Block type polymeric surfactants

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. 

More than two surfactants can be put together to form tri,- tetra- or polymeric surfactants. Trimeric or even tetrameric surfactants show properties often superior to monomeric surfactants. Besides, they are intermediate between conventional surfactants and polymeric surfactants. In a normal polymeric surfactant each monomer unit is amphiphilic. Another type of polymeric surfactant, called block copolymer [522], consists of at least two parts. One part is made of monomer type A, the other part is made of monomer B. If A is polar and B nonpolar, the blockcopolymer will be strongly surface active and show many properties of a conventional surfactant. If there are two different blocks we talk about a diblock copolymer. In the following part of this chapter we concentrate on conventional surfactants. 

Dispersion polymerization differs from emulsion polymerization in that the reaction mixture, consisting of monomer, initiator, and solvent (aqueous or nonaque-ous), is usually homogeneous. As polymerization proceeds, polymer separates out and the reaction continues in a heterogeneous manner. A polymeric surfactant of the block or graft type (referred to as protective colloid ) is added to stabilize the particles once formed. 

Another class of surfactants that are used in cosmetics and personal care products is the phosphoric acid esters. These molecules are similar to the phospholipids that are the building blocks of the stratum corneum (the top layer of the skin, which is the main barrier for water loss). Glycerine esters, in particular, triglycerides, are also frequently used. Macromolecular surfactants of the A-B-A block type [where A is PEO and B is polypropylene oxide (PPO)] are also frequently used in cosmetics. Another important naturally occurring class of polymeric surfactants is the proteins, which can be used effectively as emulsifiers. 

The most convenient polymeric surfactants are those of the block and graft copolymer type. A block copolymer is a linear arrangement of blocks of variable... 

The type of emulsion stabilized by polymeric surfactants, charged or uncharged, was not investigated in detail, although it has long been understood that it was dependent of the structure of the surfactant macromolecules. In a series of papers, Riess and his coworkers [143-145] have investigated the effect of the composition and architecture of PS-PEO-based block copolymers (PS polystyrene PEO polyethylene oxide) on stability and emul-... 

The Pluronic and Tetronic surfactants (BASF) are examples of the polyoxyethylenated (PEO) polyoxypropylene (PPO) glycol type of nonionic surfactants. The Pluronic surfactants, are PEO-PPO-PEO triblock copolymers in which the PEO portion constitutes between 10 and 80% of the copolymer [92]. Pluronic F38 and F68 (both with HLB > 24) are typical examples of these surfactants used in emulsion polymerization formulations. The Tetronic surfactants are tetrafunctional block copolymers prepared from the addition of FPO and PEO to ethylene diamine, lypical examples of the Tetronic surfactants used in emulsion polymerization are Tetronic 707 and 908 (HLB > 24). The addition of these types of surfactants enhances freeze-thaw, shear and electrolyte stability, results in low foam formation and typically decreases the water sensitivity. 

The most convenient polymeric surfactants are those of the block and graft copolymer type. A block copolymer is a linear arrangement of blocks of variable monomer composition. The nomenclature for a diblock is poly-A-block-poly-B, and for a triblock it is poly-A-block-poly-B-poly-A. One of the most widely used triblock polymeric surfactants are the Pluronics (BASF, Germany) or Synperordc PE (ICI, U.K.), which consists of two poly-A blocks of poly(ethylene oxide) (PEO) and one block of poly(propylene oxide) (PPO). Several chain lengths of PEO and PPO are available. More recently, triblocks of PPO-PEO-PPO (inverse Pluronics) became available for some specific applications. 

Polymeric surfactants of the block (AB or ABA) or graft (BA ) type are essential materials for the preparation of many systems, e.g., dyestuffs, papercoatings, inks, agrochemicals, pharmaceuticals, personal care products, ceramics, detergents, etc. 

To understand the solution behavior of polymeric surfactants of the block-and-graft type, it is essential to consider the solution properties of the more simple homopolymers. The solution behavior of homopolymers was considered in the thermodynamic treatment of Flory and Huggins. 

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