Geopolymerisation kinetics. 3. Effects of Cs and Sr salts

Alternating current impedance spectroscopy (ACIS) is used to monitor in situ the kinetics of geopolymer formation by alkali silicate activation of metakaolin at 40 °C. The evolution of conductivity and resistivity as a function of time are found to correlate very well with existing models of geopolymerisation kinetics. The dissolution of metakaolin gives a rapid increase in conductivity, with a subsequent decrease due to gelation and gel ageing. After a period of curing, the resistivity of the gel network is seen to be a sensitive probe of the development of a well-formed gel structure. This enables investigation of the changes in geopolymer formation kinetics and gel structure caused by addition of various salts of interest in nuclear waste management, in particular the nitrate, sulphate and hydroxide salts of caesium and strontium. Each of the six salts studied is shown to have specific effects on the kinetics of geopolymer formation and the pore structure of the final geopolymer gel. Caesium nitrate and caesium sulphate have very similar effects, with a slight retardation of gelation followed by significant disruption of gel structure while caesium hydroxide accelerates gel formation markedly. Strontium nitrate at high concentrations interferes significantly with the pore network development, while strontium sulphate is converted gradually to carbonate salts by atmospheric carbonation.     

Optimization of batch polymerization processes—narrowing the MWD. I. Model simulation

Several control strategies for the free-radical polymerization of methyl methacrylate are analyzed with a detailed constitutive model incorporating the gel and glass effects. Optimal temperature history, photoinitiation intensity variation, and programmed monomer and solvent additions, employed alone and in combination, represent cases simulated by this model. Solvent addition is selected for further experimental studies, due to some attractive features. The product molecular weight distribution is narrowed, while the molecular weight averages are maintained high. Model predictions of the solvent injection history exhibit a strong sensitivity to the constitutive equations for describing the gel and glass effects.     

Free radical polymerization of methyl methacrylate: Modeling and simulation under semibatch and nonisothermal reactor conditions

For the bulk free radical polymerization of methyl methacrylate (MMA), equations of the material balance can be written that are based on a kinetic diagram that considers initiation by decomposition of an AIBN initiator, propagation, and termination by disproportionation. To quantify the gel and glass effects simple empirical dependences are used between the rate constants of termination and propagation and monomer conversion. Numerical values for the empirical parameters at different temperatures and initiator concentrations are also given. Conversion history and molecular weights are obtained by simulation when an initiator or monomer are added to the reaction mass and the temperature modifies after some reaction has taken place. These intermediate operations are simulated at different moments with respect to the gel and glass effects. The validation of the model for semibatch and nonisothermal conditions are made by comparing the simulation results with literature experimental data. The most important conclusion of the article is that the empirical model proposed for the gel and glass effects can be successfully used under semibatch and nonisothermal reactor conditions. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2561–2570, 1999     

Thermoreversible hydrogel. V. Synthesis and swelling behavior of the N-isopropylacrylamide-co-trimethyl methacryloyloxyethyl ammonium iodide copolymeric hydrogels

A series of N-isopropylacrylamide/trimethyl methacryloyloxyethyl ammonium iodide (NIPAAm/TMMAI) copolymeric gels are prepared from the various molar ratios of NIPAAm, cationic monomer TMMAI, and N,N′-methylene bisacrylamide (NMBA) in this article. The influences of the amount of the cationic monomer in the copolymeric gels on the swelling behavior in water, various saline solutions, and various temperatures are investigated. Results show that the swelling ratios of copolymeric gels are significantly larger than those of pure homopolymer NIPAAm gel, and the more the TMMAI content, the higher the gel transition temperature. In the saline solution, results show that the swelling ratio of pure NIPAAm gel has not significantly changed with an increase of the salt concentration until the salt concentration is larger than 0.1M. The swelling ratios for the copolymeric gels NIPAAm/TMMAI decrease with increasing salt concentration. In various saline solutions, results show that the anionic effects are greater than cationic effects in the presence of common anion, different cations and common cation, and different anions for these hydrogels. Finally, we also tested the reversibility of the NIPAAm/TMMAI copolymeric gels. The deswelling and reswelling kinetics are dependent on the temperature, which is below or above the gel transition temperature. The gel with little TMMAI content has a good reversibility. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1793–1803, 1998     

