Chapter 14/15-
ISSUES TO ADDRESS...• What are the basic microstructural features?
1
• How do these features dictate room T tensile response?
• What dictates polymer mechanical properties?
• How does elevated temperature mechanicalresponse compare to ceramics and metals?
CHAPTERS 14/15:Polymer structures, applications, &
processing
Chapter 14/15- 2
• Polymer = many mers
C C C C C CHHHHHH
HHHHHH
Polyethylene (PE)
mer
ClCl ClC C C C C C
HHH
HHHHHH
Polyvinyl chloride (PVC)
mer
Polypropylene (PP)CH3
C C C C C CHHH
HHHHHH
CH3 CH3
mer
Adapted from Fig. 14.2, Callister 6e.
Adapted from Fig. 14.7, Callister 6e.
Polymer microstructure
The C-C bond has rotational freedom; provided there are no sterichindrance from bulky side groups (polystyrene)
Chain rotation
C C
C
Weakness of polymers is due to weak van der Waals bond betweenchains
Chapter 14/15-
Polymerization of ethylene
C CHH
HHC C
HH
HH=C C
HH
HH= X+ C C
HH
HHX * C C
HH
HHX * + C C
HH
HHX *
C CHH
HHC C C C
HHHH
HHHH
CCH
H HH
CC
CCH
HH
H
HH
HH C C
HH
HHC C C C
HHHH
HHHHCH
C CHH
HHC C C C
HHHH
HHHHC C
HH
HHC C C C
HHHH
HHHHC C
HH
HHC C C C
HHHH
HHHHC C
HH
HHC C C C
HHHH
HHHHX X
However, mistakes do happen during polymerization, leading to branching
Polymerization and termination
Addition polymerization
Chapter 14/15-
Condensation polymerization
C OH
H
H
C
O
CO H
H
H
C
OH
C CH
HH
HO HH O
C OH
H
H
C
O
C
O
C CH
HH
HO HO CO H
H
H
H
Methyl alcohol by-productRepeating unit for polyethylene terephtalate(PET)
+
Dimethyl terephtalate Ethylene glycol
Chapter 14/15-
Polymer propertiesMolecular characteristics
BranchedLinear crosslinked network
Configuration
Stereoisomers Geometrical isomers
cis transSyndiotacticIsotactic Atactic
Chemistry(Mer composition)
Size(Molecular weight)
Shape(Chain twisting, entanglement)
Structure
Chapter 14/15-
Polymer chemistry
Chapter 14/15-
Degree of polymerization
Degree of polymerization =Average molecular weight of polymer
Molecular weight of repeat unit
Mn =ΣxiMi=ΣwiMiMw
Weight-average molecular weight
Number-average molecular weight
5 10 15 20 25 30 40 (103 g/mol)35 5 10 15 20 25 30 40 (103 g/mol)35
Wei
ght f
ract
ion
Num
ber f
ract
ion
Chapter 14/15-
Chapter 14/15-
Chain stiffeness
C CHH
HC C C C
HH
HHHH
Polystyrene (PP)
CH3C C C C C C
HHH
HHHCH
C CH3
OCH3
OCH3
OCH3
O
Bulky side groups offer steric hindrance to polymer chains and kinking
Polymethyl Methacrilate (PMMA)
Chapter 14/15-
Chain crystallization
Chain-folded structure
Chain crystallinity influenced by:• Chemistry (simple mers)• Regularity in
• Structure (linear) • stereoisomerism (isotactic,
syndiotactic)• Co-polymerization (alternating and block)
Chapter 14/15-
Polyethylenes
HDPE LDPEDensity g/cm3 0.96 0.92Tensile strength (psi) 5,500 3,000Young’s modulus (psi) 180,000 40,000Applications
Propane gas Baby oil Parafin wax PolyethylenesChain length (Molecular Weight)
Plastic wrap Plastic bagsShort length branched
• low density• amorphous• weaker
Garbage cansdish pans, pipes
Some crystallinity
UHMWPE has very long molecular chains (70,000 C atoms), crystalline, inert lines sockets of artificial joints
Chapter 14/15-
Structure
• Structure – 4 Covalent chain configurations and strength:
Direction of increasing strength
Branched Cross-Linked NetworkLinear
secondarybonding
Chapter 14/15-
Chain configuration
Stereoisomers Geometrical isomers
cis transSyndiotacticIsotactic Atactic
• isotactic (side group is ordered on one side)
• syndiotactic (side group is ordered on opposite sides)
• atactic (side group is random along the chain)
CH
H
CH3C C
H
H
HC=
cis
CH
H
CH3C C
H H
HC=
trans
Natural Rubber
Gutta-percha
Chapter 14/15-
Other polymer characteristicsCo-polymers
Block
Graft
Alternating
Random
HIP – High Impact PolystyreneStyrene mer (PS)Butadiene mer (PB)
ABS – High Impact PolystyreneStyrene + Acrylonitrile (PS + PA)Butadiene + Acrylonitrile (PB + PA)
C CHH
HC C C C
HH
HHHH
C C
C
N N
N
Polyacrylonitrile
C CHH
HC C C CH
HHHH H
H
Polybutadiene
Chapter 14/15-
• Molecular weight, Mw: Mass of a mole of chains.
3
smaller M w larger M w
• Tensile strength (TS):--often increases with Mw.--Why? Longer chains are entangled (anchored) better.
