Anew technique for determining the adsorption energy of terminally adsorbed polymer

Using a new experimental technique, “Continuous Elution Method”, the desorption behavior of polystyrene(PS) and polystyrene (PS-X) functionalized by a terminal iminium ion (-X) from α-Al₂O₃ and γ-Al₂O₃ surface were investigated, and found that PS-X is forming a terminally adsorbed polymer layer on α-Al₂O₃, surface. Furthermore, it was found that the adsorption force of terminally adsorbed polymer is balanced with the desorption force which is contributed from the osmotic pressure in the adsorption layer. Based on this concept, the adsorption energy of the end-functionalized polystyrene terminally adsorbed on the α-Al₂O₃, surface was evaluated to be 4.2 ˜4.3 kT.     

An insight into the changes in the thermal analysis curves of boehmite with mechanical activation

This study deals with the changes in the thermal transformation behaviour of boehmite with mechanical activation (MA), carried out in planetary mill. Observed changes in the TG-DTG–DTA curves are: shifting of the desorption of physically adsorbed water to higher temperature, decrease in the γ-Al₂O₃ transformation temperature and its peak area, formation of α-Al₂O₃, not observed for unmilled boehmite upto 1,200 °C, for milling time ≥60 min. Reasons for such changes are explored on the basis of physicochemical changes occurring as a result of high energy milling. Structural degradation is found to increase with increase in milling time. As a consequence of structural changes, Al–OH bonds get stronger, whereas the hydrogen bonds get weaker. Stronger Al–OH bonding and enhanced surface energy increase water affinity and delays its removal. Decreased hydrogen bond strength, easy exit of dehydroxylation product (water) and displacement of Al to tetrahedral positions make the γ-Al₂O₃ transformation easier. Ease of removal of residual hydroxyls from small crystallite transition alumina from MA boehmite, as a result of shorter diffusion path, ensures α-Al₂O₃ transformation at lower temperature.     

Influence of cubic α-Fe2O3 particles on the thermal stability of poly(methyl methacrylate) synthesized by in situ bulk polymerization

Poly(methyl methacrylate)/α-Fe₂O₃ composites were prepared by in situ bulk radical polymerization of methyl methacrylate in the presence of the cubic α-Fe₂O₃ particles using 2,2′-azobisisobutyronitrile as initiator. The cubic α-Fe₂O₃ particles were synthesized by forced hydrolysis of FeCl₃ and characterized by X-ray diffraction analysis and transmission electron microscopy. The molar masses and molar mass distribution of synthesized PMMA samples were determined by gel permeation chromatography. The influence of α-Fe₂O₃ filler particles on the thermal properties of the PMMA matrix was investigated using thermogravimetry and differential scanning calorimetry. The molar mass and polydispersity of PMMA extracted from composite samples were not influenced by cubic α-Fe₂O₃ particles. The obtained composites have better thermal and thermooxidative stability than pure PMMA. On the other hand, the values of the glass transition temperature of composite samples were identical to the glass transition temperature of pure PMMA.     

Study of the Second Order Transitions and Acid-Base Properties of Polymers Adsorbed on Oxides by Using Inverse Gas Chromatography at Infinite Dilution

In Part I, we gave the details concerning inverse gas chromatography (IGC) at infinite dilution and the methods and models that will be used to characterize solid substrates. This technique proved to be an excellent technique to determine not only the glass transitions, but also beta-transition and liquid-liquid transitions of polymers adsorbed on solid substrates. In this second part, we used the IGC technique to determine the second order transitions of the systems' polymethyl methacrylate (PMMA)/SiO(2) and PMMA/Al(2)O(3), at various covered surface fractions and for various tacticities of the polymer. The maxima of the dispersive component of the surface energy gamma(s)(d) of our two systems, obtained by IGC at infinite dilution, indicated clearly the presence of transition temperatures (glass or local transitions). In general, we observed with PMMA three principal maxima that reflect the changes in motions leading to reorganization and rearrangement of the various groups or chain segments of the polymer. The change in the retention mechanism of the probes at the transition temperatures is attributed to an increased molecular mobility of the polymer segments, allowing for the penetration of the probes into the polymer layer. The study of the chemical physical properties of PMMA/SiO(2) and PMMA/Al(2)O(3) revealed an important difference in the acidic and basic behavior, in Lewis terms, of oxide covered by various concentrations of PMMA. This study also highlighted an important effect of the tacticity of the polymer on the acidic basic character of PMMA adsorbed on oxides. Copyright 2001 Academic Press.     

