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Comparison of hydrophilicity and mechanical properties of nanocomposite membranes with cellulose nanocrystals (CNCs) and carbon nanotubes (CNTs) Langming Bai, Nathan Bossa, Fangshu Qu, Judy Winglee, Guibai Li, Kai Sun, Heng Liang, and Mark R. Wiesner Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04280 • Publication Date (Web): 30 Nov 2016 Downloaded from http://pubs.acs.org on December 3, 2016

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Environmental Science & Technology

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Comparison of hydrophilicity and mechanical properties of

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nanocomposite membranes with cellulose nanocrystals (CNCs) and

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carbon nanotubes (CNTs)

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Langming Bai,†,‡,§ Nathan Bossa,‡,§ Fangshu Qu,† Judy Winglee,‡,§ Guibai Li,† Kai Sun,†,║ Heng

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Liang,*† Mark R. Wiesner* ‡,§

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†

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of Technology, 73 Huanghe Road, Nangang District, Harbin, 150090, P.R. China

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‡

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27708, United States

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State Key Laboratory of Urban Water Resource and Environment (SKLUWRE), Harbin Institute

Department of Civil and Environmental Engineering, Duke University, Durham, North Carolina

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§

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North Carolina 27708, United States

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║

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Technology, 73 Huanghe Road, Nangang District, Harbin, 150090, P.R. China

Center for the Environmental Implications of NanoTechnology (CEINT), Duke University, Durham,

Nanotechnology Innovation Center for environment and ecosystem, (NICE2), Harbin Institute of

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*Corresponding author: Tel.: +86 451 86282252; fax: +86 451 86282252; e-mail:

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[email protected]

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*Corresponding author: Phone: 919-660-5292; fax: 919-660-5219; e-mail: [email protected].

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ACS Paragon Plus Environment


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ABSTRACT

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The inherent properties of hydrophilicity and mechanical strength of cellulose nanocrystals (CNCs)

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make them a possible alternative to carbon nanotubes (CNTs) that may present fewer objections to

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application water treatment membranes. In this work, the hydrophilicity and mechanical properties

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of CNCs and CNTs nanocomposite polyethersulfone (PES) membranes were characterized and

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compared. Membrane pore geometry was analyzed by scanning electron microscopy (SEM).

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Overall porosity and mean pore radius were calculated based on a wet-dry method. Results

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showed that PES polymers were loosely packed in the top layer of both the CNC- and CNT-

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composite membranes (CNC-M) and (CNT-M). The porosity of the CNC-M was greater than that

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of the CNT-M. Membrane hydrophilicity, measured by water contact angle, free energy of

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cohesion and water flux was increased through the addition of either CNCs or functionalized

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CNTs to an otherwise hydrophobic polymer membrane. The hydrophilicity of the CNC-M was

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greater than the CNT-M. In addition, the Young’s modulus and tensile strength was enhanced for

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both the CNC-M and CNT-M. While smaller concentrations of CNTs were required to achieve an

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equal increase in Young’s modulus compared with the CNCs, the elasticity of the CNC-composite

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membranes was greater.

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INTRODUTION

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Membrane technologies have been broadly applied in water treatment to achieve disinfection, salt

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removal, organic removal, liquid-solid separation and gas transfer.1,2 Pressure-driven membrane

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process can be highly automated, require a small plant footprint and are readily adaptable to

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modular configurations at various system scales.3 They have been widely applied for water and

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wastewater treatment4, reuse and desalination throughout the world.5

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Nanomaterials, in particular carbon nanotubes (CNTs), have been incorporated into membranes to

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improve membrane permeability and selectivity as well as to increase durability of the

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membranes.6 CNTs exhibit high tensile strength with a reported Young’s modulus of 1 TPa and a

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strength of 300 GPa.7 Both multi-walled (MW) and single-walled (SW) CNTs can be used as

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reinforcing fillers for polymeric membranes by capitalizing on the load transfer to CNTs within

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the composite.8-10 Increases in the Young’s modulus of 140% over that of pure polysulfone (PSf)

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membrane have been reported at 2.0 wt % content of MWCNTs in the composite.11 Kumar et al.12

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reported that the tensile strength of a Poly(p-phenylene benzobisoxazole (PBO)/SWCNTs

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composite was about 50% higher than pure PBO. CNTs are often been functionalized, to improve

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the distribution of CNTs in the composite and/or to impart specific properties such as reactivity or

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hydrophilicity. Common functional groups include carboxylate (COOH), hydroxyl (OH), amine

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(NH2) and polyethylene glycol (PEG). Functionalized CNTs maintain the strength characteristics

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of pristine CNTs,13 while imparting the desired characteristic to the composite materials.

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Functionalized MWCNTs incorporated in PES membranes increased the membrane hydrophilicity,

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pure water flux,14 anti-fouling properties and salt rejection.15 However, some studies have reported

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that CNTs exposure increases the risk of fibrosis, genotoxicity, cytotoxicity, and oxidative stress in 3


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the lung.16,17 These possible health effects have led to concern about the use of CNTs in water

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treatment membranes where some have speculated that CNTs might be released over time. In

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addition, the energy requirements for CNT production and the some of the feed stacks involved

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suggest that CNTs might not be desirable materials to use from the standpoint of an environmental

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life cycle assessment.

