Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

P1: PBU/OVY P2: PBU/OVY QC: PBU/OVY T1: PBU Printer: Yet to Come
GTBL042-04 GTBL042-Callister-v2 July 31, 2007 16:17

Summary • 123

Figure 4.14 A

transmission

photomicrograph

(using cross-

polarized light)

showing the

spherulite structure

of polyethylene.

Linear boundaries

form between

adjacent spherulites,

and within each

spherulite appears a

Maltese cross. 525×.

(Courtesy F. P. Price,

General Electric

Company.)

composed of many different lamellar crystals and, in addition, some amorphous ma-

terial. Polyethylene, polypropylene, poly(vinyl chloride), polytetrafluoroethylene,

and nylon form a spherulitic structure when they crystallize from a melt.

SUMMARY

Hydrocarbon Molecules

Polymer Molecules

The Chemistry of Polymer Molecules

Most polymeric materials are composed of very large molecular chains with side

groups of various atoms (O, Cl, etc.) or organic groups such as methyl, ethyl, or phenyl

groups. These macromolecules are composed of repeat units, smaller structural en-

tities that are repeated along the chain. Repeat units for some of the chemically

simple polymers [i.e., polyethylene, polytetrafluoroethylene, poly(vinyl chloride),

and polypropylene] were presented.

Molecular Weight

Molecular weights for high polymers may be in excess of a million. Since all molecules

are not of the same size, there is a distribution of molecular weights. Molecular weight

is often expressed in terms of number and weight averages. Chain length may also

be specified by degree of polymerization, the number of repeat units per average

molecule.

Molecular Shape

Molecular Structure

Molecular Configurations

Copolymers

Several molecular characteristics that influence the properties of polymers were dis-

cussed. Molecular entanglements occur when the chains assume twisted, coiled, and