Effects Of Subphase Conditions On The Aggregation Behavior Of PnBa-b-PAA At The Air/Water Interface

Due to the electrostatic interaction and the conductive function of polyelectrolyte ionic groups as well as the viscoelastic properties and the hydrophilic interaction of polyelectrolyte chains,polyelectrolyte materials own the unique features such as polyelectrolyte effect,antipolyelectrolyte effect,and self-assembly,etc.(compared with ordinary polymers),which have widely attracted researchers.In this work,aggregation behavior of an amphiphilic diblock polyelectrolyte poly(n-butylacrylate)-b-poly(acrylic acid)(Pn BA-b-PAA)at the air/water interface and morphologies of its LB films were mainly characterized by the film balance technique and atomic force microscopy(AFM),respectively.Effects of subphase p H,temperature,ionic strength and kind of spreading solvents on the isotherms and hysteresis curves of the Pn BA-b-PAA monolayers and the morphologies of its LB films were systematically studied which expose the interfacial aggregation behavior of the polyelectrolyte under different subphase conditions.When tetrahydrofuran(THF)was used as the spreading solvent,the isotherms of Pn BA-b-PAA shift to the small molecular area with the increase of subphase p H,and its quasi-plateaus of “pancake to brush” structural transtion disappear gradually.Under acidic and neutral conditions,the hysteresis phenomena of Pn BA-b-PAA monolayers are both obvious,and the hysteresis degree under the former condition is slightly smaller than that under the latter condition;while the compression and expansion curves under alkaline condition are almost overlapped.With the increase of subphase temperature,the isotherms shift to the small molecular area,and the hysteresis phenomenon of compression-expansion curve increases.With the increase of ionic strength,the isotherms shift to the large molecular area,and the hysteresis degree becomes more obvious.Under acidic and neutral conditions,the LB films of Pn BA-b-PAA exhibit the large isolated circular micelles with Pn BA as the cores and PAA as the coronas;while those under alkaline condition exhibit small size micelles with closepacked structures.With the increase of subphase temperature,the hydrophobic Pn BA blocks on the water surface are more likely to aggregate into large cores due to the higher molecular mobility.Furthermore,the ringlike nanostructures with uniform size prepared under alkaline condition with medium ionic strength are observed for the first time at the air/water interface.When chloroform/ethanol mixture(1/2 in volumn)was used as the spreading solvent,the shapes of the isotherms are similar to those spreading with THF.However,with the increase of temperature,the hysteresis degree of compressionexpansion curve decreases;and the isotherm at relatively high temperature reversly shifts to the large mean molecular area.The changes of surface morphology of LB film under acidic condition are similar to those under neutral condition.With the increase of temperature,the size of the circular micelle increases;with the increase of deposition pressure,the adjacent circular micelles will coalesce into larger droplet-like aggregates.Under alkaline condition and high surface pressure,the LB films of Pn BA-b-PAA exhibit micelles with closepacked structures,and the effect of temperature on the structure of the LB films is not obvious.