Highly transparent, strong and flexible films with modified cellulose

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Highly transparent, strong and flexible films with modified cellulose nanofiber bearing UV shielding property Xun Niu, Yating Liu, Guigan Fang, Chaobo Huang, Orlando J. Rojas, and Hui Pan Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.8b01252 • Publication Date (Web): 09 Nov 2018 Downloaded from http://pubs.acs.org on November 12, 2018

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Highly transparent, strong and flexible films with modified cellulose nanofiber bearing UV shielding property Xun Niu,‡ Yating Liu,‡ Guigan Fang,§,† Chaobo Huang,†,‡ Orlando J. Rojas,⊥ Hui Pan †,‡,* †

Jiangsu Co-Innovation Center for Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University, 159# Longpan Road, Nanjing 210037, PR China.

‡

College of Chemical Engineering, Nanjing Forestry University, 159# Longpan Road, Nanjing 210037, PR China. §Institute

of Chemical Industry of Forestry Products, Chinese Academy of Forestry, Nanjing 210042, PR China.

⊥Biobased

Colloids and Materials group (BiCMat), Department of Bioproducts and Biosystems,

School of Chemical Engineering, Aalto University, FI-00076, Espoo, Finland.

*Corresponding

author: Hui Pan, E-mail address: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

This work investigates multifunctional composite films synthesized with cellulose nanofibers (CNFs) and poly (vinyl alcohol) (PVA). First, TEMPO-oxidized CNFs were modified in heterogeneous phase with benzophenone, diisocyanate and epoxidized soybean oil via esterification reactions. A thorough characterization was carried out via elemental analysis as well as FT-IR, X-ray photoelectron spectroscopies, and solid-state NMR. Following, the surfacemodified CNFs where combined with PVA to endow composite films with UV-absorbing capabilities while increasing their thermo-mechanical strength and maintaining a high light transmittance. Compared to neat PVF films, the tensile strength, Young modulus and elongation of the films underwent dramatic increases upon addition of the reinforcing phase (maximum values of ~96 MPa, ~714 MPa and ~350%, respectively). A high UV blocking performance, especially in the UVB region, was observed for the introduced multifunctional PVA films at CNF loadings below 5 wt%. The tradeoff between modified nanofibril function as interfacial reinforcement and aggregation leads to an optimum loading. The results indicate promising applications, for example, in active packaging.

KEYWORDS: cellulose nanofibrils; active packaging; functionalization; benzophenone; poly (vinyl alcohol); nanocomposite films; UV blocking, multifunctional composites

INTRODUCTION Compared to tinfoil and papers, polymer films are intensively used as packaging materials, given the inherent advantages of inexpensive cost and high transmittance along with their ability to guarantee the function and aesthetics of the material onto which they are applied.

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During


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the time between manufacture and consumption of the material, photodegradation of the polymer packaging material is a major drawback, given the typical sensitivity to ultraviolet (UV) radiation and owing to the activation of chromophores in the packaging itself and in the printed surfaces.

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In this context, the development of new and improved UV protective polymer

materials has been recognized as a necessity to fulfill their application performance. General approaches to attain UV protection have mainly relied on the incorporation of inorganic and/or organic UV absorbers in the polymers. For this purpose, inorganics such as ZnO has been mixed with chitosan and carboxymethylcellulose.

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Furthermore, silver nanoparticles

have been demonstrated to function as UV absorbers in polystyrene7 and to stabilize poly (vinyl pyrrolidone) (PVP) in low-temperature packaging. 8 Additionally, boron nitride nanotubes mixed with resins and methyl ethyl ketone10 offer opportunities to prepare novel UV protective films. However, due to the wide band gaps, the UV absorption ability of these compounds is partially limited. In addition, inorganic nanoparticles display an inherent photocatalytic effect that may cause the degradation of the polymer matrices.

