1. Poly(arylene ether)s Characteristics PEEK PES Thermal stability
Hydrolytic stability
Wide range of properties

2. Nucleophilic Aromatic Substitution
PES

3. Nucleophilic Aromatic Substitution: Mechanism
Meisenheimer Complex (an intermediate)
F > NO2 > OTs > SOPh > Cl, Br, I
- Reverse order for aliphatic substitution
NO2 never lost in aliphatic systems
- In aliphatic SN2 displacement
R-I > R-Br > R-Cl > R-F
First step is R.D.S and favored by more electron withdrawing group

4. Nucleophilic Aromatic Substitution
Important features
Activating group and leaving group combinations

5. Nucleophilic Aromatic Substitution
PES
Important features - Conversion to phenate

6. Nucleophilic Aromatic Substitution
Important features
Conversion to phenate
Dehydrating agent
Upset stoichiometry
Solvent

7. Nucleophilic Aromatic Substitution
Important features
Activating group and leaving group combinations

8. Poly(arylene ether)s
PEEK
$25 lb
Tg = 144 °C Tm = 335 °C

9. Poly(arylene ether)s
PEK O C n
PEKK

10. Composite Materials
Composites have been used in airplanes since the 1950s
Critical applications of composites started in the early 1980s
Composite materials have some fundamentally different characteristics from metals
A composite is defined as two or more materials that retain their identities in the combination while yielding properties superior to either
Common composite types include fibrous, laminate and particulate
They can employ glass, aramid or carbon fibers
Various resins are used as the "matrix" bonding individual materials together into the desired form.

11. Advantages of Composite Construction
Structural Tailorability
Fibers are able to be oriented in directions that are best for the design.
Metals have the same properties in all directions.
Lightweight Strength
The advantage of being lighter than metals is usually misunderstood
Composites indeed have lower density than most metals
But for structural stability and with other design reasons, composite airplanes usually weigh the same as metal airplanes
By using a greater amount of lighter material, structural parts like skins are relatively thicker
A composite aircraft has the feel of a more sturdy airplane, and also has better dampening (less vibration transmission)

12. Advantages of Composite Construction
Better aerodynamics
Composite manufacturing more readily allows complex curved surfaces with fewer joints, seams and rivets
Easier to get smooth surfaces for laminar flow designs which contributes to additional speed.
Stealth Potential
Ability to minimize radar cross section
Electronic transparency means antennas can be hidden inside for streamlining without loss of reception.
Simple assembly
Aircraft assembly simplified, since many of the fasteners and small parts can be replaced with larger, more integrated structures.

13. General Beliefs…
Composites are thought to be corrosion and fatigue resistant
“…Composites are not subject to corrosion from natural or man made elements…”
“…Certainly with composites, fatigue is less of an issue than with metals" Scott W. Beckwith Technical director Society for the Advancement of Material & Process Engineering

14. General Knowledge about Composites
Things that mitigate the ESC problem include:
Crystallinity
Filled systems
Crosslinking

15. Example from the Scientific Literature
“Environmental Stress Cracking and Solvent Effects in High-Performance Polymeric Composites” Dillard, Kander, et. al Composite Materials ASTM, 1996
ESC of carbon fiber-reinforced thermoplastic and thermoset composite systems were investigated

16. One hour exposure at room temperature, not under any load…
Three-point Bending Tests
Graphite fiber reinforced, thermoplastic toughened cyanate ester thermoset system
Graphite fiber reinforced, semicrystalline thermoplastic composite
Graphite fiber reinforced, amorphous thermoplastic composite
One hour exposure at room temperature, not under any load…

17. Fracture Mode – Unexposed Graphite fiber reinforced, thermoplastic toughened cyanate ester thermoset system
Matrix enveloping the fibers
Failure primarily in the matrix (good thing)
Ductile fracture

18. Fracture Mode – Solvent Exposed Graphite fiber reinforced, thermoplastic toughened cyanate ester thermoset system
One hour exposure to solvent
at room temperature
not under load…
Fibers relatively clean
Brittle, interfacial failure
Ductile fracture

19. Conclusions
“Environmental Stress Cracking and Solvent Effects in High-Performance Polymeric Composites”
10 – 30% drop in bending strength under “safe” experimental conditions
Differences in failure modes upon solvent exposure
Propensity for interfacial failure

20. Research Questions
Are the long-term prospects clear for structural, load-bearing composites immersed in jet fuel in the F-22, JSF and Comanche?
What are the most appropriate methods for long-term aging studies of environmental stress cracking of composites for such applications?
Are there effective, easily implementable methods for mitigating solvent-induced ESC in composites?