1

Literature

Polymer- och materialteknologi, 5p föreläsningar projekt

Funktionella material, 5p föreläsningar nanotillämpningar

Nanometerprogrammet

Kemiteknikprogrammet

2

Overview of the lectures in polymer chemistry

Topics:Principles of polymerisationsClassification and naming of polymersStructure and molecular weight

Step-growth polymerisationFree-radical polymerisationsIonic polymerisationsPolymerisation techniques

Conformations and solutionsDetermination of molecular weight

Chapter 1-3 in:

Lecurer: Patric JannaschInstitute of Chemistry, Division of Polymer & Materials Chemistrypatric.jannasch@polymer.lth.se

”Polymers

are now

rapidly

oozing

into eve

ry aspec

t of our

lives”

Alan G. MacD

iarmid,

Nobel Priz

e Winne

r in chem

istry,200

0

3

Polymers !

The use of plastics in different sectors

4

Plastics in the packaging industry

Plastics in the automotive industry

5

The poor reputation of plastics.....

Production, use and recycling of plastics

Incineration

6

The chemical share of the total world oil demand

Total oil demand

Chemical use

Plastics and energy

7

Incineration of plastics

plastics

copolymers

biopolymers synthetic polymers

fibers

rubbers

elastomerslatex

composites adhesives

? paints

varnishes gelshybrid

polymers

ionomers

polysaccarides

proteins

polyel

ectrol

ytes

coatings

8

Ett polymerhistoriskt perspektiv...

?

shellac

vulkat gummi

celluloid

cellulosa acetat

bakelite

urea-formaldehyd

polyvinylklorid

polystyren

polyvinylacetat

polyamid

polyeten

polyuretan

polytetrafluoreten

silikonpolymerer

polypropen

kåda, tjära,

gummi,...

Celluloid eran(1890 - 1920)

Härdplasternas epok(1910 - 1950)

Termoplasternas tid(1945- )

De funktionella (dolda)polymerernas framtid

1830

18701839

19031909

19231927

1929

1931

1938

1939

1940

1941

1943

1954

Polymer –”many parts”

Monomers, oligomers and polymers

Monomers - ”one part”

Repeatingunit

Polymerisation

n≡

Macromolecules – synonymous to ”polymers”, often used for naturally occurring polymers

Oligomers –”few parts” (2-10)

Oligomerisation

1-4

9

Repeating unit: the smallest structural unit that is repeated along the polymer chainDegree of polymerisation (DP): the number of repeating units in the polymer chainMolecular weight of a polymer chain: M = DP × M0

(where M0 is the molecular weight of the repeating unit)

CH2 CH nCH2 CH

Styrene Polystyrene, PS

Polymerisationn

1-4

Repeating unit and degree of polymerisation

Hey, we need some more monomersover here to form a polymer !

Classification of polymers according toprocessing characteristics

Thermoplastics-Soften and melt when heated and can then shaped into desired objectsCan be re-heated and re-shapedUsually have a linear or branched chain structureAre in principle solubleExamples include polystyrene, polyethylene, polypropylene

Thermosets-Polymers whose individual chains have all been chemically linked by covalentbonds during polymerisation or processing – ”curing”Cannot be melted or dissolved once curedTypical applications: composites, coatings, adhesivesExamples include epoxy- and formaldehyde-based resins,unsaturated polyesters

4

10

Classification of polymers according topolymerisation mechanism

Step-growth polymers – polymers prepared by step-growth polymerisation, i.e. by successive reactions of two molecules - monomers, oligomers and polymers

R R R R

Often have a structure like:

where...

Rand...

CH2 n

C NH

O

nis or

is O C

O

O C NH

O

or,

”aromatic polyesters”, ”alifatic polyamides”, etc.

4-8

Condensation polymers – polymers prepared by condensation polymerisation,i.e. a step-growth polymerisation where a low MW molecule is produced in each reaction

See more Table 1-3 ...

Chain-growth and addition polymers –polymers produced by chain-growth and addition polymerisationwhere the polymers grow through addition of monomer to an active center.

