1-5 polymers & ceramics 12
TRANSCRIPT
2010-2011 S1MP2004, MP2304, MP3104, MP3304
Manufacturing & Materials
A/P Zhou Wei
Web: http://www3.ntu.edu.sg/home/mwzhou/
School of MAEResearch and teaching organized into
Knowledge Domains (KD’s). MP2004 is aligned with KD of Manufacturing Engineering Division:
• Materials,• Forming,• Joining/assembly,• Machining.
Lathe
Mild Steel
Manufacturing & Materials
tool
Interplay
between
machining &
materials
Arc Welding
Mild Steel
Manufacturing & Materials
seam
Interplay
between
welding &
materials
Metals
Ceramics Polymers
Metal-PolymerCompositesMetal-Ceramic
Composites
Ceramic-Polymer Composites
Materials
Lecture 1
Revision of key concepts
Allotropic - PolymorphicCarbon.
Graphite Diamond Buckminster- fullerene
Periodic Table
BondingThree types of primary bonding:(1) Ionic (I) – metals – nonmetals (2) Covalent (C) – nonmetals-nonmetals,(3) Metallic (M) – metals.
Secondary bonding(1) Van der Waal’s (VDW) (2) Dipolar (D)(3) Hydrogen –Bonding (HB).
Nobel Gases – energetically stableGases Electron Orbits Electrons per shell
2 (K-shell)
10 (K, L-shells)
18 (K, L, M-shells)
e-
e-
e-
He42
Ne2010
Ar4018
Primary Bonding(1) I
(2) C
Na Cl Na+ Cl-
(Ne) (Ar)
Strong electrostatic force
O O O2
(Ne) (Ne)
Primary Bonding(3) M
Na
Valence e-
Na Na Na
Valence e-
(conduction e-)(Ne)
Secondary Bonding(1) VDWAs atoms vibrate may temporarily loose
electrical symmetry → dipole.
(2) DPermanent dipoles due to asymmetry.
+ -
HCl
+ -
Secondary Bonding(3) HB• Strongest secondary bonding type.• Occurs when H covalently bonded to F, O,
and N.• High melting point of H20 due to HB.
+
+-
+
+-
Defects
For effective bonding atoms (ions) need to be surrounded by maximum number of nearest neighbors.
Unit cell
∞ Single Crystal, defect free
Each atom has max. no. of nearest neighbors & optimum separaration. (no strain ). Min. energy.
⊥
Single Crystal - defectsSurface Vacancy Interstitial Substitution
Dislocation
High energy
Poly-Crystalline -diffusion
Grain boundary
High energy
Boundary EnergySolid Solution
•Atoms without max. no. of nearest neighbors in state of high energy.•Also elastic strain.
Boundary EnergyOrdered Phase at 30 °C
Boundary area=69 units
Boundary Energy
Boundary area=35 units
Ordered Phase at120 °C
Boundary energy decreases sinceoriginally high.
Boundary Energy
Boundary area=69 units
Coherent Phase at30 °C
Boundary Energy
Boundary area=69 units
Coherent Phase at 120 °C
Boundary energy unchanged sinceoriginally low.
Stress (σ) – Strain (ε)
Brittle
σ
εDuctile
ε
σ
TS
ToughnessToughness
××
Actual strength of metals less than theoretical strength. Why?
Plastic Deformation
Plastic Deformation
σ
σ
Slipplane
Six bonds break at same time!
Dislocations
σ
σ Slip plane
σ
σ
Only one bond break!
Two main deformation mechanisms:(1) Dislocation motion(2) Twinning
Plastic Deformation
Crystallography• Seven crystal systems.• Metals mainly Body Centered Cubic (BCC),
Face Centred Cubic (FCC), or Hexagonal Close Packed (HCP).
• Ceramics rarely simple structures.
a a a
cBCC FCC HCP
Slip Systems• Generally, dislocations move in directions
where atom separation is small.• All permutations of these “easy”
directions - part of slip system.• Greater number of (active) slip systems,
greater ductility of material.
BCC – [111]
Ductile or Brittle?• Energy required to move dislocation
depends exponentially on distance moved.• Almost all FCC metals are ductile (many
active slip-systems).• Almost all HCP metals are brittle (few
active slip-systems).• For BCC it depends on temperature and
metal.
A Cu-60wt% Ag alloy is prepared.
• What phases present at RT?• What are phase compositions?• What are relative amounts of each phase present?
Binary Phase Diagram Revision
×
Composition (wt % Ag)0 20 40 60 80 100
1000
800
600
400
200
α+β
α+Lβ+L
L
αβ8.0 91.271.9
Tempe
ratu
re (
°C)
×
6 wt% Ag 95 wt% Ag
α=8 wt% Ag - 92 wt% Cuβ=91.2 wt %Ag – 8.8 wt% Cu
Composition (wt % Ag)0 20 40 60 80 100
1000
800
600
400
200
α+β
α+Lβ+L
L
αβ
8.0 91.271.9
Tempe
ratu
re (
°C)
%3882.91602.91% =
−−=α
×
6 wt% Ag 95 wt% AgComposition (wt % Ag)0 20 40 60 80 100
1000
800
600
400
200
α+β
α+Lβ+L
L
αβ
8.0 91.271.9
Tempe
ratu
re (
°C)
Summary. α + β phases present at RT. α=8 wt% Ag, β=91.2 wt % Ag.. α=38%, β=62%
α-phase
β−phase
∀≡ phase (takes as long as is necessary)
• Almost no diffusion in solid state.
