Methods For Preparing And Study On Mechanical Behaviors Of Double Network Hydrogels

The main structure and organisms of creatures in nature such as plants and animals are soft materials. And one special and important kind of soft materials are hydrogels, which are composed of three-dimensional polymer networks filled with large amount water in the matrix. Owing to the similarity to soft bio-tissue and typical stimuli-response capacity, hydrogels show promising applications in drug delivery, tissues engineering, actuators, etc. However, common hydrogels are mechanically too soft and brittle, which hinders the extensive use of hydrogels. So far, hydrogels have been limited to useages where their mechanical properties are not highly concerned, such as drug delivery and water absorption. Although hydrogels have similarity with bio-tissues and organs, it is still a critical issue to fill the gap between synthetic and biological hydrogels, and to expand their applications in load-bearing systems. Therefore, it is significant to study the mechanical behaviors of hydrogels and improve the mechanical properties.Based on continuum mechanics, thermodynamics and polymer theory, firstly, we synthesied the Alginate/PAAm double network (DN) hydrogels as well as Alginate single network hydrogels cross-linked by ionic bond, and PAAm single network hydrogels cross-linked by covalent bond. We investigated the mechanical behaviors of DN hydrogels, Alginate hydrogels and PAAm hydrogels by conducting a series of uniaxial compression test during the temperature period from 24 ℃ to-50 ℃. The results indicates that the DN gels show excellent mechanical properties at room temperature, compared with other conventional hydrogels. When the temperature decreased to-6 or-10 ℃, the gel became whitish, accompanying with quick increase in Young’s modulus. However, the gel still show similar mechanical behaviors as those at room temperature. As the temperature went to-25 ℃, the cryo-hydrogel became extremely hard and tough. Furthermore, the cryo-gel started to show a yeilding behavior during the compression test. When the temperature decreased further to-50 ℃, the yeilding behavior disppeared and the frozen gel became brittle. These results indicated that incorporating a small portion of polymer network into ice led to a composite material with mechanical properties much better than those of each component. Based on these properties, the application of hydrogels may be expanded. On the other side, by mimicking the structure of articular cartilage, we prepared elastic fiber-reinforced hydrogels and investigates the mechanical behaviors of thin made of this kind of composite. The results indicated that the oriented fibers imbedded in the hydrogels will buckle when the stretch reaches the critical value. We performed theretical analysis and obtain the critical stretch.The DN hydrogels containing nearly 90 wt% water are plastic and highly extensile. Meanwhile, the frozen hydrogels are ductile enough to withstand high compressions, as well as high deformations such as bending, twisting, knotting and extensive shaping. These new results on hydrogels at low temperatures may expand the scope of hydrogels potential application. And the studies on buckling behaviors of fibers in the fiber-reinforced hydrogels are useful for the design and properties optimization of the fiber-reinforced hydrogel composites.