Molecular Dynamics Simulation of the Fracture in Polymer‐Exfoliated Layered Silicate Nanocomposites

Summary: The MD technique was used to investigate the fracture behavior in fully exfoliated layered silicate (nanoplatelet)‐polymer nanocomposites. MD results reveal that the addition of the nanoplatelets can improve the fracture strength of polymers. The interactions between the surface of the nanoplatelets and the segments of the polymer, and the relaxation time of polymer chains have significant influences on the fracture strength of the polymer. For polymers with T_g below room temperature, such as polyurethane, or close to room temperature, such as nylon, the nanoplatelets are always working for the enhancement of the mechanical properties. However, for polymers with T_g above room temperature, such as epoxy and polystyrene, the addition of the nanoplatelets is not working well for toughening these polymers. If the nanoplatelets are to enhance the mechanical properties of these polymers, it is necessary to build up a stress relaxation interface between the polymer and the nanoplatelet in order to reduce the effect of the difference between the relaxation time of nanofillers and that of polymers.

Force per area versus distance curves as a function of the difference of the relaxation times of the nanoplatelets and polymer chains.     

EPDM/PP热塑性弹性体在循环应力下的老化行为研究

本文研究了动态硫化EPDM/PP热塑性弹性体的动态疲劳老化行为,考察了其力学性能的变化,并分析了产生力学性能下降的原因。实验结果表明,随着疲劳时间的延长、疲劳振幅的增大,材料的断裂强度降低,并认为疲劳过程中完全硫化的EPDM橡胶粒子和热塑性塑料PP界面处的分子链断裂、滑移导致了断裂强度的降低;紫外光的加入,加速了材料在疲劳过程的分子链断裂、滑移速率,使材料的断裂强度有更大程度的降低;在机械疲劳老化单独作用下,材料体系几乎没有发生氧化反应,而紫外光的加入,促使了机械疲劳老化过程中氧化反应的发生。     

AFM substantiation of the fracture behavior and mechanical properties of sol–gel derived silica packed epoxy networks

Silica packed epoxy networks are prepared in two steps via in situ, solvent free sol–gel processing of tetraethoxysilane in liquid epoxy monomer and curing the mixture with a flexible diamine afterwards. The influence of filler content and processing conditions on the mechanical properties and the fracture behavior is studied by means of the static mechanical analysis and AFM characterization of the pristine and the fractured polymer surfaces, and a mechanism to enhance polymer strength and toughness is proposed. The in–situ evolution and packing of silica nanostructures into epoxy networks influences the overall morphology and performance of polymers under high stress. It is found that smaller silica domains distributed at the molecular level cause efficient crack distribution by absorbing energy and thus improve the strength and toughness of silica packed epoxy polymers.     

Fracture of polymer interfaces: what are the relevant length scales?

While it is tempting to relate directly the molecular structure of an interface (between glassy or between semi‐cristalline polymers) with its fracture toughness, these two parameters are simply the two end‐points of a complex network which needs to be understood in order to control the mechanical strength of the interface. The important mechanisms occur at three different length scales: the molecular scale (stress‐transfer across the interface), the microscopic scale (plastic deformation at the crack tip) and the macroscopic scale (loading geometry and elastic constants of the polymers). The couplings existing between these length scales in glassy polymer interfaces are reviewed in this paper in light of the latest experimental studies.     

A ductile-brittle zone in polyimides depending on the reaction temperature

The mechanical and optical properties of polyimides were studied in this paper and the influence of the variation of the reaction temperature on the physico-chemical properties of the polymers was evaluated. From this the dependence of the stress-strain diagrams on the reaction temperature, as well as the stresses and strains at fracture were experimentally determined. Moreover, the elastic moduli and Poisson's ratios, as well as the refractive index of the polymers were evaluated for different temperatures of imidization. In order to define also the behaviour of the polymers as thin membranes at fracture, simple tension tests with edge-cracked thin strips were executed up to fracture. The method of caustics was used, with the specimens loaded inK _I mode of deformation at different stress-levels to evaluate the stress intensity factors of the materials in the non-linear zone of loading. TheK _I -factor was evaluated by applying the simple Dugdale-Barenblatt model for the ductile materials, whereas for brittle samples the elastic theory was used. Interesting results concerning the physico-mechanical properties of the polyimides were derived.     