4-8

C

R1

R2

C

H

H

Classification of polymers according topolymerisation mechanism

Example - carbon main-chain polymers:

C

CH3

H

C

H

HC

H

H

C

H

HC

Cl

H

C

H

HC

OH

H

C

H

HPE PP PVC PVOH

C

H

C

H

HPS

See Table 1-2 ...

11

Classification of polymers according topolymer structure

Also:”acrylate polymers” – polymers based on

acrylate monomers”vinyl polymers” - polymers based on

vinyl monomers

Poly ”backbone group”

polyethersulfone ' "

"' ' "polyetherethersulfone

Combinations:

O R S RO

O

R O R S RO

OO

8-9

Polymer chain architectures

Copolymers:

Homopolymers:

alternating

ABA-triblock

random

graft

Terpolymer:

linear

branched network(crosslinked)

10-11

12

Conformation and configuration

Conformations:

Configuration:

The spatial positions attained by the polymer through bond rotation

random coil

helix

extendedchain

The stereochemical arrangement of atoms along the polymer chain

10-13

CH2

CH

CH

CH2

1,3-butadiene

1,2 1,4

Example:

CH2

CH

CH

CH2

CH2CH CH

CH2

CH2CH CH

CH2

trans

cis

Tacticity

isotactic

Atactic – the arrangement of the substituent is not stereoregular

Crystallisationpossible !

The tacticity of a polymer -is the result of the specific polymerisation conditionsinfluences thermal and mechanical properties, etc.

10-12

syndiotactic

- The way substituents are arranged along the backbone chain of a polymer

13

Amorphous polymers

19-20

Amorphous state:

Hydrogen bonding in polyamides:

Disordered entangledpolymer chains

Secondary bonds, or interactions, between the chainsvan der Waals forceshydrogen bonds

N CH2 N C CH2 CO

6 4OHH n

NCH2NCCH2CO

64

O H H

n

Chain flexibility –compare polydimethylsiloxane (Tg < -120oC) and polybenzimidazole (Tg > 320oC).

No long-range cooperative motions below a specific temperature- the glass transition temperature (Tg)

Semi-crystalline polymersPolymers with regular structures may crystallise by chain folding above the Tg.

Semi-crystalline polymers are characterised by a crystalline-melting temperature (Tm)

crystalline lamellas

20

spherulitic structure

folded polymer chain

crystalline lamella

amorphouspolymer

14

100% isotacticProduced by bacteriaHighly crystalline Tg = 5-10 oCTm = 175 oC

Spherulites in poly(hydroxybutyrate)

imaged by atomic force microscopy(AFM)

imaged by cross-polarised light microscopy

Spherulites

Poly(hydroxybutyrate) :

Poly(hydroxybutyrate) - a biodegradable polyester

Synthesised by bacteria, Alcaligenes eutrophus, as an energy-storage mediumThe length of the side-chain depends on the carbon substrates available to the bacteriaHigh cost of production, mainly associated with the purification process

Alcaligenes eutrophus cellcontaining domains ofpoly(hydroxybutyrate) (PHB)

0.2 µm

15

How big are polymers ?

Let= 1 cm

CH2 CH n2

This is only a 200-mer

MW of 5,600

Molecular weights of 1,000,000 are not uncommon for PE

So envisage a chain of approximately 0.3 cm in diameter and 360 m long !!

CH2 CH nCH2 CH

Ethylene Polyethylene, PE

Polymerisationn22

Molecular weights – some initial observations

How does the molecular weight of a polymer differ from that of a low-molecular weight substance?

CH4

CH3 CH3

CH3 CH2 CH3

CH3 CH2 CH2 CH3

CH2 6CH3 CH3

CH2 30CH3 CH3

CH2 30000CH3 CH3

increasingmolecular weight

16

30

44

58

114

450

420,030

gases

liquid

semi-solid ”paste”

solid

16

The importance of the molecular weight

15

*Certain proteins, e.g. enzymes, are monodisperse, i.e., all molecules have the same MW

Synthetic polymer samples are polydisperse*- have a wide range of MWs

Molecular weight

Wi = Ni × Mi

A need to defineaverage values !

