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chitosan films reinforced with polydopamine modified MMT: Effects of dopamine concentration
Accepted Manuscript Starch/chitosan films reinforced with polydopamine modified MMT: Effects of dopamine concentration Min Zhou, Qin Liu, Shuaishuai Wu, Zhenqiong Gou, Xiyu Wu, Dan Xu PII:
S0268-005X(16)30279-X
DOI:
10.1016/j.foodhyd.2016.06.030
Reference:
FOOHYD 3484
To appear in:
Food Hydrocolloids
Received Date: 15 September 2015 Revised Date:
21 June 2016
Accepted Date: 22 June 2016
Please cite this article as: Zhou, M., Liu, Q., Wu, S., Gou, Z., Wu, X., Xu, D., Starch/chitosan films reinforced with polydopamine modified MMT: Effects of dopamine concentration, Food Hydrocolloids (2016), doi: 10.1016/j.foodhyd.2016.06.030. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Starch/chitosan films reinforced with polydopamine modified MMT: Effects of
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dopamine concentration
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Min Zhou, Qin Liu, Shuaishuai Wu, Zhenqiong Gou, Xiyu Wu, Dan Xu
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College of Food Science, Southwest University, Chongqing 400715, PR China
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Correspondence: Dan Xu, College of Food Science, Southwest University,
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Chongqing 400715, China.
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Email: [email protected]
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Abstract: Montmorillonite (MMT) was modified by bio-inspired polydopamine
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(PDA), which is the self-polymerized product of dopamine (DA), to improve the
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dispersion property of MMT in polymer matrix. As revealed by FTIR spectra, XRD
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profiles and SEM micrographs, PDA has been successfully coated onto MMT surface
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thus enlarge the interlayer spacing, resulting PDA modified MMT (MMT-Dx). The
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lamellar structure of MMT-Dx was found to be highly related to the DA concentration
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applied during the modification process. The exfoliation degree of MMT-Dx increased
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along with the weight content of PDA coating, as the DA concentration increased
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from 0.5 mg/mL to 3 mg/mL. MMT-Dx was then incorporated into starch/chitosan
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(ST/CS) blended matrix at a loading level of 2 wt.% to reinforce the bio-polymers.
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Compared to neat MMT, MMT-Dx showed much higher dispersion capability as
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observed from XRD profiles and SEM micrographs. Furthermore, as DA
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concentration increased, the tensile strength and Young’s modulus of the composite
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ACCEPTED MANUSCRIPT films increased. The composite film containing MMT-Dx modified by 3 mg/mL DA
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displayed a tensile strength as high as 30.44 MPa and a Young’s modulus as high as
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1.42 GPa, which were 2.1 and 1.6 times the values of the neat ST/CS film,
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respectively. Therefore, to adopt PDA modification, MMT-Dx with adjustable
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interlayer spacing and different dispersion properties can be obtained, which provide
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multiple choices of nano-fillers for bio-polymers reinforcement; thus accelerate the
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acceptances of these bio-polymers in food packaging and related applications.
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Key words: Polydopamine; Montmorillonite; Starch; Chitosan; Nanocomposite film
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1. Introduction
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Nowadays, bio-polymers have received growing concerns all over the world, due
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to their exciting potentials for replacing petroleum-based plastics in a variety of
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applications. The large-scale application of bio-polymers in areas such as food
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packaging will be one of the most promising solutions of environment pollution
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caused by non-biodegradable plastics (Aouada, Mattoso, & Longo, 2011). When
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bio-polymers are mentioned, starch (ST) with attractive price, wide availability,
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thermoplastic property and biodegradability is absolutely at one of the best
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advantageous position in this regard (Almasi, Ghanbarzadeh, & Entezami, 2010;
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Schlemmer, Angelica, & Sales, 2010). ST films with high safety are famous for
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extending the shelf-life of various foods and maintaining their quality (Li, Ye, Liu, &
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ACCEPTED MANUSCRIPT Zhao, 2015). However, when compared with plastic films, some disadvantages, such
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as poor mechanical properties and moisture resistance, make them insufficient for
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many packaging purposes (Priya, Gupta, Pathania, & Singha, 2014). Blending with
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other polymers such as chitosan (CS) may improve the strength and moisture
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resistance, to a certain extent. CS, which is derived by deacetylation of chitin, has
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many advantages, such as good biodegradability and biocompatibility, high
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antimicrobial activity, excellent film-forming properties (Bie, et al., 2013). Moreover,
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CS may interact with ST through hydroxyl, amide and residual amide groups, thus
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reinforcing ST-based films (Akter, et al., 2014). However
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properties by blending are limited by the intrinsic properties of polymers.
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improvements on ST
Fortunately a great breakthrough has been made by bio-nanocomposites in recent
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years, which is filling bio-polymers with inorganic/organic nano-particles. When
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nano-particles with excellent mechanical and thermal properties are filled into
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bio-polymers, they can interact with polymer chains at nanoscale, leading to
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significant improvements on the properties of the resulting nanocomposites. Therefore,
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keys to prepare composite films with satisfied performances are homogeneous
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dispersion of nano-fillers in the matrix and strong interface interactions (Pan, Bao, &
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Li, 2011).
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Montmorillonite (MMT) is one of the most extensively studied nanofillers due to
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its low cost, high specific surface area, good biocompatibility, high strength and other
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outstanding properties (Arroyo, Huneault, Favis, & Bureau, 2010; Heydari,
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Alemzadeh, & Vossoughi, 2013; Maksimov, Lagzdins, Lilichenko, & Plume, 2009).
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are tactoids, intercalated layers, and randomly exfoliated lamellae (Johansson &
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Clegg, 2015). In the later two cases, polymer chains are able to interact with MMT
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lamellae at nanoscale obtaining nanocomposites with excellent properties. Therefore,
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many strategies have been explored to improve the dispersion properties of MMT
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(Chivrac, Pollet, & Avérous, 2009). Surface modification is one of the most widely
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adopted methods not only because of its diversity, but also due to its capability to
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adjust the surface properties of MMT for the purpose of enhancing compatibility with
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polymer matrix.
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Polydopamine (PDA) coating is a newly developed surface modification method,
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which is inspired by the wet adhesion of marine mussel adhesive protein. Under weak
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alkaline conditions, dopamine (DA) will self-polymerize to PDA and thus form a thin
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layer onto the surface of various materials (Wei, et al., 2010; Xuan, et al., 2013). With
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active catechol and amine groups, PDA can interact with many polar polymers
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through hydrogen bonding and provide a platform for further modification (Lee,
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Scherer, & Messersmith, 2006; Ortega-Toro, Jimenez, Talens, & Chiralt, 2014). Our
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previous study have found that PDA coating on MMT surface was able to prevent the
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regular stack of MMT lamellae and induce the exfoliated structure (Zhou & Xu, 2015).
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More importantly, PDA modified MMT (MMT-Dx) showed much better reinforcing
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effect on the ST films compared with neat MMT, which was attributed to the
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enhanced interfacial interactions thus the effective stress transfer between MMT and
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polymer matrix. However, our latest study found that the initial DA concentration
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and thus, largely affect the properties of nanocomposites filled with modified MMT
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(Zhou & Xu, 2015). Therefore, in this study, we will focus on the effects of DA
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concentration on MMT lamellae structure and furthermore on the properties of ST and
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CS blended films.
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2. Experimental
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2.1. Materials
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DA was purchased from Bio Basic Inc., Canada. MMT-Na+, with a
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cation-exchange capacity (CEC) of 1.45 meq/g, was purchased from Southern Clay
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Products Inc., USA. Corn starch (approximately 20 wt.% amylose) was provided by
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Tianjin Zhongying Health Food Co. Ltd. Chitosan (DD of 90%) was obtained from
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Weifang Seasource Biological Products Co., Ltd. Tris (hydroxymethyl) aminomethane
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(Tris) was purchased from Biosharp Co. Ltd., USA. Analytical grade glycerol was
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used as received.
