Dilhan M. Kalyon

Functionally graded electrospun polycaprolactone and β-tricalcium phosphate nanocomposites for tissue engineering applications

Fabricating functionally graded scaffolds from biodegradable polymers to enable the mimicking of native tissue is an important challenge. Here we demonstrate the fabrication and utilization of functionally graded non-woven meshes of polycaprolactone incorporated with tricalcium phosphate nanoparticles using a new hybrid twin-screw extrusion/electrospinning (TSEE) process, which allows the time-dependent feeding of various solid and liquid ingredients and their melting, dispersion, deaeration and pressurization together with electrospinning within the confines of a single process. Using this hybrid method, the concentration of tricalcium phosphate nanoparticles could be tailored to vary in a targeted/controlled manner between the two surfaces of the scaffold mesh. The graded scaffolds were seeded and cultured with mouse preosteoblast cells (MC3T3-E1). Within 4 weeks, the tissue constructs revealed the formation of continuous gradations in extracellular matrix with various markers including collagen synthesis and mineralization, akin to the type of variations observed in the typical bone-cartilage interface in terms of the distributions of concentration of Ca particles and of mechanical properties associated with this. The demonstrated hybrid method should allow much better control of the distributions of various ingredients, including the concentrations of drugs/growth factors, as well as the porosity, mechanical property, wettability, biodegradation rate distributions in tissue engineering scaffolds, aiming to mimic the elegant complex distributions found in native tissue.

Slip Effects in Capillary and Parallel Disk Torsional Flows of Highly Filled Suspensions

The shear viscosity material function of a highly filled suspension consisting of a Newtonian poly(butadiene acrylonitrile acrylic acid terpolymer) matrix, PBAN, mixed with an ammonium sulfate filler at 60% by volume was studied. Both capillary and parallel disk torsional flows were employed. The rheological characterization revealed strong slip of the suspension at the walls over a broad range of shear stresses in both types of flows. The slip velocity increased approximately linearly with the shear stress. In capillary flows, above a critical shear stress, flow took place in a pluglike manner, owing to slip at the wall. The experimental findings were further elucidated to determine the slip layer thickness and the apparent shear viscosity behavior of highly filled suspensions at high shear stress at the wall values. It was concluded that the slip effects dominate the flow of highly filled suspensions and the true flow and deformation characteristics of the highly filled suspensions may be overshadowed b...

Rheological behavior of a concentrated suspension: A solid rocket fuel simulant

The rheological behavior of a very concentrated suspension (76.5 vol %), which serves as a widely used solid rocket fuel simulant, was characterized employing both torsional and capillary flows. No comprehensive studies of the rheology of concentrated suspensions have been carried out previously at such a high solids content. The suspension exhibited shear thinning over the apparent shear rate range of 30–3000 s−1. Significant slip at the wall was observed in both torsional and capillary flows with the slip velocity increasing from about 0.001 mm/s at a shear stress of 4 Pa to as high as 60 mm/s at 100 kPa. A flow visualization technique was applied for the first time to determine the wall slip velocities in torsional flow directly, to also provide the true deformation rate and feedback on yielding. The contribution of the slip of the suspension at the wall to the volumetric flow rate in capillary flow was found to increase with decreasing shear stress, giving rise to plug flow at sufficiently low shear s...

Apparent slip and viscoplasticity of concentrated suspensions

The apparent slip flows of incompressible and viscoplastic (Herschel–Bulkley) fluids in plane Couette, capillary, and rectangular slit dies under fully developed, isothermal, and creeping flow conditions were analyzed assuming that the apparent slip layer consists solely of the binder and its thickness is independent of the flow rate. Both the drag-induced (plane Couette) and pressure-induced (capillary and slit) flows generate the same dependencies of the wall-slip velocity on the wall shear stress. Navier’s slip coefficient, which relates the wall-slip velocity to the shear stress, is similar for all three flows and is a function of the thickness of the apparent slip layer and the shear viscosity of the binder. The assumed apparent slip mechanism provides methodologies for the determination of the slip velocity values that are consistent with the traditional Mooney method and furthermore allows the determination of the true shear rate of the suspension at the wall and the yield stress. The analysis of t...

Effects of temperature and surface roughness on time‐dependent development of wall slip in steady torsional flow of concentrated suspensions

A flow visualization technique was applied to investigate the time and temperature‐dependent development of wall slip and the rheological behavior of a concentrated suspension, containing 63% by volume solid glass spheres and a poly (butadiene‐acrylonitrile‐acrylic acid) terpolymer matrix, using steady torsional flow. Flow visualization allowed the concomitant determination of the wall slip velocity and the shear viscosity of the concentrated suspension. The deformation rate, shear stress, and the wall slip velocity values during torsional flow were time dependent and asymptotically reached steady‐state values. The characteristic time necessary to reach steady state decreased with increasing shear rate and temperature. Increasing temperature also increased the wall slip velocity. The flow visualization technique was further utilized to determine the yield stress of the suspension directly, which was found to decrease with increasing temperature. Increased surface roughness prevented the wall slip of the c...

