Time-resolved synchrotron X-ray study of chain-folded crystallization of long paraffins

The examination of the crystallization of strictly uniform ultra-long n-alkanes was resumed with the view of exploring the onset of chain-folding, as laid out previously in ref. 1. The present study, largely on C₂₄₆H₄₉₄, centred on the initial stages of crystallization in the melt, registered in situ by time-resolved small-angle X-ray scattering using a synchrotron X-ray source. The salient new feature was the identification of transient initial fold lengths which were non-integer fractions (NIF) of the chain length. This NIF structure transforms subsequently into forms with integer fraction (IF) fold lengths. In the present study the latter have the extended chain (E) and once-folded (F2) configurations, while NIF has a fold length between the two. The NIF → IF transformation occurs either by lamellar thickening or thinning, or by both. It was found that the NIF state had a more disordered layer surface as compared to the final E and F2 structures, the latter being the states on which the conclusions in ref. 1 had been drawn, which accordingly should apply to the transformed material. Implications of these and several other findings for some central issues in polymer crystallization are briefly discussed. The existence of an initial NIF phase focusses attention to the importance of the fastest kinetic pathway as the determining factor for chain-folded crystal growth with particular attention to the initial chain deposition probability, a line made accessible by the present alkanes.     

Small-angle scattering from polyelectrolyte solutions. A novel method for studying counterion condensation

A new method is proposed for the study of binding of a low-molar-mass compound to a polymer chain, based on the calculation of the excess scattering amplitude from small-angle scattering data. The procedure requires the measurement of the scattering curve on an absolute scale at two polymer concentrations. Both smeared and desmeared scattering data may be used. The method is applied to aqueous solutions of poly(methacrylic acid) neutralized with NaOH. The number of Na⁺ counterions bound to the polyion has been determined as a function of the degree of neutralization.     

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.     

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     

Ultra small-angle X-ray scattering studies on structural changes in micrometers upon uniaxial stretching of segmented polyurethaneureas

Ultra small-angle X-ray scattering (USAXS) experiments were conducted in order to examine structural changes in micrometers upon uniaxial stretching of elastomeric segmented polyurethaneureas at room temperature. It was possible to stretch the sample film up to α = 7.5 without break, where α designates the stretching ratio. Around α = 6.5 the sample became turbid, while it recovered transparency when the stress was removed. To understand this curious phenomenon, we conducted the USAXS experiments for the structural analyses in micrometers. Although a set of streaks appeared in the direction parallel to the stretching direction (SD) when the sample became turbid (around α = 7.2), more interesting result is that a set of streaks appeared in the direction perpendicular to SD much earlier around α = 5.0. Close examination of the scattering intensity profile revealed evolution of multiple interference peaks, which could be ascribed to the form factor of a lamellar particle. Since the streaks disappeared upon the removal of the stress, “lamellar particles” are considered to be crazes, which develop further into cracks in a subsequent stage. From the results of USAXS measurements, it is suggested that the lamellar-shaped crazes appeared around α = 5.0 being oriented parallel to SD, and further stretching created other lamellar-shaped crazes being oriented perpendicular to SD, which are co-existing with the preceding parallel crazes.     

In situ tensile damage characterization of C/C composites through X-ray computed tomography and digital volume correlation

Carbon fiber reinforced carbon matrix (C/C) composites have been used in aerospace applications due to their excellent performance. Exploring their failure mechanisms is a subject of extensive research. Nowadays, to obtain information about changes in the failure processes, a technology known as in situ X-ray computed tomography is used. In this paper, tensile loads were applied to 3D fine-woven punctured and needle-punched C/C composites perpendicular to the punctured and needle-punched directions. In situ X-ray computed tomography was employed to observe damage development, and digital volume correlation was used to assess the laboratory X-ray computed tomographs to measure local strains. Assimilation of pores is observed in C/C composites, with cracks evolving from original micro-pores. While fine-woven punctured C/C composites present an elegant linear failure, needle-punched C/C composites present a traditional non-linear failure. This difference is due to the different structures of the preforms. Furthermore, the C/C composites are weak at the sites where they are punctured or needle-punched.     

