POLYMER

“contents of this page are everything about POLYMER COLLOID”

I. EMULSION POLYMERIZATION

Emulsion Poymerization is a heterogeneous, free radical polymerization process which has wide industrial application in the production of polymer colloids or lattices of several different types of polymer.

Types of Polymer
1. polybutadiene and butadiene-styrene copolymer
2. poly (vinyl acetate) and vinyl acetate copolymer
3. acrylate ester copolymer
4. polyvinyl chloride and chlorode copolimer
5. vinyledene chloride copolimer
6. polyethilene and ethylene copolymer, polyterfluoroethylene
7. polyacrilamide and acrylamide copolymer

Application of latices
1. synthetic rubber
2. floor coatings
3. paints
4. adhesives
5. binder for non woven fabrics
6. high impact polymer latex foam
7. additives for construction materials such as cement and concrete, flocculants and rheological modifiers

Application in biomedical
1. diagnostic test
2. immunoassays
3. biological cell-labeling
4. drug delivery systems

Types of process in emulsion polymerization
1. Batch polymerization : all ingredients are added at the start of the reaction
2. Semi Batch : one or more of the ingredients usually the monomer either neat or in emulsion is added continuously or in increments
3. Continue : All ingredients are added continuously and product latex is continously removed

Chemistry Of Free Radical Polymerization

Reaction in free radical polymerization
1) Initiation : a primary radical formed by decomposition of the initiator adds a monomer molecule to form a monomer radical
2) Propagation: the monomer radical adds monomer molecules successively to form a long polymer chain ending in a monomer radical
3) Termination: two polymer radical combine to form one polymer molecule, or disproportionate two polymer molecule
4) Transfer: a growing polymer radical react with a large variety of molecules by abstracting a hydrogen or a halogen atom from another compound to terminate the polymer radical, which add monomer molecules to grow another polymer chain

Heterogeneous Polymerizations

Types of Heterogeneous Polymerizations
1. Emulsion
*In emulsion polymerization, a water-immiscible monomer is emulsified in an aqueous continuous medium using an oil-in-water emulsifier type, and polymerized using a water soluble or an oil soluble initiator.
*The initial monomer emulsion is comprised of droplets in the size range of 1-10 micrometer.
*The final latex system is comprised of colloidal dispersion of polymer particles in water with particle size usually in the submicron range, which show no resemblance to the original monomer droplet size
2. Suspension
suspension polymerization involves the dispersion of monomer as droplets in the continuous phase, in the size range of 100=10000 micrometer in diameter.
usually a combination of a dispersing agent and stabilizing agents in addition to mechanical agitation are required to control the initial size of the monomer droplet.
Polymerization are carried out by initiating polymerization in the monomer droplets using an oil soluble initiator, each monomer droplet is considered to be small bulk polymerization system and the kinetics of polymerization usually follows bulk polymerization kinetics.
the final “bead” size usually mimics the initial monomer droplet size.
3. Inverse emulsion
Inverse emulsion polymerization involves the emulsification of a water – miscible monomer, usually in aqueous solution, in a continuous oil medium using a water in oil emulsifier.
Polymerization is carried out using an oil soluble or water soluble initiator to give a colloidal dispersion of water- swollen polymer particle in a continuous oil phase.

4. Dispersion
Dispersion polymerization involves polymerization of monomer dissolved in organic solvent using an oil-soluble initiator in the presence of polymeric stabilizer to give insoluble polymer particles in the size range of 10-10000nm dispersed in the oil medium.
The polymerization begins as solution polymerization but become heterogeneous polymerization upon stabilization of the polymer particle.

MECHANISM AND KINETICS OF EMULSION POLYMERIZATION

Emulsion polymerization reaction divided into two stages:
1. Particle nucleation
a) Begin by initiation of monomer polymerization
b) The stage is more important and controversial polymerization
Important : because it determines the number of particle present in the system and thus the rate of polymerization
controversial: because of the experimental difficulties in determining the type o nucleation mechanism taking place
for each monomer system and experimental condition.
2. Particle growth
Usually considered to take place by polymerization of monomer in the monomer-swollen particle, (by propagation)

Homogeneous Nucleation
Homogeneous nucleation theory charged free radical generated in the aqueous phase react with soluble monomer to form soluble oligomeric radical
the oligomeric radical grow by further addition of monomer units until they exceed their solubility limit in the aqueous phase and precipitate from solution
the precipitated oligomeric radical form spherical particles and adsorb surfactant molecule to form primary particles.
the oligomeric radical formed, maybe precipitate to form more primary particles or maybe captured by already formed particles.
primary polymer particles then swell with monomer and grow by propagation
primary particles may also flocculate with themselves or with growing particle depend on the effectiveness of the surfactant as a stabilizer.
Particle growth according to homogeneous nucleation mechanism may take place by both propagation and flocculation.
The number and size of the latex particles are determined by the amount of the surfactant and its effectiveness in stabilizing the primary particle and the growing particles.
homogeneous nucleation was proposed as the primary mechanism for particle formation in surfactant free emulsion polymerization system
the stabilization of the primary particles and growing particles is due to electrostastic stabilization mechanism as a result of the presence of charged initiator fragment on the particle surface.

Coagulative Nucleation Step in the coagulative mechanism
1) formation of the “precursor” particle, most likely by homogeneous nucleation
2) formation of “mature” latex particles by aggregation of the precursor particles

The coagulative mechanism assume that the precursor particles are very small in size.
Because of their small size and thus high degree of surface curvature, the precursor exhibit (show) poor colloidal stability to coagulation, and their monomer concentration are much less than in normal mature latex particles.
Consequently the grow in volume much faster by coagulative rather than by propagational processes.
Ultimately, the aggregational and propagational process of the primary and higher precursor lead to “mature” colloidally stable latex particles which are swollen with monomer fully.

MINIEMULSION POLYMERIZATION OF STYRENE

Kinetic studies of Polymerization of styrene miniemulsion (prepare using sodium lauryl sulphate surfactant and cetyl alcohol as the additive), with water soluble initiator and oil soluble initiator, showed that there are several major differences in the kinetic features between miniemulsion and conventional emulsion polymerization of styrene.

Firstly, the radical entry into styrene droplets was unusually long. The intermolecular complexes of the surfactant/additive, or liquid crystal formed at the oil/water interface provide a physical barrier, which prevent oligomeric free radical from entering the monomer droplet. As a result of this low initiator capture efficiency, not all droplet could be initiated, and the fraction becoming particles is determined by the level of initiator.

Secondly, Miniemulsion latex has a very broad particle size distribution regardless of the initiator concentration. This is contrasted with the common experience encountered in conventional emulsion polymerization where the breadth of the particle size distribution is inversely proportional to initiator concentration and polymerization temperature.

Thirdly, the characteristic of Miniemulsion polymerization is when the particle nucleation stage ends (marked by the disappearance of all monomer droplet), the polymerization rate begin to decrease immediately due to the decrease in monomer concentration in the monomer swollen particles, however in conventional emulsion polymerization of styrene exhibit a constant rate of polymerization.