AnisotropyPolycrystalline metal with equal sized grains (equiaxed) – isotropic.
Isotropic Anisotropic
KIC - materials susceptibility to brittle fracture.
Fracture Toughness, KIC
Critical stress material can withstand before fracture:
KIC-Steel ~90, KIC-Al ~25, KIC-Glass~0.8, KIC-
plastic ~1 MPa√m
aK IC
c πσ
1.1≤
2a
a
σc
σc
What is creep?• Creep - time-dependent (permanent) deformation of materials subjected to constant stress.
• Limiting factor in lifetime of devices.
• Important for metals when operating-temperature > 0.4 Tm (Tm =absolute melting temperature).
What is creep?
time
ε.
×
Secondary
Primary
Tertiary
Operation preferred in secondary (steady-state) creep.
Rupture
What is creep?Creep reduce by:
1. Increasing Tm.2. Increasing elastic modulus, E.3. Increasing grain-size.4. Alloys - have substitutional impurities with low-diffusivity at high-temperatures (such as W).
Metastable
Examples• Fe3C (cementite),• Martensite,• Amorphous materials.
EE
Stable Metastable
Lecture 2
Polymers
How Plastic Bottles are Made
http://www.youtube.com/watch?v=T01i_vp2mJE
Polymer Molecules• Atoms bonded by covalent bond. • Gigantic molecules made from repeating structural units.
Monomer unit
Polymer
-c-c-c-c-c-c-c-c-c-c-c-c-| | | | | | | | | | | |H H H H H H H H H H H H
| | | | | | | | | | | |H Cl H Cl H Cl H Cl H Cl H Cl
Polymer Molecules
Macro-Molecule (PVC)
Small-Molecule (CH4)
H – C - H|H
H|
– C - C|H
H|
|H
Cl|– C - C
|H
H|
|H
Cl|
– C - C|H
H|
|H
Cl|– C - C
|H
H|
|H
Cl|
– C - C|H
H|
|H
Cl|– C - C
|H
H|
|H
Cl|
|H
H|
– C -
5-atoms
10,000-atoms
MonomersMonomers must have multi-functional reactive bonds or chemical groups (minimum functionality is two, i.e. di-functional)
• Mono-functional:– Ethanol CH3CH2-OH
• Di-functional:– Ethylene diol HO-CH2CH2-OH
• Tri-functional– Glycerol HO-CH2-CH(OH)-CH2-OH
Condensation Reaction
Small molecule eliminated (e.g. H2O).
HO-CH2CH2-OH + HO-CH2CH2-OH→ HO-CH2CH2-O-CH2CH2-OH + H2O
HO-CH2CH2-OH + HO-CH2CH2-O-CH2CH2-OH →HO-CH2CH2-O-CH2CH2-O-CH2CH2-OH + H2O
Plastics?Plastics fall into two categories.
(1) Pure polymer (e.g. high density poly-ethylene).
(2) Multiple-blends with fillers, additives, etc.
Mers
C HHH
CH
HCR
H
HPolyethylene (PE)
General vinyl polymer – Poly vinyl mer
Polypropylene (PP)Polystyrene (PS)
when R is → ClPolyvinyl chloride (PVC)
MorphologyMaterials categorized according to their morphologies.AmorphousNo long-range order, i.e. random coils or irregular packing of chains.
Morphology
Perfectly Crystalline
Regular 3-D arrangement of molecular chains.
MorphologySemi-Crystalline
Regions of amorphous & perfectly crystalline in co-existence.
crystallite
(1) Linear
• Repeat units held by strong covalent bonds.• Chains long & flexible (1-D).• Different molecules held together by weaker secondary forces.Examples PE, PVC, PS, PMMA.
(2) Branched
•Introduced during side polymerisation.•Chains packing efficiency ↓ due to steric (size) effects.•Polymer density ↓
Thermoplastics• Polymers soften upon heating & made to flow when a stress is applied. • When cooled, they will solidify to a solid or rubbery solid.
Can be recycled.
Solidheating
Liquid (melt)cooling
Forming Processes•Widely used in manufacturing plastic products by •Injection molding•Thermoforming•Extrusion
(3a) Cross-linked
• Adjacent linear chains joined to one another by strong covalent bonds.• Achieved by growth at high temperature (irreversible).
Example Rubber elastic material cross-linked by a method known as vulcanization.
(3b) Network• Tri-functional (or more) monomer units form three or more covalent bonds resulting in a 3-D networks.• Achieved by growth at elevated temperature.
Example: Epoxies.
Thermoset• Polymers which become hard and rigid upon heating (irreversible).
• Chain motion restricted by high degrees of cross-linking.
• Harder, stronger, more brittle but with better dimensional stability compared with Thermoplastic Polymers.
Elastomers• Mechanical properties like natural rubber.• Linear or cross-linked.• Chains extended upon deformation but are prevented from flow by cross-links or physical domains.•Returns to original shape on removal of σ.
Domain
LinearChain
Classification
Homopolymer:
• One type of monomer (A) used to build polymer.
• Structure can be represented by multiple repetition of a single repeat unit.
