Study on the reinforcing effect of nanodiamond particles on the mechanical, thermal and antibacterial properties of cellulose acetate membranes

    Study on the reinforcing effect of nanodiamond particles on the mechanical, thermal and antibacterial properties of cellulose acetate membranes Habib Etemadi, Reza Yegani, Valiollah Babaeipour PII: DOI: Reference:

S0925-9635(16)30194-7 doi: 10.1016/j.diamond.2016.08.014 DIAMAT 6691

To appear in:

Diamond & Related Materials

Received date: Revised date: Accepted date:

17 June 2016 20 August 2016 29 August 2016

Please cite this article as: Habib Etemadi, Reza Yegani, Valiollah Babaeipour, Study on the reinforcing effect of nanodiamond particles on the mechanical, thermal and antibacterial properties of cellulose acetate membranes, Diamond & Related Materials (2016), doi: 10.1016/j.diamond.2016.08.014

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ACCEPTED MANUSCRIPT Study on the reinforcing effect of nanodiamond particles on the mechanical, thermal and antibacterial properties of cellulose acetate membranes Habib Etemadia, b, Reza Yegania, b,*, Valiollah Babaeipourc a

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Faculty of Chemical Engineering, Sahand University of Technology, Tabriz, Iran Membrane Technology Research Center, Sahand University of Technology, Tabriz, Iran c Department of Biological Science and Technology, Malek-Ashtar University of Technology, Tehran, Iran b

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*

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Corresponding author: Reza Yegani; E-mail address: [email protected]

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Abstract

The aim of this study was to determine the impact of detonation nanodiamond (DND) on the

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mechanical, thermal and antibacterial properties of cellulose acetate (CA) membrane. In order to achieve an efficient dispersion of DNDs in the polymeric matrix, they were functionalized via

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heat treatment. Different amounts of raw and functionalized DND; 0 to 0.75 wt.%, were added to

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the CA and various structural and characterization analyses such as scanning electron

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microscopy (SEM), transmission electron microscopy (TEM), thermal gravimetric analysis (TGA) and Fourier transform infrared (FTIR) were also carried out. Mechanical strength analysis revealed that both raw and carboxylated DND have great influence on the mechanical behavior of CA membrane particularly at 0.5 wt.% of nanoparticles (NPs) content. Application of Pukanszky’s model for tensile strength and micromechanical models for tensile modulus revealed that strong interfacial interaction and thick interphase region are formed around the NPs. In addition, the TGA results showed that the incorporation of 0.5 wt.% of the DND and DND-COOH improved the thermal stability of the CA membrane. The antibacterial tests confirmed that the nanocomposite membranes containing DND-COOH displayed greater antibacterial enhancement against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus).

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ACCEPTED MANUSCRIPT Keywords: nanodiamond, cellulose acetate, nanocomposite membrane, mechanical properties, interphase region 1. Introduction

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Membrane separation technology is one of the most efficient methods in separation science and

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processes due to its low energy consumption, easy scale-up, less or no use of chemicals, and

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absence of any harmful by-product formation [1]. Polymeric membranes have many advantages such as straightforward pore forming mechanism, higher flexibility, smaller footprints required

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for installation and considerably low costs with respect to inorganic membranes or metal

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frameworks, which make them more convenient for a wide range of applications in large scales especially in water reclamation and wastewater treatment processes [2]. Cellulose acetate (CA) is

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one of the foremost among polymer membranes which has been widely used in separation

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processes and has been nominated as one of the most applicable polymers in preparation of membranes, due to its high hydrophilicity, high biocompatibility, non-toxic nature, good

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desalting, high potential flux and relatively low cost [1, 3, 4]. However, narrow temperature range (maximum 30°C), high biofouling and microbial degradation tendency, poor mechanical and chemical stability in both acidic and basic solutions are considered as its main disadvantages, which demands for efficient modification [4]. Due to the rapid growth of nanotechnology, fabrication of nanocomposite membranes has shown great impact and efficient performance in the past decades. In this regard, the role of various nanoparticles (NPs) on the engineering features of polymeric nanocomposite membranes has been extensively examined, in many cases significant improvement in mechanical, thermal and antifouling properties has been explored. However, the effective and uniform dispersion of NPs is still a challenging subject for researchers which is greatly influenced by the intraparticle interactions. It is well known that the uniform dispersion of NPs throughout the polymeric matrix 2

ACCEPTED MANUSCRIPT and strong interfacial bonding between the NPs and the matrix are major factors that improved the mechanical and thermal properties of membranes [4-9].

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Several researches have been particularly carried out on the impact of NPs on the modification of

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CA based membranes [4, 6, 10-15]. According to these studies, at high concentration of nanoparticles e.g. silver (AgNO3), polyhedral oligomeric silsesquioxane (POSS), and organically

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modified montmorillonite (OMMt), the mechanical and thermal properties of CA membrane

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decreased due to the agglomeration of NPs as well as the weak interaction between polymer and NPs. Also, the interfacial interaction between NPs and CA matrix played a crucial role in

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improvement of matrix properties [6, 13, 14].

