polymers ansh verma

8/3/2019 Polymers Ansh Verma

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polymers- a brief up.ansh verma - 10208025, mechanical a.


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Polymers: Introduction

Polymer: molecules can be linked in repeatingunits to make longer.

High molecular weight molecule made up of asmall repeat unit (monomer).

A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A

Monomer: Low molecular weight compound thatcan be connected together to give a poymer

Oligomer: Short polymer chain

Copolymer: polymer made up of 2 or moremonomers

Random copolymer: A-B-B-A-A-B-A-B-A-B-B-B-A-A-B

Alternating copolymer: A-B-A-B-A-B-A-B-A-B-A-B-A-B

Block copolymer: A-A-A-A-A-A-A-A-B-B-B-B-B-B-B-B


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Types of Polymers

Polymer Classifications

Thermoset: cross-linked polymer that cannot bemelted (tires, rubber bands)

Thermoplastic: Meltable plastic

Elastomers: Polymers that stretch and then return totheir original form: often thermoset polymers

Thermoplastic elastomers: Elastic polymers that canbe melted (soles of tennis shoes)

Polymer Families Polyolefins: made from olefin (alkene) monomers

Polyesters, Amides, Urethanes, etc.: monomerslinked by ester, amide, urethane or other functional

groups


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Common PolyolefinsMonomer Polymer

Ethylene

H3C CH3

nRepeat unit

Polyethylene

CH3CH3

n

CH3 CH3 CH3 CH3 CH3 CH3CH3

Propylene

Polypropylene

PhCH3

n

Ph Ph Ph Ph Ph PhPhStyrene

Polystyrene

Cl

CH3

nCl Cl Cl Cl Cl ClCl

Vinyl ChloridePoly(vinyl chloride)

F2C CF2

Tetrafluoroethylene

F3C

F2C

CF2

F2C

CF2

F2C

CF2

F2C

CF2

F2C

CF2

F2C

CF2

CF3n

Poly(tetrafluoroethylene): Teflon


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Polyesters, Amides, and UrethanesMonomer Polymer

CO2HHO2CHO

OHO O

HO OH2C

H2C O

nTerephthalic

acidEthylene

glycol

Poly(ethylene terephthalate

H

Ester

HO OH

O O

4 H2N NH24Adipic Acid 1,6-Diaminohexane Nylon 6,6

HO NH

NH

H

O O

4 4n

CO2HHO2C

Terephthalic

acid

NH2H2N

1,4-Diamino

benzene

Kevlar

O

HO

OHN

HN H

n

Amide

HOOH

Ethyleneglycol

H2COCN NCO

4,4-diisocyantophenylmethaneSpandex

H2

C

H

N

H

N

O

HO

O

O

H2

C

H2

C O Hn

Urethane linkage


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Natural PolymersMonomer Polymer

Isoprenen

Polyisoprene:Natural rubber

O

H

HO

H

HO

H

HOHH

OH

OH

Poly(-D-glycoside):cellulose

O

H

O

H

HO

H

HOHH

OH

OH

H

n

-D-glucose

H3N

O

O

R

Polyamino acid:protein

H3N

OHN

R1

OHN

Rn+1

O

OH

Rn+2n

Amino Acid

BaseO

OH

OP

O

O

O

oligonucleic acidDNA

NucleotideBase = C, G, T, A

BaseO

O

OP

O

O

O

DNA

DNA


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What Makes Polymers Unique?

Really big molecules (macromolecules) likepolymers have very different properties thansmall molecules Chain entanglement: Long

polymer chains get entangled with

each other. When the polymer is melted, the chains

can flow past each other.

Below the melting point, the chains canmove, but only slowly. Thus the plastic isflexible, but cannot be easily stretched.

Below the glass transition point, the chainsbecome locked and the polymer is rigid


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Physical Properties

Stretch

Linear Polymer

The chains can be stretched, which causesthem to flow past each other. When released,the polymer will not return to its original form.

Stretch

Cross-Linked Polymer

The cross-links hold the chains together.When released, the polymer will return to it'soriginal form.

Relax


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Polymer Synthesis

There are two major classes of polymerformation mechanisms

Addition polymerization: The polymer grows bysequential addition of monomers to a reactive site

Chain growth is linear

Maximum molecular weight is obtained early in the reaction

Step-Growth polymerization: Monomers reacttogether to make small oligomers. Small oligomersmake bigger ones, and big oligomers react to give

polymers. Chain growth is exponential

Maximum molecular weight is obtained late in the reaction


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Addition Polymerization

In* AInitiation

In A* A


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Addition PolymerizationPropagation

In*A

InitiationIn A A* A


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Addition PolymerizationPropagation

AIn*A

InitiationIn A A A*


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Addition Polymerization

Propagation

nA

In A A A AnA*

A A A A Am

In A A A AnA

*A A A A Am

Combination

*A A A A Am

In A A A A n A

B A A A Am

Disproportionation

Termination

Reactive site is consumed

A

In A A A AnA

A*

Chain TransferNew reactive siteis produced

MW kpropagation

ktermination

MW

% conversion0 100

In*A

InitiationIn A A A A*


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Types of Addition Polymerizations

Ph

Anionic

C3H7 LiC4H9

Ph

Li+ Phn

C4H9

Ph Ph

Li+

n

Ph

Radical

PhCO2Ph

n

Ph

Cationic

Cl3Al OH2H

PhHOAlCl3

PhnH

Ph Phn

HOAlCl3

PhCO2

Ph

PhCO2

Ph Phn


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Step-Growth Polymerization

Stage 1

Consumptionof monomer

n n

Stage 2

Combinationof small fragments

Stage 3

Reaction ofoligomers to givehigh molecularweight polymer


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Step-Growth Polymerization

Because high polymer does not form until the

end of the reaction, high molecular weightpolymer is not obtained unless high conversion ofmonomer is achieved.

