Synchrotron Radiation Wide-angle X-ray Diffraction Research Articles

The Role of Lanthanum Stearate on Strain-Induced Crystallization and the Mechanical Properties of Whole Field Latex Rubber.

Natural rubber (NR) is extensively utilized in numerous industries, such as aerospace, military, and transportation, because of its exceptional elasticity and all-around mechanical qualities. However, commercial NR made using various techniques typically has distinct mechanical characteristics. For instance, whole field latex rubber (SCR-WF) cured with accelerator 2-Mercaptobenzothiazole exhibits poor mechanical properties. This work attempts to enhance the mechanical property of SCR-WF via the addition of lanthanum stearate (LaSt). The influence of LaSt on strain-induced crystallization (SIC) and the mechanical properties of SCR-WF were investigated. The results of crosslinking density measured by the equilibrium swelling method demonstrate that the presence of LaSt significantly increases the crosslinking density of SCR-WF with lower loading of LaSt. The results of the mechanical properties show that the introduction of LaSt can enhance the tensile strength and fracture toughness of SCR-WF. To reveal the mechanism of LaSt improving the mechanical properties of SCR-WF, synchrotron radiation wide-angle X-ray diffraction (WAXD) experiments were used to investigate the SIC behaviors of SCR-WF. We found that the LaSt leads to higher crystallinity of SIC for the strain higher than 3.5. The tube model indicates the contribution of LaSt in both crosslinking and topological constraints. This work may provide an instruction for developing SCR-WF with superior mechanical properties.

Temperature-Dependent Polymorphism and Phase Transformation of Friction Transferred PLLA Thin Films.

Poly(L-lactic acid) (PLLA) thin films with a highly oriented structure, successfully prepared by a fast friction transfer technique, were investigated mainly on the basis of synchrotron radiation wide-angle X-ray diffraction (WAXD) and Fourier transform infrared spectroscopy (FTIR). The crystalline structure of the highly oriented PLLA film was remarkably affected by friction transfer temperatures, which exhibited various crystal forms in different friction temperature regions. Interestingly, metastable β-form was generated at all friction transfer temperatures (70-140 °C) between Tg and Tm, indicating that fast friction transfer rate was propitious to the formation of β-form. Furthermore, the relative content among β-, α'-, and α-forms at different friction temperatures was estimated by WAXD as well as FTIR spectroscopy. In situ temperature-dependent WAXD was applied to reveal the complicated phase transition behavior of PLLA at a friction transfer temperature of 100 °C. The results illustrated that the contents of β- and α'-forms decreased in turn, whereas the α-form increased in content due to partially melt-recrystallization or crystal perfection. Moreover, by immersing into a solvent of acetone, β-, α'-form were transformed into stable α-crystalline form directly as a consequence. The highly oriented structure was maintained with the chain perfectly parallel to friction transfer direction after acetone treatment, evidenced by polarized FTIR and polarized optical microscopy (POM) measurements.

Stretch-Induced Reverse Brill Transition in Polyamide 46

Structural transitions of polyamide 46 (PA46) films during the tensile stretch at temperatures from 37 to 249 °C were studied using in situ synchrotron radiation wide-angle X-ray diffraction (SR-WAXD) and Fourier transform infrared (FTIR) techniques. The structural evolution during the stretch at different temperatures can be roughly divided into three regions. In region I (37 ≤ T < 180 °C), stretch enlarges the d-spacing gap between the (100) and (010/110) planes of α phase (triclinic), which is opposite to that during heating. In region II (180 ≤ T < 230 °C), stretch induces a reverse Brill transition, which is characterized by the single diffraction of γ phase (pseudohexagonal) splitting into two diffractions of α phase, while further increasing the strain drives the two split diffractions to transform to a single broad diffraction. As in situ FTIR measurements reveal the continuously increasing content of trans conformation, we speculate that stretch may induce a new form, which is an intermediate structure close to both α and γ phases (named α′ phase) at large strain. In region III (230 ≤ T ≤ 249 °C), the γ phase may skip the α phase and directly transform into the α′ phase. The nonequilibrium crystal phase diagrams of PA46 were constructed in the strain–temperature space and true stress–temperature space. Current results demonstrate that stretch plays a countereffect of heating on the Brill transition of PA46.

