Emulsion polymerization of styrene

Theoretical background

The definition for emulsion is: A stable colloidal suspension as milk, consisting of an immiscible liquid dispersed and held in another liquid by a substance called an emulsifier. Emulsion polymerization is a heterogeneous polymerization technique. Initially the monomer molecules are dispersed in water, forming small drops with diameter of about 1-10 (m. The final polymerization system is also an emulsion with even smaller particles (0.05-0.15 (m); the stability of the latter is higher when compared to the initial micro-heterogeneous system.Typically, the emulsion polymerization system comprises four basic ingredients: 1) monomer, 2) reaction/dispersion medium, 3) emulsifier, 4) initiator.(1) The monomer forms the dispersed phase. The monomer should be polymerizable through free radical polymerization.(2) The reaction medium (the continuous phase) for monomer droplets and polymer particles may be: (i) water (widely used due to inertness and other exceptional properties such as high specific heat and thermal conductivity, non-toxicity, large availability and low price) or (ii) an organic solvent (less used, if the monomer is hydrosoluble; such system is known as reversed emulsion). (3) The surfactant/emulsifier contains two ends of different solubility:

the tail - a long hydrocarbon chain soluble in nonpolar, organic compounds

the head usually a sodium or potassium salt of a carboxylic acid or sulfonic acid, water soluble.

The surfactant has different roles: (i) to reduce the interfacial tension between the monomer phase and the water phase so that, with agitation, the monomer is dispersed (or emulsified) in the water phase andto stabilize the monomer droplets in an emulsion form.(ii) to generate micelles. If these substances are used above the critical micelle concentration (CMC/cmc), they will form micelles which are ordered clusters of emulsifier molecules, with the oil-soluble part of the molecule oriented toward the center of the cluster and the water-soluble part of the molecule toward the water.(iii) to solubilize the monomer within emulsifier micelles.(iv) to stabilize the growing latex particles and to stabilize the particles of the final latex. Emulsifiers: sodium dodecyl sulfate (SDS), sodium lauryl sulfate (SLS)

SDS

SLS(4) The initiator should be soluble in the reaction medium.

Hydrosoluble initiators: AIBN, ammonium persulfate, potassium persulfateEmulsion Polymerization

In an emulsion polymerization, the surfactant is dissolved in water until the critical micelle concentration (CMC) is reached. The interior of the micelle (5-10 nm) provides the site necessary for the occurrence of the polymerization. The monomer (like styrene or methyl methacrylate) and the water soluble free radical initiator are added and the whole batch is stirred. The monomer initially forms droplets (1-10 m). The emulsion polymerization always occurs through a free radical mechanism. (Anionic and cationic chain ends would be rapidly quenched by the water.)The product of an emulsion polymerization is called latex.Once the polymerization mixture is prepared, the monomer can be found:

in large monomer droplets a very small amount dissolved in water + emulsifier in micelles. The emulsion polymerization is a complex process still not completely understood.

Schematic representation of emulsion polymerization according to Fikentscher and Harkins

Initiation takes place in two stages: first the initiator decomposes, to generate free radicals, and then the radicals migrate and they react with monomer molecules. Water soluble initiators, such as persulfates, are commonly used (this also prevents polymerization in the big monomer droplets). Once polymerization starts, the micelle is referred to as a particle. Polymer particles can grow to extremely high molecular weights, especially if the initiator concentration is low. That makes the radical concentration and the rate of termination low as well.

Monomer molecules migrate from the large monomer droplets to the micelles to sustain polymerization. Latex particles grow until the monomer drops are gone from the reaction medium. Due to the increase in size of the latex particles, more emulsifier molecules are adsorbed onto their surface and the micelles as such disappear. Practically all the monomer is consumed in emulsion polymerizations, meaning the latex can be used without purification. This is a very important practical aspect for paints and coatings.Here's the neat aspect of emulsion polymerization: each micelle can be considered as a bulk polymerization micro-reactor. Unlike traditional bulk polymerizations there is no unreacted monomer leftover, and no thermal "hot spots" form. In bulk polymerizations (no solvent, just monomer and initiator), thermal hot spots lead to degradation and discoloration; chain transfer broadens the molecular weight distribution. An increase in temperature sometimes causes the rate of polymerization to increase explosively. In emulsion polymerization the water acts as a heat exchanger for all the micro-reactors and prevents over-heating.Molecular Weight

The rate of polymerization is the same as the rate of disappearance of monomer. Monomer disappears faster when there are more particles. In order to have more particles there must be more micelles. If the surfactant concentration is increased, this ought to give more micelles. If the concentration of the initiator is kept the same, this gives more particles and less radicals. Accordingly, the number of radicals per micelle < 1. This leads to a decrease of the termination rate since there are less radicals.Decreasing the initiator concentration increases molecular weight and rate of polymerization! This is different when compared to bulk and solution polymerization. To increase the rate of polymerization for those you have to heat the reaction or increase the initiator concentration, both of which increase the rate of termination and lower the molecular weight. Polymerization type+-

Bulk Only monomer + initiator, eventually crosslinker High molecular weightsThermal hotspots, high viscosity, residual monomer

Solution The solvent allows temperature control.The solvent should be removed; chain transfer to solvent

Emulsion All the monomer is consumed; the latex can be used as such (no residual monomer);

temperature control

Experimental part

Materials:

A three necks round-bottom flask, equipped with stirring unit, thermometer and condenser Heating water bath (temperature range: 0 - 100oC) Filtration funnels, beakers and Erlenmeyer flask

Pipettes

Gradual cylinders

Solvent: distilled water: 80 ml

Monomer: styrene: 20 g (d200C = 0.905 g/cm3) Surfactant: sodium lauryl sulfate, sodium dodecyl sulfate Initiator (ammonium persulfate - APS)

CaCl2 or methanol.

Method:

The distilled water is poured in the round bottom flask. The surfactant is dissolved under stirring. The temperature is then raised to 60oC and the monomer is added under continuous stirring. When the working temperature is reached, the initiator is added (this is the zero moment of the polymerization). It is recommended to dissolve the initiator in a small amount of water (this volume should be calculated from the total amount of water to be used in this procedure). The polymerization time starts to be measured after 5 minutes from the addition of the initiator (considered as the real initial moment of the polymerization). Samples (approximately 5 ml) will be withdrawn at regular time intervals (15 minutes) with a pipette, in order to follow the monomer conversion by gravimetry. The first 4-5 samples should be removed at lower time intervals (5-10 minutes). The polymerization time needed to reach a complete monomer conversion is usually 3 h.To determine the monomer conversion each emulsion should be broken. To do this, the emulsion is poured in an aqueous solution of an electrolyte (CaCl2 or methanol, approximately 30-50 ml). The separated polymer will be filtered, washed and dried (to constant mass) and then weighed. At the end of the reaction, the whole reaction mass will be precipitated. A sample will be kept for the measurement of the average molecular weight.

Results1) The dependence conversion polymerization time should be graphically represented.

2) The curve reaction rate polymerization time should be drawn.Results:Morphology of latex particles as obtained by scanning electron microscopy: Polystyrene particles