3.1 Properties of cellulose solutions and regenerated cellulose
Cellulose is insoluble in water and most organic solvents. However, the mixture of H2O/NaOH/urea can be used to dissolve cellulose, thereby changing its field of application (Zainal et al. 2020). In this system, NaOH combines with H2O to form NaOH-H2O hydrate, which allows more water to enter the cellulose fiber bundle and promote the swelling reaction (Budtova and Navard 2016). On the other hand, urea can form hydrogen bonds with cellulose to replace the hydrogen bonds between cellulose molecules and prevent the re-agglomeration of dispersed cellulose. This not only makes the cellulose more soluble in the liquid, but also improves the stability of the cellulose solution (Yan and Gao 2008; Xiong et al. 2014). In this study, microcrystalline cellulose in H2O/NaOH/urea dissolving agent was treated with repeated stirring-freezing-thawing to form a cellulose solution, wherein the stirring can provide mechanical shearing force to destroy the molecular bundle structure of cellulose, and the freezing process caused water to become ice crystals and expand the volume, which can increase the distance between microfibers and facilitate the dissolution of cellulose (Łaszkiewicz 1998; Zhou et al. 2002).
Figure 1(a) shows the appearance of cellulose solutions prepared after one, two, and three cycles of freezing-thawing treatment. The sample after one cycle of freezing-thawing treatment shows a cloudy appearance, indicating that some of the cellulose particles are not completely dissolved, but are swollen and suspended in the solvent. Solutions with higher transparency are obtained with the increase in the number of freezing-thawing treatment. This result suggests that the repeated freezing and thawing process effectively disentangles the cellulose molecule chains and improve the solubility of cellulose.
Table 1 displays the properties of cellulose solutions prepared with different numbers of freezing-thawing treatments. All of them are alkaline solutions with pH values between 11.9 and 12.1. With the increase of the number of freezing-thawing treatment, the viscosity of the cellulose solution increase and the surface tension decrease. All the cellulose solutions have a surface tension lower than that of distilled water (about 72 dyne/cm), indicating the existence of cellulose decreases the cohesion force of water molecules. The cellulose solution prepared by three times of freezing-thawing treatment has the lowest surface tension, which shows that its cellulose dispersion effect is better.
Figure 1(b) shows the XRD curves of the original cellulose and the regenerated celluloses prepared from cellulose solutions with different freezing-thawing cycles. Original cellulose shows the reflections at 14.8°, 16.3°, 22° and 34.6°, representing (1\(\stackrel{-}{1}\)0), (110), (200), and (004), respectively, which identifies a distinctive cellulose-I pattern (Clair et al. 2006; Tendo et al. 2019). Regenerated cellulose appears to be cellulose-II pattern with the typical reflection peaks at 11.9°, 19.9°, and 21.6° corresponding to (1\(\stackrel{-}{1}\)0), (110) and (200), respectively. The conversion of crystal pattern from cellulose-I to cellulose-II after dissolved and dehydration, which confirms the result of El-Wakil and Hassan (2008) and Samayam et al. (2011).
Table 1
Properties of cellulose solution prepared with different conditions
Freezing-thawing
(times)
|
pH
|
Viscosity
(cps)
|
Surface tension
(dyne/cm)
|
1
|
12.0
|
36.7
|
55.5
|
2
|
11.9
|
41.7
|
51.8
|
3
|
12.1
|
42.4
|
44.0
|
3.2 Gelation of cellulose hydrogel
Figure 2 shows the DSC heat flow curves of the reaction of cellulose solution with epichlorohydrin. Figure 2(a) shows the DSC heat flow curves of the mixture of epichlorohydrin and cellulose solution that has an E/C equivalence ratio of 4.0/1 and being stirred for different time. A broad exothermic peak appears for all samples, indicating the crosslinking reaction occurred during the heat scanning process. Compared with different pre-stirring time, the initial crosslinking reaction temperature shifts to higher temperature and the exothermic behavior becomes less obvious with increasing stirring time. This indicates that the crosslinking reaction starts during stirring at room temperature after the cellulose solution is mixed with epichlorohydrin.
Fig 2(b) shows the exothermic behavior of the mixture of cellulose solution and epichlorohydrin under different E/C ratios. All samples show a broad exothermic peak caused by the crosslinking reaction during thermal scanning. Compared with E/C ratio of 3.0, the exothermic peak onset temperature of E/C ratio of 3.5 and 4.0 shifts to lower temperature, and the exothermic peak area is more obvious. This indicates that the more epichlorohydrin in the mixture, the easier the reaction and the higher the degree of crosslinking reaction.
Figure 3 shows the viscosity of the cellulose solution mixed with epichlorohydrin as a function of time at 50℃. The inflection points at which the viscosity rises sharply can represent the beginning of gelation of the cellulose solution. It is found in Fig. 3(a) that the cellulose solutions with different E/C ratios have similar viscosity change curves. Their gelling phenomenon occurs between 18–20 min. However, the reaction temperature has a significant effect on the gelatinization rate of the cellulose solution as shown in Fig. 3(b). It takes about 40 min for the cellulose solution to reach the gel point at 40℃, while at 50℃ and 60℃ it took only 20 min and 10 min, respectively. This result indicates that temperature can promote the cross-linking reaction between cellulose and epichlorohydrin and lead to a rapid increase in viscosity.
