Polydimethylsiloxane (PDMS) is a synthetic silicone elastomer valued for its unique properties. This polymer transitions from a highly viscous liquid to a flexible, rubbery solid during a chemical curing process. PDMS is used in various scientific and engineering fields due to its optical transparency, biocompatibility, and thermal stability. Primary applications include fabricating microfluidic devices and using soft lithography to create intricate molds. Understanding the procedures for mixing, degassing, and curing PDMS is foundational to utilizing this versatile material.
Essential Components and Chemistry
PDMS is typically supplied as a two-part kit, such as the widely used Sylgard 184 system. The first part is the base component, consisting primarily of long-chain, dimethylvinyl-terminated polydimethylsiloxane polymer molecules. This highly viscous liquid makes up the bulk of the final cured elastomer.
The second component is the curing agent, also called the cross-linker or catalyst. It contains species like dimethyl, methylhydrogen siloxane, and initiates the polymerization reaction when combined with the base. This reaction, called hydrosilylation, involves hydrogen atoms on the curing agent reacting with vinyl groups on the base polymer chains. The resulting chemical cross-links transform the liquid mixture into a cohesive, solid network.
Determining the Optimal Mixing Ratio
The mass ratio between the base polymer and the curing agent is the most important factor determining the mechanical properties of the final PDMS product. The manufacturer-recommended standard for many applications, including microfluidics, is a 10:1 ratio of base to curing agent by weight. This standard ratio yields a material with balanced properties, exhibiting good flexibility and an average Shore A hardness of around 43.
Modifying this ratio allows researchers to tailor the material’s elasticity, measured by its Young’s modulus. Increasing the proportion of the curing agent, such as a 5:1 ratio, introduces more cross-linking sites, resulting in a harder and stiffer material. The elastic modulus of PDMS is linearly proportional to the amount of cross-linker across a range of ratios from 5:1 to 33:1.
Conversely, decreasing the amount of curing agent to a 20:1 ratio produces a softer, more pliable elastomer. Research indicates that tensile strength can decrease when the ratio of curing agent is increased beyond the standard 10:1, suggesting an optimal point for mechanical performance. Accurate weighing using a precision scale is necessary to reliably achieve the desired mechanical characteristics and ensure batch-to-batch consistency.
Preparation: Weighing, Mixing, and Degassing
The preparation phase begins with the precise measurement of both components by mass once the desired ratio is determined. Careful handling is necessary due to the highly viscous nature of the components, which prevents material loss and premature air incorporation. After weighing, the two components must be thoroughly mixed until the liquid appears completely homogeneous. Stirring should be performed slowly and methodically, perhaps over 10 to 20 minutes, to minimize the introduction of air bubbles.
Despite careful stirring, the physical act of mixing inevitably traps air, appearing as microbubbles within the highly viscous pre-polymer solution. These trapped bubbles must be removed through degassing, as their presence compromises the material’s strength and optical clarity. The most common method involves placing the mixed PDMS into a vacuum chamber or desiccator connected to a vacuum pump.
Applying negative pressure causes the entrapped air bubbles to expand significantly, forcing them to migrate to the surface where they burst. For a standard 10-gram solution, traditional vacuum degassing may take 30 minutes or more, depending on the vacuum system’s strength. A technique involving periodic pressure cycling, where the vacuum is momentarily released and reapplied, can accelerate the process by forcing the bubbles to burst due to the dramatic pressure change. For small volumes, an alternative method uses a laboratory centrifuge, which leverages centrifugal force to bring the bubbles to the surface in as little as two minutes.
Curing and Post-Processing
The final stage is curing, the thermal process that transforms the liquid PDMS into its final elastomeric state. Curing time is inversely proportional to temperature, a trade-off adjusted based on the required production speed and desired material properties. A slow cure can be achieved by leaving the material at room temperature, though this process often requires 48 hours or more to fully complete.
Alternatively, the cure can be accelerated dramatically by applying heat, such as placing the PDMS in an oven at 150°C, reducing the required time to only 10 minutes. A higher curing temperature tends to yield a stiffer and more rigid final product, while slower, lower-temperature cures result in a softer, more flexible elastomer. While Young’s modulus continues to increase with higher temperatures, curing above 125°C may begin to reduce the material’s overall tensile strength.
Following the initial cure, post-curing can be implemented to improve the material’s long-term stability. Post-curing involves heating the already cured PDMS, often around 200°C, to remove trace amounts of residual uncured oligomers (volatile compounds). The removal of these volatiles enhances the final mechanical and electrical stability of the elastomer. Once solidified, the material is carefully demolded, often by gently peeling it away from the substrate, and is then ready for handling, cutting, or storing.