Effects of molecular mass and surface treatment on adsorbed poly(methyl methacrylate) on silica

The behavior of relatively monodisperse adsorbed poly(methyl methacrylate) (PMMA) samples, from 19 to 587 kDa on silica, was studied using modulated differential scanning calorimetry and FTIR. On untreated Cab? O? Sil silica, the glass transition temperatures (T_gs) were higher (by around 30 °C), and the transitions were significantly broader (by a factor of 5–6) than those for the corresponding bulk samples. While the T_gs for the bulk polymers showed the expected dependence on molecular mass, the polymers on untreated silica showed little dependence, i.e., at the same adsorbed amounts, the glass transitions were very similar. The FTIR spectra of the adsorbed PMMA (on untreated silica) showed the presence of at least two resonances, one for the bound (hydrogen bonded to surface silanols) and another for free carbonyls. Fitting of the spectra allowed the estimation of the bound fractions of carbonyls that were dependent on the adsorbed amount, but not molecular mass. On Cab? O? Sil treated with hexamethyldisilizane (HMDS), the adsorbed PMMA exhibited glass transition behavior with little molecular‐mass dependence; the T_gs for the different PMMA samples were very similar to those of the high‐molecular mass bulk polymer, but with additional broadening of about a factor of 2. FTIR spectra for the PMMA samples on the treated silica did not show significant amounts of any of the hydrogen‐bonded carbonyl groups. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 649–658, 2008     

Synthesis and characterization of nanocrystalline α-Al2O3 using Al and Fe2O3 (hematite) through mechanical alloying

In this present work, the synthesis of nanocrystalline α-Al₂O₃ using pure aluminum (Al) and Fe₂O₃ (hematite) as the precursors by mechanical alloying technique has been studied. The formation of α-Al₂O₃ nanocrystallites occurs during the solid-state reaction and through the reduction treatment. Also in this paper, effects of milling time on particle size and the lattice strain nanocrystalline α-Al₂O₃ have been investigated. Obtained powders were evaluated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential thermal analysis (DTA), thermal gravimetric analysis (TGA) and X-ray diffraction (XRD). The obtained results indicated that a reduction reaction was completed after 2 h milling in a planetary mill. The crystallite size of obtained α-alumina (α-Al₂O₃) was in general about 12 nm. Finally, the results showed appropriate homogeneity and dispersion of related nanocrystalline.     

Rapid crystal phase transformation into hexagonally shaped α-alumina using AlF3 seeds

This study focused on synthesizing highly crystalline α-Al₂O₃ at low temperatures. We found that when a small amount of hexagonal structured aluminum fluoride seeds (AlF₃) were added to aluminum precursors, the crystal phase transformation temperature required to form α-Al₂O₃ were considerably lowered and the produced α-Al₂O₃ particles showed hexagonal shapes. Furthermore, when 2.0 mol% of hexagonal structured-AlF₃ particles were added as seed to sol-solutions containing various aluminum precursors such as AIP, AlCl₃, Al(NO₃)₃ or AlOOH, rhombohedral α-Al₂O₃ particles were formed at 750 °C regardless of the use of alumina precursors. Scanning electron microscopy photos showed the α-Al₂O₃ particles produced had hexagonal shapes, and that the morphologies of these particles depended on the primitive structures of the AlF₃ seeds added. Based on the relationship between α-Al₂O₃ crystallites sizes and calcination temperatures, crystal growth in α-Al₂O₃ structure was enhanced by adding AlF₃ seeds.     

Phase transitions and surface stability of the WO3γ-Al2O3 system

The solid state transitions of the WO₃γ-Al₂O₃ system have been investigated in the temperature range 873–1323 K. The formation of α-Al₂O₃ and Al₂(WO₄)₃ phases and the thermal desorption of W(VI) attached to the γ-Al₂O₃ surface have been studied as function of the treatment time. The inhibition of the phase transition to α-Al₂O₃ and therefore the stabilization of the surface has been observed at 1323 K for samples with 7% WO₃ content. This stabilization is critically affected by the tungsten content. An explanation for the stabilization of the alumina surface is proposed.     

