Failure mechanisms in polyolefines: The role of crazing, shear yielding and the entanglement network

Macroscopic deformation and failure modes of polyolefines are reviewed in terms of deformation and failure models based on the craze initiation and propagation model of Kramer-Berger and the craze-crack transition model of Kramer-Brown. Although these models were formulated for amorphous polymers they are also valid for semi-crystalline polymers. The important role of the underlying molecular entanglement network in this approach is reflected by the strain hardening behaviour which is shown to be a robust measure for predicting slow crack growth performance. The polymer network response explains the experimentally observed presence of two Brittle-Ductile transitions, one at low temperature or high strain rates, linked with chain scission which dominates crazing, the other at elevated temperatures or low strain rates which involves disentanglement crazing. The relation between these two Brittle-Ductile transitions and the major transition temperatures for molecular mobility such as the glass transition and the crystal α relaxation temperature are discussed. Valid strategies for increasing the crack propagation resistance in polyolefines are reviewed. Finally an outlook for further research to complement the present knowledge base is formulated.     

Effect of an ultraviolet/ozone treatment on the surface texture and functional groups on polyacrylonitrile carbon fibres

The effect of an ultraviolet generated ozone treatment (UV/O₃), on the surface and near surface functionality and structure of polyacrylonitrile based carbon fibres has been studied using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, and Raman spectroscopy. Results were compared to electrochemically treated fibres. The UV/O₃ treated fibres showed increased levels of oxygen functionalities. Levels of oxygen comparable to a high level electrochemical shear treatment were achieved within 5 min of treatment (O/C 0.11 ± 0.03 for both treatments). XPS O1s/C1s ratios as high as 0.3 were produced, with saturation occurring at approximately 40 min exposure. The main functional groups introduced were, in addition to hydroxyl species, alkoxides (ca. 286.5 eV), carbonyl (288.0 eV), and carboxyl (289.5 eV). Examination of the full width half maximum of the graphite peak from XPS C1s showed some disorder was introduced to the first few layers of the fibre with treatment but the effect was not evident in the Raman, i.e. in the bulk of the fibre.     

Hierarchical Growth of ZnO Particles by a Hydrothermal Route

The crystallization of ZnO microrods by hydrothermal treatment of a suspension formed from reaction of zinc acetate and sodium hydroxide has been examined using scanning and transmission electron microscopy. Polycrystalline hexagonal ZnO microrods first appeared after 0.5 h reaction time at 120°C. These early stage rods were composed of stacks of hexagonal layers, each ~50 nm in thickness containing closely aligned assemblies of nanocrystallites <20 nm in size. Further growth of the microrods involved columns of nanoparticles extending from the basal layers of the preformed hexagonal stacks. Re‐crystallization produced single‐crystal microrods, many of which existed as twin particles.     

Orientation of amorphous poly(ethylene terephthalate) by tensile drawing,roll-drawing,and die-drawing

Orientation of initially amorphous poly(ethylene terephthalate) films and sheets was carried out by means of tensile drawing in a tensile tester, roll-drawing using a series of four rolling stations, and by die-drawing. The drawing temperature was 80 and 90°C and drawing rate ranged from 2 to 20 cm/min in the different processes. Crystallinity was observed to increase with draw ratio for all these processes. The onset of crystallinity development depends on the drawing rate. The glass transition temperature was essentially constant and crystallization temperature decreased with increasing draw ratio. The trans conformers content was observed to increase with draw ratio at the expense of the gauche conformers for the three processes. The orientation of the trans conformers increases readily from the beginning of draw and saturates rapidly. The orientation of the gauche conformers was negligible. Some differences are observed for the behavior of the 1020 and 730 cm^?1 benzene ring bands, which may be due to differences in the benzene ring configuration at the surface as a result of different deformation mechanisms for the die and roll-drawing. However, further investigations to elucidate this hypothesis are needed. The mechanical properties obtained in the longitudinal direction increased for all the processes. In the transverse direction, the roll and die-drawing processes induced a decrease in modulus and strength with increasing draw ratio, similar to that observed for uniaxial orientation. This indicates that these processes are mainly uniaxial, despite the plane strain nature of the deformation.     

Manipulating crystal growth and polymorphism by confinement in nanoscale crystallization chambers