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Cellulose nanomaterials (CNs) may be an environmentally preferable alternative to CNT for

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environmental engineering applications. They are made from renewable and sustainable resources,

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such as wood, cotton, hemp, wheat, and straw,

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have lower environmental, health and safety footprints.19 Cellulose nanomaterials have desirable

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mechanical properties, hydrophilic properties and can be produced at low cost. These attributes

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make CNs an attractive alternative as additives for improving water filtration membranes.20

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Generally, CNs can be categorized as cellulose nanocrystals (CNCs) or cellulose nanofibrils

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(CNFs). CNCs are highly crystalline with fewer amorphous regions compared to CNFs. CNCs

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exhibit excellent mechanical properties with a maximum Young’s modulus and strength reported

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to be approximately 150 and 6 GPa, respectively.20-22 CNCs-based nanocomposite membranes

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have been reported to enhance mechanical properties, specifically the Young’ modulus,23 tensile

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strength,24 elongation25 and toughness.26 It is also reported that electrospun membranes coated

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with CNCs can significantly reduce biofouling and biofilm formation.27 Chitosan ultrafiltration

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membranes blended with CNCs show great efficiency for the removal of dyes28. In addition, at an

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approximate cost of $1/g, CNCs are potentially less expensive fillers than CNTs which may cost

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from $8 to $15/g.

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It is necessary to investigate the possibility for CNCs being an alternative to CNTs in composite

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are biodegradable, non-petroleum based and

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membranes. A direct comparison between the CNCs and CNTs nanocomposite membranes has not

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been conducted to date. Previous studies usually emphasize improvements to membrane

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performance, as measured by parameters such as water flux, organic removal, anti-fouling

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properties and salt rejection with and without a given nano-filler. With respect to water treatment

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membranes, insufficient attention has been devoted to the relative effects of CNCs and CNTs on

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polymer-nanomaterials interactions, membrane morphologies, membrane hydrophilicity and

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mechanical properties. The current study addresses the need for a direct comparison between

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membranes fabricated using either CNTs or CNCs as nano-fillers.

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MATERIALS AND METHODS

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In this study, we compare CNCs and CNTs nanocomposite membranes that are fabricated and

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evaluated under the similar conditions. SEM and atomic force microscopy (AFM) were used to

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characterize and compare membrane morphologies of the CNC-M and CNT-M. The overall

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porosities and mean pore radius of CNC-M and CNT-M were calculated and compared.

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Hydrophilicity of membranes were evaluated and compared by testing contact angle, surface free

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energy of cohesion and pure water flux. Mechanical properties including Young’s modulus, tensile

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strength, elongation and toughness were investigated.

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Materials

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PES with a molecular weight of 58,000 g/mol was supplied from Goodfellow Cambridge Ltd.

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(Huntingdon, England). Dimethyl formamide (DMF, Sigma-Aldrich Chemical Company, USA)

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was used as a solvent for the polymers. Polyvinylpyrrolidone (PVP) with a molecular weight of

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360,000 g/mol, purchased from Sigma-Aldrich, was used as an additive to increase hydrophilicity 5


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and permeability of the membranes. CNCs, as slurries of 6.2 wt% solids, were obtained from the

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Process Development Center of the University of Maine (Maine, USA). According to the

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manufacture’s specifications, the CNCs were made from wood pulp with dimensions of

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approximately 5 nm in diameter and 150 to 200 nm length. Multi-wall carbon nanotubes

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(MWCNTs) with –OH groups were supplied from Cheap Tubes Inc. (Vermont, USA) with an

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outer diameter of 20 to 30 nm and lengths ranging from 10 to 30 µm. According to the

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manufacture’s specifications, functionalized CNTs were produced by catalyzed chemical vapor

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deposition. Acid chemistry was used to purify or functionalize CNTs.

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Characterization of CNCs and CNTs

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CNCs and CNTs were characterized by XRD, FTIR, and TEM. Equipment information and

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measurement procedures were described in Supporting Information (SI).

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Fabrication and characterization of the membranes

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Membrane fabrication

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Membranes were fabricated via phase inversion in a water coagulation bath. The composition of

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casting solution was listed in Table S1 and details of fabrication procedures were described in SI.

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The resultant membranes were named according to their weight concentration of CNCs or CNTs.

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For example, the 0.5 CNC-M was fabricated with 0.5 wt% of CNCs. A control PES membrane

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(named Control-M) with no CNCs or CNTs was also fabricated via the same procedures. The

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weight percentages of the membrane additives are based relative to the mass of PES.

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Morphologies of the membranes

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Morphologies of the membranes were examined by scanning electron microscope (SEM, FEI

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XL30 SEM-FEG) with an accelerating voltage set at 10 KV. To obtain top views, membrane 6



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samples were dried at room temperature. To obtain the cross-sectional views, membrane samples

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were fractured after freezing using liquid nitrogen. All the membrane specimens were sputtered

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with gold (Hummer 6.2 Vacuum Sputter) before the tests. Surface roughness of the membranes

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was tested using AFM. Details are described in supporting information.

