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lignin nanocomposite films with excellent water vapor barrier and UV-shielding properties
Journal Pre-proofs High performance PVA/lignin nanocomposite films with excellent water vapor barrier and UV-shielding properties Xiao Zhang, Weifeng Liu, Wenqiang Liu, Xueqing Qiu PII: DOI: Reference:
S0141-8130(19)36310-X https://doi.org/10.1016/j.ijbiomac.2019.09.129 BIOMAC 13374
To appear in:
International Journal of Biological Macromolecules
Received Date: Revised Date: Accepted Date:
10 August 2019 22 September 2019 29 September 2019
Please cite this article as: X. Zhang, W. Liu, W. Liu, X. Qiu, High performance PVA/lignin nanocomposite films with excellent water vapor barrier and UV-shielding properties, International Journal of Biological Macromolecules (2019), doi: https://doi.org/10.1016/j.ijbiomac.2019.09.129
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High performance PVA/lignin nanocomposite films with excellent water vapor barrier and UV-shielding properties Xiao Zhang†, Weifeng Liu*†, Wenqiang Liu†, and Xueqing Qiu*†‡ † School of Chemistry and Chemical Engineering, Guangdong Engineering Research Center for Green Fine Chemicals, South China University of Technology, Wushan Road 381, Guangzhou, Guangdong, 510640, China ‡ State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Wushan Road 381, Guangzhou, Guangdong, 510640, China *Corresponding authors E-mail: [email protected] (W. Liu); [email protected] (X. Q. Qiu).
ABSTRACT High
performance
poly(vinyl
alcohol)
(PVA)/lignin
nanomicelle
(LNM)
nanocomposite films with good vapor barrier and advanced UV-shielding properties were fabricated in this study. LNM were homogeneously distributed in the PVA matrix and strong robust hydrogen bonds were successfully constructed between LNM and PVA matrix. With only 5 wt% loading of LNM into the PVA/LNM nanocomposite, the water vapor transmission rate (WVTR) was declined by about 189% compared with pure PVA. Moreover, after introducing lignin, the PVA nanocomposite films showed improved tensile strength and toughness, excellent UV-blocking and good thermal stability. As both lignin and PVA are biodegradable, this study shows a meaningful design approach for biodegradable functional nanocomposite films using cheap and easily available biomass and biodegradable raw materials. Keywords: Poly(vinyl alcohol), Lignin nanomicelle, Water vapor barrier, UVshielding
1
1. Introduction
Nowadays, the “white pollution” resulted from non-biodegradable polymeric materials has led to great damage to the ecosystem and human health. Expanding the application of biodegradable polymeric materials is an efficient method to handle the trouble of “white pollution” [1,2]. Poly(vinyl alcohol) (PVA) is a type of water-soluble biodegradable polymer, which has attracted particular interest for advanced multifunctional packaging industry, principally owing to its non-toxic, outstanding biodegradability, high transparency, and excellent oxygen barrier performance [3]. However, the large amount of hydroxyl groups in PVA molecular chains makes it highly humidity dependence [4]. Therefore, it is essential to enhance the water vapor barrier performance of PVA film to promote its broad application. Recently, PVA nanocomposites with improved water vapor barrier performance have been reported by incorporating various nanoparticles including graphene oxide nanosheet [5], calcium carbonate nanoparticles [6], ZnO [7] and nano-SiO2 [8] etc. The barrier performance of these nanocomposite films was improved significantly in the presence of nanofillers, primarily resulting from the “winding path effect” for diffusing molecules [9]. However, the mechanical strength of the composite films was reduced dramatically resulting from the poor interfacial compatibility between the filler and the PVA matrix. Furthermore, the biodegradable feature of PVA nanocomposite films decreased in a certain degree by adding high-cost inorganic nanoparticles. Looking for green biomass resources as an alternative for nanofiller is highly desirable for fabricating full biodegradable functional polymeric materials. Lignin is the second most abundant biomass resource from plants [10,11], which has attracted unusual attention for its low-cost, non-toxic, good thermal stability, biodegradability, anti-oxidant, UV-shielding performance and favorable stiffness [12]. Although a large amount of industrial lignin is produced every year, more than 95% of industrial lignin is burned as low value fuel, leading to severe resource waste [11]. Lignin is a special type of natural aromatic biopolymer with amphiphilic molecular 2
structures. Exploiting the rational utilization of lignin for PVA composites is a feasible strategy for biomass utilization [13–15]. However, the strong hydrogen bonding and π-π interactions of lignin itself lead to serious agglomeration and irregular distribution of polar and hydrophobic groups in lignin aggregates [16], causing poor miscibility between lignin and PVA. The formation of lignin macroaggregates from ten to hundred micron scale in PVA matrix led to macroscopic phase separation[17], resulting in the composite films exhibiting poor mechanical performance. In order to improve the barrier performance of PVA/lignin composites without losing their mechanical performance, lignin needs to be homogeneously dispersed in the PVA matrix as nanoscale particles, at the same time, the interfacial compatibility between lignin and PVA matrix must be improved. Generally, the lignin structure and the content of hydroxyl groups play an important role in the miscibility between PVA matrix and lignin. Higher amount of hydroxyl groups and ether linkages could increase the miscibility [18]. Lignin is an amphiphilic biopolymer rich in hydrophilic functional groups (hydroxyl groups, carboxyl groups, etc.) and hydrophobic structure units (benzene rings, alkyl chains, etc.) [19]. The amphiphilic structure allows lignin to self-assemble into uniform hydrophilic nanomicelle in aqueous solution [17]. In this work, we adopted hydrophilic lignin nanomicelle (LNM) as a green filler to develop full biodegradable PVA nanocomposite films with advanced water vapor barrier property. The hydrophilic hydroxyl and carboxyl groups in the surface of LNM could form strong interfacial hydrogen bonding interactions with PVA matrix, and meanwhile, nano-phase separation structure was obtained in the PVA matrix, which increased the tortuosity of penetration paths and enhanced the mechanical properties of PVA composites simultaneously. The influences of LNM on the barrier property, mechanical performance, UV-shielding and thermal performance of PVA composite films were systematically investigated. PVA/LNM nanocomposite films with great mechanical performance, high water vapor barrier and excellent UV-shielding performance were fabricated. 3
2. Materials and methods
2.1 Materials Poly(vinyl alcohol) (PVA) (alcoholysis degree > 99%; Mw = 130 kDa) was purchased from Sigma-Adrich. Enzymatic hydrolysis lignin (Mw = 1.7 kDa, Đ = 1.41) from the residue of cellulosic ethanol was supplied by Shandong Longli Bio-Technology (China) Co., Ltd. The phenolic hydroxyl content (3.75 mmol/g), alcohol hydroxyl content (0.36 mmol/g) and carboxyl groups content (1.04 mmol/g) of lignin was evaluated by the P31 NMR. Sodium hydroxide (NaOH) and chlorhydric acid (HCl) were supplied by Guangdong Guanghua technology (China) Co., Ltd. 2.2 Preparation of lignin nanomicelle (LNM) Lignin was first completely dissolved in the sodium hydroxide solution, and the concentration of lignin was 5 g/L, and then the lignin solution was dialyzed (Mw=1000 of dialysis bag) until the pH of the lignin solution was balanced with the deionized water (pH = 6.8). The lignin nanomicelle (LNM) formed while the sodium salt was dialyzed out. The powder sample was obtained after rotary evaporation at 50 ºC and freeze-dried for 12 h. Lignin nanomicelle was labelled as LNM. 2.3 Preparation of PVA/LNM nanocomposite films The PVA and LNM hybrid solution (5 wt% in total) was prepared by adding PVA and LNM into deionized water successively, and was then agitated at 90 °C for 3 h to ensure the full mixing. It was poured into the Teflon mold (160 mm × 160 mm) at once. The nanocomposite films with 200 (±10) μm thickness were obtained by evaporating the moisture at room temperature. The samples were labelled according to the addition amount of LNM. For instance, LNM-5 represented that 5 wt% of LNM was added into PVA/LNM composites.
