Lecture 27, summer 2007Mechanical Properties II: Polymers

ENGR 145, Chemistry of MaterialsCase Western Reserve University

Reading assignment: Callister 16.1-16.10

Learning objectives:• Recognize distinctive aspects of polymers’ mechanical behavior:

• Temperature dependence w.r.t. Tg

• Deformation mechanism• Stress-strain curve: differences from metals & ceramics• Dependence on crystallinity and molecular weight• Viscoelasticity; relaxation modulus

Lecture 27: Mechanical Properties II: Polymers

Lecture 27, summer 2007Mechanical Properties II: Polymers

ENGR 145, Chemistry of MaterialsCase Western Reserve University

A - brittle

B - ductile (plastic)

C - elastomeric

Callister Fig. 16.1

Mechanical Properties of Polymers [Callister 16.2]

• Engineering stress – strain curves for polymers

Lecture 27, summer 2007Mechanical Properties II: Polymers

ENGR 145, Chemistry of MaterialsCase Western Reserve University

• Engineering stress – strain curve

• yield strength (σy)

• tensile strength (TS)

Callister Fig. 16.2

Note: different definitions than for metals

Note: strengthening

can be observed after yielding

Mechanical Properties of Polymers [Callister 16.2]

Lecture 27, summer 2007Mechanical Properties II: Polymers

ENGR 145, Chemistry of MaterialsCase Western Reserve University

• Effect of temperature in a linear polymer (PMMA)

Callister Fig. 16.3

increasingtemperature

Tg = 4 °C

T>Tg: plasticit

y

T≤Tg: stiffness

; brittlene

ss

Effect of Temperature: Glassy vs. Plastic [Callister 16.2]

Lecture 27, summer 2007Mechanical Properties II: Polymers

ENGR 145, Chemistry of MaterialsCase Western Reserve University

• Polymer crystals: • Platelets • Bundles • Spherulites

Callister Fig. 15.15

Callister Fig. 15.14

Semicrystalline Polymers [Callister 15.11]

Lecture 27, summer 2007Mechanical Properties II: Polymers

ENGR 145, Chemistry of MaterialsCase Western Reserve University

Callister Fig. 16.4

Microscopic Mechanism of Deformation [Callister 16.3]

Lecture 27, summer 2007Mechanical Properties II: Polymers

ENGR 145, Chemistry of MaterialsCase Western Reserve University

• Macroscopic effects of deformation

Callister Fig. 16.4

Callister Fig. 16.5

slope = tensile modulus

Mechanical Characteristics of Polymers [Callister 16.3]

necking

Lecture 27, summer 2007Mechanical Properties II: Polymers

ENGR 145, Chemistry of MaterialsCase Western Reserve University

• Factors influencing tensile strength• As molecular weight , tensile strength

• Little effect on tensile modulus

• As degree of crystallinity : • tensile strength • tensile modulus • strain to failure

TS TS A

Mn

Callister Fig. 16.6:

Polyethylene

Mechanical Characteristics of Polymers [Callister 16.3]

tensile strength at infinite molecular

weight

average molecular weight

constant

Lecture 27, summer 2007Mechanical Properties II: Polymers

ENGR 145, Chemistry of MaterialsCase Western Reserve University

• Factors influencing mechanical properties• Predeformation by drawing

• Modulus• Ductility

• Heat treatment

• Annealing near Tm to reverse effects of drawing

• Crystallization• Modulus• Yield strength• Ductility

Mechanical Characteristics of Polymers [Callister 16.3]

Callister Fig. 16.5

Lecture 27, summer 2007Mechanical Properties II: Polymers

ENGR 145, Chemistry of MaterialsCase Western Reserve University

Crystallization of a Polymer [Callister 16.5]

• Polypropylene (Tg = –18 °C; Tm = 175 °C)

• Nucleation of crystals, followed by growthfrom Callister

Lecture 27, summer 2007Mechanical Properties II: Polymers

ENGR 145, Chemistry of MaterialsCase Western Reserve University

Stress Pulse

Viscoelastic Viscous

Elastic

Viscoelasticity in Polymers [Callister 16.7]

from Callister

Lecture 27, summer 2007Mechanical Properties II: Polymers

ENGR 145, Chemistry of MaterialsCase Western Reserve University

• Viscoelasticity

• Initial strain ε0 for a given initial stress σ0

• Stress decreases from σ0 as material deforms

Relaxation modulus:

0

r

tE t

stress

0

t

t time

00

0

E

Relaxation Modulus [Callister 16.7]

Er with time as stress with time

StressRelax.mov

Lecture 27, summer 2007Mechanical Properties II: Polymers

ENGR 145, Chemistry of MaterialsCase Western Reserve University

Temperature Dependence of Relaxation Modulus [Callister 16.7]

Relaxation modulus decreases …

… with time

… with T

from Callister

Lecture 27, summer 2007Mechanical Properties II: Polymers

ENGR 145, Chemistry of MaterialsCase Western Reserve University

Thermomechanical Characteristics of Polymers [Callister 16.7]

• Five types of viscoelastic behavior• Glassy• Leathery• Rubbery• Rubbery flow• Viscous flow

from Callister