Experimental studies of block copolymer micelles

However, AmicH° and TA, determined from the temperature dependence of the Gibbs energy, are less sensitive to the association number than is AmicG° itself (Yang ef al. 1995). Assuming that Am cH° is approximately constant within a certain temperature range, eqn 3.7 can be integrated to yield 

Thus the logarithm of the erne can be plotted against inverse temperature to extract information on the micellization enthalpy. Equivalently, the logarithmic concentration can be plotted against the inverse critical micelle temperature (Alexandridis et al. 1994a Yang et al. 1994). 

The cmc is often determined from plots of the surface tension versus logarithmic concentration.The surface tension decreases rapidly with increasing concentration up to the cmc, above which there is a plateau. Examples are discussed in more detail in Section 3.3.2. 

The mass-average molar mass of the block copolymer solute, M , and the second virial coefficient, A2, can be obtained from SLS. These quantities can be determined from the concentration dependence of the scattered light intensity using the relationship (cf. Section 1.4.10) 

In dynamic light scattering (DLS), or photon correlation spectroscopy, temporal fluctuations of the intensity of scattered light are measured and this is related to the dynamics of the solution. In dilute micellar solutions, DLS provides the z-average of the translational diffusion coefficient. The hydrodynamic radius, Rh, of the scattering particles can then be obtained from the Stokes-Einstein equation (eqn 1.2).The intensity fraction as a function of apparent hydrodynamic radius is shown for a triblock solution in Fig. 3.4. The peak with the smaller value of apparent hydrodynamic radius, RH.aPP corresponds to molecules and that at large / Hs,Pp to micelles. 

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The first experimental studies of the dynamics of block copolymer micelles all invoked the theory of Aniansson and Wall (AW). 2 i and its extensions by Kahlweit et al. (K) - that were developed for micellar solutions of surfactants. Later on, theories specific to amphiphilic block copolymers were developed that drew much from AW and K theories (referred to as AWK theory below), at least for the mechanisms by which... 

Stop-flow experiments have been performed in order to study the kinetics of micellization, as illustrated by the work of Tuzar and coworkers on PS-PB diblocks and the parent PS-PB-PS triblocks [63]. In these experiments, the block copolymers are initially dissolved as unimers in a nonselective mixed solvent. The composition of the mixed solvent is then changed in order to trigger micellization, and the scattered light intensity is recorded as a function of time. The experiment is repeated in the reverse order, i.e., starting from the block copolymer micelles that are then disassembled by a change in the mixed solvent composition. The analysis of the experimental results revealed two distinct processes assigned as unimer-micelle equilibration at constant micelle concentration (fast process) and association-dissociation equilibration, accompanied by changes in micellar concentration (slow process). 

In this chapter, the focus is largely on experimental and theoretical studies of micellization in a range of solutions of model block copolymers prepared by anionic polymerization. A discussion of both neutral and ionic block copolymers is included, and features specific to the latter type are detailed. The adsorption of block copolymers at the liquid interface is also considered in this chapter. Recent experiments on copolymer monolayers absorbed at liquid-air and liquid-liquid interfaces are summarized, and recent observations of surface micelles outlined. Thus this chapter is concerned both with bulk micellization and the surface properties of dilute copolymer solutions. 

For quite some time, there have been indications for a phase-separation in the shell of polyelectrolyte block copolymer micelles. Electrophoretic mobility measurements on PS-PMAc [50] indicated that a part of the shell exhibits a considerable higher ionic strength than the surrounding medium. This had been corroborated by fluorescence studies on PS-PMAc [51-53] and PS-P2VP-heteroarm star polymers [54]. According to the steady-state fluorescence and anisotropy decays of fluorophores attached to the ends of the PMAc-blocks, a certain fraction of the fluorophores (probably those on the blocks that were folded back to the core/shell interface) monitored a lower polarity of the environment. Their mobility was substantially restricted. It thus seemed as if the polyelectrolyte corona was phase separated into a dense interior part and a dilute outer part. Further experimental evidence for the existence of a dense interior corona domain has been found in an NMR/SANS-study on poly(methylmethacrylate-fr-acrylic acid) (PMMA-PAAc) micelles [55]. 

