Thermodynamics of micellization of beta-sheet forming poly(acrylic acid)-block-poly(l-valine) hybrids

The phase behavior and thermodynamic of micellization of three hybrid poly(acrylic acid)- block-poly( l-valine), namely PAA 40-b-PLVAL 100, PAA 80-b-PLVAL 100, and PAA 80-b-PLVAL 80, were investigated. beta-sheet formation in these polymeric systems resulted in a dominant enthalpic micellization process that exhibited an upper critical solution temperature (UCST). Micelle dissociation at higher temperatures is attributed to the disruption of favorable hydrogen bonds in the micellar core. Separation of hydrogen bond contributions to the micellization thermodynamics through the addition of urea as an external denaturing agent, revealed a shift from a dominant enthalpic contribution of PLVAL segments at low degree of deprotonation (alpha), where significant beta-sheet is formed, to a balanced enthalpy and entropy contributions at high alpha. At high alpha, an enhanced "water cage" hydration of unimers was observed due to the formation of water-PLVAL hydrogen bonds. Hydrophobic forces played an indirect role in enhancing the compactness of the hydrophobic core, which enhanced the strength of hydrogen bonds in the beta-sheet structures.