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2.2. PDA modification on MMT
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MMT-Dx was prepared following the procedure reported in our earlier
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publication (Zhou & Xu, 2015). After MMT (0.2 g) was added to the Tris-HCl
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stirred for 24 h at room temperature. After filtration, washing and freeze-drying,
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MMT-D was obtained. The DA concentration used was 0.5 mg/mL, 1 mg/mL, 2
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mg/mL, 3 mg/mL and 5 mg/mL, respectively. The modified MMT was designated as
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MMT-Dx, where x represents the concentration (mg/mL) of DA used to modify
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MMT.
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2.3. Preparation of ST/CS-MMT-Dx composite films
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The composite films were prepared by solution-blending method. After 3.5 g ST and 0.8 g glycerol were dissolved in de-ionized (DI) water and plasticized at 75
for
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2 h, MMT-Dx was added. The mixture was stirred for 22 h at room temperature
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followed by ultrasonic treatment for 30 min. Then, CS solution which was prepared
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by dissolving 2.3 g CS powder in 230 mL 1% (V/V) acetic acid aqueous solution, was
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added into the above mixture and stirred for another 2 h. For all the films, the weight
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ratio of ST/CS was 60/40. The obtained homogeneous solution was poured into Petri
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dishes and dried in an oven at 50
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the solvent. Based on our previous study (Zhou & Xu, 2015), the loading level of
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MMT-Dx was kept at 2 wt.% of the total weight of ST and CS powder. The obtained
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composite films containing MMT-Dx were denoted as ST/CS-MMT-Dx. The neat
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ST/CS films and the composite films containing pristine MMT were also prepared as
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controls and denoted as ST/CS and ST/CS-MMT, respectively. The above films with
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for about 3 ~ 4 h to allow the fully evaporation of
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thickness of 30 ~ 60 µm were preconditioned at 25
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prior to test.
and 60% RH for at least 24 h
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2.4. Characterizations
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2.4.1. X-ray diffraction (XRD)
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Samples were scanned by a SHIMADZU XRD-7000 diffractometer operated from
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2θ = 3º to 40º at a scanning rate of 2º/min with Cu Kα radiation.
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2.4.2. Fourier transform infrared spectroscopy (FTIR)
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MMT, PDA and MMT-Dx were mixed with KBr and pressed to pellets, while
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composites films with thickness around 5 µm were prepared by diluted casting
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solution prior to FTIR scanning. All the pellets and films were scanned by a Nicolet
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6700 FTIR spectrometer from 450 cm -1 to 2000 cm-1 with a resolution of 4 cm -1.
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The micrographs of MMT-Dx powder and composite films were collected by a JEOL JEM-2100 scanning electron microscope after gold sputter coating.
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2.4.4. Thermo-gravimetric analysis (TGA)
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TA Q50 thermo-gravimetric analyser was used to evaluate the thermal degradation
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behavior of samples. Powder or film flakes were heated at a rate of 10
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to 800
under a nitrogen atmosphere.
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2.4.5. Tensile properties
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An auto tensile tester (XLW-PC) from Labthink Instruments Co. Ltd. was used to
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measure the tensile properties of the composite films. At least 10 samples of each kind
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of film were tested to calculate the average value. Samples with a gauge length of 50
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mm and a width of 10 mm were stretched at 25
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mm/min.
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2.4.6. Statistical analysis
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using a cross head speed of 50
The tensile properties were statistically compared between films by one-way
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analysis of variance (ANOVA). The significance of the mean values of tensile
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strength, Young’s modulus and elongation at break was determined by Duncan’s
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multiple range testing with p < 0.05.
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3. Results and discussion
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3.1. Characterizations of MMT-Dx
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3.1.1. XRD studies
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The XRD patterns of MMT-Dx and neat MMT were compared in Fig.1. As the
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001 peak of neat MMT was located at 7.00º, the interlayer distance d001 of MMT was
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calculated to be 1.26 nm according to Bragg’s equation, which was consistent with the
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reported value (Slavutsky & Bertuzzi, 2012). After modification, all of the 001 peaks
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of MMT-Dx shifted to the left indicating the enlarged d001 after modification. The
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calculated d001 of MMT-D0.5, MMT-D1 and MMT-D2 increased to 1.35 nm (2θ =
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6.54º), 1.69 nm (2θ = 5.22º) and 2.04 nm (2θ = 4.32º), respectively. According to the
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literature (Xuan, et al., 2013), the layer thickness of MMT is about 0.96 nm. Whereas,
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the bilayer of 5, 6-dihydroxyindole (DHI), which is the intermediate product of DA
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polymerization and forms nano-aggregates linking to PDA aggregates, has a thickness
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of approximately 0.34 nm. Therefore, by subtracting the MMT layer thickness, the
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interlayer distance of MMT-D0.5, MMT-D1 and MMT-D2 was 0.39 nm, 0.73 nm, and
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1.08 nm, which were approximately 1, 2 and 3 times of the thickness of DHI bilayer,
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respectively. When it came to MMT-D3, the 001 peak completely disappeared within
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the testing range, which may indicate a high extent of exfoliation of MMT lamellas.
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However, when DA concentration continued to increase, a broad peak appeared again
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at 4.28°, corresponding to the d001 of 2.06 nm. During the modification process, DA
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then polymerize to PDA coating layer. As a result, the thickness of PDA layer, which
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relies on the amount of adsorbed DA, is expected to have great effects on the lamellar
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structure of MMT-Dx. It may be speculated from the above results that with
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increasing DA concentration, more DA molecules are available in the solution and
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thus adsorbed onto MMT surface. Therefore, thick PDA layers formed between MMT
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lamellae leading to growing d001 until exfoliation structure obtained. However, when
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excess DA molecules (5 mg/mL) are applied, PDA may self-aggregate around MMT
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stacks, preventing the formation of exfoliation structure.
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3.1.2. FTIR studies
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Fig. 2 shows the FTIR spectra of MMT, MMT-Dx and PDA. In Fig. 2(a), the
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characteristic peaks of MMT are observed at 1624 cm-1, 1400 cm-1, 1022 cm-1, 910
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cm-1, 518 cm-1, and 462 cm-1, which are known as the water deformation band, Si-O
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stretching band, Al-Al-OH vibration band, Al-O-Si, and Si-O-Si deformation bands,
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respectively. In Fig. 2(g), peaks at 1400 cm-1, 1497 cm-1 and 1621 cm-1 as pointed by
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the bottom three arrows are assigned to the bending vibration of phenyl of PDA, the
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stretching vibrations of C=C and C=N, respectively, while peaks at 1264 cm−1 and
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1285 cm−1 are attributed to the catechol OH bending vibrations (Xuan, et al., 2013).
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This indicates the oxidative polymerization of DA to PDA in Tris-HCl buffer solution.
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After PDA modifications, new peaks appeared at around 1268 cm-1 and 1485 cm-1 in
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N-H shearing vibrations and the catechol OH bending vibrations of PDA. Therefore,
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it can be speculated that PDA layer have been successfully attached onto MMT
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surface. With increasing DA concentration, the intensity of these two peaks increased
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and reached the maximum for MMT-D3.
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3.1.3. SEM studies
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The SEM micrographs of MMT and MMT-Dx surface are presented in Fig. 3. It is
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observed that most lamellae of MMT stacked together without obvious single MMT
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layers. After modification with increasing DA concentration, the lamellar structure of
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MMT-Dx became more and more clear. When DA concentration was 3 mg/mL,
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exfoliated MMT lamellae were highly separated from each other as illustrated in Fig.
