Chapters 15 & 16: Microstructure

Polymer Structures, • Polymer

mer mer mer
Applications and Processing H H H H H H H H H H H H H H H H H H
C C C C C C C C C C C C C C C C C C
Issues to address...
H H H H H H H Cl H Cl H Cl H CH3 H CH3 H CH3
• What microstructural features dictate properties? Polyethylene (PE) Polyvinyl chloride (PVC) Polypropylene (PP)

• What happens during tensile deformation? • Covalent chain configurations and strength
Callister, Fig. 15.7
• How are polymers classified?
• How does tensile response depend on:
-temperature
-strain rate Linear Branched Cross-Linked Network
-time
Direction of increasing strength
Anderson 205- 16-1 Anderson 205- 16-2

Molecular Weight and Crystallinity Tensile Response: Brittle and Plastic Cases
Demo: stretch
• Molecular weight, Mw: Mass of a mole of chains semicrystalline σ(MPa)
HDPE
brittle failure
60 x near
smaller Mw larger Mw Near Failure onset of failure

40
necking plastic failure
-TS (tensile strength) often increases with Mw x
Why? Longer chains are entangled (anchored) better
20
• % Crystallinity: % of material that is crystalline unload/reload

-TS and E often

crystalline
Initial 0
0 2 4 6 8
ε
increase with
region
% Crystallinity crystalline
amorphous regions
-Annealing at Tanneal region slide
Callister,
Crystalline regions grow, so Fig. 15.11 semi-
amorphous
Callister,
crystalline crystalline Figs. 16.1,4
% Crystallinity ⇑ regions
case regions align
elongate
Anderson 205- 16-3 Anderson 205- 16-4
 Predeformation by Drawing Tensile Response: Elastomer Case
σ(MPa)
• Stretching deformation of the polymer prior to use
x
60 brittle failure
Drawing...
aligns chains to the stretching direction plastic failure
40 x
Result:
-Elastic modulus (E) increases in the stretching dir. 20 x
-Tensile strength (TS) increases in the stretching dir.
-Ductility (%EL) is reduced. Demo: demonstrate anisotropy
elastomer
0
ε
of saran wrap.
0 2 4 6 8
Annealing after drawing: Deformation final: chains
-alignment decreases is reversible! are straight,
initial: amorphous still
Compare to cold working in metals... chains are Callister, cross-linked
kinked, heavily Figs. 16.1, 16.14
cross-linked

Anderson 205- 16-5 Anderson 205- 16-6

Thermoplastics vs Thermosets Effect of T and Strain Rate: Thermoplastic

• Thermoplastics T • Decreasing T:
Callister,
rubber
σ(MPa)
viscous -increases E
-little cross linking mobile Fig. 16.9 80 4°C
liquid Tm -increases TS Data for the
-ductile liquid tough
plastic -decreases ductility semicrystalline
-soften with heating Tg 60 polymer: PMMA
20°C
polyethylene (#2) partially (Plexiglas)
polypropylene (#5) crystalline 40 40°C
crystalline
polycarbonate solid
solid
polystyrene (#6) 20
Molecular weight to 1.3
60°C
0
0 0.1 0.2 0.3
• Thermosets vulcanized rubber
• Increasing strain rate: ε
-large cross linking (10 to 50% of mers) epoxies -same effect as decreasing T
Callister,
-hard and brittle polyester resin Fig. 16.3
-do not soften with heating phenolic resin
Anderson 205- 16-7 Anderson 205- 16-8
 Time Dependent Deformation Summary
tensile test
• Stress Relaxation • General drawbacks to polymers:
εo strain
-Strain to εo and hold E, σy, Kc, Tapplication are small.
-Stress decreases w/time σ( t )
Deformation is often T & time dependent.
σ(t ) time
• Relaxation Modulus: Er(t)= ε Result: polymers benefit from composite reinforcement
o
• Data for Amorphous Polystyrene: • Thermoplastics (PE, PS, PP, PC):
Table 16.3
-Large drop in Er for T > Tg Smaller E, σy, Tapplication
105 rigid solid Larger Kc Good
Relaxation (small relax) overview
103 • Sample Tg(°C) values Easier to form and recycle
Modulus
Er(10s) transition PE (low Mw) -110 of
101 region PE (high Mw) - 90
• Elastomers (rubber): applications
in MPa
PVC + 87
Large reversible strains &
10-1
viscous liquid PS +100 • Thermosets (epoxies, polyesters): trade
-3 (large relax) PC +150 names
Callister, 10 Larger E, σy, Tapplication
Fig. 16.12 60 100 140 180T(°C)
Tg Smaller Kc
Anderson 205- 16-9 Anderson 205- 16-10
 Chapter 17: Terminology/Classification
Composite Materials • Composite Definition:
multiphase material w/significant proportions of ea. phase
• Matrix:
Issues to address... -The continuous phase
-Purpose: woven
• What are the classes and types? transfer σ to other phases fibers
protect phases from environ. 0.5µm
• Why are composites used instead of -Classification: MMC, CMC, PMC
metals, ceramics, or polymers? cross
metal ceramic polymer
• How do we estimate composite • Dispersed Phase: section
-Purpose: enhance matrix properties view
stiffness and strength?
MMC: increase σy, TS, creep 0.5µm
• What are some typical applications? CMC: increase Kc Hull, “An Intro. to Composite
Materials, Fig. 4.6
PMC: increase E, σy, TS, creep
-Classification: Particle-reinf, fiber-reinf, lamellar
Anderson 205- 17-1 Anderson 205- 17-2

