-aminopropyl)triethoxysilane (APTES). (c) Chemical structure of Zeonor polymer according to the manufacturer. R₁ to R₄ are functional groups while n & m are the number of monomer units in the polymer. (d) Generation of TMSPMA radicals at C=C bond after corona discharges while some Si-OCH₃ were transformed to Si-OH (x ≤ 3). (e) Attachment of TMSPMA to COC through the formation of covalent bond between the radical and C-H groups on the surface via grafting polymerization. (f) Generation of OH groups on PDMS surfaces by activation. (g) Bonding of COC and PDMS through the formation of covalent bond via the dehydration reaction between Si-OH groups on both COC and PDMS surfaces.

Figure 4

(a) XPS spectra of native COC (black) and TMSPMA-grafted COC (red). (b) Exploded view of the narrow band of C1s region in the XPS spectrum of the TMSPMA-grafted COC in Figure 4a.

Figure 5

Pictures of representative water droplets on the surfaces of (a) native COC, (b) corona-discharge-activated COC, (c) TMSPMA-coated COC, and (d) TMSPMA-coated COC after the second corona discharge.

Figure 6

AFM height images of the surfaces of (a) native COC, (b) corona-discharge-activated COC, (c) activated COC with TMSPMA coating (d) TMSPMA-coated COC after the second corona discharge, and (e) 3-dimensional topographic picture of (d). The height range is indicated by the scale bar. All images were obtained under a dry condition on the surface.

Figure 7

Photographs of channel intersections showing valves open (a) and closed (b). For easy visualization, a solution of a red food dye was filled in the 39 parallel flow channels (5 channels are in the field view of the microscope) and the IEF channel. When the control channel is compressed by a high-pressure N₂ source, the valves were actuated (closed) and the red solution in the valve region was displaced. (c) Photograph of the channel intersections after the solution in the central channel was replaced with another solution (dyed in blue for easy visualization) while valves were closed.