morfologi polimer 1. chemical structure of polymer has profound effect on physical properties of...
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Morfologi Polimer
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• Chemical structure of polymer has profound effect on physical properties of polymer i.e. strength, durability, transparency, heat resistance, weather resistance, spinnability, resilience and flammability.
• Polymer properties make polymer is commercially produced.
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Tacticity: How groups are arranged along polymer
R R R R R R R R R RR
isotactic
R R R R R R R R R RR
syndiotactic
Vinyl Monomers
R R R R R R R R R RR
atactic
1. Stereochemistry
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Tacticity: disubstituted monomers
R R R R R R R R R R
R R R R R R R R RR
R
R R R R R R R R R R
R R R R R R R R R
RR
isotactic
syndiotactic
H
Polymer
R
H
Polymer
R
Looksing at indicated atoms from the left
H
Polymer
R
R
Polymer
H
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2. Glass transition temperature (Tg)
The glass transition temperature (Tg) describes the temperature at which amorphous polymers undergo a second order phase transition from a rubbery, viscous amorphous solid (fresh spaghetti) to a brittle, glassy amorphous solid (3 days old Spaghetti)
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Glass-rubber-liquid
Amorphous plastics have a complex thermal profile with 3 typical states:
Log(stiffness)Pa
Temperature
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9
6
7
8
4
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Glass phase (hard plastic)
Rubber phase (elastomer)
Liquid
Leathery phase
Polystyrene
Tg
Tygon (plasticized PVC)
PDMS
polyisobutylene
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Properties Affected :• Specific Volume / Density• Specific Heat, Cp• Refractive Index• Modulus• Dielectric Constant• Permeability
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• Rigid group in backbone
• Flexible polymer backbone
• Steric Hinderance
• Long plasticizing side groups
• Symmetrical substituents
• Polar functionalities
• Plasticizers
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Factors influencing Tg
1. Chain flexibility• Silicone > Ether > Hydrocarbon > Cyclic HC
> Aromatics
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– Tg = -55 deg.C 88 deg.C•
2. Steric Bulk of Substituents
Tg = -120C 5C -24C -50C
Long side chains may act as plasticizers (C 6)
O O
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3. Molecular Symmetry Asymmetry increases chain stiffness. 4. Polar Interactions increase Tg Hydrogen bonding5. Molecular Weight up to Critical Limit6. Crosslinking Reduces Segment Mobility
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3. Crystallinity in Polymers
Sheaf-like arrangement of lamellae in a blend of polyethylenesSystem: Polyethylene (PE), Composition: LPE:BPE 3:1
An image of an alkane crystal taken by AFMSystem: Alkane, Composition: C36H74
An image of a single crystal alkaneSystem: Alkane, Composition: C294H590
Single PE spherulite AFM
Maltese cross spherulites
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natural rubber(x30,000).• Chain-folded lamellar crystallites (white lines) ~10nm thick extend radially.
Crystalline structures in polymers
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crystalline region
amorphous region
• % Crystallinity: % of material that is crystalline. --TS and E often increase with % crystallinity. --Annealing causes crystalline regions to grow. % crystallinity increases.
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Intramolecular interactions (Hf) favor crystallization & higher Tm
Hydrogen bonding20 kJ/mol
Van der Waals: 2 kJ/mole
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Molecular weight influence on Tm
• Melting temperatures of n-alkanes (up to C100) as a function of chain length.
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Methods for Inducing Crystallization in Polymers
• Slow cooling of molten polymer• Annealing between Tg and Tm
• Evaporation of solvent• Shear & disintanglement• Stretching and alignment of macromolecules
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Branching on Crystallinity
Which one will be more likely to crystallize?
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Linear crystallizes easier (HDPE = linear; LDPE = branched)
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Nucleation Rates between Tg and Tm
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Primary Crystallization
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Cry
sta
llin
ity (
%)
Cooling rate (oC/s)
10
20
30
40
00.01 0.1 1.0 10 100
Slo
w C
oolin
g
Quench
ing
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Early stages of crystallation of PEEK in the presence of a carbon fibre.
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Effects of Crystallinity
1) Strength: Stronger & Stiffer
2) Optical: Opaque (scattering by spherulites)
3) Higher density
4) Less Soluble
5) Less Permeable