Block copolymer micelles reversible dissociation

Stop-flow experiments have been performed in order to study the kinetics of micellization, as illustrated by the work of Tuzar and coworkers on PS-PB diblocks and the parent PS-PB-PS triblocks [63]. In these experiments, the block copolymers are initially dissolved as unimers in a nonselective mixed solvent. The composition of the mixed solvent is then changed in order to trigger micellization, and the scattered light intensity is recorded as a function of time. The experiment is repeated in the reverse order, i.e., starting from the block copolymer micelles that are then disassembled by a change in the mixed solvent composition. The analysis of the experimental results revealed two distinct processes assigned as unimer-micelle equilibration at constant micelle concentration (fast process) and association-dissociation equilibration, accompanied by changes in micellar concentration (slow process). 

REVERSIBLE DISSOCIATION AND FORMATION OF AZOBENZENE BLOCK COPOLYMER MICELLES... 

Whenever block copolymers are dissolved in a selective solvent that is a thermodynamical good solvent for one block and a precipitant for the other, the copolymer chains may associate reversibly to form micellar aggregates in deep analogy with the situation observed for classical low-MW surfactants, hi this respect, a CMC can be defined and experimentally measured for block copolymer micelles, as discussed in Sect. 2.1. Compared to low-MW surfactants, the values of the CMC are much lower in the case of block copolymer macrosurfactants. This motivates, e.g., the use of block copolymer micelles as nanocontainers for drug dehvery. In contrast to low-MW surfactants, these block copolymer nano containers do not dissociate into unimers whenever they are diluted in the blood stream and can therefore transport the drugs to a specifically targeted area provided that they are functionalized by suitable mo cities for site-recognition [3]. Nevertheless, macromolecular chains can encounter some dissolution problems whenever they are placed in a se-... 

An alternative strategy for design of dynamic, stimuli-responsive PE micelles is to use block copolymers with thermosensitive associating blocks, e.g., poly(A-isopropylacrylamide) [131, 134, 135] poly(A,iV-diethylacrylamide) [131-133], and poly(A,iV-dimethylacrylamide) [134]. In this case, reversible micellization-dissociation can be triggered by temperature variations that affect the solubility of the core-forming blocks. For example, in [131] it was shown that poly(acryUc acid)- Zock-poly(iV-isopropylacrylamide) copolymers can form micelles with poly(A-isopropylacrylamide) core and poly(acrylic acid) corona at pH 6 and T > 45°C, whereas at pH 4 and room temperature inverse micelles are formed. 

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