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nanocrystalline cellulose nanocomposite films
Journal Pre-proofs Preparation and characterization of polylactic acid/polyaniline/nanocrystalline cellulose nanocomposite films Xiaoyu Wang, Yanjun Tang, Xianmei Zhu, Yiming Zhou, Xinghua Hong PII: DOI: Reference:
S0141-8130(19)35738-1 https://doi.org/10.1016/j.ijbiomac.2019.09.233 BIOMAC 13530
To appear in:
International Journal of Biological Macromolecules
Received Date: Revised Date: Accepted Date:
25 July 2019 15 September 2019 23 September 2019
Please cite this article as: X. Wang, Y. Tang, X. Zhu, Y. Zhou, X. Hong, Preparation and characterization of polylactic acid/polyaniline/nanocrystalline cellulose nanocomposite films, International Journal of Biological Macromolecules (2019), doi: https://doi.org/10.1016/j.ijbiomac.2019.09.233
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© 2019 Published by Elsevier B.V.
Preparation and characterization of polylactic acid/polyaniline/nanocrystalline cellulose nanocomposite films Xiaoyu Wang1, Yanjun Tang1, 2, *, Xianmei Zhu1, Yiming Zhou1, Xinghua Hong1 1
National Engineering Laboratory of Textile Fiber Materials and Processing Technology, Zhejiang Sci-Tech University, Hangzhou 310018, China
2
Pulp and Paper Center, Zhejiang Sci-Tech University, Hangzhou 310013, China
* Correspondence:
[email protected]; Tel.: +86-571-8684-3561.
Abstract Polylactic acid (PLA) serves as an ideal matrix for preparing electrically conductive materials for electrode, electromagnetic shielding and functional biological material application.
Here,
an
environmentally
friendly
PLA/polyaniline
(PANI)/nanocrystalline cellulose (NCC) nanocomposite film was prepared. Effects of NCC loadings on the rheological behavior of PLA/PANI/NCC suspensions and the microscopic, thermal, mechanical and conductive properties of nanocomposite films were investigated. Results revealed that PLA was wrapped with PANI particles and NCC was uniformly distributed on the surface of nanocomposite films. The PLA/PANI/NCC films exhibited an electrical conductivity of up to 2.16 S∙m-1 with 1% NCC dosage. Besides, the viscosity and viscoelasticity of the PLA/PANI/NCC suspensions were decreased and the dispersion stability of the suspensions was improved with the incorporation of NCC. Furthermore, the mechanical strength of PLA/PANI/NCC
nanocomposite
films
was
significant
improved
with
the
reinforcement effect of NCC. In presence of 4% NCC loading, the tensile strength (TS) and tensile modulus (TM) of PLA/PANI/NCC films had an increase of 38.1% and 89.1%, respectively, while the elongation at break (Eb) exhibited a decrease of 27.3%, as compared to that of PLA/PANI. These results demonstrated that the as-prepared PLA/PANI/NCC nanocomposite film may have the potential to be used as a bio-based electrically conductive material.
Key words: Polylactic acid; polyaniline; nanocrystalline cellulose; conductive 1
nanocomposite film.
1. Introduction Polylactic acid (PLA), a widely used commercial biodegradable polymer [1], can be used to replace petro-based plastics and applied in package, textile, medicine and agriculture industries [2]. The various functional materials with PLA as ideal matrix have been concerned due to the biodegradable and environment-friendly nature of PLA. In recent years, imparting PLA with specific properties, such as conductive [3], antibacterial [4] and UV resistant [5], has been the research hotspot. With increasing demand for electronic devices, developing electrically conductive composites with low cost has attracted extensive attention in fields of electrode matrixes, gas/bio-sensor, heat exchangers, electromagnetic shielding [6-8]. Conducting polymers, such as polypyrrole (PPy) [9], polyaniline (PANI) [10] and their derivatives have been found to have excellent electrical characteristics and can be incorporated into the PLA matrixes. Among these conducting polymers, PANI received considerable attention because of its low-cost, simple preparation and unique doping behavior [10-12]. For instance, Razak et al. [13] incorporated PANI fillers into the conductive and highly porous PLA scaffold, which can be used for specific tissue engineering and biomedical applications. However, the application of the PLA incorporated with PANI filler was greatly limited by its hydrophobic properties, poor toughness and impact resistance [14-16]. Such limitations have been considered as the bottleneck for the application of functionalized PLA composites in electric field-application where high mechanical strength is generally required [17]. Thus, it is essential to develop feasible strategy to improve the strength of PLA incorporated with PANI filler. Nanocrystalline cellulose (NCC) can be prepared from a range of bio-based renewable sources, such as wood, cotton and bacterial cellulose by the methods of acid hydrolysis, biological enzymatic hydrolysis and mechanical treatment [18-20]. The
remarkable
mechanical
properties,
special
surface
chemistry,
good
biocompatibility and biodegradability make it an excellent reinforcing filler for 2
nanocomposite materials [21-23]. Kamal et al. [24] studied the rheological, mechanical and crystallization properties of PLA/NCC nanocomposites and found that the well-dispersed NCC nanofiller in PLA composite system can promote the mechanical properties of PLA composite in both the glassy and rubbery regions. In addition, our previous study showed that NCC greatly improved the tensile strength and reduced air permeability chitosan (CH)/guar gum (GG) nanocomposite films [25]. Thus, NCC could be used to promote the mechanical properties of PLA/PANI nanocomposite films. In this study, we developed a feasible approach to synthesize electro-conductive PLA/PANI/NCC nanocomposite film. PANI was incorporated to impart PLA with conductive property, and NCC was added to improve the mechanical strength of the films. The effects of NCC dosage on the rheological behavior of PLA/PANI/NCC suspensions were systematically investigated and the microscopic, thermal, mechanical and conductive properties of corresponding nanocomposite films were further studied. Because of the interaction of PLA, PANI and NCC within nanocomposite films, the electrical conductibility, thermal stability and mechanical property were greatly enhanced. This work could facilitate the design and optimization of PLA-based functional materials in further research.
2. Experimental 2.1 Materials and reagents Polylactic acid (PLA) pellets with a melting point of 176oC were provided by Zhejiang Haizheng Biomaterials Co., Ltd., China. Polyaniline (PANI, 100~500 nm, doped with hydrochloric acid) with a purity of 98% was purchased from Shanghai Macklin Biochemical Co., Ltd., China. Microcrystalline cellulose (MCC) powder was provided
by
Shanghai
Tonnor
Material
Science
Co.,
Ltd.,
China.
N,
N-dimethylformamide (DMF) and Polythylene glycol 6000 (PEG6000) were purchased from Shanghai Aladdin Reagent Co., Ltd., China.
