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The Science and Engineering of

Materials, 4

th

ed

Chapter 15 - Polymers

Objectives of Chapter 15

 Discuss the classification of Polymers

 Learn two main ways of creating a Polymer

 Study the effect of temperature on

Thermoplastics

 Study mechanical properties of

Thermoplastics

Chapter Outline (Continued)

 15.8 Mechanical Properties of

Thermoplastics

 15.9 Elastomers (Rubbers)

 15.10 Thermosetting Polymers

 15.11 Adhesives

 15.12 Additives for Plastics

 15.13 Polymer Processing and Recycling

Section 15.

Classification of Polymers

 Linear polymer - Any polymer in which molecules are in the form of spaghetti-like chains.

 Thermoplastics - Linear or branched polymers in which chains of molecules are not interconnected to one another.

 Thermosetting polymers - Polymers that are heavily cross-linked to produce a strong three dimensional network structure.

 Elastomers - These are polymers (thermoplastics or lightly cross-linked thermosets) that have an elastic deformation > 200%.

Table 15.1 Comparison of the three polymer categories

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning

is a trademark used herein under license.™ Figure 15.2 Three ways to represent the structure of polyethylene: (a) a solid three- dimensional model, (b) a three- dimensional “space” model, and (c) a simple two- dimensional model.

Example 15.

Design/Materials Selection for Polymer

Components

Design the type of polymer material you might select for the following applications: a surgeon’s glove, a beverage container and a pulley.

Example 15.1 SOLUTION

 The glove must be capable of stretching a great deal in order to slip onto the surgeon’s hand. This requirement describes an elastomer.

 A thermoplastic such as polyethylene terephthalate (PET) will have the necessary formability and ductility needed for this application.

 A relatively strong, rigid, hard material is required to prevent wear, so a thermosetting polymer might be most appropriate.

Section 15.

Addition Polymerization

 Addition polymerization - Process by which polymer chains are built up by adding monomers together without creating a byproduct.

 Unsaturated bond - The double- or even triple- covalent bond joining two atoms together in an organic molecule.

 Functionality - The number of sites on a monomer at which polymerization can occur.

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning

is a trademark used herein under license.™

Figure 15. Initiation of a polyethylene chain by chain- growth may involve (a) producing free radicals from initiators such as benzoyl peroxide, (b) attachment of a polyethylene repeat unit to one of the initiator radicals, and (c) attachment of additional repeat units to propagate the chain.

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning

is a trademark used herein under license.™

Figure 15. Termination of polyethylene chain growth: (a) the active ends of two chains come into close proximity, (b) the two chains undergo combina- tion and become one large chain, and (c) rearrangement of a hydrogen atom and creation of a double covalent bond by dispropor- tionation cause termination of two chains.

Example 15.2 SOLUTION (Continued)

The combination mechanism requires the number of benzoyl peroxide molecules to be:

23

23

1. 03 10 7143 ethylene / chain

215 10 ethylene molecules = ×

×

The molecular weight of benzoyl peroxide is (14 C)(12)

• (10 H)(1) + (4 O)(16) = 242 g/mol. Therefore, the amount of initiator needed to form the ends of the chains is:

1. 206 g
2. 02 10

( 0. 03 10 )(242g/mol) 23

23

×

×

Section 15.

Condensation Polymerization

 Condensation polymerization - A polymerization mechanism in which a small molecule (e.g., water, methanol, etc.) is condensed out as a byproduct.

Example 15. Condensation Polymerization of 6,6-Nylon

Nylon was first reported by Wallace Hume Carothers, of du Pont in about 1934. In 1939, du Pont’s Charles Stine reported the discovery of this first synthetic fiber to a group of 3000 women gathered for the New York World’s Fair. The first application was nylon stockings that were strong. Today nylon is used in hundreds of applications. Prior to nylon, Carothers had discovered neoprene (an elastomer).

The linear polymer 6,6-nylon is to be produced by combining 1000 g of hexamethylene diamine with adipic acid. A condensation reaction then produces the polymer. Show how this reaction occurs and determine the byproduct that forms. How many grams of adipic acid are needed, and how much 6,6- nylon is produced, assuming 100% efficiency?

Example 15.3 SOLUTION

The molecular structures of the monomers are shown below. The linear nylon chain is produced when a hydrogen atom from the hexamethylene diamine combines with an OH group from adipic acid to form a water molecule.

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.