mse 201 - polymers · 2012. 10. 2. · mech. props. • heat treatment and thermal history –...

MSE 201 - Polymers 1

Mechanical Properties of Polymers

Mechanical Properties of Polymers

Instructor: Joshua U. OtaigbeIowa State University

Why Mechanical Properties?

• Parameters that determine material response to applied stress

• Related to molecular structure• Facilitates development of materials for

specific end–use applications


Structure/Mechanical Property Relations in Polymers

• Happy and Sad balls• Silly putty• Necked sample• Gough-Joule effect

• Demos & Explain

Polymer Classifications According to Mechanical Props.

• Rubbers

4 Low stiffness, E = 106 – 107 Pa and ε up to high extensions

• Semi–crystalline polymers4 Intermediate stiffness, E = 108 – 109 Pa

and typically flexible & tough


Polymer Classifications According to Mechanical Props.

• Glasses4 High stiffness, E = 109 – 1010 Pa and typically

low ε & brittle• Fibers

4 High stiffness, E = 1010 – 1011 Pa and typically tough & strong

• Note that classif. is related to molecular architecture**

Tailorable Chemical & Morphological Features to Alter Mech. Props.

• MW and MWD– Affects flow properties during processing and

hence ultimate characteristics• Crosslinking and branching

– Vulcanize rubber to raise mechanical properties– Hardness & E increase with crosslinking– Chain branching strongly affects mechanical

properties (e.g., LDPE & HDPE)


Tailorable Chemical & Morphological Features

• Crystallinity and crystal morphology– Increasing %C increase mech. props.

• Copolymerization and/or Blending– Used to obtain props. not present in a given

polymer (e.g., LIPS & HIPS)


• Plasticization– Low MW material blended with polymer

(e.g., PVC) – To improve processability to produce softer

& tougher material• Molecular orientation

– Used fibers & biaxial drawn films**– Undesirable in injection molding &

extrusion**



• Fillers and composites– e.g., carbon or glass fibers to increase stiffness

and strength

(Above can be changed to produce polymers for specific engineering applications)!!!

Influence of Environmental Factors on Mech. Props.

• Emphasize importance of ASTM• Temperature*• Deformation rate, time & frequency*• Stress and strain amplitude*• Deformation mode (flexure, etc.)*


Influence of Environmental Factors on Mech. Props.

• Heat treatment and thermal history– Properties depend on process tempts.– In particular, rate of cooling from melt– Reheating produce annealing which modify

properties.• Surrounding atmosphere

– Water plasticizes nylon, PET– Petrol/PC helmets lead to fracture

Types of Mechanical Properties

• E (time, Temperature) for polymers (cf. metals)– Therefore, need to measure E (t, T) to

characterize bulk behavior

FF


Effect of Temperature on Stiffness of Polymers

Effect of Temperature on Stiffness of Polymers

Temperature

Log (E)

Tg Tm

Rigid

Leathery

Rubbery

Viscous

Types of Mechanical Properties• Tensile stress–strain behavior

Strain

Stre

ss

Linear Elastic Deformation


Types of Mechanical Properties

• Creep behavior– Variation of deformation with time of a material

subjected to constant loading– Very important in structural applications

2 & 3

time1

4

56

εε

Creep Behavior

– 1: Instantaneous elastic deformation due bond orientations

– 2: Delayed elastic deformation (1° creep) due to segmental motion and chain uncoiling

– 3: Viscous flow (2° creep) due to molecular slippage

– 4: Instantaneous elastic recovery due to bond recovery

2 & 3

time1

4

56

εε


Creep Behavior

– 5: Delayed elastic recovery due to segmental motion returning molecule to original configuration

– 6: Irreversible plastic deformation due to viscous flow

2 & 3

time1

4

56

εε

Creep Behavior

• D (t) = creep stress/creep strain 4 Linear viscoelasticity @ low strains

{D = f (t)}4 Non–linear viscoelasticity @ high strains

{D = f (ε, t )}

2 & 3

time1

4

56

εε


Creep Behavior in Metals (ε vs. time)

Creep Behavior in Metals (ε vs. time)

Time

Str

ain

Stress Relaxation BehaviorStress Relaxation Behavior

• Stress relaxation modulus = f(t)

St

re

ss

Time


Dynamic Mechanical Behavior• Measures material response to periodic or

varying forces• Applied force and resulting strain both vary

sinusoidally with time• Very useful for studying transition

phenomena in polymers (MSE383)

• Note: Creep & SRM give long–time behavior and DMA can give short–time behavior

Linear Viscoelasticity

• Ideal HOOKEAN response

• Ideal NEWTONIAN response

extension) (uniaxialε•=σ E

fluid) (Newtonianγ•η=τ &a


Linear Viscoelasticity

• Viscoelastic response

timeelapsed fixedFor

*icityviscoelastLinear )(

ticity viscoelasNonlinear),(

−∝⇒

−•=

−=

εσ

εσ

εσ

tf

tf

Linear ViscoelasticityLinear Viscoelasticity

t1

t2

t1 t2

t1, t2

σσ

εε

nonlinear visc.

linear visc.


