Effects of Subphase Conditions on the Interfacial Behavior of Partially Hydrophobized Double Hydrophilic Block...

At the air/water interface, amphiphilic block copolymers usually form surface micelles composed of hydrophobic block cores and hydrophilic block coronas. However, double hydrophilic block copolymers (DHBCs) do not contain hydrophobic blocks, and the resulting surface micelle structures have yet to be explored. The Langmuir film balance technique was used to explore the effects of subphase pH and temperature on the interfacial aggregation behavior of two DHBCs of poly[2-(dimethylamino)ethyl methacrylate]-b-poly[oligo(ethylene glycol)methyl ether methacrylate] partially quaternized with 1-iodohexane (Q₆) and 1-iodododecane (Q₁₂) (P(DMAEMA-co-Q_6/12DMAEMA)-b-POEGMA). Their Langmuir-Blodgett (LB) films deposited onto silicon wafers were characterized by atomic force microscopy. At the air/water interface, the two copolymers tend to form a dense network of circular micelles consisting of the tiny cores of hydrophobic carbon backbones and the short mixed shells of hydrophilic DMAEMA/QDMAEMA/OEGMA side chains, and each copolymer molecule forms two to three connected micelles/cores. These ultrafine micelle structures are successfully identified by using our newly proposed relative aggregation number method. With the increase of subphase pH, the limiting areas of the isotherms first decrease and then increase. This is due to the large steric repulsions of DMAEMA/QDMAEMA side chains on the stretch of the OEGMA side chains within the shells under acidic and alkaline conditions. With increasing temperature, the isotherms of the two block copolymers mainly first move to the smaller mean molecular areas and then to the larger ones due to the collapse of OEGMA side chains above 15 °C and the increased thermal mobility of side chains, respectively.