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Biodegradability and mechanical properties of reinforced starch nanocomposites using cellulose nanofibers
Accepted Manuscript Title: Biodegradability and mechanical properties of reinforced starch nanocomposites using cellulose nanofibers Author: Mehran Babaee Mehdi Jonoobi Yahya Hamzeh Alireza Ashori PII: DOI: Reference:
S0144-8617(15)00550-0 http://dx.doi.org/doi:10.1016/j.carbpol.2015.06.043 CARP 10032
To appear in: Received date: Revised date: Accepted date:
31-12-2014 10-6-2015 12-6-2015
Please cite this article as: Babaee, M., Jonoobi, M., Hamzeh, Y., and Ashori, A.,Biodegradability and mechanical properties of reinforced starch nanocomposites using cellulose nanofibers, Carbohydrate Polymers (2015), http://dx.doi.org/10.1016/j.carbpol.2015.06.043 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.
Hughlights
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Biodegradability and mechanical properties of reinforced thermoplastic starch nanocomposite was studied
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Compared with the neat TPS, the addition of nanofibers improved the degradation rate of the specimens
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Addition of CNFs and ACNFs significantly enhanced the mechanical properties of the nanocomposites
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Regardless of the type, the addition of nanofibers increased the thermal stability of the nanocomposites
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The moisture absorption of the TPS nanocomposite was reduced by addition of the ACNFs
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Biodegradability and mechanical properties of reinforced starch
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nanocomposites using cellulose nanofibers
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Mehran Babaee a, Mehdi Jonoobi a, Yahya Hamzeh a,*, Alireza Ashori b
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Department of Chemical Technologies, Iranian Research Organization for Science and Technology (IROST), P.O. Box 15815-3538, Tehran, Iran
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Department of Wood and Paper Science and Technology, Faculty of Natural Resources, University of Tehran, P.O. Box 31585-4313, Karaj, Iran
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Corresponding author. Tel.: +98 263 2249311; Fax: +98 26 32249311 E-mail address: [email protected] (Y. Hamzeh).
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Abstract
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In this study the effects of chemical modification of cellulose nanofibers (CNFs) on the
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biodegradability and mechanical properties of reinforced thermoplastic starch (TPS)
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nanocomposites was evaluated. The CNFs were modified using acetic anhydride and the
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nanocomposites were fabricated by solution casting from corn starch with glycerol/water
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as the plasticizer and 10 wt% of either CNFs or acetylated CNFs (ACNFs). The
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morphology, water absorption (WA), water vapor permeability rate (WVP), tensile,
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dynamic mechanical analysis (DMA), and fungal degradation properties of the obtained
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nanocomposites were investigated. The results demonstrated that the addition of CNFs
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and ACNFs significantly enhanced the mechanical properties of the nanocomposites and
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reduced the WVP and WA of the TPS. The effects were more pronounced for the CNFs
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than the ACNFs. The DMA showed that the storage modulus was improved, especially
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for the CNFs/TPS nanocomposite. Compared with the neat TPS, the addition of
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nanofibers improved the degradation rate of the nanocomposite and particularly ACNFs
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reduced degradation rate of the nanocomposite toward fungal degradation.
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Keywords: Thermoplastic starch; Fungal degradation; Water vapor permeability;
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Scanning electron microscopy (SEM).
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1. Introduction Starch is a promising biopolymer for producing biocomposite materials because it
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is renewable, completely biodegradable, and easily available at a low cost (Alissandratos
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and Halling; 2012). The development of biocomposites based on starch has been
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expanding continuously due to the fact that thermoplastic starch (TPS) can be obtained
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after the disruption and plasticization of native starch (Hulleman et al., 1998). Various
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types of plasticizers such as glycerol, water, and sorbitol have been used to prepare TPS
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(Curvelo et al., 2001). Compared to the common thermoplastic polymers, TPS has two
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main disadvantages including poor mechanical properties and a high water sensibility
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(Teixeira et al., 2009). The use of reinforcing agents in the starch matrix is an effective
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means to overcome these drawbacks and several types of biodegradable reinforcements
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such as cellulosic fibers, whiskers, and nanofibers have been utilized to develop new and
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inexpensive starch biocomposites (Curvelo et al., 2001; Petersson et al., 2007; Hietala et
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al., 2013).
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Recently, the cellulose nanofibers (CNFs) derived from renewable biomass have
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attracted much interest as an alternative to micro-sized reinforcements in composite
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materials (Jonoobi et al., 2012). The CNFs, which are generally isolated from
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lignocellulosic plants, have specific characteristics such as high modulus and high surface
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area. These CNFs show great potential for use as reinforcement in polymer matrices
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(Petersson et al., 2007). However, the processes and polymers that are suitable for
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producing composite are restricted because of the hydrophilicity of the CNFs as well as
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the formation of irreversible aggregates when dried (Siró and Plackett, 2010).
In
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addition, the uniform dispersion of the CNFs in polymers is difficult due to their high
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surface energy, and the presence of hydroxyl groups on the CNF surface. To overcome
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this problem, different chemical modifications have been used in earlier studies (Goussé
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et al., 2004; Jonoobi et al., 2010). Among the many chemical modifications, acetylation
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is one of the most promising processes because it can improve the dispersibility of
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nanofibers in a polymer matrix (Ashori et al., 2014) and the dimensional stability of the
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final composition (Khalil et al., 2001). During the acetylation, the chemical will react
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with the OH groups on the cellulose; thereby modifying the hydrophilic surface of the
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cellulose to become hydrophobic.
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Generally, biocomposites are totally biodegradable and are found to fully
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disintegrate in ideal conditions (Matthews et al., 2006). Biodegradation is governed by
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different factors including polymer characteristics, type of organism, and the nature of
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pretreatment (Gigli et al., 2012). Various microorganisms such as bacteria and fungi are
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responsible for the degradation of both natural and synthetic plastics (Gu, 2003).
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Compared to the conventional polymers, TPS is fully biodegradable in a wide variety of
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environmental conditions (Shah et al., 2008). Numerous researchers have studied the
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biodegradation condition of starch biopolymers. Nevertheless, there are few reports
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regarding the environmental biodegradability of the CNFs/starch nanocomposites.
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The main objective of this study was to investigate the effect of the addition of
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CNFs and acetylated CNFs (ACNFs) on the biodegradation and physicomechanical 5
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properties of the starch nanocomposites. Microscopic analysis was used to study the
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fungal biodegradation of the nanocomposite under various conditions. Furthermore, the
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influence of acetylation of CNFs on its dispersibility into the starch polymer matrix has
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been investigated.
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2. Materials and Methods
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2.1. Materials
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Unprocessed corn starch composed of 35% amylose and 65% amylopectin was
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supplied by the Alborz Starch Co., Iran. The CNFs used in this study were provided by
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the Institute of Tropical Forestry and Forest Products (INTROP), Malaysia, and were
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isolated from the kenaf bast fibers (Hibiscus cannabinus). The details of the CNFs
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isolation process are reported elsewhere (Jonoobi et al., 2009). In short, the fibers were
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converted to pulp fibers using NaOH-AQ (anthraquinone) followed by a three-stage
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bleaching process (DEpD). In order to isolate CNFs, mechanical isolation processes were
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used. The mechanical procedure performed to isolate CNFs was as the following:
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A water slurry with 0.5 wt% cellulose fibers was prepared using high shear mixing in a
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mechanical blender apparatus (Silverson L4RT, England) operating at 5000 rpm for 10
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min and then ground using an ultra-fine grinder (Masuko Co. Ltd, Japan (Model MKCA
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6-3)). The total grinding time was 16 minutes. During grinding, cellulose fiber fibrillation
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was obtained by passing the cellulose slurry between the static and a rotating grinding
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stones revolving at a rotational speed of about 3500 rpm and designated to give shear
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forces to the longitudinal fiber axis of the fibrous material. 6
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The selected fungus was a white rot fungus (Trametes versicolor), which was
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obtained from the National Collection of Biology Laboratory, University of Tehran, Iran.
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Glycerol, methanol, acetone, acetic anhydride (95%), pyridine, and malt extract agar
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(MEA) were purchased from the Merck Chemical Co., Germany. All the materials and
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solvents that were used were obtained from the suppliers without further purification.
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2.2. Chemical modification of the CNFs
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The CNFs used in this study were isolated from kenaf bast fibers (Hibiscus
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cannabinus). The CNFs were acetylated using the method reported by Jonoobi et al.,
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(2010), which is briefly discussed as follows. The CNFs were acetylated with acetic
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anhydride (1:20 by weight) and 5 wt% pyridine as catalyst under reflux at 100 ºC for 4 h.
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At the end of the reaction, the treated CNFs were extracted using distilled water (2:1 by
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volume) for 6 h in order to remove the unreacted acetic anhydride and acetic acid by-
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products. Subsequently, four cycles of solvent exchange were performed by
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centrifugation to obtain aqueous dispersions of the ACNFs.
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2.3. Preparation of CNFs/TPS and ACNFs/TPS nanocomposites
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The nanocomposites were prepared using the solution casting method. Glycerol
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was used on 37 wt% (based on the dry weight of starch) to plasticize the starch polymer.
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The starch and glycerol were mixed with distilled water and the suspension kept in a 7
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flask under continuous stirring, at 80 °C for 30 min for completing the gelatinization of
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the corn starch. Then, either CNFs or ACNFs (10 wt% based on dry weight of starch)
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were added and the mixture was stirred for another 30 min at 80°C. Subsequently, the
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mixture was degassed under vacuum and cast into a Teflon Petri dish followed by drying
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in an oven at 40 °C for 3 days. The obtained composite films that had a thickness of 0.4
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mm, were conditioned at 23 °C and 75% moisture for 3 days to ensure the equilibration
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of the water content in the films. The samples were cut according to the ASTM standard
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D 6287-98 to provide test specimens.
