Transcript Document
ME 260: Introduction to Engineering Materials
CHAPTER 14
Polymer Structures
Chapter 15. Polymer Structures
14.1
ME 260: Introduction to Engineering Materials
INTRODUCTION
Natural Polymers Leather, wood, rubber, cellulose, cotton, wool.
Synthetic polymers Synthesized from small organic molecules.
CHEMISTRY OF POLYMER MOLECULES
Hydrocarbons (many organic materials)= composed of hydrogen and carbon
Covalent Bonding
H
H-C-H
Similarly
H
Methane
Ethylene
Chapter 15. Polymer Structures
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ME 260: Introduction to Engineering Materials
POLYMERISATION
Mers = small structural entities making up the ‘poly’mer.
Ethylene(C2H4) gas
CAN BE CONVERTED TO POLYETHYLENE(PE) Solid
Catalyst
bonding unsatisfied for this C atom
bonding satisfied for C atom
Chapter 15. Polymer Structures
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ME 260: Introduction to Engineering Materials
RESULT
Angle =109o
Chapter 15. Polymer Structures
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ME 260: Introduction to Engineering Materials
Similarly
Unpaired electron
Methyl group
Chapter 15. Polymer Structures
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ME 260: Introduction to Engineering Materials
Chapter 15. Polymer Structures
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ME 260: Introduction to Engineering Materials
MOLECULAR WEIGHT
Number average
Not all chains same length
Weight-average
Chapter 15. Polymer Structures
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ME 260: Introduction to Engineering Materials
For copolymers
Degree of polymerization
Molecular weight of mer j
Chapter 15. Polymer Structures
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ME 260: Introduction to Engineering Materials
Linear e.g. Polyethylene,
Nylon
Branched
Lower density
Crosslinked
Network
e.g. Rubber
e.g. Epoxy
Chapter 15. Polymer Structures
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ME 260: Introduction to Engineering Materials
HOMOPOLYMERS
COPOLYMERS
Polymers composed of two
or more different ‘mer’ units
Bifunctional, trifunctional mers
Chapter 15. Polymer Structures
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ME 260: Introduction to Engineering Materials
Styrene-butadiene rubber (SBR) random copolymer
Used in automobile tires.
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ME 260: Introduction to Engineering Materials
POLYMER CRYSTALLINITY
= Packing of molecular chains so as to produce an ordered atomic array.
Small molecules (Methane, H2O) either either totally crystalline (as solids)
or amorphous (as liquids)
However POLYMERS only partially crystalline (semi-crystalline)
Max. crystallinity ~ 95%.
Chapter 15. Polymer Structures
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ME 260: Introduction to Engineering Materials
Properties greatly affected by degree of crystallinity
(crystalline = stronger and more resistant to
softening by heat.
important to quantify degree crystallinity
Crystalline polymer = higher density
Chapter 15. Polymer Structures
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ME 260: Introduction to Engineering Materials
Slower cooling rates from the melt favor higher crytallinity
(giving time for ordered configurations)
Linear Polymers Crystallization easily accomplished, since no restriction to
chain alignment
Branched polymers NEVER highly crystalline
Side branches interfere with crystallization
Network Polymers AMORPHOUS
COPOLYMERS
More Irregular and random mer less crystallinity
Alternating and block copolymers likelihood of crystallization.
Random and Graft copolymers Normally Amorphous
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