Triblock micelles

In what follows, we will discriminate between two types of ABC triblock micelles (1) those in which two blocks are insoluble in the considered solvent and then contain a compartmentalized core and a homogenous corona (2) those in which only one block is insoluble and whose corona is heterogeneous due to the presence of two types of coronal blocks. 

Stimulus-responsive ABC triblock micelles were also investigated by Patrickios et al. for copolymers containing insoluble PEVE blocks, thermore-sponsive PMVE midblocks, and water-soluble PMTEGVE outer blocks with varying block sequences [277]. While in aqueous solutions only unimers were found, and the addition of salt led to aggregates. 

ELP-based triblock copolypeptides have also been used to produce stimulus-responsive micelles, and Chaikof and coworkers envisioned the possible application of these micelles as controlled drug delivery vehicles. These amphiphilic triblock copolymers were constructed from two identical hydrophobic ELP endblocks and a hydrophilic ELP midblock. Below the transition temperature, loose and monodispersed micelles were formed that reversibly contracted upon heating, leading to more compact micelles with a reduced size [90]. 

Hvidt, S Jorgensen, EB Brown, W Schillen, K, Micellization and Gelation of Aqueous Solutions of a Triblock Copolymer Studied by Rheological Techniques and Scanning Calorimetry, Journal of Physical Chemistry 98, 12320, 1994. 

Adhikari A, Dey S, Das DK, Mandal U, Ghosh S, Bhattacharyya K (2008) Solvation dynamics in ionic liquid swollen P123 triblock copolymer micelle a femtosecond excitation wavelength dependence study. J Phys Chem B 112(20) 6350-6357... 

Recent progress in novel micellar structures, including micelles containing exotic blocks such as natural or synthetic polypeptides and metal-containing segments, micelles from ABC triblock copolymers, Janus micelles and other noncentrosymmetric micelles, micelles based on interpolyelectrolyte or other noncovalent complexes, and metallosupramolecular micelles, will be discussed in Sect. 7. 

The influence of the copolymer chain architecture on the CMC has been investigated in PEO- and PBO-containing bock copolymers, as schematically depicted in Fig. 1. From an entropic point of view, the formation of micelles should be favored for diblock architectures compared to triblock and cyclic ones, the reason being that two block junctions should reside at the... 

Fig. i Schematic representation of chain conformation in micelles from a linear PEO-PBO diblock copolymers, b linear PEO-PBO-PEO triblock copolymers, c linear PBO-PEO-PBO triblock copolymers and d cyclic PEO-PBO diblock copolymers... 

Fig. 3 TEM (top) and AFM phase contrast images (bottom) of aqueous micelles formed by a PS200-P2VP140-PEO590 ABC triblock copolymer at pH > 5 (left) and pH< 5 (right). For TEM pictures, the PS and P2VP blocks have been stained by Ru04. AFM images have been recorded with tapping mode (contrast scale black 0°, white 45°). Adapted from [47]...

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). 

Excellent reviews on micelles formed in organic solvents have been published by Hamley [2], Chu et al. [86], and Riess [14]. From these overviews it appears that a wide range of styrene-, (meth)acrylates-, and dienes-based block copolymers were investigated and that the formation of micelles in organic solvents can generally be considered as an entropy-driven process. AB diblock and ABA triblock architectures were systematically compared. All these previous investigations have been summarized by Hamley [2], We will therefore not perform an extensive review of all these systems, since this information has already been provided by others, but we will briefly outline some selected examples. 

Recent studies on PEO-PPO, PEO-PBO di- and triblock copolymers include the works of Bahadur et al. [121], who examined the role of various additives on the micellization behavior, of Guo et al. [122], who used FT-Raman spectroscopy to study the hydration and conformation as a function of temperature, of Booth and coworkers [ 123], who were mainly interested in PEO-PBO block copolymers with long PEO sequences, and of Hamley et al., who used in situ AFM measurements in water to characterize the morphology of PEO-PPO micelles [56,57]. 

