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


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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.

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

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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 CH2 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 120o 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)


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