Preparation And Properties Of Double Network Hydrogels With Molecular Stent

Hydrogels with favorable property is considered to be the most promising natural cartilage tissue replacement materials. Natural cartilage tissue can withstand high pressure and has ultra-low frictional coefficient, so, the research of toughening and sliding friction of hydrogels have great significance. In this work, we synthesize double network hydrogels comprising identical unit with linear polyelectrolyte: poly(acrylamide)/poly(2-acrylamide-2-methylpropane-sulfonic acid) as molecular stent/poly(acrylamide)(PAAm/St-PAMPS/PAAm) hydrogels and poly(acrylamide)/poly(acrylamide)/poly(2-acrylamide-2-methylpropane-sulfonic acid) as molecular stent(PAAm/PAAm/St-PAMPS) hydrogels. It is discussed that how the orders of introducing linear polyelectrolyte affects properties of PAAm/PAAm hydrogels and the fracture mechanism and microstructure of these three kinds of double network hydrogel. The sliding friction of PAAm/St-PAMPS/PAAm hydrogels in different normal pressure, solution environment and substrates was investigated.Because introducing linear polyelectrolyte of PAMPS leads to the osmotic pressure increase, the swelling degrees of PAAm/St-PAMPS/PAAm hydrogel, and PAAm/PAAm/St-PAMPS hydrogel with linear polyelectrolyte was 4 times and 1.5 times, respectively, as that of PAAm/PAAm hydrogel without linear polyelectrolyte. The PAAm/St-PAMPS/PAAm hydrogel had higher failure strain and failure stress than that of PAAm/PAAm hydrogel and PAAm/PAAm/St-PAMPS hydrogel. It could be attributed to the higher efficiency of energy dissipation of PAAm/St-PAMPS/PAAm hydrogel. Through cyclic tensile measurements, we can find the first network of PAAm/St-PAMPS/PAAm hydrogel was broken first, and energy was dissipated. Then, with an increase of strain, the second network was gradually broken. But the deformation of the double network in PAAm/PAAm/St-PAMPS and PAAm/PAAm hydrogel was homogeneous until hydrogels were totally broken. Besides, the possible physical entanglement points which may be formed by linear molecules of PAMPS that exist in the PAAm/PAAm/St-PAMPS hydrogel may lead to extra reinforcement effect as compared to PAAm/PAAm hydrogel.When strain(λ) is low, macroscopic trapped water exist between PAAm/St-PAMPS/PAAm hydrogel and glass substrates, the spreading of macroscopic trapped water changed with the increase of sliding velocity, the position, the shape and the volume of macroscopic trapped water could not be well controlled, the sliding friction of PAAm/St-PAMPS/PAAm hydrogels show poor reproducibility. When strain is high(exceed 7.4 %), PAAm/St-PAMPS/PAAm hydrogel contact completly with glass substrates, macroscopic trapped water no exist between hydrogel and glass substrates, the sliding friction of PAAm/St-PAMPS/PAAm hydrogels show well reproducibility. Meanwhile, the sliding friction of PAAm/St-PAMPS/PAAm hydrogels hardly changed with the increase of normal pressure. Because the presence of PAMPS brushes on the surface of PAAm/St-PAMPS/PAAm and PAAm/PAAm/St-PAMPS hydrogel, the sliding friction of two kinds of hydrogel was lower than that of PAAm/PAAm hydrogel when sliding velocity was low. The characteristic velocity decreases with the increases of modulus of PAAm/PAAm, PAAm/St-PAMPS/PAAm and PAAm/St-PAMPS/PAAm hydrogel. The is relate to elasticity related length ε. Under the same normal pressure, λ increases with the modulus increases. ε decreases with λ increases. When hydrogels has small ε, it was more likely to form hydrodynamic lubrication under high speed. The λ of PAAm/PAAm hydrogel was the highest, the ε was the lowest, the of PAAm/PAAm hydrogel was the highest. The absorption of SDS on the glass substrates, which shield the interaction between hydrogels and the glass substrates. When the sliding velocity was about 10-2m/s, the shielding effect of SDS was strong, the friction stress of PAAm/PAAm/St-PAMPS hydrogel obviously decreases. In Na Cl solution, the Na+ can neutralize charge between PAAm/St-PAMPS/PAAm hydrogel and the glass substrate, had the effect of shielding, the friction stress of PAAm/PAAm/St-PAMPS hydrogel obviously decreases. The teflon substrates is neutral, hydrogel had weak interaction with it, the change rule of friction stress of PAAm/PAAm/St-PAMPS hydrogel against it is similar to against glass substrates.