Study On The Relationship Between Structure And Properties Of Toughening Polyacrylamides/Chitosan Hydrogel

As a novel soft and wet material,hydrogels possess the character of crosslinked 3D network structure along with high water content?over 50 wt%?,making them promising potential applications in tissue engineering,drug release,environmental engineering,and flexible electronic devices.However,the conventional chemically-crosslinked hydrogels are generally weak and brittle due to the inhomogeneous crosslinked network and lack of energy dissipation mechanism,which hardly meets the requirement of mechanical properties in practical applications.Therefore,effective strategies have been highly desired to achieve the goal of tough and strong hydrogels.To address this issue,proposed strategies based on novel crosslinker,physical crystalline structure,nano-reinforcement and ionic coordination interaction have been demonstrated to realize the enhancement on the mechanical performance of polyacrylamide/chitosan hydrogels.A series of tough hydrogels can be achieved by combining the features of different strategies,and explored the related application in the field of strain sensor.The detailed investigation has been listed as follows.The limit of existing crosslinkers in hydrogel: few functional groups and invariable crosslinked method.Herein,hyperbranched polysiloxane?HSi?molecules with tunable molecular structure were synthesized via hydrolysis reaction and served as a novel crosslinker for fabricating polyacrylamide/chitosan?PCH?hydrogels witi excellent mechanical properties.HSi can be chemically bonded with polyacrylamide and chitosan chains,simultaneously,which is benefiting from the structure of bi-functional crosslinker?vinyl and epoxy group?.Meanwhile,a tensile strength of 302 k Pa,elongation at break of 2263%,and toughness of 3.85 MJ·m^-3 can be achieved by optimizing the molecular structure?i.e.ratio of vinyl/epoxy = 0.68?and content?1.0 vol%?of HSi with bi-functional group.To endow PCH hydrogel with higher mechanical performance,we reported a strategy to fabricate polyacrylamide/chitosan/montmorillonite nanocomposite hydrogels by simultaneously introducing lamellar montmorillonite and chitosan microcrystalline structure.It is found that benefiting from the nano-reinforcement?montmorillonite?and “sacrificed bonds”?chitosan microcrystalline structure?,polymer network structure can be strengthened effectively,thus facilitating in the formation of efficiently energy dissipation mechanism.More importantly,the mechanical performance of composite hydrogels can be modulated by changing the content of montmorillonite and degree of chitosan microcrystalline structure.As a result,the satisfactory mechanical properties of the optimal nanocomposite hydrogels were achieved at a relative high water content?80 wt%?,including a superior tensile strength of 1.91 MPa,high tensile strain of 1005% and exceptionally great toughness of 14.16 MJ·m^-3,respectively.Compared with virgin hydrogel,the increase of tensile strength and toughness is 11.7 and 3.7 times,respecitively.To acquire mechanically robust and balanced PCH hydrogel,a nanocomposite hydrogel developed by integrating chitosan decorated halloysite nanotubes?CS-f-HNTs?into dual cross-linked structure composed of chemical cross-linked network and Fe³⁺ induced ionically cross-linked network has been fabricated.The mechanical properties of nanocomposite hydrogels can be regulated by adjusting the CS-f-HNTs content,type of multivalent cations and length of soaking time.Combining the nanoparticle reinforcement with physical interactions including hydrogen bond among polymer chains and ionic coordination interaction between Fe³⁺ ions and functional groups on chitosan molecules and the copolymer chains,the hydrogel exhibits extraordinary and balanced mechanical performance,including high strength?3.06 MPa?,outstanding stretchability?2015%?and superior toughness(47.6 MJ?m^-3)in which water content remains ?80 wt%.Importantly,the increase of tensile strength and toughness is 14.5 and 87.7 times,respecitively when compared with controlled hydrogel,Moreover,the hydrogels also possessed remarkable self-recoverability and excellent anti-fatigue property.On the basis of tough and strong PCH hydrogels,we report a facile and effective strategy for fabricating mechanically robust and electrically conductive nanocomposite hydrogels via incorporating chitosan in-situ grafted magnetite nanoparticles combined with ferric ion.By controlling the f-Fe₃O₄ content and molar fraction of PAAc in copolymer chains,mechanical properties of nanocomposite hydrogel can be regulated based on the experimental results.The obtained nanocomposite hydrogel delivers a remarkable mechanical strength up to 2.33 MPa,excellent tensile strain of 1158% and high toughness of 18.18 MJ?m^-3 even at a relatively high water content?80 wt%?.In addition,the resultant nanocomposite hydrogel exhibits sensitive strain-induced resistance change under both compressive and tensile strain as well as outstanding stability and repeatability,which can accurately and repeatedly monitor both large mechanical deformation?e.g.tensile strain up to 600%?and human behaviors?e.g.,motions of joints and facial expressions?.