Stimuli-responsive Poly(ionic Liquid)brushes Based On SI-ATRP

Stimulation-responsive polymer materials are a kind of advanced materials that have been developed with the global manufacturing industry entering the industrial4.0 era.Their intelligent response to weak external stimuli enables them to adjust themselves,and then show some special functions to meet human needs.As a result,more and more scientists are turning their attention to polymer-based smart materials that can be triggered by one or more stimuli（such as ions and gases）and play an important role in broader industrial systems.Poly（ionic liquid）s（PILs）,as a new type of polymer,are of great value in the fields of carbon capture,catalysts and separation due to their flame retardancy,non-volatility,good thermal stability,high mechanical properties and flexible design of molecular skeleton.In addition,organic-inorganic composite brush-like materials are orderly arranged on the surface of an inorganic matrix due to the electrostatic interaction between polymer chains and steric hindrance effect under suitable grafting density,thus showing some special properties different from free polymers.Therefore,a well-developed surface-initiated atom transfer radical polymerization（SI-ATRP）method was used.First,a monolayer of initiator was self-assembled on the surface of planar monocrystalline silicon wafer（Si）and spherical silica nanoparticles（SiO₂ NPs）,and then modified vinylimidazole-type ionic liquids with n-butyl,amino and hydroxyl functions were initiated by SI-ATRP.PIL brushes with different ionic and CO₂ responses were synthesized.The brush-like materials were characterized by structure and the response behavior of ions and CO₂were tested.The following conclusions were drawn:（1）The surface roughness of single crystal silicon wafers increased in varying degrees after grafting with three kinds of polyionic liquids.The thickness of PIL brush grafted on Si surface increased with the prolongation of polymerization time when the monomer concentration was fixed.Additionally,the thicknesses of Si-PILs-C4,Si-PILs-NH2 and Si-PILs-OH reached the maximum at 9 h of polymerization time,and the brush thickness decreased with the prolongation of polymerization time,while the brush thickness increased with the increase of monomer concentration at fixed polymerization time.（2）The adhesion test of PIL brush shows that the material has certain ion responsiveness:when the polymerization time is certain,the flexibility of molecular chain increases with the increase of ion concentration,and the adhesion of polymer brush increases as well;When the concentration of the applied ions is fixed,the flexibility of the molecular chain increases as the polymerization time increases（the increase is more obvious when the ion concentration is lower）,but the adhesion of the polymer brush decreases（the reduction is more obvious when the ion concentration is higher）.（3）Compared with SiO₂ NPs,free polymers and their physical mixtures,brush-like PILs exhibited higher CO₂ responsiveness,which was attributed to brush-like physical adsorption,chemical adsorption of imidazole ring at position 2 C and amino functional groups.The effect of different functional groups on CO₂responsiveness is amino>n-butyl>hydroxyl,which is related to the charge effect of substituents.The effect of anions in ionic liquids on CO₂ responsiveness is hexafluorophosphate>bromide>tetrafluoroborate>bistrifluoromethane sulfonyl imide,which is related to the enthalpy and entropy changes of the system and the formation of hydrogen bonds in the process of adsorption.