Micellar clusters

Time-resolved luminescence quenching measurements using the probe Tb(pyridine-2,6-dicarboxylic acid)i and the quencher bromophenol blue show the existence of micellar clusters in AOT-based w/o microemulsions. The fast exchange appearing over several microseconds was attributed to intracluster quenching, whereas the slow exchange on the millisecond time scale was attributed to intercluster exchange [243]. 

In sodium bis(2-ethylhexyl) phosphate microemulsions, which are composed of cylindrical micelles in the dilute region, it has been observed that the formation of micellar clusters is characterized by a branched structure as the volume fraction (<1>) of the aggregates increases. At d> > 0.2, these clusters mutually overlap, forming a network expanded overall [283]. 

To illustrate how the effect of the adsorption on the modulus of the filled gel may be modelled we consider the interaction of the same HEUR polymer as described above but in this case filled with poly(ethylmetha-crylate) latex particles. In this case the particle surface is not so hydrophobic but adsorption of the poly (ethylene oxide) backbone is possible. Note that if a terminal hydrophobe of a chain is detached from a micellar cluster and is adsorbed onto the surface, there is no net change in the number of network links and hence the only change in modulus would be due to the volume fraction of the filler. It is only if the backbone is adsorbed that an increase in the number density of network links is produced. As the particles are relatively large compared to the chain dimensions, each adsorption site leads to one additional link. The situation is shown schematically in Figure 2.13. If the number density of additional network links is JVL, we may now write the relative modulus Gr — G/Gf as... 

A pearl necklace model in which the polypeptide chain forms the string of the necklace and the surfactant molecules form micelle-like clusters along the polypeptide chain, which passes through the micellar clusters in a a-helical conformation. In contrast to the rod-like particle model, this model assumes that the polypeptide chain is flexible. 

The second example concerns the multidisciplinary study of the micelliz-ing block copolymer polystyrene-( -poly(2-vinylpyridine)-ft-poly(ethylene oxide) (PS-PVP-PEO), which shows a high tendency to aggregation and the formation of micellar clusters [88,89]. It shows the application of SRM for studying the mobility and structural details of different domains in micelle-like polymeric nanoparticles. The fluorescence technique reveals interesting features of studied systems that are hardly accessible by other techniques. Section 3.3 is devoted to the development of the methodology of the solvent relaxation technique for studying nanostructured self-assembling systems. 

We found that the studied micelles containing long PEO chains, which should assure their solubility and thermodynamic stability, are surprisingly apt to a secondary aggregation and formation of micellar clusters. We suspected that the secondary aggregation of the PEO sheU is caused by a hindered and incomplete solvation. Therefore, we supplemented the LS study of micellar solutions by SRM, with the aim of obtaining detailed information on the solvation of micellar shells. 

We performed the solvent relaxation study of acidic PS-PVP-PEO solutions with the aim of (a) proving the assumption that the formation of micellar clusters is a result of an insufficient solvation of PEO units, and (b) understandinghow the solvation is affected by pH and ionic strength. 

Bronstein and coworkers have investigated PS-fe-PEO (PS -fe-PEOm, with n 10 and m 68) in aqueous solution and in mixtures of this polymer with cetyl pyridinium chloride (CPC) and SDS [20, 21, 27]. DLS experiments on the pure PS-fe-PEO solutions reveal two relaxation modes. The faster one is attributed to micelles and the slower one to micellar clusters. Addition of surfactant to these solutions significantly alters the structures found. 

Besides improving the absolute level of thickening efficiency, the interactions of the hydrophobes in aqueous solution were also seen to affect in a beneficial manner the other properties of formulations based on such polymeric thickeners. The micellar clusters formed by the assembly of hydrophobes in the aqueous phase aggregate even more strongly when simple salts are also present in solution. This arises due to the fact that the critical micelle concentration (CMC) of the surfactant forming the side chain is somewhat lower in electrolyte solutions than in water alone. This effect, which boosts viscosity, helps to offset the reduction in thickening efficiency that inorganic and other salts have on conventional polyelectrolytes. 

Finally, the structure or network of polymer chains combined with micellar clusters occurring in solution from such associative polymers also results in a modified rheological profile with respect to the viscosity versus shear rate behaviour compared with the previously known Hnear and cross-linked types. This was of particular interest in formulations such as water-based paints and will be covered in more detail in a later section on rheological profiles of acrylic thickeners. 

Tekle and Schelly have studied the electric birefringence of AOT/isooctane/water W/0 microemulsions, which revealed two distinct relaxation processes on timescales of the order of 10 and 100 ps, respectively. The fast relaxation was attributed to the polarization/alignment of the individual reverse microemulsion droplets the slow relaxation, of smaller amplitude, was assigned to the linearization/reorien-tation of the micellar clusters. The rates of both processes became slower when w, or the AOT concentration, or the temperature was increased. Transient phase separation could occur beyond some threshold values of the preceding param-... 

The authors of the above study point out that the onset of aggregation occurs at a critical concentration, but that the association transition is broad, possibly a result of the polydispersity of the samples. They also note that both end groups of one polymer molecule may participate in a single micellar cluster, but at sufficiently high polymer concentration bridging interactions of polymer chains between two micellar clusters result in gelation and a large increase in the solution viscosity. 

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