Preparation of cross-linked cellulose nanofibril aerogel with water absorbency and shape recovery

CNF was prepared using a grinding method according to Sim et al. (2015). Once-dried, commercial bleached eucalyptus kraft pulp was used as a raw material for the preparation of the CNF. Dried pulp sheets were dispersed in deionized water and refined using a laboratory Valley beater to a freeness value of 450 mL CSF. The consistency of the beaten pulp suspension was adjusted to 2 %, after which the suspension was subjected to a mechanical treatment using a grinder (Super Masscollider MKCA6-3, Masuko Sanguo Co.). The rotation speed and gap of the grinding stone were 1500 rpm and −60 µm, respectively. After 40 cycles of the grinding treatments, CNF hydrogel was obtained. Scanning electron microscope (SEM) images of the prepared CNF were shown in Fig. 1. The chemical composition of the CNF was determined by the NREL (National Renewable Energy Laboratory, USA) method (NREL/TP-510-42618). The contents of α-cellulose, hemicellulose, lignin and ash were found to be 79.4, 18.7, 1.5, and 0.5 %, respectively. To confirm esterification of cellulose with maleic acid, filter paper CNF was also prepared from pure cellulose. Advantac 5C grade filter paper and the same procedure described above was used in preparing filter paper based CNF for FT-IR spectroscopy.

Twenty wt% of sodium hypophosphite based on the oven dried weight of CNF-MA was added to the CNF-MA suspension. The CNF-MA hydrogel mixed with sodium hypophosphite of which solids content was adjusted to 1.0–3.0 wt% was poured into polystyrene cuvette molds. The CNF hydrogels in the cuvette were precooled by placing in a refrigerator at 4 °C for overnight to minimize the fracture of the frozen CNF suspension during the subsequent rapid freezing step (Zheng et al. 2014). The precooled CNF hydrogels were rapidly frozen at −196 °C using liquid nitrogen. These frozen samples were subjected to freeze-drying at −80 °C and 5 mmHg using a freeze-dryer (FD8518, Ilshin Lab) for 3 days, and CNF aerogels were obtained. For cross-linking of CNF, the CNF aerogels were placed in a convection oven and cured for 10 min at 170 °C. Cross-linked CNF aerogels were cut into cubic shapes with a dimension of 10 × 10 × 10 mm³ and conditioned in a control room at 23 °C and 50 % RH.

Maleic acid-functionalized filter paper CNF (filter paper CNF-MA) was used for FT-IR spectroscopy to identify the ester linkage between the maleic acid and cellulose. After the esterification process, the filter paper CNF-MA suspensions were treated with a 0.1 M NaOH solution at room temperature for 1 h to convert carboxylic acid to carboxylate anion. Then it was vacuum-filtered to obtain a filter paper CNF-MA film and dried in a convection oven for 2 days at 40 °C. FT-IR spectra of the filter paper CNF-MA film were obtained using a FT-IR spectrometer (Cary 660, Agilent) in ATR mode. For each spectrum, 32 scans were collected with a range of 600–4000 cm⁻¹ at a resolution of 4 cm⁻¹.

A scanning electron microscope (SEM, Auriga, Carl Zeiss) was used for the observation of the cross-linked CNF aerogels. Cross-linked CNF aerogel samples were frozen at −196 °C using liquid nitrogen, and its cross-section was prepared by fracturing the frozen sample. Then sample was attached to a sample stub with a double-sided adhesive conductive carbon tape and sputter-coated with platinum.

The specific surface area of the aerogel was determined using nitrogen adsorption/desorption measurements (ASAP 2020, Micromeritics Instruments Co.) at −196 °C. Before the adsorption measurement, the samples, dried at 105 °C for 24 h to remove moisture, were degassed at 120 °C for 4 h for the complete elimination of moisture. Both the adsorption and desorption isotherms were measured, and the surface area was determined from the adsorption curve by the Brunauer–Emmett–Teller (BET) method.

To measure the moisture uptake of the cross-linked CNF aerogel, saturated lithium chloride, calcium chloride, and sodium chloride solutions were used to control the relative humidity of air to 15, 37, and 75 % RH at 23 °C, respectively. These saturated salt solutions were placed in a humidity chamber and the weight changes of the fully dried cross-linked aerogels were measured to determine the moisture contents.

To evaluate the compressive strength and shape-recovery performance of the cross-linked CNF aerogel, a texture analyzer (TA XT Plus, Stable Micro Systems, Ltd.) with a 50 mm cylindrical probe and a 50 kg load cell was used. During the compression of cross-linked CNF aerogel, no fracture point or inflection point was observed on the stress–strain curve until 90 % of its original dimension. The compressive stress at a strain level of 90 % was determined as the compress strength. In this test, the probe speed was adjusted to 1 mm/sec.