Examples : PE, PTFE, PS, PVC, ...
A AAAA n
Copolymer• Polymer structure made up of two (or more) monomer units.•Polymer has combined properties of various monomers. Types of Copolymer
Random Copolymer - poly(A-ran-B) :
Two different units (A and B) randomly dispersed along chain
ABAABAAABBAAAB
CopolymerAlternating Copolymer - poly(A-alt-B):
ABABABABABABAB
Block Copolymer - polyA-block-polyB :
AAABBBBBAAABBBBB
CopolymerGraft Copolymer:
• chains of one kind are attached to backbone of different polymer.
• known as polyA-graft-polyB.
A AA A AA A AA
BBB
BBB
BBB
BBB
BBB
Lecture Theatre • Distribution of students, ages 19-29 years. • Can talk about number fraction of
students with age in range 23-25. i.e. number of students in this age range / total number of all students present.
• Alternatively, can talk about weight fraction of students in age range 23-25. i.e. weight of students in this age range / total weight of all students present.
Molar Mass• Measures size (or weight) of polymer
molecule. • Defined as an average value with a wide
distribution of polymer chain size.• Represents statistical nature of
polymerization. Main averages:
•Number-average molar mass, Mn•Weight-average molar mass, Mw
Number versus weight averageDistribution of molecular weight
Molecular weight
Mn
Mw
Amou
nt o
f Po
lymer
s
Poly-DispersityRatio of two measures of molecular weight called Poly-Dispersity (PD):
• PD=1.0 - all molecules have same weight! Unusual for polymers.• ↑ in poly-dispersity ⇒ ↑ width of molecular weight distribution
n
W
MMPD =
Significance of Molar Mass• Chain ends considered as defects in polymers since they have no covalent bond to transmit strength.
∴ as molecular weight ↑, tensile strength ↑ rapidly and gradually levels off.
↓ in chain ends
↑ in molecular weight ↑ in chain length
Example
Calculate:(a) Mn, (b) Mw, and (c) nw (weight averaged Degree of Polymerization.
MolecularWeight Range
(g/mol)wixi
8,000-16,00016,000-24,00024,000-32,00032,000-40,00040,000-48,00048,000-56,000
0.050.160.240.280.200.07
0.020.100.200.300.270.11
(a) Calculate Mn
Molecular wt Range Mean,Mi xi xiMi
8,000-16,000 12,000 0.05 60016,000-24,000 20,000 0.16 3,20024,000-32,000 28,000 0.24 6,72032,000-40,000 36,000 0.28 10,08040,000-48,000 44,000 0.20 8,80048,000-56,000 52,000 0.07 3,640
Mn = ΣxiMi = 33,040 g/mol
Molecular wt.Range Mean,Mi wi wiMi
8,000-16,000 12,000 0.02 24016,000-24,000 20,000 0.10 200024,000-32,000 28,000 0.20 560032,000-40,000 36,000 0.30 10,80040,000-48,000 44,000 0.27 11,88048,000-56,000 52,000 0.11 5720
Mw = ΣwiMi = 36,240 g/mol
(b) Calculate Mw
For PP, molar mass of repeat unit
= 3(mC) + 6(mH)
= (3×12 g/mol) + (6×1 g/mol) = 42 g/mol
And since , nw = 863Mwnw = m
(c) Calculate nw CH
HCC
H
HHH
Configurations & ConformationsGeometric detail of how each unit is added & shape of chain are important factors.
- Refers to placement of successive repeat units into chain
Chain Configurations:
- Chain can take on ∞ number of shapes by rotation about backbone bonds, this gives rise to different conformations.
Chain Conformations:
Molecular ConfigurationArrangements of atoms fixed by covalent bonds in molecule & cannot be altered unless bonds are broken.Configurations:
• Order (head-to-head, head-to-tail, tail-to-tail) • Stereoisomerism (Isotactic, Syndiotactic, Atactic)
Order in Molecules
Three ways adjacent bonds can be formed :
Considering vinyl molecule
• Head-to-Head• Head-to-Tail•Tail-to-Tail
CH
HCR
HHead Tail
Order in Molecules
Tail-to-Tail Head-to-Tail
CR
HC
H
HCR
HCH
HCH
HCR
HCH
HCR
H
• Head-to-tail - more energetically favourable due to lack of steric effects & polar repulsion from similar side groups.• HT configuration dominates (never 100%).
CH
HC
R
HCH
HCR
H
Head-to-head
StereoisomerismIsotactic
R groups lie on same side of zig-zag plane.
Atactic R groups on random position, no regularity
R groups alternate around plane.
Syndiotactic
C C CCC C CC
HH H
H HH
HH
HR R
H HR
HR
C C CCC C CC
HH R
H HH
HR
HR H
H HR
HH
C C CCC C CC
HH R
H HH
HH
HR H
H HR
HR
Tacticity of PS
Isotactic Syndiotactic Atactic Never 100 % tactic - in a given polymer chain, usually a combination of configurations!
Molecular Shape• Once polymer molecule formed, its configuration is fixed but single polymer bonds not “rigid”.
Some degree of freedom :
Molecule shape will affect thermal + mechanical properties.
