Study Of The Relationship Between Intermolecular Hydrogen Bond And Damping Performance Of Small Organic Molecule/Nitrile-butadiene Rubber Hybrids And Molecular Simulation

The small organic molecule/polar rubber hybrids have broad application prospects in industries of construction,transportation and modern aerospace because of their excellent damping performance.It is of great significance to in-depth study the damping mechanism of such materials.Small organic molecule/NBR hybrids were chosen in this research.By combining experiments,multi-scale molecular simulation and correlation analysis,the key factor affecting damping properties was determined.Starting from the thermodynamics and kinetics of hydrogen bond dissociation reaction,the damping mechanism of this kind of materials was studied quantitatively.Specific contents were divided into the following four parts.(1)Four important damping parameters,including maximum damping factor(tan?max),effective loss area(TA),glass transition temperature(Tg)and effective temperature region(?T),of Irganox-1035/NBR hybrids were obtained by dynamic mechanical thermal analysis(DMTA).The intermolecular interaction parameters(the type and molar concentration of hydrogen bonds(CHBs),binding energy(Ebinding),and fractional free volume(FFV))were calculated by molecular dynamics(MD)simulations.By means of grey correlation analysis and linear regression method,the priority of factors affecting damping parameters and the internal relationship between them were studied.Results showed that the intermolecular CHBs(CHBs(A))performed the most significant effect on the damping performance.Linear relationships between intermolecular interactions(CHBs(A)and Ebinding)and damping parameters(tan?max and TA)(R2>0.9)were noted.After nondimensionalization,taking C'HBs(A)and E'binding as independent variables,and tan?'max and TA' as dependent variables,multivariate linear fitting equations with R2 of 0.994 and 0.996 were constructed,respectively.(2)Based on the research contents of(1),the priority of factors affecting the damping performance of AO-80/NBR hybrids was explored.Results showed that CHBs(A)was the dominate factor for damping performance.Thermodynamic parameters of hydrogen-bonded associative dimer(HBD)dissociation reaction,including standard molar enthalpy(?H~?),standard molar Gibbs free energy(?G~?),equilibrium constant(lnK~?)and dlnK~?/dT,were simulated by quantum mechanics(QM).Intermolecular HBD dissociation reaction performed the highest ?H~?,dlnK~?/dT,and the lowest lnK~?.According to the thermodynamic theory of dissociation reaction,it can be concluded that the energy loss due to the dissociation of intermolecular hydrogen bond is much higher than that of intramolecular hydrogen bond.This is the root cause of the significant influence of CHBs(A)on damping performance.(3)The lnK? of intermolecular and intramolecular HBD dissociation reactions in three hindered phenols(AO-60,AO-70 and AO-80)/NBR systems were compared and analyzed based on the research contents of(2).Results showed that the lnK~? of intermolecular HBD dissociation reactions were lower than that of intramolecular ones in both AO-70/NBR and AO-80/NBR systems.It could be speculated that the intermolecular hydrogen bonds(HBs)will be easier formed in the two systems,and thus the phase will be more stable.The phase separation of AO-60/NBR hybrids will occur when the content of AO-60 reaches a certain value.The results of DSC and SEM characterization were in good agreement with the theoretical speculation.Four thermodynamic parameters of the intermolecular HBD dissociation reactions(R(60),R(70)and R(80))were compared.Results showed that ?G~? of three dissociation reactions were all negative when the temperature was higher than 200 K,indicating that all the intermolecular HBs can be dissociated spontaneously within the glass transition temperature range(253-313 K)of NBR.R(80)performed the highest ?H~?,dlnK~?/dT,and the lowest lnK~?,indicating that AO-80/NBR hybrids will have the best damping performance.AO-60/NBR hybrids,however,will be the worst one.The theoretical prediction were confirmed by both DMTA analysis and four damping parameters obtained.Unified linear relationship(R2=0.924)between normalized parameters(damping parameter(tan?max)and energy dissipation parameter calculated by the thermodynamic parameters)was discovered in the combined three homogeneous systems.(4)MD simulation was applied to study the relationship between the characteristic time of NBR segment motion and the lifetime of intermolecular HBs near the glass transition region of AO-60/NBR,AO-70/NBR and AO-80/NBR systems.Firstly,the following definitions were given according to the theory of dynamic mechanics of polymers associated by HBs.The time calculated according to model coupling theory is the characteristic time of chain segement motion(tc).The effective lifetime(?E)is the total duration of HBs formated by the same proton acceptor and donor.The actual lifetime(?R)is the duration of HBs in one formation-dissociation cycle.The HBs with ?E which is more than half of the observed time is the effective HBs,which will affect the dynamic mechanical performance of NBR.Then,Tg of NBR in each system was calculated by MSD curves at different temperatures.The number of effective HBs(NHBs)at various temperatures was counted.Results showed that NBR in the system with the higher NHBs and corresponding ?E performed higher Ebiding,lower FFV,and higher Tg.Based on the theory of free volume and kinetics of HBs dissociation reaction,the reason for the enhancement of Tg by intermolecular HBs was analysised.Because of the existance of intermolecular HBs,not only the FFV was reduced,but also the energy required for the start-up of polymer segment motion were enhanced.The effective HBs were divided into stable and active ones according to ?R.With the increase of temperature,the number of stable HBs decreased gradually,and both the number of active ones and the dissociation-generation cycles reached the maximum around Tg.In the three systems,the real active HBs lifetimes(?R(a))were all larger than tc around Tg.This will result in the parallel movement of the segmental motion and the active hydrogen bond dissociation and conversion,consuming a large amount of energy,thus becoming the root cause of the high damping value of the organic small molecule/polar polymer near Tg.