2.3. Characterization
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2.3.1. Degree of substitution
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The degree of substitution for the ACNFs was determined by the titration method
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described by Kim et al. (2002). Briefly, a sample of the suspension containing 100 mg of
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the ACNFs combined with 40 ml of 75% ethanol was heated at 60°C for 30 min. Then 40
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ml of 0.5 N NaOH solution was added to the sample and stirred for 15 min. The sample
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was kept at room temperature for 48 h and then the excess alkali determined with 0.5 N
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HCl using phenolphthalein as an indicator. The titration process was repeated three times
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for each sample to confirm the reproducibility of the measurement.
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2.3.2. Water absorption
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Water absorption test was performed in accordance with the ASTM standard
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E104. All the specimens for water absorption were cut from TPS, CNFs/TPS and
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ACNFs/TPS nanocomposite films with dimensions of 2 × 2 cm2 and dried at 60 °C for 24
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h. The dried samples were placed in a desiccator containing calcium sulfate (RH = 0%)
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for 3 days and afterwards the initial weight of the samples were measured. The samples
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were then transferred into a desiccator containing potassium sulfate (RH = 98%) and the
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time taken to reach a constant weight was marked. The water absorption (WA) of the
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specimens was calculated as follows:
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WA
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where W1 is the weight of the sample at the time in 98% RH and W0 is the initial sample
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weight exposure to 98% RH. An average value of the three replicates for each sample
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was reported.
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W1 W0 100 W0
2.3.3. Water vapor transmission rate
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The water vapor transmission rate was determined gravimetrically following the
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ASTM standard E 96. The samples were conditioned to an environmental chamber at
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25°C and 50% relative humidity (RH) using calcium nitrite for 24 h. Then, the specimens
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were fixed on glass cups containing 3 g of calcium sulfate that provides 0% RH. The cup
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assembly was weighed using a four-digit balance and placed into a controlled
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temperature (25 ± 0.5 °C) and humidity (98 ± 2% RH) desiccator. Cups were then
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periodically weighed at intervals of 1 and 24 h for 80 h. Weight gain was plotted as a
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function of time and the slope of the linear portion was determined using a simple linear
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regression method. Water vapor transmission rate (WVTR) and water vapor permeability
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were calculated using equations 2 and 3: G ( ) slope WVTR t A test area
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where G = weight change (g), t= time (h), A= the area exposed to moisture transfer (m2),
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G/t = the slope obtained from a chart of weight gain vs. time. Linear regression of the
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data gave a correlation coefficient > 95%.
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WVTR X P(R 1 R 2 )
WVP
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where X is the thickness of the film (m), P is the saturation vapor pressure of water at 25
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°C (Pa), R1 is the relative humidity in the desiccator (98% RH), and R2 is the relative
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humidity in the cups (0% RH).
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2.3.4. Mechanical testing
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Mechanical tests were performed according to the ASTM standard D889. The
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samples were cut into 70×10 mm2. Before testing, all the test specimens were conditioned
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for at least one week in a desiccator with 55% RH in room temperature. The tensile tests
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were conducted using an Instron testing machine (M350-10- CT) with a 500 N load cell,
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gauge length of 50 mm, and a crosshead speed of 4 mm/min. The elastic modulus was
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calculated from the initial part of the slope from the stress–strain curves. At least five
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samples were tested for each sample and the average values are presented.
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2.3.5. Dynamic mechanical analysis (DMA) The dynamic mechanical properties of the nanocomposites were performed in
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uniform moisture using the dynamic mechanical analyzer, DMA Netzsch 242, in tensile
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mode. The DMA measurements were conducted at a constant frequency of 1 Hz, a strain
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amplitude of 0.05% with a temperature range from -50 to150°C at a heating rate of 3
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obtained represented the average of the 3 samples.
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2.3.6. Biodegradation test
In order to study the fungal degradation of the biocomposites, a white rot fungus
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(T. versicolor) was used. The specimens of the neat TPS, CFN/ TPS, and ACNFs/TPS
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nanocomposites were prepared with lateral dimensions of 20.0×10.0×0.4 mm3 (length
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×width ×thickness). The samples were dried at 70 °C in an oven overnight and the initial
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weight of each specimen was recorded. The purified fungus was transferred to the Petri
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dishes containing MEA. The dishes were then kept in the laboratory incubator at 25 °C
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until the culture medium was fully covered by the fungus. The specimens were then
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transferred into the Petri dishes containing the culture medium. To prevent direct contact
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of the specimens with the culture medium, the specimens were mounted over two 2-mm
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platforms. The Petri dishes containing the fungus and the specimens were then stored in
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an incubator at 25 °C and 75% RH. The biodegradation rate of the specimens were
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evaluated for two months and at regular intervals of 10 days by weight differences before
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and after the exposure of the specimens to the white rot fungi. The fungal degradation
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rate was calculated using Equation 4.
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DE(%)
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where DE is the biodegradation rate of TPS, W0 is the initial weight of the original
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specimen, and W1 is the dry weight of the residual specimen exposed to the white rot
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fungi for a certain time.
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2.3.7. Scanning electron microscopy (SEM)
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The surface and cross-section of the specimens were examined before and after
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degradation using a scanning electron microscope, model ZEISS Ultra 55, Germany. The
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acceleration voltage was 10 kV and the specimens were sputter-coated with gold to avoid
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charging. In preparation, the specimens were removed from the culture and carefully
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rinsed with distilled water to remove fungal mycelium. Then, the specimens were dried at
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35°C for further evaluation of biodegradation efficacy.
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3. Results and discussion
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3.1. Acetylation and degree of substitution for CNFs
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The acetylation involves the replacement of hydroxyl groups of the nanofibers
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with acetyl groups in the reagent. The acetylation of the CNFs with acetic anhydride was
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carried out at 100ºC for 4 h. A long reaction time along with pyridine as a catalyst was
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used to promote the sufficient diffusion of acetic anhydride into the amorphous regions of
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the CNFs. Therefore, the bulk substitution was facilitated. The degree of the substitution
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(DS) of acetylated nanofibers determined by titration was 0.2, which confirmed the
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successful acetylation of nanofibers.
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3.2. Water absorption
The water absorption of the neat starch and the TPS nanocomposites reinforced
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with CNFs and ACNFs were studied at a relative humidity level of 98%. The moisture
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absorption of the neat starch and its CNFs and ACNFs nanocomposites increased linearly
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in the initial step while at the lateral, the rate of water uptake decreased (Figure 1).
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Moreover, the water uptake of the obtained composites reinforced with 10 wt% of CNFs
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and ACNFs was significantly reduced compared to the neat TPS. The amount of
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reduction was approximately 14% for the CNFs/TPS and 30% for the ACNFs/TPS
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composites. Similar results have been reported (Svagan et al., 2009; Hietala et al., 2013),
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which might be attributed to the amorphous and branch structure of starch with higher
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accessibility to the water than the CNFs. As expected, in regard to the ACNFs/TPS,
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further reduction in water absorption occurs due to acetylation by replacing the hydroxyl
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groups of the CNFs with acetyl groups. This confirmed that the chemical modification
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had indeed occurred on the CNFs.
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3.3. Water vapor permeability (WVP) 13
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In order to minimize the effect of water vapor, the packaging material must
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prevent, or at least decrease, moisture exchange with the environment. The WVP values
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of the TPS and TPS nanocomposites were investigated and the results are presented in
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Table 1. Similar values recently have been reported for sago starch (6.44×10-7 g/m.h.Pa
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plasticized with glycerol (Bajpai et al., 2011). It could be found that the WVP values of
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both the CNFs/TPS (8.27×10-7 g/m.h.Pa) and the ACNFs/TPS (9.45×10-7 g/m.h.Pa) were
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lower than the neat TPS (9.61× 10-7 g/m.h.Pa). On the other hand, the WVP was
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decreased with the addition of the CNFs. The addition of the CNFs to the polymer matrix
277
presumably leads to denser and less porous materials. Furthermore, a good interfacial
278
adhesion between the CNFs and starch can probably restrict the moisture diffusion rate of
279
the CNFs/TPS compared with the neat TPS. In addition, the WVP for the ACNFs/TPS
280
composite was reduced less than the CNFs/TPS. It could be explained by replacing the
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hydroxyl group with acetyl groups during acetylation of the CNFs, which reduces the
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interaction between nanofibers- nanofibers and nanofibers-matrix that causes more
283
porous network. Also, decreasing the degree of crystallinity of cellulose nanofibers
284
during the acetylation can lead to similar results. Thus, these synergistic effects resulted
285
in an increase of the passage of water vapor in the ACNFs/TPS compared to the
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CNFs/TPS (Rodionova et al., 2011). In summary, although the addition of the CNFs
287
could significantly improve barrier properties of the TPS film, the introduction of the
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acetyl groups of the CNFs can slightly increase the water vapor permeability of the
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composite.
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3.4. Mechanical properties 14
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The average values and standard deviations of the neat TPS, CNFs/TPS, and
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ACNFs/TPS tensile properties are summarized in Table 2. The tensile properties showed
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that an increased modulus and strength of both nanocomposites with 10 wt% nanofibers
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compared to the pure TPS. The Young’s modulus and tensile strength increased from
296
16.6 and 8.6 MPa to 141 and 38 MPa for the CNFs/TPS, respectively. However, the
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tensile modulus and strength of the composite with 10 wt% the ACNFs was lower than
298
the CNFs/TPS. The increment in the tensile modulus and the strength of the TPS
299
reinforced with the CNFs can be attributed to the formation of a hydrogen-bonded
300
nanofibers network and also the nanofibers entanglement (Angles and Dufresne, 2001).
