THEORY EXAMINATION (SEM–IV) 2016-17 POLYMER SCIENCE & TECHNOLOGY

POLYMER SCIENCE & TECHNOLOGY (NOE045)

Section-wise Solved Answers & Notes

SECTION – A (10 × 2 = 20 Marks)

Very short, precise answers

(a) Cross-linked polymers & Branched polymers

• Cross-linked polymers: Chains joined by covalent bonds forming a 3D network (e.g., Bakelite).
• Branched polymers: Main chain with side branches (e.g., LDPE).

(b) Tacticity in polymers

Tacticity refers to the spatial arrangement of side groups along the polymer chain (isotactic, syndiotactic, atactic).

(c) Degree of polymerization & Functionality

• Degree of polymerization (DP): Number of repeating units in a polymer chain.
• Functionality: Number of reactive sites in a monomer molecule.

(d) Repeating unit of ABS polymer

ABS consists of Acrylonitrile, Butadiene, and Styrene units combined in a copolymer structure.

(e) Why Tg is higher for high molecular weight polymers

Higher molecular weight polymers have greater chain entanglement, restricting molecular motion and increasing Tg.

(f) Decrease in elastic behaviour of PVC with impurities

Impurities disturb chain alignment and reduce intermolecular forces, decreasing elasticity.

(g) Inhibitors & their significance

Inhibitors are substances that slow down or stop polymerization by neutralizing free radicals, preventing unwanted reactions.

(h) Condensation vs Addition polymerization

(i) Alternate & Graft copolymers

• Alternate copolymers: Monomers arranged alternately (A-B-A-B).
• Graft copolymers: Side chains of one polymer grafted onto backbone of another.

(j) PTFE behaves like thermoset

PTFE is thermoplastic but shows high thermal stability and rigidity due to strong C–F bonds, resembling thermoset behavior.

SECTION – B (Any 5 × 10 = 50 Marks)

(a) Elastomers & SBR (Buna-S)

Elastomers: Polymers showing rubber-like elasticity.

SBR (Buna-S):
• Preparation: Emulsion polymerization of butadiene and styrene
• Properties: High abrasion resistance, elasticity
• Applications: Tyres, footwear, belts

(b)(i) Initiators & free radical polymerization

Initiators generate free radicals.
Example: Benzoyl peroxide decomposes to form radicals initiating addition polymerization.

(b)(ii) Molecular weight averages

Given:
10% → MW = 10,000                                    90% → MW = 1,00,000

Mn=∑wi∑(wi/Mi)=≈52,631M_n = \frac{\sum w_i}{\sum (w_i/M_i)} = \approx 52,631Mn​=∑(wi​/Mi​)∑wi​​=≈52,631 Mw=∑wiMi=91,000M_w = \sum w_i M_i = 91,000Mw​=∑wi​Mi​=91,000 

(c)(i) Emulsion polymerization

Polymerization occurs in aqueous medium using emulsifier.
Advantages: good heat control, high molecular weight.

(c)(ii) Number average DP

DPn=11−pDP_n = \frac{1}{1-p}DPn​=1−p1​

(Values calculated for given extent of reaction in exam)

(d) Kinetics of free radical polymerization

Steps:                                                                     • Initiation
• Propagation                                                        • Termination

Degree of polymerization depends on rate of initiation and termination.

(e) High performance polymers (PMMA & PC)

PMMA:                                                                 • Transparent, rigid
• Used in lenses, aircraft windows

Polycarbonate (PC):                                             • High impact strength
• Used in safety helmets, CDs

(f) Composite polymers                                          Polymers reinforced with fibers or fillers.

Processing methods:
• Hand lay-up                                                         • Filament winding

(g)(i) Condensation polymerization mechanism

Step-growth process involving elimination of small molecules like water or HCl.

(g)(ii) Numerical

Given:                                                                     Styrene = 104 g
Dp = 1000                                                              Moles of styrene = 1
Number of polymer molecules = 11000\frac{1}{1000}10001​

(h) Ziegler–Natta Catalyst

Catalyst used in coordination polymerization producing stereoregular polymers.

Advantages:                                                         • Better control
• High molecular weight                                        • Less branching

SECTION – C (Any 2 × 15 = 30 Marks)

Q3 Differentiation

(i) Thermoplastic vs Thermosetting

(ii) Suspension vs Emulsion polymerization

(iii) Buna-S vs Buna-N

Q4 (Any three)

(i) Vulcanization

Process of heating rubber with sulfur to increase strength and elasticity.

(ii) Polyamides

Nylon polymers formed by condensation polymerization; high strength and toughness.

(iii) Phenol Formaldehyde

Thermosetting polymer with high heat resistance; used in electrical fittings.

(iv) Applications of polymers                                  • Biomedical: artificial joints, syringes
• Space: lightweight composites, insulation

Q5

(i) Effect of polymer structure on properties          • Chain length → strength
• Cross-linking → rigidity                                        • Tacticity → crystallinity

(ii) PVA (Polyvinyl Alcohol)                                     • Preparation: Hydrolysis of PVC
• Properties: Water soluble, adhesive                   • Uses: Films, adhesives

(iii) Factors affecting Glass Transition Temperature

• Molecular weight                                                 • Chain flexibility
• Cross-linking                                                        • Plasticizers