# Dual Stimuli-Responsive Polysaccharide Hydrogels Manufactured by Radiation Technique
## 3 Results
### Hydrogels Formation and Swelling
This polysaccharide derivative undergoes crosslinking initiated by either the electron beam or gamma rays--with two orders of magnitude difference in the dose rate--with a comparable outcome [25]. According to the available data on the radiation-induced crosslinking of CMC, the polysaccharide with the degree of substitution of 1.32 may form a gel at a concentration of 5% or more [32]. This was also observed in the current study (Figure 2)--the GF of under 35% could be isolated from the gels formed at a relatively high dose, namely 60 kGy and above. However, in an acidic environment, the gel is created from the CMC solution of the respective concentration at a much lower dose (the so-called gelation dose), 20 kGy, and with a relatively high GF. The same characteristic was found for 10% CMC. It is apparent that, for the radical recombination reaction to occur, a high concentration of this polyion is indispensable. Then, the chains interleave or overlap and their physical proximity circumvent the electrostatic repulsion of ionized carboxylate groups. The recombination of CMC macroradicals at a neutral pH is restricted; hence, it is less efficient in competition with the single-radical-initiated scission of glycosidic bonds. This is demonstrated at a low polymer concentration in an irradiated solution [33]. The protonation of carboxylate moieties--which occurs at a low pH--removes the electrostatic barrier, and thus two macroradical reactions are not constrained, and the CMC may intra-molecularly crosslink at an even low concentration. Moreover, when exploring the reasons for more effective macroradical recombination, one may refer to the water radiolysis products dependent on the pH. At a low pH, the \(\text{e}_{\text{aq}}\)- reacts with hydronium ions to produce hydrogen atoms (H\({}^{\bullet}\)) [34]. A hydrated electron barely reacts with simple poly-carbohydrates, whereas the hydrogen atom reacts with the polymer in the same manner as the HO\({}^{\bullet}\), i.e., abstracting the hydrogen to create a macroradical. Consequently, at a low pH, there may be more radicals formed on the polymer chains--as a lower irradiation dose is necessary to attain a similar outcome to that at neutral pH. What is more, due to the abovementioned phenomenon, the temporal concentration of macroradicals may be higher, and this favors biradical reactions, as the crosslinking is. It was reported that for CMC with a high degree of substitution, 2.3, the gel was formed from solutions of concentrations as low as 0.5 and 1% at the dose of <5 kGy [35]. The data in Figure 2 demonstrate that the extent of gel formation from CMC of 1.2 degree of substitution also depends on the pH of the solution. The radiation processing of CMC aqueous solution in the presence of the crosslinker greatly increases the gel content. The application of the 25 kGy dose results in the GF of over 70% for all samples. A tendency similar to that of HPC is noted, which is that the solutions of a certain concentration of CMC (2.5, 5, or 10%) produces gels with an increasing GF when the PEGDA content increases. If the PEGDA amount is constant, the GF is reduced upon an increase in the polysaccharide content. The explanation is the same as for HPC-PEGDA systems. The decrease in GF at higher doses for lower-PEGDA concentration gels is evidence of the prevailing degradation, which was small but noticeable but also observed in the 5%-2.5% HPC-PEGDA system. The maximum irradiation doses in the experiments did not exceed 100 kGy, however, it is known that for most polymers undergoing radiation-induced simultaneous degradation and crosslinking, the GF does not increase infinitely and eventually decreases at high doses [36]. The possible crosslinking sites are exhausted, or restricted segmental mobility in the network prevents bi-radicals reactions (e.g., recombination); therefore, the insoluble fraction decreases.