Macromolecular Solids
MSE 200A Fall, 2008
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Polymeric solids
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Silicates
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Fibers
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Lipid bilayers
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Quasicrystals
– Organics – Plastics
– Rocks and minerals – Clay – Fabrics – Fiber composites – Biological membranes
J.W. Morris, Jr. University of California, Berkeley
Silicates
MSE 200A Fall, 2008
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Are the basis for rocks, clays and minerals
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Common dirt is, in fact, aluminosilicate: Si-Al-Fe oxide
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Other minerals also have complex compositions with many elements
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Study SiO2 as a simple prototype
J.W. Morris, Jr. University of California, Berkeley
Silicates
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MSE 200A Fall, 2008
++ -
M
- M+
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SiO4-4 tetrahedra join at corners to form networks: strong, hard structure
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Open structure results in several crystal structures, glasses
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Glass formation promoted by ions that terminate oxygen bonds
J.W. Morris, Jr. University of California, Berkeley
Layered Silicates: Clay
• If SiO4-4 join at three corners, they form sheets • In clay, two sheets are bound together by ions to form a sandwich
MSE 200A Fall, 2008
J.W. Morris, Jr. University of California, Berkeley
Layered Silicates: Using Clay
H2O
MSE 200A Fall, 2008
H2O
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Clay is soaked
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Clay is formed
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Clay is fired
H2O
– Water molecules fit between platelets – Water separates and lubricates plates – They slide easily over one another – Can be molded into shapes – If clay dries, shape crumbles – Firing reorganizes bonds, creates 3d network structure ⇒ clay becomes stone
J.W. Morris, Jr. University of California, Berkeley
Fibers • Fibers are used in
– Woven fabrics – Fiber matrix composites
• Inorganic fibers
– Glass (fiberglass: structures) – Graphite (Composites: stiff aircraft parts, sports equipment )
• Organic
– Fabrics (nylon, etc.: clothing) – Kevlar (“bullet proof” jackets)
MSE 200A Fall, 2008
J.W. Morris, Jr. University of California, Berkeley
Glass Fibers • Glass drawn through a dye to produce thin fibers • May be
– Chopped (cut up) and embedded in epoxy (fiberglass) – Used in long lengths to conduct light (optical fibers)
• Engineering considerations:
– Advantages: high strength, stiffness, low density – Disadvantages: brittle
MSE 200A Fall, 2008
J.W. Morris, Jr. University of California, Berkeley
Graphite Fibers •
Graphite rolled into tube to form strong, stiff fiber
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Happens naturally when certain organic fabrics are “pyrolized”: heated to decompose into graphite
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Used to make strong, stiff composites – Strong but brittle – Weak in perpendicular direction – Sometimes woven into 2d or 3d structures for multiaxial properties
MSE 200A Fall, 2008
J.W. Morris, Jr. University of California, Berkeley
Kevlar Fibers N
N
H
H
O= C
O= C
O= C
H
C =O
H
C =O
H
C =O N
N
N
N N N O= C
H
O= C
H
O= C
H
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Kevlar: benzene rings joined to make puckered sheets
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Puckered sheets form fibers – Join edgewise to form star pattern – “Accordion folds” perpendicular to axis of the fiber
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Fibers are strong, but elastic – Behave like elastomers while the accordion folds extend, then strong – Useful for body armor: “catch”projectiles
MSE 200A Fall, 2008
J.W. Morris, Jr. University of California, Berkeley
Macromolecular Solids •
Polymeric solids
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Silicates
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Fibers
– Organics – Plastics
– Rocks and minerals – Clay – Fabrics – Fiber composites
MSE 200A Fall, 2008
•
Lipid bilayers
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Quasicrystals
– Biological membranes
J.W. Morris, Jr. University of California, Berkeley
The Lipid Bilayer
MSE 200A Fall, 2008
- NIST
J.W. Morris, Jr.
University of California, Berkeley
Phospholipid Building Block • Sequence from top – – – –
NH3(CH2)2+ PO4(CH2)2CHO2(CO)2 (CH2)n
• Electronic configuration
– Polar head (+): hydrophilic (active) – Non-polar CH2 tail: hydrophobic
• Physical configuration
– Cis-double bonds may kink tail
• Lubricants have similar features – Head attaches, tail extends out
MSE 200A Fall, 2008
J.W. Morris, Jr. University of California, Berkeley
Assembly of Bilayer
• In water, phospholipids form a double layer – Polar, hydrophilic heads contact water – Non-polar, hydrophobic tails are sealed in the interior
MSE 200A Fall, 2008
J.W. Morris, Jr. University of California, Berkeley
Lipid Bilayer Structure •
Physical structure – Phospholipid sandwich – Low T: chains ordered • semi-crystalline
– Normal T: chains disordered • Glassy
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Chemical structure – Surfaces attract hydrophilic species • Polar molecules
– Bulk attracts hydrophobic species • Trapped molecules
MSE 200A Fall, 2008
J.W. Morris, Jr. University of California, Berkeley
The Lipid Bilayer
MSE 200A Fall, 2008
- NIST
J.W. Morris, Jr.
