| Rubber and plastic materials are widely used in various fields of national economy construction because of their superior properties compared to conventional materials. However polymer materials are easily affected by environmental factors (such as temperature, stress, and ozone) to the occurrence of aging failure. Therefore, study on the aging mechanism and performance of polymer materials in different environments has important practical significance. For example, diene rubber, used as sealing materials in the aerospace and automotive field, can be seriously affected by stress and ozone. Polycarbonate, used as optical materials in the aerospace and automotive field, can be seriously affected by molding process. It has been shown that multi scale analysis from the microscopic structure to macroscopic property is the key to study aging of polymer materials. Polymer materials in the actual service process will be subject to the influence of many factors. Only by studying the synergetic effect of multi aging factors can we establish a reasonable aging prediction model. In addition, it has been shown that the molding process of plastic materials can also influence the degradation behavior. Therefore, this paper focuses on aging behavior, aging mechanism, and aging model of polymer materials. At the same time, the relationship between materials microstructure and material property was explored by a novel molecular simulation method. The research content of this paper is divided into four parts.(1) Natural rubber (NR), butadiene rubber (BR) and chloroprene rubber (CR), typical representative of diene rubber, will be accelerated aging in ozone or deformed states due to their molecular chains containing isolated double bonds. Therefore, we designed the accelerated aging test of the effect of stress, ozone concentration and aging time on the performance of diene rubber. Firstly, the effects of different aging factors on the thermodynamics and morphology of diene rubber were analyzed by hardness, scanning electron microscope, and positron annihilation lifetime spectrum tests. The results demonstrated that CR had strong ozone resistance and the growth direction of aging crack was perpendicular to the direction of material force. It is noteworthy that the lower compressive strain (deformation range:20%～35%) accelerates the aging of diene rubber. Then the magnetic crosslink density, X-ray photoelectron spectroscopy, and two-dimensional Fourier transform were adopted to study chemical changes of the rubber cross-linking network and the group during aging. The results showed that both cross-link and chain scission reactions took place at the same time. The formation rate of the carbonyl was faster than that of the ozonide, which indicated that the chemical aging process was dominated by chain scission reaction. Finally, molecular simulation method was used to quantitatively study the permeation behavior of ozone molecules in deformed rubber and to study internal micro environment of diene rubber. Molecular simulation results showed that the intermolecular force between ozone molecules and molecular chains of CR was strong because CR containing polar side groups, which led to the decrease of ozone molecule permeation in diene rubber. In addition, the increase of stress resulted in an increase in compressive deformation, a decrease in molecular chain motion ability and fractional free volume and permeation capacity of ozone molecules in diene rubber, and finally weakening the ozone destruction of rubber network.(2) The aging of polymer materials is influenced by many kinds of environmental factors in service process, and there occurs physical aging and chemical aging. Therefore, we studied the aging model of diene rubber by artificial neural network (ANN). The establishment of radial basis function (RBF) neural network is based on a large number of aging test data. Regularization method was adopted to optimize the ANN model. The comparison of the experimental and prediction data showed that the prediction accuracy of the RBF-ANN was greater than 98%. The RBF-ANN can not only be used to study the effect of physical aging and chemical aging on relaxation properties, but also can be used to predict the service life of the rubber. The sensitivity analysis method was introduced to quantitatively predict the effect of compressive strain, ozone concentration, aging time, and rubber type on the relaxation properties of the rubber.(3) On the basis of the aging research of diene rubber, we further studied the structure and aging properties of different PCs, which also used in aerospace and automotive field and easily occurring aging during processing, by the combination of experiment and molecular simulation. Firstly, microstructure, processability, and mechanical properties of different PCs were analyzed by experiment and molecular simulation. The results demonstrated that OQ2720 had a moderate processability, a high tensile and shear modulus, and a low bulk modulus. This is because of its molecular chain containing more benzene ring and showing strong intermolecular force. The experimental results are in good agreement with the molecular simulation results. Because of high molding temperature and long plastic time, then we investigated the effects of molding temperature and plastic time on the molecular weight, mechanical properties and optical properties of PCs. The study showed that the yield strength changed little with the increase of plasticizing time, while the impact strength decreased rapidly with the increase of plasticizing time at the molding temperature of 300.℃ The molecular weight and transmittance decreased with the increase of the molding time and temperature. Finally, the thermal degradation kinetics of PCs was studied. The results indicated that OQ2720 had the highest half decomposition temperature and thermal degradation activation energy, showing the strongest thermal stability.(4) On the basis of the analysis of the aging properties of PCs, it can be known that the molding time and temperature affect the properties of PCs and unreasonable molding process can lead to the degradation of PCs. Therefore, we further studied the pressure-volume-temperature (P-V-T) relation and thermal properties of PCs by the comparison of Tait semi empirical equation and Simha-Somcynsky (S-S) theory equation of state (EOS), and then the relation between pressure and temperature factors and deformations of PCs was established. It has been shown that S-S EOS is more suitable to describe the P-V-T relation of PCs in rubbery state. The thermal expansion and isothermal compression coefficients of OQ2720 were small, which indicated that the size stability of OQ2720 was better. At the same time, the free volume fraction (FFV), cohesive energy density (CED) and molecular chain motion ability were analyzed by using S-S EOS and molecular simulation. It has also been shown that FFV, CED, and the intermolecular force of OQ2720 are high, and the chain movement ability is weak. The S-S EOS and molecular simulation results can not only explain that the specific volume of OQ2720 changes weakly with temperature and pressure, but also explain that the processing dimensional stability of OQ2720 is good from the microscopic view. |
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