Temperature Effects on the Fracture Dynamics and Elastic Properties of Popgraphene Membranes

Popgraphene (PopG) is a new 2D planar carbon allotrope which is composed of 5–8–5 carbon rings. PopG is intrinsically metallic and possesses excellent thermal and mechanical stability. In this work, we report a detailed study of the thermal effects on the mechanical properties of PopG membranes using fully-atomistic reactive (ReaxFF) molecular dynamics simulations. Our results showed that PopG presents very distinct fracture mechanisms depending on the temperature and direction of the applied stretching. The main fracture dynamics trends are temperature independent and exhibit an abrupt rupture followed by fast crack propagation. The reason for this anisotropy is due to the fact that y-direction stretching leads to a deformation in the shape of the rings that cause the breaking of bonds in the pentagon-octagon and pentagon-pentagon ring connections, which is not observed for the x-direction. PopG is less stiff than graphene membranes, but the Young's modulus value is only 15 % smaller.     

Relationships between molecular structure and thermomechanical properties of bio‐based thermosetting polymers

Molecular dynamics simulations are used to study highly cross‐linked epoxy networks comprised of furanyl epoxy monomer, 2,5‐bis[(2‐oxiranylmethoxy)methyl]‐furan (BOF), that is cross‐linked by two furanyl amine hardeners, 5,5'‐methylenedifurfurylamine (DFDA) and 5,5'‐ethylidenedifurfirylamine (CH₃‐DFDA). Important properties of these fully furan‐based systems, including room temperature density, glass transition temperature, and Young's modulus are found to agree with previous experimental results. We also compare the simulated and experimental values of four fully furan‐based thermosetting materials to those using the conventional resin diglycidyl ether of bisphenol A (DGEBA) cured with the two furanyl hardeners. Our simulation results predict a slight decrease in density and Young's modulus, but no impact on the glass transition temperature, upon adding the methyl group in DFDA. Detailed analyses of the MD trajectories reveal the underlying mechanisms responsible for the observed structure/property relations, which center on the lack of collinear covalent bonds in the BOF molecular structure. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 285–292     

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.     

Photoelastic analysis of matrix crack-tilted fiber bundle interaction

Matrix crack-tilted fiber bundle interaction was explored using photoelasticity. First, the isochromatic fringe patterns near the matrix crack tip, either shielded by a tilted fiber bundle or crossed by a broken fiber bundle, were observed. Then, the stress intensity factors of cracks at varying distances from the tilted fiber bundle were extracted from the isochromatic fringe patterns. Finally, finite element simulation was conducted in ABAQUS software to verify the experimental results, and the difference between photoelasticity measurement and FEM simulation were discussed. The results show that the mode I stress intensity factor of the crack near a tilted fiber bundle increases with the increase of crack length and decreases with the increase of the Young's modulus of the fiber bundle. However, the mode II stress intensity factor, which clearly increases as crack length increased and, as opposed to mode I, increases as the Young's modulus of the fiber bundle increased.     

Stability of adsorber resins under mechanical compression and ultrasonication

Different adsorber and ion exchange resins have been sonicated with ultrasound of different frequencies in a fixed bed. Afterwards, the sonicated particles were investigated by scanning electron and light microscopy. Also, different mechanical properties, like fracture load, maximum flattening and Young's modulus were measured in compression tests and compared to the results of the ultrasonication experiments. Cavitational collapses cause cracks in resins with a high fracture load and a high Young's modulus as well as a low deformation at fracture. Complete destruction of adsorbent particles due to acoustic cavitation is observed with particles of low fracture loading and low Young's modulus. No or only little destruction is observed with particles which inhibit both sufficient strength and deformation properties. On the basis of these findings it is concluded that resin particles with optimum resistance to ultrasound show both a sufficient fracture load as well as high mechanical flexibility. Copyright © 2002 John Wiley & Sons, Ltd.     

