Crystalline and amorphous orientations in injection molded polyethylene

The techniques of density, birefringence, and wide X-ray diffraction were employed to characterize the microstructure of injection molded polyethylene parts. Generally, maximum crystallinity (density) occurs at the center of the molding, while the minimum crystallinity occurs near the surface. Higher densities are observed near the gate. Raising the injection temperature tends to cause a marginal increase in the crystallinity throughout the molding. Birefringence measurements suggest that the maximum orientation occurs near the surface and that the relative orientation distribution is independent of the injection temperature. X-ray diffraction indicates that the crystallographic a-axis tends to orient in the flow direction while the b and c axes vary symmetrically about that direction. Increasing the injection temperature creates c-axis orientation near the surface, while towards the core region a-axis orientation is observed. Generally, near the surface it is the amorphous phase that makes the major contribution to the total orientation as measured by birefringence. Increasing the injection temperature tends to decrease the amorphous phase orientation near the surface. The crystalline phase contribution to the total orientation increases as distance from the surface increases, regardless of injection temperature.     

X-ray diffraction orientation studies on blown polyethylene films. II. Measurements on films from a commercial blowing unit

X-Ray diffraction pole figure measurements have been made on a series of films, blown under various conditions from three high-density polyethylenes. The results are interpreted in terms of two distinct types of orientation. The first, and probably the more normal, is the result of the type of stress crystallization process described by Keller and Machin and has the a and c axes inclined at an angle to the plane of the film. The second type of orientation is crystallographically analogous to that found in cold drawn polyethylene in having the c-axis distribution substantially along the machine direction. This is termed high-stress orientation. The type of orientation obtained is dependent both on the blowing conditions and the particular polyethylene. With an experimental Rigidex grade and with Shell LPPE 040 there are always substantially amounts of the conventional low-stress orientation although certain combinations of machine conditions predispose towards the high-stress form. This latter type forms readily in the case of Hostalen GM 9955F over a rather wide range of machine conditions and appears to be favored by slower cooling conditions.     

Characterization of the physical and molecular structure of thermally elongating poly(ethylene terephthalate) fibers

The mechanism of thermally induced elongation in poly(ethylene terephthalate) fiber spun at 3500 m min⁻¹ has been examined. This partially oriented fiber has a crystalline content of about 25% and a high degree of orientation. The effect of time and tension during heat treatment was examined, and it was found that yarns that were allowed to relax during an initial brief heat treatment at 130°C subsequently elongated by up to 5% during a long heat treatment at the same temperature. Yarns that were not allowed to relax during the brief heat treatment did not elongate on subsequent heating. The morphological and mechanical changes associated with these processes have been studied using differential scanning calorimetry, X-ray diffraction (XRD), birefringence measurement, microscopy, and tensile testing. A large increase in crystallinity was observed during the brief heat treatment, but a much smaller increase took place during the long heat treatment. XRD indicated that substantial crystal reorganization occurred during both heat treatments, but c-axis growth was most significant in those materials that elongated during long heat treatment. It is proposed that it is this c-axis growth, in conjunction with conversion of disordered amorphous material into oriented crystalline material, that is responsible for the observed elongation. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 989–995, 1997     

Structures and properties of injection moldings of crystallization nucleator-added polypropylenes. I. Structure–property relationships

Flexural test specimens were injection-molded from polypropylenes added with 0.5 wt % of calcium carbonate, talc, p-tert- dibutyl-benzoic acid monohydroxy aluminum, or p-di-methyl-benzylidene sorbitol under cylinder temperatures of 200–;320°C. Properties such as flexural modulus, flexural strength, heat distortion temperature, Izod impact strength, hardness, and mold shrinkage and higher-order structures such as crystalline texture, crystallinity, a^*-axis-oriented component fraction, and degree of crystalline orientation were measured and structure–property relationships were studied. By the addition of crystallization nucleators, the flexural modulus, flexural strength, heat distortion temperature, hardness, and mold shrinkage were increased and Izod impact strength was decreased. The degrees of crystalline orientation such as the orientation fraction OF and c-axis orientation function f_c were increased by the addition of nucleators. The degree of the increase was higher as the crystallization temperature was higher. Close relationships were observed between some properties and the degrees of crystalline orientation.     