•Rotation•Bending
Rotation & Bending
Bending
Rotation
109°
C-C bond length, d=0.154nm
Bending & RotationRotation & bending depends on :(1) Type of bond
(2) Type of main chain
• single bond - can rotate• double/triple bond - rigid
• bulky main chain restricts rotation
O C CO
C CC C
Bending & Rotation
(3) Type of side group
• bulky side groups act as stearic (size) hindrance. • restricts rotation.
C CC C
Shape• Each single bond capable of rotation.• Chain length less useful to characterise polymer shape.
r
Shape
• for linear flexible chains, distance separating chain ends is end-to-end distance, r.
• More useful descriptor for polymer shape as it describes both size & shape of chain.
End-to-End distance, , where N – number of C atoms in polymer backbone
Ndr =
Man in the Mirror
http://www.youtube.com/watch?v=KtT78oI3zDw
Lecture 3
Polymers
Crystalline Polymer
In very large molecules (polymers), ability to crystallise depends on
•Regularity of structure
•Intermolecular bonding groups.
Single Crystal of PE:
• Requires very special growth conditions.• Engineering polymers not single crystal.
Single Crystals
5 µm
Amorphous Polymer• Polymer that is completely non-crystalline.
• Structure can be thought of as simply frozen polymer liquids.
• Many engineering polymers are amorphous.
Examples: are rubber, atactic PS, epoxies.
Semi-Crystalline (SC) Polymers• Most polymer molecules partially crystallize due to size, stereo-regularity, geometrical reason.• Crystalline regions mixed within amorphous material.
Crystallite
Amorphous Most engineering
polymers SC.
SC Polymers• Bulk polymers - generally partially crystalline.• Consist of small crystalline regions (crystallites) & amorphous regions.
Reasons:• Polymer are macromolecules (long chains), hence low diffusion rates. • Macromolecules highly entangled in melt & do not have time to disentangle during solidification.
SC Polymers - spherulites
CrystalliteAmorphous
Nucleus
• 10 nm thick radial layers.• Chains oriented ⊥ to radius of structure.
Polymer CrystalinityAmorphous
polymerSC
polymerCrystalline
polymer↑ in crystallinity
Density : crystalline polymer > amorphous polymer (chains closely packed in crystalline polymer).
s, c & a refer to specimen, totally crystalline polymer & totally amorphous polymer.
% Crystallinity ( )( ) 100×
−−=
acs
asc
ρρρρρρ
Melting Temperature
• Melting - transformation of polymer crystal from solid with highly ordered structure of aligned molecular chains to a viscous liquid with highly random molecular chains.• Occurs at melting temperature, Tm.• Takes place over range of temperatures, not possible to define single melting temperature for a polymer.
Melting CharacteristicsCharacteristics of polymer melting different from other materials :
• Melting depends upon specimen history & temperature of crystallization.• Melting depends upon rate at which specimen is heated.• Differences due to peculiar morphology of polymer crystals.
Annealing
T(°C)
time0
Crystallite
Amorphous
Annealing : temperature below melting point of material
• Crystallites very thin.• Strong dependence of Tm on crystal thickness.• Surface energy ↑ with decreasing crystallite size.• Tm ↓ with decreasing size
Melting Characteristics
Melting
25% chainunbonded
5% chainunbonded
Melting
Annealing
T(°C)
time0
Crystallite
Amorphous
Annealing : as crystallites grow, their melting points increase
T(°C)
t0.1 hrs
Effect of Heating Rate
T(°C)
t 10 hrs
(a) has lower melting point compared to (b) due to annealing. This is very unusual behavior.
(a) (b)
Effect of Heating RateMelting dependent on heating rate since annealing takes place when crystalline polymer heated and melted. Decrease in heating rate
• Increase in annealing effect• Increase in lamellae thickness• Increase Tm
Annealing : heat treatment providing movement of polymer chains by diffusion (depends on temperature & time).
Glass Transition
• As polymer is cooled it becomes rubbery, then a relatively hard and/or elastic polymer glass is obtained!• Temperature at which polymer changes from rubber to glass is Glass Transition Temperature, Tg.
•Polymers that can easily deform will have low Tg.
Polymer CrystallinityAbility of polymer to crystallize depends on molecular ordering of polymer chains.
(1) Side group (bulky, polar, branches)(2) Molecular structure (type of polymer, isomerism)(3) Thermal history (cooling rate) and applied stress.
Factors affecting chain alignment:
Side Groups Chains with bulkier side groups will have less tendency towards crystallization.
Crystallize easilyCH
HCH
H
PE
CF
FCF
F
PTFE
difficult tocrystallize
PSCH CH2
Chains with hydrogen bonding promote crystallization.
CH2 CN CH2 CH2 CH2 CH2 CH2N
CH2 NC CH2 CH2 CH2 CH2CH2 C
O H O
H O H
CH2 CN CH2 CH2 CH2 CH2 CH2N
CH2 NC CH2 CH2 CH2 CH2CH2 C
O H O
H O H
Repeat unitNylon
Side Groups
Molecular StructureCrystallinity ↑ with increasing regularity (i.e. reduced randomness) in structural arrangement.
• Linear Polymers → Crystallizes easily
•Branched Polymers → Partially crystalline
•Network & Cross-linked Polymers → Amorphous
Molecular Structure
• isotactic & syndiotactic polymer - crystalline.• atactic polymers - amorphous.
