Chapter 15 - 1

Chapter 15:

Characteristics, Applications &

Processing of Polymers (1)

ISSUES TO ADDRESS...

• What are the tensile properties of polymers and how

are they affected by basic microstructural features?

• Hardening, anisotropy, and annealing in polymers.

• How does the elevated temperature mechanical

response of polymers compare to ceramics and metals?

Chapter 15 - 2 2

Mechanical Properties of Polymers –

Stress-Strain Behaviors

• Strength of polymers ~ 10% of those for metals

• Deformation strains for polymers > 10 times of those for metals

– for most metals, deformation strains < ~0.3

brittle polymer

Plastic polymer

Elastomer/rubber

elastic moduli

– less than for metals Adapted from Fig. 15.1,

Callister & Rethwisch 8e.

Chapter 15 - fig_15_02

Tensile Strength & Yield Strength for

Polymers

Chapter 15 - 4 4

• Decreasing T: -- increases E

-- increases TS

-- decreases %EL

• Increasing strain rate: -- same effects

as decreasing T.

Adapted from Fig. 15.3, Callister & Rethwisch 8e. (Fig. 15.3 is from T.S.

Carswell and J.K. Nason, 'Effect of Environmental Conditions on the

Mechanical Properties of Organic Plastics", Symposium on Plastics,

American Society for Testing and Materials, Philadelphia, PA, 1944.)

Influence of T and Strain Rate on Thermoplastics

Mechanical Property

20

4 0

6 0

8 0

0 0 0.1 0.2 0.3

4ºC

20ºC

40ºC

60ºC to 1.3

s (MPa)

e

Plots for

semicrystalline

PMMA (Plexiglas)

Chapter 15 - fig_15_04

Change in Shape for Polymers during

Tensile Test

Different from typical plastic deformation for

metals where necking concentrates locally,

polymer necking section elongate significantly

Chapter 15 - 6 6

Mechanisms of Deformation for Brittle,

Crosslinked or Network Polymers

brittle failure

plastic failure

s (MPa)

e

x

x

aligned, crosslinked

polymer Stress-strain curves adapted from Fig. 15.1,

Callister & Rethwisch 8e.

Initial

Near

Failure Initial

network polymer

Near

Failure

Chapter 15 - 7 7

Mechanisms of Deformation for

Semicrystalline Polymers

brittle failure

plastic failure

s (MPa)

x

x

crystalline

block segments

separate

fibrillar

structure

near

failure

crystalline

regions align

onset of

necking

undeformed

structure amorphous

regions

elongate

unload/reload

Stress-strain curves adapted

from Fig. 15.1, Callister &

Rethwisch 8e. Inset figures

along plastic response curve

adapted from Figs. 15.12 &

15.13, Callister & Rethwisch

8e. (15.12 & 15.13 are from

J.M. Schultz, Polymer

Materials Science, Prentice-

Hall, Inc., 1974, pp. 500-501.)

e

Chapter 15 -

fig_15_12

Elastic Deformation in

Semicrystalline Polymers

Chapter 15 -

fig_15_13

Plastic Deformation in

Semicrystalline Polymers

Chapter 15 - 10 10

Predeformation by Drawing

• Drawing…(e.g.: monofilament fishline) -- stretches the polymer prior to use

-- aligns chains in the stretching direction

• Results of (pre-) drawing:

-- increases the elastic modulus (E) in the

stretching direction

-- increases the tensile strength (TS) in the

stretching direction

-- decreases ductility (%EL) Pre-deformation for polymer has similar strengthening

effects as cold working for metals

• Annealing after drawing...

-- decreases chain alignment

-- reverses effects of drawing (reduces E and

TS, enhances %EL) Annealing for polymers has similar effect of annealing

for metals

Adapted from Fig. 15.13, Callister

& Rethwisch 8e. (Fig. 15.13 is

from J.M. Schultz, Polymer

Materials Science, Prentice-Hall,

Inc., 1974, pp. 500-501.)

Chapter 15 - 11 11

Stress-strain curves

adapted from Fig. 15.1,

Callister & Rethwisch 8e.

Inset figures along

elastomer curve (green)

adapted from Fig. 15.15,

Callister & Rethwisch 8e.

