Experimental study of transcrystallinity in UHMWPE/LLDPE composites

This study deals with the effect of a transcrystalline LLDPE (linear low-density polyethylene) layer grown on Spectra 1000 UHMWPE (ultrahigh molecular weight polyethylene) fibres. Chemical similarity between the fibre and the surrounding melt does not promote transcrystallinity as no transcrystalline microstructure appears from the surface of as-received Spectra 1000 UHMWPE fibres. However, oxygen plasma treatment of the UHMWPE fibres yields a degree of surface roughness that appears to promote easy nucleation and growth of LLDPE transcrystallinity. The kinetics of transcrystalline growth were investigated quantitatively. The growth rate increased by a factor of about 12 for a 10°C increase in supercooling, and at 105°C the maximum observed thickness of the transcrystalline layer was about one fibre diameter. The induction time was found to decrease as the crystallization isotherm increased. We discuss the possibility of using surface energy parameters to define a better criterion for the nucleation of transcrystallinity from the UHMWPE fibre substrate. Preliminary data were generated for the interfacial mechanical shear strength by means of the microbond test. It is conjectured that the combined effects of a thermal treatment and the presence/absence of a transcrystalline layer might produce significant changes in the interfacial shear strength, as illustrated here by a 43% increase observed with specimens subjected to different thermal treatments.     

Hierarchical Distribution of β-Phase in Compression- and Injection-Molded,Polypropylene-Based TPV

The distribution of the β-phase crystals of polypropylene (PP) in different zones of compression and injection moldings molded from PP-based dynamically vulcanized thermoplastic elastomer (TPV) in the presence of a β-nucleating agent (β-NA) was investigated. The influence of shear and cooling conditions and the addition of β-NA on the development of β-phase was examined and discussed based on the one-dimensional heat conduction behaviors with phase change. A hierarchical distribution of β-phase was observed in both compression- and injection-molded TPV/β-NA specimens. For the compression-molded TPV/β-NA specimens, the hierarchical distribution of β-phase induced by β-NA is dominated by the cooling conditions. For the injection-molded TPV/β-NA specimens, the occurrence of β-phase is also mainly induced by the addition of β-NA; the effect of shear on the relative content of β-phase can hardly be observed and the cooling conditions control the hierarchical distribution of β-phase. Both in compression and injection-molded TPV/β-NA specimens, the relative content of β-phase increases from the skin to the core zones, owing to the cooling rate decreasing from the skin to the core zones.     

Skin Thickness and β-Crystals Development in Injection-Molded iPP Along the Flow Direction

In this study, iPP was injection molded at 180°C, 200°C, and 220°C. According to polarization optical microscopy (POM) results, for a given part, the skin thickness steadily decreases along the flow direction. However, at the same distance from the gate, the skin thickness of the parts molded at lower melt temperature is larger than that molded at higher melt temperature. It is found that flow time (here, the time taken for melt to pass the specific position along the flow direction) and melt temperature are two significant factors leading to this phenomenon, while the gate size is another one.

The DSC and WAXD results show that the relative fraction of β-form crystals, for a specific part, decreases along the flow direction, which is mainly determined by flow time. However, for the parts molded at different molding temperatures, the fraction of the β-form crystals is mainly determined by the molding temperature, though this influence is very complex.     

Optical characterization of weakly absorbing PP,PE, and PP/PE films

The interaction between a solitary electromagnetic wave and a narrow layer with an increased electron concentration in a semiconductor superlattice in a preset uniform alternating electric field directed along the superlattice axis is investigated. The model of the electron energy spectrum in the superlattice is chosen in the strong coupling approximation taking into account the second harmonic. It is shown that, for certain relations between the amplitude and frequency of the alternating electric field, a solitary electromagnetic wave approaching the layer with an increased electron concentration can be trapped by this layer.     