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Porosity and pore size of the membranes

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The overall porosity (ε) was determined by the gravimetric method using equation (1).29

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ԑ=

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Where W1 is the weight of the wet membrane (g), W2 is the weight of the dry membrane (g), V is

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the volume of the membrane (cm3) and dw is the pure water density (0.998 g/cm3). The presented

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data are the means of triplicate measurements.

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The mean pore radius of the membrane (rm) was estimated using the filtration velocity method

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where radius is calculated using Guerout–Elford–Ferry equation (2).30

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=

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In this equation, ԑ is the overall porosity (%), ƞ is the water viscosity, 1.0016×10-3 Pa∙s, Ɩ is the

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membrane thickness (2×10-4 m), Q is the volume of the permeate water per second in m3/s, A is

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the effective area of the membrane in m2 and P is the working pressure (105 Pa).

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The specific measure of surface porosity (%) was calculated by the analysis of the top surface

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SEM images using ImageJ software (Version 1.49, National Institute of Health, USA). The images

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were processed by first conducting a segmentation procedure to isolate the voids from the

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membrane polymer. The output of this pore segmentation is a binary image where the pores and

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the solid matrix are imaged as white (binary value = 1) and black (binary value = 0) pixels,

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respectively. The porosity corresponds to the percentage of void pixel in the image. The data

W1 - W2

(1)

V × dw

2.9-1.75ԑ8ƞlQ

(2)

ԑ ×A×P

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reported are the means from five images measurements.

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Hydrophilicity of the membranes

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Contact angle

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The hydrophilicity of the membranes was quantified by contact angle measurements using a

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contact angle goniometer (Kruss EasyDrop Goniometer, Hamburg, Germany) by placing a drop of

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DI water (2 µL) on the membrane surface. The measurement range of the contact angle

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goniometer is 0-180° and the deviation is the measured value ± 0.1°. At least 10 contact angle

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measurements were performed and recorded for each type of membrane analyzed. The highest and

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lowest values were discarded and the mean value of the 8 remaining angles was reported.

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Interfacial free energy of cohesion

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Quantification of the total interfacial free energy of cohesion (GTOT ) reflects the free energy due

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to the non-electrostatic interactions of the solute in the water and the membrane phase.31,32 It is

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another method for determining the hydrophilicity/hydrophobicity of the membrane. In this study,

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GTOT is composed of Lifshitz-van der Waals force (LW) and acid–base (AB) components of

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interfacial free energy, as can be described in equation (3).33

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GTOT =GLW +GAB

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Positive values of GTOT indicate that the membrane is hydrophilic and negative values indicate

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that the membrane is hydrophobic.33,34 The values of G

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equation (4) and (5), respectively.34

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GLW = -2 γLW -γLW l

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GAB =2γ+l (2γ − γl ) + 2γl (2γ+ -γ+l ) − 4γ+ γ

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Where γLW , γ+ and γ- are surface tension parameters calculated using Young-Dupré equation

-

(3)

LW

AB

and G can be calculated by

2

(4) -

-

-

8


(5)


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(6)31 by measuring the contact angles of three probe liquids of known surface tension. In this study,

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DI water, diiodomethane and formamide were used to calculate GTOT.

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1+ cos θγTOT =2 γLW γLW +γ+ γl +γ γ+l l

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Where θ is the contact angle of the three probe liquids, γ+ is electron acceptor and γ- is electron

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donor, γTOT is the total surface tension and can be calculated by the three probe liquids with

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known surface tension parameters by equation (7) and (8).

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γTOT =γLW +γAB

(7)

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γAB =2γ+ γ-

(8)

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Membrane permeability

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To evaluate the permeability of the fabricated membranes, dead-end filtration tests were

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performed in a cell (Sterlitech™ HP4750, Sterlitech, USA) with an effective volume of 300 mL.

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The permeate volume was automatically weighed and recorded via a data acquisition system at

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interval of 1 second over a range of applied pressures of 0.1, 0.2, 0.3, 0.4 and 0.5 MPa. The water

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flux was calculated as in equation (9).

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J=V/(A×∆t)

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Where J is the permeation flux (L/m2 h) (LMH), V is the volume of the permeate (L), A is the

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membrane area (m2) and ∆t is the permeate collection interval (h).

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Mechanical properties of the membranes

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Mechanical properties of membranes were measured using a Micro-Strain Analyzer (TA

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instruments RSA III, USA). The maximum force of the Micro-strain analyzer is 35 N with force

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and strain resolution 0.0001 N and 1 nm, respectively. The specimens were cut into 10 mm wide

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and 25 mm length rectangular strips. Maximum force applied was 500 gm and speed of test was

(6)

(9)

9


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set at a rate of 0.2 mm/min at 22 °C. Each test was replicated at least 10 times. Four properties

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including Young’s modulus, tensile strength, elongation at break, and toughness were determined

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from strain-stress relationships.

