Chapter 14 - 1 ISSUES TO ADDRESS... What are the basic microstructural features ? How are polymer...
-
Upload
skyler-voyles -
Category
Documents
-
view
236 -
download
0
Transcript of Chapter 14 - 1 ISSUES TO ADDRESS... What are the basic microstructural features ? How are polymer...
Chapter 14 - 1
ISSUES TO ADDRESS...
• What are the basic microstructural features?
• How are polymer properties effected by molecular weight?
• How do polymeric crystals accommodate the polymer chain?
CHAPTER 14:POLYMER STRUCTURES
Chapter 14 - 2
Chapter 14 – Polymers
What is a polymer?
Poly mer many repeat unit
Adapted from Fig. 14.2, Callister 7e.
C C C C C C
HHHHHH
HHHHHH
Polyethylene (PE)
ClCl Cl
C C C C C C
HHH
HHHHHH
Polyvinyl chloride (PVC)
HH
HHH H
Polypropylene (PP)
C C C C C C
CH3
HH
CH3CH3H
repeatunit
repeatunit
repeatunit
Chapter 14 - 3
Ancient Polymer History
• 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 bonded to four other atoms
CnH2n+2
C C
H
H HH
HH
Chapter 14 - 5
Chapter 14 - 6
Unsaturated Hydrocarbons
• Double & triple bonds relatively reactive – can form new bonds– Double bond – ethylene or ethene - CnH2n
• 4-bonds, but only 3 atoms bound to C’s
– Triple bond – acetylene or ethyne - CnH2n-2
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
Ex: C8H18
• n-octane
• 2-methyl-4-ethyl pentane (isooctane)
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
Chemistry of Polymers
• 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 of Polymers
Adapted from Fig. 14.1, Callister 7e.
Note: polyethylene is just a long HC - paraffin is short polyethylene
Chapter 14 -10
Bulk or Commodity Polymers
Chapter 14 -11
Chapter 14 -12
Chapter 14 -13
MOLECULAR WEIGHT
molecules of #total
polymer of wttotalnM
iiw
iin
MwM
MxM
Mw is more sensitive to higher molecular weights
• Molecular weight, Mi: Mass of a mole of chains.
Lower M higher M
Adapted from Fig. 14.4, Callister 7e.
Chapter 14 -14
Molecular Weight Calculation
Example: average mass of a classN i M i x i wi
# of students mass (lb)
1 100 0.1 0.0541 120 0.1 0.0652 140 0.2 0.1513 180 0.3 0.2902 220 0.2 0.2371 380 0.1 0.204
M n M w
186 lb 216 lb
iiw MwM
iin MxM
Chapter 14 -15
Degree of Polymerization, n
n = number of repeat units per chain
ii
wiiw
niin
mfm
m
m
Mnwn
m
Mnxn
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( ) ni = 6
mol. wt of repeat unit iChain fraction
Chapter 14 -16
End to End Distance, r
Adapted from Fig. 14.6, Callister 7e.
Chapter 14 -17
• Covalent chain configurations and strength:
Direction of increasing strength
Adapted from Fig. 14.7, Callister 7e.
Molecular Structures
Branched Cross-Linked NetworkLinear
secondarybonding
Chapter 14 -18
Polymers – Molecular Shape
Conformation – Molecular orientation can be changed by rotation around the bonds– note: no bond breaking needed
Adapted from Fig. 14.5, Callister 7e.
Chapter 14 -19
Polymers – Molecular Shape
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
Chapter 14 -20
Tacticity
Tacticity – stereoregularity of chain
C C
H
H
H
R R
H
H
H
CC
R
H
H
H
CC
R
H
H
H
CC
C C
H
H
H
R
C C
H
H
H
R
C C
H
H
H
R R
H
H
H
CC
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
syndiotactic – R groups alternate sides
atactic – R groups random
Chapter 14 -21
cis/trans Isomerism
C CHCH3
CH2 CH2
C CCH3
CH2
CH2
H
cis
cis-isoprene (natural rubber)
bulky groups on same side of chain
trans
trans-isoprene (gutta percha)
bulky groups on opposite sides of chain
Chapter 14 -22
Copolymers
two or more monomers polymerized together
• random – A and B randomly vary in chain
• alternating – A and B alternate in polymer chain
• block – large blocks of A alternate with large blocks of B
• graft – chains of B grafted on to A backbone
A – B –
random
block
graft
Adapted from Fig. 14.9, Callister 7e.
alternating
Chapter 14 -23
Polymer Crystallinity
Ex: polyethylene unit cell
• Crystals must contain the polymer chains in some way – Chain folded structure
10 nm
Adapted from Fig. 14.10, Callister 7e.
Adapted from Fig. 14.12, Callister 7e.
Chapter 14 -24
Polymer Crystallinity
Polymers rarely 100% crystalline• Too difficult to get all those chains
aligned
• % Crystallinity: % of material that is crystalline. -- TS and E often increase with % crystallinity. -- Annealing causes crystalline regions to grow. % crystallinity increases.
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 -25
Polymer Crystal Forms
• Single crystals – only if slow careful growth
Adapted from Fig. 14.11, Callister 7e.
Chapter 14 -26
Polymer Crystal Forms
Spherulite surface
Nucleation site Adapted from Fig. 14.13, Callister 7e.
• Spherulites – fast growth – forms lamellar (layered) structures
Chapter 14 -27
Spherulites – crossed polarizers
Adapted from Fig. 14.14, Callister 7e.
Maltese cross
Chapter 14 -28
Core Problems:
Self-help Problems:
ANNOUNCEMENTS
Reading: