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Giridhar Madras

Publications and source records attributed to Giridhar Madras.

16 recordsLinked to original sources
PubMed2006

A distribution kinetics approach for crystallization of polymer blends.

The cluster distribution approach is extended to investigate the crystallization kinetics of miscible polymer blends. Mixture effects of polymer-polymer interactions are incorporated into the diffusion coefficient. The melting temperature, activation energy of diffusion, and phase transition enthalpy also depend on the blending fraction and lead to characteristic kinetic behavior of crystallization. The influence of different blending fractions is presented through the time dependence of polymer concentration, number and size of crystals, and crystallinity (in Avrami plots). Computational results indicate how overall crystallization kinetics can be expressed approximately by the Avrami equation. The nucleation rate decreases as the blending fraction of the second polymer component increases. The investigation suggests that blending influences crystal growth rate mainly through the deposition-rate driving force and growth-rate coefficient. The model is further validated by simulating the experimental data for the crystallization of a blend of poly(vinylidenefluoride)[PVDF] and poly(vinyl acetate)[PVAc] at various blending fractions.

Journal Article↗
PubMed2006

Novel photocatalysts for the decomposition of organic dyes based on metal-organic framework compounds.

Three novel metal-organic frameworks (MOFs) [Co(2)(C(10)H(8)N(2))][C(12)H(8)O(COO)(2)](2), 1, [Ni(2)(C(10)H(8)N(2))(2)][C(12)H(8)O(COO)(2)](2).H(2)O, 2, and [Zn(2)(C(10)H(8)N(2))][C(12)H(8)O(COO)(2)](2), 3, with three-dimensional structures have been synthesized and characterized. The structures of the three compounds appear somewhat related, formed by the connectivity involving the metal polyhedra (Co(4)N trigonal bipyramids in 1, NiO(4)N(2) octahedra in 2, and ZnO(4) tetrahedra and ZnO(3)N(2) trigonal bipyramids in 3), 4,4'-oxybis(benzoate), and 4,4'-bipyridine. The photocatalytic studies on 1-3 indicate that they are active catalysts for the degradation of orange G, rhodamine B, Remazol Brilliant Blue R and methylene blue. The compounds have also been characterized by powder X-ray diffraction, IR, thermogravitmetric analysis, UV-vis, photoluminescence, and magnetic studies.

Catalysis↗
PubMed2006

Substitution effect on the photocatalytic degradation by the series AxBi26-xMo10O68+0.5y (A = Ba, y = 0; A = Bi, La, y = 2): a kinetic study.

The compounds Ba2Bi24Mo10O68 and La2Bi24Mo10O69 were synthesized by the ceramic route and characterized by single crystal and powder X-ray diffraction, respectively. The structure of these phases is isostructural with that of Bi26Mo10O69. Along the b axis columns of [Bi12O14] units are separated by MoO4 tetrahedra and noncolumnar A (A = Ba, La) atoms. UV-visible spectra of Bi26Mo10O69, Ba2Bi24Mo10O68, and La2Bi24Mo10O69 suggest that band gap decreases on substitution of La and Ba for Bi at the noncolumnar site. The photocatalytic degradation of water pollutants such as phenol and substituted phenols was studied. All the catalysts show specificity toward nitro substituent at the para position in the aromatic ring.

Journal Article↗
PubMed2006

Cluster kinetics and dynamics during spinodal decomposition.

Spinodal decomposition (barrierless phase transition) is a spontaneous phase separation caused by conditions that force the system to become thermodynamically unstable. We consider spinodal decomposition to occur under conditions of large supersaturation S and/or small ratio of interfacial to thermal energies omega, such that the computed number of monomers in a critical nucleus xi*=(omega/ln S)3 is less than unity. The small critical nucleus size is consistent with a negligible energy barrier for initiating condensation. Thus, in contrast to conventional opinion, it is suggested that the spinodal decomposition is related to the homogeneous nucleation of metastable fluids. Population balance equations show how clusters aggregate and rapidly lead to phase separation. Different mass dependences of aggregation rate coefficients are proposed to investigate the fundamental features of spinodal decomposition. When the mass dependency is an integer, the equations are solved by the moment technique to obtain analytical solutions. When the mass dependency is a noninteger, the general cases are solved numerically. All solutions predict the two time regimes observed experimentally: the average length scale of condensed-phase domains increases as a power law with an exponent of 1/3 at early times, followed by a linear increase at longer times.

Journal Article↗
PubMed2006

Cluster kinetics model of particle separation in vibrated granular media.

We model the Brazil-nut effect (BNE) by hypothesizing that granules form clusters that fragment and aggregate. This provides a heterogeneous medium in which the immersed intruder particle rises (BNE) or sinks (reverse BNE) according to relative convection currents and buoyant and drag forces. A simple relationship proposed for viscous drag in terms of the vibrational intensity and the particle to grain density ratio allows simulation of published experimental data for rise and sink times as functions of particle radius, initial depth of the particle, and particle-grain density ratio. The proposed model correctly describes the experimentally observed maximum in risetime.

