Rate Of Gel Formation Research Articles

Tuning the Helicity of Self-Assembled Structure of a Sugar-Based Organogelator by the Proper Choice of Cooling Rate

A novel sugar-appended low-molecular-mass gelator, 4''-butoxy-4-hydroxy-p-terphenyl-beta-D-glucoside (BHTG), was synthesized. It formed thermally reversible gels in a variety of aqueous and organic solvents. Three-dimensional networks made up of helical ribbons were observed in the mixture of H(2)O/1,4-dioxane (40/60 v/v). The handedness of the ribbons depended on the rate of gel formation. Fast-cooling process led to right-handed ribbons, while slow-cooling process led to left-handed ones. A combinatory analyses of microscopic, spectroscopic, and diffraction techniques revealed that BHTG formed a twisted interdigitated bilayer structure with a d spacing of 3.1 nm in gels through a kinetically controlled nucleation-growth process. There were two kinds of molecular orientations of BHTG in the nuclei, clockwise and anticlockwise, which dictated the growth of ribbons. One was metastable and formed first during the cooling process of gel formation. It was able to gradually transform into the more stable latter one with further decreasing temperature. Fast-cooling process did not leave enough time for the nuclei to evolve from metastable to stable state and the ribbons grown from them exhibited right-handedness. However, the metastable nuclei transformed into the stable one when cooled slowly and directed the molecules of BHTG to grow into left-handed aggregates.

Physico-chemical and in vitro biological evaluation of strontium/calcium silicophosphate glass

Strontium is known to reduce bone resorption and stimulate bone formation. Incorporation of strontium into calcium phosphate bioceramics has been widely reported. In this work, calcium and calcium/strontium silicophosphate glasses were synthesized from the sol-gel process and their rheological, thermal, and in vitro biological properties were studied and compared to each other. The results showed that the gel viscosity and thus the rate of gel formation increased by using strontium in glass composition and by increasing aging temperature. In strontium-containing glass, the crystallization temperature increased and the type of the crystallized phase was different to that of strontium-free glass. Both glasses favored precipitation of calcium phosphate layer when they were soaked in simulated body fluid; however strontium seemed to retard the rate of precipitation slightly. The in vitro biodegradation rate of the strontium/calcium silicophosphate glass was higher than that of strontium-free one. The cell culture experiments carried out using rat calvaria osteoblasts showed that the incorporation of strontium into the glass composition stimulated proliferation of the cells and enhanced their alkaline phosphatase activity, depending on cell culture period.

Physical Properties of Rice Bran Wax in Bulk and Organogels

AbstractDifferential scanning calorimetry (DSC), optical microscopy, and X‐ray diffraction (XRD) were used to examine the thermal behavior, crystal structure, and crystal morphology of rice bran wax (RBX) in bulk and oil–wax mixtures, and to compare them with those of carnauba wax (CRX) and candellila wax (CLX). The RBX employed in the present study was separated from rice bran oil by winterization, filtration, refinement, bleaching, and deodorization. The RBX crystals melted in the bulk state at 77–79 °C with ΔHmelting = 190.5 J/g, which is quite large compared with CLX (129 J/g) and CRX (137.6 J/g). XRD data of the RBX crystals revealed O⊥ subcell packing and a long spacing value of 6.9 nm. Thin long needle‐shaped crystals were observed in the mixtures of RBX and liquid oils [olive oil and salad oil (canola:soy bean oil = 50:50)]; therefore, the dispersion of RBX crystals in these liquid oils was much finer than that of CRX and CLX crystals. Organogels formed when the mixture of every plant wax and liquid oil was melted at elevated temperature and cooled to ambient temperature. However, the mixture of RBX and olive oil at a concentration ratio of 1:99 wt.% formed an organogel at 20 °C, whereas the lowest concentration necessary for CRX to form an organogel in olive oil was 4 wt.% and that for CLX was 2 wt.%. Observation of the rate of gel formation using DSC and viscosity measurements indicated that the gel structure formed soon after RBX crystallized, whereas a time delay was observed between the organogel formation and wax crystallization of CRX and CLX. These results demonstrate RBX’s good organogel‐forming properties, mostly because of its fine dispersion of long needle like crystals in liquid oil phases.

