Polym. Tech.

transcript

Introduction to free-radical

polymerization

Polymerization mechanisms

- Step-growth polymerization

Polymerization mechanisms

- Chain-growth polymerization

Advantages of free-radical polymerization

- very robust technique- wide range of vinyl monomers and functionalities- wide range of operating conditions- aqueous media: emulsion polymerization

Disadvantages of free-radical polymerization

- non-selective reaction- non-trivial product control

Chain reactions

characteristics:

•each polymer chain grows fast. Once growth stops a chain is no longer reactive.

•growth of a polymer chain is caused by a kinetic chain of reactions.

•chain reactions always comprise the addition of monomer to an active center (radical, ion, polymer-catalyst bond).

•the chain reaction is initiated by an external source (energy, reactive compound, or catalyst).examples:

radical polymerizationionic polymerizationcoordination polymerizationgroup transfer polymerization

Radical polymerization

• initiation (start)• propagation (growth)• chain transfer (stop/start)• termination (stop)

During initiation active centers are being formed.During termination active centers disappear.

Concentration of active centers is very low (10-9 - 10-7 mol/L).

Growth rate of a chain is very high (103 - 104 units/s).

Chains with a degree of polymerization of 103 to 104 are being formed in 0.1 to 10 s.

Initiation

Most common method of initiation: thermal decomposition of a thermally labile compound, the initiator

These radicals are called primary radicals. Initiation takes place by addition of a monomer unit.

kd is a first order rate constant.Common values of kd are in the range: 10-4 - 10-6 s-

1.The rate of radical production is then given by:

]I[k2dt

]•R[dd

I 2 Rkd

R + M R1

The actual rate of initiation Ri is expressed in terms of the rate of radical production that leads to actual polymer chains growing!:

]I[fk2R di

where f is the efficiency factor: the fraction of radicals that really leads to initiation.Therefore, in all cases: f 1

The rate constant ki is not used in the mathematical description of the polymerization.

Examples of thermal initiators:

Ea = 140 – 160 kJ mol-1

propagation

This reaction is responsible for the growth of the polymer chain. It is the reaction in which monomer is added at the active center:Mi + M

kp Mi+1

The rate of this reaction Rp can be expressed as:

]M][•M[kR pp

Assumption:the reaction rate constant kp is independent on chain length. This appears to be reasonable above chain length i = 5 - 10

Propagation is the most important reaction for monomer consumption.

The reaction rate constant kp typically has a value in the range 102 - 104 L/mol s

Termination

Chain growth stops by bimolecular reaction of two growing radicals: termination

Traditionally two different reactions are recognized:• termination by combination (ktc)• termination by disproportionation (ktd)

Schematically these reactions can be represented as:

The general kinetic equation reads:2

tt ]•M[k2R

The reaction rate constant kt is in the range 106 - 108 L/mol s

Mi + Mj

ktcMi+j

Mi + Mj

ktdMi Mj+

Termination

Every reaction consists of two steps:1) approach of both reactants2) chemical reaction

The second step in the termination reaction is very fast. This means that the rate of approach (partially) determines the overall termination rate.

at 5 % conversion

Termination: which is faster?

1. + or +

2. + or +

in a viscous medium in a non-viscous medium

3. + or +

at 85 % conversion

Rate of polymerization

The rate of polymerization in a chain growth polymerization is defined as the rate at which monomer is consumed.

pi RRdt

Since for the production of high molar mass material Rp » Ri this equation can be re-written as:

•]M][M[kRdt

]M[dpp

From the beginning of the polymerization:• increasing number of radicals due to decomposition of the

initiator• increasing termination due to increasing radical

concentration (Rt [M·]2)• eventually a steady state in radical concentration:

•]M[k2]I[fk2

This steady state assumption leads to:

]I[fk=•]M[

From which the differential rate equation is derived:

]I[fk]M[k=R

At low conversion this means:

log(Rp) vs log[M] yields a slope = 1

log(Rp) vs log[I] yields a slope = 0.5

When the initiator decomposes slowly compared to the entire polymerization process:

e]M[]M[

dtk]M[

When ln([M]0/[M]) is plotted versus time, then the slope should equal k:

]I[fkkk

Kinetic chain length

species initiating

unitsmonomer ofnumber

k]I[fk2

•]M[2k

•]M][M[k

Also =kp [M] , with t time of growth of a polymer chainHere we find at low conversion:log(n) vs log[M] yields a straight line; slope = 1log (n) vs log[I] similar; slope = -0.5thus: increase in [I] leads to an increase in rate of polymerization and a decrease in chain length.

