Porous aerogels prepared by crosslinking of cellulose with 1,4-butanediol diglycidyl ether in NaOH/urea solution

Abstract

Cellulose aerogels based on crosslinking of cellulose with 1,4-butanediol diglycidyl ether (BDE) were homogeneously synthesized in NaOH/urea aqueous solution followed by freeze-drying. In the NaOH/urea aqueous solution, cellulose existed in a sodium alkoxide form that could react with the epoxide groups of BDE. The rheological behavior of the cellulose/NaOH/urea aqueous solution showed that the crosslinking reaction occurred immediately once the BDE was added to the cellulose/NaOH/urea aqueous solution at room temperature. The X-ray diffraction (XRD) characterization identified a transition from the crystalline structure of cellulose to an amorphous state of cellulose aerogels with increasing amount of BDE. Elemental analysis revealed the variation in carbon and oxygen elemental percentages in cellulose aerogels caused by the reaction between cellulose and BDE. The porous network of aerogels was observed by scanning electron microscopy (SEM) and the pore size of the aerogels increased as a function of BDE. The water adsorbent ability of aerogels was up to 41 g g⁻¹—even after 30 squeezing–adsorption cycles, the water adsorbent ability was still 37 g g⁻¹.

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Introduction

Renewable resources have attracted much attention because of the great importance they have in sustainable development and environmental protection. Cellulose is a nearly inexhaustible raw polymeric material with fascinating structures and properties. It is environmentally friendly and biocompatible.¹ Cellulose products^2–4 have been successfully synthesized with extensive applications because of their low cost and environmental friendliness.

In particular, cellulose aerogels have a promising future for low density and biocompatibility.⁵ They have received special interest as drug delivery vehicles,⁶ cell culture growth template,⁷ superabsorbent polymers,⁸ etc. Cellulose aerogels are generally prepared from regenerated cellulose via a freeze-drying process or a supercritical carbon dioxide process.^9,10 Using these methods, the new ultralight and highly porous cellulose materials have been prepared in recent years. Cellulose aerogels were prepared by regenerated and dried processes from cellulose/NaOH/urea aqueous solution and the density ranged from 0.06 to 0.3 g cm⁻³.¹¹ Cellulose microfibril foams with controllable channels were fabricated. This increased the concentration of microfibrils, which led to an enlargement in the compressive stress.¹² However, all of these cellulose aerogels are based on hydrogen bonds and van der Waals interactions.¹³ In aqueous systems, the cellulose chain molecules are gradually surrounded by water, which destroys the interaction of hydrogen bonds between the cellulose chains and partly obstructs the generation of the driving force to create the tight aggregates.¹⁴ Therefore, immersing the cellulose aerogels in water destroys hydrogen bonds and van der Waals interactions between cellulose. This results in a collapse of the structure and limits the application of cellulose aerogels.

Chemical crosslinking is an alternative way to improve the mechanical property of cellulose aerogels and overcome these problems. Reusable cellulose aerogels with shape recovery properties and adsorbent features were prepared by crosslinking nano-fibril cellulose. These cellulose aerogels were particularly attractive for numerous potential applications.^8,15,16 Crosslinking of nano-cellulose is one route to prepare cellulose aerogels, but the process of cellulose fiber to nano-size requires much energy, time and money. Chemical crosslinking of cellulose in its solution through a homogeneous reaction is an alternative way to prepare cellulose aerogels. However, cellulose is insoluble in water and most organic solvents due to its strong intermolecular hydrogen bonds, high crystallinity and chain rigidity.¹⁷

Only a few solvents are currently suitable for derivatization to increase the accessibility of cellulose. The N,N-dimethylacetamide (DMA)/LiCl,¹⁸ N-ethylmorpholine-N-oxide (NMMO)¹⁹ and ionic liquids (ILs)²⁰ are the most common. Various cellulose derivatives have been successfully synthesized through homogenous methods in these solvents.²¹ Nevertheless, these solvents have several problems including decomposition, high viscosity and side reactions during cellulose dissolution.²² Faced with these problems, a non-toxic and non-polluting solvent system for cellulose—the NaOH/urea aqueous solution—was developed.^23,24

