Simple theory of stress-strain properties of filled polymers

By the use of simple models of filled plastics, approximate equations are derived for the elongation to break in the case of perfect adhesion between the phases and for the tensile strength in the case of no adhesion between the polymer and filler phases. By combining these equations with equations for the modulus (assuming Hookean behavior) all the stress–strain properties can be derived, including rough estimates of the impact strength, as a function of filler concentration. Among other things, the theory predicts a very rapid decrease in elongation to break as filler concentration increases, especially for the case of good adhesion. It is also predicted for the case of good adhesion that the tensile strength of a filled polymer can be greater than that of an unfilled polymer.     

Model filled polymers

Summary Polyphenylene vinylene (PPV) coated polystyrene (PS) beads which have moderate conductivity when doped were prepared by mixing monodisperse crosslinked PS beads, the surfaces of which had been sulfonated to render them anionic, with cationic PPV precursor polymers. Two different PPV precursor polymers, poly (p-xylylidene tetrathiophenium chloride) and poly (p-xylylene--dimethylsulphonium chloride), were employed. Monodisperse crosslinked PS beads were sulfonated in the gas phase using fuming sulfuric acid to yield the surface activated monodisperse polystyrene sulfonic acid (PSSA) beads. Chemical doping with AsF₅, of pellets prepared by pressing the coated beads resulted in conductivities as high as 10^-1S/cm. The integrity of the polymer beads was determined by Scanning Electron Microscopy (SEM) and arsenic was found throughout the samples by examing fracture surfaces of the pressed coated pellets using EDXS.     

Dynamic mechanical behavior of highly filled polymers: Energy balances and damage

Dynamic tests using the Naval Weapons Center torsion–tension tester and end-bonded cylindrical propellant specimen were carried out to evaluate the effects of internal damage on propellants by subjecting samples to small tensile oscillations during low constantstrain rate tests. It is shown that microstructural damage in a propellant causes significant changes in mechanical properties. The mechanical property curves demonstrate that the response is markedly strain sensitive. So long as the maximum strain experienced by the sample during a test is not exceeded, the response of the propellant to small tensile oscillation during repeated strains below that maximum value remains relatively unchanged after the first initial dramatic change. The differences between mechanical energy balances of “undamaged” and “damaged” propellant samples were used to demonstrate microstructural damage and to estimate the extent of damage. For any one propellant, the total lost energy caused by microscopic failure of the propellant seems to be additive, constant, and independent of the mechanical path to failure.     

Ductility of filled polymers

The ductility of a calcium carbonate-filled amorphous copolyester PETG in a uniaxial tensite test was examined as a fiction other filler volume fraction. A ductile-to-quasibrittle transition occurred as the volume fraction of filler increased. This transition was from propragation of a stable neck through the entire gauge length of the specimen to fracture in the neck without propagation. The draw stress (lower yield stress) did not depend on the filler content and was equal to the draw stress of the unfilled polymer. It was therefore possible to use a simply model to predict the dependence of the fracture strain on the filler volume fraction. It was proposed that when the fracture strain decreases to the draw strain of the polymer the fracture mechanism changes and the fracture strain drops sharply. The critical filler content at which the fracture mode changes is determined primarily by the degree of strain-hardening of the polymer. © 1994 John Wiley & Sons, Inc.     

Misconceptions about filled polymers

The idea that, with filled polymers, length fraction, area fraction, and volume fraction of filler are different appears to have gained wide acceptance. The fallacy of this, except for special directions in ordered arrangements, is demonstrated. This misunderstanding has led to widespread misinterpretation of experimental results in this field.     

Fracture of particulate filled polymers

The addition of particulate mineral fillers to polymers confers certain mechanical property improvements automatically. Stiffness increases, creep diminishes and distortion at elevated temperatures is often reduced. However, the fracture energy of a polymer, as measured in impact, cracking or tearing tests, may vary quite unpredictably when filler is incorporated. In some special cases the fracture energy increases when small amounts of filler are added although it falls away again at higher volume loadings. This enhancement of polymer toughness by filler is an example of reinforcement. More generally the addition of filler causes a continuous and drastic reduction in fracture energy, resulting in a brittle, weak product. This paper seeks to explain the common degrading effect of filler on polymer fracture energy by considering the progress of a crack through the composite material. The crack travels through regions of polymer and also along the interfaces between polymer and filler. Experiment demonstrates that, although fracture of the polymer regions absorbs considerable energy, fracture of the interfaces usually requires very little. These weak interfaces do not resist cracking and are the cause of brittleness in particulate filled systems. This idea was quantified for thermoplastics such as low density polyethylene and poly (methylmethacrylate) filled with colloidal silica by twin-roll milling. Where the interfacial adhesive energy was much smaller than the polymer fracture energy, the composite toughness dropped as predicted when filler was added. The particle size, the nature or dispersion of the filler, and the crystallinity of the polymer used, had little influence on this phenomenon, as pointed out theoretically. The crucial parameters influencing the fracture energy of the filled polymer were found to be the volume fraction of filler and the interfacial adhesion between polymer and filler. By chemical treatments the adhesive energy between filler and polymer was raised until the interface was almost as tough as the polymer itself. In this case the filled polymer showed good fracture toughness, lending further support to the theory.     

