芳氧基聚磷腈玻璃化温度的分子动力学模拟

利用分子动力学模拟方法,在等温等压(NPT)系综下,模拟了芳氧基聚磷腈玻璃化转变的动力学行为。在150~340 K温度范围内,计算体系的密度、比体积、径向分布函数等状态参数,结果表明,在玻璃化温度(Tg)附近,以上性质均有不同程度的突变,芳氧基聚磷腈的玻璃化温度为245~247 K,其中,由径向分布函数获得的数值与实验结果最为接近。     

丙三醇与1,6-己二醇混合物降温凝固过程的分子动力学模拟

利用分子动力学方法对3种不同摩尔比的丙三醇与1,6-己二醇混合物进行降温计算。分别采用自扩散系数和比体积方法得到的玻璃化转变温度与实验结果都符合较好,混合物中的丙三醇与1,6-己二醇的自扩散系数均小于单质时的自扩散系数,丙三醇与1,6-己二醇的羟基比其单质状态的羟基径向分布函数中第一峰值均变大,且随着温度的降低,回转半径变短,较小的键长出现的概率增多,键角变化较小。     

环氧树脂密度与二面角扭转能的仿真计算

基于COMPASS分子力场,利用分子动力学模拟方法和Materials Studio软件建立了低固化度交联耦合的双酚A型环氧树脂交联结构模型,并利用环氧树脂交联体系模型模拟计算了不同温度下交联环氧树脂的密度和二面角扭转能,以此预测了环氧树脂的玻璃化转变温度(Tg)。结果表明,计算得到的Tg与实验值具有良好的一致性,分子动力学模拟方法可以应用于复杂聚合物体系结构与性质的研究中。     

阳离子-π动态交联高性能聚芳吲哚醚酮的构筑

玻璃化转变温度(T_g)和热稳定性是聚合物重要的物理性能,影响了材料的应用领域。交联被认为是提高T_g和热稳定性的重要方法之一。本文以4,4-二氟二苯酮和4-羟基吲哚为单体,通过亲核取代反应成功制备了线型聚芳吲哚醚酮(PINEK),并通过简易的浸泡碱性水溶液的方式实现了PINEK的阳离子-π动态交联。与传统的线型聚醚醚酮(PEEK)和PINEK相比,交联型聚芳吲哚醚酮(C-PINEK)表现出较高的T_g(T_g=251℃)和优异的热分解温度(T_d=501℃)。     

商用车C5石油树脂/CIIR复合材料减震支座阻尼机理的分子动力学模拟研究

采用分子动力学模拟和统计分析对商用车C₅石油树脂/氯化丁基橡胶(CIIR)复合材料减震支座的阻尼机理进行研究。结果表明：随着C₅石油树脂用量的增大,C₅石油树脂/CIIR复合材料的结合能(E_b)增大,在改善复合材料的阻尼性能方面分子间范德华力起主导作用;复合材料的自由体积分数(F_FV)和均方位移(D_MS)减小,C₅石油树脂起反增塑剂的作用;复合材料的玻璃化温度(T_g)升高,有效阻尼温域(ΔT)增大,C₅石油树脂可改善复合材料的阻尼性能。C₅石油树脂/CIIR复合材料的ΔT和T_g均与E_b和F_FV之间存在显著的线性关系。本研究从分子层面分析复合材料的阻尼机理,形成定量预测复合材料的阻尼性能的理论方法,为商用车减震支座复合材料的配方优化奠定了基础。     

杜仲胶/天然橡胶/低密度聚乙烯玻璃化温度的MD模拟

玻璃化转变温度(Tg)对该共混材料的形状记忆性能有着重要意义,根据共混物的主要性质搭建合理的无定形结构模型,选用COMPASS力场、在恒温恒压(NPT)系综下,采用分子动力学(MD)方法模拟计算共混聚合物在不同温度时的比体积.研究结果表明:比体积与温度的关系曲线斜率在Tg处会发生转折;模拟计算得到的Tg为384.09 K,采用差示扫描量热(DSC)法实测得到的Tg为380.33 K,两种结果在误差允许范围内基本一致,表明MD法可以用来预测共混聚合物材料的Tg.     

