Block copolymers micellar solutions

Figure 10. Variation of G with blend composition for bidisperse SI/SIS block copolymer micellar solutions with < )bc of 15 (O), 11 (A) and 7 ( ) wt%. The solid lines connect the data, and the horizontal dashed lines identify G for the solutions composed of only the SIS copolymer. Dotted lines are linear fits to the data beyond G ax- All data have been acquired at ambient temperature.

Self-consistent field theory (SCFT, see Sections 2.3.3 and 3.4,2) has recently been applied to the phase behaviour of ordered micellar solutions. Noolandi et al. (1996) compared continuum SCFT to the lattice version of this theory for triblock copolymers such as the Pluronics in aqueous solution. From a different viewpoint, this work represents an extension of the SCFT employed by Hong and Noolandi (1981, 1983) and Matsen and Schick (1994) for the phase behaviour of block copolymer melts to block copolymers in solution. The approximations introduced by the adoption of a lattice model are found to lead to some significant differences in the solution phase behaviour compared with the continuum theory, as illustrated by Fig. 4.44. For example, the continuum theory predicts ordered phases for Pluronic L64 (PE013PP03oPEO 3), whereas the lattice theory (neglecting polydispersity) predicts none. 

Within the last decade a number of studies have appeared in print on lattice Monte Carlo simulations of surfactant-water-oil mixtures, many focusing on an examination of the simulation method using micellization of short-chain amphiphiles (or block copolymers) in solution. An almost equal number of studies focus on micellar and microemulsion phase behavior. In what follows, we include a brief discussion of a few papers on block copolymers as well, as these are conceptually similar to the systems of interest in this chapter and shed light on what could be expected in short-chain surfactant systems to some extent. 

Block Copolymers In solutions, amphiphilic block copolymers self-assemble into a variety of supramolecular micellar nanostructures [163-165]. Micelles form when one block is insoluble or only sparingly soluble in the solvent, whereas the other block is soluble and swollen in the solvent. In order to reduce the interfacial energy of the system, the insoluble block segregates in the core of the micelle. When the... 

Synthesis and self-assembly in solution 2003 [41] and on soUd surfaces theories and computer simulations AB and ABA block copolymers micellar architectures co-micellization colloidal nanostructures controlled drug deUvery polyion micellar complexes metal nanopaiticles surface modification... 

In the preceding section, AB block copolymers in solution were shown to produce micellar structures. If the same AB block copolymers are studied in the absence of a solvent, the chemical bond prevents the macroscopic phase separation expected for unconnected A and B. Supramolecular structures will instead occur in which all A-type and B-type segments microsegregate in domains separated by a surface that contains the intersegmental bonds. 

Rgure 16 Schemes for the self-assemblies of an amphiphilic diblock copolymer in the presence of the solvents water and oil selective for the two blocks, (a) Micellar solution, (b) micellar cubic lyotropic liquid crystal (LLC), (c) hexagonal LLC, (d) lamellar LLC, (e) reverse hexagonal LLC, (f) reverse cubic LLC, (g) reverse micellar solution. (From P. Alexandridis et al. Langmuir 13 23, 1997, Copyright 1997 American Chemical Society.)... 

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. 

GTP was employed for the synthesis of block copolymers with the first block PDMAEMA and the second PDEAEMA, poly[2-(diisopropylamino)e-thyl methacrylate], PDIPAEMA or poly[2-(N-morpholino)ethyl methacrylate], PM EM A (Scheme 33) [87]. The reactions took place under an inert atmosphere in THF at room temperature with l-methoxy-l-trimethylsiloxy-2-methyl-1-propane, MTS, as the initiator and tetra-n-butyl ammonium bibenzoate, TBABB, as the catalyst. Little or no homopolymer contamination was evidenced by SEC analysis. Copolymers in high yields with controlled molecular weights and narrow molecular weight distributions were obtained in all cases. The micellar properties of these materials were studied in aqueous solutions. 

With block polymers of more than 20% styrene decrease of solvent quality initially worsens dispersion stability, but thereafter the stability improves. This may be due to a better anchoring of block copolymers adsorbed from a micellar solution. 

Salt effects in polyelectrolyte block copolymer micelles are particularly pronounced because the polyelectrolyte chains are closely assembled in the micellar shell [217]. The situation is quite reminiscent of tethered polymer brushes, to which polyelectrolyte block copolymer micelles have been compared, as summarized in the review of Forster [15]. The analogy to polyelectrolyte brushes was investigated by Guenoun in the study of the behavior of a free-standing film drawn from a PtBS-PSSNa-solution [218] and by Hari-haran et al., who studied the absorbed layer thickness of PtBS-PSSNa block copolymers onto latex particles [219,220]. When the salt concentration exceeded a certain limit, a weak decrease in the layer thickness with increasing salt concentration was observed. Similar results have been obtained by Tauer et al. on electrosterically stabilized latex particles [221]. 

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. 

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. 9 Schematic representation of three approaches to generate nanoporous and meso-porous materials with block copolymers, a Block copolymer micelle templating for mesoporous inorganic materials. Block copolymer micelles form a hexagonal array. Silicate species then occupy the spaces between the cylinders. The final removal of micelle template leaves hollow cylinders, b Block copolymer matrix for nanoporous materials. Block copolymers form hexagonal cylinder phase in bulk or thin film state. Subsequent crosslinking fixes the matrix hollow channels are generated by removing the minor phase, c Rod-coil block copolymer for microporous materials. Solution-cast micellar films consisted of multilayers of hexagonally ordered arrays of spherical holes. (Adapted from [33])...

PIPAAm-PBMA block copolymers form a micellar structures by selfassociation of the hydrophobic PBMA segments in water, a good solvent for PlPAAm chains below the LCST but a nonsolvent for the PBMA chains. This amphiphilic system produces stable and monodispersed micelles from polymer/A-ethylacetamide (good solvent for the both polymer blocks) solutions dialyzed against water. Hydrophobic dmgs can be physically incorporated into the iimer micelle cores with PBMA chains by hydrophobic interactions between the hydrophobic segments and dmgs. 

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). 

Similar to micellar assemblies in water, reverse micelles have also been utilized to bring about nonspecific binding interactions in organic solvents. Akiyoshi et al. (2002) have synthesized an amphiphilic block copolymer containing PEO and an amylase chain as receptor for methyl orange (MO Chart 2.2). Amylases are insoluble and methoxy-PEO (MPEO) is soluble in chloroform. Hence, an MPEO-amylase block copolymer forms reverse micelles in chloroform. Akiyoshi et al. established the capability of the buried receptors to extract the complementary analyte by studying the ultraviolet visible (UV-vis) spectra. A solution of polymer was shaken... 

Another synthetic polymer that has shown promise in recent clinical trials for the micellar encapsulation of anticancer dmgs is a block copolymer of PEG and poly (aspartic acid) [PEG-Z -P(Asp)]. Doxombicin can be covalently attached to PEG-fi-P(Asp) through the free carboxylic acid groups on aspartic acid, and the block copolymer then forms micelles in solution with the hydrophobic aspartic acid and dmg block forming the core (Yokoyama et al. 1991 Kataoka et al. 1993). As typically occurs, the hydrated PEG chains significantly increased blood circulation... 

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