Kinetic Analysis Crystallization of α-Al2O3 by Dynamic DTA Technique

The phase transformation of seeded (5 mass% Fe₂O₃ as a Fe(NO₃)₃ solution) boehmite derived alumina gel to α-Al₂O₃ was studied with DTA technique and compared with unseeded and α-Al₂O₃ seeded boehmite gels. Data for kinetic analysis of α-Al₂O₃ crystallization were obtained from quantitative DTA curves. The kinetic parameters were analysed by traditional Kissinger analysis and Friedman and Ozawa-Flynn-Wall methods using the Netzsch Thermokinetics program. Results of the comparison of values of activation energies for all three gels and methods are the process of α-Al₂O₃ transformation for originally γ-AlOOH/Fe(NO₃)₃ gels goes like that of unseeded boehmite gels,only under lower temperatures (lower about 200°C). This revised version was published online in July 2006 with corrections to the Cover Date.     

Formation of Pd–Ag nanoparticles in supported catalysts based on the heterobimetallic complex PdAg<Subscript>2</Subscript>(OAc)<Subscript>4</Subscript>(HOAc)<Subscript>4</Subscript>

The formation of Pd–Ag nanoparticles deposited from the heterobimetallic acetate complex PdAg₂(OAc)₄(HOAc)₄ on α-Al₂O₃, γ-Al₂O₃, and MgAl₂O₄ has been investigated by high-resolution trans-mission electron microscopy, temperature-programmed reduction, and IR spectroscopy of adsorbed CO. The reduction of PdAg₂(OAc)₄(HOAc)₄ supported on γ-Al₂O₃ and MgAl₂O₄ takes place in two steps (at 15–245 and 290–550°C) and yields Pd–Ag particles whose average size is 6–7 nm. The reduction of the Pd–Ag catalyst supported on α-Al₂O₃ occurs in a much narrower temperature range (15–200°C) and yields larger nanoparticles (~10–20 nm). The formation of Pd–Ag alloy nanoparticles in all of the samples is demonstrated by IR spectroscopy of adsorbed CO, which indicates a marked weakening of the absorption band of the bridged form of adsorbed carbon monoxide and a >30-cm^–1 bathochromic shift of the linear adsorbed CO band. IR spectroscopic data for PdAg₂/α-Al₂O₃ suggest that Pd in this sample occurs as isolated atoms on the surface of bimetallic nanoparticles, as is indicated by the almost complete absence of bridged adsorbed CO bands and by a significant weakening of the Pd–CO bond relative to the same bond in the bimetallic samples based on γ-Al₂O₃ and MgAl₂O₄ and in the monometallic reference sample Pd/γ-Al₂O₃.     

Influence of α-Fe2O3 nanorods on the thermal stability of poly(methyl methacrylate) synthesized by in situ bulk polymerisation of methyl methacrylate

α-Fe₂O₃ nanorods were incorporated into poly(methyl methacrylate) (PMMA) by in situ radical polymerisation of methyl methacrylate initiated by 2,2′-azobisisobutyronitrile. The α-Fe₂O₃ nanorods were synthesized by forced hydrolysis of FeCl₃ and structural characterization was performed by X-ray diffraction and transmission electron microscopy. The molar mass and the polydispersity index of synthesized PMMA samples were determined by gel permeation chromatography. The content of residual monomer was determined by ¹H NMR spectroscopy. The influence of α-Fe₂O₃ nanorods on the thermal stability of the polymer was investigated using thermogravimetry and differential scanning calorimetry. The molar mass and polydispersity index of PMMA were dependent on the content of α-Fe₂O₃ nanorods. The values of the glass transition temperature of the nanocomposites were lower compared to pure PMMA. Also, the thermal stability of nanocomposites in nitrogen and air was different from that of pure PMMA.     

