Materials Science Liquid Crystals

Liquid crystals are organic compounds that are in a state between liquid and solid forms. They are

viscous, jelly-like materials that resemble liquids in certain respects (viscosity) and crystals in other properties (light scattering and reflection). The molecules can move independently, as in a liquid, but remain somewhat organized, as in a crystal. A good analogy is a school of fish; in any region all fish swim in the same directions, but the fish can change position with respect to each other.

Liquid crystals, which are usually rod-shaped molecules, are classified as:

nematic: the molecules are oriented in one direction, with their long axes approximately parallel

smectic: the molecules are oriented in two directions; they are arranged in layers with their long axes approximately parallel

cholesteric: the molecules form nematic layers that are twisted at a slight angle to each other. The overall structure resembles a helix or a spiral. The pitch is the distance between layers that have the same orientation.

The liquid crystals found in calculator screens (LCDs), thermometer strips, mood rings, and battery test strips are all cholesteric liquid crystals. They are long thin molecules with structures that resemble cholesterol, and they are arranged in the spiral-shaped orientation shown above. When these liquid crystals are heated, the pitch changes. The wavelength of light reflected by cholesteric liquid crystals is approximately equal to the pitch, so changes in temperature can cause changes in the color of liquid crystals. Thus liquid crystals are sensors that can map regions of different temperature.

Materials

three square liquid crystal sheets of varying composition

an OilieTM sticker

hot plate

500 mL beaker

LCD from a calculator

9-volt battery and battery snap

polarizing filter

Part 1: Colors in Liquid Crystals

1. In your bag of supplies are three small liquid crystal squares labeled A, B, and C. The liquid crystals in these squares contain mixtures of the following three cholesteric molecules:

What do these liquids crystal sheets look like at room temperature? When you touch them? When you touch them to something hotter than your hands, such as a beaker of hot water? Copy the table shown below into your lab notebook, and record your results.

Sample / Chemical composition of liquid crystal by mass / Appearance of liquid crystal at room temperature / Apperance when you touch the liquid crystal / Appearance when touched to a hot beaker of water
A / 70% COC
10% CP
20% CB
B / 45% COC
45% CP
10% CB
C / 42% COC
48% CP
10% CB

2. Rank the liquid crystal samples (A – C) based on the temperature range they respond to.

3. When heating a liquid crystal, what color do you see first? last? List all of the colors you see in order (coldest to hottest).

4. In your bag of supplies there is a “temperature strip”. Hold this to your forehead or a warm beaker of water to see how it works. Record your observations.

5. The liquid crystals you have observed so far change color with temperature (they are called thermotropic liquid crystals). Some liquid crystals can also change color with pressure. Obtain an OilieTM sticker. These stickers were popular in the 1980’s. Like the thermotropic liquid crystals, they contain a mixture of cholesteric molecules. What happens when you touch these stickers?

Are the liquid crystals in the OilieTM stickers temperature sensitive? Do an experiment to find out.

Part 2: Liquid Crystal Displays

Liquid crystal displays (LCDs) contain cholesteric liquid crystals. These liquid crystals change their pitch with changes in applied voltage, which makes them useful in calculator screens, computer screens, watches, and other electronic devices.

6. Obtain an LCD panel from a calculator. Slowly rotate a polarizing filter above it. Does the orientation make a difference?

7. Examine the LCD panel and find the contact area of the panel on the bottom of the wider glass plate. Use a battery snap connected to a 9-V battery. Hold one of the leads against the contact area at one end of the panel and rub the other lead along the rest of the contact area. What do you observe?

8. Does pressure affect the LCD panel? Press on it and find out (hard enough to see something, but not so hard that you break it).

9. Using tongs, hold the LCD display in hot water (about 70 or 80 °C). What happens? Is this process reversible? Does the LCD still work after this (use the 9-volt battery to find out)?

Postlab Questions (answer in your lab notebook)

1. Based on your experimental results in part 1, which of the three cholesteric molecules (COC, CP, or CB) has the lowest melting point? How do you know?

2. Explain how the temperature strip works (what is in each rectangle, and how is it different from one rectangle to the next?)