The study on imidization degree of polyamic acid in solution and ordering degree of its polyimide film

Polyamic acid (PAA) based on pyromellitic dianhydride and 4,4′-oxydianiline (PMDA–ODA) can be chemically converted to copolyamic acid-imide (PA-I) which is a partially converted PAA at ambient temperature by treatment with a mixture of acetic anhydride and pyridine. A systematic investigation by rotation viscometer and FT-IR revealed that phase separation in PA-I solution occurred when imidization degree was about 26%. Imidization degree of PA-I solution could be controlled by using adequate acetic anhydride amount, initial PAA concentration and imidization time. PA-I solutions with different imidization degree were converted to corresponding polyimide films by thermal imidization. WAXD showed that as imidization degree in PA-I solution increased, the degree of morphologic ordering of final PI film was improved, while tensile strength and modulus increased and ultimate elongation reduced slightly.     

天然胶原的稳态流变性能和数学模拟

用旋转流变仪研究胶原溶液的稳态流变性能,讨论了浓度和温度的变化对胶原溶液流动曲线的影响。实验数据显示胶原溶液的剪切粘度随着剪切速率的增大而降低,表明胶原溶液具有明显的假塑性。同一剪切速率下,温度升高,胶原溶液的剪切粘度降低;浓度增大,剪切粘度增大。用幂律方程、Carreau方程和Cross方程对胶原溶液的流动曲线进行数学拟合,结果表明幂律方程,Carreau方程和Cross方程都能准确地描述胶原的流动曲线,其中幂律方程的粘度指数K和非牛顿指数n的变化能准确反映胶原假塑性的变化;Carreau方程和Cross方程的参数比幂律方程多,其拟合得到的曲线和实验数据吻合得更好。但在本实验条件下,Carreau方程和Cross方程预测的零剪切粘度η0和无穷剪切粘度η∞不准确。     

Characterizing mechanical properties of graphite using molecular dynamics simulation

The mechanical properties of graphite in the forms of single graphene layer and graphite flakes (containing several graphene layers) were investigated using molecular dynamics (MD) simulation. The in-plane properties, Young’s modulus, Poisson’s ratio, and shear modulus, were measured, respectively, by applying axial tensile stress and in-plane shear stress on the simulation box through the modified NPT ensemble. In order to validate the results, the conventional NVT ensemble with the applied uniform strain filed in the simulation box was adopted in the MD simulation. Results indicated that the modified NPT ensemble is capable of characterizing the material properties of atomistic structures with accuracy. In addition, it was found the graphene layers exhibit higher moduli than the graphite flakes; thus, it was suggested that the graphite flakes have to be expanded and exfoliated into numbers of single graphene layers in order to provide better reinforcement effect in nanocomposites.     

分子沉积膜的力学特性研究

在金衬底上制备了分子沉积膜。用原子力显微镜研究了它的摩擦力,结果表明沉积膜能明显地减小摩擦力。用纳米压痕仪研究它的纳米力学特性,结果表明在金表面沉积一层膜后,弹性模量、硬度和载荷都减小,说明分子沉积膜能改善金衬底的纳米力学特性。     

Preparation,mechanical and thermal properties of functionalized graphene/polyimide nanocomposites

Graphene oxide sheets with isocyanate functional groups (GONCO) were firstly synthesized and functionalized graphene/polyimide (FGS/PI) nanocomposites were subsequently prepared by typical solution casting and thermal imidization. The chemical changes of GONCO during the preparation of FGS/PI nanocomposites were carefully characterized by Fourier transfer infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses. As a result, the morphology analysis indicated that the FGS were dispersed in the PI matrix and were aligned more orderly with increasing the FGS contents. The tensile strength and the modulus of FGS/PI nanocomposites were significantly increased by 60% with a small quantity of 0.75 wt% FGS incorporated and decreased beyond that dosage. Moreover, the thermogravimetric analysis (TGA) results revealed that the thermal stability of PI was slightly improved by the incorporation of FGS.     

芳香族聚酰胺酸的制备及其热亚胺化研究

采用3,3',4,4'-二苯醚四甲酸二酐(ODPA)和4,4'-二氨基二苯醚(ODA)为单体,N,N-二甲基乙酰胺(DMAc)为溶剂,合成聚酰亚胺(PI)的前驱体聚酰胺酸(PAA).研究了单体摩尔比、反应体系的质量浓度、反应时间以及反应温度等因素对PAA相对分子质量(以PAA黏度表示)的影响.结果表明:经提纯处理后的单体等摩尔比时所得的PAA的黏度最高,相对分子质量最大;反应体系的质量浓度为10%～15%,反应温度控制在15℃,反应时间为10h左右可以得到聚合物相对分子质量较高,并且在后期亚胺化反应时成膜性良好的PAA前躯体.通过对前躯体热失重分析(TGA),结合红外光谱(IR)分析计算所得PAA热环化程度与温度的定量关系,确定出PAA亚胺化的最佳工艺条件,制得了热稳定性较好的PI,该PI在空气和氮气气氛中5%失重的温度分别达到535℃和537℃.     