Synthesis and characterization of poly(arylene ether imidazole)s

Poly(arylene ether imidazole)s were prepared by the aromatic nucleophilic displacement reaction of a bisphenol imidazole with activated aromatic dihalides. The polymers had glass transition temperatures ranging from 230 to 318°C and number-average molecular weights as high as 82,000 g/mol. Thermogravimetric analysis showed a 5% weight loss occurring ～ 400°C in air and ～ 500°C in nitrogen. Typical neat resin mechanical properties obtained at room temperature included tensile strength and tensile modulus of 14.2 and 407 ksi and fracture energy (G_lc) of 23 in. lb/in.² Titanium-to-titanium tensile shear strengths measured at 23 and 200°C were 4800 and 3000 psi, respectively. In addition, preliminary data were obtained on carbon fiber laminates. The chemistry, physical, and mechanical properties of these polymers are discussed.     

Effect of bond exchange rate on dynamics and mechanics of vitrimers

Thermoset polymers are classified amongst the most challenging materials to recycle due to the permanent crosslinks that increase their strength and stiffness compared to their thermoplastic counterparts. Vitrimers provide a promising route to achieve the recyclability of thermosets by implementing dynamic covalent bonds within the network. In this study, a hybrid molecular dynamics (MD)-Monte Carlo (MC) technique is used to simulate these adaptive networks constructed by a coarse-grained model. The model proposed in this work describes the dynamic nature of the covalent bonds while maintaining a constant crosslink density. As this framework also shows flexibility in accommodating various exchange reaction activation energy via adjusting the energy difference in MC step, the dynamic and mechanical properties of the vitrimer system are intensely affected by the number of successful bond exchanges happening at every step. In both rubbery and glassy regimes, lowering the energy barrier of the bond exchange reaction results in enhanced motion for the vitrimer segments. This enhanced mobility, in turn, directly affects the stress–strain relationship of these networks, where a higher number of exchanges results in larger deformation before fracture even at low temperatures. Furthermore, the stress distribution in vitrimers shows more homogenous distribution before failure than in the thermoset network.     

New soybean oil‐styrene‐divinylbenzene thermosetting copolymers. III. Tensile stress–strain behavior

The tensile stress–strain behavior and fracture properties of some new soybean oil based polymeric materials were investigated at room temperature. These materials were prepared by the cationic copolymerization of regular soybean oil, low saturation soybean oil (LoSatSoy oil), or conjugated LoSatSoy oil with styrene and the diene comonomers divinylbenzene, norbornadiene, or dicyclopentadiene in a process initiated by boron trifluoride diethyl etherate (BF₃ · OEt₂) or related modified initiators. These new polymeric materials exhibited tensile stress–strain behavior ranging from soft rubbers through ductile to relatively brittle plastics. The Young's moduli of these polymers varied from 3 to 615 MPa, the ultimate tensile strengths varied from 0.3 to 21 MPa, and the elongation at break varied from 1.6 to 300%. These properties are obviously related to their crosslink densities. The conjugated LoSatSoy oil polymers had higher mechanical properties than the corresponding LoSatSoy oil and regular soybean oil polymers with the same stoichiometry. Some conjugated LoSatSoy oil polymers with appropriate stoichiometries showed yielding behavior in the tensile test process. A variety of new polymer materials can thus be prepared by varying the stoichiometry, the type of soybean oil, and the crosslinking agent. These soybean oil based polymers possessed mechanical properties comparable to those of commercially available rubbery materials and conventional plastics and thus may serve as replacements in many applications. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 60–77, 2001     

Influence of Backbone Structure on Properties of Directly Polymerized Phenoxy Resins from Epichlorohydrin and Various Aromatic Dihydric Phenols Monomers

A series of phenoxy resins was directly prepared through the polymerization of each of the various aromatic dihydric phenols and epichlorohydrin.FTIR and 1H NMR spectra were recorded to characterize the structures of the re-sins.The GPC curves were used to determine the molecular weight distribution.In addition,the thermal properties of the resins were studied with differential scanning calorimetry(DSC)and thermal gravimetric analysis(TGA).The thermal stabilities of the polymers increased with the content of the benzene ring,pendant group increasing or biphenyl groups emerging.The adhesive properties of the polymers were evaluated in terms of the lap shear strength with Fe-Fe adherends.The fracture mechanisms were determined by SEM observation and it was found that there was an important participation of cohesive fracture mechanisms.Also,it has been demonstrated that the extension of these micro-cohesive mechanisms is directly correlated with the adhesive strength.According to these results,the phenoxy resin containing biphenyl groups presented a higher adhesive strength and could improve the adhesive property of the epoxy/phenoxy system to a certain extent.     