4
2.4 Fourier transform infrared spectroscopy (FTIR) Fourier transform infrared spectroscopy (FTIR) (Thermo Nicolet FTIR spectrometer, US) of the lignin sample was determined with the scanning range from 4000 to 400 cm-1. Fourier transform attenuated total reflection infrared spectra (ATR-FTIR) of the composite films were scanned between 3750 and 550 cm-1. 2.5 31P NMR spectra and particle size distribution measurement Quantitative
31P
NMR spectra were determined on AVANCE III HD 600
spectrometer according to an internal standard method, which was reported previously [20]. The particle size distribution of LNM was tested by a laser particle size analyzer (Zeta Plus, Brookhaven, US). LNM solution (1 g/L) was placed in a quartz cuvette for measurement at 25 °C. 2.6 Scanning electron microscopy (SEM) and atomic force microscope (AFM) The scanning electron microscope (SEM) (HITACHI, SU-8220, Japan) of LNM were operated at 5 kV. Samples were prepared by dropping the LNM solution (2 g/L) onto the mica slice and slowly dried at room temperature. The field-emission scanning electron microscopy (FE-SEM) was conducted to characterize the morphologies of the obtained films. The films were dried in vacuum oven, and quenched with liquid nitrogen. All samples were coated with gold before test. The microscopic surface roughness (Rq) of the pure PVA and LNM-5 was measured by Atomic force microscope (AFM, XE-100, Park Systems, Korea) adopting non-contact mode. Rq of the films was achieved by scanning a 5 μm × 5 μm area. 2.7 Liquid rheology test The hydrogen bonding interaction between PVA and LNM was evaluated by rheology test (HAAKE MARS Rheometer, Thermo Fisher, Germany). Since the PVA 5
could decompose at high temperature, only the liquid rheology tests of sample solutions were performed. The frequency dependence of the storage modulus (G’) and complex viscosity (η*) was measured within the parallel plates of 20 mm in diameter and 50 mm in height at the angular velocity range between 10-2 rad/s and 102 rad/s at 25 °C. 2.8 Differential scanning calorimetry (DSC) and X-ray diffractometer (XRD) Differential scanning calorimetry (DSC) data was determined on TA Q200 instrument (USA) under the protection of flowing N2. The samples were firstly heated to 240 °C and miantianed at 240 °C for 3 min to remove the thermal history. They were then cooled to 30 °C, isothermal for 3 min, and finally reheated to 240 °C at a scan rate of 10 °C/min. The glass transition temperature (Tg) and melting temperature (Tm) were obtained from the second heating procedure. The crystallinity of composite films from DSC (Xc-DSC) was calculated according to the equation: Xc-DSC = △Hm/△H0, where △Hm is melting enthalpy from DSC curve, △H0 is the enthalpy of pure PVA crystal (138.6 J/g) [21]. Thermogravimetric analysis (TG) was determined by thermogravimeter (TA) (Q500, US) at a heating rate of 10 °C/min from 30 °C to 700 °C under nitrogen atmosphere. The crystallization behaviors of PVA and PVA/LNM composite films were characterized using a D8 Advance X-ray diffractometer (XRD) (Bruck, Germany) equipped with Cu Kα radiation at a scanning range of 10~50°. Sample to be tested were dried by a vacuum oven at 55 °C for 8 h. The crystallinity of PVA from XRD (Xc-XRD) was calculated by the following equation: Xc-XRD = Ac/At, where At was the integration of crystallized and amorphous regions, and Ac was the integration of crystalline region. 2.9 Mechanical properties Dumbbell-shaped samples (75 mm × 4 mm) were tailored for mechanical performance test. The uniaxial tensile test was performed on a universal testing machine (MTS Systems Co. Ltd, US). The stretching speed was kept at 10 mm/min at the ambient temperature and humidity of 20 ± 5 % during the test. The tensile toughness is 6
the integral region of the stress-strain curve. 2.10 Contact angle and water vapor transmission rate (WVTR) test Contact angle of the PVA composite films were performed by static contact angle measuring instrument (Power Each JC2000C1, China). Samples to be tested need keeping flat absolutely, and dried before test. Water Vapor Transmission Rate (WVTR) was determined according to the standard GB 1037-70 sheet-cup method. A film in 6.0 cm diameter for PVA or PVA/LNM composites were prepared, and dried in vacuum oven at 50 °C for 12 h. The film was first conditioned to a constant weight at 30 °C and 50 % RH, and was then sealed to a test cup filling with calcium chloride, which was filled about 3 mm away from the tested film sample. Calcium chloride was dried in high temperature oven (120 °C) for 12 h and grinded to ultrafine powder before use. Then the sealed cup was placed in a 30 °C, 90 % RH humidity chamber for 16 h and weighed as the initial mass. Finally, the test cup was placed in the constant humidity chamber, and the interval between the two weights was 24 h. The balanced state was reached until the difference between the two mass increments was constant and less than 5 wt%. WVTR was determined using the following equation: WVTR= (△m·d)/(A·t), where △m was the mass difference between two consecutive measurements within the time of t at balanced state, A was the test area (cup mouth area), d was the thickness of the tested film, t was the two intervals between two consecutive measurements (24 h). 2.11 UV-shielding performance UV-shielding performance of PVA nanocomposite films were performed by a UVvis spectrophotometer (Shimadzu, Japan). The rectangular shaped samples (5 cm × 2 cm × 200 μm) were prepared before test.