Frustrating Strain-Induced Crystallization of Natural Rubber with Biaxial Stretch.

The supreme mechanical performance of natural rubber (NR) is commonly attributed to strain-induced crystallization (SIC). The SIC of NR during uniaxial stretch has been extensively investigated, whereas that under multiaxial deformation has been rarely reported, which is close to real service conditions (i.e., tire). In this work, the crystallization behavior of NR under biaxial stretch was studied with in situ synchrotron radiation wide-angle X-ray diffraction in combination with a custom-built biaxial stretch machine. It is observed that biaxial stretch frustrates the SIC of NR: within λx/λy < 1.6, where λx and λy are stretch ratios of two mutually perpendicular axes, no crystallization emerges even under large drawing ratio until sample fracture at ambient temperature. This finding challenges the common wisdom of the self-reinforcement mechanism of SIC in NR under multiaxial deformation in real service conditions. A theoretical SIC model is proposed, which can decouple the contributions of conformational entropy reduction ΔSf and amorphous chain orientation f to final Gibbs free energy change (ΔG) during multiaxial deformation. This model quantitatively renders a reproduction of the crystallinity during the biaxial stretch, which is well consistent with experimental results and can be further generalized for flow-induced crystallization of semicrystalline polymers.

The Effect of Epoxidation on Strain-Induced Crystallization of Epoxidized Natural Rubber.

The effect of epoxidation on strain-induced crystallization (SIC) of epoxidized natural rubber (ENR) and mechanism are studied with synchrotron radiation wide-angle X-ray diffraction (SR-WAXD) and polarized infrared spectroscopy (P-IR). WAXD results reveal that appropriate epoxidation, for example, ENR-25 epoxidized with ≈25% isoprene units, can unexpectedly enhance the SIC of natural rubber (NR), resulting in the improvement of tear resistance. On the other hand, exorbitant epoxidation, for example, ENR-40 epoxidized with ≈40% isoprene units, depresses the SIC and weakens the mechanical properties of NR remarkably. P-IR studies reveal that epoxidation can promote the orientation of chain segments along the stretching direction, which plays a determining role on SIC of NR. Accordingly, hierarchical multiscale schematic models are proposed. This insight into epoxidation on SIC of ENR strongly suggests that ENR with appropriate epoxidation degree is a promising candidate material for the fabrication of high-performance engineering rubber products.

In situ characterization of strain-induced crystallization of natural rubber by synchrotron radiation wide-angle X-ray diffraction: construction of a crystal network at low temperatures.

Strain-induced crystallization (SIC) of natural rubber (NR) at descending temperatures as low as -60 °C is systematically investigated by in situ synchrotron radiation wide-angle X-ray diffraction (SR-WAXD) measurement. The detailed structural evolution of NR during SIC is studied in the strain-temperature space, where up to four regions are defined depending on the SR-WAXD results. In region I, the molecular chains begin to be oriented under tensile loading. The onset of crystallization happens in the very beginning of region II, and the NR crystal acts as a new physical cross-linking point to form a crystal network, namely the series model. The further increment of crystallinity (> ca. 8%) leads to the transition of the crystal network from the series model to the parallel model in region III. The crystal network is finally accomplished in region IV, where the crystallinity remains almost constant. Interestingly, regions III and IV exist only in the intermediate-temperature zone II (-40 °C to -10 °C), which are missing in zones I (-10 °C to 25 °C) and III (-60 °C to -40 °C). This suggests that sufficient crystallinity (χII-III > ca. 8%) is required to form the parallel model. The new crystal network provides a deep understanding of SIC of NR considering the microscopic features, i.e. oriented amorphous component, the onset of crystallization and crystallinity evolution and its correlation with the macroscopic stress-strain curve.