Comportement mécanique de l'acier inoxydable martensitique 17-4 PH en corrosion sous contrainte et à la fragilisation par l'hydrogéne environnemental

Age hardened martensitic stainless steels have high resistances to mechanical stress, to friction corrosion and to stress-corrosion cracking (after certain ageing heat treatments). These steels have also high mechanical toughness levels. The mechanical strength of these steels increases when performing specific heat treatments in order to promote the precipitation of hardening phases such as copper-rich ε and/or chromiumrich α′. However, it is well known that the susceptibility to stress-corrosion cracking and to hydrogen embrittlement increases when the mechanical strength of the martensitic steels is high.This paper is devoted to a study the mechanical behaviour of the 17-4 PH martensitic stainless steel with respect to stress-corrosion cracking and to embrittlement by environnemental hydrogen in different ageing conditions, i.e. ageing temperatures (200 to 650 °C) and ageing times (1 and 4 hours). The two behaviours were studied by carrying out low strain rate tensile tests (ε′ = 2.7×10⁻⁶ s⁻¹) in H₂SO₄ 1N. By using differential scanning microcalorimetry, we have identified the precipitation mechanisms of ε and α′ phases, and calculated their activation energies by applying KISSINGER'S relationship.The results obtained show that the susceptibility to stress-corrosion cracking and to hydrogen embrittlement is maximum at the optimum ageing temperature for which the strength level is maximum. The optimum ageing temperature increases when the ε phase, initially coherent, becomes non-coherent. The fracture mechanism changes from cleavage to intergranular mode, respectively. Finally, the fracture mechanism by stress-corrosion cracking is the same as that provoked by hydrogen embrittlement.     

Crystal deformation in aromatic polyesters

A study has been carried out of the changes in the x-ray diffraction patterns which occur when oriented fibers or tapes of poly(trimethylene terephthalate) (3GT) and poly-(tetramethylene terephthalate) (4GT) are subjected to mechanical tensile stress. Although the polymers show very different behavior in detail, in both cases comparatively large reversible lattice strains are observed (～ several %). The diffraction pattern of 3GT changes monotonically with increasing macroscopic strain, suggesting that the lattice responds immediately to the applied stress, and deforms as though it were a coiled spring. In 4GT, on the other hand, there is no detectable change in the x-ray diffraction pattern at low macroscopic strains, i.e., low values of the applied stress. At higher stresses, changes in the pattern occur which suggest a definite change in the crystal structure. Finally at the highest values of applied stress, the lattice deformations cease to increase. A preliminary discussion is presented of the relationship of these x-ray diffraction results to the mechanical stress–strain behavior.     

The Effect of Degradation and Stabilization on the Mechanical Properties of Polymers Using Polypropylene Blends as the Main Example

Degradation can result from a variety of chemical, physical and mechanical mechanisms, most of them involving a reduction of molecular weight and thus a decrease in the mechanical performance of the degraded polymer. A clear understanding and control of these mechanisms is absolutely essential: without stabilization some polymers (e.g. PVC, polyolefins) would not survive their processing undamaged. In this paper an overview of the different degradation mechanisms, their effect on molecular chains, and the methods used to characterize the extent of degradation will be given. Subsequently we establish some fundamental relationships between the microstructure and the mechanical performance (of thermoplastic polymers) using differently aged and stabilized polypropylene (PP) - EPR compounds. In particular we investigate the influence of two types of heat stabilizers (phenolic antioxidants and hindered amine stabilizer HAS) on the degradation behaviour of test specimens thermally aged at 120 and 135°C respectively. From an investigation of the changes with aging time in structure and low-strain properties (yield stress, strain at yield, tensile modulus) and from the differences in the evolution of the fracture properties a molecular model of the chain scission mechanisms and of inter-lamellar connectivity (through tie-chain molecules) has been established, which allowed an explanation of the gradual change of the dominant deformation mechanism from cold drawing to crazing and brittle fracture.     

