Synchrotron Wide-angle X-ray Diffraction Research Articles

Structural evolution of stretch deformed HDPE/RGO nanocomposites: An in-situ synchrotron SAXS and WAXD study

We have reported that the incorporation of reduced graphene oxide (RGO) can induce epitaxial crystallization of high density polyethylene (HDPE) on RGO surfaces and then enhance mechanical properties of HDPE/RGO nanocomposites, but the underline intensifying mechanism remains unclear. Herein, to further unveil the influence of epitaxial crystallization imposing on the improvement of mechanical properties, the structural evolution of HDPE/RGO nanocomposites during the stretching process is explored as a function of RGO content by in-situ synchrotron wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS) techniques. It has shown that the introducing of RGO apparently retards the structural evolution in the tensile deformation and the hysteresis is accentuated dramatically with increasing RGO content. The epitaxial growth of HDPE upon RGO surfaces forms large sized crystals and the amount of epitaxial crystals enlarges with RGO content. The coupling of the remained existence of large sized crystals and the smaller density after stretching demonstrates that the interplay between RGO and large amount of epitaxial crystals can undergo the strong stress and thus hampers and delays the structure change and crystal fragmentation during the tensile deformation of HDPE/RGO nanocomposites. The present results not only makes clear that it is the epitaxial crystallization stemming from interfacial interaction between HDPE and RGO that plays the major role to delay the structural evolution and damage and thus lead to the enhancement of mechanical properties of HDPE/RGO nanocomposites, but reveal the underline stress-induced fragmentation-recrystallization mechanism for this tensile deformation.

Interfacial Stereocomplexation To Strengthen Fused Deposition Modeled Poly(lactide) Welds

The interlayer stiffness of fused deposition modeled (FDM) parts is significantly impaired by the slow diffusion and re-entangling of polymer chains across filament interfacial welds in the build direction. To mechanically reinforce FDM interfaces, an approach based on timing and spatial direction of stereocomplexation across weld interfaces is introduced via alternating deposition of enantiomerically opposite poly(lactides). Fundamental insight into the kinetics and spatial distribution of homo- and stereocrystallization at FDM interfaces was successfully reconstructed in 2 and 3 dimensions by micrometer resolved Fourier transform infrared microscopy and synchrotron wide-angle X-ray diffraction tomography. The rate of isothermal stereocomplexation and consequential interfacial stiffening increases with decreased absolute and relative molar masses. The spatial distribution of stereocrystals is governed by the relative molar masses and the extent of interdiffusion, under not only isothermal but also nonisothermal FDM conditions. The net local heat dosage, which depends on print speed, governs the length scales of stereocomplexation and thus mechanical reinforcement. Interfacial stereocomplexation of poly(lactides) in FDM leads to a distinct 40% increase in stiffness and nucleation of bulk filaments, aiding in thermodynamic and geometrical stability.

Quantification of isothermal crystallization of polyamide 12: Modelling of crystallization kinetics and phase composition

The crystallization of Polyamide 12 (PA12) has been investigated using a new experimental setup which allows in-situ synchrotron Wide Angle X-ray Diffraction (WAXD) during Flash-DSC measurements. The experimental results are used to parameterize and validate a new numerical model to quantify the quiescent crystallization kinetics, under isothermal conditions, of the three important crystal structures of PA12, i.e. the, γ−, α′− and mesomorphic phase. The experimental approach is based on nucleation and growth after quenching the material from the melt to an isothermal temperature, described by the Schneider rate equations and the Kolmogorov-Avrami expression for the space filling. The experimental overall crystallization rate, expressed in terms of the crystallization half-time, as well as the phase composition, are well captured by the model over a wide range of temperatures, i.e. between the glass transition and the melting temperature. It is shown that at temperature below and above 100°C different nucleation mechanisms are dominant causing the bimodal dependence of the crystallization rate. This work forms the basis for a full model for non-isothermal conditions for which transitions between different phase have to be taken into account.

