Novel SAP composites formation mechanism
The proposed reaction mechanism of novel SAP composites is shown in Fig. 1. First, the α-cellulose is swollen in the distilled water. KPS, a free radical initial agent, forms sulphate anion-radicals which can cleave H+ from the hydroxyl group of α-cellulose to form alkoxy radicals. The alkoxy radicals initiate the polymerization of AA and AM, leading to a graft copolymer of poly(AA-co-AM) onto α-cellulose chains. During the graft polymerization, the cross-linker (MBA) forms a cross-linked network between the growing polymer chains by the end vinyl groups. At the same time, modified-zeolite (MZE) has a highly active silanol (Si-OH) group, and it reacts with the carboxylic acid (-COOH) group to promote MZE particles into the composite network. Moreover, MZE serves as a cross-linked point to increase the intensity of the cross-linked network. In general, novel SAP composites were prepared by grafting copolymerization of AA and AM onto α-cellulose chains and the introduction of MZE as an inorganic filler, and similar reaction mechanism has also been illustrated in previous studies (Pourjavadi et al. 2007; Mukerabigwi et al. 2015; Dai et al. 2017).
FTIR analysis
Figure 2 displays the FTIR results of prepared samples. For MZE, the absorption peak at 970 cm− 1 is attributed to the stretching vibration of the Si-O-Si group, and the absorption peak at 522 cm− 1 is attributed to the bending vibration of Si-O-Al (Zhang et al. 2007). For α-cellulose, the stretching vibration of C-H and C-O-C groups appear at 2850 cm− 1 and 1032 cm− 1, respectively. The absorption peaks at around 3372 cm− 1 and 1310 cm− 1 are attributed to the bending and stretching vibration of –OH group (Saghir et al. 2021). The absorption peak at 1053 cm− 1 is related to β-(1,4) glycosidic bonds of cellulose. For poly(AA-co-AM), the absorption peak at 2941 cm− 1 is ascribed to the stretching vibration of the C-H group. The absorption peaks at around 1541 cm− 1 and 1415 cm− 1 appear due to the asymmetrical and symmetrical stretching vibration of the –COO− group. The absorption peak at 1669 cm− 1 is ascribed to carboxamide (Bao et al. 2011).
Compared curve (d) with the curve (a-c) in Fig. 2, the characteristic absorption peaks of MZE, α-cellulose, and poly(AA-co-AM) all appear in the spectra of novel SAP composites. The absorption peaks at 1550 cm− 1 and 1404 cm− 1 are strengthened, attributing to the asymmetric stretching vibration of a large amount of C = O groups. Additionally, the absorption peak at 1670 cm− 1 is obviously strengthened, attributing to the reaction between the –COOH group of α-cellulose-poly(AA-co-AM) and -OH group of MZE (Bao et al. 2011). The results suggest the desired product (novel SAP composites) is successfully synthesized. Moreover, the broad characteristic peak at 3450 − 3310 cm− 1 is ascribed to vibration stretching of abundant hydrophilic groups on novel SAP composites.
XRD analysis
The crystalline pattern of prepared samples was performed with XRD. The α-cellulose displays obvious peaks at 15.3° and 22.9°, which are related to cellulose I in nature (French 2014). These peaks of α-cellulose in the patterns of α-cellulose-g-poly(AA-co-AM) disappear, indicating that the original crystal structure of α-cellulose is destroyed in the process of graft polymerization, so α-cellulose forms an amorphous state to achieve better water absorbency. For SAP composites, the weak diffraction peak at 21.9°-24.1° is attributed to the change of crystalline phase (Mukerabigwi et al. 2015). Moreover, the characteristic diffraction peaks of MZE disappear in the novel SAP composite, indicating that the MZE is uniformly dispersed in the SAP composite matrix (Dai et al. 2017).
