ME 260: Introduction to Engineering Materials Chapter 16. Characterization, applications 15.1 and...

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



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



Three typical types of stress strain curves are found for polymers

Polymers can experience elongations up to 1000%Metals typically 100% maximum.



Typical values



Effect of Temperature

Tensile modulusElongationStrength

PMMA



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



DEGREE OF CRYSTALLINITYImportant since affects extent of intermolecular secondary bondingTensile modulus increases with increase in degree of crystallinityAs crystallinity increases polymer more brittle



CRYSTALLIZATION

Occurs by nucleation and growth of ordered and aligned chain folded layers from random and tangled molecules in the melt.



MELTING POINT Tm and GLASS TRANSITION TEMPERATURE Tg

Tm - occurs in crystalline polymersTg - occurs in amorphous and semi-crystalline 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???



- 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



Tg = 0.5-0.8 Tm(in K)



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



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

ME 260: Introduction to Engineering Materials Chapter 16. Characterization, applications 15.1 and...

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