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High wet-strength, thermally stable and transparent TEMPO-oxidized cellulose nanofibril film via cross-linking with poly-amide epichlorohydrin resin

One of themajor challenges towards the practical application of nanocellulose films is tomaintain their high mechanical properties under humid environment. Herein, the physicochemical properties of TEMPOoxidized cellulose nanofibril (TOCNs) films cross-linked with different dosages of polyamide epichlorohydrin resin (PAE) were investigated. The PAE/TOCNs film containing 0.5 wt% PAE was treated via an impregnation method in a 0.9 wt% PAE solution. The resulting film exhibited a high transparency of 79.5% at a wavelength of 600 nm with high tensile strength and Young's modulus of approximately 135 MPa and 11 GPa, respectively. A high wet strength of 95 MPa was also obtained due to the compact cross-linking net structure. FTIR spectra indicated that the surface carboxyl groups of the TOCNs were converted into an ester bond with the active groups of PAE, which effectively improved the thermostability of the TOCN/PAE hybrid films.


Introduction
Nanocellulose lms composed of stiff and strong cellulose nanobrils exhibit abundant outstanding performances such as high transparency, excellent mechanical properties, great thermostability, low thermal expansion, and low oxygen permeability. [1][2][3] As a novel green material, nanocellulose lms have several potential applications in the areas of transparent electronic devices and green packaging materials. 4 However, under humid conditions, the excellent performances of the lms are impaired due to the cleavage of the interbrillar hydrogen bonding. 5 Actually, the hydrophilic nature of the TOCNs causes serious problems in practical applications when TOCNs lms are preserved under humid conditions or in water. 6 Various approaches have been proposed to improve the water or moisture resistance of the nanocelluloses. Induction of hydrophobic functional groups onto the TOCN surface via acetylation, silylation, esterication, and polymer graing has been typically utilized for this purpose. [7][8][9][10] To conduct these reactions, a number of organic solvents are used, which are harmful to health, ammable, and not environmentally friendly. Moreover, these chemical reactions oen damage the crystal structure of the nanocelluloses and result in poor mechanical properties of the nanocellulose-based materials. 10 During the process of extraction of the TOCNs via chemical modication of TEMPO-mediated oxidation, abundant carboxyl groups were position-selectively formed on the TOCN surfaces. 11 Via the adsorption of cationic surfactants or quaternary alkylammonium salts, a hydrophobic layer was coated on the TOCN surfaces. 12,13 The wet tensile strength of the TOCN lms could be improved via forming covalent or physical cross-linkage between CNFs and polymer matrices, including phenolic resins, epoxy, starch, chitosan, etc., although these composite materials need multiple and time-consuming procedures for their preparations. [14][15][16][17][18] Polyamine epichlorohydrin resin (PAE), as a water-soluble thermosetting polymer, has abundant active groups including azetidinium and alkyl functional groups 19 and is commonly added to paper products to improve their wet strength. Previously, PAE cross-linked TOCN aerogels and lms with excellent performances have been prepared. 20,21 The TOCNs with abundant anionic carboxyl groups have a high adsorbing capacity for PAE. 19 Therefore, large dosages of PAE will cause aggregation of the TOCNs. This instability leads to an uneven distribution of PAE and TOCNs within the composites. Herein, a two-step process was used. First, the PAE/TOCN lms with low concentrations of PAE were produced via the solution-casting method; these lms showed high wet stability caused by the crosslinking between TOCNs and the PAE cross-linker. Then, these lms were soaked in PAE aqueous solution. During the impregnation process, the PAE cross-linker is allowed to diffuse into the TOCN lms. This allows the addition of very high PAE content within the nal hybrid lms, avoiding an uneven distribution. The cross-linking reactions were induced via a simple heating process. The entire process is green, safe, environmentally friendly, and does not involve complex chemical reactions and toxic organic solvents.
Herein, the high-wet-strength and thermal-stable TOCN lms were prepared, and their structure, hydrophobicity, mechanical properties, and thermostability were studied. The mechanism of water resistance and thermostability enhancement that results from the cross-linking between TOCNs and PAE was also illustrated.

