Preparation And Characterization Of Fast Responsive And Notch Insensitive Nanocomposite Hydrogels

In the present work, fast responsive nanocomposite cryogels (NC-C gels) with porousstructure and notch insensitive nanocomposite hydrogels (NC gels) were synthesized in theaqueous suspension of hectorite clay Laponite XLS. First, the NC-C gels with differentLaponite concentration CXLSwere prepared by freezing polymerization, in order to introducethe porous structure. Mechanical properties, deswelling and reswelling rates, and porousmorphology were studied referring to that of the normal NC gels. Second, the morehydrophilic monomer Acrylamide (AM) with smaller side group was chosen and performedin-situ copolymerization with N-isopropylacrylamide (NIPAm) to preparePoly(N-isopropylacrylamide)(PNIPAm)/Poly(acrylamide)(PAM) NC gels. The gels were cutwith different kinds of notch, then stretched to test the notch resistance. The trouser tear testwas also performed on the gels to calculate the fracture energy. The main contents and resultsare as follows:1. PNIPAm/Laponite NC-C gels were prepared by freezing polymerization at-18oC ofNIPAm monomer in the aqueous hectorite Laponite suspensions. The CXLSwas found to be akey factor determining characters of the NC-C gels, because the average interparticle distancebecame smaller than the clay diameter when CXLS>6w/v%. The NC-C gels with CXLS6w/v%were heterogeneous at the nascent state and became homogeneous after24h thawing,which endowed these cryogels self-improved mechanical strength and elongation up to900%.On the contrary, for the NC-C gels with CXLS>6w/v%, the heterogeneity cannot behomogenized after24h thawing, leading to a great reduction in the extensibility to the half ofthe corresponding normal NC gels. The deswelling and reswelling rate of the NC-C gels withCXLS>6w/v%was significantly accelerated due to the micro-porous structure formed by themolten ice crystals. This micro-porous was observed by SEM and CLSM from the cryogelswith the clay content higher than6w/v%. The extensibility of the NC-C gels was muchhigher than other cryogels with high deswelling rate, promising some possible applications.2. The swelling equilibrium ratio, phase transition temperature, notch stretching test, andtrouser tear test were studied on the PNIPAm/PAM NC gels. Because AM was morehydrophilic when compared with NIPAm, Swelling equilibrium ratio and phase transitiontemperature of PNIPAm/PAM NC gels increases with increasing AM concentration CAM.Phase transition temperature increases to40°C at the AM concentration of2w/v%. PNIPAm/PAM NC gels exhibit ultrahigh extensibility, and the elongation at break alsoincreases with increasing CAM. Notch stretching test was investigated on the PNIPAm NCgels and PNIPAm/PAM NC gels. The elongation at break of PNIPAm NC gels with notchdecreases to50%of that of un-notched samples. However, the addition of AM makes the NCgels insensitive to notch, and the elongation at break of notched samples with different notchlength almost maintain the same. The different structures between AM and NIPAM moleculesis the reason that PNIPAm/PAM NC gels become notch insensitive. With smaller side group,the polymer chain can become more flexible by copolymerizing AM into NC gels, whichmake the notch tip become blunting to suppress the stress concentration. The trouser tear testcan be changed into stretching automatically, and the tearing stroke is much larger than thesample tearing length. The fracture energy is about102J/m2,50times larger than that ofchemical crosslinked hydrogels. Thus, the temperature sensitive PNIPAm/PAM NC gelsprepared in this work shows notch insensitive properties and high fracture energy.