Characterization and modeling of direct-write fabrication of microscale polymer fibers

A new direct-write system for fabricating suspended microscale and sub-microscale polymer fibers has been developed and characterized. This system is capable of generating arrays of precisely-positioned fibers with controllable diameters in three-dimensional space. The driving mechanism behind this process harnesses the surface tension of liquid bridges to promote the controlled thinning of a macroscale polymer solution filament into the desired micro- or sub-microscale fiber. The correlation between fiber diameter and several experimental parameters including solution concentration, drawing rate, and fiber length was characterized using a series of viscous poly(methyl methacrylate) (PMMA) solutions. A dimensional analysis of the physics of the fiber drawing process was used to adapt this data into an empirical relationship describing fiber formation from a generalized polymer solution. This information was subsequently utilized to predict fiber diameter from several other non-PMMA-based polymer solutions with accuracy comparable to the intrinsic variation of the process itself, thereby eliminating the need to perform lengthy characterizations on new polymer solutions.     

Effect of phosphate ion on the textural and catalytic activity of titania–silica mixed oxide

Phosphate-modified TiO₂-SiO₂ mixed oxide catalysts have been prepared by varying the method of preparation, source and concentration of phosphate ion. The prepared catalysts were compared for their catalytic activity/selectivity in nitration of toluene. The characterisation of the catalysts was performed using X-ray powder diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), thermal analysis (TG–DTA), nitrogen adsorption–desorption methods, surface acid strength measured by Hammett indicator method, surface acid sites measured by amine titration method, and phosphate content measured by UV–VIS spectrophotometry. The XRD patterns revealed that phosphate ion stabilises the anatase phase up to 1173 K activation. FT-IR results show that phosphate species strongly bound bidentately, and that both the internal weakly H-bonded hydroxyl groups and free hydroxyl groups are present on TiO₂–SiO₂ mixed oxide support. Surface area and surface acidity are found to increase with the increase in phosphate loading up to 7.5 wt.% and thereafter the values decrease drastically. However, average pore radius and total pore volume shows the reverse order. Phosphated samples prepared using H₃PO₄ as the source of phosphate ion exhibit higher acidity, and surface area but lower porosity than the samples prepared from (NH₄)₃PO₄, though both the samples contain the same amount of phosphate (7.5 wt.%). Similar results were also observed when varying the method of preparation. TiO₂–SiO₂ samples prepared at pH=3 exhibit higher acidity and surface area but lower porosity than the samples prepared at pH=7. The acid strength of 7.5P/TiO₂–SiO₂ (H) is found to be stronger than that of 100% concentrated H₂SO₄. The material modified with phosphate ion was found to be an efficient and selective catalyst for solvent-free mono-nitration of toluene. Selectivity to the para-product is correlated with the porosity of the material.     

Ionic liquid as catalytic and reusable media for cyanoethoxycarbonylation of aldehydes

Various ionic liquids (IL 1-9) based on N-methyl N′-alkyl imidazolium salts were explored as catalytic media in cyanoethoxycarbonylation of various aldehydes. The study revealed that the alkyl chain length and counter ion of the ionic liquid are critical for the product yield. The highest product yield of cyanohydrin carbonate (up to 96%) was obtained with C₅ alkyl chain with Br⁻ as counter ion (IL 3). On the other hand PF₆⁻ as counter ion failed to catalyze the cyanoethoxycarbonylation reaction.     

Study on degradation reactions in polyethylene terephthalate containing 5‐sulpho isophthalyl moieties

Kinetics of thermal degradation occurring on polyester containing cationic dyeable comonomer units viz. 5‐sulphoisophthalate moieties are studied by measurement of changes in intrinsic viscosity and carboxyl values on the polymer after subjecting the polymer chips to temperatures in the range 275–285°C for different residence times ranging from 5 to 60 min and comparing with the homopolymer. The activation energy values for degradation are estimated from the kinetic data. Mechanical properties of the textured yarns produced from the partially oriented yarns (POY) spun under different residence times are measured. Yarn produced with higher residence time has poor mechanical properties. The SEM images of the POY show presence of particles at the surface of the yarn due to polymer degradation. The size of the particles as seen on the surface increase with increase of retention time. Addition of thermal stabilizer helps in controlling the thermal degradation. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Lattice Boltzmann method for population balance equations with simultaneous growth,nucleation, aggregation and breakage

Lattice Boltzmann method (LBM) is developed for solution of one-dimensional population balance equations (PBEs) with simultaneous growth, nucleation, aggregation and breakage. Aggregation and breakage, which act as source terms in PBEs, are included as force terms in LBM formulation. The force terms representing aggregation and breakage are evaluated by fixed pivot (FP) method. Multiscale analysis is used to derive the kinetic equations associated with LBM, whose long-time large-scale solution provides the solution of the PBE. A coordinate transformation is proposed, which allows the use of non-uniform grid for LBM to obtain accurate solution of PBE with moderate number of grid points. The performance of the proposed LBM-FP method is compared with finite volume (FV) and method of characteristics (MOC) combined with FP (MOC-FP) methods. Using benchmark examples, the proposed LBM-FP method is shown to be useful for solving PBEs due to its computational efficiency and ability to handle a wide range of problems.     

