## 3 Results and Discussion
### Hydrogel Preparation  
Hydrogels prepared by \(\gamma\)-irradiation of aqueous solutions of hydrophilic polymers or water soluble monomers present a series of advantages over chemical crosslinking hydrogels: easy process control, fortunately combining hydrogel formation with sterilization in one step, the lack of any initiators, catalysts or crosslinkers that are possibly toxic or difficult to remove, no higher temperatures needed, no wastes, etc. Moreover, unreacted monomers can be easily removed by washing with water. However, irradiation of monomer mixtures implies first the initiation of the polymerization reaction followed by crosslinking. In the case of monomers like AA and HEMA, the resulting hydrogels were not homogenous, in spite of the fairly uniform distribution of the irradiation field. Thus, poly(HEMA) chains that were not soluble in water tended to locally separate and even if eventual crosslinking with PAA chains were to take place, the resulted hydrogel would no longer be homogenous in its entire mass. This assertion was sustained not only by visual observation of the hydrogel, but also by DSC measurements that indicated two glass transitions (not shown). This last aspect may be an indication of the existence of either a mixture of two homopolymers (PAA + PHEMA) or some species of block copolymer (PAA-b-PHEMA) or a mixture of the two types of macromolecular components. However, due to the huge difference between the reactivity ratios of the two monomers, the nonhomogeneous hydrogel most likely consisted of individually synthesized PAA and PHEMA chemically linked together as a result of the irradiation process. Consequently, hydrogels prepared by this method were not further characterized by swelling equilibrium degree, FTIR or TGA. Moreover, at low absorbed doses (below 30 kGy), hydrogels were obtained only for AA solutions, while for AA/HEMA solutions the gelation dose was higher than 30 kGy, with no gel formation at lower doses.  
Based on capillary viscometry data, all polymer solutions having concentrations of about 2 wt.% and pH 3 are well above the overlapping concentration, thus intermolecular crosslinks are favored. At the same time, the volume fractions of polymers in these solutions are small enough and superabsorbent hydrogels can form. At such a low pH, hydrogen bonding between the functional groups of copolymers is also favored. Besides, as the polymer chains are already synthesized,  
Figure 2: (**a**) Specific viscosity as a function of polymer concentration (the numbers indicate the values of the slopes), and (**b**) logarithm of relative viscosity vs. polymer concentration.  
the crosslinking efficiency induced by \(\gamma\)-rays is substantially improved and hydrogels can be formed even at small absorbed doses in spite of low polymer concentration.  
The interaction of high-energy radiation with polymer solutions induces various chemical transformations both in polymer chains and water. For solutions with low or moderate concentrations, the direct action of radiation on the macromolecular backbone is less important compared with the indirect one, i.e., reactions of the polymer with the radiolysis intermediates generated by water. Hydroxyl radicals have been shown to be the main responsible species for radiochemical transformations induced by water onto the polymer chains. They abstract hydrogen atoms from macromolecules, forming macroradicals [42]. Intermolecular crosslinking appears either when two macroradicals with high enough mobility are able to react to each other, or when macroradicals could be involved in chain transfer reactions to polymer molecules.