Design Of Thermally Stiffened Hydrogels And Analysis Of Their Behavior

Thermoresponsive hydrogel is a kind of intelligent material that can change their structural and physical properties with variations in ambient temperature.The traditionally thermoresponsive hydrogels show the weak mechanical behavior due to their structure inhomogenity or lack of the energy dissipation mechanism under the deformation,and thus they are applied in the field of drug delivery,tissue engineering,and switchable window and so on.Recently,with the great demand in the engineering area,such as actuator,shape memory materials and protective device,they require not only their high mechanical strength but also the high mechanical difference before and after stimulation.Therefore,a strategy to design thermally stiffened hydrogels by using different polymer structures,and thermoresponsive groups attracts much attention and exhibits the research highlights recently.To expend the application of hydrogels,following challenges need to be addressed:(i)A simple way to fabricate thermally stiffened hydrogels with high strength and stiffness.Tough and strong the thermally stiffened hydrogels with high toughness and strength have a great potential in the engineering area.The reported thermally stiffened hydrogels are prepared based on the polymer brush method,where the thermoresponsive group are attached to the main polymer chains.While,such synthesis method is very complicated and not suitable for the large-scale-production.Furtherly,the obtained thermally stiffened hydrogels often show the low elastic modulus.(ii)The unstable structure and mechanical behavior in saline solutions.The other reported thermally stiffened hydrogels are developed based on the cooperative effects of hydrophobic interaction and ionic interaction,and these hydrogels become mechanically weak and lost the thermally stiffened features when they are immersed in saline solutions.To solve the issues mentioned above,we did the following investigations:(i)A novel thermally stiffened hydrogel was designed by the randomly copolymerization of the thermoresponseive monomer from the amino acid derivatives and acrylamide.These hydrogels not only show the high toughness(～22.3 MJ/m~3),strength(～2.5MPa),and extremely high stretchability(～2050%),but also mechanically stable in saline solutions.The high toughness of the hydrogels is comparable with the tough double-network hydrogels,nature rubbers and cartilages.(ii)The hydrogels exhibit the super high mechanical transition behavior in strength,toughness and stiffness by up to 4-,2-,and 13-folds,respectively,when the thermal treatment was applied or not,even the hydrogels show the high stiffness(～22MPa).This work provides a pathway for developing thermally stiffened materials and may be applied in the engineering field which requires the hydrogels with high stiffness and high mechanical difference before and after thermal treatment.(iii)The results show that the thermally stiffened effect is from the cooperative effects of hydrophobic interaction and hydrogen bonding interaction,which induced the structural phase separation during the thermal treatment,and thus enhance their mechanical behavior.