摘要:

To clarify whether the interfibrillar slippage occurs on plastic deformation of oriented polymers, flow creep of ultrahigh molecular weight polyethylene (UHMW PE) samples with various connectedness of microfibrils has been studied in a dead load mode at room temperature. The flow creep rate of melt-crystallized and gel-cast UHMW PE films drawn to various draw ratios, as well as of modified gel-crystallized samples (cross-linked/grafted or washed free of low molecular weight fraction) has been measured with the help of a unique laser interferometric technique (Doppler creep rate meter). The technique allows one to measure creep rates for deformation increments as small as 0.3 μ within an accuracy 1%. The interferometric technique enabled us to observe an extremely high variability of flow creep rate. It was recognized that the creep process accelerates or slows from time to time. A length of a loaded sample increased by multiple consecutive deformation jumps (or steps). The size distribution of the steps appeared to be controlled by the structure of interfibrillar regions. The influence of the latter on the variability of creep rate confirms a hypothesis that suggests a contribution of interfibrillar slippage to plastic deformation of oriented polymers. The observed phenomenon has been attributed to stick-slip motion of microfibrils and their aggregates sliding on each other under the action of applied stress. It was found that the creep rate decreases with increasing interfibrillar interaction.

ISSN:0022-2348    

DOI:10.1080/00222349908248144

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The development of crazes in polycarbonate was investigated with the method of ultra-small-angle X-ray scattering (USAXS) of synchrotron radiation. Measurements at different temperatures and with different draw rates were carried out. The two-dimensional scattering patterns were analyzed by means of a fibrillar model of the craze. The geometrical parameters of the craze as a function of the macroscopic draw ratio were determined by using a curve-fitting procedure. From the measured values of the diameter and the mean distance of the fibrils, it is possible to calculate the volume fractionvfof the fibrils directly. Additional scattering caused by submicrocracks is discussed.

ISSN:0022-2348    

DOI:10.1080/00222349908248145

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Polypropylene/CaCO3composites were homogenized in a twin-screw compounder and then injection molded into tensile bars. Six different fillers were used in a wide range of average particle sizes between 0.08 and 12 μm. Tensile and flexural properties were measured by standard techniques, while impact resistance was determined by instrumented impact testing. Structure was characterized by light and electron microscopy, while failure initiation and propagation was studied with in situ high-voltage electron microscopy. The results showed that aggregation of particulate fillers occurs when their particle size is smaller than a critical value. This critical size depends on component properties and processing conditions. Strength and impact resistance usually decrease with increasing number of aggregates. The presence of aggregation can be detected by the use of a simple semiempirical model. Comparison of samples prepared by two different technologies showed that twin-screw extrusion and injection molding leads to relatively homogeneous composites, which was indicated by smaller deviations of the properties from theoretical predictions. In spite of the acceptable dispersion, impact resistance showed a large standard deviation, probably determined by the local variation of structure. In composites containing relatively large particles, the dominating micromechanical deformation process is debonding, while in the presence of extensive aggregation of small particles, cracks are initiated inside and propagate through aggregates. Mixed-mode failure may also occur at certain intermediate panicle sizes.

ISSN:0022-2348    

DOI:10.1080/00222349908248146

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Failure mechanisms in polymer composites were investigated with the method of ultra-small-angle X-ray scattering using synchrotron radiation and acoustic emission analysis simultaneously. The composites were prepared from polystyrene filled with glass beads with different coatings. The two-dimensional scattering patterns were analyzed by means of a cylinder model, while the curves of the acoustic emission signals per stress or amplitude interval were calculated and fitted by a Weibull distribution function.

