Gel Formation Temperature Research Articles

Tightening of gelatin chemically crosslinked networks assisted by physical gelation

ABSTRACTDeveloping the use of polymers from renewable sources to build hydrogels with tailored mechanical properties has become an increasing focus of research. The impact of the thermo‐reversible physical networks of gelatin (arising from the formation of triple‐helices) on the structure formation of a chemical network, obtained by crosslinking with glutaraldehyde (a non‐catalytic crosslinker), was studied using optical rotation, oscillatory rheology, and large strain mechanical deformation. We observed a direct correlation between the storage shear modulus of the chemical network grown in the gel state (i.e., simultaneously with the physical network) and the amount of gelatin residues in the triple‐helix conformation (χ). Since χ is directly affected by temperature, the value of the storage modulus is also sensitive to changes in the temperature of gel formation. χ values as low as 12% lead to an increase of the shear storage modulus of the crosslinked gel by a factor of 2.7, when compared to a chemical network obtained in the sol state (i.e., in the absence of a physical network). Our results show that the physical network acts as a template, which leads to a greater density of the chemical crosslinks and a corresponding higher elastic modulus, beyond what is otherwise achieved in the absence of a physical network. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1850–1858

STUDY OF VISCOSITY OF AQUEOUS SOLUTIONS OF NATURAL POLYSACCHARIDES

One of the steps of synthesis of biodegradable polymers is preparation of an aqueous solution of raw materials. Formulation of biodegradable films with optimum characteristics requires to undertake a separate rheological study of each aqueous solution. An essential parameter in this step is a uniform thickness, which is achieved by means of specified viscosity parameters. Viscosity of solutions depends on many parameters, among which are composition and concentration of components in a solution, solution preparation temperature, pH of the finished medium, and presence of free ions. This article describes studies and results on study of viscosity parameters of aqueous solutions of natural polysaccharides: agar-agar, hydroxypropyl methyl cellulose and carrageenan. For aqueous solutions of carrageenan and hydroxypropyl methylcellulose, viscosity was measured at 25°C. In addition, viscosity parameters were measured for 1.5% hhydroxypropyl methylcellulose solutions at 40°C and 60°C. Depending on the gel formation temperature, viscosity of agar-agar solutions was measured at 50°C or 70°C. According to the results of experiments, it was found that viscosity of 1.0-1.5% hydroxypropyl methylcellulose aqueous solutions is in the range of 8.0-80.0 cP. Heating to 100°C at pH 6 results in irreversible destruction of hydroxypropylmethylcellulose molecules. Aqueous solutions of agar-agar are similar to hydroxypropyl methylcellulose solutions but agar-agar is able to gelate at sufficiently low concentrations (from 0.5%). Viscosity of 3.0- 5.0% carrageenan aqueous solutions varies within a wide range: 1.5-1400.0 cP. Thus, rheological properties allow to adjust viscosity of the process mixture in the production of biodegradable polymers in the desired range and in different directions.

Low-temperature synthesis and investigations on photocatalytic activity of nanoparticles BiFeO3 for methylene blue and methylene orange degradation and some toxic organic compounds

The photocatalytic BiFeO3 perovskite nanoparticles were fabricated by gel combustion method using polyvinyl alcohol and corresponding metal nitrate precursors under the optimum mild conditions such as pH 2, gel formation temperature of 80 °C, metal/polyvinyl alcohol molar ratio of 1/3, metal molar ratio Bi/Fe of 1/1 and calcination temperature at 500 °C for 2 h. The prepared sample was characterized by x-ray diffraction, field scanning electron microscopy, transmission electron microscopy, Brunauer–Emmetl–Teller nitrogen adsorption method at 77 K, energy dispersive x-ray spectroscopy, ultraviolet-visible light spectrophotometry, and thermal analysis. The effects of molar ratios of starting material and calcination temperature on phase formation and morphology were investigated. The degradation of methylene blue, methylene orange and some toxic organic compounds such as phenol and diazinon under visible light irradiation by photocatalytic BiFeO3 nanoparticles were evaluated at different parameters and conditions such as the light intensity determined from the light source to the measured sample, the addition H2O2, reaction time and the regeneration performance. Obtained results showed that the synthesized perovskite BiFeO3 nanoparticles for the optimized sample have a size smaller than 50 nm and the high mean surface area of 50 m2 g−1. Degradation efficiency was almost 90.0% for methylene blue and 80.0% for methylene orange with added H2O2 after 30 min of reaction. After the 3rd time of regeneration, the BiFeO3 nanoparticles still have 92.8% of the degradation performance for removing methylene blue. Phenol and diazinon toxic compound were degraded with the performance of 92.42% and 85.7%, respectively, for 150 min