丁羟胶玻璃化温度的模拟计算

丁羟胶的玻璃化转变温度(Tg)预测对丁羟推进剂的贮存、运输和使用具有重要的参考意义。根据丁羟胶的主要性质搭建合理的无定形端羟基聚丁二烯(HTPB)结构模型,选用COMPASS力场、在恒温恒压(NPT)系综下,采用分子动力学(MD)方法模拟计算HTPB在不同温度时的比体积。研究结果表明,比体积与温度的关系曲线斜率在Tg处会发生转折;模拟计算得到的Tg为208.00K,采用差示扫描量热(DSC)法实测得到的Tg为194.86K,两种结果在误差允许范围内基本一致,表明MD法可以用来预测丁羟胶的Tg。     

非填充氢化丁腈橡胶低温性能相关性的研究

采用玻璃化转变温度(T_g)、温度回缩(TR)、吉门扭转温度和低温压缩永久变形等表征了非填充硫化氢化丁腈橡胶的低温性能,并研究了丙烯腈含量、双键含量、交联密度和门尼粘度等对低温性能的影响。结果表明,氢化丁腈橡胶TR值和DSC T_g随ACN含量的变化而变化,其中低温牌号因存在第三单体破坏了亚甲基链段结晶,使氢化丁腈橡胶低温性能更加优异。交联密度对低温参数有不同程度的影响,但对吉门扭转值和TR10几乎没有影响;随着交联密度的增大,TR70和低温压缩永久变形显著改善。双键含量对低温性能的影响与交联密度的影响一致。门尼粘度对吉门扭转温度、TR10和DSC T_g的影响较小,门尼粘度较高时有利于TR70。     

双酚A对环氧树脂固化反应的影响研究

为优化覆铜板制作工艺,降低能耗,提高生产效率,在低分子质量环氧树脂(EP-44)与线性酚醛树脂固化体系(EPN)及EP-44/线性酚醛树脂/双氰胺协同固化体系(EPND)中添加双酚A(BPA)扩链,采用示差扫描量热(DSC)法研究了BPA对2种体系固化反应的影响。结果表明,双酚A可小幅提升两种体系的固化特征温度,降低固化反应速率,降低固化产物的玻璃化转变温度(T_g)。双氰胺可降低EPN-BPA体系的固化特征温度,提升反应速率,提高固化产物体系T_g。     

耐低温PET的合成及其性能研究

为了拓宽PET在低温环境下的使用,可以采用脂肪族二元酸合成改性PET。本文用己二酸合成耐低温PET,研究不同含量的己二酸对PET性能的影响。结果表明：随着己二酸含量的增加,耐低温PET的黏度和缩聚反应速率升高、玻璃化转变温度(T_g)和熔点(T_m)逐渐降低,T_g和T_m分别为40 ℃和180 ℃。耐低温PET的机械性能受己二酸含量影响不明显。当己二酸含量为15%时,在-20 ℃的低温环境下可以使用。     

Atomistic molecular simulations of structure and dynamics of crosslinked epoxy resin

Many excellent thermal and mechanical performances of cured epoxy resin products can be related to their specific network structure. In this work, a typical crosslinked epoxy resin was investigated using detailed molecular dynamics (MD) simulations, in a wide temperature range from 250 K to 600 K. A general constant-NPT MD procedure widely used for linear polymers failed to identify the glass transition temperature (T_g) of this crosslinked polymer. This can be attributed to the bigger difference in the time scales and cooling rates between the experiments and simulations, and specially to the highly crosslinked infinite network feature. However, by adopting experimental densities appropriate for the corresponding temperatures, some important structural and dynamic features both below and above T_g were revealed using constant-NVT MD simulations. The polymer system exhibited more local structural features in case of below T_g than above T_g, as suggested by some typical radial distribution functions and torsion angle distributions. Non-bond energy, not any other energy components in the used COMPASS forcefield, played the most important role in glass transition. An abrupt change occurring in the vicinity of T_g was also observed in the plots of the mean squared displacements (MSDs) of the crosslinks against the temperature, indicating the great importance of crosslinks to glass transition. Rotational dynamics of some bonds in epoxy segments were also investigated, which exhibited great diversity along the chains between crosslinks. The reorientation functions of these bond vectors at higher temperatures can be well fitted by Kohlrausch-Williams-Watts (KWW) function.     