Carbon-based nanomaterials are potentially useful due to their unique physical and chemical

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properties. Among them, detonation nanodiamond (DND) particles with a diamond core (sp3 carbon-carbon bond) that is covered with multiple functional groups including carboxylic acids,

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hydroxyl, ketones, ethers, and lactones, are favored for many direct applications or post modification [16, 17]. Due to the interesting characteristics of DND such as hydrophilicity [16, 18, 19], antibacterial activity [20, 21], biocompatibility [22-26], chemical stability [22], thermal stability [23] non-toxicity [22, 23, 27], superior hardness and mechanical properties, resistance to harsh environments [22, 28] and ease of surface functionalization [23, 29], it is predicted that it can be potentially used as reinforcement filler in fabrication of nanocomposite materials. The non-diamond carbon from DND surface can be easily removed via thermal or acid treatments and some desirable functional groups, such as carboxyl groups can be easily formed, which could be beneficial to dispersion capability of DND, especially in polar media [23, 30, 31]. To the best of our knowledge, literature does not report any document regarding the application of DNDs in fabrication of polymeric nanocomposite membranes. Excellent mechanical and 3

ACCEPTED MANUSCRIPT thermal properties, very high specific surface area and the presence of hydrophilic functional groups on the DND surface make it an interesting and good choice to be used as reinforcement

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agent in CA matrix to overcome the inherent disadvantages of CA membrane. DNDs have recently emerged as an important focus in the development of antibacterial and antibiofilm

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forming agents [32]. Some materials with antibacterial activity such as silver more recently

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concern about the cytotoxic effects, aggregation and loss of antibacterial activity [33]. In this case, DND can be used as a new effective agent against bacteria and prevent the biodegradability

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of CA membrane.

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In order to improve the mechanical, thermal and antibacterial properties of CA membrane, in this work, DND embedded CA nanocomposite membranes with various amounts of the DND

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contents NPs were fabricated via phase inversion method. Since the mechanical and thermal

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properties of the nanocomposite membranes depend on interaction between the polymer matrix

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and the NPs as well as on uniform dispersion NPs in polymer matrix, raw DNDs were functionalized via thermal treatment to create carboxyl groups on the surface of the DND. Fabricated membranes were analyzed by applying experimental methods such as mechanical tests, scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal gravimetric analysis (TGA) and Fourier transform infrared (FTIR) spectroscopy. Two and three phase theoretical models were also utilized to investigate the impact of DND on embedded CA membranes. 2. Experimental 2.1.Materials Cellulose acetate (Mn=30000), was used as polymer material to prepare the CA membrane, supplied by Sigma-Aldrich (Germany). The DNDs procured from Nabond Technology Co., Ltd.,

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ACCEPTED MANUSCRIPT China, having phase purity higher than 98% and average diameter of 5 nm, providing a specific surface area of about 300 m2g-1. N-N-dimethylformamide (DMF, 99.8%, Merck) and deionized

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2.2.Surface functionalization of DND via thermal oxidation

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(DI) water were used as solvent and non-solvent, respectively.

Due to the fact that the oxidation of carbon-based materials removes organic impurities by

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formation of carboxylic acid on the surface, DNDs were dried at 80 °C for 2 h in vacuum dryer

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and then warmed up to 430 °C under atmospheric condition for 1.5 h. This procedure creates

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various functional groups such as ether and carboxyl groups on DND surface [34-36]. 2.3.Preparation of neat and nanocomposite CA membranes

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Neat and nanocomposite CA flat sheet membranes were prepared by phase inversion method

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[37]. Different amounts of raw and functionalized DNDs, hereafter denoted as DND and DNDCOOH, respectively, ranging from 0-0.75 wt.% based on the total polymer weight, were used to

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prepare nanocomposite membranes. All NPs were kept in vacuum dryer at 80oC for 2hr to remove any adsorbed humidity, before adding into the solvent. In this regard, the measured amount of NPs was added to DMF and gently stirred using magnetic stirrer for 3 h at room temperature. The mixture was then sonicated (ultrasonic bath, WOSON Company, China) at 50 kHz for 3 h to better dispersion of the NPs. Then, CA polymer was added to the homogenous suspension and stirred for about 2hat 2000 rpm to achieve complete dissolution of CA. For all pure and nanocomposite membranes, the weight percent of polymer to solvent was kept constant at 17.5 wt%. The casting solutions were then kept for 24 h to remove air bubbles. After that, the dope solutions were cast on a glass plate using an automatic programmable film applicator (Coa

Test, Taiwan) with a speed of 10 mm/s using a doctor blade with gap of 200 μm. The cast films were subsequently immersed in a DI water bath for 24 h to completely induce phase separation. 5

ACCEPTED MANUSCRIPT The synthesized membranes were washed thoroughly with DI water and kept in DI water to be ready for further characterizations. The conditions under which the membranes were prepared

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are listed in Table 1. 2.4.Membrane characterizations

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2.4.1. Scanning electron microscopy (SEM)

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The surface and cross-section morphologies of the prepared neat CA and nanocomposite

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membranes were examined using a scanning electron microscope (SEM, LEO model 1455VP, UK) operating at 15 Kv. For cross sectional images, membranes were fractured in liquid

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nitrogen. All the samples were gold-coated by sputtering to produce electrical conductivity.

