Conformation and solubility of poly(ethylene oxide) in polymer-surfactant complex at high-temperature and high-salinity conditions

Abstract

The solubility of poly(ethylene oxide) (PEO) with molecular weight (MW) ranging from 300,000 to 6,000,000 g/mol was studied in aqueous solution in the presence of salt (sodium chloride, [NaCl] = 1 M) at various temperatures by cloud point measurement. All PEO solutions exhibited phase separation at temperature above 78 ± 1 °C, the cloud point was independent of PEO chain length within the selected range of MW. An anionic surfactant sodium dodecyl sulfate (SDS), and a cationic surfactant dodecyl tri-methyl ammonium chloride (DTAC) was added into the PEO solutions, respectively, to successfully eliminate the precipitation of the polymer at higher temperature (up to 90 °C). However, the effects of SDS and DTAC on the cloud point of PEO at various surfactant concentrations, as well as the rheological behaviors of PEO/SDS and PEO/DTAC solutions were completely different. This observation leads to a study of the molecular structures formed by PEO and the surfactants. The interactions between a monodisperse and short-chain PEO (MW = 20,000 g/mol) and the surfactants at molecular level were investigated via two-dimensional NMR techniques. Based on these results, two different conformations of PEO in PEO/SDS and PEO/DTAC complex were proposed and a saturation concentration of SDS binding to PEO backbone was determined. The findings in this study provide evidences that molecular interaction between a polymer and a surfactant in solution strongly affect the solubility of the polymer and the rheological property of the solution.