lecture 27, summer 2007 mechanical properties ii: polymers engr 145, chemistry of materials case...
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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