Crystallization in Microdomains of a Block Copolymer Comprising Semicrystalline Block Observed by Simultaneous Measurement of SAXS and WAXS with H v‐SALS or DSC

Abstract

Semicrystalline block copolymers provide us with a fascinating model for studying the kinetics of crystallization. We performed the simultaneous measurement of small‐ (SAXS) and wide‐angle (WAXS) x‐ray scattering (SWAXS) with differential scanning calorimetry (DSC), or SWAXS with small‐angle light scattering (H _v‐SALS). The specimen used was polyethylene‐b‐poly(ethylene propylene) (PE‐b‐PEP) with the molecular weight of 44,200. The PE block has the melting point (T _m) at 108°C. We observed the time evolution of crystallization in the lamellar microdomains of PE‐b‐PEP after a temperature drop from 180°C (?T _m) to a variety of temperatures slightly below T _m. The exothermic signal was observed by DSC right after the temperature drop, while the four‐leaf‐clover pattern of H _v‐SALS and the SAXS peaks due to the lamellar microdomains were observed several minutes after the temperature equilibration. The WAXS peaks of (110) and (200) reflection were almost simultaneously detected with the H _v‐SALS and the SAXS peaks at crystallization temperature of 100°C. With the crystallization temperature closer to T _m, the WAXS crystalline signals showed up with longer time lag after the H _v‐SALS and the SAXS peaks began to appear. Interestingly, these phenomena are interpreted as that long‐range order of density fluctuation up to the order of micrometers was generated prior to the formation of crystals with partially ordered phase rather than the instantaneous crystalline nucleation.     

FeCo基纳米晶合金高温交换耦合作用机理

研究了500和600℃真空退火后的纳米晶Fe_38.4Co₄₀Si₉B₉Nb_2.6Cu合金初始磁导率随温度的变化规律,发现较高温度(600℃)退火的FeCo基纳米晶合金,在非晶相居里温度以上较宽温度范围内磁导率没有明显的衰减,这是在双相纳米晶合金中观察到的一种新现象,其磁特性不同于Fe基纳米晶合金.为了探明这种现象的起源,估算了与剩余非晶相同成分的非晶合金的居里温度及纳米晶粒间发生交换耦合作用的参数 关键词： 交换耦合作用 非晶相居里温度 交换耦合穿透深度     

Structural characterization of Rh/pumice SMAD catalysts

The structure of Rh/pumice catalysts prepared by the SMAD (Solvated Metal Atoms Dispersion) technique at different metal loadings has been investigated by EXAFS (Extended X-ray Absorption Fine Structure Spectroscopy), XPS (X-ray Photoelectron Spectroscopy), SAXS (Small-Angle X-ray Scattering), WAXS (Wide-Angle X-ray Scattering) and TEM (Transmission Electron Microscopy). According to EXAFS and XPS, a fraction of the Rh atoms is oxidised, but a noticeable part is also present as Rh ⁰. The Rh oxidation is attributed to the interaction of the Rh atoms with the hydroxyls of the support; after the formation of the oxide, the nucleation of metallic rhodium becomes possible. The WAXS data do not show evidence of rhodium fcc crystallites; the metal-bearing particles are probably amorphous and/or very small, as results from the SAXS and TEM data analysis. The disagreement between the latter two techniques, resulting in a small-angle determination of the average size of the particles that is about half that of TEM in the catalyst with the higher Rh loading, is acknowledged and discussed. Preliminary catalytic tests are described, demonstrating the suitability of using a low surface area support for the preparation of SMAD catalysts. Received 2 February 1999     