301
In addition, the formation of hydrogen bonding results in a strong interface between two
302
phases (matrix and nanofibers) (Sreekala et al., 2008). On the other hands, the low tensile
303
modulus and strength of the TPS treated with the ACNFs, might be attributed to the
304
reduced nanofibers-to-nanofiber and nanofiber-to-matrix interactions due to the surface
305
hydrophobicity and also the reduced crystallinity of the nanofiber after acetylation
306
(Jonoobi et al., 2012). Furthermore, the elongation of the filled TPS with the CNFs was
307
lower (8%) than the neat TPS and the TPS filled with the ACNFs (15%), which could be
308
reasonably attributed to the higher rigidity of non-acetylated nanofibers. The less
309
flexibility of the CNFs compared to the ACNFs could make them more difficult to match
310
with each other and to other material (Khalil et al., 2000). Therefore, the CNFs with high
311
rigidity decreased the elongation at the break point forth CNFs/TPS composite compared
312
to the pure TPS and the TPS with the ACNFs.
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3.5. Dynamical thermal analysis (DMA) 15
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Figure 2 shows the storage modulus and the tan as a function of the temperature
316
of the prepared materials. Generally, the storage modulus decreases with increases in the
317
temperature (Soykeabkaew et al., 2012). As it can be seen from Table 3 and Figure 2A,
318
both acetylated and non-acetylated nanofibers resulted in a higher storage modulus in the
319
tested range. Table 3 shows the storage modulus of the composites at two different
320
temperatures. The storage modulus data of the TPS and its nanocomposites at room
321
temperature supported the data obtained from the tensile testing. The storage modulus of
322
the nanocomposites containing 10 wt% CNFs and ACNFs increased to 2500 and 800
323
MPa, respectively, as compared to the neat TPS at 25 °C. The results could be ascribed
324
by the formation of a network of the CNFs after evaporation of the water and the
325
formation of sufficient crosslinking of the matrix phase in the nanocomposite (Svagan et
326
al., 2007). On the other hand, the storage modulus improvement for the ACNFs/TPS is
327
less in comparison to the CNFs/TPS and this might be associated with the partial
328
destruction of the cellulose crystalline structure by acetylation and less networks
329
formation in the nanocomposite. Also, it is possible to say that the used nanofibers
330
restricted the molecular chains movement of the TPS, thereby improving its thermal
331
stability (Petersson and Oksman, 2006). Figure 2B shows how the CNFs and ACNFs
332
influenced the tan at the peak position of the TPS. The shift in the tan peak of
333
thermoplastic starch film reinforced using acetylated cellulose nanofibers was difficult to
334
estimate, because tan was not well-defined. To verify the accuracy of the data, the
335
DMA experiments were carried out in triplicate and the same curves with no sharp peaks
336
and the similar values for the temperature at the maximum in the tan peak were
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obtained by the instrument software. It is worthwhile to mention that similar curves have
338
been reported earlier for potato starch film (Zhou et al., 2009). The tan peak temperatures for the TPS and its nanocomposite are given in Table
340
3. The peak position for the TPS occurred at 49 °C and increased to 83°C and 81°C for
341
the nanocomposites based on the CNFs and the ACNFs, respectively. An increase in the
342
tan value with temperature indicates that the nanocomposites were becoming more
343
viscous after raising the temperature (Liu et al., 2010). The results confirmed an
344
improvement in dynamic mechanical properties of the TPS reinforcement by both the
345
ACNFs and the CNFs. The increase in storage modulus, together with a positive shift in
346
the tan peak for the CNFs/TPS nanocomposite could be due to the strong chemical
347
interaction between the matrix and the reinforcements that restricted the segmental
348
mobility of the polymer chains in the vicinity of the nano reinforcements (Bondeson et
349
al., 2007). However, it may be noted that the ACNFs did not improve the dynamic
350
mechanical properties of the neat TPS as much as the CNFs ones. This suggested that the
351
interface was not good enough due to the surface modification and it is in agreement with
352
the tensile testing results.
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3.6. Biodegradability of composites
355
The biodegradability of the TPS and the TPS/nanocomposites was investigated
356
undertaking white rot fungi (T. versicolor) at regular time intervals by measuring the
357
weight loss. Figure 3 shows the degradation process and the weight loss with respect to
358
the elapsed time. By increasing the decomposition time, the compactness of the films 17
Page 17 of 35
would be reduced. The films showed a rapid degradation within a period of 30 days.
360
More than 50 wt% weight loss of the total solids was observed for the composites after
361
20 days. The results indicated that the TPS was fully degraded at 30 days while the
362
complete destruction of the CNFs/TPS and the ACNF/TPS occurred in the 40 and 60
363
days, respectively. It could be observed that the CNFs/TPS and the ACNFs/TPS
364
nanocomposites had a higher degradation resistance compared to the neat TPS. This
365
might be due to the higher crystallinity and the denser structure of the CNFs, which was
366
added to the starch matrix and restricted the destructive enzymatic activities as well as the
367
enzymatic hydrolysis of the cellulose (Lai and Ontto, 1979). The crystalline structure of
368
the cellulose causes the formation of a dense layer of water, which consequently prevents
369
the spread of cellulose-degrading enzymes and other substances around the cellulose
370
((Matthews et al., 2006). The environmental degradation of cellulose can be affected by
371
several parameters such as the existence and diversity of microorganisms, the availability
372
of carbon sources, and the time span of the observation period (Gu, 2003). The micro-
373
organisms have higher activity in the presence of hydrogen and carbon groups. In the
374
case of the ACNFs/TPS, the substitution of the acetyl groups with the hydroxyl groups
375
during the chemical modification resulted in a decrease in the decomposition rate and the
376
micro-organisms activities (Glasser et al., 1994). Figure 4 shows the visual appearance of
377
samples after exposure by the white rot fungi. The samples were eroded significantly and
378
lost their original shape by over time. Also it could be seen that the color of the samples
379
became yellow to dark-brown. The results clearly demonstrated that the CNFs can
380
increase the degradation period of the nanocomposites compared to the neat TPS.
Ac ce
pt
ed
M
an
us
cr
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359
381
18
Page 18 of 35
382
3.7. Scanning electron microscopy (SEM) The changes in the morphology of the materials have been studied before and
384
after fungi treatment. The scanning of the TPS and the TPS nanocomposites are presented
385
in Figure 5. It could be observed that the surface and cross-section of the ACNFs/TPS
386
were smoother compared to the CNF/TPS and it could be due to the lower aggregation of
387
the nanofibers in the starch matrix. Figure 6 indicates the SEM of the TPS and the TPS
388
nanocomposites after exposure to the fungus after 30 days. In Figure 7, the specific
389
growth of mycelium and fungal elements can be recognized on the samples. The results
390
confirmed that the nanocomposites surface became rougher after exposure to the fungus;
391
as well, reduction in the thickness of the nanocomposites could be observed. The results
392
suggested that by changing the surface of the nanofibers from hydrophilic to
393
hydrophobic, the activity of the fungi decreased. In lower fungi activation, the surface
394
degradation of the TPS nanocomposite materials might be reduced and then, the surface
395
of the nanocomposite is found to be smoother.
pt
ed
M
an
us
cr
ip t
383
397 398
Ac ce
396
4. Conclusions
This study showed that both acetylated and non-acetylated CNFs can be used to
399
fabricate the thermoplastic starch nanocomposite. The composites were prepared
400
successfully by using the solution casting method. The effects of acetylated and non-
401
acetylated nanofibers on water absorption, tensile, dynamic mechanical properties, and
402
biodegradability of TPS were compared. The mechanical testing showed that the tensile
403
strength and modulus of both nanocomposites increased when compared to the matrix 19
Page 19 of 35
while the improvement was lower for the acetylated one. The storage modulus and the
405
tan δ peak position of both nanocomposites showed improvement when compared to the
406
matrix. Regardless of the type, the addition of nanofibers increased the Tg of the
407
nanocomposites, determined from a peak in the tan δ curve. In addition, the moisture
408
absorption of the TPS nanocomposite was reduced by addition of the ACNFs compared
409
to the non-acetylated one. Furthermore, the fungal biodegradability results showed a
410
longer decomposition period when the CNFs and the ACNFs are added to the TPS
411
matrix. But, the acetylated nanocomposite needs a longer time for degradation compared
412
to the CNFs/TPS, and it became more sustainable due to the replacement of hydroxyl
413
groups with acetyl groups.
M
414
an
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404
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415
20
Page 20 of 35
415
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416
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Bajpai, S.K., Navin, C., & Ruchi, L. (2011). Water vapor permeation and antimicrobial
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Hietala, M., Mathew, A. P., & Oksman, K. (2013). Bionanocomposites of thermoplastic
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starch and cellulose nanofibers manufactured using twin-screw extrusion. European
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Polymer Journal, 49 (4), 950-956.
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Hulleman, S. H., Janssen, F. H., & Feil, H. (1998). The role of water during plasticization of native starches. Polymer, 39 (10), 2043-2048.
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Jonoobi, M., Harun, J., Mathew, A. P., Hussein, M. Z. B., & Oksman, K. (2010).
447
Preparation of cellulose nanofibers with hydrophobic surface characteristics.
