Block micellization

Riess [14], while polyelectrolyte block micelles have been surveyed by Forster et al. [15],... 

An important group of surface-active nonionic synthetic polymers (nonionic emulsifiers) are ethylene oxide (block) (co)polymers. They have been widely researched and some interesting results on their behavior in water have been obtained [33]. Amphiphilic PEO copolymers are currently of interest in such applications as polymer emulsifiers, rheology modifiers, drug carriers, polymer blend compatibilizers, and phase transfer catalysts. Examples are block copolymers of EO and styrene, graft or block copolymers with PEO branches anchored to a hydrophilic backbone, and star-shaped macromolecules with PEO arms attached to a hydrophobic core. One of the most interesting findings is that some block micelle systems in fact exists in two populations, i.e., a bimodal size distribution. 

The smaller block copolymers contain a much higher percentage of PEO and insoluble PSt block. Micelles with Rh of 13 nm in solutions of 5800 ppm of polymer and 155 ppm of water were formed. Rh increases to 18 nm at 1030 ppm of water. It was suggested the single chains have Rh values of the order of 2 nm. 

Acrylated terminated multi-block micelle-forming biodegradable macromo-lecular hydrogels, (VI), prepared by Pathak [6], were used in drug delivery devices and as tissue coatings. 

In the second approach, metal-ion/complex was first attached to one of the polymer blocks. A thin film of the resulting polymer metal complex was then obtained by spin coating/solution casting. Alternatively, the polymer metal complex may also be dissolved in a suitable solvent system that selectively dissolves one of the blocks. Micelles or nanosized aggregates formed in this case. The micellization of amphiphilic block copolymers and their use in the formation of metal nanoparticles has been discussed previously.44 A monolayer of micelles was introduced on a substrate surface by dipping or electrostatic attraction. The substrate was then subjected to further chemical or physical treatments as mentioned earlier. The third approach involves the formation of micelles from the metal-free block copolymer in a suitable solvent system. The micelle solution was then added with metal ion, which was selectively coordinated to one of the blocks. These micelle-metal complexes can also be processed by a procedures similar to the second approach. 

An important feature of (27), (29) is the absence of a power law dependence of the aggregation number peq and / core on the length Na of the coronal block. Micelles are starlike, i.e., / corona > Rcore, provided that ... 

Arguably the most important parameter for any surfactant is the CMC value. This is because below this concentration the monomer level increases as more is dissolved, and hence the surfactant chemical potential (activity) also increases. Above the CMC, the monomer concentration and surfactant chemical potential are approximately constant, so surfactant absorption at interfaces and interfacial tensions show only small changes with composition under most conditions. For liquid crystal researchers, the CMC is the concentration at which the building blocks (micelles) of soluble surfactant mesophases appear. Moreover, with partially soluble surfactants it is the lowest concentration at which a liquid crystal dispersion in water appears. Fortunately there are well-established simple rules which describe how CMC values vary with chain length for linear, monoalkyl surfactants. From these, and a library of measured CMC values (35-38), it is possible to estimate the approximate CMC for branched alkyl chain and di- (or multi-) alkyl surfactants. Thus, most materials are covered. This includes the gemini surfactants, a new fashionable group where two conventional surfactant molecules are linked by a hydrophobic spacer of variable length (38). 

Later Yokoyama and Sugiyama proposed a mechanism that controls the growing of the pores inside PS-b-PFMA or PS-b-PFS micelles surrounded by a glassy PS matrix [91], Assuming a nucleation only inside the fluorinated block micelle they propose that the final expansion of the nanopores will be limited by the elastic forces derived from the stretching of the BCP chains due to the pore swelling (Fig. 9.29). 

The combination of amine-initiated NCA polymerization and NMP was also used to prepare amphiphilic peptide-polymer conjugates having copolymers of L glutamic acid and L-alanine as polypeptide and poly(n-butyl aciylate) or PS as polymer block. Micelles and vesicles were prepared from these block copolymers and the effects of peptidases on these particles were tested. It is possible to tune the enzymatic degradation by altering the amino acid composition in the polypeptide block [77]. 

Chain-Growth Associative Thickeners. Preparation of hydrophobically modified, water-soluble polymer in aqueous media by a chain-growth mechanism presents a unique challenge in that the hydrophobically modified monomers are surface active and form micelles (50). Although the initiation and propagation occurs primarily in the aqueous phase, when the propagating radical enters the micelle the hydrophobically modified monomers then polymerize in blocks. In addition, the hydrophobically modified monomer possesses a different reactivity ratio (42) than the unmodified monomer, and the composition of the polymer chain therefore varies considerably with conversion (57). The most extensively studied monomer of this class has been acrylamide, but there have been others such as the modification of PVAlc. Pyridine (58) was one of the first chain-growth polymers to be hydrophobically modified. This modification is a post-polymerization alkylation reaction and produces a random distribution of hydrophobic units. 

The free styrene monomer is restrained within the gel and further reaction with fumarate groups is determined by the spacial arrangement the styrene polymerizes in homopolymer blocks as it intercepts fumarate reaction sites. As individual micelles expand and deplete available fumarate sites in the short polymer chains, the remaining styrene forms homopolymer blocks that terminate at the boundaries between overlapping micelles (Fig. 4). 

Fig. 4. MiceUular gelation mechanism. A shows micelle nuclei, highly cross-linked B, boundary where micelle growth terminates in styrene block polymers.

FIG. 1 Self-assembled structures in amphiphilic systems micellar structures (a) and (b) exist in aqueous solution as well as in ternary oil/water/amphiphile mixtures. In the latter case, they are swollen by the oil on the hydrophobic (tail) side. Monolayers (c) separate water from oil domains in ternary systems. Lipids in water tend to form bilayers (d) rather than micelles, since their hydrophobic block (two chains) is so compact and bulky, compared to the head group, that they cannot easily pack into a sphere [4]. At small concentrations, bilayers often close up to form vesicles (e). Some surfactants also form cyhndrical (wormlike) micelles (not shown). 

FIGURE 22.5 OTHdC study on the temperature range of the dissociation of the micelle of a styrene-isoprene two-block polymer in n-decane. Column 3.70 fim x 300 cm. (Reprinted with permission from Ref. 14. Copyright 1989 American Chemical Society.)... 

The practical development of plant sterol drugs as cholesterol-lowering agents will depend both on structural features of the sterols themselves and on the form of the administered agent. For example, the unsaturated sterol sitosterol is poorly absorbed in the human intestine, whereas sitostanol, the saturated analog, is almost totally unabsorbable. In addition, there is evidence that plant sterols administered in a soluble, micellar form (see page 261 for a description of micelles) are more effective in blocking cholesterol absorption than plant sterols administered in a solid, crystalline form. 

Generally, the number of the shell chains in a microsphere ranges from a few hundred to a few thousand. The range of the diameter of the core is from 10-100 nm. Such a core-shell structure is very similar to the (AB)n type star block copolymers, which have many arms and spherical polymer micelles of the block or graft copolymers formed in selective solvents that are good for the corona sequence and bad for the core sequence. In fact, many theoretical investigations of the chain con-... 

This block copolymer acts as an emulsifying agent in the blends leading to a reduction in interfacial tension and improved adhesion. At concentrations higher than the critical value, the copolymer forms micelles in the continuous phase and thereby increases the domain size of the dispersed phase. 

Further examples of micellar stabilization when micelles are composed of block copolymers formed by living radical polymerization are mentioned in Section 9.9.2. 

Fig. 6. Possible structures of block copolymer micelles. Top Hairy micelle Bottom Crew-cut micelle...

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