2.2 Preparation of NCC 3
NCC suspension was prepared from sulfuric acid hydrolysis of MCC. Initially, 5.0 g of MCC was slowly added into 50 ml sulfuric acid solution (65 wt%) with vigorous magnetic stirring at 50oC for 90 min. The hydrolysis process was terminated by adding 400 ml distilled water to the reaction mixture. The suspension was diluted and centrifuged at the rate of 11000 rpm for 10 min. Then the NCC suspension was transferred to the dialysis bag (MWCO 8000) and treated in the distilled water for 48 h until neutral pH. Finally, NCC suspension was concentrated by a rotary evaporator (Buchi Rotavapor R-210, Switzerland) at 55oC, 50 mbar.
2.3 Preparation of PLA/PANI/NCC nanocomposite films PLA pellets (oven dried) were dissolved in DMF solution under vigorous magnetic stirring at 65oC for 3 h to obtain a homogeneous PLA solution, followed by adding desired amounts of hydrochloric acid-doped PANI (25 wt% related to PLA) under constant stirring. The mixed suspension was dispersed by ultrasonic treatment for 5 min at 55oC. Then NCC suspension (0~4 wt% related to PLA) was added dropwise into the above PLA/PANI mixture with constant stirring for 6 h at 65 oC. Then, PEG (10 wt% related to PLA) was finally added into the reaction mixture as cosolvent and film-forming agent. Subsequently, the resulted PLA/PANI/NCC suspension (15 ml) was casted onto the Polytetrafluoroethylene (PTFE) dish (Ф=100 mm) to form nanocomposite films. The plates were solidified at room temperature for 24 h, and then over-dried at 60oC for 6 h. Finally, the dried nanocomposite films with the average thickness of 0.08 mm were peeled from the PTFE dish for further characterization.
2.4 Structural characterization The surface morphology of the PLA, PLA/PANI and PLA/PANI/NCC films were performed by using scanning electron microscopy (SEM, ULTRA-55, Japan) with an accelerating voltage of 3.00 kV at room temperature. Before observation, the samples were mounted on a carbon disc with the help of double-sided adhesive tape and sputter-coated with a thin layer of gold. Fourier transform infrared (FTIR) spectra of 4
PLA, PLA/PANI and PLA/PANI/NCC films were recorded using a FTIR analyzer (Nicolet-5700, USA). The analysis was performed within a spectral range of 4000~500 cm-1 for each sample. X-ray diffraction (XRD) patterns of the PLA, PLA/PANI and PLA/PANI/NCC films were obtained using X-ray diffractometer (Thermo ARL XTRA, USA) in the angular range of 10~70o (2θ) at a speed of 2o/min. All the samples were measured at 40 kV and 30 mA. Thermogravimetric analyzer (TGA) of the PLA, PLA/PANI and PLA/PANI/NCC films with various amount of NCC was conducted using a TGA instrument (Perkin Elmer Pyris 1, USA). The test was conducted under a nitrogen atmosphere to prevent thermo-oxidative degradation. The temperature range and heating rate were 25~800oC and 10oC/min, respectively.
2.5 Rheological tests The rheological properties of PLA, PLA/PANI and PLA/PANI/NCC suspensions were performed on a stress-controlled rheometer (Physica MCR 301, Australia) with vane-in-cup geometry. Shear stress and shear viscosity were obtained over a shear rate range from 1 to 1000 s-1. The dynamic viscoelasticity (the storage modulus, the loss modulus, and the complex viscosity) was measured as a function of angular frequency which was varied from 1 to 100 rad/s. The oscillatory measurement was conducted at a given strain level of 1.0% within the linear viscoelastic region, as determined by dynamic strain sweep experiments.
2.6 Mechanical analysis The PLA/PANI and PLA/PANI/NCC films were treated for 24 h at 23 ± 1oC and 50 ± 2% relative humidity prior to mechanical measurement. The tensile strength (TS), tensile modulus (TM) and elongation at break (Eb) of nanocomposite films were determined according to the ASTM D882 standard method by using a Universal Testing Machine (Instro-3369, USA) equipped with a load cell of 100 N. The initial grip separation was set to 25 mm and all samples were cut into rectangles (50×10 mm) to fit the tensile grips. The samples were mounted in the film-extension grips and stretched at a rate of 1 mm/min. 5
2.7 Electrical conductivity measurements The electric resistances (R) of PLA/PANI/NCC film samples were measured using a digital multimeter (DM3068, China). Before tests, the samples were cut into rectangles (50×10 mm). The electrical resistivity (ρ) was calculated using the following formula: ρ=RS/L, where S and L are the cross-sectional area and length of the rectangles, respectively. The electrical conductivity (σ) is defined as the ratio of the current density to the electric field strength, and the value of σ is the reciprocal (inverse) of electrical resistivity (ρ).
3. Results and Discussion 3.1 Microstructure characterization The morphology of the obtained PLA, PLA/PANI and PLA/PANI/NCC films were investigated, and the SEM images were shown in Fig. 1. As can be seen in Fig 1a, the surface of the PLA film is relatively smooth and contains micropores with diameter of 1~10 μm. The pore formation could be attributed to the volatilization of organic solvent in the process of film casting [26]. For PLA/PANI/NCC nanocomposite film in Fig. 1b, it was found that PLA was wrapped with short rod-like PANI particles and the nano-sized PANI particles were partially gathered because of the high surface activity of nanoparticles and the uneven distribution of positive charge derived from the protonation of the N atoms in the molecular chain of HCl-doped PANI [27-29]. Moreover, the rod-like NCCs with a length of 200~500 nm were uniformly distributed on the surface of nanocomposite film (Fig. 1b (insert)). The uniform distribution of NCC could reinforce the PLA/PANI system. In addition, the photographs of the as-prepared PLA film and PLA/PANI/NCC nanocomposite film were taken to assess the appearance change of the film (Fig. 2). The PLA film was transparent, while the nanocomposite film was dark-colored, which further confirmed the presence of PANI.
