Solvent Compatibility of Poly(dimethylsiloxane)-Based Microfluidic Devices
Authors: Jessamine Ng Lee, Cheolmin Park, and George M. Whitesides
Publication: Lee et al., "Solvent Compatibility of Poly(dimethylsiloxane)-Based Microfluidic Devices", Analytical Chemistry (2003) 75: 6544-6554
Key words: PDMS, solvent compatibility, swelling, microfluidic
PDMS is widely used in bio-microfluidic devices due to its ease of fabrication, transparency in the UV-visible regions, chemical inertness, low polarity, low electrical conductivity and elasticity. Its low cost and bio-compatibility makes PDMS an ideal material in microdevices and MEMS.
This paper discussed the compatibility of Poly(dimethylsiloxane) (PDMS) and different solvents with respect to three aspects( the swelling of PDMS in a solvent, the partitioning of solutes between a solvent and PDMS, and the dissolution of PDMS oligomers in a solvent), which are important in PDMS's potential applications in organic synthesis.
For two materials to be soluble, their cohesive energy densities must be similar, since this energy must be overcome to separate the molecules of the solute to allow the molecules of solvent to insert. The cohesive energy δ is defined as the energy associated with the intermolecular attractive interactions within a unit volume of material. For materials such as crosslinked polymers that do not dissolve, solubility is measured by the degree of swelling.
Table 1 shows the cohesive energy δ of some frequently used solvents, while the δ of PDMS is around 7.3 cal1/2 cm-3/2, from which we can give a rough prediction for solubility.
Swelling of PDMS in organic solvents
The swelling ratio S of PDMS in different organic solvents is plotted in Fig. 1. The general trend shows a swelling peak when cohesive energies are similar for solvents and PDMS, e.g. PDMS in diisopropylamine, whose δ is almost the same as PDMS, gives the highest swelling ratio. This general trend deviates in small but important ways, e.g. acetone and methylene chloride have indistinguishable solubility parameters but methylene chloride swells PDMS much more than does acetone. This deviation is related to the difference in solvents' polarity.
In general, highly-polarized solvents (e.g. water, ethanol etc.) and fluorocarbons have low solubility and are well suitable for PDMS microfluidic systems, whereas nonpolar or slightly polar solvents have high solubility, e.g. benzene, pentane etc., and are not compatible with PDMS devices. On the other hand, high-solubility solvents can be used to extract oligomers from PDMS.
Change the surface properties of PDMS: PDMS is intrinsically hydrophobic but can change into hydrophilic when exposed to air or oxygen plasma. After plasma treatment, PDMS would gradually lose its hydrophilic property left in contact with air. This process is due to the oligomers inside PDMS moving to the surface. Thus we can use an extremely soluble solvent (diisopropylamine) to extract the soluble components from cross-linked PDMS to keep its hydrophilic property. The experimental result is shown in Fig. 2.
Deswelling of PDMS: Swelled PDMS can return to its original shape by removing the solvent. However, this process can cause PDMS to buckle if put in air. We can use decreasing soluble solvents to deswell PDMS without damaging it.
Influence of Swelling on PDMS bonded to glass: PDMS can be bonded to glass irreversably after the two surface are both treated by oxygen plasma. The bonded PDMS can be desealed by highly soluble solvents.
Swelling of PDMS in Acids and Bases: It turns out that PDMS are quite compatible with most acids and bases.
Reactive solvents: The reagents that react with PDMS include inorganic acids (concentrated sulfuric acid and trifluoroacetic acid) and some organic reagents (TBAF solution in THF, dipropylamine), which can also serve as PDMS etching agents.
Partitioning of solutes between a solvent and PDMS
The loss of solute in a PDMS microfluidic device due to its solubility in PDMS can be a big problem. The authors tested the partitioning of a few solutes in different solvents experimentally. Result shows this process is not important, it is generally more common for a solute to be miscible with a solvent than a polymer.
Extraction of PDMS oligomers into organic solvents
When extracted out by solvents, PDMS oligomers can contaminate the reactive system. Generally, the amount of extracted PDMS increases as swelling ratio increases (also solubility). This effect can be eliminated to negligible amount when first treated in high-solubility solvents.
Of these three parameters that determine the compatibility of PDMS with a solvent, the swelling of PDMS had the greatest influence. Solvents that swelled PDMS the least included water, nitromethane, dimethyl sulfoxide, ethylene glycol, perfluorotributylamine, perfluorodecalin, acetonitrile, and propylene carbonate; solvents that swelled PDMS the most were diisopropylamine, triethylamine, pentane, and xylenes. Highly swelling solvents were useful for extracting contaminants from bulk PDMS and for changing the surface properties of PDMS.