Understanding structure-mechanics relationship of high density polyethylene based on stress induced lattice distortion

The relationship between the macroscopic non-linear mechanics and the microscopic crystal structural evolution of pre-oriented high-density polyethylene (HDPE) is investigated by in situ synchrotron radiation wide-angle X-ray diffraction (WAXD) measurement over a wide temperature range from −10 to 130 °C. With the concept of stress-induced disordering of crystal, the ratio (φa/b) of lattice parameters a to b is defined as a new structural variable, which can reflect the lattice distortion and then the microscopic stress state of orthorhombic crystal (O-crystal). According to the temperature-dependent non-linear variation of φa/b with strain, the contributions of O-crystal and monoclinic crystal (M-crystal) to the macroscopic mechanics including linear elasticity, yielding, stress softening and strain hardening are clarified. It is found that M-crystal bears the main extensional stress once formed, although it survives within a limited strain window relying on temperature. By further combining the extensional phase diagram constructed in strain-temperature space, the HDPE deformation is recognized to undergo successively one-dimensional (1D) chain segments rotation of crystal, two-dimensional (2D) crystal plan shearing or slipping and three-dimensional (3D) recrystallization along with increasing strain or stress, demonstrating a multiscale structural transition and energy dissipation mechanism.

In situ synchrotron radiation X-ray diffraction studies on molecular aggregation structure of nylon 12 films during bulge testing.

It is desirable to establish a method for evaluating mechanical properties, such as modulus and strength, of micrometer and sub-micrometer thick polymer films. Bulge tests, where bulge deformation is imposed on films by the pressure of an inert gas, are suitable for satisfying this demand. However, very few studies on polymer films exist in the literature. In this study, bulge testing equipment for in situ synchrotron radiation wide-angle X-ray diffraction (WAXD) measurements is designed and used to study the relationship between the molecular aggregation structure and the mechanical properties of a crystalline nylon 12 (Ny12) film during bulge testing. Isothermally crystallized and quenched Ny12 films exhibited stress-strain curves similar to those obtained by conventional uniaxial elongation. In situ WAXD measurements during bulge testing revealed that the lattice extension of the crystallites is clearly dependent on crystallinity. Concretely, crystallites in the isothermally crystallized film show higher elastic properties than those in the quenched one. The results of the molecular aggregation structure, including the crystal structure and the amorphous chain surrounding the crystallites, of the films during bulge deformation firstly obtained in this study must be useful for designing toughened polymer films.

Microfluidic Dry-spinning and Characterization of Regenerated Silk Fibroin Fibers.

The protocol demonstrates a method for mimicking the spinning process of silkworm. In the native spinning process, the contracting spinning duct enables the silk proteins to be compact and ordered by shearing and elongation forces. Here, a biomimetic microfluidic channel was designed to mimic the specific geometry of the spinning duct of the silkworm. Regenerated silk fibroin (RSF) spinning doped with high concentration, was extruded through the microchannel to dry-spin fibers at ambient temperature and pressure. In the post-treated process, the as-spun fibers were drawn and stored in ethanol aqueous solution. Synchrotron radiation wide-angle X-ray diffraction (SR-WAXD) technology was used to investigate the microstructure of single RSF fibers, which were fixed to a sample holder with the RSF fiber axis normal to the microbeam of the X-ray. The crystallinity, crystallite size, and crystalline orientation of the fiber were calculated from the WAXD data. The diffraction arcs near the equator of the two-dimensional WAXD pattern indicate that the post-treated RSF fiber has a high orientation degree.

Effect of Draw Ratio on the Microstructure of Silk Fibroin/Graphene Oxide Hybrid Fibers

In order to fabricate high performance artificial silk, hybrid fibers were dry spun from regenerated silk fibroin (RSF) aqueous solution mixed with graphene oxide (GO) nanosheets. The influence of draw ratio on the microstructure and mechanical properties of RSF/GO hybrid fibers was studied through Fourier transform infrared spectroscopy (FT-IR), synchrotron radiation wide-angle X-ray diffraction, Raman spectroscopy and mechanical testing. The results showed that with increasing draw ratio, both amorphous phase and mesophase of the dry-spun artificial fiber gradually converted to crystals. The orientation of crystals and mesophase changed slightly at low draw ratio, but increased significantly at draw ratios of 3 and 4X. The β-sheet content of silk fibroin and the disorder degree of GO sheets in the fibers also increased with the increase of draw ratio. The initial modulus, breaking strength and breaking energy of the hybrid fibers achieved great improvement at a draw ratio of 4X. The structure evolution of the RSF/GO hybrid fibers may benefit the understanding of the structure-property relationship of other composite fibers.