MW distribution

MWs and polydispersities depend on howthe sample was synthesised and prepared

CH2 CH nCH2 CH

Styrene, M = 104.15 Polystyrene, PSPolymerisation

15-16

17

Weight-average:Σ Ni × Mi

2N

i=1Mw =Σ Ni × Mi

N

i=1

Mn

MwPolydispersity index: PDI =

Average molecular weights

W = weight of the total sampleN = total number of molecules in the sampleMi = the molecular weight of a molecule containing i monomer residuesWi = weight of all the molecules with the molecular weight Mi

Ni = number of molecules with the molecular weight Mi

Thus.... Wi = Ni × Mi ; W = ΣWi = ΣNi × Mi ; N =ΣNi

Number-average:Σ Ni × Mi

N

i=1

Σ Ni

N

i=1

Mn = =WN

See Problem 1.1, p. 18

17-18

Molecular weight averages – a silly but revealing example

Total weight: Σ Ni × Mi = (4 × 1) + (1 × 6,000,000)= 6,000,004 gram

Σ Ni × Mi

Σ NiMn =

Total number: Σ Ni = (4 + 1) = 5 animals

=6,000,004

5 = 1,200,001 =>

1,200 kg /animal

Σ Wi × Mi = (4 × 1) + (6,000,000 × 6,000,000)= 3.6 × 1013 gram2

Σ Wi × Mi

Σ WiMw = =

3.6 × 1013

6,000,004 = 5,999,996 =>

6,000 kg /animal

Number-average value Weight-average value

Calculate the”animal weight averages”

of an elephant and 4 mosquitos

4 mosquitos(each weighing 1 g)

6,000 kg elephant

18

Poly(monomer)

Often used for vinyl polymers,for example poly(vinyl chloride):

Poly(repeating unit)

Often used for step-growth polymers,for example poly(ethylene terephthalate):

O CO

CO

O CH2 CH2 n

PET

CH2 CHCl

nPVC

Commercial brand names

For example Teflon™: C C

F

F

F

Fn

Well then…Teflon, poly(tetrafluoroethylene), PTFE, or poly(difluoromethylene) ??

PTFE

Nomenclature

13-15

Polymer synthesis

In principal all chemical coupling reactions that proceed in high yieldscan be employed in polymerisations

Control of average MW, MW distribution, chain architecture, tacticity, etc.

Specific reactions and controlled reactivity, proper monomer functionality

Taylored material properties for a specific application

23

An example...low MW atactic polypropylene – waxy material with little commercial valuehigh MW isotactic polypropylene – strong material with a wide commercial use

19

Monomer functionality –has to be at least 2 in order for a (linear) polymer to formdefined only for a given reactionpolyfunctional monomers give branched or crosslinked polymer structures

Divinyl benzene4-functional in a chain-growth reaction

Pentaerythritol4-functional in a step-growth reaction

CH2 CCH2

CH2 CH2

OH

OH

OH

OH CH

CH2CHCH2

CH CH2

COOHOH

Functionalities?

Monomer functionality

CH2

O CH2

OCH

2O

thermosets

CH2 CH

COOHHOOC CH2 n

X-X

OH

CH3

CH3

Step-growth polymerizations

random reactions involving two molecules with functional end unitsthe molecules participating are monomers, oligomers, and polymers – all species can reacttypically small molecules are produced at each reaction as a by-product (condensation)high conversion of the functional groups in order to obtain high-molecular weight polymers

24

Some characteristics...

OH CH2 CH3CH

3C O CH

2CH

3

O

CH3C OH

O

+ H2O+

Condensation reaction:

Condensation polymerisation:

acetic acid ethanol ethyl acetate

+C OH

OCH2CHO

O

CH2 OH CH2 CH2HO C O

OCH2CO

O

CH2 CH2 CH2HOH

n

H2O+ (n-1)ethylene glycolmaleic acid

nn

20

Classification of step-growth polymers

24

Table 2-1

COH OH

CH3

CH3CO

Cl Cl

Cl Si ClCH3

CH3SCl ClO

Obisphenol dichlorophenylsulfonephosgene dimethyldichlorosilane

H2O

HCl

Condensate

H2OH2O

HCl

HCl

(Fig. 2-2A)

(Fig. 2-2B)

Examples of step-growth polymerisations

Non-condensation polymerisations

25-26

Condensation polymerisations

Why not a condensation polymerisation ?