碳纳米管增强陶瓷基复合材料研究进展

鉴于其优良的力学性能、独特的物理和化学性能,碳纳米管被认为是最理想的增韧材料。本文主要从制备方法和力学性能两方面综述了碳纳米管增韧陶瓷基复合材料的研究现状,并针对研究中存在的问题,提出了相应的设想和发展趋势。     

Pore structure of cement/pozzolan composites by X-ray microtomography

Microtomography, a nondestructive X-ray analysis test, was used to study the pore structure of cement/pozzolan composites. The three-dimensional picture of the overall porosity was created by stacking together sequential slices of two-dimensional microtomographic images. This picture revealed all the pores above 5 μm in diameter, showing that the largest pore was 0.5 mm in diameter and that the volume of large pores was around 2% by volume.     

Spatial distribution of lamella structure in PCL/PVB band spherulite investigated with microbeam small- and wide-angle X-ray scattering

The microbeam small- and wide-angle X-ray scattering (SAXS/WAXS) technique gives the novel information about micron-scale structural inhomogeneity of polymer crystal. By using this technique, we have studied structural inhomogeneity of lamella within the band spherulite of miscible polymer blend poly(ε caprolactone) (PCL)/poly(vinyl butyral) (PVB) and the structure development of lamella during crystallization. It is known that PCL/PVB forms very large spherulites (∼several mm in radius) with highly regular band structure because of low frequency of nucleation and that PCL/PVB crystallized at 41 °C has at least two kinds of lamella structure (150 Å, 180 Å). With an X-ray microbeam initially fixed outside near the growth front of the band spherulite, we have observed the lamella formation at the local point and have found that the larger long period grows before the appearance of the shorter long period. We have also observed that the orientation of lamella with the larger long period is different from that of lamella with the shorter long period from SAXS experiment with an X-ray microbeam scanning the band spherulite along its radial direction. Further, the discontinuity in lamella twisting was observed from scanning microbeam WAXS. Based on the experimental results, we propose two possible spatial distribution models. The result of PCL/PVB crystallized at 35 °C was also discussed.     

The effect of loading rate on the fracture toughness of fiber reinforced polymer composites

This article is a detailed review of the strain rate dependence of fracture toughness properties in polymer composite materials. An attempt is made to draw together all the strain rate studies done in the past and to elucidate the reasons given by the authors of the reviewed papers for the trends resulting from their studies to better understand the strain rate effects on the fracture toughness of fiber reinforced polymer composite materials. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 899–904, 2005     

Microstructure of high modulus solid state extruded polyethylene: 2. X-ray scattering studies of 12, 24 and 36 extrusion draw ratio

The microstructure of a series of solid state extruded polyethylene fibres was examined by wide- and small-angle X-ray scattering. By measuring absolute intensities using a two dimensional position sensitive detector, accurate values of the small-angle invariant for anisotropic samples were obtained. This measurement coupled with wide-angle X-ray scattering and d.s.c. permitted determination of the density of the noncrystalline component. The use of a two phase model for solid state extruded polyethylene is justified if consideration is given to the effective densities of the crystalline and noncrystalline phases, which change with deformation. The effective density of the crystalline phase decreases by 1% (0.999–0.990 g cm^?3) from the unextruded billet compared to a 36 draw ratio extrudate, while the noncrystalline phase density increases by 6% (0.84–0.89 g cm^?3). These changes lead to an overall decrease in the mean squared electron density fluctuation of 63%. The average axial crystallite length measured by wide-angle X-ray scattering increases with extrusion draw ratio while the SAXS long period decreases and weakens considerably. These observations and the electron microscopy results from the previous paper, (Polymer, 1982, 23, 1069), are fully consistent with the key features of the Peterlin model of fibre microstructure.     

Small-angle X-ray scattering investigations of fractal dimensions of hybrid inorganic organic network (ceramers) based on tetraethylorthosilicate and polytetramethylene oxide

Over the last 2 years the sol gel technique has become very popular because of its ability to yield multicomponent inorganic glasses at low temperatures. More recently techniques to incorporate functionlized organic species into a sol gel network of metal alkoxides have been developed and studied in our laboratory as well as others. Most of these studies have focussed on the structure-property behavior of these materials, with an emphasis on understanding the properties in terms of the level of microphase separation that occurs during the network buildup in these systems. While much has been gained in this respect, the desire exists to characterize better the structure of the inorganic phase in such systems, and in this respect, utilization of small-angle X-ray scattering profiles can provide information about this structure through fractal analysis. The work discussed here focuses specifically on the fractal character of specific ceramer materials produced as a result of the reaction between tetraethylorthosilicate and end-functionalized polytetramethylene oxide. It is shown that the cosolvent system utilized to compatibilize the reactants plays a very important role in development of the structure of the inorganic phase in these hybrid materials.     

