Week 8
Polymers
Callister Chapter 15.1-15.21
You Should Know/Be Able to:
 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.?
 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 applications for common polymers listed in Table 15.3 from the
text. Examples: ABS is used in legos because it is tough, and polycarbonate’s toughness
and amorphous structure (transparent) makes it useful as bullet-proof glass. Nylon
(polyamide) and acetal (polyoxymethylene, POM) are both engineering polymers with
sufficient strength and stiffness for use in load bearing products such as gears, but
Nylon’s hygroscopic nature is problem when used in humid or wet environments.
 Describe and explain with the in-class hands-on experiments with polymers, especially
the relations between structure and properties.
 Name and explain the two polymerization reactions that occur in polymer synthesis.
Vocabulary
Chapt 15
Addition polymerization
Condensation polymerization
Drawing
Elastomer
Fiber
Glass transition temperature
Melting temperature
Plasticizer
Relaxation modulus
Viscoelasticity
Polymer Synthesis
Addition polymerization:
A process by which monomers are attached one at a time in a chainlike fashion. The
chemical composition of the ‘mer’ is the same before and after the reaction. Example:
polyethylene below
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
C
C
C
H H
C C
H
H
H
H H
H
R
H
H
H
H
C
H
or
H
C
C
H
H
Condensation polymerization:
A step-wise intermolecular chemical reaction between 1 or more types of ‘mers,’ where a
small molecule (like water) is often a byproduct. The composition of the reactants is
different before and after the reaction.
n
Week 8
Polymers
Callister Chapter 15.1-15.21
Response to Stress
Thermoplastic Behavior
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.
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).
Effects on Thermoplastic Behavior
Effect of:
on Properties
Example
Drawing
Chains align in amorphous regions,
Fibers (nylon, Kevlar)
small packets of crystallites separate
and align. Further uncoiling is difficult.
Annealing
Heating a semicrystalline polymer can
organize, improve crystals
Heating a drawn polymer can
disorganize, weaken
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.
Types of Polymers
See Table 15.3
Week 8
Polymers
Callister Chapter 15.1-15.21
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