Preparation of cellulose ethers with alkyl halide in NaOH/urea aqueous solution have been studied as the most common chemical modification of cellulose. In the NaOH/urea aqueous system, the urea hydrates aggregate at the surface of the NaOH hydrogen-bonded cellulose to create an inclusion complex (IC) that leads to the dissolution and increases accessibility of cellulose. Cellulose exists in the form of sodium alkoxide in IC, which is a good nucleophilic reagent and easily reacts with alkyl halides such as 3-chloro-2-hydroxypropyltrimethyl ammonium chloride and epichlorohydrin.^25–27 Furthermore, sodium alkoxide is highly reactive to the epoxy groups and therefore diepoxide compounds could react with cellulose as crosslinking agents in NaOH/urea aqueous solution system. 1,4-Butanediol diglycidyl ether (BDE) is biodegradable which has a significantly lower toxicity than other ether-bond crosslinking chemistry based agents. BDE is a diepoxide-based bifunctional linker and the ability to crosslink is attributed to the reactivity of the epoxide groups present at the two ends of the molecule. BDE was usually used for crosslinking polysaccharides and polyaminesaccharides via the hydroxyl group and reactive amino group. Chitosan substrates were crosslinked with BDE via the –NH₂ groups on the chitosan backbone to modify the mechanical property of chitosan-based scaffolds.²⁸ Chitosan beads were crosslinked with BDE followed by reacting with synthetic ligands to selectively bind human serum albumin.²⁹ Dextran was crosslinked with BDE to form its nanoparticles (DN) which were then partially oxidized with sodium periodate to generate aldehyde functionalities on them.³⁰ Hyaluronic acid hydrogels (HAGs) were prepared by crosslinking hyaluronic acid (HA) with BDE which could be used as an injectable scaffold for regenerating functional tissues.³¹

In this paper, cellulose aerogels based on crosslinking of cellulose with 1,4-butanediol diglycidyl ether (BDE) were homogeneously synthesized in a NaOH/urea aqueous solution followed by freeze-drying. The effects of crosslinking agent amount and hydrogel concentration on the properties of cellulose aerogels were discussed. Crystal structure, elemental percentage, morphological features, thermal stability and rheological properties of the cellulose aerogels were analyzed by X-ray diffraction (XRD), elemental analysis, scanning electron microscopy (SEM) and rheological method, respectively. Moreover, the adsorbent ability and reusability of the cellulose aerogels in water, N,N-dimethylformamide and toluene were also investigated, respectively.

Materials and methods

Chemicals and materials

Cellulose pulp from hardwood was provided by the Hubei Chemical Fiber Group Ltd. The viscosity-average molecular weight of the cellulose pulp was 10.6 × 10⁴ g mol⁻¹ (average degree of polymerization, = 655). The raw cellulose pulp was vacuum-dried at 50 °C for 24 h to remove moisture before use. Sodium hydroxide, urea, silicone oil, 1,4-butanediol diglycidyl ether (BDE, 95% purity), N,N-dimethylformamide (DMF) and toluene were purchased from Aladdin Industrial Corporation. All chemicals were of analytical grade and used as received.

Methods

Preparation of cellulose solution. Cellulose pulp was vacuum-dried to remove moisture and precooled to −10 °C before use. The aqueous solution containing NaOH/urea/H₂O at 7 : 12 : 81 by weight was used as the solvent. Next 4.0 g of cellulose pulp was added to 96.0 g NaOH/urea/H₂O solution and mixed at −12.3 °C under stirring for 30 min to form a transparent and viscous cellulose solution.