Nghiên cứu ảnh hưởng của nhiệt độ nung, tỷ lệ mol Ce-Ti-Si đến sự hình thành pha oxit hỗn hợp CeO2-TiO2-SiO2 được tổng hợp bằng phương pháp đốt cháy gel polyvinyl ancol

The nanosized CeO2-TiO2-SiO2 mixed oxide powder has been synthesized by the gel combustion method using polyvinyl alcohol (PVA) as polymer basic and mixture of cerium (IV) nitrate, titanium (III) chloride and silane. Effects of preparative parameters on the phase formation of CeO2-TiO2-SiO2 such as Ce-Ti-Si molar ratio, calcination temperature have been investigated and discussed. The results showed that, for the optimized procedure with pH = 1, temperature of gel formation of 80 oC, molar ratio of metal over polyvinyl alcohol of 1/3, molar ratio of Ce-Ti-Si of 1-1-8. The CeO2-TiO2-SiO2 mixed oxides were obtained with the averge grain size of nanopowders up to 97 nm after calcined at 850 oC for 2 hours. Keywords: CeO2-TiO2-SiO2 mixed oxides, cerium(IV) nitrate, titanium (III) chloride, silane, polyvinyl alcohol, combustion method.

NMR studies of thermo-responsive behavior of an amphiphilic poly(asparagine) derivative in water

The thermo-responsive behavior of a unique biocompatible polymer, poly(N-substituted α/β-asparagine) derivative (PAD), has been studied with several NMR methods. The 1H and 13C solution NMR measurements of the PAD in DMSO-d6 were used to investigate the isolated polymer and perform spectral assignments. By systematic addition of D2O we have tracked structural changes due to aggregation and observed contraction of hydrophilic side chains. Solution and cross polarization/magic angle spinning (CP/MAS) 13C NMR approaches were implemented to investigate the aggregates of the PAD aqueous solution during the liquid to gel transition as the temperature was increased. At temperatures near 20 °C, all of the peaks from the PAD were observed in the 13C CP/MAS and 13C solution NMR spectra, indicating the presence of polymer chain nodes. Increasing the temperature to 40 °C resulted in a partial disentanglement of the nodes due to thermal agitation and further heating resulted in little to no additional structural changes. Deuterium T1–T2 and T2–T2 two-dimensional relaxation spectroscopies using an inverse Laplace transform, were also implemented to monitor the water–PAD interaction during the phase transition. At temperatures near 20 °C the dynamical characteristics of water were manifested into one peak in the deuterium T1–T2 map. Increasing the temperature to 40 °C resulted in several distinguishable reservoirs of water with different dynamical characteristics. The observation of several reservoirs of water at the temperature of gel formation at 40 °C is consistent with a physical picture of a gel involving a network of interconnected polymer chains trapping a fluid. Further increase in temperature to 70 °C resulted in two non-exchanging water reservoirs probed by deuterium T2–T2 measurements.

MECHANICAL PROPERTIES OF COLLAGEN FIBER AND POWDER GELS

ABSTRACT Mechanical properties and water holding capacity (WHC) of collagen gels were evaluated using two ingredients with different particle size and shape. The variables evaluated were ingredient concentration (3, 5, and 10%, w/w), NaCl addition (0 and 2% w/w), heat treatment temperature (25, 60, and 90C), gel formation temperature (10 and 25C), and the soluble/insoluble protein ratio. The collagen ingredients produced strong, highly elastic gel networks with stress at fracture values from 0.4 to 68 kPa and values for the elasticity modulus from 1.4 to 17 kPa, for the range of variables studied. The gel network strength and WHC were enhanced by a decrease in particle size, lower incubation temperatures, and by increases in soluble protein, ionic strength of the medium and heat treatment temperature. PRACTICAL APPLICATIONSCollagen has a unique molecular structure that provides ingredients of interest for incorporation into food products and other biomaterials, showing different technological properties depending on the process conditions. Once the effect of size and shape of the particle, temperature, pH, and salt addition on the mechanical properties, water holding capacity and physico‐chemical characteristics of bovine collagen gels is known, process parameters and formulations can be optimized, in order to establish desirable texture properties for food products and other biomaterials.

