Equatorial Streak Research Articles

Unique Crystalline Orientation of Poly[(R)-3-hydroxybutyrate]/Cellulose Propionate Blends under Uniaxial Drawing

Blends of poly[(R)-3-hydroxybutyrate] (PHB) and cellulose propionate (CP) with various compositions were prepared by the solvent-casting method, and their uniaxial drawing behavior was studied by means of two-dimensional wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). Although this system has been reported to be entirely miscible, the DSC results revealed that the glass transition temperatures of the blends were not composition-dependent in the whole composition range. Furthermore, these transitions were quite broad or even double in the blends with high CP content. These features are the result of differing molecular motions attributed to the difference in chemical structure between the two components of the blend. The feasible maximum draw ratio decreased with increasing CP content due to restraint of chain slippage and necking caused by the stiffness and high friction coefficient of the CP component. A remarkable feature during uniaxial drawing was that the manner of crystallization of the PHB component under strain varied from the c-axis-oriented to the a-axis-oriented crystal growth with increasing CP content. WAXD analysis demonstrated that drawn pure PHB exhibited an ordinary c-axis-oriented pattern, indicating that the (110) and (020) main reflections of PHB were concentrated on the equatorial line. On the contrary, when the CP content was 50 wt %, the diffraction texture was transformed to the a-axis orientation where the (110) reflection was located near the meridian and the (020) and (002) reflections lay on the equatorial line. In the SAXS patterns of the blends containing 30−50 wt % CP, instead of meridional spots, the equatorial streaks were clearly observed, providing direct evidence for lamellar stacking perpendicular to the stretching direction. Such a change of crystalline orientation was found to be strongly dependent on the draw ratio and annealing temperature in addition to the blend composition. In the present paper, a mechanism based on the intramolecular nucleation and confined crystal growth model was suggested to interpret this uncommon orientation behavior.

Shear-Induced Crystallization Precursor Studies in Model Polyethylene Blends by in-Situ Rheo-SAXS and Rheo-WAXD

Development of shear-induced crystallization precursor structure was studied by in-situ rheo-SAXS (small-angle X-ray scattering) and rheo-WAXD (wide-angle X-ray diffraction) techniques using binary polymer blends of high and low molecular weight polyethylenes near their nominal melting temperatures (120 °C). Two low molecular weight polyethylene copolymers, containing 2 mol % hexene, with weight-average molecular weights (Mw) of 50 000 (MB-50K) and 100 000 (MB-100K), and polydispersity of about 2, were used as the noncrystallizing matrices. A high molecular weight polyethylene homopolymer with Mw of 250 000 (MB-250K) and polydispersity of about 2 was used as the crystallizing minor component. Two series of model blends, MB-50K/MB-250K and MB-100K/MB-250K, each containing weight ratios of 100/0, 97/3, 95/5, and 90/10, were prepared by solution blending to ensure thorough mixing at the molecular level. At the chosen shear conditions (rate = 60 s-1, duration = 5 s, T = 120 °C), while no flow-induced structures were seen in pure MB-50K and MB-100K melts, the blends in both series showed distinct but different shear-induced structures. Results indicate that the high molecular weight component dominates the formation of crystallization precursor structures in the blend under shear, which can act as a template for further crystallization. A “shish-kebab” structure, detected by both SAXS and WAXD, was observed in the MB-100K/MB-250K (90/10) blend, while only a twisted lamellar structure (kebab) was seen in the rest of the blends under the same shear conditions. These findings suggest that the matrix viscosity plays an important role to influence the formation of crystallization precursor structure of the high molecular component under flow. In the MB-100K/MB-250K (90/10) blend, the length of the shish was estimated from the equatorial streak in SAXS, which showed a noticeable decrease with time, while the corresponding scattering intensity was found to increase. The evolution of the shish-kebab structure from SAXS is consistent with the appearance of the (110) peak in WAXD, which can be explained by the coil−stretch transition induced by flow.

