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Green preparation of cellulose nanocrystal and its application Xingya Kang, Shigenori Kuga, Chao Wang, Yang Zhao, Min Wu, and Yong Huang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02363 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 10, 2018
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Green preparation of cellulose nanocrystal and its
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application
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Xingya Kang,†,‡ Shigenori Kuga,† Chao Wang,† Yang Zhao,†Min Wu*,†and Yong
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Huang*,†
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Mailing address:
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†
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Zhongguancun East Road, Haidian District, Beijing 100190, People’s Republic of
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China.
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‡
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, 29
School of Packaging and Printing Engineering, Henan University of Animal
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Husbandry and Economy, 2 Longzihu North Road, Zhengdong New District,
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Zhengzhou 450046, People’s Republic of China.
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Email address of corresponding author:
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*Min Wu. E-mail:
[email protected].
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*Yong Huang. E-mail:
[email protected] 15
KEYWORDS: Cellulose nanocrystal (CNC), Ball milling, Centrifugation, Polyvinyl
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alcohol (PVA)
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ABSTRACT: A green method was used to prepare cellulose nanocrystal (CNC) with
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no use of harsh chemicals or organic solvents. This method was simple via ball
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milling cellulose with water followed by centrifugation. The diameter and length of
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CNC were 3-10 nm and 120-400 nm separately, with a aspect ratio of 20-60. The
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yield was about 20%. The thermal stability of CNC was the same as that of raw
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cellulose owing to lacking of chemical modification. So it was superior to CNC
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prepared by acid hydrolysis. Then polyvinyl alcohol (PVA)/CNC composite film was
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made by solution casting. The results showed that the tensile strength of composite
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film was 79% increased from neat PVA film when adding 5% CNC to PVA matrix. In
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addition, the composite films possessed good transparency and thermal stability.
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INTRODUCTION
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Cellulose nanocrystal (CNC) (nanowhisker) is rod-like particle with typical diameter
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of 1-50 nm and length of several hundred nanometers.1 Due to its unique
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physicochemical properties such as large specific surface area, high strength and low
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density, CNC has been an object of intense research. The traditional preparation is
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hydrolysis by strong mineral acids, which attack amorphous regions of cellulose,
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leaving crystalline region as CNC. In the post-processing, the excess acid must be
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removed by centrifugation and/or dialysis, which are laborious processes.
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On the other hand, ball milling is an effective method to produce nanocellulose.
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While dry ball milling of cellulose is known to cause its decrystallization,2-4 addition
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of liquid to the system prevents or retards the decrystallization, possibly due to
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shock-damping or specific interaction effects with cellulose. The influence depends
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heavily on the nature of the liquid added, and such effects have been reported in some
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instances.5-9
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As for water, its presence in ball milling is known to prevent decrystallization,
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leading to mere defibrillation of cellulose; therefore Zhang et al10 attempted
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preparation of cellulose nanofiber (CNF) by ball milling purified wood pulp with
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water aided by weak alkali pretreatment. Their results showed certain effectiveness,
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and the average diameter of the CNF was 100 nm. Abe et al11 bead-milled bleached
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wood pulp with 8 or 16 % NaOH solution and obtained CNF of 12-20 nm wide.
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However, the products of these two studies were not dispersed to the elementary
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fibril-level of 3-5 nm wide. While these studies used purified wood pulp as starting
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material, which were of high molecular weight and unfavorable for dispersion, Peyre
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et al12 showed that TEMPO oxidation of microgranular (microcrystalline) cellulose
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could give 5-9 nm wide CNCs by mild stirring.
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Thereupon we here examined ball milling of commercial microcrystalline
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cellulose without any pretreatment or use of harsh chemicals for preparing CNC.
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Because this procedure cannot give complete disintegration down to elementary-fibril
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level, the product was fractionated by centrifugation into the supernatant and
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precipitate. The yield of the two fractions and their effectiveness as reinforcement
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element for polyvinyl alcohol film was examined for mechanical, optical and thermal
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properties.
