Ch04 Polymers

7/30/2019 Ch04 Polymers

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MatSE 280: Introduction to Engineering Materials D.D. Johnson 2004, 2006, 2007-08

TEM of spherulite structure in natural rubber(x30,000).Chain-folded lamellar crystallites(white lines) ~10nm thick extend radially.

Chapter 4- Polymer Structures


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ISSUES TO ADDRESS...

What are the basic

Classification?

Monomers and chemical groups?

Nomenclature? Polymerization methods?

Molecular Weight and Degree of Polymerization?

Molecular Structures?

Crystallinity? Microstructural features?

Chapter 4- Polymer Structures


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Polymer= many mers

Adapted from Fig. 14.2, Callister 6e.

Polymer Microstructure

Polyethylene perspective of molecule

A zig-zag backbone structure with covalent bonds


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Covalent chain configurations and strength:

Direction of increasing strengthAdapted from Fig. 14.7, Callister 6e.


Van der Waals, HMore rigid


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Common Examples

- Textile fibers: polyester, nylon

- IC packaging materials.

- Resists for photolithography/microfabrication.

- Plastic bottles (polyethylene plastics).

- Adhesives and epoxy.

- High-strength/light-weight fibers: polyamides,polyurethanes, Kevlar

- Biopolymers: DNA, proteins, cellulose


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Thermoplastics: polymers that flow more easily when

squeezed

, pushed, stretched, etc. by a load (usually atelevated T). Can be reheated to change shape.

Thermosets: polymers that flow and can be molded

initially but their shape becomes set upon curing. Reheating will result in irreversible change or decomposition.

Other ways to classify polymers. By chemical functionality (e.g. polyacrylates, polyamides,

polyethers, polyeurethanes).

Vinyl vs. non-vinyl polymers.

By polymerization methods (radical, anionic, cationic).

Etc

Common Classification


7/52MatSE 280: Introduction to Engineering Materials D.D. Johnson 2004, 2006, 2007-08

Common Chemical Functional Groups

Saturated hydrocarbons(loose H to add atoms)

C C

H

H H

H

Ethylene(ethene)

C C

H

H C

H

H

HH

Propylene

(propene)=

1-butene

2-butenetrans cis

Acetylene(ethyne)

C CH H

Unsaturated hydrocarbons(double and triple bonds)



Alcohols Methyl alcohols

Ethers Dimethyl Ether

Acids Acetic acid

Aldehydes Formaldehyde

Aromatic

hydrocarbonsPhenol

Common Hydrocarbon Monomers



Some Common Polymers

C C

C

N

H

H H

Polyacrylonitrile (PAN)

C C

H

H H

X

C CH

H X

H

Vinyl polymers(one or more Hs of ethylene can be substituted)

Common backbone with substitutions



Monomer-based naming:

poly________

e.g. ethylene -> polyethylene

if monomer name contains more than one word:

poly(_____ ____)

e.g. acrylic acid -> poly(acrylic acid)

Monomer name goes here

Monomer name in parentheses

Note: this may lead to polymers with different names but same structure.

C C C C

H

H

H

H

H

H

H

H

C C C C

H

H

H

H

H

H

H

H

polyethylene polymethylene

Nomenclature



Polymerization Methods

H H

A. Free Radical Polymerization

1. Initiation

Free radical initiator

(unpaired electron)

C C

H

H H

H

monomer

C C

H

H H

R

H

RRadical

transferred

CC

HH

s bonds

p bond

R

H

H

C

HH

C

R

sp2 carbons

sp3 carbon





2. Propagation

C C

H

H H

H

C C

H

H H

R

H

C C

H

H H

C

H

H

H

C

H

H

R

C C

H

H H

H

C C

H

H H

C

H

H

H

C

H

H

C

H

H

C

H

H

R

H

HC

HH

C

R

H H

CC

HH

Both carbon atoms will

change from sp2 to sp3.





3. Termination

C C

H

H H

R

H

C C

H

H H

R

H

R+ C C

H

H H

R

H

R

C C

H

H H

R

H

+ C

H

H

C

H

H

C

H

H

C

H

H

R R

Intentional or unintentional molecules/impurities can also terminate.




