HTPB固体推进剂增塑剂选取分子模拟研究

固体推进剂中增塑剂要求同粘合剂体系相容性良好,并提高体系的低温性能.本文采用分子动力学(MD)方法,首先计算了端羟基聚丁二烯(HTPB)粘合剂及增塑剂癸二酸二辛酯(DOS)、己二酸二辛酯(DOA)、壬酸异癸酯(TOA)、邻苯二甲酸二丁酯(DBP)和邻苯二甲酸二辛酯(DOP)的溶度参数,以此从相容性角度选取推进剂增塑剂;计算数值基本吻合实验值,表明常用的增塑剂从相容性都能满足要求.其次模拟获取了HTPB及HTPB/增塑剂混合体系的比体积-温度关系得到了体系的玻璃化转变温度(Tg),揭示增塑剂对HTPB体系低温性能的影响.结果显示:(1)HTPB的Tg模拟值为202K,基本吻合实验值196K.(2)HTPB/DOS混合体系中,当增塑剂DOS的质量含量从12%、22%、29%到36%(摩尔含量分别为50%、66%、75%和90%)增加时,体系的Tg线性降低;TOA和DOP增塑的粘合剂体系(摩尔含量为75%)Tg也降低,而增塑剂DOA和DBP对体系的Tg影响不大.因此,基于相容性及提高粘合剂低温性能考虑,DOS、DOA和DOP作为HTPB的增塑剂优于TOA和DBP.     

HTPB与Al不同晶面结合能和力学性能的分子动力学模拟

采用分子力学(MM)和分子动力学(MD)方法, 在250、300、350、400、450 K, 对固体推进剂端羟基聚丁二烯(HTPB)和铝晶胞不同晶面结构所组成的层模型在COMPASS力场下, 进行模拟计算, 求得结合能和静态力学性能(弹性系数、模量和泊松比). 模拟结果表明, 在400 K时HTPB与Al(011)面的结合能最大, 从综合力学性能优劣上看, 各个面从优到劣的排序为(011)>(221)>(001), HTPB与Al的结合能与力学性能具有对应关系, 结合能大的力学性能优异, 结合能小的力学性能较差.     

端羟基聚丁二烯/增塑剂共混物相容性的分子动力学模拟和介观模拟

应用分子动力学(MD)和介观动力学(MesoDyn)模拟方法对固体推进剂中端羟基聚丁二烯(HTPB)与增塑剂癸二酸二辛酯(DOS)、硝化甘油(NG)的相容性进行了研究. 采用MD模拟方法在COMPASS力场下, 对纯物质、HTPB/增塑剂共混物的密度、内聚能密度、溶度参数和共混物分子间的Flory-Huggins作用参数及结合能等进行了模拟计算, 通过比较溶度参数差值(Δδ)的大小、模拟前后体系密度变化情况均可以预测HTPB与增塑剂的相容性, 结合能的分析揭示了HTPB/增塑剂共混物组分间的相互作用及本质. 将Flory-Huggins作用参数转化为MesoDyn模拟的输入参数, 采用MesoDyn模拟方法对HTPB/增塑剂共混体系的介观形貌与动力学演变过程进行了研究, 通过模拟得到的等密度图、自由能密度和有序度参数等可以判断共混体系的相容性. MD和MesoDyn模拟结果均表明: HTPB/DOS属于相容体系, 而HTPB/NG属于不相容体系, 其结论与实验结果一致.     

HTPB/增塑剂玻璃化转变温度及力学性能的分子动力学模拟

为了预测高分子粘结剂端羟基聚丁二烯(HTPB)与增塑剂癸二酸二辛酯(DOS)、硝化甘油(NG)的相容性及HTPB/增塑剂共混物的玻璃化转变温度(Tg)和力学性能,在COMPASS力场条件下采用分子动力学(MD)模拟方法对相容体系(HTPB-DOS)和不相容体系(HTPB-NG)进行了研究.结果表明,通过比较溶度参数差值(Δδ)的大小可以预测HTPB与增塑剂的相容性,即HTPB与DOS属于相容体系,而HTPB与NG不相容.通过温度-比容曲线可以得到HTPB、HTPB/DOS与HTPB/NG的Tg分别为197.54,176.30和200.03K.力学性能分析结果表明,添加DOS增塑剂后使HTPB的弹性模量(E),体积模量(K)和剪切模量(G)下降,材料刚性减弱,柔性增强,力学性能得到改善.本模拟方法可以作为预测聚合物/增塑剂共混物性能的有利工具,也可以为固体推进剂和高聚物粘结炸药的配方设计提供理论指导.     

