Anuvat Sirivat

The effect of a passive cross-stream temperature gradient on the evolution of temperature variance and heat flux in grid turbulence

The evolution of temperature variance and heat flux in decaying grid turbulence with a linear cross-stream temperature gradient is studied by producing the temperature gradient by means of two different methods: ( a ) by placing a ‘mandoline’ (Warhaft & Lumley 1978) downstream from the grid but with its wires differentially heated for the present study, and ( b ) by differentially heating ribbons of nichrome (a ‘toaster’) placed in the plenum chamber of the wind tunnel. For the former method the initial thermal/mechanical lengthscale ratio L θ / L was varied by changing the mandoline configuration. For this method it is shown that the gradient causes L θ / L to equilibrate to a value of about 0·9 regardless of its initial value, and that when this value is achieved the temperature variance increases approximately linearly with time. The toaster was used to produce a temperature gradient without the associated initial temperature variance (and initial thermal lengthscale) that is necessarily produced by the mandoline wires; for the toaster the temperature variance was produced solely by the action of turbulence against the temperature gradient. For this experiment too, the thermal variance grew linearly with time, and L θ / L was approximately the same as the equilibrium value for the mandoline experiments. The equilibrium value of the ratio of temperature-variance production to temperature-variance dissipation was approximately 1·5 for all of the experiments. The ratio of the mechanical-dissipation/thermal-dissipation timescales was also found to equilibrate, but there was considerably more scatter in the data for this parameter. The values of the equilibrium length- and timescale ratios were not affected by the magnitude of the temperature gradient, which was varied for both experiments. Good transverse homogeneity in the thermal field was achieved in all cases, in contrast with previous experiments (using heated grids).

Preparation, structure, properties and thermal behavior of rigid-rod polyimide/montmorillonite nanocomposites

Polyimide/montmorillonite nanocomposites were prepared from solution of poly(amic acid) precursors and the dodecyl-montmorillonite (DMONT) using N-methyl-2-pyrrolidone as a solvent. The cured films of the rigid-rod polyimide/DMONT hybrids as characterized by FTIR, TEM and WAXD were exfoliated nanocomposites at low DMONT content (<1 wt.%) and partially exfoliated nanocomposites at high DMONT content (containing aggregates of DMONT). The nanocomposite showed optimum properties at 3 wt.% DMONT with low water absorption, improved mechanical strength and modulus, and high dielectric strength. The clay content significantly influences thermal behavior of the nanocomposite films, such as thermal expansion coefficient (CTE), glass transition and yielding temperatures of the rigid-rod and flexible polyimide nanocomposites. It was found that the rigid polyimide matrix gave a superior thermal behavior; i.e. lower CTE, higher glass transition and yielding temperatures than the flexible ones. Effect of DMONT content was significant at high temperature operation where a side reaction of dodecylamine and a poly(amic acid) should be aware to cause a reduction in degree of imidisation and glass transition temperature and the formation of silicate layer aggregates. High DMONT content nanocomposites (aggregates were formed) showed high yielding temperatures indicating the suppression of polymer flow by the rigidity of the clay. The flexible polyimide nanocomposites showed higher thermal deformation than the rigid ones. Moreover, the rigid polyimide nanocomposite with 9 wt.% DMONT showed a thermally reversible behavior indicating that the silicate layers efficiently suppressed the deformation of the rigid polyimide.

Electrically controlled release of sulfosalicylic acid from crosslinked poly(vinyl alcohol) hydrogel

Electrically controlled drug delivery using poly(vinyl alcohol) (PVA) hydrogels as the matrix/carriers for a model drug was investigated. The drug-loaded PVA hydrogels were prepared by solution-casting using sulfosalicylic acid as the model drug and glutaraldehyde as the crosslinking agent. The average molecular weight between crosslinks, the crosslinking density, and the mesh size of the PVA hydrogels were determined from the equilibrium swelling theory as developed by Peppas and Merril, and the latter data were compared with those obtained from scanning electron microscopy. The release mechanisms and the diffusion coefficients of the hydrogels were studied using modified Franz-Diffusion cells in an acetate buffer with pH 5.5 and temperature 37 degrees C during a period of 48 h, in order to determine the effects of crosslinking ratio, electric field strength, and electrode polarity. The amounts of drug released were analyzed by UV-vis spectrophotometry. The amounts of drug released vary linearly with square root of time. The diffusion coefficients of drug-loaded PVA hydrogels decrease with increasing crosslink ratio. Moreover, the diffusion coefficients of the charged drug in the PVA hydrogels depend critically on the electric field strength between 0 and 5 V as well as on the electrode polarity. Thus, the release rate of sulfosalicylic acid can be altered and controlled precisely through electric field stimulation.