7
3. Result and discussion
3.1 Preparation and characterization of PVA/LNM nanocomposite films LNM was obtained by hydrophobic self-assembly of lignin from aqueous solution. Most of the lignin molecular chains stretched in sodium hydroxide solution [22]. As the sodium salts were dialyzed out, the solubility of lignin decreased, the hydrophilic units (hydroxy and carboxyl functional groups etc.) of lignin extended outward, and the hydrophobic chain segments (phenylpropane structure etc.) agglomerated inward forming LNM [23]. The average particle size of LNM formed by self-assembly was about 224 nm (Figure 1a). The prepared LNM uniform solution behaved evident Tyndall effect (Figure 1b). Transparent PVA/LNM complex films were then fabricated by facile solution blending and casting. Uniform distribution of LNM in the PVA matrix was achieved, as shown in Figure 1b.
Figure 1. a) Particle size distribution of LNM. b) Schematic diagram of the preparation procedure for the PVA/LNM composite film.
As shown in Figure 2a & 2b, the hydrogen bonding interactions between PVA and LNM were demonstrated by ATR-FTIR. Pristine PVA exhibited obvious hydroxyl (OH) stretching vibration peak at 3298 cm-1 (Figure 2a). As the loading content of LNM increased, an apparent redshift of hydroxyl occurred from 3298 cm-1 to 3277 cm-1 (LNM-10). In addition, a similar redshift phenomenon was also found for C-OH groups, 8
which offset from 1089 cm-1 (PVA) to 1085 cm-1 (LNM-10) (Figure 2b). The redshift in ATR-FTIR spectra sufficiently certified the robust hydrogen bonding interactions between hydroxyl groups in PVA and hydrophilic polar groups (hydroxy and carboxyl functional groups etc.) in LNM [7].
Figure 2. O-H (a) and C-O (b) stretching vibrations of PVA, LNM, and PVA/LNM nanocomposites. Storage modulus (G') c) and complex viscosity (η*) d) for PVA and PVA/LNM mixture solutions.
Rheological test was also performed to confirm the hydrogen bonding interactions between PVA and LNM (Figure 2c & 2d). The storage modulus (G′) and the complex viscosity (η*) of the mixed solution ranging from 10-2 ~ 100 rad/s increased evidently with the addition of LNM. As shown in Figure 2c, a platform for all samples appeared in the G′-ω curves ranging from 10-2 ~ 100 rad/s, suggesting the generation of strong hydrogen bonding interactions between PVA and LNM [24]. The maximum values of 9
storage modulus and the complex viscosity in the range of 10-2 ~ 100 rad/s were obtained from LNM-5, indicating the relatively concentrated hydrogen bonds-based physical cross-linking network with the loading of 5 wt% LNM. All these results proved strong hydrogen bonding interactions between PVA matrix and LNM. 3.2 Vapor barrier property of PVA/LNM nanocomposites The surface wettability of PVA/LNM nanocomposite films was evaluated by the static contact angle measurement, as shown in Figure 3a. Compared with the pure PVA film, the hydrophobicity of PVA/LNM nanocomposite films was significantly improved because of the hydrophobicity of lignin. The contact angle increased from 34.3° for pure PVA to 88.5° with only 5 wt% addition of LNM (LNM-5), and then decreased to 78.5° with the LNM loading content reached 10 wt% (LNM-10). Contrary to the variation tendency in contact angle, the water vapor transmission rate (WVTR) decreased first and then increased with the loading content of LNM. The WVTR of LNM-5 was only one third of neat PVA, indicating only 5 wt% loading of LNM could significantly improve the water vapor barrier property of the PVA film.
Figure 3. a) Static contact angle of pure PVA and PVA/LNM nanocomposite films with varied LNM loading. b) Comparison of pure PVA and PVA/LNM nanocomposite films in WVTR.
To disclose the barrier mechanism of PVA/LNM nanocomposite films, the dispersion of LNM in PVA matrix was studied by FE-SEM, as shown in Figure 4a. Compared with neat PVA, PVA/LNM nanocomposite films showed distinct LNM particles with the size about 250 nm. Moreover, the nanoparticles become denser with 10
the loading content of LNM increased. The nanoparticles became intensive while still maintaining well dispersion with 5 wt% loading content of LNM (LNM-5). The presence of regular LNM forming a microscopic phase separation structure in the PVA matrix greatly increased the tortuosity for the incursion water molecules within the nanocomposite films, which eventually increased the water vapor barrier property [14]. As shown in Figure 5, the surface roughness of LNM-5 characterized by atomic force microscope (AFM) was 3.46 nm, which was slightly rougher than pure PVA of 1.69 nm. The increase of surface roughness on nanoscale in LNM-5 caused by adding LNM might also contribute to the increase in contact angle and improve the water vapor barrier property [25]. While excessive LNM resulted in agglomeration of LNM in LNM-10. The irregular agglomeration of LNM led to macroscopic phase separation, and greatly affected the barrier property of the composite films. Figure 4b shows XRD curves of PVA/LNM nanocomposite films. Semi-crystalline PVA involved two obvious diffraction peaks centering at 19.6° and 22.8°, respectively [26]. No sharp diffraction peak was found for LNM because of its amorphous structure [12]. PVA/LNM nanocomposite films exhibited a slight decrease in crystallinity except for LNM-1 compared with pure PVA, demonstrating that introducing proper amount of LNM did not interfere the crystallization behavior of PVA matrix obviously. When the amount of LNM reached 10 wt%, the crystallinity of LNM-10 decreased obviously. These results were further verified by the DSC analysis (Figure 4c). The melting temperature (Tm) of the LNM/PVA nanocomposite films almost maintained constant (224.1 °C for pure PVA, 221.0 °C for LNM-10), but the crystallinity from DSC analysis (Xc-DSC) decreased from 29.2% for pure PVA to 22.8% for LNM-10. The glass transition temperature (Tg) of the nanocomposite increased gradually with the LNM content (Figure 4c), resulting from the strong intermolecular hydrogen bonding interactions between LNM and PVA, which restricted the flexibility of PVA chain segments intensely [27].
