Study Of Mechanism Of Physics Crosslinking In Nanocomposite Hydrogels

Environment-sensitive hydrogels have been attracted much attention for their potential application including drug delivery,enzyme immobilization and biomaterials separation and purification.However, the applications of conventional hydrogels prepared by chemical crosslink were restricted duo to their poor mechanical properties and low responsive rate.In recent years,considerable attention has been focused on polymer/clay nanocomposite hydrogels because the incorporation of layered silicate into hydrogels could improve their mechanical properties. Haraguchi’s group obtained poly(N-isopropylacrylamide)/clay nanocomposite hydrogels using an inorganic clay as a crosslinker in place of the conventional organic crosslinke.This kind of nanocomposite hydrogel exhibits high transparency,fast deswelling rate and extraordinary mechanical properties,which develop a new direction for the research of hydrogel.In this dissertation,several kinds of nanocomposite hydrogels crosslinked by inorganic clay were synthesized based on different monomers and initiators.The mechanism of forming the nanocomposite hydrogel was explored.In addition,polyelectrolytes were introduced into the poly(N-isopropylacrylamide)/clay nanocomposite hydrogels by Interpenetrating polymer network(IPN) technology,and the effect of the polyelectrolyte on the formation of the hydrogel was also studied.The main results are as follows:1.Adopting different monomers,the nanocomposite hydrogels were synthesized by using KPS as an initiator and clay as a crosslinker.The results show that the property of the monomer is a determining factor for the formation of nanocomposite hydrogels.The nonionic monomers,such as NIPA,could be polymerized to achieve nanocomposite hydrogels crosslinked by clay.The cation monomers including DMDAAC and AAc could not be formed nanocomposite hydrogels by using clay as a crosslinker.This is due to the fact that the clay platelets were seriously agglomerated in the initial solution by introducing of the cation monomers and could not act as a multifunctional crosslinker.On the other hand,KPS also played an important role in the formation of the nanocomposite hydrogel.The initiators were closely associated with the clay surfaces through ionic interactions and the free radical polymerization was initiated on the clay surface and thus the three-dimensional crosslinking network was formed.2.PVP/Clay nanocomposite hydrogel was prepared by free radical reaction using AIBN as an initiator and clay as a crosslinker.The structures of the hydrogels were characterized by means of TEM and XRD.The results show that the regular crystal structures of clay were destroyed and the clay platelets were exfoliated and dispersed randomly in the hydrogels,which acted as a multifunctional crosslinker in the hydrogels.The hydrogel had good light transmission and excellent mechanical properties.Therefore,it can be concluded that the polymerization reactions to form nanocomposite hydrogel crosslinked by clay can be initiated not only by KPS but also by AIBN.3.PDAm/Clay nanocomposite hydrogels were prepared by two diffferent initiators including KPS and AIBN.The structures and the morphology of the hydrogels were characterized.The effective crosslinking density and the chain molecular weight between crosslinks were calculated based on the theory of rubber elasticity.It was found that the effective crosslinking density of hydrogels which prepared by KPS was higher than that initiated by AIBN,whereas the molecular weight between crosslinks was lower.4.Several kinds of polyelectrolytes were introduced into the PNIPA/Clay nanocomposite hydrogels by IPN technology.The effect of polyelectrolyte on the formation of the hydrogel was also studied.The results indicate that the introducing of anionic polyelectrolyte has little influence on the dispersion of clay and the formation of the crosslinking network.The pH- and temperature-sensitive nanocomposite hydroogels can be obtained by IPN technology.In contrast,cationic polyelectrolyte would affect the dispersion of clay,and it is difficult to obtain the stable nanocomposite hydrogels.The IPN nanocomposite hydrogels exhibit good pH and temperature sensitivity,which can respond independently to both pH and temperature changes.Furthermore,the hydrogels have improved mechanical properties.