The special case with a slowly decomposing initiator leads here to:

wheretd

k]I[fk2

td kIfkM

•][2k

•][2

The number average degree of polymerization Pn of chains formed at a certain moment is dependent on the termination mechanism:* combination: Pn = 2

* disproportionation: Pn = chemistry:

CH2 CH2 C

Initiatie/terminatie via MALDI-ToF MSMethyl acrylaat met benzoine

1276.0 1310.6 1345.2 1379.8 1414.4 1449.0

Mass (m/z)

1248.5

1295.98

1312.01

1398.071382.05

1296.991313.01

1383.051399.081352.03

1438.09

1353.04

1439.101314.01

1334.041297.99 1384.06 1400.081420.10

1358.071342.041332.02 1428.101359.07

1429.101418.101343.051294.02 1333.021315.03 1385.08 1401.091360.08 1435.071349.041336.051276.101427.40

1288.0 1296.2 1304.4 1312.6 1320.8 1329.0

Mass (m/z)

1248.5

1295.98

1312.01

1296.991313.01

1314.01

1297.99

1294.02 1315.031298.99

1327.0 1335.4 1343.8 1352.2 1360.6 1369.0

Mass (m/z)

1352.03

1353.04

1334.04

1358.071354.051342.041332.02

1335.04 1359.071348.031343.051333.02 1350.03

1346.03 1360.081349.041336.05

B-MA12-A

A-MA13(-H)

A-MA13(+H)

B-MA14(-H)

B-MA14(+H)

B-MA13-B

A-MA13-A

16.03 Da

Trommsdorff effect

In radical polymerization we speak about:1) low conversion, i.e. polymer chains are in dilute solution (no contact among chains)2) “intermediate” conversion, i.e. the area in between low and high conversion3) high conversion, i.e. chains are getting highly entangled; kp decreases.

Somewhere in the “intermediate” conversion regime:* polymer chains loose mobility.* Termination rate decreases* Radical concentration increases* Rate of polymerization increases* Molar mass increases

This effect is called: gel effect, Trommsdorff effect,or auto-acceleration

In the polymerization of MMA this occurs at relatively low conversion.

Molar mass

in the ideal case:

2P (termination by combination)

(termination by disproportionation)

However, a growing chain may transfer its activity to a new chain:

This reaction is then followed by re-initiation, the start of a new chain:

Mi + T Mi + Tktr

T + M M1ki'

Chain transfer

chain transfer to:• monomer• initiator• solvent or chain transfer agent• polymer• allylic transfer

monomer, initiator and chain transfer agent are mathematically treated identically:

•][X]M[k•]M[k2•]M[kdt

]polymer[dX,tr

As derived beforethis leads to:

•]M[k2

•]M[k

•]M[k2

•]M[k

The rate of “polymer formation” is now defined as:

]T][•M[k]•M[k2]•M[kdt

]polymer[dtr

The rate of polymerization as derived before:

•]M][M[kdt

From the definition of number average degree of polymerization it follows:

dt]polymer[d

dt]M[d

]•M[k2

]•M[k

]•M][M[k

]T][•M[k]•M[k2]•M[k

]M[]T[

Chain transfer to polymer

• Intermolecular chain transfer

• Intramolecular chain transfer

Traditional approach: intermolecular, strong increase in branching density towards high conversion.

Recent results: • Hardly conversion

dependent

• Dilution results in higher degree of grafting

0 10 20 30 40 501

conversion ca. 25% conversion > 80%

[BA]0 / %(w/w)

Ziegler-natta

Summary

tMkL p ][

]][[ MMkR pp

Chain-length

Rate of polymerization

Initiator decomposition is the reaction step most strongly influenced by temperature.

MtTime of chain-growth

Chain length distribution

Crucial in the derivation of the CLD is the chance on formation of an i-mer chance on propagation:

thus: chance on chain stop: 1-pDuring the growth of a single chain p is constant

disproportionationmol fraction i-mer: p1px 1i

mass fraction i-mer: 21i

ii p1ip

which yields:

in p1P

combination

p)1i()p1(i2

p)1i()p1(x

which yields:

kk1111

kk1212

kk2121

kk2222

Copolymerization

——MM11• + M• + M11 —M—M11••

——MM11• + M• + M22 —M—M22••

——MM22• + M• + M11 —M—M11••

——MM22• + M• + M22 —M—M22••

Copolymerizationffii:: fraction of monomer fraction of monomer ii in reaction mixture in reaction mixture

ff11 = [M = [M11] / ([M] / ([M11] + [M] + [M22])])