Synchrotron X-ray Scattering and Precise Structural Analyses of Poly(ethylene terephthalate) Fibers Prepared by High Speed Melt Spinning

Precise structural analyses of high speed melt spinning fibers provide useful information of appropriate nano-structures for imparting ultra-high strength to general fibers. For quantitative and precise analyses by X-ray scattering measurements, strong X-ray source is required. In this paper, we demonstrate significance of the synchrotron X-ray scattering and precise structural analyses of poly(ethylene terephthalate) fibers prepared by high speed melt spinning, with the take-up velocity ranging 1∼9 km/min. Systematic changes in the 2d-SAXS (two-dimensional small-angle X-ray scattering) patterns were clearly observed as a function of the take-up velocity. For 3 km/min, only streaks on the equator were distinct. For 4∼6 km/min, characteristic X-shaped cross-streaks appeared with the streaks on the equator. Then, at 7∼9 km/min, a pair of broad streaked peaks parallel to the equator appeared instead of the cross-streaks, while the equatorial streaks still coexist. We ascribed the X-shaped cross-streaks to a zig-zag fibril structure, and the equatorial streaks with the broad streaked peaks to a shish-kebab structure. Then, based on the 2d-SAXS results, change in the fiber structures with an increase of the take-up velocity is considered. For 4∼6 km/min, the zig-zag fibril is dominant. However, a trivial amount of shish seems to coexist. The shape of fibril changed from zig-zag to straight with an increase of the take-up velocity, while the shish-kebab fraction was considered to increase. It is noted here that the zig-zag fibril consists of somewhat different from an ordinary fibril, but rather thicker-and-looser agglomerates of crystallites. At 9 km/min, fibril is no more the zig-zag, but a straight shish, with a well-developed kebab structure.

The interfacial properties and porous structures of polymer blends characterized by synchrotron small-angle X-ray scattering

Microvoids are induced upon uniaxial drawing of films made from immiscible polypropylene (PP)/polystyrene (PS) binary blends and ternary blends of PP, PS, and a block copolymer SEEPS. The shape of the uniaxially oriented microvoids is rod- or slit-like with a high aspect ratio. Synchrotron small-angle X-ray scattering (SAXS) is used to characterize the dimensions of these microvoids. Their scattering image is an intense azimuthally narrow equatorial streak on a two-dimensional SAXS pattern. This streak is analyzed to obtain the diameter, length and misorientation of the microvoids. The microvoids length is identified as an effective measure of the interfacial adhesion and strength between phase domains. Drawn films of binary blends of PP/PS are found to have the longest microvoids. The initial addition of the block copolymer SEEPS as a compatibilizer enhances the interfacial adhesion and shortens the length of microvoids. Further addition of compatibilizer induces the formation of aggregates of a composite PS/SEEPS dispersed phase, and this leads to reduced interfacial adhesion and a longer microvoids. Interfacial properties are also dependent on the mixing protocol used to produce the blends. The transport property of the films is determined by porosity and the degree of interconnectivity. A convenient measure of the degree of interconnectivity is proposed. The degrees of interconnectivity of these films are in accordance with the interfacial adhesion and strength. Non-equatorial streaks are observed and attributed to the microvoids with a complex orientation and geometry, which are responsible for the interconnectivity among microvoids.

Observations of structure development during crystallisation of oriented poly(ethylene terephthalate)

Two types of SAXS and WAXS experiments have been made using synchrotron radiation to observe the transformation from smectic to crystalline phases in oriented poly(ethylene terephthalate) (PET). In step-anneal experiments, PET was drawn slowly at 30 °C and then observed after annealing at 5 °C steps up to 100 °C. In the other experiments, time-resolved observations were made while drawing at 90 °C at rates up to 10 s−1. Up to 70 °C the WAXS data in the step-anneal experiments showed the smectic meridional reflection reducing in lateral width, indicating an increase in lateral long range order with annealing. Between 70 and 100 °C, there was a reduction in the intensity of the smectic reflection which correlated with an increase in the intensity of crystalline reflections. The SAXS from the step-anneal experiments showed an intense equatorial streak which has a correlation peak around 20 nm and which diminishes with annealing above 70 °C. It is concluded that this feature is a characteristic of the presence of the mesophase in oriented PET and is due to elongated domains of smectic mesophase with a length >75 nm and with an interdomain spacing of around 20 nm. Between 70 and 100 °C the SAXS data showed additional diffuse diffraction which correlated quantitatively with the crystalline phase and evolved from a cross-like appearance to a well resolved four-point pattern. The time-resolved drawing experiments were limited by the time resolution of the SAXS detector. They showed the same development of four-point diffuse SAXS patterns as was observed in the step-anneal experiments and a very weak equatorial streak. Differences in phase transformation kinetics between the two types of experiment are attributed to the different chain relaxation processes available under different conditions.

