crystallinity in polymers
DESCRIPTION
Crystallinity in Polymers. Maltese cross spherulites. Sheaf-like arrangement of lamellae in a blend of polyethylenes System: Polyethylene (PE), Composition: LPE:BPE 3:1. An image of an alkane crystal taken by AFM System: Alkane, Composition: C 36 H 74. An image of a single crystal alkane - PowerPoint PPT PresentationTRANSCRIPT
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
Thermodynamics of melting and crystallization: First order transitions
Amorphous v Crystalline Polymers Thermo-mechanical properties
Density Increase
Prop
erty
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
Thermal Transition Points of Select Polymers
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
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
Crystalline Polymers (really semicrystalline)
Polar functionality
Thermodynamic of Crystallization
For melting Sf is positive
Intramolecular interactions (Hf) favor crystallization & higher Tm
Hydrogen bonding20 kJ/mol
Van der Waals: 2 kJ/mole
Explain why Nylon 6 has a lower Tm than Kevlar
Entropic Contributions to Tm
Flexible Chains have numerous conformations
Nylon 6
Rigid Chains have fewer conformations
Kevlar example
Polymer symmetry and Melting Point
Molecular Weight Influence on Tm
• Melting temperatures of n-alkanes (up to C100) as a function of chain length.
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
Characterization of Crystalline Polymers: Diffraction
Rare to get single crystals: Powder XRD or films
Polyethylene’s Orthorhombic Unit cell
Vinyl Polymer Crystals: Substituents favor helical conformation
Characterization of Crystallinity in Polymers
Polymers generally have crystalline and amorphous contributions
Lamellar Structure of Polymer crystals
Polymer single crystals: Graduate students nightmare
Still lamellar structures
Validation of Models
Dislocations in Polymer Crystals
From singhle crystals to Aggregate structures
Polyethylene Spherulites
Spherulite Growth from Lamellar crystals
TEM of spherulite structure in natural rubber(x30,000).• Chain-folded lamellar crystallites (white lines) ~10nm thick extend radially.
Crystalline structures in polymers
crystallineregionamorphousregion
• % Crystallinity: % of material that is crystalline. --TS and E often increase with % crystallinity. --Annealing causes crystalline regions to grow. % crystallinity increases.
0unload/reload0brittle failureplastic failure204060246σ(MPa)εxx σεmi-cryσtallinε
caσε
amorphouσrεgionσεlongatε
cryσtallinεrεgionσ aligncryσtallinεrεgionσσlidε
8onσεt ofnεckingalignεd,croσσ-linkεdcaσε
nεtworkεdcaσε
InitialNεar Failurεnεarfailurε
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
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.
Micrographs of Polymer Spherultes
Seeing Maltese Crosses: Polarizing Microscopy
Polarizing Optical Microscopy
Formation of Ring Pattern: Lamellar Twisting
Microfibriallar Morphology
Polyethylene Fibers Nucleated on Si-C fibers: Shish-Kebobs
Branching on Crystallinity
Which one will be more likely to crystallize?
Linear crystallizes easier (HDPE = linear; LDPE = branched)
Nucleation Rates between Tg and Tm
Primary Crystallization
Cry
stal
linity
(%)
Cooling rate (oC/s)
10
20
30
40
00.01 0.1 1.0 10 100
Slow Cooling
Quenchin
g
Early stages of crystallation of PEEK in the presence of a carbon fibre.
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