11
Figure 4. a) FE-SEM images of pure PVA and PVA nanocomposite films. b) XRD spectra of PVA and PVA nanocomposite films. c) DSC melting curves of PVA and PVA/LNM nanocomposite films.
Figure 5. Micromorphology and roughness of PVA a) and LNM-5 b) films measured by AFM.
As illustrated in Scheme 1, water molecules permeated from one side of the film to 12
the other side through the amorphous region of the semi-crystalline polymers [28]. The water molecules were prone to seek out a path with the least resistance in the process of water vapor transmission. The crystallinity of polymer matrix and morphology of filler in matrix played a vital role in water vapor penetration. The high crystallinity and the microphase separation structure increase the diffusion path of water vapor in the PVA matrix, and the robust hydrogen bonds between filler and PVA matrix also increased the obstacle of water vapor transmission [29]. In addition, the increase in the surface roughness in nanoscale after inducing LNM into PVA is beneficial for preventing the film surface from wetting [30]. Thus, the water vapor barrier properties of PVA/LNM nanocomposites were greatly improved.
Scheme 1. Water vapor barrier mechanism of PVA and PVA/LNM composite film.
3.3 Mechanical properties The stress-strain curves of pristine PVA and PVA/LNM nanocomposites were depicted in Figure 6. The mechanical performance including tensile strength and toughness of PVA/LNM nanocomposites enhanced apparently compared with pristine PVA. Specifically, the tensile strength significantly increased to 91.3 MPa from 72.2 MPa for pure PVA with only 2 wt% loading of LNM. The tensile strength increased continuously with the amount of LNM, and reached the maximum of 94.9 MPa at the addition of 10 wt% (LNM-10) within this study. The elongation at break of the 13
composite film first increased to the maximum value of 225% in LNM-2 but then gradually declined with the LNM addition. The toughness reached the maximum value of 156.4 MJ/m3 in the sample of LNM-2, which was 35% increase compared with 115.8 MJ/m3 of PVA. Increasing the LNM content up to 10 wt% caused a gentle increase of the tensile strength but a severe decline in elongation at break, resulting from the irregular cluster of LNM particles in PVA matrix (Figure 4a). The sample LNM-5 possessed similar mechanical performance as LNM-2, with both excellent strength and toughness, which was in accordance with the dense hydrogen bond-based physical cross-linking network revealed through the solution rheological test shown in Figure 2. The nanophase separation structure in the composite films could also dissipate plenty energy during the stretching process, leading to a ductile fracture, thereby increased the strength and toughness of the composite films [7].
Figure 6. a) Stress-strain curves of pure PVA and PVA/LNM nanocomposite films (Insert: LNM2 before and after fracture). b) Comparison of pure PVA and PVA/LNM nanocomposite films for tensile strength and toughness at 20 % RH.
3.4 UV-shielding and Thermal performance PVA composite films exhibiting good ultraviolet (UV) shielding are attractive in a wide variety of applications [31,32]. As shown in Figure 7a, the neat PVA film performed high transmittance towards UV spectrum (380-190 nm), displaying no UVshielding property. The light transmittance declined successively with the LNM content, 14
especially in the UV spectrum region. Only 1 wt% addition of LNM (LNM-1) led to 100% shielding of the UVB (320-275 nm) and UVC (275-190 nm) spectrum and most of the UVA (380-320 nm) spectrum. The UVA spectrum was totally blocked once the LNM addition reached 5 wt% (LNM-5). The superior UV-shielding property was resulted from the abundant phenolic hydroxyl groups concentrated in the outer surface of LNM particles after self-assembling from aqueous solution [12,33]. Despite the brown color caused by LNM affected the visible light transmittance of PVA films (380 nm ~ 760 nm), the PVA/LNM nanocomposites retained good transparency even with the addition of 5 wt% LNM.
Figure 7. a) UV-vis transmittance curves of PVA and PVA/LNM nanocomposite films. b) TG curves of PVA and PVA/LNM nanocomposites at N2 atmosphere.
As shown in Figure 7b, TG measurement was determined to evaluate the thermal stability of PVA/LNM nanocomposite films. The maximum thermal decomposition rate temperature (Tmax) of PVA/LNM nanocomposite grew with the LNM addition from 262.3 °C of pristine PVA to 273.5 °C of LNM-10, probably because of the strong intermolecular hydrogen bonding interactions between PVA matrix and LNM which strongly restricted the flexibility of PVA chain segments. The aromatic structural units in LNM molecules also played a significant role in the improvement of thermal stability [33]. The melting temperature (200 ~ 250 °C) of PVA is extremely near to its thermal decomposition temperature, causing it hard to yield the products with involuted shapes through melt processing [34]. LNM played a major role in enhancing the thermal 15
performance of PVA nanocomposite films, putting forward a meaningful way for the realization of melt processing for PVA composite materials. 4. Conclusion In summary, high performance PVA/LNM nanocomposites with excellent water vapor barrier and UV-shielding performance were successfully prepared. Biomass lignin nanomicelle (LNM) was introduced into the PVA film to construct nanophase separation structure and strong intermolecular hydrogen bonding interactions. The PVA/LNM composite films exhibited much better water vapor barrier performance than pure PVA, benefiting from the tortuous path effect of LNM in the PVA matrix. The water vapor transmission rate (WVTR) of PVA/LNM nanocomposite films was reduced to only one third of the pristine PVA. Meanwhile, the tensile strength and toughness of PVA nanocomposites were simultaneously enhanced by introducing LNM. Moreover, adding the readily available lignin also improved the thermal stability of PVA nanocomposites and endowed PVA materials with excellent UV-shielding function. The addition of only 1 wt% LNM could achieve the full shielding of UVB and UVC and the majority of UVA shielding. Our work shows a significant strategy for fabricating full biodegradable and highly water vapor barrier PVA materials with excellent mechanical performance for a variety of potential applications such as UVshielding food or medical packaging areas.
Acknowledgments
The authors gratefully thank the National Natural Science Foundation of China (21706082), the Science and Technology Program of Guangzhou (201707020025, 201804010140), and Natural Science Foundation of Guangdong Province (2017B090903003, 2018B030311052) for the financial support.
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Highlights
This work presents an innovative design strategy for full biodegradable functional nanocomposite films using cheap and easily available biomass and biodegradable raw materials.