To evaluate the shape-recovery performance of the cross-linked CNF aerogel in the wet state, a texture profile analysis (TPA) was carried out. This test consists of two successive 70 % strain compressions of the cross-linked CNF aerogel, as shown in Fig. 2. Springiness, the recovery ratio of the deformed material after the removal of a deforming force, was obtained as in Eq. (1). T ₁ and T ₂ represent the time required for 70 % compression of the cross-linked CNF aerogel in the first and second compression cycles, respectively. When the springiness value is closer to one, the cross-linked CNF aerogel easily recovers to its original shape after compression. For the TPA test, the probe speed was adjusted to 0.5 mm/s. The time interval between the first and second compression was 5 s.

The cross-linking of CNF takes place in two separate reactions when maleic acid and hypophosphite were used. The first step is an esterification reaction between maleic acid and hydroxyl group of cellulose. In an aqueous solution, maleic acid forms a five-member cyclic anhydride by the dehydration of two carboxylic groups. This intermediate reacts with the hydroxyl group of cellulose to form an ester linkage (Fig. 3). In this esterification process, sodium hypophosphite was used as a catalyst. When the reaction temperature is below 140 °C, the reaction rate of esterification is drastically increased by the presence of sodium hypophosphite (Yang et al. 2010).

The FT-IR spectra of the filter paper CNF-MA are shown in Fig. 4. As more maleic acid was added to the filter paper CNF suspension, the peak at 1570 cm⁻¹, assigned to the C–O stretching vibration of the ester linkage, increased. When the CNF made of hardwood kraft pulp was used, the FT-IR spectra of the CNF-MA did not show any significant difference compared to that of untreated CNF. This is because the bleached hardwood kraft pulp contains nearly 20 % of hemicellulose and lignin, which gave absorption peaks at 1570 cm⁻¹.

Polycarboxylic acids such as 1,2,3,4,-butanetetracarboxylic acid (BTCA) are used as cross-linking agents for the durable press finishes of cotton fabric because they form multiple ester linkages with cellulose (Yang et al. 2010; Yao et al. 2013). Maleic acid does not crosslink cellulose. The presence of hypophosphite ion, however, allows the cross-linking of maleic acid-functionalized CNF. When the reaction temperature was 120 °C as in esterification process, sodium hypophosphite acts as a catalyst. Whereas hypophosphite ions react with carbon–carbon double bonds of maleic acid-functionalized CNF as shown in Fig. 5 (Yang et al. 2010), when the reaction temperature is higher than 140 °C.

In the case of typical CNF aerogels, the network structure is built by hydrogen bonds and physical entanglements between adjacent CNFs. When the typical CNF aerogels are exposed to moisture, interfiber hydrogen bonds are replaced with cellulose-water hydrogen bonds. As a result, the entire network structure of aerogels loses its strength.

The cross-linked and untreated CNF aerogels in cubic shapes (10 × 10 × 10 mm³) were immersed in deionized water and stirred for 1 min with a magnetic stirrer at 300 rpm. The density of aerogel was 31.5 kg/m³. As shown in Fig. 6, the CNF aerogel which had not undergone a cross-linking treatment disintegrated and dispersed in the water, while the cross-linked CNF aerogel maintained its original structure. This shows that the covalent cross-linking of CNF was formed through the treatment with maleic acid and hypophosphite.

In Table 1, the compressive strengths of the cross-linked and untreated CNF aerogel with density of 31.5 kg/m³ were compared. The cross-linked CNF aerogel exhibited greater compressive strength in both dry and wet state; this indicates that the covalent cross-linking of CNF was formed via the reaction between the cellulose-grafted maleic acid and hypophosphite. The compressive strength of the untreated aerogels, however, was around 70 % of that of the cross-linked samples, which indicates that significant parts of the CNF networks are still formed via hydrogen bonding.

The water present in the CNF hydrogel was replaced with air by rapid freezing and freeze-drying, which gave CNF aerogels without a collapse or shrink (Aulin et al. 2010). In other words, the apparent volume of the CNF aerogels is equal to the original volume of the CNF suspension in the mold. Therefore, the density of aerogel is proportional to the solids content of CNF suspension (Table 2). The specific surface area of the CNF aerogel with a density of 26.7 kg/m³ was 19.5 m²/g, which was quite lower than that of organic or inorganic aerogels. For instance, Cai et al. (2012) reported that the specific surface area of aerogels prepared with sol–gel processes were 400–654 m²/g. Our result, however, is in good agreement with previously reported values for freeze-dried CNF aerogels. Sehaqui et al. (2010) have shown that specific surface areas of 15–45 m²/g could be obtained for CNF aerogels with densities ranging from 7 to 103 kg/m³.

