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POLYMER CRYSTALS 1. Different from metals and ceramics 2. Two step process

POLYMER CRYSTALS 1. Different from metals and ceramics 2. Two step process 3. Must decide on shape of polymer chain first 4. Then pack the chains together to form crystal. POLYMER CRYSTAL Starting point is a random coil Straighten out chain before crystallization can occur. ETHANE MOLECULE.

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POLYMER CRYSTALS 1. Different from metals and ceramics 2. Two step process

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  1. POLYMER CRYSTALS 1. Different from metals and ceramics 2. Two step process 3. Must decide on shape of polymer chain first 4. Then pack the chains together to form crystal

  2. POLYMER CRYSTAL Starting point is a random coil Straighten out chain before crystallization can occur

  3. ETHANE MOLECULE Energetics of rotation Staggered has lowest energy Eclipsed has highest energy Caused by interactions between hydrogen atoms

  4. POLYETHYLENE MOLECULE Situation complicated by interactions between CH2 groups and hydrogen atoms

  5. ROTATION ABOUT C-C BOND THREE LOW ENERGY POSITIONS AS IN ETHANE BUT MINIMA HAVE DIFFERENT VALUES

  6. TRANS vs GAUCHE gauche position corresponds to 120o rotation away from linear chain, responsible for random coil generation

  7. POLYETHYLENE UNIT CELL Need all TRANS section of chain for crystal Cross-section of chain is similar to ellipsoid Results in alternating orientations along a line Rules of crystallography generate the cell shown

  8. VINYL POLYMERS (e.g. polypropylene) ADD SIDE GROUPS TO CHAIN Xs on meso diad generate high energy in trans state

  9. ISOTACTIC STATE has all meso diads Cannot exist in all trans as shown Chain must twist into gauche state for lowest energy Results in helix as does so in regular manner

  10. ISOTACTIC POLYMERS e.g polypropylene (IPP) all have this type of lowest energy state helices pack together to form crystal unit cell

  11. SYNDIOTACTIC STATE side groups point away from each other exist in trans state (e.g. SPP)

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