Unit 2: Polymers

Polymers are high-molecular-weight macromolecules built by covalently linking many small repeat units (monomers). The field originated with Staudinger's macromolecular hypothesis (1920), which established that these are true long-chain molecules, not colloidal aggregates. This unit hangs on how chain architecture governs bulk behaviour.

Defining features that later sections rely on:

• Repeat unit: the recurring structural motif, e.g. –CH2–CH2– in polyethylene.
• Polymerization type: addition (chain-growth, no by-product) versus condensation (step-growth, expels small molecules like H2O).
• Functionality: number of reactive sites per monomer; ≥2 is required for chain growth, ≥3 permits crosslinking.
• Molar mass distribution: polymers are polydisperse, described by number-average (Mn) and weight-average (Mw) masses.

II. Fundamentals and Descriptors

A. Classification of polymers

Polymers are grouped by several independent criteria that fix their processing and use.

• By origin: natural (cellulose, protein), synthetic (nylon, PVC), semi-synthetic (cellulose acetate).
• By structure: linear (HDPE), branched (LDPE), crosslinked/network (bakelite, vulcanized rubber).
• By polymerization: addition (polyethylene, PVC, PS) and condensation (nylon-6,6, polyester, bakelite).
• By monomer type: homopolymer (single monomer) and copolymer (two or more, e.g. SBR).
• By thermal response: thermoplastics (soften on heating, reusable, e.g. PE, PP) and thermosets (set irreversibly by crosslinking, e.g. bakelite, epoxy).

B. Degree of polymerization

The degree of polymerization (DP) counts how many repeat units are joined in a chain, linking molar mass to structure.

DP = M(polymer) / M0

• M(polymer): average molar mass of the polymer chain (g/mol).
• M0: molar mass of one repeat unit (g/mol).
• Worked example: polyethylene of Mn = 28,000 g/mol with M0 = 28 (–CH2CH2–) gives DP = 1000 units.
• Significance: low DP gives waxy, weak solids; high DP raises tensile strength, melt viscosity and softening point.

C. Structure-property relationship

Chain architecture and intermolecular forces determine mechanical and thermal properties.

• Chain length: longer chains increase entanglement, so strength and melting point rise with DP.
• Intermolecular forces: H-bonding (nylon) and dipole forces (PVC) stiffen and strengthen material versus weak van der Waals in PE.
• Crosslinking: raises rigidity and solvent resistance; heavy crosslinking (ebonite) yields hard, infusible solids.
• Branching: hinders packing, lowers density and strength (LDPE softer than linear HDPE).

III. Physical Structure and Thermal Behaviour

A. Molecular shape and Crystallinity

Molecular shape decides how closely chains pack, and packing sets the degree of crystallinity.

• Molecular shape: linear, regular chains (HDPE, isotactic PP) fold into ordered lamellae; irregular, branched or bulky-side-group chains resist ordering.
• Crystalline regions: tightly packed, ordered zones held by close-range forces; give higher density, opacity, strength and a sharp melting point (Tm).
• Amorphous regions: randomly coiled chains; give flexibility, transparency and toughness.
• Semi-crystalline reality: most polymers are part-crystalline; % crystallinity (e.g. HDPE ~90%, LDPE ~50%) correlates with stiffness.
• Factors favouring crystallinity: structural regularity, stereoregularity (tacticity), strong intermolecular H-bonding, minimal branching.

B. Glass transition temperature (Tg)

Tg is the temperature at which an amorphous polymer changes from a hard, glassy solid to a soft, rubbery state.

• Nature: a second-order transition affecting only amorphous regions; below Tg segmental motion is frozen, above it chain segments move.
• Contrast with Tm: Tm is melting of crystalline regions (first-order); a semi-crystalline polymer shows both Tg and Tm.
• Examples: natural rubber Tg ≈ −70 °C (rubbery at room temperature); polystyrene Tg ≈ 100 °C (glassy at room temperature).
• Practical use: service temperature must stay below Tg for rigid parts and above Tg for flexible ones.

C. Basic factors affecting Tg

Any structural feature that restricts chain-segment mobility raises Tg.

• Chain flexibility: flexible backbones (–Si–O– in silicones) lower Tg; stiff aromatic backbones raise it.
• Side groups: bulky or polar pendant groups hinder rotation and raise Tg (PS > PE).
• Intermolecular forces: strong H-bonding or polar attraction raises Tg (nylon high, PE low).
• Crosslinking: more crosslinks restrict motion and raise Tg.
• Plasticizers: added small molecules separate chains, increase mobility and lower Tg (plasticized PVC).
• Molecular weight: Tg rises with DP up to a plateau.

IV. Elastomers and Common Polymers

A. Structure, synthesis and applications of elastomers and common polymers

Elastomers are lightly crosslinked amorphous polymers (Tg below room temperature) that stretch elastically and recover; common polymers are the bulk commodity plastics.

1. Elastomers:

• Natural rubber: cis-1,4-polyisoprene; tapped as latex, then vulcanized with sulfur to crosslink chains, converting sticky gum into tough, elastic rubber (tyres, tubes).
• SBR (styrene-butadiene rubber): copolymer by emulsion polymerization; abrasion-resistant, used in tyres and footwear.
• Neoprene: polychloroprene from chloroprene; oil- and heat-resistant, used in hoses, gaskets.