The rheology of filled polymers

The flow properties of polymer melts containing fillers of various shapes and sizes have been examined. If there is no failure of either the filler or polymer in the solid state, then the modulus enhancement for randomly distributed filler is equal to the melt viscosity enhancement under medium shear stress conditions (10⁴ Nm^?2) in simple shear flow or in oscillatory shear flow. Submicron-size fillers, in particular, can form weak structures in the melt that greatly increase the low shear rate viscosity without changing the modulus of the solid proportionately. The highly pseudo-plastic nature of polymer melts at shear stresses of 10⁶ Nm^?2 means that, even without orientation of filler particles toward the flow direction, the viscosity enhancement is less than at lower shear stresses.     

Mechanical properties of particulate filled polymers

The mechanical properties of several types of inorganic fillers were investigated in a number of different thermoplastic polymers. The fillers included minerals, such as talc and silicon carbide, and metals, such as aluminum flake and stainless steel fibers. The polymers included General Electric's Noryl, acrylonitrile-butadiene-styrene terpolymer, polypropylene, and modified polypropylene. The talc and some of the aluminum flake were treated with coupling agents to improve interfacial adhesion to the polymers. The results showed that the modulus of the filled polymers was a function only of the concentration of filler used up to 40 volume percent filler. The tensile strength of the filled compositions depended very strongly on the degree of interfacial bond developed between the polymer and the filler. The interfacial bond strength depended on the effectiveness of the coupling agents and the inherent wetting ability of the polymer. Of the polymers investigated in this study, Noryl showed the greatest degree of inherent wetting to inorganic fillers. Chemical modification of polypropylene also resulted in greater adhesion to fillers. The impact strength of filled compounds had an even more complex response, because, in addition to the concentration of the filler, and strength of the polymerfiller interface, it depends on the mechanism of crack propagation.     

A new hydrostatic compression tester for anisotropic filled polymers

This paper describes a testing device designed for measuring the isotropic or anisotropic hydrostatic compression properties of filled polymers like solid propellants, including the initial void content and the bulk modulus. The propellant test samples used are either cubical, cylindrical, or rectangular specimens. Three pressure-sealed linear variable differential transformers (LVDTs) are used to monitor the dimensional changes of a cube specimen during hydrostatic pressurization simultaneously in the XYZ directions. The moduli E₁₁, E₂₂, and E₃₃ and the compression strain ratios v₂₁, v₃₂, and v₃₁ can be determined as a function of pressure. The three LVDTs can also be used to measure the length changes of three rectangular or cylindrical test specimens under hydrostatic compression only in the vertical Z direction. The bulk modulus and void content can be computed when isotropic behavior is assumed. The entire test procedure is controlled by an Apple II microcomputer via an AI13 12-bit analog input system. Some typical test results obtained with undamaged and damaged propellant samples are described.     

Estimation on thermal conductivities of filled polymers

A new thermal conduction model is proposed for filled polymer with particles, and predicted values by the new model are compared with experimental data. The model is fundamentally based on a generalization of parallel and series conduction models of composite, and further modified in taking into account that a random dispersion system is isotropic in thermal conduction. The following equation is derived from the new model; log λ = V · C₂ · log λ₂ + (1 ? V) · log(C₁ · λ₁). Therefore, when thermal conductivities of polymer and particles (λ₁, λ₂) are known, thermal conductivity of the filled polymer (λ) can be estimated by the equation, with any volume content of particles (V). The new model was proved by experimental data for filled polyethylene, polystyrene and polyamide with graphite, copper, or Al₂O₃.     

Experimental investigation and modeling of the dissolution of polymers and filled polymers

Dissolution of Polymers is important in various areas, including microlithography, controlled drug and herbicide/fertilizer delivery, and recycling. The dissolution rates of an oxetane polymer in ethyl acetate were obtained and well correlated with a quasi-stationary dissolution model. Equilibrium solubility values obtained from the mathematical model on the basis of the best fit to the dissolution data were found to be in good agreement with equilibrium solubilities obtained in independent experiments. Mass transfer coefficients were also obtained from the mathematical model on the basis of the best fit, and the calculated activation energies were typical for diffusion controlled dissolution. The dissolution of highly filled polymers in various solvents was also investigated using the oxetane polymer filled with ammonium sulfate and aluminum fillers. The dissolution rates for the highly filled polymer were well correlated with a pseudo-homogenous diffusion model.     

