Effects Of Subphase Conditions On The Interfacial Aggregation Behavior Of Block Copolymer PLA-b-PNIPAM

Environment-responsive polymers have potential applications in fields such as drug controlled release and sensors.Thermosensitive poly?N-isopropylacrylamide??PNIPAM?and its block copolymers are typical environment-responsive polymers,and the study of their aggregation behavior at the air/water interface is of great theoretical and practical significance.In this paper,the aggregation behavior of a thermosensitive amphiphilic diblock copolymer poly?lauryl acrylate?-block-poly?N-isopropylacrylamide??PLA-b-PNIPAM?at the air/water interface and the morphologies of its Langmuir-Blodgett?LB?films were characterized with the Langmuir film balance technique and atomic force microscopy,respectively.Effects of subphase pH,temperature,ionic strength,salt kind and concentration on the surface pressure-mean molecular area??-A?isotherms and hysteresis curves of the PLA-b-PNIPAM monolayers and the morphologies of its corresponding LB films were systematically studied.With the increase of subphase pH,the PLA-b-PNIPAM monolayer isotherms shift to the large molecular area and the degree of hysteresis decrease slightly.Under acidic condition,there exist two quasi-plateaus and the lower and upper plateaus are attributed to the immersion into water for the protonated amide groups and the rest of PNIPAM blocks,respectively.While,there exists only one quasi-plateau in the isotherms at neutral or alkaline condition whose low quasi-plateaus disappear due to the very low protonation degree of PNIPAM blocks at these conditions.Under acidic condition,the transition plateau pressures gradually decrease with the elevation of temperature due to the promotion of protonation and solubility of PNIPAM blocks.Under neutral and alkaline conditions,the transition plateau pressures gradually increase with the elevation of temperature.However,when the temperature reaches 35 ^oC,the transition plateau pressures are between those at 20-30 ^oC,which is consistent with the lower critical solution temperature?LCST?behavior of PNIPAM.With the increase of ionic strength,the isotherms of PLA-b-PNIPAM monolayers under acidic condition firstly shift to large molecular area,then to small molecular area,and finally to large molecular area again.All the initial LB films of PLA-b-PNIPAM transferred from different subphase pH,temperatures,and ionic strengths exhibit the tiny isolated circular micelles which coalesce and transform into large dense ones upon compression and the core diameters decrease with the increase of subphase pH.Furthermore,PLA cores usually coalesce with the elevation of temperature due to the increased molecular thermal mobility as a result of their low glass transition temperature.From the isotherms under neutral conditions with different salt kinds and concentrations,it can be seen that the addition of NaCl and Na₂SO₄ makes PNIPAM chains spread on the water surface shrink and reduce their solubility in water.While the addition of NaNO₃ and NaSCN makes PNIPAM chains more extended and increase their solubility in water.The extension degree and solubility both increased with salt concentration.At the same salt concentration,the extension degree of PNIPAM with the addition of NaSCN is greater than that with NaNO₃.Under neutral conditions with different salt kinds and concentrations,the initial morphologies of the LB films of PLA-b-PNIPAM transferred from different salts and concentrations are also tiny isolated circular micelles.Upon compression to 10 mN/m,the morphologies of the LB films do not change significantly but the core diameters increased slightly.Upon further compression to 25 mN/m,the LB films exhibit the isolated circular micelle structure with large core size except for 1 mol/L Na₂SO₄ with the dense circular micelles.