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3(e). This is in accordance with the disappearance of 001 peak in the XRD
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diffractogram of MMT-D3 as discussed before. However, when DA concentration
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further increased to 5 mg/mL, the exfoliation degree of MMT lamellae decreased.
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3.1.4. TGA studies
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TGA was utilized to estimate the content of PDA in MMT-Dx based on the
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different thermal stability of MMT and PDA. As shown in Fig. 4, MMT has excellent
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thermal stability with a residue as high as 91.3% at 800 , while PDA started to
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degrade at approximately 140
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weight loss of MMT-Dx at 800
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the order of MMT-D3 > MMT-D5 > MMT-D1 > MMT-D0.5. As the thermal stability
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of PDA was much poorer compared to MMT, therefore higher weight loss of
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MMT-Dx might indicate higher content of PDA in MMT-Dx. This was in accordance
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with our speculation from XRD diffractograms, that the thickness PDA coating layers
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formed on MMT surface increased when DA concentration increased from 0.5 to 3
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mg/ml. When DA concentration was 3 mg/ml, most of the adsorption sites on MMT
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surface were occupied by DA molecules and then polymerized to PDA, leading to the
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maximum amount of PDA coating in MMT-D3 and thus the highest degree of
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exfoliation.
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3.2.1. SEM studies
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lied between these of MMT and PDA, following
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and showed a residue of only 54.8%. Reasonably,
The micrographs of the film surface are shown in Fig. 5. In Fig. 5(a), the ST/CS
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film has a dense and smooth surface without defects due to the good compatibility
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between CS and starch (Khan, et al., 2010). With the incorporation of neat MMT,
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MMT aggregates around 1 to 3 µm were observed in the matrix as showed in Fig. 5(b).
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However, when the same amount of MMT-Dx was loaded, the aggregations became
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smaller and less, specifically for MMT-D3. This indicated that MMT-Dx were
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PDA coating layer with catechol groups might improve the compatibility between
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MMT-Dx and the hydrophilic matrix. Moreover, dispersion properties of MMT-Dx in
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the matrix were positively correlated to their exfoliation degree before they were
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added into ST/CS matrix. As revealed by section 3.1.1 and section 3.1.3, MMT-D3
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showed the highest degree of exfoliation. Therefore, after they were added into ST/CS
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matrix, lamellae in MMT-D3 were better dispersed compared with other samples.
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3.2.2. XRD studies
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Fig. 6 displays the XRD profiles of ST/CS film and composite films. In our study,
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ST/CS film showed a broad peak centered at 20°, which might be attributed to the
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crystalline structure of chitosan (Baran, Aciksoz, & Mentes, 2016). However, the
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characteristic peaks of ST at 2θ = 16.9°, 19.3° and 22.4° as we have observed in our
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previous study (Zhou & Xu, 2015) disappeared, which have been reported before
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(Mathew & Abraham, 2008). With the addition of MMT, a small peak at 3.9°
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appeared, which was attributed to the d001 diffraction of MMT corresponding to an
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inter-layer distance of 2.26 nm. In our previous study (Zhou & Xu, 2015), this peak
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shifted to 4.68° indicating an interlayer distance of 1.89 nm when only ST and
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glycerol contributed to the intercalation. The increased interlayer spacing and reduced
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peak intensity in this study might be attributed to the existence of CS chains in the
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matrix which could enter the MMT gallery via ion exchange (Monvisade &
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peaks were not observed in all the composite films within the test range, indicating
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highly exfoliations of MMT lamellae in the matrix. As we have discussed previously
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that PDA coating was not only able to enlarge the interlayer spacing of MMT but also
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enhance the interactions between MMT and the matrix polymer, good dispersion of
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MMT-Dx in the matrix with little stacked lamellae was possibly achieved under this
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circumstance. However, addition of MMT-Dx didn’t have significant effect on the
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crystalline structure of starch and CS blended matrix.
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3.2.3. TGA studies
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The TGA and differential thermogravimetric analysis (DTG) curves of ST/CS
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films and composite films are shown in Fig. 7. All the composite films displayed very
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similar thermal degradation behaviors to that of ST/CS film. There were no significant
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differences on Tonset, Tmax and char residues between ST/CS film and all the composite
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films. Therefore, addition of either MMT or MMT-Dx didn’t have significant impact
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on the thermal decomposition behaviors of the composite films.
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Table 1 shows the values of tensile strength, Young’s modulus and elongation at
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break of ST/CS film and composite films. With the addition of neat MMT, only
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film. However, when MMT-Dx was added, both tensile strength and Young’s modulus
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of the composite films increased simultaneously. Furthermore, the reinforcement
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effects were related to the DA concentration used to modify MMT, as
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ST/CS-MMT-D3 and ST/CS-MMT-D5 films showed the highest tensile strength and
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Young’s modulus. Since we have discussed earlier that the dispersion capability of
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MMT were enhanced as the DA concentration increased. Therefore, the better
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dispersed MMT-D3 and MMT-D5 would have larger contact areas with the polymer
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matrix compared with MMT-D0.5, MMT-D1, MMT-D2 and MMT, which resulted in
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more effective stress transfer between fillers and matrix (Fu, Feng, Lauke, & Mai,
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2008). That might be the reason for the improved tensile strength. Moreover, the
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catechol groups of PDA were able to interact with the polar groups of ST and CS,
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such as hydroxyl and amino groups. Therefore, the increased content of PDA coating
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from MMT-D0.5 to MMT-D3 indicated more interactions between MMT-Dx and the
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matrix. As the interfacial adhesion increased, more stress was required to produce a
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given amount of strain when the composite films were stretched by force, resulting in
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higher values of Young’s modulus. However, variations of the elongation at break of
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the composite films just showed the opposite trend that with the addition of MMT,
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and MMT-D0.5 to MMT-D3, the values gradually decreased. Tee et al. (Tee, et al.,
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2013) have reported similar results and explained that the addition of small amount of
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MMT leaded to the regular arrangement of starch chain. In this study, polymer chains
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were reasonably confined more strictly by the finer dispersed MMT-Dx lamellae. As a
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result, the ST/CS-MMT-D3 and ST/CS-MMT-D5 films presented the lowest
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elongation at break.
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4. Conclusions
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In this study, MMT have been modified by DA with five different concentrations
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resulted in distinct lamellae structure. As the DA concentration increased from 0.5 to 3
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mg/L, the resulted MMT-Dx showed increased content of PDA coating and
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exfoliation extent. Therefore, when incorporated into ST/CS matrix with the same
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loading, MMT-Dx with higher PDA content presented better dispersion properties and
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thus had greater reinforcement effects on the composite films. Therefore, complete
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exfoliation structure of MMT can be obtained via optimization of PDA modification
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conditions, which would make them good candidates as nanofillers for biopolymers
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reinforcement.
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Acknowledgements
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The author acknowledges the financial support from General Financial Grant from
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the China Postdoctoral Science Foundation (2014M552298) and the Research Fund
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for the Doctoral Program (SWU111039) of Southwest University, China.
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References
ACCEPTED MANUSCRIPT 353 Akter, N., Khan, R. A., Tuhin, M. O., Haque, M. E., Nurnabi, M., Parvin, F., et al. (2014). Thermomechanical, barrier, and morphological properties of chitosan-reinforced starch-based biodegradable composite films. Journal of Thermoplastic Composite Materials, 27(7), 933–948. Almasi, H., Ghanbarzadeh, B., & Entezami, A. A. (2010). Physicochemical properties of biodegradable
films.
Macromolecules, 46(1), 1–5.
International
Journal
of
Biological
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starch-CMC-nanoclay
Aouada, F. A., Mattoso, L. H. C., & Longo, E. (2011). New strategies in the preparation of exfoliated thermoplastic starch-montmorillonite nanocomposites. Industrial Crops and Products, 34(3), 1502–1508.