Thermal stability and glass transition behavior of PANI/γ-Al2O3 composites

Polyaniline/γ-Al₂O₃ (PANI/γ-Al₂O₃) composites were synthesized by in-situ polymerization at the presence of HCl as dopant by adding γ-Al₂O₃ nanoparticles into aniline solution. The composites were characterized by FTIR and XRD. The thermogravimetry (TG) and modulated differential scanning calorimetry (MDSC) were used to study the thermal stability and glass transition temperature (T _g) of the composites, respectively. The results of FTIR showed that γ-Al₂O₃ nanoparticles connected with the PANI chains and affected the absorption characteristics of the composite through the interaction between PANI and nano-sized γ-Al₂O₃. And the results of XRD indicated that the peaks intensity of the PANI/γ-Al₂O₃ composite were weaker than that of the pure PANI. From TG and derivative thermogravimetry (DTG) curves, it was found that the pure PANI and the PANI/γ-Al₂O₃ composites were all one step degradation. And the PANI/γ-Al₂O₃ composites were more thermal stable than the pure PANI. The MDSC curves showed that the nano-sized γ-Al₂O₃ heightened the glass transition temperature (T _g) of PANI.     

Chromatographic investigation of the effect of dissolved carbon dioxide on the glass transition temperature of a polymer and the solubility of a third component (additive)

The effect of dissolved carbon dioxide on the glass transition temperature of a polymer, PMMA, has been investigated using molecular probe chromatography. The probe solute was iso-octane, and the specific retention volumes of this solute in pure PMMA and mixtures of PMMA with CO₂ were measured over a temperature range of 0 to 180°C and CO₂ pressures from 1 to 75 atm. The amount of CO₂ dissolved in the polymer was calculated from a model fit to previously published solubility data determined chromatographically. Classical van't Hoff-type plots were used to determine the glass transition temperature of CO₂-impregnated PMMA from low pressure up to 46 atm of CO₂. Solvent-induced plasticization was observed with the glass transition temperature decreasing by about 40°C. At some pressures, glass transitions at low temperatures could not be determined from the van't Hoff plots because of the proximity of the polymer glass transition temperature to the gas–liquid transition temperature for CO₂. For these pressures, a new method was developed to determine the glass transition composition. The glass transition pressure was then calculated from the measured composition and temperature using an isotherm model. In every case, the glass transition temperature decreased linearly with increasing concentration of CO₂ in the polymer. However, at higher compositions, the glass transition pressure decreased with increasing composition and decreasing temperature. The observed retention volume of iso-octane with PMMA in a glassy state was correlated with an adsorption model developed from a theory for liquid–solid chromatography derived by Martire. This model accurately described the observed decrease in retention of iso-octane by adsorption on the surface of glassy PMMA with increasing concentration of CO₂ dissolved in the polymer. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2537–2549, 1998     

Dynamic study of ammonium dawsonite doped with yttrium transformation at elevated temperatures

Co-precipitation of alumina/YAG precursor from aluminum and yttrium nitrates solution with ammonium carbonate results in dawsonite (NH₄Al(OH)₂CO₃). Its crystallographic parameters differ from the compound precipitated without the yttrium additive. It indicates that yttrium ions become incorporated into the dawsonite structure. The DSC/TG and X-ray measurements show decomposition of dawsonite at elevated temperature resulting in γ-Al₂O₃ which transforms to δ and θ modifications at still higher temperatures. The full transformation to α-Al₂O₃ and YAG occurs at temperatures higher than 1,230 °C.     

Preparation,characterization and thermal decomposition of phenylenediammonium sulfate salts Part. XVI

Polyaniline/α-Al₂O₃ (PANI/α-Al₂O₃) composites were synthesized by in situ polymerization through ammonium persulfate ((NH₄)₂S₂O₈, APS) oxidized aniline using HCl as dopant. XRD and FTIR were used to characterize the PANI/α-Al₂O₃ composites. The thermal stabilities and glass transition temperature (T _g) of PANI/α-Al₂O₃ composites were tested using thermogravimetric (TG) method and modulated differential scanning calorimetry (MDSC) technique. The results of TG showed that the thermal stability of PANI/α-Al₂O₃ composite increased and then decreased with the increase in α-Al₂O₃ content. The derivative thermogravimetry (DTG) curves showed one step degradation of PANI when the α-Al₂O₃ content was lower than 52.5 mass%, and exhibited two steps degradation when the α-Al₂O₃ content was higher than 63.6 mass%. The MDSC curves showed that the T _g of PANI/α-Al₂O₃ composites increased and then decreased with the augment of α-Al₂O₃ for the interaction between PANI chains and the surface of α-Al₂O₃.     

