polymer synthesis chem 421 poly(arylene ether)s characteristics –thermal stability –hydrolytic...
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Polymer SynthesisCHEM 421
Poly(arylene ether)s
O O C
O
n
C O S
O
nO
CH3
CH3
O
• Characteristics
–Thermal stability
–Hydrolytic stability
–Wide range of properties
PEEK
PES
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Polymer SynthesisCHEM 421
Nucleophilic Aromatic Substitution
C O S
O
nO
CH3
CH3
O PES
C OH S
O
O
CH3
CH3
HO Cl Cl
Weak base (K2CO3)Polar, aprotic solvent (NMP)Azeotropic agent (toluene)
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Polymer SynthesisCHEM 421
Nucleophilic Aromatic Substitution:Mechanism
X
+ Y
X
Y
X
Y
X
Y
X
YY
+ X
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-FFirst step is R.D.S and favored by more electron withdrawing group
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Polymer SynthesisCHEM 421
Nucleophilic Aromatic Substitution
• Important features– Activating group and leaving group combinations
Cl
Cl S
O
O
CH3
Cl
Cl S
O
O
CH3
NaOH
350 C
NaOH
150 C
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Polymer SynthesisCHEM 421
Nucleophilic Aromatic Substitution
• Important features - Conversion to phenate
PESC O S
O
nO
CH3
CH3
O
C OH S
O
O
CH3
CH3
HO Cl Cl
Weak base (K2CO3)Polar, aprotic solvent (NMP)Azeotropic agent (toluene)
OH + K2CO32
O K + KHCO3+ OH
O K O K + H2CO3+ H2O + CO2
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Polymer SynthesisCHEM 421
Nucleophilic Aromatic Substitution
HO S
O
O
• Important features– Conversion to phenate
– Dehydrating agent
» Upset stoichiometry
– Solvent
H2O HOCl S
O
O
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Polymer SynthesisCHEM 421
Nucleophilic Aromatic Substitution
• Important features– Activating group and leaving group combinations
X S
O
O
X C
O
Cl S
O
O
F C
O
Cl S
O
O
F C
O
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Polymer SynthesisCHEM 421
Poly(arylene ether)s
HO OH C
O
O O C
O
n
F F
Ph-SO2-Ph335 CK2CO32-3 h
• $25 lb
• Tg = 144 °C Tm = 335 °C
PEEK
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Polymer SynthesisCHEM 421
Poly(arylene ether)s
PEK
C
O
OCO
n
PEKK
O C
O
n
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Polymer SynthesisCHEM 421
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.
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Polymer SynthesisCHEM 421
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)
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Polymer SynthesisCHEM 421
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.
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Polymer SynthesisCHEM 421
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. BeckwithTechnical directorSociety for the Advancement of Material &
Process Engineering
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Polymer SynthesisCHEM 421
General Knowledge about Composites
• Things that mitigate the ESC problem include:–Crystallinity–Filled systems–Crosslinking
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Polymer SynthesisCHEM 421
Example from the Scientific Literature
• “Environmental Stress Cracking and Solvent Effects in High-Performance Polymeric Composites”
Dillard, Kander, et. alComposite MaterialsASTM, 1996
• ESC of carbon fiber-reinforced thermoplastic and thermoset composite systems were investigated
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Polymer SynthesisCHEM 421
Graphite fiber reinforced, thermoplastic toughened cyanate ester thermoset systemGraphite fiber reinforced, semicrystalline thermoplastic compositeGraphite fiber reinforced, amorphous thermoplastic composite
Three-point Bending Tests
One hour exposure at room temperature,not under any load…
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Polymer SynthesisCHEM 421
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
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Polymer SynthesisCHEM 421
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
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Polymer SynthesisCHEM 421Conclusions
• “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
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Polymer SynthesisCHEM 421
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?