Unit 2: Polymers - Subjective Questions

Polymer: A polymer is a large molecule (macromolecule) built up by the repetition of small, simple chemical units called monomers, joined together through covalent bonds during a process called polymerization.

Key Terminology:

• Monomer: The simple, small molecule that acts as the building block of a polymer (e.g., ethylene for polyethylene).
• Repeat Unit: The smallest structural unit whose repetition gives the polymer chain.
• Degree of Polymerization (DP): The number of repeating (monomer) units present in a polymer chain.
• Functionality: The number of reactive sites or bonding positions present in a monomer.
  • Functionality of 2 → linear polymers
  • Functionality of 3 or more → branched or cross-linked (network) polymers.

Example: In polyethylene, ethylene is the monomer, is the repeat unit, and if 5000 units join, the DP = 5000.

Polymers can be classified on several bases:

1. Based on Source:

• Natural polymers: Occur in nature, e.g., cellulose, starch, proteins, natural rubber.
• Synthetic polymers: Man-made, e.g., polyethylene, PVC, nylon.
• Semi-synthetic polymers: Derived by modifying natural polymers, e.g., cellulose acetate, vulcanized rubber.

2. Based on Structure:

• Linear polymers: e.g., HDPE, PVC.
• Branched polymers: e.g., LDPE, glycogen.
• Cross-linked/Network polymers: e.g., bakelite, vulcanized rubber.

3. Based on Polymerization Mechanism:

• Addition (chain-growth) polymers: e.g., polyethylene, polystyrene.
• Condensation (step-growth) polymers: e.g., nylon, polyester (with elimination of small molecules like water).

4. Based on Thermal Behavior:

• Thermoplastics: Soften on heating and harden on cooling (reversible), e.g., polyethylene, PVC.
• Thermosetting plastics: Set permanently on heating (irreversible), e.g., bakelite, epoxy resins.

Molecular Shape: The geometric arrangement and regularity of polymer chains strongly influences whether a polymer can pack into ordered regions.

Crystallinity: Crystallinity refers to the degree of ordered, tightly packed arrangement of polymer chains. Polymers are usually semi-crystalline, having both crystalline (ordered) and amorphous (disordered) regions.

Factors relating shape to crystallinity:

• Linear and regular chains: Pack closely → high crystallinity (e.g., HDPE).
• Branched chains: Prevent close packing → lower crystallinity (e.g., LDPE).
• Stereoregularity: Isotactic and syndiotactic polymers crystallize easily; atactic polymers are amorphous.
• Symmetry of repeat unit: More symmetrical units pack better.
• Intermolecular forces: Strong H-bonding (e.g., nylon) promotes crystallinity.
• Bulky side groups: Hinder packing → reduce crystallinity.

Effect of crystallinity on properties:

• Higher crystallinity → greater density, strength, rigidity, higher melting point, and reduced transparency.

Glass Transition Temperature (): It is the temperature at which an amorphous polymer (or the amorphous region of a semi-crystalline polymer) changes from a hard, brittle, glassy state to a soft, flexible, rubbery state upon heating.

Explanation:

• Below : chain segments are frozen; the polymer is rigid and glassy.
• Above : chain segments gain mobility; the polymer becomes soft and flexible.

Significance:

• Determines whether a polymer behaves as a rubber or a rigid plastic at service temperature.
• Helps in selecting polymers for specific applications (e.g., a rubber must have below room temperature).
• Influences mechanical properties like flexibility, toughness, and elasticity.
• Affects processing conditions (molding, casting).

Example:

• Natural rubber: (flexible at room temperature).
• Polystyrene: (rigid at room temperature).

The Glass Transition Temperature depends on the mobility of polymer chain segments. The main factors are:

1. Chain Flexibility:

• Flexible chains (with , linkages) → low . Example: silicone rubber.
• Stiff chains → high .

2. Intermolecular Forces:

• Strong forces (H-bonding, dipole interactions) restrict motion → higher . Example: nylon, PVC.

3. Bulky Side Groups:

• Large, rigid side groups hinder rotation → increase . Example: polystyrene has higher than polyethylene.

4. Cross-linking:

• More cross-links restrict chain movement → higher .

5. Molecular Weight:

• increases with molecular weight up to a limit.

6. Plasticizers:

• Addition of plasticizers increases free volume and lowers . Example: plasticized PVC.