3.3 Chemical structure of cellulose hydrogel
Figure 4 showed the FTIR spectra of the regenerated cellulose and dehydrated cellulose hydrogels prepared with different conditions. The peak at 2896 cm-1 in the regenerated cellulose is attributed to the stretching vibration of CH2, and this absorption peak is split into two peaks at 2921 cm-1 and 2877 cm-1 in cellulose hydrogels. This is due to the introduction of a propyl structure by the interaction between epichlorohydrin and the hydroxyl group of cellulose. The peak at 1203 cm-1 is attributed to the bending vibration of C-OH at the C6 position of cellulose, and the intensity of this peak is weakened in the spectrum of cellulose hydrogel. The depletion of hydroxyl groups indicates a chemical reaction between the hydroxyl groups of cellulose and the epoxy groups of epichlorohydrin. In addition, the absorption peak at 1418 cm-1 and 1455 cm-1 due to the symmetric and asymmetric bending of CH2 are strengthened. All these phenomena confirm the chemical crosslinking reaction between cellulose and epichlorohydrin (Colom and Carrillo 2002; Oh et al. 2005; Ding et al. 2015).
3.4 Mechanical properties of cellulose hydrogel
The cellulose hydrogels prepared in this study are soft materials. Before the strain reaches about 40%, the energy applied by the external force is mainly used to generate the compressive deformation of the hydrogel (as shown in Fig. 5). Subsequently, the hydrogel gradually densified and returned more external force energy in the form of stress. After that, the stress increased significantly, and an instant rupture occurred at about 60% of the strain (as shown in Fig. 6).
Figure 6(a) shows the compression stress-strain relationship of cellulose hydrogels prepared with different E/C ratios. Since epichlorohydrin functions as a crosslinking agent in the cellulose hydrogel system, increasing the E/C ratio can form a higher degree of crosslinking, resulting in greater mechanical strength and elasticity. Hydrogels prepared with E/C = 4.0 and E/C = 3.5 have higher rupture strength and elastic modulus, but there is no significant difference between the two. However, the hydrogel prepared with E/C = 3.0 exhibited lower strength and elasticity. This phenomenon is consistent with the results of DSC reactivity analysis, in which E/C = 3.5 and E/C = 4.0 have higher, but similar reaction heat, while E/C = 3.0 is less.
Figure 3(b) has pointed out that the higher the reaction temperature, the more intense the reaction between the cellulose solution and epichlorohydrin, and the faster the viscosity of the reaction solution increases. However, Fig. 6(b) shows that the hydrogel gelled at room temperature (RT) has higher rupture strengths. When heated at 40℃, the elastic modulus increases, while the rupture strength and deformation decrease. When heated at 60℃, both the rupture strength and elastic modulus are reduced. The negative effect of temperature on compressive strength may be due to the faster reaction rate at high temperature. Excessively fast crosslinking may lead to a rapid increase in the viscosity of the reaction solution, and makes it difficult for some components to participate in the reaction due to restricted activities, or forms uneven crosslinking density. This inhomogeneous structure is prone to stress concentration when subjected to external force, resulting in low failure strength, or failure occurs when a small strain occurs.
3.5 Swelling capacity and water absorption of cellulose hydrogel
Because cellulose hydrogels are produced by mixing cellulose solution with epichlorohydrin, and forming a gelled material through a crosslinking reaction under the condition of non-dissipation of water. The volume of the hydrogel is mainly determined by the reactants. Figure 7 shows that further water absorption occurs when water-rich cellulose hydrogels are immersed in water, and the swelling capacity for water absorption is affected by the internal crosslink structure of the hydrogel. Comparing cellulose hydrogels prepared with different E/C ratios, the larger E/C ratio has a higher crosslinking density, which limits the movement between molecular chain nodes, thereby limiting the ability to swell to accommodate more water.
Figure 7(b) compares the swelling ability of hydrogels made at different temperatures. It shows that the cellulose hydrogel produced by gelation at room temperature have a lower swelling, while the hydrogels produced by heating have a higher degree of swelling. As mentioned earlier, gelation in a heating state results in a non-uniform cross-linked structure. The low cross-linking region in this non-uniform structure has larger free space, and the molecular chain is softer and can tolerate larger deformation. This facilitates the entry of water and has a higher swelling capability.
Figure 8 shows the water absorption of dehydrated cellulose hydrogel. It can be seen that dehydrated gels have excellent water re-absorption ability. It will quickly absorb water within 20 h of soaking in water, and the water absorption rate reaches equilibrium within 50 h. The trend of the effect of E/C ratio and gelation temperature on the water absorption is the same as that of the re-swelling ability of hydrogels. Among them, those prepared with E/C = 4.0 have a water absorption rate of 1000%, and those with an E/C ratio of 3.0 can reach 1500%. On the other hand, the water absorption rates for those gelled at different temperatures are between 1300% and 1600%. The above results show that the cellulose hydrogel crosslinked by epichlorohydrin can be used as an excellent water-absorbing material after dehydration.