SEM Analysis
The surface morphology of ZE, MZE, α-cellulose, α-cellulose-g-poly(AA-co-AM), and novel SAP composites is shown in Fig. 4. As can be seen from Fig. 4(a-b), MZE has a rougher and looser surface with a high specific surface area compared with ZE. It also has clear layered structure with gaps which are conducive to the absorption of water molecules. The α-cellulose shows long-strip structure, which is used as target macromolecules to be grafted and copolymerized with monomers under the action of the initiator. Compared with Fig. 4(a-d), novel SAP composites present a more comparatively coarser and undulant surface, attributing to the introduction of MZE into its surface. The coarser and undulant surface shows that novel SAP composites have a larger superficial area, which can enhance the water absorbency.
Water absorbency
The water absorbency of samples was measured in distilled water and 0.9 wt.% NaCl solution and the results are displayed in Fig. 5. As seen from Fig. 5, the water absorbency of novel SAP composites is better than that of other samples in distilled water and 0.9 wt.% NaCl solution, respectively. It is obviously observed that the water absorbency of samples in 0.9 wt.% NaCl solution is apparently lower than that of distilled water. Na+ mainly causes this phenomenon in the solution (the penetration of Na+ into cross-linked network decreases the water absorbency of these samples). After the introduction of MZE and α-cellulose, the water absorbency of novel SAP composites is increased by 93.88% (from 350.28 ± 3.16 g/g to 679.13 ± 5.49 g/g) in distilled water, while it is increased by 89.58% (from 46.65 ± 1.78 g/g to 88.46 ± 2.36 g/g) in 0.9 wt.% NaCl solution compared with poly(AA-co-AM). Combined with SEM, it is found that novel SAP composites have larger superficial area after the introduction of MZE and α-cellulose, which is able to improve the surface adsorption (Zhang et al. 2007). Moreover, both MZE and α-cellulose have large amounts of hydrophilic groups. In summary, the water absorbency of novel SAP composites is obviously improved after the introduction of MZE and α-cellulose.
Figure 6 shows the water absorbency of novel SAP composites with different MZE contents. The water absorbency of novel SAP composites is first increased and then decreased as the MZE content is increased, as displayed in Fig. 6. The highest water absorbency of these SAP composites in distilled water and in 0.9 wt.% NaCl solution was achieved almost synchronously when the MZE content is 6 wt.%. A large number of pores in MZE accommodate more water molecules with the increase of the internal pore volume. Moreover, MZE can serve as a cross-linked point, which promotes formation of cross-linked network to enhanced water absorbency (Li et al. 2015). When the content of MZE increases beyond the optimal value, MZE may exist in the form of physical filling, and it produces agglomeration, which will form steric hindrance between polymer chains to make it difficult for the reaction raw materials to evenly disperse in the novel SAP composite (Liang et al. 2009). The results show the optimal addition content of MZE in the novel SAP composite is 6 wt.%, which can obviously enhance the water absorbency. Therefore, novel SAP composites when the addition content of MZE is 6 wt.% were prepared which used for further investigation on swelling kinetic, water retention capacity and TG analysis.
Swelling kinetics
Figure 7 shows the swelling process of prepared samples in distilled water, respectively. It is obviously observed that the time needed to reach swelling equilibrium of novel SAP composites is shorter than that of poly(AA-co-AM) and α-cellulose-g-poly(AA-co-AM). The swelling rate of novel SAP composites is rapidly increased within the first 30 min and then slowly increased to 679.13 g·g− 1 until reaching its equilibrium (55 min). Moreover, the swelling rate of novel SAP composites is remarkably increased compared with α-cellulose-g-poly(AA-co-AM) (440.23 g·g− 1 at 55 min) and poly(AA-co-AM) (350.60 g·g− 1 at 55 min). The results show the addition of MZE and α-cellulose affects the swelling kinetics of SAP composites (it is related to surface area and swelling ability (Zhang et al. 2006)). The combination of α-cellulose and MZE into the polymer network can enhance swelling rate due to the –OH groups, increasing the affinity of novel SAP composites to water molecules. MZE also strengthens the cross-linked network of novel SAP composites and it provides more free volume to allow water molecules to penetrate novel SAP composites.
The swelling kinetics can be calculated by Eq. (3), which is based on the Voigt-based viscoelastic model (Kabiri et al. 2003; Irani et al. 2013).
where St (g·g− 1) is the swelling property of SAP composites at some moment, P (g·g− 1) is the power parameter, and r (min) is the rate parameter (the time required for the sample to reach 63% of its final swelling).
