2种受阻酚类防老剂对天然橡胶热氧老化防护效果多尺度分子模拟及实验研究

Protective Effect of Two Hindered Phenolic Antioxidants Against Thermal and Oxygen Aging of Natural Rubber by Multi-scale Mo-lecular Simulation and Experimental

引入多尺度分子模拟技术对比研究了2种受阻酚类防老剂2,2'-亚甲基双(4-甲基-6-叔丁基苯酚)(防老剂2246)与防老剂2,2'-亚甲基双(4-乙基-6-叔丁基苯酚)(防老剂425)对天然橡胶(natural rubber,NR)的热氧老化防护效果。量子力学模拟(QM)结果表明,2种防老剂的O-H键解离反应自由能(ΔG)均低于NR的C-H键ΔG,且防老剂425在其解离位置h处的O-H键ΔG最低,为250.08 kcal/mol,搜索2种防老剂或NR与过氧自由基CH_(3)OO·的反应过渡态,发现防老剂425的过渡态能垒(E_(barrier))最低,为5.31 kcal/mol,证明其能更快速地捕获过氧自由基,有效延缓NR分子链的氧化反应,表明了其防老剂化学反应防护机制。分子动力学(MD)模拟结果表明,防老剂425与NR之间的溶解度参数(δ)更接近,扩散系数(D)更低,即防老剂425与NR分子链更易相容,迁移性更低。蒙特卡洛(MC)模拟结果表明,氧气在3种体系中的渗透系数(P)大小顺序为P_(NR)>P_(NR/2246)>P_(NR/425),即防老剂425更容易阻隔氧气渗透,表明了其防老剂物理防护机制。为验证分子模拟结论的准确性,对不同NR复合体系进行0~7 d的热氧老化实验,采用拉力试验机、邵尔A硬度计、傅里叶红外变换光谱仪研究2种防老剂对NR的热氧老化防护效果,发现NR/425体系的力学性能保持率最好,红外图谱结果显示,防老剂425/NR复合体系的氧碳摩尔比增长率最低,表明防老剂425对NR的防护效果更好,与分子模拟结论一致,证明多尺度分子模拟有助于防老剂的选取。

The anti-oxidative effect of two types of hindered phenolic antioxidants,i.e.,2,2'-methylenebis(6-tert-butyl-4-met-hylphenol)(AOA2246)and 2,2'-methylenebis(6-tert-butyl-4-ethylphenol)(AOA425)on natural rubber(NR)was in-vestigated by means of multiscale molecular simulation and experimental validation.According to the quantum me-chanical simulation,it was found that the changes in Gibbs free energy(ΔG)of dissociation of O-H bonds of the two anti-oxidative agents were lower than that of dissociation of C-H bonds of NR when heated.In addition,theΔG of the O-H bonds cleavage in AOA425(the weakest position h)was calculated to be as low as 250.08 kcal/mol.Besides,the energy barrier(E_(barrier))for the reaction between AOA425 and CH_(3) OO was found to be only 5.31 kcal/mol,which was the lowest by searching and comparing all the transition states between the reactant(AOA2246,AOA425 or NR)and CH_(3) OO·.As a result,AOA425 was supposed to be effective in suppressing the oxidation reaction of NR in consideration of the distinguished ability of scavenging free radicals. Consequently, the protection mechanism of the hindered phenolic antioxidants was interpreted by chemical reaction mechanism. In addition, the solubility parameter of AOA425 was calculated to be close to that of NR according to the molecular dynamics simulation. The low oxygen diffusion co-efficient in NR indicated that AOA425 would play a long-term role. The oxygen permeability coefficients calculated via Monte Carlo simulation can be sequenced as P_(NR)>P_(NR/2246)>P_(NR/425). Therefore, AOA425 was found to be beneficial to shielding oxygen in the light of physical protection mechanism. Furthermore, the anti-oxidative influence of AOA425 and AOA2246 on NR was validated with the results of mechanical measurements, Shore A hardness testing, Fourier transform infrared spectra (FT-IR), and accelerated thermal oxygen aging experiments. The mechanical properties of NR were slightly decreased with the incorporation of AOA425. The infrared spectra also demonstrated that AOA425 exhib-ited better thermal oxygen aging resistance for NR in consideration of the molar ratio of carbon to oxygen. The experi-mental data showed a good agreement with the results of molecular simulation and multiscale molecular simulation would be of significance to predict the anti-oxidative performance of materials.