Effects of Added Clay on the Properties of Freeze-Casted Composites of Silica Nanoparticles

An experimental study was performed to investigate the effects of added clay particles on the structure and properties of porous, freeze-dried composites produced from silica nanoparticles. The sol–gel transition of the silica suspension arrests the sedimentation of the dense clay platelets to produce a gel, containing large clay particles dispersed throughout. No chemical additives other than sodium chloride were needed to cause the gelation. Dry composites were obtained by subjecting the gelled suspensions to rapid freezing in one of the following ways: submersion in liquid nitrogen or liquid propane, or exposure to low-temperature refrigeration. Frozen samples were then sublimated under vacuum to remove the water. Scanning electron microscope images of the dried composites showed a porous material whose pore sizes decrease with an increasing rate of freezing. The dependence on freeze rate suggests that the porous structure is the result of ice formation. Addition of the clay particles significantly increases the rate of gel formation. Furthermore, the strength of the clay/nanoparticle composites, as measured by normal compression tests, was substantially greater than that of a similar material made with only silica nanoparticles. Finally, BET surface area analysis shows a decreasing surface area with increasing volume fraction of clay at a constant volume fraction of silica.

Evolution of ordinary Portland cement hydration with admixtures by spectroscopic techniques

The various waterproofing admixtures may prevent water from penetrating concrete in different ways, but primarily by rendering it hydrophobic. Spectroscopic techniques have proved to be the appropriate tools for studying cement hydration in the presence of different admixtures. A combination of Fourier Transform-Raman and 29 Si nuclear magnetic resonance (NMR) spectroscopy were used here to monitor OPC hydration and C-S-H gel formation with two different waterproofing admixtures. The Q1/Q0 ratio was determined with the former technique to evaluate the influence of the waterproofing substances on C-S-H gel formation rate and intensity, whereas 29Si NMR was employed to establish the relative amount of C-S-H gel formed at different hydration times as well as to calculate the main chain length.

Papain‐induced Gelation of Soy Glycinin (11S)

ABSTRACT: The gelation of soy peptides produced by the action of papain enzymes on soy glycinin (11S) dispersions (4.7% w/v) was investigated. Cation‐exchange chromatography was used to fractionate crude papain. The nonbinding fraction showed no gel‐forming activity on the 11S dispersion. Two binding fractions showed gel‐forming activity, and the gel strength of both 11S gels was similar. The activity of the crude papain on 11S dispersions produced a slightly stronger gel than one formed with either of the 2 binding fractions. With the crude papain, the rate of gel formation appeared to be strongly influenced by the enzyme concentration, but the maximum gel strength was independent of enzyme concentration. When the temperature was increased, the papain treatment of 11S soy protein produced weaker gels when the measurement was made at the temperature of formation. This dependence of maximum gel strength on temperature was found to be a function of only the measurement temperature and not the gel formation temperature. The degree of protein hydrolysis at maximum gel strength was similar (∼6%) for the gels formed at different temperatures. When the temperature was increased, the elastic modulus G′, the viscous modulus G″, and the degree of viscoelasticity (G″/G′) decreased. This suggested that the gels were formed the by hydrophobic interactions among the peptides. This observation was supported by particle size measurements on samples of gels which were mixed with reagents known for their ability to disrupt hydrophilic/electrostatic, hydrophobic, or disulfide interactions.

Changes in properties of silane–water crosslinked metallocene ethylene‐octene copolymer after prolonged crosslinking time

AbstractThe silane–water crosslinking of metallocene ethylene–octene copolymer is described. Particular consideration is made on the changes in mechanical, thermal, and rheological properties of crosslinked polymers after prolonged crosslinking time. The silane grafting is carried out in a twin‐screw extruder, using a free‐radical reaction of vinyl trimethoxysilane and dicumyl peroxide. The crosslinking reaction is conducted at 70°C in the presence of water. The results show that the rate of gel formation is very high in the early stage of crosslinking process and decreases thereafter. The maximum gel content of 77% is obtained after 60 h of immersing time. Increasing the immersing time beyond 60 h, the gel content does not increase further. However, the FTIR results reveal an increase in siloxane linkages in those samples, indicating an increase in crosslink density without a simultaneous increase in gel content. This finding is supported by a rheological study where a significant increase in storage modulus is clearly observed. As a result of denser network, movement and flexibility of polymer chains are more restricted, leading to some changes in tensile properties. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 901–906, 2004