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.     

Stress transmission in silica particulate epoxy composite by X-ray diffraction

Stress on particles in silica particulate epoxy composite under constant load was investigated by X-ray diffraction. Microdeformation of the crystal lattice of silica could be detected as a shift of the X-ray diffraction peak induced by the applied stress. When a tensile stress was applied to the particulate composite, incorporated particles were found to be subjected to a stress several times larger than the applied stress. The stress concentration onto particles in particulate composite material, which was considered to result in a mechanical reinforcement of the composite, depends on the volume fraction and size of particles. Quantitative relationships between the stress concentration coefficient, particle diameters and the increment of macroscopic Young's modulus with the incorporation of filler were stated.     

Oxidation behaviour of nano-sized SiC particulate reinforced Alon composites

Silicon carbide (SiC)-aluminium oxynitride (Alon) ceramic composites exhibited improved mechanical properties, but the high temperature oxidation behaviour was unknown. The aim of this investigation was to identify oxidation characteristics and kinetics of 8 wt% SiC-Alon composites over a temperature range between 700 °C and 1200 °C in air. The Alon matrix and SiC particles near the surface were oxidized to form Al₂O₃ and SiO₂, respectively. The starting oxidation temperature of Alon was observed to be about 1000 °C. While the addition of nano-sized SiC particles resulted in a reduced starting oxidation temperature due to the large cumulative surface area and high total surface energy, the oxidation resistance at higher temperatures of 1100 °C and 1200 °C was remarkably enhanced. The oxidation kinetics changed from a linear weight gain for pure Alon into a logarithmic weight gain for the composites due to the formation of a dense protective oxidation layer arising from the presence of SiO₂.     

Sustainable conducting polymer composites: study of mechanical and tribological properties of natural fiber reinforced PVA composites with carbon nanofillers

ABSTRACT

Ball milled jute fiber (JF) was added to Polyvinyl Alcohol (PVA)/20 wt.% multi-layer graphene (MLG) composites in various proportions (0, 5, 10, 15 and 20 wt.%) to prepare sustainable and biodegradable conducting polymer composites. Also, PVA/17.5wt.%MLG/2.5wt.%MWCNT/20wt.% JF composite was prepared for comparison purpose. A dynamic mechanical analysis of the composites was conducted to analyze their viscoelastic nature. The electrical conductivity of the composites was measured to study their suitability for various applications. Jute reinforcement increased the electrical conductivity of PVA/MLG nanocomposites. The PVA/20wt.%JF/17.5wt.%MLG/2.5wt.%MWCNT hybrid composite had the highest electrical conductivity of 3.64 × 10^?4 S/cm among all the composites prepared. Multilayered structures of the hybrid composite films were made by hot-pressing, and their effectiveness in electromagnetic interference shielding was tested. The shielding effectiveness of the composites decreased with jute addition. The wear resistance of PVA/MLG/JF composites increased with an increase in the jute content up to an optimum value of 10 wt.%, and then it started deteriorating.     

Mesoscale Characterization of Supramolecular Transient Networks Using SAXS and Rheology

Hydrogels and, in particular, supramolecular hydrogels show promising properties for application in regenerative medicine because of their ability to adapt to the natural environment these materials are brought into. However, only few studies focus on the structure-property relationships in supramolecular hydrogels. Here, we study in detail both the structure and the mechanical properties of such a network, composed of poly(ethylene glycol), end-functionalized with ureido-pyrimidinone fourfold hydrogen bonding units. This network is responsive to triggers such as concentration, temperature and pH. To obtain more insight into the sol-gel transition of the system, both rheology and small-angle X-ray scattering (SAXS) are used. We show that the sol-gel transitions based on these three triggers, as measured by rheology, coincide with the appearance of a structural feature in SAXS. We attribute this feature to the presence of hydrophobic domains where cross-links are formed. These results provide more insight into the mechanism of network formation in these materials, which can be exploited for tailoring their behavior for biomedical applications, where one of the triggers discussed might be used.     

