Micelles in block copolymers

There is a substantial body of theoretical work on micellization in block copolymers. The simplest approaches are the scaling theories, which account quite successfully for the scaling of block copolymer dimensions with length of the constituent blocks. Rather detailed mean field theories have also been developed, of which the most advanced at present is the self-consistent field theory, in its lattice and continuum guises. These theories are reviewed in depth in Chapter 3. A limited amount of work has been performed on the kinetics of micellization, although this is largely an unexplored field. Micelle formation at the liquid-air interface has been investigated experimentally, and a number of types of surface micelles have been identified. In addition, adsorption of block copolymers at liquid interfaces has attracted considerable attention. This work is also summarized in Chapter 3. 

Sedimentation in a centrifugal field has been demonstrated to be a useful tool in the investigation of micellization in block copolymers, as first demonstrated by Tuzar etal. (1974). In this method, the velocity at which a solute species (unimer or micelle) is displaced under the influence of a strong centrifugal force is measured (Brown et al. 1995). For a given centrifugal force, the sedimentation velocity of a solute depends on its molecular weight, its buoyancy and friction... 

An early review of micellization in block copolymers was presented by Tuzar and Kratochvfl (1976), and these authors have recently provided an excellent review of the literature up to 1992 (Tuzar and Kratochvfl 1993). Micellar properties of block copolymers were also reviewed by Price (1982). A discussion of micellization was included in the general reviews on block copolymers by Riess et al. (1985) and Brown et al. (1989). Excellent reviews focussed on the solution properties of a particular class of copolymer, i.e. copolymers of polyoxyethylene) with poly(oxypropylene) have been presented by Alexandridis and Hatton (1995) and by Chu (1995) and Chu and Zhou (1996). Micellization and micellar association in related poly(oxyethylene)/poly(oxybutylene) copolymers have been summarized by Booth et al (1997). 

This chapter is organized as follows. The thermodynamics of the critical micelle concentration are considered in Section 3.2. Section 3.3 is concerned with a summary of experiments characterizing micellization in block copolymers, and tables are used to provide a summary of some of the studies from the vast literature. Theories for dilute block copolymer solutions are described in Section 3.4, including both scaling models and mean field theories. Computer simulations of block copolymer micelles are discussed in Section 3.5. Micellization of ionic block copolymers is described in Section 3.6. Several methods for the study of dynamics in block copolymer solutions are sketched in Section 3.7. Finally, Section 3.8 is concerned with adsorption of block copolymers at the liquid interface. 

Much recent work on micellization in block copolymers has been focussed on this industrially important type of polymer. We therefore describe experiments on micellization in aqueous solutions of poly(oxyalkylene) diblocks and triblocks in some detail. This serves to illustrate many of the important features of micellization of block copolymers, also observed in other systems such as the styrenic block copolymers covered in the following section. 

There have been very few studies on the kinetics of micellization in block copolymer solutions. Micellization in aqueous surfactant systems close to equilibrium occurs on a time-scale far below one second. Experimental results obtained by fast reaction techniques, such as temperature jumps or pressure jumps or steady-state methods such as ultrasonic absorption, NMR and ESR, show that at least... 

Nanometer size Pd colloids in block copolymer micelles of polystyrene polyvinylpyridine as catalysts have been used is a novel way by Klingelhofer for Heck reaction of C-C coupling of aryl halides with olefins. 

Instead of the familiar sequence of morphologies, a broad multiphase window centred at relatively high concentrations (ca. 50-70% block copolymer) truncates the ordered lamellar regime. At higher epoxy concentrations wormlike micelles and eventually vesicles at the lowest compositions are observed. Worm-like micelles are found over a broad composition range (Fig. 67). This morphology is rare in block copolymer/homopolymer blends [202] but is commonly encountered in the case of surfactant solutions [203] and mixtures of block copolymers with water and other low molecular weight diluents [204,205]. 

Fig. 30 Types of nanocarriers for drug delivery, (a) Polymeric nanoparticles polymeric nanoparticles in which drugs are conjugated to or encapsulated in polymers, (b) Polymeric micelles amphiphilic block copolymers that form nanosized core-shell structures in aqueous solution. The hydrophobic core region serves as a reservoir for hydrophobic drugs, whereas hydrophilic shell region stabilizes the hydrophobic core and renders the polymer water-soluble.

Colloidal catalysts in alkyne hydrogenation are widely used in conventional solvents, but their reactivity and high efficiency were very attractive for application in scC02. This method, which is based on colloidal catalyst dispersed in scC02, yields products of high purity at very high reactions rates. Bimetallic Pd/Au nanoparticles (Pd exclusively at the surface, while Au forms the cores) embedded in block copolymer micelles of polystyrene-block-poly-4-vinylpyridine... 

Micelles from Block Copolymers in Organic Solvents. 84... 