Isomerism:
Copolymer:More irregular & random monomer arrangement is less crystalline.• Alternating & Block copolymer → Crystallisation• Random & Graft copolymer → Amorphous
Important Point
•Tm only relevant to Crystalline Polymers
•Tg only relevant to Amorphous Polymers. •If both Tg & Tm are observed, polymer must be Semi-crystalline.
Factors affecting Tm High melting points associated with
• highly regular structures • rigid molecules • close packing capability • strong inter-chain attraction
Examples• Aromatic polyesters: high Tm because of relatively high chain stiffness.• Polyamides: high Tm because of inter-chain HB.
Factors Affecting Tm
Tm dependent on:
(1) Molecular Weight(2) Backbone (3) Side groups
• At Tm, whole polymer chains are able to translate from one position to another. • Factors affecting interaction between chains will affect Tm.
Molecular Weight↓ molecular weight ⇒ ↓ chain length
↑ in chain ends ⇒ ↑ in defects ⇒ ↓ in Tm
High Tm Low Tm
BackboneMost important factor affecting Tm.
Chemical Structure:
(1) Nature of backbone
• Determines stiffness of main polymer chain.• Controls ease at which rotation can occur about chemical bonds along chain.
Backbone
Flexibility decreasing groups:
⇒ ↑ Tm
• Bulky main chain:
• Unsaturated bonds:
• Polar groups:
phenyl
NHCOamide
C C C Cdiene
Flexibility increasing groups:O , O O , CO O
⇒ ↓ Tm
Backbone
Repeat Unit Tm/KCH2 CH2
CH2 CH2 O
CH2 CH2 CO O
CH2 CH2
CH2 CH2 CO NH
410340395
670603
Side GroupsType & size of side groups affects rotation about bonds in main chain.
Consider vinyl polymer:
CH2 CHR
If R = H, (a flexible group) i.e PE,
(CH2CH- H)-
Flexibility restricting groups:
CH3
⇒ ↑ Tm
Cl CN
Polar side groups
Bulky side groups
Side Groups
Repeat Unit Tm/KCH2 CHR
CH2 CH3
CH3
CH2 CH2 CH3
CH2 CH(CH3)2
460398351508450
Side group (R)
Side Groups
410H
Not as clear cut: balance betweenflexibility and size
Chain BranchingIncrease in branching • ↑ in defects (↓ in packing)• ↓ in intermolecular forces between polymer chains• ↓ in Tm
Example: HDPE has a higher Tm than LDPE.
Glass Transition, Tg• Tg depends upon rate at which temperature is changed, i.e. rate of heating /cooling.• Lower heating rate, lower Tg
•At Tg, polymer chains frozen in position become free to rotate & translate. • Factors that affect chain rotation(molecular rigidity) & move will affect Tg.
T(°C)
t0.1 hrs
Effect of Heating Rate
T(°C)
t 10 hrs
(a) has higher Tg compared to (b). This is very common behavior.
(a) (b)
Factors affecting, Tg
Tg depends on:
(1) Backbone
(2) Side groups
(3) Molecular weight
(4) Molecular structure (Isomerism and type of polymer)
(5) Additives
Nature of Backbone• Chain flexibility governed by nature of chemical groups which constitute main chain (similar to Tm).• Groups that reduce ease of rotation will increase Tg.
Repeat Unit Tg/KCH2 CH2
CH2 CH2 O O
140-270206357
353CH2 CH2
Side Groups(i) Bulky side group:• Large side groups give greatest stiffening effect.• Flexible side groups have less stiffening effect.(ii) Secondary bonding:
• Polar groups increase Tg because of restriction in rotation due to dipole interactions.
Tg(PVC)>Tg(PP)
• HB & VDW forces between chain ↓ flexibility & increase Tg.
Side GroupsRepeat Unit Tg/K
CH2 CHRCH2 CH3
CH3
CH2 CH2 CH3
CH2 CH(CH3)2
250249233323373
Side group (R)
ClOHCN
354358370
Not as clear cut: balance betweenflexibility and size
BranchingSmall number of branches
• Small number of branches tend to ↓ Tg • Increase in free volume ⇒ increase in rotation
Branching
• Long branches tend to ↑ Tg
• Long branches hinder rotation ⇒ ↑ in chain rigidity ⇒ increase Tg
Generally
Molecular Weight
↑ in molecular weight ⇒ ↑ in Tg
• Polymer of low molecular weight have greater number of chain ends in given volume compared to high molecular weight.
• Since chain ends are less restrained, they have greater mobility at a given temperature.
Molecular StructureCross-linking• Slight cross-linking will have no effect on chain rigidity.
• High density of cross-links results in decrease in chain flexibility ⇒↑ Tg
•Tg may not occur in highly cross-linked materials!
Plasticisers
• Small molecules which get in between
polymer chains.
• Molecules that space out chains from one
other ⇒Tg is lowered by use of
plasticizers.
•How do plasticizers affect Tm?
Lecture 4
Polymers
σ-ε of Brittle PolymerTypical σ-ε curve for glassy polymer, e.g. PS:
CompressionTension
Stre
ss
Strain
• Initially elastic deformation, σ proportional ε.
Typical σ-ε curve of PP (in tension):Elastic Deformation:
• Hooke’s law obeyed.
σ increase with ε up to yield point.