Rheological properties of patatin gels compared with β-lactoglobulin, ovalbumin, and glycinin

The thermal unfolding and rheological properties of patatin gels were compared with those of commonly used proteins (β-lactoglobulin, ovalbumin, glycinin). A significant difference between these proteins was observed in both the denaturation temperature (59 °C for patatin; about 20 °C lower than the other proteins) and the onset temperature of gel formation (50-60 °C, compared to 70-85 °C for the other proteins). At low ionic strength the minimal concentration was only 6% (w/v) for patatin, compared to 8-11% for the other proteins. This effect was attributed to the relatively high exposed hydrophobicity of patatin as determined by hydrophobic interaction chromatography. For gels compared at 'iso-strength', the frequency dependence was found to be close to identical, while small differences were observed in the strain at fracture. Patatin was found to form gels with comparable small-deformational rheological properties as typical food proteins. In addition, at concentrations where the elastic modulus was similar for all proteins, the frequency and strain dependence were also comparable. From this it is concluded that patatin is a promising protein to be used in food applications as a gelling agent.

Carbon nanotube reinforced supramolecular gels with electrically conducting, viscoelastic and near-infrared sensitive properties

Pristine and long-chain functionalized single-walled carbon nanotubes (SWNTs) were incorporated successfully in supramolecular organogels formed by an all-trans tri(p-phenylenevinylene) bis-aldoxime to give rise to new nanocomposites with interesting mechanical, thermal and electrical properties. Variable-temperature UV-vis and fluorescence spectra reveal both pristine and functionalized SWNTs promote aggregation of the gelator molecules and result in quenching of the UV-vis and fluorescence intensity. Electron microscopy and confocal microscopy show the existence of a densely packed and directionally aligned fibrous network in the resulting nanocomposites. Differential scanning calorimetry (DSC) of the composites shows that incorporation of SWNTs increases the gel formation temperature. The DSC of the xerogels of 1-SWNT composites indicates formation of different thermotropic mesophases which is also evident from polarized optical microscopy. The reinforced aggregation of the gelators on SWNT doping was reflected in the mechanical properties of the composites. Rheology of the composites demonstrates the formation of a rigid and viscoelastic solid-like assembly on SWNT incorporation. The composites from gel-SWNTs were found to be semiconducting in nature and showed enhanced electrical conductivity compared to that of the native organogel. Upon irradiation with a near IR laser at 1064 nm for 5 min it was possible to selectively induce a gel-to-sol phase transition of the nanocomposites, while irradiation for even 30 min of the native organogel under identical conditions did not cause any gel-to-sol conversion.

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.

Clove Oil Prevents Glycyrrhizin Gel Formation in Aqueous Solution

One persistent problem with using therapeutic concentrations of glycyrrhizin (GZ) is that, at these high concentrations, it forms a gel in an aqueous solution. We previously solved this problem by dissolving GZ in a highly concentrated phosphate buffer. Unfortunately, the resulting GZ solution has a hyperosmotic pressure that renders it unsuitable for use in patients. The aim of this study was to prepare a highly concentrated GZ solution having an osmotic pressure ratio of 1 and a pH of 7.4. By adding small amounts of oil and using a 100 mM phosphate buffer, we achieved an emulsified GZ solution that is stable at room temperature and has a physiological osmotic pressure and pH. When clove oil was used as an emulsifier, the gel formation temperature of GZ solution decreased appreciably compared to that of GZ solution without clove oil. Using scanning electron microscopy (SEM), we examined the detailed characteristics of GZ gels prepared from solutions with or without clove oil. SEM of cross sections of GZ gels revealed an irregular structure in gels prepared with clove oil, indicating that clove oil prevented the formation of the intermolecular GZ networks typically characterized by gels derived from pure GZ solutions.