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Cellulose, 17 (2), 299-307.
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an
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Jonoobi, M., Mathew, A. P., Abdi, M. M., Makinejad, M. D., & Oksman, K. (2012). A
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Jonoobi, M., Niska, K. O., Harun, J., & Misra, M. (2009). Chemical composition,
Ac ce
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ed
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crystallinity, and thermal degradation of bleached and unbleached kenaf bast (Hibiscus cannabinus) pulp and nanofibers. BioResources, 4 (2), 626-639.
456
Khalil, H. A., Ismail, H., Rozman, H., & Ahmad, M. (2001). The effect of acetylation on
457
interfacial shear strength between plant fibres and various matrices. European
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Polymer Journal, 37 (5), 1037-1045.
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Khalil, H. A., Rozman, H., Ahmad, M., & Ismail, H. (2000). Acetylated plant-fiber-
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reinforced polyester composites: a study of mechanical, hygrothermal, and aging
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characteristics. Polymer-Plastics Technology and Engineering, 39 (4), 757-781.
462
Kim, D.-Y., Nishiyama, Y., & Kuga, S. (2002). Surface acetylation of bacterial cellulose.
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gluco-pyranosides. Carbohydrate Research, 75, 51-59.
cr
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Lai, Y.-Z., & Ontto, D.E. (1979). Kinetics of base-catalyzed degradation of phenyl d-
us
464
Cellulose, 9 (3-4), 361-367.
Liu, D., Zhong, T., Chang, P. R., Li, K., & Wu, Q. (2010). Starch composites reinforced by bamboo cellulosic crystals. Bioresource Technology, 101 (7), 2529-2536.
an
463
ip t
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Matthews, J.F., Skopec, C.E., Mason, P.E., Zuccato, P., Torget, R.W., Sugiyama, J.,
469
Himmel, M.E., & Brady, J.W. (2006). Computer simulation studies of
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microcrystalline cellulose Iβ. Carbohydrate Research, 341 (1), 138-152.
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Petersson, L., & Oksman, K. (2006). Biopolymer based nanocomposites: comparing
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layered silicates and microcrystalline cellulose as nanoreinforcement. Composites
473
Science and Technology, 66 (13), 2187-2196.
475 476
pt
Petersson, L., Kvien, I., & Oksman, K. (2007). Structure and thermal properties of poly
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474
ed
471
(lactic acid)/cellulose whiskers nanocomposite materials. Composites Science and Technology, 67 (11), 2535-2544.
477
Rodionova, G., Lenes, M., Eriksen, Ø., & Gregersen, Ø. (2011). Surface chemical
478
modification of microfibrillated cellulose: improvement of barrier properties for
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packaging applications. Cellulose, 18 (1), 127-134. Shah, A. A., Hasan, F., Hameed, A., & Ahmed, S. (2008). Biological degradation of plastics: a comprehensive review. Biotechnology Advances, 26 (3), 246-265.
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Siró, I., & Plackett, D. (2010). Microfibrillated cellulose and new nanocomposite materials: a review. Cellulose, 17 (3), 459-494. Soykeabkaew, N., Laosat, N., Ngaokla, A., Yodsuwan, N., & Tunkasiri, T. (2012).
485
Reinforcing potential of micro-and nano-sized fibers in the starch-based
486
biocomposites. Composites Science and Technology, 72 (7), 845-852.
ip t
484
Sreekala, M., Goda, K., & Devi, P. (2008). Sorption characteristics of water, oil and
488
diesel in cellulose nanofiber reinforced corn starch resin/ramie fabric composites.
489
Composite Interfaces, 15 (2-3), 281-299.
us
cr
487
Svagan, A. J., Hedenqvist, M. S., & Berglund, L. (2009). Reduced water vapour sorption
491
in cellulose nanocomposites with starch matrix. Composites Science and Technology,
492
69 (3), 500-506.
M
an
490
Svagan, A.J., Azizi Samir, M.A., & Berglund, L.A. (2007). Biomimetic polysaccharide
494
nanocomposites of high cellulose content and high toughness. Biomacromolecules, 8
495
(8), 2556-2563.
ed
493
Teixeira, E.D.M., Pasquini, D., Curvelo, A.A., Corradini, E., Belgacem, M.N., &
497
Dufresne, A. (2009). Cassava bagasse cellulose nanofibrils reinforced thermoplastic
Ac ce
498
pt
496
cassava starch. Carbohydrate polymers, 78 (3), 422-431.
499
Zhou, Y.G., Wang, L.J., Li, D., Yan, P.Y., Li, Y., Shi, J., Chen, X.D., & Mao, Z.H.
500
(2009). Effects of sucrose on dynamic mechanical characteristics of maize and potato
501
starch film. Carbohydrate Polymers, 76 (2), 239-243.
502
24
Page 24 of 35
Tables Caption:
503
Table 1. Water vapor permeability (WVP) of the neat TPS and TPS
504
reinforced with CNF and ACNF composites
505
Table 2. Mechanical properties of the TPS and its nanocomposites
506
Table 3. Storage modulus and tan of the TPS and its nanocomposites
cr
ip t
502
us
507 508
Figures Caption:
510
Fig. 1. Water absorption of the TPS and TPS nanocomposites
511
Fig. 2. Storage modulus curves (A) and tan peaks (B) the TPS and its nanocomposites
512
Fig. 3. Fungal degradation of TPS and its nanocomposites
513
Fig. 4. Visual appearance of the composite influenced by white rot fungi
514
Fig. 5. SEM micrographs of the surface (top) and cross-section (down) of TPS (A)
515
TPS/CNFs (B) TPS/ACNFs (C)
516
Fig. 6. Mycelium of white rot fungi on the composites surface; TPS (A) TPS/CNFs (B)
517
TPS/ACNFs (C)
518
Fig. 7. SEM micrographs of the surface (top) and cross-section (down) of TPS (A)
519
TPS/CNFs (B) TPS/ACNFs (C) after degradation by white rot fungi
Ac ce
pt
ed
M
an
509
520
25
Page 25 of 35
521 522 523
ip t
524
cr
525 526
us
527
Table 1. Water vapor permeability (WVP) of the neat TPS and TPS
529
reinforced with CNF and ACNF composites
an
528
WVTR (g/m2.h)
9.61×10-7
CNFs/TPS
6.42
8.26×10-7
533 534 535 536 537
9.45×10-7
7.34
pt
532
ed
7.47
Ac ce
531
WVP (g/m.h.Pa)
TPS
ACNFs/TPS 530
M
Materials
538 539 540
26
Page 26 of 35
541 542 543 544
ip t
545 546
cr
547
us
548 549
555 556 557 558 559
16.6±2
8.6±1
141.0 ±35
38.0±3
29.6±6
14.7±2
M
Young modulus ( MPa)
TPS
52±2
CNFs/TPS
27±4
ACNFs/TPS
pt
50±2
Ac ce
554
Elongation at break (%)
ed
Materials
553
Tensile strength ( MPa)
Table 2. Mechanical properties of the TPS and its nanocomposites
551
552
an
550
560 561 562 563
27
Page 27 of 35
564 565 566 567
ip t
568 569
cr
570
572
576 577 578 579 580 581
49
CNFs/TPS
83
ACNFs/TPS
81
an
TPS
É 25 oC (MPa)
É 70 oC (MPa)
450
84
2500
541
800
250
M
Tan δ (oC)
ed
575
Materials
pt
574
Table 3. Storage modulus and tan of the TPS and its nanocomposites
Ac ce
573
us
571
582 583 584 585 586
28
Page 28 of 35
587 588
ip t
589 590
cr
591
us
592 593
an
594 595
M
596 597
601 602 603 604 605 606
pt
600
Fig. 1. Water absorption of the TPS and TPS nanocomposites
Ac ce
599
ed
598
607 608 609 610
29
Page 29 of 35
611 612 613
ip t
614 615
cr
616 617
us
618 619
an
620 621
M
622
626 627 628 629 630 631 632
pt
625
Ac ce
624
ed
623
633 634
Fig. 2. Storage modulus curves (A) and tan peaks (B) the TPS and its nanocomposites
635 636
30
Page 30 of 35
637 638 639 640
ip t
641 642
cr
643
us
644 645
an
646 647 648
M
649
653 654 655 656 657 658 659
pt
652
Ac ce
651
ed
650
Fig. 3. Fungal degradation of TPS and its nanocomposites
660 661 662 663
31
Page 31 of 35
664 665 666 667
ip t
668 669
cr
670
us
671 672
an
673 674
M
675 676
680 681 682 683 684 685 686
pt
679
Fig. 4. Visual appearance of the composite influenced by white rot fungi
Ac ce
678
ed
677
687 688 689 690
32
Page 32 of 35
691 692 693 694
ip t
695 696
cr
697
us
698 699
an
700 701
M
702 703
707 708 709 710 711 712 713
pt
706
Ac ce
705
ed
704
Fig. 5. SEM micrographs of the surface (top) and cross-section (down) of TPS (A), CNFs/TPS (B), and ACNFs/TPS (C)
714 715 716 717
33
Page 33 of 35
718 719 720 721
ip t
722 723
cr
724
us
725 726
an
727 728 729
M
730
734 735 736 737 738 739 740
pt
733
Ac ce
732
ed
731
741 742 743
Fig. 6. Mycelium of white rot fungi on the composites surface; TPS (A) TPS/CNFs (B)
744
and TPS/ACNFs (C) 34
Page 34 of 35
745 746 747 748
ip t
749 750
cr
751
us
752 753
an
754 755 756
M
757
761 762 763 764 765 766 767
pt
760
Ac ce
759
ed
758
Fig. 7. SEM micrographs of the surface (top) and cross-section (down) of TPS (A) TPS/ CNFs (B) and TPS/ACNFs (C) after biodegradation
768
35
Page 35 of 35
S0144-8617(15)00550-0 http://dx.doi.org/doi:10.1016/j.carbpol.2015.06.043 CARP 10032
To appear in: Received date: Revised date: Accepted date:
31-12-2014 10-6-2015 12-6-2015
Please cite this article as: Babaee, M., Jonoobi, M., Hamzeh, Y., and Ashori, A.,Biodegradability and mechanical properties of reinforced starch nanocomposites using cellulose nanofibers, Carbohydrate Polymers (2015), http://dx.doi.org/10.1016/j.carbpol.2015.06.043 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.