21

A typical pathway for a linear polymer (functional group A reacts with B):

Self-condensation:

A-A B-B

B B

B

+A-AB-B

B-B

A-A

A-AB-BB-B

A-AB-BA-A

A-B

B-B

1-mer 1-mer

2-mer

3-mer

3-mer

A-AB-BA-AB-B

A-AB-BA-AB-BA-AB-BB-B

4-mer

7-mer

poly-mer

A-BA-B1-mer 2-mer

-A-B-npoly-mer

A-A B-B+1-mer 1-mer

+

Condensation with polyfunctional monomers:

1-merB B

B

A-A A-A

A-AA-A

A-A B-B

B-B

B-BB B

B

A-A

B B

B

A-A

A-A

B B

B

Branched or crosslinked polymers(thermosets)

Reaction pathways in step-growth polymerisations

X-X

Molecular weights in step-growth polymerisations

DPn

Definitions:

- the number-average of repeating units in the polymer chain

Xn - the number-average of monomer residues units in the polymer chain

Two examples......

O CH2

O C NH

CH2

O

NH

C

O

1004 6Polyurethane:

DPn

Xn

= 100

= 200

Poly(vinyl chloride): DPn

Xn

= 100

= 100

26-27

CH2

CH

Cl

100

22

Molecular weight vs. conversion

time = 0 time = tN0 molecules (monomers) N molecules

Number-average degree of polymerisation, Xn = N0/N

A-B

A-B A-BA-B A-B

A-BA-B

A-BA-B

A-BA-B

A-B

A-B A-BA-B A-B

A-BA-B

A-B

A-B

A-B

A-BA-B

A-BA-BA-B

A-B

A-BA-BA-B

Conversion, p = (N0-N)/N0 N = N0(1-p)

polymerisation

Carothers equation

Xn = 1/(1-p)

Number-average molecular weight: Mn = Xn × M0 = M0/(1-p)where M0 is the molecular weight of the repeating unit

The weight-average degree of polymerisation, Xw = (1+p)/(1-p)27

0.50 0.75 0.90 0.95 0.992 4 10 20 100

p

XnCarothers equationXn = 1/(1-p)

The necessity to reach high conversions

The following conditions must be fulfilled in step-growth polymerisations: high conversionshigh monomer puritieshigh reaction yields stoichiometric equivalence of functional groups

(especially in A-A/B-B polymerisations)

0

20

40

60

80

100

120

0 0.2 0.4 0.6 0.8 1Conversion, p

Xn

28

23

An example: polyester by self-condensation

time = 0 time = 10 h

A-B

A-B A-BA-B A-B

A-BA-B

A-BA-B

A-BA-B

A-B

A-B A-BA-BA-B

A-BA-B

A-B

A-B

A-B

A-BA-B

A-BA-BA-B

A-B

A-BA-BA-B

polymerisationStart:

Monomer:

Concentration: 1 MConstant volume

After 10 h polymerisation:100 mL of the reaction mixtureconsumes 0.5 mmol of NaOHby titration

M0 of repeating unit = 200

Conversion: p = (N0-N)/N0 = (1 – 0.005)/1 = 0.995 i.e. 99.5 %

Concentration of molecules after 10 h: 0.0005 mol / 0.1 L = 0.005 M (each molecule consumes one NaOH)

Number-average degree of polymerisation: Xn = 1/(1-p) = 1/(1-0.995) = 200

Weight-average degree of polymerisation: Xw = (1+p)/(1-p) = (1+0.995)/(1-0.995) = 399

Number-average molecular weight: Mn = Xn × M0 = 200 × 200 = 40 000Weight-average molecular weight: Mw = Xw × M0 = 399 × 200 = 79 800

Polydispersity index: PDI = Mw /Mn = 79 800/40 000 = 1.995

COOHHO R

Chain-growth polymerisation

Polymer chains grow from reactive centers –radical, anionic, or cationic

At least three steps in common: Initiation – the active centers are formedPropagation – the polymer chains grow from the reactive centerTermination – the reactive centers are deactivated

29

CH2 CH CH2 CHn

CH2 CH CH2 CH⊕n

CH2 CH CH2 CHӨn

24

Free-radical polymerisation

Three principal steps or processes: Initiation of the monomer carrying the reactive radicalPropagation or growth of the reactive radical by sequential addition of monomerTermination of the reactive radical to give a de-activated polymer chain

All three processes:take place during the polymerisation at all times, but at different rates influence each otherdetermine the polymerisation rates and molecular weight by their relative rates

High MW polymer is obtained at very early stage

29

What is a free radical?