The phase behaviors of crystalline solids embedded within nanoporous matrices have been studied for decades. Classic nucleation theory conjectures that phase stability is determined by the balance between an unfavorable surface free energy and a stabilizing volume free energy. The size constraint imposed by nanometer-scale pores during crystallization results in large ratios of surface area to volume, which are reflected in crystal properties. For example, melting points and enthalpies of fusion of nanoscale crystals can differ drastically from their bulk scale counterparts. Moreover, confinement within nanoscale pores can dramatically influence crystallization pathways and crystal polymorphism, particularly when the pore dimensions are comparable to the critical size of an emerging nucleus. At this tipping point, the surface and volume free energies are in delicate balance and polymorph stability rankings may differ from bulk. Recent investigations have demonstrated that confined crystallization can be used to screen for and control polymorphism. In the food, pharmaceutical, explosive, and dye technological sectors, this understanding and control over polymorphism is critical both for function and for regulatory compliance. This Account reviews recent studies of the polymorphic and thermotropic properties of crystalline materials embedded in the nanometer-scale pores of porous glass powders and porous block-polymer-derived plastic monoliths. The embedded nanocrystals exhibit an array of phase behaviors, including the selective formation of metastable amorphous and crystalline phases, thermodynamic stabilization of normally metastable phases, size-dependent polymorphism, formation of new polymorphs, and shifts of thermotropic relationships between polymorphs. Size confinement also permits the measurement of thermotropic properties that cannot be measured in bulk materials using conventional methods. Well-aligned cylindrical pores of the polymer monoliths also allow determination and manipulation of nanocrystal orientation. In these systems, the constraints imposed by the pore walls result in a competition between crystal nuclei that favors those with the fastest growth direction aligned with the pore axis. Collectively, the examples described in this Account provide substantial insight into crystallization at a size scale that is difficult to realize by other means. Moreover, the behaviors resulting from nanoscopic confinement are remarkably consistent for a wide range of compounds, suggesting a reliable approach to studying the phase behaviors of compounds at the nanoscale. Newly emerging classes of porous materials promise expanded explorations of crystal growth under confinement and new routes to controlling crystallization outcomes.     

A cathode support structure for use in a magnetron oscillator experiment

A low temperature co-fired ceramic (LTCC) material system has been used to develop a protype field emission cathode structure for use in an experimental magnetron oscillator. The structure is designed for used with 30 gated field emission array (GFEA) die electrically connected through silver metal traces and electrical vias. To approximate a cylinder, the cathode structure (48 mm long and 13.7 mm in diameter) is comprised of 10 faceted plates which cover the GFEA dies. Slits in the facet plates allow electron injection. The GFEA die (3 mm × 8 mm) are placed in axial columns of 3 and spaced azimuthally around a cylindrical support structure in a staggered configuration resulting in 10 azimuthal locations. LTCC manufacturing techniques were developed in order to fabricate the newly designed cathode with seven layers wrapped to form the cylinder with electrical traces and vias. Two different cathode wrapping techniques and two different via filling techniques were studied and compared. Two different facet plate manufacturing techniques were studied. Finally, four different support stand configurations for firing the cylindrical structure were also compared with a square post stand having the best circularity and linearity measurements of the fired structure.     

Effect of silica fume and fly ash on heat of hydration of Portland cement

Results of calorimeter tests on Portland cement-silica fume-fly ash mixtures are presented. Data indicate that silica fume accelerates cement hydration at high water/cementitious ratios and retards hydration at low water/cementitious ratios. On the other hand, fly ash retards cement hydration more significantly at high water/cementitious ratios. When silica fume and fly ash are added together with cement, the reactivity of the silica fume is hampered and the hydration of the cementitious system is significantly retarded.     

Effects of large mammalian herbivores and ant symbionts on condensed tannins of Acacia drepanolobium in Kenya

Condensed tannins have been considered to be important inducible defenses against mammalian herbivory. We tested for differences in condensed tannin defenses in Acacia drepanolobium in Kenya over two years among different large mammalian herbivore treatments [total exclusion, antelope only, and megaherbivore (elephants and giraffes) + antelope] and with four different ant symbiont species on the trees. We predicted that (1) condensed tannin concentrations would be lowest in the mammal treatment with the lowest level of herbivory (total exclusion), (2) trees occupied by mutualist ants that protect the trees most aggressively would have lower levels of tannins, and (3) if chemical defense production is costly, there would be a trade-off between tannin concentrations, growth, and mechanical defenses. Mean tannin concentrations increased from total exclusion treatments to wildlife-only treatments to megaherbivore + antelope treatments. In 1997, condensed tannin concentrations were significantly lower in trees occupied by the ant Crematogaster nigriceps, the only ant species that actively removed axillary buds. Contrary to our prediction, trees occupied by ant species that protect the trees more aggressively against mammalian herbivores did not have lower overall levels of condensed tannins. There was no consistent evidence of a trade-off between tannin concentrations and growth rate, but there was a positive correlation between mean thorn length and mean tannin concentrations across species of ant inhabitants and across herbivore treatments in 1997. Contrary to our expectation, trees had higher tannin concentrations in the upper parts of the canopy where there is little herbivory by mammals.     

Quantitative X-ray powder diffraction analysis of clay minerals in Australian coals using Rietveld methods

The mineralogy of clay-rich mineral matter isolated from a range of Australian bituminous coals has been evaluated in quantitative terms from X-ray powder diffraction (XRD) patterns using a Rietveld-based data processing technique. The chemical composition of coal ash derived from this mineral matter has been calculated and compared to the directly determined composition of the ash prepared from the same coal samples. Although there are some minor differences due in part to uncertainty regarding the actual composition of several minerals, the compositions indicated by the two methods show a relatively high correlation, suggesting that the Rietveld technique provides mineralogical analyses that are consistent with independently determined chemical data. Comparison of the normalised clay mineral percentages from the Rietveld analysis to quantitative interpretations based on a peak intensities in glycolated and heat-treated oriented aggregates of the respective clay fractions also shows a high correlation, confirming mutual consistency of the two different mineralogical analysis methods. Such quantitative mineralogical data are significant to a range of coal exploration, mining and utilization activities, including seam correlation, material handling and ash and slag formation in combustion processes.