Materials
Aqueous 0.5 wt% TOCN suspensions were prepared using TEMPO-mediated oxidation as described in our previous study. 22 A cationic papermaking-grade polyamide epichlorohydrin resin (PAE) solution (Tianma Specialty Chemicals Co., China) was used without any purication.

Fabrication of the PAE/TOCN hybrid lms
The entire preparation process is shown in Scheme 1. The 0.5 wt% aqueous TOCN suspension and 0.1 wt% PAE dispersion were mixed under an ice bath under an ultrasonic condition at 300 W output power (X0-650, xianou Instruments Ltd., China). The dosages of PAE were 0.1 wt%, 0.3 wt%, and 0.5 wt% based on the oven-dry TOCN weight. The solution-casting method was performed by pouring the mixture slurry into polystyrene Petri dishes (diameter 60 mm) and drying at room temperature for few days. Aer drying, the dried hybrid lms were heated at 105 C for 30 min to induce crosslinking. All the samples were conditioned at 23 C and 50% RH for 24 h before measurements.
Then, the PAE/TOCN lms containing 0.5 wt% PAE were soaked in 0.1 wt%, 0.5 wt%, and 0.9 wt% PAE slurries for 2 h. Subsequently, the wet lms were dried at room temperature for few days. Aer drying, the lms were heated at 105 C for 30 min to induce crosslinking. All the lms were conditioned at 23 C and 50% RH for 24 h before measurements.
Analyses FE-SEM (JSM-7600F, JEOL Ltd., Japan) was carried out to characterize the cross-section of the fracture surface of the lms. The thicknesses of lms were calculated using a micrometer caliper. The packing densities of the samples were calculated from the dimension and weight of the hybrid lms aer conditioning at 23 C and 50% RH. According to the weights obtained before and aer heating at 105 C for 4 h, the moisture contents of the conditioned lms could be calculated. The optical properties of the hybrid lms were studied using a UVvis spectrophotometer (Lambda 950, PerkinElmer, USA) in the wavelength range from 200 to 1100 nm with a scan speed of 2 nm s À1 . The light transmittance of the TOCN/PAE mixture casting solutions was also tested using the same method in the wavelength range from 300 to 800 nm. Fourier transform infrared (FTIR) spectra of the hybrid lms were obtained using an FTIR spectrometer (VERTEX 80V, Bruker, Germany) at room temperature. The zeta potential of the TOCN/PAE mixture suspensions at 0.5 wt% was characterized using a Zeta potential analyzer (Zetasizer Nano ZS, Malvern Instruments Ltd., UK) at ambient temperature. The surface water resistance of the hybrid lms was measured using a dynamic CA analyzer (FTA200) at room temperature, and three measurements were conducted for each sample. Mechanical tests of dry hybrid lms were carried out using a tensile tester (H25KT, Tinius Olsen, USA) with a 500 N load cell at 1 mm min À1 at room temperature. Before the test, all the samples were conditioned at 23 C and 50% RH for at least 24 h. The lm specimens had widths and lengths of 3 mm and 50 mm, respectively. The hybrid lms were soaked in fresh water for 2 h, and excess water was removed using a blotting paper. The water content of the wet lms was calculated from the change in weight of the lms. According to TAPPI 456 OM-87, the wet mechanical properties were measured using a tensile tester (H25KT, Tinius Olsen, USA). The micrometer reading was carried out for calculating the thickness of the wet lm. The thermal stability of the hybrid lms was measured via thermogravimetric (TG) analysis using a thermogravimetric analyzer TGA (Q5000IR, TA instruments, USA). About 5 mg freeze-dried samples were weighed in a platinum pan and heated from 35 to 600 C at a heating rate of 10 C min À1 under a high-purity nitrogen ow of 40 mL min À1 .

