Shiojenn Tseng

Effects of chemical structure changes on thermal, mechanical, and crystalline properties of rigid rod epoxy resins

Effects of chemical structure changes on the thermal, mechanical, and crystalline properties of rigid rod epoxy resins have been studied for azomethine epoxy, biphenol epoxy, and tetramethyl biphenol epoxy. Rigid rod epoxies have exhibited better properties than those of the flexible bisphenol A epoxy. The chemical structures of both rigid rod epoxy and curing agent control the properties of cured rigid rod epoxies. When a flexible curing agent (methyl cyclohexane 1,2-dicarboxylic anhydride) was used, the chemical structure of rigid rod epoxy has dominated effects on the properties. Thus, the azomethine epoxy has shown the best thermal and mechanical properties among three rigid rod epoxies. While a rigid curing agent (sulfanilamide) was used, the physical properties of cured epoxies are not only dependent on the chemical structures of epoxies but also on the ease of formation of ordered network. Among the cured rigid rod epoxies, only the biphenol epoxy cured by sulfanilamide exhibits a liquid crystalline network. It has the highest glass transition temperature (219°C) and the lowest coefficient of thermal expansion (20.8 μm/m°C). However, the most thermal stable system is azomethine epoxy cured with sulfanilamide. It has a weight loss (39%) at 450°C. Their excellent thermal and mechanical properties of rigid rod epoxies are useful in composites, printed wiring boards, integrated circuit encapsulations, etc.

Influence of metal oxide nanoparticles concentration on their zeta potential.

In an attempt to estimate the zeta potential of various metal oxide nanoparticles (NPs) dispersed in water, it is interesting to observe that both the magnitude and the sign of this property depend highly upon their concentration. For example, in the case of naked TiO2 at pH 6, the zeta potential increased from -6.7 to 8.2 mV as the particle concentration varied from 0.5 to 5 mg L(-1). As a result, the isoelectric points of naked TiO2, Fe3O4, Fe(OH)3, and Al2O3-coated TiO2 could deviate ca. one, one, two, and three pH units, respectively, depending upon the particle concentration. We showed that these behaviors arise mainly from that the dissolved ambient CO2 reacts with the particle surface functional groups to form M-OCO2(-), which neutralizes or even overcompensates the particle surface charge. The surface density of M-OCO2(-), [M-OCO2(-)](s) depends upon the particle concentration; if it is sufficiently high, [M-OCO2(-)](s) becomes negligible, so is its influence on the zeta potential. We concluded that the zeta potential measurements for the tested NPs are reliable only if their concentration exceeds a certain level. This also applies to other metal oxides or hydroxides, the surface of which reacts appreciably with dissolved CO2. The results gathered are of practical significance in estimating the surface properties of unknown and/or newly synthesized NPs since conventional measurements are usually made at dilute particle concentrations.

Diffusiophoresis of a Soft Spherical Particle in a Spherical Cavity

The boundary effect on the diffusiophoresis of a colloidal particle is investigated theoretically by considering a soft spherical particle at an arbitrary position in a spherical cavity. The particle, which comprises a rigid core and an ion-penetrable layer, simulates biocolloids and particles covered by an artificial membrane layer. The diffusiophoretic behavior of the particle is governed by two types of DLP, the electrophoresis arising from the difference in the diffusivities of ionic species and diffusioosmotic flow. The influences of the thickness of the double layer, the size of the cavity, and the nature and the position of the particle on its diffusiophoretic behavior are discussed. We show that the presence of a boundary can have a profound influence on the behavior of a particle. The effect of electrophoresis can also lead to interesting diffusiophoretic behavior.

Regulating Current Rectification and Nanoparticle Transport Through a Salt Gradient in Bipolar Nanopores

Tuning of ion and nanoparticle transport is validated through applying a salt gradient in two types of nanopores: the inner wall of a nanopore has bipolar charges and its outer wall neutral (type I), and both the inner and outer walls of a nanopore have bipolar charges (type II). The ion current rectification (ICR) behavior of these nanopores can be regulated by an applied salt gradient: if it is small, the degree of ICR in type II nanopore is more significant than that in type I nanopore; a reversed trend is observed at a sufficiently large salt gradient. If the applied salt gradient and electric field have the same direction, type I nanopore exhibits two significant features that are not observed in type II nanopore: (i) a cation-rich concentration polarization field and an enhanced funneling electric field are present near the cathode side of the nanopore, and (ii) the magnitude of the axial electric field inside the nanopore is reduced. These features imply that applying a salt gradient to type I nanopore is capable of simultaneously enhancing the nanoparticle capture into the nanopore and reducing its translocation velocity inside, so that high sensing performance and resolution can be achieved.

Importance of temperature effect on the electrophoretic behavior of charge-regulated particles.

The Joule heating effect is inevitable in electrophoresis operations. To assess its influence on the performance of electrophoresis, we consider the case of a charge-regulated particle in a solution containing multiple ionic species at temperatures ranging from 298 to 308 K. Using an aqueous SiO(2) dispersion as an example, we show that an increase in the temperature leads to a decrease in both the dielectric constant and the viscosity of the liquid phase, and an increase in both the diffusivity of ions and the particle surface potential. For a particle having a constant surface potential, its electrophoretic mobility is most influenced by the variation in the liquid viscosity as the temperature varies, but for a charged-regulated particle both the liquid viscosity and the surface potential can play an important role. Depending upon the level of pH, the degree of increase in the mobility can be on the order of 40% for a 5 K increase in the temperature. The presence of double-layer polarization, which is significant when the surface potential is sufficiently high, has the effect of inhibiting that increase in the mobility. This implies that the influence of the temperature on the mobility of the particle is most significant when the pH is close to the point of zero charge.