Preparation of cross-linked cellulose aerogels. BDE in the desired amount was added to the solution and the mixture was kept at room temperature for 1 h to form the cross-linked cellulose hydrogels. The cross-linked hydrogels were separated by centrifugation and purified with water until the pH was 7.

Next, the purified hydrogels were diluted to 1.5% by adding deionized water and then the dilute hydrogels were freeze-dried in a lyophilizer for 48 h to obtain the porous cellulose aerogels. Cellu/BDE1-1.5, Cellu/BDE2-1.5, Cellu/BDE3-1.5, Cellu/BDE4-1.5 and Cellu/BDE5-1.5 represent the cellulose aerogels obtained from 1.5% hydrogels when the molar ratios between anhydroglucose unit (AGU, M = 162) of cellulose and BDE were 1 : 1, 1 : 2, 1 : 3, 1 : 4 and 1 : 5, respectively. When the molar ratio between AGU of cellulose and BDE were 1 : 3, the cellulose aerogels that were prepared from the 3.0, 2.5, 2.0, 1.5, 1.0 and 0.5% hydrogels were coded as Cellu/BDE3-3.0, Cellu/BDE3-2.5, Cellu/BDE3-2.0, Cellu/BDE3-1.5, Cellu/BDE3-1.0 and Cellu/BDE3-0.5. To compare the properties of the aerogels, regenerated cellulose (RC) without BDE was also prepared following by centrifugation and purified with water as mentioned above.

Characterization

X-ray diffraction (XRD). The X-ray diffraction patterns were measured via an X-ray diffractometer (D8 FOCUS, Bruker, Germany) with a Cu-Kα anode. The analysis was recorded at a scanning speed of 0.01° s⁻¹ in the range of 2θ = 10–50°.

Scanning electron microscopy (SEM). The surface morphology of the cellulose samples was observed using field-emission scanning electron microscopy (FE-SEM, JSM-7600F, JEOL, Japan) operated at 15 keV with a 10 nA primary beam. Samples were attached to the sample holders with conductive double side carb on tape and sputter coated with a platinum to avoid charging during the tests.

Elemental analysis. Elemental analysis was performed using a PE-2400 CHNS/O instrument (USA). In CH operating mode, the most robust and interference-free mode, the instrument showed a classical combustion principle to convert the sample elements into simple gases (CO and H₂O). The PE-2400 analyzer automatically performed combustion, reduction, homogenization, separation and detection of the gases.

Rheological properties. Dynamic rheological characterization of the cellulose materials was studied with a rotational viscometer (Mars, Hake, Germany) equipped with a temperature control system. The mixture of the cellulose solution and BDE was directly introduced to the plate and a layer of silicone oil was used to prevent water evaporation.³² Estimation of the gelation temperature T_gel was made by recording the elastic G′ and viscous G′′ moduli as a function of temperature from 20 to 90 °C at 5°C min⁻¹ while noting the temperature at which G′ = G′′. The experiments were performed on the mixture of BDE and cellulose solution at a stress of 0.01 Pa. The frequency was 1 Hz (linear viscoelastic regime) to investigate the behavior of the reaction.

Density and adsorbent ability. The density values (ρ) of the aerogels were determined by weighing them and measuring their volume after freeze-drying. The water adsorbent ability of cellulose aerogels was measured by immersing samples in 200 mL water. The saturated aerogels were weighed after sucking the surface water with filter paper. The water adsorbent ability (g g⁻¹) of cellulose aerogels was measured and calculated as:

Water adsorbent ability = (W₁ − W₀)/W₀ × 100% (1)

here, W₁ is the weight of water-saturated cellulose aerogel and W₀ is the weight of the dried cellulose aerogel, respectively.

The adsorbed water was removed by compressing the aerogels to ensure that more than 90% of the adsorbed water was removed. The compressed aerogels were immersed in water again without any pre-treatment. The aerogels were weighed to evaluate the adsorbent ability once again and the water adsorbent ability was measured for 30 squeezing–adsorption cycles. The DMF and toluene adsorbent abilities of the aerogels were measured similarly.