B. Stepwise polymerization

RC

OH

O

NH2

+R

COH

O

NH2

RC

NH

O

NH2

RC

OH

O

C. Other methods

Anionic polymerization, cationic

polymerization, coordination

polymerization

RC

O

NH

n

HO

H+

HO

H+ (n-1)

Loses water

(condensation)

Proteins (polypeptides have similar composition)

CH C

O

NH

R n

Various R groups



Molecular Weights

Not only are there different structures (molecular arrangements)

but there can also be a distribution of molecular weights(i.e. number of monomers per polymer molecule).

20 mers 16 mers

10 mers

Average molecular weight =monomermonomer

MM 3.153

101620

This is what is called numberaverage molecular weight.



Number average molecular weight:

Mn

NjMjj

Njj

mo Njj

j

Njj

NjMjj

Note: Total weight

Njj

Total # of polymer chains

Weight average molecular weight:

Mw

WjMjj

Wjj

NjMj2

j

NjMjj

Wj NjMj

In general: M

NjMj1

j

NjMj

j

MnIf = 0 then If = 1 then

Mw

Nj = # of polymer chains with lengthj

Mj= jmo mass of polymer chain with lengthj(mo

= monomer molecular weight).



Molecular Weight: Different Notations

Mn

NjMjj

Njj

Mn xiMii

xi NiNj

j

Mw

NjMj2

j

NjMjj

Mw wiMii

wi NiMi

NjMjj

In Lecture Notes In Callister Textbook



Examples

Light scattering: larger molecules scatter more light than smaller ones.

Infrared absorption properties: larger molecules have more sidegroups and light absorption (due to vibrational modes of side groups)

varies linearly with number of side groups.

Molecular WeightsWhy do we care about weight average MW?-some properties are dependent on MW (larger MW polymer chains can

contribute to overall properties more than smaller ones).

Distribution ofpolymer weights


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Polydispersity and Degree of Polymerization

Polydispersity:

MwMn 1

When polydispersity = 1, system is monodisperse.

Degree of Polymerization:

nnMnmo

Number avg degree of polymerization

nw Mwmo

Weight avg degree of polymerization


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Compute the number-average degree of polymerization forpolypropylene,given that the number-average molecular weight is 1,000,000 g/mol.

What is mer of PP?

Mer molecular weight of PP is

Example 1

C3H6

mo=3AC+6AH=3(12.01 g/mol)+6(1.008 g/mol)

= 42.08 g/mol

Number avg degree of polymerization

nnMnmo

106g/mol

42.08g/mol 23,700


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Example 2 (a, b, and c)A. Calculate the number and weight average degrees of polymerization

and polydispersity for a polymer sample with the following distribution.

Avg # of monomers/chain Relative abundance10 5

100 25

500 50

1000 30

5000 10

50,000 5

nn Mn

mom0

m0

jNjjNjj

jNjjNjj

5*10 25*100 50*500 30*100010 *5000 5 *50000

5 25 50 30 10 5 2860.4

nw Mw

mo

1

mo

(jmo )2Njj

Nj (jmo )j

j2NjjjNjj

5*102 25*1002 50*5002 30*10002 10 *50002 5 *500002

5*10 25*100 50*500 30*1000 10 *5000 5 *50000 35,800

Note: m0 cancels in all these!


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Example 2 (cont.)

B. If the polymer is PMMA, calculate number and weight averagemolecular weights.

Mw if monomer is methylmethacrylate (5C, 2O, and 8H)So m0= 5(12)+2(16)+8(1)= 100 g/mol

CH3

|-CH2-C-

|CO2CH3

Mnnnmo 2860.4(100g/mol) 286,040g/mol

Mwnwmo35,800(100g/mol) 3,580,000g/mol

Mw

Mn

3,580 ,000

286 ,040 ~12.52Polydispersity:


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Example 2 (cont.)

C. If we add polymer chains with avg # of monomers = 10 such that theirrelative abundance changes from 5 to 10, what are the new number

and weight average degrees of polymerization and polydispersity?

Add 5 more monomers of length 10 .nn =M

n

mo=

jNjj

Njj

=1 0 * 1 0 + 2 5 * 1 0 0 + 5 0 * 5 0 0 + 30 *1000 + 10 * 5000 + 5 * 50000

1 0 + 2 5 + 5 0 + 3 0 + 1 0 + 5

= 2750

Mw

Mn=

3,580,000

275000~ 13Polydispersity:

nw Mwmo

j2Njj

jNjj 35,800

Note: significant change in number average (3.8 %)

but no change in weight average!