Xn 1

1 p

Xn=Degree of polymerizationp= mole fraction monomerconversion


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Thermoplastics

No cross links between chains.

Weak attractive forces between chains

broken by warming.

Change shape - can be remolded.

Weak forces reform in new shape when


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Thermosets

Extensive cross-linking formed by

covalent bonds.

Bonds prevent chains moving relativeto each other.


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Longer chains make stronger

polymers.Critical length needed beforestrength increases.

Hydrocarbon polymers averageof 100 repeating units necessarybut only 40 for nylons.

Tensile strength measures theforces needed to snap a

polymer.

More tangles + moretouching!!!


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Crystalline polymers Areas in polymer where

chains packed in regular way.

Both amorphous and

crystalline areas in samepolymer.

Crystalline - regular chain

structure - no bulky side

groups. More crystalline polymer -

stronger and less flexible.


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Cold-drawing

When a polymer is stretched a neck forms.

What happens to the chains in the neck? Cold drawing is used to increase a polymers

strength. Why then do the handles of plastic carrier

bags snap if you fill them full of tins of beans?


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Nylon-6,6

Cl Cl

O O

4H2N NH24

Adipoyl chloride 1,6-Diaminohexane

Cl NH

NH

H

O O

4 4

NaOH

HO NH

NH

H

O O

4 4n

6 carbondiacid

6 carbondiamine

Nylon-6,6Diamine, NaOH, in H2O

Adipoyl chloridein hexane

Nylon 6,6


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Nylon-6,6

Diamine, NaOH, in H2O

Adipoyl chloridein hexane

Nylon 6,6

Since the reactants are in differentphases, they can only react at the

phase boundary. Once a layer ofpolymer forms, no more reactionoccurs. Removing the polymer allowsmore reaction to occur.


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Molecular Weight of PolymersUnlike small molecules, polymers are typically a mixture of differentlysized molecules. Only an average molecular weight can be defined.

Measuring molecular weight

Size exclusion chromatography

Viscosity

Measurements of average molecular

weight (M.W.) Number average M.W. (Mn): Total

weight of all chains divided by # ofchains

Weight average M.W. (Mw): Weighted

average. Always larger than Mn Viscosity average M.W. (Mv): Average

determined by viscositymeasurements. Closer to Mw than Mn

# of molecules

Mn

Mw

increasing molecular weight

Mv


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What the Weights MeanMn: This gives you the true average weight

Let's say you had the following polymer sample:2 chains: 1,000,000 Dalton 2,000,000

5 chains: 700,000 Dalton 3,500,000

10 chains: 400,000 Dalton 4,000,000

4 chains: 100,000 Dalton 400,000

2 chains: 50,000 Dalton 100,00010,000,000

10,000,000/23 = 435,000 Dalton

1 Dalton = 1 g/mole


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Weight Average Molecular Weight

Mw: Since most of the polymer mass is in the heavier fractions, this

gives the average molecular weight of the most abundant polymerfraction by mass.

2,000,000

10,000,000 0.20 1,000,000 200,000

3,500,00010,000,000 0.35 700,000 245,000

4,000,000

10,000,000 0.40 400,000 160,000

400,000

10,000,000 0.04 100,000 4,000

100,000

10,000,000 0.01 50,000 500

Total 609,500


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Polymer Microstructure

Polyolefins with side chains have stereocenters on every other carbon

CH3n

CH3 CH3 CH3 CH3 CH3 CH3CH3

With so many stereocenters, the stereochemistry can be complex.

There are three main stereochemical classifications for polymers.Atactic: random orientation

Isotactic: All stereocenters have same orientation

Syndiotactic: Alternating stereochemistry


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How to Determine Microstructure?13C NMR is a very powerful way to determine the microstructure of a

polymer.

13C NMR shift is sensitive to the two

stereocenters on either side on sptectrometers

> 300 MHz. This is called pentad resolution.

r mm rmr

mmrm pentad

m = meso (same orientation)r = racemic (opposite orientation)

12 1 2

13C NMR spectrum of CH3 regionof atactic polypropylene


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Why is this important?

Tacticity affects the physical properties

Atactic polymers will generally be amorphous, soft,flexible materials

Isotactic and syndiotactic polymers will be morecrystalline, thus harder and less flexible

Polypropylene (PP) is a good example

Atactic PP is a low melting, gooey material

Isoatactic PP is high melting (176), crystalline, tough

material that is industrially useful Syndiotactic PP has similar properties, but is very

clear. It is harder to synthesize

polymers ansh verma

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