Fibrils separated from polyacrylonitrile fiber by ultrasonic etching in dimethylsulphoxide solution

A novel method for the separation of polyacrylonitrile (PAN) fibrils from fibers by ultrasonic etching in a 90 wt % aqueous dimethylsulphoxide (DMSO) solution at 75 °C ± 2 °C for 6 h with a frequency of 40 kHz is demonstrated. These fibrils with a diameter of about 450 nm were systematically investigated by field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), and wide‐angle X‐ray diffraction (WAXD). It was found that the fibrils consisted of microfibrils with about 200 nm diameter, including periodic lamellae with thickness of 30–45 nm perpendicular to the fiber axis. The PAN fiber crystallinity and crystal size slightly decreased under the ultrasonic etching. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 617–619, 2010     

Research on PAN Nascent Fiber Interior Microstructure through Ultrasonic Etching and Ultrathin Sectioning

The interior microstructures of polyacrylonitrile nascent fibers is studied by the scanning electronic microscopy and the high-resolution transmission electron microscopy through ultrasonic etching and ultrathin sectioning. Due to the orientation and fold of molecular chains, the lamellae of 50–80 nm in thickness are formed. A high number of pores, ranging from dozens to two hundred nanometers in diameters exist between the lamellae, which result from residual solvent. The fibril structure is formed in the nascent fiber during the coagulation process, which are oriented along the fiber axis. An uneven tensile stress distribution leads to the formation of skin-core structures in the nascent fiber during the dry-jet wet spinning process.     

Transverse heterogeneity in PET fibers

X-ray scattering from a series of poly(ethylene terephthalate) (PET) fibers spun at differet speeds is analyzed to probe the morphology in the direction transverse to the fiber axis. Both the apparent crystal modulus, determined from the change in wide-angle X-ray scattering angle with fiber stretching, and the transverse degree of crystallinity indicate there is a substantial interfiberillar amorphous content. In the PET fiber spun at conventional speeds, only roughly one-quarter of the fiber cross-section is actually occupied by fibrils. The transverse crystallinity increases for fibers spun at speeds sufficient to cause crystallization in the spin line. The X-ray moduli and fibril diameters are correspondingly larger in these high speed spun fibers. © 1993 John Wiley & Sons, Inc.     

Longitudinal growth of polymer crystals from flowing solutions. VI. Melting behavior of continuous fibrillar polyethylene crystals

The melting behavior of continuous fibrillar crystals of high-molecular-weight polyethylene has been investigated. The macrofibers were grown from dilute solutions in xylene subjected to Couette flow in the temperature range between 103 and 118.5°C. The thermograms, as determined by differential scanning calorimetry, exhibit three melting endotherms with peak temperatures at 141, 150.5, and 159.5°C after extrapolation to zero scan speed. All peaks were found to be strongly superheatable. Reduction of fiber length, in particular by etching with fuming nitric acid, led to the disappearance of the melting peaks at 150.5 and 159.5°C. The remaining peak at 136°C appeared not to be superheatable. The heat of fusion of the fragmented fibers was 69.8 cal/g. Wide-angle x-ray diffractograms taken on a macrofiber while gradually heated at a rate of 0.35°C/min at constant length showed that the triclinic phase present in the fiber disappeared at 130°C and that the orthorhombic cell transformed into the hexagonal modification at 150°C. This hexagonal phase was still observable at 180°C. The retractive force developed on heating at constant length displays first a slight decrease followed by a maximum at 150°C. Beyond the latter temperature the stress decays abruptly corresponding to the temperature at which fracture of the fiber could be observed visually. From all these observations it is inferred that the first melting endotherm in the differential scanning calorimeter (DSC) thermograms arises from the melting of unconstrained fibrillar crystal regions which are able to shrink during fusion. Moreover, the melting of lamellar overgrowths on the elementary fibrils on shish-kebab type may contribute to this endotherm. The second melting endotherm at about 150°C is associated with the transformation of the orthorhombic into the hexagonal lattice in constrained parts of the sample. This latter “rotator” phase allows slippage of the polymer chains past each other, giving rise to stress relaxation. The third endotherm arises from melting of this hexagonal phase and the heat take-up connected with the formation of higher energy gauche states upon randomization of the chains in the melt. Almost smooth, fully constrained fibrillar crystals grown at high temperature absorb more than 15.5 cal/g during this process, indicating that the polymer chains in such fibers must be highly extended.     

