摘要:

Short carbon fiber (CF) reinforced polyamide 6 (PA6) composites were prepared by homogenization of the components in a twin-screw extruder and by injection molding. Fiber content was varied between 0 and 16 vol%, while specimens were injection molded at rates between 2.0 and 22.6 cm/s. Average fiber length and orientation were measured to characterize structure. Average fiber length decreased with increasing fiber content and processing rate. The observed structure is contradictory to those reported in the literature for short glass fiber reinforced composites. Fibers were oriented randomly relative to the mold fill direction in the skin layer, while they were oriented parallel to this direction in the middle of the specimen. The thickness of the skin decreased with increasing injection rate and decreasing fiber content. Although instrumented impact testing indicated brittle failure at all combinations of the variables, the strain energy release rate could not be determined by the usual technique using varying notch depths because of the different properties of the skin and the core. Also, the mechanism of failure seems to be different in the two layers. A minimum appears in the fracture toughness and impact resistance at low fiber contents, indicating that fibers might promote fracture initiation at such compositions. Fiber length changed in a narrow range in the studied composites; thus, properties are determined mainly by orientation. As a consequence, both increased fiber content and injection rate lead to an increase of stiffness and toughness.

ISSN:0022-2348    

DOI:10.1080/00222349908248134

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

An electrical-conducting polypropylene/polypyrrole (PP/PPy) composite was prepared by the chemically oxidative modification reaction of pyrrole on the surface of PP particles in suspension. Another type of PP/PPy composite was prepared by mixing the coated PP particles with noncoated PP particles at room temperature. The composites were processed by compression molding or by injection molding. The injection-molded composites exhibited better mechanical properties compared to compression-molded samples, while these composites showed better antistatic behavior and electrical conductivity. The differences in the behavior of the two types of composites were caused by the different structure of the PPy phase, which was studied by hot-stage optical microscopy and X-ray photoelectron spectroscopy (XPS).

ISSN:0022-2348    

DOI:10.1080/00222349908248135

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Deformation diagrams of particulate-filled polymers have been calculated on the basis of specific constitutive equations [1] for large plastic deformation of the polymer. Composite structure is represented by the Hashin polydisperse model [2]. Original finite-element (FE) code with triangular elements has been elaborated and used for the numerical solution of boundary value problems. Local achievement of a critical value by the elastic main strain was used as a fracture criterion. Engineering (force-elongation) diagrams were found to exhibit maxima for arbitrary filler fraction if the interfacial bond was perfect and for low loading at zero adhesion. Stress-strain diagrams with a yield maximum and draw minimum provide macroscopic neck-type localization. Further, the loading in the case of facilitated deboading results in the diminution of the difference between maximum and minimum drawing forces and then in the disappearance of the latter, which in turn provides the transition from localized to macrouni-form deformation. Young's modulus and the yield stress increase with filling in the case of absolute adhesion and decrease in the opposite case. Ultimate elongation sharply drops with an increase in filler fraction, and embrittlement occurs at a small fraction of inorganic particles if a perfect interfacial bond is present. Contrary, a decrease in ultimate elongation is much more gradual, and composites conserve ductile properties of the matrix up to a high portion of inclusions. The laws found qualitatively coincide with what is observed for real materials.

ISSN:0022-2348    

DOI:10.1080/00222349908248136

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Macromolecular structures, as well as aggregation of filler in polymer-based composites, often may be described properly as fractals. Scaling behavior of the elastic moduli of a modeled fractal, the Sierpinski carpet, was the subject of this study. Sheng and Tao [1] and Patlazhan [2] found that, in the case of voids in on elastic host, axial and shear moduli exhibit distinct scaling dependencies on the size of the system. Nevertheless, it is widely accepted that moduli of random isotropic fractals (percolation clusters) scale with the same exponents. Explanation of the discrepancy is one of the main targets of the paper. The self-consistent approach and position space renormalization group technique (PSRG) have been applied for this goal. The mapping, corresponding to PSRG, was constructed numerically using the finite-element method (FEM) in the cases of voids and rigid inclusions. The self-consistent approach gives scaling behavior with exponents of values of about 0.11, independent of the modulus and type of inclusion, at developed stages of the fractal. It has been shown that mappings of PSRG on the plane, for two ratios of three independent moduli, have stable fixed points. This means that different elastic moduli exhibit scaling behavior with the same exponents (0.29 for voids and 0.17 for rigid squares) for developed fractal structure. The discrepancy in the exponent values obtained in the previous simulations is caused by the analysis of the initial stages of the structure. We believe that analogous results are valid for the wide class of self-similar fractals, and the dimension is the main parameter that governs the exponents and fixed point values.

ISSN:0022-2348    

DOI:10.1080/00222349908248137

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

In this contribution, we report on a study of the self-organization and mechanical properties of polymer liquid crystals (PLCs). Both processes are computer simulated by the method of molecular dynamics. We investigated two real longitudinal PLCs (thermotropic polyesters) with macromolecules that consist of rigid and flexible parts arranged in a regular way. One rigid and one flexible part form a monomer containing 45 or 47 atoms. The total number of atoms in the macromolecules studied was 4700 (100 monomers) and 5400 (75 monomers). The self-organization was similar to that obtained earlier for a beads-on-a-string model, so compression calculations were done using this simpler model containing 1200 beads (100 monomers). Macroscopic characteristics such as the stress-strain relation, temperature change during deformation, as well as microscopic changes in structure, were investigated.

