Chapter 14
Chapter 14,15: Polymers
School of Mechanical Engineering Choi, Hae-Jin
Materials Science - Prof. Choi, Hae-Jin 1
Chapter 14
2
CHAPTER 14,15: POLYMER STRUCTURES
ISSUES TO ADDRESS... • What are the general structural and chemical characteristics of polymer molecules? • What are some of the common polymeric materials, and how do they differ chemically? • How is the crystalline state in polymers different from that in metals and ceramics ?
Materials Science - Prof. Choi, Hae-Jin
• What are the various types/classifications of polymers? • What are the polymerization and basic fabrication and processing techniques for polymers?
Chapter 14
What is a Polymer?
Poly mer many repeat unit
3
Adapted from Fig. 4.2, Callister & Rethwisch 3e.
C C C C C C H H H H H H
H H H H H H
Polyethylene (PE) Cl Cl Cl
C C C C C C H H H
H H H H H H
Poly(vinyl chloride) (PVC) H H
H H H H
Polypropylene (PP)
C C C C C C CH3
H H
CH3 CH3 H
repeat unit
repeat unit
repeat unit
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Ancient Polymers
• Originally natural polymers were used – Wood – Rubber – Cotton – Wool – Leather – Silk
• Oldest known uses
– Rubber balls used by Incas – Noah used pitch (a natural polymer)
for the ark
4 Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Polymer Composition Most polymers are hydrocarbons – i.e., made up of H and C • Saturated hydrocarbons
– Each carbon singly bonded to four other atoms – Example:
• Ethane, C2H6
5
C C
H
H H H
HH
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Unsaturated Hydrocarbons • Double & triple bonds somewhat unstable
– can form new bonds – Double bond found in ethylene or ethene - C2H4
– Triple bond found in acetylene or ethyne - C2H2
7
C CH
H
H
H
C C HH
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Isomerism • Isomerism
– two compounds with same chemical formula can have quite different structures
for example: C8H18 • normal-octane
• 2,4-dimethylhexane
8
C C C C C C C CH
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H H3C CH2 CH2 CH2 CH2 CH2 CH2 CH3=
H3C CH
CH3
CH2 CH
CH2
CH3
CH3
H3C CH2 CH3( )6
⇓
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Polymerization and Polymer Chemistry
• Free radical polymerization
• Initiator: example - benzoyl peroxide
9
C
H
H
O O C
H
H
C
H
H
O2
C C
H H
HHmonomer(ethylene)
R +
free radical
R C C
H
H
H
H
initiation
R C C
H
H
H
H
C C
H H
HH
+ R C C
H
H
H
H
C C
H H
H H
propagation
dimer
R= 2
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Chemistry and Structure of Polyethylene
10
Adapted from Fig. 4.1, Callister & Rethwisch 3e.
Note: polyethylene is a long-chain hydrocarbon - paraffin wax for candles is short polyethylene
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Bulk or Commodity Polymers
11 Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Bulk or Commodity Polymers (cont)
12 Materials Science - Prof. Choi, Hae-Jin
Chapter 14
13
Bulk or Commodity Polymers (cont)
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
MOLECULAR WEIGHT
14
• Molecular weight, M: Mass of a mole of chains.
Low M
high M
Not all chains in a polymer are of the same length — i.e., there is a distribution of molecular weights
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
MOLECULAR WEIGHT DISTRIBUTION
15
xi = number fraction of chains in size range i
molecules of #totalpolymer of wttotal
=nM
iiw
iin
MwM
MxM
Σ=
Σ=
Adapted from Fig. 4.4, Callister & Rethwisch 3e.
wi = weight fraction of chains in size range i
Mi = mean (middle) molecular weight of size range i
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Molecular Weight Calculation Example: average mass of a class
16
Student Weight mass (lb)
1 104 2 116 3 140 4 143 5 180 6 182 7 191 8 220 9 225 10 380
What is the average weight of the students in this class: a) Based on the number
fraction of students in each mass range?
b) Based on the weight fraction of students in each mass range?
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Molecular Weight Calculation (cont.) Solution: The first step is to sort the students into weight ranges.