The nanostructure of kraft pulp 1: evaluation of various mild drying methods using field emission scanning electron microscopy

Wood pulp fiber consists of carbohydrate fibrils containing crystalline cellulose microfibrils of a few nanometer width. The structure of the fibril in water is currently unclear due to the difficulty of imaging pulp fiber in water at nanometer resolution. An alternative method is to observe the sample dried with a mild drying method to preserve the structure of the wet sample. In this study, we studied softwood kraft pulp fibers which were dried with various mild drying methods and then imaged by field emission scanning electron microscopy at nanometer resolution. Both mild dried samples, as well as air dried samples, showed 10–20 nm wide fibrils, the width of which corresponded to a crystalline cellulose microfibril or bundles of them. The mild dried sample, which was critical point dried with liquid CO₂ (CPD), mainly showed 20–40 nm thick fibrils, in addition to the 10–20 nm fibrils. The existence of the thick fibril implies that the fibril itself has a swelling nature in water, although the possibility that the thick fibril was an artifact of the CPD process could not be excluded. Further investigation as to the extent that the thick fibrils found in the CPD samples reflect the nanostructure of pulp fiber in water is warranted.     

Ultrahigh modulus polyethylene. II. Effect of drawing temperature on void formation and modulus

The maximum degree of molecular orientation and deformation obtained by ultradrawing of high-density polyethylene in air is limited by formation of internal voids (both longitudinal separation of fibrils and perpendicular cracking), and thus values of Young's moduli which are achievable by ultradrawing techniques are also limited to values much below the theoretical limit for fully extended chains. Temperature has a significant effect on the critical draw ratios at which intensive void formation begins, and also on the draw ratio at which failure occurs during the ultradrawing. The temperature effect is observed only for high-density polyethylene having a wide molecular-weight distribution, and which can be drawn at higher temperatures (30–40°C below its melting point), e.g., Dow Chemical polyethylene LP51.1. As a result of ultradrawing at higher temperatures, transparent, ultrahigh modulus samples having draw ratios of order of 40 have been obtained. The higher drawing temperatures significantly reduce fibril separation, and perpendicular cracking is shifted toward higher draw ratios. Hence, with LP51.1 the highest Young's moduli (65–70 GPa) have been exhibited by the samples which were ultradrawn at 100–105°C.     

Effect of stereosequences on crystallinity and properties of zone‐drawn poly(vinyl alcohol) microfibrils

To effectively orient the molecular chains of novel syndiotactic poly(vinyl alcohol) (PVA) microfibrillar fiber (PVA fibril), a high‐temperature zone‐drawing method was adopted. The PVA fibrils were directly prepared from the saponification and in situ fibrillation without a spinning procedure. The maximum draw ratio of the PVA fibril increased with a decrease in the syndiotactic diad (r‐diad) content, indicating that the deformability of PVA molecules was lowered in higher syndiotactic PVA. Degree of crystal orientations up to 0.990 were achieved by stretching the PVA fibril with the r‐diad content of 65.1% and the original degree of crystal orientation of 0.902 at 250 °C close to its crystal melting temperature (T_m). When the same draw ratio was applied to the fibrils, a higher crystal orientation was achieved for the fibrils having higher syndiotacticity. Wide‐angle X‐ray data show that the longitudinal crystal sizes of drawn PVA fibrils were larger in higher syndiotacticities. The degree of crystal orientation, crystallinity, T_m, longitudinal crystal size, and tensile strength of the maximum drawn PVA fibril with a r‐diad content of 65.1% were 0.99, 0.97, 279 °C, 187 Å, and 4.66 N/tex, respectively. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1263–1271, 2001     

