A small-angle X-ray scattering study of powder compaction

This work demonstrated a novel and potentially important application of two-dimensional small-angle X-ray scattering (2D-SAXS) to investigate powder compaction. SAXS from powder compacts of three materials commonly used for pharmaceutical tabletting exhibited azimuthal variations, with stronger intensity in the direction of the applied compaction force, relative to the transverse direction. This implied that compaction of a (macroscopic) powder could also produce changes on the molecular (nanometre) scale, which can be probed by 2D-SAXS. Two possible explanations for this effect were suggested. A combination of anisometric (i.e. elongated or flattened) granules with anisotropic morphologies could result in azimuthal variation in X-ray scattering due to granule orientation. It is expected that this mechanism would require relatively low packing density, so may operate during die filling. Granule re-orientation appeared less likely at higher packing densities and compaction pressures, however. Under these conditions, the changes in the 2D-SAXS patterns would be consistent with the powder granules becoming relatively flattened in the compression direction, with corresponding changes in their nano-scale morphology. The magnitude of this effect was found to vary between the materials used and increased with compaction pressure. This suggested that 2D-SAXS studies could provide useful information on force-transmission within a compressed powder. Further analysis of the data also suggested differences in the compaction mechanisms (i.e. granule re-orientation, deformation or fragmentation) between the materials studied.     

A small-angle X-ray scattering study of local variations within powder compacts

This work used two-dimensional small-angle X-ray scattering (2D-SAXS) to investigate the compaction behaviour of pre-gelatinised starch (PGS) and microcrystalline cellulose (MCC), which are commonly used as pharmaceutical excipients. By analysing azimuthal variations in scattering intensity, reproducible relationships were found between the compaction pressure, relative density and changes in the shapes of 2D-SAXS patterns for each material. These results indicated differences in the compaction mechanisms between PGS and MCC.The relationships also provided a means for investigating local variations in compaction behaviour within specimens prepared using different materials and compaction conditions. Relative density results from 2D-SAXS were consistent with expectations based on the effects of friction during compaction and appeared similar to data from other methods. In addition, however, 2D-SAXS measurements revealed local variations in the effective direction in which compaction occurred, with significant radial components observed near the die walls. This appeared to be consistent with the transfer of some compaction pressure to friction on the die wall. These observations represent an important advance, since other experimental methods do not easily reveal the direction of force transmission within the powder compact.     

Small-angle X-ray scattering of long-period structures forming bundles

Small-angle X-ray scattering (SAXS) patterns have been calculated based on a structure model, which consisted of the bundles of long-period structures. The proposed model has produced various scattering patterns of polymers, such as the equatorial, layer line, four-spot, droplet-shaped and triangular scattering. The 0.5th order scattering has arisen when the disorder in or between the long-period structures was large even though the structure did not have the periodicity directly related to the scattering maximum. A slight decrease in the disorder due to slip between the long-period structures has accounted for the sudden change of the SAXS pattern of a poly(ethylene terephthalate) fiber from the four-spot to the layer line scattering which was caused by a slight tensile deformation.     

Morphological Behaviour of Thermoplastic Polyurethanes During Repeated Deformation

Summary: SAXS was used to investigate the morphological responses of two commercial thermoplastic poly(ether‐co‐urethane) elastomers during repeated uniaxial extension and stress‐relaxation at constant strain. Experimental data was analysed using a ‘globular’ morphological model, which has previously been shown to provide a good interpretation of the SAXS from these polymers. The results indicated that microdomain rotation and fragmentation coincided with and may have contributed to the strain‐softening observed during the initial deformation cycles and stress relaxation. However, these morphological changes appeared to be largely reversed, when the material was allowed to retract; consequently, they appeared insufficient to account for the dramatic changes in mechanical properties and permanent set observed between the first and second extension cycles. One possible explanation is that the mechanical properties may have been dominated by a few, larger microdomains that were too large to be observed by SAXS. Alternatively, the considerable changes in scattering intensity suggested a mechanism based on the slippage of entanglements as a result of strain‐induced segmental mixing.

