Preparation And Properties Of Poly(N-isopropylacrylamide)/Poly(Acrylamide) Interpenetrating Network Hydrogels

In the past three decades,poly(N-isopropylacrylamide)(PNIPAAm) hydrogels have attracted many scientific interests as the advanced soft materials because of temperature-responsivity,resulting from phase transition at a lower critical solution temperature(LCST).However, conventional chemically cross-linking hydrogels(OR gels)are always brittle and exhibit weak mechanical properties,leading to limit their applications.Therefore,it is necessary to improve both the mechanical properties and response rate of PNIPAAm hydrogels for wider applications.So far,several strategies have been proposed in attempts to improve the response rate of PNIPAAm hydrogels,including introducing other monomer to modify the microstructure to form an interpenetrating network(IPN).An interpenetrating network is formed when a second hydrogel network is polymerized within a pre-polymerized hydrogel.The main advantages of IPN are that relatively dense hydrogel matrices could be generated which feature stiffer and tougher mechanical properties, more widely controllable physical properties.Also,the formation of IPN structure has been found to moderate the effect of environmental changes on PNIPAAm hydrogels because of its ability to restrict the equilibrium swelling of interpenetrating phases according to the elasticity(i.e., cross-linking density).Zhang et al.successfully synthesized PNIPAAm hydrogels with IPN structure which showed improved mechanical properties,response rate to temperature and sustained release of drugs.Recently,we have successfully prepared a new kind of PNIPAAm/clay nanocomposite hydrogels using Laponite XLS,one kind of clay modified by tetrasodium pyrophosphate(Na₂P₂O₇)(i.e.ionic dispersant)as inorganic cross-linker,which showed excellent mechanical properties compared to OR gels.It is found that with increasing clay content,mechanical properties of S-N gels become better,but the response behavior of those are gradually decreased.With respect to the above considerations,this paper is focus on the superb enhancement of response rate with the basis of certain mechanical property.The results are as follows:(1)Novel PNIPAAm/PAAm IPN hydrogels based on PNIPAAm cross-linked by inorganic clay and poly(acrylamide)(PAAm)cross-linked by organic cross-linker were prepared in situ by ultraviolet(UV) irradiation polymerization.The influences of IPN on the structure and properties of PNIPAAm hydrogels were investigated.The results showed that:this kind of IPN hydrogels are temperature-sensitive,and,compared with inorganically crosslinking PNIPAAm hydrogels,IPN hydrogels exhibit enhanced dynamic mechnical properties and ultrafast response rate.(2)The effects of clay content,NIPAAm monomer content,AAm concentration and the immersed time on the properties of IPN hydrogels, such as swelling/deswelling properties,dynamic mechanical properties, optical properties were systematically studied.It was discovered that the properties of this kind of IPN hydrogels could be controlled by modulating clay content or AAm concentration.(3)Considering the controllable response rate and excellent stability of IPN hydrogels,bovine from serum albumin(BSA)was used as model drug to study the controlled release from IPN hydrogels at two different temperatures,and the effects of clay content and AAm concentration on sustained release were researched in detail.It was revealed that BSA release could be controlled by modulating temperature,clay content and AAm concentration.This controllable response rate and drug release rate of PNIPAAm/PAAm IPN hydrogels would be quite useful to widen the applications,such as controlled drug release system.