Some effects of melt-induced orientation on drawing of polypropylene monofilaments

Morphological changes accompanying the deformation of polypropylene filaments with varying degrees of melt-induced orientation are examined using wide-angle x-ray scattering (WAXS), small-angle x-ray scattering (SAXS), and electron microscopy, and their behavior is compared both to completely unoriented film samples and to very highly oriented, hard elastic filaments. Melt-oriented filaments are shown to deform predominantly by a voiding mechanism at low temperatures (<100°C), and destruction of the lamellas to produce fibrils occurs only after extensive drawing. The bimodal crystal texture of the filaments does not appear to greatly affect the low temperature deformation mechanism. High temperature (>100°C) drawing produces a fibrillar structure containing elongated voids.     

Cold Denaturation of α‐Synuclein Amyloid Fibrils

Although amyloid fibrils are associated with numerous pathologies, their conformational stability remains largely unclear. Herein, we probe the thermal stability of various amyloid fibrils. α‐Synuclein fibrils cold‐denatured to monomers at 0–20 °C and heat‐denatured at 60–110 °C. Meanwhile, the fibrils of β2‐microglobulin, Alzheimer’s Aβ1‐40/Aβ1‐42 peptides, and insulin exhibited only heat denaturation, although they showed a decrease in stability at low temperature. A comparison of structural parameters with positive enthalpy and heat capacity changes which showed opposite signs to protein folding suggested that the burial of charged residues in fibril cores contributed to the cold denaturation of α‐synuclein fibrils. We propose that although cold‐denaturation is common to both native proteins and misfolded fibrillar states, the main‐chain dominated amyloid structures may explain amyloid‐specific cold denaturation arising from the unfavorable burial of charged side‐chains in fibril cores.     

Influence of drying restraint on physical and mechanical properties of nanofibrillated cellulose films

Nanofibrillated cellulose (NFC) is a renewable and biodegradable fibril that possesses high strength and stiffness resulting from high level hydrogen bonding. Films made from NFC shrink and distort as they transition from a wet state (20 wt% solids) to a state of moisture equilibrium (90 wt% solids at 50 % RH, 23 °C). Material distortions are driven by development of moisture gradients within the fibril network and effectively reduce mechanical performance. For this study, NFC was extracted from softwood holocellulose by first employing a chemical pretreatment [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl catalyzed oxidation] followed by mechanical fibrillation using ultrasound energy. To assess the problem of film distortion, neat NFC films were dried at 50 % RH, 23 °C under one of the following three restraint conditions: fully restrained, partially restrained, and uniaxially drawn. The influence of restraint condition on the resulting physical and mechanical properties was evaluated. Raman and X-ray results showed that fibrils in the uniaxially drawn specimens tended to align with the drawing axis, whereas no in-plane orientation effects were observed for the fully or partially restrained specimens. Fully restrained specimens had a respective strength and stiffness of 222 MPa and 14 GPa in every (in-plane) direction. However, samples that were wet-drawn to a 30 % strain level had a respective strength and stiffness of 474 MPa and 46 GPa in the direction of draw. Mechanical properties for axially drawn specimens had both fibril alignment and fibril straightening contributions.     

Extensional flow-induced crystallization in polyethylene melt spinning

Polyethylene was spun into a heated chamber, with the spin-line temperature controlled in the range of 90–120°C. Within narrow limits, the stretch rate of the fiber was also controlled. Spin-line and crystallization onset conditions were characterized. Characteristics of fibers as-spun were measured via wide-angle x-ray scattering (WAXS), density, scanning electron microscopy, and differential scanning calorimetry. Spin-line data indicate that elongational flow enhances crystallization rate and that time under spin-line conditions is an important parameter. Analysis of WAXS data shows a typical “b-axis radial” orientation, the details of which change with spin-line parameters. This orientation is consistent with growth of lamellar crystals on extended-chain fibrils (shishkebab model). Other physical data are also consistent with this microstructure. Microcamera x-ray patterns show a similarity of microstructure generation in spinning and drawing.     

