A. Materials

Agarose (LP0028A, M_W = 100000 g/mol) was from British OXOID company and used without purification. Deionized water was prepared through the Purelab flex pure water equipment (ELGA, UK).

B. Preparation of agarose hydrogels

The agarose powder was mixed with deionized water to prepare 6wt.% agarose solution and refluxed to boiling point. Physically cross-linked agarose hydrogel was obtained after cooling to room temperature ((298±3) K).

C. Irradiation and characterization of samples

A. Physical properties vs. absorbed dose

Diluted aqueous solution (below 10%) of polysaccharides such as cellulose, starch, chitin/chitosan and their water-soluble derivatives have been proved as degradable type polymers under ionizing radiation. It was reported that their viscosity decreased greatly with increasing doses [11]. In this study, to elucidate the irradiation effect on agarose hydrogel, physically cross-linked hydrogels were irradiated at different doses. The phase of irradiated agarose hydrogels was closely related to the absorbed dose. As shown in Fig. 1, the phase of samples changed slightly below 50 kGy, while the samples were partially liquefied at 100 kGy and their phases showed obvious changes at 150 kGy.

Fig. 1.Full-screen View

(Color online) Phase changes of the agarose hydrogels irradiated to different doses.

To get an intuitive understanding of the changes in the physical properties, compression performance of agarose hydrogel before and after irradiation was analyzed (Fig. 2). The compression strength and strain of agarose hydrogels decreased with increasing dose, and so did the initial modulus. Similar phenomena were observed in carrageenan gel [12], and the sample strength decreased after high dose irradiation. Figure 2 indicates that the γ-rays destroyed three dimensional network structure of cross-linked agarose molecules in hydrogel, resulting in relaxation of the gel networks, with low gel rigidity and toughness. The yield point of samples appeared at about 0.05 MPa stress and about 5% strain, and it will no longer obey the elastic deformation regularity after the yield point. Therefore, physical properties of agarose hydrogel decrease with increasing dose, while the agarose hydrogel degrades under the γ-ray irradiation.

Fig. 2.Full-screen View

The stress-strain curve of agarose hydrogels irradiated to different doses.

B. Molecular weight vs. absorbed dose

For evaluating degradation degree of agarose hydrogels, GPC was employed to measure molecular weight (M_W) of irradiated agarose hydrogels. As seen in Fig. 3, M_W of the irradiated samples decreases rapidly with increasing doses up to 25 kGy, where it begins to drop slowly to 1d4 at 150 kGy. This shows that radiation degradation of agar hydrogel is dominant in early stages of the γ-ray irradiation. Degradation of polymer molecules with long chain obeys random degradation kinetics [13], i.e. 1/MW = A + kD, where k is the rate of radiation degradation, D is absorbed dose and A is a constant related to degradation of polymer molecules. The insert of Fig. 3 shows that radiation degradation of agarose is consistent with the random degradation kinetics at 0–25 kGy, and the radiation degradation rate is k=1.52×10^-6 mol/J.

Fig. 3.Full-screen View

Dose effect on molecular weight of agarose. The inset shows dosimetry response of 1/M_W below 25 kGy.

C. Chemical structure vs absorbed dose

Radiation effects on chemical structure of agarose hydrogel were studied by FTIR, UV-Vis, NMR and GC.

FTIR spectra of agarose hydrogels before and after radiation are shown in Fig. 4. The characteristic peaks of agarose hydrogels are: 3400 cm^-1, stretching vibration absorption peak of O^-H; 1078 cm^-1, the glucosidic bond; 1170 and 931 cm^-1, C^-O^-C bridge bond in L-galactose furanose dehydrated at 3,6-position [14]; 1390 cm^-1, bending vibration absorption peak of O^-H; 2920 cm^-1, vibration absorption peak of C^-H, and 1649 cm^-1, bending vibration peak of absorbed water [15]. Agarose hydrogel is decomposed gradually during irradiation, and the degree of degradation increases with dose, but the basic structure of agarose hydrogel still remains [16], so the location and intensity of the characteristic peaks does not change significantly. However, a new absorption peak at 1750 cm^-1 occurs gradually as the dose increases. It is speculated to be characteristic absorption peak of carbonyl. UV-Vis spectra of the agarose hydrogel (Fig. 5) show that the carbonyl absorption peak of irradiated samples appears at 268 nm [17, 18], and its intensity increases with dose. The carbonyl absorption peak was observed at 265 nm with irradiated chitosan or carrageenan polysaccharide solution [6, 12]. Therefore, compound with carbonyl groups is a main radiolytic product of agarose hydrogel.

Fig. 4.Full-screen View

FTIR spectra of agarose hydrogels irradiated to different doses.

Fig. 5.Full-screen View

UV-Vis spectra of agarose samples irradiated to different doses.

¹H NMR spectra of samples before and after irradiation are shown in Fig. 6. Due to the large molecular weight of agarose hydrogels, ¹H NMR spectrum cannot give the chemical structure in detail. However, a new aldehyde peak appears at 8.45 ppm, which is in accordance with the results of FTIR and UV-Vis. Zhang et al. [19] studied the radiation degradation of chitosan in aqueous solution and found that the radiolysis of chitosan was induced by ⋅H and OH. free radicals derived from radiolysis of water. Thus it can be speculated that the radiation degradation of agarose with similar structure is also induced by ⋅H and OH. free radicals, which leads to the cleavage of –C-O-C– bond in agarose molecules and the formation of carbonyl groups.

Fig. 6.Full-screen View

¹H NMR spectra of agarose samples irradiated to different doses.

The analysis of gas radiolytic products from irradiated samples is important for the application of polymeric materials and proposing the radiolysis mechanism. As seen in Fig. 7, H₂ is the main gas radiolytic product which is partly due to the combination of ⋅H and ⋅H from water radiolysis. CO₂, in fewer contents, increases with doses, indicating that internal reactions in agarose hydrogel are intensified at high doses. Trace contents of CO and CH₄ were detected. G values of the gaseous products are given in Fig. 7. They are in the order of G(H₂) > G(CO₂) > G(CO) > G(CH₄). These results indicate that the cleavage of C^-H bond is the main radiolysis path, and the degradation of the C^-C bond also appears during irradiation.

Fig. 7.Full-screen View

Gas yield of agarose hydrogels irradiated to different doses.

From the FTIR, UV-Vis, NMR and GC analyses, radiation degradation of agarose hydrogel shows similar radiolysis behavior to that of alcohol or ether [20]. The main radiation degradation process is as follows:

1) Hydrogen abstraction reaction: ⋅H or ⋅OH free radicals formed in water radiolysis attack agarose molecules, leading to H-abstraction reaction at all the carbons, and generating H₂/H₂O and agarose radicals;

2) Cleavage of glycosidic bond and formation of carbonyl: When the ⋅H or ⋅OH free radicals attack the molecules at the C1 position in β-D-galactopyranose and 3,6-anhydro-α-L-galactopyranose, or C3 in β-D-galactopyranose and C4 in 3,6-anhydro-α-L-galactopyranose. The cleavage of –C-O-C– in the main chain of agarose hydrogels leads to the formation of carbonyl group and aldehyde group (Fig. 8);

Fig. 8.Full-screen View

The main radiolysis mechanism of agarose hydrogels. (a) ⋅H or ⋅OH abstraction at C1; (b) ⋅H or ⋅OH abstraction at C4.

3) Scission reaction on main chain: Breaking C^-C bond in the main chain of agarose hydrogels is extremely difficult due to its high bond energy, hence the formation of trace methane.