2. Common polymers:

• Polyethylene (PE): addition polymer of ethylene; packaging film, bottles, insulation.
• Polyvinyl chloride (PVC): addition polymer of vinyl chloride; pipes, cables, flooring.
• Nylon-6,6: condensation of hexamethylenediamine and adipic acid; strong H-bonded fibre for textiles and gears.
• Bakelite: condensation of phenol and formaldehyde; a thermoset used in electrical switches and handles.

V. Biodegradable Polymers

A. Classifications

Biodegradable polymers are broken down by microorganisms or hydrolysis into CO2, water and biomass.

• Natural: starch, cellulose, chitosan, proteins.
• Microbial: polyhydroxyalkanoates (PHA), polyhydroxybutyrate (PHB).
• Synthetic: polylactic acid (PLA), polycaprolactone (PCL), polyglycolic acid (PGA).
• Blends: starch–polyester composites.

B. Uses

Hydrolysable ester and amide linkages let them serve where disposal or resorption matters.

• Packaging: PLA films, cups and disposable cutlery.
• Medical: PGA/PLA sutures and drug-delivery capsules that resorb in the body.
• Agriculture: mulch films and controlled-release fertilizer coatings.

C. Methods of degradation

Degradation proceeds by chain scission triggered by environment or enzymes.

• Hydrolytic: water cleaves ester bonds (PLA, PGA) into shorter fragments.
• Enzymatic/microbial: microbial enzymes digest chains (PHB, starch).
• Photodegradation: UV light breaks bonds in light-sensitive polymers.
• Oxidative: oxygen attack fragments the backbone, aiding further biodegradation.

VI. Conducting Polymers

Conducting polymers are organic polymers with conjugated backbones that conduct electricity when doped; polyacetylene, polypyrrole and polyaniline are the archetypes.

A. π-electron delocalization

Alternating single and double bonds create a delocalized π system along the chain.

• Conjugation: overlapping p-orbitals form continuous π and π* bands, like a 1-D semiconductor band gap.
• Pristine state: the undoped polymer has a filled valence band and empty conduction band, so it is only a weak semiconductor.

B. Doping mechanisms

Doping introduces charge carriers by partial oxidation or reduction, not by substitution.

• p-doping (oxidation): removes electrons using I2, FeCl3 or AsF5, creating positive carriers.
• n-doping (reduction): adds electrons using sodium naphthalide, creating negative carriers.
• Effect: conductivity of polyacetylene jumps from ~10⁻⁵ S/cm to ~10³–10⁵ S/cm on doping.

C. Charge carriers (solitons, polarons, bipolarons)

Doping generates mobile charged lattice defects that carry current.

• Soliton: a neutral or charged domain boundary in degenerate systems (trans-polyacetylene); a free radical whose ionization gives a charged, spinless carrier.
• Polaron: a radical cation (or anion), a single charge plus unpaired spin coupled to a local lattice distortion.
• Bipolaron: two like charges (dication) on one chain segment, spinless, formed at higher doping levels in non-degenerate polymers (polypyrrole, polyaniline).

D. Conductivity and transport with applications in electronics, energy, and sensors

Conduction is a combination of carrier motion along and between chains.

• Transport: intrachain hopping along the conjugated backbone plus interchain and inter-domain hopping; conductivity rises with doping and chain ordering.
• Electronics: organic LEDs, flexible transistors, antistatic coatings, EMI shielding.
• Energy: electrodes in rechargeable batteries and supercapacitors, polymer solar cells.
• Sensors: chemical and biosensors where analyte binding changes conductivity (polyaniline gas sensors).

VII. Lubricants

Lubricants are substances placed between moving surfaces to reduce friction, wear and heat.

A. Lubrication and its purpose

Lubrication interposes a film that keeps surfaces from directly contacting.

• Fluid-film lubrication: a thick film fully separates surfaces; friction is set by lubricant viscosity.
• Boundary lubrication: under high load a thin adsorbed layer protects surfaces where the film breaks down.
• Purposes: reduce friction and wear, dissipate heat, prevent corrosion, seal against contaminants, act as coolant.

B. Additives for lubricants

Additives are chemicals blended into base oil to enhance performance.

• Antioxidants: amines/phenols retard oxidation.
• Viscosity index improvers: polymers reduce viscosity change with temperature.
• Pour-point depressants: keep oil fluid at low temperature.
• Anti-wear/EP agents: ZDDP and sulfur/phosphorus compounds form protective films under high pressure.
• Detergents/dispersants: keep sludge and deposits suspended.

C. Chemical and physical properties of lubricant

Selection depends on measurable physical and chemical characteristics.

• Viscosity: resistance to flow; the primary selection parameter, temperature-dependent.
• Viscosity index (VI): measure of how little viscosity changes with temperature; high VI is desirable.
• Flash and fire point: lowest temperatures at which vapours ignite; fix safe operating range.
• Cloud and pour point: temperatures of wax appearance and cessation of flow at low temperature.
• Chemical properties: oxidation stability, acid value (neutralization number), and non-corrosiveness toward metal parts.