In Situ Liquid SAXS Studies on the Early Stage of Calcium Carbonate Formation

Calcium carbonate is a model system to investigate the mechanism of solid formation by precipitation from solutions, and it is often considered in the debated classical and nonclassical nucleation mechanism. Despite the great scientific relevance of calcium carbonate in different scientific areas, little is known about the early stage of its formation. Therefore, contactless devices are designed that are capable of providing informative investigations on the early stages of the precipitation pathway of calcium carbonate in supersaturated solutions using classical scattering methods such as wide‐angle X‐ray scattering (WAXS) and small‐angle X‐ray scattering (SAXS) techniques. In particular, SAXS is exploited for investigating the size of entities formed from supersaturated solutions before the critical conditions for amorphous calcium carbonate (ACC) nucleation are attained. The saturation level is controlled and kept constant by mixing four diluted solutions (i.e., NaOH, CaCl₂, NaHCO₃, H₂O) at constant T and pH. The scattering data are collected on a liquid jet generated about 75 s after the mixing point. The data are modeled using parametric statistical models providing insight about the size distribution of denser matter in the liquid jet. Theoretical implications on the early stage of solid formation pathway are inferred.     

The role of the amorphous phase in the re-crystallization process of cold-crystallized poly(ethylene terephthalate)

The process of re-crystallization in poly(ethylene terephthalate) is studied by means of X-ray diffraction (SAXS and WAXS) and dynamical mechanical thermal analysis. Samples cold-crystallized for 9h at the temperatures T _c = 100 fcir#circ;C and T _c = 160 fcir#circ;C, i.e. in the middle of the relaxation region and close to its upper bound, respectively, are analyzed. During heating from room temperature, a structural rearrangement of the stacks is always found at T _r ≃ T _c + 20 fcir#circ;C. This process is characterized by a decrease of the linear crystallinity, irrespective of T_c; on the other hand, the WAXS crystallinity never increases with T below T_c+30fcir#circ;C. The lamellar thickness in the low-T_c sample decreases significantly after the structural transition, whereas in the high-T_c sample the lamellar thickness remains almost unchanged. In both, high- and low-T_c, the interlamellar thickness increases above T_r. Moreover, the high-T_c sample shows a lower rate of decrease of the mechanical performance with increasing T as the threshold T_r is crossed. This result is interpreted in terms of the formation of rigid amorphous domains where the chains are partially oriented. The presence of these domains would determine i) the stabilization of the crystalline lamellae from the thermodynamic point of view and ii) the increase of the elastic modulus of the amorphous interlamellar regions. This idea is discussed by resorting to a phase diagram. An estimation of the chemical-potential increase of the interlamellar amorphous regions, due to the enhancement of the structural constraints hindering segmental mobility, is offered. Finally, previous calculations developed within the framework of the Gaussian chain model (F.J. Baltá Calleja et al., Phys. Rev. B 75, 224201 (2007)) are used here to estimate the degree of chain orientation induced by the structural transition of the stacks.     

Radio-frequency induced effects in amorphous and nanocrystalline Fe73.5Cu1Nb3Si13.5B9

The rf collapse and sideband effects are used to study the microstructure of the Fe_73.5Cu₁Nb₃Si_13.5B₉ alloy in the amorphous and nanocrystalline state. Nanocrystalline grains of -Fe(Si) are formed as a result of annealing of the amorphous alloy at 520–570 °C. The results show that the complete rf collapse of magnetic hfs in the Mössbauer spectra occurs only in the amorphous phase. The rf collapse is strongly affected by the onset of the nanocrystalline phase and is suppressed when the Fe₃B and Fe₂B phases are formed. Partial collapse allows us to follow the remaining crystalline fractions during the crystalline process. The rf sidebands disappear due to the formation of nanocrystals because of the vanishing magnetostriction.     

Study of temperature dependence of crystallisation transitions of a symmetric PEO-PCL diblock copolymer using simultaneous SAXS and WAXS measurements with synchrotron radiation

The reversible transitions of the lamellae of a crystalline-crystalline diblock copolymer from the melt to crystallites were studied using simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) measurements with synchrotron radiation. A symmetric poly(ethylene oxide)-poly( -caprolactone) diblock copolymer was chosen for this study. We showed in the course of the block copolymer crystallisation that the time-resolved integrated intensity I _int was proportional to the product of the volume fractions of the PEO and PCL phases and the scattering contrast due to the electron density difference. These results demonstrated that simultaneous SAXS/WAXS measurements could be used to monitor the crystallisation process in two domains of different sizes at the same time.     

