Advanced Materials Engineering – Polymer Module

Polymer Fundamentals

• Demand & Applications

  • Polymers are highly demanded engineering materials; novel variants replace metals/ceramics in weight-critical, corrosion-sensitive or cost-driven applications.

  • Advantages: low density, corrosion resistance, design flexibility, ease of processing, tunable properties (electrical, optical, mechanical).

• Origin of Raw Monomers

  • Fossil fuels → refining → saturated hydrocarbons (ethane, propane).

  • Cracking (heat) → unsaturated monomers (ethene, propene) → polymerization under $T$, $P$.

  • Degree of polymerization governed by reaction kinetics, catalyst, temp/pressure profile.

• Key Terminology

  • Monomer: basic building unit.

  • Mer: monomer residue within chain.

  • Polymer: high-molar-mass compound of many mers.

  • Polymerization: chemical process producing polymer from monomer.

Classification of Polymers

• By Source

  • Natural: DNA, RNA, cellulose, natural rubber, wool, sugarcane-derived polysaccharides.

  • Synthetic: nylon, polyester, PTFE, epoxy, PVC, etc.

• By Chemical Composition

  • Homopolymer: single repeating unit (PE, PTFE, PMMA…).

  • Copolymer: ≥2 repeating units; modes

  • Alternating: $ABAB\ldots$

  • Block: $A_mB_n$

  • Random: $A,B$ random sequence.

  • Graft: main chain A with B side chains.

• By Physical Structure

  • Linear → chains without branches (HDPE).

  • Branched → LDPE; lower packing, weaker secondary forces → flexible films.

  • Cross-linked → vulcanized rubber; covalent inter-chain bridges via heat/chemistry.

  • Network (highly cross-linked) → Bakelite, epoxy, polyurethane.

• Engineering Categories

  • Thermoplastics: soften on heating, harden on cooling (reversible). E.g., PE, PP, PC, PVC, PS, PMMA, nylon.

  • Thermosets: cross-link during cure, cannot remelt; strong, brittle, heat/chem resistant. E.g., phenol-formaldehyde (Bakelite), epoxy, polyester, melamine.

  • Elastomers: lightly cross-linked polymers showing high, recoverable elongation (natural rubber, SBR, silicone rubber).

Polymerization Mechanisms

• Addition (chain-growth): monomer unsaturation exploited (PE, PP, PVC). Stages: initiation, propagation, termination; free-radical example shown with $R!−!O!−!O!−!R$ initiator.

• Condensation (step-growth): functional groups react with elimination (water/alcohol); polyesters, nylon, Bakelite.

• Physical-Nature Sub-Processes

  • Bulk: neat monomer + initiator; simple, high purity; heat removal difficult; highly exothermic; used for PS, PMMA.

  • Solution: monomer dissolved; lower viscosity/easier heat control; needs solvent recovery; suitable for PAN, PVA.

  • Suspension: water-insoluble monomer droplets suspended; water cools; bead polymers (PVC). Particle size control tricky.

  • Emulsion: monomer emulsified by surfactant micelles; polymerize inside micelle; high MW possible; separation requires de-emulsifier; common for VAc, VC, acrylates.

Isomerism in Polymers

• Structural

  • Chain, positional, functional group.

• Stereoisomerism

  • Configurational (cannot interconvert via bond rotation): isotactic, syndiotactic, atactic PMMA.

  • Conformational (rotation around single bond) → gauche/anti forms.

• Optical & Geometric variants also possible.

Thermal / Mechanical Behaviour

• Glass Transition $T_g$

  • Amorphous polymer changes glassy → rubbery.

  • $T_g$ list: $\text{PS} \; +100^\circ C,\; \text{LDPE} \; −120^\circ C,\; \text{PC} \; +150^\circ C$ etc.

• Melting $T_m$ & Crystallinity

  • Semi-crystalline polymers (PE, PP, PET, PTFE) contain lamellae and amorphous tie chains; degree of crystallinity 10–80 %; affects density, stiffness, barrier properties (tortuous path concept).

  • Spherulite morphology: folded chains radiating from nucleation centre.

Viscoelasticity & Rheology

• Basic Parameters

  • Stress $\sigma,\tau$; strain $\varepsilon,\gamma$; rate $\dot{\gamma}$; moduli $E,G$; viscosity $\eta$; compliance $J$.

  • Deborah number $De = \frac{\tau<em>{relax}}{t</em>{obs}}$ determines solid- vs liquid-like response.

• Models

  • Elastic: Hookean spring $\sigma = E\varepsilon$.

  • Viscous: Newtonian dashpot $\sigma = \eta \dot{\varepsilon}$.

  • Viscoelastic analogues: Maxwell, Kelvin-Voigt, Standard Linear Solid, Jeffrey, Herschel-Bulkley etc.

• DMA outputs: Storage modulus $E'$, loss modulus $E''$, tan$\delta$.

Rubber Science

• Latex → Dry Rubber: centrifuging (↑DRC), coagulation, drying.

• Vulcanization

  • Sulphur, ZnO, stearic acid, peroxides cure at $120!−!180^\circ C$; cross-link density (mol cm⁻³) affects hardness, elasticity.