Results and discussions

General scheme for synthesis of cross-linked cellulose aerogels

Cellulose aerogels based on crosslinking of cellulose with BDE were homogeneously synthesized in NaOH/urea aqueous solution followed by freeze-drying. In the NaOH/urea aqueous solution, the urea hydrates aggregate at the surface of the NaOH hydrogen-bonded cellulose to create an inclusion complex (IC) leading to the dissolution and increasing accessibility of cellulose. Cellulose in IC exists in a sodium alkoxide form, which is a good nucleophilic reagent and easily reacts with epoxide groups.³³ BDE has two epoxide groups on the sides of the molecular chain and could react with sodium alkoxide of the cellulose as a crosslinking agent in NaOH/urea aqueous solution. BDE forms externally crosslinking with cellulose, which enhances the stability compared to those regenerated aerogels that only depend on hydrogen bonds and van der Waals interactions. A scheme for the synthesis of cross-linked aerogels is shown in Fig. 1.

Fig. 1 General scheme for synthesis of cross-linked cellulose aerogels. (a) SEM micrograph of raw cellulose pulp, (b) crosslinking reagent, (c) cross-linked cellulose aerogel, (d) SEM of aerogel and (e) crosslinking reaction between cellulose and BDE.

Rheological properties

The rheological method is a direct and reliable way to investigate the sol–gel transition behavior of cellulose/NaOH/urea aqueous solution and the BDE/cellulose/NaOH/urea mixture. Here the sample with the lowest crosslinking agent, n_AGU : n_BDE = 1 : 1, was chosen for testing. The sol–gel transition is crucial for studying the structure and predicting the properties of gels. The temperature effect on gelation of the samples was measured by the dynamic viscoelastic method.

The storage and loss modulus (G′ and G′′) were measured at a constant frequency of 1 Hz shear as a function of temperature (T) from 20 to 90 °C with a heating rate of 5 °C min⁻¹ (Fig. 2). In Fig. 2a for the cellulose/NaOH/urea aqueous solution (4 wt% cellulose), G′ is lower than G′′ from 20 to 63 °C. This shows the common viscoelastic behavior of a liquid. In this region, NaOH hydrate attracts cellulose chains through the formation of new hydrogen-bonded networks and urea hydrates self-assemble at the surface of the NaOH hydrogen-bonded cellulose to form an inclusion complex (IC).³⁴ The IC structure is not destroyed and cellulose solution still exhibits viscoelastic behavior of a liquid even though the storage modulus and the loss modulus decreased at about 40 °C. In addition, the decrease of the storage modulus and the loss modulus represents the change of a liquid's viscoelastic behavior. When the temperature is above 63 °C, the G′ value is higher than G′′ indicating the formation of an elastic gel network. This is attributed to the destruction of the IC, which leads to self-association entanglements and molecular interactions on the cellulose backbone with increasing temperature.³⁵ Therefore, the intersection temperature of 63 °C is the gel point where the storage modulus value equals the loss modulus value. However, in Fig. 2b for the BDE/cellulose/NaOH/urea mixture (n_AGU : n_BDE = 1 : 1), G′ is always higher than G′′ suggesting the formation of an elastic gel network. This suggests that cellulose reacts with BDE below 20 °C in the NaOH/urea solution and forms a gel network via the crosslinking reaction with BDE when n_AGU : n_BDE = 1 : 1.

Fig. 2 G′ and G′′ as function of temperature for (a) cellulose/NaOH/urea solution and (b) the BDE/cellulose/NaOH/urea mixture (n_AGU : n_BDE = 1 : 1).