FFii:: fraction of monomer fraction of monomer ii built into polymer built into polymer

FF11 = d[M = d[M11] / (d[M] / (d[M11] + d[M] + d[M22])])

2 frfffr

Long chain assumption (Long chain assumption (kkii, , kkdd ignored; ignored; kkpp, , kktt not ~ chain not ~ chain length)length)

Reactivity ratios independent of environmental factorsReactivity ratios independent of environmental factors

22221111

kfrkfr

frfffrk

Average copolymerisation rate: Average copolymerisation rate:

0 0.2 0.4 0.6 0.8 1f 1

MMA / BAEthene / VAcVDC / VC

Ideal copolymerisation

Composition driftComposition drift

If If ff11 ≠ F≠ F11

→ → ff11 changes changes

→ → FF11 changes changes

What does composition What does composition drift mean for the polymer drift mean for the polymer that is formed?that is formed?

Polymerization techniques

• Kinetic / mechanistic factors related to chain length, chain composition

• Technological factors e.g. heat removal, reaction rate, viscosity of the reaction mixture, morphology of the product

• Economic factors; production costs, enviromental aspects, purification steps etc.

Sometimes for one monomer several techniques of polymerizing are available. Choice of a specific technique depends on a number of factors:

Homogeneous systems• Bulk polymerization• Solution polymerization

Heterogeneous systems• Suspension polymerization• Emulsion polymerization• Precipitation polymerization• Polymerization in solid state• Polymerization in the gas phase

Polymerization techniques

Bulk polymerization Polymerization of the undiluted monomer.

Viscosity increases dramatically during conversion. Heat removal and hot spots

Advantages Disadvantages * Pure products * heat control * Simple equipment * dangerous

* No organic solvents * molecular weights very disperse

Applications Polymers through step reactions (nylon 6) PMMA-plates

Solution polymerizationMonomer dissolved in solvent, formed polymer stays dissolved. Depending on concentration of monomer the solution does not increase in viscosity.Advantages Disadvantages* Product sometimes * Contamination

directly usable with solvent* Controlled heat * Chain transfer to release solvent

* Recycling solvent Applications Acrylic coating, fibrespinning, film casting

Suspension polymerization• Water insoluble monomers are dispersed in

water.• Initiator dissolved in monomer.• Stabilization of droplets/polymer particles with

non-micelle forming emulsifiers like polyvinylalcohol or Na-carboxymethylcellulose.

• Equivalent to bulk polymerization, small droplets dispersed in water.• Product can easily be separated, particles 0.01-1mm.• Pore sizes can be controlled by adding a

combination of solvent (swelling agent) and non-solvent.

• Viscosity does not change much.

tive d

Advantages Disadvantages * Heat control simple * Contamination with * Product directly stabilizing agent

usable * Coagulation possible * Easy handling

ApplicationsIon-exchange resins, polystyrene foam, PVC

Suspension PolymerizationQ

ualita

tive d

Emulsion Polymerization

• A micelle forming emulsifier is used.• Initiator is water soluble.• The formed latex particles are much

smaller than suspension particles (0.05-2 µm).• Kinetics differ considerable from other

techniques.• Polymer is formed within the micelles and not in the monomer droplets.

Antonio de Herrera Tordesillas described an application of natural latex in 1601. A ball game as part of a religious rite.

tive d

Emulsion Polymerization

Advantages Disadvantages* Low viscosity even * Contamination of at high solid contents products with additives

* Independent control * More complicated of rate and in case of water molecular-weight soluble monomers * Direct application of

complete reactor contents

Applications of laticesPaints-Construction

* Paints / rough casting / heat insulation* Elastomeric coatings / primers* Additives for cement and concrete* Anti corrosives / wood coatings* Industrial coatings* Rheology modifiers* Roof coatings

* Fillers and levelling powders

Glues-Adhesives* Wood glues / adhesives for furniture laminates* Adhesives for floor-, wall- and

ceiling materials* Packaging- and lamination glues* Adhesion- and contact glues* Leather fibres* Glues in powder form

Textiles* Carpet backside coatings* Fleece binder* Spunbond / textile coating* Equipment of technical textiles* Pressure binder /flocculating glue

* Boxes and wallpaper* Binders for rubbed paper

* Structural Adhesives* Contact Adhesives

* Varnishes

Source: Clariant

Other* Teflon* Elastomers

Objective

Properties

Process

Molecular Microstructure & Morphology

What is the relation ?

