Sol-to-gel transition

Combining thermal- with pH-dependent gelation, Kumar et al. [86] developed a combination of methylcellulose 15% and carbopol 0.3%. This composition exhibited a sol-to-gel transition between 25 and 37 °C with a pH increase from 4 to 7.4 [203], A possible mechanism for the thermal effect could be the decrease in the degree of the methylcellulose hydration, while the polyacrylic acid can transform into a gel upon an increase in pH due to the buffering properties of the tear fluid [1]. 

The formed gels showed thermally reversible sol-to-gel transition due to the non-covalent interactions. The sol-to-gel phase diagram for C4mim/BF4 and C4py/BF4... 

When a tetraethoxysilane-water-hydrochloric add-alcohol solution of appropriate composition is held at near room temperature, hydrolysis and polycondensation occur and the solution viscosity increases. Fibers can be drawn at a viscosity about 10 Pa s, which occurs during the course of sol to gel transition. It should be pointed out that for an alkoxide solution to be spinnable (Sakka,... 

For the data given in problem 4, determine if the S3rstem will imdergo a sol-to-gel transition if the fractal aggregates have a fractal dimension of 2.15. Determine the time for gelation of this system. 

For silica gels a number of parameters have been demonstrated to have a large effect on the evolution of porosity and subsequently on the resulting silica materials [1]. Almost dense, micro- or mesoporous silica materials can be obtained depending on the experimental conditions in which hydrolysis and condensation reactions of silicon alkoxides are carried out. This is not the case for transition metal alkoxides which are very sensitive to hydrolysis. They do not cillow the adaptation of sol-to-gel transition in order to obtain controlled porous textures. Some years ago special attention was paid to the utilization of amphiphilic systems as reactive media to control hydrolysis and condensation kinetics with transition metal alkoxides [37]. In a more recent work Ayral et al. 

Thermo-Responsive Sol-to-Gel Transitions and Exploitation for Injectable Drug Delivery... 

Ti02 H2O TX-IOO /decane/water C (TIPO addition) Rheological investigation of sol-to-gel transition [90]. 

Jeong et al. studied the gelation behavior of poly(ethylene glycol)-h-poly(L-alanine) (PEG-h-L-PA) block copolymers in water [117, 118]. They suggested that the P-sheet structure of l-PA plays a critical role in developing a fibrous nanostructure as well as in the sol-to-gel transition of the copolymer solutions (Fig. 14). The conformation of l-PA in water is random coil at low concentrations. As the concentration increases, block copolymers start to aggregate and the conformation... 

Aqueous solutions of Poloxamer or Pluronic undergo sol-to-gel transition as the temperature increases (Fig. 13.4). However, the implanted gel of Poloxamer is quickly eroded and does not persist for more than a few days at most. To improve the system, end-group modified Poloxamers, and multiblock copolymers consisting of Poloxamer and biodegradable polymers have been developed. In addition, random multibloek eopolymers consisting of PEG, PPG, and a biodegradable polymer were reported. 

Aqueous solutions of chitosan-g-PEG and hyamlonic acid-g-Poloxamer undergoing thermal gelation were reported. Aqueous solutions of hydroxy-propyl methylcellulose or methyl cellulose also show thermal gelation. The hydrophobic interactions were suggested as the sol-to-gel transition mechanism. Aqueous solutions of the inclusion complexes between methylated a-... 

MonomethoxyPEG-polyalanine (mPEG-PA), polyalanine-Poloxamer-polyalanine (PA-PLX-PA) and mPEG-poly(Ala-co-Phe) (mPEG-PAF) were reported as thermogelling polypeptides.As the temperature increased, /3-sheet conformation was partially strengthened and PEG dehydrated. The secondary stmcture of polypeptides plays a role in driving the sol-to-gel transition. 

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