3.2 FTIR analysis FTIR spectroscopy can be used to analysis the change of structure characteristics of 6
the films before and after modification with PANI and NCC. The FTIR spectra of PLA, PLA/PANI and PLA/PANI/NCC films were displayed in Fig. 3. The main characteristic peaks of PLA film were located at 1754 cm-1 for C=O vibration, 1452 cm-1 for CH3 asymmetrical bending, 1383 cm-1 for C–H bending, 1181 cm-1 for C–O asymmetrical stretching and CH3 twisting, and 1083 cm-1 for C–O–C stretching. This is consistent to what has been reported by Liu et al. [30]. For PLA/PANI composite films, the absorption peaks at 1557 cm-1 and 1484 cm-1 can be assigned to C=C stretching of quinone rings and benzene rings in PANI structure, respectively [31], whereas the peaks at 1302 cm-1 and 1024 cm-1 were ascribed to the stretching vibration of benzene and C–H out-of-plane bending of proton in HCl-doped PANI, respectively [32,33]. The blue-shifted peak at 1756 cm-1 and red-shifted peak at 1179 cm-1 indicates the presence of PLA. This suggests that PANI has been successfully introduced into the PLA composites. In the case of PLA/PANI/NCC nanocomposite film, the characteristic peaks of PANI were weakened due to the NCC loading. Furthermore, a strong band appeared at approximately 3434 cm-1, which was primarily related to the stretching vibration of O–H groups from NCC [25, 34]. All of the results indicated that the as-prepared PLA/PANI/NCC nanocomposite films remained the inherent molecular structures of PLA, PANI and NCC. According to FTIR spectra, as the structure characteristics of PANI were not affected, the electrical conductivity of PLA/PANI/NCC nanocomposite films was expected to be maintained.
3.3 XRD analysis The crystal structure of PLA, PLA/PANI and PLA/PANI/NCC films were analyzed by X-ray diffraction, and the results were shown in Fig. 4. The XRD pattern of PLA exhibited characteristic diffraction peaks at 2θ = 16.8o and 18.8o, which correspond to the (200) and (100) crystal planes of PLA, respectively [34]. However, for PLA/PANI composite film, the intensity of the diffraction peaks of PLA were apparently weakened or even vanished. A new and relatively broad peak associated with HCl-doped PANI appeared between 2θ=20o and 30o [35], indicating the significant impacts of PANI particles on the crystallization behavior of PLA composites. The 7
main reason might be due to the formation of a structure similar to “quaternary ammonium salt” that resulted from the protonation of imine nitrogen atom (=N–) on PANI chain and anions (Cl–) from the doping acid forming. Besides, as observed by SEM images, the PLA was wrapped with a layer of PANI particles to form a core-shell structure and the shielding action of PANI particles coating on PLA will have an effect on the crystal form of PLA. Furthermore, PLA/PANI/NCC nanocomposite films with different amount of NCC loading were found in the similar diffraction
patterns,
suggesting
the
presence
of
PANI
inhibited
the
trans-crystallization effect caused by NCC nanoparticles [36].
3.4 Thermal analysis TGA was used to study the thermal degradation behavior of PLA, PLA/PANI and PLA/PANI/NCC films, and the results were shown in Fig. 5. The TG curve of PLA/PANI composite film (Fig. 5a) could be generally divided into three regions. The initial weight loss occurred at 25~150oC was contributed by the evaporation of absorbed water and residual DMF solvent. The second weight loss observed at 150~400oC was mainly caused by the removal of dopant (HCl) and the partial decomposition of PLA, PANI polymer to oligomers. [37]. The weight loss showed at 400~625oC was ascribed to the sustained de-doping process and the breaking of molecular chains of polymer and oligomers (PANI and PLA) into small and simple compounds such as CH4, H2 and NH3 [35, 37, 38]. The thermal stability of PLA/PANI composite film was found to be improved by the incorporation of PANI particles as the maximum weight loss rate of PLA/PANI took place at higher temperature (565oC) compared to that of PLA film (501oC) (Fig. 5b). This can be attributed to the cationization of PANI particles by the protonation of N atom on PANI chain and the encapsulation of PLA by PANI, which can stabilize the nanocomposite structure and hinder further nanocomposite decomposition. Similar results had been reported in the literatures regarding PLA/ZrO2 [39] and PANI/Al2O3 composites [40]. However, with incorporation of NCC, the PLA/PANI/NCC nanocomposites performed poor thermal stability as shown in Fig. 5b. The total weight loss of the nanocomposites with NCC 8
loading of 2% and 4% were to 63.2% and 90.1%, respectively. The reasons might be concluded as follows: 1) the nano-size effects of NCC lead to large specific surface area and high reactivity; 2) the electrostatic interaction between sulfonic groups of NCC and PANI particles [41] promoted the thermal degradation of nanocomposite films.
3.5 Steady-state rheological behavior Rheological measurement could be used to analyze the interactions between polymers and other ingredients [42-44]. Fig. 6 showed results of viscosity and shear stress as a function of shear rate for PLA solution, PLA/PANI and PLA/PANI/NCC nanocomposite suspensions. In the shear rate range of 1~100 s-1, the nanocomposite suspensions roughly exhibited a shear-thinning behavior with increasing shear rate because of ordered arrangement of rod-like NCC and PANI particles [45]. However, the nanocomposite suspensions exhibited a shear-thickening behavior in the range of 100~1000 s-1, indicating that the polymer chains were stretched under the higher shear rate to form the association between segments, leading to the increased viscosity [46]. Moreover, as compared to the viscosity of PLA solution, the viscosity of nanocomposite suspensions was increased because of the addition of PANI particles, which indicated that more solid fillers added into the composites may require a greater shearing rate to achieve the critical shear flow shear. In case of PLA/APNI/NCC suspension, the viscosity was decreased with the increase of NCC loading. At the shear rate of 144 s-1, the viscosity of the nanocomposite suspensions decreased from 1.430 (without NCC) to 1.250 (2% NCC) and 0.989 Pa۰s (4% NCC), respectively, suggesting that the presence of NCC improved the dispersion of the mixture system. This was attributed to the electrostatic repulsion between NCC nanoparticles, as reported in the work of Zhang et al. [44]. Additionally, the curves of shear stress as a function of shear rate were shown in Fig. 6b. Although all suspensions displayed a regular and consecutive increase in shear stress with increasing shear rate, the shear stress of PLA/PANI/NCC suspensions also showed a decline with the increase of NCC loading. 9
3.6 Dynamic rheological behavior Viscoelasticity measurement has been used to evaluate the flow behavior of composite suspensions [44-47]. The storage modulus (G′), loss modulus (G′′) and complex viscosity for PLA, PLA/PANI and PLA/PANI/NCC suspensions were investigated, and the results were shown in Fig. 7. It can be seen from Fig.7a, b that the G′′ was significantly higher than the G′ within the angular frequency range of 1~100 rad۰s-1, indicating a typical viscoelastic liquid-like behavior for all suspensions. The slope difference in G′ or G′′ curves can reflect the compatibility between polymers in the composite system to indicate the well-dispersion or agglomeration of particles [47]. In present work, all the G′′ curves were similar in the slopes at the same angular frequency, which implied that the existence of PLA, PANI fillers have no influence on the compatibility of PLA system. Complex viscosity is a significant parameter to study structure relationship in polymer composites [43]. Fig. 7c depicted the complex viscosity of PLA and PLA/PANI/NCC nanocomposites. It was observed that the complex viscosity exhibited an apparent increase within the range of 10~100 rad/s in the presence of PANI particles, indicating that the “core-shell” (wrap PLA with PANI particles) and “quaternary ammonium salt” (changed chemical environment of N atom in PANI main chain) structures might contribute to the intermolecular entanglements of PLA and PANI chains. However, the addition of NCC decreased the complex viscosity, suggesting that NCC can perform as a lubricant agent to smooth the molecular chains of PLA and PANI, thus increasing the flexibility of polymer chains. This result was in accordance with the variation trend in steady shear viscosity as discussed above.