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RESULTS AND DISCUSSIONS

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CNCs and CNTs characterization

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To characterize functional groups of CNCs and CNTs, FTIR analysis was performed and present

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in Figure 1a. In the case of CNCs, the peaks at around 3341 and 2900 cm-1 are corresponded to O–

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H and C–H stretching vibrations, respectively.35 The peaks observed at 1060 and 1645 cm-1 are the

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pyranose ring ether band of cellulose and O–H stretching vibration of absorbed water.36 The

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spectra of CNTs showed characteristic band at 1578 cm-1, which is attributed to the vibration of

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the carbon skeleton. The presence of CH2/CH3 groups was indicated by peaks of 2850 and 2920

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cm-1, which likely originated from defects generated in the graphitic structure.37 In addition, the

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peaks at 3343 and 1090 cm-1 are related to the O–H and C–C–O stretching vibration,

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respectively.38 FTIR results indicated hydrophilicity for both CNCs and CNTs.

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TEM images showed the morphology of CNCs (Figure 1b, c) and CNTs (Figure 1d). A single

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CNC particle showed rod-like structure with dimensions ranging from 200 to 300 nm in length

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and around 15 to 30 nm in width. CNTs exhibited tube-like structure with diameter at around 20 to

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30 nm and length in micro level.

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b

c

d

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100 nm

50 nm

100 nm

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Figure 1. FTIR of CNCs and CNTs (a); TEM of CNCs (b) and (c); TEM of CNTs (d).

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Membrane characterization

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The morphological and structural characterization of fabricated membranes was based on SEM,

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AFM, porosity and pore size investigation.

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Membrane morphologies

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To analyze the addition of CNCs and CNTs on the membrane microstructure, SEM images of the

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top surfaces and cross-sections of the investigated membranes were compared. In top surface

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images, the Control-M (Figure 2a) exhibited a smooth surface with pores uniformly distributed on

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the membrane surface. In the cases of 0.5 CNC-M (Figure 2d) and 0.5 CNT-M (Figure 2g),

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homogeneous surfaces with more pores could be found on the membrane surfaces. Well dispersed

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CNCs and CNTs leading to homogeneous casting solution are responsible for the phenomenon. In 11


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cross-sectional images (Figure 2b, e and h), all the membranes showed a typical asymmetric

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porous structure, with a skin layer as the selective barrier, a finger-like substructure and a

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sponge-like bottom support. It is generally considered that the resistance mass transfer of a porous

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UF membrane is mainly determined by the characteristics of the top layer, i.e. its pore structure,

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thickness and surface porosity. To better understand the pore structure of the top layer of the

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fabricated membranes, enlarged images of top layer were captured. As can be seen in Figure 2c,

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the Control-M exhibited a dense structure of its skin layer, probably due to the large amount of

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water intake during preevaporation.39 However, the 0.5 CNC-M (Figure 2f) and the 0.5 CNT-M

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(Figure 2i) showed a loose top layer with more porous structures. Magnified images of active

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layer of the Control-M (Figure 2j) and 0.5 CNC-M (Figure 2k) were compared. The PES polymer

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of the Control-M was densely packed, and few voids were observed; while the polymer of the 0.5

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CNC-M was loosely arranged and many pores were seen. Visually, no distinct difference was

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found in the pore structure of the top layer of the CNC-M and the CNT-M. Additionally, in both

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membrane types, there was no relationship between the visual appearance of the top layer

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structure and the additive concentration (Figure S2).

a

10 µm

500 nm

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e

d

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c

b

500 nm

f

10 µm

500 nm 12


500 nm


g

h

i

500 nm

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10 µm

j

500 nm

k

100 nm

100 nm

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Figure 2. SEM images of Control-M (a-c); 0.5 CNC-M (d-f) and 0.5 CNT-M (g-i). In each row, the first image is the top membrane surface; the second image is the cross-section of membrane; and the third image zooms in the active layer. Higher Magnification was used to observe the active layer of Control-M (j) and 0.5 CNC-M (k).

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Porosity and pore size of the membranes

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Table 1 shows the porosity and mean pore radius data for the membranes. The Control-M

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exhibited the lowest overall porosity, mean pore radius and surface porosity, respectively.

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Increasing amounts of both CNCs and CNTs led to greater membrane pore size and porosity at the

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surface (binary segmentation images in Figure S3) as well as in the bulk. This result is consistent

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with the cross-sectional SEM images (Figure 2) which present a loosely packed active layer with

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more pore structures of the CNC-M and CNT-M.

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The difference in morphologies between the nanocomposite membranes and the Control-M can be

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explained by the effects of hydrophilic CNCs and CNTs on the rate of exchange between solvent

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and non-solvent during phase inversion. During phase inversion, the casting solution is rapidly

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solidified at the interface between solvent and non-solvent due to concentration gradient of the 13


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components. Hydrophilic additives can generate weak points due to immitigable stresses produced

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by shrinkage or syneresis and lead to formation of fracture points, which eventually develop into

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pores.40 The addition of hydrophilic CNCs and CNTs could increase the demixing rate by

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enhancing the thermodynamic instability, leading to the membranes with higher porosity, mean

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pore radius and surface porosity.15 Similar results have been shown in a PES/ZnO UF membrane41

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and a graphene oxide (GO)/PSf membrane bioreactor (MBR).42 In addition, with the same

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addition content of nanomaterials, the porosity and pore size of the CNC-M were higher than the

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CNT-M. This point might be attributed to an accelerated demixing process due to the more

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hydrophilic CNC with abundant -OH groups.

