Solution—gel transition

To describe the process of 3D polymerization of a polyfiinclional monomer in sohilion, an approach has become popular based on the percolation problem. 

At Ph in the system modelled by the lattice, a property appears, which was absent at Pj Ph, ., eg. electrical conduction. If elastic links between the structural elements are meant as bonds, at Pi, the system starts to show macroscopic elasticity. 

The number of all the bonds in the infinite cluster (and the quantitative expression of the system property) rises as 

Critical index values in the problem of bond percolation (Obukhov, 1985) 

Obviously, at a high concentration of the magnetic atom.s (i 1), they form an infinite cluster. On the contrary, at a 1 the clusters of magnetic atoms arc composed of small numbers of atoms, and the lattice as a whole possesses no magnetic moment. 

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The solution-gel transition is modelled by the percolation problem, which combines node and bond percolations. 

Formally speaking, the reaction of polymerization seems most effective at P -> oo, and on the T vs V2 state diagram (Figure 3.59), the asymptote T —> oo corresponds to v n responding to P ,c. At lower uj < V2,n, the curve A of the solution-gel transition heis a positive first derivative. The specific shape of the curve A depends on the model s details. J his curve ends on the binodal curve of the two-phase gel state due to the elasticity forces of the network chains and the interaction between polymer and LMWL (see above). The numerical values of g have been determined for different types of lattice. It has also been established that the inequality f J/j < V2,c holds true (de Gennes, 1979). 

Figure 3.59. State diagram of the NP+IiMWIj system. DIG is Uio biiiodal curve of phase separation. C i.s the critical point. A is the line of the solution-gel transition, is the polymer concentration corre.sponding to the node percolation threshold l-he...

However, the development of in vivo applications for PNIPAM is limited by its non-biodegradability and the presence of amide moieties that reduce its biocompatibility. For this reason, other thermo-responsive polymers have been investigated in recent years. Poly(N-vinylcaprolactam) is a promising alternative. This polymer has a LCST between 35 and 38°C, again close to the temperature of the human body, and is characterized by high biocompatibility and low toxicity (Konak et al, 2007 Medeiros et al, 2010 Shtanko et al, 2003 Yanul et al, 2001). Additionally, amphiphilic copolymers such as Pluronics and Tetronics have been developed, based on copolymers of polyethylene oxide and polypropylene oxide. These copolymer systems exhibit a solution-gel transition at close to human body temperature that permits their application as injectable implants (Samchenko et ai,2011). 

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. 

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. 

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... 

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...

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... 

B. Silk Proteins Stability and Solubility in Solution Sol—Gel Transition. .. 25... 

Several groups used sol-gel transition to immobilize the beads packed in a capillary. For example, Dulay et al. [102] packed a slurry of ODS beads in tetraethylorthosilicate solution and heated it to 100 °C to achieve the sol-gel transition and create the monolithic structure shown in Fig. 17. This technology is extremely sensitive and even a small deviation from the optimal conditions leads to cracks in the monoliths and a rapid deterioration in the column performance. However, even the best efficiency of 80,000 plates/m achieved with these column was relatively low. Henry et al. modified the original procedure and increased the efficiencies to well over 100,000 plates/m [103,104]. 

Its aqueous solutions are not only viscous but become gels over the temperature range 20 - 90 C. The course of sol-gel transition as a function of temperature monitored by optical rotation (25) suggests the conversion of a conformationally mobile random coil in the sol state to a rigid ordered conformation in the gel state. The midpoint of transition is approximately around 45 C. Only oriented, noncrystalline fibers could be prepared, the best of them at 45 C. 

The solvent mobility in atactic polystyrene-toluene solutions has been studied as a function of temperature using NMR. The local reorientation of the solvent was studied using deuterium NMR relaxation times on the deuterated solvent. Longer range motions were also probed using the pulsed-gradient spin-echo NMR method for the measurement of diffusion coefficients on the protonated solvent. The measurements were taken above and below the gel transition temperatures reported by Tan et al. (Macromolecules, 1983. 16, 28). It was found that both the relaxation time measurements and the diffusion coefficients of the solvent varied smoothly through the reported transition temperature. Consequently, it appears that in this system, the solvent dynamics are unaffected by gel formation. This result is similar to that found in other chemically crossed-linked systems. 

The anion as is to be expected in the case of a negatively charged colloid evidently exerts but little effect. The sol-gel transition is generally examined by means of observation of the change in viscosity of the solution. The changes in viscosity as measured by the flow method in an Ostwald or similar apparatus differ as a rule... 

In Ref. 76 polymer solutions at the sol-gel transition point were treated, where it holds... 

The phase transitions, such as a phase separation of polymer solutions, a sol-gel transition, or a volume phase transition of gels, are always accompanied by conformation changes of polymers. Therefore, when the phase transitions are induced isothermally by external stimulation, the transitions cause efficient conformation changes. This contribution describes how such efficient stimuli-responsive polymer systems can be constructed. 

Figure 2 illustrates the temperature dependence of the swelling degree as a function of precursor polymer type. Methylcellulose (MC), hydroxypropyl-methylcellulose, type E (HPMC-E) and hydroxypropylmethylcellulose, type K (HPMC-K) gels have comparable effective crosslink densities of about 2 x 10 5 mol/cm3 (as determined from uniaxial compression testing), while the crosslink density of the hydroxypropylcellulose (HPC) gel is about half this [52]. The transition temperature for each gel is within several degrees of the precursor polymer lower critical solution temperature (LCST), except for the MC gel, which has a transition temperature 9 °C higher than the LCST. The sharpness of the transition was about 3%/°C, except for the HPC gel transition, which was much sharper - about 8%/°C. 

AA AAm Con A DSS DSS-gel LCST MAPTAC MBA MP MP-gel NIPA ONPG PVMA SSPG Tc TMED acrylic acid acrylamide concanavalin A dextran sulfate sodium gel containing Con A/DSS complex lower critical solution temperature [(methacrylamide)propyl]trimethylammonium chloride /V,/V -methylenebis(acrylamide) a-methyl-D-mannopyranoside gel containing Con A/MP complex /V-isopropylacrylamide O-nitrophenyl-P-D-galactopyranoside poly(vinyl methyl ether) stimulus-sensitive polymer gel transition temperature /V,/V,/V, /V -tetramethylethylenediamine... 

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