熔融NaCaF3、Na2CaF4和Na3CaF5的分子动力学模拟

通过分子动力学模拟，研究了熔盐溶液NaCaF_3、Na_2CaF_4和Na_4CaF_5体系，模拟表明，三种二元混合系的径向分布函数十分接近．由模拟所得到的摩尔混合焓很好地与实验值一致．混合焓与Na~+离子势阱深度之间表现出很好的线性关系．模拟表明，在Na_2CaF_4体系中，即NaF-CaF_2二元系处于低共熔混合组分比NaF:CaF_2＝2:1时，Na~+，Ca~(2+)和F~-离子的自扩散系数出现很大的反常．     

拉伸分子动力学模拟配体-受体相互作用

配体和受体之间的相互作用研究有助于阐明配体的作用机理, 为合理药物设计提供线索. 新发展的拉伸分子动力学模拟使原来在微秒至秒时间范围内发生的生物化学过程可以在纳秒尺度内进行模拟, 从而动态再现目前实验所无法提供的配体与受体的结合或解离过程. 文中通过详细介绍拉伸分子动力学方法对石杉碱甲与乙酰胆碱酯酶结合和解离过程以及HIV-1逆转录酶和其非核苷酸类似物抑制剂α-APA解离过程的成功模拟, 综述拉伸分子动力学模拟在研究配体和受体相互作用中的应用.     

端羟基聚丁二烯/增塑剂共混物相容性的分子动力学模拟

固体推进剂和炸药的力学性能在很大程度上依赖于配方中高分子粘结剂与增塑剂的相容性. 本文对相容和非相容两种体系进行了分子动力学(MD)模拟, 以考察分子模拟方法的实用性. 为预测固体推进剂中端羟基聚丁二烯(HTPB)与增塑剂癸二酸二辛酯(DOS)、硝化甘油(NG)的相容性, 采用MD模拟方法在COMPASS力场下, 对HTPB、DOS、NG和共混物HTPB/DOS、HTPB/NG的密度、内聚能密度及溶度参数等进行了模拟计算. 通过比较溶度参数差值(△δ)的大小、分子间径向分布函数和模拟前后体系密度变化情况均可以预测HTPB/DOS属于相容体系,而HTPB/NG属于不相容体系, 与实验结果一致. 径向分布函数分析同时揭示了HTPB/增塑剂组分之间的相互作用及本质. 本文的模拟方法可以作为预测聚合物与增塑剂相容性的有利工具, 也可以为固体推进剂和炸药的配方设计提供理论指导.     

HTPB与A1不同晶面结合能和力学性能的分子动力学模拟

采用分子力学（MM）和分子动力学（MD）方法,在250、300、350、400、450K,对固体推进剂端羟基聚丁二烯（HTPB）和铝晶胞不同晶面结构所组成的层模型在COMPASS力场下,进行模拟计算,求得结合能和静态力学性能（弹性系数、模量和泊松比）．模拟结果表明,在400K时HTPB与A1（011）面的结合能最大,从综合力学性能优劣上看,各个面从优到劣的排序为（011）〉（221）〉（001）,HTPB与A1的结合能与力学性能具有对应关系,结合能大的力学性能优异,结合能小的力学性能较差．     

石墨烯/聚乙烯复合材料及其拉伸性能的分子动力学模拟

利用分子动力学方法,模拟石墨烯/聚乙烯复合材料的微观结构和性能,并采用单轴拉伸模拟方法研究石墨烯/聚乙烯复合材料的拉伸性能.结果表明,在石墨烯/聚乙烯复合材料平衡构型中,聚乙烯基体分子在石墨烯表面处形成多层吸附层,吸附层处于动态稳定状态,层内分子可以发生扩散迁移.吸附层内聚乙烯分子发生"吸附固化"现象,分子弯曲程度减弱,发生有序排列,且在垂直于石墨烯方向的运动性能受到抑制.拉伸模拟结果表明,石墨烯能够提高聚乙烯材料的拉伸性能.在弹性区和屈服区,石墨烯阻碍了复合材料在垂直于拉伸方向的压缩变形,聚乙烯分子"吸附固化"结构保持稳定,引起体系整体应力的迅速升高.在软化区,由于石墨烯发生剧烈弯曲,"吸附固化"结构发生破坏,最终引起体系应力迅速减小.在弹性区和屈服区,体系应变主要引起了非键相互作用的改变.在软化区之后,应变主要导致了体系内分子成键相互作用的改变.应变速率能够提高复合材料的屈服应力,而不改变复合材料应力应变的整体趋势.     

