Polydimethylsiloxane is a versatile material that could not be formed into long filaments until now. While silicone filaments may have applications in weaving or additive manufacturing, we are particularly interested in them as a model foldamer. Scalable production is enabled by drawing already curing silicone through a tube furnace.
We also demonstrate a way to modify the surface of the silicone filaments using a computer controlled corona discharger. Timing in this procedure is crucial because the polymer is curing continuously. Keep a timer nearby, and be prepared for the next step in advance.
Without A Visual demonstration, the dynamic process of making filaments would not be easily reproduced. The process involves several time sensitive steps that require you to adapt to evolving conditions. To begin, connect 1/8th inch inner diameter high temperature silicone tubing to compressed air through a metering valve.
Connect the other end of the tubing to a brass extrusion adaptor with a 2.15 millimeter diameter aperture. Then, fix the cylindrical ceramic tube furnace vertically in a fume hood, two feet above the floor of the hood. Lay aluminum foil under the furnace to catch excess PDMS during extrusion.
Mount the extrusion adaptor above the tube furnace with the aperture centered on the furnace opening. Ensure that the furnace is not at an angle so that extruded filaments will not contact the furnace. Next, attach a die with a circular cut to an extruder and use a zip tie to connect high temperature silicone rubber tubing to the extruder bit.
Connect the other end of the tubing to the extrusion adaptor. Direct an infrared thermometer towards the furnace and then heat the furnace until the inner temperature is approximately 250 degrees Celsius. Then, move the furnace out from under the extrusion adapter so that the adaptor does not heat up before filament production begins.
Next, start preheating a disposable sample tube to 65 degrees celsius in a viscometer that can measure 200 to 10 thousand millipascal seconds. Check the balance, set the spinning rate to five RPM, and set the measurement rate to once per minute. While the sample tube heats, mix 18 grams of PDMS base with 1.8 grams of curing agent, in a weighing boat.
Degas the PDMS mixture in a room temperature vacuum desiccator for 15 minutes, or until no bubbles remain in the mixture. Periodically vent the desiccator to pop bubbles near the surface. Then remove the preheated sample tube from the viscometer.
Pour the degassed PDMS mixture into it, and put the tube back in the viscometer. Immediately start the measurement sequence. When the PDMS dynamic viscosity reaches four thousand millipascal seconds, note the time, and then use pliers to remove the sample tube from the viscometer.
Immediately pour the PDMS mixture into the room temperature extruder. Confirm that the furnace is at 250 degrees celsius and wait until the PDMS has been out of the viscometer for about four minutes. Then, move the furnace under the extrusion adaptor and align the inner needle of the adaptor with the tube furnace.
Twist the extruder screw by a half revolution and start collecting the filament on a wooden stick. Every three to five seconds, twist the extruder another half revolution to maintain the steady stream of PDMS. The viscosity window for drawing filaments is very narrow.
If the viscosity is too low, wait 30 seconds and try again. The PDMS will continue crosslinking at room temperature. Lay the drawn filaments across wooden racks to cure.
Let the filaments cure at room temperature for 12 hours when finished. To begin the patterning process, set up the patterning assembly in a fume hood, and send the desired pattern to the micro processor. Next, wash a cured PDMS filament with 1%sodium dodecyl sulfate.
And thoroughly rinse it with ultra pure water. Remove visible drops of water with compressed air and let the filament finish drying in the ambient air. Then, tie non conductive fishing line to the circular cut out of the non conductive filament tray.
Lay the dry PDMS filament over the central cut out of the tray and secure it in place with double sided tape. Insert the tray into a ventilated corona discharge box and ensure that it is level. Place a metal slab under the filament tray so that the filament is aligned along the edge of the slab.
Mount the corona discharge electrode about three millimeters above the filament, and connect the corona discharger to the micro controller. Fix the free end of the non conductive line to the spindle mounted on the stepper motor, ensuring that the line is taut. Run the micro controller program to pattern the filament with hydrophilic sections.
The PDMS filaments produced by this method were approximately 200 micrometers in diameter and up to 0.5 meters long. Removing the PDMS from heat before it's viscosity was high enough for drawing filaments, and letting it cross link at room temperature for four and a half minutes, extended the time window in which filaments can be spun to about four minutes. The success of corona patterning was confirmed via droplet contact angle measurements.
Water formed symmetric, barrel shaped droplets on the corona treated hydrophilic PDMS, and asymmetric, shell shaped droplets on the untreated hydrophobic PDMS. The key to this procedure is that you're manipulating the viscosity of PDMS with time and temperature. The viscosity increases with time as it crosslinks and curing accelerates at higher temperatures.
The approach of partially curing a polymer and drawing it through a tube oven, may be adapted to other temperature cured polymers. These filaments are flexible enough to adopt to many conformations, and the hydrophobic regions affectively adhere together in water. We are studying how the hydrophobicity pattern of the filaments affects their folding pathways.
PDMS can drip onto the furnace and produce a small flame. Carefully align the extruder with the furnace and quickly clear any debris from the furnace with a wooden dowel.