Using 40 lb ranges gives the following table:
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weight number of mean number weightrange students weight fraction fraction
Ni Wi xi wi
mass (lb) mass (lb)81-120 2 110 0.2 0.117
121-160 2 142 0.2 0.150161-200 3 184 0.3 0.294201-240 2 223 0.2 0.237241-280 0 - 0 0.000281-320 0 - 0 0.000321-360 0 - 0 0.000361-400 1 380 0.1 0.202
ΣNi ΣNiWi
10 1881total
number total
weight
Calculate the number and weight fraction of students in each weight range as follows:
xi =Ni
Ni∑
wi =NiWi
NiWi∑
For example: for the 81-120 lb range
x81−120 =2
10= 0.2
117.01881
011 x 212081 ==−w
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Molecular Weight Calculation (cont.)
18
Mn = xiMi∑ = (0.2 x 110 + 0.2 x 142 + 0.3 x 184 + 0.2 x 223 + 0.1 x 380) =188 lb
weight mean number weightrange weight fraction fraction
Wi xi wi
mass (lb) mass (lb)81-120 110 0.2 0.117
121-160 142 0.2 0.150161-200 184 0.3 0.294201-240 223 0.2 0.237241-280 - 0 0.000281-320 - 0 0.000321-360 - 0 0.000361-400 380 0.1 0.202
Mw = wiMi∑ = (0.117 x 110 + 0.150 x 142 + 0.294 x 184
+ 0.237 x 223 + 0.202 x 380) = 218 lb
Mw = wiMi∑ = 218 lb
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Degree of Polymerization, DP DP = average number of repeat units per chain
mMDP n=
19
iimfm
m
Σ=
=
:follows as calculated is this copolymers forunit repeat of weightmolecular average where
C C C C C C C CH
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
C C C C
H
H
H
H
H
H
H
H
H( ) DP = 6
mol. wt of repeat unit i Chain fraction Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Molecular Structures for Polymers
20
Adapted from Fig. 4.7, Callister & Rethwisch 3e.
B ranched Cross-Linked Network Linear
secondary bonding
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Polymers – Molecular Shape
Molecular Shape (or Conformation) – chain bending and twisting are possible by rotation of carbon atoms around their chain bonds – note: not necessary to break chain bonds to
alter molecular shape
21
Adapted from Fig. 4.5, Callister & Rethwisch 3e.
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Chain End-to-End Distance, r
22
Adapted from Fig. 4.6, Callister & Rethwisch 3e.
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Molecular Configurations for Polymers
Configurations – to change must break bonds • Stereoisomerism
23
E B
A
D
C C
D
A
B E
mirror plane
C CR
HH
HC C
H
H
H
R
or C C
H
H
H
R
Stereoisomers are mirror images – can’t superimpose without breaking a bond
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Tacticity Tacticity – stereoregularity or spatial arrangement of
R units along chain
24
C C
H
H
H
R R
H
H
H
CC
R
H
H
H
CC
R
H
H
H
CC
isotactic – all R groups on same side of chain
C C
H
H
H
R
C C
H
H
H
R
C C
H
H
H
R R
H
H
H
CC
syndiotactic – R groups alternate sides
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Tacticity (cont.)
25
atactic – R groups randomly positioned
C C
H
H
H
R R
H
H
H
CC
R
H
H
H
CC
R
H
H
H
CC
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
cis/trans Isomerism
26
C CHCH3
CH2 CH2
C CCH3
CH2
CH2
H
cis cis-isoprene
(natural rubber)
H atom and CH3 group on same side of chain
trans trans-isoprene (gutta percha)
H atom and CH3 group on opposite sides of chain
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Copolymers two or more monomers
polymerized together • random – A and B randomly
positioned along chain • alternating – A and B
alternate in polymer chain • block – large blocks of A
units alternate with large blocks of B units
• graft – chains of B units grafted onto A backbone A – B –
27
random
block
graft
Adapted from Fig. 4.9, Callister & Rethwisch 3e.
alternating
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Crystallinity in Polymers • Ordered atomic
arrangements involving molecular chains
• Crystal structures in terms of unit cells
• Example shown – polyethylene unit cell
28
Adapted from Fig. 4.10, Callister & Rethwisch 3e.
