Electrospun Regenerated Cellulose Nanofiber Membranes Surface

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Electrospun Regenerated Cellulose Nanofiber Membranes SurfaceGrafted with Water-Insoluble Poly(HEMA) or Water-Soluble Poly(AAS) Chains via the ATRP Method for Ultrafiltration of Water Zhao Wang, Caitlin Crandall, Vicki L Prautzsch, Rajesh Sahadevan, Todd J. Menkhaus, and Hao Fong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16116 • Publication Date (Web): 12 Jan 2017 Downloaded from http://pubs.acs.org on January 13, 2017

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ACS Applied Materials & Interfaces

Electrospun

Regenerated

Cellulose

Nanofiber

Membranes Surface-Grafted with Water-Insoluble Poly(HEMA) or Water-Soluble Poly(AAS) Chains via the ATRP Method for Ultrafiltration of Water Zhao Wang,† Caitlin Crandall,‡ Vicki L. Prautzsch,† Rajesh Sahadevan,‡ Todd J. Menkhaus,*,‡ and Hao Fong*,† †

Department of Chemistry and Applied Biological Sciences and ‡ Department of Chemical and

Biological Engineering, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701, United States

* Corresponding Authors Professor Todd. J. Menkhaus, Ph.D. Tel.: (605) 394-2422. Fax: (605) 394-1232. E-mail: [email protected] Professor Hao Fong, Ph.D. Tel.: (605) 394-1229. Fax: (605) 394-1232. E-mail: [email protected]

1 Environment ACS Paragon Plus


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Abstract: Electrospun nanofiber membranes (ENMs) have demonstrated promising applications for water purification primarily due to high water flux and low degree of fouling. However, the equivalent/apparent pore sizes of as-electrospun ENMs are in microns/sub-microns; therefore, the ENMs can only be directly utilized for microfiltration applications. To make regenerated cellulose (RC) ENMs for ultrafiltration applications, atom transfer radical polymerization (ATRP) was studied to graft polymer chains onto the surface of RC nanofibers; in specific, the monomers of 2-hydroxyethyl methacrylate (HEMA) and sodium acrylate (AAS) were selected for surface-grafting water-insoluble and water-soluble polymer chains onto RC nanofibers, respectively. With prolonging the ATRP reaction time, the resulting surface-modified RC ENMs had reduced pore sizes. The water-insoluble poly(HEMA) chains coated the surface of RC nanofibers to make the fibers thicker, thus decreasing the membrane pore size and reducing permeability. On the other hand, the water-soluble poly(AAS) chains did not coat the surface of RC nanofibers; instead, they partially filled the pores to form gel-like structures, which served to decrease the effective pore size, while still providing elevated permeability.

The surface-

modified RC ENMs were subsequently explored for ultrafiltration of ~40 nm nanoparticles and ~10 nm bovine serum albumin (BSA) molecules from water. The results indicated that the HEMA-modified RC membranes could reject/remove more than 95% of nanoparticles while could not reject any BSA molecules; in comparison, the AAS-modified RC membranes had complete rejection of the nanoparticles and could even reject ~58% of BSA molecules.

Keywords: electrospinning, nanofiber membrane, regenerated cellulose, ATRP, ultrafiltration


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1.

Introduction Water shortages and water pollutions have become global concerns due to natural disasters,

lack of infrastructure, and poverty around the world.1 To address these concerns, membrane separation methods including microfiltration,2 ultrafiltration,3,4 nanofiltration,5,6 and reverse osmosis7–9 have often been adopted for water purification/treatment, because these membrane separation processes can be readily operated with substantially less energy consumption than conventional thermal separation processes. Electrospun nanofiber membranes (ENMs) with fiber diameters typically being hundreds of nanometers have recently been investigated as innovative filtration materials, because they can efficiently remove micron/sub-micron sized particles from water.10–14 It is important to note that ENMs do not possess straight-through pores, if they are well prepared; nevertheless, small particles can permeate ENMs through different tortuous pathways due to highly interconnected pores.

As a result, the degree of membrane fouling can be significantly reduced without

considerably sacrificing particle rejection fractions; additionally, the water flux of ENMs can be substantially higher than that of conventional membranes/materials.15–19

However, the

equivalent/apparent pore sizes of as-electrospun ENMs are usually in microns/sub-microns; thus they can only be directly utilized for microfiltration applications.20 To remove/reject smallersized particles/species (with sizes from several to tens of nanometers) from water, various barrier layers have been prepared on the surfaces of ENMs;21 nevertheless, these barrier layers often contain morphological/structural defects, which require further developments to improve the separation performances of the resulting asymmetric membranes.22,23 Atom transfer radical polymerization (ATRP) is an effective surface-modification method; since it is precisely controllable, well-defined polymeric structures can be constructed.24–28



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Recent research endeavors have indicated that ENMs could be surface-modified via the ATRP method, and the resulting membranes would be promising for several applications such as biomedical materials (e.g., artificial blood vessels),29 lithium-ion batteries,30 immobilization of enzymes and/or absorption of proteins,31–33 and responsive materials.34 To the best of our knowledge, however, there has been no reported studies on applying the ATRP method to adjust/reduce the equivalent/apparent pore sizes of ENMs thus to convert the membranes from microfiltration media into ultrafiltration media. For ATRP modification of an ENM, the nanofiber surface needs to be activated/initiated first; and the common approach is to use bromo or chloro initiators (e.g., 2-bromoisobutyryl bromide or 2-chloroisobutyryl chloride) to react with hydroxyl or amino groups on the nanofiber surface.35–37 Cellulose (with three hydroxyl groups in each repeating unit of its macromolecules) is the most abundant natural polymer, which has been widely used for making textiles, composite materials, and personal care products.38 Surface-grafting of polymer chains onto cellulose fibers was first explored by Carlmark and Malmstrom;39,40 recently, cellulose fibers have been surfacemodified via ATRP with various polymer chains; and the resulting modified cellulose fibers (note that they are not electrospun nanofibers) could be used for various applications including thermo/pH-responsive membrane,41–44 biomedical materials,45 DNA immobilization/detection,46 and protein adsorption/purification.47,48 The objective of this study was to tailor/reduce the equivalent/apparent pore sizes of electrospun cellulose nanofiber membranes upon surface-grafting two types of polymer chains via the ATRP method. It is known that cellulose cannot be directly electrospun into nanofibers due to its poor solubility in common solvents. As a result, the ENM of cellulose acetate (CA) was prepared first as reported in our previous publications;32,33,49 subsequently, the CA ENM was


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converted into the ENM of regenerated cellulose (RC) upon a simple treatment of hydrolysis/deacetylation in NaOH aqueous solution. Thereafter, the ATRP method was adopted to surface-modify the acquired RC ENM; in specific, 2-hydroxyethyl methacrylate (HEMA) and sodium acrylate (AAS) were selected as the monomers for surface-grafting water-insoluble and water-soluble polymer chains on RC nanofibers, respectively. Finally, the prepared RC ENMs surface-modiﬁed with two types of polymer chains (i.e., poly(HEMA) and poly(AAS)) were evaluated for ultrafiltration applications; in this study, nanoparticles (with sizes of ~40 nm) and bovine serum albumin (BSA) molecules (with sizes of ~10 nm) were chosen for the evaluation of ultrafiltration performance.

2.

Experimental Section

2.1.