Research on architecture and composition of natural network in natural rubber

Though the superior properties of natural rubber (NR) have been attributed to its special network architecture, the currently accepted model “naturally occurring network” is far from describing its authentic network structure. In this paper, we focused on the composition of the chain entanglements in the network structure of unvulcanized NR. By using synchrotron wide-angle X-ray diffraction (WAXD), the evolution of its strain-induced crystallization (SIC) behaviors was real-time traced, and the stress-strain behaviors at various strain rates and temperatures were also tested. The results demonstrated that the entanglements can act as crosslinking points to increase the network chain density and lead to the easier SIC behavior. By applying the tube model to analyze the stress-strain curves of unvulcanized NR, we found that the contribution of the entanglement network to the stress is about an order of magnitude larger than that of the network formed by two terminal groups. Via double-quantum nuclear magnetic resonance (1H-DQ NMR), we further detected two types of entanglement in NR and deproteinized NR (DPNR), i.e. transiently trapped entanglements (TTEs) and permanently trapped entanglements (PTEs), among which TTEs are the main composition of the entanglement network. Moreover, the network formed by two terminals causes more PTEs in NR, while due to lacking the constraint of proteins, more TTEs exist in DPNR. Based on the research, we proposed a novel network model for unvulcanized NR.

Concomitant Crystallization in Propylene/Ethylene Random Copolymer with Strong Flow at Elevated Temperatures

Flow-induced crystallization of α- and γ-phases was studied for a propylene/ethylene random copolymer with 3.4 mol % ethylene at two high temperatures of 132 and 142 °C by combining a pressure-driven slit flow device with real-time synchrotron wide-angle X-ray diffraction. At 132 °C, it was found that both α- and γ-phases were generated at shear stresses ranging from 0.091 to 0.110 MPa and that the γ-phase always appeared later than the α-phase. However, for 142 °C and the same stresses, only the α-phase formed. Only upon cooling the partially crystallized copolymer did the γ-phase emerge. The lack of γ-crystals obtained at 142 °C is opposite to the behavior reported for quiescent crystallization under pressure, for which increasing temperature results in more and even pure γ-crystals. In the current study, the absence of γ-phase at 142 °C is tentatively associated with lack of epitaxial nucleation on α-lamellae and to relatively low growth rate of γ-crystals.

Lamellar Thickness Dependence of Crystal Modification Selection in the Syndiotactic Polystyrene γ-to-α/β Phase Transition Process

The syndiotactic polystyrene (sPS) γ-to-α/β phase transition was investigated using temperature-dependent synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) measurement. The sPS γ form samples, which were obtained from acetone solvent induced crystallization followed by annealing at various temperatures (Ta), exhibited a regular manner on the selection of the α or β crystal modification during the phase transition process. As the Ta was increased from 25 to 150 °C, the weight fraction of β form in bulk sPS decreased from 55% to 0%, but that of the α form increased from 45% to 100%. In other words, the α form became the dominating crystalline modification at higher Ta. The lamellar thickness (dc) of the γ form at the initial state (before heating) ranged from 4.4 to 5.8 nm, and the weight crystallinity (Xc) was between 0.52 and 0.64. However, the dc values of the γ form at the final state (before γ-to-α/β phase transition) were very close for the samples pretreated at d...

An unusual promotion of γ-crystals in metallocene-made isotactic polypropylene from orientational relaxation and favorable temperature window induced by shear

Combined effects of shear flow and non-isothermal processing, which were akin to the practical industrial processing, on the crystallization of metallocene-made isotactic polypropylene (m-iPP) were investigated by in-situ synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. The simultaneous appearance of α- and γ-crystals was observed in quiescent samples, while shear flow led to the occurrence of α-crystals prior to γ-crystals at the initial stage of crystallization. Although the local alignment of chain segments promoted by shear flow facilitated the α-crystals, the relaxation of oriented chains was experimentally confirmed to boost the formation of γ-crystals. Interestingly, the resultant γ-crystal content was higher in the sheared samples than the quiescent counterparts at a given cooling rate and further increased with the decrease of cooling rates. Such a discrepancy was not only related to shear-induced orientation and relaxation, but also was accredited to the advanced crystallization temperature window, which was favorable for the γ-crystal growth. It was quite different from the isothermal crystallization situation, where the formation of γ-crystals was depressed in presence of shear flow. The dynamic competition between α-crystal and γ-crystal formation under the effects of shear during the non-isothermal crystallization was discussed. Our work is of valuable reference to tailor the crystalline structure of m-iPP during practical processing and hence to acquire the desired performance of products.