Most experimental studies searching for dynamic micelles have focused on the proper choice of soft hydrophobic block to assure equilibrium,i.e., reversible association of the ionic/hydrophobic block copolymers. Copolymers with such soft... 

It should be emphasized that these mixed polymer brushes are very similar to block copolymer micelles whose corona is composed of mixed chains, which are presented elsewhere in this comprehensive. In the following, we will focus on mixed brashes grafted to preformed colloids. There are several experimental studies on mixed and a review of mixed SPB... 

AB diblock copolymers in the presence of a selective surface can form an adsorbed layer, which is a planar form of aggregation or self-assembly. This is very useful in the manipulation of the surface properties of solid surfaces, especially those that are employed in liquid media. Several situations have been studied both theoretically and experimentally, among them the case of a selective surface but a nonselective solvent [75] which results in swelling of both the anchor and the buoy layers. However, we concentrate on the situation most closely related to the micelle conditions just discussed, namely, adsorption from a selective solvent. Our theoretical discussion is adapted and abbreviated from that of Marques et al. [76], who considered many features not discussed here. They began their analysis from the grand canonical free energy of a block copolymer layer in equilibrium with a reservoir containing soluble block copolymer at chemical potential peK. They also considered the possible effects of micellization in solution on the adsorption process [61]. We assume in this presentation that the anchor layer is in a solvent-free, melt state above Tg. The anchor layer is assumed to be thin and smooth, with a sharp interface between it and the solvent swollen buoy layer. 

Because the essentials of micellization have been discussed in depth for poly(oxyethylene)-containing block copolymers, we do not describe experimental studies on styrenic block copolymers in solution in great detail. Instead, the features are summarized in tabular form (see Tables 3.1-3.4). Experiments on ionic block copolymers containing polystyrene are discussed in Section 3.6.2. 

There have been very few studies on the kinetics of micellization in block copolymer solutions. Micellization in aqueous surfactant systems close to equilibrium occurs on a time-scale far below one second. Experimental results obtained by fast reaction techniques, such as temperature jumps or pressure jumps or steady-state methods such as ultrasonic absorption, NMR and ESR, show that at least... 

There is a vast body of diblock copolymer studies since block choice can be such that they resemble amphiphilic surfactants. For the sake of brevity, we will skip them. Instead, we present an interesting case of triblock copolymers of poly(ethylene oxide), PEO, and poly(propylene oxide), PPO, commonly known by one of its trade names, Pluronics [117]. They have been used as non-ionic surfactants for a variety of applications such as in emulsification and dispersion stabilization. In aqueous solutions, these copolymers form micelles, and there exists a well-defined critical micelle concentration that is experimentally accessible. Several groups have investigated colloidal suspensions of these polymers [118-122], The surface properties of the adsorbed monolayers of the copolymers have been reported with respect to their structures and static properties [123-126]. 

Ajji and Utracki [1996, 1997] and in Chapter 2 Thermodynamics of Polymer Blends in this handbook. The interface thickness of A/A-B mixtures was not theoretically calculated, but experimental measurements indicate that the presence of the homopolymer leaves the domain boundary unchanged [Bates et al., 1983 Hashimoto et al., 1990 Tanaka et al., 1991 Zin and Roe, 1984 Green et al., 1989]. On the other hand, the part of the phase diagram where the concentration of the block copolymer in the mixture is low, was studied in detail [Whitmore and Noolandi, 1985b Leibler et al., 1983 Leibler, 1988]. The proposed models were similar. In both, the conditions for the formation of spherical micelles were investigated and expressions of the critical micelle concentration, were derived. For example... 

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