Structure of cellulose aerogels

Structure of the cellulose aerogels was analyzed by X-ray diffraction. Fig. 3h shows the X-ray diffraction patterns of the cellulose pulp, RC and cellulose aerogels. The raw cellulose pulp (curve a of Fig. 3h) exhibits three peaks at 2θ = 16.5, 22.7 and 34.5° typical of the cellulose I crystal.³⁶ RC sample (curve b of Fig. 3h) exhibits three peaks at 2θ = 12.8, 20.9 and 22.5° typical of the cellulose II crystal.³⁶

Fig. 3 SEM images of (a) cellulose pulp, (b) RC, (c) Cellu/BDE1-1.5, (d) Cellu/BDE2-1.5, (e) Cellu/BDE3-1.5, (f) Cellu/BDE4-1.5, (g) Cellu/BDE5-1.5 and (h) X-ray diffraction patterns of curve (a) raw cellulose pulp, curve (b) RC, curve (c) Cellu/BDE1-1.5, curve (d) Cellu/BDE2-1.5 and curve (e) Cellu/BDE3-1.5.

For the cross-linked cellulose aerogels Cellu/BDE1-1.5 and Cellu/BDE2-1.5 (curve c and d of Fig. 3h), the intensity of the diffraction peaks at 2θ = 12.8° is very weak indicating that only part of the cellulose II still existed and most of cellulose reacted with BDE. For the aerogel Cellu/BDE3-1.5 (curve e of Fig. 3h), the typical diffractions at 2θ = 12.8° for the crystalline structure of cellulose II disappears entirely indicating damage to the crystalline structure of cellulose during the dissolving and crosslinking process. In addition, with increasing amount of BDE, these initial characteristic peaks of cellulose I and II disappear in the chemically cross-linked cellulose aerogels (Cellu/BDE4-1.5 and Cellu/BDE5-1.5) and they only exhibit one peak at 2θ = 21.1° suggesting a transition from a crystalline structure to an amorphous state.

In view of the results, a crosslinking reaction of the hydroxyl groups occurred as expected for a homogeneous modification of cellulose in the NaOH/urea aqueous solution. Cellulose aerogels were synthesized by crosslinking of cellulose with BDE via covalent bonds rather than intermolecular hydrogen bonds between cellulose chains. Therefore, the cellulose crystalline region was transformed to an amorphous state of aerogels.

Elemental analysis

Elemental analysis was used to measure the elemental composition of the cellulose samples. Table 2 lists the weight fractions of O, C and H elements in cellulose samples. The O, C and H weight fractions of RC without crosslinking agent were 50.95, 41.31 and 6.01%, respectively. For the crosslinking agent BDE, the C elemental weight fraction was higher and the O elemental weight fraction was lower than RC. With increasing BDE, the O weight fraction decreased from 49.31 to 43.30% when the n_AGU : n_BDE increased from 1 : 1 to 1 : 5. The C weight fraction increased from 41.81 to 44.66% and the H weight fraction increased from 6.21 to 6.73%. There were three moles of hydroxyl groups per mole of AGU and BDE is a diepoxide-based the crosslinking agent on the two sides of molecular chain. When the BDE dosage was continually increased, the elemental composition of cellulose aerogels did not markedly changed. Therefore, a comparison of the O and C elemental composition between the RC and cellulose aerogels proved the reactions between cellulose and BDE. Taken together, the XRD and elemental analysis data indicated that the obtained cellulose aerogels were obtained by crosslinking of cellulose with BDE via covalent bonds rather than intermolecular hydrogen bonds between cellulose chains.

