Unit2 - Subjective Questions

Degree of Polymerization ():

It is defined as the number of repeating units (monomers) present in a polymer chain. It determines the length of the polymer chain.

Relationship with Molecular Weight:

The molecular weight of a polymer () is related to the degree of polymerization () and the molecular weight of the monomer unit () by the following equation:

Or,

• Low : Result in oligomers with low strength and liquid-like properties.
• High : Result in high molecular weight polymers with high tensile strength, melting point, and viscosity.

Polymers are classified into two types based on thermal behavior:

1. Thermoplastics
2. Thermosetting plastics (Thermosets)

Differences:

Crystallinity in Polymers:

Polymers contain both crystalline (ordered) and amorphous (disordered) regions. A polymer is never 100% crystalline due to the length and entanglement of chains. The degree of crystallinity refers to the fraction of the polymer that is arranged in a regular, geometric pattern.

Factors favoring crystallinity:

• Linear chains.
• Regular arrangement of side groups (Isotactic/Syndiotactic).
• Strong intermolecular forces (Hydrogen bonding).

Effect on Properties:

1. Mechanical Strength: Higher crystallinity increases tensile strength and stiffness because chains are packed closer together.
2. Density: Crystalline polymers have higher density than amorphous ones.
3. Chemical Resistance: Crystalline regions are less permeable to solvents and chemicals.
4. Optical Properties: Crystalline regions scatter light, making the polymer opaque or translucent, while amorphous polymers are often transparent.
5. Melting Point: Higher crystallinity leads to a sharper and higher melting point ().

Glass Transition Temperature ():

It is the temperature below which a polymer is hard, brittle, and glassy, and above which it becomes soft, flexible, and rubbery. It represents the onset of segmental motion of the polymer chains.

Factors Affecting :

1. Chain Flexibility: Rigid chains (e.g., aromatic rings in backbone) have higher because more energy is required for rotation. Flexible chains (e.g., -O-, -CH-) have lower .
2. Intermolecular Forces: Strong forces like hydrogen bonding or dipole-dipole interactions increase (e.g., Nylon has high due to H-bonding).
3. Side Groups (Steric Hinderance): Bulky side groups restrict rotation, thereby increasing (e.g., Polystyrene has higher than Polyethylene).
4. Cross-linking: High degree of cross-linking restricts chain mobility, significantly increasing .
5. Molecular Weight: increases with molecular weight up to a certain limit.
6. Plasticizers: Addition of plasticizers increases free volume and lowers .

1. PMMA (Polymethyl methacrylate):

• Synthesis: Prepared by free-radical polymerization of methyl methacrylate in the presence of peroxides.
  
• Properties: High optical transparency (known as Plexiglass), hard, resistant to scratches, good weather resistance.
• Applications: Aircraft windows, lenses, optical fibers, automotive tail lights, sky-lights.

2. PVC (Polyvinyl chloride):

• Synthesis: Prepared by heating vinyl chloride in an autoclave in the presence of benzoyl peroxide.
  
• Properties:
  • Rigid PVC: Hard, brittle, chemically inert.
  • Plasticized PVC: Flexible, soft.
• Applications:
  • Rigid: Pipes, door/window frames, bottles.
  • Flexible: Cable insulation, artificial leather, garden hoses, raincoats.

Preparation of Bakelite:

Bakelite is a condensation polymer formed by the reaction of Phenol and Formaldehyde.

1. Formation of intermediates: Phenol reacts with formaldehyde (HCHO) in the presence of acid or base to form o-hydroxybenzyl alcohol and p-hydroxybenzyl alcohol.
2. Formation of Novolac (Linear): If phenol is in excess and an acid catalyst is used, linear chains called Novolac are formed.
3. Formation of Bakelite (Cross-linked): If formaldehyde is in excess and an alkaline catalyst is used, or if Novolac is heated with hexamethylenetetramine, a 3D cross-linked network is formed, known as Bakelite.

Reaction Representation:

Properties:
Hard, rigid, scratch-resistant, excellent electrical insulator, high thermal stability.

Uses:

• Electrical switches, plugs, and sockets.
• Handles for utensils (cookware).
• Telephone casings.
• Laminates and adhesives.

Biodegradable Polymers:
Polymers that degrade into simple, non-toxic byproducts (like , , biomass) through the action of microorganisms (bacteria, fungi) and enzymes in the environment.

Classification:

1. Natural: Starch, Cellulose, Proteins, Chitosan.
2. Synthetic: Polylactic acid (PLA), Polyglycolic acid (PGA), Polycaprolactone (PCL), Polyhydroxyalkanoates (PHAs).

