Extruded blend films of poly(vinyl alcohol) and polyolefins: common and hard-elastic nanostructure evolution in the polyolefin during straining as monitored by SAXS

Abstract

Straining of PVA/PE and PVA/PP blends (70:30) is monitored by small-angle x-ray scattering (SAXS). Sheet-extruded films with different predraw ratio are investigated. The discrete SAXS of predrawn samples originates from polyolefin nanofibrils inside of polyolefin microfibrils immersed in a PVA matrix. PE nanofibrils deform less than the macroscopic strain without volume change. PP nanofibrils experience macroscopic strain. They lengthen but their diameter does not decrease. This is explained by strain-induced crystallization of PP from an amorphous depletion shell around the core of the nanofibril. The undrawn PVA/PE film exhibits isotropic semicrystalline nanostructure. Undrawn PVA/PP holds PP droplets containing oriented stacks of semicrystalline PP like neat precursors of hard-elastic thermoplasts. Respective predrawn films are softer than the undrawn material, indicating conversion into the hard-elastic state. Embedding of the polyolefin significantly retards neck formation. The polyolefin microfibrils can easily be extracted from the water-soluble matrix.     

PAN基原丝预氧丝炭纤维微结构的几种新的X射线衍射散射定量表征技术

介绍几种新近开发的针对PAN基原丝、预氧丝和炭丝微结构和结构缺陷的X射线衍射与散射定量表征技术,它们是:纤维材料中柱状(椭球状)纳米取向微孔体积分数X射线小角散射定量表征方法;纤维材料中柱状(椭球状)纳米取向微孔取向角的小角散射定量表征方法;预氧丝中链状相与环化相体积分数的小角衍射定量表征方法以及炭纤维中乱层石墨相与单层石墨相的X射线广角衍射定量表征方法等。     

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

Microstructutal 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 2 theta = 13. 6 degrees disappeared during the final stage of pre-oxidation, meanwhile a new peak at 2 theta = 23.6 degrees 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.     

The structure of granular amorphous carbon films with cobalt nanoparticles

The films of amorphous hydrogenated carbon containing cobalt nanoparticles are studied by small-angle X-ray scattering (SAXS). The SAXS pattern structure reveals the presence of two types of scattering centers, one of which represents cobalt particles. It is suggested that the second type of scattering centers can represent the groups of nanoparticles possessing a localized electron wavefunction and forming a cluster electron state.     

Nanostructure of atmospheric and high-pressure crystallised poly(ethylene-2,6-naphthalate

Poly(ethylene-2,6-naphthalate) (PEN) was crystallized from the glassy state at atmospheric pressure (beyond the end of primary crystallization) and from the melt at high pressure. The structure was characterized using small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), differential scanning calorimetry (DSC) and density measurements. The SAXS patterns were analysed using the interface distribution function (IDF) method. For the materials prepared at ambient pressure the crystallinity inside the layer stacks remains nearly constant during the secondary crystallization process. On the other hand, the volume filled with the stacks increases as a function of crystallization temperature (T _c) and time (t _c). For T _c > 200°C secondary crystallisation goes along with a dynamic rearrangement of the primary stacks, as concluded from variations of the layer thickness distributions in the SAXS data. For T _c < 200°C primary lamellae are stable, and both insertion of new crystal lamellae into existing stacks and generation of additional stacks is found. In contrast to PET, two different kinds of layer stacks are not observed in the PEN nano-composites. Materials prepared at 400 MPa exhibit high roughness of the crystalline domain surfaces. Depending on T _c there is a continuous transformation from the to the -crystal modification, but hardly any change of the long period. Crystal thickness increases, both at the expense of the amorphous thickness and of the volume filled with lamellar stacks. The structure of samples showing two melting peaks is discussed in terms of a dual lamellar contribution of correlated and uncorrelated nano-crystallites, respectively.     

The structure of multilayer films of DNA-aliphatic amine is preparation technique dependent

The structures of DNA-hexadecylamine and DNA-hexadecyltrimethylammonium bromide multilayer films obtained by two different approaches: layer-by-layer (LbL) adsorption and Langmuir–Blodgett (LB) technique were compared. Interlayer distance and DNA conformation in these films were determined by small-angle X-ray scattering, infrared and ultraviolet spectroscopy. The following results were obtained: (1) X-ray diffractrograms of the films indicated a distinct layer structure; (2) the interlayer distances for LbL films were less than for LB-films, made from the same components and (3) the double stranded DNA was included into the DNA-hexadecylamine film as a single-stranded formation, whereas DNA in the DNA-hexadecyltrimethylammonium film retained its double-stranded form. Structural models of LbL and LB films were created.     

