Materials Included In Kit

Additional Materials Required

Prelab Preparation

Cut the polarizing film sheets into 1" by 2" strips. The strips cut from the second sheet should have their direction of polarization orthogonal to those cut from the first sheet.

Safety Precautions

4-Cyano-4′-pentylbiphenyl is harmful by ingestion and skin exposure. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron or lab coat. The polyvinyl alcohol solution is considered non hazardous according to GHS classifications, however unpredictable reactions among chemicals are always possible. Remind students to wash their hands thoroughly with soap and water before leaving the lab. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Excess polyvinyl alcohol solution may be flushed down the drain with excess water according to Flinn Suggested Disposal Method #26b. Excess 4-cyano-4'-pentylbiphenyl may be handled according to Flinn Suggested Disposal Method #18b.

Lab Hints

• This laboratory activity was specifically written, per teacher request, to be completed in one 50-minute class period. It is important to allow time between the Prelab Homework Assignment and the Lab Activity.
• The step that has the greatest effect on the performance of the LCD is the polyvinyl alcohol layer. If the layer is too thick, the pixel will not respond when a voltage is applied. Encourage students to spread the polyvinyl alcohol solution as thin as possible.
• Due to it not being possible to have a polyvinyl alcohol layer of uniform thickness, the students’ pixels will have some regions that are always light, some that are always dark, and some that switch between light and dark in response to a voltage. These regions correspond to too much polyvinyl alcohol, not enough polyvinyl alcohol, and just the right thickness of polyvinyl alcohol.|It is a good idea to have students mark the slide to show which side is conducting.
• 4-Cyano-4′-pentylbiphenyl is only in a liquid crystal state between 18 and 35 °C. As an extension exercise, you could have students heat and cool their liquid crystal pixels and observe any changes in its properties.
• It is also possible to use capillary action to fill the cell with the liquid crystal. To do this, skip step nine, and then once the two slides are glued together apply a small drop of 4-cyano-4′-pentylbiphenyl to the edge of the slides. Capillary action will then draw the liquid crystal into the cell.

Teacher Tips

• Flinn Scientific stocks another liquid crystal kit (Liquid Crystals—How Do They Do That? Flinn Catalog No. AP7195), which demonstrates how liquid crystals can be used as temperature indicators and is a good demonstration to get students thinking about temperature-structure and structure-property relationships.

Further Extensions

Alignment to the Curriculum Framework for AP^® Chemistry 

Enduring Understanding an Essential Knowledge
Matter can be described by its physical properties. The physical properties of a substance generally depend on the spacing between the particles (atoms, molecules, ions) that make up the substance and the forces of attraction among them. (2A)
2.A.1: The different properties of solids and liquids can be explained by differences in their structures, both at the particulate level and in their supramolecular structures.

Forces of attraction between particles (including the noble gases and also different parts of some large molecules) are important in determining many macroscopic properties of a substance, including how the observable physical state changes with temperature. (2B)
2.B.1: London dispersion forces are attractive forces present between all atoms and molecules. London dispersion forces are often the strongest net intermolecular force between large molecules.
2.B.2: Dipole forces result from the attraction among the positive ends and negative ends of polar molecules. Hydrogen bonding is a strong type of dipole-dipole force that exists when very electronegative atoms (N, O, and F) are involved.
2.B.3: Intermolecular forces play a key role in determining the properties of substances, including biological structures and interactions.

Chemical and physical transformations may be observed in several ways and typically involve a change in energy. (3C)

Electrostatic forces exist between molecules as well as between atoms or ions, and breaking the resultant intermolecular interactions requires energy. (5D)
5.D.3: Noncovalent and intermolecular interactions play important roles in many biological and polymer systems.

Learning Objectives
2.1 Students can predict properties of substances based on their chemical formulas, and provide explanations of their properties based on particle views.
2.3 The student is able to use aspects of particulate models (i.e., particle spacing, motion, and forces of attraction) to reason about observed differences between solid and liquid phases and among solid and liquid materials.
2.11 The student is able to explain the trends in properties and/or predict properties of samples consisting of particles with no permanent dipole on the basis of London dispersion forces.
2.13 The student is able to describe the relationships between the structural features of polar molecules and the forces of attraction between the particles.
2.16 The student is able to explain the properties (phase, vapor pressure, viscosity, etc.) of small and large molecular compounds in terms of the strengths and types of intermolecular forces.
3.10 The student is able to evaluate the classification of a process as a physical change, chemical change, or ambiguous change based on both macroscopic observations and the distinction between rearrangement of covalent interactions and noncovalent interactions.
5.10 The student is able to identify the noncovalent interactions within and between large molecules, and/or connect the shape and function of the large molecule to the presence and magnitude of these interactions.

Science Practices
1.2 The student can describe representations and models of natural and man-made phenomena and systems in the domain.
1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively.
3.3 The student can evaluate scientific questions.
6.1 The student can justify claims with evidence.
6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices.
7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas.