Observations of Enamel Microstructure in Incipient Lesions Remineralized by NaF Dentifrices

Synchrotron Radiation (SR) Wide-angle X-ray Diffraction (WAXD) and Small-angle X-ray Scattering (SAXS) techniques were used to assess microstructure of bovine enamel white-spot lesions (WSL) evaluated in a 10-day pH cycling model comprising three different dentifrice groups: (A) 0.21% NaF plus TCP (Clinpro Tooth Crème), (B) 1.1% NaF plus TCP (Clinpro 5000), or (C) 0% NaF (Tom’s of Maine) dentifrice. Each day consisted of four 2-minute treatments, one 4-hour acid challenge (pH=5.0), and immersion in artificial saliva (pH=7.0) between these events. These specimens were also examined with cross-sectional microhardness, digital light microscopy and FE-SEM (field emission scanning electron microscope), and demonstrated the remineralization model effected changes in subsurface microstructure. X-ray diffraction data from WAXD and SAXS were collected on enamel slab cross-sections extending from 0 μm to 150 μm, in 6 μm microbeam increments. A primary outcome of this observational study was that simultaneous WAXD and SAXS measurements were able to resolve significant differences (ANOVA, Student’s t-test, p&lt;0.05) between the effects of the two fluoridecontaining dentifrices on subsurface lesion microstructure. In particular, enamel lesions treated with 1.1% NaF dentifrice group manifested an abundance of nanometer-sized crystallites, while treatment with the 0.21% NaF dentifrice produced larger apatite-like crystals. While the presence of fluoride in both cases promoted regularity in crystal size and orientation, this was not observed for lesions treated without fluoride. Altogether, our observations demonstrate the pathological processes for remineralization are markedly influenced by the presence and concentration of fluoride, the microstructural characteristics of which can be distinguished using the simultaneous WAXD and SAXS technique.

Higher-order-structure Formation in Liquid Crystal Epoxy Thermosets Investigated by Synchrotron Radiation–Wide-angle X-ray Diffraction

We report the investigation of the mesophase transformations of a liquid crystalline molecule with terminal epoxy groups from the initial stages of curing with a diamine compound. The ordered arrangement of molecules within the smectic layers in the thermoset formed at the end of the curing process was characterized by synchrotron radiation–wide-angle X-ray diffraction (SR-WAXD). Data from this experiment helps us understand the phase transitions from the nematic to smectic phases of curing liquid crystalline epoxies.

Hybrid Silk Fibers Dry-Spun from Regenerated Silk Fibroin/Graphene Oxide Aqueous Solutions.

Regenerated silk fibroin (RSF)/graphene oxide (GO) hybrid silk fibers were dry-spun from a mixed dope of GO suspension and RSF aqueous solution. It was observed that the presence of GO greatly affect the viscosity of RSF solution. The RSF/GO hybrid fibers showed from FTIR result lower β-sheet content compared to that of pure RSF fibers. The result of synchrotron radiation wide-angle X-ray diffraction showed that the addition of GO confined the crystallization of silk fibroin (SF) leading to the decrease of crystallinity, smaller crystallite size, and new formation of interphase zones in the artificial silks. Synchrotron radiation small-angle X-ray scattering also proved that GO sheets in the hybrid silks and blended solutions were coated with a certain thickness of interphase zones due to the complex interaction between the two components. A low addition of GO, together with the mesophase zones formed between GO and RSF, enhanced the mechanical properties of hybrid fibers. The highest breaking stress of the hybrid fibers reached 435.5 ± 71.6 MPa, 23% improvement in comparison to that of degummed silk and 72% larger than that of pure RSF silk fiber. The hybrid RSF/GO materials with good biocompatibility and enhanced mechanical properties may have potential applications in tissue engineering, bioelectronic devices, or energy storage.