Brill transition of nylon-6 in electrospun nanofibers

Electrospun nylon-6 fibers were prepared from its polyelectrolyte solution in formic acid with different concentrtaions. In situ Fourier transform infrared (FTIR), wide-angle X-ray diffraction and small-angle X-ray scattering (SAXS) were performed on the nylon-6 fibers heated to various temperatures until melting. For comparison, stepwise annealing of the solution-cast film having exclusively the α-form was also carried out to elucidate the structural evolution. Our results showed that Brill transition in the electrospun fibers occurs at a lower temperature than that in the solution-cast film due to the crystal size difference. Differential scanning calorimetry heating traces on the as-spun fibers exhibited a unique crystalline phase with a melting temperature of ～235?°C, higher than the equilibrium melting temperature of nylon-6. The content of high melting temperature (HMT) phase increased with increasing nylon-6 concentration; a maximum of 30?% of the fiber crystallinity was reached for fibers obtained from the 22?wt.% solution regardless of the heating rates used. Based on the SAXS and FTIR results, we speculated that the HMT phase is associated with thick α-form crystals developed from the highly oriented nylon-6 chains that are preserved in the skin layer of the as-spun fibers. A plausible mechanism for the formation of the skin/core fiber morphology during electrospinning was proposed.     

Absolute structural constraints on amyloid fibrils from solid-state NMR spectroscopy of partially oriented samples

We demonstrate that absolute, molecular-level structural information can be obtained from solid-state NMR measurements on partially oriented amyloid fibrils. Specifically, we show that the direction of the fibril axis relative to a carbonyl 13C chemical shift anisotropy (CSA) tensor can be determined from magic-angle spinning (MAS) sideband patterns in 13C NMR spectra of fibrils deposited on planar substrates. Deposition of fibrils on a planar substrate creates a highly anisotropic distribution of fibril orientations (hence, CSA tensor orientations) with most fibrils lying in the substrate plane. The anisotropic orientational distribution gives rise to distorted spinning sideband patterns in MAS spectra from which the fibril axis direction can be inferred. The experimentally determined fibril axis direction relative to the carbonyl CSA tensor of Val12 in fibrils formed by the 40-residue beta-amyloid peptide associated with Alzheimer's disease (Abeta1-40) agrees well with the predictions of a recent structural model (Petkova et al. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 16742-16747) in which Val12 is contained in a parallel beta-sheet in the cross-beta motif characteristic of amyloid fibrils.     

NMR observations of drawn polymers. VII. Nylon 66 fibers

Wide-line NMR spectra have been obtained on an oriented sample of drawn nylon 66 fibers at temperatures between ?196°C and 200°C and at alignment angles between the fiber axis and the magnetic field of 0°, 45°, and 90°. At ?196°C, 20°C, and 180°C, the complete angle dependence of the NMR spectrum has been measured. The second moments of these spectra have been compared to theoretical second moments calculated for various models of chain segmental motion in an attempt to elucidate the mechanisms involved in the low-temperature segmental motion (γ process) and the high-temperature segmental motion (α_c process). In agreement with earlier suggestions, the present results indicate that the γ process consists of segmental motion in noncrystalline regions. The overall decrease in second moment caused by the γ process is consistent with a model in which all noncrystalline segments rotate around axes nearly fixed in space. Furthermore, this decrease shows a pronounced dependence on the alignment angle. It is believed that this is due to tie molecules which become highly oriented along the fiber axis during drawing; their axes of rotation will therefore be nearly parallel to the fiber axis. The segments in noncrystalline entities such as chain folds and chain ends are less well oriented along the fiber axis and make an essentially isotropic contribution to the second moment decrease. The second moment at 180°C indicates the presence of considerable motion in the crystalline regions, and this motion is denoted the α_c process. The second moment S_c of the crystalline regions is strongly dependent on the alignment angle, the predominant feature being a relatively high value of the second moment when the fiber axis is directed parallel to the magnetic field. This is in qualitative, but not quantitative, agreement with the motional model recently advanced by McMahon, which assumes full rotation of the chains around their axes. Excellent quantitative agreement with experiment has been obtained by superimposition of rotational oscillation around the chain axis of amplitude roughtly 50°, and torsion of the chains with neighboring CH₂ groups oscillating around the C? C bond with a relative amplitude of about 40°. A model in which the chains perform rotational jumps of 60° between two equilibrium sites has also been considered (60° flip-flop motion). A distinction between this model and rotational oscillation has not been possible.     