3.7 Mechanical properties The reinforcing effect of cellulose nanoparticles on tensile strength and toughness of PLA nanocomposites was widely reported, and similar results were demonstrated. The tensile strength (TS), elongation at break (Eb) and tensile modulus (TM) of PLA/PANI nanocomposite films with various amounts of NCC were shown in Fig. 8. 10
A significant improvement in TS and TM was achieved with the addition of NCC. With 1%, 2%, 3% and 4% NCC loading, the TS of PLA/PANI composite film was increased to 20.4 MPa, 24.0 MPa, 25.7 MPa and 26.1 MPa, respectively. The improvement might be attributed to the reinforcement effect of NCC fillers as NCC might have an influence on the dispersion uniformity, chain entanglement and interfacial adhesion of the composite system. In addition, such improvement might be also resulted from the presence of PEG as film-forming chemical in all nanocomposite films, as the hydroxyl groups in PEG was helpful to form hydrogen bonds with carboxyl groups in PLA and hydroxyl groups in NCC [34, 47, 48]. Moreover, as illustrated in Fig. 8b, Eb was slightly decreased from 2.2% (without NCC) to 1.6% (with 4% NCC) with the increase of NCC loading, implying that the rod-like NCC dispersed well in the mixture system, causing a restriction in the motion of PLA and PANI chains.
3.8 Electrical conductivity The conductivity of the polymer is mainly dependent on the degree of polymer conjugation, chain length and chain ordering [50, 51]. It is thus likely that the addition of NCC into the nanocomposite will affect the electrical conductivity. To analyze such effect, the electrical conductivity of PLA/PANI/NCC nanocomposite films with different NCC loading was evaluated. As expected, the PLA film was non-conductive, while addition of PANI in PLA composite can impart PLA composite film with conductive function. With increasing loading of NCC, the electrical conductivity decreased gradually (Fig. 9). The electrical conductivity of PLA/PANI/NCC nanocomposite film was reduced from 3.42 (without NCC) to 1.5 S∙m-1 (4% NCC loading). The result could be related with the electrostatic attraction between the negative charged group on NCC [18-20] and positive charged group on HCl-doped PANI [52], which will increase the resistance of electron movement i the PANI chain and thus decreasing the electrical conductivity of PLA/PANI/NCC film. Overall, the obtained electrical conductivity of PLA/PANI/NCC nanocomposite films ranging from 2.16 to 1.5 S∙m-1 (with 1~4% NCC loading) could be used in many areas, such 11
as stimulating cell proliferation (0.1 S∙m-1) [53].
4. Conclusions Conductive PLA/PANI/NCC nanocomposite films have been successfully prepared through a solvent casting method and the characterization of nanocomposite films as a function of NCC loading was investigated. It was found that the as-prepared PLA/PANI/NCC films exhibited an electrical conductivity of up to 2.16 S∙m -1 with 1% NCC dosage. The shear viscosity, shear stress and viscoelasticity of nanocomposite suspensions decreased and the dispersion stability of suspensions was improved with the increased NCC loading. The presence of NCC exerted a significantly positive effect on the mechanical strength of the nanocomposite films. Especially, compared to PLA/PANI film, the film with 4% NCC increased 38.1% and 89.1% in TS and TM, and decreased 27.3% in Eb. The obtained PLA-based nanocomposite films showed excellent properties for potential application in the fields of electrode, electromagnetic shielding and biomaterials.
Acknowledgments This work was financially supported by the Public Projects of Zhejiang Province (Grant No. LGG19C160002), Open Foundation of Key Laboratory of Pulp and Paper Science & Technology of Ministry of Education of China (Grant No. KF201503), Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering, Technology, Zhejiang Open Foundation of the Most Important Subjects (Grant No. 2016KF01), 521 Talent Cultivation Program of Zhejiang Sci-Tech University (Grant No. 11110132521310).
Author Contributions Yanjun Tang and Xiaoyu Wang conceived and designed the experiments; Xiaoyu Wang, Xianmei Zhu and Yiming Zhou performed the experiments and analyzed the date; Xiaoyu Wang and Yanjun Tang wrote the paper. All authors reviewed the manuscript. 12
Conflicts of Interest The authors declare no conflict of interest.
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16
Research highlights
An environmentally friendly and electrically conductive PLA/PANI/NCC nanocomposite film was developed.
The thermal stability of the PLA/PANI composite films was improved with PANI incorporation.
Increased NCC loading improved the tensile strength and tensile modulus of PLA/PANI/NCC nanocomposite films.
NCC loading led to decreased viscosity and viscoelasticity of PLA/PANI/NCC nanocomposite suspensions.
17
FIGURE CAPTIONS Fig. 1 SEM images of (a) pure PLA film and (b) PLA/PANI/NCC nanocomposite film. Fig. 2 Photographs of pure PLA film and PLA/PANI/NCC nanocomposite film. Fig. 3 FTIR spectra of PLA film, PLA/PANI and PLA/PANI/NCC nanocomposite films. Fig. 4 XRD patterns of PLA film, PLA/PANI and PLA/PANI/NCC nanocomposite films. Fig. 5 TGA curves of PLA film, PLA/PANI and PLA/PANI/NCC nanocomposite films. Fig. 6 Effects on (a) viscosity and (b) shear stress of PLA solution, PLA/PANI and PLA/PANI/NCC nanocomposite suspensions at various shear rates. Fig. 7 (a) loss modulus, (b) storage modulus and (c) complex viscosity of PLA solution, PLA/PANI and PLA/PANI/NCC nanocomposite suspensions at different angular frequencies. Fig. 8 (a) TS, (b) Eb and (c) TM of PLA/PANI/NCC nanocomposite films as a function of various NCC amounts. Fig. 9 Electrical conductivity of PLA/PANI/NCC nanocomposite films as a function of different NCC amounts.
