Crystallinity in Polymers 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 System: Alkane, Composition: C 294 H 590 Single PE spherulite AFM Maltese cross spherulites

Thermodynamics of melting and crystallization: First order transitions

Amorphous v Crystalline Polymers Thermo-mechanical properties

Density Increase Property Shrinkage, Stiffness, Tensile strength, Hardness, Heat deflection, Chemical resistance Weatherability Impact strength, Ductility Low density polyethylene(LDPE) Medium density polyethylene (MDPE) High density polyethylene(HDPE) Material Density (kg/m3) % Crystallinity

Thermal Transition Points of Select Polymers

Rule of Thumb for T g ’s and T m ’s For symmetrical polymers: T g = 0.5 T m (Kelvin) For asymmetrical polymers: T g = 0.66 T m (Kelvin) Polyvinyl chloride T g = = 377 K T m = Tg/0.66 = 354/0.66 = 536 K or 263°C Experimentally T m = 273 °C Polyvinylidene chloride T g = = 255 K T m = Tg/0.50 = 255/0.5 = 510 K or 237°C Experimentally T m = 200 °C

Rule of Thumb for T g ’s and T m ’s Caution: Its just a rule of thumb: Atactic polystyrene T g = = 377 K T m = Tg/0.66 = 377/0.66 = 571 K or 298 °C Experimentally T m = 523 K or 250 °C

Crystalline Polymers (really semicrystalline) Polar functionality

Thermodynamic of Crystallization For melting  S f is positive

Intramolecular interactions (  H f ) favor crystallization & higher T m Hydrogen bonding 20 kJ/mol Van der Waals: 2 kJ/mole

Explain why Nylon 6 has a lower T m than Kevlar

Entropic Contributions to T m

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 C 100 ) as a function of chain length.

Methods for Inducing Crystallization in Polymers Slow cooling of molten polymer Annealing between T g and T m 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

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

Stress-strain curves adapted from Fig. 15.1, Callister 6e. Inset figures along plastic response curve (purple) adapted from Fig , Callister 6e. Tensile Response: Brittle & Plastic

Temperature TgTg TgTg E Higher % 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

Crystallinity (%) Cooling rate ( o C/s) Slow Cooling Quenching

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