Nanostructure evolution of oriented high-pressure injection-molded poly(ethylene) during heating

High-pressure injection-molded polyethylene (PE) rods are studied by ultra small-angle X-ray scattering from synchrotron during the heating of the polymer. Injection of a cool melt into a cold mold yields highly oriented PE rods with a core-shell structure. Samples from both the core and the shell material are studied. The two-dimensional scattering patterns are evaluated utilizing the multi-dimensional chord distribution function (CDF) analysis. From the obvious evolution of the nanostructure during successive crystallite melting, the sequence of processes occurring during crystallization is elucidated. First, nuclei form one-dimensional lattices with short-range order along the fiber axis. From this row structure, lamellae grow with wide lateral extension. An indication of an intermediate block structure is observed. Finally two steps of insertion crystallization result in two long period halvings. Increase of the mold pressure increases the lateral extension of the inserted lamellae in the shell material. In the core material a uniform row structure is absent. Extended primary lamellae form stacks with decreasing long periods before insertion crystallization takes over. But crystallites inserted in the core material do not form extended lamellae. Each of these steps leaves its footprint in the nanostructure and the corresponding scattering pattern. After CDF interpretation of the heating series, the room temperature pattern can be explained. The strong two-point pattern is associated with the primary lamellae and the intensity ridge extending along the meridian results from irregular insertion of lamellae. When the row structure is observed in the CDF, the fiber pattern exhibits equatorial scattering.Domain roughness generates a strong background scattering, which cannot be separated in one step. For the presented material it is shown that iterative background subtraction eliminates the scattering effects of the imperfect (i.e. inserted) lamellae.     

Residual stresses in alumina matrix composites reinforced with Ti(C,N)

Ti(C_0.5,N_0.5)-reinforced alumina matrix composites with an addition of 2 wt% ZrO₂ were tested to determine residual stresses of Al₂O₃ and Ti(C_0.5,N_0.5) phases. The advanced sintering technique (spark plasma sintering ―SPS) at various temperatures of 1600°C and 1700°C was used. Vickers hardness HV1, Young’s modulus E, apparent density ρ and indentation fracture toughness K_IC(HV) were evaluated. An indirectly residual stress measurement by the XRD method using the sin² ψ technique was applied. Compressive residual stresses in both phases: α-Al₂O₃ and Ti(C_0.5,N_0.5) were observed. Residual stresses of α-Al₂O₃₍₂₂₆₎ are in the range between ?204 ± 20 MPa and ?120 ± 20 MPa (for 1600 °C and 1700 °C respectively) are lower compared to Ti(C_0.5,N_0.5)_(420), for which the stresses are in the range of between ?292 ± 20 MPa and ?256 ± 20 MPa (for 1600 °C and 1700 °C respectively). The results exhibit the influence of the sintering temperature on the residual stresses of the tested phases. The residual stresses revealed at 1700°C are lower by about 40% for α-Al₂O₃₍₂₂₆₎ and much less for Ti(C_0.5,N_0.5)₍₄₂₀₎, by only about 15%. Microstructure studies using scanning electron microscopy, X-ray and electron diffraction phase analysis were used.     

Time-resolved X-ray diffraction study of SHS-produced NiAl and NiAl-ZrO<Subscript>2</Subscript> composites

NiAl and NiAl-ZrO₂ intermetallic composites were prepared from pressed elemental powders by SHS in a dynamic vacuum. The effect of diluent properties (such as conductivity and grain size) on the parameters of the SHS reaction has been investigated by time-resolved X-ray diffraction (TRXRD). By decreasing the reactivity of the green mixture, these nonreactive additives not only slow down the velocity of combustion front propagation but can also give rise to local melting of the product. Synchrotron radiation was also used to elucidate the reaction mechanism. It has been found that the combustion synthesis of NiAl is triggered by aluminum melting and proceeds via the formation of two transient intermetallic phases, NiAl₃ and Ni₂Al₃. In the final product, only the NiAl phase has been detected.