Yield:
Yield Pt
Stre
ss
StrainYield of polymers is ~ 5 – 10 % (metals ~ 0.1 %)
σ-ε of Tough Polymer
Plastic Deformation:
TensileStrength
Stre
ss
Strain
• Deformation cannot be fully recovered.• Cold drawing (extension at constant stress)• Polymer strain hardens and ruptures.
Strain hardening
Cold draw
σ-ε of Tough Polymer
• Mechanical properties controlled by molecular structure.• Material made up of network of cross-linked polymer chains with individual lengths of chain in random conformations.
Stre
ss
Strain
• Elastomeric • Rubber-like elasticity• Large recoverable ε at low σ level.
σ-ε curve (in tension):
σ-ε of Elastomers
Deformation of Single Phase Polymer Crystals
Elastic Deformation
(1) Polymer glasses: (Modulus ~106 Nm-2)
Involves bending & stretching of strong covalent bonds on polymer backbone as well as displacement of adjacent molecules which is opposed by relatively weak secondary (VDW) bonding.
•Elastic Deformation
(2) Polymer crystals:(Modulus ~1011 Nm-2 // to chain direction, Modulus ~106 Nm-2 ⊥ to chain direction)
// to chains:• Deformation involves stretching of strong covalent bonds and changing of bond angles.
Deformation of Single Phase Polymer Crystals
(2) Polymer crystals:(Modulus ~1011 Nm-2 // to chain direction, Modulus ~106 Nm-2 ⊥ to chain direction)
⊥ to chains:• Deformation opposed only by weak secondary VDW or HB.
Deformation of Single Phase Polymer Crystals
Plastic Deformation
• Polymer molecules remain relatively undistorted.• Deformation involves sliding of molecules in directions // or ⊥ to chain axis (breaking only VDW forces).
Deformation of Single Phase Polymer Crystals
Elastic Deformation
(3) Elastomers: (Modulus ~106 Nm-2)
Amorphous polymer (>Tg) will behave like viscous liquid & normally flow under its own weight.
Unstressed state:
Elastomer is amorphous & composed of molecular chains that are highly twisted, kinked and coiled.
Deformation of Single Phase Polymer Crystals
Upon application of load:• Partial uncoiling, untwisting, straightening & elongation of chains in stress direction.• Viscous flow takes place by uncoiling of chains in cross-linked network & extent of flow depends on degree of cross-linking.• Properties controlled by nature of cross-linked network.
Deformation of Single Phase Polymer Crystals
Deformation of Multi-Phase Polymers – SC Polymers
• Most SC polymers behave like composites (two-phase materials)
• Increase in modulus due to combined effect of amorphous & crystalline regions.
• Effective crystal modulus depends on (a) proportion of crystalline material, (b) size, (c) shape & (d) distribution of crystals.
Mode of Fracture:
• Thermoset - Brittle (covalent bonds in network/cross-linked structure severed.)
• Thermoplastic - Ductile & brittle
Ductile-to-brittle transition occurs when:
• ↓ in temperature (significantly < Tg)• ↑ in strain rate
Fracture
Mechanical Properties
(1) Polymer structure (Molecular weight, cross-linking, chain
stiffness)(2) Processing
(Mechanical & thermal treatment)(3) Application / Testing conditions
(temperature & strain rate dependence)
Depend on:
Degree of Crystalinity• Crystalline regions ⇒ chains aligned• Extensive secondary bonding (intermolecular bonding)
• Amorphous regions ⇒ Chains misaligned• Secondary bonding less prevalent.
Semi-crystalline polymers:
• TS ↑ with ↑ degree of crystallinity
Degree of Cross-linkingCross-linking involves formation of strong covalent bonds between individual polymer chains.
Natural Rubber:
Cross-linking ↑ strength of rubber
PropertyRaw
RubberX-linkedRubber
Tensile StrengthElongation at break (%)
3001200
3000800
Chain StiffeningPolymers with stiff chains are stronger than those with flexible chains.
Stiffen methods:
• Double or triple bonds• Bulky group on backbone
1.Stiff backbone
• Bulky side groups• Polar bonds
2.Side Groups
Testing ConditionsPlastic deformation - strong dependence on testing rate & temperature.
Temperature:At constant , yield stress decreases with ↑ temperature because chains flow/slip.
• reduction in tensile strength• enhancement of ductility• decrease in elastic modulus
↑ in temperature
ε̇
For amorphous polymer, yield stress falls ~ linearly to zero at Tg (polymer glass becomes rubber)
Temperature:
• Yield stress → 0 at Tm for SC polymer.
• Above Tg, material gains strength from crystalline regions.
SC polymer:
Testing
Temperature
SCCross-linked
TmTg
Rubbery plateau
Log
Mod
ulus
Amorphous
Strain Rate:
At constant temperature, ↑ in will lead to ↑ in yield stress due to ↓ in response time for chains.
Environment such as water, oxygen, organic solvents, etc.
Other:
Testing
ε̇
Viscoelasticity• Viscoelasticity distinctive to polymers.
• Mechanical behaviour in between that of an elastic solid and a viscous liquid.
Low temperature or high strain rates:→ Elastic
High temperature or low strain rates:
→ Viscous
Creep LoadingConstant σ applied to a polymer at time t = 0 & ε increase rapidly at first, slowing down over longer time period.
time
σ
Stress system Strain system
time
ε
Elastic solid
Polymer Viscous liquid
Stress Relaxationε in polymer held constant & σ decays slowly with time.
