The Formation Of Ordered Structures In The Solidification Of Alkyl Substituted Poly(Ionic Liquid)s Solution

The ordered nanostructures formed by poly(ionic liquid)s(PILs)are widely used for the fabrication of the smart materials and soft template.These ordered nanostructures are closely related to PIL composition including the types of cation and anion,and PIL solution property.At present,the self-aggregation of PILs in aqueous solution has been widely investigated.As well known,the family of PIL is very broad,and the PILs carrying different kinds of anion and cation are often dissolved in organic solvents instead of water.Moreover,it was reported that the ordered assembilities of PILs in organic solution seemed to have potential application in the preparation and design of new functional materials.Therefore,a systematical investigation on the self-assembly behavior of PILs in organic solution is of great importance for expanding the application of PILs and developing new functional materials.In this study,the imadazolium-based PILs with different alkyl tail and counteranion were synthesized by solution polymerization.After the processing of solution-precipitation,PILs were dissolving in the organic solvents.The structural evolution of PIL assemblies during the drying of PILs solution was studied in detail in an effort to develop a novel and facile approach for the engineering of PIL ordered microstructures.The microstructure of PILs formed in dilute solution,gel and solidification powder were characterized by a variety of techniques including transmission electron microscope(TEM),microscope,polarized optical microscopy(POM),X-ray diffraction(XRD),small-angle X-ray scattering(SAXS)and differential scanning calorime(DSC)measurements.The results were shown as following:1.Poly[C16Vim+][X](X=Br-,BF4-)formed “onionlike” multilamellar structure,while poly[C16Vim+][PF6-] formed “cloudlike” structure when PIL was dissolved in the organic solvent at low concentration.2.PILs gel displayed lyotropic liquid crystal behavior,which was dependent on the length of alkyl tails.A longer alkyl chain was helpful to form lyotropic liquid crystal.The ordered structure of lyotropic liquid crystal was related to the counteranion of PILs.A long-range order lamellar structure was observed when the anion was BF4-,as evinced as cruciate flower texture founded in POM.With Br-as counteranion,the ordering of lamellar structure was decreasedsince an incomplete cruciate flower texture was observed.In addition,the PILs gel with PF6-as counteranion formed weakly ordered lamellar structure because a birefringent random polygon-like texture was present in POM.3.During solvent evaporation,the structure of Poly[C16Vim+][Br-] gel changed from weakly ordered lamellar structure to ordered lamellar structure,and finally long-range order perforated lamellar structure.The ordering of lamellar structure of PILs with BF4-or PF6-was found to be increased at certain degree with the evaporation of organic solvents.4.The formation of lyotropic liquid crystal for poly [C16Vim+][BF4-] was relevant with the kinds of organic solvent.The stronger polar solvent DMF was beneficial to fabricate lamellar structure.However,the weak polar solvent tetrahydrofuran or chloroform facilitated the formation of hexagonal structure.5.The length of alkyl tail(n)of poly [CnVim+][X] has significant effect on the microstructures in its solid state.The poly [C16Vim+][X] seemed to be a crystalline polymer due to its longer alkyl chains.The ordering degree of crystalline was related to the counteranion for the poly [C16Vim+][X],following the order as BF4->Br->PF6-.However,the poly [CnVim+][X]was amorphous polymer when the alkyl tail was less than 12.6.From the viewpoint of the meso-structure in PIL solid powder,PILs displayed amorphous structure with entangled polymer chains when the alkyl tail was 4.However,poly [Cn Vim+][X]exhibited amorphous lamellar structure when the alkyl tail was 8 or 12.In addition,the counteranion had an effected on the ordering of amorphous lamellar structure.The ordering degree followed the order of BF4->Br->PF6-.The d-spacing of PIL microstructures decreased as the counteranion varied from Br-,BF4-to PF6-.The ordering of lamellar structure was improved with the increase in alkyl tails of PILs.7.The solid powder of poly [C16Vim+][X] maintained the same ordered lamellar structures with that its gel state.However,the counteranion with larger size such as BF4-or PF6-can form order lamellar meso-structure existed between anion and anion because of the strong interaction between anion and imidazole ring when PIL solution was completely dried.This paper investigated systematically the effect of the length of alkyl tail and the kinds of counteranion on structures of PIL assembilities,and the structural evolution during solvent evaporation.The results provide a theoretical basis and experimental guidance for the design andfabrication of PIL assembilities.