Mechanical Behavior of Polymers
Material Sciences and Engineering MatE271
Week 13
1
Goals for this unit w
Recognize different types of polymers (Ch. 13)
w
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
Material Sciences and Engineering
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Week 13
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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
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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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Week 13
6
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 ) Material Sciences and Engineering
MatE271
Week 13
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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) Material Sciences and Engineering
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Week 13
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
MatE271
Week 13
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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 Material Sciences and Engineering
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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 Material Sciences and Engineering
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Week 13
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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
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Week 13
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Stress
Tensile stress–strain behavior
Strain
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7
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
Material Sciences and Engineering
MatE271
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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Week 13
16
8
Summary w Response of different polymers to temperature w
Mechanical performance dependence on - Molecular wt., temperature, loading rate
- Read Class notes and relevant portions of Shackelford, 2001
Material Sciences and Engineering
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Week 13
17
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