Abstract
Nanocrystalline cellulose was prepared from bleached softwood kraft pulp using acid hydrolysis. Acetylated nanocellulose (AcNCC) and polylactic acid (PLA) were mixed together at different proportions under certain humidity conditions using a miscible method to prepare highly regular porous AcNCC/PLA composite films. The composites were characterized using scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, and Fourier transform infrared spectroscopy. The PLA/Ac-NCC-4 showed a uniform pore diameter distribution with diameter of 1.0 μm to 5.0 μm. The mechanical properties and thermal stability of the PLA composites were improved with the addition of AcNCC. The pore structure was regular and well distributed. When the AcNCC loading was 4%, the tensile strength and Young’s modulus of the composites were 2.07 and 2.41 times higher than that of the pure PLA, respectively. The composites also exhibited high visible light transmission.
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Preparation and Characterization of Acetylated Nanocrystalline Cellulose-reinforced Polylactide Highly Regular Porous Films
Mingcong Xu,a Rue Yang,b Qiongtao Huang,b Xin Zhao,c Chunhui Ma,a Wei Li,a,b,* Jian Li,a and Shouxin Liu a,*
Nanocrystalline cellulose was prepared from bleached softwood kraft pulp using acid hydrolysis. Acetylated nanocellulose (AcNCC) and polylactic acid (PLA) were mixed together at different proportions under certain humidity conditions using a miscible method to prepare highly regular porous AcNCC/PLA composite films. The composites were characterized using scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, and Fourier transform infrared spectroscopy. The PLA/Ac-NCC-4 showed a uniform pore diameter distribution with diameter of 1.0 μm to 5.0 μm. The mechanical properties and thermal stability of the PLA composites were improved with the addition of AcNCC. The pore structure was regular and well distributed. When the AcNCC loading was 4%, the tensile strength and Young’s modulus of the composites were 2.07 and 2.41 times higher than that of the pure PLA, respectively. The composites also exhibited high visible light transmission.
Keywords: Nanocrystalline cellulose; Acetylation; Polylactic acid; Highly regular porous film
Contact information: a: Key laboratory of Bio-based Material Science and Technology of Ministry of Education, Northeast Forestry University, Hexing Road 26, Harbin 150040, P. R.; b: Post-Doctoral Research Center, Yihua Lifestyle Technology Co., Ltd., Shantou, 515834, P. R. China; c: Key Laboratory of Pulp and Paper Science & Technology of Ministry of Education/Shandong Province, Qilu University of Technology, Jinan, 250353, China;
* Corresponding authors: liwei19820927@126.com; liushouxin@126.com
INTRODUCTION
Nanocrystalline cellulose (NCC) is the most common polymer material found in nature. The low manufacturing costs, good thermal stability, excellent mechanical properties, such as a large surface area and high modulus, and other advantages have resulted in increasing interest in this material in recent years (Habibi et al. 2010; Li et al. 2011, 2013a). The abundance of hydroxyl groups on the surface of NCC makes it hydrophilic and insoluble in organic solvents, which limits its application with enhanced organic polymers. The NCC surface can be modified to make it miscible in chloroform and other organic solvents, while retaining the original characteristics of the nanocellulose. This can improve its dispersibility in organic polymers and enhance its effects (Lin et al. 2011; Li et al. 2013b; Habibi 2014; Lam et al. 2017).
Polylactic acid (PLA) is a synthetic polymer with excellent biocompatibility and biodegradability. It is non-toxic, non-irritating, has a high strength, high plasticity, is easily processed, and can be decomposed easily by a variety of microorganisms, plants, and enzymes to ultimately form water and carbon dioxide (Nakagaito et al. 2009; Kowalczyk et al. 2011; Fortunati et al. 2012; Frone et al. 2013; Ghalia and Dahman 2017). Therefore, PLA is considered to be one of the most promising biodegradable polymer materials. However, the shortcomings of PLA include brittleness, poor thermal stability, and a low crystallization rate, which limit its applications in practice. Modified NCC is used in the preparation of enhanced PLA composite films by enhancing the mechanical properties and thermal stability and mitigating the brittleness of PLA (Shi et al. 2012; Haafiz et al. 2013; Xie et al. 2014; Fortunati et al. 2014). The properties of PLA can be effectively improved by adding acetylated NCC (AcNCC) to its matrix to prepare enhanced PLA composite film materials (Bilbao-Sáinz et al. 2010; Pankaj et al. 2014; Robles et al. 2015).