Table 1 Summary of cellulose aerogels and their physical properties^a

Sample	Cellulose/g	BDE/g	nAGU : nBDE	Hydrogel concentration/%	ρ/g cm^−3	Water adsorbent ability/g g^−1
a AGU represents anhydroglucose unit of cellulose, ρ represents the density.
RC	4.0	0	1 : 0	1.5	0.6511	5
Cellu/BDE1-1.5	4.0	5.1	1 : 1	1.5	0.6357	10
Cellu/BDE2-1.5	4.0	10.1	1 : 2	1.5	0.0167	35
Cellu/BDE3-1.5	4.0	15.2	1 : 3	1.5	0.0162	41
Cellu/BDE4-1.5	4.0	20.2	1 : 4	1.5	0.0157	37
Cellu/BDE5-1.5	4.0	25.3	1 : 5	1.5	0.0154	34
Cellu/BDE3-3.0	4.0	15.2	1 : 3	3.0	0.0244	25
Cellu/BDE3-2.5	4.0	15.2	1 : 3	2.5	0.0211	29
Cellu/BDE3-2.0	4.0	15.2	1 : 3	2.0	0.0189	36
Cellu/BDE3-1.0	4.0	15.2	1 : 3	1.0	0.0149	39
Cellu/BDE3-0.5	4.0	15.2	1 : 3	0.5	0.0141	38

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Table 2 The weight fractions of the O, C and H elements in the cellulose samples

Samples	O/%	C/%	H/%
RC	50.95	41.31	6.01
Cellu/BDE1-1.5	49.31	41.81	6.21
Cellu/BDE2-1.5	46.96	42.61	6.47
Cellu/BDE3-1.5	45.17	43.45	6.57
Cellu/BDE4-1.5	44.30	43.88	6.76
Cellu/BDE5-1.5	43.30	44.66	6.73

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The morphology and network structure

SEM is a useful technique to study the morphology of cellulose materials. Here, a FW177 high speed disintegrator (Taisite Instrument Company, Tianjin, China) was used for dispersing of the hardwood to cellulose pulp and cellulose pulp was used as the starting material to make cross-linked aerogels. As shown in Fig. 3, the pristine cellulose pulp (Fig. 3a), which is estimated to be about 20–100 μm in diameter, formed a fibrillar network structure. Fibrillar joints in the network were apparent. The cellulose pulp of the RC sample (Fig. 3b) is severely coagulated to form a dense and film-like mass-this feature is consistent with the re-forming of hydrogen bonds during regeneration. The water adsorption ability of RC was 5 g g⁻¹. Versus the RC, the porous internal network of cellulose aerogels is clearly visible with film slices and the pore sizes are on the range of dozens of micrometers.

The obtained aerogels exhibited a porous structure, which is a very important factor inconsideration of the density and adsorbent ability. The Cellu/BDE aerogels with different amounts of BDE and different hydrogels concentrations were prepared. The influence of crosslinking agent on the morphology of cellulose aerogels (1.5% hydrogels concentration) was visualized by SEM (Fig. 3). Cellu/BDE1-1.5 (Fig. 3c) shows a highly porous network with a small pore size. The density of Cellu/BDE1-1.5 was 0.6357 g cm⁻³ and 1.0 g dried aerogel could adsorb 10.0 g water (Table 1). When the amount of crosslinking agent is low, covalent bonds destroy the hydrogen bonds of cellulose II during this regenerated process leading to the small pore size feature of the porous structure. Cellu/BDE2-1.5 and Cellu/BDE3-1.5 (Fig. 3d and e) show a sheet form with an enlarged pore size. The water adsorbent abilities of Cellu/BDE2-1.5 and Cellu/BDE3-1.5 were 35 and 41 g g⁻¹, respectively. With increasing amounts of crosslinking agent, the crosslinking junction points aggregate together to form crosslinking junction zones and forms sheets like as shown in Fig. 3f and g. The water adsorbent abilities of Cellu/BDE4-1.5 and Cellu/BDE5-1.5 decreased to 37 and 34 g g⁻¹, respectively. This could be attributed to the over abundance of crosslinking agent and the pore volume did not increase further. Ultimately, the water adsorbent ability of Cellu/BDE3-1.5 was the best.