填料特性对聚酰亚胺复合材料摩擦学性能的影响

采用不同特性玻璃质刚性填料(玻璃纤维、玻璃纤维粉、5μm 和20μm 玻璃微珠) 填充改性热塑性聚酰亚胺( TPI) , 利用MPX-2000 摩擦试验机测定了干摩擦、水润滑和油润滑条件下材料的摩擦磨损性能, 考察了不同形态、尺度填料的影响, 用扫描电子显微镜(SEM) 观察磨损表面形貌, 分析材料磨损机理。结果表明, 大尺寸填料的单位个体与基体的界面面积和结合强度大于小尺寸填料, 其磨损率比小尺度填料的材料低。在水和油起到良好冷却作用后, 球形颗粒易出现应力集中, 疲劳裂纹向四周扩展、交汇, 产生疲劳磨损, 其程度随颗粒尺寸增大而提高, 表现为20μm 玻璃微珠填充材料磨损率最大。      

Strain- and strain-rate-dependent mechanical properties and behaviors of Cu<Subscript>3</Subscript>Sn compound using molecular dynamics simulation

This work aims at investigating the mechanical properties and behaviors of orthorhombic Cu₃Sn crystals at room temperature through molecular dynamics (MD) simulation. The focuses are placed on the tensile stress–strain behaviors and properties of the Cu₃Sn single crystal and also their dependence on applied strain and strain rate. An attempt to characterize the deformation evolution of the Cu₃Sn nanostructure during the stress–strain test is also made. In addition, the elastic properties of bulk polycrystalline Cu₃Sn are estimated, as a function of strain rate and applied strain, by using the monocrystal results. The effectiveness of the MD model is demonstrated through comparison with the nanoindentation results and also published theoretical and experimental data. The calculated orthotropic elastic and shear moduli and Poisson’s ratio of Cu₃Sn single crystal reveal not only high anisotropy, but also the great effects of applied strain and strain rate only as the strain rate exceeds a threshold value of about 0.072% ps⁻¹. Specifically, raising the strain rate increases the orthotropic elastic properties and also the ultimate tensile and shear strengths of the nanocrystal, whereas increasing the applied strain reduces them.     

无机组分对聚酰亚胺杂化薄膜电性能的影响

初步探讨了聚酰亚胺薄膜在电场作用下的电学行为,采用溶胶-凝胶工艺制备了聚酰亚胺/二氧化硅纳米杂化薄膜,并对薄膜进行浸水24 h处理,利用原子力显微镜对制备的薄膜进行表面形貌表征,讨论了无机组分SiO2和水对薄膜电性能的影响.结果表明:无机组分的引入及两相间的界面形态将对杂化薄膜的电学性能产生重大的影响;偶联剂的引入使得两相间产生紧密的微相结合,并对电性能产生一定的影响.     

Effect of rapeseed oil on the rheological,mechanical and thermal properties of plastic lubricants

A strong difference in the physico-chemical properties of the plastic lubricants studied was found in this study through pressure drop, thermal analysis, vibration damping, texture hardness and rheological measurements. Oxidation aging of the lubricant sample containing rapeseed oil additive was proposed. Its higher thermal sensitivity was simultaneously confirmed by frequency dependent complex shear modulus of elasticity measurements as well as by rheological testing. Rapeseed oil modified lubricant showed a higher decrease in both storage and moduli losses due to a temperature increase from 16 to 26 ° C compared to the rapeseed oil free sample. Simultaneously, the flow curves were shifted to the higher shear stresses (for plastic lubricant without rapeseed oil additive) typical for rheopectic fluids. For the rapeseed oil modified lubricant, the flow curves were shifted to the lower shear stresses, indicating its thixotropic fluid behaviour. The synthetic lubricant without rapeseed oil additive exhibited higher dissipative rheological behaviour as reflected by decreasing first resonance frequency peak position compared to the rapeseed oil modified lubricant as obtained from vibration damping measurements. It was found that the synthetic lubricant exhibited better vibration damping properties and mechanical energy dissipation into heat due to its higher viscous friction than the rapeseed oil modified lubricant under experimental conditions.