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For an asymmetric monomer

T H T H+

T H T H

T H H T

H T T H

C

H

F

C

H

H

C

F

H

C

H

H

C

H

F

C

H

H

C

H

H

C

F

H

e.g. poly(vinyl fluoride):

H to T T to T

H to H

Random arrangement

e.g. PMMA

C

C

CH3

C

H

H

C

C

CH3

C

H

H

O OCH

3 OCH

3

O

C

C

CH3

C

H

H

C

C

CH3

C

H

H

OO

CH3

OCH

3

O

H to T H to TH to T

Exclusive H to T arrangement (Why?)

Sequence isomerism


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Regularity and symmetry of side groups affect properties

Stereoisomerism: (can add geometric isomerism too)

Polymer Molecular Configurations

IsotacticOn one side

SyndiotacticAlternating sides

AtacticRandomly placed

- Conversion from one stereoisomerism to another is notpossible by simplerotation about single chain bond; bonds must be severed first, then reformed!

PolymerizeCan it crystallize?

Melting T?


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Regularity and symmetry of side groups affect properties

Polymer Geometrical Isomerism

cis-structure trans-structure

with R= CH3 to form rubber

Cis-polyisoprene trans-polyisoprene

HH

-Conversion from one isomerismto another is notpossible by simplerotation about chain bond because double-bond is too rigid!

-See Figure 4.8 for taxonomy of polymer structures


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Polymer Structural Isomerism

Some polymers contain monomers with more than 1 reactive site

e.g. isoprene

CH2

CCH

CH2

CH3

trans-isoprene

trans-1,4-polyisoprene

CH

2

CCH

C

H2

CH3

1

42

trans-1,2-polyisoprene

n

C

H2

C

CH

CH2

CH3

n

3

3,4-polyisoprene

C

H2

CH

C

CH2

CH3

n

Note: there are also cis-1,4- and cis-1,2-polyisoprene


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Covalent chain configurations and strength:

Direction of increasing strengthAdapted from Fig. 14.7, Callister 6e.


Van der Waals, H More rigid

Short branching

Long branchingStar branching Dendrimers


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Random, Alternating, Blocked, and Grafted

CoPolymers

Synthetic rubbers are often copolymers.

e.g., automobile tires (SBR)

Styrene-Butadiene Rubber random polymer


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Molecular Structure

How do crosslinking and branching occur in polymerization?

1. Start with or add in monomers that have more than 2 sites that bond

with other monomers, e.g. crosslinking polystyrene with divinyl benzene

stryene polystyrene

Control degree of

crosslinking by

styrene-divinyl

benzene ratio

+

styrene

divinyl benzene crosslinked polystyrene

Monomers with trifunctional groups lead to network polymers.


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Molecular Structure

Branching in polyethylene (back-biting)

CH2

CH2 R

CH2

CH

2

CH2

CH

2

CH2

C

H

H

RC

CH

2

CH2

CH2

C

H

HH

H

Same as

Radical moves to a different carbon

(H transfer) RC

CH

2

CH2

CH2

C

H

H

H

H

Polymerization continues from this carbon

Process is difficult to avoid and leads to (highly branched) low-density PE .When there is small degree of branching you get high-density PE.


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Example 3Nitrile rubber copolymer, co-poly(acrylonitrile-butadiene), has

Calculate the ratio of(# of acrylonitrile) to (# of butadiene).

Mn106,740 g/mol

nn 2000

3 C = 3 x 12.01 g/mol

3 H = 3 x 1.008 g/mol

1 N = 1 x 14.007 g/mol

m0= 53.06 g/mol

4 C = 4 x 12.01 g/mol

6 H = 6 x 1.008 g/mol

m0= 54.09 g/mol

1,4-addition product

moMnnn

106,740

2000 53.57g/molWe need to use an

avg. monomer MW:

mo f1m1 f2m2 f1(m1m2)m2

f1

m0 m2m1m2

53.37 54.09

53.06 54.09 0.7 f2 1 f1 0.3

f2

f1

0.7

0.37 : 3


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Crosslinking in elastomers is called vulcanization, and is achieved byirreversible chemical reaction, usually requiring high temperatures.

