1111

Description of Materials

1. Polymeric Materials

2. Ceramics

3. Composite Materials

2222

1. polymeric materials(1) introduction

elastomers (or rubbers)

polymers thermoplastics

plasticsthermosetting plastics(or thermosets)

thermoplastics can be reheated and formed into new shapes consist of very long main chains of carbons

covalently bonded together long molecular chains are bonded to each

other by secondary bondsthermosetting plastics set by chemical reaction cannot be reformed by reheating consist of a network of carbons covalently

bonded into a thermoset network structure engineering plastics

333333

(2) polymerization reactionschain growth polymerization small molecules (monomers) covalently bond to form very long chains (polymers)

ex. polyethylene

n : degree of polymerization (DP)ex. a particular type of polyethylene has a

molecular mass of 150,000 g/mol, what is its degree of polymerization?

molecular mass of polymer 150000DP = =

mass of monomer 28= 5357

44444

(a) chain polymerization steps(i) initiation: a radical is neededex.

one of free radicals react with ethylene molecule to form new longer chain free radical

555555

(ii) propagation process of extending polymer chain by addition of monomersenergy of system is lowered by polymerization

RCH2CH2 + CH2=CH2 RCH2CH2CH2CH2

(iii) termination by addition of termination free radical combining of two chains

R(CH2CH2)m + R(CH2CH2)n R(CH2CH2)m(CH2CH2)n R

(iv) average molecular weightaverage molecular weight determined byspecial physical-chemical techniques

average molecular weight of thermoplastic fi MiMm = fi

Mi = mean molecular weight of eachmolecular range selected

fi = weight fraction of the material having molecular weights of a selected molecular weight range

666666

ex. calculate the average molecular weight Mm for a thermoplastic material that has the mean molecular weight fractions fi for molecular weight ranges list below

fi Mi 19550Mm = = = 19550 g/mol fi 1.00(b) functionality of a monomer

functionality number of active bonds in a monomer bifunctional : two active bonds for the

polymerization of long chains ex. ethylene

trifunctional: three active bonds to form a network polymeric material ex. phenol C6H5OH

7777

(c) structure of noncrystalline linear polymerszig-zag configuration in ethylene due to 109oangle between carbon covalent bonds

on a larger scale the polymer chains are randomly entangled

entanglement increases tensile strength

branching decreases tensile strength

(d) vinyl and vinylidene polymersvinyl polymers one of the hydrogen atom is replaced by another atom or group of atoms

8888

ex.

vinylidene polymers both hydrogen of carbon are replaced by another atom or group of atoms

ex.

99999

(e) homopolymers and copolymershomopolymers polymer chain is made up of

single repeating units AAAAAAAAcopolymers polymer chains made up of two

or more repeating units random copolymers different monomers

randomly arranged in chainsABBABABBAAAAABA

alternating copolymers definite ordered alterations of monomers ABABABABABAB

block copolymers different monomersarranged in long blocksAAAAA.BBBBBBBB

9

101010101010

graft copolymers one type of monomergrafted to long chain of another AAAAAAAAAAAAAAAAAAA

B BB BB B

ex. a copolymer consists of 15 wt% polyvinyl acetate (PVA) and 85 wt% polyvinylchloride (PVC), determine the mole fraction of each component

No. of mole of PVA = 15/86 = 0.174 moleNo. of mole of PVC = 85/62.5 = 1.36 molemole fraction of PVA = 0.174/1.534 = 0.113mole fraction of PVC = 1.36/1.534 = 0.887

11111111

ex. determine the mole fraction of vinyl chloride and vinyl acetate in a copolymer having a molecular weight of 10,520 g/mol and a degree of polymerization of 160

MWav = fvc MWvc + fva MWva = fvc MWvc + (1 - fvc) MWvaMWav(polymer) 10520MWav = = = 66.75 g/mol DP 160

66.75 = fvc (86) + (1 - fvc)(62.5) fvc = 0.86 fva = 0.14

ex. a PVC-PVA copolymer has a ratio of 10:1vinyl chloride to vinyl acetate and amolecular weight of 16000 g/mol, what is its degree of polymerization

MWav = 10/11(62.5) + 1/11(86) = 64.6 g/molDP = 16000/64.6 = 248

1212121212

(f) other methods of polymerizationstepwise polymerization monomers chemically react with each other to produce linear polymers and a small molecule ofbyproduct condensation polymerization

network polymerization chemical reaction takes place in more than two reaction sites(3D network)