Study on the gas permeabilities in styrene-butadiene rubber by molecular dynamics simulation

In this research, molecular dynamics (MD)simulations were used to study the transport properties of small gas molecules in the butadiene-styrene copolymer(SBR). The condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS) force field was applied. The diffusion coefficients were obtained from MD (NVT ensemble) and the relationship between gas permeability; the chemical structure and free volume of butadiene-styrene copolymer were investigated. The results indicated that the diffusion coefficient of oxygen declined with increasing styrene content. The fraction of free volume (FFV) in butadiene-styrene copolymer was calculated. It was concluded that diffusion coefficient increased as the FFV increases, which is in accordance with the analysis of the small molecular hop through the free volume in polymer matrix. Subsequently, the glass transition temperatures of these copolymers were calculated by MD. The result showed that the glass transition temperature increased with increasing styrene content in polymer.     

Study on the gas permeabilities in styrene-butadiene rubber by molecular dynamics simulation

In this research, molecular dynamics (MD) simulations were used to study the transport properties of small gas molecules in the butadiene-styrene copolymer (SBR). The condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS) force field was applied. The diffusion coefficients were obtained from MD (NVT ensemble) and the relationship between gas permeability; the chemical structure and free volume of butadiene-styrene copolymer were investigated. The results indicated that the diffusion coefficient of oxygen declined with increasing styrene content. The fraction of free volume (FFV) in butadiene-styrene copolymer was calculated. It was concluded that diffusion coefficient increased as the FFV increases, which is in accordance with the analysis of the small molecular hop through the free volume in polymer matrix. Subsequently, the glass transition temperatures of these copolymers were calculated by MD. The result showed that the glass transition temperature increased with increasing styrene content in polymer.     

Improvement of the glass transition temperature in novel molybdenum carbide doped polyaniline nanocomposites

Several studies have revealed that the glass transition temperature (T_g) of polymer nanocomposites (PNCs) may change upon the loading of even a small number of nanoparticles (NPs). However, the exact phenomenon behind such shifting in T_g is still not well known. This study uses experimental and MD simulation approaches to explore the physics of the T_g of polyaniline (PANI) nanocomposite with molybdenum carbide (Mo₂C) nanoparticles at different weight percentage loading of NPs. The experimental study shows that, at a lower wt-% loading of NPs, the T_g of the resulting PNCs decreases slowly, and above a particular loading wt-%* (30 wt-%), the depression in T_g becomes more serious. The SEM images show that for wt-% < wt-%*, NPs remain diffuse in the polymer matrix, and for wt-%t > wt-%* a large number of NPs surround the polymer. The MD simulation reveals a slow increase in the diffusion of polymer and NPs for wt-% < wt-%* and a sudden increase for wt-%t > wt-%*. The measurement of the radius of gyration of the polymer reveals swelling of the polymer for wt-% < wt-%* and contraction of the polymer for wt-%t > wt-%*. Moreover, the variations of the diffusion constants and the radius of gyration of the polymer are consistent with the T_g behaviour of PNCs.     

Thermal properties of poly(lactic acid) fumed silica nanocomposites: Experiments and molecular dynamics simulations

Poly(lactic acid) (PLA) fumed silica nanocomposites were prepared by twin-screw extruder. Thermal properties were investigated by experiments and molecular dynamics simulations. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used and 1.34 °C increase of the glass transition temperature (T_g) and 12 °C improvement of thermal stability were observed for PLA–silica nanocomposites as compared to neat PLA. Molecular dynamics simulations (NPT ensemble) were carried out using modified OPLS-AA force field, and T_g and root-mean-square radii of gyration (R_g) were calculated. A good agreement between the simulation results and experiments was obtained.     

