Workshop 2

Question 1: Which techniques would we use to assess the amount of crystalline material in a PET object?

  • DSC (Differential Scanning Calorimetry): Quantitative through ΔH (accurate).

    • WAXS (Wide-Angle X-ray Scattering): Gives absolute crystallinity value, but may have inaccuracies due to background.

    • Density measurements: Very accurate.

Question 2: Crystal growth rate G of polymers and its temperature dependency.

  • a) Factors leading to the bell-shaped dependency:

    • Nucleation vs. Diffusion: At low T, nucleation is high but diffusion is low; at high T, diffusion is high but nucleation is low. The maximum growth rate occurs where both processes are balanced.

  • b) Microstructures at TA​ and TB​:

    Microstructures

    • At TA​: Many nuclei lead to small crystals/spherulites due to low diffusion.

    • At TB​: Few nuclei lead to large crystals due to high diffusion.

  • c) Crystal growth rate for different molecular weights (Mw):

    • Higher Mw makes crystallization harder due to more entanglements, reducing the growth rate.

Question 3: Why does the glass transition temperature (Tg) of polyamides depend on air humidity?

Water acts as a plasticizer, interacting with hydrogen bonds in polyamides, which decreases Tg​.

Question 4: DSC plots for PET films.

  • a) Quenched PET film: Shows a glass transition (Tg), cold crystallization, and melting.

  • b) Annealed PET film: Shows reduced cold crystallization and increased melting due to pre-existing crystallinity.

  • c) Fully crystallized PET film: No cold crystallization peak; only melting is observed.

Question 5: Factors affecting Tg.

    1. Chain flexibility & molecular structure.

    2. Branching and cross-linking.

    3. Polymer molar mass (MM) / chain length.

    4. Chemical composition.

    5. Additives.

Question 6: Processing precautions to minimize amorphous components during solidification.

    • Slow solidification rate and high crystallization temperature.

    • Post-deposition procedures like thermal annealing.

    • Heterogeneous nucleation (e.g., nucleating agents, epitaxy).

    • Crystallization from dilute solutions to reduce entanglements.

    • Stress and pressure.

Question 7: Temperature range and type of polymer for toughness.

Use semicrystalline polymers above Tg for toughness. Below Tg​, polymers are brittle; above Tg​, they exhibit plastic deformation.

Question 8: Importance of Tg​ and Tm​ in structural applications.

    • Tg​ and Tm​ determine the application temperature range.

    • Semicrystalline polymers are tough between Tg​ and Tm.

    • Amorphous polymers can be used as elastomers above Tg​.

Question 9: Modifying Young’s Modulus of an elastomeric polymer.

    • Increase cross-linking.

    • Increase molecular weight (MM).

Question 10: Crystallinity in conductive polymers.

  • A) Properties maximized: Carrier mobilities.

  • B) How to increase crystallinity:

    • Use high molecular weight (MM).

    • Use small molecules to improve electrical conductivity.

    • Use nucleating agents and epitaxial crystallization.

    • Use high-boiling point solvents for slow crystallization.

Question 11: Block-copolymers of PS and PB.

  • i) Why PS and PB are selected: PS is brittle, PB is an elastomer with low Tg​. Combining them optimizes mechanical properties.

  • ii) Block copolymers vs. random copolymers: Block copolymers create physically crosslinked structures (e.g., rubbery PB domains crosslinked by PS), offering better mechanical properties than random copolymers.

Question 12: Increasing Tg​ of a polymer.

    • Cross-linking.

    • Making a miscible blend with a high Tg polymer.

      Polymer blends

Question 13: Stress-strain curve of PMMA below and above Tg​.

Below Tg​, PMMA is brittle. Above Tg​, it exhibits plastic deformation, with higher temperatures leading to more ductile behaviour.

Question 14: Di-block copolymer of Nylon 6 and Nylon 10.

    • The block copolymer will display two melting temperatures due to microphase separation (Strong Segregation Limit, χABN>>10).

    • The block copolymer will have a higher degree of crystallinity compared to a random copolymer because the blocks can phase separate and crystallize independently.

Question 15: DSC thermograms with and without nucleating agents.

Sample II contains the nucleating agent, as it crystallizes at higher temperatures, indicating assisted nucleation.

Question 16: Behaviour of amorphous polymer chains under stress.

    • Before stress: Random coil configuration.

    • After stress:

      • Elastic response: Chains stretch.

      • Viscoelastic response: Chains begin to slide past each other.

      • Viscous flow: Chains flow, leading to permanent deformation