Temperature dependence of the formation and melting of pectin–Ca 2+ networks: a rheological study

Small deformation dynamic rheometry was used to characterise the calcium-induced gelation of low-methoxyl pectins, at two different pH values. The gelation kinetics was interpreted with basis on the change of the storage modulus with time, taken as a measure of changes in cross-linking density within the pectin–calcium network. The temperature influence on the rate of gel formation and ageing was evaluated, as well as the temperature sensitivity of the cured gels. The formation and the softening/melting of pectin–calcium networks were also studied under non-isothermal conditions. The structural diversity of the two pectin samples had a great influence on the gelation kinetics and thermal behaviour of these pectin–calcium networks, due to differences in the steric arrangement or environment and/or availability of the chelating groups. An association mechanism is suggested to predominate under conditions of low availability of dissociated carboxylic groups, due to a low pH, higher degree of methylation, or steric constraints introduced by acetylation or neutral side chains, different from the classical ‘egg-box’ model, and where non-ionic hydrogen bonding and hydrophobic interactions may co-exist and cooperate with coordinative binding of calcium-ions.

Effect of CTBN rubber inclusions on the curing kinetic of DGEBA–DGEBF epoxy resin

The curing kinetics of an epoxy resin matrix, based on diglycil ether of bisphenol A and F (DGEBA–DGEBF), associated with an anhydride hardener, at different carboxyl-terminated copolymer of butadiene and acrylonitrile liquid rubber (CTBN) concentration (0–10 phr) are studied using a differential scanning calorimetry (DSC) and a stress-controlled rheometer in isothermal and dynamic conditions. The aim of this work is to correlate the presence of the rubber phase with the transition phenomena that occur during the curing process. The CTBN rubber induces a catalytic effect on the polymerization of the pure resin clearly observed by a significant enhancement of the curing rate. Calorimetric and rheological analysis also evidences that gelation and vitrification times take place not punctually but in a wide range of time. Rheological data show that the presence of rubbery phase induces a higher rate of gel formation during the early stages of the reactions, confirming the calorimetric results. Finally the results are compared with theoretical models evidencing a good fitting between experimental and predictive data.

Synthesis and gelation of DOPA-modified poly(ethylene glycol) hydrogels.

3,4-Dihydroxyphenylalanine (DOPA) residues are known for their ability to impart adhesive and curing properties to mussel adhesive proteins. In this paper, we report the preparation of linear and branched DOPA-modified poly(ethylene glycol)s (PEG-DOPAs) containing one to four DOPA endgroups. Gel permeation chromatography-multiple-angle laser light scattering analysis of methoxy-PEG-DOPA in the presence of oxidizing reagents (sodium periodate, horseradish peroxidase, and mushroom tyrosinase) revealed the formation of oligomers of methoxy-PEG-DOPA, presumably resulting from oxidative polymerization of DOPA endgroups. In the case of PEG-DOPAs containing two or more DOPA endgroups, oxidative polymerization resulted in polymer network formation and rapid gelation. The amount of time required for gelation of aqueous PEG-DOPA solutions was found to be as little as 1 min and was dependent on the polymer architecture as well as the type and concentration of oxidizing reagent used. Analysis of reaction mixtures by UV-vis spectroscopy allowed the identification of reaction intermediates and the elucidation of reaction pathways. On the basis of the observed reaction intermediates, oxidation of the catechol side chain of DOPA resulted in the formation of highly reactive DOPA-quinone, which further reacted to form cross-linked products via one of several pathways, depending on the presence or absence of N-terminal protecting groups on the PEG-DOPA. N-Boc protected PEG-DOPA cross-linked via phenol coupling and quinone methide tanning pathways, whereas PEG-DOPA containing a free amino group cross-linked via a pathway that resembled melanogenesis. Similar differences were observed for the rate of gel formation as well as the molecular weight between cross-links ((-)M(c)), calculated using equilibrium swelling and the Flory-Rehner equation.