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Table 1. Porosity and mean pore radius of Control-M and Nanocomposite-M Polymer Solution

Overall porosity (%)

Surface porosity (%)

Mean pore radius (nm)

Control-M Nanocomposite-M Loading 0.5% 1.0% 2.0%

65 ± 1.0

15.1 ± 0.1

54.2 ± 0.7

CNC/CNT 76 ± 0.5/71 ± 1.0 77 ± 0.5/75 ± 0.3 79 ± 0.3/78 ± 0.3

CNC/CNT 17.7 ± 0.2/15.5 ± 0.1 19.6 ± 0.3/17.9 ± 0.1 20.5 ± 0.2/18.9 ± 0.2

CNC/CNT 60.9 ± 0.4/56.3 ± 0.7 81.2 ± 0.7/67.1 ± 0.3 81.5 ± 0.3/71.1 ± 0.2

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Surface roughness

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Membrane surface roughness was characterized by three-dimensional (3D) AFM images (Figure

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S4). For better evaluation of the surface variations, mathematical analysis of the AFM images was

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performed; the parameters are tabulated in Table S2. Parameters Rq, Ra, and Rmax represent the root

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mean square of the height deviations, average plane roughness and the maximum roughness,

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respectively. As evident from the results in Table S2, the Control-M exhibited the lowest surface

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roughness with the Ra value of 4.9 nm. Membranes with higher concentrations of CNCs and CNTs

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had greater surface roughness due to the enhancement of solvent and non-solvent exchange by

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well dispersed hydrophilic CNCs and CNTs.14,43 With the same addition concentration, CNC-M 14



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were smoother than the CNT-M. As mentioned above, the length of the CNC rod is orders of

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magnitude smaller than that of the CNTs, leading to the better miscibility of CNCs in casting

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solution. Hence, the difference between surface roughness of CNC-M and CNT-M may be due to

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better miscibility and dispersion between the CNCs and the solvent, resulting in a more

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homogeneous casting solution. It should be noted that surface roughness has an effect on the

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contact angle measurement and thus influences the evaluation of hydrophilicity of the membrane

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surface.44 However, the surface roughness of the CNC-M and CNT-M only increased slightly, and

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as a result, the effect of surface roughness on the hydrophilicity of the membranes was not

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investigated in this study.

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Hydrophilicity of the membranes

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Contact angle

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The hydrophilicity of the membrane surface can be evaluated by water contact angle

292

measurements. The contact angle was measured immediately after placing a drop of DI water onto

293

the membrane surface. Lower contact angle measurements indicate that the membrane is

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hydrophilic while higher contact angle merriments indicate that the membrane is hydrophobic. As

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can be seen in Figure 3a, the Control-M showed the highest contact angle of 56.7 °, indicating it is

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the most hydrophobic of all the investigated membranes. Compared with the Control-M, water

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contact angles of the CNC-M and the CNT-M declined which suggested more hydrophilic surfaces

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of the composite membranes due to the hydrophilic –OH groups of CNC and CNT, which is

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confirmed by FTIR results. The increase in hydrophilicity may be due to the migration of

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nanomaterials to the membrane surface during the phase inversion process.29 It can be found that

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the contact angles of both the CNC-M and the CNT-M decreased as the increasing of 15


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nanomaterials content; and with the same addition content, the CNC-M showed a more

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hydrophilic surface than the CNT-M.

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Addition of 0.5 wt% of nanomaterials decreased the contact angle of the 0.5 CNC-M and the 0.5

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CNT-M from 56.7 ° to 50.8 ° and 56.1 °, respectively. The considerable decrease in contact angle

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of the 0.5 CNC-M indicated that in low concentrations, CNCs were more effective in increasing

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the membrane hydrophobicity. However, with the addition of 1.0 wt%, contact angle of the

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CNT-M was comparable with that of the CNC-M. The large increase in hydrophilicity of the 1.0

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CNT-M was probably due to the increased amount of CNT which were sufficient to anchor more

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hydrophilic PVP of the membranes during the phase inversion process.45 In the case of 2.0 wt%

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nanomaterials addition, the contact angle of the 2.0 CNC-M was less than that of the 2.0 CNT-M,

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probably due to the greater hydrophilic of CNCs. In addition, possible agglomeration of CNTs in

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high concentrations may have decreased the effective surface of CNTs, leading to the reduction of

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functional groups of membrane surface.46

315 316 317 318 319

Figure 3. (a) Contact angle of Control-M and Nanocomposite-M (error bars represent the standard deviations from the means) (n=8). (b) Pure water flux of Control-M and Nanocomposite-M at different TMP levels (error bars represent the standard deviations from the means) (n=3).

