CHEMISTRY 161 – EXPERIMENT 10
POLYMERS: The Organic Phenomenon
INTRODUCTION
In the 1967 movie The Graduate, Benjamin (played by Dustin Hoffman) was given one word of advice by a family friend …‘Plastics’. Indeed, it is sometimes said that we are living in the age of plastics. Much of the clothing you wear, the CD’s and DVD’s you use, many parts of cars and computers, packaging and all types of coatings and adhesives are based on polymers. Roughly half of the chemists in the U.S. work on the development of polymers! Presently the starting materials to make polymers come from fossil fuels-natural gas, oil and coal. As these become more scarce chemists are working on plants and crops as new sources.
Polymers are a class of molecules characterized by their high molar mass and by the presence of simple repeating structural units called monomers. We will prepare a number of polymer types and investigate some of their properties in a series of experiments.
Polymers are classified based on a variety of characteristics, for example
Composition: polymers that consist of only one monomer are called homopolymers, while polymers that consist of two or more monomers are called copolymers.
R-M1- (M1)n –M-R R-M1-(M1-M2)n-M2-R
Homopolymer Copolymer
Here R represents end groups and M1 and M2 represent particular monomers.
Origin: polymers can also be classified as synthetic or natural. Synthetic or ‘man-made’ polymers can be grouped into one of three categories:
v plastics- polymers that can be molded,
v elastomers- polymers that show elasticity like a rubber band, and
v fibers- thread like strands of solid polymer.
Important natural polymers include: proteins, polysaccharides and nucleic acids.
Preparation: polymers can be prepared by either addition polymerization, where the monomer (s) used to make the polymer are unsaturated (contain one or more multiple bonds) or condensation polymerization, where two monomers react to form a chemical bond and release a small molecule such as H2O. We will discuss this further as we prepare polymers using both types of polymerization.
Crosslinking; if one views a group of polymer molecules as individual strands then the concept of crosslinking involves chemically bonding these strands together. This leads to important differences in physical properties. For instance, plastic polymers that are NOT crosslinked are often thermoplastic (T/P) because they can be softened repeatedly by heating, while crosslinked plastic polymers are called thermoset (T/S) and cannot be softened by heating.
Glass Transition Temperature: In the solid state polymers can exist in two ‘states’. The glass state involves crystalline regions, as well as amorphous regions which is due to the very large molar mass of polymers. However, as the temperature is increased the amorphous regions begin to flow, giving the polymer different properties-more like a rubber. Hence this second solid ‘state’ is called the rubber state. The transition from the glass state to the rubber state is characteristic of the polymer in question and is called the glass transition state (Tg). For instance polystyrene has a Tg of about 100 deg C. Polystyrene utensils placed in a dishwasher will undergo a transition to the rubber state and will flow. Once the utensils cool they are frozen in their new shape.
SAFETY PRECAUTIONS
Many of the chemicals used in this experiment are toxic and MUST be handled properly. It is VERY important that you use the latex gloves supplied to you AND your safety glasses. In addition please dispose of the spent solvents and other chemicals used in the proper waste containers. Also, the solvents used are flammable. Be careful!
PART I:
Investigation of a Polystyrene Product-Styrofoam
Styrofoam is a ‘puffed-up’ form of polystyrene formed by polymerizing styrene in the presence of a blowing agent like pentane. The heat of polymerization causes the agent to vaporize and is temporarily trapped forming a bubble or cell. The final material has many of these cells resulting in a ‘foam-like’ material. The foam serves as an insulator because air is not very effective in transferring heat or cold. Styrofoam is a very good insulating material.
PART II:
Preparation of a Nylon
(Condensation Polymerization-T/P)
DuPont researchers led by Dr. Wallace Carothers, invented nylon 66 (pronounced "six-six") polymer in the 1930s. Nylon, the generic name for a group of synthetic fibers, was the first of the miracle yarns made entirely from chemical ingredients through the process of polymerization. Nylon 66 polymer chip can be extruded through spinnerets into fiber filaments or molded and formed into a variety of finished engineered structures.