Typical σ−ε Plot for a Ductile PolymerTypical σ−ε Plot for a Ductile Polymer

σσ

εε

strain hardening(over)

Practical Demo. of Strain HardeningPractical Demo. of Strain Hardening


Typical σ−ε Plot for Typical Polymers @ 20°C & low rates

Typical σ−ε Plot for Typical Polymers @ 20°C & low rates

PMMA; PSE ~ 2-3 GPa εε = 3 %

σσ

xtalline polymerE ~ 100 MPaεε = 500-600% %

RubberE ~ 2 MPaεε = 600% %

εε

Time & Tempt. Effects on σ−ε Behavior

Time & Tempt. Effects on σ−ε Behavior

speed of testing

εε εε

σσ

Temp

PIsoP @T > Tgσσ


Typical Stress/Strain behavior of Elastomers

• High extensibility• Rubber elasticity

–Gough-Joule–Thermodynamic

QuickTime™ and aGraphics decompressor

are needed to see this picture

Temperature

Log (E)

Tg Tm

Rigid

Leathery

Rubbery

Viscous

Tempt. Effects on Modulusof a Typical Polymer


(1)

(2)

(3)(4)

(5)



• Region 1 (T < 90°C - glassy region)– 109 < E <109.5 Pa. Polymer is glassy, hard,

brittle; main-chain segments frozen-in; no rotational motion

• Region 2 (90-120°C - transition region)– 105.7 < E <109 Pa. Polymer is leathery & T-

dependent, hard, brittle; main-chain segments begin to undergo rotational and short-range motions


are needed to see this picture.

(1)

(2)

(3)(4)

(5)

Temp. Effects on Modulusof a Typical Polymer

• Region 3 (120-150°C - rubbery plateau)– 105.4 < E <105.7 Pa. Polymer is rubbery– Short-range chain motions are now extremely

rapid; long-range motions are retarded • Region 4 (150-180°C-rubbery flow reg.)

– 104.5 < E <105.4 Pa. Polymer is elastic, rubbery & liquid-like long-range molecular motions set in



(1)

(2)

(3)(4)

(5)



• Region 5 (T > 180°C-liquid flow region)– E <104.5 Pa. – Whole scale motion of molecules with no

elastic recovery

(Above regions present in polymers to greater or lesser extent)



(1)

(2)

(3)(4)

(5)

Recap• Polymers exhibit unique mech. props.• Mech. props related to molecular structure

and can be tailored!!• Polymers are viscoelastic• Time & temp. effects important• 5 regions discernible in E vs. T plot• Creep & SR more important in polymers

relative to other materials


Temperature Effects on Polymers-Transition Temperatures

Temperature Effects on Polymers-Transition Temperatures

• Transitions in polymers include:– Glass transition (Tg)**– Secondary transitions (TBD)– Crystal melting & Tm

• Tg is most important transition in amorphous polymers– Also important to a lesser degree in semi-

crystalline polymers

Glass Transition

– Main chain motions in amorph. regions– Properties change from rubbery (or leathery) to

glassy (brittle)– Reversible process– T>Tg, get long-range segmental motions &

rotation of molecules about bonds– Determines maximum use tempt.**


Examples of Propertiesthat Change @ Tg

– Specific volume* – Thermal expansion coefficients*– Mechanical damping*– Mechanical properties* (garden hose)**– Electrical properties– Refractive index, – etc., etc.

Show movie over

Tg MovieTg Movie


Stiffness vs. TempStiffness vs. Temp

Stiffness drops by 3 orders of magnitude

PIsoP PS

Sti

ffn

ess

Tempt-70°C 100°C

Chemical Structure & Tg

• Different chemical structures lead to varying energy barrier to rotation.

• Chain Flexibility– Flexible chains give low Tg’s (e.g. ether

linkages, siloxane bonds, etc.– Rigid groups (e.g. aromatic rings) give high

Tg’s


• Increase Tg– bulky & rigid– steric hindrance– Polarity– H-bonding

Effect of Side Groupson Tg

• Decrease Tg– flexible– symmetry

Effect of Side Groups on TgEffect of Side Groups on Tg

HH

3

C C

H

CH

X

CH 3X =

Tg °C = – 10 100 115

n


Effect of Side Groups on TgEffect of Side Groups on Tg

HHC C

H X n

X =

Tg °C = 135 145

Structural Effects & Tg

– Increases in MW increases Tg– Crosslinking increases Tg– Plasticization decreases Tg


Boyer & Beaman RelationsBoyer & Beaman Relations

– Symmetrical molecules

– Assymetrical molecules

– Typical range for both is 0.5–0.8

T g

Tm=

23

T g

Tm=

12

Empirical Relation (Fox)Empirical Relation (Fox)

– For random copolymers & miscible blends (e.g. PS/PPO)

1Tg

=w

1Tg

1

+w

2Tg

2where 1 is homopolymer 1and 2 is homopolymer 2

Get 2 Tg’sforimmiscible blends (e.g.SAN + BR)


Measurement of Transitions

– Dilatometry (measures ∆v versus T)– Refractometry (measures ∆ refractive index

vs. T)– DTA (measures ∆ heat capacity)– DSC (measures ∆ heat flow)**– DMA (measures internal friction), etc.

Preferential Orientationof Amorphous Polymers

– Shear-induced orientation of molecular chains (processing & solid-phase forming)

– Random conformation changed to non-random (or aligned) conformation

– Leads to anisotropic physical properties


End of LectureUnit 2.3

• Read– Lecture notes and Shackelford, Ch. 9

• Optional additional reading– http://www.iastate.edu/mse383

• To dig deeper– Consider taking MSE–Polymers

mse 201 - polymers · 2012. 10. 2. · mech. props. • heat treatment and thermal history –...

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