Colloidal Stabilisation

Emulsion polymers are colloids, meaning that they consist of small, discrete particles dispersed in a continuous liquid media. Colloids have many unique and interesting properties as a result of their small size (typically less than 1 pm) and large interfacial area (typically greater than 10 crcficrcf latex). Because interfacial effects dominate, surface science becomes paramount in understanding how emulsion polymers are formed and stabilised (42). 

Surfactant molecules are amphiphilic (121), meaning that they contain both hydrophilic (water-compatible) and hydrophobic (water-incompatible) sections (151, 280). The amphiphilic nature of surfactant molecules causes them to partition between the water and monomer/polymer phases by adsorbing at the droplet/particle interface. Once adsorbed at the interface, the surfactant keeps the droplets (or particles) separated by causing them to be repelled from one another through the electrostatic and/or steric stabilisation mechanisms (296,377). The same surfactant may act as both an electrostatic and a steric stabiliser, and is known as an electrosteric stabiliser (214, 398). 

In the electrostatic stabilisation mechanism, surfactant molecules dissociate in solution to provide an ionic charge when they adsorb at the droplet surface. The surface-charged particles repel one another electrostatically. Anionic surfactants (377, 391) carry a negative ionic charge, whereas cationic surfactants 

In the steric stabilisation mechanism, the surfactant molecules are water-soluble, polymeric chains that have some conformational mobility (109). When two particles covered with nonionic surfactant approach one another, the adsorbed layer is compressed, thereby limiting the mobility of the stabiliser chains. There is an associated, thermodynamically-undesirable increase in free energy that causes the particles to be repelled from one another. 

Being surface active (375), surfactants lower the interfacial tension between the water and monomer phases. The decrease in interfacial tension allows smaller droplets to be formed more easily during dispersion of the monomer in the water phase. In addition, the thermodynamic driving force for coalescence is lowered as a consequence of the reduction in interfacial tension. For this reason, and especially because of the electrostatic and steric stabilisation mechanisms, emulsions prepared with surfactants are more colloidally stable. 

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In latex compounding, same as protective colloid. Stabilisers are incorporated in synthetic raw rubbers to protect the rubbers from oxidation during storage. Since the chemical behaviour of these protective materials is not significantly different from that of antioxidants in a vulcanisate, it has been proposed that the use of stabiliser in this sense be dropped in favour of antioxidant. 

The dispersion stability, rheology, and consolidation of numerous aqueous and non-aqueous Si3N4 suspensions have been studied extensively [251-257]. Recently a novel class of dispersants for Si3N4 powders in non-aqueous media has been designed and its interactions with the powder surface have been characterised systematically on the basis of surface chemistry and fundamentals of colloidal stabilisation [255, 258]. 

There has been considerable recent interest in the self - assembly and surface activity of amphiphilic polymers and copolymers. Their interfacial and bulk solution properties have shown a rich pattern of behavior, and the ability to tailor their properties offers a wide range of potential applications. Their bulk aggregation behavior make them candidates, for example, for dye transportation and drug delivery whereas their surface properties make them useful as colloid stabilisers, anti -foaming agents and emulsifiers. This behavior can be illustrated in Fig. 3.24. 

Finally, it has been shown that in some cases, the use of a lower critical solution temperature colloidal stabiliser can control ink behaviour on the substrate. Above 37°C, the solubility of the stabiliser decreases causing a dramatic increase in viscosity. Lines printed using this approach did not display deviations at their starts and ends, and bulges in the line were prevented. 

In a typical procedure monomers and initiator are dissolved in the appropriate solvent or solvent mixture together with a colloidal stabiliser, which is usually a polymer. The mixture is purged with inert gas to remove oxygen, then heated to initiate polymerisation, usually with gentle agitation or stirring. After completion of polymerisation the particles are recovered by filtration, sedimentation or centrifugation and washed. 

Unfortunately, to date, this technique has received little attention from the molecular imprinting community and only one report of a dispersion polymerisation method had appeared until very recently [26]. This is probably better classified as a precipitation polymerisation, since random aggregates were produced rather than beads. No colloidal stabilisers were included in this procedure. The aggregates were made in situ in chromatography columns, which avoided the need to grind and sieve the polymer and pack the columns. Due to the rather polar nature of the solvent mixtures used (cyclohexanol, dodecanol, isopropanol), good imprints were only achieved for compounds which interact strongly with functional monomer... 

Stabilisation leads to colloid stabilisation due to the high and charge of the adsorbed organics. Multivalent... 

Figure 4.19 Postulated structures (A) stable hematite colloids in absence of organics, (B) reaction limited aggregation (RLAJ, (C) diffusion limited aggregation (DLAJ, (D) SPO aggregates with organics, (E) OPS colloids stabilised with organics, (F) OPS colloids stabilised with NOM, (G) OPS colloids stabilised with organics and destabilised with calcium.

