Investigation On Surface Activity And Structure Of Stimulus-responsive Polymer Surfactants At The Air/solution Interface

Polymer surfactants are surface-active polymers containing hydrophilic and hydrophobic parts.The surface activity can be adjusted by changing the types,numbers,arrangement and distribution of hydrophilic/hydrophobic groups.The chain structure can be designed according to the application fields.The introduction of stimulus-responsive groups into polymer surfactants leads to the responsiveness,which is stimulated by p H,CO₂/N₂,temperature,light,etc.The,stimulus-responsive polymer surfactants is very useful in various applications such as emulsion stability,foam stability,precise control of emulsion polymerization,and separation of products.The surface activity dependson the chain arrangement of the surfactant at the air/solution interface.In situ characterization of the structure of surfactant at the air/solution interface is of great significance for elucidating the interface behavior of polymer surfactants.Among these stimuli-responsive systems,light and heat systems do not need to add other substances to the solution,which prevents the affecting factors from other aspects.In this study,polymer surfactants with temperature-responsive and light-responsive groups were synthesized respectively.Stimulus-responsive surface activity is realized by adjusting the surface structure through temperature or light.The influence of concentrationand temperature on the surface activity of temperature-responsive system was studied.The surface behavior of light-responsive polymer surfactants were studied and the surfactant were applied in emulsification/demulsification and emulsion polymerization.The structure change of the surfactant at air/solution interface were investigated by sum frequency generation（SFG）spectroscopy and polarization modulated infrared reflection absorption spectroscopy（PM-IRRAS）in situ.The conclusions are summarized as follows:1.The macromolecular surfactants poly（propyleneoxide）-block-poly（N-isopropylacryla mide）(PPO₄₃-b-PNIPAM_x)with different block lengths of PNIPAM were synthesized through a standard ATRP procedure by varying the feeding amounts of NIPAM.The effects of polymer concentration and structure on the surface activity and the temperature responsiveness of the surface activity were studied.（a）Different from commercial Pluronic surfactants,three surface tension reduction processes appeared in the decrease of surface tension with concentration.In the first process,the amounts of adsorbed polymers increased and the surface tension decreased sharply.In the second process,the decrease of surface tension was caused by the rearrangement of methyl groups with increasing adsorption amount.In the third process,the increasing adsorbed amount resulted in the rearrangement of isopropyl groups,which changed from a lying down or horizontal conformation to a standing up or vertical conformation.（b）The surface tension of PPO₄₃-b-PNIPAM_x was thermally responsive,which was reversible during the heating-and-cooling cycles.At low concentrations（0.001 mg/m L）,the surface tension is 15 m N/m larger at 10 ^oC than that at 40 ^oC.While at higher concentration（1mg/m L）,the surface tension changed 9 m N/m.At lower concentration,the low surface tension at 40 ^oC was caused by the increasing adsorption amount and ordered arrangement of methyl groups,while,the standing up conformation of isopropyl groups at higher concentrations resulted in the low surface tension observed at 40 ^oC.2.PNIPAM_x-AZO and PS₉₆-AZO with azobenzene（AZO）at the end of the main chain and PEO₄₅-CD withβ-cyclodextrin at the end of polyethylene glycol were synthesized.The hydrophilic/hydrophobic ratio were adjusted through the host-guest interaction.In this way,the polymer surfactants PNIPAM_x-AZO@CD-PEO₄₅ and PS₉₆-AZO@CD-PEO₄₅ were prepared.The emulsification and demulsification of water/petroleum ether system with PNIPAM_x-AZO@CD-PEO₄₅ as surfactant was adjusted by temperature and light.PS nanosphereswere synthesized through emulsion polymerization using PS₉₆-AZO@CD-PEO₄₅as surfactants and the PS nanospheres were precipitated when irradiated with ultraviolet light.（a）The hydrophobic AZO is modified on the PNIPAM molecular chain to increase the hydrophobicity.By the host-guest interaction with PEO₄₅-CD,the hydrophobic AZO enters the cavity of the CD,which reduces the hydrophobicity of PNIPAM_x-AZO@CD-PEO₄₅ and increases the surface tension of the solution.After being irradiated with ultraviolet light,PNIPAM_x-AZO@CD-PEO₄₅ disassembled and the adsorption amount of PNIPAM_x-AZO at the air/solution interface increased,which led to the decreasing of surface tension.The results shows that the surface tension of PNIPAM_x-AZO@CD-PEO₄₅ solution decreased about 11m N/m after being irradiated with ultraviolet light.The change of surface tension in the ultraviolet/visible light irradiation cycles was reversible.Through in-situ characterization of the air/solution interface,we found that the decrease of surface tension is caused by the increasing adsorption amount of PNIPAM_x-AZO,which is more hydrophobic after ultraviolet light irradiation.（b）Water/petroleum ether emulsion with PNIPAM₂₁-AZO@CD-PEO₄₅ as surfactant was prepared,which is an oil-in-water（O/W）type stable Pickering emulsion.When irradiated by ultraviolet light,the AZO molecule changes from trans to cis and removes from the CD,which makes the PNIPAM₂₁-AZO@CD-PEO₄₅self-assembled particles unstable.As a result,the water/petroleum ether emulsionis demulsified.Because the surface tension decrease with increasing temperature,the demulsification of the emulsion system also can be caused by increasing temperature.The emulsification/demulsification of the emulsion system can be well adjusted inthe heating-and-cooling cycles.The emulsification/demulsification of water/petroleum ether system can be adjusted both by light and temperature.（c）PS nanospheres were prepared using PS₉₆-AZO@CD-PEO₄₅ as emulsifier in the emulsion polymerization.Micelle were formed in PS₉₆-AZO@CD-PEO₄₅solution.The size of the micelles increased when styrene monomer was added.After emulsion polymerization,stable PS nanospheres solution was formed.When irradiated with ultraviolet light,PS nanospheres precipitated.The destruction of the host-guest interaction between PS₉₆-AZO and PEO₄₅-CD leads to the removing of PEO₄₅-CD from the surface of PS nanospheres,which makes the PS nanospheres unstable.The light-responsive surfactant can not only make the demulsification of the emulsion polymerization product easier,but also can be used to prepare PS nanospheres with non-polar surfactant on the surface,which is useful inpreparing particles with different surface structure.