Gelation dose

This is one of the most universal techniques for obtaining hydrogels from water-soluble polymers. Crosslinked PEO, PVA, PAAm, PAAc and its salts, as well as some polymer blends were obtained by this method. Although all polymers mentioned above have their own specific features, in most cases the gelation doses do not exceed 1-2 Mrad, i.e. they are substantially lower than for the same polymer in bulk. This is due to the fact that in aqueous media crosslinking occurs indirectly, namely because of the OH radical formation and their attack on the macromolecules. There exists a developed theory of these processes [73],... 

Neither of these explanations is entirely satisfactory. In particular the former would not allow for significant radiation protection by small concentrations of additive in fact, the gelation dose can be doubled by only 1% of additive in solid polymer and considerably less in liquid polymer. The second explanation would lead to almost complete protection since the additive concentration vastly exceeds the polymer radical concentration, and radicals would react with the additive before meeting another radical. In practice partial protection is observed. 

With large single crystals of polyethylene, the gelation dose is some 10 times greater than for the same polymer grown under conventional conditions (14). This is not caused by any inherent difference in the effect of radiation since both the radical concentration (deduced from ESR measurements) and hydrogen production are similar. We must therefore assume that most of the links produced in the single crystal are internal links which do not influence solubility. This is understandable in a crystal where each molecule folds backward on itself. 

In certain water-soluble polymers such as polyvinylpyrrolidone, the minimum dose required for gel formation shows a curious dependence on concentration. As this is reduced to about 1%, the gelation dose decreases in spite of the fact that the polymer molecules are further apart. This is ascribed to the higher contribution made to polymer radi-... 

At lower concentrations the gelation dose increases sharply until no network is formed, even at the highest doses. This competing effect is ascribed to internal linking—links occur between different monomers in the same chain but are ineffective for network formation. 

The formation of a polymer networks starts with an increase in molecular weight and formation of branched structures. At a typical extent of reaction, the gelation point is that point at which a network is first formed. The extent of reaction as well as the cross-linking density in radiation-induced cross-linking processes is determined by the radiation dose. The term dose means the quantity of radiation applied to or absorbed accidentally by a given volume or mass of sample. The absorbed dose is measured in Gray (Gy), 1 Gy = 1J kg-1. Therefore, the formation of a polymeric network needs a certain dose, the gelation dose Dg, which can be determined by sol-gel analysis. ... 

If the absorbed dose D is smaller than the gelation dose (D < Dg), then no cross-linking occurs. If the absorbed dose equals the gelation dose, then a network is formed for the first time. For D > Dg, the amount of polymer chains that have reacted with the network increases, and therefore the gel fraction increases and the sol fraction decreases. 

Fig. 16 Structural changes during irradiation. Below the gelation dose Dg, an increasing of the molecular weight occurs. Mw becomes infinite at Dg. Above Dg, the formation of macroscopic networks starts (gel content >0)...

The gelation dose as a function of polymer concentration is given in Fig. 59 for polymethylvinylether in various solvents. Complete agreement exists between these curves and the theory of Henglein and Schnabel [442, 443]. 

Fig. 9 Sol-gel analysis of PVP gel. (a) Gel content versus radiation dose (e-beam) (b) Charlesby-Pinner plot (with regression line).The gelation dose is determined to Dg = 94 kGy from the intercept follows po/ijo = 103. Reprinted from Burkert et al. (2007a), p. 1326. Copyright (2007), with permission from Elsevier...

An advantage of radiochemical induced cross-linking is the possibility to combine polymers with different properties. For that purpose, the polymers were separately dissolved in water. The solutions were mixed and irradiated. (Gottlieb et al. 2005) describes the synthesis of temperature-sensitive hydrogel blends of PVME (as thermo-sensitive polymer) and the radiation-cross-linkable polymer PVP (a polymer that is applied in pharmaceutics). The experiments show that the gelation dose of the blend is between the gelation doses of the two pure polymers. 

In order to achieve lower gelation doses for saturated linear aliphatic polyesters such as polycaprolactone, certain functional groups including unsaturation can be incorporated. In this regard, and as will be discussed here, e-caprolactone has been copolymerized with a relatively small amount of allyl glycidyl ether. Therefore, the randomly distributed allyl glycidyl ether units(2-6 mole percent) in the synthesized caprolactone-allyl... 

As discussed below solution-state NMR methods cannot be successfully applied to materials irradiated to above their gelation dose. This situation naturally does not arise for polymers for which chain scission is the dominant reaction on irradiation. The two systems of this type which have been most extensively studied are the poly(methacrylate)s and poly(isobutylene). 

The gelation dose (D ) represents the minimum required dose to initiate gelation process. Pq/Qo is the parameter that allows the determination of the final results of irradiation and is equal to half of the scission yield/cross-linking yield ratio [0.5 x If Po/qo is lower, cross-linking occurs... 

Gelation dose, Dg, can be determined from Charlesby-Pinner diagram as the minimal dose where gel fraction is stUl zero (s + = 2) [08Z1] or from equation (4 )... 

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