18
Fig. 1
19
Fig. 2
20
Transmittance
1557 1484
1302 1024
1754 PLA PLA/PANI PLA/PANI/NCC
2500
2000
1452
1383 1181 1083
1500
1000
Wave numbers (cm-1)
Transmittance
3434
4000
PLA PLA/PANI PLA/PANI/NCC 3500
3000
2500
2000
1500
Wave numbers (cm-1) Fig. 3
21
1000
Intensity
Pure PLA PLA/PANI/NCC(0%) PLA/PANI/NCC(2%) PLA/PANI/NCC(4%)
10
20
30
40
50
2 theta (degree) Fig. 4
22
60
70
Fig. 5
23
Fig. 6
24
25
Fig. 7
26
27
Fig. 8
28
Fig. 9
29
S0141-8130(19)35738-1 https://doi.org/10.1016/j.ijbiomac.2019.09.233 BIOMAC 13530
To appear in:
International Journal of Biological Macromolecules
Received Date: Revised Date: Accepted Date:
25 July 2019 15 September 2019 23 September 2019
Please cite this article as: X. Wang, Y. Tang, X. Zhu, Y. Zhou, X. Hong, Preparation and characterization of polylactic acid/polyaniline/nanocrystalline cellulose nanocomposite films, International Journal of Biological Macromolecules (2019), doi: https://doi.org/10.1016/j.ijbiomac.2019.09.233
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© 2019 Published by Elsevier B.V.
Preparation and characterization of polylactic acid/polyaniline/nanocrystalline cellulose nanocomposite films Xiaoyu Wang1, Yanjun Tang1, 2, *, Xianmei Zhu1, Yiming Zhou1, Xinghua Hong1 1
National Engineering Laboratory of Textile Fiber Materials and Processing Technology, Zhejiang Sci-Tech University, Hangzhou 310018, China
2
Pulp and Paper Center, Zhejiang Sci-Tech University, Hangzhou 310013, China
* Correspondence:
[email protected]; Tel.: +86-571-8684-3561.
Abstract Polylactic acid (PLA) serves as an ideal matrix for preparing electrically conductive materials for electrode, electromagnetic shielding and functional biological material application.
Here,
an
environmentally
friendly
PLA/polyaniline
(PANI)/nanocrystalline cellulose (NCC) nanocomposite film was prepared. Effects of NCC loadings on the rheological behavior of PLA/PANI/NCC suspensions and the microscopic, thermal, mechanical and conductive properties of nanocomposite films were investigated. Results revealed that PLA was wrapped with PANI particles and NCC was uniformly distributed on the surface of nanocomposite films. The PLA/PANI/NCC films exhibited an electrical conductivity of up to 2.16 S∙m-1 with 1% NCC dosage. Besides, the viscosity and viscoelasticity of the PLA/PANI/NCC suspensions were decreased and the dispersion stability of the suspensions was improved with the incorporation of NCC. Furthermore, the mechanical strength of PLA/PANI/NCC
nanocomposite
films
was
significant
improved
with
the
reinforcement effect of NCC. In presence of 4% NCC loading, the tensile strength (TS) and tensile modulus (TM) of PLA/PANI/NCC films had an increase of 38.1% and 89.1%, respectively, while the elongation at break (Eb) exhibited a decrease of 27.3%, as compared to that of PLA/PANI. These results demonstrated that the as-prepared PLA/PANI/NCC nanocomposite film may have the potential to be used as a bio-based electrically conductive material.
Key words: Polylactic acid; polyaniline; nanocrystalline cellulose; conductive 1
nanocomposite film.
1. Introduction Polylactic acid (PLA), a widely used commercial biodegradable polymer [1], can be used to replace petro-based plastics and applied in package, textile, medicine and agriculture industries [2]. The various functional materials with PLA as ideal matrix have been concerned due to the biodegradable and environment-friendly nature of PLA. In recent years, imparting PLA with specific properties, such as conductive [3], antibacterial [4] and UV resistant [5], has been the research hotspot. With increasing demand for electronic devices, developing electrically conductive composites with low cost has attracted extensive attention in fields of electrode matrixes, gas/bio-sensor, heat exchangers, electromagnetic shielding [6-8]. Conducting polymers, such as polypyrrole (PPy) [9], polyaniline (PANI) [10] and their derivatives have been found to have excellent electrical characteristics and can be incorporated into the PLA matrixes. Among these conducting polymers, PANI received considerable attention because of its low-cost, simple preparation and unique doping behavior [10-12]. For instance, Razak et al. [13] incorporated PANI fillers into the conductive and highly porous PLA scaffold, which can be used for specific tissue engineering and biomedical applications. However, the application of the PLA incorporated with PANI filler was greatly limited by its hydrophobic properties, poor toughness and impact resistance [14-16]. Such limitations have been considered as the bottleneck for the application of functionalized PLA composites in electric field-application where high mechanical strength is generally required [17]. Thus, it is essential to develop feasible strategy to improve the strength of PLA incorporated with PANI filler. Nanocrystalline cellulose (NCC) can be prepared from a range of bio-based renewable sources, such as wood, cotton and bacterial cellulose by the methods of acid hydrolysis, biological enzymatic hydrolysis and mechanical treatment [18-20]. The
remarkable
mechanical
properties,
special
surface
chemistry,
good
biocompatibility and biodegradability make it an excellent reinforcing filler for 2
nanocomposite materials [21-23]. Kamal et al. [24] studied the rheological, mechanical and crystallization properties of PLA/NCC nanocomposites and found that the well-dispersed NCC nanofiller in PLA composite system can promote the mechanical properties of PLA composite in both the glassy and rubbery regions. In addition, our previous study showed that NCC greatly improved the tensile strength and reduced air permeability chitosan (CH)/guar gum (GG) nanocomposite films [25]. Thus, NCC could be used to promote the mechanical properties of PLA/PANI nanocomposite films. In this study, we developed a feasible approach to synthesize electro-conductive PLA/PANI/NCC nanocomposite film. PANI was incorporated to impart PLA with conductive property, and NCC was added to improve the mechanical strength of the films. The effects of NCC dosage on the rheological behavior of PLA/PANI/NCC suspensions were systematically investigated and the microscopic, thermal, mechanical and conductive properties of corresponding nanocomposite films were further studied. Because of the interaction of PLA, PANI and NCC within nanocomposite films, the electrical conductibility, thermal stability and mechanical property were greatly enhanced. This work could facilitate the design and optimization of PLA-based functional materials in further research.
2. Experimental 2.1 Materials and reagents Polylactic acid (PLA) pellets with a melting point of 176oC were provided by Zhejiang Haizheng Biomaterials Co., Ltd., China. Polyaniline (PANI, 100~500 nm, doped with hydrochloric acid) with a purity of 98% was purchased from Shanghai Macklin Biochemical Co., Ltd., China. Microcrystalline cellulose (MCC) powder was provided
by
Shanghai
Tonnor
Material
Science
Co.,
Ltd.,
China.
N,
N-dimethylformamide (DMF) and Polythylene glycol 6000 (PEG6000) were purchased from Shanghai Aladdin Reagent Co., Ltd., China.
