[email protected] engr-45_lec-30_polymer-apps.ppt 1 bruce mayer, pe engineering-45: materials...

[email protected] • ENGR-45_Lec-30_Polymer-Apps.ppt1

Bruce Mayer, PE Engineering-45: Materials of Engineering

Bruce Mayer, PELicensed Electrical & Mechanical Engineer

[email protected]

Engineering 45

PolymerApplicatio

ns



Learning Goals – Polymer Apps

Learn How Microstructure affects Room Temperature Tensile Stress Behavior

Understand Hardening, Anisotropy, and Annealing in Polymers

How the elevated-temperature mechanical-response for PolyMers compares to Ceramics and Metals



PolyMer Tensile σ-ε Response

PolyMers Exhibit 3 Basic Types of Tensile ResponseA. Brittle → Glass-

Like• Linear-Elastic• Very Small Strain

at Fracture

B. Elastic-Plastic → Metal-Like• Well-Defined

Yielding• Significant Strain

at Fracture

C. Elastomeric → Rubber-Like• Completely

Elastic; all the way to fracture

• Very Large Strains



Tensile Response – Brittle & Plastic

0

unload/reload

0

brittle failure

plastic failure

20

40

60

2 4 6

s (MPa)

e

x

x

semicrystalline

case

amorphous regions

elongate

crystalline regions align

crystalline regions

slide

8

onset of necking

aligned, cross- linked case

networked case

Initial

Near Failure

near failure



Tensile Response – Elastomers

Compare Elastomers to responses of other polymers:• BRITTLE response (aligned, cross-linked & networked case)• PLASTIC response (semi-crystalline case)

initial: amorphous chains are kinked, heavily cross-linked.

final: chains are straight,

still cross-linked

0

20

40

60

0 2 4 6

s (MPa)

e 8

x

x

x

elastomer

plastic failure

brittle failure

Deformation is reversible!



T & StrainRate - ThermoPlastics

DEcreasing Temp• Increases E• Increases TS• Decreases %EL

INcreasing Strain Rate...• Same effects as decreasing Temperature20

40

60

80

00 0.1 0.2 0.3

4°C

20°C

40°C

60°C

to 1.3

s (MPa)

e

Data for the semicrystalline polymer: PMMA (Plexiglas)



PreDeformation by Drawing Drawing

• stretches the polymer prior to use• aligns chains to the stretching direction

Results of drawing• Increases the elastic modulus (E) in the

stretching direction• Increases the tensile strength (TS) in the

stretching direction• Decreases ductility (%EL)



Vulcanization Chemically Induced Cross-Linking Process in

Elastomers is called VULCANIZATION• An Irreversible Chemical Reaction Performed at

Elevated Temperature Thru a Cross Linking Agent; Typically Sulfur

Typically Requires More than ONE S-atom per Cross-Link

Vulcanization at the 1-5 wt%-S level improves Elastomer Properties Including Wear & Strength (e.g., Tires)



Time Dependent Deformation

Stress Relaxation Test• Rapidly deform to

Strain ε0, and Hold

• Measure Hold-Stress as a Function of Time

time

strain

tensile test

o

t( )

The Hold-Stress Decreases with Time Due to “unkinking” of the PolyMer chains

/0)( tet

• Where– σ0 Stress at time-0

– Time Constant; i.e., the time required for the stress to drop by 63%



Relaxation Modulus

Given Stress Relaxation

time

strain

tensile test

o

t( )

Next Pick a BaseLine time, Say 10s, and Vary Temperature to

or

ttE

)(

)(

Define a Time-Dependent RELAXATION Modulus Ttempo

r

sTE

)10()(10



Relaxation Modulus vs Temperature

Glassy State• Material is RIGID and

BRITTLE• ELASTIC Stretching of

Bonds

Leathery State• some Sliding of chains

over one another• some Permanent

deformation• deformation will be time-

dependent and not totally recoverable



Relaxation Modulus vs Temp cont.1

Rubbery Plateau• deforms in a

rubbery manner• both elastic and

viscous behavior• straightening out of

polymer chains gives large strains

• strain is reversible due to crosslinks and entanglements



Relaxation Modulus vs Temp cont.2

Rubbery flow and Viscous flow• high temperature states• polymer chains slide

over each other• permanent deformation

is possible (molding)



Glass Transition Temperature

Notice on the Er

vs T curve the almost VERTICALSlope at the Centerof the Leathery, orTough Regime

This marks the Transition from a Brittle, amorphous State to a ViscoElastic Condition



The Glass Transition Temp

Temperature

Sp

ecifi

c V

olu

me =

1/ρ

(cu

-m/k

g)

Tg Tm

Note Change in

Slope

NonCrystalline Material• T < Tg → Rigid, Glass-Like• Tg < T < Tm → Rubbery

or Leathery• T > Tm → Melted, Liquid



Structure-Property Relationships

Ease of MOVEMENT of molecular chains affects properties• crystallinity (ability to pack efficiently)• Tg (onset of

large-scale molecular motion)

• Glass-forming ability

• Strength vs. Flexibility



Structure-Property Relns cont.

STRUCTURAL factors that inhibit molecular motion:• Complexity of the Mer• Size of Side Groups• Branching,

Crosslinking• Configuration• Bonding • Entanglements

Possible organization of PolyEthylene polymer chains

CrystallineRegion

Amorphous Tie Region



WhiteBoard Work

Prob Similar to 15.24 Vulcanize PolyIsoPrene with Sulfur

• Given – 57 wt%-S combined with the polymer– Six Sulfur atoms per CrossLink on Average

• Determine CrossLinks per Isoprene Mer

Natural rubber (cis-polyisoprene) before vulcanizing

with sulfur



IsoPrene Polymerization



σ-ε vs. T for FluoroPolymer

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