Journal Article↗
PubMed2005

Kinetics of nonisothermal polymer crystallization.

We adopt a cluster size distribution model to investigate the kinetics of nonisothermal polymer crystallization. The time dependencies of polymer concentration, number and size of crystals, and crystallinity (in Avrami plots) are presented for different cooling rates. The incubation period is also investigated at different cooling rates and initial temperatures. The relationship between cooling rates and incubation time is presented graphically and compared with experimental measurements. The initial temperature (relative to melting point) has a significant effect on nonisothermal crystallization. A comparison of moment and numerical solutions of the population balance equations shows the influence of Ostwald ripening. Agreement between modeling results and experimental measurements at different cooling rates supports the application of the distribution kinetics model for nonisothermal crystallization.

Journal Article↗
PubMed2005

Temperature effects for isothermal polymer crystallization kinetics.

We adopt the cluster size distribution model to investigate the effect of temperature on homogeneous nucleation and crystal growth for isothermal polymer crystallization. The model includes the temperature effects of interfacial energy, nucleation rate, growth and dissociation rate coefficients, and equilibrium solubility. The time dependencies of polymer concentration, number and size of crystals, and crystallinity (in Avrami plots) are presented for different temperatures. The denucleation (Ostwald ripening effect) is also investigated by comparing moment and numerical solutions of the population balance equations. Agreement between the model results and temperature-sensitive experimental measurements for different polymer systems required strong temperature dependence for the crystal-melt interfacial energy.

Journal Article↗
PubMed2005

Synthesis, structure, and photocatalysis in a new structural variant of the Aurivillius phase: LiBi4M3O14 (M = Nb, Ta).

Two new compounds, LiBi4Nb3O14 and LiBi4Ta3O14, have been synthesized by the solid-state method, using Li2CO3, Bi2O3, and M2O5 (M = Nb, Ta) in stoichiometric quantities. These compounds crystallize in the monoclinic C2/c space group with a = 13.035(3) A, b = 7.647(2) A, c = 12.217(3) A, beta = 101.512(4) degrees , V = 1193.4(5) A3 , and Z = 4 and a = 13.016(2) A, b = 7.583(1) A, c = 12.226(2) A, beta = 101.477(3) degrees , V = 1182.6(5) A3, and Z = 4, respectively. These are isostructural and the structure along the b axis consists of layers of [Bi2O2]2+ units separated by layers of LiO4 tetrahedra and NbO6 octahedra hence depicting an unusual variation in the Aurivillius phase isolated for the first time. The presence of lithium has been confirmed by 7Li NMR studies. ac impedance measurements and variable temperature (7)Li NMR studies indicate oxygen ion conductivity in these materials. The UV-visible spectra suggest a band gap of 3.0 eV for LiBi4Nb3O14 and 3.5 eV for LiBi4Ta3O14, respectively, and the associated studies on degradation of dyes and phenols render these materials suitable for photocatalysis.

Journal Article↗
PubMed2005

Distribution kinetics of polymer crystallization and the Avrami equation.

Cluster distribution kinetics is adopted to explore the kinetics of polymer crystallization. Population balance equations based on crystal size distribution and concentration of amorphous polymer segments are solved numerically and the related dynamic moment equations are also solved. The model accounts for heterogeneous or homogeneous nucleation and crystal growth. Homogeneous nucleation rates follow the classical surface-energy nucleation theory. Different mass dependences of growth and dissociation rate coefficients are proposed to investigate the fundamental features of nucleation and crystal growth. A comparison of moment solutions with numerical solutions examines the validity of the model. The proposed distribution kinetics model provides a different interpretation of the familiar Avrami equation.

Journal Article↗
PubMed2004

Cluster kinetics of density relaxation in granular materials.

Of the many complex processes of granular materials, vibrational settling and compaction are common phenomena that have attracted much attention. In this work, we investigate vibrational, or tapping, compaction, and propose that the underlying kinetics involves clusters fragmenting and aggregating, and individual grains attaching and dissociating at cluster surfaces. The periodic vibrations cause cluster breakage and interchange between individual free grains and the clusters. The population balance equations for the concurrent kinetics are solved by a moment method, yielding easily solved differential equations. The compaction ratio defined in terms of the mass moments agrees well with experimental data [Knight et al., Phys. Rev. E. 51, 3957 (1995); Nowak et al., ibid. 57, 1972 (1998)] and other models. A change in tapping acceleration can produce reversible or irreversible transitions between densities, depending on the number of clusters that have evolved.

Journal Article↗
PubMed2004

Synthesis and structure of nanocrystalline TiO2 with lower band gap showing high photocatalytic activity.