     

Microstructures and mechanical properties of aligned electrospun carbon nanofibers from binary composites of polyacrylonitrile and polyamic acid

High mechanical performance carbon nanofibers are highly required for the carbon nanofiber-reinforced composites, and it is necessary to develop novel precursors for the preparation of carbon nanofibers. In this work, blends of poly(acrylonitrile-butyl acrylate mono-butyl itaconate) (co-PAN) and polyamic acid (PAA) were electrospun into aligned nanofibers and the nanofibers were converted to carbon nanofibers by thermal imidization, pre-oxidation and high-temperature carbonization. FT-IR spectroscopy was applied to monitor the chemical structures of the nanofibers before and after pre-oxidation. Tensile tests were used to characterize the mechanical properties of electrospun carbon nanofibers (ECNFs). The microstructures of ECNFs were investigated by high-resolution TEM and Raman spectroscopy. The results indicated that the ECNFs derived from blend of co-PAN/PAA with molar ratio of 6/4 and with carbonization temperature of 1400 °C possessed the highest tensile strength of 1212 MPa, which could be attributed to the ordered graphitic structures in ECNFs.     

Effects of acid solution of different components on the pore structure and mechanical properties of coal

To explore the influence of acid solution with different components on the pore structure and mechanical properties of coal, nuclear magnetic resonance (NMR), scanning electron microscope (SEM), energy dispersion spectrum (EDS) and uniaxial compression experiments were used to analyze the acid-treated coal samples. The results show that acid treatment can obviously improve the connectivity of coal, increase the porosity of coal, and is conducive to the circulation and diffusion of gas, but the promotion effect of acid solutions of different components on pores with different pore sizes is different. After acid treatment, the number of mineral particles on the surface of the coal samples are obviously reduced, and the space filled with mineral particles is gradually exposed, thus increasing the density of pores and fractures of the coal samples. The energy spectrum analysis shows that acid solution added HF has a good removal effect on kaolinite minerals, iron minerals and calcite minerals. For the mechanical properties of the coal samples, acid treatment can reduce the strength and elastic modulus of coal samples, but increase its toughness. In addition, the fracture evolution of the acid-treated coal samples mainly goes through four stages: initial stage, transition stage, expansion stage and destruction stage. Because most of the energy accumulated in the acid-treated coal samples is used to damage the coal body, the acid-treated coal samples are easy to form fracture network, and is mostly broken in blocks when unstable.     

The effect of formulated molecular weight on temperature resistance and mechanical properties in polyimide based composites

This experimental study examines the role of formulated molecular weight between crosslink sites on the temperature resistance and mechanical properties of composites based on a polyimide containing a diphenyl thioether unit (PTI). The composites are fabricated by in situ polymerization of monomer reactants (PMR) using three monomeric ingredients: bis(3,4-dicarboxyphenyl) sulfide dianhydride (TDPA); 4,4-methylene dianiline (MDA); and the monomethyl ester of norbornene anhydride (NE). By changing monomeric molar ratio, three formulations are prepared, in which formulated molecular weight between crosslink sites varies from 1487 to 3446 g mol^–1. Unidirectional composite laminates from each formulation and T300 carbon fibres are compression moulded and cut into a series of test specimens. By measuring the glass transition temperature (T _g), Mode I interlaminar fracture toughness (G _IC) and other mechanical properties at room and elevated temperatures, the influences of formulated molecular weight on the temperature resistance and mechanical properties of PTI-based composites are investigated.     

Residual stress in CIGS thin film solar cells on polyimide: simulation and experiments

Sputtering technique is used to prepare Cu(In,Ga)Se₂ (CIGS) thin film solar cells on a UPILEX-S 50S polyimide substrate in order to investigate the residual stress in the Mo back contact and CIGS absorber layer after selenized annealing with various the thickness ratios of the Mo contact, R_Mo. A comparison between the results of numerical simulation and those obtained from X-ray diffractometry, indicate the existence of compressive residual stress in both the Mo contact and the CIGS absorber layer. The residual stress in the Mo contact was inversely proportional to the residual stress in the CIGS absorber layer. Residual stress decreased with an increase in the thickness ratio of the Mo contact. The empirical formulae for the residual stresses as a function of R_Mo in the Mo and CIGS films were deduced, from the results of this study.     

升温速率对聚酰亚胺薄膜聚集态结构与性能的影响

采用五种升温速率1℃/min、3℃/min、5℃/min、7℃/min、10℃/min对PMDA-ODA型聚酰亚胺薄膜的聚集态结构和性能进行了系统化研究。TGA和FTIR的研究发现酰亚胺化反应分为三个阶段,每个阶段影响失重的主要因素不同。聚酰亚胺薄膜的聚集态结构依赖于升温速率对溶剂含量与酰亚胺化程度的影响。当升温速率适中时,才能使溶剂残留率、酰亚胺化程度、分子链运动达到最恰当的匹配,导致分子链形成规整有序排列。形成规整有序结构的最佳升温速率为5℃/min。     