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Polymer Crystallinity • Crystalline regions
– thin platelets with chain folds at faces – Chain folded structure
29
10 nm
Adapted from Fig. 4.12, Callister & Rethwisch 3e.
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Polymer Crystallinity (cont.) Polymers rarely 100% crystalline • Difficult for all regions of all chains to
become aligned
30
• Degree of crystallinity expressed as % crystallinity. -- Some physical properties depend on % crystallinity. -- Heat treating causes crystalline regions to grow and % crystallinity to increase.
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.)
crystalline region
amorphous region
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Polymer Single Crystals • Electron micrograph – multilayered single crystals
(chain-folded layers) of polyethylene • Single crystals – only for slow and carefully controlled
growth rates
31
Adapted from Fig. 4.11, Callister & Rethwisch 3e.
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Semicrystalline Polymers • Some semicrystalline
polymers form spherulite structures
• Alternating chain-folder crystallites and amorphous regions
• Spherulite structure for relatively rapid growth rates
32
Spherulite surface
Adapted from Fig. 4.13, Callister & Rethwisch 3e.
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Photomicrograph – Spherulites in Polyethylene
33
Adapted from Fig. 4.14, Callister & Rethwisch 3e.
Cross-polarized light used -- a maltese cross appears in each spherulite
Materials Science - Prof. Choi, Hae-Jin
Chapter 14
Point Defects in Polymers • Defects due in part to chain packing errors and impurities such
as chain ends and side chains
34
Adapted from Fig. 5.7, Callister & Rethwisch 3e.
Adapted from Fig. 5.7, Callister & Rethwisch 3e.
Chapter 14
35
Mechanical Properties of Polymers – Stress-Strain Behavior
35
• Fracture strengths of polymers ~ 10% of those for metals • Deformation strains for polymers > 1000% – for most metals, deformation strains < 10%
brittle polymer
plastic elastomer
elastic moduli – less than for metals Adapted from Fig. 7.22,
Callister & Rethwisch 3e.
Chapter 14
36
Mechanisms of Deformation—Brittle Crosslinked and Network Polymers
36
brittle failure
plastic failure
σ (MPa)
ε
x
x
aligned, crosslinked polymer Stress-strain curves adapted from Fig. 7.22,
Callister & Rethwisch 3e.
Initial Near
Failure Initial
network polymer
Near Failure
Chapter 14
37
Mechanisms of Deformation — Semicrystalline (Plastic) Polymers
37
brittle failure
plastic failure
σ (MPa)
x
x
crystalline block segments
separate
fibrillar structure
near failure
crystalline regions align
onset of necking
undeformed structure amorphous
regions elongate
unload/reload
Stress-strain curves adapted from Fig. 7.22, Callister & Rethwisch 3e. Inset figures along plastic response curve adapted from Figs. 8.27 & 8.28, Callister & Rethwisch 3e. (Figs. 8.27 & 8.28 are from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp. 500-501.)
ε
Chapter 14
38
Predeformation by Drawing • Drawing…(ex: monofilament fishline) -- stretches the polymer prior to use -- aligns chains in 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) • Annealing after drawing... -- decreases chain alignment -- reverses effects of drawing (reduces E and TS, enhances %EL) • Contrast to effects of cold working in metals!
Adapted from Fig. 8.28, Callister & Rethwisch 3e. (Fig. 8.28 is from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp. 500-501.)
Chapter 14
39
Mechanisms of Deformation—Elastomers
• Compare elastic behavior of elastomers with the: -- brittle behavior (of aligned, crosslinked & network polymers), and -- plastic behavior (of semicrystalline polymers) (as shown on previous slides)
Stress-strain curves adapted from Fig. 7.22, Callister & Rethwisch 3e. Inset figures along elastomer curve (green) adapted from Fig. 8.30, Callister & Rethwisch 3e. (Fig. 8.30 is from Z.D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd ed., John Wiley and Sons, 1987.)