Particle-Reinforced Particle-Reinforced: Elastic Modulus

• Examples: • Ec depends on volume fractions Vp and Vm:
-spheroidite steel particles: upper limit:
matrix: cementite (Fe3C)
(brittle)
Ec = VmEm + VpEp “rule of mixtures”
ferrite (α)
E(GPa)
(ductile) Callister,
Fig. 10.10 350
60µm lower limit:
Cu matrix 300
-WC/Co cemented carbide w/tungsten 250 1 Vm Vp
matrix: particles: = +
particles 200 Ec Em Ep
cobalt WC
Callister,
(ductile) (brittle, hard) 150
Fig. 17.3
Vm: 10-15vol%! Callister,
Fig. 17.4 0 20 40 60 80 100 vol% tungsten
600µm
-Automobile tires (Cu) (W)
matrix: particles: • Application to other properties
rubber C
(stiffer)
Electrical conductivity, σe: replace E by σe.
(compliant)
Callister, Thermal conductivity, k: replace E by k.
0.75µm Fig. 17.5 Anderson 205- 17-3 Anderson 205- 17-4
 Fiber-Reinforced-I Fiber-Reinforced-II very stiff,
strong C fibers
• Aligned Continuous: • Discontinuous, random 2D:
less stiff
Carbon-Carbon
view onto less strong
fibers: plane C matrix
Process: fiber/pitch,
γ’(Ni3Al) followed by burnout at up
matrix: to 2500°C
brittle fibers lie
α (Mo)
in plane
ductile Uses: disk brakes, gas
turbine exhaust flaps,
fracture nose cones
surface Fig. 4.24a,b: Matthews & Rawlings

2µm • Other variations:

Metal: γ’(Ni3Al)-α(Mo) Glass w/SiC fibers Discontinuous, random 3D
Process: glass slurry
Process: eutectic solidification Discontinuous 1D
Eglass = 76GPa
Fig. 3.5, Matthews & Rawlings:
Composite Materials: Engineering ESiC = 400GPa
and Science Figs. 4.22, 11.20,
Matthews & Rawlings
Anderson 205- 17-5 Anderson 205- 17-6

Critical fiber length Fiber Composite Properties

• For effective stiffening and strengthening:
fiber strength fiber diameter σ d
σ d • Valid when fiber length >> 15 τf
fiber length > 15 τfshear strength of c
c
fiber/matrix interface Elastic modulus in fiber direction:
• Example: For fiberglass, fiber length > 15mm is needed. Ec = EmVm + KEfVf
• Why? Longer fibers carry stress more efficiently
efficiency factor
Shorter, thicker fiber: Longer, thinner fiber:
σ d σ d aligned 1D: K=1 (anisotropic)
fiber length > 15 τf fiber length < 15 τf random 2D: K=3/8 (2D isotropy)
c c
fiber efficiency: Poor fiber efficiency: Better random 3D: K=1/5 (3D isotropy)
σ(x) σ(x) TS in fiber direction:
(TS)c = (TS)mVm + (TS)fVf (aligned 1D)

Anderson 205- 17-7 Anderson 205- 17-8

 Laminar Composites Composite Benefits
• CMCs: Increased toughness • PMCs: Increased E/ρ
• Stacked and bonded fiber-reinforced sheets Force E(GPa)
-stacking sequence: e.g., 0/90 particle-reinf 103
ceramics
line of
-benefit: balanced, in-plane stiffness 102 PMCs
constant E/ρ
fiber-reinf 10
metal/
1 metal alloys
un-reinf Fig. 4.1, Matthews
& Rawlings .1 G=3E/8 polymers Ashby &
• Sandwich panels Bend displacement .01 K=E Jones, p. 7
.1 .3 1 3 10 30
-low density honeycomb core Density, ρ [Mg/m3]
-4
-benefit: small weight, large bending stiffness εss (s-1)10
6061 Al • MMCs
face sheet
adhesive layer 10-6 Increased
honeycomb creep resistance
6061 Al
10-8
w/SiC Fig. 3.27, Matthews & Rawlings
whiskers σ(MPa)
10-10
Callister, Fig. 17.16 Anderson 205- 17-9 20 30 50 100 200 Anderson 205- 17-10