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EXPERIMENTAL SECTION
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Materials. Microcrystalline cellulose (MC, DP=262, Sigma Aldrich), CF11 (DP=200,
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Whatman), bleached kraft pulp cellulose (DP=1162, Kinleith), polyvinyl alcohol
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(degree of polymerization was 1700, degree of deacetylation was 99%, Aladdin,
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analytical grade). All the materials were used directly.
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Preparation of CNC. Three kinds of cellulose material with different DP were used:
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microcrystalline cellulose, CF11 and bleached kraft pulp. 1.1 g of cellulose and 20.9 g
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of deionized water were added to a 45 mL zirconia pot containing seven zirconia balls
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(10 mm). Ball milling was carried out at 300 rpm for 0.5 to 16 h by a Fritsch P7
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planetary mill. After ball milling, cellulose slurry was centrifuged at 4000 rpm for 5
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min to give supernatant and precipitation. Both fractions were freeze-dried and
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weighed. Samples were marked as “S/P-milling time”. “S” means supernatant and “P”
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means precipitation. For example, “S-6” means the supernatant after 6-hour milling.
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Preparation of PVA-CNC composite films. In this section, CNC was referred to S-6
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from MC. 5 g of PVA was dissolved in 95 g of deionized water at 90°C oil bath under
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stirring. Different amounts of CNC was dispersed in the PVA solution at room
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temperature. Weight percent of CNC in composite films was 1, 3, 5, 7, 10%,
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respectively. The mixed solution was sonicated for 20 min and cast in the glass petri
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dish to dry in ambient air for 1 day. PVA/P-6 film was also prepared for comparison.
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P-6 was also prepared from MC.
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Characterization. CNC was dispersed in water to make 0.01wt% suspension and
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dropped on carbon film for TEM test (JEOL 2100) at 200 kV, or on freshly cleaved
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mica for AFM characterization (Bruker Multimode 8) in scanasyst mode.
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The freeze-dried sample was ground with KBr for FTIR (Varian 3100) in
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transmission mode. The FTIR spectra of PVA/CNC composite films were measured in
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reflection mode.
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XRD was measured by an X’Pert PRO X-ray diffractometer (Bruker AXS GmbH).
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The degree of crystallinity was calculated by eq 1, where I002 is the intensity of the
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crystalline region of cellulose (2θ = 22.2°) and Iam is the intensity of amorphous refion
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(2θ = 18.6°).
( (%) ) =
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× 100
(1)
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Thermogravimetry of cellulose samples and PVA/CNC composite film was done by
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TGA (Q 50) in nitrogen atmosphere from room temperature to 600 oC with heating
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rate of 10 oC/min。
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The composite films were cut into 5 mm wide and 30 mm long strip for tensile test
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by a universal testing machine (MTS Sintech-1). The gauge of the strips was 20 mm
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and the strain rate was 25 mm/min. The samples were conditioned in measuring
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environment for 48 h before testing. Measurement was done for more than five test
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pieces.
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The surface and cross-section of PVA/CNC films were examined by SEM (JEOL-4800). The cross-section of samples were fractured in liquid nitrogen.
Light transmittance of PVA/CNC films were measured using UV−vis-NIR
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spectrometer (Varian Cary 5000) from 200 to 800 nm.
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RESULTS AND DISCUSSION
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The morphology and size of CNC. Figure 1 showed TEM images of supernatant and
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precipitate of microcrystalline cellulose ball milled with water for 6 h. The
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supernatant consisted of rod- or needle-like particles, mostly 10 nm wide and several
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hundred nanometers long (Figure 1a). The results were similar for CF11 as shown in
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Figure 2a. Though the length was that of typical CNC, the width was much larger
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than that of elementary fibrils, indicating that ball milling without surface-modifying
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agent was not enough for ultimate dispersion. Still, dispersion of these particle in
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supernatant was very stable at least for one week, showing flow birefringence (Figure
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3).