Preparation and Characterization of Poly(Vinyl Alcohol) (PVA)/SiO2, PVA/Sulfosuccinic Acid (SSA) and PVA/SiO2/SSA Membranes: A Comparative Study

Abstract

New organic–inorganic nanocomposites based on PVA, SiO₂ and SSA were prepared in a single step using a solution casting method, with the aim to improve the thermomechanical properties and ionic conductivity of PVA membranes. The structure, morphology, and properties of these membranes were characterized by Raman spectroscopy, small- and wide-angle X-ray scattering (SAXS/WAXS), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), water uptake (Wu) measurements and ionic conductivity measurements. The SAXS/WAXS analysis showed that the silica deposited in the form of small nanoparticles (～ 10?nm) in the PVA composites and it also revealed an appreciable crystallinity of pristine PVA membrane and PVA/SiO₂ membranes (decreasing with increasing silica loading), and an amorphous structure of PVA/SSA and PVA/SSA/SiO₂ membranes with high SSA loadings. The thermal and mechanical stability of the nanocomposite membranes increased with the increasing silica loading, and silica also decreased the water uptake of membranes. As expected, the ionic conductivity increased with increasing content of the SSA crosslinker, which is a donor of the hydrophilic sulfonic groups. Some of the PVA/SSA/SiO₂ membranes had a good balance between stability in aqueous environment (water uptake), thermomechanical stability and ionic conductivity and could be potential candidates for proton exchange membranes (PEM) in fuel cells.     

Magnetic Behavior of Nanocrystalline FeNbCrBCu Alloys at Elevated Temperatures

The magnetic behavior of nanocrystalline Fe_73.5Nb_4.5Cr₅B₁₆Cu₁ alloys is investigated in a series of specimens with different volume fractions of crystalline phase. It is shown that the Curie temperature of amorphous phase firstly decreases after structural relaxation in amorphous state and then rapidly increases during the first stages of crystallization. The strikingly different behavior of coercivity at elevated temperatures is observed for the samples with low and high volume fractions of nanocrystalline particles.     

Initial stages in the decay of the amorphous phase in the bulk metallic glass Zr-Cu-Ti

Using methods of x-ray diffraction analysis, differential scanning calorimetry, dilatometry, and transmission electron microscopy, we have investigated the initial stages of decay of the amorphous phase in a bulk metallic glass based on zirconium. We found that crystallization of the bulk metallic glass proceeds in several stages, where in the first stage the bulk conversion effect is equal to more than 1.6%, or about 80% of the total bulk crystallization effect. We showed that the first stage of decay of the amorphous phase in the bulk metallic glass Zr₂₉Ti₁₁Cu₆₀ leads to the formation of a nanocrystalline structure with a grain size of 1–5 nm. We have analyzed the change in the shape of the diffraction maximum during the formation of the very fine nanocrystalline structure. Fiz. Tverd. Tela (St. Petersburg) 41, 1129–1133 (July 1999)     

Thermal imidization of polyamic acid–fullerene composites: 'in situ' X-ray diffraction study

'In situ' wide angle (WAXS) and small angle X-ray scattering (SAXS) studies were carried out to characterize the structural changes occurring during the conversion of a polyamic acid into the polyimide in the presence of fullerene, C₆₀. The variations in crystallinity, long period, and the SAXS invariant of the samples are discussed in the light of the various temperature ranges studied. The conversion of the polyamic acid into the polyimide is analyzed according to the different structural steps. Results reveal that C₆₀ affects the degree of order of the composite in the molecular direction, giving rise to slightly smaller crystals than those found in pure polyamic acid. The difference in the SAXS data between the pure PAA and the composite are discussed through the interaction of C₆₀ molecules with the polyamic acid, which seems to hinder the imidization reaction up to 185°C.     