A free radical:

is any atom with at least one unpaired electron in the outermost shell,which is capable of independent existence

is easily formed when a covalent bond between two entities is brokenand one electron remains with each newly formed species (homolyticbond cleavage)

is normally highly reactive due to the presence of unpaired electron(s).

R1-R2

homolytic

heterolytic ionic species

radical speciesR1•+ R2•

R1- + R2

+

energy

25

The first initiation step

Dissociation of an initiating compound (”initiator”) to form radical species:

I~I 2 Ikd

Dissociation rate constant, kd = A exp(-Ea/RT) , 1st order reaction

Initiators contain labile bonds - activated by heat or irradiation: R-O-O-Rperoxide

R-S-S-Rdisulfide

R-N=N-Razo

30-31

The second initiation step

Addition of a single monomer to the radical:

I + Mka IM Monomer association rate constant, ka

CH2 CH

C O O

O

C

O

C O

O

2

C O

O

+ CH2 CHC O

O

Initiation of polymerisation of styrene using BPO:

ka

kd

31

The initiator dissociation is the rate determining step !

26

Propagation

IM + Mkp IMM

Additional monomers are added to the initiated monomer species:

Propagation rate constant, kp

IMM + n Mkp

IM(M)nM

CH2 CH+CH2 CHC O

O kpCH2 CHC O

O

CH2 CH

CH2 CHC O

O

CH2 CHn

CH2 CH

kp

n

31-32

TerminationTermination:

creation of (”dead”) polymer chainsoccurrs when two propagating molecules meet and react at their free-radical endsmay occurr either by combination or disproportionation

IM(M)nM + M(M)mMI IM(M)nM-M(M)mMIktc

CH2 CHC O

O

CH2 CHn

ktcHC 2CH CO

O

CH2CHm

+

CH2 CHC O

O

CH2 CHn

HC 2CH CO

O

CH2CHm

Termination by combination:

One polymer chain formed with DP = m + n + 2 32-33

27

Termination

I(M)nM + M(M)mIktd

CH2 CHC O

O

CH2 CHn

ktdHC 2CH CO

O

CH2CHm

+

Termination by disproportionation:

Two polymer chains formed !

I(M)nM + M(M)mI

CH2 CHC O

O

CH2 CHn 2 + HC 2CH CO

O

CHCHm

DP = n + 1 DP = m + 1

33

Chain transferChain transfer processes:

are due to the high reactivity of the radicalsoccurr by hydrogen abstraction by the radical species from another moleculecreate (”dead”) polymer chains and new radical speciesdecrease the degree of polymerisation

I(M)nM + H-Sktr

I(M)nM-H + S

ktr

One polymer chain and one radical specie formed

CH2 CHC O

O

CH2 CHn

+ H S

CH2 CHC O

O

CH2 CHn 2 +

DP = n + 1

S

initiation

propagation

34

28

Ri = d[I ]/dt = 2 × f × kd × [I~I]Ri = d[I ]/dt = 2 × f × kd × [I~I]

I~I 2 Ikd

intiating

lostInitiator efficiency, f = initiating / total

Rt = - d[IMx ]/dt = 2 × kt × [IMx ]2Rt = - d[IMx ]/dt = 2 × kt × [IMx ]2

IM(M)nM + M(M)mMI Polymerkt

Kinetics of free-radical polymerisations

Rate of termination = rate of radical disappearance:

Rate of initiation = rate of radical formation:

34-36

Rate of polymerisation

Rp = kp × [IMn ] × [M]Rp = kp × [IMn ] × [M]

Difficult to measure [IMn ] !