X-ray diffraction orientation studies on blown polyethylene films. III. High-stress crystallization orientation

X-Ray diffraction orientation measurements have been made on a wide range of films blown from three high-density polyethylenes, to determine more precisely the conditions which lead to the high-stress crystallization type of orientation. The most extensive measurements relate to films from Hostalen GM 9955F; the results show that there is a very wide range of orientational behavior. Under very low-stress conditions there is almost pure a-axis orientation; with very high stress there is substantial c-axis orientation, both with reference to the machine direction. Commercial blowing conditions give rather high stress and the a axis is inclined at 60° to 70° to the machine direction in the sheet-normal—machine-direction plane. Calcium stearate, used to improve the surface finish, increases the stress for a given set of machine conditions and, of these, a high draw ratio and a low extrusion temperature are most effective in promoting high-stress crystallization. The less extensive results for an experimental Rigidex grade and Shell LPPE 040 fit into this overall pattern; for a given set of blowing conditions they have lower stress than the Hostalen polymer. Commercial blowing conditions give an a-axis inclination of about 45°.     

Orientation mechanism during crystallization of apatite-type lanthanum silicate thin films from an amorphous precursor

Control of the crystallization and orientation of apatite-type lanthanum silicate (LSO) plays an important role in designing and improving the LSO synthesis process. The mechanisms that determine the c-axis preferential orientation of LSO thin films synthesized by chemical solution deposition and their correlation with preparation conditions were investigated. Crystallization was found to be initiated preferentially at the surface of the precursor thin films. It was also found that the orientation of LSO thin films was largely governed by the orientations of the LSO nuclei that formed at the surface of the precursor thin films. In addition, the c-axis orientation was influenced by the atmosphere used during crystallization and the Si/La ratio in the precursor thin films. An oxygen atmosphere during annealing and lower Si/La ratios reduced the degree of c-axis orientation.     

Epitaxial aluminum nitride thin films grown by pulsed laser deposition in various nitrogen ambients

The effect of nitrogen ambient pressure on growth of AlN films has been examined. High-quality epitaxial AlN films were grown on (0001) sapphire substrates using pulsed laser deposition from a sintered AlN target in low nitrogen ambient of 9.0×10⁻⁵ Torr. The orientation of AlN films can be controlled by nitrogen pressure. AlN films are c-axis oriented when grown in a nitrogen pressure of 9.0×10⁻⁵ to 4.0×10⁻² Torr. Film orientation converted to a-axis as nitrogen pressure increased to 4.0×10⁻¹ Torr. The X-ray rocking curves of the AlN (0002) peak became narrower with decreasing ambient pressure and yielded a full width at half-maximum of 0.078°. The N/Al composition ratio increases with nitrogen pressure.     

Effects of substrate temperature on thermal stability of Al-doped ZnO thin films capped by AlOx

Effects of substrate temperature on the thermal stability of Al-doped ZnO (AZO) films have been studied. Degradation of electrical properties of AZO films by annealing under flowing N₂ gas depends on their crystallinity controlled by the substrate temperature. A thin AlO_x capping layer was employed to passivate the thermal degradation of the AZO layer. A strong correlation between Zn desorption and reduction in carrier concentration was observed. Thermal desorption of Zn was prevented by the AlO_x layer, retaining carrier concentration. With the AlO_x capping layer, the reduction in Hall mobility was prevented in samples with good c-axis orientation, while the reduction in Hall mobility was still observed in poor c-axis oriented films. However, the reduction was smaller than that in bare AZO films. The dependence of Hall mobility evolution on the substrate temperature, and therefore, on crystallinity, strongly suggests the impact of grain boundary scattering on thermal degradation. An increase in optical mobility, which was evaluated from optical spectra using the Drude model, with annealing temperatures, supports the conclusion that an increase in grain boundary scattering by annealing caused the degradation of Hall mobility. The increase in grain boundary scattering induced by Zn desorption was prevented by the capping layer, while contributions of domain alignment and other segregation of defects to the grain boundary scattering, which depend on the substrate temperature retained, leading to different evolutions of Hall mobility.     