Reinforced and Ultraviolet Resistant Silks from Silkworms Fed with Titanium Dioxide Nanoparticles

As the perfect combination of strength and luster, silkworm silks have been widely used in many fields but still need improvements. This paper demonstrates an in vivo uptake of titanium dioxide (TiO2) nanoparticles by silkworms, leading to the direct production of intrinsically modified silk. The nanoparticles can be easily incorporated into the silk gland of silkworm by using this method due to the interactions between TiO2 and silk fibroin molecules. Infrared spectra indicate that TiO2 nanoparticles confine the conformation transition of silk fibroin from random coil/α-helix to β-sheet. Results of synchrotron radiation wide-angle X-ray diffraction and small-angle X-ray scattering suggest that modified silks have lower crystallinity, higher mesophase content, and higher Herman’s orientation functions of crystalline region and mesophase region than control group. The breaking strength and elongation at break of the modified silk can be improved up to 548 ± 33 MPa and 16.7 ± 0.8%, respectively, by adding 1...

Strain-induced crystallization of polyimide fibers containing 2-(4-aminophenyl)-5-aminobenzimidazole moiety

Structural evolution induced by cyclization reaction and heat-drawing of polyimide (PI) fibers containing the 2-(4-aminophenyl)-5-aminobenzimidazole moiety was studied by means of synchrotron radiation wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). WAXD results indicate that the drawing process induces an increase in the crystallinity and crystal orientation of PI fibers, leading to improvement of the mechanical properties. SAXS results reveal that lamellar structures formed upon heat-drawing. A mechanism for strain-induced phase transitions has been proposed to correlate the structural information with WAXD/SAXS patterns. Dynamic mechanical analysis (DMA) of the obtained fibers shows an obvious α relaxation process above room temperature, which is found to be crystallinity dependent, the intensity of tan δ decreases significantly after heat-drawing. The stretched fiber exhibits a negative value of the linear coefficient of thermal expansion (CTE), while the unstretched PI fiber exhibits a positive value. Moreover, heat-drawing increases the thermal stability of the prepared fibers. These improvements may be related to the aggregation structure of the high performance fibers.

Chemical structure and interlayer distance correlation of graphite oxide in the heating process as revealed by in situ Fourier transform infrared spectroscopy and wide-angle X-ray diffraction techniques.

The thermal reduction behavior of graphite oxide (GO) film in an air atmosphere during a continuous heating process was monitored in situ using temperature-dependent infrared (IR) spectroscopy and synchrotron radiation wide-angle X-ray diffraction (WAXD) techniques. The results show that most of the water adsorbed by the GO sheets is removed by heating them to 130 °C. The dehydration process leads to a slight decrease of the interplane distance of the GO sheets. The IR data suggest that the thermal reduction occurs starting at 160 °C. The synchronous change of the of hydroxyl and carbonyl stretching mode (ν(-OH) and ν(C=C)) bands of GO between 160 and 210 °C suggest that the recovery of conjugated structure is mainly due to the reduction of -OH groups in this temperature region, in which the d spacing has not been affected. When the temperature reaches 210 °C, the rapid reduction of C=O groups together with the removal of the residual -OH and ether (C-O-C) groups leads to the sudden collapse of the GO sheets. Based on these findings, we present a schematic of the thermal stability of GO film in a continuous heating process, in which the thermal-induced chemical and crystallographic structural changes of the GO film have been correlated.

The changes of crystalline structure of cellulose during dissolution in 1-butyl-3-methylimidazolium chloride

The morphology and crystalline structure changes of cellulose during dissolution in 1-butyl-3-methylimidazolium chloride [(BMIM)Cl] were investigated by optical microscopy and synchrotron radiation wide-angle X-ray diffraction (WAXD). Neither swelling nor dissolution of cellulose was observed under the melting point of [BMIM]Cl. While the temperature was elevated to 70 °C, the swelling phenomenon of cellulose happened with the interplanar spacing of (\( 1\bar{1}0 \)) and (020) planes increased slightly. With the temperature further going up to 80 °C, cellulose was dissolved gradually with the crystallinity (Wc,x) and crystalline index (CrI) of cellulose decreased rapidly, which indicated the crystalline structure of cellulose was destroyed completely and transformed into amorphous structure.

Polarized Light Scattering and Synchrotron Radiation Wide-Angle X-ray Diffraction Studies on Smectic Liquid Crystal Formation of Main-Chain Polyester

Polarized Light Scattering and Synchrotron Radiation Wide-Angle X-ray Diffraction Studies on Smectic Liquid Crystal Formation of Main-Chain Polyester