Small‐angle X‐ray scattering investigation of carbon nanotube‐reinforced polyacrylonitrile fibers during deformation

Structural changes during deformation in solution‐ and gel‐spun polyacrylonitrile (PAN) fibers with multi‐ and single‐wall carbon nanotubes (CNTs), and vapor‐grown carbon nanofibers were investigated using synchrotron X‐ray scattering. Previously published wide‐angle X‐ray scattering (WAXS) results showed that CNTs deform under load, alter the response of the PAN matrix to stress, and thus enhance the performance of the composite. In this article, we find that the elongated scattering entities that give rise to the small‐angle X‐ray scattering (SAXS) in solution‐spun fibers are the diffuse matrix‐void interfaces that follow the Porod's law, and in gel‐spun fibers these are similar to fractals. The observed smaller fraction of voids in the gel‐spun fibers accounts for the significant increase in the strength of this fiber. The degree of orientation of the surfaces of the voids is in complete agreement with those of the crystalline domains observed in WAXS, and increases reversibly upon stretching in the same way as those of the crystalline domains indicating that the voids are integral parts of the polymer matrix and are surrounded by the crystalline domains in the fibrils. The solution‐spun composite fibers have a larger fraction of the smaller (<10 nm) voids than the corresponding control PAN fibers. Furthermore, the size distribution of the voids during elongation changes greatly in the solution spun PAN fiber, but not so in its composites. The scattered intensity, and therefore the volume fraction of the voids, decreases considerably above the glass transition temperature (T_g) of polymer. Implications of these observations on the interactions between the nanotubes and the polymer are discussed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2394–2409, 2009     

CHARACTERIZATION OF BLEND ACRYLIC FIBER''''S STRUCTURES

The fine structures, paracrystal, porous and phase structures of blend acrylic fibers have bencharacterized using techniques based on X-ray diffraction, gas adsorption, density, electron and opticalmicroscopy, mercury penetration method and IR spectroscopic analysis. The existence of a fibril-void structure, with voids and fibrils of the order of 10~2.to 10~3A diameter has been confirmed for thewet-spun blend fibers. It was found that the blend acrylic fiber in the present work is a kind of ma-trix-fibril fiber; phase separation takes place in it. Further studies about the effects of composition andsequence distribution of copolymers on structure of the blend fibers were made; and it was discoveredthat void volume in the blend fibers largely depends on the interface area between two polymer phases,and pore size and number on the particle size and number of the dispersing phase. An appropriatestructural model about the blend fibers has been described.     

The thermal degradation of polyoxymethylene produced by solid-state polymerization

The thermal degradation of polyoxymethylene produced by the solid-state polymerization of trioxane crystals has been studied. Changes in small-angle and wide-angle x-ray patterns, weight loss, and density have been measured in specimens heated in air at 175°C, 185°C, and 195°C. The polyoxymethylene contained material in two different crystallographic orientations, which behaved differently. Material in the so-called “twin” orientation melted preferentially at 185°C and 195°C, and at 185°C resolidified on cooling in parallel to the main orientation, in a lamellar structure. At 195°C, randomly oriented material was produced on resolidification. At 175°C, there was no evidence of melting or the formation of a lamellar structure, but the twin material was preferentially degraded so that only material in the main orientation remained after a time of 1500 min. The more rapid loss by degradation of the twin material was attributed to its having a lower molecular weight than the material in the main orientation.     