SC
Arroyo, O. H., Huneault, M. A., Favis, B. D., & Bureau, M. N. (2010). Processing and properties of PLA/thermoplastic starch/montmorillonite nanocomposites. Polymer Composites, 31(1), 114–127.
Baran, T., Aciksoz, E., & Mentes, A. (2016). Highly efficient, quick and green synthesis of biarlys with
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chitosan supported catalyst using microwave irradiation in the absence of solvent. Carbohydrate Polymers, 142, 189–198.
Bie, P., Liu, P., Yu, L., Li, X., Chen, L., Xie, F., et al. (2013). The properties of antimicrobial films derived from poly(lactic acid)/starch/chitosan blended matrix. Carbohydrate Polymers, 98(1), 959–966.
Chivrac, F., Pollet, E., & Avérous, L. (2009). Progress in nano-biocomposites based on polysaccharides and nanoclays. Materials Science and Engineering: R: Reports, 67(1), 1–17.
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Fu, S.Y., Feng, X.Q., Lauke, B., & Mai, Y.W. (2008). Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of particulate-polymer composites. Composites Part B-Engineering, 39(6), 933–961. Heydari, A., Alemzadeh, I., & Vossoughi, M. (2013). Functional properties of biodegradable corn starch nanocomposites for food packaging applications. Materials & Design, 50, 954–961.
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Johansson, C., & Clegg, F. (2015). Effect of clay type on dispersion and barrier properties of hydrophobically modified poly(vinyl alcohol)–bentonite nanocomposites. Journal of Applied Polymer Science, 132(28), No. 42229. Khan, M. A., Rahman, M. A., Khan, R. A., Rahman, N., Islam, J. M. M., Alam, R., et al. (2010).
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Preparation and characterization of the mechanical properties of the photocured chitosan/starch blend film. Polymer-Plastics Technology and Engineering, 49(7), 748–756.
Lee, H., Scherer, N. F., & Messersmith, P. B. (2006). Single-molecule mechanics of mussel adhesion. Proceedings of the National Academy of Sciences of the United States of America, 103(35), 12999–13003.
Li, J., Ye, F., Liu, J., & Zhao, G. (2015). Effects of octenylsuccination on physical, mechanical and moisture-proof properties of stretchable sweet potato starch film. Food Hydrocolloids, 46, 226–232. Maksimov, R. D., Lagzdins, A., Lilichenko, N., & Plume, E. (2009). Mechanical properties and water vapor permeability of starch/montmorillonite nanocomposites. Polymer Engineering and Science, 49(12), 2421–2429. Mathew, S., & Abraham, T. E. (2008). Characterisation of ferulic acid incorporated starch–chitosan
ACCEPTED MANUSCRIPT blend films. Food Hydrocolloids, 22(5), 826–835. Monvisade, P., & Siriphannon, P. (2009). Chitosan intercalated montmorillonite: Preparation, characterization and cationic dye adsorption. Applied Clay Science, 42(3–4), 427–431. Ortega-Toro, R., Jimenez, A., Talens, P., & Chiralt, A. (2014). Properties of starch-hydroxypropyl methylcellulose based films obtained by compression molding. Carbohydrate polymers, 109, 155–165. Pan, Y., Bao, H., & Li, L. (2011). Noncovalently functionalized multiwalled carbon nanotubes by films. Acs Applied Materials & Interfaces, 3(12), 4819–4830.
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Chitosan-grafted reduced graphene oxide and their synergistic reinforcing effects in chitosan Priya, B., Gupta, V. K., Pathania, D., & Singha, A. S. (2014). Synthesis, characterization and antibacterial activity of biodegradable starch/PVA composite films reinforced with cellulosic fibre. Carbohydrate Polymers, 109, 171–179. characterization
of
thermoplastic
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Schlemmer, D., Angelica, R. S., & Sales, M. J. A. (2010). Morphological and thermomechanical starch/montmorillonite
Structures, 92(9), 2066–2070.
nanocomposites.
Composite
Slavutsky, A. M., & Bertuzzi, M. A. (2012). A phenomenological and thermodynamic study of the
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water permeation process in corn starch/MMT films. Carbohydrate Polymers, 90(1), 551–557. Tee, T.T., Sin, L. T., Gobinath, R., Bee, S.T., Hui, D., Rahmat, A. R., et al. (2013). Investigation of nano-size montmorillonite on enhancing polyvinyl alcohol-starch blends prepared via solution cast approach. Composites Part B-Engineering, 47, 238–247.
Tuhin, M. O., Rahman, N., Haque, M. E., Khan, R. A., Dafader, N. C., Islam, R., et al. (2012). Modification of mechanical and thermal property of chitosan-starch blend films. Radiation Physics and Chemistry, 81(10), 1659–1668.
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Wei, Q., Zhang, F., Li, J., Li, B., & Zhao, C. (2010). Oxidant-induced dopamine polymerization for multifunctional coatings. Polymer Chemistry, 1(9), 1430–1433. Xuan, Y., Jiang, G., Li, Y., Wang, J., & Geng, H. (2013). Inhibiting effect of dopamine adsorption and polymerization on hydrated swelling of montmorillonite. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 422, 50–60.
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Zhou, M., & Xu, D. (2015). Starch-MMT composite films: Effects of bio-inspired modification on MMT. Starch-Starke, 67(5-6), 470–477.
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Fig. 1. XRD profiles of (a) MMT, (b) MMT-D0.5, (c) MMT-D1, (d) MMT-D2, (e)
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MMT-D3, and (f) MMT-D5
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Fig. 2. FT-IR spectra of (a) MMT, (b) MMT-D0.5, (c) MMT-D1, (d) MMT-D2, (e)
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MMT-D3, (f) MMT-D5 and (g) PDA
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Fig. 3. Surface micrographs of (a) MMT, (b) MMT-D0.5, (c) MMT-D1, (d) MMT-D2,
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(e) MMT-D3, and (f) MMT-D5
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Fig. 4. TGA (I) and DTG (II) curves of (a) MMT, (b) MMT-D0.5, (c) MMT-D1, (d)
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MMT-D2, (e) MMT-D3, (f) MMT-D5 and (g) PDA
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Fig.
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ST/CS-MMT-D0.5, (d) ST/CS-MMT-D1, (e) ST/CS-MMT-D2, (f) ST/CS-MMT-D3,
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and (g) ST/CS-MMT-D5
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Fig. 6. XRD profiles of films: (a) ST/CS, (b) ST/CS-MMT, (c) ST/CS-MMT-D0.5, (d)
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ST/CS-MMT-D1, (e) ST/CS-MMT-D2, (f) ST/CS-MMT-D3, and (g) ST/CS-MMT-D5
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Fig. 7. TGA (I) and DTG (II) curves of films: (a) ST/CS, (b) ST/CS-MMT, (c)
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ST/CS-MMT-D1, (d) ST/CS-MMT-D3, and (e) ST/CS-MMT-D5
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Table 1
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Tensile properties of the composite films Tensile strength
Young’s modulus
Elongation at
(MPa)
(GPa)
Break (%)
ST/CS
14.49 ± 1.43e
0.93 ± 0.06d
ST/CS-MMT
15.44 ± 1.13e
1.14 ± 0.08c
ST/CS-MMT-D0.5
19.27 ± 1.20d
1.18 ± 0.07c
ST/CS-MMT-D1
24.00 ± 1.53c
1.19 ± 0.04c
ST/CS-MMT-D2
26.82 ± 1.79b
1.36 ± 0.05b
6.70 ± 0.14cd
ST/CS-MMT-D3
30.44 ± 1.80a
1.42 ± 0.06ab
5.95 ± 0.57cd
ST/CS-MMT-D5
32.62 ± 1.83a
1.51 ± 0.08a
5.77 ± 0.55d
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14.65 ± 0.21a
11.03 ± 1.10b
10.70 ± 1.41b 7.11 ± 0.56c
Mean values in the same column with different letters are significantly different (p <
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0.05)
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Highlights:
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1. MMT was modified by polydopamine using different initial dopamine
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concentration 2. Modified MMT (MMT-Dx) showed different lamellae structure and dispersibility
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3. MMT-Dx can better reinforce starch/chitosan composite films compared to neat
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S0268-005X(16)30279-X
DOI:
10.1016/j.foodhyd.2016.06.030
Reference:
FOOHYD 3484
To appear in:
Food Hydrocolloids
Received Date: 15 September 2015 Revised Date:
21 June 2016
Accepted Date: 22 June 2016
Please cite this article as: Zhou, M., Liu, Q., Wu, S., Gou, Z., Wu, X., Xu, D., Starch/chitosan films reinforced with polydopamine modified MMT: Effects of dopamine concentration, Food Hydrocolloids (2016), doi: 10.1016/j.foodhyd.2016.06.030. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Starch/chitosan films reinforced with polydopamine modified MMT: Effects of
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dopamine concentration
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Min Zhou, Qin Liu, Shuaishuai Wu, Zhenqiong Gou, Xiyu Wu, Dan Xu
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College of Food Science, Southwest University, Chongqing 400715, PR China
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Correspondence: Dan Xu, College of Food Science, Southwest University,
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Chongqing 400715, China.
