1. Introduction to Materials Science, Chapter 14, Polymer Structures University of Virginia, Dept. of Materials Science and Engineering 1 Chapter Outline: Polymer Structures  Hydrocarbon and Polymer Molecules  Chemistry of Polymer Molecules  Molecular Weight and Shape  Molecular Structure and Configurations  Copolymers  Polymer Crystals  Optional reading: none Greek polumeres, which means `having many parts' Google Polymers http://www.psrc.usm.edu/macrog/index.htm "I am inclined to think that the development of polymerization is perhaps the biggest thing chemistry has done, where it has had the biggest impact on everyday life." -Lord Todd (b. 1907), president of the Royal Society of London

2. Introduction to Materials Science, Chapter 14, Polymer Structures University of Virginia, Dept. of Materials Science and Engineering 2  Polymer:large molecule consisting of repeated chemical units (`mers') joined like beads on a string. Polymers usually contain many more than five monomers; some contain hundreds or thousands of monomers in each chain.  Polymers are natural, such as cellulose or DNA, or synthetic, such as nylon or polyethylene. Polymers: Introduction Many of the most important current research problems involve polymers. Living organisms are mainly composed of polymerized amino acids (proteins) nucleic acids (RNA and DNA), and other biopolymers. Our brains are mostly a complex polymer material soaking in salty water! Silk fibre produced by silk worms in a cocoon to protect it while it metamorphoses into a moth.

3. Introduction to Materials Science, Chapter 14, Polymer Structures University of Virginia, Dept. of Materials Science and Engineering 3  Most polymers are organic, and formed from hydrocarbon molecules  Each C atom has four e - that participate in bonds, each H atom has one bonding e - Hydrocarbon molecules (I) Methane, CH 4 Propane, C 3 H 8 Ethane, C 2 H 6 Examples of saturated (all bonds are single ones) hydrocarbon molecules:

4. Introduction to Materials Science, Chapter 14, Polymer Structures University of Virginia, Dept. of Materials Science and Engineering 4 Double and triple bonds can exist between C atoms (sharing of two or three electron pairs). These bonds are called unsaturated bonds. Unsaturated molecules are more reactive Hydrocarbon molecules (II) Ethylene, C 2 H 4 Acetylene, C 2 H 2 H-C  C-H Isomers are molecules that contain the same atoms but in a different arrangement. An example is butane and isobutane: Butane  C 4 H 10  Isobutane

5. Introduction to Materials Science, Chapter 14, Polymer Structures University of Virginia, Dept. of Materials Science and Engineering 5 Other organic groups can be in a polymer. R represent radical: organic groups that remain a unit during reactions (e.g. CH 3, C 2 H 5, C 6 H 5 ) Hydrocarbon molecules (III)

6. Introduction to Materials Science, Chapter 14, Polymer Structures University of Virginia, Dept. of Materials Science and Engineering 6 Polymer molecules  Polymer molecules are very large: macromolecules  Polymers are long and flexible chains with a string of C atoms as a backbone.  Side-bonding: C to an H or a radical  Double bonds possible in both chain and side bonds  Repeat unit (“unit cell”) is a mer  A single mer is called a monomer

7. Introduction to Materials Science, Chapter 14, Polymer Structures University of Virginia, Dept. of Materials Science and Engineering 7 Plastics Some ubiquitous `mers' are ethylene, styrene and acrylamide. Each polymerized to make: polyethylene (the soft clear plastic that plastic bags are made of), polystyrene (stiffer, usually white plastic that the covers for soft-drink cups are made of) polyacrylamide (very tough, clear plastic that compact discs are made from). On bottom of recyclable plastic bottle - you will see PE (polyethylene) or PS (polystyrene) Examples of what happens to polymers when they solidify: chains are entangled and packed together to make light, tough, flexible materials

8. Introduction to Materials Science, Chapter 14, Polymer Structures University of Virginia, Dept. of Materials Science and Engineering 8 A way to think about some of these materials is to think of what a big glob of cooked spaghetti is like. Stretch it a bit, it is kind of elastic, but if you pull hard, the noodles start to slide past one another and the whole glob starts to permanently deform. Does this remind you of what happens to a PE plastic bag when you stretch it? Think about what must be happening to the microscopic spaghetti that the bag is made up of! If you heat up PE or PS to moderate temperatures, if the chains have not been chemically stuck together (`cross-linked') they will melt, and turn into goopy liquids, which are called polymer melts. Some polymers are melts even at room temperature. Remembering that paper is made of cellulose, which is a polymer of biological origin, if you look around the room that you are in, you will see that a good fraction of the stuff in it is made of polymers. And of course, you are, too!

9. Introduction to Materials Science, Chapter 14, Polymer Structures University of Virginia, Dept. of Materials Science and Engineering 9 Chemistry of polymer molecules (I)  Ethylene (C 2 H 4 ) a gas at room T and P  Ethylene-->polyethylene (solid) though active mer formed by reaction with initiator / catalytic radical (R. )  (. ) denotes unpaired electron (active site) 3. Termination: two active chain ends meet or active chain end meet with radical (R. ) 2. Rapid propagation ~1000 mer units in 1-10 ms: Polymerization: 1.Initiation reaction:

10. Introduction to Materials Science, Chapter 14, Polymer Structures University of Virginia, Dept. of Materials Science and Engineering 10 Chemistry of polymer molecules (II) Replace hydrogen in polyethylene  polytetraflouroethyl ene (PTFE) – Teflon Replace every fourth hydrogen in polyethylene with Cl  polyvinyl chloride Replace every fourth hydrogen in polyethylene with CH 3 methyl group  polyproplylene

11. Introduction to Materials Science, Chapter 14, Polymer Structures University of Virginia, Dept. of Materials Science and Engineering 11 Chemistry of polymer molecules (III)  When all mers are the same, molecule is called homopolymer  When there is more than one type of mer present, the molecule is a copolymer  Mer units that have 2 active bonds to connect with other mers are called bifunctional  Mer units that have 3 active bonds to connect with other mers are called trifunctional. Form 3-D molecular network structures. Phenol-formaldehyde ( trifunctional) Polyethilene ( bifunctional)

12. Introduction to Materials Science, Chapter 14, Polymer Structures University of Virginia, Dept. of Materials Science and Engineering 12 Molecular weight (I)  Final molecular weight (chain length) controlled by relative rates of initiation, propagation, termination steps  Formation of macromolecules during polymerization results in distribution of chain lengths and molecular weights  Average molecular weight obtained by averaging masses with fraction of times they appear (number-average molecular weight) or with mass fraction of the molecules (weight- average molecular weight). number-average: weight-average: w i is weight fraction of chains of length i x i is number fraction of chains of length i