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On the other hand, traditional organic UV

absorbing agents have shown excellent UV-blocking performance. Combinations of various polymer films with benzophenone (BP), 11 benzotriazole 12,13 or hydroxyphenyl triazine 13,14 have been reported recently. Among the commonly used organic UV blockers, BP derivatives have been widely employed to prevent the photodegradation of packaging polymers and their contents by direct mixing with the BP derivatives.12-14 The latter exerts a protective effect via the n→π* and π→π* transitions from the benzene ring structures as well as other intramolecular chargetransfer transitions from the carbonyl and hydroxyl groups.16 At the same time, related photoreactive groups can maintain a high chemical inertness in the absence of UV light.15-17 However, concerns have emerged because small molecules like BP derivatives can easily


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permeate through the packaging and may migrate to the contained foodstuff as containment. 15-18 To address these issues and for the safer use of UV-absorbing molecules, recent researches have focused on the preparation of various polymeric UV filters that prevent the migration of organic UV blockers. Examples of these blockers include bioinspired hollow dopamine−melanin nanoparticles (Dpa-h NPs) that enhance the UV-shielding properties of poly(vinyl alcohol) (PVA).19 Also, octyl methoxycinnamate (OMC) used in the core of nanocapsuled PVA nanofibers with combined anti-UV properties,

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and BP chemically grafted onto TEMPO-

oxidized cellulose nanofibers to prepare photoactive films.20 Recently, the unique advantages of cellulose nanofibers (CNFs) have found its applications on packaging due to its biodegradability and low cytotoxicity. In addition, CNFs are ideally suited as reinforcing agents due to their excellent mechanical property and low weight. The large number of hydroxyl groups on CNFs also facilitates their miscibility with hydrophilic polymers; alternatively, the surface of CNFs can be modified to improve its functionality. For this purpose, CNFs have been demonstrated to work as multifunctional agents in poly(ethylene glycol) (PEG),21,22 PVA,23,24 poly(dimethyl siloxane) (PDMS),25 poly(lactic acid),26,27 and biopolymers,28,29, among others. In this study, CNFs were first modified by chemical grafting of BP derivatives. Then, the modified CNFs were incorporated into a PVA matrix to act as a nanosized UV absorber and as mechanical reinforcement in biodegradable composite films. FT-IR, element analysis, and XPS were used to demonstrate the successful preparation of nanosized CNF-based UV absorber. The morphology, thermal stability, optical and mechanical performances of the obtained CNF/PVA nanocomposite films were determined. The UV absorbing ability of the nanocomposite film was also demonstrated by UV-vis assays and compared with that of neat PVA films.

EXPERIMENTAL


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Materials. A CNF suspension in water (0.5 wt%) was acquired from Yu Yue Nanotechnology (Shanghai, China). 2-Hydroxy-4-methoxybenzophenone (98+%, HMBP) and 2, 4tolylene diisocyanate (80%, TDI) were purchased from Alfa Aeser (Karlsruhe, Germany). Epoxidized soybean oil (ESBO) was purchased from Macklin Reagent Co. Ltd. (Shanghai, China). Ethanol, acetone, sodium carbonate and dimethylsulfoxide (DMSO) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All chemicals were used directly. A CNF aqueous suspension (10 g) was solvent exchanged with acetone and ethanol twice and redispersed in DMSO (20 mL) with 5 minutes’ centrifugation at 12500 rpm. The dispersion was homogenized with an IKA T25 homogenizer at 15000 rpm for 10min. The CNF/DMSO mixture was vigorously stirred before further reaction. Heterogeneous modification of CNF with BP derivatives. The HMBP and TDI crosslinker system (herein, referred to as BTC) was prepared according to the literature.30 Briefly, 4.5 mg HMBP was mixed with 2 mL DMSO in a 5 mL glass vial under continuing stir. For a molar ratio of TDI/HMBP exceeding 2, a volume of 0.03 mL TDI was transferred to the vial followed by vigorous stir for 2 h. Then, the solution containing HMBP and TDI crosslinker system (BTC) was combined with the CNF/DMSO suspension and reacted at room temperature for 30 min under continuing stir (Figure 1d). The mixture was subsequently washed with acetone three times to eliminate the excess crosslinker. In the final step, the obtained BTC-modified CNF (herein, referred to as BT-CNF) was redispersed in distilled water (20 mL) and homogenized (IKA T25 homogenizer) for 10 min at 15000 rpm. The CNF suspension in water (10 g) was adjusted to pH 9.0 with dropwise addition of 1 wt% Na2CO3. This mixture was solvent exchanged with acetone and ethanol twice and redispersed in


