Small diblock copolymer micelles

Finally, we have designed and synthesized a series of block copolymer surfactants for C02 applications. It was anticipated that these materials would self-assemble in a C02 continuous phase to form micelles with a C02-phobic core and a C02-philic corona. For example, fluorocarbon-hydrocarbon block copolymers of PFOA and PS were synthesized utilizing controlled free radical methods [104]. Small angle neutron scattering studies have demonstrated that block copolymers of this type do indeed self-assemble in solution to form multimolecular micelles [117]. Figure 5 depicts a schematic representation of the micelles formed by these amphiphilic diblock copolymers in C02. Another block copolymer which has proven useful in the stabilization of colloidal particles is the siloxane based stabilizer PS-fr-PDMS [118,119]. Chemical... 

Since they act as surfactants, copolymers are added in only small amounts, typically from a thousandth parts to a few hundredth parts. Theoretically, Leibler [30] showed that only 2% of a diblock copolymer may thermodynamically stabilize an 80%/20% incompatible blend with an optimum morphology (submicronic droplets). However, in practice kinetic control and micelle formation interfere in this best-case scenario. To a some extent, compatibilization increases with copolymer concentration [8,31,32], Beyond a critical concentration (critical micellar concentration cmc) little or no improvement is observed (moreover, for high amounts, the copolymer can act as a plasticizer). Copolymer molecular weight influence is similar to that of the concentration effect. For example, in a PS/PDMS system [8,31,32], when the copolymer molecular weight increases, domain size decreases to a certain extent. Hu et al. [31] correlated their experimental results with theoretical prediction of the Leibler s brush theory [30]. Leibler distinguishes two regimes to characterize the behaviour of the copolymer at the interface... 

The pH-induced micellization of a DMAEMA-6-DEAEMA diblock copolymer has been studied in detail using dynamic light scattering, small-angle neutron scattering, and fluorescence spectroscopy [166], The DMAEMA constitutes the corona of the micelle, whereas the DEAEMA forms the core. Pyrene was used as a probe to determine the nature of the DEAEMA blocks. It was shown that the hydrophobicity of the micellar cores increased progressively as the solution pH was adjusted from pH 7 to 9. In the presence of an electrolyte, it was possible to observe both individual chains (unimers) and micelles under certain conditions. The critical micellization pH depended on both the copolymer concentration and also the background electrolyte concentration. 

Diblock copolymers can self-assemble into micelles when immersed in a selective solvent of small molecules, or in a matrix comprised of a homopolymer. The micelles are easily detected by techniques such as light scattering (f ) and transmission electron microscopy ( ), but the critical micelle concentration (cmc) may be so small that other techniques are required for its determination. Techniques based on the ability to detect fluorescence from very dilute systems have been used for this purpose ( ). 

For diblock copolymers, the found behavior is even more diverse. This arises from the large variety of the different blocks having different Tg, charge, etc. However, screening the available results at least allows the identification of some similarities in the group of BCPs made of PEO and an apolar chain (e.g., PB or PS). In this case, the decomposition of the BCP micelles often leads to rather monodis-perse mixed micelles containing more than one BCP molecule. Above a certain threshold in the surfactant concentration, additional surfactant leads to the formation of small surfactant micelles coexisting with the mixed ones. 

Recently, novel polymeric and peptide self-assembling systems have been developed for drug delivery. These carriers mimic the characteristics of conventional Upid systems and, in some cases, demonstrate improved drug delivery qualities. In this case, diblock copolymers are commonly used to construct micellar assanbUes. Self-assembly of block copolymers is often spontaneous and can be performed in the presence of small molecules that form micelles encapsulated with drugs, with diameters up to hundreds of nanometers (Lee and Yuk 2007 Branco and Schneider 2009). In addition to micelles, amphiphilic block copolymers can self-assemble into vesicles in water. Polymersomes have been used for delivery of small hydrophobic... 

To prepare micelle solutions, diblock copolymers were first dissolved in a small amount of THF, a common solvent for both blocks. Hexane, a precipitant for PFDMS, was added dropwise afterward with gentle stirring, and the solutions were monitored in a light-scattering apparatus. When a strong increase in lightscattering intensity indicated the onset of the aggregate formation, the addition of hexane was stopped. The solution was left to stir for 3 days at 23 °C to equilibrate the micellar structures. All micelle solutions have a final polymer concentration of 1 mg/mL. 

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