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Polydimethylsiloxane


Polydimethylsiloxane (PDMS) is a commonly used silicon-based organic polymer. Due to its unique mechanical, chemical, and optical properties, it has become integrated into many microfluidic and optical devices.

Procedures

Polydimethylsiloxane can be purchased as a two-part kit. The kit consists of a base and a cross-linking agent. The two parts are in a viscous liquid form until mixed and cross-linking occurs. The cross-linking procedure will occur without aid once the two parts are mixed. However, the procedure can be greatly accelerated with heat. The mixing ratios and curing procedures used during development determine the mechanical, chemical, and optical properties of the final solid.

Mechanical Properties

When cross-linked, PDMS acts like a rubbery solid. In this state, the polymer does not permanently deform when under stress or strain. Rather, the elastic polymer will return to its original shape when released. The elastic property of PDMS is highly dependent on the amount of cross-linking agent (often methyltrichlorosilane) integrated into the polymer. The higher the concentration of cross-linking agent, the more solid the polymer ultimately becomes. With little or no cross-linking agent, the polymer will remain a viscous liquid. Since the curing process changes PDMS from a liquid into an elastic solid, PDMS is commonly used in microfabrication molds. PDMS has been used as walls for microfluidic channels and as a silicon wafer bonding agent.

Chemical Properties

PDMS is generally considered to be chemically inert. It is also notably hydrophobic, meaning that water cannot easily penetrate its surface. This property has led extended use of PDMS in microfluidics. However, most organic solvents can still penetrate the PDMS surface, limiting its versatility. PDMS has also increasingly been used in extraction processes, where PDMS is used to remove organic contaminants from water for analysis. As organic solvents are absorbed into the polymer, the volume of the polymer must increase, or swell, to account for the volume of the introduced chemicals. The solubility parameter of each chemical determines the amount of swelling that occurs. Neither chemical absorption nor physical swelling are permanent. The absorbed chemicals can just as easily diffuse out of the polymer as they can diffuse in. The diffusion mechanics are dependent on equilibrium states between the polymer and the surrounding medium. Therefore, absorbed chemicals will remain in the polymer as long as a similar concentration of that chemical exists in the surrounding medium at the PDMS surface. If the concentration in the medium decreases, then diffusion mechanics will cause the absorbed chemical to naturally flow out of the PDMS until a new equilibrium is met.

Optical Properties

PDMS is optically clear at a wide range of wavelengths. In addition, the curing time and temperature used during cross-linking can determine the refractive index (RI) of the bulk. Since the polymer can be easily molded, it has been used to form lenses and waveguides. Also, the effective RI and absorption spectrum of PDMS are changed when organic compounds are physically absorbed into the polymer. These properties have formed the basis for several fiber-optic based chemical sensors. Through monitoring changes in refractive index or absorption spectrum, chemical concentrations absorbed into a volume of PDMS may be known and characterized.

PDMS Film Recipe

The BYU photonics laboratory uses the following procedure to create PDMS films

1. Mix the PDMS base and curing agent in a 10:1 ratio measured by weight.
2. De-gas the polymer in a desiccator for approximately 30 minutes. This removes any air bubbles resulting from the mixing process.
3. Spin the polymer on a silicon wafer at 4000 rpm for 60 seconds. This spin recipe results in an approximate 2.5 Ám layer.
4. Cure the PDMS-coated wafer at 110░ C for 1.5 hours to promote cross-linking. Also, store the wafer at room temperature for an additional 24 hours before use.




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