Thermoreversible hydrogels. II. Effect of some factors on the swelling behavior of N,N-dimethylacrylamide and n-butoxymethyl acrylamide copolymeric gels

A series of thermoreversible hydrogels are prepared from the various molar ratios of N,N-dimethylacrylamide (DMA), n-butoxymethyl acrylamide (nBMA), and N,N′-methylenebisacrylamide (NMBA). The influences of the amount of DMA in the copolymeric gels, temperature, and polymerization media on the swelling behaviors in water are investigated. Results indicate that the higher the DMA content in the hydrogel systems the higher the swelling ratio and the gel transition temperature. The effects of the gel thickness on the swelling ratio for DMA/nBMA copolymeric gels indicate that the equilibrium swelling time and diffusion coefficient for the thinner gel (1.5 mm) from the dried state to the completely swollen state are obviously faster than are those for the thicker gels (2.0 and 3.5 mm). The effects of the different polymerization media on the swelling ratio for DMA/nBMA copolymeric gels also show that the larger the solvent molecular size and the poor miscibility of the monomer and solvent the higher the swelling ratio and the diffusion coefficient. The drug release in these copolymeric gel systems are also investigated. © 1997 John Wiley & Sons, Inc. J Appl Polm Sci 65:909–916, 1997     

Dilation of portland cement grains during early hydration and the effect of applied hydrostatic pressure on hydration

An optical interference method shows that at early stages of hydration there is a significant increase in size of individual cement grains. The effect of applied hydrostatic pressure on cement pastes is an increase in the degree of hydration. Explanations for these effects are given in terms of the osmotic gel membrane model of hydration.     

Analysis of esterification mixtures of terephthalic acid and ethylene glycol by gel chromatography

A quantitative gel permeation chromatography analysis of polycondensation mixtures of terephthalic acid with ethylene glycol was carried out on a styrene-divinylbenzene copolymer (4% of crosslinking agent) using tetrahydrofuran as eluent. Adsorption effects on the gel were suppressed by addition of acetic acid. A calibration curve for the identification of linear and cyclic oligomers present in the reaction mixture is given.     

Hydration and Strength Development in Tricalcium Silicate Pastes Seeded with Afwillite

The formation of afwillite in tricalcium silicate pastes initiated by ding with crystals was studied to determine the effects on development of tensile strength. Afwillite forms in pastes only when fresh surfaces of both tricalcium silicate and afwillite seed are exposed by initial grinding. Higher early strengths reflect increased rates of hydration in seeded pastes, but strengths at later ages are lower than those of untreated pastes. At all ages tensile strength is lower at a given degree of hydration in seeded pastes than in unseeded ones; this behavior is considered to result from changes in pore-size distribution which occur as afwillite forms. Afwillite initially forms concomitantly with calcium silicate hydrate gel but later arises from conversion of the gel.     

Thermoreversible hydrogels. I. Synthesis and effect of a hydrophobic monomer on swelling behaviors of thermoreversible gels prepared by copolymerizing N-alkoxyalkylacrylamide with butyl acrylate

A series of copolymeric gels were prepared from N-alkoxyalkylacrylamide and n-butyl acrylate (BA) at various feed ratios. The effect of the content of BA in the copolymer on the gel behaviors is discussed. The respective crosslinked copolymer exhibits a gel transition behavior, collapsing and shrinking above gel transition temperature but swelling and reexpanding below gel transition temperature. By utilizing this character, these copolymeric gels could be used for drug release or drug delivery systems. The drug released from the copolymeric gels was plotted as M_t/M_∞ versus t, where M_t/M_∞ is the fraction of drug released at given time t. In this experiment, crystal violet and caffeine were chosen as model drugs. The deswelling-kinetics experiments with caffeine showed that a water pocket was formed within the gel matrix when the gel deswelled rapidly. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1477–1484, 1997     

27Al and 29Si MAS–NMR studies of structural changes in hybrid aluminosilicate gels