Methods of Degradation:

1. Hydrolysis: Breakdown of the polymer chain by reaction with water, often accelerated by acids or bases. Common in polyesters and polyamides.
2. Enzymatic Degradation: Enzymes secreted by microorganisms catalyze chain scission.
3. Oxidation: Degradation initiated by oxygen or ozone, leading to chain breaking.
4. Photo-degradation: Breakdown caused by UV light/sunlight.

Uses: Sutures, drug delivery systems, compostable packaging.

Nylon-6,6:

It is a polyamide obtained by the condensation polymerization of a diamine and a dicarboxylic acid.

Monomers:

1. Hexamethylene diamine:
2. Adipic acid:

Synthesis:
When equimolar amounts of hexamethylene diamine and adipic acid are heated under high pressure and temperature (), water molecules are eliminated to form Nylon-6,6.

Equation:

Properties & Uses:

• High tensile strength, abrasion resistance, and high melting point.
• Used in bristles for toothbrushes, textile fibers (carpets, clothing), gears, and bearings.

Conducting Polymers:
Organic polymers that conduct electricity. While most polymers are insulators, intrinsically conducting polymers (ICPs) possess electrical, magnetic, and optical properties similar to metals or semiconductors. Examples: Polyacetylene, Polyaniline, Polypyrrole.

-electron Delocalization:

• The backbone of conducting polymers consists of a conjugated system (alternating single and double bonds, e.g., ).
• In a conjugated system, the -orbitals of the carbon atoms overlap continuously.
• This allows the -electrons to be delocalized (move freely) over the entire polymer chain rather than being confined to a single bond.
• However, conjugation alone is usually not enough for high conductivity; the polymer band gap must be lowered, typically through doping to create mobile charge carriers.

Doping in Polymers:
The process of oxidizing or reducing a neutral conjugated polymer to introduce charge carriers and increase conductivity is called doping.

1. p-doping (Oxidative Doping):

• Mechanism: Removal of electrons from the polymer backbone (oxidation) usually by treating with a Lewis acid (e.g., ).
• Result: Creates a positive charge (hole) on the chain.
• Example: Polyacetylene treated with Iodine.
  

2. n-doping (Reductive Doping):

• Mechanism: Addition of electrons to the polymer backbone (reduction) usually by treating with a Lewis base or alkali metal (e.g., ).
• Result: Creates a negative charge on the chain.
• Example: Polyacetylene treated with Sodium.
  

In conducting polymers, charge transport occurs via specific defects or quasiparticles rather than simple free electrons.

1. Solitons:

• Found in polymers with degenerate ground states (e.g., trans-polyacetylene).
• It is a topological defect or a domain wall separating two phases of opposite oscillation.
• Types: Neutral soliton (radical), Positive soliton (cation), Negative soliton (anion).

2. Polarons:

• Found in polymers with non-degenerate ground states (e.g., Polypyrrole, Polythiophene).
• Formed when an electron is removed (oxidation) or added (reduction).
• It creates a radical-ion associated with a lattice distortion. A polaron has spin .

3. Bipolarons:

• Formed upon further oxidation/reduction of a polaron.
• Two polarons combine to lower energy, forming a pair of like charges (e.g., dications) stabilized by lattice distortion.
• Bipolarons have zero spin and are the primary charge carriers in highly doped conducting polymers.

1. Electronics:

• OLEDs (Organic Light Emitting Diodes): Used in display screens (TVs, phones) due to their ability to emit light when voltage is applied.
• Organic Transistors: Used in flexible electronic circuits and RFID tags.
• Wiring: Lightweight electromagnetic shielding.

2. Energy Storage:

• Batteries: Used as electrode materials (cathode/anode) in rechargeable batteries due to reversible doping/undoping capability.
• Supercapacitors: High surface area and redox activity allow for high energy density storage.
• Solar Cells: Used in organic photovoltaic cells to convert sunlight into electricity.

3. Sensors:

• Chemical Sensors: Conductivity changes upon exposure to specific gases (e.g., ) or volatile organic compounds.
• Biosensors: Used to detect glucose or DNA by coupling the polymer with biological enzymes; the reaction alters the polymer's conductivity.

Vulcanization:
A chemical process involving the heating of raw natural rubber with sulfur (or other curing agents) at to improve its physical properties.

Mechanism:

• Raw rubber (Polyisoprene) consists of long flexible chains that slide past each other (plastic flow).
• Sulfur forms cross-links (disulfide bridges ) between the reactive allylic carbon atoms of adjacent isoprene chains.
• These cross-links anchor the chains, preventing slippage.