Deformation and microstructure of extended-chain polydiacetylene crystals

The mechanical properties (Young's modulus, ultimate tensile strength, deformation processes) of extended-chain polydiacetylene crystals are investigated. The properties observed are similar to those of metal and ceramic whiskers. The elastic modulus is strain-dependent and the ultimate tensile strength increases with decreasing crystal size. The maximum tensile strength observed was 1700 Nmm^?2. The ultimate tensile strength seems to be controlled by the presence of a small number of defects near the surface at which fracture nucleates. Irreversible deformation of the crystals was observed to occur by crack propagation normal and parallel to the direction of the macromolecules. The observed mechanical behavior corresponds to exceptionally high per-chain properties. The per-chain modulus obtained for these crystals is nearly as high as that of diamond. A chain-aligned polyethylene fiber with the same per-chain mechanical properties would have an ultimate strength as high as 0.9 × 10⁴ Nmm^?2.     

Micromechanical characterization of the interphase layer in semi‐crystalline polyethylene

The interphase layer in semi‐crystalline polyethylene is the least known constituent, compared to the amorphous and crystalline phases, in terms of mechanical properties. In this study, the Monte Carlo molecular simulation results for the interlamellar domain (i.e. amorphous+ interphases), reported in (Macromolecules 2006, 39, 439–447) are employed. The amorphous elastic properties are adopted from the literature and then two distinct micromechanical homogenization approaches are utilized to dissociate the interphase stiffness from that of the interlamellar region. The results of the two micromechanical approaches match perfectly. Interestingly, the dissociated interphase stiffness lacks the common feature of positive definiteness, which is attributed to its nature as a transitional domain between two coexisting phases. The sensitivity analyses reveal that this property is insensitive to the non‐orthotropic components of the interlamellar stiffness and the uncertainties existing in the interlamellar and amorphous stiffnesses. Finally, using the dissociated interphase stiffness, its effective Young's modulus is calculated, which compares well with the effective interlamellar Young's modulus for highly crystalline polyethylene, reported in an experimental study. This satisfactory agreement along with the identical results produced by the two micromechanical approaches confirms the validity of the new information about the interphase elastic properties in addition to making the proposed dissociation methodology quite reliable when applied to similar problems. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1228–1243     

参杂缺陷石墨烯的高分子复合材料导热特性分子动力学模拟

Polymers are widely used advanced materials composed of macromolecular chains, which can be found in materials used in our daily life. Polymer materials have been employed in many energy and electronic applications such as energy harvesting devices, energy storage devices, light emitting and sensing devices, and flexible energy and electronic devices. The microscopic morphologies and electrical properties of the polymer materials can be tuned by molecular engineering, which could improve the device performances in terms of both the energy conversion efficiency and stability. Traditional polymers are usually considered to be thermal insulators owing to their amorphous molecular chains. Graphene-based polymeric materials have garnered significant attention due to the excellent thermal conductivity of graphene. Advanced polymeric composites with high thermal conductivity exhibit great potential in many applications. Therefore, research on the thermal transport behaviors in graphene-based nanocomposites becomes critical. Vacancy defects in graphene are commonly observed during its fabrication. In this work, the effects of vacancy defects in graphene on thermal transport properties of the graphene-polyethylene nanocomposite are comprehensively investigated using molecular dynamics (MD) simulation. Based on the non-equilibrium molecular dynamics (NEMD) method, the interfacial thermal conductance and the overall thermal conductance of the nanocomposite are taken into consideration simultaneously. It is found that vacancy defects in graphene facilitate the interfacial thermal conductance between graphene and polyethylene. By removing various proportions of carbon atoms in pristine graphene, the density of vacancy defects varies from 0% to 20% and the interfacial thermal conductance increases from 75.6 MW·m⁻²·K⁻¹ to 85.9 MW·m⁻²·K⁻¹. The distinct enhancement in the interfacial thermal transport is attributed to the enhanced thermal coupling between graphene and polyethylene. A higher number of broken sp² bonds in the defective graphene lead to a decrease in the structure rigidity with more low-frequency (< 15 THz) phonons. The improved overlap of vibrational density states between graphene and polyethylene at a low frequency results in better interfacial thermal conductance. Moreover, the increase in the interfacial thermal conductance induced by vacancy defects have a significant effect on the overall thermal conductance (from 40.8 MW·m⁻²·K⁻¹ to 45.6 MW·m⁻²·K⁻¹). In addition, when filled with the graphene layer, the local density of polyethylene increases on both sides of the graphene. The concentrated layers provide more aligned molecular arrangement, which result in better thermal conductance in polyethylene. Further, the higher local density of the polymer near the interface provides more atoms for interaction with the graphene, which leads to stronger effective interactions. The relative concentration is insensitive to the density of vacancy defects. The reported results on the thermal transport behavior of graphene-polyethylene composites provide reasonable guidance for using graphene as fillers to tune the thermal conduction of polymeric composites.     