纳米多孔配位聚合物的研究进展

纳米多孔材料可用作催化剂、气体储存材料和光电子器件,是目前新型多孔材料的研究热点之一.借助自组装技术,选择羧酸类、联吡啶类和吡啶羧酸类化合物作为桥联配体,可获得一维、二维和三维的多孔配位聚合物.简要综述了该类纳米多孔配位聚合物的设计原理、合成路线和应用前景,评价了其合成方法的优缺点.由于其特殊的结构和性质,纳米多孔配位聚合物将发展成为具有光、电、磁等性质的多功能材料.     

两亲性超支化聚合物研究进展

介绍了两亲性超支化聚合物的合成方法,利用长链烷基和聚乙二醇对超支化聚合物端基接枝改性;或者对超支化聚合物改性引入活性位点,再利用其引发乙烯基单体,通过自由基聚合、开环聚合得到两亲性超支化聚合物。阐述了不同结构的两亲性超支化聚合物在溶液中的独特性质,如核壳型单分子胶束以及不同胶束形态的聚集体。详细介绍了两亲性超支化聚合物在药物输送载体、材料改性以及染料分子的封装等领域的应用现状,指出采用新的改性技术、聚合技术制备具有特殊性能的两亲性超支化聚合物以及探索其在生物医药领域的研究为今后的主要发展方向。     

Young's modulus degradation in fatigue of filled polymers for household appliances

Abstract

Monotonic and fatigue tests in simple bending were carried out on a particulate composite made of polypropylene charged with CaCO₃ particles, fixing the maximum displacement. Under monotonic conditions, the material underwent a considerable loss in modulus with increasing the displacement, without exhibiting a typical fracture in two pieces. The same phenomenon was found in fatigue, suggesting that a failure criterion should be based on the modulus degradation, rather than on fracture. To model the modulus variation as a function of the number of cycles and the maximum displacement, an exponential law was assumed. From the experimental results, the parameters appearing in the model could be found having available two sets of data, concerning two distinct values of the maximum displacement. The analysis of the model revealed that a simplified form of the exponential law, needing only two experimental constants, is effective in predicting the modulus degradation. This finding considerably shortens the characterisation stage, yet allowing a satisfactory correlation with the experimental results.     

Further developments in crude oil processing

The processing conditions for obtaining high oil quality and yield in continuous caustic refining of the following crude oils are given: tallow, palm, fish, rapeseed, and sunflower. pH Monitoring of reaction mixture assures proper caustic dosage. pH Set point range for various crudes is noted.     

Recent application developments of water-soluble synthetic polymers

In the history of man-made macromolecules, water-soluble polymers have primarily maintained passive roles; examples include the uses of water-soluble polymers for viscosity control and as binders. The importance of water on earth has increased research into the development of active roles for water-soluble polymers. These expanding roles span from medical applications, such as drug delivery to environmental applications, such as the removal of heavy metals. The development of water-soluble polymers brings significant benefits to the structural engineering and production of nanomaterials and electronic materials. The current limits of the structure–property relationship have been challenged to meet these rapidly-developing application areas.     

Surface fracture in injection molding of filled polymers

In injection molding certain polymers, fracture of the polymer stream sometimes occurs at the mold surface. This phenomenon has been found to be a tearing apart of the polymer surface layer accompanied by downstream slip of the flowing melt at the polymer/mold interface. Fracture occurs early in mold filling and is initiated usually at the gate to the mold cavity. Analysis of the fracture mechanism indicates that fracture is caused by: (1) high shearing stress in the melt as it fills the mold; (2) poor polymer/mold adhesion; and (3) low polymer surface cohesive strength.     

Surface interactions and some properties of filled polymers

Inverse chromatography was applied to evaluate interaction parameters for polyethylene (PE), polyvinyl chloride (PVC), and CaCO₃, these parameters being based on retention volumes of proton-donor, -acceptor, and neutral vapors. The acid/base characteristics of CaCO₃ were controllably altered by exposing the particulate to microwave plasmas sustained by acidic and basic vapors. It was shown that the ease-of-dispersion of fillers in the polymer matrixes related with the acid–base interaction balance in the polymer-filler pair, and varied widely with the surface treatment given to the filler. Mechanical properties at large deformation of the filled polymers and their durability also were shown to depend on surface interactions. Optimization of properties in PVC compounds was favored when strong acid-base interactions could take place with the plasma-modified filler. In the case of PE, properties were superior when unmodified filler was used; imparting strongly acidic or basic surface properties to the filler diminished its “compatibility” and usefulness with this nonpolar matrix.