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2.4.2. Transmission electron microscopy (TEM)

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TEM analysis was performed with a Philips CM120 transmission electron microscope operating at 120 keV. The sample membranes were embedded with epoxy, and cross sections of

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approximately 50 nm were obtained by sectioning with a Leica Ultracut UCT ultramicrotome. 2.4.3. Fourier Transform Infrared (FTIR) spectroscopy The chemical structure of NPs and membranes was studied by Fourier transform infrared spectroscopy (FTIR) with a VERTEX 70 FTIR spectrometer (Bruker, Germany) in the range of 400 – 4000 cm -1. The sample pellet of nanoparticles for FTIR test was prepared by mixing the particles with KBr. Membrane samples were placed active-face down on the ATR crystal, and held in place by a clamp. A background was run before each sample set. Automatic baseline correction and scale normalization were performed for each set of data. 2.4.4. Membrane porosity

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ACCEPTED MANUSCRIPT The membrane porosity ε(%) can be defined as the volume of pores divided by the total volume of the porous membrane. The porosity of different membranes was calculated through the

( Ww  Wd ) / D w  100 ( Ww  Wd ) / D w  Wd / D p

(1)

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(%) 

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following Eq. (1) [38]:

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where ε is the porosity of membrane (%), W w is the wet sample weight (g), Wd is the dry sample weight (g), Dw (0.998 gcm-3) and Dp (1.3 gcm-3) are the density of the water (water has been

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selected due to hydrophilicity property of CA) and polymer at 25°C, respectively. Three

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samples were measured for each membrane and the average value of membrane porosity was

2.4.5. Mechanical properties

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reported.

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Tensile tests were performed by a universal testing machine (STAM-D, SANTAM, Iran)

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at room temperature with crosshead speed of 10 mm/min. All samples for mechanical testings were in rectangular shape. The tensile strength was extracted from the relevant stress–strain curves. The reported results were the average of at least three tests. 2.4.6. Thermal properties

The thermal stabilities of fabricated membranes were investigated by thermogravimetric analysis (TGA). TGA measures the weight loss of a material due to the formation of organic volatile species as the function of a programmed temperature [39]. The thermal behavior of the membranes was determined using a thermogravimetric analyzer (Pyris Diamond TG/DTA, PerkinElmer) at a heating rate of 10°C/min from room temperature to 700 °C. 2.4.7. Antibacterial activity

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ACCEPTED MANUSCRIPT The antibacterial activities of membranes were tested against both Gram-positive and Gramnegative bacteria, Staphylococcus aureus (S. aureus) and Escherichia coli(E. coli), respectively,

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according to JIS L 1902–2002 method [40]. 3 mL of Luria Bertani (LB) broth was prepared in test tubes with 10 mL capacity. All tubes were autoclaved at 121 °C for 15 min. Each tube was

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inoculated aseptically with 30  L of bacterial suspension. The flat sheet membranes were cut

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into small sheets with dimensions of 1 cm  1 cm, washed with 75% ethanol to kill any bacteria

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on their surfaces and finally immersed in each tube. The positive control for the bacterial growth was inoculated with same concentration of bacterial suspension without adding flat sheet

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membranes. The negative control contains only LB broth without inoculation of bacterial suspension. All tubes were incubated at 35 °C in an incubator for 24 h. After incubation, optical

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density (OD) at 600 nm was measured by spectrophotometer (Bio Quest CE2501).

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3. Results and discussion

3.1.Membrane characterization

3.1.1. FTIR analysis of DND treatment and membranes The surface chemistry of the raw and thermally treated nanodiamonds; DND and DND-COOH, respectively, was determined by FTIR spectra. As shown in Fig. 1, in case of DNDs, the absorption peaks at 2922 and 2860 cm-1correspond to the asymmetric and symmetric stretching vibration of C-H band, respectively. Also, the absorption bands at 1341 cm-1 can be attributed to the deformation vibration of C-H band in alkyl group [41]. The absorption peak at 3423 cm1

corresponds to the stretching vibration of O-H, while that at 1634 cm-1corresponds to the

deformation vibration of O-H band [36]. The spectrum revealed another bands from oxygen

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ACCEPTED MANUSCRIPT containing functional groups, at 1711 and 1132 cm-1which are attributed to the stretching vibration of carbonyl, C=O and ether, C-O-C groups, respectively [42].

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Comparing the FTIR spectra of raw and thermally treated DND particles in Fig.1 reveals that the

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variety of surface functional groups in raw DND has been converted into their oxidized derivatives. After oxidation, for example, C-H bands in raw DND are completely disappeared

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and C=O vibrations bands are shifted from 1711 to 1796 cm−1, indicating a conversion of

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ketones, aldehydes and esters groups into the carboxylic acids, anhydrides, or cyclic ketones

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groups [43].

The surface chemistry of neat and nanocomposte CA membranes investigated using FTIR

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analysis. Fig. 2 shows the FTIR spectra of neat, CA/DND and CA/DND-COOH nanocomposite

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membranes. The main characteristics bands of CA were assigned as follows. The absorption peak at 1734 cm−1corresponds to the C=O functional groups, the characteristic absorption peak at

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3481 cm−1can be attributed to the presence of hydroxyl group (OH) and the peaks at 1379 and 1050 cm−1correspond to the CH3 and ether C−O−C functional groups, respectively [44]. Also, the absorption peak at 2961 cm-1corresponded to C–H vibrations of alkyl groups [–(CH)n–] [4]. Comparing the FTIR spectra of neat and nanocomposite CA membrane in Fig. 2 confirms that the characteristics peak of OH group was shifted from 3481 cm−1 in neat CA membrane to 3423 and 3407 cm−1 in CA/DND and CA/DND-COOH nanocomposite membranes. Similar shifts are observed for the peaks located at 1734, 1379 and 1050 cm−1. According to data reported by Sui et al., shifts in the peaks positions can be attributed to the formation of hydrogen bonding between NPs and matrix [45]. 3.1.2. Membrane morphologies

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ACCEPTED MANUSCRIPT SEM images were taken to determine the effects of the DND and DND-COOH on the morphology of the CA membrane. Fig. 3 depicts the SEM images of the surface of the

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membranes.

