Study On Preparation And Properties Of Dual Physically Crosslinked Self-healing Agarose/Polyacrylic Acid-Fe³⁺ Hydrogels With High Toughness

Most of the organisms in nature are made of soft materials,the hydrogel is a kind of special soft material with three-dimensional network.As polymer material,because of the hydrophilic,biocompatible,soft and tough these properties,hydrogels have great application prospects in the human cartilage tissue material replacement areas.Because of its unique three-dimensional structure and high water retention,double-network hydrogels have attracted much attention in the field of biological tissue engineering applications.However,the traditional double-network hydrogels are crosslinked by chemical crosslinking,and the preparation process is complicated.The irreversibility of cross-linking and the poor self-repairing performance of chemical crosslinked hydrogels restrict its application in biological tissue engineering.Therefore,it is of great significance to prepare high strength hydrogels with self-repairing properties and to study the mechanical properties and the self-repairing properties.Based on the concept of dual physical cross-linked hydrogel,Agar was crosslinked by hydrogen bond to form the first network,under the ion complexation,polyvalent metal ions Ca2+/Al³⁺/Fe³⁺ cations were used to crosslink the PAAc molecular chains to form the second network.Agar/PAAc-Fe³⁺ double network?DN?hydrogels with high toughness and great self-healing properties were prepared by "one-pot" method.The mechanical properties and self-repairing properties of the Agar/PAAc-Fe³⁺ DN hydrogels were tested by a tensile tester and a rotational rheometer,then the damage mechanism and the self-healing mechanism were discussed referred to experimental results.The microscopic three-dimensional morphology of the Agar/PAAc-Fe³⁺ DN hydrogel was observed by the scanning electron microscopy?SEM?.Indications of the experimental results are as follows.The tensile strength 320.7 kPa and elongation at break 1130%of Agar/PAAc-Fe³⁺DN gels are 12 times and 9 times respectively those of Agar/PAAc-Ca2+ DN gels.The tensile strength 320.7 kPa of Agar/PAAc-Fe³⁺ DN gels is 5 times as large as that of Agar/PAAc-Al³⁺ DN gels,elongation ratios of them are the same.The dual physically cross-linked double-network structure can greatly improve the mechanical properties of hydrogels,all of polyvalent metal ions Ca2+/Al³⁺/Fe³⁺ can reinforce hydrogels,trivalent metal ions can form greater coordination??COO?3M?,and the three-dimensional structure of the polymer network crosslinked by the trivalent metal ions is more compact than that crosslinked by the divalent metal ions,so the trivalent metal ions have better enhancement effect than the divalent metal ions.Compared with Ca³⁺ and Fe³⁺,the ionic radius of Fe³⁺ is 6.45 A which is larger than that of the Al³⁺ 5.35A,then the charge effect of Fe³⁺ is stronger than those of Ca2+ and Al³⁺,so Agar/PAAc-Fe³⁺ has the better mechanical properties.The damage behavior of Agar/PAAc-Fe³⁺ DN hydrogel conforms to the principle of "sacrificial bond".In the process of stretching from small strain to large strain,the internal damage behavior of hydrogels can be divided into three phases:?i?destruction of the Agar networks,?ii?destruction of the PAAc networks,and?iii?Agar network fragments are further broken into "small clusters" and scattered in the PAAc network.The damaged Agar/PAAc-Fe³⁺ DN hydrogel were processed in 50? environment for 15 mins,and the repair rate can reach to 100%.With the increase of processing time,crosslinking points were not only formed between Agar network fragments and the PAAc molecular chains,but also formed among Agar clusters under the influence of the temperature environment,the reconstruction of hydrogen bonds between Agar network fragments increased cross-linking density,which is beneficial to the recovery of mechanical properties.High temperature environment is conducive to the movement of Fe³⁺,which accelerates the reconstruction of ion complexation bonds to a certain extent,the high efficiency repairing and structural optimization of dual networks make the mechanical performance of initial sample is better than that of the repaired sample.Compared with the Fe³⁺ ion environment,Agar/PAAc-Fe³⁺ DN hydrogel has a faster self-repairing efficiency at 50 ?.Agar/PAAc-Fe³⁺ DN hydrogel has excellent dynamic self-repairing properties.When the microstructure of Agar/PAAc-Fe³⁺ DN hydrogel was destroyed,it can be self-repaired in a short time and the repair rate can reach 90%.The dual physically crosslinked Agar/PAAc-Fe³⁺ DN hydrogel has great self-healing properties,without adding any chemicals and external stimuli,the cutting-off sample can be self-healed to be intact and exhibit great mechanical properties.The dual physically crosslinked Agar/PAAc-Fe³⁺ DN hydrogel is pH-sensitive,and the swelling degree of the hydrogel increased with the increase of the pH value?in the range of pH=3?13?of solution,the mechanical performance of the hydrogel became worse with the increase of the pH value.