Strain system Stress system
time
ε σ
time
Elastic solid
Polymer
Viscous liquid
Viscoelastic Stress Relaxation
Size of σ relaxation modulus dependent on temperature & time:
• E(t) decreases with increasing t.• E(t) decreases with increasing T.
• σ required to maintain a constant ε found to decrease with time.• Under isothermal condition, at constant ε0.time dependent stress relaxation modulus given by: ( ) ( )
0εσ ttEr =
Viscoelastic Creep• Deformation significant even at room temperature & under modest σ < yield strength.
• Under isothermal condition, constant stress applied, σ0, time dependent creep modulus given by:
( ) ( )ttE oc ε
σ=time dependent strain
Viscoelastic Creep
Creep modulus dependent on:
Temperature - Ec(t) → 0 with ↑ temperature ⇒ creep ↑
Crystallinity – Ec(t) ↑ with ↑ crystallinity⇒ creep ↓
Temperature Dependency of ErFive regions:(1) Glassy• At lowest T, material glassy & brittle.
• Er ~ equal to elastic modulus.
(2) Leathery (Glass transition)
• Er ↓ by 103 in 20 oC.• Deformation is time dependent & not totally recoverable on release of applied load.
Temperature (°C)
E r
Glassy
Leathery
RubberyRubber flow
Viscous flow (L) TmTg
104
103
102
10
1
10-1
10-2
10-3
10-460 80 100 120 140 160 180 200
(3) Rubbery Plateau
• Deforms in rubbery manner.• Consist of viscous & elastic components
(4) Rubbery Flow• Gradual transition into soft rubbery state(5) Viscous Flow
• Modulus ↓ dramatically with ↑ temperature• Chain segment vibration is independent of rotational motion.
Temperature Dependency
Example 1Is it possible to have a PolyMethyl MethAcrylate (PMMA) homopolymer with the following molecular weight data and a weight-average degree of polymerization of 585? Please justify your answer.
Example 1
Example 1MW Range
(g/mol)Mi wi Mi wi
8,000-20,000 14,000 0.01 140
20,000-32,000 26,000 0.05 1,300
32,000-44,000 38,000 0.12 4,560
44,000-56,000 50,000 0.25 12,500
56,000-68,000 62,000 0.27 16,740
68,000-80,000 74,000 0.21 15,540
80,000-92,000 86,000 0.09 7,740
molgwwMi
ii /520,58=Σ
Σ
Example 1Weight-average degree of polymerisation is 585, ∴molecular weight of this polymer mer is 100 g/mol.
molgnMmw
w /100585
58,520 ===
Example 1
Examine the mer.
( ) ( ) ( )( )100
21681512=
×+×+×=m
Yes, possible!
Example 2
An experimental poly-propylene sample has only ten polymer chains present. Four molecules have a degree of polymerization of 2, three molecules have a degree of polymerization of 3, and three molecules have a degree of polymerization of 7.
Example 2Sketch the mer unit.
C
H
Weight of one PP mer is (12g/mol × 3) + (1g/mol × 6) = 42 g/mol
Example 2Calculate number average molecular weight.
two mers
Example 2Calculate number average molecular weight.
Mean Molecular Weight, Mi
Number Fraction, xi
xiMi
2 × 42 g/mol = 84 g/mol 0.4 33.63 × 42 g/mol= 126 g/mol 0.3 37.87 × 42 g/mol= 294 g/mol 0.3 88.2
∑ xiMi = 159.6 g/mol
CERAMICSCERAMICS
Lecture 5Lecture 5
CeramicsMetal-Nonmetal Compounds
Oxides nitrides carbides borides silicates
Insulator of heat & electricity
Resistant to high temperatureand corrosion
Hard butvery brittle
Atomic Bonding: Ionic, Ionic+Covalent, Covalent
Ceramic Structure• Composed of at least two elements.• Bonding: ionic to covalent (very strong)• More complex than metals.
Crystalline
Amorphous
Crystal Structure
Ceramics withionic bonding
Metallic ions (+) - Cation
TwoFactors:
(1) Magnitude of electrical charge (positive = negative charges to maintain electrically neutral)
Nonmetallic ions (-) - Anion
(2) Relative size of cations to anions (Cation must make contact with surrounding anions)
Examples: (1) Charge effect (reflected in chemical formula)
CaF2 Ca2+
2F-Al2O3
2Al3+ 3O2-
Electroneutrality must be maintained in a ceramic material
Crystal Structure
(2) Size Effect: Cation contact with surrounding anions(a) Ionic radius: Cation rC < Anion rA(b) Cation has maximum contacting anion neighborsExamples:
A A
AA
A A
AA
Unstable Stable
A A
AA
Stable
Crystal Structure
(c) larger cation radius, more contacting
anions. Smaller anion radius, more
contacting cation
(d) rC/rA ratio: determines Coordination
Number (CN)
Crystal Structure
CN & rC/rA RatioCN rC/rA Geometry CN rC/rA Geometry
2 < 0.155
3 0.155- 0.225
4 0.225- 0.414
6 0.414 - 0.732
8 0.732- 1.000
Coordination Number & rC/rA Ratio
•Consider simple AX-type structure such as NaCl, MgO, FeO, CsCl, ZnS, SiC.