Structure development during melt spinning of linear polyethylene fibers

Apparatus has been developed for studying the development of crystallinity and orientation during the melt spinning of synthetic fibers. Tension in the fiber and temperature, diameter, and x-ray diffraction patterns are measured as a function of distance from the spinneret for a running monofilament. Measurements are presented for linear polyethylene over a range of spinning variables together with other investigations carried out on the final as-spun fibers. These data indicate that the development of crystallinity in polyethylene is controlled by a balance between increased crystallization kinetics caused by the stress in the fiber and a tendency for increased supercooling with change in any spinning variable that increases cooling rates in the fiber. The type of crystalline orientation observed, its development during the spinning process, and the changes observed with changes in spinning conditions suggest a model for the as-spun fiber structure in which varying amounts of row nucleation and twisting of lamellar, folded-chain crystal overgrowths occur depending on the spinning conditions. As-spun fiber birefringence was shown to depend primarily on the crystalline orientation. Mechanical properties correlated well with c-axis crystalline orientation function and spinline stress.     

Electron spin resonance of transparent alumina ceramics fabricated by spark plasma sintering

Transparent α‐alumina ceramics are fabricated using spark plasma sintering. Paramagnetic defects related to the optical properties of the ceramics have been investigated using electron spin resonance (ESR) analyses. An isotropic ESR signal at g = 2.003 (S = 1/2) with a linewidth of 0.5 mT is formed during sintering. The g = 2.003 signal intensity has a weak correlation with the absorbance in the visible region but does not correlate with the real in‐line transmission (RIT) at 650 nm. An ESR signal with a fine structure attributed to Fe³⁺ was detected in both the α‐Al₂O₃ starting powder and the sintered ceramic samples. The degree of c‐axis orientation of the grains has been determined using the Fe³⁺ signal intensity, which depends on the angle between the directions of the c‐axis and the applied magnetic field. The ESR analysis indicated that the c‐axis tends to be oriented in the direction of the sintering pressure. The degree of c‐axis orientation was found to correlate with the RIT in highly densified ceramics.     

Control of molecular orientation

I. Amorphous polymers . The mechanical performance of a glassy amorphous polymer is strongly dependent upon molecular orientation. The pattern of molecular orientation is governed by the kinematics (and temperature) of mechanical forming operations. Three types of controllable orientation are: (a) uniaxial, (b) biaxial, and (c) “crossed.” The optimum pattern of orientation in a part is one which is appropriate for the mechanical stresses encountered in service. For a fiber subjected to tensile and bending loads, uniaxial orientation is appropriate. A shell structure, subjected to multiaxial stresses, requires either biaxial or crossed orientation for maximum performance. As a rule, the maximum achievable multidirectional strength in such a structure is less than the maximum strength of a uniaxially oriented fiber. II. Crystalline polymers . Oriented crystalline polymer structures can be created in two distinct ways. An isotropic polycrystalline polymer can be deformed below the melting point, with extensive reorganization of the crystal morphology, or an oriented amorphous melt can undergo crystallization to yield oriented crystalline polymer. Performance of an oriented semicrystalline polymer depends upon orientation of the amorphous portion as well as orientation of the crystallites. As with amorphous polymers, orientation can be uniaxial, biaxial, or crossed. “Orientation” usually denotes c-axis orientation only, but drawing followed by rolling can result in double orientation—orientation of a-axis, b-axis, and c-axis.     

