Transition, sol-gel

Disc-like particles can also undergo an Onsager transition—here tire particles fonn a discotic nematic, where tire short particle axes tend to be oriented parallel to each other. In practice, clay suspensions tend to display sol-gel transitions, witliout a clear tendency towards nematic ordering (for instance, [22]). Using sterically stabilized platelets, an isotropic-nematic transition could be observed [119]. 

Olivares et al. (2006), studies performed viscometers very dilute gelatin solutions with concentrations between 10-5 and 10-3 g/cm3, where either intermolecular aggregation or intramolecular folding are possible, respectively, and the sol-gel transition is not observed. 

Djabourov M., Leblond J., Papon P. Gelation of aqueos gelatin solutions. II. Rheology of the sol-gel transition. J. Phys. France 49 (1988b) 333-343. 

Djabourov M., Grillon Y., Leblond J. The sol-gel Transition in gelatin viewed by Diffusing colloidal probes. Polymer Gels and Networks 3 (1995) 407-428. 

Matsumoto A, Chen J, Collette AL et al (2006) Mechanisms of silk fibroin sol-gel transitions. J Phys Chem B 110 21630-21638... 

Classical models of gel formation (or sol-gel transitions [4]) by Flory [394], Gordon, Ma-cosko and Miller [228,245], and others (see Ref. 4 for a more complete review) considered... 

Adam, M Lairez, D, Sol-gel transition. In Physical Properties of Polymeric Gels Cohen Addad, JP, ed. Wiley Chichester, UK, 1996 88. 

The polymer is capable of forming left-handed, double helices, and undergoes a sol-gel transition where a network is formed through cooperative association of the helices. Fig. 12 shows the c.d. spectra of agarose in aqueous solution at various temperatures. As the temperature is increased. 

Then, the ionic selectivity is discussed and related to the mechanism of crosslinking with divalent counterions. The sol-gel transition is then examined for LM and HM pectins and the mechanisms described in these two cases. The physical properties of the gels are related to the microstructure of the polymers and few data are examined. 

In the one phase region, when the sample was seen to flow easily, it was said that the system was still a sol. When the meniscus was seen not to deform under it own weight, the system was considered a gel. The sol-gel transition was taken at the onset of meniscus deformation when the tube is held horizontal. Syneresis and precipitation were detected by the presence of water at the gel surface or by the existence of large turbid aggregates which could be centrifugated. 

The sol-gel transition has been determined visually, with calcium and copper, for different pectins under different external conditions. As shown in Figure 5 for sample C44 the homogeneous gel phase is situated between the two transition lines. The extension of this phase was found to depend mainly on the DE, temperature and nature of the cation. With calcium the amount of cation required to get a gel increased with the degree of esterification and above 50% it became impossible to get a gel [8]. 

Figure 5. Sol-gel transition curve for sample C44 in O.IM NaCl, pH 7, 20°C, with copper ( ) and calcium (O) and phase separation curve with copper ( ) and calcium ( ).

Contrary to the phase separation curve, the sol/gel transition is very sensitive to the temperature more cations are required to get a gel phase when the temperature increases and thus the extension of the gel phase decreases [8]. The sol/gel transition as determined above is well reproducible but overestimates the real amount of cation at the transition. Gelation is a transition from liquid to solid during which the polymeric systems suffers dramatic modifications on their macroscopic viscoelastic behavior. The whole phenomenon can be thus followed by the evolution of the mechanical properties through dynamic experiments. The behaviour of the complex shear modulus G (o)) reflects the distribution of the relaxation time of the growing clusters. At the gel point the broad distribution of... 

Statistical analysis did not reveal any significant interactions between Na and K. Three-dimendonal plots for each deagn showed that the data of each mixtiu e design could be easily described by the data belonging to the border lines of the defined fields, because minimum and maximum values were located there. Thus, without loss of information, figure 1 gives a representative idea of the influence, which the studied cations exert on the sol / gel transition of the three pectins. The curvature in the dotted curves reveal the interactions between Ca and a monovalent cation. 

