## 3 Results and Discussion
### DSC Measurements  
Molecular modifications induced by radiation were further investigated by DSC. The associated thermograms plotted in Figure 5 for the samples in dry state also provided general information about the copolymerization reaction. That a single T\({}_{\text{g}}\) was detected for both synthesized copolymers mainly suggested the statistical character of the copolymer microstructure on molecular scale, despite very different reactivity ratios of comonomers.  
T\({}_{\text{g}}\) for **PAA** was approximately 108 \({}^{\circ}\)C (see Figure 5a), while its copolymers with HEMA had higher glass transition temperatures (125 \({}^{\circ}\)C for **PAH18**, and 114 \({}^{\circ}\)C for **PAH9**). This is a little unusual considering that T\({}_{\text{g}}\) of poly(HEMA) is about 100 \({}^{\circ}\)C [43]. According to the Fox equation, the two copolymers should exhibit a lower glass transition temperature than **PAA**. However, a positive deviation from the Fox equation indicates some attractive intermolecular interactions between polymer chains. Moreover, T\({}_{\text{g}}\) for **PAH18** is significantly higher than T\({}_{\text{g}}\) for **PAH9** and this fact cannot be explained by the different molar ratio between monomers in the initial substrate, but by a different molar ratio of monomer units in copolymer, mainly enriched in HEMA units.  
Figure 4: FTIR spectra for **(a) PAH18**; **(b) PAH18** after absorption of 40 kGy and **(c)** 50 kGy; **(d) PAH9** after absorption of 30 kGy, **(e)** 40 kGy, and **(f)** 50 kGy.  
Figure 5: DSC curves for (a) PAA; (b) PAH18; (c) PAH9.  
After irradiation, the glass transition temperature of all polymers increased. However, while for **PAA** a further increase in the absorbed dose maintained the T\({}_{\text{g}}\) almost unchanged, for copolymers the transition temperature diminished. This effect could be observed for **PAH9** after absorption of 50 kGy and for **PAH18** samples irradiated with 40 and 50 kGy. Thus, in spite of an increased crosslinking degree (see Table 1), T\({}_{\text{g}}\) lowered, but nevertheless it remained higher than T\({}_{\text{g}}\) of un-crosslinked polymers. This could be a direct consequence of removing some functional groups by irradiation to such an extent that the intermolecular interactions diminished. Because the mass between crosslinks was still high, even if the resulting network had more chemical crosslinking points at a higher absorbed dose, the number of intra or intermolecular interactions became smaller, allowing some parts of the polymer chains to move more freely between the neighboring two crosslinkages. Accordingly, a schematic representation of hydrogel formation by irradiation was proposed and displayed in Figure 6.  
Indeed, increasing the absorbed dose to 70 kGy for a 2 wt.% PAA aqueous solution (results not shown), M\({}_{\text{c}}\) became 3512 g/mol, corresponding to a crosslink density of 142 mol/m\({}^{3}\), while the glass transition appeared at 120 \({}^{\circ}\)C. However, it is noteworthy that increasing the energy transferred to a polymer mass may significantly potentiate the possibility of scission of main chains as well, so the degradation reactions may occur more frequently than crosslinking ones. Consequently, the gel fraction for this particular sample of PAA was only 0.88.