Epoxy‐functionalized, low glass‐transition temperature latex. I. Synthesis, characterizations, and polymer interdiffusion

AbstractResults are presented on the synthesis and characterization of epoxy‐functionalized poly(butyl acrylate‐co‐methyl methacrylate) latex dispersions. Two pairs of latex dispersions, of low and high molecular weights, were prepared by semicontinuous emulsion polymerization. In each pair, one latex was labeled with phenanthrene, a donor for nonradiative energy‐transfer (ET) experiments, and the other was labeled with anthracene, the acceptor. Films were prepared from 1:1 mixtures of donor‐ and acceptor‐labeled latex, and ET experiments were used to study the rate and extent of polymer diffusion in these films. A complicating feature of these experiments was that, even in the absence of an externally added crosslinking agent, the films began to gel as soon as they were formed. The rate of gel formation increased with increasing temperature but never reached 100% under the conditions of these experiments. The presence of a gel fraction caused the rate of polymer diffusion to slow down. An important parameter affecting the rate of diffusion is the amount of time the films are held at relatively low temperatures (4 and 25 °C) before being annealed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2609–2625, 2002

Xerogels Modified with 4-(2-Pyridylazo)-Resorcinol and Cetylpyridinium Chloride: Determination of Zinc in Solutions by Solid-Phase Spectrophotometry

Silicon dioxide-based xerogels modified with 4-(2-pyridylazo)-resorcinol (PAR) and cetylpyridinium chloride (CP) were synthesized. The concentration of PAR and CP in the hydrolyzed mixture in the ranges (0.6–3.10) × 10–3 and (5–12) × 10–3 M, respectively, does not affect the rate of gel formation as well as the specific surface area and average pore diameter of xerogels obtained upon drying in a microwave oven with a power of 600 Wt. It was found that the retention of PAR can be significantly improved in the presence of CP. Reactions of zinc with modified xerogels were studied by solid-phase spectrophotometry at different concentrations of PAR and CP, pH of the solution, and equilibrium time. A procedure was developed for determining 0.1–4.5 mg/L zinc in solutions by solid-phase spectrophotometry. Results of determining zinc in acid soil extracts and milk ash are given.

Effects of Preparation Temperature on Gelation Properties and Molecular Structure of High‐Amylose Maize Starch

ABSTRACTIn this study, 3% aqueous high‐amylose maize starch (Hylon VII) dispersions were heated to temperatures of 140–165°C. The onset and rate of gel formation was observed using a small‐strain oscillation rheometer as a function of temperature from 90 to 25°C. The gel formation clearly began earlier in high‐amylose starch paste preheated at lower temperatures, but the rate of gelation was slower and the resulting gel was weaker in comparison with starch pastes preheated at higher temperatures. In addition, the structure of the final gels was studied using large deformation compression measurements. The most rigid gel structure on the basis of small and large deformation tests was obtained for high‐amylose starch gel preheated to 150–152°C, depending on the type of measurement. The rate of gelation was also fastest in that temperature range. High‐amylose gels heated to higher temperatures lost their rigidity. The molecular weight distribution of starch molecules was measured by size‐exclusion chromatography. Heating caused extensive degradation of amylopectin, which had a great effect on amylose gel formation and the final gel properties of high‐amylose maize starch. Micrographs of Hylon VII gels showed that phase separation of starch components visible in light microscopy occurred on heating to higher temperatures.

