Effects of high hydrostatic pressure on the mechanical behavior of a crystalline polymer–polyoxymethylene

The true stress-true strain behavior of polyoxymethylene, n(-CH₂O), as an example of a bulk semi-crystalline polymer, has been investigated for constant hydrostatic environmental pressures from 1 atmosphere to 8 kilobars with the principal objectives of elucidating the factors controlling flow and fracture. Experiments were conducted in uniaxial tension at room temperature and constant strain rate. The tensile observations were supplemented by measurements of bulk compressibility and stress relaxation behavior at pressure. In contrast with metals and inorganic compounds, the modulus, yield stress and fracture stress of POM increase strongly with pressure by a factor of approximately three at 8 kilobars. The modulus increase is shown from the stress relaxation measurements to be associated with a pressure-induced increase in the β-transition temperature which points to the potential usefulness of the concept of pressure-temperature super-position of mechanical behavior. The characteristics of the pressure dependence of the yield stress demonstrate that yield criteria based on continum mechanics considerations, including the Mohr or Coulomb-Navier criterion, are not valid for general deformation (non-plane strain) conditions in this polymer. The concept of a critical volume change determining the initiation of yielding is suggested to be applicable to semi-crystalline polymers. Comparison with analogous changes in yield stress with temperature points to an increasing contribution to the control of yielding by the initially disordered regions with increasing pressure or decreasing temperature. The fracture behavior observed at pressure eliminates the concepts of a critical stress as a fracture criterion for POM and of a simple reduction in normal stress at points of stress concentration as the principal effect of the applied pressure on fracture.     

Network model for the viscoelastic behavior of polymer nanocomposites

A theoretical network model reproducing some significant features of the viscoelastic behavior of unentangled polymer melts reinforced with well dispersed non-agglomerated nanoparticles is presented. Nanocomposites with low filler volume fraction (∼10%) and strong polymer-filler interactions are considered. The model is calibrated based on results obtained from discrete simulations of the equilibrium molecular structure of the material. This analysis provides the statistics of the network of chains connecting fillers, of dangling strands having one end adsorbed onto fillers, and that of the population of loops surrounding each nanoparticle. The network kinetics depends on the attachment-detachment dynamics of grafted chains of various types and is modeled by using a set of convection equations for the probability distribution functions. The overall viscoelastic response depends strongly on the lifetime of the polymer-filler junctions. The largest reinforcement is observed at low strain rates and low frequency oscillations. A solid like behavior is predicted for systems in which the polymer molecules interact strongly with the nanoparticles, effect which is associated with the behavior of the network of bridging segments.     

Large deformation mechanical behavior of flexible nanofiber filled polymer nanocomposites

In the present work, the large deformation behavior of high aspect ratio flexible nanofiber reinforced polymer composites is investigated. Simple or successive tensile tests are performed at room temperature, i.e. in the rubbery state. By studying two different types of fibers, namely cellulose nanofibrils and carbon nanotubes, with two processing routes, the role of entanglements and of interactions existing between fibers—within the nanofiber network that can be formed in the material—on the composite properties is highlighted. For cellulosic nanofillers, strong hydrogen bonds between fibers lead to a spectacular reinforcement effect combined with a decrease of the composite ultimate strain and an irreversible damage of composite properties after first deformation (rigid network). When such strong interactions between fillers are limited (soft entangled network or simple contacts between non-entangled fibers) the resulted reinforcement is less important and no decrease of the deformation at break is observed. For carbon nanotube fillers, the evolution of the filler network during tensile test is finally highlighted by in situ electrical measurements.     

Influence of rheological properties on the sagging of polypropylene and abs sheet for thermoforming applications

The isothermal sagging resistance of different grades of conventional and a high melt strength (HMS) PP has been correlated with the rheological characteristics of the polymers, such as dynamic shear properties, melt strength, and zero shear viscosity. A thermoforming grade of acrylonitrile‐butadiene‐styrene (ABS) was used as a reference material. At 190°C, ABS had the highest viscosity and elastic modulus in the frequency range measured, showing that this polymer is highly elastic. HMS PP had a greater shear thinning behavior than conventional PP because of its broader molecular weight distribution. The tan δ of the polymers showed that conventional PP had a higher tendency to flow than HMS PP and ABS when heated above 172°C. This was confirmed with sagging experiments performed in an air circulating oven, where the rate of sagging decreased as the melt strength and the zero shear viscosity of the polymer increased.     

