polymer study

CHAPTER 14/15: POLYMER STUDIES

Issues:• What are the basic microstructural features of a polymer?

• How are polymer properties affected by molecular weight?

• How do polymeric materials accommodate the polymer chain?

• What are the tensile properties of polymers and how are they affected by basic microstructural features?

• Changing Polymer Properties: Hardening, anisotropy, and annealing in polymers.

• How does the elevated temperature mechanical response of polymers compare to ceramics and metals?

• What are the primary polymer processing methods?

Chapter 14 – Polymers

What is a polymer? Poly mer

many repeat unit

Adapted from Fig. 14.2, Callister 7e.

C C C C C CHHHHHH

HHHHHH

Polyethylene (PE)ClCl Cl

C C C C C CHHH

HHHHHH

Polyvinyl chloride (PVC)HH

HHH H

Polypropylene (PP)

C C C C C CCH3

HH

CH3CH3H

repeatunit

repeatunit

repeatunit

Ancient Polymer History

• Originally many natural polymers were used– Wood – Rubber– Cotton – Wool– Leather – Silk

• Oldest known uses of “Modern Polymers”– Rubber balls used by Incas– Noah used pitch (a natural polymer)

for the ark – as had all ancient mariners!

Polymer CompositionMost polymers are hydrocarbons – i.e. made up of H and C

(we also recognize Si-H ‘silicones’)• Saturated hydrocarbons

– Each carbon bonded to four other atoms

C C

H

H HH

HH

CnH2n+2

Unsaturated Hydrocarbons• Double & triple bonds relatively reactive – can form new bonds

– Double bond – ethylene or ethene - CnH2n

• 4-bonds, but only 3 atoms bound to C’s

– Triple bond – acetylene or ethyne - CnH2n-2

C CH

H

H

H

C C HH

Isomerism

• Isomerism– two compounds with same chemical formula can have quite

different structures Ex: C8H18

• n-octane

• 2-methyl-4-ethyl pentane (isooctane)

C C C C C C C CH

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H H3C CH2 CH2 CH2 CH2 CH2 CH2 CH3=

H3C CH

CH3

CH2 CH

CH2

CH3

CH3

H3C CH2 CH3( )6

Chemistry of Polymers• Free radical polymerization

• Initiator: example - benzoyl peroxide

C

H

H

O O C

H

H

C

H

H

O2

C C

H H

HHmonomer(ethylene)

R +

free radical

R C C

H

H

H

H

initiation

R C C

H

H

H

H

C C

H H

HH

+ R C C

H

H

H

H

C C

H H

H H

propagation

dimer

R= 2

Chemistry of Polymers• Free radical polymerization (addition polymerization)

• Initiator: example - benzoyl peroxide

C

H

H

O O C

H

H

C

H

H

O2

C C

H H

HHmonomer(ethylene)

R +

free radical

R C C

H

H

H

H

initiation

R C C

H

H

H

H

C C

H H

HH

+ R C C

H

H

H

H

C C

H H

H H

propagation

dimer

R= 2

Condensation Polymerization

• Some of the original monomer’s materials are shed (condensed out) during polymerization process

• Process is conducted in the presence of a catalyst• Water, CO2 are commonly condensed out but other

compounds can be emitted including HCN or other acids

Water is “Condensed out” during polymerization of Nylon

Bulk or Commodity Polymers

NOTE: See Table 15.3 for commercially important polymers – including trade names

MOLECULAR WEIGHT

molecules of #totalpolymer of wttotal

nM

iiw

iin

MwM

MxM

Mw is more sensitive to higher molecular weights

• Molecular weight, Mi: Mass of a mole of chains.