Mechanical Properties of a Single Electrospun Fiber and Its Structures

Summary: A method to measure the Young's modulus of a single electrospun polyacrylonitrile (PAN) fiber is reported. The Young's modulus can be calculated from the force‐displacement curves obtained by the bending of a single fiber attached to an atomic force microscopy (AFM) cantilever. It is suggested that the high modulus of electrospun fibers is caused by the orientation of molecular chains, which is confirmed by wide‐angle X‐ray diffraction (WAXD) measurements. The communication will provide a basic understanding of the relationship between mechanical properties and structures of electrospun fibers.

A PAN fiber was attached to a contact mode cantilever to facilitate the measurement of force‐displacement curves and Young's modulus.     

Dynamics response of polyethylene polymer nanocomposites to shock wave loading

The shock response of polyethylene polymer modified by nanoparticles (NP) is investigated using a coarse‐grained molecular dynamics simulation. The u_s‐u_p Hugoniot analysis yields a linear relationship under the range of particle velocity investigated, in agreement with previous simulation and experimental results. NP addition improves the mechanical properties of the composites, as reflected by the increased Young's modulus and yield strength especially in the case of shorter chain length of polymer. This is directly related to the increased shock impedance with NP volume fraction, as demonstrated by the enhanced pressure in the shocked state, slightly reduced microscopic deformation, and increased shock velocity. The layered structure with alternate soft and hard regions, with NP addition only in the hard regions, leads to significantly enhanced microscopic deformation in the soft regions. It is also important that the shock impedance difference between the soft and hard region to be large enough to facilitate the energy absorption through plastic deformation in the soft regions. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1292–1302     

A study on the orientation structure and mechanical properties of polyacrylonitrile precursors

The polyacrylonitrile precursors were made through the two‐stage drawing process. The orientation structure was examined through wide‐angle X‐ray diffraction (WAXD). The orientation factors and the modulus were measured through the sound velocity method. The mechanical properties, such as the Young's modulus, the tensile strength and the breaking elongation ratio were obtained by the single fiber tensile test. The results showed that the Young's modulus and the strength of the precursors increased with draw ratio, which is accordant with the enhancement of the micromolecular orientation degree. Therefore the orientation factors obtained from the experiments were compared with the theoretical curves which were predicted through the Crawford and Kolsky's model. The physical meaning of the parameters m and n were analyzed. A good agreement of the orientation factor between the experimental data and the theoretical curve was achieved. Copyright © 2005 John Wiley & Sons, Ltd.     

Comparing the mechanical response of di‐, tri‐, and tetra‐functional resin epoxies with reactive molecular dynamics

The influence of monomer functionality on the mechanical properties of epoxies is studied using molecular dynamics (MD) with the Reax Force Field (ReaxFF). From deformation simulations, the Young's modulus, yield point, and Poisson's ratio are calculated and analyzed. Comparison between the network structures of distinct epoxies is further advanced by the monomeric degree index (MDI). Experimental validation demonstrates the MD results correctly predict the relationship in Young's moduli. Therefore, ReaxFF is confirmed to be a useful tool for studying the mechanical behavior of epoxies. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 255–264     