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SEM images of the surface of the neat CA membrane show non porous structure (Fig. 3a), which is typical for CA membrane and has been reported in many literature [11, 46, 47]. As shown in

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Fig. 3b and 3c, in presence of 0.5 wt% of both raw and functionalized DNDs, porous structure on

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the membrane surfaces are appeared.

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SEM images of the cross-sections of the selected membranes; neat and nanocomposite CA membranes with specific DND contents; CA/DND (0.5 wt.%) and CA/DND-COOH (0.5 wt.%),

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are shown in Fig. 4. All samples show asymmetric structure consisting of superior dense and

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inferior finger-like layers. It can be seen that in the presence of DND, the length of finger-like pores increases, the thickness of dense layer decreases and micro-cavities appears (see Fig. 4b).

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The highly hydrophilic nature of CA strongly interacts with water as non-solvent and decrease its diffusion rate, which consequently retards the coagulation rate during the phase inversion, resulted in the formation of denser skin layer [47]. By adding DND and DND-COOH particles, the number as well as the length of macrovoids increase, while the size of macrovoids decreases after increasing DND and DND-COOH contents. The raw and functionalized DNDs are not discernible in the cross sectional SEM images due to their relatively lower content in the nanocomposite membranes. On the other hand, any DND and DND-COOH clusters or agglomerates are also not observed in the SEM images of the membrane cross section (Figs. 4b and 4c) confirming the homogeneous dispersion of DND and DNDCOOH in the CA membrane.

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ACCEPTED MANUSCRIPT However, the information about the dispersion of nanoparticles cannot be obtained from the SEM images. Therefore, TEM was used to investigate the dispersion of nanoparticles in CA

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membranes, and the result was shown in Fig. 5. In lower amounts of functionalized nanoparticles, i.e. 0.5 wt.% DND-COOH, the formation of large aggregations was prevented and

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therefore a relatively uniform distribution of the DND-COOH particles was achieved as shown

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in Fig. 5a. Meanwhile, by increasing the amount of nanoparticles, i.e. 0.75 wt.% DND, more

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agglomerations were formed (see Fig. 5b).

The effect of DND and DND-COOH addition on the porosity of prepared membranes is

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presented in Fig. 6. The results revealed that the nanocomposite membranes exhibited higher porosity than that of the pure CA membrane. It can be seen from Fig. 6 that by increasing the

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NPs concentration, the porosity increases from 76.7% to 80.5% and 81.4% for CA, CA/DND

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(0.5 wt.%) and CA/DND-COOH (0.5 wt.%) membranes, respectively. As reported in literature

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[48], mixing hydrophilic NPs with the matrix of polymer could increase the amorphous nature of membranes. Together with the fast exchange of solvent and nonsolvent in the phase inversion process, the overall porosity of nanocomposite membranes was improved. Higher contents of DND as well as DND-COOH do not yield further increase in the porosity of nanocomposite membranes. This finding can be contributed to the particles agglomeration [49]. 3.1.3. Mechanical stability Fig. 7displays the variation of tensile strengths of neat CA membrane and CA/DND as well as CA/DND-COOH nanocomposite membranes with various NPs contents under wet and dry conditions. For wet samples, shown in Fig. 7a, the tensile strength of neat CA membrane is about 5.2MPa. In comparison with pure CA membrane, when DND contents of both raw DND and

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ACCEPTED MANUSCRIPT DND-COOH increase up to 0.5 wt%, the tensile strength of nanocomposite membranes increases about 7% and 12%, respectively.

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It can be seen that the increase in the tensile strength of DND-COOH embedded CA membrane

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is more pronounced than raw DND embedded CA membrane. This can be due to the treatment of the DND surface which reduces the tendency of DND particles to join each other and form

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particle aggregates. As schematically proposed in Fig. 8, there is a good potential to make strong

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interactions e.g. hydrogen bonding between DND and CA molecules. Any further increase in DNDs contents, up to 0.75 wt%, results in a significant loss in tensile strength. It can be

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attributed to the formation of NPs aggregates/agglomerates in the polymeric matrix which has

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been already explained in Fig. 5.