Lower M higher M


Molecular Weight Calculation

Example: average mass of a classN i M i x i wi

# of students mass (lb)1 100 0.1 0.0541 120 0.1 0.0652 140 0.2 0.1513 180 0.3 0.2902 220 0.2 0.2371 380 0.1 0.204

M n M w

186 lb 216 lb

iiw MwM

iin MxM

i ii

i iall i

N Mw N M

ii

iall i

Nx N

Degree of Polymerization, n

n = number of repeat units per chain

ii

wiiw

niin

mfm

m

mMnwn

mMnxn

unit repeat of weightmolecular average where

C C C C C C C CH

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

C C C C

H

H

H

H

H

H

H

H

H( ) ni = 6

mol. wt of repeat unit iChain fraction

• Covalent chain configurations and strength:

Direction of increasing strength


Molecular Structures

Branched Cross-Linked NetworkLinear

secondarybonding

Polymers – Molecular Shape

Conformation – Molecular orientation can be changed by rotation around the bonds– note: no bond breaking needed


Polymers – Molecular ShapeConfigurations – to change must break bonds

• Stereoisomerism

EB

A

D

C C

D

A

BE

mirror plane

C CR

HH

HC C

H

H

H

R

or C C

H

H

H

R

TacticityTacticity – stereoregularity of chain

C C

H

H

H

R R

H

H

H

CC

R

H

H

H

CC

R

H

H

H

CC

C C

H

H

H

R

C C

H

H

H

R

C C

H

H

H

R R

H

H

H

CC

C C

H

H

H

R R

H

H

H

CC

R

H

H

H

CC

R

H

H

H

CC

isotactic – all R groups on same side of chain

syndiotactic – R groups alternate sides

atactic – R groups random

cis/trans Isomerism

C CHCH3

CH2 CH2

C CCH3

CH2

CH2

H

ciscis-isoprene

(natural rubber)

bulky groups on same side of chain

transtrans-isoprene (gutta percha)

bulky groups on opposite sides of chain

Copolymers

two or more monomers polymerized together

• random – A and B randomly vary in chain

• alternating – A and B alternate in polymer chain

• block – large blocks of A alternate with large blocks of B

• graft – chains of B grafted on to A backbone

A – B –

random

block

graft


alternating

Polymer Crystallinity

Ex: polyethylene unit cell

• Crystals must contain the polymer chains in some way – Chain folded structure



amorphousregion

Polymer Crystallinity

• % Crystallinity: how much is crystalline.

-- TS and E often increase with % crystallinity. -- Annealing causes crystalline regions to grow. % crystallinity increases.

Adapted from Fig. 14.11, Callister 6e.(Fig. 14.11 is from H.W. Hayden, W.G. Moffatt,and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc., 1965.)

crystalline region

Polymers rarely exhibit 100% crystalline• Too difficult to get all those chains aligned

Mechanical Properties• i.e. stress-strain behavior of polymers

brittle polymer

plasticelastomer

FS of polymer ca. 10% that of metals

Strains – deformations > 1000% possible (for metals, maximum strain ca. 100% or less)

elastic modulus – less than metal


Tensile Response: Brittle & Plasticbrittle failure

plastic failure

(MPa)

x

x

crystalline regions

slide

fibrillar structure

near failure

crystalline regions align

onset of necking

Initial

Near Failure

semi-crystalline

case

aligned,cross-linkedcase

networkedcase

amorphousregions

elongate

unload/reload

Stress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along plastic response curve adapted from Figs. 15.12 & 15.13, Callister 7e. (Figs. 15.12 & 15.13 are from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp. 500-501.)

Predeformation by Drawing

• Drawing…(ex: monofilament fishline) -- stretches the polymer prior to use -- aligns chains in the stretching direction• Results of drawing: -- increases the elastic modulus (E) in the stretching direction -- increases the tensile strength (TS) in the stretching direction -- decreases ductility (%EL)• Annealing after drawing... -- decreases alignment -- reverses effects of drawing.• Comparable to cold working in metals!

Adapted from Fig. 15.13, Callister 7e. (Fig. 15.13 is from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp. 500-501.)

• Compare to responses of other polymers: -- brittle response (aligned, crosslinked & networked polymer) -- plastic response (semi-crystalline polymers)

Stress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along elastomer curve (green) adapted from Fig. 15.15, Callister 7e. (Fig. 15.15 is from Z.D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd ed., John Wiley and Sons, 1987.)