σ (MPa)
ε initial: amorphous chains are kinked, cross-linked.
x
final: chains are straighter,
still cross-linked
elastomer
deformation is reversible (elastic)!
brittle failure
plastic failure x
x
Chapter 14
40
Polymer Types – Fibers Fibers - length/diameter >100 • Primary use is in textiles. • Fiber characteristics:
– high tensile strengths – high degrees of crystallinity – structures containing polar groups
40
• Formed by spinning – extrude polymer through a spinneret (a die containing many small orifices) – the spun fibers are drawn under tension – leads to highly aligned chains - fibrillar structure
Chapter 14
41
Polymer Types – Miscellaneous • Coatings – thin polymer films applied to surfaces – i.e.,
paints, varnishes – protects from corrosion/degradation – decorative – improves appearance – can provide electrical insulation
41
• Adhesives – bonds two solid materials (adherands) – bonding types:
1. Secondary – van der Waals forces 2. Mechanical – penetration into pores/crevices
• Films – produced by blown film extrusion • Foams – gas bubbles incorporated into plastic
Chapter 14
42
Advanced Polymers
• Molecular weight ca. 4 x 106 g/mol • Outstanding properties
– high impact strength – resistance to wear/abrasion – low coefficient of friction – self-lubricating surface
• Important applications – bullet-proof vests – golf ball covers – hip implants (acetabular cup)
42
UHMWPE
Adapted from chapter-opening photograph, Chapter 22, Callister 7e.
Ultrahigh Molecular Weight Polyethylene (UHMWPE)
Chapter 14
43
Polymer Formation • There are two types of polymerization
– Addition (or chain) polymerization – Condensation (step) polymerization
Chapter 14
44 44
Addition (Chain) Polymerization
– Initiation
– Propagation
– Termination
Chapter 14
46 46
Polymer Additives Improve mechanical properties, processability,
durability, etc. • Fillers
– Added to improve tensile strength & abrasion resistance, toughness & decrease cost
– ex: carbon black, silica gel, wood flour, glass, limestone, talc, etc.
• Plasticizers – Added to reduce the glass transition temperature Tg below room temperature – Presence of plasticizer transforms brittle polymer to a ductile one – Commonly added to PVC - otherwise it is brittle
Chapter 14
47 47
Polymer Additives (cont.) • Stabilizers
– Antioxidants – UV protectants
• Lubricants – Added to allow easier processing – polymer “slides” through dies easier – ex: sodium stearate
• Colorants – Dyes and pigments
• Flame Retardants – Substances containing chlorine, fluorine, and boron
Chapter 14
48 48
Processing of Plastics • Thermoplastic
– can be reversibly cooled & reheated, i.e. recycled – heat until soft, shape as desired, then cool – ex: polyethylene, polypropylene, polystyrene.
• Thermoset – when heated forms a molecular network (chemical reaction) – degrades (doesn’t melt) when heated – a prepolymer molded into desired shape, then chemical reaction occurs – ex: urethane, epoxy
Chapter 14
49 49
Processing Plastics – Compression Molding Thermoplastics and thermosets • polymer and additives placed in mold cavity • mold heated and pressure applied • fluid polymer assumes shape of mold
Fig. 14.29, Callister & Rethwisch 3e. (Fig. 14.29 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley & Sons, 1984.)
Chapter 14
50 50
Processing Plastics – Injection Molding
Fig. 14.30, Callister & Rethwisch 3e. (Fig. 14.30 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 2nd edition, John Wiley & Sons, 1971.)
Thermoplastics and some thermosets • when ram retracts, plastic pellets drop from hopper into barrel • ram forces plastic into the heating chamber (around the
spreader) where the plastic melts as it moves forward • molten plastic is forced under pressure (injected) into the mold
cavity where it assumes the shape of the mold
Barrel
Chapter 14
51 51
Processing Plastics – Extrusion
Fig. 14.31, Callister & Rethwisch 3e. (Fig. 14.31 is from Encyclopædia Britannica, 1997.)
thermoplastics • plastic pellets drop from hopper onto the turning screw • plastic pellets melt as the turning screw pushes them
forward by the heaters • molten polymer is forced under pressure through the
shaping die to form the final product (extrudate)
Chapter 14
52 52
Processing Plastics – Blown-Film Extrusion
Fig. 14.32, Callister & Rethwisch 3e. (Fig. 14.32 is from Encyclopædia Britannica, 1997.)
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