Poly(arylene ether)s

1. Poly(arylene ether)s PEEK PES • Characteristics • 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 O O C n PEK PEKK

10. Composite Materials • Composites have been used in airplanessince the 1950s • Critical applications of composites startedin the early 1980s • Composite materials have some fundamentallydifferent characteristics from metals • A composite is defined as two or more materialsthat retain their identities in the combinationwhile 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. 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 – UnexposedGraphite 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 ExposedGraphite 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?