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The precipitate fractions of MC and CF11 consisted of incompletely dispersed long
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fibers (Figure 1b and 2b). Though their width was not much larger than that of
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supernatant fractions, their longer length would cause extensive entanglement and
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aggregation, which would lead to sedimentation on centrifugation. This factor was
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likely to be the cause of centrifugal separation of the tow fractions.
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On the other hand, pulp cellulose ball milled with water for 6 h was totally
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sedimented by centrifugation, the supernatant also consisted of long fiber aggregates
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(Figure 4). This behavior confirmed the role of cellulose DP in preparation of CNC
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via this new method.
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More accurate width determination was possible by AFM. The CNC made from
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MC had width of 3-12 nm and lengths 120-400 nm, with the aspect ratio of 20-60
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(Figure 5a). Figure 5b was the AFM image of the thin nanofiber in the precipitate,
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showing width of 3-8 nm. Though entangled by the long lengths, the fibers were well
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dispersed into near-elementary fibril levels. Figure 6 showed supernatant and
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precipitate fractions from CF11. They also had widths of 2-10 nm.
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The yield of CNC. Figure 7 showed the yield of CNC from MC and CF11 obtained
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by different ball milling time. The yield of CNC increased rapidly in the first 4 h, then
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nearly levelled off after 8 h. Conventional CNC preparation by mineral acids was
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known to give low yield, less than 30%.13 Therefore, together with eliminating
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necessity of harsh treatment and laborious purification, use of ball milling for CNC
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preparation can be a practical method.
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Crystallinity of CNC. The crystallinity of CNC affects its properties in application.
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Figure 8 showed the XRD patterns of supernatant CNC and precipitate samples ball
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milled with water for 0.5 h and 6 h. All the patterns were that of cellulose I, and there
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was little apparent difference between the samples, including the original cellulose,
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but the degree of crystallinity were reduced by ball milling as shown in Table 1. This
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obviously reflects mechanical damage by ball milling, probably accompanying
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decrease in degree of polymerization. On the other hand, there was no additional
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sharp peaks from inorganic contamination which often encountered in dry ball
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milling.
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It is known that acid treatment for CNC preparation can cause enhancement of
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crystallinity due to removal of the amorphous regions of cellulose14. In contrast, such
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effect was not observed in the case of ball milling. Also, the difference between the
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supernatant and precipitate fractions was negligibly small, indicating the ball milling
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action gave similar effects to both fractions except for the fiber length and aggregating
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tendency.
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The surface groups of CNC. The FTIR spectra of both fractions of ball milled
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cellulose were almost the same with each other, but slightly different from that of
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original cellulose (Figure 9). The main difference was in i) sharpness of OH stretching
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band at 3000-3700 cm-1, indicating enhanced homogeneity in hydrogen bonding
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patterns; ii) weakening of CH stretching band at 2800-3000 cm-1; iii) weakening of
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multiple peaks at 1250-1500 cm-1. Interpretation of these changes needs further
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studies.
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Thermogravimetric analysis of CNC. Figure 10 showed the thermogravimetric
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curves of S-6 and P-6 obtained from the two kinds of starting cellulose. Because no
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new functional groups were introduced by ball milling, the thermal decomposition
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behavior was nearly the same as the original cellulose. Only small decrease in the
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decomposition temperature, 5-10 K, was noted (Table 2). This was in sharp contrast to
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the CNC prepared by sulfuric acid treatment, which started to decompose at about
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220°C due to presence of sulfate ester of surface hydroxyls.15-16 Here was another
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advantage of ball mill-derived CNC over acid hydrolysis-derived CNC in practical
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application.
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PVA/CNC composite films. The supernatant and precipitate fractions from
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microcrystalline cellulose ball milled with water for 6 h were used for fabrication of
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nanocomposite with PVA via water-based mixing. Figure11a showed the effect of
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CNC addition on the mechanical properties of the films. The tensile strength
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increased at first, and then decreased. The maximum of 85 MPa was attained at 5%
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CNC, which was 79% increase from neat PVA film. The decrease at higher CNC
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addition was likely to result from inhomogeneous dispersion of CNC in the film. The
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elongation at break decreased linearly with increase of CNC content, giving 31%
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decrease from the neat PVA. Thus addition of CNC made the film stronger and stiffer.