Nanocrystal formation in gas-atomized amorphous Al85Ni10La5 alloy

An Al₈₅Ni₁₀La₅ amorphous alloy, produced via gas atomization, was selected to study the mechanisms of nanocrystallization induced by thermal exposure. High resolution transmission electron microscopy results indicated the presence of quenched-in Al nuclei in the amorphous matrix of the atomized powder. However, a eutectic-like reaction, which involved the formation of the Al, Al₁₁La₃, and Al₃Ni phases, was recorded in the first crystallization event (263°C) during differential scanning calorimetry continuous heating. Isothermal annealing experiments conducted below 263°C revealed that the formation of single fcc-Al phase occurred at 235°C. At higher temperatures, growth of the Al crystals occurred with formation of intermetallic phases, leading to a eutectic-like transformation behaviour at 263°C. During the first crystallization stage, nanocrystals were developed in the size range of 5 ～ 30 nm. During the second crystallization event (283°C), a bimodal size distribution of nanocrystals was formed with the smaller size in the range of around 10 ～ 30 nm and the larger size around 100 nm. The influence of pre-existing quenched-in Al nuclei on the microstructural evolution in the amorphous Al₈₅Ni₁₀La₅ alloy is discussed and the effect of the microstructural evolution on the hardening behaviour is described in detail.     

Grain growth process of two-phase nanocrystalline soft magnetic materials

In order to clarify the origin of the high thermal stability of the microstructure in bcc-Fe/amorphous two-phase nanocrystalline soft magnetic materials, we have investigated the changes in the magnetic and microstructural properties upon isothermal annealing at 898 K for an Fe₈₉Zr₇B₃Cu₁ alloy by means of transmission electron microscopy, Mössbauer spectroscopy and DC magnetometry. The mean grain size was found to remain almost unchanged at the early stage of annealing. However, rapid grain coarsening was evident at an annealing time of 7.2 ks where the intergranular amorphous phase begins to crystallize into Fe₂₃Zr₆. The grain growth process with a kinetic exponent of 1.6 is observed for the growth process beyond this annealing time, reflecting the disappearance of the intergranular amorphous phase. Our results confirm that the thermal stability of the bcc-Fe/amorphous two-phase nanocrystalline soft magnetic alloys is governed by the residual amorphous phase.     

Microstructure and electrical conductivity of YSZ thin films prepared by pulsed laser deposition

Yttria-stabilized zirconia (YSZ) is the most common solid electrolyte material used e.g. in ceramic fuel cells. Thin films of YSZ were deposited on c-cut sapphire single crystals by pulsed laser deposition using a KrF excimer laser focused on a polycrystalline 8 mol% Y₂O₃-stabilized ZrO₂ target. Depending on the substrate temperature and the oxygen background pressure during deposition, different microstructures are obtained. XRD and high-resolution SEM revealed the formation of dense amorphous films at room temperature. At 600°C preferentially (111) oriented polycrystalline films consisting of densely agglomerated nm-sized grains of the cubic phase resulted. Grain size and surface roughness could be controlled by varying the oxygen background pressure. RBS and PIXE evidenced congruent transfer only for a low number of pulses, indicating a dynamical change of the target stoichiometry during laser irradiation. The in-plane ionic conductivity of the as-deposited crystalline films was comparable to bulk YSZ. The conductivity of initially amorphous YSZ passes a maximum during the crystallization process. However, the relative changes remain small, i.e. no significant enhancement of ionic conductivity related to the formation of a nanocrystalline microstructure is found.     

纳米超微晶Fe73.5Cu1Mo3Si13.5B9合金显微结构和磁性的研究

对纳米晶Fe_73.5Cu₁Mo₃Si_13.5B₉合金的原始制备态和各退火态样品进行了室温Mossbauer谱研究,结果表明晶化态的合金存在α-Fe(Si)微晶相和晶界的非晶相。晶相和非晶相内场和面积随退火温度的变化是退火时Cu,Mo,B等成分的扩散和在各相中的再分配引起的。最佳磁性能对应非晶相中的铁量占合金铁总量的30％左右,超微晶合金的双相无规各向异性模型表明,一定量的非晶相对保持纳米晶优异的软 关键词：     

Structure evolution of a zirconium-based amorphous alloy upon megaplastic strain in the bridgeman chamber

