Study On Structure And Mechanical Properties Of HA/Chitosan Composite Hydrogels

Bio-based polyelectrolyte complex hydrogels are formed by mixing the oppositely charged biopolymers.They are the most promising materials for drug release,tissue engineering and electronic devices due to their unique amphiphilic structure,good biocompatibility and biodegradability.In comparison to synthetic polyelectrolytes polymers,bio-based polyelectrolyte complex hydrogels made from charged biopolymers with an ultra-high molecular weight and a long persistence length exhibited special mechanical features,such as elastic-plastic transformation behavior.Most works focused on the synthetic polyelectrolyte complex hydrogels,whereas there have been relatively few reports on the relation between structural and mechanical performance of natural polyelectrolyte complex hydrogels.In this study,carbohydrate polymers including negatively charged sodium hyaluronate(HA)and positively charged chitosan were used to create physically cross-linked and dual physicalchemical cross-linked HA/Chitosan hydrogels.The dynamic behavior,crack propagation behavior and thermal rheological behavior of bio-based polyelectrolyte complex HA/Chitosan hydrogels were systematically investigated.The following is a summary of the main works:(1)Dynamic behavior of hydrogels.The effects of salt concentration and ambient temperature on the structure and mechanical behaviors of physically crosslinked hydrogels were systematically investigated.The Young’s modulus and yield stress of hydrogel decreased from 148 k Pa to 98 k Pa,and 41 k Pa to 23 k Pa with increasing of temperature from 25 ℃to 75 ℃,respectively.And increasing of salt concentration from 0 to 0.1 mol/L results in the reduced yield stress from 41 k Pa to 20 k Pa.The topology of hydrogel network did not change with temperature,but exhibited a remarkable salt concentration dependence,which caused the disentanglement of polymer chains in the gel network.Dynamic light scattering and rheological analysis suggest that the chain aggregations are physical crosslinked by surrounded semi-flexible chains,and their diffusion is highly hindered by the topological entanglements and ionic associations.Both the relaxation behavior of polymer chains in the gel network and the diffusion behavior of molecular chain aggregates had Arrhenius type dependence,and the flow activation energy was similar(～ 47 k J/mol).The synergistic effect of aggregations diffusion and chain dynamics causes the slow macroscopic stress relaxation behavior of hydrogels before yielding,independent of applied strain.After yielding,the amplitude of strain accelerates the stress relaxation,resulting in chain disentanglements and slipping.(2)Crack propagation behavior of hydrogels.By altering the stretch velocity,we first examined the correlation between the crack propagation speed and fracture toughness of the notched physically crosslinked hydrogel and dual cross-linked hydrogels.It was found that viscous energy dissipation dominated the fracture energy of the physically crosslinked HA/Chitosan hydrogel,whereas the viscoelastic energy dissipation around the crack tip dominated the fracture energy of the dual cross-linked hydrogels,which was different from synthetic polyelectrolyte hydrogels.Secondly,we performed the fracture test by stretching the sample to the described strain,followed by adding a pre-crack length,and investigated the impact of interactions between the polymer chains on the fracture behavior of the dual cross-linked hydrogels.The interaction between polymer chains was adjusted by the change of salt and tannic acid(TA)concentration,whose effect on the crack propagation of hydrogels was systematic explored.Salt can reduce the interaction between polymer chains,resulting in the decrease of mechanical strength of hydrogels.The fracture stress decreased from 500 k Pa to 200 k Pa after equilibrated them in 0.2 mol/L salt solution.However,TA can enhance the interaction between polymer chains,leading to the increasing of mechanical performance.The fracture stress of hydrogel after equilibrated them in 1 wt% TA solution was enhanced from 500 k Pa to 1.8 MPa.Two regions,“fast mode” and “slow mode” were observed in the crack velocity dependency of the energy release rate.In the "fast mode" region,the increasing of chain interaction will substantially reduce the crack growth rate.In the "slow mode" region,the critical strain energy release rate increase with the chain interaction,whereas the crack growth rate is unaffected by it.(3)Thermorheological behavior of hydrogels.The linear rheological behavior,and the tensile mechanical behavior of dual cross-linked hydrogel under various temperature and strain rate were systematically explored.We observed that the time-temperature superposition principle is obeyed not only for the small strain rheology but also for the nonlinear fracture properties.The two processes possessed the same shift factors.Furthermore,it was discovered that such method works for a variety of dynamic hydrogel systems(including those with ionic and hydrogen bonds),regardless of the nature of the dynamic bonds or the stiffness of the main chains.