  • Rheograph: $M<em>L$, $M</em>H$, scorch time $t<em>{s2}$, optimum cure $T</em>{90}$, cure rate.

• Compounding Ingredients (phr)

  • Polymer 100, fillers (Hi-Sil 233 50), plasticizer oil 5, ZnO 4, stearic acid 2, antioxidants 1, accelerators (MBTS 1.5, TMTM 0.5), sulphur 2, DEG 2.

  • Equipment: internal mixer (rotor, ram), two-roll mill (friction ratio 1:1.25, nip/bank control).

• Mechanical Behaviour

  • Hyperelasticity, incompressibility $\nu=0.5$; hysteresis (energy lost as heat); Mullins effect (stress softening under cyclic loading).

Polymer Processing

• Injection Molding (IM)

  • Components: hopper, barrel & screw (feed, compression, metering), heaters; clamping toggle, mold & cooling.

  • Design criteria: uniform wall, ribs (<70 % wall), boss/ribs spacing, fillets/radii (1.5× thickness outer, 0.5× inner), draft (>$1^\circ$), undercuts, surface texture.

  • Mold types: two-plate, three-plate, stack; hot runner vs cold; gate designs (sprue, pin-point, edge, tab, fan, diaphragm, tunnel, thermal, valve).

  • Feed system analysis: pressure drop $\Delta P = f(k,n,\dot{Q},R,L)$; balance flow; optimize runner size subject to $\Delta P_{max}$; runner volume ≤30 % (cold) /100 % (hot) of cavity volume.

• Extrusion

  • Extruder screw (L/D 20:1–30:1; compression ratio 1.5–3); sections: feed, transition, metering.

  • Outputs: drag flow, pressure flow, leakage; viscosity via $\tau = \mu \dot{\gamma}$; design exercises included for PVC, PP pipes.

  • Processes: pipe, film blowing, profile.

• Other Processing

  • Film blowing parameters: MFI, density, bubble ratio, frost line height.

  • Blow molding (EBM, IBM): parison formation, inflation, cooling; comparisons (tooling cost, capacity, flash).

  • Thermoforming: vacuum, pressure, mechanical; heating, forming, trimming; materials ABS, LDPE, PVC.

  • Compression & transfer molding; rotational molding (large hollow parts).

Degradation Phenomena

• Mechanisms & Agents

  • Photodegradation (UV + O₂), thermo-oxidative, hydrolytic, biodegradation, mechanical.

  • Oxidation primary route; ozone cracking in unsaturated rubbers (SBR, NR).

  • Plastics lifetimes 20–500 yr; color change, embrittlement.

• Resin Identification Codes $1\text{–}7$ (PET, HDPE, PVC, LDPE, PP, PS, OTHER).

Polymer Rheology Tools

• Capillary rheometer (Poiseuille flow) equations $\tau = \frac{\Delta P R}{2L},\; \dot{\gamma} = \frac{4Q}{\pi R^3}$; MFI correlated to viscosity/intrinsic viscosity.

Molecular Metrics

• Degree of polymerization $DP=\frac{M<em>n}{m}\; \text{or}\; \frac{M</em>w}{m}$.

• Molecular weights: $M<em>n = \frac{\sum n</em>i M<em>i}{\sum n</em>i},\; M<em>w = \frac{\sum n</em>i M<em>i^2}{\sum n</em>i M<em>i},\; PD = \frac{M</em>w}{M_n}$.

• Crystallinity index (XRD): $\text{CI}=\frac{A<em>{cryst}}{A</em>{cryst}+A_{amorph}}$.

Computational Materials Science & FEA Basics

• Process Chain: Physical problem → mathematical model (PDEs) → numerical model (mesh) → preprocessing (geometry, material, BCs) → solution → post-processing (contours).

• Elements, Nodes, Mesh size: discretization converts continuum to finite element network.

• Simulation Types: static, dynamic, thermal, flow (CFD), electromechanical.

• Requirements: CAD model, property data, boundary conditions, solver (ANSYS, SolidWorks), high-RAM PC (≥32 GB).

• Future Trends: automotive crash/thermal simulations, civil structural analysis, tire curing, molecular optimization.

Key Equations & Data (selected)

• Newton’s law $\tau = \mu \dot{\gamma}$; Power-law $\tau = A \dot{\gamma}^n + \tau_0$.

• Hooke $\sigma=E\varepsilon$; Poisson $\nu=0.5$ incompressible.

• Deborah number $De=\tau<em>{relax}/t</em>{obs}$.

• Sprue/runner diameter balance $D<em>{down}=D</em>{up} / \sqrt[3]{n}$ for equal velocity.

• Cure kinetics: $t<em>{90}$ (90 % torque rise), $CR = (M</em>H−M<em>L)/(t</em>{90}−t_{s2})$.

These bullet-point notes consolidate all major and minor concepts, equations, examples, processing methods, degradation pathways, rheological/thermal behaviour, and computational analysis techniques discussed across Lectures 1–13 of ME 4301 Advanced Materials Engineering (Polymer section).