Influence of the isotopic composition of Li, Rb, and Ag on the absorption of their resonance lines

For the resonance lines of the two-isotope elements Li, Rb, and Ag an investigation is made of the influence of the isotopic composition (b) of an absorbing layer on the magnitude of absorption (A). It is assumed that the contours of the superfine structure (SS) components are of Doppler type and that the source of the line spectrum contains elements of natural isotopic composition. It is shown that A=f(b) for the LiI 670.8 nm line is characterized by a one-to-one dependence on b, and for the RbI 780 nm line it passes through a maximum. For the AgI 383.3 nm line with overlapping SS components the dependence of A on b is practically absent. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 2, pp. 251–252, March–April, 2000.     

Charged double layer at the boundary between a symmetric plasma and a wall

It is shown that the features of formation and structure of a stationary charged layer at the interface between a symmetric plasma and a wall depend to a considerable extent on the ratios τ_± (for both plasma components) of the thermal velocity to the velocity of the plasma flow at the beginning of the layer, as well as on the polarity and magnitude of electric potential φ_W of the wall. The conditions under which a double layer-type structure can be formed in the near-wall layer of the symmetric plasma are formulated. The charge double layer contains two sublayers with opposite polarities of charge for a fixed polarity of the wall. The charge polarity of sublayer adjoining the plasma is opposite to the polarity of the wall, while the sublayer adjoining the wall has a charge of the same polarity as that of the wall. The ranges of symmetric plasma parameters for which the double layer exists are determined. It is found that a structure of this type can be formed nor for all parameters of the plasma, for which a charged layer exists; the possibility of formation of this structure depends on τ_± as well as on the sign of the potential of the wall.     

CRYSTALLIZATION BEHAVIOR OF PP IN A NEW GENERATION OF COMPOSITE MATERIAL

The microstructure of isotactic polypropylene (iPP) has been investigated during different steps of the processing of commingled PP/unidirectional glass fiber composites. From wide angle x-ray scattering and differential scanning calorimetry analysis, complemented by density measurements, it can be concluded that the crystalline phase in both un-reinforced PP and composite materials is only constituted of the α phase. The morphology of the crystalline phase and the two-dimensional geometric arrangement of fibers within the matrix of commingled composites reinforced by 45, 60, and 75 wt% of fibers have also been investigated using optical microscopy. The composites exhibit heterogeneous morphology whatever the fiber content is. Moreover, large spherulites can be distinguished, but the presence of a transcrystalline phase around the fibers cannot be detected.     

Effect of the thickness of a dicarbocyanine dye layer on the conformational composition and spatial orientation of layer components

The conformational and aggregate structure and the spatial orientation of polyatomic organic molecules on solid substrates are investigated by the example of molecular layers of symmetric and asymmetric dicarbocyanine dyes. It is shown that the structure of a dicarbocyanine dye layer is determined by two mechanisms: (i) the change in the symmetry of the intramolecular charge distribution in the monomeric components of the layer as a result of the intermolecular interaction with the substrate, which leads to closer values of the equilibrium concentrations of two monomers with different spatial structure (rotational stereoisomers), and (ii) the association of monomer molecules with formation of dimers and J aggregates. The relative position of the spectra of the layer components, the spatial orientation of the components, and the concentration ratio of different monomeric forms depend on the structure and the electron-donating ability of terminal groups. In going from a monomolecular layer to multilayered structures, the asymmetry of the intramolecular charge distribution induced by the substrate and the ratio of different stereoisomeric forms first change and then become stabilized.     

Phase and structure state of titanium loaded by spherically converging shock waves

The phase and structural states of titanium spheres loaded by spherical converging shock waves of various intensities were studied layer by layer by means of X-ray diffraction, optical, and transmission electron microscopy. It was established that defects of different types (twins, dislocations, and adiabatic shear bands) are produced during high-rate deformation occurring in materials under such method of pulsed loading. The amount and distribution of the defects depend on the loading intensity. The presence of the ω-phase is revealed only in the layers near the external surface of the titanium sphere after low-intensity loading.     

