ANWSER
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Question 1:
(a) List and explain the important polymer Isomerism
Polymer isomerism refers to the different ways monomer units can be arranged in a polymer chain. The important types include:
– Structural Isomerism: Different arrangements of atoms in the polymer backbone (e.g., linear vs. branched polyethylene).
– Stereoisomerism: Differences in spatial arrangement (e.g., isotactic, syndiotactic, and atactic configurations in polypropylene).
– Geometric Isomerism (Cis-Trans): Occurs in polymers with double bonds (e.g., natural rubber (cis) vs. gutta-percha (trans)).
– Tautomeric Isomerism: Dynamic equilibrium between two forms (rare in polymers).
(b) Explain polyurethanes
Polyurethanes are a class of polymers formed by the reaction of diisocyanates with polyols. They exhibit a wide range of properties, from flexible foams to rigid plastics, depending on the monomers used.
(c) Give five (5) advantages of polyurethanes
1. Versatility: Can be tailored for flexibility, rigidity, or elasticity.
2. Durability: Resistant to abrasion, oils, and solvents.
3. Lightweight: Ideal for insulation and cushioning.
4. Thermal Insulation: Used in foams for refrigeration and construction.
5. Chemical Resistance: Stable in harsh environments.
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Question 2:
(a) Discuss the phases of solubility
1. Swelling: Solvent penetrates the polymer, causing expansion but no dissolution.
2. Dissolution: Polymer chains separate and disperse in the solvent, forming a homogeneous solution.
(b) List and explain five (5) polymers and their dissolution solvents
1. Polyethylene (PE): Dissolves in hot xylene or toluene.
2. Polyvinyl Chloride (PVC): Soluble in tetrahydrofuran (THF) or cyclohexanone.
3. Polystyrene (PS): Dissolves in benzene or chloroform.
4. Polyacrylonitrile (PAN): Soluble in dimethylformamide (DMF).
5. Nylon 6,6: Dissolves in formic acid or m-cresol.
(c) Briefly discuss how chemical structure affects the solubility of polymers
– Polarity: Polar polymers dissolve in polar solvents (e.g., PAN in DMF).
– Crystallinity: Highly crystalline polymers (e.g., PE) resist dissolution unless heated.
– Branching: Linear polymers dissolve more easily than branched ones.
– Molecular Weight: Higher MW polymers dissolve slower due to entanglement.
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Question 3:
(a) Differentiate between thermoplastic and thermoset
– Thermoplastic: Softens on heating and hardens on cooling (reversible, e.g., PET, PVC).
– Thermoset: Irreversibly hardens when heated (cannot be remolded, e.g., epoxy, Bakelite).
(b) Give five (5) characteristics of thermoset polymer
1. High Thermal Stability: Resists deformation at high temperatures.
2. Rigidity: Strong and inflexible.
3. Chemical Resistance: Impervious to solvents.
4. Dimensional Stability: Minimal shrinkage or expansion.
5. Crosslinked Structure: Covalent bonds prevent melting.
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Question 4:
(a) List and describe the mechanical properties of polymer
1. Tensile Strength: Resistance to breaking under tension.
2. Elasticity: Ability to return to original shape after deformation.
3. Hardness: Resistance to surface indentation.
4. Impact Resistance: Ability to absorb energy without fracturing.
5. Creep: Slow deformation under constant stress.
(b) Briefly explain how mechanical behaviour of a polymer can be assessed by the stress-strain properties
– Elastic Region: Linear slope (Young’s modulus) shows stiffness.
– Yield Point: Onset of plastic deformation.
– Plastic Region: Permanent deformation occurs until fracture.
(c) Give five (5) properties of polyurethanes
1. High elasticity.
2. Excellent abrasion resistance.
3. Good adhesion to substrates.
4. Variable hardness (Shore A to D).
5. Resistance to ozone and oxidation.
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Question 5:
(a) With equations only explain the thermodynamics of rubber elasticity
– Entropic Elasticity: \( \Delta G = \Delta H – T\Delta S \) (where \(\Delta S\) dominates).
– Retractive Force: \( f = \left( \frac{\partial G}{\partial L} \right)_{T,P} \).
– Ideal Rubber Behavior: \( f = \frac{3nkT}{L_0} \left( \alpha – \frac{1}{\alpha^2} \right) \), where \(\alpha = \frac{L}{L_0}\).
(b) Explain simple identification tests
– Burn Test: Observe flame color/smell (e.g., PE melts with blue flame, PVC emits HCl gas).
– Density Test: Float/sink in specific liquids (e.g., PP floats in water).
– Solubility Test: Dissolve in solvents (e.g., PS in acetone).
(c) Define rubber Elasticity
Rubber elasticity is the ability of elastomers to undergo large, reversible deformations due to entropic uncoiling of polymer chains when stretched, returning to their original shape upon release.
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