Yong Lak Joo

Silica nanofibers from electrospinning/sol-gel process

The fabrication of materials with one-dimensional nanoscale structures is of great promise for the fundamental understanding of the roles of dimensionality and size in an optical, electrical, and mechanical properties with reference to applications in semiconductor mechanical and chemical industries. Polymer nanofibers are of considerable interest for various kinds of applications including filters, reinforcing agents, biomedical materials, and fiber templates to prepare nanotubes [1–6]. Recently, there has been a growing interest in one-dimensional, inorganic nanosized materials such as carbon nanotubes, carbide nanorods, silica and titania nanofibers/nanotubes [7–11]. These one-dimensional nanomaterials exhibit some novel physical and chemical properties due to their peculiar structure and size effect, and are of great importance in nanodevices and mesoscopic theoretical research. Electrospinning technique is an effective method to produce nanofibers [12–16]. The electrospinning process involves the application of a strong electrostatic field to a capillary connected with a reservoir containing a polymer solution or melt. Under the influence of the electrostatic field, a pendant droplet of the polymer solution at the capillary tip is deformed into a conical shape (Taylor cone). If the voltage surpasses a threshold value, electrostatic forces overcome the surface tension, and a fine charged jet is ejected. The jet moves towards a ground plate acting as counter electrode. Due to the extensional viscosity of the polymer solution and the presence of entanglements, the jet remains stable and does not transform into spherical droplets as expected for a liquid cylindrical thread. The solvent begins to evaporate immediately after the jet is formed. The result is the deposition of a thin polymer fiber on a substrate located above the counter electrode. The sol-gel method has widely been used as an alternative technology for the preparation of a wide variety of forms including monoliths, powders, coatings, and fibers [17–20]. The typical sol-gel method is hydrolysis and condensation of tetraethyl orthosilicate (TEOS), Si(OCH2CH3)4. In recent years, there have been efforts to synthesize metal oxide (silica or titania) nanofibers and nanotubes by the sol-gel template method [11, 21, 22]. Meanwhile, micron-scale silica fibers have been achieved by extruding the spinnable sol through an orifice [23–25]. In the present work, we study formation of silica nanofibers using the sol-gel method and electrospinng technique. We note that the TEOS solution used in this study does not contain any gelator or binder to help spinnability. Kobayashi and coworkers synthesized titania fibers via the sol-gel method from a physical gel of titanium tetraisopropoxide by a low molecular weight organogelator [10]. Zhang and coworkers synthesized silica and titania nanorods with the sol-gel method and anodic alumina template membrane [11, 21]. The silica sol was prepared from tetraethyl orthosilicate (TEOS), distilled water, ethanol, and HCl. The sol composition in molar ratio was 1:2:2:0.01 (TEOS:ethanol:water:HCl). First, TEOS was mixed with ethanol in a beaker. The HCl/water solution was then added drop by drop to the TEOS/ethanol solution under vigorous stirring. The solution was heated at 80 ◦C for 30 min and then cooled down to room temperature. The silica sol was placed in a pasteur pipet and the electrode was directly connected with the solution. A tubular shaped counter electrode with a diameter of 22 cm was located below the reservoir. The winding drum was rotated at speed of 10 rpm during the electrospinning. The fibers were collected on aluminum foil covered the tubular layer. The distance between the tip of the capillary and the counter electrode (tip-tocollector distance, TCD) was 10 cm and the applied voltages ranged from 10 kV to 16 kV. The morphology and diameter of silica fiber were measured with SEM (S-2350 of Hitachi). The composition of silica fiber was determined with FTIR (Travel IR of SensIR Technol.). The structure of silica fiber was analyzed with XRD (DMAX 2000 of Rigaku Denki). The thermal property was analyzed with a thermogravimetric analyzer of TGA 2050 of TA Instrument. TGA analysis was performed at 50–800 ◦C with 20 ◦C/min in air. It has been known that silica fibers obtained by the conventional technique through the sol-gel process are affected by composition of sol and ripening condition [20, 24]. In the current study, silica nanofibers were obtained successufully by electrospinning technique and

Electrospinning of viscoelastic Boger fluids: Modeling and experiments

An experimental and theoretical investigation of electrospun Newtonian and viscoelastic jets is presented. In particular, the effect of electrical conductivity and viscoelasticity on the jet profile during the initial stage of electrospinning is examined. In the theoretical study, the fluid is described as a leaky dielectric with charges only on the jet surface and viscoelastic models for polymer solutions such as Oldroyd-B and FENE-P are fully coupled with the fluid momentum equations and Gauss’ law. A theoretical model for the jet is derived using a thin filament approximation, and the resulting differential equations governing electrically charged, stable polymeric jets are solved numerically. Two different experimental systems are considered: Newtonian solutions of glycerol containing trace amounts of lithium chloride salt, and viscoelastic PIB/PB Boger fluid solutions. The experimental jet profiles from electrospinning experiments are compared with the model predictions. Our results reveal that incre...

