Research On Swelling And Friction Of NBR Based On Molecular Dynamics Simulation

Nitrile butadiene rubber(NBR)is often used as stator rubber in submersible screw pump.Due to the complex downhole conditions,the phenomenon of swelling and degumming of stator rubber often occurs,which affects the service life of screw pump.At present,although domestic and foreign scholars have carried out researches on the properties of NBR,there are few reports on revealing the micro mechanism of NBR from the atomic scale.Consequently,in this thesis,molecular dynamics theory was used to explore the effects of crosslinking density and swelling aging on the properties of NBR,and reveal the corresponding mechanism from the micro scale.The specific research contents are as follows:Firstly,the molecular models of NBR with different crosslinking densities of 0%,10%,15% and 20% were established using Materials Studio.The glass transition temperature and mechanical properties of NBR with different crosslinking densities were calculated,and the micro parameters such as mean square displacement and unit cell pressure were analyzed.The simulation results show that the cross-linking network will restrict the movement of molecular chain,so that the glass transition temperature of NBR increases with the increase of crosslinking density.However,when the cross-linking density is too high,the presence of the crosslinking bond is easy to cause uneven stress transfer,leading to a fact that the mechanical properties of NBR first increase and then decrease with the increase of cross-linking density.When the cross-linking density is 10%,the mechanical properties of NBR are the best.Secondly,the molecular models of NBR with different swelling volumes were constructed,and the glass transition temperature and mechanical properties of the corresponding molecular models were determined.The tensile process of these models was simulated and analyzed by using LAMMPS,and the micro mechanism of the effect of swelling on the mechanical properties of NBR was explored.The simulation results show that the glass transition temperatures of nitrile rubber with swelling volume of 0%,5%,10%,and 15% are248.29 K,240.58 K,237.98 K,and 232.05 K,respectively;the maximum tensile strength is12.4 MPa and 11.7 MPa,respectively,11.2 MPa and 10.7 Mpa.Combining the calculation of the potential energy and radial distribution function during the stretching process,it is shown that the introduction of cyclohexane reduces the intermolecular forces of the NBR,which leads to the decrease of the mechanical properties of the NBR.Finally,the molecular friction model between the swelling NBR and metal atoms was developed.The effects of swelling and normal pressure on the tribological performances of NBR were explored.The simulation results were experimentally verified by friction tests.The results show that the shear resistance of nitrile rubber after swelling decreases with the increase of the swelling volume.The metal friction pair is more likely to take away the rubber molecules from the surface of the rubber matrix.The results also demonstrate that with the increase of normal pressure,the friction coefficient and wear rate of rubber matrix increase gradually.When the normal pressure increases initially,the friction coefficient and wear rate increase slowly.However,with the increase of normal pressure,the contact area between iron atoms and rubber increases,the temperature of the internal system increases continuously,the flexibility of molecular chain decreases,and the friction coefficient and wear rate increase faster.