Preparation Of PH-sensitive Hydrogels And Their Ultrafast Structural Dynamics Studies

Gel materials are widely used in environmental control,advanced materials and precision medicine because of their special properties in liquids and solids(sensitive response to external stimuli actions,and a small amount of additives can significantly change the properties of materials).Hydrogel materials are also popular in the study of soft materials due to their special high water content(up to 90%)and environmental friendliness.In this dissertation,2-(dimethylamino)ethyl methacrylate/acrylic acid copolymerization hydrogel(the following referred to DMAEMA/AA copolymer hydrogel)was prepared by aqueous solution polymerization,and their pH stimulated responsiveness was studied.The synthesis process of DMAEMA/AA copolymer hydrogel was studied.And the ultrafast response of the three-dimensional network of hydrogel and the ultrafast structural dynamics at the molecular level under the environmental(pH)stimulation were studied by infrared spectroscopy and ultrafast multiple-dimensional vibrational spectroscopy.The main research contents and results are as follows:1.DMAEMA/AA copolymer hydrogels were synthesized in aqueous solution using ammonium persulfate as redox initiator and N,N-Methylene bisacrylamide as crosslinking agent.The equilibrium swelling degree(ESR)of gel in deionized water was taken as the main index,and the effects of dosage of initiator,crosslinking agent,monomer ratio and monomer mass percentage on the swelling properties of hydrogels were investigated.The results showed that the optimized conditions for the synthesis of the hydrogel were as follows:the initiator dosage was 0.4%of the total monomer mass,the amount of crosslinking agent was 1.4%of the total monomer mass,mass ratio of monomers DMAEMA:AA=4:1,monomer mass percentage was 30%.The dynamical swelling behaviors of DMAEMA/AA copolymer hydrogel and its environmental stimulus responsibility of pH were studied.When pH was 7,the hydrogel had the smallest equilibrium swelling degree,swelling rate factor and kinetic factor.When pH<7,the above properties increased with pH decreasing;when pH>7,the above properties increased with the increase of pH.We studied the pH stimuli response of storage modulus of DMAEMA/AA copolymer hydrogels.When the medium pH was less than 7 and greater than 9,storage modulus increased,while the gel storage modulus changed little from 7 to 9.2.The DMAEMA/AA copolymer hydrogel with a certain amount of NaSCN was synthesized by deuterated water instead of deionized water in the same way.SCN-ion was used as probe to study the molecular dynamics of the probe ion in the DMAEMA/AA copolymer hydrogel system under different pH environments.The weak interaction in soft materials is revealed from the molecular micro level.The results of vibration population relaxation shows that when pH<7,the vibration life of antisymmetric stretch of SCN-becomes shorter and the vibration population relaxation becomes faster.When pH>7,the vibration life of antisymmetric stretch of SCN-first becomes shorter,the vibration population relaxation becomes faster,then the vibration life becomes longer,and the vibration population relaxation becomes slower.The results of rotational dynamics of SCN-shows that the rotational time constant of the slow component increases with the decrease of pH when pH is less than 7,and increases with the increase of pH when pH is more than 7.When pH=7,the rotational dynamics of SCN-is fastest.The degree of equilibrium swelling of pH sensitive hydrogel varies with the pH due to the influence of the special group(amino group,carboxyl group)on the polymer network structure in different pH environments.Although previous scientists have done a lot of research on various types of pH sensitive hydrogels and synthesized many types of pH sensitive hydrogels,their pH response mechanisms,especially from microscopic and molecular dynamics structures,have rarely been reported.The purpose of this study is to provide useful structural and kinetic theories for designing better environmental responsive hydrogels and pH sensitive hydrogels.