L10: Polymer Crystallization, Morphology, and Mechanical Properties

Categorization of Polymers

• Polymers can be categorized based on:

  • Geometry: Linear, branched, cross-linked, etc.

  • Position of chemical groups: Atactic, isotactic, syndiotactic.

2. Mechanism of Crystallization

• Crystallization involves nucleation and growth.

• Nucleation: Formation of small crystalline regions.

• Growth: Expansion of these regions into larger crystals.

3. Morphology and Structure of Polymer Crystals

• Common structures include spherulites, lamellae, and fibrils.

• Spherulites: Radial growth of crystals from a central nucleus.

4. Characterization of Crystals

• SAXS (Small-Angle X-ray Scattering): Used to study larger-scale structures (e.g., lamellar thickness).

• WAXS (Wide-Angle X-ray Scattering): Used to study atomic-scale crystal structures.

5. Formation of Spherulites

• Spherulites form when polymer chains crystallize radially from a central nucleus, often observed under polarized light.

6. Effect of Processing Parameters and Molar Mass

• Processing parameters: Cooling rate, pressure, and solvent evaporation rate affect crystallization.

• Molar mass (Mw): Higher Mw leads to more entanglements, making crystallization harder.

7. Thermodynamics and Kinetics of Crystallization

• Thermodynamics: Governed by Gibbs free energy.

• Kinetics: Influenced by temperature, nucleation rate, and growth rate.

8. Factors Affecting Crystallization and Melting

• Crystallization: Affected by temperature, pressure, and molecular weight.

• Melting: Affected by crystal size, lamellar thickness, and molecular weight.

9. Determining % Crystallinity

• Techniques include DSC (Differential Scanning Calorimetry), WAXS, and density measurements.

10. Glass Transition Temperature (Tg​)

• Definition: The temperature at which an amorphous polymer transitions from a glassy to a rubbery state.

• Factors affecting Tg​: Chain flexibility, molecular weight, cross-linking, and additives.

• Behaviour above and below Tg​: Below Tg​, polymers are rigid; above Tg​, they become flexible.

11. Information from DSC

• DSC provides data on:

  • Melting temperature (Tm​).

  • Glass transition temperature (Tg​).

  • Crystallization temperature (Tc​).

  • Heat of fusion (ΔHf).

12. Flory-Huggins Theory

• Describes the thermodynamics of polymer solutions and blends.

• Predicts phase separation in polymer blends based on the interaction parameter (χ).

13. Phase Separation in Immiscible Polymers

• Immiscible polymers phase separate into distinct domains.

• DSC can confirm phase separation by showing multiple Tg values.

14. Effect of Crystalline and Amorphous Structures on Mechanical Properties

• Crystalline regions: Provide strength and rigidity.

• Amorphous regions: Provide flexibility and toughness.

15. Stress-Strain Curves

• Elastic region: Linear relationship between stress and strain.

• Plastic region: Permanent deformation occurs after the yield point.

• Fracture point: Material breaks.

16. Effect of Temperature on Stress-Strain Behaviour

• Below Tg​: Polymers are brittle.

• Above Tg​: Polymers become ductile and can undergo plastic deformation.

17. Effect of Molar Mass on Mechanical Properties

• Mechanical properties (e.g., tensile strength, Young’s modulus) improve with increasing molar mass up to a certain point.

• Toughness and elongation at break show a step-like increase when molar mass exceeds the entanglement threshold (Me​).

Summary of Conductive Polymers

1. Structure of Conductive Polymers

• Conductive polymers have a backbone of alternating single and double bonds (conjugated system).

• Examples: Polyacetylene, P3HT (Poly(3-hexylthiophene)), PEDOT (Poly(3,4-ethylenedioxythiophene)).

2. Effect of Small Molecules on Electrical Conductivity

• Small molecules (e.g., solvents, additives) can form eutectic mixtures with polymers, affecting crystallization and conductivity.

• Eutectic behaviour: Both polymer and small molecule crystallize simultaneously, leading to fine-grained structures.

3. Effect of Additives on Nucleation and Growth

• Additives like 2-methylpentane (2-MP) and ultrasound can enhance nucleation and growth of polymer crystals.

4. Effect of Epitaxial Crystallization

• Epitaxy: Oriented growth of polymer crystals on a substrate, leading to improved electrical properties.

5. Effect of Solidification Rate

• High-boiling point solvents: Slow evaporation leads to higher crystallinity and better carrier mobility.

• Low-boiling point solvents: Fast evaporation leads to lower crystallinity and reduced carrier mobility.

6. Doping in Conductive Polymers

• Doping: Introduction of charge carriers (e.g., via oxidation or reduction) to increase conductivity.

• Example: PEDOT:PSS (Poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid (PSS).

7. Electrical Conductivity and Doping

• Undoped polymers: Insulators or semiconductors.

• Doped polymers: Can achieve metallic conductivity.

Mechanical Properties of Polymers

1. Stress-Strain Behaviour

• Brittle materials: Fracture without significant plastic deformation.

• Plastic materials: Exhibit yielding and plastic deformation before fracture.

• Elastomers: Show large elastic deformation with low stress.

2. Yield Point and Tensile Strength

• Yield point (σy): Stress at which plastic deformation begins.

• Tensile strength (TS): Maximum stress before fracture.

3. Modulus of Elasticity

• Tensile modulus: Slope of the stress-strain curve in the elastic region.

4. Molecular Weight Dependence

• Mechanical properties (e.g., melting temperature, Young’s modulus) increase with molecular weight up to a plateau.

• Toughness and elongation at break improve significantly when molecular weight exceeds the entanglement threshold (Me​).