Effects of shearing conditions on crystalline orientation and relaxation in polyethylene

The effects of shearing conditions (i.e., shear temperature and shear rate) on the degree of orientation of polyethylene (Marlex 6006) and to what extent the induced orientation could be relaxed were examined in this study. Two types of samples were prepared: namely, SIC (shear-induced crystallization) and non-SIC samples. The SIC samples show induction times and possess a high degree of c-axis orientation along the shear direction. The induced orientation of SIC samples can be relaxed to a small extent but does not reach a steady value. Non-SIC samples do not show induction times and they show low degrees of c-axis orientation. The induced orientation of non-SIC samples can be relaxed to a steady state value with an activation energy of 90 kJ/mole. Our results also indicate that, when the shear temperature is at and above 145°C, polyethylene can be sheared up to 200X without introducing any significant molecular orientation even at very high shear rates.     

Wide angle X-ray diffraction investigation of crystal orientation in miscible blend of poly(ε-caprolactone)/poly(vinyl chloride) crystallized under strain

The orientation of poly(ε-caprolactone) crystals in miscible poly(ε-caprolactone)/poly(vinyl chloride) (PCL/PVC) blends, melt crystallized under strain, has been studied by wide angle X-ray diffraction (WAXD). At low draw ratios or low PVC contents, all the observable (hk0) crystal reflections orient towards the meridional direction in WAXD patterns, indicating the presence of ring-fibre orientation. With the increase of draw ratio or PVC content, additional crystal orientation with the crystal a-axis parallel to the stretching direction is found to superimpose on the WAXD pattern of ring-fibre orientation. Both the ring-fibre orientation, which dominates the WAXD pattern, and the a-axis orientation are characterized by the perpendicular orientation of the crystal c-axis to the stretching direction. The unusual PCL orientation is a consequence of the combined effects of both the stretching and the presence of PVC in the PCL/PVC blends.     

Unit cell variations of polyethylene crystal with temperature and pressure

The unit cell dimensions for the drawn and the extended chain crystal samples of polyethylene (PE) have been measured at temperatures between 20° and 130°C under hydrostatic high pressures up to 4000 kg/cm², by the use of high pressure and high temperature X-ray diffraction apparatus. A clear difference was found in the variation of a, b and c dimensions of orthorhombic unit cell in PE with temperature and pressure. The temperature changes of linear compressibility for each axis direction of the unit cell and for cell volume were determined on the bases of the variation of cell dimensions. For the a-axis direction, a drastic increase of the compressibility was observed above ～90°C but for the b- and c-axis directions, it was constant for all the temperature region of the measurement. The values of Grüneisen constant, γ, were evaluated at various temperatures from the compressibility data for both samples. The value of γ of the ECC sample was nearly constant below 90°C and gradually increased above 90°C; in the drawn sample, however, a rather steep increase was observed above 90°C.     

Fabrication of c-axis oriented zinc oxide by electrophoretic deposition in a rotating magnetic field

The fabrication of c-axis oriented zinc oxide was attempted by electrophoretic deposition (EPD) in a rotating magnetic field. The EPD was conducted in a small container which was placed on a turntable arranged in a superconducting magnet. The suspension was rotated at 0–90 rpm in a 12 T magnetic field during the deposition. The deposits were dried and then sintered at 1400 °C for 2 h. The degree of the c-axis orientation was evaluated by the Lotgering factor calculated from the X-ray diffraction data.     

Microstructural characterization of injection-molded articles

The techniques of density, X-ray diffraction, and infrared dichroism measurements were employed to study anisotropic behavior, as a result of processing, in injection-molded parts. Data pertaining to the distributions of crystallinity and orientation indicated that significant changes occur at or near the surface of the molding. Generally, maximum density (crystallinity) is observed in the core of the molding and near the gate, while minimum density (crystallinity) is observed near the surface. X-ray diffraction suggests a complex pattern of orientation for the three crystallographic axes. The crystallographic a-axis tends to be oriented in the flow direction as indicated by both X-ray diffraction and infrared dichroism measurements. Generally, raising the molding temperature has only marginal influence on crystallinity and orientation. However, the two resins included in the study exhibited substantial differences in the distribution of these properties. The distributions of crystallinity and orientation are attributable to the complex interactions between resin properties and process conditions.     

