Polymer Structures

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CHAPTER 14 Polymer Structures
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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
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POLYMERISATION
Mers small structural entities making up the
polymer.
CAN BE CONVERTED TO POLYETHYLENE(PE) Solid
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RESULT
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Similarly
Unpaired electron
Methyl group
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MOLECULAR WEIGHT
Number average
Not all chains same length
Weight-average
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For copolymers
Degree of polymerization
Molecular weight of mer j
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Branched
Linear
e.g. Polyethylene, Nylon
Lower density
Crosslinked
Network
e.g. Rubber
e.g. Epoxy
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HOMOPOLYMERS
COPOLYMERS
Polymers composed of two or more different mer
units
Bifunctional, trifunctional mers
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Styrene-butadiene rubber (SBR) random
copolymer Used in automobile tires.
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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.
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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
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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