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Email: [email protected]
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Abstract: Montmorillonite (MMT) was modified by bio-inspired polydopamine
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(PDA), which is the self-polymerized product of dopamine (DA), to improve the
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dispersion property of MMT in polymer matrix. As revealed by FTIR spectra, XRD
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profiles and SEM micrographs, PDA has been successfully coated onto MMT surface
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thus enlarge the interlayer spacing, resulting PDA modified MMT (MMT-Dx). The
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lamellar structure of MMT-Dx was found to be highly related to the DA concentration
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applied during the modification process. The exfoliation degree of MMT-Dx increased
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along with the weight content of PDA coating, as the DA concentration increased
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from 0.5 mg/mL to 3 mg/mL. MMT-Dx was then incorporated into starch/chitosan
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(ST/CS) blended matrix at a loading level of 2 wt.% to reinforce the bio-polymers.
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Compared to neat MMT, MMT-Dx showed much higher dispersion capability as
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observed from XRD profiles and SEM micrographs. Furthermore, as DA
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concentration increased, the tensile strength and Young’s modulus of the composite
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displayed a tensile strength as high as 30.44 MPa and a Young’s modulus as high as
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1.42 GPa, which were 2.1 and 1.6 times the values of the neat ST/CS film,
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respectively. Therefore, to adopt PDA modification, MMT-Dx with adjustable
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interlayer spacing and different dispersion properties can be obtained, which provide
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multiple choices of nano-fillers for bio-polymers reinforcement; thus accelerate the
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acceptances of these bio-polymers in food packaging and related applications.
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Key words: Polydopamine; Montmorillonite; Starch; Chitosan; Nanocomposite film
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1. Introduction
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Nowadays, bio-polymers have received growing concerns all over the world, due
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to their exciting potentials for replacing petroleum-based plastics in a variety of
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applications. The large-scale application of bio-polymers in areas such as food
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packaging will be one of the most promising solutions of environment pollution
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caused by non-biodegradable plastics (Aouada, Mattoso, & Longo, 2011). When
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bio-polymers are mentioned, starch (ST) with attractive price, wide availability,
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thermoplastic property and biodegradability is absolutely at one of the best
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advantageous position in this regard (Almasi, Ghanbarzadeh, & Entezami, 2010;
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Schlemmer, Angelica, & Sales, 2010). ST films with high safety are famous for
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extending the shelf-life of various foods and maintaining their quality (Li, Ye, Liu, &
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as poor mechanical properties and moisture resistance, make them insufficient for
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many packaging purposes (Priya, Gupta, Pathania, & Singha, 2014). Blending with
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other polymers such as chitosan (CS) may improve the strength and moisture
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resistance, to a certain extent. CS, which is derived by deacetylation of chitin, has
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many advantages, such as good biodegradability and biocompatibility, high
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antimicrobial activity, excellent film-forming properties (Bie, et al., 2013). Moreover,
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CS may interact with ST through hydroxyl, amide and residual amide groups, thus
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reinforcing ST-based films (Akter, et al., 2014). However
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properties by blending are limited by the intrinsic properties of polymers.
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improvements on ST
Fortunately a great breakthrough has been made by bio-nanocomposites in recent
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years, which is filling bio-polymers with inorganic/organic nano-particles. When
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nano-particles with excellent mechanical and thermal properties are filled into
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bio-polymers, they can interact with polymer chains at nanoscale, leading to
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significant improvements on the properties of the resulting nanocomposites. Therefore,
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keys to prepare composite films with satisfied performances are homogeneous
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dispersion of nano-fillers in the matrix and strong interface interactions (Pan, Bao, &
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Li, 2011).
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Montmorillonite (MMT) is one of the most extensively studied nanofillers due to
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its low cost, high specific surface area, good biocompatibility, high strength and other
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outstanding properties (Arroyo, Huneault, Favis, & Bureau, 2010; Heydari,
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Alemzadeh, & Vossoughi, 2013; Maksimov, Lagzdins, Lilichenko, & Plume, 2009).
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are tactoids, intercalated layers, and randomly exfoliated lamellae (Johansson &
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Clegg, 2015). In the later two cases, polymer chains are able to interact with MMT
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lamellae at nanoscale obtaining nanocomposites with excellent properties. Therefore,
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many strategies have been explored to improve the dispersion properties of MMT
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(Chivrac, Pollet, & Avérous, 2009). Surface modification is one of the most widely
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adopted methods not only because of its diversity, but also due to its capability to
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adjust the surface properties of MMT for the purpose of enhancing compatibility with
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polymer matrix.
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Polydopamine (PDA) coating is a newly developed surface modification method,
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which is inspired by the wet adhesion of marine mussel adhesive protein. Under weak
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alkaline conditions, dopamine (DA) will self-polymerize to PDA and thus form a thin
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layer onto the surface of various materials (Wei, et al., 2010; Xuan, et al., 2013). With
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active catechol and amine groups, PDA can interact with many polar polymers
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through hydrogen bonding and provide a platform for further modification (Lee,
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Scherer, & Messersmith, 2006; Ortega-Toro, Jimenez, Talens, & Chiralt, 2014). Our
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previous study have found that PDA coating on MMT surface was able to prevent the
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regular stack of MMT lamellae and induce the exfoliated structure (Zhou & Xu, 2015).
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More importantly, PDA modified MMT (MMT-Dx) showed much better reinforcing
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effect on the ST films compared with neat MMT, which was attributed to the
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enhanced interfacial interactions thus the effective stress transfer between MMT and
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polymer matrix. However, our latest study found that the initial DA concentration
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and thus, largely affect the properties of nanocomposites filled with modified MMT
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(Zhou & Xu, 2015). Therefore, in this study, we will focus on the effects of DA
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concentration on MMT lamellae structure and furthermore on the properties of ST and
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CS blended films.
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2. Experimental
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2.1. Materials
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DA was purchased from Bio Basic Inc., Canada. MMT-Na+, with a
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cation-exchange capacity (CEC) of 1.45 meq/g, was purchased from Southern Clay
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Products Inc., USA. Corn starch (approximately 20 wt.% amylose) was provided by
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Tianjin Zhongying Health Food Co. Ltd. Chitosan (DD of 90%) was obtained from
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Weifang Seasource Biological Products Co., Ltd. Tris (hydroxymethyl) aminomethane
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(Tris) was purchased from Biosharp Co. Ltd., USA. Analytical grade glycerol was
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used as received.
