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Polymers
What is a polymer?
Poly mermany repeat unit
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
repeat
unit
repeat
unit
repeat
unit
Ancient Polymer History
• Originally many natural polymers were used
– Wood – Rubber
– Cotton – Wool
– Leather – Silk
• Oldest known uses of “Modern Polymers”
– Rubber balls used by Incas
– Noah used pitch (a natural polymer)
for the ark – as had all ancient mariners!
Polymer Composition
Most polymers are hydrocarbons– i.e. made up of H and C
(we also recognize Si-H ‘silicones’)
• Saturated hydrocarbons
– Each carbon bonded to four other atoms
C C
H
H HH
HH
CnH2n+2
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
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
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
Chemistry of Polymers
• Free radical polymerization (addition 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
Condensation Polymerization
• Some of the original monomer’s materials are shed
(condensed out) during polymerization process
• Process is conducted in the presence of a catalyst
• Water, CO2 are commonly condensed out but other
compounds can be emitted including HCN or other acids
Water is “Condensed out”
during polymerization of Nylon
Bulk or Commodity Polymers
NOTE: See Table 15.3 for commercially important
polymers – including trade names
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.
Molecular Weight Calculation
Example: average mass of a class
N i M i x i wi
# of students mass (lb)
1 100 0.1 0.054
1 120 0.1 0.065
2 140 0.2 0.151
3 180 0.3 0.290
2 220 0.2 0.237
1 380 0.1 0.204
M n M w
186 lb 216 lb
iiw MwM
iin MxM
i ii
i i
all i
N Mw
N M
ii
i
all i
Nx
N
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
• Covalent chain configurations and strength:
Direction of increasing strength
Molecular Structures
Branched Cross-Linked NetworkLinear
secondarybonding
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.
Polymers – Molecular Shape
Configurations – to change must break bonds
• Stereoisomerism
EB
A
D
C C
D
A
BE
mirror
plane
C C
R
HH
HC C
H
H
H
R
or C C
H
H
H
R
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
cis/trans Isomerism
C C
HCH3
CH2 CH2
C C
CH3
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
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
alternating
Polymer Crystallinity
Ex: polyethylene unit cell
• Crystals must contain the
polymer chains in some way
– Chain folded structure
• Thermoplastics:-- little crosslinking
-- ductile
-- soften w/heating
-- polyethylene
polypropylene
polycarbonate
polystyrene
• Thermosets:-- large crosslinking
(10 to 50% of mers)
-- hard and brittle
-- do NOT soften w/heating
-- vulcanized rubber, epoxies,
polyester resin, phenolic resin
Thermoplastics vs. Thermosets
Callister, Fig. 16.9
T
Molecular weight
Tg
Tmmobile liquid
viscousliquid
rubber
tough plastic
partially crystalline solid
crystalline solid
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
– commonly added to PVC - otherwise it is brittle
Polymer Additives
• Stabilizers
– Antioxidants
– UV protectants
• Lubricants
– Added to allow easier processing
– “slides” through dies easier – ex: Na stearate
• Colorants
– Dyes or pigments
• Flame Retardants
– Cl/F & B
Processing of Plastics
• Thermoplastic –
– can be reversibly cooled & reheated, i.e. recycled
– heat till soft, shape as desired, then cool
– ex: polyethylene, polypropylene, polystyrene, etc.
• Thermoset
– when heated forms a network
– degrades (not melts) when heated
– mold the prepolymer then allow further reaction
– ex: urethane, epoxy
Polymer Types: Elastomers
Elastomers – rubber
• Crosslinked materials
– Natural rubber
– Synthetic rubber and thermoplastic elastomers
• SBR- styrene-butadiene rubberstyrene
– Silicone rubber
butadiene
Recycling
Most plastics are produced from crude oil, coal or gas.
Many of them are not biodegradable and have become a visible part of our environmental litter.