Molecular Dynamics Simulation Of The Fracture Properties Of Double-crosslinked Single-network Hydrogels

Hydrogels are soft materials composed of water and a three-dimensional polymer network.The cross-linked network in the network includes covalent chemical cross-linking or non-covalent physical cross-linking,which has high water content,good biocompatibility,strong plasticity and biodegradability,and also has very good mechanical properties.performance.It has important application prospects in the fields of drug delivery,tissue engineering,biosensors,and biomimetic smart materials.Good mechanical properties are the basis for materials to exert their various properties.However,due to the limitations of existing experimental methods and instrumental characterization methods,it is difficult to observe the evolution process of hydrogel microstructure,and the existing toughening mechanism needs to be supplemented.We use the method of coarse-grained molecular dynamics to further reveal the toughening mechanism of hydrogels through the changes of the microscopic network structure of the hydrogels during the tensile fracture process.Therefore,we have carried out the following two targeted works:(1)Molecular dynamics simulation of fracture properties of physicochemical double-crosslinked single-network hydrogels based on noncovalent and covalent bondsWe simulate the chemical cross-linking network by random cross-linking of covalent bonds,and simulate the physical cross-linking network by adjusting the interaction strength between the physical cross-linking points.Influence of interaction strength between crosslinks on gel fracture properties.We found that this synergy works best at intermediate physicochemical network ratios.At the same time,The larger the proportion of chemical cross-linking networks,the greater the degree of bond breaking of the cross-linking network,and the material showed a certain degree of brittleness.For the physical network,the physically cross-linked clusters are broken and reorganized during the stretching process,which consumes more energy and increases the fracture energy.Through stress decomposition,it is found that at small strains,the stress is mainly borne by the physical network,while at large strains,the stress is mainly borne by the chemical network,which indicates the synergistic effect of the two in enhancing the mechanical properties of the hydrogel.Finally,the physical network separates the weak areas and prevents small holes from merging during the stretching process,so that the number of holes is more and more dispersed,which effectively disperses stress and prevents stress concentration.(2)Molecular dynamics simulation of the fracture properties of physical double-network hydrogels based on non-covalent bondsThis work constructs a first network with weak interactions and a second network with strong interactions,and explores the toughening performance and internal mechanism of the dual-network system compared with the singlenetwork system;the first network crosslinks interact with each other.Influence of strength and proportion of first network crosslink density on gel fracture properties.The results show that compared with the single-crosslinked network system,the maximum stress,elongation at break and breaking energy of the dual network system are significantly improved,which is due to the interpenetration between the two networks,which enhances the degree of entanglement of molecular chains.,the entanglement points can also act as physical cross-linking points,thus improving the resistance to deformation by external forces.At the same time,the stronger the physical cross-linked network of interaction energy,the more complete the polymer network is,which is conducive to stress transfer,and at the same time increases the energy value dissipated by the polymer network through molecular chain slip and disentanglement.We found that an excessively high proportion of strongly interacting second networks will bring negative effects,and the attraction between networks will gradually merge the second network clusters,which is not conducive to the energy dissipation of the second cross-linked network.The fracture energy is the largest when the proportion of the first network is 70%,because the second network with low crosslink density participates in energy dissipation while maintaining the integrity of the polymer network,and the two mechanisms reach an optimal equilibrium state,so as to have strong mechanical properties.