Polymer-Impregnated mortars I. Effect of polymer state on mechanical behavior

Mortar specimens were impregnated with methyl methacrylate, n-butyl acrylate, styrene, and crosslinking agents in various combinations. After polymerization of the monomers in situ, studies of mechanical properties such as Young's modulus and compressive strength were made. In one experiment, various ccpolymers of methyl methacrylate and n-butyl acrylate were prepared and tested as a function of temperature. Excellent reinforcement was obtained with any combination of monomers as long as the resulting polymer was at a temperature below its glass transition temperature. This suggests that the modulus of the reinforcing polymer is crucial, glassy behavior being required. The addition of crosslinking agents such as TMPTMA increased the high temperature strength, however.     

A coarse-grained model for the mechanical behavior of multi-layer graphene

Graphene is the strongest and highest weight-to-surface ratio material known, rendering it an excellent building block for nanocomposites. Multi-layer graphene (MLG) assemblies have intriguing mechanical properties distinct from the monolayer that remain poorly understood due to spatiotemporal limitations of experimental observations and atomistic modeling. To address this issue, here we establish a coarse-grained molecular dynamics (CG-MD) model of graphene using a strain energy conservation approach. The model is able to quantitatively reproduce graphene’s mechanical response in the elastic and fracture regimes. The hexagonal symmetry of graphene’s honeycomb lattice is conserved, and therefore the anisotropy in the non-linear large-deformation regime between the zigzag and armchair directions is preserved. The superlubricity effect, namely the strong orientational dependence of the shear rigidity between graphene layers, is also captured. We demonstrate the applicability of the model by reproducing recent experimental nanoindentation results in silico. Our model overcomes the limitations of current CG-MD approaches, in accurately predicting the fracture properties, the interlayer shear response, and the intrinsic anisotropy of MLG. Additionally, our fast, transferable force-field can be straightforwardly combined with existing coarse-grained models of polymers and proteins to predict the meso-scale behavior of hybrid carbon nanomaterials.     

A predictive model for the mechanical behavior of particulate composites

A method for predicting the stress-strain and volumetric behavior of particulate composites from constituent properties has been developed for large values of strain. This approach allows a simple model for systems in which damage occurs without resorting to complicated constitutive equations. An energy balance derived from the first law of thermodynamics and the equations of linear elasticity calculates critical strain values at which filler particles will dewet when subjected to uniaxial tension and superimposed pressure. Calculations of critical strains over the entire strain history using reevaluated material properties accounting for the damage yield highly nonlinear stress-strain and volumetric curves. Experimentally observed dependences on particle size, filler concentration, matrix and filler properties, and superimposed pressure are correctly predicted. The method has no adjustable parameters, and allows several idealized models of the dewetting process to be examined. Comparisons of model predictions to experimental data show good agreement.     

Size-dependent mechanical behavior of nanoscale polymer particles through coarse-grained molecular dynamics simulation

Anisotropic conductive adhesives (ACAs) are promising materials used for producing ultra-thin liquid-crystal displays. Because the mechanical response of polymer particles can have a significant impact in the performance of ACAs, understanding of this apparent size effect is of fundamental importance in the electronics industry. The objective of this research is to use a coarse-grained molecular dynamics model to verify and gain physical insight into the observed size dependence effect in polymer particles. In agreement with experimental studies, the results of this study clearly indicate that there is a strong size effect in spherical polymer particles with diameters approaching the nanometer length scale. The results of the simulations also clearly indicate that the source for the increases in modulus is the increase in relative surface energy for decreasing particle sizes. Finally, the actual contact conditions at the surface of the polymer nanoparticles are shown to be similar to those predicted using Hertz and perfectly plastic contact theory. As ACA thicknesses are reduced in response to reductions in polymer particle size, it is expected that the overall compressive stiffness of the ACA will increase, thus influencing the manufacturing process.     