Morphological Distribution in Micro-Injected Polypropylene Parts in the Presence of β-Nucleating Agent

Micro-injection molding is attracting much attention nowadays. Characterization of the morphological distribution in parts prepared by micro-injection molding is thus of growing importance. The morphological features of micro-parts may strongly differ from those of the macro-parts prepared by conventional injection molding, resulting in specific physical properties. In the present study, β-nucleated isotactic polypropylene micro-parts (μPPB) with 200 μm thickness, as well as macro-parts (PPB) with 2000 μm thickness, were prepared. Polarized light microscopy (PLM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and wide-angle X-ray diffraction (WAXD) were used to investigate their morphological features. The results show that the morphology distribution in μPPB had many differences from that of the PPB. The one-dimensional WAXD and DSC analysis showed similar results; the degree of crystallinity of the μPPB was higher than that of the PPB. However, the content of β-crystals of μPPB was lower than that of the PPB. This can be explained by the restraining effect for the formation of β-crystals in β-nucleated isotactic polypropylene (iPP) under the strong shear field. The through-the thickness-morphology of both μPPB and PPB exhibited a “skin-core” structure from PLM observations, but the former had a large fraction of shear layer in comparison to the latter implied. The SEM observations showed that the shear layer of μPPB consisted of a highly oriented shish-kebab structure, while that of the core layer consisted of deformed spherulites structure. The two-dimensional WAXD pattern of the core layer of PPB, showing full Debye rings, indicated an overall random orientation of the iPP chains, while the arcing indicated a pronounced orientation in the shear layer. The more pronounced arcing of the μPPB indicated a more pronounced orientation.     

Effect of Stretching on β-Phase Content of Isotactic Polypropylene Melt Containing β-Nucleating Agent

Pure isotactic polypropylene (iPP) and that containing 0.2 wt% of a β-nucleating agent (β-NA) were extruded through a slit die. Simultaneously, the extruded melt was stretched at the die exit with different stretching rates (SR). The change of β-phase content with different SR was investigated by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The results indicate that, for pure iPP, the content of β-phase first increases with increasing SR till it reaches a maximum and then it gradually decreases. However, for the case of β-nucleated iPP, it decreases monotonously with increasing SR. The spatial confinement is considered as the best explanation for the suppression of β-phase in the nucleated iPP melt upon stretching.     

The influence of strain-reducing layer on strain distribution and ground state energy levels of GaN/AlN quantum dot

This article deals with the strain distributions around GaN/AlN quantum dots by using the finite element method. Special attention is paid to the influence of Al_0.2Ga_0.8N strain-reducing layer on strain distribution and electronic structure. The numerical results show that the horizontal and the vertical strain components are reinforced in the GaN quantum dot due to the presence of the strain-reducing layer, but the hydrostatic strain in the quantum dot is not influenced. According to the deformation potential theory, we study the band edge modifications and the piezoelectric effects. The result demonstrates that with the increase of the strain reducing layer, the transition energy between the ground state electron and the heavy hole increases. This result is consistent with the emission wavelength blue shift phenomenon observed in the experiment and confirms that the wavelength shifts toward the short wavelength range is realizable by adjusting the structure-dependent parameters of GaN/AlN quantum dot.     

Aggregation,fracture initiation,and strength of PP/CaCO3 composites

Polypropylene/CaCO₃ composites 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.     

Bound to continuum intersubband transition optical properties in the strain reducing layer-assisted InAs quantum dot structure

In this paper, the impact of wetting layer, strain reducing layer and dot height on the electronic, linear and nonlinear optical properties of bound to continuum states transitions are investigated in a system of InAs truncated conical shaped quantum dot covered with the In_xGa_1−x As strain reducing layer. The electronic structure, containing two main states of S and wetting layer states (WL), was calculated by solving one electronic band Hamiltonian with effective-mass approximation. The results reveal that the presence of the strain reducing layer in the structure extends the quantum dot emission to longer wavelength which is reported as a red-shift of the photoluminescence (PL) peak in the experimental measurement. This study also highlights the possibility of improving the intersubband optical properties based on the significant size-dependence of the three layer dot matrix by employing the strain reducing and wetting layers. According to this simulation, relatively tall dots on the thick wetting layer introduce the optimized structure size for practical applications to meet the SRL assisted enhanced dot structure.     

