Chapter 14 - Polymers Polymers are long chains of repeating organic molecules. Molecules are formed by covalent bonds with secondary forces between molecules. Types of Polymers: A. Elastomers -Polymers that have rubberlike properties; at room temperature, stretches under a low stress to at least twice its length and then quickly returns to almost its original length upon removal of the stress. B. Plastics - A large and varied group of synthetic materials which are processed by forming or molding into shape.

Polymerization -Process by which small molecules are joined to create long chains of a polymer. a. Initiation: An ethylene molecule (C2H4) is activated so that the double C bond is replaced by a single bond. b. Growth: Individual “mer units” of ethylene join to form the “polymer” polyethylene [ (C2H4)n ] consisting of many “mers” c. Termination: when two chains combine -- or impurities terminate the polymer chain.

Polymer Molecular Structure • Polymer = many mers • Chain configurations and strength: Direction of increasing strength

Molecular Weight • Molecular weight, Mw: Mass of a mole of chains. Average Molecular weight a) number-average molecular weight -- based on the number fraction of chains within various size ranges. weight-average molecular weight -- based on the weight fraction of chains within the size ranges. Polymer characteristics (e.g., melting temperature, and tensile strength increases with molecular weight) are affected by the magnitude of the molecular weight. Longer chains are entangled (anchored) better.

Example: Assume that the molecular weight distribution shown are for polyvinyl chloride. For this material, compute (a) the mer molecular weight, (b) the number-average molecular weight and degree of polymerization, (c) the weight-average molecular weight and degree of polymerization. Mol Wt Range 5,000-10,000 10,000-15,000 15,000-20,000 20,000-25,000 25,000-30,000 30,000-35,000 35,000-40,000 Mean Mi (g/mol) 7,500 12,500 17,500 22,500 27,500 32,500 37,500 xi 0.05 0.16 0.22 0.27 0.20 0.08 0.02 xi = fraction of the total number of chains in the ith range.

mer unit has 2 C atoms, 3 H atoms, and 1 Cl atom Mer molecular weight: mer unit has 2 C atoms, 3 H atoms, and 1 Cl atom m = 2(12.01) + 3(1.01) + 35.45 = 62.50 g/mol b) Mean Mi (g/mol) 7,500 12,500 17,500 22,500 27,500 32,500 37,500 xi 0.05 0.16 0.22 0.27 0.20 0.08 0.02 xiMi 375 2000 3850 6075 5500 2600 750 ------------- Mn=21,150

c) Mean Mi (g/mol) 7,500 12,500 17,500 22,500 27,500 32,500 37,500 wi 0.02 0.10 0.18 0.29 0.26 0.12 0.03 wiMi 150 1250 3150 6525 7150 3900 1225 ------------- Mw=23,350 wi = weight fraction of the molecules in the size range.

Copolymers Combination of different mers Random Alternating Block Graft

Mechanical Behavior of Polymers --elastomer (heavily kinked cross-linked amorphous chains) --brittle response (aligned, cross linked & networked case) --plastic response (semi-crystalline case)

Tensile Properties: Brittle and Plasic

Plastics 1. Thermoplastics 2. Thermosetting plastics (thermosets) - can be repeatedly softened when heated and harden when cooled into a different shape, without changing the final basic structure or properties of the polymer. - little cross-linking; carbon chains are held together only by weak Van der Waals bonds. 2. Thermosetting plastics (thermosets) - formed into a permanent shape and cured or “set” by a chemical reaction cannot be remelted and reformed into another shape but decompose upon being heated to too high a temperature ---> cannot be recycled. - large cross linking (10 to 50% of mers); network of carbon atoms covalently bonded to form a rigid solid --> hard and brittle; stronger than thermoplastics. - epoxies, vulcanized rubber, polyester resin (used for matrix material for fiberglass), phenolics (used for friction materials and for printed circuit boards).

T AND STRAIN RATE: THERMOPLASTICS • Decreasing T... --increases E --increases TS --decreases %EL • Increasing strain rate... --same effects as decreasing T.

SUMMARY • General drawbacks to polymers: • Thermoplastics: -- E, sy, toughness, Tapplication are generally small. -- Deformation is often T and time dependent. -- Result: polymers benefit from composite reinforcement. • Thermoplastics: -- Smaller E, sy, Tapplication -- Ductile -- Easier to form and recycle • Elastomers (rubber): -- Large reversible strains! • Thermosets (epoxies, polyesters): -- Larger E, sy, Tapplication Table 15.3 Callister 6e: Good overview of applications and trade names of polymers.

It’s Clicker Time! 1. Which of the following increases the resistivity of a metal? Decreasing the temperature Decreasing the concentration of impurities Cold working Decreasing the specimen diameter

2. Insulators have low electrical conductivity due to: absence of valence electrons overlapping valence and conduction bands wide energy band gap partially filled valence band

3. For a given semiconductor, which of the following electrical properties increases with temperature? a) Resistivity b) intrinsic conductivity c) extrinsic conductivity d) none of the above

4. Which of the following elements can be used to form a p-type extrinsic semiconductor with GaAs? (Ga is from group IIIA, and As is from group VA.) Ca (group IIA) Ge (group IVA) Sb (group VIA) In (group IIIA)

5. Which of the following determines the coordination number of a ceramic’s crystal structure?   a) cation-anion mass ratio b) cation-anion radius ratio c) cation-anion electronegativity ratio d) none of the above

6. Which of the following is true regarding ceramics?   Ceramics are strong in compression and weak in tension due to covalent and ionic bonds. Ceramics are brittle due to the presence of Van der Waals forces. Ceramics have good thermal and electrical conductivities due to the absence of free electrons. none of the above.

7. Which of the following is true regarding polymers? Polymers are elastic. Ductility is independent of temperature. Polymers have good thermal and electrical conductivities due to the presence of Vander Waals forces. Melting temperature increases with molecular weight.

8. Which of the following will increase the thermal shock resistance of a ceramic material?   increasing the modulus of elasticity increasing the material’s ability to change its dimensions with temperature increasing the material’s ability to reduce internal temperature gradients none of the above

9. Which of the following materials can be repeatedly softened when heated and hardened when cooled into different shapes without changing its basic structure and properties? Thermosets Thermoplastics Elastomers ceramics

10. Which of the following polymer structure is the strongest? Linear Branched Network Cross-linked