Mixed polymer-surfactant systems

MIXED POLYMER-SURFACTANT SYSTEMS Bioadhesive Gels... 

There is a lack of systematic experimental studies. Only on the basis of a large data base can further improvements in the theories be made. Thus, the more effective instruments and newly developed techniques should enable surface scientists to produce more systematic data. There is also a deficiency in respect to relaxation theories for polymers and mixed polymer/surfactant systems. First ideas about dilational properties of composite adsorption layers were published by Lucassen (1992). A theory has also been developed by Johnson Stebe (1994) to differentiate exchange of matter from dilational viscosity. This calls for new experimental developments. 

For a mixed polymer-surfactant system, the behaviour is completely analogous. One difference is that the degree of polymerisation of a micelle, unlike a polymer, is not fixed but may vary with the conditions (temperature, electrolyte concentration, etc.), as we... 

A series of similar, mixed polymer, surfactant systems has been studied by small angle neutron scattering techniques in order to determine the size of the aggregates. The results obtained are consistent with the rheological interpretation presented here and will be reported separately. ... 

Successful attempts have been made to modify/minimize preeipitation in polyelectrolyte/oppositely charged surfactant systems. Laurent and Scott (65) reported such an effect with the addition of simple salts and defined a critical electrolyte concentration (c.e.c.) at which precipitation is totally inhibited. (See Chapter 5 and also Section III.E below.) Likewise, Dubin et al. (66,67) have found inhibitory effects on adding nonionic surfactants to these mixed polymer/surfactant systems, presumably a result of mixed micelle formation. 

The general effects observed in mixed polymer/surfactant systems have already been described in Chapter 4. Here only brief summaries and some special effects will be presented. 

In addition to thin-film measurements, solution properties of these mixed polymer/surfactant systems were investigated. For the PAMPS/CiaEs and PAMPS/Ci2SO Na solutions small-angle X-ray scattering (SAXS) measurements provided a clear correspondence between the polymer bulk correlation length and polymer concentration, in agreement with... 

Kunieda H, Yamagata M. Three phase behavior in a mixed nonionic surfactant system. Colloid Polym Sci 1993 271 997-1004. 

Obviously many variables were not investigated, such as the effect of the type of nonionic surfactant, broader ranges of surfactant concentration, and broader ranges of monomer concentration. However, the results of this work are sufiBcient to demonstrate that the block copolymer architecture can be modified in MHAP by modifying the anionic surfactant concentration and type. These results have supported but not proved the block copolymer theory for this type of polymer-surfactant system. This model system is also a simple means of studying some aspects of mixed surfactant systems, an area of much current interest (e.g., ref 37). 

Acrylamide was successfully polymerized in a supercritical inverse emulsion composed of an ethane-propane mixture as the continuous phase, water and acrylamide as the dispersed phase, and a mixed nonionic surfactant system as the emulsifier [86], AIBN [2,2 -azobis(isobutyronitrile)] was the initiator. The polymerization was subsequently repeated in supercritical CO2 [87]. The C02-philic surfactant used to produce the inverse emulsion was an amide, end-capped poly(hexafluoropropylene oxide). The process yielded polymers of average molecular weights from 5 x 10 to 7 x 10. ... 

These findings are confirmed by study of the thermodynamic parameters of mixing of the cured epoxy resin with OP-20. At 6-7% content of surfactant, corresponding to the maximum surface tension of the polymer, a kink and an area of decrease of the Flory-Huggins parameter are observed in the dependence of X2,s on the surfactant concentration. This anomalous dependence can be explained in terms of the rearrangement of the intermolecular bonds in the polymer-surfactant system. With ED-20 initial resin, there are no extrema on the curve. Alteration of the macromolecular conformation affects the supermolecular structure of the polymer. Adding surfactant to ED-20 resin changes the form and causes a noticeable decrease of the size of the polymeric supermolecular formations. 

Self-assembly and micelle formation have, however, a broader significance than this. Mixed polymer-surfactant solutions have many applications and it has become more and more evident that an important role of the polymer chains in many systems is to promote micelle formation. A macromolecular cosolute will be much more effective in reducing the CMC than a low-molecular-weight one. This is discussed in some detail in Chapter 20. 