Formation and microstructural characterization of coaxial carbon cylinders consisting of aligned carbon nanotube stacks

The macroscopic coaxial carbon cylinders (dia. approximately 0.5 cm with varying lengths approximately 2-5 cm) consisting of aligned carbon nanotube (CNT) stacks have been prepared by spray pyrolysis of benzene-ferrocene solution in argon atmosphere at approximately 850 degrees C-900 degrees C temperature. The coaxial carbon cylinders of CNT stacks have been formed directly inside the quartz tube. We attempted to prepare superimposed multi carbon cylinder configurations, each consisting of ordered and aligned CNTs stacked over each other. For this, we have terminated the spray of precursor after run of about 25 minutes, for a short interval (approximately 5 min), and then the solution was sprayed again over the already deposited hot CNT stack. Gross structural characterization of CNTs was done through X-ray diffraction technique (XRD). Microstructural characterizations of as prepared coaxial carbon cylinders with CNT stacks were done by scanning electron microscopic (SEM) techniques. SEM studies show that the CNTs are well aligned along the periphery of the cross section of coaxial carbon cylinder, each consisting of CNTs of the type described in the above. Comparisons have been made between the present macroscopic coaxial carbon cylinders with CNT stacks studied earlier by several other workers. Plausible explanation for the synthesis of CNT stacks will be put forward.     

Carbon fibers modified with carbon nanotubes

Carbon nanotubes were used to modify a polyacrylonitrile (PAN) polymer solution before the manufacture of the carbon fiber precursor. The modified PAN fibers were spun from a dimethylformamide solution containing a small amount of single-walled carbon nanotubes. The fibers were characterized by thermogravimetry and optical and scanning electron microscopy. Structure, morphology, and selected properties of the composite polymeric fibers and the fibers after carbonization are characterized. The mechanical properties of the fibers are examined. It is found that nanotubes in the PAN solution have a strong tendency to form agglomerates that inhibit suitable macromolecular chain orientation of the carbon fiber precursor. Fibers manufactured from such a solution have similar mechanical properties to those from a pure PAN precursor, and after carbonization the resultant carbon fibers are very weak. A comparison of pure carbon fibers and those containing nanotubes reveals slight differences in their structural ordering.     

Nanostructured CeO2 thin films: A SAXS study of the interface between grains and pores

The analysis of the interface between grains and pores in thin films of CeO₂, prepared by two different sol-gel procedures, was performed using grazing incidence small-angle X-ray scattering. The existence and the average thickness of an interfacial layer, formed during one of the preparation procedures, were examined by analyzing the depletion of scattering intensity in the Porod's scattering region.     

X-ray photoelectron spectroscopy studies of sodium modified carbon films suitable for use in humidity sensors

Carbon films obtained using the spray pyrolysis technique are used in the construction of humidity sensitive resistors and capacitors. Both the electronic and surface properties of these films are dependent on several factors such as the carbonization temperature and duration. An addition of sodium containing salts (sodium acetate) to the basic precursor i.e., furfuryl alcohol, produces carbon films that have very specific surface properties. X-ray photoelectron spectroscopy studies revealed that the sodium atoms are located in the mother carbon network. These sodium atoms are associated with oxygen forming structures corresponding to Na2O. These spots act as centres of durable CO2 adsorption. This results in an intensive oxidation of the surface leading to a state where the surface layer contains more than 45% oxygen. Such an intensively oxidized surface is highly hydrophilic which results in an intensive water vapour adsorption that follows an I-type adsorption isotherm. The electronic conduction in the sodium modified carbon films is extremely sensitive to water adsorption and it increases by thousands of times after an exposure to humid air. This revised version was published online in November 2006 with corrections to the Cover Date.     

