Polymeric steric stabilisation

Other types of surfactants are the polymeric (steric) stabilisers, such as partially hydrolysed polyvinyl acetate. Also oligomeric species formed in situ, when SO radicals react with some monomer units in the aqueous phase, will have surface active properties, and can even form a colloidally stable latex Electrosteric stabilisers combine steric and electrostatic functionalities for example, inclusion of acrylic acid in a recipe results in chains with blocks comprised largely of poly(acrylic acid) which for in the aqueous phase, then pick up enough hydrophobic monomer to enter the particle and continue polymerisation in the particle interior. The hydrophilic component remains in the aqueous phase and provides colloidal stability both sterically and, imder the appropriate conditions of pH, electrostatically. This mode of stabilisation is very common in surface coatings, because it gives excellent freeze-thaw stability. 

Polymers are also essential for the stabilisation of nonaqueous dispersions, since in this case electrostatic stabilisation is not possible (due to the low dielectric constant of the medium). In order to understand the role of nonionic surfactants and polymers in dispersion stability, it is essential to consider the adsorption and conformation of the surfactant and macromolecule at the solid/liquid interface (this point was discussed in detail in Chapters 5 and 6). With nonionic surfactants of the alcohol ethoxylate-type (which may be represented as A-B stmctures), the hydrophobic chain B (the alkyl group) becomes adsorbed onto the hydrophobic particle or droplet surface so as to leave the strongly hydrated poly(ethylene oxide) (PEO) chain A dangling in solution The latter provides not only the steric repulsion but also a hydrodynamic thickness 5 that is determined by the number of ethylene oxide (EO) units present. The polymeric surfactants used for steric stabilisation are mostly of the A-B-A type, with the hydrophobic B chain [e.g., poly (propylene oxide)] forming the anchor as a result of its being strongly adsorbed onto the hydrophobic particle or oil droplet The A chains consist of hydrophilic components (e.g., EO groups), and these provide the effective steric repulsion. 

Interaction between Particles Containing Adsorbed Nonionic and Polymeric Sur ctant Layers (Steric Stabilisation)... 

Interaction between Oil or Water Droplets Containing an Adsorbed Polymeric Surfactant Steric Stabilisation... 

As most nanoemulsions are prepared using nonionic and/or polymeric surfactants, it is necessary to consider the interaction forces between droplets containing adsorbed layers (steric stabilisation). As this was described in detail in Chapter 10, only a summary will be given here [15, 16]. 

The aromatic nuclei of the bisphenol A segment have a high affinity for the aromatic nuclei of styrene - ACN copolymer styrene units, the polyether chains having a strong interaction with the liquid poly ether medium. As an immediate consequence, the structure 6.15 assures a good steric stabilisation of polymeric dispersions in liquid polyether polyols (see the structure in Figure 6.6). 

Another commonly used application-specific method is discrete particle encapsulation (DPE). In this method, selected chemicals are used to form a thin polymeric shell around each nanoparticle providing the characteristics a user needs. Then a second thin-shell coating is added, so the nanoparticle will disperse in the best needed format. This shell contains spacer molecules that prevent the nanoparticles from coming into contact with each other. The result is steric stabilisation for nanoparticles in non liquid solvents and polymers, and electrosteric stabilisation for those needing to disperse in a fluid. 

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. 

As mentioned, dispersant design and mode of action are well understood by surface and colloid scientists [43, 112-115]. In that field, the term dispersant refers to any additive that reduces the interparticle interactions, thereby encouraging dispersion of the particles. This is achievable via a number of mechanisms using low molecular weight, oligomeric, or polymeric additives [128]. Steric stabilisation is most relevant to mineral fillers in polymers because it is the main way to achieve colloidal stability in low polarity solvents. This stabilisation mechanism operates by strong adsorption of a layer of organic additive that physically prevents close interparticle approach. 

Several examples of block and graft copolymers may be quoted. Triblock polymeric surfactants [ Pluronics (BASF) or Synperonic PE (ICI)] - two poly-A blocks of PEO and one block poly-B of poly(propylene oxide) (PPO) several chain lengths of PEO and PPO are available. Triblocks of PPO-PEO-PEO (inverse Pluronics ) are also available. Polymeric triblock surfactants can be applied as emulsifiers and dispersants. The hydrophobic PPO chain resides at the hydrophobic surface, leaving the two PEO chains dangling in aqueous solution (providing steric stabilisation). 

A-B, A-B-A block and BAn graft type polymeric surfactants are used to stabilise emulsions and suspensions [18]. B is the anchor chain that adsorbs very strongly at the 0/W or S/L interface, whereas the A chains are the stabilising chains that provide steric stabilisation. These polymeric surfactants exhibit surface activity at the 0/W or S/L interface. The adsorption and conformation of these polymeric surfactant at the interface has been described in detail in reference 18. 

The principal consideration in selecting an appropriate soluble polymeric component for the steric barrier is that it should be freely soluble in the dispersion medium to be usedo This had been recognised empirically for some years before Napper (4) undertook quantitative studies which showed that sterically stabilised latexes flocculated at or around the theta point for the soliible polymer in the dispersion medium. Later studies have shown that both upper and lower critical flocculation temperatures may occur in organic solventso... 

In most cases, a mixture of a polymeric stabiliser such as poly(vinyl alcohol) or Pluronic (an A-B-A block copolymer of PEO, A and PPO) with an anionic surfactant, such as sodium dodecyl sulphate is used. In this case, the stabiHsing mechanism is the combination of electrostatic and steric mechanism, referred to as electrosteric. 

Three main mechanisms of stabilisation can be considered (i) electrostatic, as produced by ionic surfactants (ii) steric, as produced by nonionic polymeric surfactants of the A-B, B-A-B, A-B-A or AB graft copolymers (where A is the anchor chain and B is the stabilising chain and (iii) electrosteric, as produced by polyelectrolytes. 

Polymers can be effective stabilisers of NPs through electronic and steric effects. Dendrimers are highly branched polymers only a few nanometers in diameter. Dendrimers are defined by a central core, interior polymeric branches and an exterior functionalized surface. Their uniform size, dispersity and water solubility (due to outer hydrophilic and metal encapsulating functional groups) make them ideal candidates as polymeric stabilizers for NP catalysed cross-coupling reactions. The encapsulated NPs are primarily eonfined sterically... 

---