Surface degradation of α-naphthalene sulfonate-doped polypyrrole during XPS characterization

The surface compositions of non-aging chemically synthesized polypyrrole samples doped with a-naphthalene sulfonate (PPy/ a-NS � ) were investigated by X-ray photoelectron spectroscopy (XPS). The power of the X-ray source used was 300 W. Employing liquid nitrogen cooling of the sample holder, the spectra in the regions of C 1s, O 1s, N 1s and S 2p were found to be reproducible with a gradual change in S 2p spectrum when the accumulated X-ray exposure time was less than 1050 min. Beyond this accumulated X-ray exposure time, dramatic changes were observed in all spectral regions. The decreases in the amount of dopant and HSO4 � or SO4 2� co-dopant species correspond to the decrease in the amount of polaron charge carrier species of PPy (–NH � þ –). We found increases in the amount of the imine-like nitrogen (=N–) and hydrocarbon, and the presence of new sulfurcontaining species. These results suggested the deprotonation of the polaron by the dopant and the desulfonation of the protonated dopant. After desulfonation, these new sulfur-containing species were removed whereas the carbons of the naphthalene rings were still adhering to the surface of the PPy pellet. This degradation became more severe when the liquid nitrogen cooling system of the spectrometer was not used. The results indicate that the degradation was mainly induced by heat from the X-ray beam. # 2002 Elsevier Science B.V. All rights reserved.

Electrical conductivity response of polypyrrole to acetone vapor: effect of dopant anions and interaction mechanisms

Abstract The chemically prepared undoped polypyrrole (PPy) and the PPy doped with seven dopant anions, at a dopant to monomer molar ratio of 1/12, were used as chemical sensors to detect acetone vapor. The negative changes in specific conductivity, Δ σ , of PPys upon exposed to acetone vapor were observed. The magnitudes of Δ σ depend critically on the type of the dopants which induces different properties amongst doped PPys. For the sulfonate doped PPy, Δ σ depends exponentially on the doping level (N + /N) and on the initial specific conductivity. It also depends linearly on the proportions of the bipolaron species and of the order aggregation of PPy. However, it depends inversely on the proportion of the imine-like nitrogen defect (N). An environmental scanning electron microscope and an X-ray diffractometer, for instances, were used to investigate the interaction between PPy and acetone molecules. The swelling or the disordering effect, the hydrogen-bonding formation, and the reduction of charge carrier species are suggested to be the mechanisms causing the PPy conductivity changes when exposed to acetone vapor.

Electrical conductivity response of dodecylbenzene sulfonic acid-doped polypyrrole films to SO2-N2 mixtures

Abstract The effects of using dodecylbenzene sulfonic acid as a dopant on electrical conductivity of polypyrrole films were investigated in N 2 atmosphere and SO 2 –N 2 mixtures. For a film with a higher doping level, the time for the specific electrical conductivity σ dc to reach its equilibrium value when exposed to SO 2 was reduced; and consistently, the short time response (d σ dc /d t ) at zero exposure time increased. The temporal response of conductive polypyrrole films was improved with the films having higher doping levels. On the other hand, the gas sensitivity or change in the specific electrical conductivity from its base value when exposed to SO 2 was independent of dopant concentration, if it was below a critical doping level. Above this critical doping level, the change in the specific electrical conductivity climbed to a maximum value and then declined slightly as the doping level was increased. The dependence of the specific electrical conductivity on dopant concentration can be well correlated with changes in morphology observed. The conductive polymer chains changed from assuming three dimensional random-coil structures to rod-like fabrillar structures.

Electrically controlled release of salicylic acid from poly(p-phenylene vinylene)/polyacrylamide hydrogels.

The apparent diffusion coefficients, Dapp, and the release mechanisms of salicylic acid from salicylic acid-loaded polyacrylamide hydrogels, SA-loaded PAAM, and salicylic acid-doped poly(phenylene vinylene)/polyacrylamide hydrogels, SA-doped PPV/PAAM, were investigated. In the absence of an electric field, the diffusion of SA from the SA-doped PPV/PAAM is delayed in the first 3 h due to the ionic interaction between the anionic drug (SA anion) and the PPV. Beyond this period, SA is dissolved in and can diffuse into the buffer solution through the PAAM matrix. The Dapp of the SA-doped PPV/PAAM is higher than that of the SA-loaded PAAM, and the former increases with increasing electric field strength due to combined mechanisms: the expansion of PPV chains inside the hydrogel; the reduction reaction under a negative potential driving the anionic SA through the PAAM matrix; and the expansion of the matrix pore. The Dapp of SA from the SA-loaded PAAM and the SA-doped PPV/PAAM apparently obey the scaling behavior: Dapp/D0 = (drug size/pore size)m with the scaling exponent m equal to 0.50 at 0.1 V for both SA-loaded PAAM and SA-doped PPV/PAAM. Thus, the presence of the conductive polymer and the applied electric field can be combined to control the drug release rate at an optimal desired level.