Effect of electroosmotic flow on the electrophoresis of a membrane-coated sphere along the axis of a cylindrical pore.

The electrophoresis of a membrane-coated spherical particle along the axis of a long, charged cylindrical pore was analyzed theoretically under the conditions of low surface potential and weak applied electrical field. The influences of the thickness of the double layer, the pore radius, the nature of the membrane layer, and the charged conditions of the particle and the pore on the electrophoretic behavior of the particle were investigated. The occurrence of the electroosmotic flow and the presence of the membrane layer were found to have a significant influence on the mobility of the particle. For instance, because the direction of electrophoresis does not depend solely on the applied electrical field and the nature of the charge carried by the particle, care must be taken in interpreting electrophoresis data. Furthermore, the magnitude of the electrophoretic mobility of a particle can show a local maximum as the concentration of electrolytes varies, which is of practical significance if electrophoresis is employed, for example, as a separation tool.

Ionic Current Rectification in a pH-Tunable Polyelectrolyte Brushes Functionalized Conical Nanopore: Effect of Salt Gradient.

The behavior of ionic current rectification (ICR) in a conical nanopore with its surface modified by pH-tunable polyelectrolyte (PE) brushes connecting two large reservoirs subject to an applied electric field and a salt gradient is investigated. Parameters including the solution pH, types of ionic species, strength of applied salt gradient, and applied potential bias are examined for their influences on the ionic current and rectification factor, and the mechanisms involved are investigated comprehensively. The ICR behavior depends highly on the charged conditions of the PE layer, the level of pH, the geometry of nanopore, and the thickness of the double layer. In particular, the distribution of ionic species and the local electric field near the nanopore openings play a key role, yielding profound and interesting results that are informative to device design as well as experimental data interpretation.

Influence of boundary on the effect of double-layer polarization and the electrophoretic behavior of soft biocolloids

The electrophoresis of a soft particle comprising a rigid core and a charged porous membrane layer in a narrow space is modeled. This simulates, for example, the capillary electrophoresis of biocolloids such as cells and microorganisms, and biosensor types of device. We show that, in addition to the boundary effect, the effects of double-layer polarization (DLP) and the electroosmotic retardation flow can be significant, yielding interesting electrophoretic behaviors. For example, if the friction coefficient of the membrane layer and/or the boundary is large, then the DLP effect can be offset by the electroosmotic retardation flow, making the particle mobility to decrease with increasing double layer thickness, which is qualitatively consistent with many experimental observations in the literature, but has not been explained clearly in previous analyses. In addition, depending upon the thickness of double layer, the friction of the membrane layer of a particle can either retard or accelerate its movement, an interesting result which has not been reported previously. This work is the first attempt to show solid evidence for the influence of a boundary on the effect of DLP and the electrophoretic behavior of soft particles. The model proposed is verified by the experimental data in the literature. The results of numerical simulation provide valuable information for the design of bio-analytical apparatus such as nanopore-based sensing applications and for the interpretation of relevant experimental data.

Electrophoresis of a Charge-Regulated Soft Sphere in a Charged Cylindrical Pore

The boundary effect on the electrophoresis of a soft spherical particle, which comprises a rigid core and a porous layer, along the axis of a cylindrical pore is analyzed under the conditions of low surface potential and weak applied electric field. The porous layer of the particle is of charge-regulated nature where the distribution of fixed charge depends on the degree of dissociation/association reaction of the ionizable function groups contained in the porous layer. The pore might be charged, implying that the effect of electroosmotic flow can play a role. The influences of the nature of the porous layer of a particle, the thickness of the double layer, the pH of the bulk liquid, and the relative size of a pore on the electrophoretic behavior of the particle are investigated. Several unexpected and interesting results are obtained. For instance, if the pH value of the bulk liquid or the thickness of the membrane layer takes a medium large value, the electrophoretic mobility of the particle has a local maximum as the thickness of the double layer varies.

A comparison of the parameter estimating procedures for the Michaelis-Menten model

The performance of four parameter estimating procedures for the estimation of the adjustable parameters in the Michaelis-Menten model, the maximum initial rate Vmax, and the Michaelis-Menten constant Km, including Lineweaver & Burk transformation (L-B), Eadie & Hofstee transformation (E-H), Eisenthal & Cornish-Bowden transformation (ECB), and Hsu & Tseng random search (H-T) is compared. The analysis of the simulated data reveals the followings: (i) Vmax can be estimated more precisely than Km. (ii) The sum of square errors, from the smallest to the largest, follows the sequence H-T, E-H, ECB, L-B. (iii) Considering the sum of square errors, relative error, and computing time, the overall performance follows the sequence H-T, L-B, E-H, ECB, from the best to the worst. (iv) The performance of E-H and ECB are on the same level. (v) L-B and E-H are appropriate for pricesly measured data. H-T should be adopted for data whose error level are high. (vi) Increasing the number of data points has a positive effect on the performance of H-T, and a negative effect on the performance of L-B, E-H, and ECB.