Triblock polymeric surfactants

The above polymeric triblocks can be applied as dispersants, whereby the assumption is made that the hydrophobic PPO chain resides at the hydrophobic surface, leaving the two PEO chains dangling in aqueous solution and hence providing steric repulsion. Although these triblock polymeric surfactants have been widely used in various appHcations suspensions, some doubt has arisen as to how effective these can be. It is generally accepted that the PPO chain is not sufficiently hydrophobic to provide a strong anchor to a hydrophobic surface. 

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. 

The most convenient polymeric surfactants are those of the block-and-graft copolymer type triblock polymeric surfactants—Pluronics (BASF) and Synperonic PE (ICI)—two... 

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). 

Poly(alkylene oxide)-based (PEO-PPO-PEO) triblock and diblock copolymers are commercially successful, linear non-ionic surfactants which are manufactured by BASF and ICI. Over the last four decades, these block copolymers have been used as stabilisers, emulsifiers and dispersants in a wide range of applications. With the development of ATRP, it is now possible to synthesise semi-branched analogues of these polymeric surfactants. In this approach, the hydro-phobic PPO block remains linear and the terminal hydroxyl group(s) are esteri-fied using an excess of 2-bromoisobutyryl bromide to produce either a monofunctional or a bifunctional macro-initiator. These macro-initiators are then used to polymerise OEGMA, which acts as the branched analogue of the PEO block (see Figures 2 and 3). 

Gadelle et al. (1995) investigated the solubilization of various aromatic solutes irbfftRSS-b-PEO (ABA)/PPO-bPEO-bPPO (BAB) triblock copolymers. According to the experimental results, they indicated two different solubilization processes. To understand better the mechanism for solubilization in the polymeric surfactant solutions, it was postulated that (1) the addition of apolar solutes promotes micellization of the polymeric surfactant molecules, (2) the central core of the polymeric micelles contains some water molecules, and (3) solubilization is initially a replacement process in which water molecules are displaced from the micellar core bythesolubilizate. Adetailed discussion of the solubilization process can be found in the next section and the pharmaceutical application section of this chapter. 

The hydrophobic (tail) part of the surfactant is typically a single alkyl chain in small-molecule surfactants, while polymeric surfactants have hydrophobic regions composed of less soluble polymers. Thus triblock copolymers of the type poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (EO PO ,EO ) form micelles with poly(propylene oxide) in the hydrophobic core, and the poly(ethylene oxide) strands on the outside of the micelle. These triblock copolymers, particularly the... 

Fig. 5. (a) Photograph of highly concentrated CO2-U1-water (C/W) emulsion (97%, v/v, CO2) stabihzed by a triblock copolymer surfactant. A water-soluble dye, methyl orange, was included in the aqueous phase, (b) Emulsion-templated poly(acrylamide) material synthesized by polymerization of a concentrated CAV emulsion (90%, v/v, CO2) stabihzed with a diblock surfactant [45]. Reproduced with permission copyright 2005 American Chemical Society. 

PEO-PPO, and later on PEO-PBO copolymers, represent the link between classical low molecular weight non-ionic surfactants and polymeric surfactants. These commercially available products (formerly known as POLOXAMERS, PLURICARE, PLURONICS, SYN-PERONICS), mainly with di- and triblock structures can form, depending on temperature and concentration, true solutions, micelles of different shapes and physical gels. Their micellization behavior has been studied quite extensively and the experimental as well as the theoretical results were summarized in the review articles of Nace [10], Chu and Zhou [116], Ahngren etal. [147], Hamley [11], Booth and co-workers [79,148] and Wanka etal. [149]. 

Polymeric surfactants can be homopolymers, random amphiphilic copolymers, and of the A-B (diblock), A-B-A (triblock), and BA (graft) types [23]. The A chain is referred to as the stabilizing chain (soluble in the medium), and the B chain is referred to as the anchor chain (insoluble in the medium with strong affinity to the surface). The simplest type of a polymeric surfactant is a homopolymer, such as PEO and poly(vinyl pyrrolidone) (PVP). Homopolymers are not the most suitable surfactants and it is better to use polymers with some groups that have affinity to the surface. The most employed copolymers are random amphiphilic copolymers, like poly(vinyl alcohol), diblocks of polystyrene-block-poly(vinyl alcohol) (PS-b-PVA), poly(ethylene oxide)-block-polystyrene (PEO-b-PS), and triblocks of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-b-PPO-b-PEO, Pluronic) (PPO resides at the hydrophobic surface, leaving the two PEO chains dangling inaqueous solution), andpoly(ethyleneoxide)-block-polystyrene-block-poly(ethylene oxide) (PEO-b-PS-b-PEO). The graft copolymer is referred to as a comb stabilizer Atlox 4913,... 

Hypeimer CF-6, and Inulin. In general, the abilities of polymeric surfactants to decrease surface and interfacial tension are mnch lower than those of the low-molecular-weight surfactants. Block copolymers exhibit low CMC and lower diffnsion coefficient with respect to classical surfactant. Triblock copolymers are mnch more efficient than the diblock copolymers with the same composition and molecnlar weight [24]. 

Thin liquid films have proven their advantages in the study of interaction forces in foam, emulsion, and wetting films stabilized by various types of surfactants see, for example. Refs. l-3]. DLVO and non-DLVO surface forces that stabilize these films have been established in many cases the relation between surface forces and film stability has also been found. Recently, several authors have reported results of model experiments with thin liquid films (foam, emulsion and wetting films) stabilized by polymeric surfactants. In our laboratories all three types of films from aqueous solutions of A-B-A triblock copolymers [4—6] or AB hydrophobically modified inulin [8-10] have been studied. The corresponding disperse systems (foams, emulsions, suspensions) stabilized by AB polymeric surfactants have also been studied extensively see, for example. Refs. [11-14]. It was supposed that the stabilizing forces are steric surface forces but they have not been directly proven and quantitatively studied. 

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. 

Fig. 10 Aggregation numbers 2 as function of degree of polymerization of insoluble block for uncharged block copolymers. Open symbols different diblock-, triblock-, graft-, and star polymers. Filled symbols low-MW surfactants. Reprinted with permission from [211]. Copyright (2002) Wiley...

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