Interfacial Aggregation Behavior of Double Hydrophilic Block Copolymer of PDMAEMA-b-POEGMA

The detailed micelle/aggregate structures of double hydrophilic diblock copolymers (DHBCs) at the air/water interface are not well understood and need to be further explored. The Langmuir film balance technique and atomic force microscopy were used to study the effects of subphase pH and temperature on the interfacial aggregation behavior of one DHBC of poly[2-(dimethylamino)ethyl methacrylate]-b-poly[oligo(ethylene glycol) methyl ether methacrylate] (PDMAEMA-b-POEGMA) and the structures of its Langmuir-Blodgett (LB) films, respectively. At the air/water interface, the PDMAEMA-b-POEGMA copolymer forms a dense network structure of circular micelles with the hydrophobic carbon backbones of PDMAEMA and POEGMA blocks as the tiny cores and their hydrophilic side chains as the short shells, and each copolymer molecule forms two connected micelles/cores. This ultrafine core-shell micelle structure is successfully identified by using our newly proposed relative aggregation number method, which is different from the isolated core-shell-petal and core-shell-corona structures presented in our previous DHBC systems. With the increase of subphase pH, the isotherms of the copolymer first move toward smaller mean molecular areas (mmA) and then move toward larger ones. Under alkaline conditions, the monolayer exhibits the largest hysteresis degree, whereas that under neutral conditions exhibits the smallest one. As the temperature rises, the isotherms under acidic conditions move to larger mmA due to the increased thermal mobility of the OEGMA side chains. Under neutral and alkaline conditions, the isotherms at 20 °C appear at the left of those at 10 °C due to the collapse of the OEGMA side chains above 15 °C.