Quiz 7 and 8 Polymers
You Should Know/Be Able to:
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State how the microstructure of polymers differs from metals and ceramics
State how the mechanical and physical properties of polymers differ from
metals and ceramics, and explain those differences from a microstructural
perspective
State the differences between thermoplastic, thermosetting, and
elastomeric polymers with respect to microstructure and general properties.
• For thermoplastics, state what bonding is present within the chains and
between the chains, and explain how this affects mechanical and physical
properties.
• For thermosets, state what bonding is present within the chains and
between the chains, explain the effect of the bonding and structure on
properties, and describe how those properties differ from the
thermoplastics.
• For elastomers, state what bonding is present within the chains and
between the chains, and explain how the microstructure affects
mechanical and physical properties
State what happens microstructurally for a thermoplastic in the elastic and
plastic stages of a tensile test
Describe the problems associated with specifying properties of a
viscoelastic material for use in load bearing applications, including the
effects of temperature and strain rate. Explain why don't we see creep
problems in plastic telephones, coke bottles, etc.?
Name and explain the effects of chain length (degree of polymerization),
chain stiffness, side group size, and degree of crystallinity on the
mechanical and physical properties of thermoplastics.
Name and explain the effect of crosslinking on the mechanical and physical
properties of thermosets and elastomers.
Define the term “entropy spring” and explain what happens microstructurally
Name or recognize circumstances in which polymers would be a good choice
over metals and ceramics and when the use of polymers is contraindicated.
Name two significant attributes for common polymers listed on page 6 (e.g.
ABS and polycarbonate are notable for toughness. Amorphous
polycarbonate can be transparent, while the "multiphase" ABS cannot; Nylon
(polyamide) and acetal (polyoxymethylene, POM) are both engineering
polymers with sufficient strength and stiffness for use in load bearing
products such as gears. Nylon has the disadvantage of being hygroscopic).
Describe and explain with the in-class hands-on experiments with polymers,
especially the relations between structure and properties
Quiz 7&8 Review
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Polymers (many repeating units) Long chain (macro) molecules with primary bonds
along the chains. Bonds between chains can be primary or secondary. The
microstructure can be viewed as spaghetti. Strength mostly depends on how easy it
is to move strands of spaghetti relative to each other.
Comparison of Properties with Metals and Ceramics
• low stiffness (1-3 orders of magnitude less than metals, ceramics)
• lower strength (order of magnitude) than metals, ceramics
• ductility from 0-600% (ceramics≈0%, ceramics 0-50%)
• some have extraordinary (400%) elastic deformation (elastomers) (metals
and ceramic are less than a few percent)
• much lower maximum operating temperatures than most metals and ceramics
• easily manufactured into complex shapes (injection molding, extrusion, blow
molding)
• can have good specific strength (esp. fibers) due to low density (Kevlar,
Spectra have specific strength higher than steel)
• can modify appearance - color, lustre, texture (less easy with metals,
ceramics)
Reason for Property differences with Metals and Ceramics
• Secondary bonds between chains are relatively weak and allow chain motion
under stress, leading to lower strength and stiffness than materials with all
primary bonds
• Weak secondary bonds result in softening (for thermoplastics) at relatively
low temperatures reducing temperature of use, but making processing much
easier.
Polymerizing (addition polymerization)
H
R
Free Radical
breaks double
bond and bonds
with Carbon
H
C
C
H
H
Unsaturated
(double bond)
allows chain
to form.
When double
bond is broken,
carbon has an
open bonding site
and chain forms
H
R
H
H
H
H
H
C
H
C
C
C
C
C
H
H
H
H
H
or
H
H
C
C
H
H
n
Molecular shape
• Linear - Easier to crystallize (many thermoplastics)
• Branched - Harder to crystallize (amorphous) (some thermoplastics)
• Crosslinked - Primary bonds between chains (thermosets and elastomers)
• Network - Trifunctional mers form amorphous 3-D networks (thermosets)
Molecular configuration
• Isotactic - side group on same side
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(easiest to crystallize)
Syndiotactic - side group on alternate sides
Quiz 7&8 Review
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H
R
H
R
H
R
C
C
C
C
C
C
H
H
H
H
H
H
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H
•
R
H
H
H
R
C
C
C
C
C
C
H
H
H
R
H
H
H
R
H
R
H
H
C
C
C
C
C
C
H
H
H
H
H
R
Atactic - side group at random locations
(very hard to crystallize)
Copolymers (similar to alloying in metals, but with mers rather than elements)
• Random - different mers occupy different sites along the chain
!-!-!-!-!-!-!-!-!-!-!-!-!
• Alternating - different mers alternate along the chain
!-!-!-!-!-!-!-!-!-!-!-!-!
• Block - different mers alternate in groups
!-!-!-!-!-!-!-!-!-!-!-!