Curing behavior and mechanical behavior of fully and semi-interpenetrating polymer networks based on polyurethane and acrylics

Polyurethane (PU) was made by reacting stoichiometric equivalent of trimethylol propane (TMP) and desmodur L. Fully interpenetrating polymer networks (fully IPN's) of various compositions based on PU and poly(ethylene glycol) diacrylate (PEGDA) were prepared by blending various ratios of PU/PEGDA, and cured by benzoyl peroxide (BPO). Semi-interpenetrating polymer networks based on PU and poly(ethylene glycol) monomethyl ether of acrylate (PEGMEA) were prepared in a similar way. Shift of exothermic peaks during IPN formation were examined with dynamic DSC. Viscosity increases were investigated with a Brookfield RVT type viscometer. Dynamic mechanical properties were probed via a rheometric dynamic spectroscopy (RDS).Expermintal results revealed a good compatibility of both IPN systems, as evidenced from the single damping peak of the RDS curves for each composition. Shifts of exothermic peaks to higher temperatures during the formation of fully IPN were observed, especially for the composition of PU/PEGDA = 50/50, which showed an exothermic peak at the highest temperature. Experimental results also revealed delayed viscosity increases and decreased gel fractions for all fully IPN's. On the contrary, the semi-IPN did not exhibibt similar phenomena. All these findings supported an effect of network interlock during fully IPN formation. The existence of a network not only provided a sterically hindered environment, but also restrained the chain mobility of the growing network, and vice versa, thus retarding the curing rates of both networks. Network interlock also broadened the width of the half damping peak, T_1/2, and subsequently led to improved mechanical properties such as the impact resistance and Young's modulus of fully IPN material.     

A qualitative model for the mechanical behavior of pretreated asbestos-epoxy composites

The aim of the present work was to investigate the effect of fiber pretreatment, fiber content, coating concentration, and also pretreatment procedure on the tensile properties of asbestos-epoxy composites. These composites comprise materials of considerable industrial importance. Asbestos exhibits unique properties. Occasionally it is difficult to disperse asbestos properly because the individual fibrils are very small and tend to agglomerate, but there is no evidence that asbestos is not easily wetted out by all systems. However, the higher potential aspect ratio of asbestos fibers compared with glass is not always realized in practice because the fibers break in length to fine diameters. Accordingly, the very considerable new surface created makes successful wetting difficult or impossible. For this reason, asbestos was precoated with a well adhered film of poly(hexamethylene adipamide), a polymer especially compatible with the epoxy phase. The pretreatment procedure followed was based on the principles of the interfacial polymerization involving the serial application to the asbestos of two immiscible solutions of hexamethylenediamine and adipoyl chloride. Accordingly, epoxy-nylon-chrysotile composites were made while varying pretreated fiber volume fraction, concentration of the polyamide coating, and reactants application order. Results obtained from tensile measurements proved quite interesting: Relative modulus of elasticity and relative ultimate strength exhibited a similar behavior when correlated with the ratio of polyamide to asbestos concentration. The corresponding curves pass through distinct maxima at which severe improvement of the material performance is effected. A thorough interpretation of these results is also included.     

The mechanical behavior of polymer coatings: Solvent removal in polyamic acid films

The development of the mechanical properties and stresses during the formation of polyamic acid (PAA) coatings has been studied using methods such as a “propagating wave technique.” The shrinkage stress due to solvent removal in PAA coatings under a one dimensional constraint was determined to be 8 MPa. A coupling between the residual stress and solvent removal process has been observed.     

Application of a constitutive equation to polymer melts

Yamamoto's integral constitutive equation in which the memory function is dependent on the second invariant of the rate of deformation tensor at past times has been found to be successful in predicting many of the nonlinear viscoelastic functions from the linear viscoelastic data for melts of linear polyethylenes, polypropylenes, and polystryene but not for those of branched polyethylenes with high level of long-chain branching. A specific functional form for the rate-dependent relaxation spectrum is used and is based on the physical meaning resulting from the molecular entanglement theory of Graessley on steady shearing flow. No arbitrary constant is involved in such an interconversion scheme. The data examined are dynamic storage modulus and loss modulus, steady flow viscosity, first normal stress difference, and parallel superimposed small oscillations on steady shear flow. The theory predicts that in such parallel superimposed experiments, storage modulus G′(ω,\documentclass{article}\pagestyle{empty}\begin{document}$ {\rm \dot \gamma } $\end{document}) divided by the square of frequency shows a maximum under finite shear and that G′(ω,\documentclass{article}\pagestyle{empty}\begin{document}$ {\rm \dot \gamma } $\end{document}) would itself become negative at a frequency whose value is about one third the superimposed rate of shear. The experiments are in line with such predictions. Possible reasons for the failure of the theory for branched polyethylenes are considered, and a possible approach is suggested so that the interconversion scheme may be successful for such resins.     