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DMSO using a centrifuge. The CNF-DMSO suspension was transferred to a three-necked flask. Then, 5.0 g HMBP was directly added to the flask and reacted with CNF suspension at 120 oC for 1 h under continuous magnetic stir. The flask was kept in a nitrogen atmosphere during the reaction (Figure 1b). Upon the end of the reaction, the obtained mixture was washed with plenty amount of acetone to remove unreacted HMBP. And the resulting HMBP-modified CNF (herein, referred to as B-CNF) was collected by centrifugation. Finally, the B-CNF was redispersed in distilled water (20 mL) and homogenized (IKA T25 homogenizer) for 10 min at 15000 rpm. HMBP and ESBO-modified CNF (herein, referred to as BE-CNF) was prepared following the same procedure as that used for B-CNF, except that 1 mL ESBO and 5.0 g HMBP were added together to the flask, and the reaction time lasted for 20 h (Figure 1c).


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Figure 1. Schematic illustration of the surface modification of hydroxyl and carboxylic groups in TEMPO-oxidized CNF (a) with BP derivatives: B-CNF (b), BE-CNF (c) and BT-CNF (d). The synthesis of BTC and the structure of ESBO are also shown (see the bottom of the figure). Preparation of CNF/PVA Nanocomposite Films. Modified CNF/PVA nanocomposite films were obtained through a casting method. PVA (0.375 g) was mixed with distilled water at ambient temperature to make a solution of 0.025 mg/mL concentration. The solution was then mixed with B-CNF, BT-CNF or BE-CNF at given loadings (0 wt%, 5 wt%, 10 wt%, and 20 wt%) and degassed by sonication for 10 min to get a better dispersion of the modified CNFs. The


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mixture (15 mL) was subsequently cast onto an ultra-flat plastic Petri dishes and dried in an convection oven at 60 °C for 20 h. The average thickness of resulting composite films were ~25 μm. Fourier Transform Infrared Spectroscopy (FT-IR). FT-IR spectroscopy was used to detect the changes in the chemical structure of the CNFs before and after modification. CNF and modified CNF (B-CNF, BE-CNF, and BT-CNF) aqueous suspension were freeze-dried to powder and then mixed with KBr to prepare FT-IR testing tablets. FT-IR spectra were performed on a FT-IR spectrometer (Nicolet 380, Thermo Electron, Waltham, MA, USA) from 4000 to 800 cm−1 at a resolution of 4 cm−1 and 16 scans per sample were collected. Elemental Analysis (EA). The carbon, hydrogen, nitrogen, and oxygen contents in the unmodified CNF and modified CNF (B-CNF, BE-CNF, and BT-CNF) were evaluated by a cube elemental analyzer (Vario Macro, Elementar, Fulda, German). For each sample, duplicate measurements were implemented. X-ray Photoelectron Spectroscopy (XPS). The XPS measurements of CNF and modified CNF samples (B-CNF, BE-CNF, and BT-CNF) were carried out with an electron spectrometer (Kratos Analytical AXIS Ultra, Kyoto, Japan) equipped with 100 W monochromatic A1 Kα irradiation and effective charge neutralization. Reported elemental concentrations were normalized. The C−C/C−H contribution to the C1s signal was shifted to 285.0 keV. The program XPS PEAK 41 was used for the decomposition of C1s peaks with Gaussian functions after subtraction of a Shirley background. Solid-state Nuclear Magnetic Resonance (NMR). NMR spectra were collected using a 600 MHz wide bore spectrometer (Bruker Avance III) operating at 14.1 T. The contact time for CP