An experimental investigation of fracture by cavitation of model elastomeric networks

A new methodology to investigate the failure of elastomers in a confined geometry has been developed and applied to model end-linked polyurethane elastomers. The experimental in situ observations show that the elastomers fail by the growth of a single cavity nucleated in the region of maximum hydrostatic stress. Tests carried out at different temperatures for the same elastomer show that the critical stress at which this crack grows is not proportional to the Young's modulus E but depends mainly on the ratio between the mode I fracture energy G_IC and E. A reasonable fit of the data can be obtained with a model of cavity expansion by irreversible fracture calculating the energy release rate by finite elements with a strain hardening constitutive equation. Comparison between different elastomers shows that the material containing both entanglements and crosslinks is both tougher in mode I and more resistant to cavitation relative to its elastic modulus. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48:1409–1422, 2010     

Enhanced crystallization,thermal properties,and hydrolysis resistance of poly(l-lactic acid) and its stereocomplex by incorporation of graphene nanoplatelets

Poly(d-lactic acid) (PDLA) and graphene nanoplatelets were used as nucleating agents for poly(l-lactic acid) (PLLA). The graphene (1 wt%) shows a more pronounced effect than PDLA in facilitating PLLA crystallization. Graphene effect on crystallization of stereocomplex (SC) polylactide is also demonstrated. Although medium molecular weight PLLA was blended with a limited content (1 wt%) of low molecular weight PDLA in the presence of graphene (0.5 phr), SC melting temperature is slightly increased without the use of high molecular weight polylactide pair. Also, optimal graphene content (0.5 phr–1.5 phr) promotes crystallization of PLLA homocrystals in the three-component system (PLLA/PDLA/graphene). Graphene additionally enhances Young's modulus and barrier property to thermal degradation of both PLLA and SC systems. Furthermore, PLLA/graphene is more resistant to hydrolysis than PLLA. Likewise, PLLA/PDLA/graphene is more stable than PLLA/PDLA during hydrolysis.     

Mechanical properties of the novel organometallic polymer poly(ferrocenyldimethylsilane)

A study of the mechanical properties of poly(ferrocenyldimethylsilane) [Fe(η‐C₅H₄)₂SiMe₂]_n, 3 , a novel organometallic polymer, has been performed on thin films of this material. The Young's modulus and Poisson's ratio of film samples (15 × 1 × 1 mm) of 3 were measured in quasi‐static tension using a video extensometer. For 3 , the values of the Young's moduli (E) and Poisson's ratios (ν) were similar between axes in the plane and independent of the splicing direction used during sample preparation. The mean and standard deviation of the Young's modulus and Poisson's ratio were 0.78 ± 0.08 GPa and 0.37 ± 0.06 GPa, respectively. Thermomechanical analysis of 3 revealed a steady decrease of E from a room temperature value of approximately 0.70 GPa. Additionally, it was found that at 150 °C, 3 was unable to support even small stresses, consistent with the onset of a melt transition (ca. 135 °C). A mathematical model based on molecular geometry is developed to describe the results. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2280–2288, 2005     

Stress distribution in poly‐p‐phenylenebenzobisoxazole (PBO) fiber as viewed from vibrational spectroscopic measurement under tension. I. Stress‐induced frequency shifts of Raman bands and molecular deformation mechanism

To clarify the relationship between a molecular deformation mechanism and a high Young's modulus of poly‐p‐phenylenebenzobisoxazole (PBO), Raman spectra were measured for fibers subjected to a tensile stress along the chain axis. The stress‐induced frequency shift of the observed Raman bands could be reproduced reasonably by the normal‐mode calculation under a quasi‐harmonic approximation. The frequency position at zero stress and the shift factor of Raman bands were predicted for a PBO chain that agreed with the actually evaluated values. On the basis of these analyses, the molecular deformation mechanism of the PBO chain has been discussed in detail. The crystalline modulus of the PBO chain was calculated theoretically to be 458 GPa, in good agreement with the X‐ray observed value of 460 GPa. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1269–1280, 2002     