The structure evolution of the Zr₅₀Ni₁₈Ti₁₇Cu₁₅ massive metal glass upon megaplastic strain (MPS) in the Bridgeman chamber is investigated in a wide interval of strain degrees at room temperature. It is demonstrated that upon MPS with n = 1/2 rotation of the Bridgeman anvil, nanocrystalline phases are precipitated in the alloy. With increasing strain, these phases are dissolved. It is suggested that nucleation centers of nanocrystalline phases (nanoclusters) are formed in the amorphous matrix upon MPS under compression without shear in the Bridgeman chamber. These phases grow in the shear bands transforming into the nanocrystalline phase upon MPS.     

Temperature dependence of the Mössbauer spectra of amorphous and nanocrystallized Fe86Zr7Cu1B6

Mössbauer measurements have been performed on amorphous and nanocrystalline alloy ribbons of nominal composition Fe₈₆Zr₇Cu₁B₆. The nanocrystalline samples were obtained by annealing the as-quenched alloy at different temperatures in the range between 650 and 870 K. Mössbauer spectra of the as-quenched amorphous sample have been recorded at 77 K, room temperature and above the Curie temperature (330 K) at 360 K. We have also performed Mössbauer measurements at room temperature in the nanocrystalline alloys to characterize the phases that appear after the annealing and their relative concentration. The as-quenched sample spectra reveal the existence of two inequivalent sites for Fe. Such a feature is also observed in the remaining amorphous phase of the annealed samples. In the first steps of crystallization, -Fe precipitates and its concentration increases with the annealing temperature. The experimental results suggest that the composition of the whole amorphous phase does not suffer large changes during crystallization.     

Quantitative description of the T1 formation kinetics in an Al–Cu–Li alloy using differential scanning calorimetry,small-angle X-ray scattering and transmission electron microscopy

The object of the present study is to design a methodology to follow the kinetics of T₁ precipitation, in an AA2198 alloy, in terms of precipitate size, morphology (thickness, diameter) and volume fraction, during a two-temperature isothermal heat treatment. We used in situ small-angle X-ray scattering (SAXS) as a way to measure the evolution of the T₁ mean thickness and diameter during the heat treatment. Transmission electron microscopy (TEM) was then performed in order to calibrate these evolutions. Furthermore, we demonstrate that the volume fraction evolution can be described successfully using a simple analysis of the differential scanning calorimetry (DSC) thermograms. The latter was calibrated by selected observations in high angular annular dark field scanning transmission electron microscopy (HAADF-STEM). Microstructure evolution during DSC heating ramps was analysed using in situ SAXS: the T₁ phase transformation is found to consist in a two-step thickening process explained by two consecutive diffusion stages. The enthalpy of formation of the T₁ phase is deduced from the DSC measurements.     

X-ray photoelectron spectroscopy investigations of Si in non-stoichiometric SiNx LPCVD multilayered coatings

Si nanocrystals were formed in the non-stoichiometric Si-enriched SiN_x low-pressure chemical vapor deposited (LPCVD) coatings on Si wafers treated by various modes. The coating structure as a function of technological conditions was investigated by ellipsometry and X-ray photoelectron spectroscopy (XPS) depth profiling. It was found that nanocomposites on base of SiN_x films enriched by Si have a complex multilayered structure varying in dependence of deposition and annealing parameters. Analysis of the XPS spectra and Si 2s peaks shows the existence and quantity of four chemical structures corresponding to the Si–O, Si–N states, nanocrystalline and amorphous Si. The XPS results show evolution of the chemical structure of silicon nitride and formation of Si nanocrystals. It was found:

• The LPCVD technology of nanocrystals formation allows to get enough high concentration of Si nanocrystals on different depths from the sample surface.

• The volume fraction of nanocrystalline and amorphous Si is changed with depth; this relation depends from SiN_x composition and annealing parameters.

• XPS detects these two phase compositions of Si nanoparticles in SiN_x and SiO₂ layers. The ellipsometry, HR-TEM, and XPS results are in good agreement.