J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
Microstructure
• Crystalline solids – Basic crystal structure – Defects in crystals
• Amorphous solids; glasses – Random distributions of atoms or molecules
• Macromolecular solids – Materials built of distinguishable molecules – Polymers – Silicates – Biological membranes
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
Amorphous Solids (Glasses)
• Solids with essentially random atomic arrangements
• “Configurational glasses” – atoms or molecules don’t fit
• “Chemical glasses” – bonds forbid order
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
Amorphous Solids (Glasses)
• Materials with nearly random atom configurations – “frozen liquids”
• Used for: – Isotropic properties
• Window glass – Microstructural uniformity
• Metallic glass (“Metglass”) magnetic materials – Formability
• Thermoplastics
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
Amorphous Solids (Glasses)
• Amorphous structures are never preferred by nature
• Liquid cooled too quickly to crystallize is a glassy solid at T<Tg – Example of a structural
“instability”
• Glass can often be reheated to soften it (thermoplastic)
v
T
liquidglass
crystal
Tg mT
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
Chemical Glasses
• Atoms are bonded into an irregular pattern
• Example: window glass SiO2 – Si (+4) bonded to four O (-2) in tetrahedral coordination – Creates SiO4
-4 tetrahedra – Bonded at corners (Si - O - Si) to generate solid SiO2 – Bonds tend to create frozen, irregular patter
= Oxygen
= Silicon
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
Chemical Glasses: Glassformers
• Glass-formers disrupt bonding, inhibit crystallization – Ionic species such as Na, Pb, Mg in silica – Bond to oxygen, seal tetrahedra
• Glass-formers may also contribute properties – Optical clarity of “leaded crystal” – Color of tinted glass (pink panes of Beacon Street)
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
Configurational Glasses
• Thermoplastic polymers – Long-chain polymers tangle like
strands of spaghetti
• Metallic glasses – Unusual magnetic, chemical or
mechanical properties – Always require glass-formers
(metalloids, rare earths) – Require rapid cooling
• Quenched ribbons or sheets • Bulk materials
– Soft magnets (transformers)
– “liquid metal” (golf clubs) - R. Busch, JOM, 52, 39 (2000)
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
Macromolecular Solids
• Polymeric solids – Organics – Plastics
• Silicates – Rocks and minerals – Clay
• Fibers – Fabrics – Fiber composites
• Lipid bilayers – Biological membranes
• Quasicrystals
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
Chain polymers
• Carbon bonded to H and to itself forms the simplest polymer chain
C
H
H
HH
Methane
H C C H
H H
H HEthane
H C C C H
H H H
H H HPropane
HH C C C
H H H
H H H
^ ^ ^ C C C
H H H
H H HPolyethylene
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
Chain polymers
• The chain polymer is elaborated by – replacing H with an atom or molecule
(R) – Replacing H by a “double bond”
between carbons
• The basic block (“mer”) multiplies itself into a “polymer” – E.g., by breaking double bond to bond
blocks into chain
C C
H H
H R
C C
F
F F
F
C C
H
H
CH3
C O
O CH3Tetraflourethylene (teflon) Methyl methacrylate
(plexiglass or Lucite)
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
Order in Polymers: Sidegroup Order
C C
H H
H R
C C
H H
H R
C C
H H
H R
C C
H H
H R
C C
H
HH
R
C C
H H
H R
C C
H H
H R
C C
H
HH
R
C C
H
HH
R
C
CH3
C
H
C
H
H
C
H
H
C
CH3
C
H
C
H
H
C
H
H
isotactic (one side)
syndiotactic (alternate side)
atactic (random)
Cis-polyisoprene (rubber)
Trans-polyisoprene (gutta percha)
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
Order in Polymers: Copolymers
random copolymer
A, B = mers
regular copolymer
block copolymer
branched copolymer
...AAABABAABAAAABAB.. ..ABABABABAB..