The word "nylon" is now used to represent all synthetic polyamides. The various nylons are described by a numbering system that indicates the number of carbon atoms in the monomer chains. Nylons from diamines and dicarboxylic acids are designated by two numbers, the first representing the diamine and the second the dicarboxylic acid. Thus nylon 6-10 is formed by the reaction of hexamethylenediamine and sebacoyl chloride.
Many diamines and diacids (or diacid chlorides) can be reacted to make other condensation products that are described by the generic name "nylon." For example, nylon 6-6, can be prepared by substituting adipoyl chloride (which has (CH2)4 in the center) for sebacoyl chloride (which has (CH2)8 in the center).
Note that nylon polymer chain is formed via condensation polymerization with the loss of HCl as the result of the reaction. Even though condensation polymerization often results in thermosetting materials, nylon is an example of a thermoplastic polymer. Nylon 6-10 softens above 50 °C and melts into a liquid above 227 °C.
In this experiment, hexamethylenediamine and sebacoyl chloride will be used to create a thread of nylon 6-10. The single thread will be recovered, washed, dried, and measured.
Part III
Preparation of a Polyurethane
(Condensation Polymerization -T/S)
Research into the production and use of urethane polymers was first done in Germany by C. A. Wurtz in 1848. Urethane polymers are used as polymers, elastomers, and as adhesives, but their principle use is in the foam form. The term urethane foam has been adopted by the polymer industry to describe urethane foam and similar foam materials.
Polyurethanes are produced by the condensation reaction between isocyanate and an alcohol. The reaction can produce thermosetting materials or elastomers such as synthetic rubber.
This laboratory exercise uses a Polyurethane Foam System marketed by Flinn Scientific, Inc. The foam produced by this system is ridged polyurethane foam that is used in furniture, packaging, insulation, and flotation devices, among others. In this system, the foam is produced by mixing equal parts of two liquids, called Part A and Part B. The resulting lightweight foam expands to about thirty times its original liquid volume and will become rigid in about five minutes.
Part A is a viscous cream-colored liquid containing polypropylene glycol [HO(C3H6O)nH], a silicone surfactant, and a catalyst. The hydroxyl (-OH) end of the polypropylene polymer is the reactive site. The silicone surfactant reduces the surface tension between the Part A and Part B liquids, and the catalyst speeds up the condensation reaction without being chemically changed itself.
Part B is a dark brown viscous liquid containing diphenylmethane diisocyanate [(C6H5)2C(NCO)2] and higher oligomers (dimmers, trimers or tetramers) of diisocyanate. When the polypropylene glycol (Part A) is mixed with the diisocyanate (Part B), an exothermic condensation polymerization reaction occurs, producing polyurethane.
During the course of the polymerization reaction, a small amount of water reacts with some of the diisocyanate to produce carbon dioxide gas, thus causing the solution to foam and expand in volume. Because of multiple sites of unsaturation in the reactants, the resulting polymer is highly cross-linked, causing it to become rigid within minutes.
A simplified two-step reaction shows the process involved.
Part IV
Preparation of Crosslinked Polyvinyl Acetate
(Effect of Crosslinking)
White glue (Elmer’s Glue is one example) contains polyvinyl acetate (PVA). PVA is a thermoplastic polymer that is partially soluble in water. The milky-white suspension of PVA strands in water is known as “Latex”. Naturally occurring liquid latex can be found in milkweed plants, rubber tress, pine trees, aloe plants, and many desert plants. This latex is used to mend and repair any damage to the outer covering of the plant (it glues itself back together). This natural phenomenon has been imitated by materials scientists to develop “self-healing” materials. Liquid latex can be added to other polymers and the latex automatically seals scratches and cracks in plastic parts. PVA can also be used as packaging materials for medicines, vitamins, etc. Once the PVA capsule is swallowed, the PVA dissolves and the medicine is released.
