Monday June 30, 2008 : Making PDMS

Jennifer, Jennifer and I start out the day with Chelsea Davis and Doug Farmer, who outline our program. Yuri will also be working with us. Doug is a fourth year, Chelsea is a second year, and Yuri is a first year PhD student.

PDMS is made of a pre-polymer (Dow Corning Sylgard 184) and a cross linking (curing) agent. We will be using a 10:1 ratio; ten parts of pre-polymer and one part cross linking agent. We make a 10:1 and a 20:1 sample of PDMS just to see the difference between the two. The 10:1 is much less flexible than the 20:1. This makes sense because there is less cross linker per ml in the 20:1 than in the 10:1, so the 10:1 should be stiffer.

When making PDMS, it is important that time is allowed for bubbles trapped during the mixing process to rise to the surface and be released. This can be done by either using a desicator with a vacuum pump attached (this can be a bit dangerous as too much vacuum applied too fast causes the PDMS to overflow its container), or by letting the PDMS sit for a while (an hour or so) then blowing the bubbles off of the surface. The PDMS is cured into the desired mold, and cured to hardness in one of three ways. The PDMS can sit a room temperature for 24 hrs, or it can cure in a 70 C oven for 3 hours, or it can cure in the 110 C oven for 20 minutes. Each process results in a slightly different stiffness of the product (heat increases the chance of the cross linker coming in contact with the pre-polymer). Once the PDMS has cured, remove it from the mold using a razor blade: the deisred side of the PDMS is DOWN inside the mold, not the top surface. The top surface will have a meniscus, while the bottom surface will be shaped by the mold. The removed, molded PDMS should be placed on a glass slide; the side of the PDMS that was down in the mold is now up on the glass slide. Store the PDMS in a petri dish until you are ready to use it. It will be useful for about 6 hrs.

We were given three papers to read, one concerning how PDMS cross links and how exposure of PDMS to UVO (Ultaviolet light and ozone) creates a glassy surface on the PDMS. This occurs because the methyl groups, normally found attached to the Si as a side branch in the Si-O-Si-O chain is replaced by oxygen, which then becomes a hydroxyl group. This makes the surface similar to SiO2, or glass. This new surface is hydrophilic (oxygen present) whereas the CH3 was hydrophobic. This changes the properties of the PDMS in other ways as well, affecting elasticity.

The UVO oven needs to be turned on about five minutes prior to use. Place the sample, on a glass slide, in the oven, then close the drawer tightly. Set the timer for the desired exposure, then watch to see if the blue light comes in in the top center of the drawer. If it is on, the UVO exposure is underway. The oven will shut itself off with the timer expiration.

In the afternoon, we learn how to use the VEECO DEKTAK 150 PROFILOMETER. It is mounted on a vibration free table, as the instrument is sensitive to vibration and this could substantially affect results. The instrument is quite sensitive; the tip used to measure depth/thickness has a 12.5 micron radius. The stage of the instrument is motorized for optimum control and to prevent damage to the instrument.

SEE PROFILOMETER Dektak 150 PROCEDURE For Operating Instructions.

In the afternoon, Doug taught us about contact angle and how it can be used to measure surface energy. Hydrophilic materials have low surface energy and a low contact angle. Hydrophobic materials have high surface energy and higher contact angles.