Unexpected Strength and Toughness Reinforcement of the Injection-Molded Isotactic Polypropylene Parts with Oriented β-Crystals

In this work, three profiles of injection temperatures were adopted to tune the morphology of a β-nucleating agent and further affect the resulting β-crystal morphology of isotactic polypropylene (iPP) during an injection molding process. When the middle injection temperature profile was adopted (the maximum temperature was 220 °C), abundant oriented β-crystals with the c-axis perpendicular to the injection direction were presented throughout the whole injection-molded part, and their existence and distribution were confirmed by synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). These oriented β-crystals give rise to simultaneously enhanced tensile and impact strength of the β-nucleated iPP part. In a novel outcome, this work transformed the usually mechanically weak and soft component (i.e., the β-crystals) of iPP into one that added to its self-reinforcement and overall strength. This finding provides a new idea that a soft structure can be utilized to reinforce itself.

Strain-induced structural evolution during drawing of poly(ethylene terephthalate) fiber at different temperatures by in situ synchrotron SAXS and WAXD

In situ synchrotron small angle X-ray scattering (SAXS) and wide angle X-ray diffraction (WAXD) were carried out to study the lamellar and crystalline structural evolutions of poly(ethylene terephthalate) (PET) fiber during cold (20 °C) and hot (200 °C) drawing. The stress-strain curves were divided into two zones, i.e. linearly stress developing zone (zone I) and yield zone around fracture (zone II). In zone I during cold drawing, the two-dimensional (2-D) SAXS pattern transformed from a four-spot pattern to the coexistence of two- and four-spot patterns. The disappearance of lamellar peaks in zone II indicated the destruction of periodic lamellar structures, which was caused by the surface fragmentation of crystalline layers. In contrast, the 2-D SAXS patterns always existed during hot drawing, and the lamellar inclination decreased with strains. The lamellar and fibrillar structures were not destroyed, and crystallization occurred in the interlamellar amorphous layers. Fragmentation of crystalline surface mostly occurred at the end of zone II near fracture process. WAXD data suggested that the crystallinity and crystallite size perpendicular to (010) plane decreased, while the crystallite size perpendicular to (100) plane was rather stable during both cold and hot drawing.

WAXD/SAXS study and 2D fitting (SAXS) of the microstructural evolution of PAN-based carbon fibers during the pre-oxidation and carbonization process

Microstructural evolution in polyacrylonitrile (PAN) fibers at different temperatures during pre-oxidation and carbonization under stretching was studied by synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). Microvoids were characterized by the classical SAXS method, and were compared with simulation results obtained by fitting 2D SAXS patterns to a model based on a dilute system of cylindrical microvoids randomly distributed and preferentially orientated along the fiber axis and having a log-normal size distribution. The WAXD results showed that the crystal size, d-spacing and preferred orientation decreased during pre-oxidation, and increased during carbonization. A diffraction peak for PAN fibers at 2θ=13.6° disappeared during the final stage of pre-oxidation, meanwhile a new peak at 2θ=23.6° appeared, whose intensity increased during carbonization, indicating the formation of the graphite structure. The average length of the microvoids increased, and new microvoids were formed, which became oriented along the fiber axis as the fiber manufacturing process proceeded. The length of microvoids from simulation results is consistent with that from the classical method, indicating that the model is valid to describe the microvoid structure of fibers.

Structural properties of paeonol encapsulated liposomes at physiological temperature: Synchrotron small-angle and wide-angle X-ray diffraction studies

Structural properties of paeonol encapsulated liposomes at physiological temperature: Synchrotron small-angle and wide-angle X-ray diffraction studies

Low Temperature Reaction of Molecular Zinc Oxide Precursors in Ionic Liquids Leading to Ionogel Nanoparticles with Shape Anisotropy

The majority of particle synthesis methods are based on nucleation and growth processes in solvents. Whereas the role of capping agents has been investigated extensively regarding control particle size and shape, the unique role of the solvent is understood to a much lesser extent. Compared to other polar solvents, e.g. water, ionic liquids (ILs) are unique because their properties can be fine‐tuned precisely by appropriate choice and modification of cation and anion. This makes ILs also interesting for particle synthesis. We present the generation of zinc oxide (ZnO) in imidazolium ILs starting from molecular precursors. A hydrolytic, sol‐gel based synthesis route is suitable to achieve nanocrystalline ZnO. We find by in‐situ synchrotron wide angle X‐ray diffraction that in ILs an unusual ZnO phase with α‐boron nitride structure acts as an intermediate prior to crystallization of the thermodynamically stable Wurtzite. This special mechanism leads to organic‐inorganic hybrid IL/ZnO nanoparticles with plate‐like morphology. Because of the large content of IL embedded in the ZnO matrix the novel particles gain ionogel properties, e.g. ion conductivity probed by impedance spectroscopy.

