Solid (cured) PDMS is a cross-linked polymer, which means it cannot be truly dissolved back into a liquid the way you’d dissolve sugar in water. However, you can swell it with compatible solvents, chemically break it down, or, if you’re working with uncured PDMS, thin it before curing. The right approach depends on whether your PDMS is already cured and what you’re trying to achieve.
Why Cured PDMS Won’t Dissolve Like a Normal Solid
Once PDMS cures (for example, Sylgard 184 after heat treatment), its polymer chains become permanently cross-linked into a three-dimensional network. Solvents can penetrate that network and cause it to swell, sometimes dramatically, but the chains remain connected. You end up with a swollen gel rather than a true solution. If you evaporate the solvent, the PDMS returns roughly to its original shape.
This distinction matters because “diluting” cured PDMS really means either swelling it to soften and expand it, or chemically degrading the cross-links to break it apart entirely. Each route uses different chemicals and gives different results.
Swelling Cured PDMS With Organic Solvents
The most practical way to soften and expand cured PDMS is to soak it in a solvent whose solubility parameter is close to that of PDMS itself (about 7.3 cal½cm-3/2). The closer the match, the more the solvent penetrates the polymer network. Solvents in the 7.3 to 9.5 range work best.
Here are some common solvents ranked by how much they swell cured PDMS, expressed as a swelling ratio (the swollen size divided by the original size):
- Diisopropylamine: 2.13 (highest swelling)
- Triethylamine: 1.58
- Pentane: 1.44
- Xylenes: 1.41
- Chloroform: 1.39
- Diethyl ether: 1.38
- Tetrahydrofuran (THF): 1.38
- Hexane: 1.35
- Toluene: 1.31
Diisopropylamine more than doubles the volume of a PDMS piece, while toluene expands it by about 31%. For quick, aggressive swelling, ether shows the highest compatibility within the first two minutes. Xylene overtakes it at around the five-minute mark. If you submerge a thin PDMS sheet in toluene, expect visible curling within about five minutes as the solvent absorbs unevenly.
Solvents that barely swell PDMS at all include water, dimethyl sulfoxide (DMSO), ethylene glycol, and acetonitrile. These have solubility parameters far from the 7.3 target and are essentially useless for this purpose.
Chemically Breaking Down Cured PDMS
If you need to actually remove or destroy cured PDMS rather than just soften it, chemical degradation is the route. A mixture of 30% potassium hydroxide (KOH) by weight, 20% isopropanol (IPA), and 50% deionized water, heated to 70°C, can etch away cured PDMS completely in about 20 minutes. This solution attacks both modified and unmodified PDMS, so it is not selective. It is useful when you need to strip PDMS off a substrate or clean a mold, but it destroys the material rather than preserving it in a diluted form.
Fluoride-based reagents like tetrabutylammonium fluoride (TBAF) can also cleave the silicon-oxygen bonds in the PDMS backbone. This is a more specialized approach typically used in research settings to digest PDMS microfluidic devices for analysis of their contents.
Thinning Uncured PDMS Before Curing
If your goal is to make PDMS thinner or less viscous before it cures, you have two main options: adding a compatible solvent to the uncured mixture, or blending in a low-viscosity silicone oil.
Using a Solvent
You can mix uncured PDMS (the base and curing agent already combined) with a solvent like toluene or hexane to reduce viscosity. This makes it easier to spin-coat very thin films or fill fine features in a mold. The solvent evaporates during or before the curing step, leaving behind a thinner PDMS layer. The tradeoff is that forming the network in the presence of solvent reduces the stiffness of the final material. The storage modulus drops because the solvent dilutes polymer chains during cross-linking, reduces physical entanglements, and increases the formation of loop defects (chains that connect back to themselves instead of to neighboring chains). High solvent loadings also make the cured PDMS noticeably less tough and more fragile.
Using Silicone Oil
A cleaner alternative is blending uncured PDMS with a low-viscosity PDMS-based silicone oil. Because the oil is chemically similar, it mixes easily and does not introduce volatile organic compounds. Researchers have found that a mass ratio of silicone oil to PDMS between 0.25 and 0.35 works well for spin-coating ultra-thin membranes (down to roughly 12 micrometers thick at a ratio of 0.3). Below 0.25, the mixture may still be too viscous to form the features you need. Above 0.35, the excess oil can interfere with layer bonding if you are assembling multilayer structures. The silicone oil remains in the final product, so expect a softer, more flexible result compared to standard Sylgard 184.
Choosing the Right Approach
Your method depends on what state your PDMS is in and what you need the end result to look like:
- Cured PDMS you want to soften or expand: Soak in hexane, toluene, or xylenes. Diisopropylamine gives the most swelling but is less commonly stocked.
- Cured PDMS you want to remove entirely: Use the KOH/IPA/water mixture at 70°C.
- Uncured PDMS you want thinner for coating: Add toluene or hexane for a volatile diluent, or blend in silicone oil at a 0.25 to 0.35 ratio for a non-volatile option.
Keep in mind that any solvent-based approach with cured PDMS is reversible. Once the solvent evaporates, the PDMS returns to roughly its original dimensions and stiffness. If you need a permanent change in the material’s properties, you need to either reformulate the uncured mixture or chemically degrade the cured piece and start over.