SLIME – Observations of a Polymer

CHEM 1405 Laboratory Experiment

Introduction

Polymers are long molecules that are formed through polymerization reactions which combine many smaller molecules called monomers. The polymerization reactions chemically connect the monomers in chains of varying length. For example, in this experiment, you will be using polyvinyl alcohol, which is a polymer built from vinyl alcohol, which has the chemical formula, C₂H₄O. The formula of polyvinyl alcohol is written (C₂H₄O)_n.

The number of repeating monomers (“n” in the formula above) in a polymer is called the degree of polymerization. In the polymer made in this lab, n will be around 2,300.

Depending upon the monomers used, polymers can be linear, branched, and cross-linked as shown in Figure 1. As polymers become more branched or cross-linked, their properties change. More cross-linking yields a more rigid polymer is an example of such a change in properties.

Figure 1: Illustration of types of polymers

Polymers can be either natural or synthetic. Natural polymers include things like proteins, nucleic acids, silk and wool. Nylon, Teflon, plastics and Plexiglas are examples of synthetic polymers.

Experimental Overview

Good science depends upon good observations. In order to understand what is going on in an experiment, we must pay careful attention to how our chemicals behave and what changes are taking place. We quantify many things by measuring them- mass, volume, density, etc. are all things for which we record numbers and make calculations. Equally important are the qualitative observations we make- What did it look like? What did it smell like? What was its texture? Was it a solid, a liquid, or a gas?

This experiment will introduce you to the laboratory balance, the Bunsen burner and the glassware typically used in the lab. It will also give you practice in making observations.

Experimental Procedure

Prior to the lab: Watch the balance, Bunsen burner and glassware videos found on eCampus.

1. Obtain approximately 25mL of the Polyvinyl Alcohol (PVA) solution in a clean 250mL beaker and set it aside.

2. Weigh out approximately 1g of Borax into a clean 50mL beaker.

3. Place about 25mL of deionized water (DI Water) in a clean 100mL beaker. Heat the water almost to boiling using a Bunsen burner and add the Borax slowly to the water while stirring. When dissolved in water, Borax (sodium tetraborate) dissociates into hydrated sodium (Na+) and tetraborate (B₄O₇^2-) ions.

4. Add a few drops of food coloring to the PVA solution. Then, using a pipet, add approximately 5mL of the Borax solution, drop by drop, to the PVA while constantly stirring. The tetraborate ions react with the hydroxyl groups (-OH) of the PVA forming cross links with the probable elimination of water. Hydrogen bonding is also involved in forming the cross links which break and reform as the slime flows.

5. After you are finished adding the 5mL of the Borax solution, a viscoelastic gel should have formed which can be removed from the beaker by hand. The cross-linking is responsible for the rigidity of the slime. The amount of cross-linking is light compared to natural rubber in which sulfur forms numerous cross-links between the polymer chains locking it into a very rigid structure.

6. Viscoelastic gels have characteristics of both liquids and solids. Note the new physical properties of the cross-linked product compared to the properties of the original PVA solution.

Safety

Although these materials are relatively non-toxic, the Slime must not be ingested.

Testing the Material and Reporting the Findings

Once you have made your Slime, observe its characteristics and behavior. For this lab, you must make at least 15 observations about your product. These observations can be simple and include things like color and texture. They can also be more detailed, perhaps comparing them to the properties of solids, liquids and gases (Does your Slime have constant volume? Does it have constant shape?) or to the original materials that went into it. Observations can also include what happens when you manipulate it. For example, some common tests for materials include the following:

1. Tensioning: Try stretching the material. Stretch it slowly. Then stretch it quickly. Describe the response on the Data and Results page. In Material Science, this test is known as tensioning. The force applied is called tensile stress and the amount of stretch induced is called the tensile strain.

2. Shear: Lay the material on the table and press down on top of it. Report the results. This is testing shear which is the resistance of a material to deformation when a force is applied parallel to the surface on which the object is resting.

3. Creep: Incline the material on a board and see how it moves. Report the results. This is known as Creep which is the slow change of shape of the material by flow when it is subjected to a constant force such as gravity.

4. Impact Strength: Roll the gel into a ball and throw it straight down against the table. Report your results. This activity demonstrates Impact Strength and it is a measure of the resistance of a material to breakage.

Note anything else about the material that might help characterize it.

Name:____________________________________________Date: _______________ Section:__________

SLIME – Observations of a Polymer

Pre-Lab Questions

1. Watch videos on the use of the balance, Bunsen burner and glassware.

2. Choose an object on your desk or in your study area or backpack. Make 8 observations about this object.

3. Choose an object from your pantry or refrigerator, from the supermarket, or from a restaurant. Make 12 observations about this object.

Name:___________________________________________Date: _______________ Section:__________

SLIME – Observations of a Polymer

Observations and Results

Observations

Observations (continued)

Observations (continued)