Evolution of the structural orientation in polyacrylonitrile precursors during stabilization revealed by in-situ synchrotron wide-angle X-ray diffraction and polarized infrared spectroscopy

Evolution of the orientation structures of polyacrylonitrile (PAN) precursors during thermal stabilization was investigated on the basis of in situ temperature-dependent measurements including synchrotron wide-angle X-ray diffraction and polarized infrared spectroscopy. The results indicated that the Hermans orientation factor of PAN crystallites increased firstly and then decreased in the process of stabilization, while the orientation of molecular chains showed a two-stage decrease. These were mainly attributed to the thermal relaxation of molecular chains and the cyclization reactions, which also resulted in the physical shrinkage as well as the chemical shrinkage apparently observed from the thermal mechanical curves of the precursor.

Structure Evolution of Polyamide 1212 during the Uniaxial Stretching Process: In Situ Synchrotron Wide-Angle X-ray Diffraction and Small-Angle X-ray Scattering Analysis

Structure evolution of polyamide 1212 (PA1212) during the uniaxial stretching process at room temperature was investigated by in situ synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. Variation mechanism of polymorphic phase and long spacing dac was discussed in depth. Deformation induced crystal–crystal transition in PA1212 from triclinic α-form to mesomorphic phase was observed by WAXD analysis, and it could be ascribed to the lamellar slippage mechanism. From 2D-WAXD patterns, crystal transition in PA1212 took place earlier in meridian direction (parallel to stretching direction) than equatorial direction. It could be explained by the fact that the lamellae with the normal perpendicular to the stretching direction tended to experience lamellar slippage and fragmentation, where polymer chains in (010) lattice plane consisting of sheets of hydrogen bonds were easy to experience conformational transformation. This new crystal transition behavior from α-for...

Strain-induced crystallization of blends of natural rubber and ultra high cis polybutadiene as studied by synchrotron X-ray diffraction

Strain-induced crystallization of a series of natural rubber (NR) and ultra high cis polybutadiene (UBR) blend was studied by the in situ synchrotron wide angle X-ray diffraction. X-ray diffraction peaks from the oriented rubber were decomposed into the three phases (isotropic amorphous (IA), oriented amorphous (OA), and crystalline (CR)) to probe the structure changes upon stretching. Overall chain orientation of the blend upon stretching was speeded up, which was found to be associated with the UBR chains. The strain-induced crystallization was retarded by the UBR chains. It was found out that 100% UBR did not show crystallization and the crystallization in the blend was only associated with NR chains. Furthermore, the early oriented UBR chains do not play a role as nucleation sites for the NR chains to crystallize. The crystallite size became larger with the UBR content, which was attributed to the small number of nuclei formation by the presence of UBR chains.

Poly(alanine): Structure and Stability of the D and L-Enantiomers.

High-performance, biobased materials can potentially be manufactured from polymerized α-amino acids (α-polypeptides). This paper reports on the synthesis, structure, and properties of both polyalanine enantiomers (PLAla and PDAla). The molecular structure of the polypeptide chains, their molecular weight, and polydispersity were investigated by (1)H NMR, MALDI-TOF, and size-exclusion chromatography. The secondary structure and crystalline order were probed via Fourier transform infrared spectroscopy, circular dichroism, and (synchrotron) wide-angle X-ray diffraction. The phase behavior and thermal stability were assessed by differential scanning calorimetry and thermogravimetric analysis. The kinetically trapped PAla chain conformation in the solid state, after synthesis or solvent treatments, is the α-helical shape. Upon heating, crystals from the α-helices convert into more stable crystals from β-sheets at a temperature higher than 210 °C. This temperature is close to where polymer degradation sets in. The β-sheet crystals combine melting with thermal degradation at temperatures above 330 °C. In the presence of superheated water, the conversion from α-helices to β-sheets happens at lower temperatures, allowing for a conversion without degradation.