Viscoelastic material functions of noncolloidal suspensions with spherical particles

We have characterized various steady and time-dependent material functions of suspensions of a non-Newtonian binder, poly(dimethyl siloxane), incorporated with 10%–60% by volume of hollow and spherical glass beads. The material functions included storage and the loss moduli, shear stress and first normal stress difference growth and relaxation, relaxation modulus upon step strain and creep and recovery behavior. Both constant shear stress and shear rate experiments were carried out using multiple rheometers over a broad temperature range (−35 to 40 °C) while following sample fracture and wall slip effects. With increasing volume fraction, φ, of the noncolloidal particles, the strain range, over which linear viscoelastic behavior is observed, became narrower and the relaxation time of the suspension increased. Increasing solid content gave rise to the development of the yield stress and the dependence of large amplitude oscillatory shear properties on time and deformation history. The yield stress values i...

Flow instabilities in capillary flow of concentrated suspensions

The development of flow instabilities during the capillary flow of two concentrated suspensions filled with 76.5 and 65.6% by volume solids was investigated. The flow instabilities manifested themselves by the development of concentration gradients as a result of the filtering of the binder, superimposed on the bulk motion of the suspension. The effects of apparent shear rate, capillary diameter and the surface roughness of the particles were investigated. The use of the comparison of the filtration rate with the bulk velocity of the suspension during flow is shown to be promising for the prediction of the apparent shear rate at which filtration-based flow instabilities occur.

Membranes of polyvinylidene fluoride and PVDF nanocomposites with carbon nanotubes via immersion precipitation

Microporous polyvinylidene fluoride (PVDF) and PVDF nanocomposite membranes were prepared via an isothermal immersion precipitation method using two different antisolvents (ethanol and water). The structure and morphology of the resulting membranes were investigated by wide angle X-ray diffraction (WAXD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). The effects of the type of the antisolvent and the presence of multiwalled carbon nanotubes (MWNTs) on membrane morphology and the crystal structure developed within the membranes were studied. The crystallization of the PVDF upon immersion precipitation occurred predominantly in the α-phase when water is used as the antisolvent or in the absence of the carbon nanotubes. On the other hand, β-phase crystallization of the PVDF was promoted upon the use of ethanol as the antisolvent in conjunction with the incorporation of the MWNTs. The morphology and the total crystallinity of the PVDF membranes were also affected by the incorporation of the MWNTs and the antisolvent used, suggesting that the microstructure and the ultimate properties of the PVDF membranes can be engineered upon the judicious selection of crystallization conditions and the use of carbon nanotubes.

Percolation in magnetic composites

Electric and magnetic properties of composite materials consisting of low density polyethylene filled with powdered ferromagnetic materials were investigated. The volume fractions of the fillers were varied from 10% up to the theoretical maximum packing fractions, i.e. between 0.70 and 0.77, so that the percolation phenomenon could be investigated. The ferromagnetic fillers used were HyMu 800 (a nickel-iron-molybdenum alloy), MnZn ferrite and NiZn ferrite. The particle sizes and size distributions of the fillers were well characterized by image analysis techniques. Based on the particle size distribution the maximum loading levels of fillers as permitted by geometric considerations were calculated. The properties of the composites characterized included: volume and surface resistivities, dielectric constants, electrical loss factors and magnetic permeabilities.

Wall slip and extrudate distortion of three polymer melts

A high-density polyethylene (HDPE), a poly(dimethylsiloxane) (PDMS), and an oxetane based alternating block thermoplastic elastomer (TPE) were subjected to steady torsional and capillary flows. HDPE and PDMS exhibited wall slip in steady torsional flow. The shear stress ranges at which wall slip became apparent during steady torsional flow for HDPE and PDMS coincided with the wall shear stress ranges in capillary flow at which distortions of the extrudates emerging from capillary die were first noted. In contrast, TPE did not exhibit wall slip in steady torsional flow (for shear rates up to 200 s−1 and strains up to 25). None of the three polymers exhibited any overshoots of shear stress or first normal stress difference under steady flow conditions, indicating that the often-reported stress overshoots could be artifacts of wall slip. Furthermore, the extrudates of the TPE were relatively smooth and exhibited only minor and nonperiodic surface blemishes. The absence of extrudate distortions under the pres...