The water vapor barrier performance, mechanical properties as well as the UVshielding performance of PVA film were significantly improved by lignin nanomicelle.
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S0141-8130(19)36310-X https://doi.org/10.1016/j.ijbiomac.2019.09.129 BIOMAC 13374
To appear in:
International Journal of Biological Macromolecules
Received Date: Revised Date: Accepted Date:
10 August 2019 22 September 2019 29 September 2019
Please cite this article as: X. Zhang, W. Liu, W. Liu, X. Qiu, High performance PVA/lignin nanocomposite films with excellent water vapor barrier and UV-shielding properties, International Journal of Biological Macromolecules (2019), doi: https://doi.org/10.1016/j.ijbiomac.2019.09.129
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© 2019 Published by Elsevier B.V.
High performance PVA/lignin nanocomposite films with excellent water vapor barrier and UV-shielding properties Xiao Zhang†, Weifeng Liu*†, Wenqiang Liu†, and Xueqing Qiu*†‡ † School of Chemistry and Chemical Engineering, Guangdong Engineering Research Center for Green Fine Chemicals, South China University of Technology, Wushan Road 381, Guangzhou, Guangdong, 510640, China ‡ State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Wushan Road 381, Guangzhou, Guangdong, 510640, China *Corresponding authors E-mail: [email protected] (W. Liu); [email protected] (X. Q. Qiu).
ABSTRACT High
performance
poly(vinyl
alcohol)
(PVA)/lignin
nanomicelle
(LNM)
nanocomposite films with good vapor barrier and advanced UV-shielding properties were fabricated in this study. LNM were homogeneously distributed in the PVA matrix and strong robust hydrogen bonds were successfully constructed between LNM and PVA matrix. With only 5 wt% loading of LNM into the PVA/LNM nanocomposite, the water vapor transmission rate (WVTR) was declined by about 189% compared with pure PVA. Moreover, after introducing lignin, the PVA nanocomposite films showed improved tensile strength and toughness, excellent UV-blocking and good thermal stability. As both lignin and PVA are biodegradable, this study shows a meaningful design approach for biodegradable functional nanocomposite films using cheap and easily available biomass and biodegradable raw materials. Keywords: Poly(vinyl alcohol), Lignin nanomicelle, Water vapor barrier, UVshielding
1
1. Introduction
Nowadays, the “white pollution” resulted from non-biodegradable polymeric materials has led to great damage to the ecosystem and human health. Expanding the application of biodegradable polymeric materials is an efficient method to handle the trouble of “white pollution” [1,2]. Poly(vinyl alcohol) (PVA) is a type of water-soluble biodegradable polymer, which has attracted particular interest for advanced multifunctional packaging industry, principally owing to its non-toxic, outstanding biodegradability, high transparency, and excellent oxygen barrier performance [3]. However, the large amount of hydroxyl groups in PVA molecular chains makes it highly humidity dependence [4]. Therefore, it is essential to enhance the water vapor barrier performance of PVA film to promote its broad application. Recently, PVA nanocomposites with improved water vapor barrier performance have been reported by incorporating various nanoparticles including graphene oxide nanosheet [5], calcium carbonate nanoparticles [6], ZnO [7] and nano-SiO2 [8] etc. The barrier performance of these nanocomposite films was improved significantly in the presence of nanofillers, primarily resulting from the “winding path effect” for diffusing molecules [9]. However, the mechanical strength of the composite films was reduced dramatically resulting from the poor interfacial compatibility between the filler and the PVA matrix. Furthermore, the biodegradable feature of PVA nanocomposite films decreased in a certain degree by adding high-cost inorganic nanoparticles. Looking for green biomass resources as an alternative for nanofiller is highly desirable for fabricating full biodegradable functional polymeric materials. Lignin is the second most abundant biomass resource from plants [10,11], which has attracted unusual attention for its low-cost, non-toxic, good thermal stability, biodegradability, anti-oxidant, UV-shielding performance and favorable stiffness [12]. Although a large amount of industrial lignin is produced every year, more than 95% of industrial lignin is burned as low value fuel, leading to severe resource waste [11]. Lignin is a special type of natural aromatic biopolymer with amphiphilic molecular 2
structures. Exploiting the rational utilization of lignin for PVA composites is a feasible strategy for biomass utilization [13–15]. However, the strong hydrogen bonding and π-π interactions of lignin itself lead to serious agglomeration and irregular distribution of polar and hydrophobic groups in lignin aggregates [16], causing poor miscibility between lignin and PVA. The formation of lignin macroaggregates from ten to hundred micron scale in PVA matrix led to macroscopic phase separation[17], resulting in the composite films exhibiting poor mechanical performance. In order to improve the barrier performance of PVA/lignin composites without losing their mechanical performance, lignin needs to be homogeneously dispersed in the PVA matrix as nanoscale particles, at the same time, the interfacial compatibility between lignin and PVA matrix must be improved. Generally, the lignin structure and the content of hydroxyl groups play an important role in the miscibility between PVA matrix and lignin. Higher amount of hydroxyl groups and ether linkages could increase the miscibility [18]. Lignin is an amphiphilic biopolymer rich in hydrophilic functional groups (hydroxyl groups, carboxyl groups, etc.) and hydrophobic structure units (benzene rings, alkyl chains, etc.) [19]. The amphiphilic structure allows lignin to self-assemble into uniform hydrophilic nanomicelle in aqueous solution [17]. In this work, we adopted hydrophilic lignin nanomicelle (LNM) as a green filler to develop full biodegradable PVA nanocomposite films with advanced water vapor barrier property. The hydrophilic hydroxyl and carboxyl groups in the surface of LNM could form strong interfacial hydrogen bonding interactions with PVA matrix, and meanwhile, nano-phase separation structure was obtained in the PVA matrix, which increased the tortuosity of penetration paths and enhanced the mechanical properties of PVA composites simultaneously. The influences of LNM on the barrier property, mechanical performance, UV-shielding and thermal performance of PVA composite films were systematically investigated. PVA/LNM nanocomposite films with great mechanical performance, high water vapor barrier and excellent UV-shielding performance were fabricated. 3
2. Materials and methods
2.1 Materials Poly(vinyl alcohol) (PVA) (alcoholysis degree > 99%; Mw = 130 kDa) was purchased from Sigma-Adrich. Enzymatic hydrolysis lignin (Mw = 1.7 kDa, Đ = 1.41) from the residue of cellulosic ethanol was supplied by Shandong Longli Bio-Technology (China) Co., Ltd. The phenolic hydroxyl content (3.75 mmol/g), alcohol hydroxyl content (0.36 mmol/g) and carboxyl groups content (1.04 mmol/g) of lignin was evaluated by the P31 NMR. Sodium hydroxide (NaOH) and chlorhydric acid (HCl) were supplied by Guangdong Guanghua technology (China) Co., Ltd. 2.2 Preparation of lignin nanomicelle (LNM) Lignin was first completely dissolved in the sodium hydroxide solution, and the concentration of lignin was 5 g/L, and then the lignin solution was dialyzed (Mw=1000 of dialysis bag) until the pH of the lignin solution was balanced with the deionized water (pH = 6.8). The lignin nanomicelle (LNM) formed while the sodium salt was dialyzed out. The powder sample was obtained after rotary evaporation at 50 ºC and freeze-dried for 12 h. Lignin nanomicelle was labelled as LNM. 2.3 Preparation of PVA/LNM nanocomposite films The PVA and LNM hybrid solution (5 wt% in total) was prepared by adding PVA and LNM into deionized water successively, and was then agitated at 90 °C for 3 h to ensure the full mixing. It was poured into the Teflon mold (160 mm × 160 mm) at once. The nanocomposite films with 200 (±10) μm thickness were obtained by evaporating the moisture at room temperature. The samples were labelled according to the addition amount of LNM. For instance, LNM-5 represented that 5 wt% of LNM was added into PVA/LNM composites.