Vulcanization

Sulfur compounds are added to form chains that bond adjacent

polymer backbone chains and crosslinksthem. Unvulcnaized rubber is soft and tacky an poorly resistant to wear.

e.g., cis-isoprene Stress-strain curves

+ (m+n) S (S)n

(S)m

Single bonds

Double bonds

See also sect. in Chpt. 8


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Molecular weight, Mw: Mass of a mole of chains.

Tensile strength (TS):--often increases with Mw.

--Why? Longer chains are entangled (anchored) better.

% Crystallinity: % of material that is crystalline.--TS and E often increase

with % crystallinity.

--Annealing causes

crystalline regions to grow.

% crystallinity increases.

crystalline

region

amorphousregion


Molecular Weight and Crystallinity


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Polymer Crystallinity

polyethylene Some are amorphous.

Some are partially crystalline (semi-crystalline). Why is it difficult to have a 100% crystalline polymer?

%crystallinity c (s a )

s (ca )

100%

s = density of specimen in question

a

= density of totally amorphous polymer

c = density of totally crystalline polymer


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%crystallinityMcrystalline

Mtotal100%

cVc

sVs100%

c

s

fc100%

Volume fraction of crystalline component.

MtotalMcrystallineMamophous

MsM

cM

asVs cVc aVa

s cVcVs

aVaVs

cfc afa cfc a(1 fc) fc(c a) a

Using definition of volume fractions:

fc VcVs

faVaVs

fc

s a

c a

Substituting in fc into the original definition: %crystallinity c(s a )

s (c a )100%


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Polymer Crystallinity

Degree of crystallinity depends on processing conditions (e.g.

cooling rate) and chain configuration.

Cooling rate: during crystallization upon cooling through MP,polymers become highly viscous. Requires sufficient time for

random & entangled chains to become ordered in viscous liquid.

Chemical groups and chain configuration:

More Crystalline

Smaller/simper side groups

Linear

Isotactic or syndiotactic

Less Crystalline

Larger/complex side groups

Highly branched

Crosslinked, network

Random


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Semi-Crystalline Polymers

Fringed micelle model: crystalline region embedded in amorphous region.

A single chain of polymer may pass through several crystalline regions aswell as intervening amorphous regions.

fc s a

c a

Crystalline volume fractions Important


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Chain-folded model: regularly shaped platelets (~10 20 nm thick)

sometimes forming multilayers.Average chain length >> platelet thickness.


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Spherulites: Spherical shape composed of aggregates of chain-folded crystallites.

Natural rubber

Cross-polarized light through

spherulite structure of PE.


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Diblock copolymers

Representative polymer-polymer

phase behavior with different

architectures:A) Phase separation with mixed

LINEAR homopolymers.

B) Mixed LINEAR homopolymers and

DIBLOCK copolymergives

surfactant-like stabilized state.

C) Covalent bond between blocks in

DIBLOCK copolymergive

microphase segregation.F. Bates, Science1991.


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Thermoplastics:--little cross linking

--ductile

--soften w/heating

--polyethylene (#2)

polypropylene (#5)

polycarbonatepolystyrene (#6)

Thermosets:--large cross linking (10 to 50% of mers)

--hard and brittle--do NOT soften w/heating

--vulcanized rubber, epoxies,

polyester resin, phenolic resin

Callister,Fig. 16.9

T

Molecular weight

Tg

Tmmobileliquid

viscousliquid

rubber

toughplastic

partiallycrystallinesolid

crystallinesolid


Thermoplastics vs Thermosets

Tm: melting over wide range of T

depends upon history of sample

consequence of lamellar structure

thicker lamellae, higher Tm.

Tg: from rubbery to rigid as T lowers

P ki f P l


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Packing of spherical atoms as in ionic and metallic crystals led to

crystalline structures.

How polymers pack depend on many factors:

long or short, e.g. long (-CH2-)n.

stiff or flexible, e.g. bendy C-C sp3.

smooth or lumpy, e.g., HDPE.

regular or random single or branched

slippery or sticky, e.g. C-H covalent (nonpolar) joined via vdW.

Analogy:Consider dried (uncooked) spaghetti (crystalline) vs.cooked and buttered spaghetti (amorphous).

pile of long stiff spaghetti forms a random arrangement.

cut into short pieces and they align easily.

Candle wax more crystalline than PE, even though same chemicalnature.

Packing of Polymers

Wh t A E t d P ti ?


