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

GTBL042-08 GTBL042-Callister-v3 October 4, 2007 11:51

2nd Revised Pages

276 • Chapter 8 / Deformation and Strengthening Mechanisms

tensile strength increases with increasing molecular weight. Mathematically,TSis a

function of the number-average molecular weight according to

TS=TS∞−

A

Mn

(8.11)

For some polymers,

dependence of

tensile strength on

number-average

molecular weight whereTS∞is the tensile strength at infinite molecular weight andAis a constant. The

behavior described by this equation is explained by increased chain entanglements

with risingMn.

Degree of Crystallinity

For a specific polymer, the degree of crystallinity can have a rather significant influ-

ence on the mechanical properties, since it affects the extent of the intermolecular

secondary bonding. For crystalline regions in which molecular chains are closely

packed in an ordered and parallel arrangement, extensive secondary bonding or-

dinarily exists between adjacent chain segments. This secondary bonding is much

less prevalent in amorphous regions, by virtue of the chain misalignment. As a con-

sequence, for semicrystalline polymers, tensile modulus increases significantly with

degree of crystallinity. For example, for polyethylene, the modulus increases approx-

imately an order of magnitude as the crystallinity fraction is raised from 0.3 to 0.6.

Furthermore, increasing the crystallinity of a polymer generally enhances its

strength; in addition, the material tends to become more brittle. The influence of chain

chemistry and structure (branching, stereoisomerism, etc.) on degree of crystallinity

was discussed in Chapter 4.

The effects of both percent crystallinity and molecular weight on the physical

state of polyethylene are represented in Figure 8.29.

Predeformation by Drawing

On a commercial basis, one of the most important techniques used to improve me-

chanical strength and tensile modulus is to permanently deform the polymer in ten-

sion. This procedure is sometimes termeddrawing(also described in Section 8.17),

and corresponds to the neck extension process illustrated schematically in Figure

7.25, with the corresponding oriented structure shown in Figure 8.28d. In terms of

property alterations, drawing is the polymer analog of strain hardening in metals.

It is an important stiffening and strengthening technique that is employed in the

Molecular weight

(Nonlinear scale)

Percent crystallinity

(^00500) 2,000 5,000 20,000 40,000
25
50
75
100
Grease,
liquids
Soft
waxes
Brittle
waxes
Tough
waxes
Hard plastics
Soft plastics
Figure 8.29 The
influence of degree
of crystallinity and
molecular weight on
the physical
characteristics of
polyethylene. (From
R. B. Richards,
“Polyethylene—
Structure,
Crystallinity and
Properties,”J. Appl.
Chem.,1,370, 1951.)