2D‐SAXS patterns of poly(ether‐co‐urethane)s during second uniaxial extension.     

Real time SAXS/stress-strain studies of thermoplastic polyurethanes at large strains

Simultaneous small angle X-ray scattering (SAXS) and force measurements have been recorded during tensile deformation of two contrasting polyurethane elastomers. The elastomers comprise the same hard and soft chemical segments; in Sample A, the length of the hard blocks is randomised while in Sample B the hard blocks are monodisperse. During deformation of Sample A, the SAXS halo from the mesophase structure deforms to an ellipse with intensification on the meridian. In Sample B, the halo transforms into a four point pattern. The ellipse patterns of A are interpreted in terms of a model based on particles located on a statistical lattice which is subjected to an affine deformation scheme. According to this model, the SAXS patterns of A are consistent with the hard phase regions behaving as embedded particles which separate from each other in an affine manner and which are not impeded by interconnections during the mechanical yield process. In B, the interconnection of the hard phase prevents affine deformation of the structure and involves the formation of a four point ‘lattice’ structure which then subsequently deforms in an affine manner. The differences in behaviour are linked with the segment sequencing which result in the phase regions of Sample A having a lower volume fraction and are consistent with variation in applied stress.     

Deformation-induced highly oriented and stable mesomorphic phase in quenched isotactic polypropylene

Changes of structure and morphology of quenched isotactic polypropylene (i-PP) film during tensile deformation at room temperature have been investigated by applying in situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. SAXS patterns show that the isotropic diffuse scattering peak in the initial quenched i-PP film disappears after the film experiences necking and a strong streak, which accompanies obvious film stress-whitening, appears when strain reaches 600% and above. Explanation to the former is the dropping of density contrast between amorphous and mesomorphic phases due to deformation and that to the latter is the formation of microfibrils and/or microvoids in the film. WAXD patterns indicate that i-PP chains in the mesomorphic phase become highly oriented along the drawing direction with the strain value more than 5% and the three-fold helical chain conformation keeps in the deformed mesomorphic phase. The mesomorphic structure in the initial quenched i-PP film does not transform into crystal phase although high orientation appears during tensile deformation and the highly oriented mesomorphic phase is considered to be quite stable. Thermal behaviors of the initial quenched i-PP and deformed i-PP with the strain value of 650% are examined by modulated differential scanning calorimetry (MDSC). MDSC results show that the highly oriented i-PP chains in deformed mesomorphic phase are stable, which brings about restriction to chain mobility and prohibits from phase transformation of mesophase to crystal phase in the temperature range normally measurable for the initial quenched i-PP. A model about tensile deformation of the quenched i-PP is outlined to distinctly illuminate the changes of structure and morphology in both the amorphous and mesomorphic phases.     

Real-time in situ light scattering and X-ray scattering studies of polyethylene blown film deformation

A series of linear low-density polyethylene blown films were studied using the techniques of time-resolved, small-angle X-ray scattering (SAXS) using a synchrotron source and a time-resolved, small-angle light scattering. Scattering patterns and the load-extension curve were obtained simultaneously during deformation. It was found that the initial orientation of the film, with respect to the tensile axis, was important in determining the operative elastic deformation modes. Films drawn parallel to the machine direction (MD) showed evidence for lamellar separation, whereas interlamellar shear occurred in films drawn parallel to the transverse direction. In films drawn at 45° to MD, lamellar stack rotation was observed via SAXS. In all cases, the yield point corresponded to the activation of crystallographic deformation and the onset of the disruption of crystalline lamellae. In films drawn parallel to MD, the SAXS showed a distinct 4-point pattern upon macroscopic yield, indicating lamellar corrugation. Regardless of the initial orientation, a fibrillar morphology was achieved at some strain after yield that coexisted with the fragmenting lamellar morphology. Comparison of results from deformed spherulitic bulk samples showed that the study of oriented blown film containing a stacked lamellar morphology may be used, to a first approximation, as a model for the deformation of different regions of spherulites in unoriented spherulitic samples. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 321–339, 1998     

