Molecular Gels Based On Calix[4]arene Derivatives And Dynamic Covalent Bond Gels

Being two fascinating types of soft materials,molecular gels and dynamic covalent bond(DCB)gels are of great interest because their 3D network structures are built via dynamic and reversible non-covalent or dynamic covalent bonds.The non-covalent interactions used in molecular gels include ?-? stacking,hydrogen bonding,metal-ligand coordination,host-guest complexation,etc.The bonds in the 3D network used for DCB gels include imine bonds,acylhydrazone bonds,boronate ester bonds,disulphide bonds,etc.It is the dynamic and reversible nature of the non-covalent and dynamic covalent bonds that contribute to the formation of these network structures.These features also endow molecular gels and DCB gels undergo significant gel-sol phase transition or volume phase transition in respond to certain physical and chemical stimuli(such as temperature,electric/magnetic fields,solvent composition,light intensity,pressure,shear-force,pH,ions,and specific chemical compositions).Thus,both the two types of gels provide great potential for fundamental and applied research.Calixarene derivatives are the third generation of macrocyclic compounds in supramolecular chemistry.They have shown various applications in the sensing and analysis for the last few decades because of their facile modification,accessible conformation and unique molecular or ion recognitionability.However,molecular gels based on calixarenes are rarely reported.Calixarenes-based DCB gels have never been reported.To expand the applications of the calixarene derivatives,it is necessary to develop and synthesize new calixarene derivatives-based molecular gels and DCB gels.Based on literature study on DCB gels and calixarene-based molecular gels as well as the research work conducted in our lab in the fields of molecular gels and gel-emulsions,low-molecular mass gelators(LMMGs)and building blocks for DCB gels based on calix[4]arene derivatives were designed and prepared in this dissertation.New W/O gel-emulsions and molecular gels with high mechanical strength and smart thixotropic properties have been produced by using calix[4]arene derivatives.Meanwhile,dynamic acylhydrazone bond-based gels have been successfully developed.The gels show unusual mechanical properties to resist slicing,sustain high compression and withstand extensive stretching.They also have the abilities of self-healing and temperature/pH-responsiveness.This dissertation mainly includes the following two parts.In the first section,three calix[4]arene derivatives(CMA,CDA and CTA)appended with one,two or three carboxyl acid structures were synthesized.The gelation behaviors of the compounds and calix[4]arene itself were evaluated in common organic solvents,organoalkoxysilanes and monomeric liquids.It was revealed that compound with more carboxyl groups shows stronger gelation ability.However,the as created gels,in particular the CTA/trimethoxyphenylsilane(PTMS)gel,exhibit superior mechanical strength with a storage modulus(G')greater than 1.9 × 106 Pa and a yield stress exceeding 3,600 Pa at a concentration of 6.0%(w/v),showing not only thermo-reversible phase transition properties,but also fast and reversible thixotropic properties at room temperature.Moreover,the gel could be used as substrate for sensing film fabrication,injection molding and melting-free deposition molding.The objects from the molding and fabrication could be turned into permanent structures through further hydrolysis and condensation reactions.It is believed that the LMMGs based organoalkoxysilane gels have the potential to be used as smart materials for 3D printing and pre-cursors to a functionality-oriented solid matrix.Further studies demonstrated that the mixtures of monomeric liquids and water were effectively emulsified and gelled with introduction of CTA.The as produced gel-emulsions could be further used as templates for tunable preparation of porous monoliths.Preliminary test showed that these monoliths are good adsorbents of heavy metal ions such as Cr(?),and organic liquids such as benzene.Moreover,the materials can be reused via simple washing and drying process and have potential application in water purification.Based on the results of the first work,it is realized that calix[4]arene derivatives with four functional groups are favorable for gel networks.However,the inherently weak nature of non-covalent interactions may cause molecular gels have limited lifetimes and poor mechanical properties.DCB are intrinsically robust and reversible,it reasonable to predict that calix[4]arene-based DCB gels would be mechanically stable and responsive to external stimuli.Thus,we have designed and synthesized a calix[4]arene derivative bearing four hydrazide moieties(CTH)(serve as cross-linking points)and five linear aldehyde-terminated polyethylene glycols(PEG-CHOs)with different average molecular weight(serve as long arm cross-linker)to prepare dynamic acylhydrazone bond gels.The gelation behaviors of CTH with the five PEG-CHOs(nCTH:nPEG-cHOs = 1:2)in the mixture solvent of ethanol/water(3/1,v/v)were studied.It was found that higher CTH concentration of solution is beneficial for gel formation.The evolution of intrinsic viscoelastic properties of the dynamic crosslinking process of above five gel systems were tracked by optical microrheology.Gel G4208(30%,w/v)prepared from CTH and PEG4208(Mn?4208)exhibit excellent compressive properties whose compressive stress is as high as 6.41 MPa with a fracture strain of 88.21%.As is revealed by SEM images,frozen-dried' G4208 showed highly interconnected and homogeneous porous network structures.FTIR measurements showed that the networks were built by the formation of acylhydrazone bonds.Moreover,the gel system has self-healing ability and the gel-sol transition of the G4208 triggered by pH or temperature was reversible,which should be attributed to the formation and destruction of the dynamic acylhydrazone bond.Furthermore,a hydrogel,HG4208,was obtained from swollen of xerogel of G4208(naturally dried at room temperature)in pure water.Interestingly,the dynamic feature of acylhydrazone bond in HG4208 can be activated by the aniline(in ethanol atmosphere)and the self-healing process can be accelerated accordingly.Given the versatile properties,this novel calix[4]arene-based DCB gelsmight have potential applications in the fields of drug delivery,tissue engineering,electronic skin,flexible and stretchable energy storage devices.