Note Miniemulsion 2015

7/24/2019 Note Miniemulsion 2015

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MINIEMULSION POLYMERIZATION


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Introduction

Miniemulsions are a special class of emulsions that are stabilized against

coalescence (by a surfactant) and Ostwald ripening (by an osmotic pressure

agent).

Miniemulsions are produced by the combination of a high shear which is

provided by a sonicator or a mechanical homogenizer to break the

emulsion into submicron monomer droplets with a size ranging from 50 to500 nm.

Surfactant/co-stabilizer is added to retard monomer diffusion/droplet

coalescence caused by Brownian motion settling or Stokes law creaming

by prevents Ostwald ripening . Therefore, when a liquid emulsion is

subjected to high shear, small droplets will obtain.


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The droplet surface area in these systems is very large where most of the

surfactant is adsorbed at the droplet surface.

Particle nucleation is primarily via radical (primary or oligomeric) entryinto monomer droplets, since little surfactant is present in the form of

micelles.

The important feature is that the reaction proceeds by polymerization of

the monomer in the droplets, thus there is no true Interval II inminiemulsion.

A polar droplets can be dispersed in a polar liquid to give direct

miniemulsions (oil-in-water), whereas polar droplets in a non-polar liquid

leads to inverse miniemulsions (e.g., water-in-oil, w/o).


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a) Macroemulsion polymerization versus b) miniemulsion polymerization.

b

a

Interval I

Interval II

Interval III

Monomerswollenmicelle ~100

Monomer swollenpolymar particle ~

500

Monomer swollenpolymar particle ~500

Emulsifier

Continuous aqueous phase



Monomerswollenpolymarparticle ~1000

Emulsifiedmonomerdroplet~ 10

Emulsifiedmonomerdroplet ~ 10

Interval I-

Particle nucleation

Interval II-

Particle growth

Interval III-

Monomer droplet

depletion

NO TRUE interval II

in miniemulsion

Particle of same size

Particle size increase


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Example of polymers produced by miniemulsion polymerization.

Polymer class Polymerization Year

polystyrene radical 1973

polyvinyl chloride radical 1984

silicone anionic 1994

polyethylene catalytic 2000

epoxy polyaddition 2000

polyurethane polyaddition 2001

saturated polyester polycondensation 2003

polyamide anionic 2005

polyimide polycondensation 2009


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Ostwald ripening

Ostwald ripening is a phenomenon which smaller particlesin solution dissolve and deposit on the larger particles inorder to reach more thermodynamically stable wherein thesurface to area ratio is minimized.

The small particle disappears while the size of large particle

increases at the end of process.

Small particle

Large particle Larger particle size

Small particle

disappears


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Ostwald ripening occurs because molecules on the surfaceof particles are more energetically unstable than those at thecentre.

The unstable surface molecules often migrate into solution,thus shrinking the particle over time and increasing thenumber of free molecules in solution.

The surface particle

gone after 7.8 sec,

migrates into solution


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When the solution is supersaturated with the molecules of the shrinking

particles, those free molecules will redeposit on the larger particles.

Small particles decrease in size until they disappear and large particles grow

even larger, thus resulting a larger mean diameter of a particle size

distribution (PSD).

Ostwald ripening is often found in oil-in-water emulsions where oil

molecules will diffuse through the aqueous phase and join larger oildroplets.

Over time, this causes emulsion instability and eventually phase separation.

Free molecule of particles

Larger particle


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Factors affect miniemulsion

Miniemulsions are produced by the combination of a high shear device (to

break up the emulsion into submicron monomer droplets) and a water-

insoluble, monomer-soluble component (to retard monomer diffusion

from the submicron monomer droplets) where both steps are necessary

to effect predominant droplet nucleation.

1. Surfactant/co-stabilizer

The vast majority of miniemulsion polymerizations have been stabilized

with anionic surfactants due to their compatibility with neutral or

anionic (acid) monomers and anionic initiators.

Co-stabilizer used either cetyl alcohol (CA) or hexadecane (HD) to

retard Ostwald ripening in submicron monomer droplets.

Both CA and HD possess requisite properties for a co-stabilizer i.e. high

monomer solubility, low water solubility and low molecular weight.


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High monomer solubility will give a large Flory Huggins interaction

parameter between the co-stabilizer and the monomer, thus giving a higher

volume fraction of co-stabilizer in the droplet.Low molecular weight co-stabilizer will give a high ratio of co-stabilizer

molecules to monomer molecules in the droplet.

These factors will enhance swelling or retard monomer loss via Ostwald

ripening.

2.

Choice of Initiator

Most miniemulsion polymerizations have been run using water-soluble

initiators. However, a number of researchers have looked at the

possibility of using an oil-soluble initiator instead.

In most cases the oil-soluble initiator is used as a finishing initiator to

increase final monomer conversion.


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3. Shear Device

High shear device (or sonication) is needed to break up the emulsion into

submicron monomer droplets.

In the absence of a high-shear device, miniemulsion systems revert tomacroemulsion polymerizations, indicating that the presence of a co-

stabilizer alone is not sufficient to cause predominant droplet nucleation.

Firstly, a coarse pre-emulsion is formed by vigorous stirring of the

monomer, water, surfactant to form a macroemulsion.

For reasons of practicality, the costabilizer is dissolved in the monomerbefore pre-emulsification.

Then, the coarse pre-emulsion particle is subjecting the system to high

shear device to form submicron droplet.

The intensity of shearing depends on the rotor speed, normally from 535

krpm for most modern equipment.

However, at higher speeds the shearing action generates a significant

amount of heat, which may harm the sample being emulsified or the

machine itself.


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Advantages of miniemulsion

1. The most prominent advantage is no step of monomerdiffusion through the aqueous phase - from the monomer

droplets to the polymer particles during the course of

polymerization because the monomer droplets are

directly polymerized.

2. The micellar nucleation in conventional emulsion

polymerization is extremely sensitive to a large number

of factors such as amount of surfactant, amount of

initiator, agitation speed, temperature of the

polymerization reaction, mode of addition of the

monomers, etc. These factors are eliminated in

miniemulsion.

Note Miniemulsion 2015

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