Effect of seeding layer on orientation control of potassium niobate thin film by CSD

This paper focuses on the orientation control of the KN thin film on Si wafer by chemical solution deposition (CSD). We selected the PbO layer and PZT layers as the seeding layer in order to control the crystal orientation of the resulting KN thin film. Crystalline phase in KN thin film was identified by XRD, and the degree of c-axis orientation was calculated from XRD analysis. The resultant KN thin film was orthorhombic perovskite single phase. As a result, highly c-axis oriented thin film (about 90%) was deposited by using PbO seeding layer. The dielectric constant of the resultant KN thin film was measured by impedance analyzer. The dielectric constant of highly c-axis oriented KN thin film was compared with that of the c-axis of KN single crystal.     

Electrochemical self-assembly synthesis of zinc oxide nanoparticles and lamellar-structured organic/inorganic hybrids by electrodeposition in surfactant solution

A cathodic electrodeposition was potentiostatically performed in an anionic surfactant solution under various electrodeposition conditions to investigate the influence of the condition on the structure of electrodeposited products, c-axis oriented ZnO films, ZnO nanoparticles shaped like ellipsoidal grains, and lamellar-structured organic/inorganic hybrids. The single phase of the ZnO nanoparticles was obtained only at a specific electrodeposition condition which was confirmed to be the boundary condition for two phases of the c-axis oriented ZnO and the lamellar hybrids. The origin of synthesis of the ZnO nanoparticles is probably a competitive formation reaction between the products of the two phases.     

c-axis oriented ZnO formed in a rotating magnetic field with various rotation speeds

A rotating magnetic field was used to fabricate c-axis oriented zinc oxide. The influence of rotating speed on orientation structure was also examined. The aligned axes had the largest diamagnetic susceptibility, which axis was difficult to align with a static magnetic field. In c-axis oriented ZnO, the degree of orientation (Lotgering factor) in the green compact ranged from 0.2 to 0.5 along c-axis. The Lotgering factor increased with rotating speed. For all samples with the rotating magnetic field, the degrees of orientation increased up to above 0.9 after sintering at 1573 K.     

Optically transparent self-reinforced poly(ethylene terephthalate) composites: molecular orientation and mechanical properties

Self-reinforced composites have been fabricated by compaction of oriented polyethylene terephthalate (PET) fibers under pressure at temperatures near, but below, their melting point. The originally white fiber bundles, which were about 40% crystalline, show increased crystallinity (55%) but optical translucency after processing. Differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) were used to study the crystallization and orientation of the fibers, revealing that the degree of crystallinity was somewhat insensitive to compaction conditions while the melting point increased substantially with increasing compaction temperature. Crystalline orientation, gauged using the Hermans orientation parameter from WAXD data, indicated that no significant loss in orientation of the crystalline fraction occurs due to compaction. Mechanical characterization revealed a stepwise decrease in flexural modulus (9.4-8.1 GPa) and concomitant increase in transverse modulus and strength on increasing the compaction temperature from 255 to 259 °C. This transition in behavior was accompanied by a loss of optical transparency and a change in the distribution of amorphous fraction from fine intrafibrillar domains to coarse interfibrillar domains as seen with electron microscopy. We argue then that the mechanical properties of PET compactions are influenced more by orientation of the amorphous phase than that of the crystalline phase. The impact properties of compacted materials, characterized using an unnotched Charpy test method, showed remarkable impact resistance after compaction, with impact toughness decreasing as compaction temperature was increased.