crystallinity in polymers sheaf-like arrangement of lamellae in a blend of polyethylenes system:...
TRANSCRIPT
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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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Thermodynamics of melting and crystallization: First order transitions
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Amorphous v Crystalline Polymers Thermo-mechanical properties
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Density Increase
Pro
pert
y
Shrinkage, Stiffness, Tensile strength, Hardness, Heat deflection, Chemical resistance
Weatherability
Impact strength, Ductility
Low density polyethylene (LDPE) 915-929 45-65Medium density polyethylene (MDPE) 930-939 65-75High density polyethylene (HDPE) 940-965 75-90
Material Density (kg/m3) % Crystallinity
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Thermal Transition Points of Select Polymers
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Rule of Thumb for Tg’s and Tm’s
For symmetrical polymers: Tg = 0.5 Tm (Kelvin)
For asymmetrical polymers: Tg = 0.66 Tm (Kelvin)
Polyvinyl chloride Tg = 81 + 273 = 377 K
Tm = Tg/0.66 = 354/0.66 = 536 K or 263°C
Experimentally Tm = 273 °C
Polyvinylidene chloride Tg = -18 + 273 = 255 K
Tm = Tg/0.50 = 255/0.5 = 510 K or 237°C
Experimentally Tm = 200 °C
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Rule of Thumb for Tg’s and Tm’s
Caution: Its just a rule of thumb:
Atactic polystyrene Tg = 104 + 273 = 377 K
Tm = Tg/0.66 = 377/0.66 = 571 K or 298 °C
Experimentally Tm = 523 K or 250 °C
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Crystalline Polymers (really semicrystalline)
Polar functionality
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Thermodynamic of Crystallization
For melting Sf is positive
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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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Explain why Nylon 6 has a lower Tm than Kevlar
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Entropic Contributions to Tm
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Flexible Chains have numerous conformations
Nylon 6
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Rigid Chains have fewer conformations
Kevlar example
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Polymer symmetry and Melting Point
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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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Characterization of Crystalline Polymers: Diffraction
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Rare to get single crystals: Powder XRD or films
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Polyethylene’s Orthorhombic Unit cell
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Vinyl Polymer Crystals: Substituents favor helical conformation
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Characterization of Crystallinity in Polymers
Polymers generally have crystalline and amorphous contributions
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Lamellar Structure of Polymer crystals
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Polymer single crystals: Graduate students nightmare
Still lamellar structures
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Validation of Models
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Dislocations in Polymer Crystals
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From singhle crystals to Aggregate structures
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Polyethylene Spherulites
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Spherulite Growth from Lamellar crystals
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TEM of spherulite structure in natural rubber(x30,000).• Chain-folded lamellar crystallites (white lines) ~10nm thick extend radially.
Crystalline structures in polymers
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crystallineregionamorphousregion
• % 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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0unload/reload0brittle failureplastic failure204060246σ( )MPaεxx -semicrystalline
case
amorphousregionselongate
crystalline regions align
crystallineregions
slide
8 onset ofnecking
,aligned-cross
linkedcase
networkedcase
Initial Near Failurenearfailure
Stress-strain curves adapted from Fig. 15.1, Callister 6e. Inset figures along plastic response curve (purple) adapted from Fig. 15.12, Callister 6e.
Tensile Response: Brittle & Plastic
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TemperatureTg Tg
EHigher % S-Cryst
Cooling rates for semi-crystallines are important!
Amorphous polymer properties do not depend on cooling rate.
Amorphous
Semicrystalline polymer properties depend on final degree of crystallinity, and hence the rate of cooling.
Lower % S-Cryst
Achieved using slower cooling rates.
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Micrographs of Polymer Spherultes
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Seeing Maltese Crosses: Polarizing Microscopy
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Polarizing Optical Microscopy
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Formation of Ring Pattern: Lamellar Twisting
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Microfibriallar Morphology
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Polyethylene Fibers Nucleated on Si-C fibers: Shish-Kebobs
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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
Slow Cooling
Quenchin
g
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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
Smaller interchain distancesStronger intermolecular forces