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

122 • Chapter 4 / Polymer Structures

~10 nm

Figure 4.12 The

chain-folded structure

for a plate-shaped

polymer crystallite.

here are the individual chain-folded lamellar crystals that are separated by amor-

phous material. Tie-chain molecules that act as connecting links between adjacent

lamellae pass through these amorphous regions.

As the crystallization of a spherulitic structure nears completion, the extremities

of adjacent spherulites begin to impinge on one another, forming more or less planar

boundaries; prior to this time, they maintain their spherical shape. These boundaries

are evident in Figure 4.14, which is a photomicrograph of polyethylene using cross-

polarized light. A characteristic Maltese-cross pattern appears within each spherulite.

The bands or rings in the spherulite image result from twisting of the lamellar crystals

as they extend like ribbons from the center.

Spherulites are considered to be the polymer analogue of grains in polycrys-

talline metals and ceramics. However, as discussed above, each spherulite is really

Interspherulitic

boundary

Nucleation site

Tie molecule

Amorphous

material

Lamellar

chain-folded

crystallite

Direction of

spherulite growth

Figure 4.13 Schematic

representation of the detailed

structure of a spherulite.