Graft polymeric surfactant

Here we review the results obtained [4—10] with two types of non-ionic polymeric surfactants (i) A-B-A triblock copolymers and (ii) novel graft polymeric surfactants based on inulin. 

For INUTEC SPl, the graft polymeric surfactant based on hydrophobically modified inulin, the critical dectrolyte concentration, Cdo, that separates DLVO from non-DLVO interactions is 5 x 10 moldm [8j. The effect of the electrolyte on the electrostatic component of disjoining pressure is dearly seen, and the film thickness remains constant h =ll nm) above where steric interactions are acting. 

Tethering may be a reversible or an irreversible process. Irreversible grafting is typically accomplished by chemical bonding. The number of grafted chains is controlled by the number of grafting sites and their functionality, and then ultimately by the extent of the chemical reaction. The reaction kinetics may reflect the potential barrier confronting reactive chains which try to penetrate the tethered layer. Reversible grafting is accomplished via the self-assembly of polymeric surfactants and end-functionalized polymers [59]. In this case, the surface density and all other characteristic dimensions of the structure are controlled by thermodynamic equilibrium, albeit with possible kinetic effects. In this instance, the equilibrium condition involves the penalties due to the deformation of tethered chains. 

Y. Sela, Y. Magdassi, and N. Garti Polymeric Surfactants Based on PolysUoxanes-Graft-Poly(Oxyethylene) for Stabilization of Multiple Emulsions. Colloids Surfaces 83, 143 (1993). 

Short chain amphiphiles can be incorporated into the backbone of the polymer chains. The resulting graft macromolecules are able to form both intrachain and interchain aggregates. Polymeric surfactants assemble into a variety of intrachain micelles. These polymeric surfactants and/or amphiphilic polymacromonomers can also form mixed aggregates which incorporate free monomeric (macromonomer with a very small hydrophobic group) surfactants. 

Most dispersion polymerizations in C02, including the monomers methyl methacrylate, styrene, and vinyl acetate, have been summarized elsewhere (Canelas and DeSimone, 1997b Kendall et al., 1999) and will not be covered in this chapter. In a dispersion polymerization, the insoluble polymer is sterically stabilized as colloidal polymer particles by the surfactant that is adsorbed or chemically grafted to the particles. Effective surfactants in the dispersion polymerizations include C02-soluble homopolymers, block and random copolymers, and reactive macromonomers. Polymeric surfactants for C02 have been designed by combining C02-soluble (C02-philic) polymers, such as polydimethylsiloxane (PDMS) or PFOA, with C02-insoluble (C02-phobic) polymers, such as hydrophilic or lipophilic polymers (Betts et al., 1996, 1998 Guan and DeSimone, 1994). Several advances in C02-based dispersion polymerizations will be reviewed in the following section. 

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. 

In the next section, therefore, we review recent studies of simpler cases, i.e., homopoly(macromonomers), star- and comb-shaped polymers, followed by some interesting properties of the graft copolymers to be used as polymeric surfactants, surface modifiers, and compatibilizers for blends. 

Diblock copolymers consisting of soluble and insoluble parts (Fig. 2b) act much as grafted chains once they are adsorbed on the surface. However, the thermodynamics of the initial solution, consisting primarily of micelles, and the conformation of the insoluble blocks on the surface affect the coverage in ways not well understood (e.g., Munch and Gast, 1988 Marques et al., 1988 Gast, 1989). Many dispersants or polymeric surfactants are synthesized in this way (Reiss et al, 1987). 

After the reaction terminated the latex was post-stabilized with 0.5 phr Triton X-202 (Rohm and Haas surfactant) to inhibit aggregation and wras stored at 40 °F under dry nitrogen. No antioxidant could be added at this time because it would destroy the catalyst necessary for the final grafting polymerization. 

Emulsion polymerization has become an important process for the production of a large number of industrial polymers in the form of polymer colloids or latexes. They are the base of adhesives, paints and especially of waterborne coatings. An interest has been developed in recent years in emulsion polymerization systems in which the classical low molecular weight surfactaints are replaced by polymeric surfactants, either hydrophilic-hydrophobic block and graft copolymers (1-4) or functionalized oligomers (5). 

Polymerization of the alkoxyallene with macromonomers having a poly (ethyleneglycol) group by [(7r-allyl)Ni(OCOCF3)]2/PPh3 produces a graft copolymer with narrow molecular weight distribution [129]. The products serve as polymeric surfactants in the polymer blend system of polystyrene and poly(methyl methacrylate). 

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... 

Another effective graft copolymer is hydrophobicaUy modified inuUn this is a linear polyfructose chain A (with degree of polymerisation >23) onto which several alkyl chains have been grafted. The polymeric surfactant adsorbs onto several alkyl chains via multipoint attachment. 

Most reports on emulsion polymerisation have been limited to commercially available surfactants which, in many cases, are relatively simple molecules such as sodium dodecyl sulphate and simple nonionic surfactants. However, studies on the effects of surfactant structure on latex formation have revealed the importance of the structure of the molecule. Block and graft copolymers (polymeric surfactants) are expected to be better stabilisers when compared to simple surfactants. The use of these polymeric surfactants in emulsion polymerisation and the stabilisation of the resulting polymer particles is discussed below. 

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. 

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