Structure-property correlations in stretched LLDPE films using oblique incidence small-angle X-ray scattering

This paper demonstrates the use of oblique incidence small-angle X-ray scattering (SAXS) to characterize microstructure in linear low-density polyethylene films subjected to biaxial deformation in a TM Long stretcher. Differences in tear strength and impact properties were found when two different deformation temperatures were used. SAXS was used to characterize the nanometer scale microstructure of these films. Since the microstructure was expected to be anisotropic, SAXS data were collected in normal incidence, T (transverse) oblique incidence and M (reference direction) oblique incidence. The sections through the three-dimensional SAXS pattern collected in these three orientations are combined to provide clues to the nature of the complete 3-D SAXS pattern, which can be interpreted to characterize the changes wrought by deformation.     

A SAXS–WAXD study on the mesomorphic-α transition of isotactic polypropylene

The smectic-α transition of iPP was studied by wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS). WAXD and SAXS patterns were taken at different temperatures during the transition. Two different approaches were taken in the analysis of SAXS data: the correlation function and a fitting method based on theoretical distribution models. Up to 80°C just smectic phase was observed, whereas beyond that temperature α lamellae appeared and the two populations of lamellae were found to coexist in the sample. The new α phase population stemmed from within the preexistent smectic stacks, according to a lamellar insertion model. α lamellae thickened to a larger extent than the smectic phase, that just underwent thermal expansion. Results were consistent with a mechanism of transformation involving a rearrangement of the chains without a melting-recrystallization process. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Structural changes during the deformation of thermoplastics in relation to impact resistance

Plain and rubber-modified polymers deform by shearing, crazing, and voiding. The structural changes during deformation were studied by density measurements, small-angle X-ray scattering (SAXS), and electron microscopy (EM). The consequences of these deformation processes for impact resistance are briefly discussed.     

Temperature-resolved SAXS studies of morphological changes in melt-crystallized poly(hexamethylene terephthalate) and its melting upon heating

Temperature-resolved small-angle X-ray scattering (SAXS) on poly(hexamethylene terephthalate) (PHT) samples crystallized from the melt yields direct information about the morphological changes in lamellar crystals and interlamellar amorphous layers upon melt-crystallization and subsequent heating to melting. Absolute intensities of these SAXS patterns were further analyzed via one-dimensional correlation and interface distribution functions. These analyses indicate that melt-crystallization at low temperature produces lamellar crystals having diverse thicknesses whereas crystallization at high temperature tends to favor growth of thick lamellar crystals with a nearly uniform distribution of thickness. When heating the PHT samples in the melting temperature region, the melting of the lamellar crystals was found to correlate well with the sequential-melting features. When these crystals are heated to higher temperatures, structural alterations from stacked lamellae to isolated lamellar crystals evolve with increasing extent of sequential melting, but, upon re-crystallization during extended annealing, the isolated lamellar crystals can pass through a reversible transition back to stacked lamellae.     

Biaxial orientation of polypropylene by hydrostatic solid state extrusion. Part I: Orientation mechanism and structural hierarchy

The structure which results from solid state extrusion using biaxial orientation is analyzed for oriented polypropylene. Structural changes on the spherulitic, lamellar, and macromolecular level during orientation are investigated using optical microscopy (OM), small angle X-ray scattering (SAXS), and wide angle X-ray scattering (WAXS). The results show that polypropylene spherulites undergo stepwise biaxial affine deformation and deform homogeneously into a disc-like morphology. During this spherulitic flattening process, lamellar rotation into the planar direction occurs prior to lamellar break-up at a baxial draw ratio of about 1.5. On the macromolecular level, the crystalline c-axis orients in the plane concurrently with the lamellar break-up, while the crystalline b*-axis gradually orients normal to the plane. Amorphous chains are also oriented preferentially in the plane of deformation. A hierarchical model is proposed to illustrate the nature of the orientation in the flattened spherulites.     