320

Interfacial free energy of cohesion

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The interfacial free energy of cohesion is the interaction free energy when two surfaces of the 16



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same material are immersed in water and brought into contact.47 The values of free energy can

323

provide a quantitative insight in terms of hydrophilicity/hydrophobicity of the membranes. A

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negative value for G131 represents the thermodynamically unstable state, while positive value

325

represents a stable state.48 Table 2 lists the surface energy parameters and the interfacial free

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energy of cohesion derived from multiple contact angle measurements. As can be seen, the

327

2 G 131 of the Control-M (-27.64 mJ/m ) was negative, indicating its hydrophobic surface nature.

328

By contrast, the G 131 of the nanocomposite membranes exhibited positive values, indicating

329

their hydrophilic characteristics due to the addition of hydrophilic CNCs and CNTs. Similar

330

results were reported for a PES UF membrane blended with cellulose fibrils43 and a CNT/PES

331

composite membrane.49 The free energy values of both types of nanocomposite membranes were

332

greater as the he loading content of the nanomaterials increased (Table 2). A comparison between

333

the G 131 values with the same amount of nanomaterial shows that, the G131 of CNC-Ms

334

was greater than that of CNT-Ms, suggesting higher hydrophilicity of the CNC-M. The free energy

335

results were in good agreement with water contact angle data shown in Figure 3a, demonstrating a

336

more hydrophilic surface of the CNC-Ms.

17


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Table 2. Surface energy parameters and interfacial free energy of cohesion (unit: mJ/m2) Surface Control-M Nanocomposite-M Loading 0.5% 1.0% 2.0%

γLW

γ+

γ-

γAB

γTot

GLW 131

GAB 131

G 131

43.16

2.18

13.08

10.67

53.83

-7.22

-20.42

-27.64

CNC/CNT 49.41/45.97 48.78/48.37 46.74/46.88

CNC/CNT 0.63/0.23 0.86/0.29 1.26/0.04

CNC/CNT 36.68/33.06 41.67/37.05 50.35/38.19

CNC/CNT 9.58/5.55 11.99/6.6 15.92/2.48

CNC/CNT 58.99/51.52 60.77/54.97 62.66/49.36

CNC/CNT -5.38/-8.91 -10.71/-10.44 -9.38/-9.48

CNC/CNT 17.16/12.79 23.17/18.71 32.14/21.92

CNC/CNT 11.78/3.88 12.46/8.27 22.86/12.44

18



337

Pure water flux

338

The variation of pure water flux under different transmembrane-pressure (TMP) levels of the

339

Control-M and the Nanocomposite-M is presented in Figure 3b. The flux increased linearly with

340

the TMP (details in Figure S5) indicating that the membrane pores were not compressed or

341

narrowed during the filtration process. The flux of the Control-M increased from around 242 to

342

1240 LMH as TMP rise from 0.1 to 0.5 MPa. The flux of the nanocomposite membranes was

343

greater for membranes with higher CNC or CNT contents. The maximum fluxes were 3524 and

344

2833 LMH at TMP of 0.5 MPa with the 2.0 CNC-M and 2.0 CNT-M, respectively. The increase in

345

flux of the CNC-M and the CNT-M was due to the enhanced porous structure and improved

346

hydrophilicity due to the addition of the nanomaterials. A comparison of membranes with the same

347

amount of added nanomaterials shows that, the pure water flux of a CNC-M was greater than that

348

of a CNT-M. This finding is consistent with the results of pore structure analysis (Table 1) and

349

hydrophilicity evaluation (Figure 3a and Table 2). Interestingly, even though 2.0 CNC-M

350

presented greater hydrophilic than the 1.0 CNC-M, the fluxes of the two membranes were similar.

351

As mentioned above, the permeability of membranes depends on both pore structure and

352

hydrophilicity of the membranes. As tableted in Table 1, the mean pore radius of the 1.0 CNC-M

353

and the 2.0 CNC-M was quite narrow, 81.2 and 81.5 nm, respectively. Consequently, it can be

354

deduced that in this case, pore structure played a major role influencing the flux enhancement. In

355

summary, the flux of the investigated membranes showed apparent regularity with CNC-M >

356

CNT-M > Control-M, in accordance with membrane pore structure and hydrophilicity

357

measurements.

19


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358

Mechanical properties of the membranes

359

Evaluation of mechanical properties of the investigated membranes was conducted by

360

micro-tensile tests as shown in Figure 4a. Under applied stress, membranes first underwent

361

reversible elastic deformation, as shown in the initial linear part of the stress-strain curves.

362

Subsequently, membranes endured irreversible inelastic deformation with the increased applied

363

stress, as shown in the curvature portion of the curves. Eventually, membranes attained their

364

fracture strength, resulting in breakage. In practical operation, inelastic deformation caused by

365

excessive stress leads to the fatigue damage of the membranes, resulting in membrane failure for

366

the purposes of industrial application. For better comparison the mechanical properties of the

367

CNC-M and CNT-M, Young’s modulus, tensile strength, elongation and toughness were extracted

368

from strength-strain curves and investigated specifically.