Assumption: the radicals obtain a constant steady-state concentration

At any given time during polymerisation:The number of radicals formed = the number of radicals terminated

Ri = RtRi = Rt

increased [IMn ] increased Rt decreased [IMn ]

2 × f × kd × [I~I] = 2 × kt × [IMx ]2

Rate of polymerisation = rate of disappearance of monomer = rate of propagation

[IMx ] = ×f × kd

kt[I~I]

½½

34-36

29

Rate of polymerisation

Rate of polymerisation, Rp = kp × [IMn ] × [M]

[IMx ] = ×f × kd

kt[I~I]

½½

From steady-state:

Rp = kp × × [M] f × kd

kt[I~I]

½½

×

Rp is proportional to:the monomer concentrationthe square root of the initiator concentration

37

H2

CH2

CH

C O

NH

C CH

C O

NH

CH2 N

CH2

CH2CH2

CH2

OHCH2

OH

CH2

OH OH CH2 CH2 OH

Rocagil BT/2:

N-methylolacrylamide

(40% in water)

bisacrylamide(0.5%)

ethylene glycoltriethanol amine

Na2S2O8

CH2 CH

C O

NH

OH

CH2

Component 1

Component 2sodium persulfate in water Functionality?

RocagilTM - a bad example......

Use: sealing material in, e.g., crack zones in tunnels to limit the flow of water

What happens to Rp if the reaction mixture is diluted by a factor 10?

30

Degree of polymerisation

If termination by combination: Xn = 2 × ν

If termination by disproportionation: Xn = ν

Kinetic chain length, ν– the average number of monomer additions before the chain is terminated

ν = Rp/Rtν = Rp/Rt

I(M)nM + M(M)mI I(M)nM + M(M)mI

IM(M)nM + M(M)mMI IM(M)nM-M(M)mMI

Combination gives twice as high MW as disproportionation !

37

Chain-transfer

Rtr = ktr × [IMx ] × [H-S]Rtr = ktr × [IMx ] × [H-S]

Rate of chain transfer:

I(M)nM + H-Sktr

I(M)nM-H + S

1/Xn = 1/(Xn)0 + C × [H-S]/[M]1/Xn = 1/(Xn)0 + C × [H-S]/[M]

C = ktr/kp , the chain transfer coefficient

(Xn)0 = Xn in the absence of chain transferXn

[H-S]/[M]

Degree of polymerisation:

38

(Xn)0

31

Ionic polymerisation

Vinyl monomers with electron-withdrawing groups:

Vinyl monomers with electron-donating groups:

More complex reaction mechanisms than in free-radical polymerisationStrongly influenced by solvent, temperature, and impurities

The propagating species have to be able to stabilise an anion or a cation:

CH2 C

C H3

C H3

e.g. isobutylene

CH2 C

C

CH3

O

O

CH3

e.g. methyl methacrylate Anionic polymerisation

Cationic polymerisation

45

Anionic polymerisation

Initiator: strongly nucleophilic agents, e. g. : CH3

C H2

C H2

C H2

Li

BuLi

Anionic polymerisation of polystyrene:

CH3

C H2

C H2

C H2

Li CH2 CH+ CH2 CH LiCH3

C H2

C H2

C H2

⊕Ө

CH2 CH CH2nCH

3C H

2C H

2C H

2CH LiӨ ⊕

initiation

deliberatetermination

”living” polystyrene by addition ofstyrene to the anionic center

”dead” polymer with very low PDI

Xn = [M]0/[I]0

In the absence of termination or chain-transfer:

46-47

32

Cationic polymerisation

Catalysts: Lewis acidsCo-catalyst: water BF3 + H2O H⊕ [BF3OH]Ө

CH2 C

C H3

C H3

+ CH3 C

C H3

C H3

⊕ [BF OH]Ө3

Cationic polymerisation of isobutylene:

initiation

CH2 C

C H3

C H3

propagation

C

C H3

C H3

⊕ [BF OH]Ө3C H2C

C H3

C H3

C H2CH3 C

C H3

C H3n

C

C H3

C H3

H ⊕ [BF OH]Ө3

C HC

C H3

C H3

C H2CH3 C

C H3

C H3n

+

chain-transfer

Usually no active termination reactions

MW is instead limited by chain-transfer reactions:

H⊕ [BF3OH]Ө

47-48

Polymerisation processes

Bulk polymerisationSolution polymerisationSuspension polymerisationEmulsion polymerisation