Hughlights
1 2 -
Biodegradability and mechanical properties of reinforced thermoplastic starch nanocomposite was studied
5 6
-
Compared with the neat TPS, the addition of nanofibers improved the degradation rate of the specimens
7 8
-
Addition of CNFs and ACNFs significantly enhanced the mechanical properties of the nanocomposites
9 10
-
Regardless of the type, the addition of nanofibers increased the thermal stability of the nanocomposites
11 12
-
The moisture absorption of the TPS nanocomposite was reduced by addition of the ACNFs
an
us
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3 4
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13
1
Page 1 of 35
13 14
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15 16
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17
Biodegradability and mechanical properties of reinforced starch
19
nanocomposites using cellulose nanofibers
an
20
us
18
21
M
22 23
Mehran Babaee a, Mehdi Jonoobi a, Yahya Hamzeh a,*, Alireza Ashori b
24 25
a
26 27
b
pt
Department of Chemical Technologies, Iranian Research Organization for Science and Technology (IROST), P.O. Box 15815-3538, Tehran, Iran
30 31 32 33
Ac ce
28 29
ed
Department of Wood and Paper Science and Technology, Faculty of Natural Resources, University of Tehran, P.O. Box 31585-4313, Karaj, Iran
34 35
_________________
36 37
Corresponding author. Tel.: +98 263 2249311; Fax: +98 26 32249311 E-mail address: [email protected] (Y. Hamzeh).
2
Page 2 of 35
38
Abstract
40
In this study the effects of chemical modification of cellulose nanofibers (CNFs) on the
41
biodegradability and mechanical properties of reinforced thermoplastic starch (TPS)
42
nanocomposites was evaluated. The CNFs were modified using acetic anhydride and the
43
nanocomposites were fabricated by solution casting from corn starch with glycerol/water
44
as the plasticizer and 10 wt% of either CNFs or acetylated CNFs (ACNFs). The
45
morphology, water absorption (WA), water vapor permeability rate (WVP), tensile,
46
dynamic mechanical analysis (DMA), and fungal degradation properties of the obtained
47
nanocomposites were investigated. The results demonstrated that the addition of CNFs
48
and ACNFs significantly enhanced the mechanical properties of the nanocomposites and
49
reduced the WVP and WA of the TPS. The effects were more pronounced for the CNFs
50
than the ACNFs. The DMA showed that the storage modulus was improved, especially
51
for the CNFs/TPS nanocomposite. Compared with the neat TPS, the addition of
52
nanofibers improved the degradation rate of the nanocomposite and particularly ACNFs
53
reduced degradation rate of the nanocomposite toward fungal degradation.
54
Keywords: Thermoplastic starch; Fungal degradation; Water vapor permeability;
55
Scanning electron microscopy (SEM).
cr
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56
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39
3
Page 3 of 35
56
1. Introduction Starch is a promising biopolymer for producing biocomposite materials because it
58
is renewable, completely biodegradable, and easily available at a low cost (Alissandratos
59
and Halling; 2012). The development of biocomposites based on starch has been
60
expanding continuously due to the fact that thermoplastic starch (TPS) can be obtained
61
after the disruption and plasticization of native starch (Hulleman et al., 1998). Various
62
types of plasticizers such as glycerol, water, and sorbitol have been used to prepare TPS
63
(Curvelo et al., 2001). Compared to the common thermoplastic polymers, TPS has two
64
main disadvantages including poor mechanical properties and a high water sensibility
65
(Teixeira et al., 2009). The use of reinforcing agents in the starch matrix is an effective
66
means to overcome these drawbacks and several types of biodegradable reinforcements
67
such as cellulosic fibers, whiskers, and nanofibers have been utilized to develop new and
68
inexpensive starch biocomposites (Curvelo et al., 2001; Petersson et al., 2007; Hietala et
69
al., 2013).
cr
us
an
M
ed
pt
71
Recently, the cellulose nanofibers (CNFs) derived from renewable biomass have
Ac ce
70
ip t
57
72
attracted much interest as an alternative to micro-sized reinforcements in composite
73
materials (Jonoobi et al., 2012). The CNFs, which are generally isolated from
74
lignocellulosic plants, have specific characteristics such as high modulus and high surface
75
area. These CNFs show great potential for use as reinforcement in polymer matrices
76
(Petersson et al., 2007). However, the processes and polymers that are suitable for
77
producing composite are restricted because of the hydrophilicity of the CNFs as well as
78
the formation of irreversible aggregates when dried (Siró and Plackett, 2010).
In
4
Page 4 of 35
addition, the uniform dispersion of the CNFs in polymers is difficult due to their high
80
surface energy, and the presence of hydroxyl groups on the CNF surface. To overcome
81
this problem, different chemical modifications have been used in earlier studies (Goussé
82
et al., 2004; Jonoobi et al., 2010). Among the many chemical modifications, acetylation
83
is one of the most promising processes because it can improve the dispersibility of
84
nanofibers in a polymer matrix (Ashori et al., 2014) and the dimensional stability of the
85
final composition (Khalil et al., 2001). During the acetylation, the chemical will react
86
with the OH groups on the cellulose; thereby modifying the hydrophilic surface of the
87
cellulose to become hydrophobic.
an
us
cr
ip t
79
M
88
Generally, biocomposites are totally biodegradable and are found to fully
90
disintegrate in ideal conditions (Matthews et al., 2006). Biodegradation is governed by
91
different factors including polymer characteristics, type of organism, and the nature of
92
pretreatment (Gigli et al., 2012). Various microorganisms such as bacteria and fungi are
93
responsible for the degradation of both natural and synthetic plastics (Gu, 2003).
94
Compared to the conventional polymers, TPS is fully biodegradable in a wide variety of
95
environmental conditions (Shah et al., 2008). Numerous researchers have studied the
96
biodegradation condition of starch biopolymers. Nevertheless, there are few reports
97
regarding the environmental biodegradability of the CNFs/starch nanocomposites.
Ac ce
pt
ed
89
98 99
The main objective of this study was to investigate the effect of the addition of
100
CNFs and acetylated CNFs (ACNFs) on the biodegradation and physicomechanical 5
Page 5 of 35
properties of the starch nanocomposites. Microscopic analysis was used to study the
102
fungal biodegradation of the nanocomposite under various conditions. Furthermore, the
103
influence of acetylation of CNFs on its dispersibility into the starch polymer matrix has
104
been investigated.
ip t
101
2. Materials and Methods
107
2.1. Materials
us
106
cr
105
Unprocessed corn starch composed of 35% amylose and 65% amylopectin was
109
supplied by the Alborz Starch Co., Iran. The CNFs used in this study were provided by
110
the Institute of Tropical Forestry and Forest Products (INTROP), Malaysia, and were
111
isolated from the kenaf bast fibers (Hibiscus cannabinus). The details of the CNFs
112
isolation process are reported elsewhere (Jonoobi et al., 2009). In short, the fibers were
113
converted to pulp fibers using NaOH-AQ (anthraquinone) followed by a three-stage
114
bleaching process (DEpD). In order to isolate CNFs, mechanical isolation processes were
115
used. The mechanical procedure performed to isolate CNFs was as the following:
116
A water slurry with 0.5 wt% cellulose fibers was prepared using high shear mixing in a
Ac ce
pt
ed
M
an
108
117
mechanical blender apparatus (Silverson L4RT, England) operating at 5000 rpm for 10
118
min and then ground using an ultra-fine grinder (Masuko Co. Ltd, Japan (Model MKCA
119
6-3)). The total grinding time was 16 minutes. During grinding, cellulose fiber fibrillation
120
was obtained by passing the cellulose slurry between the static and a rotating grinding
121
stones revolving at a rotational speed of about 3500 rpm and designated to give shear
122
forces to the longitudinal fiber axis of the fibrous material. 6
Page 6 of 35
123
The selected fungus was a white rot fungus (Trametes versicolor), which was
125
obtained from the National Collection of Biology Laboratory, University of Tehran, Iran.
126
Glycerol, methanol, acetone, acetic anhydride (95%), pyridine, and malt extract agar
127
(MEA) were purchased from the Merck Chemical Co., Germany. All the materials and
128
solvents that were used were obtained from the suppliers without further purification.
cr
ip t
124
2.2. Chemical modification of the CNFs
an
130
us
129
The CNFs used in this study were isolated from kenaf bast fibers (Hibiscus
132
cannabinus). The CNFs were acetylated using the method reported by Jonoobi et al.,
133
(2010), which is briefly discussed as follows. The CNFs were acetylated with acetic
134
anhydride (1:20 by weight) and 5 wt% pyridine as catalyst under reflux at 100 ºC for 4 h.