The strands of PVA can, however, be “cross-linked” to form new materials with different useful properties. Among other changes the “sticky” glue looses its ability to stick to surfaces. In this experiment we will use hydrated sodium borate (Na2B4O7·10H2O), the primary component of the mineral “Borax, to cross-link the strands of PVA by a “carbon – oxygen – boron – oxygen – carbon” linkage as illustrated below:
As the number of cross-links increases, the material becomes more ridged and strong.
PROCEDURE
PART I:
Place a Styrofoam cup, bottom down, in a beaker or petri dish which contains a small amount of acetone. Using a glass stirring rod observe the reaction and record what happens to the cup.
PART II:
1. Using a 10-mL graduated cylinder, place 10-mL of 2% w/w 1,6-Hexanediamine in 0.4 M NaOH solution into a 100-mL plastic beaker.
2. Using a 10-mL graduated cylinder CAREFULLY place 10 ml of 2% v/v Sebacoyl Chloride in Hexane in the 100-mL plastic beaker by flowing it down the side of the beaker.
TRY NOT TO MIX THE SOLUTIONS
3. Hexane is lighter than water and it will remain as a separate layer on top of the beaker. The polymer condensation reaction will occur at the interface between the aqueous and hexane layers. QUICKLY DO parts (4) and (5)!
4. Using a paper clip shaped as a hook at one end , insert the hooked end into the beaker and “fish out” a strand of nylon polymer filament.
5. Wrap the end of the nylon filament around a popsicle stick and carefully turn it to “wind-up” a single long filament of nylon.
6. The filament will end when all of the reactants have been removed from the solutions.
7. Carefully, unwind the filament into a 250-mL beaker containing DI water and stir gently with a glass-stirring rod to wash the nylon.
8. Remove the nylon filament from the water and dry the filament with a paper towel.
9. Using a ruler measure the length of the filament that you generated.
10. Report the length of the filament in your lab notebook and lab report.
PART III:
1. Put on the Latex Exam Gloves.
2. Place 20 mL of Part-A into a small plastic cup that has been marked to contain 20 mL of liquid.
3. Place 20 mL of Part-B into a second small plastic cup that has been marked to contain 20 mL of liquid.
4. Place a sheet of Aluminum Foil on the workbench.
5. Place the first cup containing Part-A onto the Aluminum Foil.
6. Carefully pour the contents of the second cup containing Part-B into the cup containing Part-A. Use the wooden Popsicle stick to scrape out any remaining solution of Part-B in the cup.
7. Use the Popsicle stick to mix the solutions thoroughly. The cup will become warm as the exothermic condensation reaction progresses. When the solution begins to foam, remove the Popsicle stick, wipe off excess solution on the lip of the cup and allow the cup to rest on the Aluminum Foil.
8. Observe the foam as it expands to about 30 times its original volume.
9. Allow the foam to harden for at least 15 minutes before handling.
10. The foam may continue to react for up to 24 hours.
11. Test a small piece of the foam in acetone, toluene and methanol. (FLAMMABLE SOLVENTS!)
PART IV:
1. Using a 25 mL graduated cylinder pour 20 mL of 4% PVA solution into a small clean plastic cup. Note wash out the graduated cylinder after this, as the PVA solution is tacky.
2. Use a 10-mL graduated cylinder to measure 10 mL of 5% Borax solution.
3. Add the 10 mL of 5% Borax Solution into the plastic cup.
4. Stir the mixture using a popsicle stick until the polymer forms into a clump.
5. After about 15 minutes of air-dry time put and using latex exam gloves, manually knead the clump of polymer until it firms to the consistency of “Play Dough” or “Silly Putty”.
6. Roll the polymer into a ball.
7. Using a ruler drop the polymer ball from a height of 25 cm from the bench top and record the maximum height of the bounce.
8. Calculate the ratio of the bounce height to the drop height of 25 cm.
9. This ratio is known as the “Coefficient of Restitution” for an elastomer. Things that are “bouncier” have a larger coefficient. “Superballs” have a coefficient of Restitution > 0.9