As shown in Fig. 7, the fitted curves of swollen samples are consistent with experimental results. The r is 9.57 min, and P for novel SAP composites is 681.16 g·g− 1, the r is 17.22 min, and P is 465.34 g·g− 1 for α-cellulose-g-poly(AA-co-AM), and r and P for poly(AA-co-AM) are 23.79 min and 397.1 g·g− 1, respectively. The P-value reflects better water absorbency, and the r value reflects the swelling rates (Wang et al. 2009). It is observed that the r is the minimum and the P is the maximum for novel SAP composites. The results indicate that novel SAP composites can provide a better water absorbency and a greater swelling rate after the introduction of MZE and α-cellulose.
Water retention capacity
The water retention capacity of samples was evaluated at 50°C by using the auto mass measurement experiment, displayed in Fig. 8. It is obviously noticed that the absorbed water is gradually decreased as the time is increasing. Moreover, novel SAP composites significantly maintain more water compared with the other two samples at the same time. At 5 hs, the weight loss of water in swollen novel SAP composites is about 51.1% while α-cellulose-g-poly(AA-co-AM) is about 73.9% and poly(AA-co-AM) is about 92.8%. By contrast with poly(AA-co-AM), the retained water time of novel SAP composites is increased from 6.5 hs to 11.2 hs, increasing the retained water time by 71.79%. Results show the water retention capacity of novel SAP composites is evidently improved. The introduction of MZE and α-cellulose can resist shrinkage of SAP composite matrix as dried. Moreover, more absorbed water is fixed in the cross-linked network of novel SAP composites, and the release of this water needs more energy, contributing to the enhanced water retention capacity (Li et al. 2004).
Thermal behavior analysis
The effect of the introduction of MZE on the thermal performance of novel SAP composites was further examined by TGA, and the analysis is represented in Fig. 9 and Table 1.
For α-cellulose-g-poly(AA-co-AM), four mass loss stages are observed. The first stage in a temperature range of 55–105°C with about 12% mass loss is ascribed to the evaporation of water moisture (Seki et al. 2014). The second loss stage between 105°C and 346°C is attributed by decomposition of α-cellulose (Fu et al. 2016; Etminani-Isfahani et al. 2020). The third mass loss stage between 346°C and 410°C is attributed by the decomposition of some short or straight chains of α-cellulose-g-poly(AA-co-AM) (Liang et al. 2009). The fourth mass loss stage between 410°C and 510°C is ascribed to the degradation of grafted chains in the cross-linked network (Bee et al. 2014). The TG curve of novel SAP composites contains four similar stages compared with α-cellulose-g-poly(AA-co-AM). However, Tonset of novel SAP composites is increased from 63.5°C to 63.8°C, and the residual rate is increased from 32.9–34.5%.
It is obviously observed in Table 1 that the T10%, T50% and Tpeak (the temperatures at a maximum decomposition rate of each step) of novel SAP composites are increased compared with α-cellulose-g-poly(AA-co-AM), respectively. The above results indicate that the addition of MZE can slightly improve the thermal stability of novel SAP composites. MZE has higher thermal stability which can serve as a cross-linked point to enhance the intensity of cross-linked networks which delays the thermal decomposition of novel SAP composites, and it has strong interactions with polymers in the novel SAP composites (Li et al. 2015; Etminani-Isfahani et al. 2020), both resulting in improvement of thermal stability. However, the addition content of MZE is only 6 wt.% now and its effect on the thermal stability of novel SAP composites is not significant yet.
Table 1
Different characteristic temperatures in TG and DTG experiments
Samples
|
TG
|
DTG
|
Tonset/°C
|
T10%/°C
|
T50%/°C
|
Residual rate/%
|
Tpeak /°C
|
α-cellulose-g-poly(AA-co-AM)
|
63.5
|
75.1
|
461.8
|
32.9
|
64
|
318
|
369
|
439
|
novel SAP composites
|
63.8
|
77.2
|
476.7
|
34.5
|
63
|
330
|
372
|
450
|