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was 3 ms, MAS speed was 12.5 kH, and a recovery delay was 3 s. All 13C chemical shifts spectra were normalized with a reference TMS at 0 ppm. Scanning Electron Microscopy (SEM). The cross-sections of the composite film samples were inspected with SEM (Hitachi, Japan). The samples were fractured by frozen with liquid nitrogen. Then the cross sections of fractured surfaces were sputter-coated with a thin layer of gold. The SEM images were obtained on an S-4700 scanning electron microscope operated at a accelerating voltage of 15-kV. Atomic Force Microscopy (AFM). The topographies morphology and roughness of the nanocomposite films were observed with AFM (Dimesion Edge, Bruker, Corp., Germany) . The microscope was operated and imaged in tapping mode under ambient conditions. X-ray Diffraction (XRD). XRD characterization of CNF samples were acquired with an Ultima IV X-ray diffractometer (RigaKu, Tokyo, Japan) under the acceleration voltages of 40 kV and a current of 30 mA at room temperature. The diffraction signals were recorded in the 2Ɵ range of 5−30° at a 5° min−1 scan rate. Thermogravimetric Analysis (TGA). Perkin Elmer simultaneous thermal analyzer (STA 6000) was used to analyses the thermal stability of the composite films. Approximately 8 mg of sample (CNFs and modified CNFs (B-CNF, BE-CNF, and BT-CNF) was put into the testing pan and heated to 700 °C with a rate of 10 °C/min from ambient temperature. Two measurements were performed for each sample. UV-Shielding Performance of PVA and Nanocomposite Films. The UV absorption performance of the nanocomposite films was evaluated by a UV-Vis absorption spectrometer. The transmittance data were recorded to calculate the blocking percentage (Eq. 1, 2):


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400

UVA blocking (%) = 100 ―

∫320𝑇(𝜆)𝑑𝜆 400

∫320𝑑𝜆

(%)

(1)

The blocking percentage in the UVB (290−320 nm) was calculated by 320

UVB blocking (%) = 100 ―

∫290𝑇(𝜆)𝑑𝜆 320

∫290𝑑𝜆

(%)

(2)

where T(λ) is the average value of spectral transmittance of the material, dλ is the bandwidth, and λ is the wavelength. Mechanical Properties of the Composite Films. The tensile strength, Young’s modules, and elongation at break of the composite films were collected with a mechanical testing machine (Shenzhen, China). Samples in rectangle (40×5 mm2) were cut from random positions on the composite films. The gauge length was set at 10 mm. A 500 N loading cell and a crosshead speed of 10 mm/min were used. The average value of 10 replications per sample was reported.

RESULTS AND DISCUSSION Synthesis and Characterization of Modified CNFs. The modification of CNFs with the different BP derivatives is shown in Figure 1. Three types of surface-modified CNFs were prepared. The HMBP derivative reacts with the hydroxyl groups on TEMPO-CNF through esterification. Additionally, EBSO with multiple long fatty acid chains shows excellent promise as an inexpensive, renewable material for packaging applications, such as a plasticizer, stabilizer, and diluent.

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The epoxide rings from ESBO and hydroxyl groups from the CNFs react and

can further promote the forming of a crosslinking structure between the ESBO-modified CNFs and the polymer, thus contributing to an improved thermal stability. Importantly, the HMBP and TDI crosslinker system (BTC) has been shown to be effective to improve the reaction rates of


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cellulose modification as well as the mechanical properties and chemical stability of the resulting cellulose; the diisocyanate could provide connections between cellulose and other polymers possessing a hydroxyl or amino group. 34,35 The FT-IR spectrum of neat CNF (Figure 2) displays typical cellulose peaks at 3420 cm−1 (OH stretching), 2883 cm−1 (C-H symmetrical stretching), 1423 cm−1 (H-C-H in-plane bending vibration), 1165 cm−1 (C-O-C vibrations of the glycosidic bridges) and 1632 cm−1 (cellulose absorbed water). The results suggested that TEMPO oxidation treatment did not dramatically change the chemical structure of cellulose.36 Compared with the spectrum of neat CNFs, the peak intensity of O-H stretching was decreased and shifted to a lower wavenumber, 3360 cm-1 (in BCNF, BE-CNF, and BT-CNF) after modification. The intramolecular hydrogen bonding in cellulose is around 3455 cm-1–3410 cm-1 (O2–H–O6) and 3375 cm-1–3340 cm-1(O3–H–O5).37,38 For all modified CNFs, the changes in the hydrogen bonds is mainly attributed to the chemical surface modification of O-H group39 and the decreased concentration of O-H due to their reaction with the BPs. Upon CNF surface modification, several new absorption peaks were identified in the spectra. Peaks at 1601 cm−1 (from carboxyl groups in the salt form)40,41 were found in both of the modified, B-CNF and BE-CNF, indicating the successful grafting of BP on the CNF surface. In addition, in the BE-CNF sample, the vibration of the grafted ESBO groups was identified between 1150 and 1240 cm-1, and the stretch of the carbonyl bands of ESBO was identified at 1750 cm-1. The peak at 2900 cm-1and 3005 cm-1 appeared when ESBO and BTC were attached to the CNF surface, confirming the presence of long hydrocarbon chains (-CH-, -CH2- and -CH3).42 However, when considering the reaction scheme, as illustrated in Figure 1, three reactions for the


