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: C36H74
An image of a single crystal alkane
System: Alkane, Composition: C294H590
Single PE spherulite AFM
Thermodynamics of melting and crystallization:
First order transitions
Amorphous v Crystalline Polymers Thermo-mechanical properties
Material
Density (kg/m3)
Low density polyethylene (LDPE)
915-929
Medium density polyethylene (MDPE)
930-939
High density polyethylene (HDPE)
940-965
% Crystallinity
45-65
65-75
75-90
Property
Shrinkage, Stiffness, Tensile strength,
Hardness, Heat deflection, Chemical
resistance
Weatherability
Impact strength, Ductility
Density Increase
Thermal Transition Points of Select Polymers
Rule of Thumb for Tg’s and Tm’s
For symmetrical polymers: Tg = 0.5 Tm (Kelvin)
Polyvinylidene chloride Tg = -18 + 273 = 255 K
Tm = Tg/0.50 = 255/0.5 = 510 K or 237°C
Experimentally Tm = 200 °C
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
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
Van der Waals:
2 kJ/mole
Hydrogen bonding
20 kJ/mol
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 nalkanes (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
Crystalline structures in polymers
TEM of spherulite structure in natural rubber(x30,000).
• Chain-folded lamellar crystallites (white lines) ~10nm thick extend radially.
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.
Tensile Response: Brittle & Plastic
Stress-strain curves adapted from Fig. 15.1, Callister 6e. Inset figures along plastic response curve
(purple) adapted from Fig. 15.12, Callister 6e.
Amorphous polymer properties do not depend on cooling rate.
Semicrystalline polymer properties depend on final degree of
crystallinity, and hence the rate of cooling.
Achieved using
slower cooling rates.
Higher % S-Cryst
E
Lower % S-Cryst
Cooling rates for
semi-crystallines are
important!
Amorphous
Tg
Temperature
Tg
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
Quenching
30
20
10
Slow Cooling
Crystallinity (%)
40
0
0.01
0.1
1.0
Cooling rate (oC/s)
10
100
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 distances
Stronger intermolecular forces