Upload
madeline-mason
View
219
Download
1
Tags:
Embed Size (px)
Citation preview
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
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)
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
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
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
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
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
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
Polymer SynthesisCHEM 421
Poly(arylene ether)s
PEK
C
O
OCO
n
PEKK
O C
O
n
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.
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)
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.
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
Polymer SynthesisCHEM 421
General Knowledge about Composites
• Things that mitigate the ESC problem include:–Crystallinity–Filled systems–Crosslinking
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
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…
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
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
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
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?