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Fig. 7b shows the variation in the tensile strengths of pure and nanocomposite CA membranes in dry conditions. The obtained results show similar trend in respect with DNDs contents. The

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reason why tensile strengths of all membranes immersed in water are much lower than in dry mode can be explained using the plasticizing effects of water on the hydrophilic polymeric membrane [50]. In addition, these results also deduce that swollen condition may hinder chain orientation along the stress direction and thus results in the lower tensile strength of the wet membranes [51]. When DNDs and DND-COOH content increases to 0.75 wt.%, decline in tensile strength is observed which can be attributed to the impact of particle aggregate in higher concentration in case of both DNDs. In other words, lower filler content would be beneficial for the efficient dispersion of both DND and DND-COOH particles. This analysis also supports our speculation that smaller DNDs provide higher surface area which results in more interfacial surface between the filler and the matrix. This explanation will be correct when NPs aggregation does not occur. 12

ACCEPTED MANUSCRIPT However, the quality of the filler dispersion in the matrix is of paramount importance, since it determines the surface area of the nanoparticles available for interaction with the matrix and

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ultimately influences the mechanical properties of the resulting nanocomposites [52, 53]. In order to investigate the impact of interfacial interaction between DNDs and CA polymer,

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Pukanszky’s model was employed to analyze the interfacial interaction in CA/DND and

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CA/DND-COOH nanocomposite membranes under dry conditions. The model is expressed as

1  f exp( B f ) 1  2.5 f

(2)

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c  m

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follows[54]:

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where σc and σm are the tensile strength of the nanocomposite membrane and the matrix (CA),

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respectively,  f the volume fraction of the filler and B a characterizing parameter to evaluate the

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interfacial interaction, including the interlayer thickness and interfacial strength. The filler volume fractions were calculated based on the rule of mixtures, i.e.,

f 

m w f  m w f  (1  w m ) f

(3)

in which wf,  m and  p represent the mass fraction of filler, matrix density and filler density, respectively. The interaction parameter B is expressed by the following relationship [54]:

B  (1   f A f ) ln(

i ) m

(4)

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ACCEPTED MANUSCRIPT where Af ,  and  i are the specific surface area, the thickness and the tensile strength of the interphase, respectively. According to this model, the interphase is defined as the region that lies

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between the reinforcing agents and the bulk of the polymer and may have different properties from the polymer matrix due to the interaction between polymeric chains and the reinforcing

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agents. For any polymer/reinforcement system having established interfacial interaction,

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parameter B stands above zero. Fig. 7b compares the experimental data and the Pukanszky’s

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model predictions with different values of B at dry conditions. The obtained results revealed that the values of B at dry condition for CA/DND-COOH and CA/DND nanocomposite membranes

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decrease from 60 and 40 at 0.5 wt.% content to zero at 0.75 wt.% content, respectively. The good interfacial interaction and efficient dispersion of raw and functionalized DNDs could

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be responsible for such behavior, which according to the Eq. 4, it can be explained in terms of

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specific surface area of DND (~300 m2 g -1). However, the larger B values observed in CA/DND

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and CA/DND-COOH nanocomposite membranes are then ascribed to the formation of thick interphase regions, which can be explained in terms of reversible work of adhesion of polymer and DNDs. Reversible work of adhesion (WPN ) can be expressed by [54]:

WPN   CA   DND   PN

(5)

where  CA and  DND are the surface tensions of CA and DND, respectively, and  PN is the interfacial tension between CA and DND. The surface tension of CA;  CA as well as the polar p and dispersive components,  CA and  dCA have been already measured and obtained values are

equal with 45.9, 14.66 and 31.24 mJ/m2, respectively [55]. For DND, these parameters are taken p d from literature [56] as  DND = 42.9 mJ/m2,  DND =11.5 mJ/m2 and  DND = 31.4mJ/m2. Interfacial

interaction can be calculated according to Owens-Wendt equation as follows [54]: 14

ACCEPTED MANUSCRIPT p p d d  PN   CA   DND  2  CA  DND  2  CA  DND

(6)

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 PN was calculated and the obtained value was equal with 0.2 mJ/m2. Using Eq. 5, the work of

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adhesion was calculated as 88.6 mJ/m2. This value is high enough to promote the thick interphase area as well as the strong interfacial interaction. The surface tension and WPN of CA

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and some particles (which commonly used as filler to fabrication of nanocomposite membranes)

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are listed in Table 2. According to the results summarized in Table 2, it can be concluded that the

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work of adhesion for DNDs is higher than that of other particles and fillers with CA polymer. Table 3 shows the surface tension and WPN of DND and other polymers which are commonly

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used in the fabrication of membranes. It is observed that these values are lower than the value of

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the work of adhesion of the CA/DND system. Since the higher work of adhesion corresponds with the better interfacial adhesion [54], the results suggest that the interaction between CA and

respectively.

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DND is higher than CA/NPs and polymer/DND systems, which are listed in Tables 2 and 3,

Table 4 shows tensile modulus and elongation at break of membranes in dry and wet conditions. Tensile modulus is calculated from the initial linear region of the tensile stress–strain curves. In both dry and wet conditions, tensile modulus and elongation at break of CA/DND (0.5 wt.%) and CA/DND-COOH (0.5 wt.%) membranes were larger than other samples due to the reinforcing effect of the DND and DND-COOH and the presence of good interaction between NPs and polymer matrix. For all membranes, membranes in wet conditions had lower tensile modulus and higher elongation at break than those in dry conditions. It may be due to the plasticizing effect of water on the CA membrane.