Benefits (Properties of Vulcanized Rubber):

1. Reduced Stickiness: Unlike raw rubber, it is not sticky.
2. Higher Tensile Strength: Much stronger than raw rubber.
3. Elasticity: Returns to original shape after stretching (memory).
4. Temperature Stability: Usable over a wider temperature range ( to ).
5. Resistance: Better resistance to oxidation, wear, and abrasion.

Lubricant:
A substance introduced between two moving surfaces to reduce friction, heat, and wear by creating a protective film between them.

Fluid Film (Hydrodynamic) Lubrication:

• Condition: Occurs when the moving surfaces are completely separated by a thick continuous film of lubricant (thickness ).
• Mechanism:
  1. The lubricant is pumped or drawn between the surfaces by the motion of the machine parts.
  2. This creates a pressure wedge that supports the load.
  3. The metal-to-metal contact is entirely prevented.
  4. The resistance to motion is due only to the internal viscosity of the lubricant, not the surface friction.
• Application: Used in delicate instruments, watches, clocks, and light machines running at high speeds.

1. Boundary (Thin Film) Lubrication:

• Condition: Occurs when the load is heavy or speed is low, and a continuous fluid film cannot be maintained.
• Mechanism: The lubricant forms a very thin adsorbed film (monolayer) on the metal surfaces due to physical or chemical adsorption. This film reduces direct metal-to-metal contact.
• Lubricants used: Vegetable oils or fatty acids with polar groups are effective as they adsorb strongly to metal surfaces.

2. Extreme Pressure (EP) Lubrication:

• Condition: Occurs under very high load and high temperature where even boundary films might fail/decompose.
• Mechanism: Special additives (EP additives) containing active elements like Chlorine, Sulfur, or Phosphorus are added.
• Action: At high temperatures generated by friction, these additives react chemically with the metal surface to form metallic chlorides, sulfides, or phosphides. These solid layers serve as lubricants and have high melting points, preventing welding/seizing of surfaces.

Lubricants often require additives to improve performance. Common additives include:

1. Anti-oxidants: Prevent the oxidation of oil at high temperatures, preventing sludge formation (e.g., aromatic amines, phenols).
2. Viscosity Index (VI) Improvers: Long-chain polymers (e.g., polyisobutylene) that help the oil maintain viscosity at high temperatures (prevent thinning).
3. Pour Point Depressants: Prevent wax crystallization at low temperatures, keeping the oil fluid (e.g., alkyl naphthalene).
4. Corrosion Inhibitors: Protect metal surfaces from acid attack (e.g., alkaline earth sulfonates).
5. Anti-wear/Extreme Pressure Agents: Form protective chemical films under high load (e.g., tricresyl phosphate, sulfurized fats).
6. Detergents/Dispersants: Keep engine parts clean by suspending carbon/dirt particles in the oil.

Viscosity:
It is the property of a fluid that determines its resistance to flow. It is the most important property of a lubricant.

• Importance: If viscosity is too low, the oil film may break under load. If too high, it causes excessive internal friction and power loss.

Viscosity Index (VI):
An arbitrary number indicating the effect of temperature changes on the viscosity of an oil.

• Low VI: Viscosity changes rapidly with temperature (bad).
• High VI: Viscosity changes very little with temperature (good).
• Importance: A good lubricant should have a high VI to function effectively in both cold starts (not too thick) and hot operating conditions (not too thin).

1. Flash Point: The lowest temperature at which the lubricant gives off enough vapor to ignite momentarily when a flame is brought near it. It indicates volatility and fire safety.
2. Fire Point: The lowest temperature at which the lubricant vapors burn continuously for at least 5 seconds when ignited. Usually higher than the flash point.
3. Cloud Point: The temperature at which the lubricant becomes cloudy or hazy due to the crystallization of dissolved waxes or separation of impurities upon cooling.
4. Pour Point: The lowest temperature at which the oil ceases to flow or pour. It is crucial for machines operating in cold environments.

1. Neutralization Number (Acid Value):

• Definition: The number of milligrams of KOH required to neutralize the free acid present in 1 gram of oil.
• Significance:
  • It indicates the acidity of the oil.
  • Fresh oil should have a low acid value.
  • A high acid value in used oil indicates oxidation and degradation (formation of corrosive acids), suggesting the oil needs changing.

2. Saponification Number:

• Definition: The number of milligrams of KOH required to saponify the fatty oil present in 1 gram of lubricant.
• Significance:
  • Mineral oils do not saponify, while vegetable/animal oils do.
  • It helps determine if a mineral oil has been compounded with fatty oils.
  • It gives an idea about the quality and type of fatty oil used in the blend.