Block copolymer micelles exchange kinetics

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. 

It has been shown that block copolymer micelles are dynamic structures, although they can be kinetically frozen. Unimers can thus escape from micelles and be exchanged with other micelles or be adsorbed on another interface... 

Willner, L. Poppe, A. Allgaier, J. Monkenbusch, M. Richter, D. Time-resolved SANS for the determination of unimer exchange kinetics in block copolymer micelles. Europhysics Letters 2001, 55, 661-613. 

Exchange Kinetics in Block Copolymer Micelles Halperin and Alexander Theory... 

Experimental work on the equilibrium kinetics in block copolymer micelles is very rare in comparison with structural investigations. This is most likely due to the challenging problems in accessing the chain exchange by suitable experimental techniques or systems. Early experiments include studies using size-exclusion... 

Fig. 24 Logarithmic chain exchange kinetics of PS-PB block copolymer micelles (top) in DMF at 20° C (stars) and of inverted micelles (bottom) with swollen PS cores in n-hexadecane at 20°C (circles) and 30°C (triangles). Lines depict fits with a logarithmic time decay. Reprinted with permission from [101], Copyright (2006) by the American Physical Society...

In principle, what has just been stated for surfactant micelles also holds for the larger and more complex self-assembhes that surfactants and amphiphific block copolymers can form microemulsion droplets, vesicles, and mesophases. The lifetimes of these assembhes are much longer than for micelles, mainly when they involve block copolymers. Nevertheless, exchanges and other processes can also take place. Vesicles and lyotropic mesophases can be considered as permanent objects. However, vesicles can be transformed into micelles, and vice versa. Likewise, a lyotropic mesophase may be transformed into another mesophase or in a micellar solution by an appropriate change brought to the system. The kinetics of these transformations is of basic as well as of practical interest. 

Fig. 2 Illustration of two important mechanisms involved in various kinetic processes in micellar systems, (a) Unimer exchange, single surfactant/block copolymer chains are interchanged one by one via the solvent medium, (b) Fusion/fission, where two micelles fuse or are fragmented to...

The main part is then devoted to the equilibrium exchange kinetics of selected PEP-PEO micellar systems. We report on TR-SANS measurements in pure water that, independently of block copolymer molecular weight, composition, and temperature, revealed frozen micelles. This review further concerns the effect of tuning the kinetics by addition of co-solvents, i.e., reduction of y. The relaxation behavior of some selected systems revealing chain exchange dynamics that can be resolved by TR-SANS wfll be presented, followed by a discussion of the main observation, namely, the unexpected appearance of a pseudo-logarithmic time decay of the relaxation function. 

The influence of micellar morphology on the exchange kinetics in diblock copolymer micelles has been investigated by Lund et al. [104]. The studied system was a short chain PEPl-PEOl copolymer with symmetric block composition in water/ DMF mixtures as selective solvents for PEO. The morphological behavior of this system has already been described. The main features are illustrated in Eig. 20d. 

In this review, we have provided a selective overview of theoretical and experimental studies on kinetic processes in block copolymer micellar systems. We have demonstrated the strengths of time-resolved small-angle scattering techniques by highlighting recent examples from the literature. Most of the available literamre in this field is either related to equihbrium exchange kinetics or micellization kinetics. 

Subsequently, another MA uptakes these chains from the solvent phase and thus becomes larger. The rate of the chain uptake process is evidently proportional to the critical micellar concentration (cmc) or the concentration of unimolecular chains that are solubilized under a given set of solvation conditions. If a solvent is sufficiently poor for the insoluble block(s), the cmc can be very low. Thus, the chain-exchange kinetics can be slow for block copolymers. Fusion or fission of micelles would not occur to a significant extent for MAs whose cores are protected from contacting one another by thick repulsive coronas. 

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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