Fluorescent Composite Hydrogel of Carboxymethyl Celluose-Eu((cid:0))/Polyvinyl Alcohol and Its Application in Functional Paper

A novel carboxymethyl cellulose-Europium((cid:0))/polyvinyl alcohol (CMC-Eu((cid:0))/PVA) uorescent hydrogel was prepared by a green and facile method. The hydrogel formed a physical cross-linking network under mild reaction conditions without using volatile organic chemical reagents. A porous structure was formed by the hydrogen bonding and other interaction between the PVA chains and the free hydroxyl on CMC-Eu((cid:0)) during the composite gel transformation process. The addition of CMC-Eu((cid:0)) improved the tensile and compressive mechanical properties of PVA hydrogel. When the CMC-Eu((cid:0)) content was 15%, the maximum tensile stress of the composite hydrogel was 47.25±10.35 kPa and the compressive stress was 10.14±1.90 kPa. Meanwhile, the CMC-Eu((cid:0))/PVA hydrogels exhibited a 5 D 0 → 7 F 2 characteristic emission peak of Eu 3+ at 615 nm, and emitted stable red uorescence under UV irradiation at 254 nm. Moreover, the hydrogel was applied in making uorescent paper as an internal sizing agent. When the amount of hydrogel was 1%, the tensile strength of uorescent paper reached 3.52 kN/m, which is promising in the application of anti-counterfeiting.

All the above studies involved organic solvents, nonbiodegradable synthetic polymers, or complex processes. But with the demand for sustainable development, the concept of green chemistry and green economy gradually interiorize. The research and application of natural polymers have become a hot topic in recent years Hua et al. 2010;Zhao et al. 2016 The object of this work is to prepare a exible and recognizable uorescent composite hydrogel by PVA and CMC-Eu( ) using environmental-friendly method. It is expected that adding CMC-Eu( ) into the composite hydrogels enhanced the mechanical properties. Such hydrogel could use as an internal sizing agent in papermaking to prepare a uorescent paper with excellent mechanical and uorescent properties.

Experimental Section
Materials PVA was purchased from Aladdin with a polymerization degree of 1799 and 98-99% (mol/mol) hydrolyzed. Food-grade CMC with a degree of substitution of 0.92 was supplied by Changshu Wealthy Science and Technology Co. Ltd. Europium oxide (Eu 2 O 3 ) powder was purchased from Aladdin. NaOH, HCl, and AgNO 3 were purchased from Guangzhou Chemical Reagent Factory. KBr was supplied by KERMEL Chemical Reagent Company in Tianjin, dried in mu e furnace at 400℃ for 2h and then cooled to room temperature. All the reagents were analytical grade and used without further puri cation. CTMP bamboo pulp with 36.8°SR of pulp-beating degree was provided by Guangzhou paper group Ltd. was dissolved in 100 ml of deionized water at room temperature. A 15 mL EuCl 3 solution was diluted with 40 mL of deionized water and the pH was adjusted to 7 with 1 M/L NaOH solution, followed by adding to the CMC solution dropwise (a drop rate of 1~2 drops/3s). The resulting solution was placed on a constant temperature magnetic stirrer for 30 min. The reaction mixture was then dialyzed in distillate water for 3 days until all chlorides were eliminated (texting by AgNO 3 ). Finally, the mixture was oven-dried at 55 °C to obtain CMC-Eu( ) composite, which was ground into powder for later use.
Preparation of CMC-Eu( )/PVA composite hydrogel PVA with 98% ~ 99% hydrolyzed and a polymerization degree of 1799 was used in this work. PVA solution was prepared by dissolving the PVA particles in distilled water (8 wt%) at 80℃ for 1.5 ~ 2 h with continuous stirring. CMC-Eu( ) suspension was prepared by dispersing the dry sample in distillated water under magnetic stirring for 1h. The two liquids were blended together to give CMC-Eu( )/PVA precursor blend. Pure PVA hydrogel and CMC-Eu( )/PVA (CEP) hydrogel were prepared by freezing at -20 ℃ for 20 h followed by exposing at room temperature for 4 h. This freeze-thaw process was repeated three times. The weight ratio of CMC-Eu( ) to PVA was 1:5, 1:10, 1:15, and the resulting hydrogels were designated as CEP-5, CEP-10, and CEP-15, respectively.

Preparation of handsheets
The diameter of handsheet was 20 cm, and the quantitative value was 90 g/m 2 . The handsheets were prepared by RAPID-KOETHEN (RK3AKWT). The hydrogel CEP-15 was cut into a cube of about 2-5 mm in length, 1-2 mm in width, and 2 mm in thickness. The hydrogel cubes was added into the pulp and well mixed before papermaking process. The contents of CEP-15 were 0%, 1%, and 3%, respectively, named as f control, FP-1, and FP-3.

