Cononsolvency of Poly[2-(methacryloyloxy) ethylphosphorylcholine] in Ethanol-Water Mixtures: A Neutron Reflectivity Study

Molecular mechanisms underlying the cononsolvency, are-entrant coil-to-globule-to-coil conformational transition of polymerchains in mixtures of two good solvents, of poly[2-(methacryloyloxy)-ethyl phosphorylcholine] (PMPC) in ethanol-water binary mixtureswere complementarily investigated. This was accomplished by followinga statistical mechanical model for competitive hydrogen bondingcombined with the cooperative solvation concept as well as neutronreflectivity (NR) experiments employing contrast variation in thecononsolvents. The experimental re-entrant aggregation of the PMPCchains in ethanol-water mixed solvents, obtained on the basis ofturbidity was accurately reproduced by theoretical calculations. Thecalculation proved the relatively strong cooperativity of ethanol and thepreferential interaction of water, while the total coverage of solvents was the lowest at an ethanol volume fraction (fethanol) of 0.90. Atthis level, the cononsolvency was the most significant, and the collapsed PMPC chains were solvated with more water than the bulkmixed solvent. The ethanol-water cononsolvency for the PMPC brushes on a planar silicon wafer was investigated by NRexperiments, and the solvent composition involved in the collapsed PMPC brush was addressed according to the contrast variationstudy with mixed solvents of water, deuterium oxide, ethanol-d5, and ethanol-d6. The collapsed PMPC brushes atfethanol= 0.90contained more water than the bulk solvent. The preferential distribution of water in the collapsed PMPC brush was consistent withthe simulation results. Therefore, the molecular mechanism for the cononsolvency of PMPC in ethanol-water mixed solvents basedon competitive hydrogen bonding coupled with cooperative solvation was experimentally rationalized