Thermoreversible gelation of a polystyrene–poly(ethylene/butylene)–polystyrene triblock copolymer in n‐octane/4‐methyl‐2‐pentanone solutions

AbstractThe thermoreversible gelation of a triblock copolymer polystyrene‐block‐poly(ethylene/butylene)‐block‐polystyrene in n‐octane and two solvent mixtures of n‐octane and 4‐methyl‐2‐pentanone with a high n‐octane content has been studied. n‐Octane and 4‐methyl‐2‐pentanone are selective solvents for the middle poly(ethylene/butylene) block and the end polystyrene blocks, respectively. The influence of the solvent composition on the sol–gel transition and the mechanical properties of the gels was studied. The gel formation temperature increased with the copolymer concentration and the n‐octane content in the solvent system. The mechanical properties of the different gels were studied through oscillatory shear measurements. The concentration dependence of the elastic storage modulus showed an exponent close to that expected for gels in good solvents (2.25) that possess a structure similar to those of chemical networks.© 2002 Society of Chemical Industry

Curdlan gels as protein drug delivery vehicles

The use of curdlan, a natural β-1,3-glucan, in protein drug delivery vehicles was studied by carrying out in vitro release studies with curdlan gels containing bovine serum albumin (BSA) as a model protein. Addition of urea (8 M) decreased the gel formation temperature to 37 °C. Curdlan was hydroxyethylated in order to form gels under mild conditions such as physiological temperature and pH. In gels formed in 8 M urea solution, urea was almost released after 2 h while BSA was completely released after 45–100 h. The total time for complete release of BSA increased with curdlan concentration within gels. The strength of hydroxyethylated curdlan gels (385.7 dyne cm−2) was weaker than that of curdlan gels formed in 8 M urea solution (6277 dyne cm−2).

Role of galactomannan composition on the binary gel formation with xanthan

The influence of the galactomannan characteristic ratios ( M/ G) on the temperature of gelation ( T g) and the gel strength of mixtures of galactomannan with xanthan is reported. Two galactomannans were investigated: one highly substituted from the seeds of Mimosa scabrella ( M/ G=11), and the other, less substituted, from the endosperm of Schizolobium parahybae, with ( M/ G=30) [Ganter JLMS, Zawadzki-Baggio SF, Leitner SC, Sierakowski MR, Reicher F. J Carbohydr Chem 1993;12:753]. The xanthan:galactomannan systems (4:2 g l −1, in 5 mM NaCl) showed a temperature of gel formation ( T g) of 24°C for that of S. parahybae [Bresolin TMB, Milas M, Rinaudo M and Ganter JLMS. Int J Biol Macromol 1998;23:263] and 20°C for the galactomannan of M. scabrella, determined by viscoelastic measurements and microcalorimetry. A T g of 40–50°C was found by Shatwell et al. [Shatwell KP, Sutherland IW, Ross-Murphy SB, Dea ICM. Carbohydr Polym 1991;14:29] for locust bean gum-LBG ( M/ G=43). Lundin and Hermansson [Lundin L, Hermansson AM. Carbohydr Polym 1995;26:129] reported a difference of 13°C for T g of two LBG samples with M/ G=3 (40°C) and 5 (53°C), in mixtures with xanthan. It appears that the more substituted galactomannans have lower temperatures of gelation in the presence of xanthan. The mechanism of gelation depends also on the M/ G ratio. For the lower values it involves only disordered xanthan chains in contrast to M/ G ratios higher than 3. In addition, the presence of the galactomannan from M. scabrella increased slightly the temperature of the conformational change ( T m) of xanthan probably due to the ionic strength contribution of proteins (3.9%) present in the galactomannan. On the other hand, the galactomannans from S. parahybae, with 1.5% of proteins and M. scabrella, with 2.4% of protein, did not show this effect, the T m of xanthan alone or in a mixture being practically unchanged.

Gelation Mechanism of Curdlan by Dynamic Viscoelasticity Measurements

ABSTRACTGelation of curdlan was classified into two forms: thermo‐reversible and thermo‐irreversible gelation. The temperature dependence of the rheological properties indicated that the temperature at which the transition to thermo‐irreversible gel system occurred was concentration‐dependent, while thermo‐reversible gel formation temperature was independent of concentration. An increase in concentration lowered the irreversible gel formation temperature from the generally reported 80°C and decreased the amount of thermo‐reversible component in the final gel. This indicates that an increase in the curdlan concentration promoted hydrophobic interactions between micelles at junction zones at a lower temperature than 80°C.

Xanthan–galactomannan interactions as related to xanthan conformations

The influence of xanthan conformation on the physicochemical behaviour of their mixtures with galactomannan from Schizolobium parahybae mannose:galactose ratio (M/G=3), was studied by viscoelastic measurements, differential scanning calorimetry (DSC) and chiroptical (circular dichroism) methods. The results suggested a more effective interaction of the galactomannan with disordered xanthan segments, which are more abundant in low salt concentrations but are still present in lower proportion at temperatures lower than the temperature of xanthan conformational transition ( T m). The dependence of ellipticity with temperature in a circular dichroism (CD) spectra suggested an ordering of the xanthan chains induced by galactomannan at the temperature of gel formation ( T g≈25°C), under conditions where xanthan alone exhibits a disordered conformation. The lower T g value found (≈25°C) compared with that (60°C) usually described in the literature is certainly related to the M/G ratio and the galactosyl unit distribution along the mannan main chain.

