3 research outputs found
Hybrid Silica–PVA Nanofibers via Sol–Gel Electrospinning
We report on the synthesis of polyÂ(vinyl alcohol) (PVA)–silica
hybrid nanofibers via sol–gel electrospinning. Silica is synthesized
through acid catalysis of a silica precursor (tetraethyl orthosilicate
(TEOS) in ethanol–water), and fibers are obtained by electrospinning
a mixture of the silica precursor solution and aqueous PVA. A systematic
investigation on how the amount of TEOS, the silica–PVA ratio,
the aging time of the silica precursor mixture, and the solution rheology
influence the fiber morphology is undertaken and reveals a composition
window in which defect-free hybrid nanofibers with diameters as small
as 150 nm are obtained. When soaked overnight in water, the hybrid
fibers remain intact, essentially maintaining their morphology, even
though PVA is soluble in water. We believe that mixing of the silica
precursor and PVA in solution initiates the participation of the silica
precursor in cross-linking of PVA so that its −OH group becomes
unavailable for hydrogen bonding with water. FTIR analysis of the
hybrids confirms the disappearance of the −OH peak typically
shown by PVA, while formation of a bond between PVA and silica is
indicated by the Si–O–C peak in the spectra of all the
hybrids. The ability to form cross-linked nanofibers of PVA using
thermally stable and relatively inert silica could broaden the scope
of use of these materials in various technologies
Hybrid Carbon Silica Nanofibers through Sol–Gel Electrospinning
A controlled sol–gel synthesis
incorporated with electrospinning
is employed to produce polyacrylonitrile–silica (PAN–silica)
fibers. Hybrid fibers are obtained with varying amounts of silica
precursor (TEOS in DMF catalyzed by HCl) and PAN. Solution viscosity,
conductivity, and surface tension are found to relate strongly to
the electrospinnability of PAN–silica solutions. TGA and DSC
analyses of the hybrids indicate strong intermolecular interactions,
possibly between the −OH group of silica and −CN of
PAN. Thermal stabilization of the hybrids at 280 °C followed
by carbonization at 800 °C transforms fibers to carbon–silica
hybrid nanofibers with smooth morphology and diameter ranging from
400 to 700 nm. FTIR analysis of the fibers confirms the presence of
silica in the as-spun as well as the carbonized material, where the
extent of carbonization is also estimated by confirming the presence
of −CC and −CO peaks in the carbonized
hybrids. The graphitic character of the carbon–silica fibers
is confirmed through Raman studies, and the role of silica in the
disorder of the carbon structure is discussed
Fibrillar Structure of Methylcellulose Hydrogels
It is well established that aqueous
solutions of methylcellulose
(MC) can form hydrogels on heating, with the rheological gel point
closely correlated to the appearance of optical turbidity. However,
the detailed gelation mechanism and the resulting gel structure remain
poorly understood. Herein the fibrillar structure of aqueous MC gels
was precisely quantified with a powerful combination of (real space)
cryogenic transmission electron microscopy (cryo-TEM) and (reciprocal
space) small-angle neutron scattering (SANS) techniques. The cryo-TEM
images reveal that MC chains with a molecular weight of 300 000
g/mol associate into fibrils upon heating, with a remarkably uniform
diameter of 15 ± 2 nm over a range of concentrations. Vitrified
gels also exhibit heterogeneity in the fibril density on the length
scale of hundreds of nanometers, consistent with the observed optical
turbidity of MC hydrogels. The SANS curves of gels exhibit no characteristic
peaks or plateaus over a broad range of wavevector, <i>q</i>, from 0.001–0.2 Å<sup>–1</sup>. The major feature
is a change in slope from <i>I</i> ∼ <i>q</i><sup>–1.7</sup> in the intermediate <i>q</i> range
(0.001 – 0.01 Å<sup>–1</sup>) to <i>I</i> ∼ <i>q</i><sup>–4</sup> above <i>q</i> ≈ 0.015 Å<sup>–1</sup>. The fibrillar nature
of the gel structure was confirmed by fitting the SANS data consistently
with a model based on the form factor for flexible cylinders with
a polydisperse radius. This model was found to capture the scattering
features quantitatively for MC gels varying in concentration from
0.09–1.3 wt %. In agreement with the microscopy results, the
flexible cylinder model indicated fibril diameters of 14 ± 1
nm for samples at elevated temperatures. This combination of complementary
experimental techniques provides a comprehensive nanoscale depiction
of fibrillar morphology for MC gels, which correlates very well with
macro-scale rheological behavior and optical turbidity previously
observed for such systems