Polymers
• Macromolecule that is formed by linking of
repeating units through covalent bonds in
the main backbone
• Properties are determined by molecular
weight, length, backbone structure, side
chains, crystallinity
• Resulting macromolecules have huge
molecular weights
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Polymers
• Terminology:
–
–
–
–
–
–
–
mer: a unit
monomer: one unit
dimer: two units
trimer: three units
tetramer: four units
polymer: many units
pre-polymer: growing
towards being a polymer
– oligomer: few units fixed in
size
– homopolymer: polymer of
fixed mer type
HOMOPOLYMER
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Polymers
• Terminology (contn):
– copolymer: polymers of two mer types
• random · · ·-B-A-B-A-B-B-A-· · ·
• alternating· · ·-A-B-A-B-A-B-A-· · ·
• block · · ·-A-A-A-A-B-B-B-· · ·
– heteropolymer: polymers of many mer types
COPOLYMER
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Polymers: Molecular Weight
• i: degree of polymerization (# of monomer units)
Mi = i x M m
Mi: molar mass of polymer molecule i
Mm: molecular weight of monomer
• Typically all chains are not equally long but display a
variation
– monodisperse: equal chain lengths is specific to proteins
– polydisperse: unequal length specific to most synthetic
molecules
• Therefore we need to define an “average” molecular
weight
– number average, Mn
– weight average, Mw
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Polymers: Molecular Weight
• number average, Mn
• weight average, Mw
Ni: # of molecules with degree of polymerization of i
Mi: molecular weight of i
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Polymers: Molecular Weight
• Ratio of Mw to Mn is known as the polydispersity index
(PI)
• PI is a measure of the breadth of the molecular weight
• PI = 1 indicates Mw = Mn, i.e. all molecules have equal
length (monodisperse)
• PI = 1 is possible for natural proteins whereas synthetic
polymers have 1.5 < PI < 5
• At best PI = 1.1 can be attained with special techniques
• EXERCISE: Draw the molecular weight distribution for PI
= 1, PI = 2, and PI = 4
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Polymers: Molecular Weight
• Biomedical applications:
25,000<Mn<100,000 and
50,000<Mw<300,000
• Increasing molecular weight increases
physical properties; however, decreases
processibility
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Polymers
• Types of polymers:
– Thermoplastic: Polymers that flow when heated; thus,
easily reshaped and recycled. This property is due to
presence of long chains with limited or no crosslinks.
(polyethylene, polyvinylchloride)
– Thermosetting: Decomposed when heated; thus, can
not be reformed or recycled. Presence of extensive
crosslinks between long chains induce decomposition
upon heating and renders thermosetting polymers
brittle. (epoxy and polyesters)
– Elastomers: Intermediate between thermoplastic and
thermosetting polymers due to presence of some
crosslinking. Can undergo extensive elastic
deformation (natural rubber, silicone)
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Polymers
H
R
H
R
H
R
—C—C—
—C—C— • • • •
• • • • —C—C—
H
Structure
H
H
H
H
H
Source-Based Name
Application
R = -H
Polyethylene
Plastic
R = -CH3
Polypropylene
Rope
R = -Cl
Poly(vinyl chloride)
"Vinyl"
X = -H, R = -C2H5
Poly(ethyl acrylate)
Latex paints
X = -CH3, R = -CH3
Poly(methyl methacrylate)
Plastic
R = -H
Polybutadiene
Tires
R = -CH3
Polyisoprene
Tires
X = -F, R = -F
Polytetrafluoroethylene
Teflon®
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Polymers
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Polymers
• Polymers can be either amorphous or semi-crystalline
• Tacticity, i.e. arrangements of substituents around the backbone,
determines the degree of crystallinity
• Atactic polymers are amorphous
• Isotactic and syndiotactic may crystallize
• Crytallinity depends on:
–size of side groups (smaller, ↑crystallinity)
–regularity of chain
• Increased crystallinity enhances mechanical properties
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Polymer Synthesis
• Two common methods of polymerization
– Condensation polymerization (or stepwise addition)
– Addition reaction (or chain polymerization)
• Condensation: Two monomers react to form a covalent
bond usually with elimination of a small molecule such
as water, HCl, methanol, or CO2. Reaction continues
until one type of reactant is used up.
• Addition: Monomers react through stages of initiation,
propagation, and termination.
– initiators such as free radicals, cations, anions opens the double
bond of the monomer which becomes active and bonds with
other such monomers
– rapid chain reaction propagates in this fashion
– reaction is terminated by another free radical or another polymer
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Polymer Synthesis: Condensation
• phenol-formaldehyde:
results in condensation
of a water molecule
• nylon (polyamide): an
organic acid reacts with
an amine to form an
amide. HCl condenses
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Polymers Synthesis: Addition
Termination may occur by:
•two radicalized polymers reacting
•another radicalized monomer
•one initiator (alkoxy radical, •OR, in this case)
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Condensation vs. Addition
• Addition:
– Difficult to control molecular weight
– Undesirable branching products
• Condensation:
– Molecular weight closely controlled
– Polydispersity ratios close to unity can be obtained
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Stress
Polymers: Deformation
I
II III IV
Strain
I. Chain unfolding, unwinding, unwrapping or uncoupling (low energy)
II. Chain sliding (low energy)
III. Bond stretching, side group ordering (high energy)
IV. Bond breaking (high energy)
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Stress
Polymers: Deformation
Ceramics
Metals
Polymers
Strain
•Lower elastic modulus, yield and ultimate properties
•Greater post-yield deformability
•Greater failure strain
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Polymers: Viscoelasticity
• Dependency of stress-strain behavior on time
and loading rate
• Due to mobility of chains with each other
• Crosslinking may affect viscoelastic response
Stress
increasing loading
rate
Strain
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Polymers: Thermal Properties
decreasing temperature
or increasing crystallinity
log(Modulus)
Stress
• In the liquid/melt state enough thermal energy
for random motion (Brownian motion) of chains
• Motions decrease as the melt is cooled
• Motion ceases at “glass transition temperature”
• Polymer hard and glassy below Tg, rubbery
above Tg
Tg
Tm
semicrystalline
crosslinked
linear amorphous
Strain
Temperature
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Viscoelastic Deformation
Glassy Materials and Tg
Rigid Brittle Solid below Tg
Viscous Deformable liquid
above Tg
F
A

d
v
dx
η is the viscosity
of the fluid

Q 

 o  exp 

 R T 
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Viscoelastic Modulus versus Temperature
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Fracture of Polymers
•Thermoset and Thermoplastic materials below Tg behave as brittle solids
and fail by cracking. The cracks are sharp.
•Crystalline Thermoplastic Resins above Tg yield and undergo ductile failure.
•Noncrystalline Thermoplastic resins above Tg undergo Crazing and
Cracking.
• Crazing - Orientation of polymer chains across the opening of a “cracklike” feature. The work of crazing is 1000 times larger than the surface work
to create a crack.
•Cracking then occurs down the middle of the craze.
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Craze Formation
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Crack Propagation Through Crazed Area
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Cracking Example in Polymers
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