Post on 11-Oct-2020
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
MSE 201 - Polymers 2
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
MSE 201 - Polymers 3
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)
MSE 201 - Polymers 4
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)
Tailorable Chemical & Morphological Features
• 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**
MSE 201 - Polymers 5
Tailorable Chemical & Morphological Features
• 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.)*
MSE 201 - Polymers 6
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
MSE 201 - Polymers 7
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
MSE 201 - Polymers 8
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
MSE 201 - Polymers 9
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
MSE 201 - Polymers 10
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
MSE 201 - Polymers 11
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
MSE 201 - Polymers 12
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.
MSE 201 - Polymers 13
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
MSE 201 - Polymers 14
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
MSE 201 - Polymers 15
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
Tempt. Effects on Modulusof a Typical Polymer
(1)
(2)
(3)(4)
(5)
MSE 201 - Polymers 16
Tempt. Effects on Modulusof a Typical Polymer
• 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
QuickTime™ and aGraphics decompressor
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
QuickTime™ and aGraphics decompressor
are needed to see this picture.
(1)
(2)
(3)(4)
(5)
MSE 201 - Polymers 17
Tempt. Effects on Modulusof a Typical Polymer
• 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)
QuickTime™ and aGraphics decompressor
are needed to see this picture.
(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
MSE 201 - Polymers 18
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.**
MSE 201 - Polymers 19
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
MSE 201 - Polymers 20
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
MSE 201 - Polymers 21
• 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
MSE 201 - Polymers 22
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
MSE 201 - Polymers 23
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)
MSE 201 - Polymers 24
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
MSE 201 - Polymers 25
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