Cross-sectional images of a CNF aerogel with a density of 26.7 kg/m³ are shown in Fig. 7. This CNF aerogel exhibits a highly porous internal structure with open pores. It was noted that the network structure of the CNF aerogel was built with very thin nanopapers. Svagan et al. (2008) and Chen et al. (2011) have shown that randomly distributed nanofibers are forced out by the growing ice crystals and develop film-like cellulose nanopaperlets during the freeze-drying process. This nanopaper structure is commonly found in cellulose aerogels prepared by the freeze-drying of a CNF suspension (Cervin et al. 2012; Aulin et al. 2010; Sehaqui et al. 2010; Syverud et al. 2011).

A water-absorption test was carried out by immersing the CNF aerogels into deionized water. The CNF aerogels absorbed water instantly by wetting and capillary action due to the hydrophilic and porous nature of CNF aerogels. Because hydrogen bonding and mechanical entanglement of fibers are the main source of structural rigidity of the CNF aerogel, its network structure is destroyed and irreversibly collapsed upon the absorption of water. On the other hand, the cross-linked CNF aerogel retained its original shape and volume, and its water-absorbing capacity corresponded to its internal pore volume (Fig. 8).

As shown in Fig. 9, the equilibrium moisture content of cross-linked CNF aerogel was drastically increased with the relative humidity of air, and it was significantly greater for the typical handsheets prepared with the same bleached eucalyptus kraft pulp. At 75 % RH condition, the moisture contents of the cross-linked CNF aerogel was 31 %, while that of the handsheet was 11 %. The large specific surface area of the cross-linked CNF aerogel resulted in greater adsorption of water and gave higher equilibrium moisture content.

In Fig. 10, the compressive strength of cross-linked CNF aerogel was plotted as a function of the moisture content. The density of the cross-linked CNF aerogel was 31.5 kg/m³. The compressive strength was drastically decreased as the moisture content was increased. The adsorption of water reduces the interfiber hydrogen bond of adjacent celluloses (Kang and Kim 2012; Kim et al. 2006). The reduction of compressive strength indicated that a considerable portion of the CNF network structure is still consisted of hydrogen bond even after the cross-linking treatment.

Cross-linking of CNF aerogel caused difference in the shape-recovery behavior after compressive deformation. Compressed CNF aerogels remained in compressed shape in dry state irrespective of the cross-linking treatment (Fig. 11). After wetting, the compressed cross-linked CNF aerogels showed substantially greater shape recovery characteristics. The cross-linked CNF aerogel recovered nearly 70 % of the original shape within a few seconds after wetting. On the other hand, untreated CNF aerogel remained compressed even after wetting with water. The shape recovery property of cross-linked CNF aerogel in the wet state was examined with the use of TPA test (Fig. 12). The cross-linked CNF aerogel recovered its original volume after wet compression and the springiness value was 0.78. On the other hand, the untreated CNF aerogel gave the springiness of 0.33.

Zhang et al. (2012) described that the shape-recovery of the cross-linked CNF aerogels stems from the interaction between the absorbed water and CNF. Compression of the fiber network of CNF aerogel takes place by bending of individual fibers or breaking of interfiber bonds, and by the loss of both friction between adjacent fibers and physical entanglement of the fibers. Since the longitudinal compression of cellulose fibrils can be neglected, only the bending of the fiber can be taken into account during the compressive deformation of CNF aerogels. The elastic modulus of the crystal region of cellulose ranges from 130 to 250 GPa (Zimmermann et al. 2004; Sakurada et al. 1962), while that of the amorphous region is reported to be 12.2 (Wang et al. 2014) or 10.4 GPa (Chen et al. 2004). Due to this substantial difference in the elastic moduli in the crystalline and amorphous regions of cellulose, the bending of CNFs under compression is confined to the amorphous region. When the compressed cross-linked CNF aerogel absorb water, the amorphous region swells partially, causing the CNF to expand to a more stretched form and recovers its original shape. And the presence of connecting points in the cross-linked CNF aerogels caused the stretching of CNF transforms into structural expansion. In other words, the network structure of cross-linked CNF provides improved mechanical stability both in dry and wet states. As a result, the cross-linked CNF aerogel maintains its original interfiber network structure during the compression and recovery processes. When the external force is removed, the deformed individual fibers recover their original shape, and this expanding force allows the entire network return back closer to its original shape.