Recently, Beletskaya and coworkers reported the use of Pd colloids stabilised by block-copolymer micelles formed by polystyrene-co-poly(ethylene oxide) and cetyl-pyridinium chloride as surfactant [37]. The material exhibited moderate to high activities and high recyclability for the Heck reaction between aryl iodides and acrylates. 

Polymers in solution have an enormous effect on the adhesion between surfaces. Such polymers are used as lubricants, as thickeners, as colloid stabilisers, as binders, glues, and inks. Also they are ubiquitous in biological systems. Their practical significance is large but understanding their effects remains to be explored to a great extent. This section describes some experimental observations of the adhesion forces and draws a schematic theoretical picture of the effects. 

The results are reported of a study of the mechanism of stabilisation during vinyl acetate-ethylene emulsion copolymerisation using various colloidal stabilisers. These stabilisers included PVAl, aUcylphenol ethoxylate and a diisocyanate chain extended polyethylene glycol. The effects of these stabilisers on emulsion characteristics, film properties and applications behaviour are discussed. 5 refs. (217th ACS National Meeting, Anaheim, Calif., 21-25 March, 1999)... 

TEM images of particles deposited from the standard sol yielded an (number) average diameter of 1.5 nm (mass-weighted mean 2.2 nm), and a coefficient of variation (o/) of 38 %, but some cluster coalescence within the spedmen was observed. This caused bands of much coarser particles (<10 nm) at locally high concentrations to be present in the specimen. Only the well separated clusters were measured for the distribution presented here. The addition of the colloid-stabilising agent gelatin to the hydrosol prevented the formation of these bands of presumably... 

Auxiliaries sueh as initiators, ehain transfer agents or sometimes emulsifiers or colloidal stabilisers either become part of the product or are separated. 

This example illustrates a graft polymerisation in which colloid stabilisation and both Redox and thermal initiation techniques are employed. The reaction in effect takes place in two stages, with the vinyl acetate polymerisation taking place before the acrylic monomers are polymerised. 

Another technique for overcoming the solids/viscosity conundrum is demonstrated in a patent by American Cyanamid. Here a conventional water soluble acrylic is prepared as a dispersion and is then used as a colloid stabiliser for further emulsion polymerisation. This polymerisation occurs within the dispersion droplets that have already been formed. This allows higher solids solutions to be formed with low viscosities, and low cosolvent contents ... 

A radical polymerisation can be carried out with a range of polymerisation techniques. Those with only a single phase present in the system are bulk and solution polymerisations, involving the monomer, a solvent if present and the initiator. By definition, the formed polymer in a bulk or solution polymerisation remains soluble (either in the monomer or the solvent). A precipitation polymerisation is one in which the system starts as a bulk or solution polymerisation, but the polymer precipitates from the continuous phase to form polymer particles which are not swollen with monomer. A precipitation polymerisation when the polymer particles swell with monomer is called dispersion polymerisation apart from polymerisation in the continuous phase, the polymer particles have an additional locus of polymerisation, and the particles in these systems are colloidally stabilised. Precipitation polymerisation is often performed in an aqueous medium (e.g. acrylonitrile polymerisation in water). Dispersion polymerisation is usually performed in organic solvents that are poor solvents for the formed polymer (supercritical or liquid carbon dioxide may also be used as a continuous medium for dispersion polymerisation). 

An emulsion polymerisation system comprises water, an initiator (usually water soluble), a water-insoluble monomer and a colloidal stabiliser, which may be added or maybe formed in situ. The main locus of polymerisation is within the monomer-swollen latex particles, which are either formed at the start of polymerisation or may be added initially (in which case one has a seeded emulsion polymerisation). The term emulsion polymerisation is a misnomer (arising for historical reasons the process was originally developed with the aim of polymerising emulsion droplets, although, in fact, this does not occur). The starting emulsion is not thermodynamically stable. An inverse emulsion polymerisation is one where the continuous phase is organic in combination with an aqueous discrete phase containing a water-soluble monomer (e.g. acrylamide). Two variants of emulsion polymerisation are... 

Used as 1-3% aq. solns. as a protective colloid (stabilisation of pseudo-solns., e.g. Se or Te sol.) indirect detn. of Cu(//) (oxidn. of I to I2 which forms coloured complex with the reagent). Used in textile and... 

The fatty acid monoethanolamides and diethanolamides, which are prepared by the amidation of fatty acids with mono-ethanolamine and diethanolamine, have a superfatting, colloid-stabilising and skin-protective action in addition, they are good dispersants for the dyeing of textiles and also have an influence on the hydrotropic properties of detergent combinations [29]. 

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