Date post: | 15-Jan-2016 |
Category: |
Documents |
View: | 217 times |
Download: | 0 times |
ME 260: Introduction to Engineering Materials
Chapter 16. Characterization, applications 15.1 and processing of Polymers
CHAPTER 15CHAPTER 15
Characterization, applications and processing of Polymers
ME 260: Introduction to Engineering Materials
Chapter 16. Characterization, applications 15.2 and processing of Polymers
Mechanical Characteristics of Polymers v.sensitive to
TEMPERATURE STRAIN RATE ENVIRONMENT (presence of water, O2, solvents)
Modulus of Elasticity =Tensile modulus = modulus
Stress at which fracture occurs
For polymers: 7 MPa- 4GPaMetals: 48-410GPa
ME 260: Introduction to Engineering Materials
Chapter 16. Characterization, applications 15.3 and processing of Polymers
Three typical types of stress strain curves are found for polymers
Polymers can experience elongations up to 1000%Metals typically 100% maximum.
ME 260: Introduction to Engineering Materials
Chapter 16. Characterization, applications 15.4 and processing of Polymers
Typical values
ME 260: Introduction to Engineering Materials
Chapter 16. Characterization, applications 15.5 and processing of Polymers
Effect of Temperature
Tensile modulusElongationStrength
PMMA
ME 260: Introduction to Engineering Materials
Chapter 16. Characterization, applications 15.6 and processing of Polymers
FACTORS INFLUENCING MECHANICAL PROPERTIES OF POLYMERS
Extensive chain entanglement or significant molecular bonding = increase in modulus and strength
Effect of Molecular weight
TS = TS – A Mn
Tensile strength at infinite molecular weight
Constant
TS increases with increase in molecular weight, WHY????Increasing chain entanglement
ME 260: Introduction to Engineering Materials
Chapter 16. Characterization, applications 15.7 and processing of Polymers
DEGREE OF CRYSTALLINITYImportant since affects extent of intermolecular secondary bondingTensile modulus increases with increase in degree of crystallinityAs crystallinity increases polymer more brittle
ME 260: Introduction to Engineering Materials
Chapter 16. Characterization, applications 15.8 and processing of Polymers
CRYSTALLIZATION
Occurs by nucleation and growth of ordered and aligned chain folded layers from random and tangled molecules in the melt.
ME 260: Introduction to Engineering Materials
Chapter 16. Characterization, applications 15.9 and processing of Polymers
MELTING POINT Tm and GLASS TRANSITION TEMPERATURE Tg
Tm - occurs in crystalline polymersTg - occurs in amorphous and semi-crystalline polymers
ME 260: Introduction to Engineering Materials
Chapter 16. Characterization, applications 15.10 and processing of Polymers
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???
ME 260: Introduction to Engineering Materials
Chapter 16. Characterization, applications 15.11 and processing of Polymers
- C – C -
- C -
H
H H
H
- C – C -
H
H
H
H
H
H
Larger side group
Polyethylene MER
Polypropylene MER
e.g. Polypropylene (Tm = 175 oC), Polyethylene(Tm = 115 oC)
Tg affected in similar way to Tm
ME 260: Introduction to Engineering Materials
Chapter 16. Characterization, applications 15.12 and processing of Polymers
Tg = 0.5-0.8 Tm(in K)
ME 260: Introduction to Engineering Materials
Chapter 16. Characterization, applications 15.13 and processing of Polymers
ME 260: Introduction to Engineering Materials
Chapter 16. Characterization, applications 15.14 and processing of Polymers
THERMOPLASTIC AND THERMOSETTING POLYMERS
Classification according to mechanical response at elevated temperatures
THEMOPLASTIC THEMOSET
Repeatable and reversible processes
Secondary bonding decreases with increasedTemp. due to increased molecular motion
MOST LINEAR & SLIGHTLY BRANCHED POLYMERS
Heating = harden polymer cross-linking
10-50% of mers x-linkedX-lined and network polymerse.g. Epoxy
ME 260: Introduction to Engineering Materials
Chapter 16. Characterization, applications 15.15 and processing of Polymers
POLYMER TYPES
PLASTICS- ELASTOMERS (RUBBERS)- FIBERS, COATINGS, ADHESIVE, FOAMS, FILMS.
If plastic is x-linked and used above Tg = good elastomer
PLASTICS
Majority of polymerse.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