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Addition of the precipitate fraction to PVA was also effective, but the extent of
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improvement was lower than that of CNC; the maximum strength was obtained at 3%
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loading, with 40% increase in tensile strength (Figure 11b). This was because of the
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aggregates of precipitate.
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SEM of fractured cross section of the composite film (Figure 12) showed gradual
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roughening with increased loading of CNC, suggesting influence of aggregation of
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CNC particles. Still, the images showed no indication of phase separation or fibril
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plucking, indicating good adhesion between CNC and PVA matrix. This adhesion is
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likely to be based on hydrogen bonding and van der Waals forces.
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Influence of CNC addition on transparency was not significant, as shown in
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Figure 13. Transmittance was 91.9% at 550 nm for neat PVA, and linearly decreased
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to 85.1% for 10% CNC composite film. This behavior was understandable in view of
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the closeness of refractive indexes of the two materials, about 1.58 of cellulose vs.
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1.49-1.52 of PVA.
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Figure 14 showed thermal properties of PVA/CNC composite films. There were
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three thermal decomposition stages for neat PVA. The first stage was below 150°C,
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evaporation of adsorbed water; the second stage was 250-350°C, decomposition of
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PVA with weight loss of ~75%. The last stage was above 400°C, which was the
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decomposition of residual char. The thermal decomposition temperature of pure PVA
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film was 240°C, and those of composite films were about 255°C, as shown in Figure
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14a. Therefore, it could be considered the thermal stability of PVA/CNC composite
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films improved by addition of CNC. Differential TGA (Figure 14b) showed the
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influence of CNC addition clearly as a shoulder at ca. 350°C, which agreed with the
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main decomposition range of pure cellulose (data not shown). This behavior indicated
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that decomposition of the two components took place nearly independently.
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CONCLUSIONS
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Cellulose nanocrystal was prepared by ball milling cellulose with water followed by
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centrifugation. This method was simple and green with no use of harsh chemicals or
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organic solvents, offering a new route of cellulose nanocrystal preparation. The
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diameter of CNC was 3-10 nm and the length was 120-400 nm, so the aspect ratio
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was 20-60. Though complete dispersion into 3-5 nm width was not possible,
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supernatant fraction after centrifugation, with typical yield of 20%, making maximum
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surface area and aspect ratio beneficial for composite additive. The thermal stability
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of CNC was higher than CNC prepared by acid hydrolysis due to lack of chemical
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modification. The PVA/CNC composite film made by solution casting showed
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significant enhancement of mechanical strengths, while showing no detrimental effect
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on transparency. Furthermore, the thermal stability of composite films were improved
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to some extent. Cellulose is abundantly reproducible and biodegradable; the
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physicochemical procedure here does not involve harsh/harmful chemicals or drastic
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conditions. Therefore the approach is expected to be useful for providing an
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environment-friendly and sustainable new materials.
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Figure 1. TEM of S-6 (a) and P-6 (b) of microcrystalline cellulose.
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Figure 2. TFM image of S-6 (a) and P-6 (b) obtained from CF 11.
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Figure 3. The flow birefringence of CNC
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Figure 4. TFM image of S-6 obtained from pulp cellulose.
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Figure 5. AFM images of S-6 (a) and P-6 (b) obtained from MC.
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Figure 6. AFM images of S-6 (a) and P-6 (b) obtained from CF11.
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Figure 7. The yield of CNC of different ball milling time obtained from MC and
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CF11.
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Figure 8. XRD spectra of CNC and precipitation of different ball milling time
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obtained from MC (a) and CF 11 (b).
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Figure 9. FTIR spectra of S-6 and P-6 obtained from MC (a) and CF 11 (b).
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Figure 10. TGA of S-6 and P-6 obtained from MC and CF11.