Answers to Prelab Questions

1. Cholesteryl benzoate (see Figure 5) is a derivate of cholesterol and was the first material identified as having liquid crystal properties. Identify the features of this molecule that make it a suitable liquid crystal.
   {12359_PreLabAnswers_Figure_5}

   The fused rings of the cholesterol moiety provide the molecule with rigidity, while the benzoate group introduces polarity to the molecule. The dipole–dipole interactions associated with this molecule, combined with the overall shape, result in an ordered liquid-crystal phase.

2. The structures of three different benzene derivatives are shown below, along with their melting and boiling points (see Figure 6). With reference to their intermolecular interactions, explain any trends in boiling and melting points.
   {12359_PreLabAnswers_Figure_6}

   All three molecules have similar molecular masses and sizes. For this reason, the strength of the London-dispersion force interactions should be comparable. Changing the aldehyde group into an alcohol functionality results in a molecule that is now able to hydrogen bond. Since hydrogen bonding is a stronger interaction than dipole–dipole, the melting point and boiling point increase. The carboxylic acid functional group is a weak electrolyte that results in some ionic character now existing within the molecule; this further increases both the melting point and the boiling point.

3. When a material is in the liquid crystal phase it tends to be more viscous than when in the liquid phase, explain why this is.

   When in a liquid crystal phase, the molecules are semi-ordered. This ordering increases the number and hence strength of the intermolecular interactions making the liquids more viscous.

4. Polarimeters measure the rotation of light by an optically active sample. Because the degree of rotation depends on the wavelength of light used it is very common for monochromatic light from the sodium D line at 589 nm to be used in these measurements. What is the energy of a photon of light with wavelength of 589 nm?
   {12359_PreLabAnswers_Equation_1}
5. A student investigated the rotation of plane polarized light by a fructose solution and summarized the results in the table below. By generating a graph of the data, comment on the type of relationship that exists between path length and degree of rotation.
   {12359_PreLabAnswers_Table_2}

   Plotting the provided data gives the graph below. This graph is consistent with a direct relationship between path length and the degree of rotation. Students should be aware that even though the gradient of the trend line is negative this is still a direct relationship.

   {12359_PreLabAnswers_Figure_11}
6. When material is in a liquid crystal state it tends to be cloudy. However, when heated to a liquid state it becomes transparent. Explain why this is.

   When in the liquid crystal state, the molecules are semi-ordered and interact with light, scattering it and giving a cloudy appearance (similar to how a fine suspension of solids, or colloids, results in a cloudy solution). When heated to a liquid phase, the ordering of the molecules breaks down and interacts with light in the same manner as a regular liquid, resulting in a clear solution.

7. How does the entropy of the liquid crystal phase compare with the entropy of the solid and liquid phases?

   Entropy can be linked to how disordered a material is. Solids are highly ordered and as such have a low entropy. Liquids are much more random and have higher entropies. Liquid crystals, being halfway between a solid and a liquid, have entropy greater than that of a solid, but lower than a liquid.

8. A material transitioning from a liquid to a solid phase has its entropy decrease. Explain how this is not a violation of the second law of thermodynamics.

   The second law of thermodynamics states that for an isolated system, the total entropy cannot decrease. In this example, the system not only involves the material that is freezing but also its surroundings. Because freezing is an exothermic process, energy will be transferred into the surroundings, which will in turn increase the entropy of the surroundings and maintain the second law of thermodynamics.

Sample Data

Example Procedure

1. Obtain two strips of polarizing film. If the direction of polarization for these strips is the same, the resulting pixel will switch from off to on. if the direction of polarization is opposed, then the pixel will switch from on to off.
2. Tape one strip of film to each side of the cell.
3. Obtain a 9-V battery and battery connector.
4. Attach the battery connector to the 9-V battery.
5. Clip one lead from the battery connector to one of the pieces of copper tape.
6. Touch the other battery connector clip to the other piece of copper tape, and observe how the LCD responds.

References

AP^® Chemistry Guided-Inquiry Experiments: Applying the Science Practices; The College Board: New York, NY, 2013

Introduction

Capture the concepts and hit the ground running on exam day with this lab! Encompassing Big Idea 2, the structure-property relationships of chemicals. Liquid crystal displays (LCD) are now a common place piece of technology, used in a wide range of devices. An LCD exploits the switching of chemical alignment, in response to electrical current, to toggle individual pixels between opaque and transparent states. In this lab, students will build their own twisted nematic liquid crystal pixel, for use in an investigation into the rotation of polarized light. A prelab homework assignment guides you through the necessary concepts to ensure success on lab day. You will find it fun, engaging and challenging!

Concepts

• Intermolecular forces
• Polarized light
• Phase transitions

Background

Crystalline solids are materials where the molecules are arranged in a regular repeating array. As a solid is heated it will eventually undergo a phase change, usually to a liquid state. In the liquid state, the ordered arrangement of the molecules is lost. However, for some chemicals there exists a liquid crystal state, where the material is able to flow and take the shape of its container but the molecules are still aligned in an ordered fashion (see Figure 1). There are various different types of liquid crystals, the one that you will use in this lab is called a nematic liquid crystal. Nematic liquid crystals are rod shaped molecules that despite being liquid maintain an overall alignment. This alignment results in the material interacting with plane polarized light in a similar way to a polarizing filter.