The surface morphology of Cellu/BDE3 with different concentrations is shown in Fig. 4. The density of Cellu/BDE3-3.0 aerogel decreased from 0.0244 g cm⁻³ to 0.0141 g cm⁻³ of Cellu/BDE3-0.5. The dense and porous structure and wall structure of Cellu/BDE3-3.0 and Cellu/BDE3-2.5 (Fig. 4a and b) are clearly observed. The structure of Cellu/BDE3-2.0 and Cellu/BDE3-1.5 (Fig. 4c and d) were small pieces and the total pore volume increased. The water adsorbent ability increased from 25 to 41 g g⁻¹ when the concentration of the hydrogel decreased from 3.0 to 1.5% due to the large pore volume. The wall structure mostly disappears, and the fiber-like structure could be observed in the fracture surface of the Cellu/BDE3-1.0 and Cellu/BDE3-0.5 (Fig. 4e and f), which are quite different from those of the higher concentration series. The fiber-like structure is likely formed by weak self-association of the cellulose chains at lower hydrogel concentrations. The interactions between cellulose are weakened over long distances and the low concentration leads to more separate fiber-like cellulose that decreases the water retention ability of the pores. Compared with Cellu/BDE3-1.5, the water adsorbent ability of Cellu/BDE3-1.0 and Cellu/BDE3-0.5 decreased slightly to 39 and 38 g g⁻¹.

Fig. 4 SEM images of cellulose aerogels (a) Cellu/BDE3-3.0, (b) Cellu/BDE3-2.5, (c) Cellu/BDE3-2.0, (d) Cellu/BDE3-1.5, (e) Cellu/BDE3-1.0 and (f) Cellu/BDE3-0.5.

Adsorbent properties

Hydrogels formed based on cellulose/NaOH/urea aqueous solution by heating higher than 63 °C and the aerogels were prepared followed by purification and freeze-drying. The aerogels prepared in this method was coded as RC₀. The water adsorbent ability of RC₀ made in this method was 7 g g⁻¹, which was similar with the water adsorbent ability of RC. The cellulose pulp is insoluble in water due to its strong intermolecular hydrogen bonds, high crystallinity and chain rigidity. The cellulose pulp dispersed in water and was freeze-dried into aerogel which led to self-association entanglements and molecular interactions on the cellulose backbone. The aerogel made in this method were coded as CP-aerogel and the water uptake ability of the CP-aerogel was 17 g g⁻¹ (reaching adsorption equilibrium in less than 2 s). The water storability of CP-aerogel was lower than the crosslinked aerogels. In addition, RC, RC₀ and CP-aerogel were entirely re-dispersed in the water system by shaking for a 5 s. The cellulose-water hydrogen bonds replaced the hydrogen bonds and van der Waals interactions between intermolecular cellulose.¹⁴ The samples of RC, RC₀ and CP-aerogel were easily destroyed by submersing it in water which limited their application. The chemically crosslinked cellulose aerogels still remained their original structure even upon vigorous stirring, which demonstrated the strong covalent bond interactions after chemical crosslinking.

The cross-linked aerogels show typical adsorbent properties. Fig. 5 shows that the adsorbent ability of aerogels increased when the n_AGU : n_BDE changed from 1 : 1 to 1 : 3, however the adsorbent ability decreased with further addition of BDE. Insufficient crosslinking agent resulted in collapsed pores due to the disconnection between cellulose and BDE, while over-crosslinking would lead to pore shrinkage. The adsorbent ability was also affected by the hydrogel concentration, which led to diverse pore sizes and porous networks as shown in SEM analysis. The adsorbent ability increased when the hydrogel concentration decreased from 3.0 to 1.5% due to the large pore volume. It decreased slightly when the hydrogel concentration decreased to 1.0 and 0.5% because of the decreased water-retaining ability of the pores. The adsorbent tests indicated that the aerogels reached adsorption equilibrium in water, DMF and toluene for 5 s, 15 s and 30 min, respectively. The adsorbent ability of aerogels was water > DMF > toluene. The water adsorbent ability was up to 41 g water per g dried aerogel. The polarity of DMF was similar to water and the aerogels adsorbed 38 g DMF per g dry aerogel. The toluene adsorbent ability of cellulose aerogels was low due to toluene with a low polarity index that could barely penetrate into the cellulose aerogels.