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ACCEPTED MANUSCRIPT In order to obtain deep insight into the reinforcing efficiency of DND and DND-COOH in CA matrix under dry condition, the experimental tensile modulus was compared with the predictions

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obtained by two micromechanical models entitled Halpin-Tsai (H-T) and Takayanagi models, which are widely used for micro/nanocomposite structures. The schematic illustration of particle-

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polymer geometry is shown in Fig.9. The H-T model is expressed as follows for spherical

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particles [67]:

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E c 1  2 f  Em 1   f

(7)

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where Ec is the tensile modulus of the nanocomposite membrane, Em the modulus of the matrix

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(8)

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Ef 1 Em  Ef 2 Em

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(0.227 GPa, see Table 4), and ηcan be written as:

in which Ef is the tensile modulus of DND (1220 GPa [68]). In addition, Takayanagi’s model is expressed as follows [69]:

f Ec  (1  f  ) 1 E Em 1  f  f ( f ) Em

(9)

As illustrated in Fig. 10, H-T and Takayanagi’s models are not able to predict the experimental data for DND and DND-COOH embedded in CA membrane. It is obvious that all experimental points are located above the theoretical curves. When the dispersed particle approaches a very small size, the specific surface area becomes very large, resulting in very large volume fraction of the interphase region, comparable or even larger than that of the dispersed phase [70]. 16

ACCEPTED MANUSCRIPT Therefore, the analysis performed based on the two-phase micromechanical models suggests that the tensile modulus is significantly affected by interphase region. Another model which was used

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to fit the experimental data was proposed by Ji et al. [70]. This model is proposed for spherical particles and presented by the following equation:

(10)

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[(r  ) / r ]3 f  f Ec  (1  [(r  ) / r ]3 f  (k  ) Em 1  [(r  ) / r ]3 f  [(r  ) / r ]3 f ln k f  ) 1 (k  ) E 1  [(r  ) / r ]3 f  { [(r  ) / r ]3 f  f }  f ( f ) 2 Em

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Eq. 10 is a three phase model in which matrix, filler and interphase regions are considered as individual phases. The obtained results are shown in Fig. 10. In the Eq. 10, r and k parameters

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represent the radius of dispersed particles and the ratio of the interphase modulus on the surface

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of particle [Ei(0)] to that of matrix, respectively assuming a linear gradient change in the

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interphase modulus from the bulk matrix to the surface of particle. k parameter is usually ranged in 1< k < (Ef/Em) domain and defined as follows [70]:

k

E i (0) Em

(11)

The modulus of the interface region with thickness of τ can be expressed as [70]: Ei 

1 (E i (0)  E m ) 2

(12)

In order to appropriately use the Eq. 10, proper values for k and τ parameters are required. In this study, it is assumed that the interphase region is a spherical shell surrounding a single DND or DND-COOH particles. In Fig. 10a, assuming τ = 6 nm and k = 20, the theoretical curve agreed very well with experimental data for CA/DND nanocomposite membranes, while these values change to τ = 8 nm and k = 25 for CA/DND-COOH nanocomposite membranes as shown in Fig. 17

ACCEPTED MANUSCRIPT 10b. Therefore, the interphase thickness in CA/DND-COOH membrane is 1.33 times larger than CA/DND membrane. Increase in interphase thickness observed for the CA-DND-COOH

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membrane in comparison to CA-DND is probably due to well interaction between hydrophilic COOH groups on the DND-COOH and CA matrix. This result confirms that the interphase plays

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a key role in the mechanical behavior of DNDs and CA polymer. Thus, according to Eqs. 11 and

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12, these values offer average interphase modulus of 2.38 GPa and 2.95 GPa for CA/DND and CA/DND-COOH nanocomposite membranes, respectively, in which the tensile modulus of neat

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CA membrane was experimentally found to be 0.227 GPa. However, due to presence of

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aggregation, this assumption is an ideal case. According to Zhang et al. [26] even at low DND content in polymer matrix, mechanical properties significantly improved which show that DND

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3.1.4. Thermal stability

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has good affinity and is uniformly distributed in the matrix.

The thermal stabilities of neat and selected CA nanocomposite membranes are illustrated in Fig. 11. Similar to the results, elucidated by Dasgupta [4], three degradation steps can be observed in the diagram. Volatilization of the volatile matter occurred at 30 °C to 330 °C. The main thermal degradation of CA chains started at 330 °C to 400 °C. The final step which represents the carbonization of the products to ash started at 400 °C.

It is obvious that the presence of DNDs has positive effect on the thermal stability of the CA polymer matrix and improves thermal behavior of the CA membrane. Thermal resistance of CA/DND-COOH membrane is even better than the CA/DND membrane which can be mainly attributed to the homogeneous distribution and increased the tensile strength of functionalized DNDs embedded nanocomposite membranes. Three main essential temperatures including the 18

ACCEPTED MANUSCRIPT onset temperature, temperatures corresponding to 10 wt.% mass loss (T10) and the percent of mass residue in Tmax for selected membranes are presented in Table 5. For pure CA membrane,

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the onset temperature is 243 °C, while for the both nanocomposite membranes it has been increased to 274 °C. T10 of pure CA membrane is also smaller than that of nanocomposite

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membranes. From Table 5, it can be seen that the mass residues in Tmax(%) of both raw and

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functionalized DNDs embedded CA nanocomposite membrane are higher than that of neat CA membrane. It confirms that addition of DNDs to the CA membrane improves the thermal

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stability. As reported by Dasgupta et al. improvement of interfacial adhesion between the

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polymer matrix and NPs improves the thermal stability of nanocomposite membranes [4]. The enhanced stability may also be attributed to the high thermal stability of DND, which

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prevents quick heat transfer; thereby it restricts the continuous decomposition of the

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nanocomposite membranes. Similar TGA curves of selected CA/DND and CA/DND-COOH