ISSN:0022-2348    

DOI:10.1080/00222349908248138

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Polymers possess a very large inherent capacity for property modifications. The bridge between structure or morphology and mechanical properties is created by the micromechanical processes of deformation and fracture, the “micromechanics.” Developments mainly in electron microscopy (EM) (scanning, transmission, and high-voltage electron microscopy) and scanning force microscopy (SFM) opened up a wide range of experiments previously impossible, including the in situ study of micromechanical processes. These new techniques are reviewed and used to study micromechanical properties of amorphous and semi-crystalline polymers and several toughened polymers. On the basis of the detailed knowledge of micromechanical mechanisms, a new method of polymer modification becomes a realistic possibility, a method of micromechanical construction of new polymeric systems.

ISSN:0022-2348    

DOI:10.1080/00222349908248139

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The deformation processes in crystalline polymers have been studie ever since the discovery of chain folding in 1957. Since then, scientists have been intrigued by the different steps of the transformation of the folded-chain lamellar structure of single crystals or of macroscopically isotropic, often spherulitic, polymers into fibrous morphologies (see Refs. 1 and 2 for early reviews). The importance of molecular tilt, of inter- and intralamellar slip, and of micronecking were rapidly recognized [1–4]. In this paper, we discuss the analogies and differences with respect to crazing of glassy amorphous polymers. Obviously, there is an extensive body of literature on the micromechanics of crazing (see the reviews in Refs. 5–9). On the basis of these studies, it has been established that crazes in amorphous polymers are well-defined regions with approximately planar boundaries that extend perpendicular to the direction of maximum principal tensile stress and that contain highly stretched and voided material [7]. However, crazelike features have also been observed in many semicrystalline polymers (polyethylene [PE], isotactic polypropylene [IPP], isotatic polystyrene [IPS], polyoxymethylene [POM], polyamide 6 and 66 [PA6 and PA66], polycarbonate [PC], polyethylene terephthalate [PET], polybutylene terephthalate [PBT], polyvinylidene fluoride [PVDF], and polyether ether ketone [PEEK]). They are designated in the literature [3–10] as micronecks, true crazes, fibrillar deformation zones (DZs), or simply as crazes since they correspond well to the above definition.

ISSN:0022-2348    

DOI:10.1080/00222349908248140

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Using (Me5Cp)2ZrCl2and Et(Ind)2ZrCl2activated by methylaluminoxane (MAO), ethene/1-hexene copolymers of markedly different densities were produced under the same conditions. Binary blends were produced by melt blending of a high-density ethene/1-hexene copolymer with an elastomeric ethene/1-hexene copolymer. Transmission electron microscope (TEM) investigations of ultrathin sections of samples stained with ruthenium tetroxide revealed the morphology of the different blends. Depending on the blend composition, the degrees of segregation differed. An analysis of the TEM micrographs shows that increasing the segregation of the elastomeric blend component seems to be accompanied by an increase of the mean thickness of the crystalline lamellae of the matrix. Corresponding to the TEM results, typical morphological structures were also revealed by scanning force microscope (SFM) investigations using the tapping mode and a force modulation mode. Furthermore, these SFM techniques were applied to study in situ local deformations and changes in the morphological structures in a certain specimen position while the external stress was successively increased. Results of these experiments, as well as those from additional TEM in situ tensile tests of thin sections, show that the deformation that appears homogeneously down to the micron range is strongly inhomogeneous in the submicron range.

ISSN:0022-2348    

DOI:10.1080/00222349908248141

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Many different molecular processes that cause plastic deformation in polymers can be investigated by transmission electron microscopy (TEM). A short review of those deformation mechanisms is given in this paper using model morphologies and correlating them to the corresponding mechanical behavior.

ISSN:0022-2348    

DOI:10.1080/00222349908248142

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

When polyethylene (PE) is deformed to large strains, the stress originates from both the viscous forces associated with the plastic deformation of the crystallites by slip and fragmentation processes and the entropic elastic forces arising from the stretching of the entangled amorphous regions. The dependencies of the relative weights of these processes on crystallinity were analyzed in a comprehensive study ofAseries of samples: high-density polyethylene (HDPE), low-density polyethylene (LDPE), and ethylene-vinylacetate copolymers. The comparison was based on measured true stress-strain curves at constant strain rates, on the recovery properties of the samples studied in tensile tests with included unloading-reloading loops, and on wide-angle X-ray spectroscopy (WAXS) measurements carried out to determine the related texture changes. It was found that, in spite of the large changes in the gross mechanical properties from solid-like to rubberlike behavior, there always exist four characteristic points at which the deformation behavior changes. These may be associated with (1) the onset of isolated slip processes, (2) a change into a collective activity of the slips, (3) the beginning of crystallite fragmentation, and (4) chain disentanglement. Increasing crystallinity leads to increasing stresses at these points: the related strains, however, remain essentially constant. The crystal texture is a function of the imposed strain only. Experiments support the novel picture of a granular substructure of the crystalline lamellae as a basic structural feature.

ISSN:0022-2348    

DOI:10.1080/00222349908248143

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor