Materials Letters 202 (2017) 78–81
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Electrospinning polyvinylidene fluoride/expanded graphite composite membranes as high efficiency and reusable water harvester Zhao-Xia Huang a,b, Xiaoxiao Liu a, Shing-Chung Wong a,⇑, Jin-ping Qu b,⇑ a
Department of Mechanical Engineering, The University of Akron, Akron, OH 44325-3903, USA National Engineering Research Center of Novel Equipment for Polymer Processing, Key Laboratory of Polymer Processing Engineering of the Ministry of Education, South China University of Technology, Guangzhou 510640, China b
a r t i c l e
i n f o
Article history: Received 6 March 2017 Received in revised form 14 May 2017 Accepted 15 May 2017 Available online 17 May 2017 Keywords: Electrospinning PVDF Expanded graphite Water harvester Reusable
a b s t r a c t Reusable polyvinylidene fluoride (PVDF) membranes with inclusions of expanded graphite (EG) were developed by electrospinning methodology for water harvesting studies. Scanning electron microscopy (SEM) was used to perform morphological studies and they demonstrated that the inclusions of EG slightly increased the diameters of the electrospun fibers. Static water contact angle was measured to evaluate wetting phenomena. Hydrophobic surfaces were obtained for both samples with and without EG inclusions, while the composite membrane was found to have relatively high water contact angle. A setup was used to evaluate the water-retaining performance of electrospun composite samples. An improvement of 63.4% in water harvesting efficiency was obtained when EG was included. The morphology and wettability of membranes containing EG after water harvesting experiments remained the same as before. And the results suggested the composite membranes were reusable. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction High efficiency fresh water harvesters derived from nanotechnology are pressingly needed because of the scarcity of the former in landlocked regions and environmental pollutions . Artificial techniques were invented to harvest water from rain and fog . Inspired by naturally occurring water-harvesting systems, electrospun membranes were used for water condensation . We had previously studied expanded graphite nanocomposites [4,5]. In this work, inclusions of EG were found to enhance the performance of electrospun PVDF membranes. The effects of EG on morphology and wettability of composite membranes were determined. A simple water extraction experiment was employed to evaluate the water harvesting efficiency (WHE). The reusability was studied by comparing the morphology and wettability of samples before and after water extraction experiments. 2. Experimental 2.1. Materials PVDF powders (Kynar 761) were purchased from Elf Atochem North America Inc. Expandable graphite flakes (Grade 160-80 N) ⇑ Corresponding authors. E-mail addresses: [email protected]
(S.-C. Wong), [email protected]
(J.-p. Qu). http://dx.doi.org/10.1016/j.matlet.2017.05.067 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
were supplied from GrafTech International. N,N-dimethylformamide (DMF) and acetone were from Sigma-Aldrich Corporation and used as received. 2.2. Fabrication of PVDF/EG hybrid membranes 2.2.1. Fabrication of EG EG was obtained by irradiating expandable graphite in a commercial microwave under power of 1200 W for 10 s. Followed by dissolving and sonication in DMF for 2 hr for further exfoliation and dispersion into polymer solutions. 2.2.2. Electrospun PVDF/EG hybrid membranes A mixture of DMF/acetone in ratio of 70/30 by weight was used to form a polymer blend solution based on optimization studies performed separately. The loading of EG was 1.5 wt%. A virgin PVDF solution was prepared as a control sample. The resulting solutions were stirred overnight at 70 °C. A standard electrospinning setup was used to electrospin membranes. The final solution was fed into a 5-ml syringe fitted with 21-gauge needle connected with high voltage. An aluminum foil collector was placed 20 cm away from the needle. Electrospinning was performed at voltage of 18 kV with flow rate of 1.5 mL/hr till the basis weight of sample reached 15 g/m2 . As-spun samples were placed in oven at 70 °C till a complete evaporation of solvents.
Z.-X. Huang et al. / Materials Letters 202 (2017) 78–81
2.3. Characterizations Morphology of the electrospun membranes was observed using a scanning electron microscopy (SEM, JSM-6510LV, JEOL). Water contact angle measurement was performed using a contact angle meter (Dms-200, Kyowa). 2.4. Water harvesting evaluation A laboratory-made setup was used to evaluate WHE of samples (see Fig. 1(A)) . Evaluation was carried out at a temperature of 23 °C and related humidity of 40%. A commercial humidifier was used to supply moist airflow. During evaluation, sample was cut into 4 4 cm and placed 5 cm away from the nozzle of the humidifier to harvest water. A vial was placed under sample to collect the condensed water droplets. The weight of harvested water was measured after 1 hr. WHE was defined as weight of collected water per hour per unit area . For each sample, five tests were performed to calculate average WHE. Before testing, samples were placed inside a vacuum oven to ensure full desiccation. 3. Results and discussions Figs. 2(A) and (B) show the SEM micrographs of as-spun membranes. Bead-free nanostructures are observed in samples both with and without EG. The mean diameters of samples were
measured using ImageJ based on at least 150 counts of fibers under 5–10 SEM micrographs. The fiber diameter distributions are shown on the right side of Figs. 2(A) and (B). The average diameter of virgin PVDF membrane is 319 nm, while a mean diameter of 373 nm is determined in sample with EG inclusions. An increase of 17% in fiber diameter could be due to the inclusion of EG that affected the viscosity and electrical conductivity of PVDF/EG solutions, which subsequently changed the fibers as discussed by other investigators . Furthermore, the comparison between the high magnification (5000) SEM micrographs of samples with and without EG showed some swelling behavior. This could be attributed to the larger EG particles encapsulated by the polymer solutions and, thereby, solutions were solidified into composite fibers . Figs. 3A and B show water droplets (5 lL) placed on the surface of membranes. Both samples with and without EG inclusions show hydrophobicity. To quantitatively analyze the wetting phenomena, the contact angles (CA) of water droplets on the membranes were measured. CA value of each sample is shown in corresponding photo-micrograph. Comparison is made of membranes between virgin PVDF and composite membrane. Electrospun PVDF membrane has a CA of 128.6 ± 0.9°, while the CA of composite membrane increases to 134.7 ± 1.3°. The increase in CA is attributed to the presence of EG, which can increase the surface roughness and decrease the surface energy of nanofibers . Moreover, the EG containing samples produced swelling behavior of electrospun composite samples which also contributed to the increase in CA.