Tensile Response: Elastomer Case(MPa)

initial: amorphous chains are kinked, cross-linked.

x

final: chainsare straight,

stillcross-linked

elastomer

Deformation is reversible!

brittle failure

plastic failurex

x

• Thermoplastics: -- little crosslinking -- ductile -- soften w/heating -- polyethylene polypropylene polycarbonate polystyrene

• Thermosets: -- large crosslinking (10 to 50% of mers) -- hard and brittle -- do NOT soften w/heating -- vulcanized rubber, epoxies, polyester resin, phenolic resin

Adapted from Fig. 15.19, Callister 7e. (Fig. 15.19 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.)

Thermoplastics vs. Thermosets

Callister, Fig. 16.9

T

Molecular weight

Tg

Tmmobile liquid

viscous liquid

rubber

tough plastic

partially crystalline solidcrystalline

solid

• Decreasing T... -- increases E -- increases TS -- decreases %EL

• Increasing strain rate... -- same effects as decreasing T.

Adapted from Fig. 15.3, Callister 7e. (Fig. 15.3 is from T.S. Carswell and J.K. Nason, 'Effect of Environmental Conditions on the Mechanical Properties of Organic Plastics", Symposium on Plastics, American Society for Testing and Materials, Philadelphia, PA, 1944.)

T and Strain Rate: Thermoplastics

20

40

60

80

00 0.1 0.2 0.3

4°C

20°C

40°C

60°C to 1.3

(MPa)

Data for the semicrystalline polymer: PMMA (Plexiglas)

Melting vs. Glass Transition Temp.

What factors affect Tm and Tg?

• Both Tm and Tg increase with increasing chain stiffness

• Chain stiffness increased by1. Bulky sidegroups2. Polar groups or

sidegroups3. Double bonds or

aromatic chain groups

• Regularity (tacticity) – affects Tm only


• Stress relaxation test:-- strain to and hold.-- observe decrease in stress with time.

or

ttE

)()(

• Relaxation modulus: • Sample Tg(C) values:PE (low density)PE (high density)PVCPSPC

- 110- 90+ 87+100+150

Selected values from Table 15.2, Callister 7e.

Time Dependent Deformation

time

straintensile test

o

(t)

• Data: Large drop in Er

for T > Tg. (amorphouspolystyrene)

Adapted from Fig. 15.7, Callister 7e. (Fig. 15.7 is from A.V. Tobolsky, Properties and Structures of Polymers, John Wiley and Sons, Inc., 1960.)

103

101

10-1

10-3

105

60 100 140 180

rigid solid (small relax)

transition region

T(°C)Tg

Er (10s) in MPa

viscous liquid (large relax)

Polymer Fracture

fibrillar bridges microvoids crack

alligned chains


Crazing Griffith cracks in metals

– spherulites plastically deform to fibrillar structure – microvoids and fibrillar bridges form

Polymer Additives

Improve mechanical properties, processability, durability, etc.

• Fillers– Added to improve tensile strength & abrasion resistance,

toughness & decrease cost– ex: carbon black, silica gel, wood flour, glass, limestone,

talc, etc.

• Plasticizers – Added to reduce the glass transition temperature Tg

– commonly added to PVC - otherwise it is brittle

Polymer Additives• Stabilizers

– Antioxidants– UV protectants

• Lubricants – Added to allow easier processing – “slides” through dies easier – ex: Na stearate

• Colorants – Dyes or pigments

• Flame Retardants– Cl/F & B

Processing of Plastics

• Thermoplastic – – can be reversibly cooled & reheated, i.e. recycled– heat till soft, shape as desired, then cool– ex: polyethylene, polypropylene, polystyrene, etc.

