Molecular Dynamics Simulation Of Corrosion Resistance And Mechanical Properties Of Rubber Materials Under High Pressure And Low Temperature

Pressure and temperature are important factors affecting the performance of rubber materials.With the development of the times,the application environment of rubber materials is becoming more and more harsh,so the research on rubber materials under extreme environment（high pressure and low temperature,etc.）can not be delayed.The reinforcing effect of nano-SiO₂,which is a common reinforcing agent for rubber materials,can also be affected by high pressure and low temperature.In this paper,the effects of pressure,temperature and nano-SiO₂ particle size on the microstructure and mechanical properties of EPDM and PVMQ are investigated using molecular dynamics simulations,and the corrosion resistance of the two rubber materials under the set pressure and temperature conditions are also investigated.Details are as follows:The microscale models of EPDM and PVMQ and the two-layer structural models of the two rubbers with salt solution respectively were constructed using molecular dynamics simulations,and the microstructural characteristics of the two rubber materials under high-pressure and low-temperature environments were analyzed,while the diffusion behavior of the salt solution in the two rubbers was investigated and characterized by parameters such as glass transition temperature and binding energy.The results showed that the glass transition temperature of PVMQ molecular system was always lower than that of EPDM molecular system,while the free volume fraction was always larger than that of EPDM molecular system and the slope of MSD curve was also larger than that of EPDM molecular system,which indicated that PVMQ molecular system had better low-temperature resistance,regardless of atmospheric pressure or high-pressure conditions.In addition,in the bilayer structure of the two rubbers with salt solution respectively,the average binding energy of both systems is positive and both have certain corrosion resistance.The MD method was used to construct EPDM molecular models and nano-SiO₂/EPDM composites with particle sizes of 1.2 nm,1.5 nm,1.8 nm and 2.0 nm,respectively,and the effects of pressure and doping of nano-SiO₂ with different particle sizes on the microstructure and mechanical properties of the EPDM molecular system under low temperature conditions were analyzed.The results showed that the glass transition temperature and cohesive energy density of the doped nano-SiO₂ system were increased to different degrees,and the increase was more obvious when the nano-SiO₂particle size was 1.2 nm,and the increase was 11 K.Under 230 K,the free volume fraction and mean square displacement of the composite system decreased as the pressure increased,and the decrease was greater when the doped nano-SiO₂ particle size was 1.2 nm.The magnitude of the reduction is greater for the doped nano-SiO₂ particle size of 1.2 nm.Meanwhile,the doping of nano-SiO₂ can improve the mechanical properties of EPDM molecular system to different degrees,and the enhancement effect decreases with the increase of nano-SiO₂ particle size.Among them,the bulk modulus of the composite system at 50 MPa increased by 8.91%,the shear modulus by 95.78%and the Young’s modulus by 46.88%when the particle size of nano-SiO₂ was 1.2 nm;the bulk modulus of the composite system at 100 MPa increased by 5.98%,the shear modulus by 198%and the Young’s modulus by 34.65%.The all-atomic RDF of the composite system after doping with nano-SiO₂ is not much different from that of the EPDM molecular system,and the characteristics are basically the same.The nano-SiO₂/PVMQ composites with particle sizes of 1.2 nm,1.5 nm,1.8 nm and 2.0 nm were constructed using similar means to the nano-SiO₂/EPDM composite model.The effects of high pressure and low temperature environment and nano-SiO₂particle size on the structural properties and mechanical properties of the PVMQ system were mainly analyzed and compared with the EPDM composite system,and the following results were obtained:the smaller the particle size of nano-SiO₂ filler under high pressure conditions,the more obvious the increase of glass transition temperature and cohesive energy density of the PVMQ system,but compared with the nano-SiO₂/EPDM composite system However,the glass transition temperature of the nano-SiO₂/PVMQ composite system is always lower,and the free volume fraction and mean square displacement of the nano-SiO₂/PVMQ composite system are both higher under the same conditions.The bulk modulus,shear modulus and Young’s modulus of the nano-SiO₂/PVMQ composite system were improved,and the smaller the particle size of nano-SiO₂,the more obvious the improvement effect was,but the overall nano-SiO₂/EPDM composite system showed better mechanical properties.