L6: Elastomers and thermodynamics of polymer mixtures

1. Elastomers

  • Definition: Elastomers are amorphous polymers that can sustain large deformations and return to their original shape after stress is removed.

  • Characteristics:

    • Highly amorphous.

    • Operate above their glass transition temperature (Tg​).

    • High molar mass and light crosslinking are required for network formation.

    • Behaviour is governed by entropy (chain mobility).

  • Examples:

    • Natural rubber (cis-polyisoprene): Obtained from the Hevea brasiliensis tree.

    • Vulcanized rubber: Crosslinked with sulphur to improve properties (e.g., elasticity, durability).

    • Synthetic elastomers: Polybutadiene, styrene-butadiene rubber (SBR), silicone, polyurethanes.

2. Effect of Crosslinking on Elastomers

  • Crosslinking: Increases the modulus (stiffness) of the polymer, especially in the rubbery plateau region.

  • Modulus-Temperature Relationship:

    • Below Tg​: Polymer is glassy and stiff.

    • Above Tg​: Polymer becomes rubbery and flexible.

    • High molar mass (M > M_e): Polymer exhibits a rubbery plateau.

    • Low molar mass: Polymer behaves like a liquid unless crosslinked.

3. Control of Tg and Tm​

  • Relationship between Tg​ and Tm​: Tg​ is typically between 0.5 and 0.8 of Tm​ (in Kelvin).

  • Strategies to modify Tg​ and Tm​:

    • Copolymers: Random copolymers reduce Tm​ more than Tg​, bringing them closer together.

    • Example: Nylon 6,6 and nylon 6,10 copolymers for fibres.

4. Random Copolymers

  • Effect on Tm​ and Tg​: Adding a fraction of polymer B to polymer A disrupts crystallinity, reducing Tm​ while Tg​ is less affected.

  • Practical Example: Copolymers of nylon 6,6 and nylon 6,10 are used to reduce the gap between Tg and Tm for fibre applications.

5. Block Copolymers

  • ABA Triblock Copolymers:

    • Example: Polystyrene-block-polybutadiene-block-polystyrene (SBS).

    • Behavior: Polystyrene domains act as physical crosslinks, providing elastomeric properties.

    • Applications: Used in car tires (SBR) and thermoplastic elastomers.

6. Thermodynamics of Polymer Mixtures (Flory-Huggins Theory)

  • Flory-Huggins Theory: Describes the thermodynamics of polymer solutions and blends.

  • Key Concepts:

    • Entropy of Mixing (ΔSmix): Favours mixing due to increased spatial arrangements.

    • Interaction Energy (ΔGinteraction​): Determines whether mixing or phase separation occurs.

    • Flory-Huggins Interaction Parameter (χ):

      • χ>0.5: Poor solvent, phase separation.

      • χ<0.5: Good solvent, mixing.

      • χ=0.5: Theta conditions, random coil conformation.

  • Equation for Free Energy of Mixing:

    • ϕ1,ϕ2​: Volume fractions of components.

    • χ12​: Interaction parameter between components.

7. Phase Separation in Polymer Blends

  • Miscible Blends: Homogeneous mixture with a single Tg​.

  • Immiscible Blends: Phase separation with distinct Tg values for each component.

  • Microphase Separation: Occurs in block copolymers (e.g., ABA triblock copolymers) where incompatible blocks form separate domains.

  • Macrophase Separation: Large-scale phase separation in immiscible polymer blends.

8. Flory-Huggins Interaction Parameter (χχ)

  • Definition: Describes the interaction energy between polymer segments and solvent or other polymers.

  • Applications:

    • Polymer Solutions: Determines whether a solvent is good (χ<0.5) or poor (χ>0.5).

    • Polymer Blends: Determines miscibility (χ<0.5) or immiscibility (χ>0.5).

    • Block Copolymers: Microphase separation occurs when χABN>>10 (Strong Segregation Limit).

9. Lower and Upper Critical Solution Temperatures (LCST and UCST)

  • LCST: Below a certain temperature, the mixture phase separates (e.g., polymer/solvent or polymer/polymer mixtures).

  • UCST: Above a certain temperature, the mixture phase separates.

  • Example: Changing temperature or solvent can induce phase separation.

10. Summary of Key Points

  • Elastomers: Amorphous polymers with high elasticity, operating above Tg​.

  • Crosslinking: Increases stiffness and improves elastomeric properties.

  • Copolymers: Random copolymers reduce Tm​ and bring Tg​ and Tm closer together.

  • Block Copolymers: Microphase separation leads to unique morphologies (e.g., lamellae, cylinders, spheres).

  • Flory-Huggins Theory: Describes the thermodynamics of polymer mixtures, including entropy of mixing and interaction energy.

  • Phase Separation: Determined by the Flory-Huggins interaction parameter (χ)