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CNF modification led to reactions with the hydroxyl or the carboxylic acid groups in the CNF structure. These reactions result in a change in the carbonyl peak due to the change in the environment of the two hydroxyl groups. In BT-CNF, the peak at 1710 cm−1are ascribed to the carbonyl stretching (amide I), especially, the carboxyl groups in the acid conditions.43 A new NH bending (amide II) peak at 1523 cm−1 was also detected.44 The distinct broadening of the peak around 1500 cm−1 is attributed to the overlap of the absorption bands of N-H bending with the C=C stretching vibration of benzene.

Figure 2. FT-IR spectra of B-CNF, BE-CNF, BT-CNF, and CNF samples. The modified CNFs were further characterized by EA and XPS. The elemental analysis results are presented in Table 1. The slightly higher content of carbon in the modified CNFs (BCNF, BE-CNF, BT-CNF) (39%, 39%, 40%) than that of neat CNFs (38%) is likely due to the grafting of the BPs on the CNF. Compared to the unmodified CNF, the highest carbon content


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was found in the BT-CNF sample. The apparent increase in the nitrogen content in BT-CNF was directly related to the isocyanate groups grafted on the cellulose backbone. In addition, the trace amount of nitrogen detected in the unmodified CNF sample is explained by the presence of impurities or other contaminants in the instrument or samples. The XPS spectra and deconvolution results of the C1s signal for each sample are presented in Figure 3 and Table S1, respectively. The deconvoluted carbon of the unmodified CNF displayed four typical peaks, namely, C-C (C1, 285.0 eV), C-O (C2, 286.6 eV), O-C-O or C=O (C3, 287.8 eV), and O-C=O (C4, 289.2 eV), where the presence of C3 and C4 were due to the TEMPO oxidation process. The decomposition result (Table S1) reveals that the intensity of C4 (O-C=O) increases from 0.011 to 0.016, to 0.031 and to 0.018 for B-CNF, BE-CNF, and BTCNF, respectively, which again identified the occurrence of esterification on the CNF. 45 What’s more, the significant increased C4 peak of BE-CNF is primarily attributed to the presence of ESBO with extra O-C=O structure. The C1/C3 ratio corresponds to the number of aliphatic carbons per glucose unit. This value changed from 0.8029 (neat CNF) to 0.6875, 0.6210 and 0.6271 (B-CNF, BE-CNF and BT-CNF, respectively), confirming the increase of carbon content per glucose unit that caused by the grafting of BP derivatives on modified CNFs. 46

Table 1. Elemental analysis results of CNF, B-CNF, BE-CNF, and BT-CNF samples. C%

H%

N%

CNF

38.27

6.217

0.92

B-CNF

39.05

6.304

0.97

BE-CNF

39.32

6.501

0.97

BT-CNF

39.72

6.476

1.20


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Figure 3. High-resolution C 1s XPS deconvolution spectra of CNF, B-CNF, BE-CNF, and BTCNF samples. The CNF and modified CNF samples were also characterized by solid-state

13C

single pulse

MAS NMR (Figure S1). Signals of six carbons in the cellulose were obtained in the range of 60−110 ppm for all CNF sample, 44 Carboxylate carbons in salt state (C6*) was detected at 173 ppm. The pattern of these signals and NMR chemical shift remained generally unchanged along with the appearance of some new weak signals after the modification with BPs, indicating the modified CNF remain their cellulose skeletons while the modifications mostly occurred at the