Hydrogel characterization
The surface and cross-section images were recorded with EVO18 SEM (Carl Zciss, Germany). The hydrogel samples were freeze-dried and then sprayed with gold, EHT=10.00 kV. Mesoporous were tested by WBL-820 Surface Area and Porosities Analyzer from Shanghai Instrument Co., Ltd. The FT-IR analysis was performed on a TENSOR spectrometer (Bruker, German), using KBr pellets in the spectral range of 4000-400 cm −1 (1/36 resolution, baseline correction, 32 scans). Before FT-IR analysis, the samples were freeze-dried for 24 h and ground into powder. The XRD analysis was carried out by a D8 Advance X-ray diffractometer (Bruker, German). The sample was cut into a suitable size. The surface was smoothed. A back-pressurizing method was used to load the sample. Cu target, Kα ray, tube pressure 40 kV, tube current 40 mA, diffraction angle range 5-60°, scanning step length 0.04°, scanning speed 0.2 s/step. The degree of crystallinity (X c ) was calculated according to the method described in the literature (Costa-Junior et al. 2009). Thermogravimetric analysis was implemented by using a German TG209F3. The samples were scanned from room temperature to 700 ℃ at a heating rate of 10 ℃/min under an N 2 atmosphere. Crystallinity and thermal analysis process were characterized by differential scanning calorimetry in the US TA Q200 differential scanning calorimetry analyzer. Under the N 2 atmosphere, the heating rate was 10 °C/min, and the temperature scanning range was 25~300 °C. Before the differential scanning calorimetry analysis, the samples were freeze-dried for 24 hours and about 10 mg was sampled for analysis. The degree of crystallinity (X cr ) was calculated from the following E q 1: Where △H m was determined by integrating the area under the melting peak over the range of 190-240 ℃; W PVA was the weight fraction of PVA; △Hc was the heat required for melting a 100 % crystalline PVA sample, 138.6 J/g. (Mallapragada and Peppas 1996) The mechanical properties were determined using a US INSTRON 5565 analyzer with the loading rate kept at a strain rate of 50 mm/min at room temperature. The tensile specimen was a rectangle with a width of 10 mm, length of 70 mm, and thickness of 2 mm. The compressed sample was a cylindrical sample with a diameter of 32 mm and a thickness of 12 mm. The compression cycle rate was 15 mm/min with 5 cycles. Photoluminescence spectra were recorded using a Fluorolog-3 uorescent spectrophotometer (JY, America). The excitation slit width and the emission slit width were 10 nm and 15 nm, respectively. The emission and excitation spectra were obtained at detection wavelengths of 394 nm and 615 nm, respectively.

Handsheets characterization
The surface and cross-section images of the handsheets were studied by EVO18 SEM (Carl Zciss, Germany). The tensile strength and elongation at break of handsheet were measured by a L&W CE062 tensile tester according to GB/T 12914-2008. The tear strength of handsheet was measured by a L&W