Dynamic mechanical behavior of a polyimide

The dynamic mechanical properties of a polyimide (poly-4,4′-oxydiphenylene pyromellitimide) were studied from about 4 to 800°K. at audio frequencies. A prominent relaxation associated with absorbed water content occurs near 230°K. Below 270°K., the modulus undergoes a corresponding increase in value with increasing water content. Above 550°K. the onset of a major relaxation process is observed in the modulus data, and a maximum in the internal friction is observed at about 675°K. Minor relaxation peaks are also noted throughout the temperature range. Relatively minor differences are noted in the mechanical relaxation spectra for the polyimide when treated (after drying) with dimethylformamide, dimethylacetamide, and dimethylsulfoxide, whereas the treatment with water after drying has a marked effect on the relaxation behavior. A reactor radiation dose of 3000 Mrad also causes only minor alterations in the dynamic mechanical spectra.     

Flow behavior of stampable sheet with a rib part

The effect of die design on the flow characteristics of compression molded three-layer long glass fiber reinforced polypropylene composite, known as “stampable sheet,” was investigated. The flow behavior of each layer was observed by optical and soft X-ray photographs. The fiber flow length into the rib part was measured by an image processing system. The results showed that during compression resin exudation occurred and that this depended on the die's rib width. Thus, the larger the rib width, the lower the amount of resin exuded. Resin exudation was found to occur earlier in a die with zero fillet radius. The fiber flow length into the rib part was enhanced by using a die with a large fillet radius. The results were confirmed by a short-shot molding process. In this process, the stamping operation was stopped short of making a complete part. A finite element method was used to study the flow process. The results agree with the experimental results confirmed by short-shot molding.     

Prediction of the mechanical properties of a crosslinking polymer from a combination of the measured kinetics and a model for network growth

The mechanical properties of a crosslinking isocyanate–hydroxy system were predicted by a combination of the measured curing kinetics and a model for polymer network growth. The kinetic parameters were determined from FTIR using the linear rising temperature method (the activation energy = 52 kJ/mol, reaction order = 2.9). This data, combined with the network model, was used to predict the rise in elastic modulus as measured by dynamic mechanical analysis (DMA). The agreement between the predicted and experimental results was excellent over the early part of the curing, but the model failed at higher isocyanate conversions. In addition a novel method for obtaining the activation energy for the reaction directly from DMA results is presented; the value obtained from this method was in excellent agreement with that obtained by FTIR.     

Aspects of interactions in complex polymer coating formulations

Acid–base interaction parameters have been measured by inverse gas chromatography for mixed stationary phases of film‐forming polymers and pigments. The quantities of adsorbed polymer required fully to coat the pigment surfaces were established, and rheological measurements were used to evaluate the thickness of polymer barriers generated by the adsorption process. Both the barrier thickness and the critical amount of polymer needed to overcoat the pigments were found to be dependent on acid–base interactions. Acid–base considerations also determined the rate of material redistribution when a third component was added to premixtures of two‐component polymer/pigment combinations. Time‐dependent variations in the surface energies of polymer films were attributed to the component redistribution process. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1378–1386, 2000     

一维导电聚合物纳米材料的制备及其应用研究进展

一维导电聚合物纳米材料由于具有结构低维、环境稳定性良好、成本低廉、易于合成等优点而备受关注。本文综述了一维导电聚合物纳米材料的制备方法,包括硬模板法、软模板法、界面聚合法、稀溶液聚合法、快速混合聚合法和电化学方法,讨论了各种制备方法的优缺点,并展望了一维导电聚合物纳米材料的应用前景。