• % Crystallinity: % of material that is crystalline.--TS and E often increase
with % crystallinity.--Annealing causes
crystalline regionsto grow. % crystallinityincreases.
crystalline region
amorphous region
Adapted from Fig. 14.11, Callister 6e.(Fig. 14.11 is from H.W. Hayden, W.G. Moffatt,and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc., 1965.)
Molecular weight & crystallinity
Chapter 14/15- 4
0
unload/reload
0
brittle failure
plastic failure
20
40
60
2 4 6
σ(MPa)
ε
x
x
semi- crystalline
caseamorphous
regions elongate
crystalline regions align
crystalline regions
slide
8
onset of necking
aligned, cross- linked case
networked case
Initial
Near Failure
near failure
Stress-strain curves adapted from Fig. 15.1, Callister 6e. Inset figures along plastic response curve (purple) adapted from Fig. 15.12, Callister 6e. (Fig. 15.12 is from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp. 500-501.)
Tensile response: brittle & plastic
Chapter 14/15- 5
• Drawing...--stretches the polymer prior to use--aligns chains to the stretching direction
• Results of drawing:--increases the elastic modulus (E) in the
stretching dir.--increases the tensile strength (TS) in the
stretching dir.--decreases ductility (%EL)
• Annealing after drawing...--decreases alignment--reverses effects of drawing.
• Compare to cold working in metals!
Adapted from Fig. 15.12, Callister 6e. (Fig. 15.12 is from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp. 500-501.)
Predeformation by drawing
Chapter 14/15-
Rubber
Cis-isoprene
CCH3
=H
HC C
H H
HH
HC C
Trans-isoprene
CCH3
=H
HC C
H
HH
HC C
H
Crowding causes kinking and chain rotation
Linear chain is stiffer CH
H
CH3C C
H H
HC=
CH
H
CH3C C
H
H
HC=
Chapter 14/15- 6
• Compare to responses of other polymers:--brittle response (aligned, cross linked & networked case)--plastic response (semi-crystalline case)
Stress-strain curves adapted from Fig. 15.1, Callister 6e. Inset figures along elastomer curve (green) adapted from Fig. 15.14, Callister 6e. (Fig. 15.14 is from Z.D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd ed., John Wiley and Sons, 1987.)
Tensile response: elastomer case
initial: amorphous chains are kinked, heavily cross-linked.
final : chains are straight,
still cross-linked
0
20
40
60
0 2 4 6
σ(MPa)
ε 8
x
x
x
elastomer
plastic failure
brittle failure
Deformation is reversible!
Chapter 14/15- 7
• Thermoplastics:--little cross linking--ductile--soften w/heating--polyethylene (#2)
polypropylene (#5)polycarbonatepolystyrene (#6)
• Thermosets:--large cross linking
(10 to 50% of mers)--hard and brittle--do NOT soften w/heating--vulcanized rubber, epoxies,
polyester resin, phenolic resin
Callister, Fig. 16.9
T
Molecular weight
Tg
Tmmobile liquid
viscous liquid
rubber
tough plastic
partially crystalline solidcrystalline
solid
Adapted from Fig. 15.18, Callister 6e. (Fig. 15.18 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.)
Thermoplastics vs thermosets
Chapter 14/15- 8
• Decreasing T...--increases E--increases TS--decreases %EL
• Increasingstrain rate...
--same effectsas decreasing T.
Adapted from Fig. 15.3, Callister 6e. (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.)
20
40
60
80
00 0.1 0.2 0.3
4°C
20°C
40°C
60°C to 1.3
σ(MPa)
ε
Data for the semicrystalline polymer: PMMA (Plexiglas)
T and strain rate: thermoplastics
Chapter 14/15-
Melting vs Glass transition temperature
Tg Tm
Den
sity
or s
peci
fic v
olum
e
Temperature
Temperature at which upon cooling a noncrystallinepolymer transforms into from a supercooled liquid to a rigid glass
Supercooledliquid
Liquid
Glass
CrystallineSemi-crystalline
Chapter 14/15- 9
• Stress relaxation test:
Er (t ) =
σ(t )εo
--strain to εο and hold.--observe decrease in
stress with time.
• Relaxation modulus:
• Data: Large drop in Erfor T > Tg. (amorphous
polystyrene)
• Sample Tg(C) (Glass transition) values:PE (low Mw)PE (high Mw)PVCPSPC
-110- 90+ 87+100+150
103
101
10-1
10-3
105
60 100 140 180
rigid solid (small relax)
viscous liquid (large relax)
transition region
T(°C)Tg
Er(10s) in MPa
Adapted from Fig. 15.7, Callister 6e. (Fig. 15.7 is from A.V. Tobolsky, Properties and Structures of Polymers, John Wiley and Sons, Inc., 1960.)
Selected values from Table 15.2, Callister 6e.
time dependent deformation
time
straintensile test
εo
tσ( )
Chapter 14/15- 10
• General drawbacks to polymers:-- E, σy, Kc, Tapplication are generally small.-- Deformation is often T and time dependent.-- Result: polymers benefit from composite reinforcement.
• Thermoplastics (PE, PS, PP, PC):-- Smaller E, σy, Tapplication-- Larger Kc-- Easier to form and recycle
• Elastomers (rubber):-- Large reversible strains!
• Thermosets (epoxies, polyesters):-- Larger E, σy, Tapplication
-- Smaller Kc
Table 15.3 Callister 6e:
Good overviewof applicationsand trade namesof polymers.
Summary
Chapter 14/15-
Reading:
Core Problems:
Self-help Problems:
0
ANNOUNCEMENTS