3.1.2. Characterization
Figure 2 presents the FTIR spectra of the series SN20 (CS), DN20.2 (CS/PEI25) and TN20.2 (CS/PEI25/PEI18) (from
Figure 1). As expected, the main bands of CS are visible in the spectrum of SN20 cryogel, as follows: 2924 and 2876 cm
−1, attributed to C-H and -CH
2 stretching vibrations in CS; 1651 cm
−1, assigned to the stretching vibration of C=O bond in the secondary amide bond and to the Schiff base; 1566 and 1410 cm
−1, attributed to the –NH bending vibration in amide II, and to C-H and -CH
2 stretching, respectively; 1323 cm
−1, assigned to N-H stretching in amide (amide III); 1259 cm
−1, assigned to the C-N stretching modes; the large band at 1076 cm
−1 indicates the C-O-C anti-symmetric stretching in CS [
49,
50,
51]. By the construction of the second network (PEI25), significant changes in the position of the main bands are obvious in the spectrum of the DN20.2 composite. The sharp band located at 1462 cm
−1 supports the presence of PEI, while the bands at 1364 and 1302 cm
−1 indicate CH
3 symmetrical deformation mode and the amide III [
49]; the bands at 1097 and 943 cm
−1 were assigned to the C-O-C anti-symmetric stretching, and the wagging of the saccharide structure of CS. The construction of the third network (PEI18) in TN20.2 did not lead to dramatic changes in the spectrum, just small blue or red shifts; for example, the band at 1566 cm
−1 in SN20 was blue-shifted at 1572 cm
−1 in TN20.2, the band at 1462 cm
−1 was blue-shifted to 1466 cm
−1, the band at 1364 cm
-1 in DN20.2 was shifted at 1358 cm
-1 and the band at 943 cm
−1 appears as a shoulder after the construction of the third network, while the band at 897 cm
−1 in SN20, assigned to the wagging of saccharide structure, was red-shifted at 893 cm
−1 in DN20.2 and at 868 cm
−1 in TN20.2.
The FTIR spectra of SN5, DN5 and TN5.2 sponges are compared in
Figure S1. The bands located at 2924 and 2677 cm
−1 in SN5, at 2924 and 2822 cm
−1 in DN5, and at 2926 and 2822 cm
−1 in TN5.2 were attributed to C-H and -CH
2 stretching vibrations; the bands situated at 1632, 1651 and 1631 cm
−1 in SN5, DN5 and TN5.2, respectively, were assigned to the stretching vibration of C=O bond in the secondary amide bond and to the Schiff base. The large bands located at 1076 cm
−1 in SN5, 1115 cm
−1 in DN5 and 1159 cm
−1 in TN5.2 indicate the C-O-C anti-symmetric stretching in CS [
49,
50,
51]. The sharp bands located at 1442 cm
−1 in SN5, 1462 cm
−1 in DN5 and 1451 cm
−1 in TN5.2 were assigned to methylene C-H bending vibrations [
51]. The presence of PDMAEMA as the third network is supported by the band at 1731 cm
−1 attributed to the C-O bond in the ester group and the strong band at 1451 cm
−1.
Figure S2 presents the FTIR spectra of DN20.3 and TN20.3 composite sponges. As can be seen, these spectra are similar because the second and the third networks were constituted of PEI18 cross-linked with EGDGE.
SEM images in
Figure 2 support the morphological changes which occurred during the successive construction of the networks. The pore sizes and the wall thickness become smaller and bigger, respectively, after the addition of the second and third networks, both for SN5 and SN20 as the first network. It is obvious that the compactness of the composites was higher when the first network was SN20, the theoretical cross-linking degree being four times higher compared with that of SN5. The size of pores and the wall thickness for some representative composite cryogels are presented in
Table S1 and show the dramatic decrease in the pore sizes by the construction of the second network (DN5 compared with SN5 and DN20.2 compared with SN20). The pore size and pore wall thickness further decreased and increased, respectively, by the construction of the third network (
Table S1).
The elemental analyses obtained from EDX spectra, and given in
Table S2, support the structure of the composites. As can be seen, the value of nitrogen content in the SN20 cryogel is lower than in the SN5 cryogel because a higher content of GA had as a consequence increasing carbon content and decrease of nitrogen and oxygen content. The construction of the PEI network led to the increase in carbon and nitrogen content and to the decrease in oxygen content (DN5 and DN20.2). The strong difference between the values of elements in the case of the composites TN5.1 and TN5.2 is attributed to the third network, which was PEI in the first case and PDMAEMA for TN5.2. The difference between the element content in the case of DN20.2 and TN20.2 was not so significant because the second and the third networks were constituted of PEI.
