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2012-03-22
1
Chapter 4 -
Chapter 4: Polymer Structures
School of Mechanical EngineeringChoi, Hae-Jin
Materials Science - Prof. Choi, Hae-Jin 1
Chapter 4 - 2
CHAPTER 4: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
2012-03-22
2
Chapter 4 -
What is a Polymer?
Poly mermany repeat unit
3
Adapted from Fig. 4.2, Callister & Rethwisch 3e.
C C C C C CHHHHHH
HHHHHH
Polyethylene (PE)ClCl Cl
C C C C C CHHH
HHHHHH
Poly(vinyl chloride) (PVC)HH
HHH H
Polypropylene (PP)
C C C C C CCH3
HH
CH3CH3H
repeatunit
repeatunit
repeatunit
Materials Science - Prof. Choi, Hae-Jin
Chapter 4 -
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
4Materials Science - Prof. Choi, Hae-Jin
2012-03-22
3
Chapter 4 -
Polymer CompositionMost 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 4 - 6Materials Science - Prof. Choi, Hae-Jin
2012-03-22
4
Chapter 4 -
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 4 -
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
ß
Materials Science - Prof. Choi, Hae-Jin
2012-03-22
5
Chapter 4 -
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 4 -
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
2012-03-22
6
Chapter 4 -
Bulk or Commodity Polymers
11Materials Science - Prof. Choi, Hae-Jin
Chapter 4 -
Bulk or Commodity Polymers (cont)
12Materials Science - Prof. Choi, Hae-Jin
2012-03-22
7
Chapter 4 - 13
Bulk or Commodity Polymers (cont)
Materials Science - Prof. Choi, Hae-Jin
Chapter 4 -
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
2012-03-22
8
Chapter 4 -
MOLECULAR WEIGHT DISTRIBUTION
15
xi = number fraction of chains in size range i
molecules of #totalpolymer of wttotal
=nM
iiw
iin
MwM
MxM
S=
S=
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 4 -
Molecular Weight CalculationExample: average mass of a class
16
Student Weightmass (lb)
1 1042 1163 1404 1435 1806 1827 1918 2209 225
10 380
What is the averageweight of the students inthis 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
2012-03-22
9
Chapter 4 -
Molecular Weight Calculation (cont.)Solution: The first step is to sort the students into weight ranges.
Using 40 lb ranges gives the following table:
17
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
SNi SNiWi
10 1881total
numbertotal
weight
Calculate the number and weight fraction of students in each weight range as follows:
�
xi =Ni
Niå
�
wi =NiWi
NiWiå
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 4 -
Molecular Weight Calculation (cont.)
18
�
Mn = xiMiå = (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 = wiMiå = (0.117 x 110 + 0.150 x 142 + 0.294 x 184
+ 0.237 x 223 + 0.202 x 380) = 218 lb
�
Mw = wiMiå = 218 lb
Materials Science - Prof. Choi, Hae-Jin
2012-03-22
10
Chapter 4 -
Degree of Polymerization, DPDP = average number of repeat units per chain
mMDP n=
19
iimfm
m
S=
=
: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 iChain fractionMaterials Science - Prof. Choi, Hae-Jin
Chapter 4 -
Molecular Structures for Polymers
20
Adapted from Fig. 4.7, Callister & Rethwisch 3e.
Branched Cross-Linked NetworkLinear
secondarybonding
Materials Science - Prof. Choi, Hae-Jin
2012-03-22
11
Chapter 4 -
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 4 -
Chain End-to-End Distance, r
22
Adapted from Fig. 4.6, Callister & Rethwisch 3e.
Materials Science - Prof. Choi, Hae-Jin
2012-03-22
12
Chapter 4 -
Molecular Configurations for Polymers
Configurations – to change must break bonds• Stereoisomerism
23
EB
A
D
C C
D
A
BE
mirror plane
C CR
HH
HC C
H
H
H
R
or C C
H
H
H
R
Stereoisomers are mirrorimages – can’t superimposewithout breaking a bond
Materials Science - Prof. Choi, Hae-Jin
Chapter 4 -
TacticityTacticity – 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
2012-03-22
13
Chapter 4 -
Tacticity (cont.)
25
atactic – R groups randomlypositioned
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 4 -
cis/trans Isomerism
26
C CHCH3
CH2 CH2
C CCH3
CH2
CH2
H
ciscis-isoprene
(natural rubber)
H atom and CH3 group on same side of chain
transtrans-isoprene (gutta percha)
H atom and CH3 group on opposite sides of chain
Materials Science - Prof. Choi, Hae-Jin
2012-03-22
14
Chapter 4 -
Copolymerstwo 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 4 -
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
2012-03-22
15
Chapter 4 -
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 4 -
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
amorphousregion
Materials Science - Prof. Choi, Hae-Jin
2012-03-22
16
Chapter 4 -
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 4 -
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
2012-03-22
17
Chapter 4 -
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