e media strikes again! - materials science · e media strikes again! j.w. morris, ... creates 3d...

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

e media strikes again!


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


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


MSE 200A Fall, 2008


•  Materials with nearly random atom configurations –  “frozen liquids”

•  Used for: –  Isotropic properties

•  Window glass –  Microstructural uniformity

•  Metallic glass (“Metglass”) magnetic materials –  Formability

•  Thermoplastics


MSE 200A Fall, 2008


•  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


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


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)


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)


MSE 200A Fall, 2008

Macromolecular Solids

•  Polymeric solids –  Organics –  Plastics

•  Silicates –  Rocks and minerals –  Clay

•  Fibers –  Fabrics –  Fiber composites

•  Lipid bilayers –  Biological membranes

•  Quasicrystals


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


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)


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)


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)


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


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


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


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


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


MSE 200A Fall, 2008

Macromolecular Solids

•  Polymeric solids –  Organics –  Plastics

•  Silicates –  Rocks and minerals –  Clay

•  Fibers –  Fabrics –  Fiber composites

•  Lipid bilayers –  Biological membranes

•  Quasicrystals


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


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++


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


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


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)


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


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


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 =

e media strikes again! - materials science · e media strikes again! j.w. morris, ... creates 3d...

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