7. Crystallinity:

• Higher crystallinity generally raises .

Elastomers: Elastomers are polymers that can be stretched to several times their original length and return to their original shape when the stress is released. They have low (below room temperature) and are lightly cross-linked with weak intermolecular forces.

Structure of Natural Rubber:

• Natural rubber is cis-1,4-polyisoprene.
• The cis configuration causes coiled, irregular chains giving elasticity.

Synthesis:

• Obtained from latex of rubber trees (Hevea brasiliensis).
• Produced synthetically by polymerization of isoprene.

Vulcanization:

• Process of heating rubber with sulphur (3–5%).
• Sulphur forms cross-links (sulphide bridges) between polymer chains.
• Improves strength, elasticity, hardness, and resistance to temperature and solvents.

Applications:

• Tyres, tubes, footwear, gaskets, seals, conveyor belts, insulation materials.

Buna-S (Styrene-Butadiene Rubber, SBR):

• Synthesis: Copolymerization of butadiene and styrene (in ratio ~3:1) in presence of sodium (Na) catalyst.
  
• Properties: High abrasion resistance, good mechanical strength, resistant to oxidation.
• Applications: Automobile tyres, footwear soles, cable insulation, floor tiles.

Buna-N (Nitrile Rubber, NBR):

• Synthesis: Copolymerization of butadiene and acrylonitrile.
  
• Properties: Excellent resistance to oil, heat, and chemicals due to polar nitrile groups.
• Applications: Oil seals, gaskets, fuel hoses, conveyor belts handling oils.

Note: "Bu" = Butadiene, "Na" = Sodium catalyst, "S" = Styrene, "N" = Nitrile.

Nylon-6,6:

• Type: Condensation polymer (polyamide).
• Synthesis: Condensation of hexamethylenediamine and adipic acid.
  
• Properties: High tensile strength, elasticity, abrasion resistance, H-bonding gives crystallinity.
• Applications: Fibers for textiles, ropes, tyre cords, bristles, gears, bearings.

PVC (Polyvinyl Chloride):

• Type: Addition polymer.
• Synthesis: Addition polymerization of vinyl chloride ().
  
• Properties: Rigid, chemically resistant, good insulator; softened using plasticizers.
• Applications: Pipes, electrical cable insulation, raincoats, flooring, upholstery.

Biodegradable Polymers: These are polymers that can be broken down (degraded) into simpler, environmentally acceptable products such as , water, and biomass by the action of microorganisms, enzymes, moisture, or other natural agents.

Classification:

1. Based on Origin:

• Natural biodegradable polymers: Cellulose, starch, chitosan, proteins, polyhydroxyalkanoates (PHA).
• Synthetic biodegradable polymers: Polylactic acid (PLA), polyglycolic acid (PGA), polycaprolactone (PCL), poly(β-hydroxybutyrate) (PHB).

2. Based on Degradation Mechanism:

• Photodegradable: Degrade under UV light.
• Hydro-biodegradable: Degrade by hydrolysis then microbial action.
• Oxo-biodegradable: Degrade by oxidation followed by microbial action.

Examples:

• PHBV (poly(3-hydroxybutyrate-co-3-hydroxyvalerate)) – packaging.
• PLA – used in surgical sutures and packaging.
• Nylon-2-nylon-6 – biodegradable polyamide.

Methods of Degradation:

1. Biodegradation (Microbial):

• Microorganisms (bacteria, fungi) secrete enzymes that break polymer chains into smaller molecules and finally into , , and biomass.

2. Hydrolytic Degradation:

• Water molecules attack hydrolyzable bonds (ester, amide) breaking the chain. Common in PLA, PGA.

3. Photodegradation:

• UV radiation breaks chemical bonds; carbonyl groups absorb light and initiate chain scission.

4. Oxidative Degradation:

• Oxygen reacts with polymer forming peroxides that cause chain cleavage.

5. Thermal Degradation:

• Heat causes bond breaking and decomposition.

Uses of Biodegradable Polymers:

• Medical: Surgical sutures, drug delivery systems, tissue engineering scaffolds, implants.
• Packaging: Biodegradable bags, films, food containers.
• Agriculture: Mulch films, controlled release of fertilizers/pesticides.
• Disposable items: Cutlery, cups.

These polymers help reduce plastic pollution and environmental impact.

