Covalent Cross-linked Polymer Gels With Reversible Sol-Gel Transition And Self-healing Properties

Physical gels are susceptible to the external environment and easy to perform a sol-gel transition in response to temperature, pH and solvent, but their stability and mechanical strength are poor. Chemical gels have good stability and mechanical strength, but it is difficult for them to perform a sol-gel transition. Integrating the stimuli-responsibility of the physical gels and stability of the chemical gels into a single system to yield a robust but smart gel is difficult and is a frontier in the field of gel research. The application of dynamic covalent chemistry into polymer gels provides an possible method to overcome this difficulty. As one of the dynamic covalent bonds, acylhydrazone was adopted for the synthesis of polymer gels in this paper. Acylhydrazone is a covalent bond and it is stronger than molecular forces such as hydrogen bond. Gels which are cross-linked with this bond may have good stability and mechanical strength like chemical gels and can perform a sol-gel transition in response to the changing of pH because of the reversible of acylhydrazone at certain pH values. Furthmore, gels had the self-healing property.The main contents and results of this paper are as follows:1.Through esterification and nucleophilic substitution and hydrazinolysis of ester, acylhydrazine- terminated poly(ethylene oxide) was obtained with almost 100% degree of terminal benzoate functionality. Based on tris(hydroxymethyl)ethane, tris[(4-formylphenoxy) methyl]ethane was abtained through esterification and nucleophilic substitution.2.Dynamic covalent cross-linked poly(ethylene oxide) gels with good elasticity and mechanical properties were prepared by condensation of acylhydrazine-terminated poly(ethylene oxide) and tris[(4-formylphenoxy) methyl]ethane. Both of the concentration of gelators and the amount of catalyst had the effect on gel formation. With the concentration of gelators increased, the time for gel formation was decreased. Addition of acetic acid can dramatically shorten the time for gel formation.3.The reversible gel-sol transition was studied in detail.Experimental results indicated that the rate for forming of acylhydrazone was rapid than hydrolysis of acylhydrazone when pH>4 and the order was changed when pH<4.The gelator solution was in gel state when pH>4 and changed to sol state when pH<4. The gels had good stability and gels with gelator concentration of 12% can suffer more than eight reversible gel-sol phase transitions.4.It was reversible for the forming of acylhydrazone bond under mild conditions with acidic catalysis. The gels cross-linked by acylhydrazone were dynamers. When two plates of gels were transferred together, they can self-heal and merge together.