In situ small angle X‐ray scattering study on structural evolution of crosslinked polytetrafluoroethylene during deformation

The structural evolution of virgin and crosslinked polytetrafluoroethylene (PTFE) during stretching was studied by in situ synchrotron small‐angle X‐ray scattering (SAXS). Both yield and tensile stress of crosslinked PTFE increased with increasing crosslinking density. During stretching, for virgin PTFE, amorphous chains gradually turned to tensile direction at early stage, perpendicularly arranged lamellar stacks appeared at high strains (>140%). While for crosslinked PTFE, lamellar structure was observed even at lower strains; with increasing irradiation dose, the lamellar structure became obvious and the long period decreased. Four‐point SAXS patterns were observed only in 3000kGy‐dosed PTFE during deformation, which indicated that an alternately tilted lamella arrangement called herringbone structure was formed. Radiation dose induces crosslinked networks formed, which can carry part of local stress during deformation, resulting in the increase of yield and tensile stress. Crosslinking density is an important factor on structural evolution. In addition, a deformation mechanism of different crosslinked PTFE is proposed. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39883.     

Small‐angle X‐ray scattering study of modified porous suspension–poly(vinyl chloride) particles

Small‐angle X‐ray scattering (SAXS) was applied to investigate the microstructure of unmodified and modified porous commercial suspension‐type poly(vinyl chloride) (PVC) particles. The modified PVC particles were prepared by an in situ stabilizer‐free polymerization/crosslinking of particles absorbed with a monomer/crosslinker/peroxide solution. The modifying polymers include styrene with or without divinyl benzene (DVB) as a crosslinker and methyl methacrylate (MMA) with or without ethylene glycol dimethacrylate (EGDMA) as a crosslinker. The SAXS method was used to highlight the effect of polystyrene (PS) on the microstructure of PVC particles and to evaluate the characteristic lengths, both in the PVC/PS and the PVC/XPS (PS crosslinked with 0 and 5% DVB, respectively) systems. A model is suggested, where during the synthesis modification process, swelling of PVC by styrene and styrene polymerization occur simultaneously. PVC swelling by styrene causes destruction of the PVC subprimary particles, whereas styrene polymerization leads to phase separation resulting from incompatibility of the polymers. It was further suggested that because of PVC swelling by styrene, structure of the subprimary particles is lost. Therefore the characteristic lengths of PVC/PS and PVC/XPS, as calculated from the SAXS measurements, were attributed to the size of the phase‐separated PS and XPS inclusions, respectively. The SAXS method also shows that PMMA and XPMMA do not influence the PVC microstructure. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 95: 1024–1031, 2005     

Domain and segment orientation behavior of PBS–PTMG segmented block copolymers

Segment and domain orientation behaviors of a series of poly(butylene succinate) (PBS) –poly(tetramethylene glycol) (PTMG) segmented block copolymers containing different amounts of hard segment were studied with synchrotron small‐angle X‐ray scattering (SAXS) and infrared dichroic methods. Copolymers used in this work consist of PBS as a hard segment, and poly(tetramethylene oxide) (PTMO) of molecular weight 2000g/mol as a soft segment. As hard‐segment content increased, phase‐separated morphology changed from a phase of continuous soft matrix containing isolated hard domain to one of continuous hard matrix. Upon stretching, domains responded differently depending on their initial orientation. Based on SAXS results, two major domain deformation modes, that is, lamellar separation and shear compression, were suggested. The orientation behavior of the hard and soft segments was examined with infrared dichroic method. Upon drawing, the orientation function of the crystalline hard segment decreased at low‐draw ratios. It was interpreted in terms of rotation of long axis of hard domain along the stretching direction. The lowest value of the orientation function of PBS30 was approximately −0.5, that is, theoretical minimum. This result seems to indicate that for PBS30 containing about 30% hard segment, rotation of hard domain occurs without appreciable interdomain interaction, which is consistent with the morphological model suggested on the basis of SAXS results. Plastic deformation of the hard domain due to domain breakup was found to occur at low‐draw ratios for the sample containing higher hard‐segment content. Domain mechanical stability was tested by drawing a sample up to three different maximum draw ratios. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 699–709, 2000     