369

370 20



371 372 373

Figure 4. (a) Stress-strain curves of the membranes. Mechanical properties of the membranes: (b) Young’s modulus; (b) Tensile strength (d) Elongation; and Toughness (e).

374 375

Young’s modulus and tensile strength

376

Young’s modulus is used as an indication of stiffness of the onset at elastic deformation, which is

377

calculated from the initial slope of linear portion (approximately initial 2.0 % of strain) of the

378

stress-strain isotherm. The values of Young’s modulus of the investigated membranes are shown in

379

Figure 4b. Young’s modulus of the Control-M is the lowest of all the membranes with a value of

380

44.7 MPa. CNC-M and CNT-M exhibited an increasing trend of Young’s modulus for membranes

381

with higher nanomaterials contents. The Young’s modulus of the 2.0 CNC-M and the 2.0 CNT-M

382

is 150% and 185% of the Control-M, respectively, indicating enhanced stiffness of the CNC-M

383

and CNT-M. Similar trends were reported in a distillation polyvinylidene fluoride (PVDF) hollow

384

fiber (HF) membrane incorporating 2.0 wt % of CNCs, and a proton exchange membrane (PEM)

385

with functionalized CNTs, where the Young’s modulus increased to 145.8% and 195% of the

386

control membrane, respectively.23,50 The tensile strength is the maximum stress that membrane

387

samples can withstand while being stretched before breaking and can be used as a parameter to

388

evaluate strength of the membrane. As shown in Figure 4c, tensile strength of the Control-M is the

389

lowest among the investigated membranes with a value of 1.70 MPa. The tensile strength of the 21


Page 22 of 31

Page 23 of 31


390

CNC-M and the CNT-M increased in membranes with higher concentration of nanomaterials.

391

Specifically, tensile strength was reinforced to 150 and 156% of the Control-M for the 2.0 CNC-M

392

and the 2.0 CNT-M, respectively. Similar behaviors have been reported in a CNC/Chitosan

393

nanocomposite membrane and an aramid nanofibers (ANFs)/CNT membrane.51,52

394

The increase of stiffness and strength of the nanocomposite membranes can be explained by the

395

properties of CNCs and CNTs materials themselves and their dispersion ability in casting solution.

396

The Young’s modulus and tensile strength of individual CNCs particle are reported to range from

397

20 to 150 GPa53 and 2 to 6 GPa20, respectively. The Young’s modulus of CNTs is reported to be 1

398

TPa54 which is orders of magnitude greater that the CNCs, and tensile strength of individual CNTs

399

was reported as 11 to 63 GPa.20 Dispersion of nanomaterials is the most significant factor for

400

nanocomposites exploiting mechanical properties of CNCs and CNTs55. The well-dispersed CNCs

401

and CNTs in casting solution can overcome surface interactions including Van der Waals

402

interaction, the hydration force, depletion, et al.56 In addition, CNTs can nucleate the

403

crystallization of the polymer55, leading to strengthening and load transfer from nanomaterials to

404

composite membranes. Evidence can be found in recent publications, which investigated complete

405

and incomplete cracked surfaces of nanocomposite membranes. CNCs were observed at the

406

cracked edge of the membrane,57 acting as the final barrier maintaining the membrane integrity.

407

Unbroken CNT fibers were seen in the cracked part of the membrane, indicating not only

408

polymers but also CNTs underwent stretched, breakage and pull-out during crack growth.52

409

Figure 4b and c also show that, with the same weight percentage of nanomaterials, the Young’s

410

modulus and tensile strength of the CNT-M was greater than the CNC-M due to the stronger

411

properties of CNTs. However, Young’s modulus of the 1.0 CNC-M and the 0.5 CNT-M was quite 22



412

comparable, 52.7 and 52.5MPa, respectively. Additionally, Young’s modulus of the 2.0 CNC-M

413

and the 1.0 CNT-M were in the same range with values of 67.0 and 66.7 MPa, respectively.

414

Similar phenomenon can be also found in tensile strength results where the 1.0 CNC-M and the

415

0.5 CNT-M exhibited similar tensile strengths with a mean value of 2.27 MPa. The tensile strength

416

of the 2.0 CNC-M (2.55 MPa) was higher than that of the 1.0 CNT-M (2.38 MPa). Therefore, it

417

can be deduced that in our study, Young’s modulus and tensile strength of a CNT-M are

418

approximately those of a CNC-M having half the nano-filler content.

419

It should be noted that the CNC-M and CNT-M had Young’s moduli and tensile strengths that fall

420

far below those of the individual building blocks (CNCs and CNTs). Young’s modulus and tensile

421

strength were insufficient to capture the full mechanical behavior of the composite membranes.58

422

This may be attributed to the relatively low concentrations of nano-filler and the intrinsic

423

mechanical properties of the PES polymer which are comparatively weak.

424

Elongation

425

The elongation at break, corresponding to the expansion percentage of the initial span of a

426

membrane sample, is used as an indication of ductility of membrane. As shown in Figure 4d, the

427

elongation of the Control-M was 31.4%. The elongation of the CNC-M exhibited a modest

428

increasing tendency as the increase of CNCs and reached to 33.9% of the 2.0 CNC-M. By contrast,

429

the elongation of the CNT-M decreased for membranes with higher weight loading percentages.