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surface of CNF. The weak peaks around 120-160 ppm and 10-40 ppm in the spectrum of B-CNF should be assigned to the benzophenone moiety and the side chains, respectively. For BE-CNF, the resonance signals of 10−40 ppm are ascribed to the long fatty chains of ESBO. And for BTCNF, the resonance signals at 20−40 ppm are assigned to the aliphatic side chains of BPs. However, the crosslink between the HMBP and TDI formed complexed and multiple carbon environments, which resulted in more complicated signals in the NMR spectra. Although the signals are weak owing to the modifications only occurred on the surface of CNF, the above NMR results qualitatively revealed the bonding between BP derivatives and CNF. Morphology analysis of the CNF/PVA Nanocomposite Films. The surface and fracture section of pure PVA film and CNF/PVA nanocomposite films at given CNF loading were observed by AFM and SEM (Figure 4 and Figure 5, respectively). As a reference, the AFM image of pure PVA clearly displays a homogeneous and uniform surface (Figure 4a). Meanwhile, it can be obviously seen that with the incorporation of CNFs, the surfaces of films tend to be slightly rougher (Figure 4b, 4c and 4d). However, the existing of a small quantity of fibers did not affect the smooth structure of the material.47 The cross-section of neat PVA film and CNF/PVA nanocomposite films were characterized with SEM as well. Pure PVA film exhibited a uniform and smooth cross-section (Figure 5b). For nanocomposites with 1 wt% modified CNFs, small CNF nanoparticles have a good distribution in the PVA matrix and the cross-sections became slightly rougher than for the neat PVA film (Figure 5c, 5f and 5i). However, no apparent agglomerations or clusters were observed at the total fractured surfaces, demonstrating a film with a dense, uniform, and compact structure. With the increased CNF content, up to 10%, fibrils were randomly oriented and formed a network structure. Obviously, individual CNF lumps protruded from the surface of the smooth PVA structure (Figures 5e, 5h


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and 5k), indicating that aggregation occurred at a high CNF content where these nanofibers were bound together by hydrogen bonding.

Figure 4. AFM images of (a) a pure PVA film and composite films: (b) 5 wt% B-CNF/PVA, (c) 5 wt% BE-CNF/PVA and (d) 5 wt% BT-CNF/PVA.


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B-D

Figure 5. Photographs of a 5 wt% BE-CNF/PVA composite film (a). Cross-sectional SEM images of a neat PVA film (b) and composite films at CNF loadings of 1, 5, 10 wt% (from left to


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right) of B-CNF/PVA (c, d, e), BT-CNF/PVA (f, g, h), and BE-CNF/PVA (i, j, k). The arrows are added to indicate CNF agglomerates. XRD was performed to analyze the impact of the incorporation of modified CNFs on the crystalline structure of the PVA films. Figure 6 shows the XRD patterns of PVA films modified with B-CNF, BE-CNF and BT-CNF at the concentration of 1, 5 and 10 wt%. The pure PVA film exhibited an intense diffraction peak around 19.50o and a weak one at 22.9°, assigned to the strong interaction of intermolecular and intramolecular hydrogen bonding between the PVA chains. 48,49 The diffractograms of the modified CNF/PVA films showed a superposition of two portions (Figure 6), revealing the interaction between modified CNF and PVA matrix.

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A weak peak

around 23o is assigned to the overlapping of the (200) plane of cellulose I at 22.6o and the weak PVA diffraction peak at 22.9°. Furthermore, for all three composite films, the intensity of the weak peak at 2θ =22.6° was observed to increase with CNF loadings in the PVA matrix, as expected. Notably, the intensity of the peak at 2θ = 22.6° is relatively low for B-CNF (Figure 6a), which suggests that the B-CNF in PVA may contain partly internal fiber conjunction due to the weak interactions between B-CNF and PVA.52 For the composite film containing BE-CNF, this peak becomes more visible (Figure 6b), implying that the composites contained an increased amount of individual cellulose nanofibrils, due to the better diffusion of BE-CNF in the PVA matrix. In contrast, for BT-CNF films, there was a less obvious increased intensity of the peak at 22.6o with a further rise in BT-CNF content from 5% to 10 wt% (Figure 6c). This result may be because that the modified CNFs at a relatively low content works as a nucleating agent. In contrast, at higher CNF contents, the free movement of PVA chains is suppressed by the rigid chain structure of BTC, thus imposing restrictions on their ability to fold and limiting the