Effect of hydrolytic degradation on the microstructure of quenched,amorphous poly(glycolic acid): an X-ray scattering study of hydrated samples

The effect of hydrolytic degradation on the microstructure of unoriented, quenched poly(glycolic acid) (PGA) was investigated using simultaneous small- and wide-angle X-ray scattering (SAXS/WAXS). Samples were analysed immediately after removal from the degradation media in order to prevent dehydration. Analysis showed that the material initially contained a small degree of crystallinity. On degradation, the material rapidly crystallized, developing a broadly similar morphology to samples crystallized from the melt. The behaviour of these new structures on degradation was similar to that observed in the precrystallized samples previously reported. The crystal density remained constant and little change was seen in the lateral extent of the crystal lamellae. Both the crystallinity and SAXS scattering power (or invariant) increased during the first 30 days which may be due to the preferential removal of amorphous material and further crystallization of amorphous chains. The crystallization of amorphous material was facilitated by plasticization due to the ingress of water and the cleavage of amorphous chains. In both quenched and precrystallized material, the average lamellar spacing fell and then rose during degradation. It is not possible to interpret this unambiguously from the SAXS data alone. It may be partially the consequence of a two-stage removal of amorphous material. Alternatively, the behaviour may be explained by changes in the osmotic potential of the amorphous layer on degradation, together with insertion crystallization. © 1999 Society of Chemical Industry     

Polymer crystallization from the metastable melt: The formation mechanism of spherulites

One of the most popular crystalline morphologies is a spherulite. An evidence is reported that the spherulite is crystallized through a dense packing state of small particles appearing in a droplet, which is caused by the primary phase separation of the melt in the metastable region of a phase diagram proposed by Olmsted et al. [Olmsted PD, Poon WCK, McLeish TCB, Terrill NJ, Ryan AJ. Phys Rev Lett 1998; 81: 373]. According to this phase diagram, the crystallization from the metastable state causes the nucleation and growth (N & G) of nematic domains, here named droplets, in the isotropic matrix. As a next step, the secondary phase separation of spinodal decomposition (SD) type into smectic and amorphous domains occurs inside the droplet where entanglements are excluded from the smectic to the amorphous domain; then such an SD structure turns into a densely packing structure of many small particles owing to surface tension. At this final stage of the induction period a long period peak of small-angle X-ray scattering (SAXS), so-called SAXS before WAXS, appears, which may be due to the average distance between these small particles. Furthermore, it is considered that crystalline lamellae are formed by radial and azimuthal fusion of these small particles inside the droplet, resulting in a spherulite. Such a type of crystallization occurs most commonly when flexible polymers are crystallized under the usual conditions. This tentative concept of spherulitic growth, which is completely different from a theory by Keith and Padden [Keith HD, Padden FJ. J Appl Phys 1963; 34: 2409], would give a new insight into problems of spherulites.     

Assessing organo-clay dispersion in polymer layered silicate nanocomposites: A SAXS approach

A SAXS method for the quantitative assessment of the morphology of polymer layered silicate nanocomposites is proposed. Fitting the SAXS patterns, the number of clay layers, the periodicity of the layers in the tactoids, the thickness of the regions interposed between the clay platelets and their distributions can be measured. A good agreement with TEM data was obtained, avoiding the inconsistencies with microscopical observations often reported in the literature.     

Volume strain changes of plasticized poly(vinylidene fluoride) during tensile and creep tests

The volume strain changes of plasticized poly(vinylidene fluoride) during tensile and creep tests were characterized. A negative volume strain was observed. This phenomenon was ascribed to compaction taking place during mechanical testing, which further delayed the onset of plastic instability and, therefore, the cavitation process. It was suggested that this compaction was caused by the orientation of amorphous chain segments leading to material densification. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1784–1791, 2004