Analysis of small-angle X-ray scattering from fibers: Structural changes in nylon 6 upon drawing and annealing

The changes in the fibrillar and the lamellar structure in nylon 6 fibers resulting from drawing and annealing were studied by a detailed analysis of their two-dimensional small-angle scattering patterns. The scattering object that gives to rise the diffuse equatorial scattering in the angular range of Q = 0.02 to 0.3 Å⁻¹ is assumed to be a fibril. There are two distinct regimes in the equatorial diffuse scattering. The scattering at Q < 0.1 Å⁻¹ is dominated by scattering due to the longitudinal dimension of the fibril, and that at Q > 0.1 Å⁻¹ to the lateral dimensions/organization of the fibril. The interfibrillar regions, unlike the interlamellar regions that are essentially made of amorphous chain segments, may have microvoids in addition to amorphous chain segments. The intensity distribution within the lamellar reflections was used to obtain the lamellar spacings and the dimension of the lamellar stacks. The length of the fibrils is between 1000 and 3000 Å, the higher values being more prevalent at lower draw ratios. The fibril length is larger than the length of the lamellar stack, and approaches the latter at higher draw ratios. Annealing does not change the lengths of the fibrils, but the length of the lamellar stack increases. The fibrils form crystalline aggregates with a coherence length of ∼200 Å at higher draw ratios. The diameter of the fibrils (50–100 Å) determined from the lamellar reflection using both the Scherrer equation and the Guinier law are consistent with the lateral size of the crystallites derived from wide-angle x-ray diffraction. The longitudinal correlation of the lamellae between the neighboring fibrils improves upon drawing and decreases upon annealing. The degree of fibrillar and lamellar orientation is about the same as the crystalline orientation. Lamellar spacing increases upon drawing (from ∼60 to 95 Å) and annealing (from ∼85 to 100 Å). This is accompanied by an increase in the width of the amorphous domains from 30 to 50 Å in drawn fibers, and from 45 to 55 Å in annealed fibers. The diameter of the fibrils decreases slightly upon drawing and increases considerably upon annealing. © 1996 John Wiley & Sons, Inc.     

Occurrence of rippling during the deformation of oriented polyethylene

Rippling is another mode, in addition to kink-band formation, by which oriented polyethylene can deform and results in a profuse and irregular waviness in the fibrils. For the medium-density and high-density polyethylenes investigated, rippling tended to occur only at strain rates below about 1 min^?1 at 25°C. Above this rate, kink bands tended to form. It is suggested that rippling results from easy slip between the fibrils of the oriented polymers and from the resistance of the fibrils to shortening under a compressive stress. The applied shear stress is reduced by the easy slip to a simple compression along the fibrils, and this distorts the fibril into the series of waves that constitutes rippling. Stress–strain measurements confirm that fibril slip is considerably easier under the rates at which rippling occurs than at the rates at which kink bands form.     

Atomic layer deposition (ALD) as a coating tool for reinforcing fibers

Layers of alumina were deposited on to bundled carbon fibers in an atomic layer deposition (ALD) process via sequential exposure to vapors of aluminium chloride and water, respectively. Scanning electron microscopic (SEM) images of the coated fibers revealed that each individual fiber within a bundle was coated evenly and separately, fibers are not bridged by the coating. SEM and transmission electron microscopic (TEM) images indicate that the coating was uniform and conformal with good adhesion to the fiber surface. Average deposition rate, measured from SEM images, was 0.06 nm per cycle at 500 °C. SEM also revealed that at deposition temperatures of 500 °C few of the fibers were damaged. At temperatures of 300 °C, no damaged fibers were observed, the average deposition rate decreased down to 0.033 nm per cycle. Oxidation resistance of the alumina-coated fibers was characterized by thermogravimetric analysis (TGA). The alumina coating improved oxidation resistance of the carbon fiber significantly. Oxidation onset temperature was 600 °C for fibers coated with a 45 nm thick alumina. Uncoated fibers, on the other hand, started to oxidize at temperatures as low as 250 °C.