碳纤维三向织物/环氧树脂复合材料的制备与力学性能

选择不同纱线间距[即二经(纬)纱之间的中心距]尺寸的碳纤维三向织物, 采用热压成型技术制备了碳纤维三向织物/环氧树脂复合材料. 研究了纱线间距及样品裁剪角度等对力学性能的影响, 并与碳纤维二向织物/环氧树脂复合材料的力学性能进行对比. 结果表明, 随着纱线间距尺寸从2 mm增加到6 mm, 0°方向断裂强度从221.7 MPa下降到148.1 MPa, 撕裂强力从1000 N下降到600 N; 90°方向断裂强度从50.0 MPa下降到22.1 MPa, 撕裂强力从330 N下降到100 N; 顶破强力从424 N下降到216 N. 这些力学性能的逐渐降低是单位面积的碳纤维增强体含量减少和织物的孔洞增大共同作用的结果. 纱线间距为2 mm的碳纤维三向织物复合材料在0°(以纬纱为基准), 30°, 45°, 60°和90°方向的断裂强度分别为221.7, 48.5, 44.3, 227.7和50.0 MPa, 即断裂强度在0°和60°方向大于在30°, 45°及90°方向. 由三向织物的编织原理可知, 0°与60°方向完全相同, 因此其断裂强度相似, 且样品中有一组纱线与外加载荷平行, 对形变破坏具有一定的约束作用, 而30°, 45°和90°方向样品中三组纱线所在的方向均和外加载荷存在一定的夹角, 有效分散外加载荷的能力减弱. 对比碳纤维三向织物/环氧树脂复合材料与碳纤维二向织物/环氧树脂复合材料的断裂强度、 撕裂强力和顶破强力发现, 前者的综合性能明显优于后者, 碳纤维三向织物/环氧树脂复合材料的断裂强度、 撕裂强力、 顶破强力分别为221.7 MPa, 1000 N和424 N, 三向织物的优势得到凸显, 为后续制备多种类型的碳纤维三向织物复合材料及应用奠定了基础.     

A Guiding Principle for Strengthening Crosslinked Polymers: Synthesis and Application of Mobility‐Controlling Rotaxane Crosslinkers

Three component mobility controlling vinylic rotaxane crosslinkers with two radically polymerizable vinyl groups ( RC_R s) were synthesized to prove that the mobility of the components of the RC_R s plays a crucial role in determining the properties of rotaxane‐crosslinked polymers (RCPs). RC_R s (R=H, Me, or Et) were obtained from living ring‐opening polymerization. RCP_Et was prepared using RC_Et , which exhibits the lowest component mobility. The low component mobility is reflected in inferior mechanical strength and stretching ability in tensile stress tests compared to components with good (R=Me) and high (R=H) mobility. However, RCP_Et exhibited significantly higher stress and strain values than the corresponding covalently crosslinked polymers ( CCP_R s). These results indicate that a suitable component mobility substantially enhances the mechanical strength of RCPs. This behavior could serve as a guiding principle for the molecular design of advanced RCs.     

嵌段共聚物相容剂对聚合物共混体系力学性能影响的连续介观模拟

结合介观动力学方法和三维弹簧格子模型,研究了嵌段共聚物相容剂对相容性较差的聚合物二元共混体系力学性能的影响.在适当范围内不断增加嵌段共聚物相容剂的用量,研究了相容剂含量对体系杨氏模数及拉伸强度的影响,同时也对不同体系材料的破碎位点进行了分析.结果表明,未加入相容剂的二元共混体系在拉伸模拟中表现出较低的拉伸强度,而适量添加相容剂可以显著提升材料的拉伸强度,随着相容剂含量的增加,共混体系的破碎位点会发生转移并最终改善材料的整体性能.而相容剂的加入对体系杨氏模数的影响较小.该连续模拟方法为关联聚合物复合体系的微观结构和宏观力学性能提供了一条高效的途径.     

