Unit 2: Polymers

1. Basics of Polymer Chemistry

Definitions

• Polymer: A large molecule (macromolecule) formed by the repeated linking of small structural units called monomers via covalent bonds.
• Monomer: A simple, low molecular weight molecule capable of combining with others to form a polymer.
• Polymerization: The chemical process of linking monomers together to form a polymer.
• Functionality: The number of bonding sites or reactive functional groups present in a monomer.
  • Bifunctional: Forms linear polymers (e.g., Ethylene).
  • Trifunctional/Polyfunctional: Forms branched or cross-linked polymers (e.g., Phenol).

2. Classification of Polymers

A. Based on Source

1. Natural Polymers: Found in plants and animals (e.g., Starch, Cellulose, Proteins, Natural Rubber).
2. Synthetic Polymers: Man-made in laboratories (e.g., Polyethylene, PVC, Nylon, Teflon).
3. Semi-synthetic Polymers: Chemically modified natural polymers (e.g., Cellulose acetate, Rayon, Vulcanized rubber).

B. Based on Structure

1. Linear Polymers: Long, continuous chains. High density and melting point due to close packing (e.g., HDPE, PVC).
2. Branched Polymers: Chains with side branches. Lower density and melting point due to irregular packing (e.g., LDPE).
3. Cross-linked (Network) Polymers: Chains linked by covalent bonds in a 3D network. Hard, rigid, and brittle (e.g., Bakelite, Melamine).

C. Based on Mode of Polymerization

1. Addition (Chain Growth) Polymerization:
   • Monomers add to the chain without eliminating by-products.
   • Requires unsaturated monomers (containing double/triple bonds).
   • Mechanism: Free radical, Cationic, or Anionic.
   • Example: Ethylene Polyethylene.
2. Condensation (Step Growth) Polymerization:
   • Monomers combine with the elimination of small molecules (water, HCl, alcohol).
   • Requires monomers with bifunctional or polyfunctional groups.
   • Example: Hexamethylenediamine + Adipic acid Nylon 6,6 + .

D. Based on Molecular Forces

1. Elastomers: Weak intermolecular forces (Van der Waals); elastic properties (e.g., Rubber).
2. Fibers: Strong hydrogen bonds/dipole interactions; high tensile strength (e.g., Nylon, Polyester).
3. Thermoplastics: Intermediate forces; soften on heating, harden on cooling; recyclable (e.g., PVC, Polyethylene).
4. Thermosetting Plastics: Extensive cross-linking during molding; infusible and insoluble once set; cannot be remolded (e.g., Bakelite).

3. Degree of Polymerization (DP)

• Definition: The number of repeating units () in a polymer chain.
• Formula:
  
• Significance:
  • Low DP: Oils or oligomers.
  • High DP: Solid polymers with high mechanical strength.
  • As DP increases, viscosity, boiling point, and mechanical strength increase up to a threshold.

4. Structure-Property Relationship

Molecular Shape

• Linear chains: Allow close packing, leading to high density, crystallinity, and tensile strength.
• Branched chains: Disrupt packing, lowering density and melting points.

Crystallinity

Polymers are rarely 100% crystalline; they are semi-crystalline (mix of amorphous and crystalline regions).

• Crystalline Regions: Ordered arrangement of chains (lamellae). Provides strength, stiffness, and chemical resistance.
• Amorphous Regions: Disordered/random arrangement. Provides flexibility and impact resistance.
• Factors aiding crystallinity: Linear structure, simple monomer structure, high intermolecular forces (H-bonding).

5. Glass Transition Temperature ()

Definition

The temperature at which a polymer transitions from a hard, glassy, brittle state to a soft, rubbery, flexible state.

• Below : Polymer is glassy/brittle (segmental motion is frozen).
• Above : Polymer is rubbery/flexible.
• Note: is for amorphous regions; Melting Point () is for crystalline regions.

Basic Factors Affecting

1. Chain Flexibility: Rigid chains (aromatic rings in backbone) High . Flexible chains Low .
2. Intermolecular Forces: Strong forces (H-bonding, polar groups) restrict motion High .
3. Side Groups (Steric Hindrance): Bulky side groups hinder rotation High .
   • Example: Polystyrene (bulky phenyl group, ) vs. Polyethylene (small H atom, ).
4. Cross-linking: Restricts segmental motion Increases .

6. Structure, Synthesis, and Applications of Common Polymers

A. Elastomers (Synthetic Rubbers)

1. Styrene-Butadiene Rubber (SBR / Buna-S)

• Synthesis: Copolymerization of Styrene (25%) + 1,3-Butadiene (75%).
• Properties: High abrasion resistance, high load-bearing capacity.
• Applications: Automobile tires, footwear soles, conveyor belts.

2. Nitrile Rubber (NBR / Buna-N)

• Synthesis: Copolymerization of Acrylonitrile + 1,3-Butadiene.
• Properties: Excellent resistance to oils, fuels, and solvents due to polar CN groups.
• Applications: Oil seals, fuel tanks, hoses, gaskets.

B. Common Plastics

1. Polyvinyl Chloride (PVC)

• Monomer: Vinyl Chloride ().
• Structure: Linear chain with Chlorine pendant groups.
• Applications: Pipes, cable insulation, raincoats, flooring.