Dispersity of the PAE/TOCN mixture suspensions
The zeta potential values and light transmittance of the TOCN/ PAE suspensions are listed in Table 1. TOCNs can be dispersed in water to form very stable transparent aqueous solutions with the help of strong repulsion forces between the negative charges on the nanober surfaces. The zeta potential values reect the surface charge state and the stability of the disperse system. 23 According to Table 1 content from 0 to 0.5 wt%. It is probably because TOCNs carry a stronger negative charge on their surface and have a stronger affinity for cationic PAE. As shown in the inset in Fig. 1, TOCN suspensions with slight addition of cationic PAE presented excellent light transmittance. To further evidence the transparency of the TOCN/PAE suspensions, the transmittance spectra of diluted suspensions in the range of 200-1100 nm were obtained using a UV instrument. A slight decrease in the transmittance was observed aer adding cationic PAE; however, the TOCN/PAE suspensions still exhibited a high transparency of approximately 85% at a wavelength of 600 nm.

Structure of the TOCN/PAE hybrid lms
As shown in Scheme 2, TOCN lms with a compact structure were formed via the bonding of interbrillar hydrogen. Under humid conditions, these lms exhibited poor mechanical properties because water molecules were adsorbed on the TOCN surface and acted as a competitive agent for interbrillar hydrogen bonding. In phase 1, TOCN/PAE lms had great wet stability. However, within these lms, a cross-linked network could not be formed because of the slight addition of PAE, and the resultant wet strength for these lms was too weak to be measured. In phase 2, the formation of the water-insoluble network effectively limited the water absorption and protected interbrillar hydrogen bonding, endowing the TOCN/PAE lms with high wet strength.
The fundamental properties of the hybrid lms are shown in Table 2. The density values showed an increasing tendency with the increasing PAE content, which was the result of the compact cross-linking structure within the hybrid lms. The highest moisture content of 10.2% was found for the TOCN lms (sample 1), and the other lms (sample 2-7) had relatively low moisture contents that ranged from 7.3% to 7.8%. The result indicated that the cross-linking of the TOCN lms with PAE possessed great water resistance.
To analyze the nanostructure of the hybrid lms, the fracture surfaces were characterized via FE-SEM. The lamellar structure can be obviously observed in Fig. 2(a)-(c). Compared to the neat TOCN lms (sample 1), TOCN/PAE hybrid lms (sample 4, 7) had denser packed layers. Fig. 2(d) shows the light transmittance of the hybrid lms with a thickness of approximately 35 mm. All lms had high optical transmittance of 70-90% at a wavelength of 600 nm (Table 2). A trend was suggested, where the hybrid lms had decreasing transparency with the increasing PAE content. This is probably because of the UV absorption of PAE and the yellowing phenomenon during the heat-induced cross-linking process. 24,25 To ascertain the cross-linking between TOCNs and PAE cross-linker, FTIR analysis was performed to analyze the changes in the chemical structure, and the spectra are shown in Fig. 2(e). The adsorption bands observed in the region of 3650-3000 cm À1 and 2900 cm À1 were related to O-H stretching vibration and C-H stretching vibrations, respectively. 26 The peaks at 1429, 1110, and 896 cm À1 indicated the typical structure of cellulose found in all samples, which revealed that there were no signicant structural changes aer the cross-linking reaction. 27 As the PAE content increased, a peak was observed around 1633 cm À1 , corresponding to the amide group belonging to PAE. 19 Compared to the neat TOCN lm, a new band was observed around 1718 cm À1 , which was assigned to the C]O vibrations from the ester functions in the crosslinking network. 20,28 The existence of ester functions in the hybrid lms strongly proved the covalent bond linkage between PAE and TOCNs aer thermal initiation.