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molecular motion.53 A similar phenomenon has been recorded for cellulose nanocrystals and PVA composites.52 The results indicate the different trends in the overlapping peak, which are likely attributed to different interactions between PVA and the modified CNF. These interactions are compactly associated with the structure of the three types of modified CNF.


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Figure 6. X-ray diffractograms of (a) B-CNF/PVA composite films, (b) BE-CNF/PVA composite films, and (c) BT-CNF/PVA composite films, at different CNF contents. Thermal stability and reprocessing ability of CNF/PVA nanocomposite films. The TGA thermograms of the composites with the various modified CNF are presented in Figure 8. Three degradation stages were observed for pure PVA films. The first recorded degradation temperature, namely under 110 oC, is due to the evaporation of water in the film. The two subsequent major weight losses, from 200–500 °C, are attributed to the structural decomposition of PVA, namely, the degradation of the side chains of PVA in the range from 200 to 400 °C and the decomposition of the PVA backbone at 360 to 500 °C.54,55 With the functionalization of CNF with B, BE, BT, the weight loss of the composites still occurred in three main steps, which are assigned to the decomposition of the PVA matrix, given its dominant (> 90%) content in the composite film. The incorporation of modified CNF in the PVA matrix had a significantly positive effect on the thermal stability of the films: the onset temperature of the second degradation stage of the CNF/PVA film is higher than that of PVA, regardless of the type of CNF modification. In addition, all the degradation temperatures of the composite film with modified CNF shifted to the higher temperature as CNF loadings increased from 0 wt% to 10 wt%, confirming the crucial role of the CNF to enhance the thermal stability of the composite films. It is known that the cellulose’s crystalline regions contribute to the improvement of the thermal stability of the polymer matrix. Additionally, the surface of the modified CNFs contains three types of stable benzene units with high molecular weight, which would also improve the thermal stability of CNF.56 Consequently, the degradation of PVA could probably be delayed by the degradation of the added CNF component. Furthermore, char residues formed during the decomposition of


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cellulose, which could indeed delay the formation of methyl-terminated polyenes from the interaction between the PVA residual fragments formed during depolymerization.57 A better dispersion in the composite films of modified CNFs could also have a positive effect on the thermal degradation processes by enhancing the interactions, such as van der Waals interactions and hydrogen bonding between PVA chains. 58


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Figure 7. Thermogravimetric profiles of PVA and composite films with given CNF content: (a) B-CNF/PVA, (b) BE-CNF/PVA, and (c) BT-CNF/PVA. Influence of the CNFs on PVA’s UV−vis properties. The absorbance spectra of pure PVA and the CNF/PVA composite films collected in the region of UV-vis (200−800 nm) are presented in Figure 7. PVA shows a low absorbance in this region, indicating no UV absorbing ability. After the incorporation modified CNFs, the presence of a new peak at 250−300 nm was observed on the spectra, which is consistent with the well-known absorbing characteristic peak of the BP derivatives. The UV−vis transmittance values at 550 nm along with the UVA and UVB blocking percentages of neat PVA and CNF/PVA films are presented in Table S2. The transmittance at 550 nm of the pure PVA film was approximately 97%. The transmittance remained in the 9397% range when B-CNF or BE-CNF was added at loadings up to 10 wt% in the composite films. This confirmed that the incorporation of B-CNF or BE-CNF at < 10 wt% has a neglectable influence on the transparency of the composite films. For the BT-CNF/PVA films, ~10 % decrease in transparency was observed when the content of BT-CNF was increased to 10 wt%. This result may be attributed to the fact that BT-CNF tended to agglomerate in the PVA matrix,


