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Crystallinity in Polymers

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

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Crystallinity in Polymers

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  1. 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

  2. Thermodynamics of melting and crystallization: First order transitions

  3. Amorphous v Crystalline Polymers Thermo-mechanical properties

  4. Material Density (kg/m3) % Crystallinity Low density polyethylene (LDPE) 915-929 45-65 Medium density polyethylene (MDPE) 930-939 65-75 High density polyethylene (HDPE) 940-965 75-90 Shrinkage, Stiffness, Tensile strength, Hardness, Heat deflection, Chemical resistance Property Weatherability Impact strength, Ductility Density Increase

  5. Thermal Transition Points of Select Polymers

  6. 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

  7. 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

  8. Crystalline Polymers (really semicrystalline) Polar functionality

  9. Thermodynamic of Crystallization For melting Sf is positive

  10. Intramolecular interactions (Hf) favor crystallization & higher Tm Van der Waals: 2 kJ/mole Hydrogen bonding 20 kJ/mol

  11. Explain why Nylon 6 has a lower Tm than Kevlar

  12. Entropic Contributions to Tm

  13. Flexible Chains have numerous conformations Nylon 6

  14. Rigid Chains have fewer conformations Kevlar example

  15. Polymer symmetry and Melting Point

  16. Molecular Weight Influence on Tm • Melting temperatures of n-alkanes (up to C100) as a function of chain length.

  17. 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

  18. Characterization of Crystalline Polymers: Diffraction

  19. Rare to get single crystals: Powder XRD or films

  20. Polyethylene’s Orthorhombic Unit cell

  21. Vinyl Polymer Crystals: Substituents favor helical conformation

  22. Characterization of Crystallinity in Polymers Polymers generally have crystalline and amorphous contributions

  23. Lamellar Structure of Polymer crystals

  24. Polymer single crystals: Graduate students nightmare Still lamellar structures

  25. Validation of Models

  26. Dislocations in Polymer Crystals

  27. From singhle crystals to Aggregate structures

  28. Polyethylene Spherulites

  29. Spherulite Growth from Lamellar crystals

  30. Crystalline structures in polymers • TEM of spherulite structure in natural rubber(x30,000). • Chain-folded lamellar crystallites (white lines) ~10nm thick extend radially.

  31. • % Crystallinity: % of material that is crystalline. --TS and E often increase with % crystallinity. --Annealing causes crystalline regions to grow. % crystallinity increases.

  32. 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.

  33. 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 Amorphous Tg Tg Temperature Cooling rates for semi-crystallines are important!

  34. Micrographs of Polymer Spherultes

  35. Seeing Maltese Crosses: Polarizing Microscopy

  36. Polarizing Optical Microscopy

  37. Formation of Ring Pattern: Lamellar Twisting

  38. Microfibriallar Morphology

  39. Polyethylene Fibers Nucleated on Si-C fibers: Shish-Kebobs

  40. Branching on Crystallinity Which one will be more likely to crystallize?

  41. Linear crystallizes easier (HDPE = linear; LDPE = branched)

  42. Nucleation Rates between Tg and Tm

  43. Primary Crystallization

  44. 40 Quenching 30 Slow Cooling 20 Crystallinity (%) 10 0 100 0.01 0.1 1.0 10 Cooling rate (oC/s)

  45. Early stages of crystallation of PEEK in the presence of a carbon fibre.

  46. Effects of Crystallinity Strength: Stronger & Stiffer Optical: Opaque (scattering by spherulites) Higher density Less Soluble Less Permeable Smaller interchain distances Stronger intermolecular forces

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