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Would you expect melting ofnylon 6,6to be lower than PE?

What Are Expected Properties?

N

|

H

H

|

C

|

H

6

N

|

H

O

||C

H

|

C

|

H

4

N

|

H

O

||C

N

|

H

H

|

C

|

H

6

N

|

H

O

||

C

H

|

C

|

H

4

N

|

H

O

||

C

+

++

+

++

H

C

H

H

C

H

H

C

H

H

C

H

+

+

+

+

+

+

nylon 6,6 polyethylene

a) What is the source of intermolecular cohesion in Nylon vs PE?

b) How does the source of linking affect temperature?

Hydrogen bonds

Van der Waals bonds

With H-bonds vs vdW bonds, nylon is expected to have (and does) higher melting T.

Wh t A E t d P ti ?


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Which polymer more likely to crystallize? Can it be decided?


Linear syndiotactic polyvinyl chloride Linear isotactic polystyrene

Linear and syndiotactic polyvinyl chloride is more likely to crystallize.

The phenyl side-group for PS is bulkier than the Cl side-group for PVC.

Generally, syndiotactic and isotactic isomers are equally likely to crystallize.

For linear polymers, crystallization is more easily accomplished as chain

alignment is not prevented.

Crystallization is not favored for polymers that are composed of

chemically complex mer structures, e.g. polyisoprene.

Wh t A E t d P ti ?


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Linear and highly crosslink

cis-isoprene

Not possible to decide which might crystallize. Both not likely to do so.

Networked and highly crosslinked structures are near impossible to

reorient to favorable alignment.

H+

+ H20

Networked

Phenol-Formaldehyde

(Bakelite)

Wh t A E t d P ti ?


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alternatingPoly(Polystyrene-Ethylene)

Copolymer

random

poly(vinyl chloride-tetra-fluoroethylne)

copolymer

Alternating co-polymer more likely to crystallize than random ones, as they are

always more easily crystallized as the chains can align more easily.

D t t


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Soap is a detergent based on animal or vegetable product, some

contain petrochemicals

Detergents

grease

water

detergent

What properties of soap molecules do you need to remove grease?green end must be hydrophilic. Why?

Opposite end must be hydrocarbon. Why?

Water must be like oxygen (hoard

electrons and promote H-bonding)

greasee.g., oxy-clean

Si l l El l + B SLIME!


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Simple polymer: Elmers glue + BoraxSLIME!

Chemistry Elmers glueis similar to poly (vinyl alcohol) with formula:

Borax is sodium tetraborate decahydrate (B4Na2O7 10 H2O).

The borax actually dissolves to form boric acid, B(OH)3.

This boric acid-borate solution is a buffer with a pH of about 9 (basic).Boric acid will accept a hydroxide OH- from water.

B(OH)3 + 2H2O B(OH)4- + H3O

+ pH=9.2

OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH

this is a SHORT, n=15 chain of poly(vinyl alcohol)

Hydrolyzed molecule acts in a condensation reaction

with PVA, crosslinking it.

Simple polymer: Elmers glue + Borax SLIME!


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Simple polymer: Elmers glue + Borax SLIME!

Hydrolyzed molecule acts in a condensation reaction with PVA,

crosslinking it.

B(OH)3 + 2H2O B(OH)4- + H3O

+ pH=9.2

Crosslinkingties chains via weak non-covalent(hydrogen) bonds, so it flows slowly.

Crosslinked

Range of Bonding and Elastic Properties


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Range of Bonding and Elastic Properties

Is slime a thermoset or thermoplastic, or neither?

Thermoset

bondingThermoplastic

bonding

Induced dipolar bondsform crosslinks

Slime?

Stiffness increases

Where is nylon?

Covalent bonds form

crosslinks H-bonds form

crosslinks

Summary


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Polymers are part crystalline and part amorphous.

The more lumpy and branched the polymer, the less denseand less crystalline.

The more crosslinking the stifferthe polymer. And, networked

polymers are like heavily crosslinked ones.

Many long-chained polymers crystallize with a Spherulite

microstructure - radial crystallites separated by amorphous

regions.

Optical properties: crystalline -> scatter light (Bragg)amorphous -> transparent.

Most covalent molecules absorb light outside visible spectrum, e.g.PMMA (lucite) is a high clarity tranparent materials.

Summary

Ch04 Polymers

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