4
2.4 Fourier transform infrared spectroscopy (FTIR) Fourier transform infrared spectroscopy (FTIR) (Thermo Nicolet FTIR spectrometer, US) of the lignin sample was determined with the scanning range from 4000 to 400 cm-1. Fourier transform attenuated total reflection infrared spectra (ATR-FTIR) of the composite films were scanned between 3750 and 550 cm-1. 2.5 31P NMR spectra and particle size distribution measurement Quantitative
31P
NMR spectra were determined on AVANCE III HD 600
spectrometer according to an internal standard method, which was reported previously [20]. The particle size distribution of LNM was tested by a laser particle size analyzer (Zeta Plus, Brookhaven, US). LNM solution (1 g/L) was placed in a quartz cuvette for measurement at 25 °C. 2.6 Scanning electron microscopy (SEM) and atomic force microscope (AFM) The scanning electron microscope (SEM) (HITACHI, SU-8220, Japan) of LNM were operated at 5 kV. Samples were prepared by dropping the LNM solution (2 g/L) onto the mica slice and slowly dried at room temperature. The field-emission scanning electron microscopy (FE-SEM) was conducted to characterize the morphologies of the obtained films. The films were dried in vacuum oven, and quenched with liquid nitrogen. All samples were coated with gold before test. The microscopic surface roughness (Rq) of the pure PVA and LNM-5 was measured by Atomic force microscope (AFM, XE-100, Park Systems, Korea) adopting non-contact mode. Rq of the films was achieved by scanning a 5 μm × 5 μm area. 2.7 Liquid rheology test The hydrogen bonding interaction between PVA and LNM was evaluated by rheology test (HAAKE MARS Rheometer, Thermo Fisher, Germany). Since the PVA 5
could decompose at high temperature, only the liquid rheology tests of sample solutions were performed. The frequency dependence of the storage modulus (G’) and complex viscosity (η*) was measured within the parallel plates of 20 mm in diameter and 50 mm in height at the angular velocity range between 10-2 rad/s and 102 rad/s at 25 °C. 2.8 Differential scanning calorimetry (DSC) and X-ray diffractometer (XRD) Differential scanning calorimetry (DSC) data was determined on TA Q200 instrument (USA) under the protection of flowing N2. The samples were firstly heated to 240 °C and miantianed at 240 °C for 3 min to remove the thermal history. They were then cooled to 30 °C, isothermal for 3 min, and finally reheated to 240 °C at a scan rate of 10 °C/min. The glass transition temperature (Tg) and melting temperature (Tm) were obtained from the second heating procedure. The crystallinity of composite films from DSC (Xc-DSC) was calculated according to the equation: Xc-DSC = △Hm/△H0, where △Hm is melting enthalpy from DSC curve, △H0 is the enthalpy of pure PVA crystal (138.6 J/g) [21]. Thermogravimetric analysis (TG) was determined by thermogravimeter (TA) (Q500, US) at a heating rate of 10 °C/min from 30 °C to 700 °C under nitrogen atmosphere. The crystallization behaviors of PVA and PVA/LNM composite films were characterized using a D8 Advance X-ray diffractometer (XRD) (Bruck, Germany) equipped with Cu Kα radiation at a scanning range of 10~50°. Sample to be tested were dried by a vacuum oven at 55 °C for 8 h. The crystallinity of PVA from XRD (Xc-XRD) was calculated by the following equation: Xc-XRD = Ac/At, where At was the integration of crystallized and amorphous regions, and Ac was the integration of crystalline region. 2.9 Mechanical properties Dumbbell-shaped samples (75 mm × 4 mm) were tailored for mechanical performance test. The uniaxial tensile test was performed on a universal testing machine (MTS Systems Co. Ltd, US). The stretching speed was kept at 10 mm/min at the ambient temperature and humidity of 20 ± 5 % during the test. The tensile toughness is 6
the integral region of the stress-strain curve. 2.10 Contact angle and water vapor transmission rate (WVTR) test Contact angle of the PVA composite films were performed by static contact angle measuring instrument (Power Each JC2000C1, China). Samples to be tested need keeping flat absolutely, and dried before test. Water Vapor Transmission Rate (WVTR) was determined according to the standard GB 1037-70 sheet-cup method. A film in 6.0 cm diameter for PVA or PVA/LNM composites were prepared, and dried in vacuum oven at 50 °C for 12 h. The film was first conditioned to a constant weight at 30 °C and 50 % RH, and was then sealed to a test cup filling with calcium chloride, which was filled about 3 mm away from the tested film sample. Calcium chloride was dried in high temperature oven (120 °C) for 12 h and grinded to ultrafine powder before use. Then the sealed cup was placed in a 30 °C, 90 % RH humidity chamber for 16 h and weighed as the initial mass. Finally, the test cup was placed in the constant humidity chamber, and the interval between the two weights was 24 h. The balanced state was reached until the difference between the two mass increments was constant and less than 5 wt%. WVTR was determined using the following equation: WVTR= (△m·d)/(A·t), where △m was the mass difference between two consecutive measurements within the time of t at balanced state, A was the test area (cup mouth area), d was the thickness of the tested film, t was the two intervals between two consecutive measurements (24 h). 2.11 UV-shielding performance UV-shielding performance of PVA nanocomposite films were performed by a UVvis spectrophotometer (Shimadzu, Japan). The rectangular shaped samples (5 cm × 2 cm × 200 μm) were prepared before test.
