Star-like micelle

Many micellar catalytic applications using low molecular weight amphiphiles have already been discussed in reviews and books and will not be the subject of this chapter [1]. We will rather focus on the use of different polymeric amphiphiles, that form micelles or micellar analogous structures and will summarize recent advances and new trends of using such systems for the catalytic synthesis of low molecular weight compounds and polymers, particularly in aqueous solution. The polymeric amphiphiles discussed herein are block copolymers, star-like polymers with a hyperbranched core, and polysoaps (Fig. 6.3). 

Fig. 6.3 Different types of micelles and micelle analogous structures a) amphiphilic block copolymers, b) star-like polymers with a hyperbranched core, c) polysoaps.

K-casein aggregates in aqueous medium are more complicated than those of p-casein, being composed of star-like sub-micelles, where each submicelle contains nine K-casein chains and the total degree of association may reach about 140 (Thum el al., 1987a). Payens and Vreeman (1982) used sedimentation measurements to infer a critical micelle concentration for K-casein of 0.5 mg/ml. 

Xu et al. (1992) used light scattering to characterize micelles formed by a wide range of PS-PEO di- and tri-block copolymers in dilute solution in water. Although full analysis of the data was complicated by the tendency of the micelles to undergo secondary association, they did find that the micellar radius scaled as eqn 3.14, in agreement with the predictions of Halperin (1987). With values of p and RB from the star-like micelle model, Xu et al. (1992) were able to compute % parameters for the interactions of PEiO with water and with PS, in... 

We inferred that the unusual structures of PFS-b-PDMS and PFS-b-PI were due to the crystalline nature of the PFS block [11,12], In order to verify this conclusion, we slightly modified the poly(ferrocenyldimethylsilane) block by replacing one methyl group linked to the silicon by an ethyl group, leading to a non-crystallizable poly(ferrocenylethylmethylsilane) block (PFEMS) [11], We found that PI and PDMS diblock copolymers with the non-crystallizable PFEMS block form only star-like micelles in n-alkane solvents. Moreover, further studies performed on PI copolymerized with polyferrocenylphos-phine (PFP) which cannot crystallize [14] show only the... 

We also noticed that the formation of these structures is sensitive to the temperature history of the sample. For example, the sample shown in Fig. 4a, cooled from 61 °C to room temperature (ca. 23 °C) over 2h and then aged, exhibited long nanotube-like structures [8,15], In contrast, a sample of the same solution of PFS40-PDMS480 in w-decane heated at 61 °C for 30 min and then rapidly quenched on the TEM grid, exhibited a mixture of star-like and short rod micelles (Fig. 5). 

Triblock copolymers of ABA-type have basically the same behavior in giving a star-like morphology (Fig. 6a) when dissolving into a selective solvent for the outer A blocks. However, the micellization of ABA-type triblock copolymers in selective solvent for the middle B blocks deals with a much more complex situation [42], A relatively low concentration of the copolymer in the selective solvent and/or a low molecular weight of the A blocks can lead to isolated flower-like micelles (Fig. 6b) with the middle B block being looped - referred to as petals - and with the two outer A blocks taking part of the same micellar core. However, if the copolymer concentration or the A block molecular weight is increased, a micelle association... 

Fig. 6 Self-assembly of ABA triblock copolymers under different conditions, (a) Star-like micelle, (b) Flower-like micelle, (c) Micelle assembly...

Recently, a versatile class of poly(ethylene propylene)/poly(ethylene oxide) block copolymer micelles were introduced they were stable due to a combination of high block incompatibility, kinetically frozen core, and high interfacial tension between core and solvent [53, 58]. Moreover, by using a co-solvent of varying composition, the aggregation number was controlled and soft spheres from star-like to micelle-like could be obtained. Another way is core stabilization via chemical crosslinking, say by UV radiation [59-64]. 

Well-characterized systems. This depends on the appropriate chemistry and subsequent characterization (typical issues here are the polydispersity, control of grafting density, reproducibility of procedure to obtain identical particles). One frequent problem here is that the price one pays for such systems is tlie availability of small amounts (sometimes only fractions of 1 g) of material. For example, multiarm star polymers are in many ways unique, clean, soft colloids [ 19,23], but their nontrivial synthesis makes them not readily available. On the other hand, recent developments witli block copolymer micelles from anionically synthesized polymers [54-58] and arborescent graft copolymer synthesis [40] appear to have adequately addressed this issue for making available different alternative star-like systems. 