2.2 Preparation of NCC 3
NCC suspension was prepared from sulfuric acid hydrolysis of MCC. Initially, 5.0 g of MCC was slowly added into 50 ml sulfuric acid solution (65 wt%) with vigorous magnetic stirring at 50oC for 90 min. The hydrolysis process was terminated by adding 400 ml distilled water to the reaction mixture. The suspension was diluted and centrifuged at the rate of 11000 rpm for 10 min. Then the NCC suspension was transferred to the dialysis bag (MWCO 8000) and treated in the distilled water for 48 h until neutral pH. Finally, NCC suspension was concentrated by a rotary evaporator (Buchi Rotavapor R-210, Switzerland) at 55oC, 50 mbar.
2.3 Preparation of PLA/PANI/NCC nanocomposite films PLA pellets (oven dried) were dissolved in DMF solution under vigorous magnetic stirring at 65oC for 3 h to obtain a homogeneous PLA solution, followed by adding desired amounts of hydrochloric acid-doped PANI (25 wt% related to PLA) under constant stirring. The mixed suspension was dispersed by ultrasonic treatment for 5 min at 55oC. Then NCC suspension (0~4 wt% related to PLA) was added dropwise into the above PLA/PANI mixture with constant stirring for 6 h at 65 oC. Then, PEG (10 wt% related to PLA) was finally added into the reaction mixture as cosolvent and film-forming agent. Subsequently, the resulted PLA/PANI/NCC suspension (15 ml) was casted onto the Polytetrafluoroethylene (PTFE) dish (Ф=100 mm) to form nanocomposite films. The plates were solidified at room temperature for 24 h, and then over-dried at 60oC for 6 h. Finally, the dried nanocomposite films with the average thickness of 0.08 mm were peeled from the PTFE dish for further characterization.
2.4 Structural characterization The surface morphology of the PLA, PLA/PANI and PLA/PANI/NCC films were performed by using scanning electron microscopy (SEM, ULTRA-55, Japan) with an accelerating voltage of 3.00 kV at room temperature. Before observation, the samples were mounted on a carbon disc with the help of double-sided adhesive tape and sputter-coated with a thin layer of gold. Fourier transform infrared (FTIR) spectra of 4
PLA, PLA/PANI and PLA/PANI/NCC films were recorded using a FTIR analyzer (Nicolet-5700, USA). The analysis was performed within a spectral range of 4000~500 cm-1 for each sample. X-ray diffraction (XRD) patterns of the PLA, PLA/PANI and PLA/PANI/NCC films were obtained using X-ray diffractometer (Thermo ARL XTRA, USA) in the angular range of 10~70o (2θ) at a speed of 2o/min. All the samples were measured at 40 kV and 30 mA. Thermogravimetric analyzer (TGA) of the PLA, PLA/PANI and PLA/PANI/NCC films with various amount of NCC was conducted using a TGA instrument (Perkin Elmer Pyris 1, USA). The test was conducted under a nitrogen atmosphere to prevent thermo-oxidative degradation. The temperature range and heating rate were 25~800oC and 10oC/min, respectively.
2.5 Rheological tests The rheological properties of PLA, PLA/PANI and PLA/PANI/NCC suspensions were performed on a stress-controlled rheometer (Physica MCR 301, Australia) with vane-in-cup geometry. Shear stress and shear viscosity were obtained over a shear rate range from 1 to 1000 s-1. The dynamic viscoelasticity (the storage modulus, the loss modulus, and the complex viscosity) was measured as a function of angular frequency which was varied from 1 to 100 rad/s. The oscillatory measurement was conducted at a given strain level of 1.0% within the linear viscoelastic region, as determined by dynamic strain sweep experiments.
2.6 Mechanical analysis The PLA/PANI and PLA/PANI/NCC films were treated for 24 h at 23 ± 1oC and 50 ± 2% relative humidity prior to mechanical measurement. The tensile strength (TS), tensile modulus (TM) and elongation at break (Eb) of nanocomposite films were determined according to the ASTM D882 standard method by using a Universal Testing Machine (Instro-3369, USA) equipped with a load cell of 100 N. The initial grip separation was set to 25 mm and all samples were cut into rectangles (50×10 mm) to fit the tensile grips. The samples were mounted in the film-extension grips and stretched at a rate of 1 mm/min. 5
2.7 Electrical conductivity measurements The electric resistances (R) of PLA/PANI/NCC film samples were measured using a digital multimeter (DM3068, China). Before tests, the samples were cut into rectangles (50×10 mm). The electrical resistivity (ρ) was calculated using the following formula: ρ=RS/L, where S and L are the cross-sectional area and length of the rectangles, respectively. The electrical conductivity (σ) is defined as the ratio of the current density to the electric field strength, and the value of σ is the reciprocal (inverse) of electrical resistivity (ρ).
3. Results and Discussion 3.1 Microstructure characterization The morphology of the obtained PLA, PLA/PANI and PLA/PANI/NCC films were investigated, and the SEM images were shown in Fig. 1. As can be seen in Fig 1a, the surface of the PLA film is relatively smooth and contains micropores with diameter of 1~10 μm. The pore formation could be attributed to the volatilization of organic solvent in the process of film casting [26]. For PLA/PANI/NCC nanocomposite film in Fig. 1b, it was found that PLA was wrapped with short rod-like PANI particles and the nano-sized PANI particles were partially gathered because of the high surface activity of nanoparticles and the uneven distribution of positive charge derived from the protonation of the N atoms in the molecular chain of HCl-doped PANI [27-29]. Moreover, the rod-like NCCs with a length of 200~500 nm were uniformly distributed on the surface of nanocomposite film (Fig. 1b (insert)). The uniform distribution of NCC could reinforce the PLA/PANI system. In addition, the photographs of the as-prepared PLA film and PLA/PANI/NCC nanocomposite film were taken to assess the appearance change of the film (Fig. 2). The PLA film was transparent, while the nanocomposite film was dark-colored, which further confirmed the presence of PANI.