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2.2. PDA modification on MMT
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MMT-Dx was prepared following the procedure reported in our earlier
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publication (Zhou & Xu, 2015). After MMT (0.2 g) was added to the Tris-HCl
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stirred for 24 h at room temperature. After filtration, washing and freeze-drying,
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MMT-D was obtained. The DA concentration used was 0.5 mg/mL, 1 mg/mL, 2
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mg/mL, 3 mg/mL and 5 mg/mL, respectively. The modified MMT was designated as
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MMT-Dx, where x represents the concentration (mg/mL) of DA used to modify
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MMT.
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2.3. Preparation of ST/CS-MMT-Dx composite films
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The composite films were prepared by solution-blending method. After 3.5 g ST and 0.8 g glycerol were dissolved in de-ionized (DI) water and plasticized at 75
for
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2 h, MMT-Dx was added. The mixture was stirred for 22 h at room temperature
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followed by ultrasonic treatment for 30 min. Then, CS solution which was prepared
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by dissolving 2.3 g CS powder in 230 mL 1% (V/V) acetic acid aqueous solution, was
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added into the above mixture and stirred for another 2 h. For all the films, the weight
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ratio of ST/CS was 60/40. The obtained homogeneous solution was poured into Petri
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dishes and dried in an oven at 50
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the solvent. Based on our previous study (Zhou & Xu, 2015), the loading level of
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MMT-Dx was kept at 2 wt.% of the total weight of ST and CS powder. The obtained
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composite films containing MMT-Dx were denoted as ST/CS-MMT-Dx. The neat
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ST/CS films and the composite films containing pristine MMT were also prepared as
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controls and denoted as ST/CS and ST/CS-MMT, respectively. The above films with
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for about 3 ~ 4 h to allow the fully evaporation of
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thickness of 30 ~ 60 µm were preconditioned at 25
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prior to test.
and 60% RH for at least 24 h
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2.4. Characterizations
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2.4.1. X-ray diffraction (XRD)
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Samples were scanned by a SHIMADZU XRD-7000 diffractometer operated from
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2θ = 3º to 40º at a scanning rate of 2º/min with Cu Kα radiation.
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2.4.2. Fourier transform infrared spectroscopy (FTIR)
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MMT, PDA and MMT-Dx were mixed with KBr and pressed to pellets, while
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composites films with thickness around 5 µm were prepared by diluted casting
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solution prior to FTIR scanning. All the pellets and films were scanned by a Nicolet
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6700 FTIR spectrometer from 450 cm -1 to 2000 cm-1 with a resolution of 4 cm -1.
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The micrographs of MMT-Dx powder and composite films were collected by a JEOL JEM-2100 scanning electron microscope after gold sputter coating.
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2.4.4. Thermo-gravimetric analysis (TGA)
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TA Q50 thermo-gravimetric analyser was used to evaluate the thermal degradation
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behavior of samples. Powder or film flakes were heated at a rate of 10
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under a nitrogen atmosphere.
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2.4.5. Tensile properties
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An auto tensile tester (XLW-PC) from Labthink Instruments Co. Ltd. was used to
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measure the tensile properties of the composite films. At least 10 samples of each kind
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of film were tested to calculate the average value. Samples with a gauge length of 50
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mm and a width of 10 mm were stretched at 25
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mm/min.
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using a cross head speed of 50
The tensile properties were statistically compared between films by one-way
172
analysis of variance (ANOVA). The significance of the mean values of tensile
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strength, Young’s modulus and elongation at break was determined by Duncan’s
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multiple range testing with p < 0.05.
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3. Results and discussion
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3.1. Characterizations of MMT-Dx
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3.1.1. XRD studies
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The XRD patterns of MMT-Dx and neat MMT were compared in Fig.1. As the
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001 peak of neat MMT was located at 7.00º, the interlayer distance d001 of MMT was
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calculated to be 1.26 nm according to Bragg’s equation, which was consistent with the
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reported value (Slavutsky & Bertuzzi, 2012). After modification, all of the 001 peaks
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of MMT-Dx shifted to the left indicating the enlarged d001 after modification. The
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calculated d001 of MMT-D0.5, MMT-D1 and MMT-D2 increased to 1.35 nm (2θ =
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6.54º), 1.69 nm (2θ = 5.22º) and 2.04 nm (2θ = 4.32º), respectively. According to the
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literature (Xuan, et al., 2013), the layer thickness of MMT is about 0.96 nm. Whereas,
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the bilayer of 5, 6-dihydroxyindole (DHI), which is the intermediate product of DA
191
polymerization and forms nano-aggregates linking to PDA aggregates, has a thickness
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of approximately 0.34 nm. Therefore, by subtracting the MMT layer thickness, the
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interlayer distance of MMT-D0.5, MMT-D1 and MMT-D2 was 0.39 nm, 0.73 nm, and
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1.08 nm, which were approximately 1, 2 and 3 times of the thickness of DHI bilayer,
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respectively. When it came to MMT-D3, the 001 peak completely disappeared within
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the testing range, which may indicate a high extent of exfoliation of MMT lamellas.
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However, when DA concentration continued to increase, a broad peak appeared again
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at 4.28°, corresponding to the d001 of 2.06 nm. During the modification process, DA
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then polymerize to PDA coating layer. As a result, the thickness of PDA layer, which
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relies on the amount of adsorbed DA, is expected to have great effects on the lamellar
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structure of MMT-Dx. It may be speculated from the above results that with
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increasing DA concentration, more DA molecules are available in the solution and
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thus adsorbed onto MMT surface. Therefore, thick PDA layers formed between MMT
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lamellae leading to growing d001 until exfoliation structure obtained. However, when
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excess DA molecules (5 mg/mL) are applied, PDA may self-aggregate around MMT
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stacks, preventing the formation of exfoliation structure.
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3.1.2. FTIR studies
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characteristic peaks of MMT are observed at 1624 cm-1, 1400 cm-1, 1022 cm-1, 910
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cm-1, 518 cm-1, and 462 cm-1, which are known as the water deformation band, Si-O
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stretching band, Al-Al-OH vibration band, Al-O-Si, and Si-O-Si deformation bands,
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respectively. In Fig. 2(g), peaks at 1400 cm-1, 1497 cm-1 and 1621 cm-1 as pointed by
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the bottom three arrows are assigned to the bending vibration of phenyl of PDA, the
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stretching vibrations of C=C and C=N, respectively, while peaks at 1264 cm−1 and
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1285 cm−1 are attributed to the catechol OH bending vibrations (Xuan, et al., 2013).
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This indicates the oxidative polymerization of DA to PDA in Tris-HCl buffer solution.
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After PDA modifications, new peaks appeared at around 1268 cm-1 and 1485 cm-1 in
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N-H shearing vibrations and the catechol OH bending vibrations of PDA. Therefore,
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it can be speculated that PDA layer have been successfully attached onto MMT
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surface. With increasing DA concentration, the intensity of these two peaks increased
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and reached the maximum for MMT-D3.
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3.1.3. SEM studies
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The SEM micrographs of MMT and MMT-Dx surface are presented in Fig. 3. It is
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observed that most lamellae of MMT stacked together without obvious single MMT
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layers. After modification with increasing DA concentration, the lamellar structure of
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MMT-Dx became more and more clear. When DA concentration was 3 mg/mL,
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exfoliated MMT lamellae were highly separated from each other as illustrated in Fig.
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3(e). This is in accordance with the disappearance of 001 peak in the XRD
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diffractogram of MMT-D3 as discussed before. However, when DA concentration
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further increased to 5 mg/mL, the exfoliation degree of MMT lamellae decreased.