ISSN:0022-2348    

DOI:10.1080/00222349908248147

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The failure mechanism in polystyrene (PS) filled with glass beads was investigated with the methods of ultra-small-angle X-ray scattering (USAXS) and acoustic emission (AE) analysis simultaneously. The interface in the composites was tailored using carboxyl-terminated PS (PS-COOH). Monolayers of PSCOOH were prepared on the glass surface in a wide range of grafting densities, from single chains to a polymer brush. The layers were characterized by ellipsometry, scanning probe microscopy, and the wetting method. The morphology of the layers is in good agreement with the theoretical predictions. USAXS experiments identify submicrovoids (SMVs) at the interface in the composites even before a tensile deformation. The uniaxial loading of the specimens is followed by a debonding (dewetting) process that is identified by simultaneous increase of integral scattering intensity and AE signals. The stress of debonding is proportional to the grafting density of PS-COOH. The amplitude of the AE

ISSN:0022-2348    

DOI:10.1080/00222349908248148

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The deformation behavior at room temperature of injection-molded and blow-molded samples of high-density polyethylene (HDPE) differing in orientation (with respect to the melt flow direction) and in filler content (untreated and surface-treated kaolin, respectively) were characterized by the stretching calorimetry technique. Samples with longitudinal and transversal orientations were examined. Strain-softening and strain-hardening phenomena were observed, respectively, in the range of inelastic strains above the yield point. The depression of yield stresses for the filled composites compared to those for the corresponding neat polymers is associated with the onset of debonding of the matrix polymer from the filler surface. The analysis of the energy balance of the debonding process suggests that the fraction of a debonded interface is smaller in samples containing a coupling agent and larger filler particles.

ISSN:0022-2348    

DOI:10.1080/00222349908248149

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Formation of an adhesive contact between a polymer melt (or solution) and reinforcing fibers is considered from the viewpoint of kinetics. A two-stage model of this process has been proposed, and an expression for the interfacial bond strength as a function of time and temperature is derived. Experimental data on bond strength in adhesive joints between thermoplastic polymers and reinforcing fibers formed under various conditions were obtained, and the concept of activation energy was used to analyze them. Since the process is controlled by the stage having the larger activation energy, the adhesive contact formation between fibers and polymer solutions is governed by the rate of adhesive bonding, whereas that between fibers and polymer melts is governed by the rate of the melt spreading.

ISSN:0022-2348    

DOI:10.1080/00222349908248150

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The viscoelastic properties of a compatibilized epoxy/thermoplastic polymer blend were investigated. Mechanical spectrometry revealed an additional loss peak at about 333 K at 1 Hz in the compatibilized ternary blend spectrum that is detailed as a “micromechanical transition.” Such an interphase relaxation does not correspond to a relaxation of one of the pure components. Mechanical modeling was used in a reverse mode to investigate the actual characteristics of the interphase (i.e., volume content and mechanical properties).

ISSN:0022-2348    

DOI:10.1080/00222349908248151

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

We investigated the fragmentation of thin coatings on substrates under uniaxial tension. Theoretical models based on linear elasticity predict a power-law dependence of the mean fragment length <L> on the strain e to which the substrate is subjected: namely, one has <L> ∞ε−αwith α ≤ 1/2. Experimentally, such a relation is obeyed in the fragmentation of thin SiOxcoatings on polyethylene terephthalate (PET) substrates. In our experiments, we used brittle paints to coat rubber substrates: the ensuing mean fragment lengths also obey power laws in ε: we found, however, α = 0.71 ± 0.03. Such values of α can be reproduced theoretically by taking into account the nonlinearity of the coupling between coating and substrate.

ISSN:0022-2348    

DOI:10.1080/00222349908248152

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Microfracture surface structure of segmented polyurethanes (PUs) with different chemical and physical structures was tested in manually broken samples far below their glass transition. To observe the microfracture surface, transmission electron microscopy (TEM) was utilized. Some correlations between swelling degree (thus physical structure) and the appearance of the microfracture surface pattern were found in chemically different PUs. A radial microfracture topology was observed for cross-linked PU networks.

ISSN:0022-2348    

DOI:10.1080/00222349908248153

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor