Polymer III

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Polymer III

• Polymer Failure Environmental Effects

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Molecular Motions

• Vibration of atoms about equilibrium
• Motion of a few atoms or rotation/vibration of
  side groups
• Cooperative movement of segments (40-50 atoms
  long).
• Translational motion of entire molecules.

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Deformation

• Two Processes in Thermoplastics
• Crazing (amorphous)
• b. Shear Yielding or Shear Flow

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Deformation Processes

• Crazing
• Amorphous
• Volume increase
• Extremely small
• Solvents assist

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Craze Process Zone in Fracture
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Deformation Process

• Shear Yielding or Shear Flow
• Shear stress induced
• Translation of molecules
• Largely irreversible
• Absorbs lots of energy

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Shear Yielding in Semicrystalline

• Semicrystalline yield is not reversible
• Further yield is possible in which remaining
  crystallites also break up.

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Molecular Orientation

• Failure process depends on molecular orientation
• Generally more orientation greater toughness

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Fracture in Polymers

• Polymers will undergo slow stable crack growth
  (ie. KapplltK1C)
• Fracture process zone dominated by either crazing
  or shear yielding.
• Surface morphology can be complex

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Fracture Process Zone PE
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PE Fracture Surface at Different Strain Rates
A
B
C
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Crack Tip in Polycarbonate

• Very tough amorphous polymer
• Shear yield dominates
• Shear Bands ahead of crack tip

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Thermoset Deformation Process

• 1. Crazing suppressed
• 3-D polymer network prevents alignment
• Free segments too short
• 2. Shear Yielding only option
• a. Molecular translation restricted by network
• b. High defect sensitivity prevents high shear
  strains in tension

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Fatigue in Polymers
1. Macroscopically brittle 2. Same mechanisms
(shear yield, crazing) 3. Viscoelasticity
complicates picture
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Toughness in Polymers

• Toughness enhanced by improved molecular mobility
  (plasticizers, temperature, free volume, lower
  crystallinity)
• Second phase toughening common (rubber, TP in TS)
• Most fillers reduce toughness

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Effect of 2nd Phase Tougheners
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Environmental Effects on Polymers

• Permeable to many gases H2O vapor

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Environmental Effects on Polymers

• Permeable to many gases H2O vapor
• Inorganic Acids Bases fairly resistant

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Environmental Effects on Polymers
3. Organic Solvents can be very
sensitive -Crosslinks, crystallinity improves
resistance -Polarity of polymer solvent
important -Solvents swell, dissolve, crack
polymers
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Environmental Effects on Polymers

• 4. Moisture absorption
• -Polar polymers (PC, Nylon)
• -lowers modulus and yield strength
• -problem in processing

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Moisture Absorption in Delrin
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Environmental Effects on Polymers
5. UV Visible light -Chain scission -UV
stabilizers may be added
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Environmental Effects on Polymers

• 6. Heat
• -Chemical Aging
• -Physical Aging