A hydrophobically modified water-soluble polymer (HM-polymer) can be viewed as a modified surfactant. It forms micelles, or hydrophobic microdomains, on its own at very low concentrations (intramolecularly, at infinite dilution) and these micelles can solubilize hydrophobic molecules. Furthermore, an HM-polymer and a surfactant in general have a strong tendency to form mixed micelles in a similar way as two surfactants. Two stoichiometries are important for HM-polymer-surfactant systems, i.e. the alkyl chain stoichiometry and the charge stoichiometry. 

Nordskog et al. observed a behavior similar to that found by Bronstein et al. for mixed systems of PEO62- -PB40 with DTAB [29]. In this work, worm-like BCP micelles were characterized by SANS, DLS, and Cryo-TEM, Upon addition of the cationic surfactant, the apparent hydrodynamic radius (/ h) calculated for these block copolymer solutions started to decrease until a plateau value of approximately 15 nm was reached. Further addition of surfactant did not lead to any further change in size of the mixed polymer/surfactant micelles. Using SANS it was possible to show that above a certain threshold concentration, excess micelles of the surfactant were formed. Below this threshold, all added surfactant is incorporated into mixed BCP/DTAB micelles and, hence, the change in size is due to structure reorganization induced by the DTAB. 

Before dealing with the adsorption behavior of polymer-surfactant systems, we outline briefly, the main features of polymer and surfactant association in aqueous solution. In another section, we analyze the major differences existing between the adsorption of a polymer and a surfactant at the solid surface. Further sections will discuss in more detail the adsorption pattern of both the polymer and the surfactant when they are mixed together in solution. 

In this area, recent unrelated efforts of the groups of Bhattacharya and Fife toward the development of new aggregate and polymer-based DAAP catalysts deserve mention. Bhattacharya and Snehalatha [22] report the micellar catalysis in mixtures of cetyl trimethyl ammonium bromide (CTAB) with synthetic anionic, cationic, nonionic, and zwitterionic 4,4 -(dialkylamino)pyridine functional surfactant systems, lb-c and 2a-b. Mixed micelles of these functional surfactants in CTAB effectively catalyze cleavage of various alkanoate and phosphotriester substrates. Interestingly these catalysts also conform to the Michaelis-Menten model often used to characterize the efficiency of natural enzymes. These systems also demonstrate superior catalytic activity as compared to the ones previously developed by Katritzky and co-workers (3 and 4). 

The goals of this work have been to determine the effect of polymers on the phase behavior of aqueous surfactant solutions, prior to and after equilibration with oil, to understand the mechanism of the so-called "surfactant-polymer interactions (SPI) in EOR, to develop a simple model which will predict the salient features of the phase behavior in polymer-microeraulsion systems, and to test the concept of using sulfonate-carboxylate mixed microemulsions for increased salt tolerance. 

Colic, M., Fisher, M.L., and Fuerstenau, D.W, Electrophoretic behavior and viscosities of metal oxides in mixed surfactant systems. Colloid Polym. Sci., 276, 72, 1998. 

Because the main driving force for surfactant self-association in polymer-surfactant mixed systems is the hydrophobic effect, the binding of surfactants to polyelectrolytes exhibits a similar dependence on the length of the alkyl chain as known for free micellization. Surfactants with longer hydrocarbon chains bind more strongly to polyions than those with shorter chains, and the binding starts a lower surfactant concentrations. In this context, a convenient parameter to characterize polyelectrolyte-surfactant systems is the critical aggregation concentration, cac, which is a counterpart of the well-known critical micellization concentration, cmc, but applies to solutions of surfactants in the presence of a polymer. It is defined as the... 

As explained before, when surfactant, water, and monomer(s) are mixed, the colloidal system obtained consists of monomer-swollen micelles (if the surfactant concentration exceeds its CMC) and monomer droplets dispersed in an aqueous phase that contains dissolved molecules of surfactant and a small amount of the sparingly water-soluble monomer(s). When free radicals are generated in the aqueous phase by action of an initiator system, then the emulsion polymerization takes place. Its evolution is such that the colloidal entities initially present tend to disappear and new colloidal entities (polymer latex particles) are bom by a process called nucleation. For convenience, we first focus on the particle nucleation mechanisms, a very important aspect of emulsion polymerization. 

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