Kinetics of strain-induced crystallization in natural rubber: A diffusion-controlled rate law

The kinetics of strain-induced crystallization in natural rubber were studied by synchrotron wide-angle X-ray diffraction (WAXD) in a strain jump setup over a broad temperature range. The increase of crystallinity after a strain jump was observed on a millisecond time scale, until a steady state plateau was approached. A novel model, based on chain dynamics scaling laws for diffusion in polymer networks was developed to interpret the time and temperature dependence of the strain-induced crystallization. In contrast to established thermal crystallization models, this model for the strain-induced crystallization proposes that the formation of stretched crystallizable chain segments takes place before the attachment to a growing crystallite.

Direct evidence for the non-additive gelatinization in binary starch blends: A case study on potato starch mixed with rice or maize starches

Potato starch (PS) was blended with maize (MS), waxy maize (WMS), rice (RS) or waxy rice (WRS) starch in different ratios. Gelatinization of the blends was studied at intermediate water content (1:1 starch dry matter:water). In the pure state, PS gelatinizes at slightly lower temperature than the other components. The pure systems as well as the blends display bimodal gelatinization with so-called G (low temperature) and M1 (high temperature) endotherms in Differential Scanning Calorimetry (DSC). Gelatinization of the blend, however, cannot be understood as a proportional superposition of the gelatinization of the pure components. Time-resolved synchrotron wide angle X-ray diffraction (WAXD) was used to uncover the gelatinization of each starch in the blend separately, thereby exploiting the fact that PS contains B-type and the other species A-type crystals. Very clearly, PS gelatinizes more predominantly within the G, and the other component within the M1 endotherm in the blends than when gelatinized alone. This can be understood in terms of competition for water, needed to induce gelatinization. Finally, a novel DSC based method was proposed to approach gelatinization of each starch component within the blend separately. The results are qualitatively in agreement with those obtained from the more rigorous WAXD based method.

Simultaneous Synchrotron WAXD and Fast Scanning (Chip) Calorimetry: On the (Isothermal) Crystallization of HDPE and PA11 at High Supercoolings and Cooling Rates up to 200 °C s(-1).

An experimental setup, making use of a Flash DSC 1 prototype, is presented in which materials can be studied simultaneously by fast scanning calorimetry (FSC) and synchrotron wide angle X-ray diffraction (WAXD). Accumulation of multiple, identical measurements results in high quality, millisecond WAXD patterns. Patterns at every degree during the crystallization and melting of high density polyethylene at FSC typical scanning rates from 20 up to 200 °C s(-1) are discussed in terms of the temperature and scanning rate dependent material crystallinities and crystal densities. Interestingly, the combined approach reveals FSC thermal lag issues, for which can be corrected. For polyamide 11, isothermal solidification at high supercooling yields a mesomorphic phase in less than a second, whereas at very low supercooling crystals are obtained. At intermediate supercooling, mixtures of mesomorphic and crystalline material are generated at a ratio proportional to the supercooling. This ratio is constant over the isothermal solidification time.

In-situ synchrotron wide-angle X-ray diffraction as a rapid method for cocrystal/salt screening

The purpose of this work was to explore in-situ synchrotron wide-angle X-ray diffraction (WAXD) as a rapid and accurate tool to screen and monitor the formation of cocrystal/salts during heating. The active pharmaceutical ingredients (caffeine, carbamazepine and lamotrigine) were respectively mixed with the coformer (saccharin), and then heated by the hot stage. Real-time process monitoring was performed using synchrotron WAXD to assess cocrystal formation and subsequently compared to differential scanning calorimetry (DSC) measurements. The effect of heating rates and cocrystal growth behavior were investigated. Synchrotron WAXD was fast and sensitive to detect cocrystal formation with the appearance of characteristic diffraction rings, even at the heating rate of 30°C/min, while DSC curves showed overlapped peaks. Unlike the indirect characterization of DSC on endo/exothermic peaks, synchrotron WAXD can directly and qualitatively determine cocrystal by diffraction peaks. The diffraction intensity–temperature curves and the corresponding first-derivative curves clearly exhibited the growth behavior of cocrystal upon heating, providing useful information to optimize the process temperature of hot melt extrusion to continuously manufacture cocrystal. The study suggests that in-situ synchrotron WAXD could provide a one-step process to screen cocrystal at high efficiency and reveal the details of cocrystal/salts growth behavior.