New sol–gel synthetic route to transition and main-group metal oxide aerogels using inorganic salt precursors

We have developed a new sol–gel route to synthesize several different transition and main-group metal oxide aerogels. The approach is straightforward, inexpensive, versatile, and it produces monolithic microporous materials with high surface areas. Specifically, we report the use of epoxides as gelation agents for the sol–gel synthesis of chromia aerogels and xerogels from simple Cr(III) inorganic salts. The dependence of both gel formation and its rate was studied by varying the solvent used, the Cr(III) precursor salt, the epoxide/Cr(III) ratio, as well as the type of epoxide employed. All of these variables were shown to affect the rate of gel formation and provide a convenient control of this parameter. Dried chromia aerogels were characterized by high-resolution transmission electron microscopy (HRTEM) and nitrogen adsorption/desorption analyses, results of which will be presented. The results presented here show that rigid monolithic metal oxide aerogels can be prepared from solutions of their respective metal ion salts (Fe 3+, Al 3+, In 3+, Ga 3+, Zr 4+, Hf 4+, Ta 5+, Nb 5+, and W 6+), provided the formal oxidation state of the metal ion is greater than or equal to +3. Conversely, when di-valent transition metal salts are used precipitated solids are the products.

Use of Epoxides in the Sol−Gel Synthesis of Porous Iron(III) Oxide Monoliths from Fe(III) Salts

Iron oxide-based porous solids were prepared by a sol−gel process using Fe(III) salts in various solvents. It was observed that the addition of propylene oxide to Fe(III) solutions resulted in the formation of transparent red-brown monolithic gels. The resulting gels were converted to either xerogels by atmospheric drying or aerogels by supercritical extraction with CO2(l). Some of the dried materials were characterized by nitrogen adsorption and desorption analysis and transmission electron microscopy (TEM). The results of those analyses indicate that the materials have high surface areas (∼300−400 m2/g), pore sizes with mesoporic dimensions (2−23 nm), and a microstructure made up of 5−10 nm diameter clusters of iron(III) oxide. The dependence of both gel formation and its rate was studied by varying the epoxide/Fe(III) ratio, the Fe(III) precursor salt, amount of water (H2O/Fe(III)) present, and the solvent employed. All of these variables were shown to affect the rate of gel formation and provide a con...

Irradiation of crosslinked, poly(vinyl alcohol) blended hydrogel for wound dressing

In order to obtain a more ideal hydrogel wound dressing, crosslinked hydrogel films blended with polyvinyl alcohol (PVA), polyvinyl pyrrolidone, kappa-carrageenan (KC), and powder silk were prepared by electron beam, and their physiochemical properties were investigated as a combination of function factors. The experimental results showed that the gel fraction of the hydrogel films depended mainly on irradiation dose and the monomer concentration of the polymers, the properties of hydrogel could be greatly extended or improved by blending homopolymers. The rate of gel formation of the hydrogel was raised, and the water evaporation from hydrogel could be retarded after mixing with KC, while the tensile strength of hydrogel films were obviously increased after mixing with silk. Toxicity and healing effect of PVA/PVP/KC/silk blended hydrogel films as wound dressings were evaluated. The irradiated blended hydrogel showed satisfactory properties for wound dressing, the hydrogel did not induceany acute general toxic effects, and it is effective for fast healing of wound.

High-pressure treatment of milk: effects on casein micelle structure and on enzymic coagulation.

High isostatic pressures up to 600 MPa were applied to samples of skim milk before addition of rennet and preparation of cheese curds. Electron microscopy revealed the structure of rennet gels produced from pressure-treated milks. These contained dense networks of fine strands, which were continuous over much bigger distances than in gels produced from untreated milk, where the strands were coarser with large interstitial spaces. Alterations in gel network structure gave rise to differences in rheology with much higher values for the storage moduli in the pressure-treated milk gels. The rate of gel formation and the water retention within the gel matrix were also affected by the processing of the milk. Casein micelles were disrupted by pressure and disruption appeared to be complete at treatments of 400 MPa and above. Whey proteins, particularly beta-lactoglobulin, were progressively denatured as increasing pressure was applied, and the denatured beta-lactoglobulin was incorporated into the rennet gels. Pressure-treated micelles were coagulated rapidly by rennet, but the presence of denatured beta-lactoglobulin interfered with the secondary aggregation phase and reduced the overall rate of coagulation. Syneresis from the curds was significantly reduced following treatment of the milk at 600 MPa, probably owing to the effects of a finer gel network and increased inclusion of whey protein. Levels of syneresis were more similar to control samples when the milk was treated at 400 MPa or less.

