Polymer_Degradation_Flashcards
(24 cards)
What is the difference between physical ageing, chemical ageing, and plasticization in polymers?
Physical ageing is reversible structural relaxation below Tg (no chemical change); chemical ageing is irreversible degradation with bond scission; plasticization is reversible softening due to solvent penetration, increasing chain mobility.
How is thermal stability in a vacuum related to bond dissociation energy?
It is governed by the weakest bond in the polymer; higher bond dissociation energy means higher thermal stability, but toxic degradation products can still form at lower temperatures.
What is meant by triggers for degradation often combining?
Degradation mechanisms like heat, oxygen, moisture, and UV often act together (e.g., thermo-oxidation, photooxidation), accelerating polymer breakdown.
Why can extrusion reduce molecular weight in polymers?
High shear, temperature, and oxygen during extrusion cause chain scission through thermal, mechanical, and oxidative degradation.
Name five structural features that promote polymer degradation.
1) Ester/amide groups (hydrolysis) 2) Tertiary C-H bonds (radical formation) 3) C=C bonds (ozone/UV attack) 4) Polar groups (water uptake) 5) Bulky side groups (promote unzipping).
How can optical or electron microscopy reveal polymer degradation?
Optical microscopy shows surface cracks and discoloration; SEM reveals microvoids and fibrillation; TEM shows submicron damage, but none detect chemical changes or molecular weight.
How does SEC help quantify polymer degradation?
It measures molecular weight distribution; degradation shifts curves to lower Mw and broadens MWD, but cannot assess insoluble cross-linked fragments.
Why does cross-linking complicate SEC data interpretation?
Cross-linked polymers become insoluble and do not elute in SEC, leading to underestimation of degradation severity.
How do tensile strength, elongation at break, and Tg change with degradation?
Tensile strength and ETB decrease (especially ETB); Tg can increase (cross-linking) or decrease (scission or plasticization).
How is the melting region of semicrystalline polymers affected by degradation?
Lamellar thickness and crystallinity decrease, broadening the melting peak and lowering Tm; recrystallisation may occur if mobility increases.
What is the relationship between chain entanglements, degradation, and viscosity?
Degradation shortens chains, reducing entanglements and viscosity; lower Mw = fewer entanglements = lower melt viscosity.
How is FTIR used to identify chemical degradation products lost during TGA?
FTIR detects new or lost functional groups; for PVC, loss of C–Cl (~600–700 cm⁻¹) and formation of C=C (~1600 cm⁻¹) confirm HCl elimination.
Describe the random chain scission mechanism in PP and PE.
Backbone C–C bonds break randomly, reducing Mw; oxygen accelerates this by promoting radical formation in autoxidation.
How is PP/PE degradation hindered and quantified?
Free radical scavengers inhibit degradation; FTIR tracks oxidation via increased carbonyl index (~1720 cm⁻¹).
What are the two stages of PVC degradation?
1) Dehydrochlorination forms conjugated double bonds; 2) Cross-linking and gelation create insoluble networks and char.
How does PMMA degrade and why does it depolymerise cleanly?
It unzips from chain ends to monomer (methyl methacrylate) due to low packing efficiency from bulky side groups.
Why must PA, PET, and PLA be dried before processing?
Water causes hydrolytic chain scission during melting, reducing Mw; drying time and temperature must be carefully optimised.
What is the degradation concern with PHB?
It thermally degrades to crotonic acid, causing bubbling and processing instability; mitigated by copolymerisation or neutralisers.
How does cross-linking affect PEEK’s Tg and crystallisation?
Cross-linking raises Tg but reduces crystallisation ability by limiting chain mobility.
Why does irradiation initially help UHMWPE but later reduce strength?
Irradiation improves wear resistance via cross-linking, but long-term oxidative degradation causes embrittlement and strength loss.
Why are natural rubbers ozone-sensitive and how is ozonolysis prevented?
Ozone attacks C=C bonds forming cracks; prevention includes antiozonants (e.g. waxes) and saturated elastomers.
How do morphology and molecular weight affect PCL biodegradation?
Amorphous, low-Mw PCL degrades faster; crystalline, high-Mw PCL resists enzymatic attack.
What happens to PET during accelerated weathering?
UV, heat, and moisture cause yellowing, surface cracks, increased brittleness, and carbonyl formation (~1710–1740 cm⁻¹ in FTIR).
How does PVC’s char formation contribute to fire resistance?
PVC cross-links and forms a char layer during degradation, acting as a thermal barrier and slowing combustion.