Synthesis and characterization of uncapped gamma-Fe2O3 nanoparticles prepared by flame pyrolysis of ferrocene in ethanol

Nanoparticles of iron(III) oxide were synthesized by spontaneous combustion of ferrocene in ethanol solution using a simple spirit lamp. X-ray powder diffraction and electron diffraction analysis of the powder suggested the formation of gamma-Fe2O3 (Maghemite phase) having lattice constant 8.3539 +/- 0.0209 A. Transmission Electron Micrograph suggested the formation of spherical particles with an average diameter of 24.7 +/- 1.6 nm. A sextet with an isomeric shift of 0.328 mm s(-1) seen in the Mossbauer spectrum recorded at room temperature, further supports the formation of gamma-Fe2O3. The particles were dispersed freely in the polar solvents like ethanol, dimethyl sulfoxide, and water. Infra Red spectrum gave bands at 400, 432, 565, and 638 cm(-1), which confirms the presence of gamma-Fe2O3 phase.     

CNT loading into cationic cholesterol suspensions show improved DNA binding and serum stability and ability to internalize into cancer cells

Methods which disperse single-walled carbon nanotubes (SWNTs) in water as 'debundled', while maintaining their unique physical properties are highly useful. We present here a family of cationic cholesterol compounds (Chol+) {Cholest-5en-3β-oxyethyl pyridinium bromide (Chol-PB+), Cholest-5en-3β-oxyethyl N-methyl pyrrolidinium bromide (Chol-MPB+), Cholest-5en-3β-oxyethyl N-methyl morpholinium bromide (Chol-MMB+) and Cholest-5en-3β-oxyethyl diazabicyclo octanium bromide (Chol-DOB+)}. Each of these could be easily dispersed in water. The resulting cationic cholesterol (Chol+) suspensions solubilized single-walled carbon nanotubes (SWCNTs) by the non-specific physical adsorption of Chol+ to form stable, transparent, dark aqueous suspensions at room temperature. Electron microscopy reveals the existence of highly segregated CNTs in these samples. Zeta potential measurements showed an increase in potential of cationic cholesterol aggregates on addition of CNTs. The CNT-Chol+ suspensions were capable of forming stable complexes with genes (DNA) efficiently. The release of double-helical DNA from such CNT-Chol+ complexes could be induced upon the addition of anionic micellar solution of SDS. Furthermore, the CNT-based DNA complexes containing cationic cholesterol aggregates showed higher stability in fetal bovine serum media at physiological conditions. Confocal studies confirm that CNT-Chol+ formulations adhere to HeLa cell surfaces and get internalized more efficiently than the cationic cholesterol suspensions alone (devoid of any CNTs). These cationic cholesterol-CNT suspensions therefore appear to be a promising system for further use in biological applications.     

Optimization of the polycondensation stage of poly(ethylene terephthalate) reactors

A general kinetic scheme for the polycondensation step of the PET formation has been used to establish the mole balance equations of various functional groups in batch reactors. An objective function has been defined which aims to attain a desired degree of polymerization in the shortest time, has a specified level of diethylene glycol group content, and minimizes the other side products. Using the control vector iteration procedure, an optimum temperature profile has been calculated. The computations suggest that a high temperature should be used initially which must be lowered as the time of reaction increases for limiting the formation of side products.     

Inverse dynamics of articulated flexible structures: Simultaneous trajectory tracking and vibration reduction

This paper addresses the problem of inverse dynamics for articulated flexible structures with both lumped and distributed actuators. This problem arises, for example, in the combined vibration minimization and trajectory control of space robots and structures. For such flexible structures, closed loop passive joint based controllers have been shown to be effective in trajectory control by Paden et al. Crucial to the development of such closed loop controllers, which are robust to external perturbations, is the problem of dynamic inversion which yields the nominal state trajectories and the feedforward inputs. In this paper we propose a new inverse dynamics scheme for computing the nominal lumped and distributed feedforward inputs for tracking a prescribed trajectory.     

Condensation polymerisations with unequal reactivity in segregated continuous-flow stirred tank reactors

A parametric study of the molecular weight distribution and its moments has been made on the irreversible condensation polymerisation of ARB type monomers which do not obey the equal reactivity hypothesis. The reactions are simulated for a continuous-flow stirred-tank reactor with distributed residence times, the mean residence time being the operating variable. Two different kinetic schemes have been proposed which represent the limiting cases of the actual situations (models I and II) which are usually encountered and the effect of segration in a CSTR has been studied. The molecular weight distributions, number average chain lengths and the polydispersity indices have been computed. Results are found to lie intermediate between batch reactors and homogeneous, continuous-flow stirred tank reactors. Plots between the poly dispersity index and the monomer conversion with the reactivity ratio, R, and the number average chain length as parameters indicate that results are far more sensitive to variations in R for model II than for model I.