Aggregation Behavior Of Triblock Terpolymer PnBA-b-PNIPAM-b-PDMAEA At The Air/Water Interface

The ternary triblock copolymers have broad application prospects in biology,medicine,optics,and electronics.At the air/water interface,the interfacial aggregation behavior of triblock copolymers have attracted researchers’attention due to their more complex structures compared with linear diblock copolymers.In this work,the Langmuir film balance technology was used to study the subphase p H value,temperature,ion strength,and ion-specificity effects on the aggregation behavior of a poly[n-butyl acrylate-block-N-isopropylacrylamide-block-2-（dimethylamino）ethyl acrylate]（PnBA-b-PNIPAM-b-PDMAEA）triblock terpolymer at the air/water interface.Atomic force microscopy（AFM）was used to observe the morphologies of its Langmuir-Blodgett（LB）films.This work reveals the conformations of the two hydrophilic blocks of the terpolymer under different subphase conditions,which will bring new insights on the interface properties of the linear triblock terpolymers.The effects of subphase p H value and temperature on the interface behavior of triblock copolymer PnBA-b-PNIPAM-b-PDMAEA were studied under the absence of salt conditions.Under different subphase p H and temperature conditions,the terpolymer tends to form circular micelles composed of hydrophobic PnBA cores and hydrophilic PNIPAM-PDMAEA coronas at the air/water interface.The surface pressure-molecular area（π-A）isotherms of the copolymer monolayers under acidic and alkaline conditions are on the right side under neutral condition.Under acidic conditions,PNIPAM blocks have the largest stretching degrees at the interface because the fully protonated PDMAEA blocks generate electrostatic repulsion underwater.However,under alkaline conditions,the non-protonated PNIPAM and PDMAEA blocks have large stretching degrees at the interface.Under different subphase p H conditions,the hysteresis degrees of the monolayers depend on the initial stretching degrees of the two hydrophilic blocks at the interface.Therefore,the hysteresis degrees of monolayers under acidic and alkaline conditions are greater than that under neutral conditions.Under acidic conditions,the isotherms shift towards a smaller area with the increase of temperature due to the increased solubility of PNIPAM blocks.Under neutral conditions,PDMAEA blocks are partially protonated and the isotherms at different temperatures are close.However,under alkaline conditions,the isotherms significantly exhibit the lower critical solution temperature（LCST）behavior of PNIPAM blocks.The effects of ionic strength and ion-specificity on the interface behavior of the triblock copolymer PnBA-b-PNIPAM-b-PDMAEA were studied under acidic and alkaline conditions,respectively.Under acidic conditions,PNIPAM blocks are hardly protonated,and the PDMAEA blocks are fully protonated.At low ionic strength of Na Cl solution,the isotherm of the copolymer monolayer is mainly affected by the salting-out effect of PDMAEA blocks.At moderate and high ionic strengths of Na Cl solution,the isotherms of the copolymer monolayers do not change significantly.The isotherms in the cases of Na₂SO₄solutions shift to large MMA with the increase of ionic strengths.Under alkaline conditions,the stretching degrees of hydrophilic blocks increase with the increase of the concentrations of Na₂SO₄,Na SCN,and Na NO₃,while the change of Na Cl concentration has little effect on the stretching degree of hydrophilic blocks.At all salt concentrations,the influence of salt type on the stretching degree of hydrophilic blocks does not conform to the Hofmeister series.Under different ionic strengths and concentrations,the LB films of the terpolymer present isolated circular micelles with small PnBA cores.