Light propagates as a transverse wave, with electronic and magnetic components perpendicular to the direction of propagation. For unpolarized light the orientation of the electronic and magnetic components is random. However, when unpolarized light passes through a polarizing filter only the waves that are aligned with the direction of polarization pass through, resulting in the light becoming polarized. If this polarized light were to encounter another polarizing filter, set at 90° to the first then all the light would be blocked out (see Figure 2). When polarized light encounters a polarizer that is at an angle other than 90°, only some of the light will be absorbed and the rest will be transmitted with this new orientation. Layers of nematic liquid crystals are able to twist on top of each other. As polarized light passes through a twisted nematic liquid crystal, it is rotated and its direction of polarization is changed by the same amount as the degree of the twist. Through the use of alignment layers, the degree of twisting can be controlled. When a twisted nematic liquid crystal with a 90° twist is placed between two cross polarized filters, the cell will still appear bright even though the two filters are set at 90°. The final property of nematic liquid crystals that makes them able to be used in the fabrication of an LCD is that they will align themselves to an electric current. This provides a way to switch the cell between an on and off state (see Figure 3). In order for a molecule to act as a nematic liquid crystal, it needs to contain a rigid section to provide the general rod like shape, as well as a polar group in order to promote alignment. The nematic liquid crystal you will be using in this lab is 4-pentyl- 4′-cyanobiphenyl (see Figure 4). This molecule is about 20 Å long and exists in a liquid crystal phase from 18–35 °C. The two benzene rings provide the overall rigid linear shape, with the nitrile group giving rise to dipole–dipole interactions that help the molecules to align with each other.

Experiment Overview

The purpose of this activity is to complete the homework assignment prior to lab to promote conceptual understanding. On lab day, you will follow the instructions for constructing a twisted nematic liquid crystal pixel, and then use your own procedure to investigate how your pixel interacts with plane polarized light.

Prelab Questions

1. Cholesteryl benzoate (see Figure 5) is a derivate of cholesterol and was the first material identified as having liquid crystal properties. Identify the features of this molecule that make it a suitable liquid crystal.
   {12359_PreLab_Figure_5}
2. The structures of three different benzene derivatives are shown below, along with their melting and boiling points (see Figure 6). With reference to their intermolecular interactions, explain any trends in boiling and melting points.
   {12359_PreLab_Figure_6}
3. When a material is in the liquid crystal phase it tends to be more viscous than when in the liquid phase, explain why this is.
4. Polarimeters measure the rotation of light by an optically active sample. Because the degree of rotation depends on the wavelength of light used it is very common for monochromatic light from the sodium D line at 589 nm to be used in these measurements. What is the energy of a photon of light with wavelength of 589 nm?
5. A student investigated the rotation of plane polarized light by a fructose solution and summarized the results in the table below. Generate a graph of the data, and comment on the type of relationship that exists between path length and degree of rotation.
   {12359_PreLab_Table_1}
6. When material is in a liquid crystal state it tends to be cloudy. However, when heated to a liquid state it becomes transparent. Explain why this is.
7. How does the entropy of the liquid crystal phase compare with the entropy of the solid and liquid phases?
8. A material transitioning from a liquid to a solid phase has its entropy decrease. Explain how this is not a violation of the second law of thermodynamics.
9. Read through the ten steps for constructing a twisted nematic liquid crystal pixel below. Then write a detailed procedure for investigating the effect you expect it to have on plane polarized light. Two pieces of polarizing film and a 9-volt battery will be provided for use in this investigation.

Procedure

Construction of the Twisted Nematic Liquid Crystal Pixel

1. Take two conducting glass slides.
2. Using a multimeter, determine which side of the slide is conductive, and which is not.
3. Place a strip of transparent tape on one end of the conductive surface of each slide (see Figure 7).
   {12359_Procedure_Figure_7}
4. Place a small drop of polyvinyl alcohol solution (~100 μL) on each slide and spread it out to form a thin layer.
5. After about 5 minutes remove the tape and place the slides in a 110 °C oven for 10 minutes.
6. Carefully remove the slides from the oven and leave to cool.
7. Once cooled rub one of the slides with the micro fiber cloth along its long axis for 10 minutes. Rub the other slide along the short axis (see Figure 8). This rubbing aligns the polymer molecules.
   {12359_Procedure_Figure_8}
8. Attach a 3 cm strip of copper tape to each end of the slides, so that it overhangs the end.
9. Place a 20 μL drop of 4-cyano-4′-pentylbiphenyl in the center of one slide, then place a drop of super glue in each corner of the polyvinyl alcohol region of the slide (see Figure 9).
   {12359_Procedure_Figure_9}
10. Sandwich the super glue and 4-cyano-4′-pentylbiphenyl between the two polyvinyl alcohol layers by taking the other slide and pressing it down onto the first. (see Figure 10). The twisted nematic cell is now complete.
    {12359_Procedure_Figure_10}