135
At the end of the reaction, the treated CNFs were extracted using distilled water (2:1 by
136
volume) for 6 h in order to remove the unreacted acetic anhydride and acetic acid by-
137
products. Subsequently, four cycles of solvent exchange were performed by
138
centrifugation to obtain aqueous dispersions of the ACNFs.
140
ed
pt
Ac ce
139
M
131
2.3. Preparation of CNFs/TPS and ACNFs/TPS nanocomposites
141
The nanocomposites were prepared using the solution casting method. Glycerol
142
was used on 37 wt% (based on the dry weight of starch) to plasticize the starch polymer.
143
The starch and glycerol were mixed with distilled water and the suspension kept in a 7
Page 7 of 35
flask under continuous stirring, at 80 °C for 30 min for completing the gelatinization of
145
the corn starch. Then, either CNFs or ACNFs (10 wt% based on dry weight of starch)
146
were added and the mixture was stirred for another 30 min at 80°C. Subsequently, the
147
mixture was degassed under vacuum and cast into a Teflon Petri dish followed by drying
148
in an oven at 40 °C for 3 days. The obtained composite films that had a thickness of 0.4
149
mm, were conditioned at 23 °C and 75% moisture for 3 days to ensure the equilibration
150
of the water content in the films. The samples were cut according to the ASTM standard
151
D 6287-98 to provide test specimens.
2.3. Characterization
154
2.3.1. Degree of substitution
ed
153
M
an
152
us
cr
ip t
144
The degree of substitution for the ACNFs was determined by the titration method
156
described by Kim et al. (2002). Briefly, a sample of the suspension containing 100 mg of
157
the ACNFs combined with 40 ml of 75% ethanol was heated at 60°C for 30 min. Then 40
158
ml of 0.5 N NaOH solution was added to the sample and stirred for 15 min. The sample
159
was kept at room temperature for 48 h and then the excess alkali determined with 0.5 N
160
HCl using phenolphthalein as an indicator. The titration process was repeated three times
161
for each sample to confirm the reproducibility of the measurement.
Ac ce
pt
155
162 163
2.3.2. Water absorption
8
Page 8 of 35
Water absorption test was performed in accordance with the ASTM standard
165
E104. All the specimens for water absorption were cut from TPS, CNFs/TPS and
166
ACNFs/TPS nanocomposite films with dimensions of 2 × 2 cm2 and dried at 60 °C for 24
167
h. The dried samples were placed in a desiccator containing calcium sulfate (RH = 0%)
168
for 3 days and afterwards the initial weight of the samples were measured. The samples
169
were then transferred into a desiccator containing potassium sulfate (RH = 98%) and the
170
time taken to reach a constant weight was marked. The water absorption (WA) of the
171
specimens was calculated as follows:
172
WA
173
where W1 is the weight of the sample at the time in 98% RH and W0 is the initial sample
174
weight exposure to 98% RH. An average value of the three replicates for each sample
175
was reported.
178
cr
us an
ed
M
(1)
pt
177
W1 W0 100 W0
2.3.3. Water vapor transmission rate
Ac ce
176
ip t
164
The water vapor transmission rate was determined gravimetrically following the
179
ASTM standard E 96. The samples were conditioned to an environmental chamber at
180
25°C and 50% relative humidity (RH) using calcium nitrite for 24 h. Then, the specimens
181
were fixed on glass cups containing 3 g of calcium sulfate that provides 0% RH. The cup
182
assembly was weighed using a four-digit balance and placed into a controlled
183
temperature (25 ± 0.5 °C) and humidity (98 ± 2% RH) desiccator. Cups were then
184
periodically weighed at intervals of 1 and 24 h for 80 h. Weight gain was plotted as a
9
Page 9 of 35
185
function of time and the slope of the linear portion was determined using a simple linear
186
regression method. Water vapor transmission rate (WVTR) and water vapor permeability
187
were calculated using equations 2 and 3: G ( ) slope WVTR t A test area
(2)
ip t
188
where G = weight change (g), t= time (h), A= the area exposed to moisture transfer (m2),
190
G/t = the slope obtained from a chart of weight gain vs. time. Linear regression of the
191
data gave a correlation coefficient > 95%.
us
cr
189
192
WVTR X P(R 1 R 2 )
WVP
194
where X is the thickness of the film (m), P is the saturation vapor pressure of water at 25
195
°C (Pa), R1 is the relative humidity in the desiccator (98% RH), and R2 is the relative
196 197
humidity in the cups (0% RH).
198
2.3.4. Mechanical testing
(3)
pt
ed
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an
193
Mechanical tests were performed according to the ASTM standard D889. The
200
samples were cut into 70×10 mm2. Before testing, all the test specimens were conditioned
201
for at least one week in a desiccator with 55% RH in room temperature. The tensile tests
202
were conducted using an Instron testing machine (M350-10- CT) with a 500 N load cell,
203
gauge length of 50 mm, and a crosshead speed of 4 mm/min. The elastic modulus was
204
calculated from the initial part of the slope from the stress–strain curves. At least five
205
samples were tested for each sample and the average values are presented.
Ac ce
199
206
10
Page 10 of 35
207
2.3.5. Dynamic mechanical analysis (DMA) The dynamic mechanical properties of the nanocomposites were performed in
209
uniform moisture using the dynamic mechanical analyzer, DMA Netzsch 242, in tensile
210
mode. The DMA measurements were conducted at a constant frequency of 1 Hz, a strain
211
amplitude of 0.05% with a temperature range from -50 to150°C at a heating rate of 3
212
o
213
obtained represented the average of the 3 samples.
ip t
208
215
us
an
214
cr
C/min. The test specimen dimensions were approx. 30.0×5.0×0.4 mm3. Each value
2.3.6. Biodegradation test
In order to study the fungal degradation of the biocomposites, a white rot fungus
217
(T. versicolor) was used. The specimens of the neat TPS, CFN/ TPS, and ACNFs/TPS
218
nanocomposites were prepared with lateral dimensions of 20.0×10.0×0.4 mm3 (length
219
×width ×thickness). The samples were dried at 70 °C in an oven overnight and the initial
220
weight of each specimen was recorded. The purified fungus was transferred to the Petri
221
dishes containing MEA. The dishes were then kept in the laboratory incubator at 25 °C
222
until the culture medium was fully covered by the fungus. The specimens were then
223
transferred into the Petri dishes containing the culture medium. To prevent direct contact
224
of the specimens with the culture medium, the specimens were mounted over two 2-mm
225
platforms. The Petri dishes containing the fungus and the specimens were then stored in
226
an incubator at 25 °C and 75% RH. The biodegradation rate of the specimens were
227
evaluated for two months and at regular intervals of 10 days by weight differences before
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216
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Page 11 of 35
228
and after the exposure of the specimens to the white rot fungi. The fungal degradation
229
rate was calculated using Equation 4.
230
DE(%)
231
where DE is the biodegradation rate of TPS, W0 is the initial weight of the original
232
specimen, and W1 is the dry weight of the residual specimen exposed to the white rot
233
fungi for a certain time.
W0 W1 100 W0
us
cr
ip t
(4)
2.3.7. Scanning electron microscopy (SEM)
M
235
an
234
The surface and cross-section of the specimens were examined before and after
237
degradation using a scanning electron microscope, model ZEISS Ultra 55, Germany. The
238
acceleration voltage was 10 kV and the specimens were sputter-coated with gold to avoid
239
charging. In preparation, the specimens were removed from the culture and carefully
240
rinsed with distilled water to remove fungal mycelium. Then, the specimens were dried at
241
35°C for further evaluation of biodegradation efficacy.
pt
Ac ce
242
ed
236
243
3. Results and discussion
244
3.1. Acetylation and degree of substitution for CNFs
245
The acetylation involves the replacement of hydroxyl groups of the nanofibers
246
with acetyl groups in the reagent. The acetylation of the CNFs with acetic anhydride was
12
Page 12 of 35
carried out at 100ºC for 4 h. A long reaction time along with pyridine as a catalyst was
248
used to promote the sufficient diffusion of acetic anhydride into the amorphous regions of
249
the CNFs. Therefore, the bulk substitution was facilitated. The degree of the substitution
250
(DS) of acetylated nanofibers determined by titration was 0.2, which confirmed the
251
successful acetylation of nanofibers.
ip t
247
us
253
cr
252
3.2. Water absorption
The water absorption of the neat starch and the TPS nanocomposites reinforced
255
with CNFs and ACNFs were studied at a relative humidity level of 98%. The moisture
256
absorption of the neat starch and its CNFs and ACNFs nanocomposites increased linearly
257
in the initial step while at the lateral, the rate of water uptake decreased (Figure 1).
258
Moreover, the water uptake of the obtained composites reinforced with 10 wt% of CNFs
259
and ACNFs was significantly reduced compared to the neat TPS. The amount of
260
reduction was approximately 14% for the CNFs/TPS and 30% for the ACNFs/TPS
261
composites. Similar results have been reported (Svagan et al., 2009; Hietala et al., 2013),
262
which might be attributed to the amorphous and branch structure of starch with higher
263
accessibility to the water than the CNFs. As expected, in regard to the ACNFs/TPS,
264
further reduction in water absorption occurs due to acetylation by replacing the hydroxyl
265
groups of the CNFs with acetyl groups. This confirmed that the chemical modification
266
had indeed occurred on the CNFs.