Conducting Polymers: These are organic polymers that conduct electricity due to the presence of conjugated -electron systems along the polymer backbone. Example: polyacetylene, polyaniline, polypyrrole.

-Electron Delocalization:

• Conducting polymers have alternating single and double bonds (conjugation) along the chain.
• The overlapping -orbitals form delocalized -electron clouds spread over the chain.
• These delocalized electrons can move along the backbone, providing a pathway for charge transport.

Requirement for Conductivity:

• A conjugated backbone alone gives only semiconducting behavior because there is an energy gap between the valence band () and conduction band ().
• To achieve high conductivity, doping is required, which introduces charge carriers (holes or electrons).

Example — Polyacetylene:

• Pure (undoped) polyacetylene is a semiconductor.
• On doping, its conductivity increases by several orders of magnitude, approaching metallic values.

Doping: Doping is the process of introducing charge carriers into a conjugated polymer by partial oxidation or reduction, drastically increasing its electrical conductivity.

Types of Doping:

1. p-Type Doping (Oxidative Doping):

• The polymer is oxidized (electrons removed) using oxidizing agents like , , , .
• Creates positive charge carriers (holes) on the polymer chain.
• Example:

2. n-Type Doping (Reductive Doping):

• The polymer is reduced (electrons added) using reducing agents like sodium naphthalide, alkali metals (Na, K).
• Creates negative charge carriers (extra electrons).
• Example:

Effect:

• Doping generates charge carriers such as solitons, polarons, and bipolarons that facilitate conduction.
• Conductivity can increase from to .

Note: Doping in conducting polymers differs from semiconductor doping — it involves redox reactions and counter-ions rather than substitutional atoms.

When conducting polymers are doped, various charge carriers are generated that carry current along the chain.

1. Solitons:

• Formed in polymers with a degenerate ground state, e.g., trans-polyacetylene.
• A soliton is a structural defect (a radical, positive, or negative charge) that separates two equal-energy resonance structures.
• Types: neutral soliton (radical), positive soliton (cation), negative soliton (anion).
• They move freely along the chain, carrying charge.

2. Polarons:

• Formed in polymers with a non-degenerate ground state, e.g., polypyrrole, polythiophene.
• A polaron is a radical cation (or radical anion) — a combination of a charge and an unpaired spin, along with local lattice distortion.
• Introduces new energy states within the band gap.

3. Bipolarons:

• At higher doping levels, two polarons combine to form a bipolaron.
• A bipolaron is a doubly charged species (dication or dianion) with no unpaired spin.
• More stable and contributes strongly to conduction at high doping.

Role in Conduction:

• These carriers move along and hop between chains under an applied field, giving rise to electrical conductivity.

Conductivity and Transport Mechanism:

• Conduction in conducting polymers occurs through movement of charge carriers (solitons, polarons, bipolarons).
• Intra-chain transport: Charge moves along the conjugated backbone via delocalized -electrons.
• Inter-chain transport: Charge hops between neighbouring chains.
• Inter-particle/domain transport: Charge moves across crystalline and amorphous domains.
• Overall conductivity depends on doping level, chain alignment, and crystallinity.
• Conductivity follows a hopping mechanism where carriers jump between localized states.

Applications:

1. Electronics:

• Light-emitting diodes (OLEDs), field-effect transistors, antistatic coatings, electromagnetic shielding.

2. Energy:

• Rechargeable batteries, supercapacitors, solar cells (organic photovoltaics), fuel cell electrodes.

3. Sensors:

• Gas sensors, biosensors, chemical sensors — conductivity changes in presence of analytes enable detection.

Examples of Materials: Polyaniline, polypyrrole, polythiophene, PEDOT:PSS.

Lubricant: A lubricant is a substance (solid, liquid, or semi-solid) introduced between two moving/sliding surfaces to reduce friction and wear.

Lubrication: Lubrication is the process of applying a lubricant between contacting surfaces in relative motion to minimize friction, reduce wear, and dissipate heat.

Purposes and Functions of Lubricants:

• Reduce friction: Forms a film between surfaces, preventing direct metal-to-metal contact.
• Reduce wear and tear: Protects surfaces from abrasion and damage.
• Dissipate heat: Acts as a coolant, carrying away heat generated by friction.
• Reduce power loss: Improves mechanical efficiency.
• Prevent corrosion: Forms a protective layer against moisture and oxidation.
• Act as a seal: Prevents leakage of gases (e.g., in engines) and keeps out dust/dirt.
• Reduce noise and vibration: Provides smooth, quiet operation.