Results And Discussion
Morphology of CMC-Eu( )/PVA hydrogel Scheme 1 shows the fabrication and crosslinking structure of CEP hydrogel. The CMC-Eu( ) particles were well dispersed with good anti-sedimentation stability in the PVA solution system (Scheme 1b). The blend was able to produce hydrogel after 3 freeze-thaw cycle, which showed a stable shape and su cient elasticity when touching with tweezers (Scheme1c). The SEM images of the surfaces and cross-sections showed the hydrogels were highly porous (Scheme 1e and Figure 1), which was caused by the alignment of molecular chains during freeze-thaw cycle and by evaporating water during freeze-drying. The crosssection of the PVA hydrogel showed a valley shape with a few pores. The tropistic arrangement of PVA hydrogel owes to the highly regular crystalline structure of PVA (Gonzalez et al. 2014;Ma et al. 2009). The bers loosely interweaved with many pores in CEP-5 hydrogel, while the CEP-10 had more uniform ber interweaving and pore size. The CEP-15 hydrogel with large and small pores showed stacked lamellar (see Figure 1). As measured by N 2 physisorbption, the average mesopore diameters were around 2.67-2.78 nm for all hydrogel samples. The results indicate that the hydrogels have a network structure containing macropores and mesopores.
Chemical structural and thermal analysis Figure 2 shows the FT-IR spectra of CMC-Eu( ) and all hydrogels. The spectra of CEP hydrogels were similar to that of PVA hydrogel. Two C-H stretching vibration peaks at 3000~2800 cm -1 were observed in CEP hydrogels and PVA. And the spectra of CEP hydrogels contained a weak peak at 1715 cm -1 , owing to stretching C=O and C-O of CH 3  Besides, the spectra of PVA, CMC-Eu( ), and CEP also show some differences. The bands at 3600~3000 cm -1 were corresponding to the stretching of -OH involved in the intramolecular and intermolecular hydrogen bonds. This peak of CEP was located at 3427~3435 cm -1 , which was of lower wavenumber than that of CMC-Eu( ), and higher wavenumber than that of PVA hydrogel. The -C=O vibration appeared at 1601 cm -1 of CEP hydrogels showed only one peak, which differed from that of PVA. The peak at 1095 cm -1 originated from -CO-vibration became wider after adding CMC-Eu( ). In the spectra of CEP hydrogel, the peaks at 850 cm -1 from C-C stretching vibration weakened slightly compared with that of  The thermogravimetric and differential scanning calorimetry (DSC) curves are shown in Figure 4. The thermogravimetric curves of CEP are similar to that of PVA (see Figure 4a). The weight loss before 205 ℃ is because of dehydration of hydrogels. It is worth noting that the weight will not change with the increase of temperature during 205 ~ 240 ℃. The DSC thermograms show a peak at 205 ~ 240 ℃, which is related to the melting process with the crystallization of PVA (Yang et al. 2004). According to E q 1 of Mallapragada's method (Mallapragada and Peppas 1996), crystallinity can be calculated and listed in Table 1. The crystallinity of hydrogels calculated from DSC curves were 47.60% (PVA), 42.78% (CEP-5), 45.20% (CEP-10), and 45.62% (CEP-15). The melting temperatures of the hydrogels crystallization zone also were obtained by Mallapragada's method (Mallapragada and Peppas 1996). The melting temperatures of CEP hydrogels were slightly lower than that of PVA. Melting temperatures is also related to the crystallinity of PVA (Abitbol et al. 2011;Butylina et al. 2016). The results of FT-IR, XRD, and DSC indicate that the PVA chains have hydrogen bonds, entanglement, and van der Waals forces with unreacted CMC segments of CMC-Eu( ).

Mechanical performance
Tensile stress-strain curves of all hydrogels are shown in Figure 5a. The curves display typical tensile behaviors, which are broken in the linear elastic region without an obvious yield phenomenon and plastic deformation. The tensile strength (σ t ) and toughness of CEP hydrogels were signi cantly higher than those of PVA. Speci cally, the σ t and strain of CEP-10 were 44.91±2.69 kPa (Table 2) and 90%, more than quadruple and double of the results from PVA hydrogel, respectively.
After stretching, the bers in all hydrogels were oriented in the direction of the external force and compact, as shown in Figure 5b. The results of XRD and DSC suggested that the crystallinity of all hydrogels did not change much. Even though the CMC-Eu( ) interacted with PVA chains in amorphous to form a stable gel network structure, the mechanical properties did not increase accordingly as the increased of CMC-Eu( ) content in the CEP sample. The porous network structure of the hydrogels might enhance their strength and toughness. The tensile property of the CEP hydrogel was better than that of PVA hydrogel. Moreover, the tensile properties of the CEP hydrogels are different due to their different porous sizes and ber arrangement. The tensile strain of CEP-5 was better than the other samples because the loose network structure increased the relaxation time of ber movement. As shown in Figure 5b, CEP-5 was stretched at. However, the orientation of the bers was poor, leading to lower strength. The condensed and uniform pore distribution of CEP-10 improved strength and toughness. Therefore. the strength was greater than CEP-5 and the toughness was greater than CEP-15. CEP-15 retained layered orientation after stretching, so it had a large strength, but a small strain due to uneven pore distribution.  (Butylina et al. 2016). In addition, the hydrogels showed decent compression fatigue resistance. The compression stress-strain curves of hydrogels at 60% strain under ve loading-unloading cycles are shown in Figure 6b. The shape and strength remained intact after ve cycles. The results suggest that no substantial plastic deformation or strength degradation occurred in the hydrogels, indicating an outstanding recovery behavior, as well as resistance to compressing for application in exible materials.
The mechanical properties of CEP hydrogels are displayed through knotted stretching and close bending. In the experiment of knotted stretching of the hydrogel, no fracture was observed (Figure 7a). Additionally, the hydrogel was able to totally recovered after bending to 180° and then releasing the pressure ( Figure  7b). However, the PVA hydrogel was cracked in the test. The hydroxyl group of CMC-Eu( ) and the free hydroxyl group in the PVA are linked by hydrogen bonds, which enhance the interfacial bonding strength of the network structure of the CEP hydrogel, thus improving the mechanical properties (Pan and Xiong 2009). The CEP hydrogels have deformation diversity, good compressive fatigue resistance, and toughness. And the hydroxyl groups on CMC-Eu( ) and PVA might make it possible to combine well with the cellulosic ber, thus the CEP hydrogel can be used as an internal sizing agent, which will not affect the mechanical properties and the application of the as-prepared paper.

