Solvent-Responsive Self-Assembly of Amphiphilic InvertiblePolymers Determined with SANS.

Amphiphilicinvertible polymers (AIPs) are a new class of macromoleculesthat self-assemble into micellar structures and rapidly change structurein response to changes in solvent polarity. Using small-angle neutronscattering (SANS) data, we obtained a quantitative description ofthe invertible micellar assemblies (IMAs). The detailed compositionand size of the assemblies (including the effect of temperature) weremeasured in aqueous and toluene polymer solutions. The results showthat the invertible macromolecules self-assemble into cylindricalcore–shell micellar structures. The composition of the IMAsin aqueous and toluene solutions was used to reveal the inversionmechanism by changing the polarity of the medium. Our experimentsdemonstrate that AIP unimers self-assemble into IMAs in aqueous solution,predominantly through interactions between the hydrophobic moietiesof macromolecules. The hydrophobic effect (or solvophobic interaction)is the major driving force for self-assembly. When the polarity ofthe environment is changed from polar to nonpolar, poly(ethylene glycol)(PEG) and aliphatic dicarboxylic acid fragments of AIP macromoleculestend to replace each other in the core and the shell of the IMAs.However, neither the interior nor the exterior of the IMAs consistsof fragments of a single component of the macromolecule. In aqueoussolution, with the temperature increasing from 15 to 35 °C, theIMAs’ mixed core from aliphatic dicarboxylic acid and PEG moietiesand PEG-based shell change the structure. As a result of the progressivedehydration of the macromolecules, the hydration level (water content)in the micellar core decreases at 25 °C, followed by dehydratedPEG fragments entering the interior of the IMAs when the temperatureincreases to 35 °C. [ABSTRACT FROM AUTHOR]