Aggregation behavior of triblock terpolymer poly[n-butyl acrylate-block-N-isopropylacrylamide-block-2-(dimethylamino)ethyl acrylate] at the air/water interface

Langmuir film balance technique was used to study the subphase pH and temperature effects on the aggregation behavior of a dual-responsive triblock terpolymer poly[n-butyl acrylate-block-N-isopropylacrylamide-block-2-(dimethylamino)ethyl acrylate] (PnBA-b-PNIPAM-b-PDMAEA) at the air/water interface. Atomic force microscopy was used to observe the morphologies of its Langmuir-Blodgett (LB) films. At the interface, the terpolymer tends to form isolated circular micelles composed of hydrophobic PnBA cores and hydrophilic PNIPAM-PDMAEA coronas. The limiting areas in the surface pressure-molecular area isotherms under acidic and alkaline conditions are larger than that under neutral conditions due to the larger interfacial stretching degrees of PNIPAM blocks, which results from the strong electrostatic repulsions of the fully protonated PDMAEA blocks underwater in the acidic case. In our minds, the hysteresis degrees of the compression-expansion cycles under different subphase pH conditions should mainly depend on the initial stretching degrees of the hydrophilic blocks. Under acidic conditions, the isotherms shift toward small areas with the increase of temperature due to the increased solubility of PNIPAM blocks. However, under alkaline conditions, the isotherms significantly exhibit the lower critical solution temperature behavior of PNIPAM blocks, which is a good example to build up the relationship between the interfacial and solution properties of block copolymers.