Investigation of degradation mechanisms in mechanical properties of polyimide films exposed to a low earth orbit environment

The degradation of the mechanical properties of polyimide films was evaluated by means of tensile tests after exposure to a low earth orbit (LEO) environment. Polyimide films irradiated with atomic oxygen (AO), ultraviolet (UV) light, and electron beam (EB) rays using ground simulation facilities were also evaluated similarly and compared. In these experiments tensile stress (7.0 MPa or less) was applied to the samples in order to assess its effects on mechanical properties. The mechanical properties of the flight samples decreased concomitantly with increased exposure duration. The fracture surfaces exhibited characteristic radiated patterns initiating from the exposed surfaces which showed a rough texture. In the AO-irradiated samples the mechanical properties degraded and the surface texture developed as the AO fluence increased; similar fracture surfaces appeared in the flight samples. In contrast, UV and EB irradiation had little impact on mechanical properties. Based on these results, the eroded surfaces by AO irradiation served as the starting points of the rupture, resulting in degradation of mechanical properties of polyimide films exposed to a LEO environment. The tensile stress states induced no difference in evaluations.     

Effect of Pickering emulsion on the mechanical performances and fracture toughness of epoxy composites

The improvement of mechanical properties and toughness of nanoparticles for epoxy composites was mostly dependent on the disperse state of nanoparticles in epoxy matrices. When the content of nanoparticles was higher than a threshold value, it was easy to aggregate and then affect the improvement effect. Pickering emulsion was prepared using SiO₂ nanoparticles as emulsifier and functional monomer as oil phase. The influence of Pickering emulsion on the curing process was investigated. The effect of Pickering emulsion on the mechanical properties, toughness, and glass transition temperature (Tg) was studied. Impact and tensile fracture surface were observed by scanning electron microscopy (SEM). Results from differential scanning calorimeter (DSC), tensile, impact, and fracture toughness tests are provided. The results indicated that the introduction of Pickering emulsion can eliminate the residual stress and accelerate curing reaction. Epoxy composites were capable of increasing tensile strength by up to 29.9%, impact strength of three‐fold, fracture toughness of 35%, and Tg of 20.7°C in comparison with the reference sample. SEM images showed that SiO₂ nanoparticles exhibit a good dispersion in epoxy matrix. The increases in mechanical properties, toughness, and Tg of epoxy composites were attributed to the “Second Phase Toughness” mechanism.     

Broad line NMR study of molecular motion in uniaxially stretched polymers

Capability of NMR technique to study fracture and elementary destruction acts in polymers is discussed. Tensile stress influences macromolecular mobility As a rule mobility of macromolecules decreases under deformation. So far deformation remains reversible, this effect is reversible, too. As a result of chain micro-Brownian motion hindering under stress the amorphous layers can pass from high-elastic to solid state even at temperatures rather higher than T_g measured under initial conditions. It is mechanical vitrification (not structure changes) that is regarded as the general causes of draw process interruption (destruction of polymer). Deformation-induced electron emission was detected. It is caused by ionization of stressed chains and tunnel transitions of electrons into deep traps. Influence of molecular mobility on the traps and their destroying under stress are considered.     

Polymer Alloys—Their Structure,Morphology, and Properties

Polymeric materials with novel properties for new technological applications are increasingly obtained by combining existing polymers, while the synthesis of new monomers has receded into the background. These polymer combinations or “alloys” (polyblends) are characterized by their chemical composition, the conformation of the chain molecules, and the morphology, i.e. the state of order at supramolecular level. Multiphase constitution is a typical characteristic of these substances, with a decisive influence on their macroscopic properties. The morphology of multiphase polymer alloys can be controlled to a limited extent via the chemical composition of their components when homopolymers are mixed in the melt or as dispersions. Graft copolymerization, on the other hand, makes it possible to achieve the desired morphology at a given chemical composition. Furthermore, transprent two-phase polymer alloys can be obtained under certain conditions. In multiphase polymers the reduction of stress without fracture, caused by mechanical loading will be treated using models. Certain combinations of properties such as hardness and toughness are connected with the coexistence of disperse and continuous phases. Equilibrium thermodynamical criteria for liquid mixtures wil be used to explain demixing phenomena in polymers. In the last few years it has become possible to determine the chain conformation experimentally using neutron scattering.     

Environmental Stress Cracking of Polymers Monitored by Fatigue Crack Growth Experiments

This article describes fatigue crack growth experiments to investigate the degradation of the durability of polymers due to fluid environments. The degrading effect of media causing stress cracking can be observed on the fracture surfaces of tested samples by scanning electron microscopy. Strategies to improve environmental stress cracking like changes in molecular weight, orientation, toughening with rubber particles of different sizes are discussed. Fatigue crack growth experiments can be employed as a very fast and effective screening method.