Aluminosilicate gels with stoichiometric and nonstoichiometric compositions were synthesized by means of colloidal sol-gel method and their mullitization behavior was studied by X-ray diffraction (XRD), ²⁷Al and ²⁹Si magic angle spinning nuclear magnetic resonance (MAS–NMR) experiments. Particular attention was given to the structural changes of matrix accompanying the formation of mullite. The various coordinated Al occupancies were clarified by simulating the ²⁷Al MAS–NMR spectra with Gaussian lines. The results demonstrate that the so-synthesized aluminosilicate gel is a hybrid gel containing a mixture of a single-phase gel and a diphasic gel. The mullitization of so-formed hybrid gel exhibits a consecutive one-step conversion process, but not a two-step process, much similar to that of a true diphasic mullite gel. The mullite formation from hybrid aluminosilicate gel mainly depends on the nature of dominant matrix part, but not on the nature of minor matrix part in gel. During the formation process of mullite, amorphous Si-rich phase appears as a transitional phase. The effects of gel composition and heating rate on the phase transformation behavior of hybrid aluminosilicate gel were also discussed here.     

Thermoreversible hydrogels. IV. Effect of some factors on the swelling behavior of N-tetrahydrofurfurylacrylamide

A thermoreversible hydrogel, poly(N-tetrahydrofurfurylacrylamide) [poly-(NTHFAAm) gel], was prepared from N-tetrahydrofurfurylacrylamide, which was synthesized from N-tetrahydrofurfurylamine and acryloyl chloride (through acylation), with N,N′-methylenebisacrylamide, a crosslinker, in various aqueous solutions. The influences of temperature, gel thickness, and polymerization media on the swelling behaviors in water were investigated. The effect of the gel thickness on the swelling ratio for NTHFAAm gel indicated that the equilibrium swelling time and diffusion coefficient for the thinner gel were faster than those for the thicker gels. The effects of different polymerization media on the gel swelling ratio showed that the larger the solvent molecular size and the poorer the miscibility of the monomer and solvent, the higher the swelling ratio and diffusion coefficient. The drug release profiles in the various gels were also investigated. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1597–1603, 1998     

Detection of ASR in opened fractures of damaged concrete

Indisputable evidence of alkali-silica reaction (ASR) in field concrete depends on positive identification of alkali-silica gel, the reaction product of ASR. Experimental results are given on the unambiguous detection of gel by microscopic techniques using fractured concrete surfaces.     

Preparation of Silane-Grafted and Moisture Crosslinked Low Density Polyethylene. Part II: Electrical,Thermal and Mechanical Properties

Among different polyethylene cross-linking methods, such as peroxide, irradiation, and silane cross-linking, silane-based methods are the most suitable methods for producing cable insulation and hot water pipe materials due to process simplicity and superior properties of its product. Some electrical, thermal and mechanical properties of silane-grafted water-cross-linked polyethylene were investigated. The effects of silane grafting and gel content on volume resistivity, tensile properties and melting behavior of low density polyethylene (LDPE) were studied. Results indicated that volume resistivity increased with increasing gel content. Stress at break increased with increasing grafting level and gel content. Elongation at break increased with grafting and decreased with gel content. High temperature tensile properties showed that cross-linked polyethylene (XLPE) is more stable than LDPE at high temperature. In differential scanning calorimetry (DSC) analysis a broad endothermic peak appeared for XLPE due to phase separation. Melting point and crystalline percentage decreased with increased grafting level and gel content. Incorporation of carbon black into XLPE reduced the volume resistivity and degree of crystallization.     

A new direction in the creation of ceramic materials with a specified phase composition and structure. Part I. Preparation of binders for ceramic production by the sol-gel method. 3. Hydrolysis and gel formation in ethylsilicate binders without organic solvents

Results of a physicochemical study of hydrolysis and gel formation in ethylsilicate binders without organic solvents are presented. The effects of the kind and amount of acid catalyst and the proportion of water to ethylsilicate on the thermal effect of the hydrolysis reaction, the sol-gel transformation, and the properties of the binder are investigated. IR spectra of gels of hydrolysates prepared by rapid and slow hydrolysis are presented. The technological aspects of production of binders are considered. The preceding papers of the series have been published in Nos. 1–3,1999.     