A critical micelle temperature or CMT is a very useful value for PEO-PPO-PEO copolymers. This arises from the fact that micellization in these copolymers is due to the dehydration of the PPO block with increasing temperature. The value of the CMT ranges from 20 to 50 °C in commercially available PEO-PPO-PEO copolymers. The CMT increases whenever the copolymer concentration is increased [19]. 

The more recently developed cryo-TEM technique has started to be used with increasing frequency for block copolymer micelle characterization in aqueous solution, as illustrated by the reports of Esselink and coworkers [49], Lam et al. [50], and Talmon et al. [51]. It has the advantage that it allows for direct observation of micelles in a glassy water phase and accordingly determines the characteristic dimensions of both the core and swollen corona provided that a sufficient electronic contrast is observed between these two domains. Very recent studies on core-shell structure in block copolymer micelles as visualized by the cryo-TEM technique have been reported by Talmon et al. [52] and Forster and coworkers [53]. In a very recent investigation, cryo-TEM was used to characterize aqueous micelles from metallosupramolecular copolymers (see Sect. 7.5 for further details) containing PS and PEO blocks. The results were compared to the covalent PS-PEO counterpart [54]. Figure 5 shows a typical cryo-TEM picture of both types of micelles. 

The use of block copolymers to form a variety of different nanosized periodic patterns continues to be an active area of research. Whether in bulk, thin film, or solution micelle states, block copolymers present seemingly unlimited opportunities for fabricating and patterning nanostructures. The wealth of microstructures and the tunability of structural dimensions make them a favorable choice for scientists in a variety of research fields. As reviewed here, they can function as nano devices themselves, or act as templates or scaffolds for the fabrication of functional nanopatterns composed of almost all types of materials. However, there are still two obvious areas which require more work control of the long-range 3D nanostructure via more user-friendly processes and the identification of new materials with different functional properties. 

Riess, G., Micellization of block copolymers. Progress in Polymer Science 2003, 28, 1107-1170. 

Application of amphiphilic block copolymers for nanoparticle formation has been developed by several research groups. R. Schrock et al. prepared nanoparticles in segregated block copolymers in the sohd state [39] A. Eisenberg et al. used ionomer block copolymers and prepared semiconductor particles (PdS, CdS) [40] M. Moller et al. studied gold colloidals in thin films of block copolymers [41]. M. Antonietti et al. studied noble metal nanoparticle stabilized in block copolymer micelles for the purpose of catalysis [36]. Initial studies were focused on the use of poly(styrene)-folock-poly(4-vinylpyridine) (PS-b-P4VP) copolymers prepared by anionic polymerization and its application for noble metal colloid formation and stabilization in solvents such as toluene, THF or cyclohexane (Fig. 6.4) [42]. 

Several routes have been developed to control the formation of nanoparticles in block copolymer systems. They include several steps (i) preparation of block copolymers (ii) loading of the precursor polymer (iii) micellization (iv) chemical... 

In addition to utilizing natural ODNs in block copolymers, ODN mimics have also been studied using polymers with the capability of forming hydrogen bonds. Bazzi and colleagues (Bazzi and Sleiman 2002 Bazzi et al. 2003) have reported diblock and triblock amphiphilic polymers with diamidopyridine (DAP) and dicarboximide moieties as molecular recognition units and studied their micelle formation properties (Chart 2.4). 

Nuclear magnetic resonance (NMR) has been used to study segmental motions in block copolymer solutions. The mobility of protons in polymer chains in dilute solutions has been probed using high-resolution H NMR. Association of chains into micelles leads to a reduction in mobility in the core, which leads to a broadening of the respective NMR lines that has been studied for a number of systems, as described by Tuzar and Kratochvil (1993). The sol-gel transition in concentrated solutions has been located via ]H transverse relaxation time experiments, as outlined in Chapter 4. 

The critical micelle concentration (cmc) in block copolymer solutions can be determined by measurement of the surface tension (y) as a function of concentration. The method detects completion of the Gibbs monolayer at the air/water interface, and is a secondary indicator of the onset of micellization. The cmc for solutions of monodisperse polymers is indicated by a fairly sharp decrease in y versus log(c). 

Among other approaches, a theory for intermolecular interactions in dilute block copolymer solutions was presented by Kimura and Kurata (1981). They considered the association of diblock and triblock copolymers in solvents of varying quality. The second and third virial coefficients were determined using a mean field potential based on the segmental distribution function for a polymer chain in solution. A model for micellization of block copolymers in solution, based on the thermodynamics of associating multicomponent mixtures, was presented by Gao and Eisenberg (1993). The polydispersity of the block copolymer and its influence on micellization was a particular focus of this work. For block copolymers below the cmc, a collapsed spherical conformation was assumed. Interactions of the collapsed spheres were then described by the Hamaker equation, with an interaction energy proportional to the radius of the spheres. 

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