Surface Micelle Structures and Monolayer Compression Moduli of Double Hydrophilic Block Copolymer

At the air/water interface, amphiphilic block copolymers are well-known to form typical surface micelles composed of hydrophobic block cores and hydrophilic block coronas. However, the micelle/aggregate structures of the double hydrophilic block copolymers (DHBCs) need to be explored because of the lack of hydrophobic blocks. Influences of subphase pH and temperature on the interfacial aggregation behavior of a thermoresponsive DHBC of poly­(N-isopropylacrylamide)-block-poly­[oligo­(ethylene glycol) acrylate] (PNIPAM-b-POEGA) and the structures of its Langmuir–Blodgett (LB) films were studied with the Langmuir monolayer technique and atomic force microscopy, respectively. All initial LB films of PNIPAM-b-POEGA show circular micelles with a sunflower-like structure composed of a small PNIPAM backbone core, a short amide group shell, and several large POEGA petals, the structure of which is proposed in the DHBCs for the first time. Two kinds of novel methods based on the methylene group number in a core or the core diameter were successfully used to evaluate the micelle structure. With the increase of subphase pH, the isotherms move toward the larger mean molecular areas due to the gradually increased stretching degrees of POEGA blocks at the air/water interface. With the rise in temperature, the limiting molecular areas of the isotherms initially increase and then decrease above the lower critical solution temperature. As compared to those of other copolymers, the maximum monolayer compression moduli of PNIPAM-b-POEGA are very low, which indicates that the DHBC monolayers are highly compressible. Furthermore, at low temperature, hysteresis levels in the continuous compression–expansion cycles decrease stage by stage, whereas those at high temperature change slightly.