Lecture Connections: Polymers

Lecture Connections

Polymers: Slime

Chapter & Section	Keyword	Explanation
1.1	Chemical reaction	In a chemical reaction, an actual chemical change occurs: the slime is chemically different from the starting substances.
1.3	properties	The characteristics of a substance that allow us to recognize and distinguish it from other substances.
12.2	polymers	Molecules substances of high molecular mass formed by joining together individual molecules called monomers

Elasticity in Polymers

Polymers are either natural or synthetic. Natural polymers such as proteins, DNA, starch, and cellulose occur in nature. Synthetic polymers are chemically prepared. The physical properties of polymers can vary tremendously. For example, some polymers are extremely flexible and fluid, whereas others are very hard and stress-resistant. One way to classify polymers is based on differences in elasticity. Elasticity refers to the ability of a polymer to stretch and return to its original shape. Elastomers are highly flexible and very elastic. One factor that affects the elasticity is the type of monomer group. Rubber, for example, is very elastic and is made from isoprene monomers.

In contrast, polymeric fibers have a much lower degree of elasticity. The chains of a fiber are in a highly ordered arrangement, making it more difficult to stretch. Nylon, Dacron, and acrylics (Orlon) are all fibers. In between elastomers and fibers are plastics, which demonstrate intermediate characteristics.

Another important factor affecting the elasticity of the polymer is the substituent or chemical group attached to the monomer. A large substituent on the monomer creates brittle polymers. For example, polymers made from ethylene are usually much more elastic than the brittle polymers synthesized from styrene (see below). The extra bulky group in styrene greatly affects how the polymers fit together.

Cross Linking

Cross linking may increase the strength of a polymer. Cross linking is a type of bonding that occurs between two chains of polymer. As the amount of cross linking increases, the polymer become less flexible. Often, an agent is needed to promote cross linking. When you make Slime you will use a cross linking agent. The diagram below depicts cross linking. The wavy lines between polymer 3 and 4 represents cross linking bonds.

Cross Linking Poly(vinyl alcohol) to form Slime

In this lab experiment, you will synthesize Slime from poly(vinyl alcohol). Poly(vinyl alcohol) or PVA is a polymer with a chemical formula (CH(OH)CH₂CH(OH)CH₂)_n. PVA is used in many applications such as coating grease-proof paper, artificial sponges, and thickening food. However, PVA is not Slime! Slime is produced by cross-linking PVA polymers with borate anions, [B(OH)^4-]. When borax is added to water, borate anions form.

The reaction that forms Slime is shown below. The oxygen atoms on the borate anion can interact with the alcohol group (OH group) of PVA. This weak interaction is called hydrogen bonding and is also responsible for the 3-dimensional structures of proteins. These weak interactions can break and reform continuously, if the correct amount of water is present. In the picture below, the dotted lines represent weak hydrogen bonds forming an extended lattice throughout Slime.

Slime is very flexible when it contains enough water. Yet when it is at rest, the Slime becomes rigid. Though each hydrogen bond alone is very weak, all together the hydrogen bonds form a rigid structure. However, this network of hydrogen bonding is easily disrupted by deformation through handling, squeezing, stirring, and pouring. As Slime dries out, the amount of water decreases and the slime becomes hard. However, if too much water is added, the weak hydrogen bonding linkages between -OH and borate become separated so that the Slime no longer gels. This situation is not easily reversed until enough water has evaporated from the Slime.