Orientation-induced crystallization in isotactic polypropylene melt by shear deformation

Development of orientation-induced precursor structures (nuclei) prior to crystallization in isotactic polypropylene melt under shear flow was studied by in-situ synchrotron small-angle X-ray scattering (SAXS) and rheo-optical techniques. SAXS patterns at 165°C immediately after shear (rate = 60 s−1, ts = 5 s) showed emergence of equatorial streaks due to oriented structures (microfibrils or shish) parallel to the flow direction and of meridional maxima due to growth of the oriented layer-like structures (kebabs) perpendicular to the flow. SAXS patterns at later times (t = 60 min after shear) indicated that the induced oriented structures were stable above the nominal melting point of iPP. DSC thermograms of sheared iPP samples confirmed the presence of two populations of crystalline fractions; one at 164°C (corresponding to the normal melting point) and the other at 179°C (corresponding to melting of oriented crystalline structures). Time-resolved optical micrography of sheared iPP melt (rate = 10 s−1, ts = 60 s, T = 148°C) provided further information on orientation-induced morphology at the microscopic scale. The optical micrographs showed growth of highly elongated micron size fibril structures (threads) immediately after shear and additional spherulities nucleated on the fibrils at the later stages. Results from SAXS and rheo-optical studies suggest that a stable scaffold (network) of nuclei, consisting of shear-induced microfibrillar structures along the flow direction superimposed by layered structures perpendicular to the flow direction, form in polymer melt prior to the occurance of primary crystallization. The scaffold dictates the final morphological features in polymer.

In-Situ Synchrotron WAXD/SAXS Studies of Structural Development during PBO/PPA Solution Spinning

Combined wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) studies using synchrotron radiation were carried out to investigate the structural development during in-situ solution spinning of poly(p-phenylenebenzobisoxazole) (PBO) in poly(phosphoric acid) (PPA). WAXD patterns indicated that the structure in the extruded PBO/PPA dope before coagulation (in water) possessed lyotropic “biaxial nematic” nematic order involving PBO−PPA complex as the mesogenic unit. A different biaxial nematic order in pure PBO was found to develop immediately upon coagulation in water. When the coagulation time exceeded 30 min, the complex PBO/PPA structure completely disappeared, and well-oriented PBO crystals were formed. The meridian peaks exhibited long streaks, indicating a translational disorder of PBO molecules in the fiber direction. SAXS patterns of the PBO fiber after coagulation showed equatorial streaks with no further maxima observed, which is attributed to a microvoid structure.

Synchrotron studies of polymers at DND-CAT

Two examples are presented that illustrate the capabilities of DND-CAT instrumentation for the study of the effects of processing on polymers. Firstly, a thermoplastic elastomer, Hytrel®, was stretched while 2-D data were collected simultaneously. The Hytrel® data show that the yield point of the stress-strain curve is associated with the sudden appearance of a four-point pattern. At higher deformations, strain-induced crystallization and the destruction of the hard segment domains lead to a substantial decrease of the contrast as monitored by the SAXS invariant. Prior to breakage, the extent and intensity of an equatorial streak develops as the material fibrillates. Secondly, SAXS and WAXS data were collected from quenched and annealed Poly(ethylene terephthalate) (PET) samples mounted on a DSC cell, to characterize the pre-melting shoulder in this material. Results show that substantial melting and re-crystallization occurs within the range of this shoulder in the quenched sample.

