Transcript Document

ME 260: Introduction to Engineering Materials
CHAPTER 15
Characterization, applications
and processing of Polymers
Chapter 16. Characterization, applications
15.1
ME 260: Introduction to Engineering Materials
Mechanical Characteristics of Polymers v.sensitive to
TEMPERATURE
STRAIN RATE
ENVIRONMENT
(presence of water, O2, solvents)
Stress at which fracture occurs
Modulus of Elasticity =
Tensile modulus = modulus
For polymers: 7 MPa- 4GPa
Metals: 48-410GPa
Chapter 16. Characterization, applications
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ME 260: Introduction to Engineering Materials
Three typical types of stress strain curves are found for polymers
Polymers can experience elongations
up to 1000%
Metals typically 100% maximum.
Chapter 16. Characterization, applications
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ME 260: Introduction to Engineering Materials
Typical values
Chapter 16. Characterization, applications
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ME 260: Introduction to Engineering Materials
Effect of Temperature
Tensile modulus
Elongation
Strength
Chapter 16. Characterization, applications
PMMA
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ME 260: Introduction to Engineering Materials
FACTORS INFLUENCING MECHANICAL PROPERTIES OF POLYMERS
Extensive chain entanglement or significant molecular bonding
= increase in modulus and strength
Effect of Molecular weight
Constant
TS = TS – A
Mn
Tensile strength at infinite molecular weight
TS increases with increase in molecular weight, WHY????
Increasing chain entanglement
Chapter 16. Characterization, applications
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ME 260: Introduction to Engineering Materials
DEGREE OF CRYSTALLINITY
Important since affects extent of intermolecular secondary bonding
Tensile modulus increases with increase in degree of crystallinity
As crystallinity increases polymer more brittle
Chapter 16. Characterization, applications
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ME 260: Introduction to Engineering Materials
CRYSTALLIZATION
Occurs by nucleation and growth of ordered and aligned chain
folded layers from random and tangled molecules in the melt.
Chapter 16. Characterization, applications
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ME 260: Introduction to Engineering Materials
MELTING POINT Tm and GLASS TRANSITION TEMPERATURE Tg
Tm - occurs in crystalline polymers
Tg - occurs in amorphous and semi-crystalline polymers
Chapter 16. Characterization, applications
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ME 260: Introduction to Engineering Materials
MELTING POINT
From ordered to disordered state.
Requires rotation and movement of ordered molecules
CHAIN STIFFNESS
Controlled by ease of rotation about chemical bonds along he chain
Double bond chains and aromatic groups  reduces chain flexibility
 increases Tm
Size and type of side group  affect chain rotational freedom and flexibility
Tm increases with increase in Molecular weight (chain length).
Why is there a range of Tm rather than a single temperature???
Chapter 16. Characterization, applications
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ME 260: Introduction to Engineering Materials
e.g. Polypropylene (Tm = 175 oC), Polyethylene(Tm = 115 oC)
Polyethylene MER
H H
Polypropylene MER
H
H
- C–C-
- C – C-
H H
H- C - H H
H
Larger side group
Tg affected in similar way to Tm
Chapter 16. Characterization, applications
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ME 260: Introduction to Engineering Materials
Tg = 0.5-0.8 Tm(in K)
Chapter 16. Characterization, applications
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ME 260: Introduction to Engineering Materials
Chapter 16. Characterization, applications
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ME 260: Introduction to Engineering Materials
THERMOPLASTIC AND THERMOSETTING POLYMERS
Classification according to mechanical response at elevated temperatures
THEMOPLASTIC
Repeatable and reversible
processes
Secondary bonding decreases with increased
Temp. due to increased molecular motion
MOST LINEAR & SLIGHTLY BRANCHED
POLYMERS
THEMOSET
Heating = harden polymer
 cross-linking
10-50% of mers x-linked
Chapter 16. Characterization, applications
X-lined and network polymers
e.g. Epoxy
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ME 260: Introduction to Engineering Materials
POLYMER TYPES
PLASTICS- ELASTOMERS (RUBBERS)- FIBERS, COATINGS, ADHESIVE,
FOAMS, FILMS.
If plastic is x-linked and used above Tg = good elastomer
PLASTICS
Majority of polymers
e.g. Polyethylene, Polypropylene, PVC, Polystyrene, fluorocarbons, epoxies,
polyesters.
Brittle and flexible, linear , branched, thermoplastic, thermosetting etc.
Fluorocarbons = low coeff. Of friction, extremely resistant o chemical attack
 Used as coating in cookware.
Optical applications = PMMA, polystyrene
Chapter 16. Characterization, applications
15.15