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Figure 11. The mechanical properties of PVA/CNC (a) and PVA/P-6 (b) composite
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films.
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Figure 12. The morphology of cross-section of PVA/CNC films.
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Figure 13. The optical transparency of PVA/CNC films. (a) Photograph of PVA/CNC
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films; (b). Light transmittance of PVA/CNC films.
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Figure 14. Thermal degradation of PVA/CNC films under nitrogen. (a) TGA; (b)
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DTGA.
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Table 1. The crystalline degree of CNC and precipitation of different ball milling
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time.
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Table 2. Thermal decomposition temperatures of S-6 and P-6 obtained from MC and
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CF11.
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AUTHOR INFORMATION
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Corresponding Author
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*Min Wu. E-mail:
[email protected]. Tel: +86-10-82543500.
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*Yong Huang. E-mail:
[email protected]. Tel: +86-10-82543478.
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGEMENTS
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This study was supported by the National Natural Science Foundation of China
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(No.51373191,
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Professorships.
51472253),
and
Chinese
Academy
of
Sciences
Visiting
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cellulose I to cellulose II polymorph by a ball-milling method with a specific amount of water. Cellulose 2004, 11 (2), 163-167. (6). Ago, M.; Endo, T.; Okajima, K., Effect of solvent on morphological and structural change of cellulose under ball-milling. Polymer Journal 2007, 39 (5), 435-441. (7). Rao, X.; Kuga, S.; Wu, M.; Huang, Y., Influence of solvent polarity on surface-fluorination of cellulose nanofiber by ball milling. Cellulose 2015, 22 (4), 2341-2348. (8). Zhao, M.; Kuga, S.; Jiang, S.; Wu, M.; Huang, Y., Cellulose nanosheets induced by mechanical impacts under hydrophobic environment. Cellulose 2016, 23 (5), 2809-2818. (9). Nuruddin, M.; Hosur, M.; Uddin, M. J.; Baah, D.; Jeelani, S., A novel approach for extracting cellulose nanofibers from lignocellulosic biomass by ball milling combined with chemical treatment. Journal of Applied Polymer Science 2016, 133 (9) (10). Zhang, L.; Tsuzuki, T.; Wang, X., Preparation of cellulose nanofiber from softwood pulp by ball milling. Cellulose 2015, 22 (3), 1729-1741. (11). Abe, K., Nanofibrillation of dried pulp in NaOH solutions using bead milling. Cellulose 2016, 23 (2), 1257-1261. (12). Peyre, J.; Pääkkönen, T.; Reza, M.; Kontturi, E., Simultaneous preparation of cellulose nanocrystals and micron-sized porous colloidal particles of cellulose by TEMPO-mediated oxidation. Green Chem. 2015, 17 (2), 808-811. (13). Wang, Q.; Zhao, X.; Zhu, J. Y., Kinetics of Strong Acid Hydrolysis of a Bleached Kraft Pulp for Producing Cellulose Nanocrystals (CNCs). Industrial & Engineering Chemistry Research 2014, 53 (27), 11007-11014. (14). Yu, H.; Qin, Z.; Liang, B.; Liu, N.; Zhou, Z.; Chen, L., Facile extraction of thermally stable cellulose nanocrystals with a high yield of 93% through hydrochloric acid hydrolysis under hydrothermal conditions. Journal of Materials Chemistry A 2013, 1 (12), 3938. (15). Rosa, M. F.; Medeiros, E. S.; Malmonge, J. A.; Gregorski, K. S.; Wood, D. F.; Mattoso, L. H. C.; Glenn, G.; Orts, W. J.; Imam, S. H., Cellulose nanowhiskers from coconut husk fibers: Effect of preparation conditions on their thermal and morphological behavior. Carbohydrate polymers 2010, 81 (1), 83-92. (16). Wang, N.; Ding, E.; Cheng, R., Thermal degradation behaviors of spherical cellulose nanocrystals with sulfate groups. Polymer 2007, 48 (12), 3486-3493.
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Cellulose nanocrystal and the flow birefringence phenomenon.
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