•Determining factor: size effect.
Unit cells -2D
Unit cells -2D
Cation
AnionUnit cell – 1 anion+1 cation
1 2
1 2
4 3
• Face of unit cell (UC) – shared between two neighboring units cells.• Edge – shared 4 UC; Corner – shared 8 UC
Cesium Chloride - Structure
Coordination No.= 8;rC/rA: 0.732-1.0
Simple cubic of Cl-with Ce+ at body centre
Unit cell:1 formula unit CsCl
Cl-Ce+
Cesium Chloride - Structure
Coordination No.= 8;rC/rA: 0.732-1.0
Simple cubic of Cl-with Ce+ at body centre
Unit cell:1 formula unit CsCl
Cl-Ce+
Unit cell
CsCl crystal
ion81
Rock Salt (NaCl) -Structure Coordination No. = 6; rC/rA: 0.414-0.732
e.g. NaCl, MgO, MnS, LiF, FeO
Unit cell: 4 Cl- & 4 Na+
(4 formula units)
Cl-Na+
Zinc Blende (ZnS) - Structure Coordination No.= 4; rC/rA: 0.225-0.414
All ions tetrahedrallycoordinated
e.g. ZnS, ZnTe, SiC
Unit cell:4 formula unit
S-Zn+
•Many ceramic compounds have more than one type of cation (e.g. BaTiO3)
•Crystal structure becomes very complex
•But (1) Charge effect and (2) Size effect are still determining factors
AmXp-Type Structure
Ceramic Density
AC
AC
NVAAn ∑ ∑+
=)('
ρ
Formula units in unit cell
Atomic weight of allcations + anions
Unit cell volume Avogadro’s Number6.023x1023
ImperfectionsPoint Defects Interstitials: Cation
Vacancy: Cation & Anion
Cation Vacancy Cation Interstitial
Anion V acan cy
Cation impurity Cation Interstitial Impurity
Anion im
pur ityImperfections
Impurities(solid solution)
Cation: Interstitial & SubstitutionalAnion: Substitutional
(1) Frenkel defect: Cation-vacancy/ Cation interstitial pair(2) Schottky defect: Cation-vacancy/ Anion vacancy pair
Point Defects
Frenkel Schottky
Iron Oxide: Fe present in Fe2+ + Fe3+ state;Wustite-FeO: Fe2+ in perfect structure.
Point Defects
O2-
Fe2+
Destruction of neutrality: Fe3+ formation
Maintain neutrality: 2Fe3+& 1 Fe2+ vacancy
Imperfections
Fe2+ vacancy
Fe3+Fe3+
Impurities: example, NaCl (Na+, Cl-)
If impurity Ca2+ substitute for Na+, electro-
neutrality disrupted by extra + charge
To maintain neutrality: (1) Remove 1 Na+ (form a vacancy)(2) Add 1 Cl- (interstitial): highly unlikely (Cl- too big)
ImperfectionsElectro-neutrality MUST be maintained
ZrO2-CaOTe
mpe
ratu
re (
°C)
Composition (wt% CaO)
Liquid
3000
2500
2000
1500
1000
5000 10 20 30
ZrO2
CaZrO3
Cubic ZrO2
+CaZrO3
C+L L+CaZrO3
CaZr4O9
+CaZrO3
Cubic ZrO2
C+T
M +CaZr4O9
T
C+MM
ZrO2-CaO System• One eutectic, two eutectoid reactions• ZrO2: three crystal structures:
Monoclinic Tetragonal Cubic
• T → M (on cooling): volume expansion: 4% → severe cracking• Pure ZrO2 is useless.
T & % CaO dependent
Crack Prevention: add CaO (or Y2O3 or MgO) to enable formation of Cubic Phase.
Stabilising
Partially stabilising
+ 3-7%CaOC+T: > 1000oCNo M on coolingT & C stabilizedNo cracking
Fully stabilising
add more CaOCubic at high TCubic on cooling
ZrO2-CaO System
Partially Stabilised Zirconia (PSZ)
•Stabilising agents: CaO, MgO, Y2O3
•Very tough: better than other ceramics•Transformation toughening – why?
PSZ -Transformation toughening RT: C-phase + T-phase (Metastable)T-phase → M-phase by elastic energy causing expansion & compressive σ – impede crack growth.
Crack
Stress-induced T → M
C T MC T
Mechanical Properties
Fracture•Brittle•Low fracture toughness (KIC)•KIC < prediction - due to small flaws (voids, pores, cracks)
PlasticsCeramics
Composites
Metals
0.1 1 10 100KIC
• Ceramics have extremely high melting points; most not melted and formed (e.g. casting).
• Many ceramics structures processed from powder.
• Powder compressed and heated (sintered) to give net-shape structure with high density.
Ceramic Processing
• Presence of voids, defects, cracks.• Complex crystallography and small
number of slip systems.• Very strong ionic (or covalent)
bonding.• Energetically expensive for
dislocations to move.
Why are ceramics brittle?
Example 1
It is possible for ceramic to have more than one type of cation (represented by A and B) with their chemical formula designated as AmBnXp. Determine m, n, and p for barium titanate (BamTinOp) by examination of the unit cell.
Ba2+
Example 1
O2-
Ti4+
There’s 1 Ti ion, 1 Ba ion 3 O ions. →Must be BaTiO3.