Trennen von Gasgemischen durch Adsorption

Separation of gas mixtures by adsorption . Gas mixtures are separated primarily by absorption and rectification. Rectification requires at least (v + 1)-fold of the enthalpy of the distillate as heat energy (v is the reflux ratio). In the case of absorption, the resulting solution is frequently subjected to thermal regeneration with the energy requirement depending inter alia on the boiling point of the solvent. In view of the high energy requirements of these processes and the development of new adsorbents, possibilities of separating by adsorption with subsequent regeneration of the adsorbents, above all by change in pressure, are investigated. The separation of gas mixtures in adsorber beds (e.g. molecular sieves, silica gel, activated charcoal and coke, or alumina) depend upon various separating effects: steric effects, equilibrium effects, kinetic effects. A survey is given of proposed and practical industrial separation processes utilizing adsorption.     

Characterization and properties of macromolecules. VIII. A study of experimental variables in preparative-scale gel permeation chromatography

Three preparative-scale gel permeation chromatography columns were constructed and filled with Corning porous-glass packing materials. Each column was packed with a different pore-size materials, CPG 10–2000, CPG 10–350, CPG 10–120, all of which had been treated with hexamethyldisilazane. An additional preparative-scale column packed with Styragel, nominal porosity 5 × 10⁴ Å, was added to complete the column bank. Polystyrene standards were used to investigate the effects of molecular weight and sample concentration upon elution volume. A commerical polystyrene sample was fractionated using this system to study the effects of sample concentration and flow rate on fractionation efficiency. These fractions were analyzed by high-resolution analyticalscale gel permeation chromatography. Where possible, the results have been compared with similar studies that used Porasil (Spherosil) and Styragel columns.     

Free radical polymerizations associated with the trommsdorff effect under semibatch reactor conditions. I: Modeling

Several important polymerizations [e.g., poly(methyl methacrylate) (PMMA), polystyrene] exhibit gel and glass effects during polymerization. These are associated with the decrease of the diffusivities of the macroradicals and monomer molecules with increasing viscosities of the reaction mass. A new model has been developed to account for the gel and glass effects, both under batch and semibatch reactor operations. The free volume theory of Vrentas and Duda has been used to account for the changes in the diffusion coefficients. The model parameters are tuned using experimental data on the isothermal bulk and solution polymerizations of MMA in batch reactors. Conversion histories and molecular weights have been obtained for cases where solvent, initiator or monomer are added to (or removed from) the reaction mass after some reaction has taken place. In addition, studies involving step changes in temperature hav been carried out. It is found that the conversion vs. time behavior after such step changes depend upon a complex interplay of three factors: Polymer concentration, molecular weight, and the temperature at the point these changes are effected.     

Overview of concentration polarization in ultrafiltration

Concentration polarization mechanisms are briefly outlined. A morphological analysis of ways to prevent the detrimental influence on membrane fluxes is given. The potentials of these different approaches are analysed and some examples of module designs resulting from the various approaches are given.Particular emphasis is given to a study of the physical-mechanical properties of the gel layer in the ultrafiltration of milk whey in a rotating ultrafiltration module.     

Solution properties of xanthan. Light scattering and viscometry on dilute and moderately concentrated solutions

Elastic and quasi-elastic light-scattering, viscometric and rheological studies are given for solutions of the microbial polysaccharide Xanthomonas campestris (xanthan) in aqueous 0.62N NaCl for polymer concentrations from 0.03 to 2.2 g kg⁻¹. The observed negative ∂ln[η]/∂ln T is interpreted as a decrease of the persistence length with increasing T. The behaviour in moderately concentrated solutions (2<[η]c<25) reveals intermolecular association, leading to gel formation in the extreme case. The effect of the association on the viscometric and light-scattering data is discussed. It is concluded that the early stages of association involve structure with the chain axes nearly parallel, but that larger, particulate-like structures develop with increasing c, eventually leading to gel formation under certain conditions.     

Rationalisation of the sizes of small molecules in gel permeation chromatography

Published data on the sizes of small molecules measured by gel permeation chromatography are reconsidered after subjecting them to a recalculation procedure which gives molecular sizes in consistent units. Almost half the results are rejected as being unreliable. A set of molar volume values for n-alkanes is recommended as a standard which should be used to calibrate gel permeation chromatographs irrespective of operating conditions. Molar volumes of some simple gases are given.