13131313

(3) industrial polymerizationraw materials: natural gas,

petroleum and coal

polymerization

granules, pellets, powders or liquids

bulk polymerization monomer and activator mixedin a reactor and heated and cooled as desired solution polymerization monomer dissolved in non-reactive solvent and catalyst suspension polymerization monomer and catalyst suspended in water emulsion polymerization monomer and catalyst suspended in water along with emulsifier

1414141414

(4) crystallinity and stereoisomerism in some thermoplastics

(a) solidification of thermoplastics there is no sudden change in specific

volume on cooling in noncrystallinethermoplastics

glass transition temperature Tgabove Tg, show viscous or rubbery behaviorbelow Tg, show glass-brittle behavior polyethylene -110oC polypropylene -18oC PVA 29oC polystyrene 75~100oCPVC 82oC polymethyl methacrylate 72oC

in partly crystalline thermoplastics, suddendecrease in specific volume occurs due tomore efficient packing of polymer chains

151515

(b) structure of partly crystalline thermoplastics the longest dimension of crystalline region

is 5~50 nm fringed-micelle modellong polymer chains of 5000 nm wandering successively through a series of disordered and ordered region

folded-chain modelsections of molecular chains folding on themselves, a transitionfrom crystalline to non-crystalline regions canbe formed

15

1616161616

(c) stereoisomerism in thermoplastics stereoisomer same chemical composition but

different structural arrangements atactic stereoisomer pendent methyl group of

polypropylene is randomly arranged on either side of main carbon chain

isotactic stereoisomer pendent methyl group is always on same side of the carbon chain

syndiotactic stereoisomer pendant group regularly alternates from one side of the chain to the other side

171717

(5) general purpose thermoplastics polyethylene, polyvinyl chloride (PVC)

polypropylene and polystyrene account for most plastic materials sold

some properties of general-purpose thermoplastics are considered:density: relatively low (~ 1)tensile strength: relatively low impact strength dielectric strength: generally good insulatormaximum-use temperature: relatively low

(54 ~ 149oC)

181818

(a) polyethylene mp : 110 ~ 137oC clear-to-whitish translucent

thermoplastic material types of polyethylene

low-density: has a branched-chain structurehigh-density: has a straight-chain structurelinear-low-density

applications: containers, insulation, chemical tubing, bottles, water pond liners18

191919

(b) polyvinyl chloride and copolymers mp : ~ 204oC PVC is amorphous, does not

recrystallize chlorine atoms produce large

dipole moments and also hinder electrostatic repulsion

PVC homopolymer has high strength (7.5 to 9 KSI) and is brittle

compounding of PVC modifies and improves properties(i) plasticizers impart flexibility

phthalate esters are commonly used(ii) heat stabilizers prevent thermal

degradation typical stabilizers are organometallic compounds of Pb and Sn

(iii) lubricants aid in melt flow of PVCwaxes, fatty esters and metallic soaps

(iv) fillers lower the cost. calcium carbonate

(v) pigments give color, opacity and weatherability

202020

(c) polypropylene mp : 167 ~ 177oC methyl group substitute every

second carbon atom in carbon polymer chain

high heat deflection temperature low density, good chemical resistance,

moisture resistance and heat resistance good surface hardness and dimensional

stability applications: housewares, appliances,

packaging, laboratory ware, bottles, etc.

(d) polystyrene mp : 150 ~ 243oC phenyl ring present on every

other carbon atom very inflexible, rigid, and brittle low processing cost and good dimensional

stability poor weatherability and easily attacked by

chemicals. applications: automobile interior parts, dials

and knobs of appliances and housewares

21212121

(e) polyacrylonitrile often in the form of fibers high strength good resistance to moisture

and solvents applications: sweaters and blankets comonomer for SAN and ABS resins.

(f) styrene-acrylonitrile (SAN) random amorphous copolymer of styrene

and acrylonitrile better chemical resistance, high heat-

deflection temperature, toughness and load bearing characteristics than polyester alone applications: automotive instrument lenses,

dash components, knobs, blender and mixer bowls

2222222222

(g) ABSABS : acrylonitrile + butadiene + styrene

good impact and mechanical strength, withease of processing

ABS can be considered a blend of a glassy copolymer (SAN) and rubbery domains (butadiene polymer)

applications: pipe and fittings, automotive parts, computer and telephone housings

232323232323

(h) polymethyl methacrylate (PMMA) mp : 160oC an acrylic commonly

known as Plexiglas. rigid and relatively strong completely amorphous and

very transparent applications: glazing of aircraft, boats,

skylights, advertising signs etc.