7
3. Result and discussion
3.1 Preparation and characterization of PVA/LNM nanocomposite films LNM was obtained by hydrophobic self-assembly of lignin from aqueous solution. Most of the lignin molecular chains stretched in sodium hydroxide solution [22]. As the sodium salts were dialyzed out, the solubility of lignin decreased, the hydrophilic units (hydroxy and carboxyl functional groups etc.) of lignin extended outward, and the hydrophobic chain segments (phenylpropane structure etc.) agglomerated inward forming LNM [23]. The average particle size of LNM formed by self-assembly was about 224 nm (Figure 1a). The prepared LNM uniform solution behaved evident Tyndall effect (Figure 1b). Transparent PVA/LNM complex films were then fabricated by facile solution blending and casting. Uniform distribution of LNM in the PVA matrix was achieved, as shown in Figure 1b.
Figure 1. a) Particle size distribution of LNM. b) Schematic diagram of the preparation procedure for the PVA/LNM composite film.
As shown in Figure 2a & 2b, the hydrogen bonding interactions between PVA and LNM were demonstrated by ATR-FTIR. Pristine PVA exhibited obvious hydroxyl (OH) stretching vibration peak at 3298 cm-1 (Figure 2a). As the loading content of LNM increased, an apparent redshift of hydroxyl occurred from 3298 cm-1 to 3277 cm-1 (LNM-10). In addition, a similar redshift phenomenon was also found for C-OH groups, 8
which offset from 1089 cm-1 (PVA) to 1085 cm-1 (LNM-10) (Figure 2b). The redshift in ATR-FTIR spectra sufficiently certified the robust hydrogen bonding interactions between hydroxyl groups in PVA and hydrophilic polar groups (hydroxy and carboxyl functional groups etc.) in LNM [7].
Figure 2. O-H (a) and C-O (b) stretching vibrations of PVA, LNM, and PVA/LNM nanocomposites. Storage modulus (G') c) and complex viscosity (η*) d) for PVA and PVA/LNM mixture solutions.
Rheological test was also performed to confirm the hydrogen bonding interactions between PVA and LNM (Figure 2c & 2d). The storage modulus (G′) and the complex viscosity (η*) of the mixed solution ranging from 10-2 ~ 100 rad/s increased evidently with the addition of LNM. As shown in Figure 2c, a platform for all samples appeared in the G′-ω curves ranging from 10-2 ~ 100 rad/s, suggesting the generation of strong hydrogen bonding interactions between PVA and LNM [24]. The maximum values of 9
storage modulus and the complex viscosity in the range of 10-2 ~ 100 rad/s were obtained from LNM-5, indicating the relatively concentrated hydrogen bonds-based physical cross-linking network with the loading of 5 wt% LNM. All these results proved strong hydrogen bonding interactions between PVA matrix and LNM. 3.2 Vapor barrier property of PVA/LNM nanocomposites The surface wettability of PVA/LNM nanocomposite films was evaluated by the static contact angle measurement, as shown in Figure 3a. Compared with the pure PVA film, the hydrophobicity of PVA/LNM nanocomposite films was significantly improved because of the hydrophobicity of lignin. The contact angle increased from 34.3° for pure PVA to 88.5° with only 5 wt% addition of LNM (LNM-5), and then decreased to 78.5° with the LNM loading content reached 10 wt% (LNM-10). Contrary to the variation tendency in contact angle, the water vapor transmission rate (WVTR) decreased first and then increased with the loading content of LNM. The WVTR of LNM-5 was only one third of neat PVA, indicating only 5 wt% loading of LNM could significantly improve the water vapor barrier property of the PVA film.
Figure 3. a) Static contact angle of pure PVA and PVA/LNM nanocomposite films with varied LNM loading. b) Comparison of pure PVA and PVA/LNM nanocomposite films in WVTR.
To disclose the barrier mechanism of PVA/LNM nanocomposite films, the dispersion of LNM in PVA matrix was studied by FE-SEM, as shown in Figure 4a. Compared with neat PVA, PVA/LNM nanocomposite films showed distinct LNM particles with the size about 250 nm. Moreover, the nanoparticles become denser with 10
the loading content of LNM increased. The nanoparticles became intensive while still maintaining well dispersion with 5 wt% loading content of LNM (LNM-5). The presence of regular LNM forming a microscopic phase separation structure in the PVA matrix greatly increased the tortuosity for the incursion water molecules within the nanocomposite films, which eventually increased the water vapor barrier property [14]. As shown in Figure 5, the surface roughness of LNM-5 characterized by atomic force microscope (AFM) was 3.46 nm, which was slightly rougher than pure PVA of 1.69 nm. The increase of surface roughness on nanoscale in LNM-5 caused by adding LNM might also contribute to the increase in contact angle and improve the water vapor barrier property [25]. While excessive LNM resulted in agglomeration of LNM in LNM-10. The irregular agglomeration of LNM led to macroscopic phase separation, and greatly affected the barrier property of the composite films. Figure 4b shows XRD curves of PVA/LNM nanocomposite films. Semi-crystalline PVA involved two obvious diffraction peaks centering at 19.6° and 22.8°, respectively [26]. No sharp diffraction peak was found for LNM because of its amorphous structure [12]. PVA/LNM nanocomposite films exhibited a slight decrease in crystallinity except for LNM-1 compared with pure PVA, demonstrating that introducing proper amount of LNM did not interfere the crystallization behavior of PVA matrix obviously. When the amount of LNM reached 10 wt%, the crystallinity of LNM-10 decreased obviously. These results were further verified by the DSC analysis (Figure 4c). The melting temperature (Tm) of the LNM/PVA nanocomposite films almost maintained constant (224.1 °C for pure PVA, 221.0 °C for LNM-10), but the crystallinity from DSC analysis (Xc-DSC) decreased from 29.2% for pure PVA to 22.8% for LNM-10. The glass transition temperature (Tg) of the nanocomposite increased gradually with the LNM content (Figure 4c), resulting from the strong intermolecular hydrogen bonding interactions between LNM and PVA, which restricted the flexibility of PVA chain segments intensely [27].