Determination of Kinetic Parameters of Phase Transition on the Basis of DTA Measurements

A simple method of determination of kinetic parameters by analysis of DTA(t) function was developed for the case of systems undergoing transitions without mass change and when kinetic equation describing transition rate is known. The presented method also permits the determination of transition rate dα/dt (or α(t )) when the kinetic equation of transition is unknown. The developed method was tested using DTA data of crystallization of 2CaO×Al₂ O₃ ×1.95SiO₂ glass pure and doped with Cr³⁺ and Nd³⁺ . This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis of α-alumina from a less common raw material

A nanostructured α-Al₂O₃ with particle size lower than 100 nm was obtained from a hazardous waste generated in slag milling process by the aluminium industry. The route developed to synthesize alumina consisted of two steps: in the first one, a precursor of alumina, boehmite, γ-AlOOH was obtained by a sol–gel method. In the second step, the alumina was obtained by calcination of the precursor boehmite (xerogel). Calcination in air was performed at two different temperatures, i.e. 1,300 and 1,400 °C, to determine the influence of this parameter on the quality of resulting alumina. X-Ray diffraction patterns and transmission electron microscopy images of calcined powers revealed beside corundum the presence of transition aluminas and some rest of amorphous phase in the sample prepared at 1,300 °C. The increase of the calcinations temperature to 1,400 °C favors the formation of an almost single-phase corundum powder. The transition of θ- to α-Al₂O₃ was followed by means of infrared spectroscopy, since it is accompanied by the disappearance of the IR band frequencies associated with tetrahedral sites (AlO₄ sites), giving rise to a spectrum dominated by Al³⁺ ions in octahedral sites (AlO₆) characteristic of corundum.     

Determination of Polycyclic Aromatic Hydrocarbons in Vegetables by Headspace SPME-GC

Inverse gas chromatography (IGC) at infinite dilution is a powerful technique to characterize the superficial and interfacial properties of solid substrates as oxides, polymers or polymers adsorbed on oxides. It can also be used to determine the physicochemical properties and the transition phenomena of polymers. In this paper, IGC was used to determine the changes, as a function of temperature, of the specific free enthalpy ??G _a ^SP and deduce the specific entropy ??S _a ^SP of poly (methyl methacrylate) (PMMA) adsorbed on alumina or on silica for different tacticities of PMMA. The study of the surface properties of PMMA/SiO₂ and PMMA/Al₂O₃, revealed an important difference in the physicochemical behaviour of oxides covered by various concentrations of PMMA. This study also highlighted an important effect of the tacticity of the polymer on the specific entropy of PMMA adsorbed on oxides.     

Study of CuCl2 Supported on SiO2 and Al2O3

The nature and stability of surface species of CuCl₂ supported on α-Al₂O₃, γ-Al₂O₃, and SiO₂ were investigated by using X-ray diffraction techniques and reflectance spectroscopy. No specific chemical interaction of CuCl₂ is observed on an inert α-Al₂O₃ support, as opposed to hydrated carriers as SiO₂ and γ-Al₂O₃. On these supports the coordination sphere of Cu²⁺ consists of surface groups (OH^? or O^? at drying and activation, resp.), H₂O and Cl^?, with the H₂O ligands decreasing in concentration in the process of impregnation, drying and calcination. γ-Al₂O₃ samples, calcined at 400°C, show γ-Cu₂(OH)₃Cl as opposed to CuAl₂O₄ at higher temperatures. The absence of Cu₂(OH)₃Cl on SiO₂-supported samples is related to the acid-base characteristics of the carriers. The various supports can be arranged in the following order of stability of the complexes formed: γ-Al₂O₃ > SiO₂ ? -Al₂O₃.     

Liquid phase hydrogenation of naphthalene in the presence of CO over supported Ni catalyst

The liquid phase hydrogenation of naphthalene was performed in the presence of CO over a commercial Ni/SiO₂-Al₂O₃ catalyst. Naphthalene was hydrogenated even in the presence of CO at elevated temperatures, accompanying the hydrogenation of CO. Two activation energy values were obtained for the naphthalene hydrogenation depending on the reaction temperature. FT-IR measurement of the adsorbed CO was also carried out. Hydrogenation of the adsorbed CO created sites active for naphthalene hydrogenation.