Preparation And Characterization Of Natural Polymer Reinforced Hydrogels

Hydrogel is a kind of hydrophilic polymer with three-dimensional network structure.Due to the high cross-linking density of the whole network,the polymer can be highly swollen in water without dissolving,and maintain a certain shape.Those characteristics make hydrogels widely used in food,chemical industry,medicine,flexible devices and other fields.However,practical use of hydrogels also confronts with many problems,such as poor stability,non-degradability and weak mechanical properties.Improving the mechanical properties of hydrogels are one of the most important issues of hydrogels.Preparing composite hydrogels with markedly improved strength has become an extensive research area.Natural polymers are one of the most widely used materials for the preparation of composite hydrogels due to their sustainable ability,non-toxicity,biodegradability and abundant in resources.In this paper,surface-modified microcrystalline cellulose,alginate,chitosan and chitosan fiber were used as raw materials to prepare a variety of high strength composite hydrogels.The composition,structure and properties of the composite hydrogels were characterized and analyzed in detail.The results show the introduction of natural polymer could greatly improve the mechanical properties of composite hydrogels...The research contents are summarized as followsMicrocrystalline cellulose surface-Modified with acrylamide for reinforcement of hydrogelsIn this part,microcrystalline cellulose particles(MCC)were modified with acrylamide under microwave irradiation,and the product was named as acrylamide-modified MCC(AM-MCC).The successful grafting of AM on MCC was verified by Fourier transform infrared spectroscopy,and the grafting ratio was determined by elemental analysis.The functional groups or polyacrylamide(PAM)chains modified on the surface of the particles facilitated their even dispersion in water.So,the AM-MCC particles were then dispersed in aqueous solution of monomer of N,N-dimethylacrylamide(DMAA),which was later polymerized to prepare a poly(N,N-dimethylacrylamide)(PDMAA)-based composite hydrogel.The microstructure of the composite hydrogel was carefully studied with Fourier transform infrared spectroscopy and scanning electron microscopy.The mechanical properties of the composite hydrogel were investigated through compressive and tensile tests.The results indicate great improvement in the mechanical properties upon addition of the AM-MCC particles for the composite hydrogel.And when the AM-MCC loading content was 5.6%,the composite hydrogels exhibited the optimal mechanical properties,the compressive and tensile strength improved 1.8 and 2.2 times,respectively,compared with pure PDMAA hydrogel.The analyses suggest that the evenly distributed AM-MCC particles can act as a physical cross-linker to connect the neighboring polymer chains to strengthen the composite hydrogel,on the basis of their strong interactions,including hydrogen bonding and chain entanglements.However,over-loading(more than 5.6 wt%)of the AM-MCC would lead to aggregation of the AM-MCC particles to destroy the uniform microstructure of the composite hydrogel,resulting in a reduction of the reinforcement effect.Moreover,the recoverability,cyclic,and swelling behaviors have been measured to indicate potential applications of the composite hydrogels.Therefore,this work provides an inexpensive but active filler of AM-MCC,which can be well-dispersed in water media and is suitable to reinforce hydrogels.Preparation and characterization of AM-MCC reinforced PAA self-healing composite hydrogelIn this part,PAA/AM-MCC/Fe3+ composite hydrogel was prepared by one-pot method.The introduced AM-MCC particles not only exhibit traditional filler enhancement effect,and numerous H-bonds would form between the PAM polymer chains on AM-MCC surface and PAA polymer of gel matrix,which would also contribute to the high mechanical strength of composite hydrogels.The DMA results show AM-MCC could significantly improve the elastic property of composite gel,but with little effects on its viscosity property.The results of mechanical tests show AM-MCC could improve the compression strength and tensile strength of composite hydrogel.Besides,the composite also exhibit self-healing ability because of the reversible multi-interactions within the gel,including H-bonds,metal-functional group interaction,etc.Among the prepared gels,Gel-3 show the most efficient self-healing efficiency,its breaking elongation and strength could reach 90%and 48%of its initial state,respectively,after self-healing at room temperature for 24 h.Apart from that,the composite hydrogels also show responsiveness to the concentration of surrounding ion.When the ion concentration is high,the PAA polymer chains in hydrogel matrix would form much more complex entanglement,which would help improve the effective chain density and mechanical strength of the composite gel.Therefore,the mechanical strength of composite gel could be controlled by controlling the surrounding ion concentration.Comparison of the mechanical behavior of composite hydrogel with interpenetrating and semi-interpenetrating network structureIn this part,hydrogels with IPN and semi-IPN microstructures were respectively prepared by incorporating natural polymers of sodium alginate(SA)and chitosan(CS)into PDMAA network,where the SA was thereafter cross-linked while the CS was not.The mechanical properties of the two types of hydrogels were carefully measured and compared.The results demonstrate that the mechanical strength can be markedly improved upon addition of the natural polymers,based on the interactions of the components.The toughening of the semi-IPN hydrogel results from the chain entanglement between the trapped CS and the PDMAA network.In addition to the chain entanglement,the presence of sacrificial ionic cross-linkings and the higher chain density may contribute to the much higher toughening effect for the IPN hydrogel of PDMAA/SA.The semi-IPN hydrogel displays satisfactory anti-fatigue property under all of the investigated strains,which is attributed to higher energy dissipation efficiency of semi-IPN network.Under relatively low strain,the IPN hydrogel is more or less stable.However,at higher strains such as 50%and 75%,the stress and toughness of the IPN hydrogel decrease obviously,and then reduce gradually.The sharp decrease in mechanical strength after the first compressing results from the un-recoverable destruction and/or deformation of the ionically cross-linked SA network,and the thereafter high level of strength but gradually decreasing may too be due to the dynamically and rapidly breaking and re-forming of the Ca2+-cross-linked SA network.Based on those findings,the stronger IPN hydrogel and the more stable semi-IPN hydrogel may find proper applications in specific fields.Self-reinforced chitosan film materialIn this part,chitosan fibers(CFs)were used as strengthen material to prepare CFs enhanced chitosan film.Because of the excellent consistency between CFs and chitosan film matrix,the composite films show uniform and smooth structure without phase separation.Moreover,the introduced CFs could largely improve the mechanical properties of chitosan film,its breaking strength and Young’s modulus improves by 2.9-folds and 3.1-folds,respectively.The enhancing effect of CFs even surpasses nanocellulose,chitin whisker,etc.Apart from that,the composite film is also composed of pure chitosan,indicating good anti-bacteria property,biocompatibility,biodegradability of the composite film and potential applications in food and package fileds.