CL-20/HMX共晶及其为基PBX界面作用和力学性能的MD模拟研究

为提高共晶炸药的实际使用价值, 改善其安全性和力学性能, 以CL-20/HMX共晶炸药为基, 分别添加2种高聚物粘结剂Estane 5703(聚氨基甲酸乙酯)和HTPB(端羟基聚丁二烯), 共构建两种共晶基高聚物粘结炸药(PBX)模型, 进行细致的295 K NPT分子动力学(MD)模拟研究. 通过两种PBX模型及其与该共晶炸药的MD模拟结果比较表明, 与基炸药之间的结合能Estane 5703大于HTPB, 预示含少量Estane 5703的PBX稳定性和相容性更佳; 对相关函数g(r)揭示粘结剂与基炸药界面相互作用的方式, 以基炸药中H分别与Estane 5703中羰基O和HTPB中端羟基O之间的氢键较强. 与CL-20/HMX共晶炸药相比, 少量粘结剂Estane 5703或HTPB的加入, 使弹性系数C_ij下降, 拉伸模量(E)、体积模量(K)和剪切模量(G)均显著减小, 而泊松比(ν), 柯西压(C₁₂－C₄₄)和K/G值明显增大, 表明PBXs体系刚性减小, 延展性增强, 力学性能大为改善. 少量粘结剂包覆使PBXs致钝, 主要归因于其隔热、吸热并使体系变“软”的缓冲作用, 而界面作用造成的分子结构引发键键长变化变为次要因素.     

Influence of isotherm inflection on the loading dependence of the diffusivities of n-hexane and n-heptane in MFI zeolite. Quasi-elastic neutron scattering experiments supplemented by molecular simulations

Quasi-Elastic Neutron Scattering (QENS) experiments were carried out to determine (a) Fick diffusivity, D (b) self-diffusivity, Dself, and (c) 1/Gamma, the inverse of the thermodynamic correction factor, for n-hexane (nC6) and n-heptane (nC7) in MFI zeolite (all silica silicalite-1) at 300 K for a variety of loadings. These experimental results are compared with configurational-bias Monte Carlo (CBMC) and molecular dynamics (MD) simulations of, respectively, the adsorption isotherms and diffusivities. For n-hexane, the CBMC simulated isotherm shows a slight inflection at a loading=4 molecules per unit cell; this inflection manifests, also, in the loading dependence of 1/Gamma, obtained from QENS. The trend in the loading dependence of the Fick D and Dself of nC6 obtained from QENS matches the MD simulation results. For nC7 the CBMC simulated isotherm shows a strong inflection at a loading=4 molecules per unit cell. At this loading=4, 1/Gamma tends to zero and there is a very good match between QENS and molecular simulations for the loading dependence of 1/Gamma. Both MD simulations and QENS data on the Fick diffusivity shows a sharp maximum at a loading in the region of=4. For both nC6 and nC7 the simulated values of diffusivity are about an order of magnitude higher than those determined from QENS.     

Vibrationally induced dissociation of sulfuric acid (H2SO4)

One of the important reactive steps in Earth's atmosphere is the decomposition of H(2)SO(4) to H(2)O and SO(3). However, because the UV spectrum of H(2)SO(4) was not found up to 140 nm, alternative mechanisms, including vibrationally induced dissociation, were proposed. Using adiabatic reactive molecular dynamics (ARMD) simulations with validated force fields for the product and educt channels, it is shown through explicit atomistic simulation that by exciting the ν(9) (OH-stretching-) mode, photodissociation can occur on the picosecond time scale. With the potential energy surfaces used in the present work, ν(9) = 4 is sufficient for this process. From a statistically significant number of trajectories (several thousands), vibrationally induced dissociation times are found to follow Gamma-distributions with most likely reaction times between 40 and 200 ps by depositing energies ranging from 40 to 60 kcal/mol, corresponding to 4 and 6 vibrational quanta in the OH stretching vibration. Because ARMD simulations allow multiple and long-time simulations, both nonstatistical, impulsive H-transfer and statistical, IVR-regimes of the decomposition reaction can be discussed in detail at an atomistic level.     

正十六烷体系凝固过程的分子动力学模拟

采用分子动力学模拟的方法,根据体系热容和自扩散系数的突变,计算了正十六烷体系以及正十六烷和降凝剂α-辛基萘、α-十二烷基萘及α-十六烷基萘等α-烷基萘混合体系的凝点,凝点降低结果与实验值相一致.用径向分布函数及末端距离分布对凝固温度附近体系的微观结构变化进行了研究.结果表明,凝固过程中正十六烷分子倾向于转变成为全反式的构象,以利于分子间平行有序的排列,这和熵减小的原理相一致.结合正十六烷的构象变化,探讨了烷基萘降凝的机理.     