膜厚对四面体非晶碳膜机械性能的影响

采用过滤阴极真空电弧技术以相同的工艺条件在p(100)单晶硅衬底上制备了不同厚度的四面体非晶碳薄膜,并利用表面轮廓仪测试薄膜的厚度和应力,利用纳米压入仪测试薄膜的硬度、杨氏模量和临界刮擦载荷.试验表明,在一定的扫描波形条件下,薄膜大约以0.7 nm/s的沉积速率稳定生长.随着膜厚的增加,薄膜的应力持续降低,当膜厚超过30nm时,应力将低于5GPa;当膜厚超过300nm时,硬度和杨氏模量分别将近70GPa和750GPa,已经十分接近体金刚石的性能指标.另外,随着膜厚增加所产生的应力变化,也导致了可见光拉曼光谱非对称宽峰的峰位逐渐向低频偏移.     

Effects of hydrostatic extrusion on the mechanical and thermal properties of polypropylene

The structure change in spherulites for hydrostatically extruded polypropylene (PP) was studied by the use of internal friction measurements and thermophotometry tests. The onset temperature of the -loss peak of the PP sample decreases with increasing reduction in area, R. For the extrudates below R=50%, the peak temperature of -loss shifts to lower temperature. The and absorptions for the extrudates up to R=50% become broad and overlap with each other. The intensity of the -loss peak, _max, is maximum for the extrudate with R=50%. The results of tan , damping, and the intensity of the -loss peak indicate that the mechanism of molecular chain deformation is divided into two stages, below and above R=50%. The results are due to spherulitic changes, i.e. the shape of spherulites changed from spherical to elliptical in the extrudates above R=50% and the spherulite with R=50% changed from coarse structure to a finer one by the imposition of hydrostatic pressure.     

Influences of magnetized hydroxyapatite on the growth behaviors of osteoblasts and the mechanism from molecular dynamics simulation

To investigate the influence of magnetized hydroxyapatite on the growth and differentiation of osteoblasts, hydroxyapatite (HA) and magnetized hydroxyapatite (mHA) were synthesized and characterized. The cell viability, differentiation, and morphologies of osteoblasts were investigated in vitro, respectively. The results showed that compared to HA, cells cultured with mHA had better cell viability, and both HA and mHA were beneficial to the early differentiation of osteoblasts. Furthermore, the interaction mechanism between mHA and osteoblasts was elucidated using a molecular dynamics simulation. The simulation results indicated that when cultured with osteoblasts, HA adsorbed bovine serum protein onto its surface from the medium immediately, which was beneficial to the adhesion and proliferation of osteoblasts. The main driving force for the adsorption of bovine serum was the electronic properties of HA crystal faces. The (211) crystal face of HA had the highest electron density among its all crystal faces, thus mainly contributing to the protein adsorption of HA. Nevertheless, the (211) crystal face of mHA still had a relatively higher electron density than that of HA, thus possessing better protein adsorption than that of HA, and in turn promoting the biological functions of osteoblasts.     

Effects of hydroxyapatite dosage on mechanical and biological behaviors of polylactic acid composite materials

Due to the widespread use of bioresorbable polylactic acid (PLA) based materials in biomedical applications, there is an increasing need to obtain independent control of key properties such as mechanical strength, degradation rate and bioactivity in order to further enhance the performance of the PLA based medical implants. Because PLA composites containing hydroxyapatite (HAp) particles can augment cell growth and mechanical properties, in this present study, the effects of HAp dosages on various mechanical strengths and cell proliferation of PLA composites were further investigated. The information resulting from this study can assist in the optimizing of PLA composite formulas for specific medical applications.     

Development of polyimide–nanosilica filled EPDM based light rocket motor insulator compound: Influence of polyimide–nanosilica loading on thermal,ablation, and mechanical properties

Thermal protection materials are necessary to protect structural components of launch vehicles during lift-off of launching system. The present study deals with the development of a novel thermally protected light rocket motor insulator compound (RMIC) of polyimide–silica filled EPDM nanocomposites. The insulation compound prepared for the studies comprised of aromatic polyimide and nanosilica particles. The addition of these materials in rocket insulator compound enhanced the multifunctional thermal and insulation characteristics. EPDM when grafted with maleic anhydride, contributed polarity in the non-polar EPDM matrix. Nanosilica contributes specifically better erosion resistance. SEM and TEM micrograph of EPDM nanocomposites exhibits good dispersion of nanosilica in polyimide–EPDM matrix. Nanocomposite formation was characterised by FTIR. Density, co-efficient of thermal expansion, thermal conductivity, ablation rate, specific heat, maximum thermal degradation, char yield and mechanical properties of the RMIC have been measured. This developmental study may find wide scope for commercial exploitation.