In-Situ Simultaneous Synchrotron Small- and Wide-Angle X-ray Scattering Measurement of Poly(vinylidene fluoride) Fibers under Deformation

Poly(vinylidene fluoride) (PVDF) fibers spun at different take-up speeds (10.6−61.0 m/min) were subjected to stretch−hold deformation at room temperature and in-situ simultaneous synchrotron small- and wide-angle X-ray scattering measurements. Crystal transformation from α to β form and morphological changes in lamellar and fibrillar structures were analyzed in detail. All fibers were found to yield at an early stage of deformation, resulting in alternating necked and unnecked regions along the fiber. From the two-dimensional (2-D) wide-angle X-ray diffraction (WAXD) patterns, mixed α and β (minor fraction) forms were found to coexist in the undrawn fibers. Deformation assisted in the conversion from the α-form into the β-form. In necked regions, more α to β transformation took place than in unnecked regions. The overall crystallinity index and unit cell parameters of the α form did not change significantly prior to necking. From the 2-D small-angle X-ray scattering (SAXS) patterns, two kinds of equatorial streaks were observed. The first kind originated from fiber of high take-up speed (61.0 m/min) under zero or small strain, and the second kind came from highly deformed fibers (all take-up speeds) in both necked and unnecked regions. These two kinds of equatorial streaks were attributed to the formation of microfibrils and microvoids, respectively. The dimensions of the lengths of microfibrils and microvoids were estimated by Ruland's method. Meridianal scattering maxima from a lamellar morphology were observed in the SAXS patterns in fibers under zero or low strain. The long period of the lamellar structure, estimated using correlation function analysis, increased with strain. Results from SAXS and WAXD analysis suggest that the formation of defects during yielding and plastic flow facilitates the α to β crystal phase transformation, and a phase, similar to conformationally disordered phase, whose density is close to that of crystal, is induced out of the amorphous phase in lamellar structure with application of strain.

Small-Angle X-ray Scattering of Spider Dragline Silk

As part of a general study of the structure of spider silk fibers, major ampullate gland silk fibers were collected from Nephila clavipes spiders, and SAXS patterns were obtained from loops of fibers under a variety of conditions. Two orders of lamellar reflection were seen, with a long spacing of 8.4 nm. This increased reversibly by 4% when the fiber was stretched by 10% and shrank to 5.8 nm when the fiber itself shrank 45% on wetting. A strong equatorial streak had a bimodal orientation distribution similar to that seen in NMR and WAXD. The sharper component had a lateral size scale (radius of gyration) of 2.5 nm and a misorientation of 10° FWHM, similar to the orientation of the crystals. The minimum breadth of the streak indicates that the scattering objects are 0.1 μm long. There is an isotropic central scattering, probably caused by voids. On wetting, the lamellar peaks became more intense; more dramatically, the equatorial scattering strengthens and extends to higher angles, almost halving the lateral size scale to 1.4 nm. An equatorial maximum appears at a spacing of 6 nm, indicating a degree of order in the fibrillar structure.

Full-pattern analysis of two-dimensional small-angle scattering data from oriented polymers using elliptical coordinates

Two-dimensional small-angle scattering (SAS) data from oriented polymers are parameterized by profile fitting the intensity distribution to a product of two orthogonal functions. Elliptical cylindrical coordinates were found to best describe the observed small-angle scattering data. Each of the essential features of the small-angle X-ray/neutron scattering from uniaxially oriented polymers—the equatorial streak, lamellar reflections and interfibrillar interference peaks—is described completely by a single function in the elliptical coordinates. The parameters of the fit are used to describe the fibrillar and the lamellar structures. The analysis is illustrated with data from nylon 6 fibres, and the results are compared with those from a previous analysis of the same data as a series of one-dimensional scans. The method has enabled us to follow the changes in the weak equatorial scattering attributed to fluid-like organizations of the fibrils. The elliptical coordinates can be used to describe the wide variety of small-angle patterns that have been reported in the literature. The applicability of the elliptical coordinate system to SAS data is shown to be a natural consequence of the scattering object being elongated along the flow or the draw direction. Ellipticity could be used as a quantitative measure of the shape and the orientation distribution of the scattering object.

On the small-angle X-ray scattering of rigid-rod polymer fibres

Small-angle X-ray scattering was carried out on as-spun and heat-treated poly(p-phenylene benzobisoxazole) (PBO) fibre. While as-spun PBO fibre exhibits only an equatorial streak, generally attributed to elongated voids or fibrils, the heat-treated fibre shows in addition a radial four-point pattern. The long period and radial spacing of this pattern are 28 and 15 nm, respectively. The spacings do not vary with heat-treatment temperature. As the angle defined by the plane normal to the fibre axis and the X-ray beam is increased from 0° to 30° to 60°, the four-point pattern gradually changes to a two-point pattern. The measured long period in the tilted fibre is a projection of that in the untilted fibre. Various possible morphological models are discussed.

