Exchange between block copolymer solution

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... 

The Role of E ab- The previous simulations employed E as = Eaq. Here we break this equality in order to evaluate the importance of the repulsion between the two blocks, represented by Eab- The cmc deduced from simulations with E ab>0 and Eas = 0 45 are presented in Table I. The value of E as must be positive if micelle formation is to occur in dilute solution. After equilibrium is established there is a fluctuation in Nfree due to the exchange of diblock copolymers between the aggregates and the pool of free chains (5). This fluctuation in Nfree Corresponds to about 0.0007 for the evaluated by Equation (4). 

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. 

Micellar assemblies continuously undergo the formation and breakdown in structure and how quickly this occurs depends on the interaction parameter and also the cmc value. The cmc value for natural lipids in aqueous solution range from pmol to picomolar concentrations and this equates to lipid exchange rates of hours between aggregates. However, for block copolymers much more stable and kinetically trapped aggregates are formed that thus have significantly lower cmc values. 

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. 

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... 

Interfacial electron-transfer reactions between polymer-bonded metal complexes and the substrates in solution phase were studied to show colloid aspects of polymer catalysis. A polymer-bonded metal complex often shows a specifically catalytic behavior, because the electron-transfer reactivity is strongly affected by the pol)rmer matrix that surrounds the complex. The electron-transfer reaction of the amphiphilic block copol)rmer-bonded Cu(II) complex with Fe(II)(phenanthroline)3 proceeded due to a favorable entropic contribution, which indicated hydrophobic environmental effect of the copolymer. An electrochemical study of the electron-transfer reaction between a poly(xylylviologen) coated electrode and Fe(III) ion gave the diffusion constants of mass-transfer and electron-exchange and the rate constant of electron-transfer in the macromolecular domain. 

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