Loading cell with LC mixture and photopolymerization
This procedure describes how to load the cell with the reaction mixture
by heating, and then photo-crosslinking the mixture to form a film. The
entire procedure is run in a darkroom.
Materials List
Hot-stage |
Cell |
Metal
Spatula |
Dry reaction
mixture |
Dark room |
Light source |
>535nm
filter |
Red light |
|
|
|
|
Procedure
please note, all photographs are for reference, due to the presence
of the light sensitive photo-initiator, the procedure needs to be run in the dark
- The material needs to be in liquid state to fill the cell. A hot stage is a cPlace the cell on the hot-stage and heat to the mixture's isotropic
phase transition temperature
- Unwrap the vial containing the LC mixture. It should be a solid, if any
liquid is visible, return it to the vacuum to remove all solvent.
- Place the vial on or against the hot stage to melt the contents. Tilt the vial so the liquid
collects in one section of the vial. It is not a problem if the material solidifies again
- Using a metal spatula, transfer a small amount of the mixture to the bottom edge of the cell against
the gap. (see images to right).The solid should immediately melt when
the spatula is held against the cell. Although it is very difficult to see, the
material will be pulled into the cell by capillary action. Turning the
spoon backwards when filling the cell, shown in the last picture for
step four, make the material flow into the cell more quickly.
- One very useful technique is to collect some solid on the spatula, hold the spatula against the hotplate until it melts,
and the load the liquid into the cell. Try to spread the liquid along the entire length of the opening.
- Continue to add solid until the cell is full. The two images to the
right show the cell becoming progressively fuller. When working in the
dark, this will be almost impossible to see. Holding a red light at an
angle may help to see how the cell is filling.
- Lower the temperature until clouding is observed in the cell, that
is any part of the cell no longer remains clear but has a hazy or cloudy
appearance.
Record this temperature.
- Heat the cell until it becomes clear again. Record this clearing
point temperature.
- Continue to heat the cell 10 oC above the clearing point
- Program the hot stage to decrease the temperature 5o C below the
clearing point at 0.5o/min for nematic systems, 0.3oC/
min for smectic systems.
- Cover the cell with a piece of aluminum foil and let cool
- Next, the sample is irradiated with a single light source (>535nm, not the overhead lights)
~ approximately 3mW/cm 2 for
2 hours. Be sure that the cell is not exposed to ambient light
- After exposure, the cell can be immediately removed from hot stage and is ready for optical
characterization
Image gallery
Step 1: Cell sitting on hotplate. To the left is a red LED light that does not effect the photoinitiator
Step 3: Unmelted material. After the solvent is removed, some of the material may be stuck on the sides of the
glass. After it melts, most will roll to the bottom of the vial
Step 3: Melted material.
Step 4: Cell alignment before loading
Step 4: Side view of cell assembly. The orange circle is a bead of the material laying on the bottom ledge of the
cell. When melted, it will flow into the cell via capilary action
Step 4: Loading the cell using a metal spatula
Step 4: Using the backside of the spatula to load the material into the cell
Step 5: Melting the material on the spatula before loading on the cell. The material may cool and harden on the
spatula, making it difficult to load the cell. If this happend, use the hotstage to melt the material on
the surface of the spatula before attempting to fill the cell. Here, orange solid on the spatula is the unmeltaed material
Step 5: Material melted on the back of the spatula
Step 6: Melted materials flowing into the cell. Here we see the initial stages of the melted materials flowing
into the cell. Looking closes at the right hand edge of the cell, a faint orange layer is visible in the cell. The next
picture shows the further progress
Step 6: Melted material flowing into the cell. This image shows the cell a little over 1/2 full.
It required approximately five minutes to fill the cell this far.
Step 12. Illumination setup for photo crosslinkng the cell. Here the illumination source is from the right, which is then reflected off a mirror
to the cell surface, shown in the left hand side of the image.
Step 12. A close up view of the cell under illumination.