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.

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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 Energetics of rotation Staggered has lowest energy Eclipsed has highest energy Caused by interactions between hydrogen atoms

POLYETHYLENE MOLECULE Situation complicated by interactions between CH 2 groups and hydrogen atoms

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

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

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

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

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

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

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