Fluorescent handsheets characterization
Therefore, we fabricated some uorescent handsheets using different amounts of CEP-15 hydrogels, as it gave the strongest uorescent intensity. As shown in Figure 9, under sunlight, it is hard to identify the hydrogel dots in FP-1 whereas the hydrogel dots in the FP-3 are bigger and more obvious. The red spots on FP-1 and FP-3 are clearly with bare eyes under UV light. The hydrogel dots of FP-1 are evenly dispersed, and they are within the size range of hydrogels cubes added, while some sizes of hydrogel dots on FP-3 are larger than the sizes of hydrogel cubes added, indicating the hydrogel cubes added aggregated in FP-3 during the paper making. It is worth to mention that the CEP hydrogel can apply to made uorescent paper as a conventional internal sizing agent. And this utilization can take advantage of the existing process and equipment. Figure 10 shows the SEM images of the surfaces and cross-sections of the handsheets. By adding the uorescent hydrogels, the bers on the surfaces of uorescent handsheets organize directionally.
Compared with the cross-section of the control, the bers of uorescent handsheets were compact and order in Figure 10b. Especially, the sizes and distributions of the bers both on the surface and crosssection of FP-1 were more even. Figure 11 shows the uorescent emission spectra of CEP-15 hydrogel and the handsheets. The CEP-15 hydrogel, FP-1, and FP-3 showed characteristic emission of Eu 3+ ions. The uorescence emission peaks at 591nm and 615nm belong to the 5 D 0 → 7 F 1 transition and 5 D 0 → 7 F 2 transition of Eu 3+ ions, respectively.
The uorescent property of CEP-15 in handsheet did not change, indicating the uorescent is stable and this functional paper can be potentially used as anti-counterfeiting paper. However, the uorescent intensity of electric dipole transition at 615 nm in the handsheets became weaker because of concentration quenching (Lakowicz 2006).
The mechanical properties of the handsheets are shown in Figure 12. Adding CEP-15 hydrogel enhances not only the tensile strength but also the elongation at the break of the uorescent handsheets. Especially, FP-1 reached 3.52 kN/m of tensile strength and 1.598% of elongation at break, respectively. This might be due to the more compact and orientational of the bers on the handsheets. Moreover, the bers on FP-1 were organized more even (see Figure 10). However, the tear strength and fold resistance of the uorescent handsheets were weaker, especially for the FP-3, than that of control. This might be due to the differences between bers and pieces of the hydrogels. The regions with the hydrogels could cause phase separation on the uorescent handsheets. The burst strength increased from 182.50 to 209.25 kPa owing to the excellent mechanical properties of CEP-15 hydrogel, compared with the control. These results demonstrate that the mechanical properties of the uorescent handsheets are better with adding 1% of CEP hydrogel than adding 3%.

Conclusions
In summary, we have prepared a CEP hydrogel by a facile, green, and economical method that does not need organic solvents and under mild reaction conditions. The results show that CMC-Eu( ) and PVA chains were cross-linked by hydrogen bonding and other interactions to form a porous network structure. The composite hydrogel was similar to the PVA hydrogel crystalline form. The stretchability and exibility of the CEP hydrogel were 44.91 ± 2.69 kPa of the tensile strength (CEP-10) and 99.08% of the tensile strain (CEP-5). Besides, the CEP hydrogel shows the characteristic red uorescence of Eu 3+ under UV light. We fabricated the uorescent paper by simply blending the CEP hydrogel and bamboo pulp. The asprepared paper showed similar uorescent to the CEP hydrogel, therefore it is a promising material for anti-counterfeiting in labels and packaging.   FT-IR spectra of CMC-Eu( ), PVA hydrogel, and CMC-Eu( )/PVA hydrogels.        Fluorescence spectra of CEP-15 hydrogel (dry) and all handsheets.

Figure 12
Mechanical properties of all handsheets : tensile strength; elongation at break; tear strength; fold resistance; burst strength.

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