The values of
SR and
EWC, presented in
Table S3, clearly indicate the differences between SN5 and SN20 caused by the differences in cross-linking degrees. Thus, for SN5, the values of
SR and
EWC were 64.69% and 98.45%, respectively, while for SN20, the values of these characteristics were 45.25% and 97.79%. The values of
SR and
EWC dramatically decreased with the construction of the second network (DN5, DN20.1, DN20.2 and DN20.3), while the decrease was less significant when the third network was added (TN5.1, TN5.2, TN20.1, TN20.2 and TN20.3).
Uniaxial stress–strain compression tests were first performed to evaluate the mechanical strength of the SN cryogels, namely SN5 and SN20. The stress–strain curves presented in
Figure 3a,b show typical elastic behavior characteristic of cryogels, with sustained compression values beyond 70% and without any deformation or failure of the gel networks. Moreover, after the removal of the load, the SN5 and SN20 cryogels reabsorbed the water released during compression and regained their original shape. Consequently, in order to further evaluate their resilience and robustness, the dynamic stress–strain behavior of these cryogels was carried out for five successive loading cycles at 95% maximum strain. The results in
Figure 3c,d and
Table S4 (Supplementary Materials) demonstrate the remarkable mechanical stability of the SN5 and SN20 cryogels upon successive compression experiments; the values of the maximum sustained compression, the compressive strength and the compressive moduli remained almost unchanged after five successive cycles of compression. It should be pointed out that SN5 and SN20 cryogels exhibited unique mechanical features (high elasticity, non-brittleness, shape recovery), which support their further use as matrices/scaffolds for construction of multi-network composite cryogels. By employing PEI18 and EGDGE to prepare the second network, stiffer DN cryogels were obtained, which can sustain 53% (sample DN5) and 31.18% (sample DN20.3) compression before fracture, respectively (
Figure 4 and
Table 2). The crack development in DN cryogels at low strain can be associated with the drastically decreased values of the swelling ratio of the DN cryogels in comparison with those of SN cryogels (see
Table S4, Supplementary Materials).
However, the compressive moduli of DN cryogels, calculated as the gradient of the initial linear portion in the stress–strain curve (
Figure 4c), increased with the addition of the second network. For instance, DN5 and DN20.3 cryogels exhibited an elastic modulus about five and thirty times higher than that of the starting networks (SN5 and SN20), respectively. Thus, increasing the polymer content led to more dense and rigid networks. Moreover, the construction of the third network based on PEI18 or PDMAEMA matrices led to a further improvement of the elastic modulus of the composite cryogels (samples TN5.1 and TN5.2;
Table 2). Furthermore, the TN5.1 cryogels remained mechanically stable and sustained 69.02% compression at a compressive nominal stress of 538.2 kPa (
Figure 4a and
Table 2). Thus, the use of PEI18 to prepare the third network increased the flexibility of the TN5.1 network and hampered its failure. On the other hand, when SN20 cryogels, having a higher cross-linking density (GA concentration 20 mole %), were used as starting networks for the construction of the TN networks (sample TN20.3), the mechanical properties were completely changed. A cross-linker content of 20% in the first network (TN20.3) induced an enhancement in the elastic modulus compared to a cross-linker content of 5% (TN5.1), but the fracture of the TN20.3 networks occurred at lower compression ratio (21.91%), while the TN5.1 cryogels were robust without deformation or fracture at higher strain.
Additionally, the uniaxial compression data proved that the PDMAEMA chains had a beneficial effect on the maximum sustained compression and compressive strength of DN cryogels (
Figure 4 and
Table 2). DN20.1 cryogels demonstrated mechanical stability and sustained 76.3% compression at a compressive nominal stress of 370.49 kPa, whereas DN20.2 and DN20.3 cryogels were broken at about 13.30% and 31.18% compression and compressive nominal stress of 14.45 kPa and 24.75, respectively (
Figure 4b and
Table 2). Nevertheless, the presence of a third network based on PEI18 determined a significant reinforcement of the TN20.2 cryogel network, which sustained 76.15% compression, at a compressive nominal stress of 282.35 kPa. In addition, the remarkable mechanical stability of TN5.1 and TN20.2 can be also associated with the well-interconnected networks of small pores observed for these cryogels (see SEM micrographs,
Figure 2). An improvement of the mechanical strength with the decrease in pore sizes has also been previously reported for other porous hydrogels [
34,
52,
53]. Finally, it should be emphasized that the mechanical properties of SN, DN and TN cryogels can be modulated by controlling (i) the cross-linker ratio of the first network, (ii) the nature and molar mass of the polycation used to prepare the second and third networks and (iii) the order of the network construction.