Confined assembly of asymmetric block-copolymer nanofibers via multiaxial jet electrospinning.

Multiaxial (triaxial/coaxial) electrospinning is utilized to fabricate block copolymer (poly(styrene-b-isoprene), PS-b-PI) nanofibers covered with a silica shell. The thermally stable silica shell allows post-fabrication annealing of the fibers to obtain equilibrium self-assembly. For the case of coaxial nanofibers, block copolymers with different isoprene volume fractions are studied to understand the effect of physical confinement and interfacial interaction on self-assembled structures. Various confined assemblies such as co-existing cylinders and concentric lamellar rings are obtained with the styrene domain next to the silica shell. This confined assembly is then utilized as a template to guide the placement of functional nanoparticles such as magnetite selectively into the PI domain in self-assembled nanofibers. To further investigate the effect of interfacial interaction and frustration due to the physically confined environment, triaxial configuration is used where the middle layer of the self-assembling material is sandwiched between the innermost and outermost silica layers. The results reveal that confined block-copolymer assembly is significantly altered by the presence and interaction with both inner and outer silica layers. When nanoparticles are incorporated into PS-b-PI and placed as the middle layer, the PI phase with magnetite nanoparticles migrates next to the silica layers. The migration of the PI phase to the silica layers is also observed for the blend of PS and PS-b-PI as the middle layer. These materials not only provide a platform to further study the effect of confinement and wall interactions on self-assembly but can also help develop an approach to fabricate multilayered, multistructured nanofibers for high-end applications such as drug delivery.

A purely elastic instability in Dean and Taylor–Dean flow

The linear stability of the viscoelastic flow of an Oldroyd‐B fluid between rotating cylinders with an applied, azimuthal pressure gradient is considered. It is found that this Taylor–Dean flow is unstable in certain flow parameter regimes even in the limit of vanishingly small Reynolds number. The critical conditions and the structure of the vortex flow at the onset of instability are presented. These are determined in the limit as the channel width to radius of curvature becomes small. The present results reveal that the instability is a stationary mode when the pressure gradient becomes the dominant flow driving force, while it is an oscillatory instability when the shearing by the cylinder rotation is dominant. In addition, it is found that the direction of the pressure gradient controls the characteristics of the instability: A pressure gradient applied along the cylinder rotation destabilizes the flow, while if applied against the rotation, the flow is substantially stabilized. The mechanism of thes...

Controlling nanoparticle location via confined assembly in electrospun block copolymer nanofibers.

Coaxial nanofibers with poly(styrene-block-isoprene) (PS-b-PI)/magnetite nanoparticles as core and silica as shell are fabricated using electrospinning.1-4 Thermally stable silica helps to anneal the fibers above the glass transition temperature of PS-b-PI and form ordered nanocomposite morphologies. Monodisperse magnetite nanoparticles (NPs; 4 nm) are synthesized and surface coated with oleic acid to provide marginal selectivity towards an isoprene domain. When 4 wt% nanoparticles are added to symmetric PS-b-PI, transmission electron microscopy (TEM) images of microtomed electrospun fibers reveal that NPs are uniformly dispersed only in the PI domain, and that the confined lamellar assembly in the form of alternate concentric rings of PS and PI is preserved. For 10 wt% NPs, a morphology transition is seen from concentric rings to a co-continuous phase with NPs again uniformly dispersed in the PI domains. No aggregates or loss of PI selectivity is found in spite of interparticle attraction. Magnetic properties are measured using a superconducting quantum interference device (SQUID) magnetometer and all nanocomposite fiber samples exhibit superparamagnetic behavior.