(Fig. 15.15 is from Z.D.

Jastrzebski, The Nature

and Properties of

Engineering Materials,

3rd ed., John Wiley and

Sons, 1987.)

Mechanisms of Deformation of

Elastomers/Rubbers s (MPa)

e

initial: amorphous chains are kinked, cross-linked.

x

final: chains are straighter,

still cross-linked

elastomer

deformation is reversible (elastic)!

brittle failure

plastic failure x

x

Chapter 15 - 12 12

• Relaxation:

-- strain in tension to eo

and hold.

-- observe decrease in

stress with time.

o

r

ttE

e

s

)()(

• Relaxation modulus: modulus change (decreases) with time

or

Relaxation & Creep for

Semicrystalline Polymers

time

strain

tensile test

eo

σ(t)

• Creep:

-- stress in tension to σo

and hold.

-- observe increase in

strain with time.

time

stress

tensile test

σo e (t)

)()( 0

ttEr

e

s

Chapter 15 - fig_15_05

Strain vs. Time Behaviors

Pure Elastic (Metal/Ceramics)

Viscous (honey, pitch)

Viscoelastic (typical plastics above glass transition Tg)

Chapter 15 -

fig_

15

_0

7

(Relaxation) Modulus vs. Temperature

Rigid and brittle

(as glass)

Viscoelastic

(as typical

plastics)

Elastomer

(as rubber)

Viscous liquid

(polymer melts)

Chapter 15 -

fig_15_11

Fatigue in Polymer

Chapter 15 - 16 16

Fracture of Polymers (1)

fibrillar bridges microvoids crack

aligned chains

Adapted from Fig. 15.9,

Callister & Rethwisch 8e.

Craze formation prior to cracking

– during crazing, plastic deformation of spherulites

– and formation of microvoids and fibrillar bridges

Chapter 15 -

fig_

15

_1

0

Fracture of Polymers (2)

Chapter 15 - 18 18

Melting & Glass Transition Temps. Tm – Melting Temperature:

For crystalline material the temperature at

which phase transition between (free

flowing) liquid and crystalline solid happen

Tg – Glass transition temperature

For glassy material, the temperature

at which the highly viscous liquid

transforms to rigid amorphous solid,

accompanied by sudden change in other

physical/chemical properties

Tm > Tg

Adapted from Fig. 15.18,

Callister & Rethwisch 8e.

What factors affect Tm and Tg?

• Both Tm and Tg increase with increasing

chain stiffness

• Chain stiffness increased by presence of

1. Bulky side-groups

2. Polar groups or side-groups

3. Chain double bonds and aromatic

chain groups

Chapter 15 -

Examples for Polymers with

Different Tg and Tm • Rubbers for auto tires:

– Tg of about -50 oC https://www.perkinelmer.com/CMSResources/Images/44-

141192APP_007771B_15_Characterization_of_Car_Tire_Rubber.pdf

• High performance nylon for under-the-

hood applications

– Tm about 250 oC

– Tg about 50 oC http://fiberpolymer.invista.com/en/advantages.html

19

Chapter 15 - 20 20

• Thermoplastics: -- little crosslinking

-- ductile

-- soften with heating easy reshaping and recycle

-- polyethylene

polypropylene

polycarbonate

polystyrene

• Thermosets: -- significant crosslinking

(10 to 50% of repeat units)

-- hard

-- do NOT soften with heating usually not recycled

-- vulcanized rubber, epoxies, polyester resin, phenolic resin

Thermoplastics vs. Thermosets

Chapter 15 - 21

• Polymers mechanical properties: -- E, sy, Kc, Tapplication are generally small.

-- Deformation is often time and temperature dependent.

• Thermoplastics (PE, PS, PP, PC):

-- Smaller E, sy, Tapplication -- Larger Kc

-- Easier to form and recycle

• Elastomers (rubber):

-- Large reversible strains!

• Thermosets (epoxies, polyesters):

-- Larger E, sy, Tapplication

-- Smaller Kc

Table 15.3 Callister &

Rethwisch 8e:

Good overview

of applications

and trade names

of polymers.

Summary