Effects Of Spreading And Subphase Conditions On The Interfacial Aggregation Behavior Of PnBA-b-PAA

Due to the unique environment-responsive property of polyelectrolytes,they have potential applications in fields like drug delivery,non-linear optical materials and optical electronic devices.In this paper,aggregation behaviors and morphologies of LB films of two kinds of diblock polyelectrolytes poly(n-butylacrylate)-b-poly(acrylic acid)(PnBA-b-PAA)at the air/water interface were systematically investigated by a Langmuir balance technique and atomic force microscopy(AFM),respectively.Effects of spreading solution concentration and volume,subphase temperature,pH and ionic strength,and copolymer component on the surface pressure-mean molucular area(?-A)isotherms and their morphologies of LB films were investigated.Under alkaline subphase condition,surface micelles formed spontaneously after the evaporation of the solvent when the PnBA-b-PAA solution is spread on the interface.For the same spreading volume,with the increase of spreading solution concentration,?-A isotherms shift toward small mean molecular area(mma).With the rise in subphase temperature,the isotherms shift toward small mma;With the rise in subphase ionic strength,the isotherms shift to high mma due to the salting out effect.All the morphologies of LB films exhibit isolated circular micelles.For small spreading volume,the micelle core diameters first decrease and then increase with the increase of spreading concentration,which are attributed to the predominant interfacial chain entanglements of PAA and higher local polymer concentration;for large spreading volume,the micelle core diameters first increase and then decrease,which are attributed to the predominantly enhanced surface polymer concentration and entanglements of PAA chains.For small spreading volume,the core diameters first decrease and then increase with the rise in subphase temperature.However,for the large spreading volume,the micelle core diameters increase due to enhanced thermal mobility of polymer molecules and disentanglements of PAA chains.With the rise in subphase ionic strength,the core diameters first increase and then decrease due to the predominantly enhanced electrostatic shielding effect and salting out effect,respectively.With the increase of subphase pH,PAA chains dissolve into subphase and the isotherms shift toward small mma.For acidic subphase condition with or without NaCl,the isotherms are almost identical because PAA chains are in an electrically neutral state and do not dissolve into subphase before compression.However,under neutral and alkaline subphase condition,the isotherms shift toward relatively larger mma due to the decreased electrostatic repulsion among PAA chains caused by the introduction of Na~+.For acidic subphase added with NaCl,the LB films of PnBA-b-PAA exhibit totally circular micelles with larger core diameter and amount compared with those without NaCl,which is attributed to the complexation between Na~+and carboxyl groups,leading to a certain amount of electrostatic repulsion among PAA chains and causing increased dissolution of PAA chains into subphase.For neutral and alkaline subphase added with NaCl,the LB films of PnBA-b-PAA also exhibit circular micelles with larger core diameter and amount compared with those without NaCl,which is attributed to the electrostatic shielding effect caused by Na~+.