Observations of purely elastic instabilities in the Taylor–Dean flow of a Boger fluid

An experimental and theoretical investigation of the stability of the viscoelastic flow of a model Boger fluid between rotating cylinders with an applied pressure gradient is presented. In our theoretical study, a linear stability analysis based on the Oldroyd-B fluid model which predicts the critical conditions and the structure of the vortex flow at the onset of instability is developed. Our results reveal that certain non-axisymmetric modes are more unstable than the previously studied axisymmetric mode when the shearing by the cylinder rotation is the dominant flow-driving force. This is consistent with recent results presented by Beris & Avgousti on the stability of elastic Taylor-Couette flow

Highly parallel time integration of viscoelastic flows

Abstract A highly parallel time integration method is presented for calculating viscoelastic flows with the DEVSS-G/DG finite element discretization. The method is a synthesis of an operator splitting time integration method that decouples the calculation of the polymeric stress by solution of a hyperbolic constitutive equation from the evolution of the velocity and pressure fields by solution of a generalized Stokes problem. Both steps are performed in parallel. The discontinuous finite element discretization of the hyperbolic constitutive equation leads to highly-parallel element-by-element calculation of the stress at each time step. The Stokes-like problem is solved by using the BiCGStab Krylov iterative method implemented with the block complement and additive levels method (BCALM) preconditioner. The solution method is demonstrated for the calculation of two-dimensional (2D) flow of an Oldroyd-B fluid around an isolated cylinder confined between two parallel plates. These calculations use extremely fine finite elements and expose new features of the solution structure.

Effect of shear on nanoparticle dispersion in polymer melts: A coarse-grained molecular dynamics study

Coarse-grained, molecular dynamics (MD) simulations have been conducted to study the effect of shear flow on polymer nanocomposite systems. In particular, the interactions between different components have been tuned such that the nanoparticle-nanoparticle attraction is stronger than nanoparticle-polymer interaction, and therefore, the final equilibrium state for such systems is one with clustered nanoparticles. In the current study, we focus on how shear flow affects the kinetics of particle aggregation at the very initial stages in systems with polymers of different chain lengths. The particle volume fraction and size are kept fixed at 0.1 and 1.7 MD units, respectively. Through this work, shear has been shown to significantly slow down nanoparticle aggregation, an effect that was found to be a strong function of both polymer chain length and shear rate. To understand our findings, a systematic study on effect of shear on particle diffusion and an analysis of relative time scales of different mechanisms causing particle aggregation have been conducted. The aggregation rate obtained from the time scale analysis is in good agreement with that determined from the aggregation time derived from the pair correlation function monitored during simulations.

Two-dimensional numerical analysis of non-isothermal melt spinning with and without phase transition

A model and simulation method are developed for two-dimensional non-isothermal melt spinning of a visco elastic melt. The visco elastic stress is evaluated from a non-isothermal Giesekus constitutive equation developed by application of the pseudo-time method to the isothermal form of the model [J. Non-Newt. Fluid Mech. (2001)]. The crystallization kinetics is described with the model proposed by Nakamura et al. [J. Appl. Polym. Sci. 17 (1973) 1031], whereas the crystallization rate, which is a function of both temperature and molecular orientation, is evaluated according to the equation proposed by Ziabicki [Fundamentals of Fiber Formation, Wiley, New York, 1976]. The set of non-linear governing equations is solved by using the DEVSS-G/SUPG finite element method. Melt spinning is simulated for two different polymers: amorphous polystyrene and fast-crystallizing Nylon-6,6. The analysis demonstrates that although the kinematics in the thread-line are approximately one-dimensional, the radially non-uniform thermal history, caused by the leading order variation of the temperature gradient ∂T/∂r, gives rise to radially non-uniform visco elastic stresses. This stress gradient results in radially non-uniform molecular orientation and a strong radial variation in crystallinity for Nylon-6,6. The radially non-uniform stress profiles obtained from the simulations are in good agreement with experimental results for melt spinning of polystyrene. Simulations of Nylon-6,6 show that the thermally-induced crystallization depends strongly on the choice of the Avrami index n, and a sharp increase in crystallinity due to stress-induced crystallization is predicted only when the molecules are highly oriented in the drawing direction at high drawing speeds. The significant influences of visco elasticity, air drag, and operating conditions on non-isothermal melt spinning dynamics also are predicted.

The effects of inertia on the viscoelastic Dean and Taylor–Couette flow instabilities with application to coating flows

The effects of inertia on the elastic instabilities in Dean and Taylor–Couette flows are investigated through a linear stability analysis. The critical conditions and the structure of the vortex flow at the onset of these instabilities are presented. The results reveal that the purely elastic Dean flow is destabilized by inertial effects. It is also found that inertia destabilizes elastic Taylor–Couette flow if the rotation of the inner cylinder is the flow driving force, while it stabilizes the flow driven by rotation of the outer cylinder. The mechanism of destabilization or stabilization of these viscoelastic instabilities is investigated through an examination of the disturbance‐energy equation. It is shown that Dean flow is destabilized by two separate mechanisms: a purely elastic mechanism discussed previously (i.e., energy production due to the coupling of a perturbation velocity to the polymeric stress gradient in the base state) [see Phys. Fluids A 3, 1691 (1991)] and a purely inertial mechanism ...