University of California, Berkeley
Surface and Trans-Membrane Proteins
• Proteins adsorbed on surface – Membrane coating – Catalytic functions
• Proteins that penetrate wall – “Channel proteins”
• Permit ion transport • Participate in “nerve firing” – Electric field controls – Chemical species influence
– Mixed proteins
• Hydrophobic body pins • Hydrophilic ends react
MSE 200A Fall, 2008
J.W. Morris, Jr. University of California, Berkeley
Embedded Molecules: Cholesterol
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Membrane elasticity – Critical to integrity
• No brittle fracture
– Flexibility in volume change
• Pulses in blood flow • Ion exchange through channels
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Cholesterol
– Molecules imbed in bilayer – Stiffen membrane – May be
• Beneficial: some strengthens wall • Harmful: too much overhardens wall
MSE 200A Fall, 2008
J.W. Morris, Jr. University of California, Berkeley
Macromolecular Solids •
Polymeric solids
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Silicates
– – – –
Organics Plastics
Rocks and minerals Clay
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Fibers
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Lipid bilayers
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Quasicrystals – Ordered, but non-periodic – Ex.: semi-infinite spiral
– – –
Fabrics Fiber composites Biological membranes
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MSE 200A Fall, 2008
Unique origin of pattern
J.W. Morris, Jr. University of California, Berkeley
Thermochemical Properties • Materials respond to
– Thermal stimuli (temperature) – Chemical stimuli (composition or environment) – Electromagnetic stimuli (electric or magnetic fields) – Mechanical stimuli (mechanical forces)
• Consider the first two together
– Response to thermal or chemical stimuli defines thermochemical properties
MSE 200A Fall, 2008
J.W. Morris, Jr. University of California, Berkeley
Thermochemical Properties • Essential features:
– Thermodynamics: what material wants to do (forces) – Kinetics: what it can do, and how quickly
• Study
– Thermodynamics
• Properties • Equilibrium phase diagrams
– Kinetics
• Continuous: heat and mass diffusion • Structural phase transitions
– Environmental interactions • Wetting and catalysis • Corrosion
MSE 200A Fall, 2008
J.W. Morris, Jr. University of California, Berkeley
Thermodynamics • The conditions of equilibrium and stability
– Equilibrium ⇒ no desire for change – Deviation from equilibrium ⇒ driving force for change – Beyond limits of stability ⇒ must change
• Internal equilibrium
– T, P, {µ} are constant – Deviation drives heat and mass diffusion
• Global equilibrium
– Thermodynamic potential is minimum – Deviation drives structural phase transformations
MSE 200A Fall, 2008
J.W. Morris, Jr. University of California, Berkeley
First Law of Thermodynamics
• Defines “internal energy”, E • Energy is conserved dE = dW + dQ dW = work done (chemical + mechanical +electromagnetic) dQ = heat transferred (thermal work) – Energy transferred to one material is taken from another
MSE 200A Fall, 2008
J.W. Morris, Jr. University of California, Berkeley
Second Law of Thermodynamics • Defines “entropy”: S • Entropy is associated with
– Evolutionary time (most fundamental) – Heat – Randomness (information)
• When a system is isolated, S can only increase – Any system is isolated when its surroundings are included
MSE 200A Fall, 2008
J.W. Morris, Jr. University of California, Berkeley
A Simple “Adiabatic” System
• Simple system in thermally insulated container • Can do mechanical work
– Reversibly, with a frictionless piston – Irreversibly, with a paddle wheel – But no thermal interaction because of insulation
• This is called an “adiabatic” system
– An isolated system is one example of an adiabatic system
MSE 200A Fall, 2008
J.W. Morris, Jr. University of California, Berkeley
Change of State in an Adiabatic System
• Moving piston generates E-V curve • Turning paddle wheel
– Raises E at constant V – Changes the reversible E-V curve
Reversible curve from moving piston
E
• Paddle work is irreversible
– System moves to new E-V curve – System can never return
Irreversible change from paddle wheel
V MSE 200A Fall, 2008
J.W. Morris, Jr. University of California, Berkeley
The Measure of Time in an Adiabatic System Future
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The E-V curve divides states into
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States (E,V) below are the past
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States (E,V) above are in the future
E
Past
Present
– Past (below current curve) – Present (on current curve) – Future (above current curve)
– System cannot do work on paddle wheel – These states are unattainable
V
– Can be reached by paddle + piston – But system can never return
Reversible curve from moving piston
E
• Irreversible change from paddle wheel
The current (E,V) curve is the present
– System can sample these states at will
V MSE 200A Fall, 2008
J.W. Morris, Jr. University of California, Berkeley
Entropy = Time (State of Evolution)
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States (E,V) on a reversible curve have a common property: call it entropy (S)
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Assign a numerical value of S to each curve such that S is continuous
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Then S = S(E,V) measures the evolutionary time of the state (E,V)
curves of constant “entropy” S
E S
– S can only increase – S divides past (S’S)
V
MSE 200A Fall, 2008
J.W. Morris, Jr. University of California, Berkeley