Tire morphology of a micelle is primarily determined by the composition of the copolymer and the incompatibility between the blocks and the solvent.. Symmetric block copolymers produce micelles in which the core and the corona have comparable volume, leading to colloidal particles akin to the sterically stabilized particles described above. By contrast, very asymmetric copolymers form star-like particles... 

In another study, Selb and Gallot investigated the conformational properties of poly(styrene-g-4-vinyl-N-ethylpyridinium bromide) in water/methanol/LiBr mixtures [306]. The graft copolymers did not show intermolecular association in contrast to the linear block copolymers. Viscometric results showed that these graft copolymers also form compact, star-like monomolecular micelles with polystyrene cores and poly(4-vinyl-V-ethylpyridinium bromide) coronas, which resemble the polymolecular micelles of diblock materials. 

A very large number of morphologies can be found in the world of polymers and copolymers. Polymers can be linear, branched, comb-type, star-like, micelles, macrocyclic or cross-linked, when chains are linked together for copolymers the order can be random, alternating, in block or graft as illustrated in Figure 10.1. The order of the repeating units has to be specified, as different orders result in different properties. 

The salt-controlled behavior of PE coronae of kinetically frozen star-like micelles was examined experimentally [52, 58]. A good correspondence between the theoretical (—1/5) and the observed (—0.18 in [52], and —0.2 in [58]) values of the exponent was found. 

The coupling between the ionization of an annealing polyion and its conformation is expected for other branched macroions as well. Recently, this effect was unambiguously demonstrated for thennoresponsive spherical star-like micelles of diblock copolymers with a polybasic (PDMAEMA) corona [134]. Due to the... 

IPECs Based on Star-like Micelles of Ionic Amphiphilic Block Co- and Terpolymers. . 146... 

This review deals with recently obtained experimental results on IPECs based on branched PE species, specifically including PE stars, star-like micelles generated in aqueous solutions of ionic amphiphilic block co- and terpolymers, and cylindrical PE brushes. In addition, we will also present the results of molecular dynamics (MD) simulations performed for some of these systems, which enable the possible structural organization of the formed macromolecular co-assemblies to be revealed. 

We should emphasize that the trends discussed in this section are also expected to manifest for IPECs based on other types of branched PE species acting as HPEs. In particular, they can be related to IPECs derived from star-like micelles of ionic amphiphilic block copolymers. These are considered from the experimental point of view in Sect. 4. 

The results of experimental and theoretical research on water-soluble (nonstoichio-metric) IPECs based on nonlinear (branched) polyionic species (HPE) complexed with oppositely charged linear PEs (GPE) demonstrated that the main feature of such macromolecular co-assemblies is their pronounced compartmentalized structure, which results from a distinctly nonuniform distribution of the linear GPE chains within the intramolecular volume of the branched HPE. In the case of star-shaped PEs or star-like micelles of ionic amphiphilic block copolymers, this com-partmentalization leads to the formation of water-soluble IPECs with core-corona (complex coacervate core) or core-shell-corona (complex coacervate shell) structures, respectively. Water-soluble IPECs based on cylindrical PE brushes appear to exhibit longitudinally undulating structures (necklace) of complex coacervate pearls decorated by the cylindrical PE corona. 

From this formalism, the dependence of the various micellar parameters on, for example, molecular weight, composition, interfacial tension, etc. can be estimated. The results seem to compare rather well with experimental data, see, e.g., [44,45]. For example, for intermediate and star-like micelles, the aggregation number would... 

The theory proposed by Halperin and Alexander (H-A theory) [60] is based on the structural scaling description of polymeric micelles outlined in Sect. 2.1.2. Using a combination of scaling theory and Kramers rate theory for diffusirai in an external potential [61], the expulsion rate for both crew-cut and star-like spherical micelles was derived. Moreover, Halperin and Alexander discussed different scenarios of chain exchange between micelles. 

Fig. 4 Illustration of the chain expulsion process of a single chain from a star-like micelle with core radius and corona thickness D and a corresponding schematic free energy profile, F(y), along the reaction coordinate. In the calculatimis given in the text the reference state is chosen according to F(P + 1) = 0 so thatF = for the expulsion...

The above-mentioned expression is valid whenever the core is large compared to the corona. In the opposite limit, as in star-like micelles, the diffusion through the corona has to be considered instead. Using the Langevin equation to describe the... 

---