Viscoelastic Properties of Carbopol 940 Gels and Their Relationships to Piroxicam Diffusion Coefficients in Gel Bases

PurposeThis study was conducted to determine the effect of formula compositions on viscoelastic properties of piroxicam gels using Carbopol 940 as a gelling agent and to determine the relationships between viscoelastic properties of Carbopol 940 gel bases and diffusion coefficients of piroxicam in gel bases.MethodsPiroxicam gels (1.0% w/w) were prepared by using Carbopol 940 as a gelling agent and varying Carbopol 940 concentrations, glycerin, and sodium chloride contents. The in vitro release of piroxicam from gel bases to the receiving media, isotonic phosphate buffer solution (pH 7.4), were carried out using Franz-modified cell. The piroxicam diffusion coefficients were obtained by Higuchis equation. Rheological property measurements of gel samples were performed via a cone and plate fluid rheometer. Relationships between viscoelastic properties of gel samples and piroxicam diffusion in gel bases were analyzed by Pearsons test at a p value of less than 0.05.ResultsAll piroxicam gels exhibited predominantly elastic solid behavior whose magnitude depended on Carbopol 940 concentration. Preparations containing good solvent exhibited more elastic solid characters. In contrast, the piroxicam gels containing higher sodium chloride contents possessed more viscous fluid behavior. Analyzed by Pearsons test at a p value of less than 0.05, piroxicam diffusion coefficients were directly proportional to loss tangent, but were inversely proportional to storage modulus, loss modulus, complex modulus, and viscosity.ConclusionsThere is a potential for predicting drug diffusion coefficients from their correlations to rheological parameters. This could be beneficial to the formulation design of transdermal drug delivery systems including mucoadhesive drug delivery systems.

Influence of weak elasticity of dispersed phase on droplet behavior in sheared polybutadiene/poly(dimethyl siloxane) blends

The contribution of weak droplet-phase elasticity is investigated for blends of polybutadiene in poly(dimethyl siloxane) in a simple shearing flow with droplet-phase Weissenberg number, Wid, up to around unity. The elasticity of the polybutadiene-dispersed phase is varied by adding various amounts of high-molecular-weight polybutadiene into low-molecular-weight polybutadiene Newtonian fluid. To isolate the contribution of elasticity, the experiments are conducted at fixed viscosity ratio by varying the experimental temperature to counteract the small effect of high-molecular-weight polymer on droplet viscosity. Droplet deformation and relaxation are measured using an optical flow cell mounted on an optical microscope. As the droplet-phase elasticity increases, the steady-state shape deformation at fixed capillary number, Ca, decreases and the critical capillary number for droplet breakup increases. For a 20% dispersed phase blend, the steady-state capillary number calculated from the volume-averaged dropl...

Controlled transdermal iontophoresis of sulfosalicylic acid from polypyrrole/poly(acrylic acid) hydrogel

A conductive polymer-hydrogel blend between sulfosalicylic acid-doped polypyrrole (PPy) and poly(acrylic acid) (PAA) was used as a carrier/matrix for the transdermal drug delivery under applied electrical field. PAA films and the blend films were prepared by solution casting with ethylene glycol dimethacrylate (EGDMA) as a cross-linking agent, followed by the blending of PPy particles and the PAA matrix. The effects of cross-linking ratio and electric field strength on the diffusion of the drug from PAA and PPy/PAA hydrogels were investigated using a modified Franz-diffusion cell with an acetate buffer of pH 5.5 and at 37 degrees C, for a period of 48h. The diffusion coefficient of the drug is calculated using the Higuchi equation, with and without an electric field, at various cross-linking ratios. The drug diffusion coefficient decreases with increasing drug size/mesh size ratio, irrespective of the presence of the conductive polymer as the drug carrier. The diffusion coefficient, at the applied electric field of 1.0V, becomes larger by an order of magnitude relative to those without the electric field.