Gel formation and the efficiency of anodic film growth on aluminium

The development of gel layers during anodizing, and their influence on the efficiency of growth of anodic films, has been examined for formation of films at constant current density on aluminium in saturated potassium antimonate electrolyte and 0.1 M sodium molybdate, sodium silicate and sodium tungstate electrolytes. The gels are produced immediately above the growing anodic films by the reaction of H + ions, generated at the film/gel interface, with the electrolyte anions to form uniform layers of hydrated oxide. The gels can enhance the efficiency of film growth by reducing, or eliminating, field-assisted ejection of Al 3+ ions from the film to the electrolyte. The thicknesses of the gel layers increase at constant rates with thickening of the anodic films, although the efficiencies of gel formation per se are relatively low. The thickest gels are obtained following anodizing in antimonate electrolyte possibly reflecting a more favorable rate of gel formation, relative to its rate of dissolution, than that for other gel layers. The gel layers may shrink and crack on drying, most noticeably for the gels formed in antimonate and tungstate electrolytes. The gel layers formed in molybdate electrolyte appear, by comparison, highly resistant to cracking.

Gelation kinetics of scleraldehyde–chitosan co-gels

Abstract The gelation kinetics of modified scleroglucan carrying various fractions of reactive aldehyde groups in the side-chains, scleraldehyde, and chitosan was determined using oscillatory shear rheometry to study the dynamic viscoelastic properties as a function of reaction time. The experimentally determined initial increase in the storage modulus per unit time, ΔG′/Δt, for various scleraldehyde concentrations, CSc, degree of aldehyde substitution of scleraldehyde, DAld, and chitosan concentrations, CChi, revealed the existence of a lower critical concentration for gelation for both the two polysaccharides, CSc,0 and CChi,0. The obs erved power law coefficients m and n, ΔG′/Δt∼(DAldCSc)mCnChi, were both significantly larger than one in the employed relative concentrations ranges CSc/CSc,0∈(1–20) and CChi/CChi,0∈(1–10), which was accounted for by the total wastage reactions and network statistics. The rate of gel formation showed a maximum near pH 7 due to a delicate balance among several competing effects: (1) The free-amine concentration of the chitosan increases in the pH range 4.5–6.5 (pKa of the amino group), (2) the Schiff-base junction formation is acid catalyzed, and (3) the solubility of chitosan decreases when the pH exceeds 7. The increased aggregation resulting from increasing the pH from 7 to 7.6 yielded an increasing fraction of reactive groups involved in wastage reactions, yielding an increase both in CChi,0 and the power law coefficient n from 1.7 to 2.6. The temperature dependence of ΔG′/Δt could be accounted for by incorporating an apparent activation energy Ea=43 kJ mol-1. The temperature dependence of G′ of the fully cured scleraldehyde–chitosan co-gels was found to give rise to a shift from ΔG′/ΔT>0 Pa/min to ΔG′/ΔT

Pulsed e − beam irradiation of polymers—A comparison of dose rate effects and let effects

Linear Energy Transfer (LET) effects in radiation chemistry are ascribed to the high density of active species in the track structure, resulting in overlapping of spurs. We studied the possibility of spur overlapping in electron beam irradiation at extremely high dose rate, both theoretically and experimentally. Considering differences in overlap mode leads to the concept of a “threshold dose rate”, above which spur overlapping may occur, and an “overlapping dose”, which is necessary to cause overlapping of spurs even at higher dose rate than the threshold, depending on lifetime and effective volume of reactive intermediates. Using Sandia's pulsed power e − beam system, we irradiated cellulose triacetate (CTA), polymethylmethacrylate (PMMA), polycarbonate (PC) and high density polyethylene (HDPE) at a dose rate as high as 4 × 10 10 Gy/s at room temperature in the absence of oxygen. Comparison of the e − beam results with data obtained from gamma irradiation at 0.5 Gy/s showed no dose rate effects based on discoloration sensitivity for CTA, or on scission probabilities of PMMA and PC. For HDPE, the results indicated a slightly lower rate of gel formation under e − beam irradiation, implying that the crosslinking efficiency may be somewhat reduced at the high dose rate.