Exchange between block copolymer micelles

Exchange of unimers between two different types of block copolymer micelles has often been referred to as hybridization. This situation is more complex than for the case described above because thermodynamic parameters now come into play in addition to the kinetic ones. A typical example of such hybridization is related to the mixing of micelles formed by two different copolymers of the same chemical nature but with different composition and/or length for the constituent blocks. Tuzar et al. [41] studied the mixing of PS-PMAA micelles with different sizes in water-dioxane mixtures by sedimentation velocity measurements. These authors concluded that the different chains were mixing with time, the driving force being to reach the maximum entropy. 

Similarly to surfactant micelles, amphiphilic block copolymer micelles are not frozen objects, at least when the copolymer molecular weight is relatively low. The same processes as those discussed in Chapter 3 — that is, exchange of surfactants between micelles and bulk phase and micelle formation/break-down — also occur in micellar solutions of copolymers. In view of the structure of amphiphilic copolymers, it is readily realized... 

These are stable micelles that are formed with polymeric surfactants. Amphiphilic block copolymers such as the pluronics (polyoxyethylene-polyoxypropylene block copolymers) are able to self-assemble into polymeric micelles and hydrophobic drugs may be solubilized within the core of the micelle or, alternatively, conjugated to the micelle-forming polymer. Although micelles are rather dynamic systems that continuously exchange units between the micelle structure and the free units in solution, those composed of polyoxyethylene - poly(aspartic acid) have been found sufficiently... 

In low-molecular weight surfactants, above the CMC, a constant exchange occurs between aggregated material and that which is free (i.e., unimers) in the aqueous phase. Similarly, with block copolymers, a dynamic equilibrium exists between micelles and nonaggregated chains. The average residence time of a single block copolymer chain within a micelle is of importance because this will dictate the stability of the system. Researchers have used fluorescence techniques to probe such characteristics as the exchange rate between unimers and polymeric micelles [168]... 

Stop-flow experiments have been performed by Tuzar and Kratochvil [7] and more recently by Kositza et al. [128]. In analogy to low molar weight surfactants, it could be shown that two relaxation processes have to be considered for block copolymer micellar systems the first in the time scale of tens of microseconds, associated to unimer exchange between micelle and bulk solution, and the second, in the millisecond range, attributed to the rearrangement of the micelle size distribution. Major differences were observed between A-B diblock and A-B-A triblock copolymers, which could be explained by the fact that the escape of a unimer, which has to disentangle from the micellar core, might be much easier in a diblock than in a triblock structure. 

Hybridization of micellar systems was more extensively studied by fluorescence techniques. As mentioned by Webber [117], the chain exchange between micelles can be characterized by mixing micelles composed of block copolymers that are similar or identical except that they are tagged with different fluorophores. Thus, when two micelle populations, the one ta ed with donor and the other with acceptor groups, are mixed and the donor is excited, primarily donor fluorescence is observed. As the chromophore tagged chains are exchanged between micelles the donor will sensitize the fluorescence of the acceptor. 

The stopped-flow method has been used to study the kinetics of micelle formation/breakdown in surfactant solutions (see Chapter 3), of the exchange process in micellar solutions of amphiphihc block copolymers (see Chapter 4, Sections IV and V), and also of colhsions between droplets in microemulsions (see Chapter 5, Section VI.F). It has been also used to study the kinetics of the vesicle-to-micelle transformation (see Chapter 6) and of various types of chemical reactions performed in micelles or microemulsion droplets (see Chapter 10). The stopped-flow method has also been used to study the rate of dissolution of oil or water in microemrdsions (see Chapter 5, Section VII.C). In such studies the syringe that contains the oil or water to be solubilized is of a much smaller diameter than that containing the microemulsion. 

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