The vast majority of block copolymer micelles has been constructed from AB diblock copolymers. However, ABC triblock copolymers have attracted a great deal of interest due to the huge number of different morphologies that have been observed so far in bulk and because the introduction of a third block may introduce interesting new functionalities. Although many investigations have... 

Micelles of type (1) were the first investigated examples of ABC triblock copolymer micelles. These micelles are generally characterized by the so-called onion, three-layer, or core-shell-corona structures, i.e., the first insoluble A block forms the micellar core, the second insoluble B block is wrapped around the core, and the third soluble C block extends in the solution to form the micellar corona (Fig. 18). To the best of our knowledge, there are no known examples of ABC block copolymer micelles with A and C insoluble blocks and a B soluble block. 

Core-shell-corona micelles were formed by PEHA-PMMA-PAA triblock copolymers in water, as demonstrated by Kriz et al. [266]. Ishizone et al. [267] synthesized ABC triblock copolymers containing 2-(perfluorobutyl)ethyl methacrylate, tBMA, and 2-(trimethylsilyloxy) ethyl methacrylate with various block sequences. These copolymers were converted into amphiphilic sys-... 

Fig. 18 Schematical representation of different types of micelles formed by ABC triblock copolymers. Core-shell-corona micelles with insoluble core and shell (a), core-shell-corona micelles with radially compartmentalized corona (b), and Janus micelles with laterally compartmentalized corona (c)...

Crew-cut micelles were prepared from PS-PMMA-PAA triblock copolymers with a large PS block [270]. The morphology of these micelles was found to be dependent on the starting nonselective solvent (dioxane, THF, or DMF), as discussed previously in Sect. 6. 

For some applications, it is desirable to lock the micellar structure by cross-Hnking one of the micellar compartments, as discussed previously in Sect. 2.6. Cross-Hnked core-shell-corona micelles have been prepared and investigated by several groups as illustrated by the work of Wooley and Ma [278], who reported the cross-linking of PS-PMA-PAA micelles in aqueous solution by amidation of the PAA shell. Very recently, Wooley et al. prepared toroidal block copolymer micelles from similar PS-PMA-PAA copolymers dissolved in a mixture of water, THF, and 2,2-(ethylenedioxy)diethylamine [279]. Under optimized conditions, the toroidal phase was the predominant structure of the amphiphilic triblock copolymer (Fig. 19). The collapse of the negatively charged cylindrical micelles into toroids was found to be driven by the divalent 2,2-(ethylenedioxy)diethylamine cation. 

Fig. 19 TEM image of toroidal micelles from a PAA-PMA-PS triblock copolymer (A). This sample was cast from a solution with 0.1 wt% PAA99-PMA73-PS66 triblock copolymer, a THF water volume ratio of 1 2, and an amine acid molar ratio of 0.5 1 by addition of 2,2-(ethylenedioxy)diethylamine. The cast film was negatively stained with uranyl acetate. A schematical representation of theses micelles is also shown (B). Reprinted with permission from [279], Copyright (2004) American Association for the Advancement of Science...

As introduced previously, type 2 ABC triblock copolymer micelles are formed by triblock copolymers containing an insoluble A block while the B and C blocks are soluble in the considered solvent. The insoluble blocks can be located either between the two soluble blocks (BAC structure) or at one end of the triblock (ABC or ACB structures). Micelles of the latter type were discussed above for, e.g., PS-P2VP-PEO pH-responsive micelles and are indeed considered as core-shell-corona, onion, or three-layer structures since the heterogeneity in the micellar corona is observed in the radial direction (Fig. 18). Micelles formed by BAC triblock copolymers are different from the previous case because they can give rise in principle to a heterogenous corona in the lateral dimension (Fig. 18). This could induce the formation of noncentrosymmetric micelles as discussed in Sect. 7.3. 

Early examples of micelles formed by BAC triblock copolymers have been reported by Patrickios and coworkers, who studied the formation of micelles from polyampholytic PDMAEMA-PMMA-PMAA and PDMAEMA-poly(2-phenylethylmethacrylate)-PMAA copolymers as a function of pH [285-287]. 

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