Supercritical ammonia: a molecular dynamics simulation and vibrational spectroscopic investigation

Combining infrared spectroscopy and molecular dynamics simulations, we have investigated the structural and dynamical properties of ammonia from liquid state (T = 220 and 303 K) up to the supercritical domain along the isotherm T = 423 K. Infrared spectra show that the N-H stretching and bending modes are significantly perturbed which is interpreted as a signature of the change of the local environment. In order to compare the experimental spectra with those obtained using molecular dynamics simulation, we have used a flexible four sites model which allows to take into account the anharmonicity in all the vibration modes particularly that of the inversion mode of the molecule. A good agreement between our experimental and calculated spectra has been obtained hence validating the intermolecular potential used in this study to simulate supercritical ammonia. The detailed analysis of the molecular dynamics simulation results provides a quantitative insight of the relative importance of hydrogen bonding versus nonhydrogen bonded interactions that governs the structure of fluid ammonia.     

Head-group-modification-controlled mixing behavior of binary cationic surfactants: conductometric,viscometric, and NMR studies

The mixed-micelle formation by hexadecyltrimethylammonium bromide (HTAB) with hexadecyltriphenylphosphonium bromide (HTPB) and hexadecyltributylphosphonium bromide (HTBB), and by hexadecylpyridinium bromide (HPyBr) with HTPB and HTBB has been studied with the help of conductivity, viscometry, and NMR studies. The conductivity studies showed close-to-ideal mixing in all cases with weak synergistic interactions. The relative viscosity is more sensitive than the conductivity and demonstrated that the micellization of HTAB, HPyBr, HTPB, and HTBB proceeded through the appearance of a minimum, whereas their mixtures showed a strong maximum in each case. NMR measurements suggested that bulky head groups of HTPB and HTBB induce significant steric hindrance in the mixed state upon mixing with HTAB and HPyBr. The appearance of the maximum shown by the relative viscosity in the mixed-micellization process is due to steric compulsions, which may also lead to some structural transitions.     

Vibrational energy relaxation dynamics of Si---H stretching modes on stepped H/Si(111) 1×1 surfaces

The vibrational energy relaxation rates of excited Si---H stretching modes on the monohydride steps of miscut H/Si(111) 1×1 surfaces are calculated using Bloch-Redfield theory combined with classical molecular dynamics (MD) simulation. The structure and vibrational frequencies of the surface are first investigated using the Car-Parrinello ab initio MD method. The calculated Si---Si---H bending frequencies and relaxed structures are then used to refine the empirical potential for the classical MD simulations. The lifetime of the excited Si---H stretching mode at the step is found to be shorter than the modes on the terrace. Both the magnitude and the trend of the calculated results agree well with the experimental measurement on the 9° monohydride stepped surface. The vibrational relaxation rate of the Si---H stretching modes on the 15° monohydride stepped surface are also calculated and predicted to have a slightly shorter lifetime than for the 9° surface.     

Photodissociation of CH3I: a full-dimensional (9D) quantum dynamics study

The photodissociation of methyl iodide in the A band is studied by full-dimensional (9D) wave packet dynamics calculations using the multiconfigurational time-dependent Hartree approach. The potential energy surfaces employed are based on the diabatic potentials of Xie et al. [J. Phys. Chem. A 2000, 104, 1009] and the vertical excitation energy is taken from recent ab initio calculations [Alekseyev et al. J. Chem. Phys.2007, 126, 234102]. The absorption spectrum calculated for exclusively parallel excitation agrees well with the experimental spectrum of the A band. The electronic population dynamics is found to be strongly dependent on the motion in the torsional coordinate related to the H(3)-C-I bend, which presumably is an artifact of the diabatic model employed. The calculated fully product state-selected partial spectra can be interpreted based on the reflection principle and suggests strong coupling between the C-I stretching and the H(3)-C-I bending motions during the dissociation process. The computed rotational and vibrational product distributions typically reproduce the trends seen in the experiment. In agreement with experiment, a small but significant excitation of the total symmetric stretching and the asymmetric bending modes of the methyl fragment can be seen. In contrast, the umbrella mode of the methyl is found to be too highly excited in the calculated distributions.     

二元合金Ag_(50)Rh_(50)从液态到非晶的分子动力学计算

本文对贵重金属银铑合金Ag50Rh50的液态结构和激冷过程进行了分子动力学模拟研究原子间作用势采用紧束缚势模拟在施加了周期性边界条件的常压状态下进行。采用了偶关联函数、键对分析技术和键取向序参数以分子动力学模拟计算方法揭示了Ag50Rh50的液态结构存在原子偏聚特征以及在快速凝固过程形成原子偏聚的不均匀非晶并与同族过渡金属进行了非晶形成能力的比较。