| Natural rubber(NR)has been widely applied in military,energy,technology and industrial fields due to its unique high elasticity and excellent comprehensive properties.However,under the influences of environmental factors such as oxygen(O2)and heat,the existence of high level unsaturated isolated double bonds and side methyl groups in NR molecular chains makes it easily age during service,and the performance of the product is deteriorated.Thus,it is significantly important to prolong service life of NR products by taking effective measures.The addition of antioxidant is one of the most convenient methods.In this topic,the most widely used N,N'-substituted p-phenylenediamines(PPDs type)were selected and added to the NR matrix.Through the exper:imental methods,the anti-aging effects of various antioxidants were comprehensively studied.In addition,multi-scale molecular simulation methods were introduced to analyze factors affecting the anti-aging effects of antioxidants.The physical and chemical protection mechanisms of antioxidants were performed on the molecular scale.The contents mainly include the following two parts:(1)In the first part of the research contents,antioxidants 6PPD,OPPD,and 7PPD,the example of p-phenylenediamines,were chosen to study the factors affecting anti-aging behavior of antioxidants in NR composites.In experimental test research,macro properties and micro structures were investigated by mechanical testing and Fourier transform infrared spectroscopy(FTIR)in NR composites.Meanwhile,the thermo-gravimetric(TG)curves at different heating rates were analyzed by Kissinger method and Flynn-Wall-Ozawa(FWO)method,and the thermal-oxidative degradation kinetic parameters(Ea)were determined.The results showed that,the anti-aging efficiency of three antioxidants was in order of 7PPD>OPPD>6PPD.Combined with quantum mechanics(QM)methods and molecular dynamics(MD)simulations,the four factors affecting the anti-aging behavior of antioxidants was summarized.For the chemical influence factors of antioxidants,the free energy of hydrogen dissociation reaction of different structures was calculated by QM method.The result indicated all three antioxidants contain a dissociation energy value of higher activity N-H bond lower than the minimum dissociation energy value of C-H bond in NR,thus the three antioxidants could all preferentially react with peroxyl radicals in chemical protection and significantly delay aging process of NR composites.By MD simulations,the physical factors such as the permeability of O2,the solubility,and the mobility of antioxidant were characterized by the permeation coefficient(P),the solubility parameter(?),and the mean square displacement(MSD),respectively.The simulation predictions were consistent with the experimental results.To clarify the relative importance of four factors in the choice of antioxidants,the gray relational analysis(GRA)method was employed.It was concluded that in the same class of antioxidants such as PPDs system,physical factors are more important than chemical factors.(2)In the second part of the research contents,we mainly studied the protective effect of three structurally similar diaryl-p-phenylenediamine antioxidants in NR composites.Firstly,macro properties and micro structures of NR composites were analyzed by mechanical testing,FTIR,and X-ray photoelectron spectroscopy(XPS).The preliminary conclusion obtained by the experiment is that the anti-aging efficiency of the three antioxidants in the NR composites is:C>B>A.Combined with molecular simulations,the N-H bond dissociation energy of three antioxidants,the permeability for 02,the solubility,and migration of antioxidants were characterized.The result of the simulation prediction is that antioxidant C has the highest anti-aging efficiency,which is very consistent with the experimental results.Finally,through studying two p-phenylenediamine systems,we obtained that,in a structurally similar antioxidant system,the focus of assessing anti-aging effect is on physical factors.The conclusion has important guiding significance for the design and synthesis of high-efficiency amine antioxidants. |
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