Materials with an ordered porous structure are widely applied in the fields of biotechnology, tissue engineering, and efficient separation (Shimomura 1993). Under high humidity conditions, water vapor is condensed on the polymer surface because of solvent evaporation (Jenekhe and Chen 1999). Water droplets accumulate on the polymer surface because of the temperature gradient in the solution under the action of capillary flow and reflux, which then results in an ordered porous polymer film (Srinivasarao et al. 2001; Zhao et al. 2003). The ability to form stable droplets is the key to the formation of a regular pore structure in polymer films (Widawski et al. 1994).
In this study, NCC was first acetylated and modified. Then, different amounts of AcNCC and PLA were dissolved in a chloroform solution. The PLA composite films with a porous structure were prepared using a water-assisted method. The structure, mechanical properties, thermal stability, and transmittance of the PLA/AcNCC cellular porous composite films prepared using different ratios of AcNCC and PLA were analyzed.
EXPERIMENTAL
Methods
Preparation of the NCC
The bleached softwood kraft pulp (BSKP) (Mudanjiang Hengfeng Paper Co. Ltd., Mudanjang, China) was ground to a 40 mesh to 60 mesh size with a wood flour-grinding machine (JL064, Shanghai Jiading instrument Co. Ltd., Shanghai, China). Ten grams of wood powder were added to 64 wt.% of H2SO4 at 45 °C, and the mixture was then stirred for 0.5 h, after which the samples became a pale-yellow liquid. After acid hydrolysis, the suspension was diluted with 500 mL of distilled water to terminate the reaction. The excess acid was removed by centrifugation/washing at 8000 rpm, and this process was repeated five times until the pH value of the suspension was approximately 3 to 5. The suspension was added to a dialysis bag and was washed with distilled water dialysis for 15 d until the pH value of the suspension was constant. Finally, the suspension was concentrated and freeze-dried before further use.
Preparation of the AcNCC
To prepare the AcNCC, 2 g of NCC were added to an appropriate amount of glacial acetic acid and underwent swelling for 3 h. The swollen NCC was filtered through filter paper and placed in a reactor. Then, 10 mL of acetic anhydride and 16 mL of acetic acid were added, and the resulting mixture was heated to 45 C. While the mixture was stirred, 0.8 mL of a 5 M sulfuric acid solution was added. The AcNCC was obtained after the reaction was performed for 0.5 h. Finally, the obtained AcNCC was freeze-dried before further use.
Preparation of the highly regular porous composite films
To prepare the highly regular porous composite films, 5 g of PLA and 50 mL of chloroform were added to a conical flask and the PLA was completely dissolved using magnetic stirring. The prepared AcNCC powder was added to the chloroform and ultrasonication was applied to the resulting mixture for 3 h to obtain a 2 wt.% solution. The PLA and PLA/AcNCC solutions were combined, the mixtures were degassed, and the PLA/AcNCC films were generated with a saturated NaCl solution (relative humidity = 75%) via the solution casting method. The AcNCC/PLA composites were prepared by adding AcNCC at mass fractions of 0%, 2%, 4%, 8%, and 12%, which were denoted as PLA, PLA/AcNCC-2, PLA/AcNCC-4, PLA/AcNCC-8, and PLA/AcNCC-12, respectively. After the solvent was volatilized, the resulting PLA films were dried at 50 °C for 1.5 h to remove the residual solvent and the films were placed in a desiccator in preparation for the experiments.