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nanocomposite membranes confirms the influential impact of both raw and functionalized DNDs; DND and DND-COOH, on the thermal stability of CA membrane. This behavior can also be explained in terms of efficient interaction between DNDs and CA matrix; interaction parameter, i.e. B, according to Eq. 4.Interestingly, the obtained results confirm that incorporation of DND-COOH particle improves the degradation temperature of CA membrane better than that of DND particles in the final step as well as in mass residue in Tmax. 3.1.5. Antibacterial activity The antibacterial behaviors of neat and nanocomposite membranes are presented in Fig.12. Optical density (OD) for each bacterial culture; E. coli and S. aureus, was measured when membranes were immersed cultivation medium for 24 h. As can be seen from this figure, DND and DND-COOH can efficiently improve the antibacterial properties of CA membrane. 19

ACCEPTED MANUSCRIPT The results reveal that the antibacterial activities of both nanocomposite membranes decrease at high loading of NPs, where agglomeration occurs. It is well known that NPs with smaller

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diameter depict higher antibacterial activity than the NPs with larger diameter [71]. Also, dispersion is an important parameter which cause greater bacteria contact and can potentially

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increase cell wall damage [72]. According to Fig. 12, it can be observed in the antibacterial tests

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that the E. coli was more sensitive to the both of CA/DND and CA/DND-COOH nanocomposite membranes than S. aureus, as shown in Figs. 12a and 12b, respectively. This phenomenon could

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be attributed to the structure of the cells. The cell wall of the gram negative bacteria s usually

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thinner than that of the gram positive ones [73]. Compared with CA/DND membranes, CA/DND-COOH membranes has exhibited excellent antibacterial activity against E.coli and S.

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aureus, due to well dispersion of DND-COOH in CA matrix and consequently decrease in the

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size of aggregation of NPs.

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More recently, Wheling et al. [21] studied the antibacterial activity of partially oxidized DND. Their experiments suggested that the antibacterial activity of DND is linked to the presence of partially oxidized and negatively charged surfaces. According to recent studies, carbon based NPs containing -OH and –COOH functional groups on the surface, exhibit improved antibacterial activity to both gram-positive and gram-negative bacteria [74]. A schematic mechanism for antibacterial activity of CA/DND-COOH nanocomposite membranes is proposed in Fig. 13. It is speculated that the COOH groups can form covalent bonds with adjacent proteins and molecules on cell walls or bind to intracellular components. This coupling inhibits vital enzymes and proteins, leading to a rapid collapse of the bacterial metabolism and finally cell death. On the other hand, physical interaction between NPs and bacteria can disrupt the bacterial cell wall (especially E. coli). Jee et al. [75] showed that the

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ACCEPTED MANUSCRIPT mechanism of the DND interaction with bacteria might be physical in nature, or due to a highly reactive surface, might hit the bacterial outer membrane and cause defects in the cell membrane

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resulting in cell death.

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4. Conclusions

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In this study, the effect of detonation detnanodiamond (DND) and heat treated DND (DNDCOOH) on the mechanical, thermal and antibacterial properties of CA membrane was studied.

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Phase inversion process with DMF as solvent was used to prepare flat sheet membranes. The

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presence of small amount of DND and DND-COOH particles in the membrane can significantly improve the mechanical, thermal and antibacterial properties of the membrane. The main

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conclusions are listed as follows:

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(1) SEM images of the surfaces of membranes depicted the increase in the number of pores

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by the addition of DND and DND-COOH into polymer matrix. The cross-section SEM images indicated the size of pores decrease and length of macrovoids increase with the addition of DND and DND-COOH particles. (2) Tensile test showed that the both NPs enhanced considerably the strength and modulus of the CA membrane. However, tensile strength and tensile modulus of CA/DND and CA/DND-COOH nanocomposite membranes showed a maximum at 0.5 wt.% loading of NPs. (3) Theoretical analysis based on Pukanszky’s model and the estimation of adhesion work based on surface energy revealed that the strong interfacial interaction was established at CA/DND interface region. The presence of interphase layer was also examined using micromechanical models for tensile modulus.

21

ACCEPTED MANUSCRIPT (4) In comparison with pure CA, TGA measurements showed a lower degradation rate of all nanocomposite membranes. However, CA/DND-COOH nanocomposite membrane

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showed a mass loss slightly lower than CA/DND nanocomposite membrane in the step of carbonization of degraded products.

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(5) The antibacterial performance against both Gram-negative bacteria (Escherichia coli; E.

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coli) and Gram-positive bacteria (Staphylococcus aureus; S. aureus) proved these nanocomposite membranes can act as good antibacterial substrates.

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(6) The results showed the CA/DND-COOH nanocomposite membrane had the highest

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values in mechanical, thermal and antibacterial properties with respect to CA/DND nanocomposite membrane, due to the better dispersion of DND-COOH particle compared

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Acknowledgement

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to DND particle in the CA matrix.

The authors gratefully acknowledge the financial support from Sahand University of Technology with grant number of 30/15975. References

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ACCEPTED MANUSCRIPT Figure Captions: Fig. 1.FTIR spectra of DND and DND-COOH particles.

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Fig. 2. FTIR spectra of pure CA membrane, CA/DND (0.5 wt.%) and CA/DND-COOH (0.5 wt.%) nanocomposite membranes.

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Fig. 3. SEM micrographs of top surface of: (a) CA, (b) CA/DND (0.5 wt.%) and (c) CA/DNDCOOH (0.5 wt.%) membranes.

