Influence of hydrophilic dyes on the phase transition of a thermoresponsive hyperbranched polymer

Abstract

The influences of hydrophilic dyes, including Congo Red (CR), Methyl Orange (MO), Eosin Y (EY), Fluorescein Sodium (FS) and Methylene Blue (MB), on the phase transition temperature (T_cp) of thermoresponsive hyperbranched polyethylenimine with many isobutyramide groups (HPEI-IBAm) were studied systematically. The cationic MB always showed a minor influence on the T_cp of both the cationic and the neutral HPEI-IBAm. According to the T_cp value at the 1 : 1 molar ratio of the dye to HPEI-IBAm, the ranking of these anionic dyes in reducing the T_cp of the cationic HPEI-IBAm (at pH 8) was CR > EY ≈ MO > FS. These anionic dyes could raise or have negligible influence on the T_cp of the neutral HPEI-IBAm (at pH 10) and their ranking in raising the T_cp of the neutral HPEI-IBAm was CR > EY > MO > FS. The pivotal interaction forces between HPEI-IBAm and the different hydrophilic dyes were interpreted through the combination of 2D ¹H NMR characterization, zeta potential measurement and comparison with the influences of these dyes on the T_cp of the control thermoresponsive linear polymers. Cationic HPEI-IBAm was mixed with a mixture of anionic CR and cationic MB. The heating-induced cationic HPEI-IBAm precipitates the adsorbed CR molecules more efficiently from the CR/MB mixture that have a different effect on the T_cp. Moreover, the neutral HPEI-IBAm could separate the mixture of CR and FS molecules having a pronounced different efficiency in raising the T_cp.