(i) fluoroplasticsmonomers have one or more atoms of Fpolytetrafluoroethylene (PTFE) a crystalline polymer soften at 370oC useful mechanical properties at

a wide temperature range exceptionally resistant to chemicals high impact strength, low tensile strength good wear and creep resistance applications: chemically resistant pipe,

parts, molded electrical components, nonstick coating

24242424

polychlorotrifluoroethylene (PCTFE) mp : 218oC

less crystalline and more moldable applications: chemical processing equipments, electrical applications, gaskets, O rings, seals, electrical components

24

25252525

(6) engineering thermoplasticslow densitylow tensile strengthhigh insulationgood corrosion resistance

25

26262626

(a) polyamides (Nylons) () main chain structure incorporates

repeating amide group amide linkage

produced by stepwise polymerization of a dibasic organic acid with a diamineex. Nylon 6, 6 mp: 250~266oC

also produced chain polymerization of ring compound containing both acid and amineex. Nylon 6, 6 mp: 216~225oC

properties of nylonNHO type of hydrogen bonding between molecular chain results in high strength, high heat-deflection temperature and good chemical resistance

26

272727

flexibility of carbon chain produces molecular flexibility which leads to low melt viscosity, easy processibility and high lubricity

applications: electrical equipments, gears, auto parts, packaging

(b) polycarbonate basic repeating unit

mp: 270oC

high tensile strength, impact strength, toughness and dimensional stability

high heat deflection temperature 27

2828

good electrical insulating property resistance to corrosion applications: safety shields, cams and gears,

helmets, aircraft components, boat propellers, housings for power tool and computer terminals

(c) phenyl oxide based resins basic repeating unit

produced by oxidative coupling of phenolicmonomers

high rigidity, strength, chemical resistance, dimensional stability and heat deflection temperature

wide temperature range, low creep high modulus

applications: electric connectors, TV tuners, small machine housing, dashboards and grills

2929292929

(d) acetals basic repeating unit

polyoxymethylenemp: 175oC

the strongest (68.9 Mpa) and stiffest in flexure (2820 Mpa) thermoplastics

excellent long term load carrying capacity and dimensional stability

two basic types: homopolymer (du Pont Delrin), and copolymer (Celeanese Celcon)

homopolymer is harder and rigid than copolymer

low wear and friction but flammable applications: fuel systems, seat belts,

window handles of automobiles, couplings, impellers, gears and housing

30303030

(e) thermoplastic polyesters two important polyesters:

polyethylene terephthalate (PET)

polybutylene terephthalate (PBT)

phenylene ring provides rigidity good strength and resistant to most

chemicals good insulator: independent of temperature

and humidity applications: switches, relays, TV tuner

components, circuit boards, impellers, housing and handles

313131

(f) polysulfone basic repeating unit mp : 315oC

phenylene rings provide high strength and rigidity

para-oxygen atom provides high-oxidation stability

can be used for long timse at 150~174oC high heat-deflection temperature, high

tensile strength, low tendency to creep applications: electrical connectors, cores,

circuit boards, pollution control equipments

(g) polyphenylene sulfide basic repeating unit

mp : 228oC rigid and strong highly crystalline. no chemical can dissolve it below 200oC applications: chemical process equipment,

emission control equipment, electrical connectors

323232323232

(h) polyetherimide

high heat and creep resistance and rigidity good electric insulation. applications: high voltage circuit breaker

housing, coils etc.

(i) polymer alloys polymer alloys mixture of structurally

different homopolymers or copolymers optimizes properties

some degree of compatibility needed.