High-modulus polyethylene fiber structure as shown by x-ray diffraction

High-quality X-ray diffraction patterns were obtained from small bundles and single fibers of polyethylene at both small and wide angles using a synchrotron X-ray source. The equatorial streak in small angle (SAXS) can be used to give the length, width, and misorientation of the scattering objects, while the shape of wide-angle spots gives the same information about the crystals. The SAXS objects are 150-200 nm long, longer than the crystal length. This is in accord with microfibril models where the crystals are not continuous. The fibril interfaces are significantly less well oriented than thecrystals within them

Small-angle X-ray scattering of aramid fibre

Aramid fibres show elongated microvoids parallel to the direction of the fibre axis, resulting in an equatorial streak in the small-angle X-ray scattering (SAXS) pattern. The scattered intensity in the equatorial direction has been measured for three samples of the same fibre: an untreated yarn specimen, yarn in which Ag2S was precipitated and a single filament of the latter. In the case of the untreated yarn, the interpretation of the SAXS pattern is hindered by the presence of residual water in the voids, resulting in a three-phase system. The SAXS pattern of the yarn with precipitated Ag2S follows the theoretical pattern (adapted for the two-dimensional case) for a system with random pores, but the interface between aramid and Ag2S shows fractal properties. The single filament with Ag2S shows a SAXS pattern analogous to that of the fibre with Ag2S, indicating that multiple scattering and interfilament scattering is negligible. It was also found that the Ag2S precipitates in the fibre with preferred orientation.

Small-angle X-ray diffraction of Kevlar using synchrotron radiation

A synchrotron source of X-rays has been used to obtain high-resoultion small-angle X-ray scattering (SAXS) patterns from commercial poly( p-phenylene terephthalamide) (Kevlar) fibres. In Kevlar 49, detailed analysis of the equatorial streak, often called the ‘void scattering’, shows that it comes from the crystals in the fibre and not from any voids. The scattering objects producing the SAXS pattern have the same length, width and orientation distribution as the crystals. In Kevlar 149, there is less scatter overall, a bimodal distribution of sizes and orientation, and an off-axis streak at ±25° to the equator. Similar off-axis small-angle scattering has recently been seen in other rigid-rod polymer fibres, but the fibre structure producing it is not understood.

Structure and properties in ultra-high speed spinning of poly(butylene terephthalate).

Ultra-high speed spinning of poly (butylene terephthalate) (PBT) has been carried out for take-up velocity up to 14, 000m/min. The structure and properties of as-spun fibers were investigated and compared with poly (ethylene terephthalate) (PET).Orientation of molecules and crystallization proceed largely at relatively low velocities up to 3, 000m/min. Increase in birefringence becomes small above 3, 000m/min, and the skin-core structure developes above 10, 000m/min, though the core part is small in comparison with PET. Density and maximum birefringence in the cross section of the fiber increase up to 14, 000 m/min. SAXS shows a typical meridian reflection and a weak equatorial streak at ultra-high take-up velocities, which suggests the regular arrangement of crystallites perpendicular to the fiber axis. Generation of microvoids in PBT fiber is not so remarkable in comparison with PET fiber. The size of crystallites and the amount of the α crystal form increase with increasing take-up velocity, Melting temperature rises about 21°C in the range of take-up velocity from 1, 000 to 14, 000m/min.As PBT has more flexible chains, crystallization of the polymer occurs more easily along spinline than that of PET. This may explain that the more stable fine structure appeares in the ultra-high take-up velocities.

Linear equatorial streaks.

To the Editor. —I read with great interest the description by Fountain and Schlaegel of linear equatorial streaks in the February issue of theArchives(1981;99;246-248), for I have collected six such cases during the past two years. In my experience, these streaks seem to be nondiagnostic, for they can occur in normal eyes, in high myopia, in ocular toxoplasmosis, and in presumed ocular histoplasmosis (POHS). I, therefore, do not believe it appropriate to change the POHS triad to a tetrad on the basis of this as yet unexplained but apparently nonspecific ocular sign.

X-RAY DIFFRACTION STUDIES ON FROG MUSCLES

X-RAY DIFFRACTION STUDIES ON FROG MUSCLES