Types: Solid (graphite, MoS₂), liquid (mineral/synthetic oils), semi-solid (greases).

Additives are chemical compounds added to base lubricants to improve their performance and impart desired properties.

Common Lubricant Additives and Functions:

• Antioxidants: Prevent oxidation and formation of sludge/acids. Example: amines, phenols.
• Viscosity Index (VI) Improvers: Maintain viscosity over a wide temperature range. Example: polyisobutylene, polymethacrylates.
• Pour Point Depressants: Lower the temperature at which oil ceases to flow. Example: alkyl naphthalenes.
• Anti-wear / Extreme Pressure (EP) Additives: Form protective films under high load/pressure. Example: ZDDP (zinc dialkyl dithiophosphate), chlorinated/sulphurized compounds.
• Corrosion/Rust Inhibitors: Protect metal surfaces from corrosion. Example: metallic soaps, phosphites.
• Detergents and Dispersants: Keep engine surfaces clean and hold contaminants in suspension. Example: metal sulphonates.
• Anti-foaming Agents: Prevent foam formation. Example: silicone polymers.
• Emulsifiers/Demulsifiers: Control mixing with water.

These additives extend the life of the lubricant and improve efficiency of machinery.

Physical Properties:

• Viscosity: The resistance to flow; the most important property. Determines the load-carrying capacity and film thickness.
• Viscosity Index (VI): Measures change of viscosity with temperature. High VI is desirable (viscosity stays stable).
• Flash Point and Fire Point: Lowest temperatures at which vapours ignite momentarily (flash) or continuously (fire). Indicate safety and volatility.
• Cloud Point and Pour Point: Temperatures at which oil becomes cloudy and ceases to flow, respectively. Important for cold-weather use.
• Oiliness: Ability to stick to metal surfaces under high pressure.
• Volatility: Tendency to evaporate; low volatility preferred.

Chemical Properties:

• Oxidation Stability: Resistance to reacting with oxygen; prevents sludge and acid formation.
• Acid Value / Neutralization Number: Amount of acidic substances present; indicates degradation.
• Saponification Value: Indicates presence of fatty (vegetable/animal) oils.
• Corrosion Stability: Ability not to corrode metal surfaces.
• Emulsification / Demulsification: Behaviour towards water contamination.

These properties help select the right lubricant for specific operating conditions.

The properties of polymers are determined by their molecular structure. Understanding this structure-property relationship helps in designing polymers for specific applications.

1. Crystallinity:

• Higher crystallinity → increased strength, rigidity, density, melting point, and chemical resistance, but reduced transparency and flexibility. Example: HDPE (crystalline, strong) vs LDPE (less crystalline, flexible).

2. Cross-linking:

• Increased cross-linking → higher hardness, strength, thermal stability, and reduced elasticity/solubility. Example: vulcanized rubber, bakelite.

3. Intermolecular Forces:

• Strong forces (H-bonding, dipole-dipole) → higher strength, higher , higher melting point. Example: nylon (H-bonding) is strong and high-melting.

4. Molecular Weight:

• Higher molecular weight → greater tensile strength, toughness, and viscosity.

5. Branching:

• More branching → lower density, lower crystallinity, and reduced strength.

6. Chain Stiffness and Side Groups:

• Bulky/rigid groups increase and rigidity.

Summary: Linear, regular, high-molecular-weight, crystalline, and strongly bonded polymers are stronger and more rigid, while branched or amorphous polymers are softer and more flexible.

Degree of Polymerization (DP): It is the number of repeating monomer units present in a single polymer chain. It is a measure of the length of the polymer chain.

Importance of DP:

• Determines molecular weight of the polymer.
• Higher DP → higher tensile strength, toughness, melting point, and viscosity.
• Below a critical DP, the material lacks useful mechanical strength.
• Governs processability and end-use properties.

Calculation:

Given:

• Molecular weight of polyethylene =
• Molecular weight of ethylene monomer =

Answer: The degree of polymerization = 2500, meaning about 2500 ethylene units are joined in each polymer chain.

Addition (Chain-Growth) Polymerization vs Condensation (Step-Growth) Polymerization:

Addition Example:

Condensation Example:

The key distinction is that condensation releases small molecules while addition does not.