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Fig. 4. The SEM images of the cross-sectional morphologies of the (a) CA, (b) CA/DND (0.5 wt.%) and (c) CA/DND-COOH (0.5 wt.%) membranes.

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Fig. 5. TEM images of distribution of nanoparticles in CA membrane: (a) 0.5 wt.% of DNDCOOH (b) 0.75 wt.% of DND.

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Fig. 6. Porosity values for pure CA and its nanocomposite membranes.

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Fig. 7. Experimental tensile strength fitted by Pukanszky’s model for CA/DND and CA/DNDCOOH nanocomposite membranes as a function of NP concentration at a (a) wet and (b) dry conditions.

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Fig. 8.Schematic illustration of the interaction between CA and DND particles.

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Fig. 9. Schematic illustration of interaction between NP and matrix, (a) without (two phase) and (b) with interphase region (three phase). Fig. 10. Experimental data and theoretical predictions of tensile modulus by two and three phase models for (a) CA/DND and (b) CA/DND-COOH nanocomposite membranes. Fig. 11.Thermal degradation of pure CA, CA/DND and CA/DND-COOH nanocomposite membranes. Fig. 12. Antibacterial activity of (a) CA/DND and (b) CA/DND-COOH nanocomposite membranes against E. coli and S. aureus. Fig. 13. Antibacterial mechanism illustration of the CA/DND-COOH nanocomposite membrane.

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Table 1. Composition of prepared nanocomposite membranes.

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Table Captions:

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Table 2. Surface tension and WPN of CA and some NPs (which commonly used as filler to fabrication of nanocomposite membranes).

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Table 3. Surface tension and WPN of DND and some polymer (which commonly used to fabrication of membranes). Table 4. The results of tensile modulus and elongation at break of membranes in dry and wet conditions.

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Table 5. Thermal degradation temperatures of membranes obtained by TGA analysis.

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Table 1.Composition of prepared nanocomposite membranes.

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Solvent (82.5 wt.%) DMF 82.5 82.5 82.5 82.5 82.5 82.5 82.5

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CA CA/DND (0.25 wt.%) CA/DND (0.5 wt.%) CA/DND (0.75 wt.%) CA/DND-COOH (0.25 wt.%) CA/DND-COOH (0.5 wt.%) CA/DND-COOH (0.75 wt.%)

Polymer and NPs (17.5 wt.%) Polymer (CA) NPs 100 0 99.75 0.25 99.50 0.50 99.25 0.75 99.75 0.25 99.50 0.50 99.25 0.75

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Membrane

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Table 2. Surface tension and WPN of CA and some NPs (which commonly used as filler to fabrication of nanocomposite membranes).

19.7 3.8

18.4

26.9

40 31.6

0 8.1

Ref. no.

(mJ/m ) 38 25.2

WPN of CA/NP (mJ/m2)

[57] [58]

81.8 66.6

45.3

[59]

87.6

[60] [61]

70.7 84.6

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18.3 21.4

   d  p 2

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 p (mJ/m2)

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Organoclay Titanium Dioxide (TiO2) Multiwall Carbon Nanotube (MWCNT) Silver Nitrate (AgNO3)  - Alumina (Al2O3)

 d (mJ/m2)

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Particle

30

40 39.7

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30.2 27

0.3 4

(mJ/m2) 30.5 31

40.4

3.6

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1 0.6 4.3

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   d  p

 d (mJ/m2)

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WPN of Polymer/DND (mJ/m2) 68.9 71.8

[61]

84.1

[64] [65] [66]

76.8 82.8 76

ACCEPTED MANUSCRIPT Table 4. The results of tensile modulus and elongation at break of membranes in dry and wet conditions. Wet 16.2 (±1.1) 17.9 (±1.2) 19.1 (±0.4) 17.2 (±1.2) 18.2 (±1.1) 21.5 (±0.9) 18.3 (±1.4)

Dry 227 (±12) 254 (±14) 275 (±5) 252 (±13) 276 (±11) 311 (±18) 271 (±16)

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Dry 7.6 (±1.4) 9.7 (±1) 12.8 (±0.3) 11.6 ± 1.1 11.9 (±0.9) 13.7 (±1.2) 11(±1.3)

Tensile Modulus (MPa)

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CA CA/DND (0.25 wt.%) CA/DND (0.5 wt.%) CA/DND (0.75 wt.%) CA/DND-COOH (0.25wt.%) CA/DND-COOH (0.5 wt.%) CA/DND-COOH (0.75wt.%)

Elongation at Break (%)

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Wet 135 (±7) 142 (±5) 189 (±3) 135 (±8) 167 (±7) 182 (±10) 143 (±9)

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Table 5. Thermal degradation temperatures of membranes obtained by TGA analysis. Onset temperature (°C)

T10(°C)

Mass residue in Tmax(%)

CA CA/DND (0.5 wt.%) CA/DND-COOH (0.5wt.%)

243 274 274

307 325 324

2 7.2 8

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 Cellulose acetate (CA)/nanodiamond (NDs) nanocomposite membrane was fabricated.  The reinforcement effect of NDs was theoretically and experimentally investigated.  A strong interfacial adhesion between CA and neat as well as functionalized NDs was achieved.  Fabricated nanocomposite membranes exhibited good thermal and antibacterial properties.

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