430

The elongation of the 2.0 CNT-M decreased to only 16.0%. Similar behavior can be found in a

431

poly (vinyl alcohol)/CNC membrane24 and an aromatic polyamide (PA)/CNT membrane.59

432

The difference of intrinsic properties of the CNCs and CNTs are responsible for the opposite

433

results of elongation. The elongation of individual CNCs was investigated by Sinko et al. using a 23


Page 24 of 31

Page 25 of 31


434

potential of mean force (PMF) method.26 From the stress-strain curve of their study, the elongation

435

at break of the CNC was ca. 40%. However, CNTs are generally considered to be hard and stiff,

436

but brittle as well. The elongation at break of individual CNT was reported as ca. 12% by a

437

nanostressing stage combining SEM and AFM.7 Well-dispersed of CNCs and CNTs resulted in the

438

composite membranes following the elongation properties of the nanomaterials, leading to the

439

different elongation trends of the CNC-M and the CNT-M.

440

Toughness

441

Toughness is defined as the amount of energy a material absorbs before it fails,60 expressed as

442

follow:

443

U= 0 σ dԑ

444

Where U is the energy per volume absorbed, σ is the stress, and ԑ is the failure stain. The value for

445

toughness of the investigated membranes was determined by integrating the area under

446

stress-strain curve from zero load to maximum extension. As shown in Figure 4e, the toughness of

447

the Control-M was about 0.438 MJ/m3. The toughness of the CNC-M increased with increasing

448

CNC content and attained a value of 0.688 MJ/m3 of the 2.0 CNC-M. However, the toughness of

449

the CNT-M showed an opposite trend, decreasing to 0.342 MJ/m3 of the 2.0 CNT-M. The CNC-M

450

was stronger and more stretchable because of the enhanced tensile strength and elongation

451

captured from CNCs, resulting in the improvement of toughness. However, the strength of the

452

CNT-M was increased accompanied with the decreasing of elongation, lead to a drop of the

453

toughness of the CNT-M. The better performance in toughness of the CNC-M indicated that they

454

can endure more energy or work required for rupture.

ԑ

(11)

24



Page 26 of 31

455

IMPLICATIONS

456

In this study, the hydrophilicity and mechanical properties of nanocomposite membranes were

457

investigated to compare the effects of CNCs and CNTs. The porosity, hydrophilicity and pure

458

water flux of the CNC-M was greater than the CNT-M. The most promising finding was that the

459

CNC-M could attain same level in Young’s modulus and tensile strength as the CNT-M by

460

doubling the CNCs content. Considering the advantages of CNCs compared to CNTs of abundant

461

existence, environmental friendly, biocompatible, biodegradable, renewable

462

hydrophilic,18-20 the enhancement of mechanical properties of the CNC-M is promising for

463

environmental application of nanocomposite membranes with CNCs. Since the cost of CNCs and

464

CNTs are $1/g and $8 to $15/g, respectively,20 even when doubling the concentration of CNCs, the

465

cost of the CNC-M is still lower than the CNT-M. This reveals that it is achievable to widely

466

utilize CNC in environmental engineering applications, especially water filtration membranes. To

467

better understand and utilize CNCs and CNTs in membrane application, different membrane types

468

for specific aims need to be further explored.

469

ASSOCIATED CONTENT

470

Supporting Information

471

Equipment information and measurement procedures of TEM, FTIR and XRD for CNTs and

472

CNCs characterization; composition of casting solution for the Control-M and Nanocomposite-M

473

(Table S1); methods of AFM characterization for Control-M and Nanocomposite-M; XRD of

474

CNCs and CNTs (Figure S1); SEM images of 1.0 CNC-M, 2.0 CNC-M, 1.0 CNT-M and 2.0

475

CNT-M (Figure S2); SEM images after binary segmentation of the Control-M and

476

Nanocomposite-M (Figure S3); AFM images of the Control-M and Nanocomposite-M (Figure S4); 25


and more

Page 27 of 31


477

Surface roughness parameters of Control-M and the Nanocomposite-M (Table S2); Linear

478

relationship between pure water flux (LMH) and TMP (MPa) of the Control-M and

479

Nanocomposite-M (Figure S5).

480

ACKNOWLEDGEMENTS

481

This work was partially funded through the Center for the Environmental Implications of

482

NanoTechnology (CEINT). We gratefully acknowledge the valuable help of Dr. Jingjing Li and Dr.

483

Marielle DuToit in the experimental analysis and discussion. This research was jointly supported

484

by the National Natural Science Foundation of China (51378140), the National Science

485

Foundation for the Outstanding Youngster Fund (51522804), Program for New Century Excellent

486

Talents in University (NCET-13-0169), HIT Environment and Ecology Innovation Special Funds

487

(HSCJ201603). The first author also thanks China Scholarship Council (CSC) for providing the

488

living cost during his study at Duke University.

489

26



490

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