2. Teflon (Polytetrafluoroethylene - PTFE)

• Monomer: Tetrafluoroethylene ().
• Properties: Chemically inert, high thermal stability, low coefficient of friction, non-stick.
• Applications: Non-stick cookware coatings, bearings, gaskets, chemical storage liners.

3. PMMA (Polymethyl Methacrylate / Plexiglass)

• Properties: High optical transparency, weather resistance.
• Applications: Aircraft windows, lenses, automotive tail lights, skylights.

4. Kevlar (Aromatic Polyamide)

• Synthesis: Condensation of terephthaloyl chloride + p-phenylenediamine.
• Structure: Rigid aromatic backbone with inter-chain hydrogen bonds.
• Properties: Extremely high tensile strength, heat resistance, lightweight.
• Applications: Bulletproof vests, aerospace components, high-performance cables.

7. Biodegradable Polymers

Classification

1. Natural: Starch, cellulose, collagen, chitosan.
2. Synthetic: Polylactic acid (PLA), Polycaprolactone (PCL), Polyglycolic acid (PGA).
3. Microbial: Polyhydroxyalkanoates (PHAs).

Methods of Degradation

1. Hydrolysis: Breakdown of ester/amide bonds by water (common in PLA/PGA).
2. Enzymatic Degradation: Microorganisms secrete enzymes (proteases, cellulases) to digest polymer chains.
3. Photo-degradation: Light energy breaks polymer bonds.

Applications

• Medical: Dissolvable surgical sutures, drug delivery systems, bone pins.
• Packaging: Compostable bags, food containers.
• Agriculture: Mulch films that degrade into soil.

8. Conducting Polymers (ICPs)

Usually, organic polymers are insulators. Intrinsically Conducting Polymers (ICPs) possess electrical conductivity similar to metals/semiconductors.

Mechanism: Conjugation and delocalization

• Conducting polymers have a Conjugated Backbone: Alternating single () and double () bonds (e.g., ).
• -electron Delocalization: The overlap of p-orbitals allows electrons to move along the polymer chain, reducing the band gap between the Valence Band (VB) and Conduction Band (CB).

Doping Mechanisms

Conductivity is drastically increased (from to S/cm) by Doping (introducing impurities).

1. p-doping (Oxidative): Removing electrons from the polymer backbone using oxidizing agents (). Creates positive charges (holes).
2. n-doping (Reductive): Adding electrons to the polymer backbone using reducing agents (Na, Li). Creates negative charges.

Charge Carriers

When doped, structural deformations occur, creating specific charge carriers:

1. Solitons: Associated with degenerate ground states (e.g., Polyacetylene). A domain wall separating two phases of orientation.
2. Polarons: A radical ion (spin 1/2) associated with lattice distortion. Formed in non-degenerate ground states (e.g., Polypyrrole).
3. Bipolarons: A di-cation or di-anion (spin 0) formed by the combination of two polarons. Responsible for high conductivity.

Conductivity and Transport

• Transport occurs via intra-chain movement (along the backbone via delocalization) and inter-chain hopping (jumping from one chain to another).
• Conductivity range: S/cm (Neutral) to S/cm (Doped).

Applications

• Electronics: OLEDs (Organic LEDs), flexible displays, transistors (OFETs).
• Energy: Polymer solar cells, electrode materials for supercapacitors and batteries.
• Sensors: Chemical sensors (conductivity changes upon exposure to specific gases/chemicals).

9. Lubricants

Lubrication and Purpose

Definition: A substance introduced between two moving surfaces to reduce friction.
Purpose:

1. Reduce friction and wear.
2. Dissipate heat (coolant).
3. Prevent corrosion.
4. Seal gaps (e.g., piston rings).

Mechanisms of Lubrication

1. Fluid Film (Hydrodynamic) Lubrication: Thick oil film completely separates surfaces. Used in high speed, low pressure.
2. Boundary Lubrication: Thin oil film (monolayer) adsorbed on surfaces. Asperities (surface peaks) may touch. Used in low speed, high pressure.
3. Extreme Pressure (EP) Lubrication: Under high heat/pressure, additives react chemically with metal to form solid protective layers (chlorides/sulfides).

Additives for Lubricants

To improve performance, additives are mixed with base oils:

• Anti-oxidants: Prevent oxidation of oil (e.g., Aromatic amines).
• Anti-wear agents: Form protective films (e.g., ZDDP).
• Viscosity Index Improvers: Prevent oil from thinning at high temps (e.g., Polymethacrylates).
• Detergents: Keep engine parts clean from sludge.

Chemical and Physical Properties

1. Viscosity: Resistance to flow. Most important property. Must be optimal (too low = friction; too high = energy loss).
2. Viscosity Index (VI): Rate of change of viscosity with temperature. High VI is desirable (viscosity changes little with heat).
3. Flash and Fire Point:
   • Flash Point: Lowest temp where oil vapor ignites momentarily.
   • Fire Point: Temp where oil burns continuously. (Safety indicators).
4. Cloud and Pour Point:
   • Cloud Point: Temp where oil becomes hazy (wax crystallization).
   • Pour Point: Lowest temp where oil flows. (Important for cold weather operation).
5. Aniline Point: Lowest temp at which oil mixes with aniline. High aniline point Low aromatic content (Good, prevents rubber seal damage).
6. Neutralization Number (Acid/Base Value): Measures acidity/alkalinity. Indicates age/degradation of oil.