11
Figure 4. a) FE-SEM images of pure PVA and PVA nanocomposite films. b) XRD spectra of PVA and PVA nanocomposite films. c) DSC melting curves of PVA and PVA/LNM nanocomposite films.
Figure 5. Micromorphology and roughness of PVA a) and LNM-5 b) films measured by AFM.
As illustrated in Scheme 1, water molecules permeated from one side of the film to 12
the other side through the amorphous region of the semi-crystalline polymers [28]. The water molecules were prone to seek out a path with the least resistance in the process of water vapor transmission. The crystallinity of polymer matrix and morphology of filler in matrix played a vital role in water vapor penetration. The high crystallinity and the microphase separation structure increase the diffusion path of water vapor in the PVA matrix, and the robust hydrogen bonds between filler and PVA matrix also increased the obstacle of water vapor transmission [29]. In addition, the increase in the surface roughness in nanoscale after inducing LNM into PVA is beneficial for preventing the film surface from wetting [30]. Thus, the water vapor barrier properties of PVA/LNM nanocomposites were greatly improved.
Scheme 1. Water vapor barrier mechanism of PVA and PVA/LNM composite film.
3.3 Mechanical properties The stress-strain curves of pristine PVA and PVA/LNM nanocomposites were depicted in Figure 6. The mechanical performance including tensile strength and toughness of PVA/LNM nanocomposites enhanced apparently compared with pristine PVA. Specifically, the tensile strength significantly increased to 91.3 MPa from 72.2 MPa for pure PVA with only 2 wt% loading of LNM. The tensile strength increased continuously with the amount of LNM, and reached the maximum of 94.9 MPa at the addition of 10 wt% (LNM-10) within this study. The elongation at break of the 13
composite film first increased to the maximum value of 225% in LNM-2 but then gradually declined with the LNM addition. The toughness reached the maximum value of 156.4 MJ/m3 in the sample of LNM-2, which was 35% increase compared with 115.8 MJ/m3 of PVA. Increasing the LNM content up to 10 wt% caused a gentle increase of the tensile strength but a severe decline in elongation at break, resulting from the irregular cluster of LNM particles in PVA matrix (Figure 4a). The sample LNM-5 possessed similar mechanical performance as LNM-2, with both excellent strength and toughness, which was in accordance with the dense hydrogen bond-based physical cross-linking network revealed through the solution rheological test shown in Figure 2. The nanophase separation structure in the composite films could also dissipate plenty energy during the stretching process, leading to a ductile fracture, thereby increased the strength and toughness of the composite films [7].
Figure 6. a) Stress-strain curves of pure PVA and PVA/LNM nanocomposite films (Insert: LNM2 before and after fracture). b) Comparison of pure PVA and PVA/LNM nanocomposite films for tensile strength and toughness at 20 % RH.
3.4 UV-shielding and Thermal performance PVA composite films exhibiting good ultraviolet (UV) shielding are attractive in a wide variety of applications [31,32]. As shown in Figure 7a, the neat PVA film performed high transmittance towards UV spectrum (380-190 nm), displaying no UVshielding property. The light transmittance declined successively with the LNM content, 14
especially in the UV spectrum region. Only 1 wt% addition of LNM (LNM-1) led to 100% shielding of the UVB (320-275 nm) and UVC (275-190 nm) spectrum and most of the UVA (380-320 nm) spectrum. The UVA spectrum was totally blocked once the LNM addition reached 5 wt% (LNM-5). The superior UV-shielding property was resulted from the abundant phenolic hydroxyl groups concentrated in the outer surface of LNM particles after self-assembling from aqueous solution [12,33]. Despite the brown color caused by LNM affected the visible light transmittance of PVA films (380 nm ~ 760 nm), the PVA/LNM nanocomposites retained good transparency even with the addition of 5 wt% LNM.
Figure 7. a) UV-vis transmittance curves of PVA and PVA/LNM nanocomposite films. b) TG curves of PVA and PVA/LNM nanocomposites at N2 atmosphere.
As shown in Figure 7b, TG measurement was determined to evaluate the thermal stability of PVA/LNM nanocomposite films. The maximum thermal decomposition rate temperature (Tmax) of PVA/LNM nanocomposite grew with the LNM addition from 262.3 °C of pristine PVA to 273.5 °C of LNM-10, probably because of the strong intermolecular hydrogen bonding interactions between PVA matrix and LNM which strongly restricted the flexibility of PVA chain segments. The aromatic structural units in LNM molecules also played a significant role in the improvement of thermal stability [33]. The melting temperature (200 ~ 250 °C) of PVA is extremely near to its thermal decomposition temperature, causing it hard to yield the products with involuted shapes through melt processing [34]. LNM played a major role in enhancing the thermal 15
performance of PVA nanocomposite films, putting forward a meaningful way for the realization of melt processing for PVA composite materials. 4. Conclusion In summary, high performance PVA/LNM nanocomposites with excellent water vapor barrier and UV-shielding performance were successfully prepared. Biomass lignin nanomicelle (LNM) was introduced into the PVA film to construct nanophase separation structure and strong intermolecular hydrogen bonding interactions. The PVA/LNM composite films exhibited much better water vapor barrier performance than pure PVA, benefiting from the tortuous path effect of LNM in the PVA matrix. The water vapor transmission rate (WVTR) of PVA/LNM nanocomposite films was reduced to only one third of the pristine PVA. Meanwhile, the tensile strength and toughness of PVA nanocomposites were simultaneously enhanced by introducing LNM. Moreover, adding the readily available lignin also improved the thermal stability of PVA nanocomposites and endowed PVA materials with excellent UV-shielding function. The addition of only 1 wt% LNM could achieve the full shielding of UVB and UVC and the majority of UVA shielding. Our work shows a significant strategy for fabricating full biodegradable and highly water vapor barrier PVA materials with excellent mechanical performance for a variety of potential applications such as UVshielding food or medical packaging areas.
Acknowledgments
The authors gratefully thank the National Natural Science Foundation of China (21706082), the Science and Technology Program of Guangzhou (201707020025, 201804010140), and Natural Science Foundation of Guangdong Province (2017B090903003, 2018B030311052) for the financial support.
16
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Highlights
This work presents an innovative design strategy for full biodegradable functional nanocomposite films using cheap and easily available biomass and biodegradable raw materials.
The water vapor barrier performance, mechanical properties as well as the UVshielding performance of PVA film were significantly improved by lignin nanomicelle.
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