3.2 FTIR analysis FTIR spectroscopy can be used to analysis the change of structure characteristics of 6
the films before and after modification with PANI and NCC. The FTIR spectra of PLA, PLA/PANI and PLA/PANI/NCC films were displayed in Fig. 3. The main characteristic peaks of PLA film were located at 1754 cm-1 for C=O vibration, 1452 cm-1 for CH3 asymmetrical bending, 1383 cm-1 for C–H bending, 1181 cm-1 for C–O asymmetrical stretching and CH3 twisting, and 1083 cm-1 for C–O–C stretching. This is consistent to what has been reported by Liu et al. [30]. For PLA/PANI composite films, the absorption peaks at 1557 cm-1 and 1484 cm-1 can be assigned to C=C stretching of quinone rings and benzene rings in PANI structure, respectively [31], whereas the peaks at 1302 cm-1 and 1024 cm-1 were ascribed to the stretching vibration of benzene and C–H out-of-plane bending of proton in HCl-doped PANI, respectively [32,33]. The blue-shifted peak at 1756 cm-1 and red-shifted peak at 1179 cm-1 indicates the presence of PLA. This suggests that PANI has been successfully introduced into the PLA composites. In the case of PLA/PANI/NCC nanocomposite film, the characteristic peaks of PANI were weakened due to the NCC loading. Furthermore, a strong band appeared at approximately 3434 cm-1, which was primarily related to the stretching vibration of O–H groups from NCC [25, 34]. All of the results indicated that the as-prepared PLA/PANI/NCC nanocomposite films remained the inherent molecular structures of PLA, PANI and NCC. According to FTIR spectra, as the structure characteristics of PANI were not affected, the electrical conductivity of PLA/PANI/NCC nanocomposite films was expected to be maintained.
3.3 XRD analysis The crystal structure of PLA, PLA/PANI and PLA/PANI/NCC films were analyzed by X-ray diffraction, and the results were shown in Fig. 4. The XRD pattern of PLA exhibited characteristic diffraction peaks at 2θ = 16.8o and 18.8o, which correspond to the (200) and (100) crystal planes of PLA, respectively [34]. However, for PLA/PANI composite film, the intensity of the diffraction peaks of PLA were apparently weakened or even vanished. A new and relatively broad peak associated with HCl-doped PANI appeared between 2θ=20o and 30o [35], indicating the significant impacts of PANI particles on the crystallization behavior of PLA composites. The 7
main reason might be due to the formation of a structure similar to “quaternary ammonium salt” that resulted from the protonation of imine nitrogen atom (=N–) on PANI chain and anions (Cl–) from the doping acid forming. Besides, as observed by SEM images, the PLA was wrapped with a layer of PANI particles to form a core-shell structure and the shielding action of PANI particles coating on PLA will have an effect on the crystal form of PLA. Furthermore, PLA/PANI/NCC nanocomposite films with different amount of NCC loading were found in the similar diffraction
patterns,
suggesting
the
presence
of
PANI
inhibited
the
trans-crystallization effect caused by NCC nanoparticles [36].
3.4 Thermal analysis TGA was used to study the thermal degradation behavior of PLA, PLA/PANI and PLA/PANI/NCC films, and the results were shown in Fig. 5. The TG curve of PLA/PANI composite film (Fig. 5a) could be generally divided into three regions. The initial weight loss occurred at 25~150oC was contributed by the evaporation of absorbed water and residual DMF solvent. The second weight loss observed at 150~400oC was mainly caused by the removal of dopant (HCl) and the partial decomposition of PLA, PANI polymer to oligomers. [37]. The weight loss showed at 400~625oC was ascribed to the sustained de-doping process and the breaking of molecular chains of polymer and oligomers (PANI and PLA) into small and simple compounds such as CH4, H2 and NH3 [35, 37, 38]. The thermal stability of PLA/PANI composite film was found to be improved by the incorporation of PANI particles as the maximum weight loss rate of PLA/PANI took place at higher temperature (565oC) compared to that of PLA film (501oC) (Fig. 5b). This can be attributed to the cationization of PANI particles by the protonation of N atom on PANI chain and the encapsulation of PLA by PANI, which can stabilize the nanocomposite structure and hinder further nanocomposite decomposition. Similar results had been reported in the literatures regarding PLA/ZrO2 [39] and PANI/Al2O3 composites [40]. However, with incorporation of NCC, the PLA/PANI/NCC nanocomposites performed poor thermal stability as shown in Fig. 5b. The total weight loss of the nanocomposites with NCC 8
loading of 2% and 4% were to 63.2% and 90.1%, respectively. The reasons might be concluded as follows: 1) the nano-size effects of NCC lead to large specific surface area and high reactivity; 2) the electrostatic interaction between sulfonic groups of NCC and PANI particles [41] promoted the thermal degradation of nanocomposite films.
3.5 Steady-state rheological behavior Rheological measurement could be used to analyze the interactions between polymers and other ingredients [42-44]. Fig. 6 showed results of viscosity and shear stress as a function of shear rate for PLA solution, PLA/PANI and PLA/PANI/NCC nanocomposite suspensions. In the shear rate range of 1~100 s-1, the nanocomposite suspensions roughly exhibited a shear-thinning behavior with increasing shear rate because of ordered arrangement of rod-like NCC and PANI particles [45]. However, the nanocomposite suspensions exhibited a shear-thickening behavior in the range of 100~1000 s-1, indicating that the polymer chains were stretched under the higher shear rate to form the association between segments, leading to the increased viscosity [46]. Moreover, as compared to the viscosity of PLA solution, the viscosity of nanocomposite suspensions was increased because of the addition of PANI particles, which indicated that more solid fillers added into the composites may require a greater shearing rate to achieve the critical shear flow shear. In case of PLA/APNI/NCC suspension, the viscosity was decreased with the increase of NCC loading. At the shear rate of 144 s-1, the viscosity of the nanocomposite suspensions decreased from 1.430 (without NCC) to 1.250 (2% NCC) and 0.989 Pa۰s (4% NCC), respectively, suggesting that the presence of NCC improved the dispersion of the mixture system. This was attributed to the electrostatic repulsion between NCC nanoparticles, as reported in the work of Zhang et al. [44]. Additionally, the curves of shear stress as a function of shear rate were shown in Fig. 6b. Although all suspensions displayed a regular and consecutive increase in shear stress with increasing shear rate, the shear stress of PLA/PANI/NCC suspensions also showed a decline with the increase of NCC loading. 9
3.6 Dynamic rheological behavior Viscoelasticity measurement has been used to evaluate the flow behavior of composite suspensions [44-47]. The storage modulus (G′), loss modulus (G′′) and complex viscosity for PLA, PLA/PANI and PLA/PANI/NCC suspensions were investigated, and the results were shown in Fig. 7. It can be seen from Fig.7a, b that the G′′ was significantly higher than the G′ within the angular frequency range of 1~100 rad۰s-1, indicating a typical viscoelastic liquid-like behavior for all suspensions. The slope difference in G′ or G′′ curves can reflect the compatibility between polymers in the composite system to indicate the well-dispersion or agglomeration of particles [47]. In present work, all the G′′ curves were similar in the slopes at the same angular frequency, which implied that the existence of PLA, PANI fillers have no influence on the compatibility of PLA system. Complex viscosity is a significant parameter to study structure relationship in polymer composites [43]. Fig. 7c depicted the complex viscosity of PLA and PLA/PANI/NCC nanocomposites. It was observed that the complex viscosity exhibited an apparent increase within the range of 10~100 rad/s in the presence of PANI particles, indicating that the “core-shell” (wrap PLA with PANI particles) and “quaternary ammonium salt” (changed chemical environment of N atom in PANI main chain) structures might contribute to the intermolecular entanglements of PLA and PANI chains. However, the addition of NCC decreased the complex viscosity, suggesting that NCC can perform as a lubricant agent to smooth the molecular chains of PLA and PANI, thus increasing the flexibility of polymer chains. This result was in accordance with the variation trend in steady shear viscosity as discussed above.