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3.1.4. TGA studies
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TGA was utilized to estimate the content of PDA in MMT-Dx based on the
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different thermal stability of MMT and PDA. As shown in Fig. 4, MMT has excellent
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thermal stability with a residue as high as 91.3% at 800 , while PDA started to
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degrade at approximately 140
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weight loss of MMT-Dx at 800
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the order of MMT-D3 > MMT-D5 > MMT-D1 > MMT-D0.5. As the thermal stability
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of PDA was much poorer compared to MMT, therefore higher weight loss of
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MMT-Dx might indicate higher content of PDA in MMT-Dx. This was in accordance
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with our speculation from XRD diffractograms, that the thickness PDA coating layers
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formed on MMT surface increased when DA concentration increased from 0.5 to 3
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mg/ml. When DA concentration was 3 mg/ml, most of the adsorption sites on MMT
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surface were occupied by DA molecules and then polymerized to PDA, leading to the
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maximum amount of PDA coating in MMT-D3 and thus the highest degree of
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exfoliation.
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lied between these of MMT and PDA, following
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and showed a residue of only 54.8%. Reasonably,
The micrographs of the film surface are shown in Fig. 5. In Fig. 5(a), the ST/CS
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film has a dense and smooth surface without defects due to the good compatibility
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between CS and starch (Khan, et al., 2010). With the incorporation of neat MMT,
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MMT aggregates around 1 to 3 µm were observed in the matrix as showed in Fig. 5(b).
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However, when the same amount of MMT-Dx was loaded, the aggregations became
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smaller and less, specifically for MMT-D3. This indicated that MMT-Dx were
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PDA coating layer with catechol groups might improve the compatibility between
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MMT-Dx and the hydrophilic matrix. Moreover, dispersion properties of MMT-Dx in
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the matrix were positively correlated to their exfoliation degree before they were
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added into ST/CS matrix. As revealed by section 3.1.1 and section 3.1.3, MMT-D3
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showed the highest degree of exfoliation. Therefore, after they were added into ST/CS
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matrix, lamellae in MMT-D3 were better dispersed compared with other samples.
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3.2.2. XRD studies
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Fig. 6 displays the XRD profiles of ST/CS film and composite films. In our study,
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ST/CS film showed a broad peak centered at 20°, which might be attributed to the
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crystalline structure of chitosan (Baran, Aciksoz, & Mentes, 2016). However, the
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characteristic peaks of ST at 2θ = 16.9°, 19.3° and 22.4° as we have observed in our
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previous study (Zhou & Xu, 2015) disappeared, which have been reported before
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(Mathew & Abraham, 2008). With the addition of MMT, a small peak at 3.9°
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appeared, which was attributed to the d001 diffraction of MMT corresponding to an
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inter-layer distance of 2.26 nm. In our previous study (Zhou & Xu, 2015), this peak
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shifted to 4.68° indicating an interlayer distance of 1.89 nm when only ST and
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glycerol contributed to the intercalation. The increased interlayer spacing and reduced
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peak intensity in this study might be attributed to the existence of CS chains in the
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matrix which could enter the MMT gallery via ion exchange (Monvisade &
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peaks were not observed in all the composite films within the test range, indicating
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highly exfoliations of MMT lamellae in the matrix. As we have discussed previously
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that PDA coating was not only able to enlarge the interlayer spacing of MMT but also
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enhance the interactions between MMT and the matrix polymer, good dispersion of
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MMT-Dx in the matrix with little stacked lamellae was possibly achieved under this
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circumstance. However, addition of MMT-Dx didn’t have significant effect on the
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crystalline structure of starch and CS blended matrix.
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3.2.3. TGA studies
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The TGA and differential thermogravimetric analysis (DTG) curves of ST/CS
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films and composite films are shown in Fig. 7. All the composite films displayed very
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similar thermal degradation behaviors to that of ST/CS film. There were no significant
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differences on Tonset, Tmax and char residues between ST/CS film and all the composite
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films. Therefore, addition of either MMT or MMT-Dx didn’t have significant impact
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on the thermal decomposition behaviors of the composite films.
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Table 1 shows the values of tensile strength, Young’s modulus and elongation at
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break of ST/CS film and composite films. With the addition of neat MMT, only
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film. However, when MMT-Dx was added, both tensile strength and Young’s modulus
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of the composite films increased simultaneously. Furthermore, the reinforcement
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effects were related to the DA concentration used to modify MMT, as
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ST/CS-MMT-D3 and ST/CS-MMT-D5 films showed the highest tensile strength and
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Young’s modulus. Since we have discussed earlier that the dispersion capability of
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MMT were enhanced as the DA concentration increased. Therefore, the better
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dispersed MMT-D3 and MMT-D5 would have larger contact areas with the polymer
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matrix compared with MMT-D0.5, MMT-D1, MMT-D2 and MMT, which resulted in
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more effective stress transfer between fillers and matrix (Fu, Feng, Lauke, & Mai,
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2008). That might be the reason for the improved tensile strength. Moreover, the
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catechol groups of PDA were able to interact with the polar groups of ST and CS,
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such as hydroxyl and amino groups. Therefore, the increased content of PDA coating
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from MMT-D0.5 to MMT-D3 indicated more interactions between MMT-Dx and the
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matrix. As the interfacial adhesion increased, more stress was required to produce a
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given amount of strain when the composite films were stretched by force, resulting in
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higher values of Young’s modulus. However, variations of the elongation at break of
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the composite films just showed the opposite trend that with the addition of MMT,
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and MMT-D0.5 to MMT-D3, the values gradually decreased. Tee et al. (Tee, et al.,
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2013) have reported similar results and explained that the addition of small amount of
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MMT leaded to the regular arrangement of starch chain. In this study, polymer chains
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were reasonably confined more strictly by the finer dispersed MMT-Dx lamellae. As a
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result, the ST/CS-MMT-D3 and ST/CS-MMT-D5 films presented the lowest
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elongation at break.
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4. Conclusions
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In this study, MMT have been modified by DA with five different concentrations
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resulted in distinct lamellae structure. As the DA concentration increased from 0.5 to 3
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mg/L, the resulted MMT-Dx showed increased content of PDA coating and
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exfoliation extent. Therefore, when incorporated into ST/CS matrix with the same
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loading, MMT-Dx with higher PDA content presented better dispersion properties and
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thus had greater reinforcement effects on the composite films. Therefore, complete
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exfoliation structure of MMT can be obtained via optimization of PDA modification
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conditions, which would make them good candidates as nanofillers for biopolymers
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reinforcement.
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Acknowledgements
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The author acknowledges the financial support from General Financial Grant from
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the China Postdoctoral Science Foundation (2014M552298) and the Research Fund
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for the Doctoral Program (SWU111039) of Southwest University, China.
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References
ACCEPTED MANUSCRIPT 353 Akter, N., Khan, R. A., Tuhin, M. O., Haque, M. E., Nurnabi, M., Parvin, F., et al. (2014). Thermomechanical, barrier, and morphological properties of chitosan-reinforced starch-based biodegradable composite films. Journal of Thermoplastic Composite Materials, 27(7), 933–948. Almasi, H., Ghanbarzadeh, B., & Entezami, A. A. (2010). Physicochemical properties of biodegradable
films.
Macromolecules, 46(1), 1–5.
International
Journal
of
Biological
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starch-CMC-nanoclay
Aouada, F. A., Mattoso, L. H. C., & Longo, E. (2011). New strategies in the preparation of exfoliated thermoplastic starch-montmorillonite nanocomposites. Industrial Crops and Products, 34(3), 1502–1508.
SC
Arroyo, O. H., Huneault, M. A., Favis, B. D., & Bureau, M. N. (2010). Processing and properties of PLA/thermoplastic starch/montmorillonite nanocomposites. Polymer Composites, 31(1), 114–127.
Baran, T., Aciksoz, E., & Mentes, A. (2016). Highly efficient, quick and green synthesis of biarlys with
M AN U
chitosan supported catalyst using microwave irradiation in the absence of solvent. Carbohydrate Polymers, 142, 189–198.