Sodium chloride's effect on self‐assembly of diphenylalanine bilayer

Understanding self‐assembling peptides becomes essential in nanotechnology, thereby providing a bottom‐up method for fabrication of nanostructures. Diphenylalanine constitutes an outstanding building block that can be assembled into various nanostructures, including two‐dimensional bilayers or nanotubes, exhibiting superb mechanical properties. It is known that the effect of the ions is critical in conformational and chemical interactions of bilayers or membranes. In this study, we analyzed the effect of sodium chloride on diphenylalanine bilayer using coarse‐grained molecular dynamics simulations, and calculated the bending Young's modulus and the torsional modulus by applying normal modal analysis using an elastic network model. The results showed that sodium chloride dramatically increases the assembling efficiency and stability, thereby promising to allow the precise design and control of the fabrication process and properties of bio‐inspired materials. © 2016 Wiley Periodicals, Inc.     

Micellar structure and mechanical properties of block copolymer‐modified epoxies

Amphiphilic block copolymers provide a unique means for toughening epoxy resins because they can self‐assemble into different inclusion shapes before epoxy curing. The two examples reported here are spherical micelles and vesicles, which form in blends containing epoxy and symmetric or asymmetric poly(ethylene oxide)–poly(ethylene‐alt‐propylene) (PEO–PEP) block copolymer with PEO volume fractions of 0.5 and 0.26, respectively. The vesicles and spherical micelles were characterized by transmission electron microscopy and small‐angle X‐ray scattering (SAXS), respectively. SAXS data from the spherical micelles were fit to the Percus–Yevick model for a liquid‐like packing of spheres with hard‐core interactions. Mechanical properties of spherical‐micelle‐modified and vesicle‐modified epoxies in the dilute limit are compared. The glass‐transition temperature and Young's (storage) modulus were tested with dynamic mechanical spectroscopy, and compact‐tension experiments were performed to determine the critical plane‐strain energy release rate for fracture. Vesicles were most effective in improving the epoxy fracture resistance. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2996–3010, 2001     

Effects of Moisture on the Mechanical Properties of Microcrystalline Cellulose and the Mobility of the Water Molecules as Studied by the Hybrid Molecular Mechanics–Molecular Dynamics Simulation Method

A hybrid molecular mechanics–molecular dynamics simulation method has been performed to study the effects of moisture content on the mechanical properties of microcrystalline cellulose (MCC) and the mobility of the water molecules. The specific volume and diffusion coefficient of the water increase with increasing moisture content in the range studied of 1.8–25.5 w/w%, while the Young's modulus decreases. The simulation results are in close agreement with the published experimental data. Both the bound scission and free‐volume mechanisms contribute to the plasticization of MCC by water. The Voronoi volume increases with increasing moisture content. It is related to the free volume and the increase enhances the mobility of the water molecules and thus increases the coefficient of diffusion of the water. Moreover, with increasing moisture content, the hydrogen bonding per water molecule between MCC–water molecules decreases, thus increasing the water mobility and number of free water molecules. © 2019 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 454–464     

Mechanical properties of crosslinked polyolefin-based materials

The effect of crosslinking on mechanical properties of various polyethylene-based materials is compared. In virgin polyethylene, higher strength results from formation of spatial network, especially at increased temperatures. On the other hand, decreased crystalline portion leads to lower Young's modulus values compared with the uncrosslinked polymer. Crosslinking of LDPE/PP blends leads to a dramatic increase in elongation at break and impact resistance. The reason is seen in an in situ formation of very efficient compatibilizers via co-crosslinking on the phase boundary. In LDPE filled with silica, the main effect consists in higher elongation at break and increased toughness, although the effect is lower than that in LDPE/PP blends. Increased resistance to the crack growth rate was demonstrated to be a reason for the observed behaviour. In LDPE filled with organic fillers, formation of direct covalent bonds on the interface and a significant increase in adhesion is suggested to be the reason for the enormous increase in Young's modulus and tensile strength observed.