Synthesis,Characterization And Application Of Nvoel Stimuli-sensitive Smart Hydrogels

In the field of smart polymer materials, environment sensitive hydrogels based on themonomer N-isopropylacrylamide(NIPA) is becoming a research focus. There existsimultaneously hydrophobic and hydrophilic groups in the chemical structure of PNIPAhydrogels which exhibit a sharp volume phase transition in water at about 33℃ and this turningtemperature is called Lower Critical Solution Temperature(LCST). The response rate ofvolume phase transition is one of the most important parameters for appreciating hydrogels'properties. However, hydrogels prepared through conventional methods respond to externalstimuli with a slow rate. Since the rapid response rate is the most essential function for theirapplications including artificial muscle and bio-sensor, the response rate has to be improved. In the first part of this dissertation, PNIPA/porogent hybrids were prepared using SiO2with two different diameters(0.1mm,3μm) as porogents. Porous PNIPA hydrogels wereobtained through just immersing hybrids in the hydrofluoric acid solution for a period of timewithout other treatments. LCST as phase transition temperature of the series of poroushydrogels were determined all at about 33℃. Porous structures with different degree wereobserved via SEM. The effect of each reaction factor on the properties about swelling wasinvestigated and found that the type and amount of porogent displayed a strong influence on theswelling ratio (SR) of porous hydrogels and porous hydrogels had much faster deswelling andreswelling rate compared to conventional ones.Thermal-and pH-sensitive porous P(NIPA-co-AA) hydrogels were synthesized throughintroducing ionizable group at the presence of SiO2 (3μm) as porogents. The re-swellingkinetics of copolymer hydrogels previously soaked under different acidic and neutralconditions were studied. Although different pre-swelling history exhibited no effect on theequilbrium swelling ratio with same composition, the neutral re-swelling kinetics with acidicpre-swelling corresponded to neither first nor second-order kinetics. Mechanically speaking,the sigmoidal kinetics derived from this series of hydrogels might be the result of severalconsecutive and/or simultaneous processes: namely, one that follows a first-order kinetics witha very small rate constant;the second one, an overall auto-catalytic process with its two rateconstants. However, acidic re-swelling kinetics with neutral pre-swelling exhibited an obviousover-shooting effect featured a whole process which included two swelling-deswellingprocesses. It was demonstrated that the re-swelling kinetics with opposite pre-swelling historyboth attributed to the function of hydrogen bonds and one is dynamic disruption of hydrogenbond, and the other is dynamic formation of hydrogen bond.In the last section of this thesis, water-soluble macromolecules poly(ethylene glycol)(PEG)with different molecular weight as a kind of general porogent were used to successfully prepareporous P(NIPA-co-AA) hydrogels. The aggregated state of PEGs was investigated throughLLS so as to explorer their pore-forming mechanism and PEG-6000 was selected as the mostsuitable porogent based on the research above. It was found that PEG-6000 couldn't be totallywashed out only through long time immersion of hydrogel/porogent hybrid in deionized water.Based on the characterization results derived from LLS, the hydrogel network could be furtherenlarged through immersion in pH=9 alkaline buffer solution so that water molecules couldenter into hydrogel to wash out residual porogent. Regular porous structure could be foundfrom the SEM observation. It was displayed from the investigation of hydrogel's thermal-andpH-sensitive deswelling that its pH-sensitive deswelling was similar to a pulsed release process.This process was confirmed in the preliminary study of drug controlled release and this type ofhydrogel have a potential application in some special drug release.