聚合物共混制备纳米和多孔碳纤维及性能研究

聚丙烯腈(PAN)和聚甲基丙烯酸甲酯(PMMA)共混膜的结构和尺寸可由两组分比例和分子量调整。以PAN为碳前驱体,PMMA为热分解聚合物,并控制m(PAN)/m(PMMA)为30/70和70/30,通过湿法纺丝制备了PAN/PMMA共混纤维。以m(PAN)/m(PMMA)为30/70和70/30的共混纤维为原丝经碳化后获得了纳米碳纤维(CNFs)和多孔碳纤维(PCFs)。利用扫描电镜观察了所得CNFs和PCFs的形貌,发现单根CNFs的直径为50～150nm,PCFs中孔的直径为0.1～1μm。由CNFs和PCFs的拉曼光谱分析了不同碳化温度对CNFs和PCFs石墨化程度的影响,结果表明随碳化温度升高,石墨化程度也增加,同时电导率也随之提高。     

Morphology and mechanical property relationship in linear low-density polyethylene blown films

Linear low-density polyethylene (LLDPE) blown films fabricated under two different processing conditions, namely a non-stalk bubble configuration and a stalk bubble configuration, were investigated. Morphological characterization was performed using small-angle X-ray scattering, transmission electron microscopy, infrared dichroism, and differential scanning calorimetry. The findings on crystal orientation characteristics of the films suggest that modification on the widely accepted row orientation model of Keller and Machin may be needed. In comparison to the conventional non-stalk bubble geometry for LLDPE film blowing, the stalk bubble configuration can produce a more randomly orientated lamellar texture, resulting in less anisotropy in mechanical properties and a higher dart impact resistance. A good correlation between mechanical properties and morphological features was found.     

聚丙烯腈基炭纤维中微孔的演变规律

利用二维小角X射线散射技术(SAXS),结合纤维孔结构解析理论及分形原理得到了炭纤维形成过程中纺丝、预氧化、低温和高温炭化等四个阶段样品的微孔结构信息。结果表明:原丝中孔隙沿纤维轴向择优取向,呈长梭状,其长轴与短轴的平均尺寸分别为24.3nm和19.2nm,长径比为1.27。遗留到预氧化阶段的原丝孔洞使得预氧化纤维出现高达1.85的长径比极大值,这可能与原丝线性结构向预氧丝耐热梯形结构转化有关。炭化阶段微孔尺寸迅速减小,长轴、短轴分别达到3.56nm和2.85nm左右,长径比也减小至1.24。分形状态研究表明:表面分形维数DS值介于2.42～2.88之间,且随工艺的进行逐渐增大,低温炭化阶段变化幅度较大,说明各级产品在微观结构上越来越复杂,亦证明低温炭化是促进炭纤维微观结构转变的重要的工艺段。     

Continuous Nanoscale Carbon Fibers with Superior Mechanical Strength

Continuous nanoscale carbon fibers can be developed by stabilization and carbonization of highly aligned and extensively stretched electrospun polyacrylonitrile copolymer nanofiber precursor under optimal tension. These carbon fibers, with diameters of tens of nanometers, are expected to possess a superior mechanical strength that is unlikely to be achieved through conventional approaches. This is because i) the innovative precursor, with a fiber diameter approximately 100 times smaller than that of conventional counterparts, possesses an extremely high degree of macromolecular orientation and a significantly reduced amount of structural imperfections, and ii) the ultrasmall fiber diameter also effectively prevents the formation of structural inhomogeneity, particularly sheath/core structures during stabilization and carbonization.     

Small-angle neutron scattering studies of hot-wire CVD a-Si:H

Several a-Si:H and a-Si:D films prepared by hot-wire chemical vapor deposition have been examined by small-angle neutron scattering (SANS) to search for H non-uniformity in this material. The SANS measurements were supplemented by small-angle X-ray scattering measurements. The differences in H/D detection sensitivity of these two techniques allow distinction of the scattering mechanisms. Two- or three-phase models are used to interpret the results quantitatively. Significant H non-uniformity, as well as a small fraction of microvoids, was found in the best-quality material. Samples grown with higher deposition rates or lower substrate temperatures have much larger void fractions. The size scale of the heterogeneity spans a range from 2 nm to more than 50 nm, with the largest features assigned to surface roughness.     

Sublimation and deposition of carbon during internal resistance heating of carbon fibers

A carbon fiber tow subjected to internal resistance heating often breaks when its surface temperature is raised higher than about 2200°C. In this study, the cause of the tow breakage was investigated. Internal resistance heating gives rise to a large temperature distribution in the tow cross section, the temperature being at a maximum in the central region of the tow cross section. Because of this temperature distribution, carbon filaments in the tow are sublimated from the central region of the tow cross section, leaving a growing cavity; as a result the tow breaks. Some of the sublimated carbon deposits on the filaments in the low temperature region surrounding the sublimating region, forming a laminar texture around the filament surface. The structure of the deposited carbon was investigated by reflected light microscopy, by scanning electron and transmission electron microscopy, and by electron and wide-angle X-ray diffraction. The deposited carbon was found to be composed of carbon layer stacks with low in-plane stacking regularity.     