Properties of omnidirectional photonic band gap in one-dimensional staggered plasma photonic crystals

In this paper, the properties of the omnidirectional photonic band gap (OBG) realized by one-dimensional (1D) photonic crystals (PCs) with a staggered structure which is composed of plasma and isotropic dielectric layer have been theoretically studied by the transfer matrix method (TMM). From the numerical results, it has been shown that such OBG is insensitive to the incident angle and the polarization of electromagnetic wave (EM wave), and the frequency range and central frequency of OBG can be effectively controlled by adjusting the plasma frequency, the average thickness of plasma layer, the average thickness of dielectric layer and staggered parameters, respectively. The frequency range of OBG can be notably enlarged with increasing the plasma frequency, average thickness of plasma layer, respectively. Moreover, the bandwidth of OBG can be narrowed with increasing the average thickness of dielectric layer. Changing staggered parameters of dielectric and plasma layer means that the OBG can be tuned. It is shown that 1D plasma dielectric photonic crystals (PPCs) with such staggered structure have a superior feature in the enhancement of frequency range of OBG compared with the conventional 1D binary PPCs. This kind of OBG has potential applications in filters, microcavities, and fibers, etc.     

Principles of the alloying of steel in a beam of relativistic electrons

It is shown that it is possible to significantly strengthen steel by alloying a surface layer melted by the energy from a beam of relativistic electrons. A study was made of the effect of different treatment parameters (accelerating voltage, beam current, specimen velocity and temperature, etc.) on the structure, depth, hardness, and wear resistance of the alloyed layer. Several types of alloying mixtures were developed based on carbides of tungsten, chromium, and boron, and including special additions and modifiers. The proportions of the components was optimized. The technology of alloying in a beam of relativistic electrons is compared with vacuum electron-beam alloying. Heat treatment is used to additionally improve the structure of the layers.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 115–125, March, 1996.     

Transcrystalline interphases in natural fiber-PP composites: effect of coupling agent

The interest in lignocellulosic fiber composites has been growing in recent years because of their high specific properties. In this work, a new technique was used to prepare specimen to observe the transcrystalline zones in kenaf fiber-polypropylene composites. A maleated polypropylene (MAPP) coupling agent was used to improve the stress-transfer efficiency in the composites. Transcrystallinity was observed for both the uncoupled and coupled composites, although the rate of growth was higher for the coupled composites. Dynamic mechanical spectroscopy was used to observe the relaxations of the composites. The peak temperature of the β-relaxation, associated with the glass-rubber transition of the amorphous molecules, of the coupled composites was higher than that of the uncoupled composites. Restricted molecular mobility due to covalent interactions between the MAPP and the lignocellulosic surface may account for the shift to higher temperatures. It appears that during compounding the extractives sheared from the fiber surface is an important factor in determining the β-relaxation of these composites. The intensities of the α-transition, related to molecular mobility associated with the presence of crystals, is proportional to the fiber volume fraction. Thus it is possible that the molecules responsible for the α-transition are predominantly in the transcrystalline zone. These 'rigid' amorphous molecules in the transcrystalline zone do play a role in composite behavior and need to be considered when tailoring interphases.     

Shearing of planar smectic C chevrons

We have theoretically investigated chevron formation in smectic C materials and the transformation of this chevron structure to a tilted layer structure as the cell is sheared. We find a series of transition temperatures at which the behaviour of the cell critically changes. As the cell is cooled from the smectic A phase past the first critical temperature there is a second order transition which forms two tilted layer states with lower energy than the smectic A bookshelf structure. Although these low energy tilted structures exist the bookshelf structure is the stable state for zero shear. However, upon further cooling this bookshelf structure becomes unstable to the formation of a chevron state. Now when the cell is sheared the chevron structure smoothly transforms into the tilted layer structure. As each further critical temperature is passed an additional multiple chevron solution is formed which although a high energy, unstable state may be observed transiently. For sufficiently low temperatures the transition from chevron to tilted layer becomes first order. This first order transition occurs as the chevron interface merges with the surface alignment region to form the tilted layer structure. Received 28 December 1998 and Received in final form 8 April 1999     

Structure and impact resistance of short carbon fiber reinforced polyamide 6 composites

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.