Fig. 1. (A) Schematic illustrating the laboratory water harvesting setup. (B) The weight of water harvested for 1 hr and the corresponding WHE.
Fig. 2. SEM micrographs (x3000) and corresponding fiber diameter distribution graph of every sample. The inserted graphs in SEM micrograph is high magnification (5000) of samples. The average fiber diameter of sample is shown. (A) As-spun virgin PVDF membrane; (B) as-spun composite membrane; (C) virgin PVDF membrane after multicycle experiment; (D) composite membrane after multi-cycle experiment.
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Fig. 3. Photographs of water droplets (about 5 lL) placed on the membranes. Contact angle with water (WCA) was calculated based on the photographs and shown in figures. (A) As-spun virgin PVDF membrane; (B) as-spun composite membrane; (C) virgin PVDF membrane after multi-cycle experiment; (D) composite membrane after multi-cycle experiment (Scale bar = 1 mm).
According to the theoretical framework performed by Extrand and coworker , it could be argued that a membrane with higher CA usually shows better WHE (see Support Information). Thus, we pursued this water harvesting experimentation to determine the potential of this composite membrane. Fig. 1B shows the weight of extracted water droplets and WHE. The results showed that the electrospun PVDF membrane collected 1.65 ± 0.05 g H2O in 1 hr and converted into a WHE of 103.1 ± 2.9 mg/cm2/hr. As to electrospun PVDF/EG membrane, the values rose to 2.70 ± 0.36 g and 168.5 ± 22.4 mg/cm2/hr, respectively, indicating an improvement of 63.4% than virgin PVDF membrane. Thus, based on the water harvesting mechanism (see Support Information), the significant increase of 63.4% in WHE could be attributed to the increase in CA. Moreover, the condition of harvested water as shown using optical microscopy indicated no observable contaminants. Recently, Nørgaard and coworkers  determined that desert beetle has WHE at 21.4 mg/cm2/hr. In comparison to this natural water harvester, our electrospun membrane harvester shows 7.8 times higher in WHE. In a study by Baji and coworkers , hierarchical electrospun PVDF nanofibers showed WHE of 81 mg/cm2/hr. Hashaikeh and coworkers  applied the lubricant on the surface of electrospun PVDF membrane and obtained WHE of 118 mg/cm2/hr. Although the direct comparison could not be performed herein because of differences in empirical parameters, the WHE measured in our sample is 108% and 42.7% higher than the values reported by Baji and coworkers and Chase and coworkers, respectively. To investigate the reusability of our composite membranes, a multi-cycle water harvesting empirical set-up was used. Figs. 2 (C) and (D) show the SEM micrographs and fiber diameter distributions, of samples after 5 cycles of testing in water harvesting experimentation. The results indicate that for PVDF virgin membranes, the diameter slightly increased to about 406 nm, which could be attributed to the damage or rupture of nanofibers during the experiments, as suggested by Kim and coworkers . While for composite membranes, no obvious change could be found. The CA of samples after water harvesting experiments were measured
and shown in Figs. 3(C) and (D). The wettability of samples does not change after 5 cycles of experiments. Moreover, the WHE of samples during the multi-cycle tests did not show observable deviation from original value. The combination of both morphology and wettability analyses indicated that composite membrane was effective in reusability. 4. Conclusions Reusable electrospun PVDF membranes with and without EG inclusions were successfully fabricated in our work to study water harvesting capacity. The morphology of fiber membranes indicated that bead-free nanofibers were prepared under appropriate processing parameters. The fiber diameter distribution counting indicated that the presence of EG could increase the fiber diameter caused by EG particle encapsulation. The composite membrane showed more hydrophobicity in comparison with the control sample. An increase of WHE of 63.4% from 103.1 mg/cm2/hr of virgin PVDF membrane to 168.5 mg/cm2/hr of composite membrane was observed. The morphology, wettability and WHE of composite membrane showed no observable deviation from original morphology after multi-cycle testing, the fact of which suggested its reusability. Acknowledgements We would like to acknowledge the University of Akron, Ohio, United States, for supporting this research. The Key Program of National Natural Science Foundation of China (Grant No. 51435005), the National Natural Science Foundation of China (Grant No. 51505153), the National Instrumentation Program (Grant No. 2012YQ230043), the PhD Start-up Fund of Natural Science Foundation of Guangdong Province, China (Grant No. 2016A030310429) provided support for one of us (ZXH), who would like to thank the China Scholarship Council, for supporting his stipend.
Z.-X. Huang et al. / Materials Letters 202 (2017) 78–81
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