Supramolecular structure of gel-forming systems of pectins of different origin

Two pectin preparations (citrus ‘Copenhagen Pectin’ and that derived from pumpkin) that formed opalescing suspensions in aqueous media were investigated. The values of concentration and mean size of supramolecular particles determined by a spectroturbidimetric method were used as indicators of the structural heterogeneity of the preparations. According to these criteria, the pumpkin pectin preparation had a higher degree of structural heterogeneity than that of the citrus pectin. The process of gel formation in the systems studied did not cause any significant turbidimetric effects that could have pointed to the formation of particles of a new phase. Resuspended fractions of the supramolecular particles, which were obtained from the pumpkin preparations by centrifugation, formed gels in acid solutions of sucrose. In this case, the temperature of gel formation was close to that recorded for the initial preparations. However, it was about 10°C below the temperature of gel formation registered for the supernatants of pumpkin pectin preparations that remained after removal of the supramolecular particles by centrifugation.

The effects of solvent type on the concentration dependence of the compression modulus of thermoreversible isotactic polystyrene gels

AbstractSolutions of isotactic polystyrene in either trans‐decalin or 1‐chlorodecane were transformed into gels by quenching from a high temperature (ca. 180°C) to −20°C. The relaxation modulus in compression of these gels was measured over a range of concentrations of from 0.04 g/g to 0.40 g/g. At 22°C, the gels show a double logarithmic stress relaxation rate, m, which is higher than for PVC and gelatin gel systems. 120 s isochronal modulus concentration diagrams exhibit non‐power law behavior, i.e., not only is the general trend such that the double logarithmic slope decreases with increasing concentration, but there are also regions in which abrupt changes in modulus occur over narrow ranges in concentrations. These features in the concentration dependence of the modulus are less pronounced than those found previously1 in isotactic polystyrene/cis‐decalin gels. The behavior is interpreted to be inconsistent with a fringed micelle picture of the gel structure. Preliminary results are reported indicating that polymer fraction and temperature of gel formation can significantly affect the modulus of the gels.

Physical properties of elastic duplicating materials.

Agar has been used for a number of years in the preparation of elastic duplicating materials. These duplicating materials have been used mainly in partial denture prosthesis for the preparation of a duplicate refectory cast of the original stone cast of the patient's mouth. Since moderately large quantities of duplicating materials are used in these procedures, the low cost and reversibility of the agar materials are a definite advantage. The mercaptan and silicone rubber elastic impression materials have not been used extensively in duplicating procedures because of their high cost and the fact that these materials, once set, are irreversible. Recently, a plastic duplicating material has been developed which is a reversible polyvinyl chloride plastic gel. Because of its chemical and physical nature, this material has elastic properties somewhat different from the agar compounds. The agar duplicating compounds are composed primarily of agar and water but may also contain borax to increase the strength, salts to accelerate the setting of the investment, and other ingredients. Cellulose fibers, waxes, and fillers, which may be found in impression compounds, generally are omitted from duplicating compounds, since the duplicating materials are often stored in the sol state for extended periods and these ingredients tend to separate under this condition of storage. Since the duplicating compounds may be stored at temperatures of 130°-150° F., their resistance to thermal degradation is an important property. The basic composition of the agar impression compounds is much the same as for the agar duplicating compounds. The principal difference is that the impression compounds may contain 10-15 per cent agar, while the duplicating materials may contain only 5 per cent agar and, as just indicated, need not contain waxes, fillers, and other modifiers common to impression materials. In fact, agar impression materials have been used as duplicating compounds by simply diluting the impression material with an equal amount of water. The properties of the agar impression compounds have been studied by numerous investigators. Skinner and Kern' determined the linear-dimensional changes in air and subsequently in water of various agar impression compounds. They also measured the compressive properties of proportional limit, maximum flexibility, and crushing strength, as well as the plastic flow properties of these agar materials. Paffenbarger2 reported the compressive properties of strength, deformation, set, and stiffness of a number of agar compounds. The temperature of gel formation and the impression-