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an
254
267 268
3.3. Water vapor permeability (WVP) 13
Page 13 of 35
In order to minimize the effect of water vapor, the packaging material must
270
prevent, or at least decrease, moisture exchange with the environment. The WVP values
271
of the TPS and TPS nanocomposites were investigated and the results are presented in
272
Table 1. Similar values recently have been reported for sago starch (6.44×10-7 g/m.h.Pa
273
plasticized with glycerol (Bajpai et al., 2011). It could be found that the WVP values of
274
both the CNFs/TPS (8.27×10-7 g/m.h.Pa) and the ACNFs/TPS (9.45×10-7 g/m.h.Pa) were
275
lower than the neat TPS (9.61× 10-7 g/m.h.Pa). On the other hand, the WVP was
276
decreased with the addition of the CNFs. The addition of the CNFs to the polymer matrix
277
presumably leads to denser and less porous materials. Furthermore, a good interfacial
278
adhesion between the CNFs and starch can probably restrict the moisture diffusion rate of
279
the CNFs/TPS compared with the neat TPS. In addition, the WVP for the ACNFs/TPS
280
composite was reduced less than the CNFs/TPS. It could be explained by replacing the
281
hydroxyl group with acetyl groups during acetylation of the CNFs, which reduces the
282
interaction between nanofibers- nanofibers and nanofibers-matrix that causes more
283
porous network. Also, decreasing the degree of crystallinity of cellulose nanofibers
284
during the acetylation can lead to similar results. Thus, these synergistic effects resulted
285
in an increase of the passage of water vapor in the ACNFs/TPS compared to the
286
CNFs/TPS (Rodionova et al., 2011). In summary, although the addition of the CNFs
287
could significantly improve barrier properties of the TPS film, the introduction of the
288
acetyl groups of the CNFs can slightly increase the water vapor permeability of the
289
composite.
Ac ce
pt
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an
us
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ip t
269
290 291
3.4. Mechanical properties 14
Page 14 of 35
The average values and standard deviations of the neat TPS, CNFs/TPS, and
293
ACNFs/TPS tensile properties are summarized in Table 2. The tensile properties showed
294
that an increased modulus and strength of both nanocomposites with 10 wt% nanofibers
295
compared to the pure TPS. The Young’s modulus and tensile strength increased from
296
16.6 and 8.6 MPa to 141 and 38 MPa for the CNFs/TPS, respectively. However, the
297
tensile modulus and strength of the composite with 10 wt% the ACNFs was lower than
298
the CNFs/TPS. The increment in the tensile modulus and the strength of the TPS
299
reinforced with the CNFs can be attributed to the formation of a hydrogen-bonded
300
nanofibers network and also the nanofibers entanglement (Angles and Dufresne, 2001).
301
In addition, the formation of hydrogen bonding results in a strong interface between two
302
phases (matrix and nanofibers) (Sreekala et al., 2008). On the other hands, the low tensile
303
modulus and strength of the TPS treated with the ACNFs, might be attributed to the
304
reduced nanofibers-to-nanofiber and nanofiber-to-matrix interactions due to the surface
305
hydrophobicity and also the reduced crystallinity of the nanofiber after acetylation
306
(Jonoobi et al., 2012). Furthermore, the elongation of the filled TPS with the CNFs was
307
lower (8%) than the neat TPS and the TPS filled with the ACNFs (15%), which could be
308
reasonably attributed to the higher rigidity of non-acetylated nanofibers. The less
309
flexibility of the CNFs compared to the ACNFs could make them more difficult to match
310
with each other and to other material (Khalil et al., 2000). Therefore, the CNFs with high
311
rigidity decreased the elongation at the break point forth CNFs/TPS composite compared
312
to the pure TPS and the TPS with the ACNFs.
Ac ce
pt
ed
M
an
us
cr
ip t
292
313 314
3.5. Dynamical thermal analysis (DMA) 15
Page 15 of 35
Figure 2 shows the storage modulus and the tan as a function of the temperature
316
of the prepared materials. Generally, the storage modulus decreases with increases in the
317
temperature (Soykeabkaew et al., 2012). As it can be seen from Table 3 and Figure 2A,
318
both acetylated and non-acetylated nanofibers resulted in a higher storage modulus in the
319
tested range. Table 3 shows the storage modulus of the composites at two different
320
temperatures. The storage modulus data of the TPS and its nanocomposites at room
321
temperature supported the data obtained from the tensile testing. The storage modulus of
322
the nanocomposites containing 10 wt% CNFs and ACNFs increased to 2500 and 800
323
MPa, respectively, as compared to the neat TPS at 25 °C. The results could be ascribed
324
by the formation of a network of the CNFs after evaporation of the water and the
325
formation of sufficient crosslinking of the matrix phase in the nanocomposite (Svagan et
326
al., 2007). On the other hand, the storage modulus improvement for the ACNFs/TPS is
327
less in comparison to the CNFs/TPS and this might be associated with the partial
328
destruction of the cellulose crystalline structure by acetylation and less networks
329
formation in the nanocomposite. Also, it is possible to say that the used nanofibers
330
restricted the molecular chains movement of the TPS, thereby improving its thermal
331
stability (Petersson and Oksman, 2006). Figure 2B shows how the CNFs and ACNFs
332
influenced the tan at the peak position of the TPS. The shift in the tan peak of
333
thermoplastic starch film reinforced using acetylated cellulose nanofibers was difficult to
334
estimate, because tan was not well-defined. To verify the accuracy of the data, the
335
DMA experiments were carried out in triplicate and the same curves with no sharp peaks
336
and the similar values for the temperature at the maximum in the tan peak were
Ac ce
pt
ed
M
an
us
cr
ip t
315
16
Page 16 of 35
337
obtained by the instrument software. It is worthwhile to mention that similar curves have
338
been reported earlier for potato starch film (Zhou et al., 2009). The tan peak temperatures for the TPS and its nanocomposite are given in Table
340
3. The peak position for the TPS occurred at 49 °C and increased to 83°C and 81°C for
341
the nanocomposites based on the CNFs and the ACNFs, respectively. An increase in the
342
tan value with temperature indicates that the nanocomposites were becoming more
343
viscous after raising the temperature (Liu et al., 2010). The results confirmed an
344
improvement in dynamic mechanical properties of the TPS reinforcement by both the
345
ACNFs and the CNFs. The increase in storage modulus, together with a positive shift in
346
the tan peak for the CNFs/TPS nanocomposite could be due to the strong chemical
347
interaction between the matrix and the reinforcements that restricted the segmental
348
mobility of the polymer chains in the vicinity of the nano reinforcements (Bondeson et
349
al., 2007). However, it may be noted that the ACNFs did not improve the dynamic
350
mechanical properties of the neat TPS as much as the CNFs ones. This suggested that the
351
interface was not good enough due to the surface modification and it is in agreement with
352
the tensile testing results.
354
cr
us
an
M
ed
pt
Ac ce
353
ip t
339
3.6. Biodegradability of composites
355
The biodegradability of the TPS and the TPS/nanocomposites was investigated
356
undertaking white rot fungi (T. versicolor) at regular time intervals by measuring the
357
weight loss. Figure 3 shows the degradation process and the weight loss with respect to
358
the elapsed time. By increasing the decomposition time, the compactness of the films 17
Page 17 of 35
would be reduced. The films showed a rapid degradation within a period of 30 days.
360
More than 50 wt% weight loss of the total solids was observed for the composites after
361
20 days. The results indicated that the TPS was fully degraded at 30 days while the
362
complete destruction of the CNFs/TPS and the ACNF/TPS occurred in the 40 and 60
363
days, respectively. It could be observed that the CNFs/TPS and the ACNFs/TPS
364
nanocomposites had a higher degradation resistance compared to the neat TPS. This
365
might be due to the higher crystallinity and the denser structure of the CNFs, which was
366
added to the starch matrix and restricted the destructive enzymatic activities as well as the
367
enzymatic hydrolysis of the cellulose (Lai and Ontto, 1979). The crystalline structure of
368
the cellulose causes the formation of a dense layer of water, which consequently prevents
369
the spread of cellulose-degrading enzymes and other substances around the cellulose
370
((Matthews et al., 2006). The environmental degradation of cellulose can be affected by
371
several parameters such as the existence and diversity of microorganisms, the availability
372
of carbon sources, and the time span of the observation period (Gu, 2003). The micro-
373
organisms have higher activity in the presence of hydrogen and carbon groups. In the
374
case of the ACNFs/TPS, the substitution of the acetyl groups with the hydroxyl groups
375
during the chemical modification resulted in a decrease in the decomposition rate and the
376
micro-organisms activities (Glasser et al., 1994). Figure 4 shows the visual appearance of
377
samples after exposure by the white rot fungi. The samples were eroded significantly and
378
lost their original shape by over time. Also it could be seen that the color of the samples
379
became yellow to dark-brown. The results clearly demonstrated that the CNFs can
380
increase the degradation period of the nanocomposites compared to the neat TPS.