Fig. 5 The water, DMF and toluene adsorbent ability of cellulose aerogels, (a) cellulose aerogels with various crosslinking agent, (b) cellulose aerogels with various hydrogels concentration.

The reusability of Cellu/BDE3-1.5 aerogels was further tested by numerous cycles of squeezing and re-adsorption (Fig. 6). The adsorbed water in the cellulose aerogel was squeezed out with an external force, and then filter papers sandwiched the squeezed aerogel to adsorb the water pressed out from the aerogel until no more water was removed. The Cellu/BDE3-1.5 aerogels showed excellent and stable water adsorbent ability; the water adsorbent ability was still 37 g g⁻¹ after 30 squeezing–adsorption cycles. The DMF adsorbent ability of Cellu/BDE3-1.5 also remained nearly constant even after 30 repeated processes. In contrast, the toluene adsorbent ability decreased significantly from 16 to 3 g g⁻¹ after 30 cycles. Even with 2 h of readsorption, the Cellu/BDE3-1.5 immersed in toluene did not increase further.

Fig. 6 The adsorbent abilities of Cellu/BDE3-1.5 as a function of squeezing–readsorption cycles in (a) water, (b) DMF and (c) toluene. The insets illustrate the squeezing–readsorption of the corresponding aerogels in corresponding solvent.

An effort was made to further research the mechanism of the aerogels re-adsorbent ability for water, DMF and toluene. The ring-shape aerogel that adsorbed water was first folded lengthwise and then twisted into a small ball to remove adsorbed water. The aerogel was immersed in water again and the ball quickly expanded and swelled to almost its original ring-shape. Cellulose modified with BDE is a polar material with an abundance of hydroxyl groups. It creates hydrodynamic forces and the water penetrated into the aerogel due to these pores storability inside. The little ball that had been squeezed out DMF was immersed into DMF again and DMF also penetrated into the inside pores. This causes the little ball to swell to a ring-like shape. The adsorbent ability of the aerogel could only be reused when the squeezed ball could swell to its original shape. On the contrary, toluene with a low polarity index hardly penetrated into the squeezed aerogel ball. Toluene could not trigger the pores to recover their original volume because of the low polarity. Consequently the toluene adsorbent ability of the aerogel clearly decreased during the repeated squeezing–adsorption processes (Fig. 6c).

Conclusion

In summary, BDE reacted with cellulose as a crosslinking agent in the NaOH/urea aqueous solution. The rheological properties of the BDE/cellulose/NaOH/urea mixture had features of an elastic gel network at room temperature. This indicates the occurrence of a crosslinking reaction and the transition of cellulose from a sol to a gel. The cellulose aerogels were prepared by freeze-drying a BDE/cellulose/NaOH/urea mixture. There was a transition from a crystalline structure of cellulose to an amorphous state of cellulose aerogels as well as a variation of carbon elemental percentage in cellulose aerogels. This is because of the crosslinking of cellulose with BDE. The morphology of the cellulose aerogels determined the density and water adsorbent ability and depended on the crosslinking agent amount and the crosslinking hydrogels concentrations. The water adsorbent ability of Cellu/BDE3-1.5 was 41 g water per g dried aerogel. It was still 37 g g⁻¹ after 30 cycles of repeated squeezing–adsorption. The toluene adsorbent ability decreased significantly from 16 to 3 g g⁻¹. The polarity of the solvents was influenced by the swelling processes and re-adsorbent ability. The results showed that the chemical cross-linked aerogels exhibited excellent re-usability in water even after 30 applications.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

The authors express their gratitude for the National Natural Science Foundation of China (31200446); Foundation of Jiangsu Province Biomass Energy and Material Laboratory (JSBEM-S-201504); Natural Science Foundation of Jiangsu Province of China (BK20140973). The authors also thank LetPub (http://www.letpub.com) for its linguistic assistance during the preparation of this manuscript.

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