3.7 Mechanical properties The reinforcing effect of cellulose nanoparticles on tensile strength and toughness of PLA nanocomposites was widely reported, and similar results were demonstrated. The tensile strength (TS), elongation at break (Eb) and tensile modulus (TM) of PLA/PANI nanocomposite films with various amounts of NCC were shown in Fig. 8. 10
A significant improvement in TS and TM was achieved with the addition of NCC. With 1%, 2%, 3% and 4% NCC loading, the TS of PLA/PANI composite film was increased to 20.4 MPa, 24.0 MPa, 25.7 MPa and 26.1 MPa, respectively. The improvement might be attributed to the reinforcement effect of NCC fillers as NCC might have an influence on the dispersion uniformity, chain entanglement and interfacial adhesion of the composite system. In addition, such improvement might be also resulted from the presence of PEG as film-forming chemical in all nanocomposite films, as the hydroxyl groups in PEG was helpful to form hydrogen bonds with carboxyl groups in PLA and hydroxyl groups in NCC [34, 47, 48]. Moreover, as illustrated in Fig. 8b, Eb was slightly decreased from 2.2% (without NCC) to 1.6% (with 4% NCC) with the increase of NCC loading, implying that the rod-like NCC dispersed well in the mixture system, causing a restriction in the motion of PLA and PANI chains.
3.8 Electrical conductivity The conductivity of the polymer is mainly dependent on the degree of polymer conjugation, chain length and chain ordering [50, 51]. It is thus likely that the addition of NCC into the nanocomposite will affect the electrical conductivity. To analyze such effect, the electrical conductivity of PLA/PANI/NCC nanocomposite films with different NCC loading was evaluated. As expected, the PLA film was non-conductive, while addition of PANI in PLA composite can impart PLA composite film with conductive function. With increasing loading of NCC, the electrical conductivity decreased gradually (Fig. 9). The electrical conductivity of PLA/PANI/NCC nanocomposite film was reduced from 3.42 (without NCC) to 1.5 S∙m-1 (4% NCC loading). The result could be related with the electrostatic attraction between the negative charged group on NCC [18-20] and positive charged group on HCl-doped PANI [52], which will increase the resistance of electron movement i the PANI chain and thus decreasing the electrical conductivity of PLA/PANI/NCC film. Overall, the obtained electrical conductivity of PLA/PANI/NCC nanocomposite films ranging from 2.16 to 1.5 S∙m-1 (with 1~4% NCC loading) could be used in many areas, such 11
as stimulating cell proliferation (0.1 S∙m-1) [53].
4. Conclusions Conductive PLA/PANI/NCC nanocomposite films have been successfully prepared through a solvent casting method and the characterization of nanocomposite films as a function of NCC loading was investigated. It was found that the as-prepared PLA/PANI/NCC films exhibited an electrical conductivity of up to 2.16 S∙m -1 with 1% NCC dosage. The shear viscosity, shear stress and viscoelasticity of nanocomposite suspensions decreased and the dispersion stability of suspensions was improved with the increased NCC loading. The presence of NCC exerted a significantly positive effect on the mechanical strength of the nanocomposite films. Especially, compared to PLA/PANI film, the film with 4% NCC increased 38.1% and 89.1% in TS and TM, and decreased 27.3% in Eb. The obtained PLA-based nanocomposite films showed excellent properties for potential application in the fields of electrode, electromagnetic shielding and biomaterials.
Acknowledgments This work was financially supported by the Public Projects of Zhejiang Province (Grant No. LGG19C160002), Open Foundation of Key Laboratory of Pulp and Paper Science & Technology of Ministry of Education of China (Grant No. KF201503), Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering, Technology, Zhejiang Open Foundation of the Most Important Subjects (Grant No. 2016KF01), 521 Talent Cultivation Program of Zhejiang Sci-Tech University (Grant No. 11110132521310).
Author Contributions Yanjun Tang and Xiaoyu Wang conceived and designed the experiments; Xiaoyu Wang, Xianmei Zhu and Yiming Zhou performed the experiments and analyzed the date; Xiaoyu Wang and Yanjun Tang wrote the paper. All authors reviewed the manuscript. 12
Conflicts of Interest The authors declare no conflict of interest.
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Research highlights
An environmentally friendly and electrically conductive PLA/PANI/NCC nanocomposite film was developed.
The thermal stability of the PLA/PANI composite films was improved with PANI incorporation.
Increased NCC loading improved the tensile strength and tensile modulus of PLA/PANI/NCC nanocomposite films.
NCC loading led to decreased viscosity and viscoelasticity of PLA/PANI/NCC nanocomposite suspensions.
17
FIGURE CAPTIONS Fig. 1 SEM images of (a) pure PLA film and (b) PLA/PANI/NCC nanocomposite film. Fig. 2 Photographs of pure PLA film and PLA/PANI/NCC nanocomposite film. Fig. 3 FTIR spectra of PLA film, PLA/PANI and PLA/PANI/NCC nanocomposite films. Fig. 4 XRD patterns of PLA film, PLA/PANI and PLA/PANI/NCC nanocomposite films. Fig. 5 TGA curves of PLA film, PLA/PANI and PLA/PANI/NCC nanocomposite films. Fig. 6 Effects on (a) viscosity and (b) shear stress of PLA solution, PLA/PANI and PLA/PANI/NCC nanocomposite suspensions at various shear rates. Fig. 7 (a) loss modulus, (b) storage modulus and (c) complex viscosity of PLA solution, PLA/PANI and PLA/PANI/NCC nanocomposite suspensions at different angular frequencies. Fig. 8 (a) TS, (b) Eb and (c) TM of PLA/PANI/NCC nanocomposite films as a function of various NCC amounts. Fig. 9 Electrical conductivity of PLA/PANI/NCC nanocomposite films as a function of different NCC amounts.
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Fig. 1
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Fig. 2
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Pure PLA PLA/PANI/NCC(0%) PLA/PANI/NCC(2%) PLA/PANI/NCC(4%)
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2 theta (degree) Fig. 4
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Fig. 5
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Fig. 9
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