Nanocrystalline TiO2 was synthesized by the solution combustion method using titanyl nitrate and various fuels such as glycine, hexamethylenetetramine, and oxalyldihydrazide. These catalysts are active under visible light, have optical absorption wavelengths below 600 nm, and show superior photocatalytic activity for the degradation of methylene blue and phenol under UV and solar conditions compared to commercial TiO2, Degussa P-25. The higher photocatalytic activity is attributed to the structure of the catalyst. Various studies such as X-ray diffraction, Raman spectroscopy, Brunauer-Emmett-Teller surface area, thermogravimetric-differential thermal analysis, FT-IR spectroscopy, NMR, UV-vis spectroscopy, and surface acidity measurements were conducted. It was concluded that the primary factor for the enhanced activity of combustion-synthesized catalyst is a larger amount of surface hydroxyl groups and a lowered band gap. The lower band gap can be attributed to the carbon inclusion into the TiO2 giving TiO(2-2x)C(x) VO2**.

Catalysis↗
PubMed2004

Photocatalytic degradation of organic compounds over combustion-synthesized nano-TiO2.

The photocatalytic degradation of various organics such as phenol, p-nitrophenol, and salicylic acid was carried out with combustion-synthesized nano-TiO2 under UV and solar exposure. Under identical conditions of UV exposure, the initial degradation rate of phenol with combustion-synthesized TiO2 is 2 times higher than the initial degradation rate of phenol with commercial Degussa P-25 TiO2. The intermediates such as catechol (CC) and hydroquinone (HQ) were not detected during the degradation of phenol with combustion-synthesized TiO2, while both the intermediates were detected when phenol was degraded over Degussa P-25. This indicates that the rates of secondary photolysis of CC and HQ occur extremely faster than the rates at which they are formed from phenol and further implies that the primary hydroxylation step is rate limiting for the combustion-synthesized TiO2 aided photodegradation of phenol. The degradation rates of salicylic acid and p-nitrophenol were also investigated, and the rates were higher for combustion-synthesized titania compared to Degussa P-25 TiO2. Superior activity of combustion-synthesized TiO2 toward photodegradation of organic compounds can be attributed to crystallinity, higher surface area, more surface hydroxyl groups, and optical absorption at higher wavelength.

Anti-Infective Agents↗
PubMed2003

Distribution kinetics of Ostwald ripening at large volume fraction and with coalescence.

Condensation phase transitions from metastable fluids occur by nucleation with accompanying particle growth and eventual Ostwald ripening. During ripening the subcritical particles dissolve spontaneously while larger particles grow and possibly coalesce if their volume fraction is large enough. The classical diffusion-influenced rates are also affected by large particle concentrations and are here described by mass-dependent rates. We represent the kinetics of ripening through growth, dissolution, and biparticle coalescence by a new population dynamics equation for the particle size distribution (PSD). Numerical solutions of the scaled governing equations show that coalescence plays a major role in influencing the PSD when the scaled mass concentration (volume fraction) or number concentration is relatively large. The solution describes the time range from initial conditions to the final narrowing of polydispersity. We show that the time dependence of the average particle mass in the asymptotic period of ripening has a power-law increase dependent on rate expressions for particle growth and coalescence at large values of volume fraction.

Journal Article↗
PubMed2002

Numerical and similarity solutions for reversible population balance equations with size-dependent rates.

Population balance equations (PBEs) for reversible aggregation-fragmentation processes are important to particle agglomeration and dissolution, polymerization and degradation, liquid droplet coalescence and breakup, and floc coagulation and disintegration. Moment solutions provide convenient solutions to the PBEs, including steady state and similarity solutions, but may not be feasible for complex forms of size-dependent rate coefficients and stoichiometric kernels. Numeric solutions are thus necessary not only for applications, but also for the study of the mathematics of PBEs. Here we propose a numerical method to solve PBEs and compare the results to moment solutions. The numeric results are consistent with known steady state and asymptotic long-time similarity solutions and show how processes can be approximated by self-similar formulations.

Journal Article↗
PubMedDate not supplied

Modeling of lipase catalyzed ring-opening polymerization of epsilon-caprolactone.

Enzymatic ring-opening polymerization of epsilon-caprolactone by various lipases was investigated in toluene at various temperatures. The determination of molecular weight and structural identification was carried out with gel permeation chromatography and proton NMR, respectively. Among the various lipases employed, an immobilized lipase from Candida antartica B (Novozym 435) showed the highest catalytic activity. The polymerization of epsilon-caprolactone by Novozym 435 showed an optimal temperature of 65 degrees C and an optimum toluene content of 50/50 v/v of toluene and epsilon-caprolactone. As lipases can degrade polyesters, a maximum in the molecular weight with time was obtained due to the competition of ring opening polymerization and degradation by specific chain end scission. The optimum temperature, toluene content, and the variation of molecular weight with time are consistent with earlier observations. A comprehensive model based on continuous distribution kinetics was developed to model these phenomena. The model accounts for simultaneous polymerization, degradation and enzyme deactivation and provides a technique to determine the rate coefficients for these processes. The dependence of these rate coefficients with temperature and monomer concentration is also discussed.

Caproates↗