Notes

Outline

Polymers The Structure of Matter Edward Goo University of Southern California

Naturally Occuring Polymers Silk - used in clothing Asphalt and pitch - used for waterproofing for the last 5000 years. Amber, rosin, gum arabic - used by the Egyptians to varnish sarcophagi. Rubber - waterproof garments, eraser.

Silkworm Caterpillar and Cocoon

Rubber Tree

History of Polymers 1832 Christian Schonbein made cellulose nitrate by dissolving natural fibers in nitric acid. 1839 - Charles Goodyear discovered the vulcanization of rubber by adding sulfer and heating increased its stiffness.

Charles Goodyear

History of Polymers In 1868 John and Isaiah Hyatt added camphor to cellulose nitrate and produce celluloid.  Rayon was invented in 1884 by Count Louis Hilaire de Chardonnet.

Bakelite The first truly synthetic polymers was Bakelite that was made by reacting phenol and formaldehyde. Discovered in 1905 by Leo Baekeland.

Bakelite

Bakelite

Nylon In 1934 E. I. Du Pont’s Wallace Caruthers invented nylon.

Monomers Polymers are made up of units called monomers. Monomers join to form large polymer molecules. Functionality of the monomer refers to the number of bonds the monomer may form.

Functionality A monomer must be able to form at least two bonds or have a functionality of two. A bifunctional monomer will form long chains. A trifunctional monomer will allow branching to occur.

Polyethylene Made up of bifunctional monomer CH2. Discovered in 1935 by accident.

Polyethylene Used in many household items. Many food containers are polyethylene. Ethylene is CH2 = CH2. More pounds of ethylene is produced than any other organic chemical.

Polytetrafluorethylene Better known as Teflon, it was developed at Dupont in 1938. It is made of the bifunctional monomer CF2.

Polyvinyl Chloride Commonly known as PVC it has Cl as a side group. Made up of the monomer:

Polyvinyl Chloride The vinyl group is CH2 = CH - Vinyl chloride is CH2 = CHCl Vinyl chloride is an established human carcinogen. It is used for pipes. It may also be made rubber-like and is used in squeeze bottles.

Plasticizers Plasticizers are low molecular weight liquids that are added to polymers to make them more soft and flexible. Exposure to sunlight and heat will cause polymers to become brittle due to the removal of the plasticizers.

Polystyrene Polystyrene like polyethylene is used in household products. Polystyrene may be expanded into a foam to make styrofoam. Polystyrene is similar to PVC except that the Cl is replaced by a benzene ring.

Styrofoam and Polystyrene Containers

Styrofoam Insulation

Polystyrene monomer

Polypropylene Polypropylene is obtained by polymerizing propene CH3CH=CH2. Polypropylene had a high melting temperature allowing it to be used with hot foods. Polypropylene is also very strong.

Polymethyl methacrylate Commonly know as Plexiglas or Lucite it is a very strong and tough polymer. It is used in applications such a canopies for jet fighters. The monomer look like:

Polyisopropene This is the chemical name for rubber. The one structure for the isoprene monomer is:

Polyisopropene The large methyl group interferes with the adjacent hydrogen atoms. The rubber molecule is bent or kinked to better accommodate the large methyl group. When rubber is stretched the kinked molecules are straightened out. However the molecules revert back to the kinked shape when the force is remove.

Steric Hinderance The interference by the large methyl group with the neighboring atoms is known in general as steric hinderance. Steric hinderance is simply the repulsion that occurs from placing atoms too close together.

Distribution of Side Groups Atactic - random distribution Isotactic - all on the same side Syndiotactic - regularly alternating sides Strong interactions between the side groups and neighboring atoms leads to isotactic or syndiotactic arrangements.

Distribution of Side Groups

Polyisopropene It is possible to place a methyl group and the adjacent hydrogen on opposite sides to reduce the steric hinderance. A polymer made with such a monomer would not have the kinks and would be much stiffer. Gutta-percha is such a polymer and is very stiff.

Rubber

Gutta-percha The gutta-percha molecule is much straighter due to the lack of steric hinderance. Therefore gutta-percha is stiffer than rubber.  Cotterill 1985

Cross-linking The elasticity of the rubber assumes that the long chain molecule is free to move. Cross-linking is when atoms form bonds between the long chains. Cross-linking will increase the stiffness of the polymer.

Arrangement of the Monomers

Vulcanization of Rubber Charles Goodyear discovered that the addition of sulfer and heat to rubber increased its stiffness. Sulfer atoms forms cross-links between the polyisopropene molecules.

Thermoplastic Polymers Melt upon heating to a liquid state. They are recyclable. Polyethylene, polystyrene and polyvinyl chloride are thermoplastic. Polymers that are not heavily cross-linked are thermoplastic.

Thermosetting Polymers Thermosetting polymers will melt once and solidify into a highly cross-linked polymer. Once solidified they may not be remelted. They are not recyclable. Polymethyl methacrylate is a thermosetting polymer.

Non-moldable Polymers These polymers cannot be melted at all. Therefore they cannot be fabricated by any molding process. Cellulose is a non-moldable polymer.

Addition Polymerization Monomers simply add together to form the polymer. Most of the simple monomers create polymers via addition polymerization. Polyethylene, polystyrene, polypropylene and Teflon are all made by addition polymerization.

Condensation Polymerization Monomers join together by a reaction that results in the production of waste products. Nylon is produced by condensation polymerization where the water is the waste product.

Condensation Polymerization of Nylon

Silicones Silicones are silicon containing organic compounds. They have the advantage that they are more stable to heat and oxygen. Silicone is made up by alternating silicon and oxygen atoms with organic groups such as a methyl group bonded to the silicon.

Silicones This shows the bonding in silicone.

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