The choice of process is influenced by:The nature of the specific polymerisationHow the polymer is to be used – solution, emulsion, solid?Cost and environmental issues

Industrial scale processes

53

33

Bulk polymerisation

Homogeneoussystem of

monomer andinitiator/catalyst

Advantages:High-purity polymerHigh yieldEasy product recoveryCast polymerisation possibleContinous process possible

Disadvantages:Difficult to coolRemoval of residual monomer difficultBroadened MW distribution

Exothermic reactions: 40-80 kJ/molPoor thermal conductivity

The autoacceleration process:

Increase intemperature

Polymerisation

Increase inpolymerisation rate

Difficultiesto cool

Increase inviscosity

Increase inconversion

Suitable for free-radical polymerisations and step-growth polymerisations

53-54

Bulk polymerisation of styrene

BP Chemicals,Trelleborg

CH2 CH n

CH2 CH

Styrene

Polystyrene

n

∆

styrene +

initiator

80 oC

~30% conversion

~100% conversion

100 oC

200 oC

extruder

rotatingknife

polystyrenepellets

54

34

Solution polymerisationSuitable for free-radical and ionic polymerisations

Solvent:Monomer and initiator solubleAcceptable chain-transfer characteristicsSuitable melting and boiling pointsSafe (toxicity, flashpoint)Environmentally acceptableLow cost

Advantages:Easy to cool (low viscosity)Water can be used as solvent (radical)

Disadvantages:Small yield/reactor volumeSolvent handling/recovery/removal

Especially advantageous if the polymeris to be used as a solution (in water)

Homogeneoussystem ofmonomer,initiator

and solvent

54

Suspension polymerisation

Suitable for free-radical polymerisations

monomer (water-insoluble)initiator (water-insoluble)water (non-solvent)protective colloid

Advantages:Easy to cool (low viscosity)Water is used as mediumEasy to recover product by filtration orcentrifugation

Disadvantages:Small yield/reactor volumeLow purityOnly batch production possible

water

droplets or beads ofpolymer + monomer + initiator(50µm < d < 200µm)

Heterogeneoussystem

protective colloid(at interface)

rapidstirring

55

35

Emulsion polymerisation

Suitable for free-radical polymerisations

monomer (water-insoluble)initiator (water-soluble)water surfactant (soap)+ other

Advantages:Easy to cool (low viscosity)Water is used as mediumHigh polymerisation rate and high MWs

Disadvantages:Small yield/reactor volumeLow purityOnly batch production possibleRecovery of the polymer difficult

Heterogeneoussystem

Especially advantageous if the polymeris to be used as a latex

55-57

Fig. 2-9

Conformation, solutions and molecular weight

Polymers in solutions are used in:Polymerisation processesFilm castingFiber formationCoating and paintingPolymer characterisationsBiomedical and food applications

87

Important issues:Solubility – will the polymer dissolve or swell?Relations between polymer structure and solution properties,f.i., MW and viscosity

36

Conformation and chain dimensions

The polymer chains usually form ”random” coils in solutions The average end-to-end distance (r) can be used to descibe the coil size.

These coils constantly change their shape (conformation)

The dimension of the coils is determined by:MWthe flexibility of the polymer chainpolymer-solvent interactionstemperature

∆t ∆t ∆t ∆t

r

X-X

”hydrodynamic volume”

The freely jointed polymer chain

The freely jointed chain model with n freely jointed links of length l:

r<r2> = n × l2

ln = 23

Example: a polymer chain with 10 000 segments of 2.5 Å length : <r2> = n × l2 = 104 × 2.52 = 62 500 Å2 => r = 250 Å

- A simple model to describe polymer coil sizes

88-90

37

Polymer dissolution

swelling

solidpellet

swollenpellet polymer in solution

dissolution

Crosslinked polymers can swell but do not dissolve !Rule of thumb: ”like-dissolves-like”A given polymer is only dissolved by a certain number of solvents

Dissolution of a solid polymer sample:

In some cases solubility is desired, in other cases it should be avoided

A need to predict polymer solubility

δi = Eicoh = ∆Ei

v/Vi

1/2 1/2

Solubility parameters (δ) can be usedto predict polymer solubility

...where Eicoh = cohesive energy density

∆Eiv = molar energy of vaporisation

Vi = molar volume

Polymer solubility

Solubility if:δpolymer ≈ δsolvent

113-115

Same degree of interactions insolvent and polymer per volume unit

38

Solubility parameters

Is polystyrene soluble in toluene ?In hexane or methanol ?