• Branched - one mer along the chain, other on the branches
Thermoplastic
Thermoset
Elastomer
(softens upon heating)
(cures upon heating)
(significant elastic
deformation)
• primary bonds along
• primary bonds along
• primary bonds along
chain
chain
chain
• few primary bonds
• primary bonds
• secondary bonds
between chains
between chains
between chains
• highly “coiled” chains
• usually can recycle (as • difficult to recycle
• difficult to recycle
• simple shapes can be
same product
• “vulcanized” rubber
pressed, some
• usually easier to
• Latex rubber,
injection molded
fabricate (injection
Neoprene, silicone
• Phenolic (bakelite,
mold, extrude, blow
rubber
Epoxy, Polyester
mold)
(Bondo)
• Polyethylene (PE),
Nylon (PA), Delrin
(POM), Lexan (PC),
Acrylic (Plexiglas)
(PMMA)
Response to Stress
Quiz 7&8 Review
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Stress
First PeakYield Strength
Highest Peak Ultimate Tensile
Strength
Drawing - Chain coiling and uncoiling
- Crystalline packets separate and align
Bond stretching (secondary and primary)
Strain
Note: In polymers, yield is not doe to planer slip from dislocation motion, but
typically from chain uncoiling. Therefore, strengthening procedures used
for metals are inappropriate. Rather, strengthening of thermoplastics is by
prevention/impeding of chain motion.
Crystallinity
• No polymers are fully crystalline
• Many thermoplastics are amorphous (Lexan, Plexiglass, polystyrene)
• Some thermoplastics are partly crystalline (nylon, acetal, PET)
• 3-D order comes from chain folding and alignment
• Crystalline regions alternate with amorphous regions
• Thermosets are never crystalline
Effect of:
Molecular Weight
(degree of
polymerization)
Side Groups
Bonding (secondary)
Crystallinity
Crosslinking
Drawing
Quiz 7&8 Review
on Properties
Longer chains cause more
entanglement and impede
uncoiling, therefore higher
strength and stiffness
Bulky side groups on the chain
impede uncoiling
Highly polar molecules have
stronger secondary bonds,
impede uncoiling
In crystalline portions molecules
packed more tightly and
secondary bonds are stronger
Primary bonds between chains
impede uncoiling
Chains align in amorphous regions,
small packets of crystallites
separate and align. Further
uncoiling is difficult.
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Example
Octane" LDPE"
HDPE " UHMWPE
Polystyrene
PVC, PTFE (Teflon)
Nylon, Acetal
Thermoset, epoxy
Fibers (nylon, Kevlar)
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Annealing
Heating a semicrystalline polymer
can organize, improve crystals
Heating a drawn polymer can
disorganize, weaken
Viscoelasticity
A significant design challenge of polymers is that the material properties are time
dependent and temperature dependent. This can create any number of problems.
The relationship between Elastic modulus and temp for a thermoplastic is shown
below.
Log E
Glassy
Example: Vinyl garden hose may be
flexible above Tg (summer) and rigid
below Tg (winter). Note that the
vertical axis is log scale. Modulus can
change by orders of magnitude in a small
temperature range.
Leathery
Rubbery
Temp
Tg, Glass Transition Temp
Glass Transition Temperature – Transition from flexible to glassy. Chain coiling is
prevented by temperature (thermal contraction means less room for motion)
Melting Temperature – Transition from Crystalline to Amorphous (NOT necessarily
the solid-liquid transition, e.g. liquid crystal polymers)
Volume
Volume
Volume
Tg
Amorphous
Temp
Tm
Crystalline
Temp
Tg
Tm
Temp
Semicrystalline
Creep – Progressive strain over time at constant stress (polymers, esp. those above
Tg, and metals at temps high w.r.t. melting)
Stress Relaxation – Progressive loss of tension over time at constant deflection
(polymer bolts, guitar strings)
Rules of Thumb
• Try not to have polymers under constant load
• Use glass-filled grades for load bearing
Quiz 7&8 Review
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Crystallinity
• No polymers are fully crystalline
• Many thermoplastics are amorphous (Lexan, Plexiglass, polystyrene)
• Some thermoplastics are partly crystalline (nylon, acetal, PET)
• 3-D order comes from chain folding and alignment
• Crystalline regions alternate with amorphous regions
• Thermosets are never crystalline
Elastomer Behavior
• Large deformation from chain uncoiling
• No chain sliding due to light cross linking (keep the same neighbor chains)
• Full elastic recovery due to entropy (chains are at lowest energy when
randomly coiled, and will spontaneously return to that approximate amount of
coiling if sufficient thermal energy is available for random chain motion.)
• There will be a difference between the elastic load and unload curve on a
stress-strain diagram. This hysteresis represents the energy lost in the
process (damping).
Manufacturing
In general, polymers can be easily formed into shape with few secondary
heat-treating or finishing steps. This can lead to a relatively inexpensive
part despite relatively high material cost.
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Compression Molding – Heat and pressure in a die to form and cure a
thermoset (electrical switchplates,
Injection Molding – Hot plastic (usually thermoplastic) forced into a mold
which can have significant complexity. (Process is similar to die casting of
metals) (toys, gears, phone housings)
Extrusion – Hot plastic (usually thermoplastic) is forced through a die to
make parts with constant cross section
Blowmolding - Inflating a parison to make bottles (bottles, gas cans, gas
tanks)
Types of Polymers
See Table 16.3
Quiz 7&8 Review
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