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forming lumps (see SEM images in Figure 5). As a result, the formation of CNF aggregates would increase the light scattering and reduce the transmittance to some extent. All composite films synthesized with the three types of modified CNF absorbers revealed an increased ability to block UV light below 400 nm with CNF loading (from 1 wt% to 10 wt%), confirming that the modified CNF endowed the composite films with UV shielding functions.42 For B-CNF films, when the nanofiller increased to 10 wt%, approximately 74% of UVB light were shielded while the transmittance of the film remained at > 90%. BE-CNF/PVA nanocomposite films showed even wider UV-absorbing bands and higher UV-protection ability while simultaneously maintaining a higher transmittance than B-CNF/PVA film. For instance, the BE-CNF/PVA film with 5 wt% BE-CNF loading could block 87% UVB light below 320 nm. When 10 wt% of BE-CNF was added, approximately 92% of the UVB light was efficiently filtered. In addition to the excellent UV-shielding property, the BE-CNF/PVA films at 10 wt% were more transparent than the counterparts, the B-CNF or BT-CNF films. The superior light transmitting property of BE-CNF/PVA films might be attributed to the grafting of ESBO onto the CNF, which has a co-functional effect both as a modifying agent for CNFs and as a plasticizer, thus enhancing the interaction between the polymer and the reinforcing phase. 31,33 When BT-CNF was increased to 10 wt%, approximately 92% of UVB light and 52% of UVA light were shielded, showing superior UV absorbing property that attributed to the multiaromatic ring structures of BTC.


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Figure 8. UV−vis absorption spectra of composite films of given CNF loadings: (a) BCNF/PVA, (b) BE-CNF/PVA, and (c) BT-CNF/PVA.


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Mechanical performance of CNF/PVA nanocomposite films. The tensile strength, tensile modulus, and elongation at break properties of the CNF/PVA composite films are reported in Figure 9. The mechanical properties, especially the flexibility, are extremely significant for the application of packaging films. The tensile strength and Young’s modulus of the CNF/PVA nanocomposites were remarkably influenced by the modified CNF type and content (Figure 9a). In general, a low CNF loading (5%. In contrast, ESBO acts as lubricant and plasticizer in composites comprising CNF carrying grafted ESBO (BE-CNF), in such case where the negative effect of aggregation is offset.

Figure 9. Tensile test results of B-CNF/PVA, BE-CNF/PVA and BT-CNF/PVA composite films, at different CNF contents (a) tensile strength and Young’s modulus and (b) Elongation at break.

CONCLUSIONS We presented a novel approach to introduce a UV-shielding function and improved mechanical properties to PVA film by addition of BP-modified CNF (B-CNF, BE-CNF and BTCNE) as UV absorber and reinforcement phase. The grafting of the respective BP derivative on CNF took place through esterification reactions by three methods. Importantly, the addition of the UV absorber showed a remarkable improvement in the thermal stability of the PVA matrix. These CNF based UV absorbers (B-CNF, BE-CNF and BT-CNE) can primarily block the UV irradiation through the whole region of 200−400 nm, and can maintain high transparency (up to


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85%) in the visible region. Moreover, flexible and strong modified CNF/PVA nanocomposite films are demonstrated, showing promise in practical applications, such as in packaging.


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Supporting Information. C1s deconvoluted Peaks of XPS Survey Scans of CNF, B-CNF, BE-CNF, and BT-CNF. Percentage of blocking from UV-A and UV-B and T550 (transmittance of various composite films at 550nm) values of samples. Tonset (onset temperature) and Tmax (degradation temperature) of samples results obtained from TGA/DTG data analysis.

ACKNOWLEDEGEMENTS Corresponding Author * Hui Pan (Email: [email protected]), Tel: (86) 25-85427233 ORCID 0000-0001-5074-8314. Funding Sources The authors are grateful for the financial support by the Forestry Industry Research Special Funds for the National Key Basic Research Program of China (2017YFD0601005), the Forestry Industry Research Special Funds for Public Welfare Projects (201504602), the project of the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions, and Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP). Notes The authors declare no competing financial interests.


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Highly transparent, strong and flexible films with modified cellulose

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