Liquid-liquid Phase Separation And Structure Modulation Of Cationic Polyelectrolyte Solution

Liquid-liquid phase separation(LLPS)normally occurs after mixing oppositely charged polyelectrolytes aqueous solutions,leading to the formation of fluidic,low-surface tension complex coacervates.Most of these polyelectrolyte LLPSs result from the electrostatic attraction between oppositely charged polyelectrolyte chains.By contrast,there are few reports on LLPS between likely charged polyelectrolytes due to electrostatic repulsion between the same charges,which limits the composition diversity of the two phases formed by LLPS.In addition,it is difficult to accomplish in situ cross-linking curing and material forming of coacervates.These factors restrict the functionality and application scope of coacervates.To these ends,cationic polyelectrolytes containing“imidazolyl-acetonitrile”units were designed,and on basis of which,LLPS between likely charged polyelectrolytes were constructed through the synergy of ion shielding effect and“cation-π”interactions.The microstructures of coacervates were tuned and cured in salty water and natural seawater through p H-responsive crosslinking reactions,leading to the formation of macroporous hydrogel membranes featuring excellent performance in photothermal desalination of seawater.The main contents of this thesis are as follows:(1)Poly(1-benzyl-3-vinylimidazolium chloride)(PILben)was synthesized,and LLPS occurred when PILben aqueous solution was mixed with poly(dimethyl diallyl ammonium chloride)(PDDA)aqueous solution.The upper and lower phases were the dilute and condensed solutions of PILben/PDDA polymers,respectively.The effects of polyelectrolyte concentration,solution temperature and salt concentration on the phase behavior of the PILben/PDDA mixture were investigated and LLPS phase diagrams were obtained.The critical PDDA concentration,i.e.,at which the LLPS of PILben/PDDA mixture occurred,was negatively correlated with the concentration of PILben and salt in the mixed solution,while it was positively correlated with the solution temperature.The results of two-dimensional NMR and Raman spectroscopy indicated the existence of cation-πinteractions between the benzene rings(in PILben)and the cations(in PILben and PDDA),which drove the chain condensation to form coacervates despite electrostatic repulsion between likely charged PILben-PDDA.(2)The effect of covalent crosslinking on the phase transitions of polyelectrolyte solutions was investigated to facilitate the curing and solidification of liquid microstructures in polyelectrolyte LLPS systems.Firstly,poly(1-cyanomethyl-3-vinylimidazolium bromide)(PILCN1)was synthesized.It was found that the“imidazolyl-acetonitrile”group can activate nitrile groups,which enabled cyclization reactions of nitriles in alkaline atmosphere and aqueous solution at room temperature.The effects of chemical structures of cationic polyelectrolytes and the types of counter ions on crosslinking reactions were studied.The nitrile crosslinking reaction could occur in both the imidazolium-type and triazoium-type polyelectrolytes,regardless of the counter ions such as tetrafluoroborate,hexafluorophosphate,bis(trifluoromethane)sufonimide and tetraphenylboron.The solid-state PILCN1 membrane was treated with an ammonia atmosphere at room temperature,during which the nitrile crosslinking occurred and improved its mechanical strength and stability.In addition,the straightforward conversion of PILCN1 solution into gel was rendered through the nitrile crosslinking,resulting in gel membranes with good tensile strength(6.2 MPa)and flexibility.(3)The LLPS between PILCN1 and PDDA solutions was constructed and tuned into bi-continuous coacervates,which were rapidly cured at room temperature.Firstly,PILCN1and PDDA aqueous solutions were mixed to investigate the effect of polymer concentration on the phase behavior of PILCN1/PDDA mixture.It was found that the critical PDDA concentration for LLPS was negatively correlated with the concentration of PILCN1.Self-condensation of PILCN1 occurred due to the cation-πinteraction,according to the elemental analysis,Raman spectroscopy and two-dimensional NMR results.During this course,PILCN1 was enriched in the bottom phase while PDDA was enriched in the top phase.The LLPS system could be transformed into a bi-continuous phase through mechanical shaking.Then the PILCN1-rich phase was rapidly cured into solid phases in alkali environments,and the PDDA liquid phase was removed by water-washing,resulting in the formation of sponge-like macroporous hydrogel.Composite macroporous hydrogels with good hydrophilicity,salt-resistance and high photothermal conversion properties were prepared through the selective incorporation of CNT.Under irradiation of 1 k W/m~2,the hydrogel exhibited a seawater evaporation rate of 2.5 kg/m~2h,and both its structure and performance remained stable during a 10-day operation.(4)A post-modified polyelectrolyte(PECHIA)was synthesized by the graft modification of polyepichlorohydrin(PECH)with 1-imidazolyl acetonitrile.When immersed in natural seawater,PECHIA underwent LLPS and formed a PECHIA-rich phase dispersed with seawater droplets.With an increasing soaking time,the PECHIA-rich phase gradually solidified under weak alkaline conditions of seawater,forming porous structures.The effects of salt concentration,p H and polyelectrolyte concentration on the structure evolution of PECHIA solution were studied.It was found that the cation-πinteraction between PECHIA is strengthened by increasing salt concentration,which slows down the kinetics of PECHIA LLPS.In addition,the crosslinking rate of nitriles was increased at higher p H,which would shorten the solidification time of PECHIA phase.By modulating the concentration of PECHIA,the phase structures of PECHIA can be changed from“continuous PECHIA-dispersed seawater”to“bi-continuous PECHIA-seawater”,and“continuous seawater-dispersed PECHIA”structures.On basis of these properties,template-free preparation of PECHIA hollow fibers,hollow spheres and microcapsules were rendered at room temperature.