Mechanical Behavior of
Polymers
Material Sciences and Engineering
MatE271
Week 13
1
Goals for this unit
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Recognize different types of polymers (Ch. 13)
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Understand the mechanical characteristics
Why design with plastics?
- Lightweight, resilient, Corrosion resistant
- Transparent, color fastness, ease of processing
- Favorable balance of properties
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Week 13
2
1
What is Polymer?
- From the Greek polus (many), and meros (parts)
- Contain C H O Cl N S - Natural & Synthetic
- About 80 commercial SYNTHETIC polymers
(cf: inorganic polymers)
- 180 including alloys, blends
- 16000 including different grades (polymers
and compounds)
Material Sciences and Engineering
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3
Week 13
Monomer Units in Some Polymers
Mer-unit
Polyethylene
Polypropylene
Polystyrene
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4
2
Degree of Polymerization
• • D.P. = polymer chain length (# repeating units)
– e.g., D.P. (PVC) = 1000
– M(PVC) = M(PVC) = D.P. x Mo = 1000 x 62.5
D.P. is more convenient than MW for polymers
1000 < MW < 1,000,000 (typical values)
10 < n, D.P. < 10,000 (where Mo = 100
Material Sciences and Engineering
MatE271
Week 13
5
Classification of Polymers
- Thermoplastics (Acrylic) soften by heating
- Thermosets (Epoxy) Hardened by heating
- Elastomers (rubber silicon) flexible flexible@low tempts
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3
Molecular Classifications
• NATURAL (silk, wool, DNA, cellulose) vs.
cellulose) vs. SYNTHETIC **
• ARCHITECTURE
– FINITE (thermoplastics)
may be “linear” or "branched" "linear" or
"branched")
– INFINITE ( (thermosets)
usually crosslinked )
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Week 13
7
Polymers vs. Plastics
- Polymer denotes virgin material
- Plastic denotes polymer + additives
Common additives
- Antistatics (alkyl sulphonics)
- Coupling agents
- Fillers and extenders (CaCO3 , silica, & clay)
- Lubricants (wax or soaps)
- Flame retardants (Cl, Br, P, H)
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8
4
Melting and Glass Transition Temperatures
Temperature at which melting (or glass transition)
occurs is determined as it is for glasses:
Specific. Vol.
Specific. vol. vs. T.
Temperature:
Material Sciences and Engineering
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Week 13
9
Polymer Classifications According to
• Rubbers
Mechanical Properties
- Low stiffness, E = 1 – 10 MPa, high εf to fracture
• Semi–crystalline polymers
- Intermediate stiffness, E = 100 - 1000 MPa
Typically flexible & tough
• Glasses
- High stiffness, E = 1 – 10 GPa, typically low εf & brittle
• Fibers
- High stiffness, E = 10 – 100 GPa, typically tough & strong
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Week 13
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5
Tailorable Chemical & Morphological Features
• MW and D.P. - Affects flow properties during processing
and hence ultimate characteristics
• Crosslinking and branching
– Vulcanize rubber to raise mechanical properties
– Hardness & E increase with crosslinking
– Chain branching strongly affects mechanical properties
• Molecular orientation
– Used fibers & biaxial drawn films
– Undesirable in injection molding & extrusion
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Week 13
11
Polymers Mechanical properties
All polymer mechanical properties are
Time and Temperature dependent
Deformation evolves through
chain unfolding and sliding
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Week 13
12
6
Effect of Temperature on Stiffness of Polymers
E
Rigid
Leathery
Rubbery
Viscous
Temperature
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Week 13
14
Stress
Tensile stress–strain behavior
Strain
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Variation of deformation with
time of a material subjected to
constant loading
1. Instantaneous elastic def.
due bond orientations
Strain
Creep behavior (under constant load)
loading unloading
3
2
4
1
2. Delayed elastic deformation (I- creep)
due to segmental motion and chain uncoiling
Time
3. Viscous flow (II- creep) due to molecular slippage
4. Instantaneous elastic recovery due to bond recovery
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15
Week 13
Time and temperature effects on σ−ε behavior
σ
Speed of testing
σ
T>Tg
Temperature
ε
ε
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8
Summary
w Response of different polymers to temperature
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Mechanical performance dependence on
- Molecular wt., temperature, loading rate
- Read Class notes and relevant portions of
Shackelford, 2001
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9