Strength development and diffusion in symmetric and asymmetric pairs of amorphous COCs in the vicinity of Tg: A new modified temperature–pressure dependent model

The strength development and healing of symmetric and asymmetric interfaces of amorphous cyclic olefin copolymers (COC) was studied by assessment of diffusion and lap-shear strength measurement of samples bonded at different temperatures, pressures and healing times. Amorphous COCs comprise copolymers of norbornene and ethylene whereby COCs with differing norbornene content have different glass transition temperature (T_g) and molecular weight. For similar thermal bonding conditions near Tg, the symmetric pairs have larger bond strength than asymmetric pairs. The activation energy calculated using Arrhenius model was independent of molecular weight and norbornene content. The free volume of COCs increased with norbornene content. In the asymmetric pairs, the adherend consisting of the COC grade with lower T_g and larger free volume dominated the diffusion compared to the other COC adherend. The penetration depth for molecular diffusion calculated using the Flory–Huggins theory was consistent with that estimated using the experimental values of the radius of gyration and bond strength. A new modified model that incorporates the combined effects of pressure, temperature and healing time could be used to predict the bond strength of both symmetric and asymmetric COC joints. Moreover, the new model facilitates determination of more reliable values of the activation energy.     

Developing predictive models for polycyanurates through a comparative study of molecular simulation and empirical thermo-mechanical data

Using a parameter set (RDA-DR2.21_Inv) previously published by our group, both the atom-to-atom connectivities and geometries and the physical and mechanical properties of poly[bis-4-(4-cyanatophenoxy)phenyl sulphone] have been simulated with remarkable accuracy. Molecular dynamics simulation, carried out on the polymer structure at a variety of experimental temperatures, yields a transition temperature of ca. 510-520 K, which can be equated with the empirical glass transition temperature (T_g) for the polycyanurate. The same dicyanate was prepared experimentally and characterised using dynamic mechanical analysis (yielding an empirical T_g of ca. 519 K) and thermo-mechanical analysis (yielding an empirical T_g of 508-528 K). The effect of incomplete cure of the monomer or development of structural defects in the network on the thermo-mechanical properties is also examined using molecular simulation.     

Monitoring the reaction progress of a high‐performance phenylethynyl‐terminated poly(etherlmide). Part II: Advancement of glass transition temperature

The cure schedule for carbon fiber‐reinforced, phenylethynyl‐terminated Ultem™ (GE Plastics) composites was studied in an attempt to optimize the resultant glass transition temperature, T_g. Reaction progress and possible matrix degradation were monitored via the T_g. On the basis of previous research, matrix degradation induced T_g reduction was expected for increases in cure time or temperature beyond approximately 70 minutes at 350°C. Using the central composite design (CCD) of experiment technique, composite panels, neat resin, and polymer powder‐coated tow (towpreg) were cured following various cure schedules to allow for the measurement of the glass transition temperatures resulting fronm cure time and temperature variations. The towpreg and neat resin specimens were cured in a differential scanning calorimeter. The glass transition temperatures of all specimens were measured via differential scanning calorimetry; the composite glass transition temperatures were also measured with dynamic mechanical thermal analysis. The composite panels and towpreg specimens showed similar trends in T_g response to cure schedule variations. Composite and towpreg glass transition temperatures increased to a plateau with increasing cure time and temperature, whereas, the neat resin showed an optimal T_g followed by T_g reduction with increasing cure time and temperature. The optimal neat resin T_g occurred within a cure time and temperature significantly below that required to maximize the composite and towpreg glass transition temperatures.     

Cooperative behavior of poly(vinyl alcohol) and water as revealed by molecular dynamics simulations

An aqueous poly(vinyl alcohol) (PVA) model has been extensively studied by using the molecular dynamics (MD) simulation method. The employed molecular and force field models are validated against the available data in the literature. In particular, the glass transition temperature (T_g) is determined from the specific volume versus temperature, which compares well with the experimental observations. The diffusion coefficients of water (H₂O) through the PVA matrix follow the Arrhenius equations at both temperature regions separated by T_g, indicating the existence of free and bound water defined by hydrogen bonds (HBs). It has also been confirmed that HBs occur between PVA and H₂O, between PVA and PVA, between H₂O and H₂O, and all of them play the key roles in the glass transition. The local dynamics suggested by the decorrelations of various bond vectors can be well described by the Williams-Landel-Ferry (WLF) equation. This work demonstrates the cooperative behavior of PVA and H₂O which is responsible for the glass transition of the whole binary system.