115

Predicting solubility parameters

δ = Σ Fii=1

V

...where V is the molar volume of the polymer repeating unit or solvent = M/ρ

Solubility parameters can be predicted using ”Molar Attraction Constants” (Fi)for the different chemical entities in the polymer repeating unit or solvent:

Values of Fi can be found in tables (see Table 3-2)

C

H

C

H

HPS

The density of polystyrene is 1.04 g/cm3.What is the value of δ for polystyrene?Compared to experimental value?

39

Measurements of molecular weight

128

Important to characterise:Average molecular weightsMolecular weight distributions

Common molecular-weight averages: Mn, Mw and Mz

Distributions can be:”Narrow””Broad”UnimodalBimodaletc.

Methods to measure molecular weights

Primary (absolute) methods - no calibration necessaryMembrane osmometry - MnScattering - MwSedimentation - Mz

Secondary (relative) methods - calibration necessary*Chromatography – Mn, Mw, Mz, etc., + distributionIntrinsic viscosity measurements - Mv

* using samples of known MWs and narrow distributions

128

40

Membrane osmometry

129-131

Osmotic pressure, Π:Results from the different chemical potential of the solvent molecules in thepure solvent and in the polymer solution Depends on molecular weight and concentration (g/cm3)

For ideal solutions:

Π RTc/M=

ΠMn

= RTc1

+ A2c +A3c2 + ...

For high-MW polydisperse polymers in solution:Semi-permeable

A2, A3, describe deviation from ideality

Π = ρgh

Plots of osmometry data

Π/c = RT Mn

1+ A2c +A3c2 + ...

Measurements are:simpletime-consumingcritically dependent on the membrane

130-131

RTMn

41

Intrinsic viscosity measurements

The intrinsic viscosity, [η]:

[η] = K × Mva

... where K and a are empirical constants specific forpolymer, solvent, and temperature.

Mv is the viscosity-average molecular weight

Mark-Houwink-Sakuradaequation

Mn < Mv < Mw

Why do polymers in solution have a higher viscosity than the solvent itself?

139

Variation of the laminar flow rateof a liquid in a capillary tube

velo

city

Internal friction forces leads toresistance to flow - viscosity

Viscosity of dilute polymer solutions

The viscosity depends on the radius of thehydrodynamic sphere occupied by the polymer

– and hence on the MW

42

ηi /c = ηred = [η] + kH[η]2c

Huggins equation

Viscosity measurements

ηi = η - ηsηs

=t - ts

ts

ts = efflux time for pure solventt = efflux time for a solution with polymer concentration, cηi = relative viscosity-incrementηred = reduced viscosity

Ubbelohde capillaryviscometer

1. ts and t for solutions with different c are measured2. ηi are calculated for different values of c3. ηi/c are plotted vs. c

Procedure:

139-141

Plot of ηi/c vs. c

Intercept with y-axis = [η]

Evaluation of viscosity data

Mv can be calculated using M-H-S eqn.using tabulated values of K and a(Table 3-6)

<r2> = M [η]Φ

2/3

The mean-square end-to-end distance can be estimated by:

... where Φ is a ”universial” constant, Φ ≈ 2.1 × 1021 dL mol-1 cm-3

r

140-142

[η]

43

Gel-permeation chromatography (GPC)

...or size-exclusion chromatography (SEC)

Principle of separation:

Principle of operation:

142-143

Evaluation of gel-permeation chromatograms

143

Elution volume dependent on:Polymer characteristicsSolventTemperaturePumping rateColumn packing and sizeMolecular weight

Standard samples needed of:known MWsnarrow MW distributions

.... usually prepared by anionic polym.

44

Universal GPC calibration

144Can be done if the Mark-Houwink parameters are known for the polymer samples

- in the same solvent and at the same temperature

Hydrodynamic volume ~ <r3> ~ 1/Elution volume

r

<r2> = M [η]Φ

2/3

<r3> = M [η]Φ