## 3 Results and Discussion
### TG Analysis  
Thermal degradation of the synthesized polymers and the corresponding xerogels were studied by thermogravimetry. For the un-crosslinked polymers, the degradation process took place in three generally well-defined steps for **PAA** and four steps for **PAH18** and **PAH9**. Thus, according to some experimental data reported elsewhere on **PAA**[44; 45; 46; 47], the three steps were ascribed to water loss, 5%, (up to 185 \({}^{\circ}\)C), the reaction between functional groups with anhydride formation and also decarboxylation, about 35% weight loss (range 220-290 \({}^{\circ}\)C), and finally main chain degradation and depolymerization, 45% weight loss (above 325 \({}^{\circ}\)C). Because PHEMA generally decomposes with temperature in a single step [23; 24; 48], the analysis of thermal behavior of an AA-HEMA copolymer was much simplified. Thus, for both copolymers, the presence of HEMA units led to a new degradation step assigned to esterification reactions between functional groups. For **PAH18**, this new step of weight loss could be identified on the first derivative thermogravimetric (DTG) curve as a broad pick (with an onset temperature of 180 \({}^{\circ}\)C and the temperature at maximum reaction rate of 216 \({}^{\circ}\)C) partially overlapped with the sharp pick assigned to decarboxylation/anhydride formation within PAA  
Figure 6: Schematic representation of hydrogel formation by \(\gamma\)-irradiation. sequences. A similar situation was observed for **PAH9** on DTG trace, but with a much lower onset temperature of 145 \({}^{\circ}\)C and a temperature of maximum reaction rate of 181 \({}^{\circ}\)C. The difference of 35 \({}^{\circ}\)C between the two pick temperatures (on DTG curves) is somewhat surprising and deserves further investigation, even though we suspect that a higher proportion of hydroxyl groups as in **PAH9** means more possible hydrogen bonds established with carboxyl groups and favorable to inter/intramolecular esterification. On the other hand, increasing the number of HEMA units in the copolymers raised the temperature at which 50% of the sample was still present, _T50_, which was higher for **PAH18** and **PAH9** (382 and 388 \({}^{\circ}\)C, respectively, when compared to 371 \({}^{\circ}\)C for **PAA**). As the thermal degradation was performed in nitrogen atmosphere, some residue, around 10-15%, was detected at the end of the thermal treatment. Irradiation of **PAA** practically did not alter at all the onset temperature corresponding to the reaction of functional groups (even though the temperature at maximum reaction rate rose by a few degrees Celsius), while the associated weight loss decreased just a little bit (31%). At the same time, the degradation reactions started at a temperature higher than that for the non-irradiated **PAA** by ca. 14 \({}^{\circ}\)C (see Figure 7). Overall, the absorbed dose in the studied range did not have a significant influence on the thermal stability of the polymers. Irradiation of both copolymers induced more significant changes than in the case of **PAA** (see Figure 8). Thus, the step corresponding to the esterification reaction was no longer visible after diminishing the number of the functional groups as a result of irradiation. Only one step of weight loss corresponding to both decarboxylation and reaction between the remaining functional groups was detected. At the same time, the weight loss corresponding to the reaction between functional groups became smaller as the irradiation dose increased, while the onset temperature increased as well. In other words, such a modified copolymer containing fewer carboxyl and hydroxyl groups, which are most likely located far from each other along the macromolecular backbone, is expected to maintain its integrity better at a certain temperature in the range of 230-350 \({}^{\circ}\)C.  
The absorbed dose also had a significant influence (more important for **PAH9**) on the depolymerization and chain degradation step, unlike the case of **PAA**.