Despite the importance of initiators, synthesis conditions, and diluents on the properties of a gel, composition is, of course, the most important variable. When growing polymeric chains are first initiated, they tend to grow independently. As the reaction proceeds, different chains become connected through cross-links. At a critical conversion threshold, called the gel point or the sol-gel transition, enough growing chains become interconnected to form a macroscopic network. In other words, the solution gels. The reaction is typically far... 

During our early experiments on chemical gels, when first observing the intermediate state with the self-similar spectrum, Eq. 1-5, we simply called it viscoelastic transition . Then, numerous solvent extraction and swelling experiments on crosslinking samples showed that the viscoelastic transition marks the transition from a completely soluble state to an insoluble state. The sol-gel transition and the viscoelastic transition were found to be indistinguishable within the detection limit of our experiments. The most simple explanation for this observation was that both phenomena coincide, and that Eqs. 1-1 and 1-5 are indeed expressions of the LST. Modeling calculations of Winter and Cham-bon [6] also showed that Eq. 1-1 predicts an infinite viscosity (see Sect. 4) and a zero equilibrium modulus. This is consistent with what one would expect for a material at the gel point. 

Fig. 3.1 Ti me dependences of the complex viscosity measured at a frequency of 1 Hz. An aqueous solution was prepared by dissolving 30wt.% of precursor and 0.2wt.% of N-[(trimethox-ysilyl)propyl]-N,N,N-trimethylammonium chloride in 0.01 M sulfuric acid. The stages ofthe sol-gel processes discussed in the text are I - condensation and sol formation, II - sol-gel transition, III - gel maturation. (Unpublished results).

Fig. 3.7 Schematic drawings demonstrating the main features of two-stage (A) and one-stage (B) procedures leading to a difference in the morphology of the fabricated materials. (A) Sol nanoparticles initially prepared in the first stage (1, see also Figure 3.3) can self-assemble into a three-dimensional network when they are in direct contact with each other. Forthis reason, a gel formed after cross-linking (sol-gel transition) has a smaller volume (2). (B) The initial stage (1) is represented by a solution of entangled biopolymer macromolecules. The...

The next stage is a sol-gel transition that is accounted for by the formation of a three-dimensional network from cross-linked sol nanoparticles. It is apparent that it can happen only when the particles are in direct contact. The association of sol nanoparticles, as shown by Figure 3.7A, will inevitably result in shrinkage of the volume because of a decrease of the distance between them. The larger the initial distance and the denser the arrangement of particles in the gel, that is the larger the difference between the initial and final states in the system, the larger the syneresis. 

The first phase in the process is the formation of the sol . A sol is a colloidal suspension of solid particles in a liquid. Colloids are solid particles with diameters of 1-100 nm. After a certain period, the colloidal particles and condensed silica species link to form a gel - an interconnected, rigid network with pores of submicrometer dimensions and polymeric chains whose average length is greater than one micrometer. After the sol-gel transition, the solvent phase is removed from the interconnected pore network. If removed by conventional drying such as evaporation, so-called xerogels are obtained, if removed via supercritical evacuation, the product is an aerogel . 

In 1997, Kim and coworkers first developed biodegradable IP systems using a triblock copolymer of PEG and PLLA, PEG-b-PLLA-b-PEG, and demonstrated sustained release of drugs from the hydrogel [127]. After this achievement, many kinds of biodegradable amphiphilic block copolymers (including multiblock copolymers) exhibiting temperature-responsive sol-gel transition have been reported [137, 308-318]. In this review, only several recent results are introduced. 

Jeong and coworkers have reported peptide-based thermo-gelling systems using PEG-b-polyAla as an injectable cellular scaffold [315]. The polymer aqueous solution undergoes sol-gel transition as temperature increases. The fraction of the p-sheet structure of the poly Ala dictated the population and thickness of fibrous nanostructure in the hydrogel, which affected the proliferation and protein... 

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