chapter 14 - 1 issues to address... what are the general structural and chemical characteristics of...
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Chapter 14 - 1
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 ?
Chapter 14:Polymer Structures
Chapter 14 - 2
What is a Polymer?
Poly mer many repeat unit
Adapted from Fig. 14.2, Callister & Rethwisch 8e.
C C C C C C
HHHHHH
HHHHHH
Polyethylene (PE)
ClCl Cl
C C C C C C
HHH
HHHHHH
Poly(vinyl chloride) (PVC)
HH
HHH H
Polypropylene (PP)
C C C C C C
CH3
HH
CH3CH3H
repeatunit
repeatunit
repeatunit
Chapter 14 - 3
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
Chapter 14 - 4
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
C C
H
H HH
HH
Chapter 14 - 5
Chapter 14 - 6
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
C C
H
H
H
H
C C HH
Chapter 14 - 7
Isomerism
• Isomerism– two compounds with same chemical formula can
have quite different structures
for example: C8H18
• normal-octane
• 2,4-dimethylhexane
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
Chapter 14 - 8
Polymerization and Polymer Chemistry
• Free radical polymerization
• Initiator: example - benzoyl peroxide
C
H
H
O O C
H
H
C
H
H
O2
C C
H H
HH
monomer(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
Chapter 14 - 9
Chemistry and Structure of Polyethylene
Adapted from Fig. 14.1, Callister & Rethwisch 8e.
Note: polyethylene is a long-chain hydrocarbon- paraffin wax for candles is short polyethylene
Chapter 14 -10
Bulk or Commodity Polymers
Chapter 14 -11
Bulk or Commodity Polymers (cont)
Chapter 14 -12
Bulk or Commodity Polymers (cont)
Chapter 14 -
VMSE: Polymer Repeat Unit Structures
13
Manipulate and rotate polymer structures in 3-dimensions
Chapter 14 -14
MOLECULAR WEIGHT• 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
Chapter 14 -15
xi = number fraction of chains in size range i
molecules of #total
polymer of wttotalnM
MOLECULAR WEIGHT DISTRIBUTION
Mn xiMi
Mw wiMi
Adapted from Fig. 14.4, Callister & Rethwisch 8e.
wi = weight fraction of chains in size range i
Mi = mean (middle) molecular weight of size range i
Chapter 14 -16
Molecular Weight Calculation
Example: average mass of a class
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 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?
Chapter 14 -17
Molecular Weight Calculation (cont.)
Solution: The first step is to sort the students into weight ranges. Using 40 lb ranges gives the following table:
weight number of mean number weightrange students weight fraction fraction
Ni Wi xi wi
mass (lb) mass (lb)
81-120 2 110 0.2 0.117121-160 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
Ni NiWi
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
100.2
117.01881
011 x 212081 w
Chapter 14 -18
Molecular Weight Calculation (cont.)
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 0.117121-160 142 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
Chapter 14 -19
Degree of Polymerization, DP
DP = average number of repeat units per chain
iimfm
m
:follows as calculated is this copolymers for
unit 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 fraction
m
MDP
n
Chapter 14 -20
Adapted from Fig. 14.7, Callister & Rethwisch 8e.
Molecular Structures for Polymers
Branched Cross-Linked NetworkLinear
secondarybonding
Chapter 14 -21
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
Adapted from Fig. 14.5, Callister & Rethwisch 8e.
Chapter 14 -22
Chain End-to-End Distance, r
Adapted from Fig. 14.6, Callister & Rethwisch 8e.
Chapter 14 -23
Molecular Configurations for Polymers
Configurations – to change must break bonds• Stereoisomerism
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
Chapter 14 -24
TacticityTacticity – stereoregularity or spatial arrangement of R
units along chain
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
Chapter 14 -25
Tacticity (cont.)
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
Chapter 14 -26
cis/trans Isomerism
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
Chapter 14 -
VMSE: Stereo and Geometrical Isomers
27Chapter 7 - 19
Manipulate and rotate polymer structures in 3-dimensions
Chapter 14 -28
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 –
random
block
graft
Adapted from Fig. 14.9, Callister & Rethwisch 8e.
alternating
Chapter 14 -29
Crystallinity in Polymers
• Ordered atomic arrangements involving molecular chains
• Crystal structures in terms of unit cells
• Example shown– polyethylene unit cell
Adapted from Fig. 14.10, Callister & Rethwisch 8e.
Chapter 14 -30
Polymer Crystallinity
• Crystalline regions – thin platelets with chain folds at faces– Chain folded structure
10 nm
Adapted from Fig. 14.12, Callister & Rethwisch 8e.
Chapter 14 -31
Polymer Crystallinity (cont.)
Polymers rarely 100% crystalline• Difficult for all regions of all chains to
become aligned
• 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
Chapter 14 -32
Polymer Single Crystals
• Electron micrograph – multilayered single crystals (chain-folded layers) of polyethylene
• Single crystals – only for slow and carefully controlled growth rates
Adapted from Fig. 14.11, Callister & Rethwisch 8e.
Chapter 14 -33
Semicrystalline Polymers
Spherulite surface
Adapted from Fig. 14.13, Callister & Rethwisch 8e.
• Some semicrystalline polymers form spherulite structures
• Alternating chain-folded crystallites and amorphous regions
• Spherulite structure for relatively rapid growth rates
Chapter 14 -34
Photomicrograph – Spherulites in Polyethylene
Adapted from Fig. 14.14, Callister & Rethwisch 8e.
Cross-polarized light used -- a maltese cross appears in each spherulite
Chapter 14 -35
Core Problems:
Self-help Problems:
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