Chemical gels

Figure 17 summarizes the avadable sol—gel processes (56). The process on the right of the figure involves the hydrolysis of metal alkoxides in a water—alcohol solution. The hydrolyzed alkoxides are polymerized to form a chemical gel, which is dried and heat treated to form a rigid oxide network held together by chemical bonds. This process is difficult to carry out, because the hydrolysis and polymerization must be carefully controlled. If the hydrolysis reaction proceeds too far, precipitation of hydrous metal oxides from the solution starts to occur, causing agglomerations of particulates in the sol. 

A popular cross-linking agent for chitosan is glutaraldehyde, as proposed by Muzzarelli et al. [217]. Chitosan networks were obtained by reaction with glutaraldehyde in lactic acid solution (pH 4-5) at molar ratio amino groups/carbonyl functions about 10-20 reduction gave stable chemical gels. 

The chemical gel point defines the instant of LST of chemically crosslinking polymers. Before the crosslinking polymer has reached its gel point it consists of a distribution of finite clusters. It is called a sol since it is soluble in good solvents. Beyond the gel point, it is called a gel . The gel is an infinitely large... 

For describing the observed molecular weight effects in chemical gels, we propose to distinguish three regions based on the precursor molecular... 

The gelation transition is observable for Ng > 10. Otherwise, the material behaves as a liquid (Ng < 1). Little is known about materials near Ng = 1. For the following, we consider only materials with iVg 1 and treat them just like chemical gels. The expression T(n + 1, (t — t )/Xpg)/T(n + 1) in Eq. 5-2 approaches a value of one in this case of Ng g> 1, and the critical gel equation, Eq. 4-1, is recovered. However, much work is needed to understand the role of non-permanent physical clusters on network formation and rheological properties. 

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. 

Typically forms true chemical gel during film formation... 

Chemical food preservation, 12 85-86 Chemical formula, defined, 21 336 Chemical fossils, 18 571 Chemical gas scavengers, 12 77 Chemical gel stabilization, 23 71 Chemical-grade limestone, 15 27 Chemical-grade propylene, product specification for, 20 1111 Chemical hazards, 21 833-846... 

Number-average molar masses were determined using a vapor pressure osmometer (VPO) (Hitachi 117 Molecular Weight Apparatus) at 54.8 0.1°C in toluene (Fisher Scientific, certified A.C.S.) which was distilled from freshly crushed CaH2. The VPO apparatus was calibrated with pentaerythritol tetrastearate (Pressure Chemical). Gel permeation chromatographic (GPC) analyses were performed in tetrahydrofuran by HPLC (Perkin-Elmer 601 HPLC) using six y-Styragel columns (106, 105, 10l, 103, 500, and 100 A) after calibration with standard polystyrene samples. 

Physical gels, as exemplified by gelatin gels, exhibit many common features with chemical gels. Among them, we found the topological disorder of the network formed by the polymer chains, and the formal similarity of the process of gelation with a percolation problem. 

Figure 26-36 (a) A chemical gel contains covalent cross-links between different polymer chains, (b) A physical gel is not cross-linked but derives its properties from physical entanglement of the polymers. 

If you aren t set up to wash cloth diapers at home, conventional disposable diapers aren t a good option. One widely quoted study (published in Archives of Environmental Health and conducted by Anderson Laboratories back in 1999) found mice exposed to VOC chemicals emitted by conventional disposables had asthmalike reactions. They also contain chlorine and have high-tech chemical gel cores that activate when your baby pees to lock in moisture. The Children s Health Environmental Coalition says this absorbent material—sodium polyacrylate— could cause respiratory and skin irritations in occupational settings (where exposure is higher than with diaper use). We wonder how safe can that much chemical activity that close to a baby s genitals be twenty-four hours a day ... 

The term gels refers to a range of materials which are grouped under the general definition A dilute mixture of two or more components which form a separate uninterrupted phase throughout the system [147,148]. This classification includes chemical gels—where covalent bonds create the continuous network—and physical gels which are maintained by non-covalent interactions, which concern us here. 

Cross-Linked (Chemical) Gels, Structure and Properties... 

Chemical gels are covalently cross-linked polymer networks, featuring very high viscosity and well-defined pore structure. Polyacrylamide is the most widely used chemical gel material, usually cross-linked with N,N-methylene-bisacryl-amide (BIS). The pore size of the gel is determined by the relative concentration of monomer and cross-linker used during polymerization (%T, total monomer concentration and %C, cross-linker concentration as a percent of the total monomer and cross-linker concentration [34]). Highly cross-linked ( 5%C) poly-... 

Experimental detection of the gel point is not always easy since the equilibrium shear modulus is technically zero at the gel point and any applied stress will eventually relax, but only at infinite time. From the classical theory, the attributes of the gel point are an infinite steady-shear viscosity and a zero equilibrium modulus at zero frequency limit (Figure 6-3) (Flory, 1953). These criteria have been widely employed to detect the gel point of chemical gels. However, because continuous shearing affects gel formation, accurate information from viscosity measurement is not possible in the close vicinity of the gel point. Further, information regarding the transition itself could only be obtained by extrapolation, thereby introducing uncertainties in the determination of the gelation moment. 

This chapter is devoted to the properties of polymeric gel-forming liquids. Particulate gels are discussed in Chapter 7. The structure of a polymeric gel is sketched in Fig. 5-1. Since this book is devoted to materials that are in some sense liquid, or at least liquefiable, we shall not say much about hard, irreversible, chemical gels such as cured epoxies or vulcanized rubber, but shall focus instead on chemical pre-gels and thermally reversible physical gels, both of which can be considered borderline fluids. This chapter is confined to a brief overview. Much more detail can be found in Winter and Mours (1997), and volume 101 of the Faraday Discussions. ... 

Further discussion of models of the elasticity of gels is beyond the scope of the present work the interested reader can find a thorough description of the elasticity and viscoelasticity of polymer chemical gels in Ferry (1980), Treloar (1975), and Flory (1953). 

However, as remarked earlier, for many gelling systems, particularly those with relatively large precursor molecules, the exponent n can be much less than the theoretical value n = 2/3. Winter and Mours (1996) provide a thorough sunmiary of these and other rheological studies of chemical gels. 

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