Influence of Gel Stage from Cellulose Dissolution in NaOH-Water System on the Performances of Cellulose Allomorphs-Based Hydrogels

Abstract

Novel hydrogels were prepared starting from different cellulose allomorphs (cellulose I, II, and III), through a swelling stage in 8.5% NaOH aqueous solution, followed by freezing at low temperature (−30 °C), for 24 h. After thawing at room temperature, the obtained gels were chemical cross-linked with epichlorohydrin (ECH), at 85 °C. The swelling degrees of the hydrogels were investigated, and a complex dependence on the type of the cellulose allomorph was found. Moreover, the gel stage has been shown to play a key role in the design of hydrogels with different performances, following the series: H-CII > H-CI > H-CIII. The correlations between the allomorph type and the morphological characteristics of hydrogels were established by scanning electron microscopy (SEM). The hydrogel H-CII showed the biggest homogeneous pores, while H-CIII had the most compacted pores network, with small interconnected pores. The rheological studies were performed in similar shear regimes, and a close correlation between the strength of the gel structure and the size of the gel fragments was observed. In the case of hydrogels, it has been shown that H-CII is softer, with a lower resistance of the hydrogel (G′) above the oscillation frequencies tested, but it maintains its stable structure, while H-CIII has the highest modulus of storage and loss compared to H-CI and H-CII, having a stronger and more rigid structure. The X-ray diffraction (XRD) method showed that the crystalline organization of each type of allomorph possesses a distinctive diffraction pattern, and, in addition, the chemically cross-linking reaction has been proved by a strong decrease of the crystallinity. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy provided clear evidence of the chemical cross-linking of cellulose allomorphs with ECH, by the alteration of the crystal structure of cellulose allomorphs and by the formation of new ether bands.

Enhanced Superabsorbency of Cellulose-Based Hydrogels in NaOH Solution: Synthesis, Characterization, and Performance Evaluation

“…The higher the NaOH concentration, the more cellulose polymer chains are disrupted, resulting in shorter bonds and smaller cross-linking, decreasing pore size. Consequently, the water absorption capacity is reduced [26]. The swelling degree in PBS solution exhibits a significantly higher value compared to distilled water, while following the same trend of reduction with increasing NaOH fraction, as depicted in Figure 4 (b).…”

“…It can be observed that increasing the NaOH fraction in the hydrogel leads to a decrease in swelling degree, attributed to the reduction in pore size. Higher weight loss is also observed, indicating a weakening of crosslinking strength between polymer chains [26].…”

“…The small pore size leads to a lower absorption capacity, which will be confirmed by the results of the swelling test. [26].…”

“…Some solvents that are commonly used such as cuoxam, cuan, cadoxen, N-Methylmorpholine-N-Oxide (NMMO), Lithium Chloride/N,N-Dimethylacetamide (LiCl/DMAc), and ionic liquids exhibit properties of volatility, toxicity, and high cost, which limit their usefulness [25]. The NaOH-water-based system is widely recognized as one of the most popular and advantageous solvents for cellulose due to its affordability, ease of recyclability, simple, and environmentally friendly nature [26], [27]. The NaOH-water-based system is widely recognized and used as a solvent in the pulp industry [27].…”

Efficient Removal of Heavy Metals from Aqueous Solution Using Licorice Residue-Based Hydrogel Adsorbent

“…In comparison, glutaraldehyde raised concerns regarding the potential presence of toxic residues after the reaction [46]. Moreover, the use of citric acid or cellulose crosslinking required an elevated temperature to achieve higher crosslinking efficiency, which may lead to cellulose degradation [47]. Based on the aforementioned factors, EPI is considered a superior crosslinker for hydrogel preparation.…”