Effects of conformational change of polyacrylonitrile on the aging behavior of the solutions in N,N-dimethyl formamide

Effect of anions on the phase transition temperature of two structurally isomeric polymers: poly(N-isopropylacrylamide) and poly(2-isopropyl-2-oxazoline)

In chaotropic solution, the different lower critical solution temperature (LCST) increments of two structural isomers, namely, poly(N-isopropylacrylamide) (PNIPAAm) and poly(2-isopropyl-2-oxazoline) (PiPOx), is studied.

Ultra-thin carbon nanofibers based on graphitization of near-field electrospun polyacrylonitrile

A novel manufacturing means for arrays of carbon fibers with diameters less than 10 nm has been developed. The method improves dramatically upon the state of the art of making carbon fibers by pyrolysis of near field electrospun (NFES) polyacrylonitrile (PAN) fibers. The conductive nature of the PAN ink, touch and retract mode of the droplet at the ejector needle tip and a rotating drum for jet initiation as well as a tight control of the ink flow and droplet shape with a porous paper enable continuous near field electrospinning at an ultra-low voltage of 35 V - far below current low-voltage NFES practice. By pyrolyzing the thus obtained thinner PAN nanofibers on carbon scaffolds, a dramatic further shrinkage of the diameter leads to ultrathin carbon nanofibers e.g. from an ∼245 nm polymer fiber to an ∼5 nm carbon fiber i.e. a 4900% reduction in fiber diameter. To increase and survive the thinning during the pyrolysis process, PAN stabilization at a relatively low temperature (115 °C) and carbon support scaffolds are implemented to provide just the right amount of tension of the suspended polymer nanofibers. A small inter-fiber spacing (<8 μm) of the ultrathin carbon fibers in an array is achieved by simultaneously controlling the linear speed of the spinneret and the rotational speed of the collector. In one application, we demonstrate aligned ultra-thin carbon fibers freely suspended and in good ohmic contact with carbon scaffolds on a silicon substrate.

Ultrastructures and Mechanics of Annealed Nephila clavipes Major Ampullate Silk

The semicrystalline protein structure and impressive mechanical properties of major ampullate (MA) spider silk make it a promising natural alternative to polyacrylonitrile (PAN) fibers for carbon fiber manufacture. However, when annealed using a similar procedure to carbon fiber production, the tensile strength and Young's modulus of MA silk decrease. Despite this, MA silk fibers annealed at 600 °C remain stronger and tougher than similarly annealed PAN but have a lower Young's modulus. Although MA silk and PAN graphitize to similar extents, annealing disrupts the hydrogen bonding that controls crystal alignment within MA silk. Consequently, unaligned graphite crystals form in annealed MA silk, causing it to weaken, while graphite crystals in PAN maintain alignment along the fiber axis, strengthening the fibers. These shortcomings of spider silk when annealed provide insights into the selection and design of future alternative carbon fiber precursors.

Effect of Interfacial Interaction on the Conformational Variation of Poly(vinylidene fluoride) (PVDF) Chains in PVDF/Graphene Oxide (GO) Nanocomposite Fibers and Corresponding Mechanical Properties

Poly(vinylidene fluoride) (PVDF)/graphene oxide (GO) nanocomposite fibers were dry-jet wet spun at the GO concentrations of 0, 1, and 2 wt % with respect to the polymer. The as-spun fibers were drawn in the draw ratio (DR) range of 2-6.5, and the correlation between the PVDF chain conformation and the mechanical properties of the fibers upon drawing has been studied by two-dimensional correlation spectroscopy of Fourier-transformed infrared, wide-angle X-ray diffraction, differential scanning calorimetry, and tensile testing. The PVDF/GO nanocomposite fibers exhibited that the mobile PVDF crystals due to the conformational defects and kinks were nucleated because of the polar interaction between PVDF chains and functional groups of GO, whereas the control PVDF fiber showed the conventional conversion of crystal polymorphs (α and γ phases to β phase). As a result, the nanocomposite fiber showed dramatically improved toughness (enhanced by 1123% at a DR of 2 and 120% at a DR of 6.5) as compared to that of the control fiber. Furthermore, the tensile strength and modulus of the PVDF/GO (2 wt %) fiber were 394 MPa and 4.6 GPa, respectively, whereas those of the control PVDF fiber were 295 MPa and 3.9 GPa, respectively.

Rheological Behavior of Amino-Functionalized Multi-Walled Carbon Nanotube/Polyacrylonitrile Concentrated Solutions and Crystal Structure of Composite Fibers

The rheological behavior of amino-functionalized multi-walled carbon nanotubes (amino-CNTs)/polyacrylonitrile (PAN) concentrated solutions in the dimethyl sulphoxide solvent and the effects of the amino-CNTs on the PAN precursor fibers by wet-spinning method were investigated. The amino-CNT/PAN concentrated solutions prepared by in situ solution polymerization with homogeneous dispersion of amino-CNTs have higher complex viscosity, storage modulus and loss modulus as compared to the control PAN concentrated solutions containing 22% PAN polymer by mass. The composite fibers with amino-CNTs of 1 wt % have lower degree of crystallization, crystal size and crystal region orientation compared to the control PAN precursor fibers. However, the amino-CNT/PAN composite fibers with diameter of about 10.5 μm exhibit higher mechanical properties than the control PAN precursor fibers with diameter of about 8.0 μm. Differential scanning calorimetry analysis demonstrated that the cyclization reaction in composite fibers have broad exothermic temperature range and low exothermic rate. These results indicate that the addition of amino-CNTs into PAN precursor fibers is beneficial to controlling the process of thermal stabilization and obtaining the higher performance of composite fibers.

Investigation of the Brill transition in nylon 6,6 by Raman, THz-Raman, and two-dimensional correlation spectroscopy

The Brill transition is a phase transition process in polyamides related with structural changes between the hydrogen bonds of the lateral functional groups (CO) and (NH). In this study, we have used the potential of Raman spectroscopy for exploring this phase transition in polyamide 6,6 (nylon 6,6), due to the sensitivity of this spectroscopic technique to small intermolecular changes affecting vibrational properties of relevant functional groups. During a step by step heating and cooling process of the sample we collected Raman spectra allowing us from two-dimensional Raman correlation spectroscopy to identify which spectral regions suffered the largest influence during the Brill transition, and from Terahertz Stokes and anti-Stokes Raman spectroscopy to obtain complementary information, e.g. on the temperature of the sample. This allowed us to grasp signatures of the Brill transition from peak parameters of vibrational modes associated with (CC) skeletal stretches and (CNH) bending, and to verify the Brill transition temperature at around 160°C, as well as the reversibility of this phase transition.