Molecular Dynamics To The Influences Of Plasma Modified On Polyimide

Polyimide(PI)is applied to the inter-turn insulation of variable frequency motor due to its excellent performance.Under the action of high frequency and steep pulse,the inter-turn insulation of variable frequency motor often fails prematurely.Studies have shown that the surface of plasma-modified PI film can improve its corona resistance,but at the same time the thermodynamic properties of the material will change,and the change of thermodynamic properties will affect the service life of the inter-turn insulation of the variable frequency motor.Therefore,it is important to study the changes in thermodynamic properties of plasma-modified PI films.In this paper,molecular dynamics simulation software was used to build a molecular model based on PMDA-ODA type PI molecules,and the bond strength of each chemical bond was analyzed by bond level.A molecular chain with a degree of polymerization of 15 was constructed and a PI unit cell was constructed to calculate the elastic modulus and glass transition temperature of the model.In order to simulate the role of introducing groups in the PI film during the initial stage of plasma treatment,a polar group was grafted at the end of the molecular chain to construct a unit cell containing different percentages of grafted molecular chains,and the elastic modulus and glass transformation temperature of each unit cell were calculated.Studies have shown that: after the introduction of polar groups,the groups form bonds with each other forming net structure which improves the mechanical properties of the material;The introduction of polar groups enhances the interaction between molecules and the mobility of the segments is weakened,so the glass transformation temperature will also increase.In order to simulate the molecular chain breakage and the formation of small molecules after plasma treatment for a long time,PI cell cells containing different percentages of PI decomposition products and grafting group molecular chains were constructed,and the elastic modulus and glass transition temperature of each unit cell were calculated.Studies have shown that when the molecular chain breaks and generate small molecular substances,the intermolecular force is weakened,and the mechanical properties of the material are degraded.When the molecular chain breaks to form a small molecular substance,the mobility of the segment is enhanced and the glass transition temperature of the material is lowered.In order to verify the simulation,the PI film was prepared by in-situ polymerization on the basis of economy and convenience,and measure the elastic modulus and glass transition temperature of the PI film with different plasma processing time.By comparing the experimental and simulation results,it is found that the experimental data change trend is almost the same as the simulation when the plasma treatment time is long.When the processing time is short,the numerical change of the experimental measurement results is smaller than the simulation.This is because the plasma mainly acts on the surface of the PI film without changing the matrix properties of the material at the initial stage of the treatment,so the numerical value of the experimental measurement is small.The simulation model directly changes the matrix properties of the material,so the simulation data changes significantly;After a long time of treatment,the plasma will etch the material matrix,so the thermodynamic properties of the material decrease with the increase of the plasma processing time.The simulation model is equivalent to the damage material structure by adding small molecular substances in the model,so the experimental data and the simulation data have the almost same change trend.