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​:

  

  • 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.

    

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