33333333

(7) thermosetting plasticshigh thermal rigidityhigh dimensional stability resistance to creep and deformation light weight electric and thermal insulation

(a) phenolics low cost, good electric and heat insulating

properties and mechanical properties produced by polymerization of phenol and

formaldehyde33

34

two-stage (novolac) phenolic resinsin the first stage, a brittle thermoplatic resin is producedaddition of hexamethylenetetramine (as basic catalyst) creates methylene cross-linkages to form thermosetting material

various types of phenolic molding compounds are manufactured:

(i) general-purpose compounds: usually wood flour filled to increase impact resistance

(ii) high-impact-strength compounds: filledwith cellulose and glass fibers to provide impact strength

(iii) high electrical insulating compounds:mineral (Mica) filled to increase electrical resistance

(iv) heat-resistant compounds: mineral (asbetos) filled to withstand long-term exposure to 150~180oC

applications: wiring devices, telephone relay system, auto transmission parts, plywood lamination, adhesives, shell molding

34

3535353535

(b) epoxy group epoxy group

general chemical structure:

cross-linking (curing) of two linear epoxymolecules with ethylene diamine:

good adhesion, chemical resistance and mechanical properties

high molecular mobility, low shrinkage during hardening

applications: protective and decorative coating, drum lining, high voltage insulators and laminates

3636

(c) unsaturated polyesters ester linkage reaction of alcohol with acid

unsaturated polyester resin can be formed by reaction of diol with diacid that contains reactive double carbon-carbon bonds

36

37

linear unsaturated polyesters are usually cross-linked with vinyl-type molecules:

low viscosity and can be reinforced with low viscosity materials

open mold lay up or spray up techniques are used to process many small parts

compression molding is used for big parts applications: automobile panels and body

parts, boat hulls, pipes, tanks

37

383838

(d) amino resins (ureas and melamines) formed by reaction of formaldehyde with

compounds having NH2 group

melamine reacts with formaldehyde:

combined with cellulose fillers to produce low cost products with good rigidity, strength, and impact resistance

applications: electrical wall plates, molded dinnerware, buttons, control buttons, knobs, flooring

39

(8) elastomers (rubbers) ()polymers whose dimensions can be greatly changed when stressed and which return to original dimensions when the deforming stress is removed

(a) natural rubber produced from latex of Havea Brasiliensis tree

repeating structural unit: cis-1,4-polyisoprene

vulcanization heating rubber with sulfurand lead carbonate

39

404040

cross-linking of cis-1,4-polyisoprene chains by sulfur atoms

cross-linking of molecules restricts molecular movement and increases tensile strength

properties of natural rubber

41414141

(b) synthetic rubberstyrene-butadiene rubber (SBR)

most widely used synthetic rubber greater elasticity than natural rubbers

tougher and stronger, war resistant disadvantage : absorbs organic solvents and

swellnitrile rubbers 55~82% butadiene + 45~18% acrylonitrile resistance to solvents and wear less flexiblepolychloroprene (neoprene) increased resistance to

oxygen, ozone, heat and weather

low temperature flexibility, high cost

42424242

vulcanization of polychloroprene elastomersinstead of S, Zn and MgO are used

(c) silicone rubbers based on Si and O in the main chain basic repeating structural unit

polydimethyl siloxane

wide temperature range used in gaskets, electric insulation

4343

ex. how much sulfur must be added to 100 g of polyisoprene rubber to cross-link 5% of the mers?MW for isoprene : 68 g/mol(100/1.68) 0.05 32 = 2.35 g

ex. a butadiene-styrene rubber is made by polymering 1 monomer of styrene and 8 monomers of butadiene, how much S is required to react with 100 g of this rubber to cross-link 20% of the cross-link sites?wt% of butadiene in copolymer =

(8 54)/ (8 54 + 104) = 0.806amount of S required for 20% cross-linking =

100 0.806 32 = 9.55 g

54

44444444

(9) deformation of thermoplastics elastic deformation below Tg

stretching out of the covalent bonds within the molecular chains

plastic deformation above Tgsliding of molecular chains past each other by the breaking and remaking of secondrydipole bonding forces

combination of elastic deformation and plastic deformationuncoiling of the linear polymers

45454545

tensile stress vs. strain for PMMA

(b) strengthening of thermoplasticsseven factors to determine the strength of thermopalstics : increasing average molecular mass increases

strength up to a certain critical mass. as degree of crystallinity increases, tensile

strength, tensile modulus of elasticity and density of material increase

chain slippage during permanent deformation can be made more difficult by introduction of pendant atomic groups to main carbon chain