• Thermoset– when heated forms a network– degrades (not melts) when heated– mold the prepolymer then allow further reaction– ex: urethane, epoxy

Processing Plastics - Molding• Compression and transfer molding

– thermoplastic or thermoset

Adapted from Fig. 15.23, Callister 7e. (Fig. 15.23 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed.,John Wiley & Sons, 1984. )

Processing Plastics - Molding

• Injection molding– thermoplastic & some thermosets

Adapted from Fig. 15.24, Callister 7e. (Fig. 15.24 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 2nd edition,John Wiley & Sons, 1971. )

Processing Plastics – Extrusion

Adapted from Fig. 15.25, Callister 7e. (Fig. 15.25 is from Encyclopædia Britannica, 1997.)

Polymer Types: Elastomers

Elastomers – rubber• Crosslinked materials

– Natural rubber– Synthetic rubber and thermoplastic elastomers

• SBR- styrene-butadiene rubber styrene

– Silicone rubber

butadiene

Polymer Types: Fibers

Fibers - length/diameter >100• Textiles are main use

– Must have high tensile strength– Usually highly crystalline & highly polar

• Formed by spinning– ex: extrude polymer through a spinnerette

• Pt plate with 1000’s of holes for nylon• ex: rayon – dissolved in solvent then pumped through die head to make fibers

– the fibers are drawn – leads to highly aligned chains- fibrillar structure

Polymer Types• Coatings – thin film on surface – i.e. paint, varnish

– To protect item– Improve appearance– Electrical insulation

• Adhesives – produce bond between two adherands– Usually bonded by:

1. Secondary bonds2. Mechanical bonding

• Films – blown film extrusion

• Foams – gas bubbles in plastic

Blown-Film Extrusion

Adapted from Fig. 15.26, Callister 7e. (Fig. 15.26 is from Encyclopædia Britannica, 1997.)

Advanced Polymers• Ultrahigh molecular weight

polyethylene (UHMWPE)– Molecular weight

ca. 4 x 106 g/mol– Excellent properties for

variety of applications • bullet-proof vest, golf ball

covers, hip joints, etc.

UHMWPE

Adapted from chapter-opening photograph, Chapter 22, Callister 7e.

The Stem, femoral head, and the AC socket are made from Cobalt-chrome metal alloy or ceramic, AC cup made from polyethylene

ABS, Acrylonitrile-Butadiene-StyreneMade up of the 3 materials: acrylonitrile, butadiene and styrene. The material is located under

the group styrene plastic. Styrene plastics are in volume one of the most used plastics.

Properties

The mechanical properties for ABS are good for impact resistance even in low temperatures. The material is stiff, and the properties are kept over a wide temperature range. The hardness and stiffness for ABS is lower than for PS and PVC.

Weather and chemical resistance

The weather resistance for ABS is restricted, but can be drastically improved by additives as black pigments. The chemical resistance for ABS is relatively good and it is not affected by water, non organic salts, acids and basic. The material will dissolve in aldehyde, ketone, ester and some chlorinated hydrocarbons.

Processing

ABS can be processed by standard mechanical tools as used for machining of metals and wood. The cutting speed need to be high and the cutting tools has to be sharp. Cooling is recommended to avoid melting of the material. If the surface finish is of importance for the product, the ABS can be treated with varnish, chromium plated or doubled by a layer of acrylic or polyester. ABS can be glued to it self by use of a glue containing dissolvent. Polyurethane based or epoxy based glue can be used for gluing to other materials.

ABS – A Polymerized “Alloy”

A Processing Movie:

Calloway Golf:

http://www.callawaygolf.com/Global/en-US/Products/GolfBalls.html

• General drawbacks to polymers: -- E, y, Kc, Tapplication are generally small. -- Deformation is often T and time dependent. -- Result: polymers benefit from composite reinforcement.

• Thermoplastics (PE, PS, PP, PC): -- Smaller E, y, Tapplication

-- Larger Kc

-- Easier to form and recycle

• Elastomers (rubber): -- Large reversible strains!

• Thermosets (epoxies, polyesters): -- Larger E, y, Tapplication

-- Smaller Kc

And Remember:Table 15.3 Callister 7eIs a Good overview of applications and trade names of polymers.

Summary

polymer study

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