MEE433BMech412

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MEE433B/Mech412

• Polymers and Plastics

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What are they?

• Polymer A solid or liquid, nonmetallic (usually
  organic) compound of high molecular weight
  composed of small repeating mer units.
• Plastic A solid polymeric material of high
  molecular weight - may contain fillers and
  additives to enhance properties.

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Background

• Polymers Carbon based organic compounds
• Natural
• Wood, cotton, leather, silk
• Ships, sails, rope, early aircraft
• Synthetic
• First introduced this century
• Developed into useful engineering materials since
  WWII

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• Natural Polymers
• Cotton aircraft skins, tent fabric
• Animal Proteins adhesives
• Hemp rope
• Natural Rubber belts, tires, hoses, tubing
• Wood countless structural uses
• Synthetic Polymers
• Lightweight (densities abt 1 g/cm3)
• ductile
• easily formed
• corrosion resistant
• cheap

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What are Polymers...

• Organic compounds - usually with carbon backbones
  - and hydrogen (Hydrocarbons)
• Ethylene C2H4

H
H
R

-
R-C
C
H
H
Polyethelene, PE
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Polymer Chains and Mers
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Common Polymer Chains
F
F
F
F
F
F
Polytetraflouroethylene PTFE - Teflon
C
C
C
C
C
C
F
F
F
F
F
F
H
H
H
H
H
H
Polyvinylchloride PVC
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
Polypropylene PP
C
C
C
C
C
C
H
H
H
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Homopolymers and Copolymers

• HomopolymerAll reapeating units along the chain
  are the same.
• Copolymer chain composed of two or more
  different mer units
• Various structures are possible
• Properties are enhanced

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Mer molecular weight

• The sum of the atomic weights for all atoms in
  the mer unit represent the Mer molecular weight,
  m (g/mol)
• Try it! Find the mer molecular wt. for PVC

H
H
H
H
H
H
H
H
H
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Molecular Weight
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Distribution of Molecular Weights
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Degree of Polymerization
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Effect of Molecular Weight

• Melting temperature
• Increases with increasing molecular weight up to
  about 100,000 g/mol.
• 100 g/mol (gas), 1000 g/mol (waxy-solid),
• 10,000 g/mol (solid)
• Strength
• Increases with increasing molecular weight
• Ductility
• Tends to decrease with increasing molecular weight

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Polymer Structure

• Thermoplastics
• Chain-like molecular structure
• Carbon backbone with strong bonds, week
  VanderWalls bonds between chains.
• Soften with tempearture
• Have a melting point
• Thermosets
• Have a 3-D covalent-bonded structure
• Do not soften with temperature
• Char rather than melt

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Polymer Structures
Branched
Linear
Networked
Crosslinked