Characterization
Transmission electron microscopy (TEM) observations were made with a FEI/Philips Tecnai G2 (Philips, Eindhoven, The Netherlands) operated at 80 kV. A drop of diluted NCC suspension was deposited on 300 mesh screen carbon-coated grids (the drop size of the suspension was 2 mm in diameter) and allowed to dry at room temperature (25 C). The sample was stained with phosphotungstic acid (2 wt.%) for 30 s.
Fourier transform infrared (FT-IR) spectra were measured using a Nicolet iS10 FT-IR instrument (Thermo Scientific, Waltham, USA). The spectra were measured in the attenuated total reflectance mode and the data was recorded over the range of 650 cm-1 to 4000 cm-1 with a resolution of 1 cm-1.
Thermogravimetric analysis (TGA) was performed using a TGA-Q50 TG analyzer (TA, New Castle, DE, USA). The temperature programs for the dynamic tests were run over a temperature range of 25 C to 600 C at a heating rate of 10 C/min.
Microstructural analysis was performed using scanning electron microscopy (SEM) (QUANTA 200, FEI, Hillsboro, OR, USA). The sample surfaces were coated with a thin layer of gold using a BAL-TEC SCD 005 sputter coater (Leica, Wetzlar, Germany) to provide electrical conductivity.
Mechanical properties of the PLA and PLA/AcNCC composite films were analyzed using an RGT-20A instrument (Shenzhen Reger Instrument, Shenzhen, China). To characterize each type of film, three samples were fabricated with a length of 100 mm ± 2 mm, width of 10 mm ± 0.3 mm, and thickness of 0.1 mm ± 0.05 mm. The optical transmittance was measured over the range of 300 nm to 800 nm using ultraviolet-visible spectroscopy.
RESULTS AND DISCUSSION
TEM Analysis
Figure 1 shows the TEM images and length distributions of the NCC and AcNCC. The NCC showed a rod-like structure with a rod diameter of 10 nm to 20 nm and length of 75 nm to 150 nm. Because of the acidic conditions of the acetylation modification, the prepared AcNCC maintained a rod-like structure with a diameter of 10 nm to 20 nm and decreased length of 45 nm to 100 nm. Moreover, the length distribution of the AcNCC was more uniform than that of the NCC.
Fig. 1. TEM images and length distribution of the NCC (a) and AcNCC (b)
FT-IR Analysis
Figure 2a shows the infrared spectrum of the NCC. Characteristic peaks for cellulose -OH were observed at 3330 cm-1. This was because the NCC had a broad absorption peak formed from the superposition of multiple -OH stretching vibrational absorption peaks.
Fig. 2. FT-IR spectra of the NCC (a) and AcNCC (b)
The peak at 2882 cm-1 indicated that the corresponding C-H symmetric expansion of the C-H was absorbed by methyl methylene (-CH2-). Figure 2b shows the infrared spectrum for the AcNCC. An absorption peak for -OH stretching vibration at 3330 cm-1 was not present, but a notable C=O stretching vibration absorption peak at 1735 cm-1 and characteristic C-O stretching vibration peaks at 1216 cm-1 were observed, which indicated that the NCC reacted with acetic anhydride to consume the hydroxyl groups and produced acyl groups. Therefore, AcNCC was successfully prepared through this method.
Figure 3 shows infrared spectra for AcNCC, PLA, and PLA/AcNCC-4 composite film. The adsorption band in the PLA spectrum of Fig. 3 at 1750 cm-1 corresponded to a C=O stretching vibration peak. The adsorption bands at 1202 cm-1 was a characteristic C-O peak, which illustrated the structural characteristics of the PLA. A comparison of the PLA and PLA/AcNCC-4 spectra indicated that the PLA combined with the AcNCC. The same number of absorption peaks was observed for the composite film and pure PLA because of the low AcNCC content in the composite films. These results showed that the PLA and AcNCC were compatible. There were no other new peaks in the infrared spectrum of the PLA/AcNCC-4 composite membrane, which indicated that the AcNCC and PLA in the composite film were physically combined.