Molecular Design And Computational Simulation Study Of Silicone Elastomer With A Wide Temperature Range

Elastomeric materials are widely used in a variety of applications,with silicone elastomer（SR）being the most promising class of material for extreme temperature environments.However,existing SRs are not sufficient for normal use in extreme high and low temperature environments,the preparation of new elastomeric materials with excellent wide service temperature range performance is essential.At the same time,it is crucial to investigate the links between structures and properties and explain the mechanisms at the microscopic atomic scale.In this paper,a novel structure of SR has been designed through molecular dynamics（MD）simulations to focus on its high and low temperature resistance and thermal decomposition properties.The work is conducted in two aspects:（1）A novel silicone elastomer model is developed by an all-atom molecular dynamics method,and a novel SR molecular chain is designed based on the structure of the polydimethylsiloxane（PDMS）.Dimethylsiloxane acts as a repeating unit of the polymer backbone,with a silicon-oxygen bond（-Si-O-）as the dominant structure to create large sized side chains,with grafted long side chains replacing two methyl groups of the same silicon atom in the backbone.A series of pure SR systems in different lengths of side chains and various grafting densities were established.Firstly,to investigate its cold resistance,three kinds of methods were applied to evaluate the glass transition temperature（T_g）of SR,i.e.calculating the volume,the non-bonding interaction energy and the transition rate of the torsion angle.The predictions of T_gby the three methods are generally consistent.Then,its heat resistance was investigated by a method based on the rate of change of Mean square displacement（MSD）as a function of temperature to characterize the thermal decomposition temperature（T_d）of SR.Pure SR has the widest temperature range with T_gbelow-140°C and T_dabove 420°C when dimethylsiloxane and siloxane long side chains are copolymerised in a ratio of 3:1 and the unilateral side chain length contains three（-Si-O-）repeating units.This is due to the side chains increasing the flexibility of the molecular chains,leading to a lower T_g.The effective inter-chain interaction and the high bond energy of the（-Si-O-）bond act to increase the stability of the molecular chain,leading to an increase in T_d.Finally,the thermal decomposition characteristics of the new SR were investigated by reactive force field molecular dynamics（Reax FF-MD）simulations,including the effect of temperature on the pyrolysis process,the product distribution and the product formation regimes.The results of the temperature-dependent simulations show that temperature has a significant effect on product distribution and decomposition rates.The dominant final products of thermal decomposition are CH₄,H₂,C₂H₄and siloxane.（2）With the introduction of Si O₂nanoparticles,a series of new SR/Si O₂nanocomposites are established,containing various mass fractions（Φ）of Si O₂nanoparticles.The results of the study on the cold and heat resistance of the composite system show that the cold and heat resistance of the composite system is significantly better than that of the pure system.The temperature range is greatest whenΦis between 5.17and 12.01wt%,with T_gand T_dreaching about-150°C and above 450°C respectively.Predictions of the packing structure,energy and conformational transitions of the composites at 300K for T_gare consistent with the results of the characterisation of T_g.Characterisation of the systems’MSD at 300K shows that the incorporation of NPs improved the motility of the molecular chains.By studying the heterogeneous distribution of nanocomposites,we found that systems with a higher content of NPs have the reduced rotational space potential resistance of the molecular chains,so T_gdecreases.The thermal decomposition activation energy of the composite system reaches 110k J/mol,which is higher than that of the pure system（about 103k J/mol）,indicating that the thermal stability of the SR molecular chains in the composite is better than that of the pure system.The study of the product distribution of the composite system shows that the dominant new product generated by the SR/Si O₂nanocomposites is H₂O.The gradual increase in filler content inhibits the production of CH₄.In summary,our findings provide some reference and guidance for the subsequent experimental preparation of novel wide-temperature domain silicone elastomers.