6. Material Properties

Question 1

Elastic response to tensile stress

Answer 1

Full recovery from deformation…
- Metals
- Ceramics
- Polymer Fibres
- Glassy, amorphous, semi-crystalline polymers (below Tg)
- Hard materials w/ high modulus (small deformation)

Question 2

Rubber elastic response to tensile stress

Answer 2

Full recovery…
- Low crosslink density polymer networks
- Soft solid materials
- Low modulus and high reversible deformation

Question 3

Viscoelastic response to tensile stress

Answer 3

Non-reversible deformation…
- Soft materials
- Polymer melts
- Semi-crystalline polymers
(*outside their elastic range)
Initially react elastically to stress but flow and deform irreversibly under prolonged stress

Question 4

Energy elasticity in hard materials (and in relation to temperature)

Answer 4

Stretching increases inter-atomic distances, potential energy increases - E drops with temperature; increased strain with same applied tensile stress

Question 5

Key concepts in stress-strain behaviour

Answer 5

• E (modulus) = stress (sigma) / strain (epsilon)
• STIFF: high modulus, small deformation, vs FLEXIBLE
• HARD withstand high tensile stress vs SOFT
• STRONG absorb high impact energy before breaking (area under curve), vs WEAK

Question 6

Behaviours above elastic limit

Answer 6

• Plastic deformation:
  Strain does not return to 0 on removal of tensile stress, original shape not fully restored, above ‘yield point’
• Brittle fracture

Question 7

Deformation of semi-crystalline polymer above Tg

Answer 7

1. Necking: rapid decrease in stress, local decrease in cross-section of sample, plastic deformation
2. Cold drawing:
   Stress constant, plastic deformation, viscous flow and breaking of crystalline regions, then chain orientation and re-crystallisation in drawing direction
3. Strain hardening:
   Stress increase, flow stopped due to oriented, re-crystallised chains, now significantly stiffer and stronger in the drawing direction, 2nd elastic region

Question 8

Elastomers

Answer 8

• Very low E modulus
• Fully elastic strain under low tension
• Stretching is exothermic, contraction is endothermic
• Contract under tension when heat is applied (unstrained rubber shows normal thermal expansion properties)
• Get stiffer w/ increasing temp. (E incr.)

Question 9

Helmholtz free energy (deltaA)

Answer 9

DeltaA = Delta’U’ - TDeltaS
U = Internal energy
F = (dA/dL)^V,T

Question 10

Elastomer strain

Answer 10

S = k*ln(omega)
(omega = amount of available chain conf.s)
Since elastomers strain via conformational change i.e entropy/S
Random coil therefore large entropy; chain uncoils, increasingly app-rotational orientation until fully extended

Question 11

Time dependency of polymer properties

Answer 11

Rate of conformational change in polymers is far slower than in small molecules
-> observed mechanical response depends on rate of deformation