Bie, P., Liu, P., Yu, L., Li, X., Chen, L., Xie, F., et al. (2013). The properties of antimicrobial films derived from poly(lactic acid)/starch/chitosan blended matrix. Carbohydrate Polymers, 98(1), 959–966.
Chivrac, F., Pollet, E., & Avérous, L. (2009). Progress in nano-biocomposites based on polysaccharides and nanoclays. Materials Science and Engineering: R: Reports, 67(1), 1–17.
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Fu, S.Y., Feng, X.Q., Lauke, B., & Mai, Y.W. (2008). Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of particulate-polymer composites. Composites Part B-Engineering, 39(6), 933–961. Heydari, A., Alemzadeh, I., & Vossoughi, M. (2013). Functional properties of biodegradable corn starch nanocomposites for food packaging applications. Materials & Design, 50, 954–961.
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Johansson, C., & Clegg, F. (2015). Effect of clay type on dispersion and barrier properties of hydrophobically modified poly(vinyl alcohol)–bentonite nanocomposites. Journal of Applied Polymer Science, 132(28), No. 42229. Khan, M. A., Rahman, M. A., Khan, R. A., Rahman, N., Islam, J. M. M., Alam, R., et al. (2010).
AC C
354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395
Preparation and characterization of the mechanical properties of the photocured chitosan/starch blend film. Polymer-Plastics Technology and Engineering, 49(7), 748–756.
Lee, H., Scherer, N. F., & Messersmith, P. B. (2006). Single-molecule mechanics of mussel adhesion. Proceedings of the National Academy of Sciences of the United States of America, 103(35), 12999–13003.
Li, J., Ye, F., Liu, J., & Zhao, G. (2015). Effects of octenylsuccination on physical, mechanical and moisture-proof properties of stretchable sweet potato starch film. Food Hydrocolloids, 46, 226–232. Maksimov, R. D., Lagzdins, A., Lilichenko, N., & Plume, E. (2009). Mechanical properties and water vapor permeability of starch/montmorillonite nanocomposites. Polymer Engineering and Science, 49(12), 2421–2429. Mathew, S., & Abraham, T. E. (2008). Characterisation of ferulic acid incorporated starch–chitosan
ACCEPTED MANUSCRIPT blend films. Food Hydrocolloids, 22(5), 826–835. Monvisade, P., & Siriphannon, P. (2009). Chitosan intercalated montmorillonite: Preparation, characterization and cationic dye adsorption. Applied Clay Science, 42(3–4), 427–431. Ortega-Toro, R., Jimenez, A., Talens, P., & Chiralt, A. (2014). Properties of starch-hydroxypropyl methylcellulose based films obtained by compression molding. Carbohydrate polymers, 109, 155–165. Pan, Y., Bao, H., & Li, L. (2011). Noncovalently functionalized multiwalled carbon nanotubes by films. Acs Applied Materials & Interfaces, 3(12), 4819–4830.
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Chitosan-grafted reduced graphene oxide and their synergistic reinforcing effects in chitosan Priya, B., Gupta, V. K., Pathania, D., & Singha, A. S. (2014). Synthesis, characterization and antibacterial activity of biodegradable starch/PVA composite films reinforced with cellulosic fibre. Carbohydrate Polymers, 109, 171–179. characterization
of
thermoplastic
SC
Schlemmer, D., Angelica, R. S., & Sales, M. J. A. (2010). Morphological and thermomechanical starch/montmorillonite
Structures, 92(9), 2066–2070.
nanocomposites.
Composite
Slavutsky, A. M., & Bertuzzi, M. A. (2012). A phenomenological and thermodynamic study of the
M AN U
water permeation process in corn starch/MMT films. Carbohydrate Polymers, 90(1), 551–557. Tee, T.T., Sin, L. T., Gobinath, R., Bee, S.T., Hui, D., Rahmat, A. R., et al. (2013). Investigation of nano-size montmorillonite on enhancing polyvinyl alcohol-starch blends prepared via solution cast approach. Composites Part B-Engineering, 47, 238–247.
Tuhin, M. O., Rahman, N., Haque, M. E., Khan, R. A., Dafader, N. C., Islam, R., et al. (2012). Modification of mechanical and thermal property of chitosan-starch blend films. Radiation Physics and Chemistry, 81(10), 1659–1668.
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Wei, Q., Zhang, F., Li, J., Li, B., & Zhao, C. (2010). Oxidant-induced dopamine polymerization for multifunctional coatings. Polymer Chemistry, 1(9), 1430–1433. Xuan, Y., Jiang, G., Li, Y., Wang, J., & Geng, H. (2013). Inhibiting effect of dopamine adsorption and polymerization on hydrated swelling of montmorillonite. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 422, 50–60.
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Zhou, M., & Xu, D. (2015). Starch-MMT composite films: Effects of bio-inspired modification on MMT. Starch-Starke, 67(5-6), 470–477.
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Fig. 1. XRD profiles of (a) MMT, (b) MMT-D0.5, (c) MMT-D1, (d) MMT-D2, (e)
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MMT-D3, and (f) MMT-D5
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Fig. 2. FT-IR spectra of (a) MMT, (b) MMT-D0.5, (c) MMT-D1, (d) MMT-D2, (e)
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MMT-D3, (f) MMT-D5 and (g) PDA
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Fig. 3. Surface micrographs of (a) MMT, (b) MMT-D0.5, (c) MMT-D1, (d) MMT-D2,
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(e) MMT-D3, and (f) MMT-D5
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Fig. 4. TGA (I) and DTG (II) curves of (a) MMT, (b) MMT-D0.5, (c) MMT-D1, (d)
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MMT-D2, (e) MMT-D3, (f) MMT-D5 and (g) PDA
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Fig.
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ST/CS-MMT-D0.5, (d) ST/CS-MMT-D1, (e) ST/CS-MMT-D2, (f) ST/CS-MMT-D3,
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and (g) ST/CS-MMT-D5
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Fig. 6. XRD profiles of films: (a) ST/CS, (b) ST/CS-MMT, (c) ST/CS-MMT-D0.5, (d)
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ST/CS-MMT-D1, (e) ST/CS-MMT-D2, (f) ST/CS-MMT-D3, and (g) ST/CS-MMT-D5
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Fig. 7. TGA (I) and DTG (II) curves of films: (a) ST/CS, (b) ST/CS-MMT, (c)
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ST/CS-MMT-D1, (d) ST/CS-MMT-D3, and (e) ST/CS-MMT-D5
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Table 1
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Tensile properties of the composite films Tensile strength
Young’s modulus
Elongation at
(MPa)
(GPa)
Break (%)
ST/CS
14.49 ± 1.43e
0.93 ± 0.06d
ST/CS-MMT
15.44 ± 1.13e
1.14 ± 0.08c
ST/CS-MMT-D0.5
19.27 ± 1.20d
1.18 ± 0.07c
ST/CS-MMT-D1
24.00 ± 1.53c
1.19 ± 0.04c
ST/CS-MMT-D2
26.82 ± 1.79b
1.36 ± 0.05b
6.70 ± 0.14cd
ST/CS-MMT-D3
30.44 ± 1.80a
1.42 ± 0.06ab
5.95 ± 0.57cd
ST/CS-MMT-D5
32.62 ± 1.83a
1.51 ± 0.08a
5.77 ± 0.55d
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14.65 ± 0.21a
11.03 ± 1.10b
10.70 ± 1.41b 7.11 ± 0.56c
Mean values in the same column with different letters are significantly different (p <
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Highlights:
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1. MMT was modified by polydopamine using different initial dopamine
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concentration 2. Modified MMT (MMT-Dx) showed different lamellae structure and dispersibility
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3. MMT-Dx can better reinforce starch/chitosan composite films compared to neat
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