Tensile strength of the TOCN/PAE hybrid lms
The mechanical properties of the dried TOCN/PAE lms are shown in  29 which was attributed to the lower density of the resulting lms. In phase 1, the tensile strength slightly decreased as the PAE content increased from 0% to 0.5%. In fact, the changes were not obvious because only a very small amount of PAE was added. In phase 2, an impregnation method with the PAE dispersion was used to treat the lms. The obtained hybrid lms showed higher mechanical performances than the neat TOCN lms. The stiffness of these lms was improved probably by the formation of TOCN crosslinking with the PAE cross-linker. The moisture content of the wet hybrid lms was measured aer soaking in water for 2 h ( Table 2). The highest moisture content of 1954% was found for the neat TOCN lms with a hydrophilic nature. As the amount of PAE increased, the moisture content of the hybrid lms dramatically decreased, ranging from 17% to 189%. The results indicated that the crosslinking structures within the PAE/TOCN hybrid lms effectively limited the water absorption and protected the interbrillar hydrogen bonding. Wet TOCN/PAE hybrid lms with high water a Measured aer soaking the lms in water for 2 h. b TOCN/PAE hybrid lms were prepared via solution casting, with the PAE content of 0%, 0.1%, 0.3%, and 0.5%, and the samples were denoted 1, 2, 3, and 4, respectively. c TOCN/PAE hybrid lms were produced by a impregnation treatment of sample 4 with 0.1 wt%, 0.5 wt%, and 0.9 wt% PAE slurry, respectively, and the samples were denoted 5, 6, and 7.  content usually had a lower tensile strength and higher strainto-failure. These properties were expected because of plasticization induced by water. 30,31 Fig. 3 shows the stress-strain curves of the wet TOCN/PAE lms (samples 5, 6, and 7), and the characteristic data are listed in Table 4. The wet neat TOCN lms (sample 1) and TOCN/PAE lms with low PAE content (samples 2, 3, and 4) could not be subjected to the wet tensile measurement because these lms were too weak under the water-swollen state. 5 Aer soaking with 0.9 wt% PAE solution, a high wet strength of 95 MPa was obtained for the hybrid lms (sample 7) with the formation of water-insoluble networks. These results are superior to the values of the previous report. 21,32 Water resistance of the PAE/TOCN hybrid lms surface Thermal stability of the PAE/TOCN hybrid lms The regular wood bers have thermal stability degradation at around 300 C, 33 whereas for TOCNs, degradation begins at 200-220 C. 34 It is known that the existence of carboxyl groups on the surface of TOCNs causes this decrease in thermal    stability. 1 The TG and the corresponding DTG curves of the lms samples are shown in Fig. 5. The slight weight loss caused by the evaporation of absorbed moisture within 30 and 100 C was not taken into account for better comparison of the thermal stability of the samples. Moreover, the characteristic data for the TG and DTG curves are given in Table 5. As a result of carboxyl groups being converted into ester bonds, the thermal stability of the TOCN lms was slightly improved. For the neat TOCN lms (sample 1), the 10% weight loss occurred at about 239 C. Owing to the water-insoluble polymer network structure, the T 10% values of sample 7 lms were improved to 248 C. The T 10% values of sample 4 lms were similar to those of the neat lms, indicating the existence of unprotected carboxyl groups within the hybrid lms for which similar results were observed via FTIR analysis. As shown in Table 5, char yield (CY) obviously decreased with the improvement of weight ratios of PAE in the lms, from 32.8 to 30.6%, which was the result of the decrease in the proportion of TOCNs in the hybrid lms.
During the whole heating process, two obvious decomposition behaviors were observed (Fig. 5(b)). It likely represents the decomposition of the carboxyl groups and cellulose molecules. 35 For the neat TOCN lms (sample 1), the Stage I T max occurred at 252 C with a maximum weight loss rate (WLR) of 6.03% min À1 and the corresponding data of Stage II were 306 C and 5.18% min À1 . While for the sample 7 lms, the Stage I T max appeared at 276 C with a maximum weight loss rate (WLR) of 3.23% min À1 and the corresponding data of Stage II were 327 C and 7.15% min À1 .

Conclusion
In this study, the PAE/TOCN hybrid lms were prepared by the covalent bond linkage between TOCNs and PAE. The waterinsoluble net structure limits the water swelling of the lms and protects the interbrillar hydrogen bonding. Under humid conditions, the hybrid lms exhibit high mechanical performances. Moreover, the surface carboxyl groups of the TOCNs were converted into ester bonds with the azetidinium groups of PAE. This change is benecial to improve the thermostability of the TOCN lms. As a result, modied TOCNs lms had a reduced moisture uptake and higher surface water contact angle and wet tensile properties as compared to the neat TOCN lms. In addition to the expected improvement of wet strength, the enhancement of thermostability and dry tensile strength were also observed. The results observed in this study will improve the applications of TOCN-based lms under high humid conditions.