AAAAAAAAAABBBBBBAAAAAAAAAABBBBBBAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAA
AAAAAA
AAAAAAAAAAAAAAAAAABBBBBB
BBBBBB
(a)
(b) (c)
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
Order in polymers: Crystallization
• Long-chain polymers crystallize by aligning or folding chains
• Chain polymeric solids are usually only partially crystallized (10-90%)
fringed micelles folded chain crystals
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
Thermoplastic Polymers (Glasses)
• To suppress crystallization: – Use long chains, irregular sidegroups to inhibit crystallization – Cool quickly to freeze in glassy state
• Stable configurational glasses soften on heating – Thermoplastic forming
v
T
liquidglass
crystal
Tg mT
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
Network Polymers
• Blocks join by bonds to form a three-dimensional network
• The resulting network polymers are – Easy to cast into complex shapes – Dimensionally stable and strong – “Thermosetting” - decompose rather than soften on heating
OH
H
H
H
H
H
OH
H
H
H
H
H
O
C
H H
OH
H
H
H
OH
H H
H
H
H
C
H H
OH
H H
C
H HC
H H
H HC Phenol-formaldehydem(Bakelite): The first “blockbuster” commercial polymer
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
Network Polymers: Crosslinking
• Chain polymers connected by chemical bonds form 3d networks
• Example: vulcanization of rubber – Sulpher (S2) bridges cis-ordered groups – Cross-linking turns rubber from gum to engineering elastomer – The breakthrough that made the automotive industry:
• No rubber tires, no highways, no cars
C
CH3
C
H
C
H
H
C
H
H
C
CH3
C
H
C
H
H
C
H
H
C
CH3
C
H
C
H
H
C
H
H
C
CH 3
C
H
C
H
H
C
H
H
C
CH 3
C
H
C
H
H
C
H
H
C
CH3
C
H
C
H
H
C
H
HS S
C
CH3
C
H
C
H
H
C
H
H
C
CH3
C
H
C
H
H
C
H
H
C
CH 3
C
H
C
H
H
C
H
H
C
CH 3
C
H
C
H
H
C
H
HS-S
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
Network Polymers: Elastomers
• Elastomers are kinked polymers that stretch like springs – Rubber is kinked by the cis-configuration at the C=C double bond
• They must be cross-linked to provide a reference frame – Otherwise chains simply slide over one another – For example, natural rubber is a gum – Vulcanized rubber is an elastomer: it recovers its shape after a stretch
S S
S S
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
Macromolecular Solids
• Polymeric solids – Organics – Plastics
• Silicates – Rocks and minerals – Clay
• Fibers – Fabrics – Fiber composites
• Lipid bilayers – Biological membranes
• Quasicrystals
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
Silicates
• Are the basis for rocks, clays and minerals
• Common dirt is, in fact, aluminosilicate: Si-Al-Fe oxide
• Other minerals also have complex compositions with many elements
• Study SiO2 as a simple prototype
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
Silicates
• SiO4-4 tetrahedra join at corners
to form networks: strong, hard structure
• Open structure results in several crystal structures, glasses
• Glass formation promoted by ions that terminate oxygen bonds
M+--- M++
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
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
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
Layered Silicates: Using Clay
• Clay is soaked – Water molecules fit between
platelets
• Clay is formed – Water separates and lubricates
plates – They slide easily over one another – Can be molded into shapes
• Clay is fired – If clay dries, shape crumbles – Firing reorganizes bonds, creates 3d
network structure ⇒ clay becomes stone
H2O H2O H2O
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
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)
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
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
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
Graphite Fibers
• Graphite rolled into tube to form strong, stiff fiber
• Happens naturally when certain organic fabrics are “pyrolized”: heated to decompose into graphite
• Used to make strong, stiff composites – Strong but brittle – Weak in perpendicular direction – Sometimes woven into 2d or 3d
structures for multiaxial properties
J.W. Morris, Jr. University of California, Berkeley
MSE 200A Fall, 2008
Kevlar Fibers
• Kevlar: benzene rings joined to make puckered sheets
• Puckered sheets form fibers – Join edgewise to form star pattern – “Accordion folds” perpendicular to axis of the fiber
• Fibers are strong, but elastic – Behave like elastomers while the accordion folds extend, then strong – Useful for body armor: “catch”projectiles
C = ON
H
NH
CO =
CO =N
H
C = ON
H
NH
CO =
CO =N
H
C = ON
H
NH
CO =
CO =