45

46

strength can be increased by bonding highly polar atoms on the main carbon chain

strength can be increased by introduction of O and N atoms into main carbon chain

introduction of phenylene ring into main polymer chain in combination with other elements (As, O, N, S) increases strength

adding plastic fibers increases the strengthpolymer tensile strengthPE 2.5~5 ksiPVC 6~11 ksipolyoxy methylene (acetal) 9~10 ksinylon 6, 6 9~12 ksinylon 6, 6 (with 40% glass fiber) 30 ksiphenylene oxide- based material 7.8~9.6 ksithermoplastic polyester 10 ksipolycarbonate 9 ksi

(c) strengthening of thermosetting plastics can be made by reinforcements andcreation of covalent bonds by chemicalreaction during setting

474747

(d) effects of temperature on strength thermoplastics soften as temperature

increases strength dramatically decreases after Tg

thermosets also become weaker but not viscous

thermosets are more stable at high temperature than thermoplastics

4848

(10) creep and fracture of polymerscreep deformation of polymer under a constant

applied load at a constant temperature continues to increase with time

ex. creep curve for polystyrene at 77oF

creep increases with increased applied stress and temperature.

creep rate is relatively low below Tgat Tg, deformation is more easily and is referred to as viscoelastic behaviorabove Tg, deformation is viscous flow

the creep of polymer is measured by the creep modulus which is the ratio of initial applied stress o to creep strain (t)a high value for creep modulus implies a low creep rate

49

glass fiber reinforcements decreases creep stress relaxation decrease in stress with

time under constant strainstress relaxation due to breaking and reforming of secondary bonds

decrease in stress with time is given by

= 0 e t/1 = C e Q/RT

: stress after time t 0 : initial stress : relaxation time T : temperature R : molar gas constant

ex. a stress of 1100 psi is applied to an elastomer at constant strain, after 40 days at 20oC, the stress decreases to 700 psi(a) what is the relaxation time? (b) what will be the stress after 60 days t 20oC?(a) = 0 e t/ 700 = 1100 e 40/

ln(700/1100) = -40/ = 40ln(1100/700) = 88.5 days

(b) = 1100 e t/88.5 = 1100 e 60/88.5 = 559 psi49

505050505050

ex. relaxation time for an elastomer at 25oC is 40 days, while at 35oC relaxation time is 30 days, calculate the activation energy for this stress-relaxation process.

1 1 Q 1 1 = C e Q/RT = exp[ ( )] 2 R T1 T240 Q 1 1

= exp[ ( - )]30 8.314 298 308Q = 22,000 J/mol = 22 kJ/mol

fracture of polymers two extreme modes: brittle or ductilefor thermosetting plastics, primarily brittle modefor thermoplastics, depending on temperature:

below Tg primarily brittle modeabove Tg primarily ductile mode

surface energy required to fracture a glassy polymer is about 1000 times greater than that would be required for the fracture just involved simple CC bond breaking

a craze in glassy thermoplastic is formed in a highly stressed region and consist of alignment of molecular chains combined with a high density of interdispersed voids

51

the structure of a craze near the end of a crack in a glassy thermoplastic

above Tg, thermoplastics can exhibit plastic yielding before fracture, molecular linear chains uncoil and slip past each other and gradually align closer together

5252

(11) polymers in biomedical applicationsbiopolymer majority are thermoplasticsadvantages : low density, ease of forming, and can be made for maximum biocompatiblerecent development : biodegradable polymers

(a) cardiovascular artificial heart valves

flange and leaflets are made from biometals(Ti or Co-Cr alloy)sewing ring made from PTFE or PET polymers are only materials that make connection of the valve to heart tissuePTFE is used as vascular graft to bypassclogged arterieshydrophobic polymer (polypropylene) membranes are used to oxygenate blood during bypass surgery

53

(b) opthalmiceye glasses, contact lenses and intraocularimplants are made of polymers hydrogel is used to make soft contact lenses

absorbs water and allows snug fit oxygen permeable made of poly-HEMA

hard lenses made from PMMA not oxygen permeable

mixed with siloxanylalkyl metacrylate andmetacrylic acid to make permeable and hydrophilic

intraocular lens are made of PMMA(c) drug-dilevery

biodegradable polymer polylactic acid (PLA), polyglycolic acid (PGA)

(d) suture used to close wounds and incisionsnonabsorbable suture : polypropylene, nylon, polyethylene terephthalaate, polyethyleneabsorbable suture : polyglycolic acid

54545454

(e) orthopaedicbone cement a structure material to fill the space between the implant and the bonePMMA is the primary material used in this purpose