Differential Scanning Calorimetry

GlasovergangstemperaturenTg [°C]

Polyvinylacetaat + 29

Polyvinylpropionaat + 7

Poly(VeoVa 10) - 2

Polyvinylidenchloride + 80

Polyacrylonitril + 100

Polyethyleen (-125)

Polystyreen + 100

Polymethylmethacrylaat +105

Poly 2-ethyl-hexylacrylaat -85

Poly n-butylacrylaat -54

Polyacrylzuur (kristallijn) + 166

Poly methacrylzuur + 185Poly acrylamide (kristallijn) + 153

Polyhydroxy-ethyl-methacrylaat + 55

Monomer: Tg [°C] Methyl - Acrylaat + 8

Ethyl - Acrylaat - 22 n-Butyl - Acrylaat - 54 2-Ethylhexyl - Acrylaat - 85

Methyl - Methacrylaat + 105 Ethyl - Methacrylaat + 65 n-Butyl - Methacrylaat + 20 2-Ethylhexyl - Methacrylaat - 10 Decyl - Methacrylaat - 70 n-Butyl - Acrylaat - 54 iso-Butyl - Acrylaat - 43 sek.-Butyl - Acrylaat - 20 tert.-Butyl - Acrylaat + 41 n-Butyl - Methacrylaat + 20 iso-Butyl - Methacrylaat + 48 sek.-Butyl - Methacrylaat + 60 tert.-Butyl - Methacrylaat + 107

Glasovergangstemperaturen

Monomeer – Indeling in groepen

Harde monomeren

Styreen

Methylmethacrylaat

Vinylchloride

(Vinylacetaat)

(Vinylpropionaat)

Acrylonitril

Zachte monomeren

Acrylzuuresters

Butadieen

Ethyleen

Versaticsäurevinylester

Malein- und Fumaarzuur-esters met C > 4

Geladen monomerenAcrylzuur

Methacrylzuur

Maleinezuur

Fumaarzuur

Natrium-Etheensulfonaat

Natrium-Etheenphosphaat

Vernettende monomeren

Polyvinyl- und Polyallyl- verbindingen

N-Methylol-verbindingen

Aktieve halogeenhoudendeverbindingen

Spherulieten in de microstructuur van een polymeer.

De 3M aspecten van de PE proef. Links de toepassing, een beker (macro), met in het midden de morfologie van dit polymeer (meso). Rechts de atomaire ketens zoals die in het polymeer voorkomen (micro).

Copolymeren

Block-Copolymeer

Graft polymeer

Statistisch Copolymeer

Alternerend Copolymeer

Naast een molmassaverdeling bezit een copolymeer ook Naast een molmassaverdeling bezit een copolymeer ook een samenstellingsverdelingeen samenstellingsverdeling

FFmonomer 1monomer 1

Approach : Raman spectroscopy as on-line sensor

lasermodel

Properties CopolymerCompositionDistributionMonomer

Composition

Reactivity

Monomer Partitioning

Kinetics

In-situ Raman spectroscopy gives

on-line information on concentrations

at break

Mechanical properties

Example: 25% styrene - 75% methyl acrylate

0 0.5 1

Polymeerveroudering

• Plafondtemperatuur• Degradatie• Stabilisatie

Polymeerveroudering

-Ketenbreuk

-Depolymerisatie

-Crosslinking

-Bindingsverandering

-Zijgroepverandering

Plafondtemperatuur

• Temperatuur waarbij in evenwicht– 1 mol/l monomeer aanwezig is– Of [M]= concentratie zuiver monomeer

Methylmethacrylaat Tc=220 CAlpha-methyl styreen Tc= 61 C

Degradatie

• Thermische degradatie– Willekeurige ketenbreuk– Keteneind depolymerisatie (unzipping)

Degradatie

• Oxidatieve degradatie– Radicaalvorming door reacties met zuurstof

Degradatie

• Stralingsdegradatie– Fotolyse (UV)– Radiolyse

Degradatie

• Chemische degradatie– NO2

Degradatie

• Mechanochemische degradatie– Kogelmolen– Spuitgieten– Vermalen

Degradatie

• Biologische degradatie– Controlled drug release

Composieten

Nanotubes for conductive coatings

Encapsulated pigment

Armoured latex particle

Encapsulated clay platelet

Polym. Tech.

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