Influence of heat treatment conditions on the structure of hollow carbon fibers prepared from solid PVA fibers using iodine pretreatment

The influence of heating conditions on the structure of hollow carbon fibers (H-CFs) during their fabrication from solid poly(vinyl alcohol) (PVA) fibers is reported. The hollow structure of PVA-derived carbon was formed by selective iodination and subsequent stabilization of precursor PVA fiber close to the fiber surface followed by carbonization. The broadening of X-ray diffraction peaks due to disorder and the small size effects of the (002) plane were strongly reduced by increasing the heat treatment temperature (HTT) from 800 to 3000 °C, but the asymmetric shape of (10) and (110) reflections suggests turbostratic layer stacking. The increase of HTT to 3000 °C increased the degree of graphitization evident from the decrease of interplanar spacing from 0.360 to 0.338 nm and the intensity ratio of D to G bands in Raman spectra from 0.93 to 0.58. The crystallite size, orientation and electrical conductivity of the resultant H-CFs were also improved with higher HTT. Besides, the size of the hollow core was also influenced by the HTT and both wall thickness and carbon yield decreased with higher HTT. The core of the H-CFs could be easily filled with polymer by bulk polymerization of monomer.     

SAXS analysis of the effect of H2 dilution on microstructural changes of HWCVD deposited a-SiC : H

Hydrogenated amorphous silicon carbon (a-SiC : H) films were deposited by hot wire chemical vapor deposition (HWCVD) and their porosity was investigated by small-angle X-ray scattering (SAXS), XRR and FTIR. SAXS measurement was analyzed by Guiner plot, Porod plot, Debye plot and scaling factor. The measurement of the SAXS results assumed a distribution of spherical pores. This analysis suggested that the maximum of pore size distributions occur for radius of gyration of 5–6 Å. As the H₂ flow rate increases, the pore size distribution narrows and the volume occupied by the pores decreases. A direct relation between the atomic density of a-SiC : H films deposited by HWCVD and the pore volume fraction was also obtained. The low scattering intensity observed for the films deposited by HWCVD showed that they were compact and homogeneous regardless of the H₂ dilution.     

Conditions and features of matrix and bulk carbonization of the organic precursors

Comparative research of matrix and bulk carbonization of some organic precursors (sucrose, acetonitrile) in silica mesoporous materials SBA-15 and KIT-6 was conducted. X-ray diffraction, nitrogen adsorption analysis, Raman spectroscopy were used for determination of the structural-sorption characteristics of the obtained materials. It was shown that the carbon mesoporous materials CMK-8 obtained in the mesopores of KIT-6 had higher adsorption characteristics because of features of three-dimensional cubic structure, larger pore volume and framework’s wall thickness. It was established that partially graphitized spatially well-organized carbon materials were formed as a result of pyrolysis of acetonitrile in the silica matrices SBA-15 and KIT-6. It was conditioned by the absence of considerable spatial limitations for growth of graphite structures on the initial stage of the synthesis when the pores of the matrix were not filled up with the organic precursor. Product of bulk carbonization of sucrose is compact carbon microporous framework with low sorption characteristics (micropore volume is 0.09 cm³/g).     

SiC fibers with controllable thickness of carbon layer prepared directly by preceramic polymer pyrolysis routes

Continuous SiC fibers with different thickness of carbon layer were prepared through three preceramic polymer pyrolysis routes. To make the carbon layer thickness controllable, a simple improvement by using a ceramic bushing was adopted to retard the deposition of the pyrolytic carbons. Auger electron spectroscopy (AES) analysis reveals that the carbon layer thickness varies from less than 5 nm to 40 nm. The specific resistivity of the fibers increases by 5 orders of magnitude as the carbon layer thickness decreases. All of the fibers exhibit a tensile strength of around 1.8 GPa which is independent of the carbon layer thickness. The formation process of the carbon layer is discussed in three steps: the decomposition, the carbonization and the deposition. The as-received fibers have a potential application as the reinforcement of functional materials.