Ac ce
pt
ed
M
an
us
cr
ip t
359
381
18
Page 18 of 35
382
3.7. Scanning electron microscopy (SEM) The changes in the morphology of the materials have been studied before and
384
after fungi treatment. The scanning of the TPS and the TPS nanocomposites are presented
385
in Figure 5. It could be observed that the surface and cross-section of the ACNFs/TPS
386
were smoother compared to the CNF/TPS and it could be due to the lower aggregation of
387
the nanofibers in the starch matrix. Figure 6 indicates the SEM of the TPS and the TPS
388
nanocomposites after exposure to the fungus after 30 days. In Figure 7, the specific
389
growth of mycelium and fungal elements can be recognized on the samples. The results
390
confirmed that the nanocomposites surface became rougher after exposure to the fungus;
391
as well, reduction in the thickness of the nanocomposites could be observed. The results
392
suggested that by changing the surface of the nanofibers from hydrophilic to
393
hydrophobic, the activity of the fungi decreased. In lower fungi activation, the surface
394
degradation of the TPS nanocomposite materials might be reduced and then, the surface
395
of the nanocomposite is found to be smoother.
pt
ed
M
an
us
cr
ip t
383
397 398
Ac ce
396
4. Conclusions
This study showed that both acetylated and non-acetylated CNFs can be used to
399
fabricate the thermoplastic starch nanocomposite. The composites were prepared
400
successfully by using the solution casting method. The effects of acetylated and non-
401
acetylated nanofibers on water absorption, tensile, dynamic mechanical properties, and
402
biodegradability of TPS were compared. The mechanical testing showed that the tensile
403
strength and modulus of both nanocomposites increased when compared to the matrix 19
Page 19 of 35
while the improvement was lower for the acetylated one. The storage modulus and the
405
tan δ peak position of both nanocomposites showed improvement when compared to the
406
matrix. Regardless of the type, the addition of nanofibers increased the Tg of the
407
nanocomposites, determined from a peak in the tan δ curve. In addition, the moisture
408
absorption of the TPS nanocomposite was reduced by addition of the ACNFs compared
409
to the non-acetylated one. Furthermore, the fungal biodegradability results showed a
410
longer decomposition period when the CNFs and the ACNFs are added to the TPS
411
matrix. But, the acetylated nanocomposite needs a longer time for degradation compared
412
to the CNFs/TPS, and it became more sustainable due to the replacement of hydroxyl
413
groups with acetyl groups.
M
414
an
us
cr
ip t
404
Ac ce
pt
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415
20
Page 20 of 35
415
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Himmel, M.E., & Brady, J.W. (2006). Computer simulation studies of
470
microcrystalline cellulose Iβ. Carbohydrate Research, 341 (1), 138-152.
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468
Petersson, L., & Oksman, K. (2006). Biopolymer based nanocomposites: comparing
472
layered silicates and microcrystalline cellulose as nanoreinforcement. Composites
473
Science and Technology, 66 (13), 2187-2196.
475 476
pt
Petersson, L., Kvien, I., & Oksman, K. (2007). Structure and thermal properties of poly
Ac ce
474
ed
471
(lactic acid)/cellulose whiskers nanocomposite materials. Composites Science and Technology, 67 (11), 2535-2544.
477
Rodionova, G., Lenes, M., Eriksen, Ø., & Gregersen, Ø. (2011). Surface chemical
478
modification of microfibrillated cellulose: improvement of barrier properties for
479 480 481
packaging applications. Cellulose, 18 (1), 127-134. Shah, A. A., Hasan, F., Hameed, A., & Ahmed, S. (2008). Biological degradation of plastics: a comprehensive review. Biotechnology Advances, 26 (3), 246-265.
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Siró, I., & Plackett, D. (2010). Microfibrillated cellulose and new nanocomposite materials: a review. Cellulose, 17 (3), 459-494. Soykeabkaew, N., Laosat, N., Ngaokla, A., Yodsuwan, N., & Tunkasiri, T. (2012).
485
Reinforcing potential of micro-and nano-sized fibers in the starch-based
486
biocomposites. Composites Science and Technology, 72 (7), 845-852.
ip t
484
Sreekala, M., Goda, K., & Devi, P. (2008). Sorption characteristics of water, oil and
488
diesel in cellulose nanofiber reinforced corn starch resin/ramie fabric composites.
489
Composite Interfaces, 15 (2-3), 281-299.
us
cr
487
Svagan, A. J., Hedenqvist, M. S., & Berglund, L. (2009). Reduced water vapour sorption
491
in cellulose nanocomposites with starch matrix. Composites Science and Technology,
492
69 (3), 500-506.
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an
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Svagan, A.J., Azizi Samir, M.A., & Berglund, L.A. (2007). Biomimetic polysaccharide
494
nanocomposites of high cellulose content and high toughness. Biomacromolecules, 8
495
(8), 2556-2563.
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Teixeira, E.D.M., Pasquini, D., Curvelo, A.A., Corradini, E., Belgacem, M.N., &
497
Dufresne, A. (2009). Cassava bagasse cellulose nanofibrils reinforced thermoplastic
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498
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496
cassava starch. Carbohydrate polymers, 78 (3), 422-431.
499
Zhou, Y.G., Wang, L.J., Li, D., Yan, P.Y., Li, Y., Shi, J., Chen, X.D., & Mao, Z.H.
500
(2009). Effects of sucrose on dynamic mechanical characteristics of maize and potato
501
starch film. Carbohydrate Polymers, 76 (2), 239-243.
502
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Tables Caption:
503
Table 1. Water vapor permeability (WVP) of the neat TPS and TPS
504
reinforced with CNF and ACNF composites
505
Table 2. Mechanical properties of the TPS and its nanocomposites
506
Table 3. Storage modulus and tan of the TPS and its nanocomposites
cr
ip t
502
us
507 508
Figures Caption:
510
Fig. 1. Water absorption of the TPS and TPS nanocomposites
511
Fig. 2. Storage modulus curves (A) and tan peaks (B) the TPS and its nanocomposites
512
Fig. 3. Fungal degradation of TPS and its nanocomposites
513
Fig. 4. Visual appearance of the composite influenced by white rot fungi
514
Fig. 5. SEM micrographs of the surface (top) and cross-section (down) of TPS (A)
515
TPS/CNFs (B) TPS/ACNFs (C)
516
Fig. 6. Mycelium of white rot fungi on the composites surface; TPS (A) TPS/CNFs (B)
517
TPS/ACNFs (C)
518
Fig. 7. SEM micrographs of the surface (top) and cross-section (down) of TPS (A)
519
TPS/CNFs (B) TPS/ACNFs (C) after degradation by white rot fungi
Ac ce
pt
ed
M
an
509
520
25
Page 25 of 35
521 522 523
ip t
524
cr
525 526
us
527
Table 1. Water vapor permeability (WVP) of the neat TPS and TPS
529
reinforced with CNF and ACNF composites
an
528
WVTR (g/m2.h)
9.61×10-7
CNFs/TPS
6.42
8.26×10-7
533 534 535 536 537
9.45×10-7
7.34
pt
532
ed
7.47
Ac ce
531
WVP (g/m.h.Pa)
TPS
ACNFs/TPS 530
M
Materials
538 539 540
26
Page 26 of 35
541 542 543 544
ip t
545 546
cr
547
us
548 549
555 556 557 558 559
16.6±2
8.6±1
141.0 ±35
38.0±3
29.6±6
14.7±2
M
Young modulus ( MPa)
TPS
52±2
CNFs/TPS
27±4
ACNFs/TPS
pt
50±2
Ac ce
554
Elongation at break (%)
ed
Materials
553
Tensile strength ( MPa)
Table 2. Mechanical properties of the TPS and its nanocomposites
551
552
an
550
560 561 562 563
27
Page 27 of 35
564 565 566 567
ip t
568 569
cr
570
572
576 577 578 579 580 581
49
CNFs/TPS
83
ACNFs/TPS
81
an
TPS
É 25 oC (MPa)
É 70 oC (MPa)
450
84
2500
541
800
250
M
Tan δ (oC)
ed
575
Materials
pt
574
Table 3. Storage modulus and tan of the TPS and its nanocomposites
Ac ce
573
us
571
582 583 584 585 586
28
Page 28 of 35
587 588
ip t
589 590
cr
591
us
592 593
an
594 595
M
596 597
601 602 603 604 605 606
pt
600
Fig. 1. Water absorption of the TPS and TPS nanocomposites
Ac ce
599
ed
598
607 608 609 610
29
Page 29 of 35
611 612 613
ip t
614 615
cr
616 617
us
618 619
an
620 621
M
622
626 627 628 629 630 631 632
pt
625
Ac ce
624
ed
623
633 634
Fig. 2. Storage modulus curves (A) and tan peaks (B) the TPS and its nanocomposites
635 636
30
Page 30 of 35
637 638 639 640
ip t
641 642
cr
643
us
644 645
an
646 647 648
M
649
653 654 655 656 657 658 659
pt
652
Ac ce
651
ed
650
Fig. 3. Fungal degradation of TPS and its nanocomposites
660 661 662 663
31
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664 665 666 667
ip t
668 669
cr
670
us
671 672
an
673 674
M
675 676
680 681 682 683 684 685 686
pt
679
Fig. 4. Visual appearance of the composite influenced by white rot fungi
Ac ce
678
ed
677
687 688 689 690
32
Page 32 of 35
691 692 693 694
ip t
695 696
cr
697
us
698 699
an
700 701
M
702 703
707 708 709 710 711 712 713
pt
706
Ac ce
705
ed
704
Fig. 5. SEM micrographs of the surface (top) and cross-section (down) of TPS (A), CNFs/TPS (B), and ACNFs/TPS (C)
714 715 716 717
33
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718 719 720 721
ip t
722 723
cr
724
us
725 726
an
727 728 729
M
730
734 735 736 737 738 739 740
pt
733
Ac ce
732
ed
731
741 742 743
Fig. 6. Mycelium of white rot fungi on the composites surface; TPS (A) TPS/CNFs (B)
744
and TPS/ACNFs (C) 34
Page 34 of 35
745 746 747 748
ip t
749 750
cr
751
us
752 753
an
754 755 756
M
757
761 762 763 764 765 766 767
pt
760
Ac ce
759
ed
758
Fig. 7. SEM micrographs of the surface (top) and cross-section (down) of TPS (A) TPS/ CNFs (B) and TPS/ACNFs (C) after biodegradation
768
35
Page 35 of 35