Materials

Electrospun RC nanofiber membrane with fiber diameter of ~500 nm and membrane thickness of ~60 µm was first prepared according to our previously reported studies.32,33,49 Prior to surface grafting of polymer chains via ATRP, the RC membrane was dried in a vacuum oven (27 mm Hg) at 70 °C for 24 h. Hexane, 2-bromoisobutyryl bromide (2-BIBB), pyridine, methanol, copper (I) bromate (CuBr), copper (II) bromate (CuBr2), sodium acrylate (AAS), 2-hydroxyethyl methacrylate (HEMA), and 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA) were purchased from Sigma-Aldrich and used as received. The 1 g/L filtration mixture was made by using ~40 nm Nanobead NIST traceable particles (Polysciences, Catalog No: 64004) in deionized water for the larger particle size. For the smaller particle size, the 0.5 g/L filtration solution was made by using ~10 nm bovine serum albumin (BSA, lyophilized powder, Catalog No: A7906, Sigma-Aldrich) in 20 mmol phosphate buffer solution (PBS) at the pH value of 7.



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2.2.

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Initiation of Electrospun RC Nanofiber Membrane

An electrospun RC nanofiber membrane (3 × 3 cm) was first immersed in 20 mL hexane for 20 min. The membrane was then placed into a 200 mL beaker containing 60 µL pyridine and 100 mL hexane in an ice bath for 15 min. Subsequently, 300 µL of 2-BIBB was diluted with 10 mL hexane; and the diluted 2-BIBB was then added into a beaker dropwise. The reaction was allowed to proceed for 3, 6, and 9 h at room temperature of ~25 °C. Thereafter, each initiated RC (RC-Br) membrane was thoroughly rinsed with hexane, deionized water, and ethanol. The initiated RC membranes with different reaction times were denoted as RC-Br-3h, RC-Br-6h, and RC-Br-9h, respectively. As depicted in Figure 1, the initiation reagent (i.e., 2-BIBB) can react with hydroxyl groups on the surface of electrospun RC nanofibers; upon the experimental results, the RC-Br-9h membrane with the highest degree of initiation was selected for the subsequent surface-grafting of polymer chains via the respective ATRP reactions. 2.3.

Surface-Grafting of Poly(HEMA) or Poly(AAS) via ATRP

25 mL methanol, 25 mL deionized water, and 110 µL HMTETA were added into a 100 mL pear-shaped flask. Subsequently, 1 mL HEMA or, 1 g AAS and 2 g NaCl, was added into the flask to prepare the respective polymer chains of poly(HEMA) and poly(AAS). During the experiment, the mixture was first degassed to thoroughly remove oxygen through three freezepump-thaw cycles by using a Schlenk line; 30 mg CuBr and 6 mg CuBr2 were then added into the flask. Thereafter, the mixture was degassed again and was magnetically stirred for 1 h until all of the CuBr was dissolved. Subsequently, the RC-Br membrane was placed into the flask and the mixture was degassed two additional times to thoroughly remove oxygen. The system was then sealed with argon at room temperature for different reaction times.

Finally, the

HEMA/AAS-modified RC-Br ENMs (RC-HEMA/AAS) were thoroughly rinsed with deionized


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water and then ethanol to completely remove the unreacted monomers as well as the polymer chains that were not grafted on the electrospun RC nanofibers before being dried in air. The reaction times for RC-HEMA were set at 20, 40, and 60 min, while the reaction times for RCAAS was set at 15, 30, and 45 min; in this study, these polymer chains with varied reaction times were denoted as RC-HEMA-20/40/60 and RC-AAS-15/30/45, respectively. 2.4.

Characterization of Different Membranes

FTIR spectra of different nanofiber membranes were acquired by using a Tensor 27 Fourier transform infrared spectrophotometer (Bruker, Germany) equipped with a Smart Orbit diamond attenuated total reflection (ATR) accessory. The wavenumber range was from 4000 to 400 cm−1, and a sample was scanned 32 times. X-ray photoelectron spectroscopy (XPS) was employed to characterize the chemical compositions, and the XPS measurements were performed on a Kratos AXIS HSi spectrometer using a monochromatized Al KR X-ray source (1486.6 eV photons). Additionally, a Zeiss Supra 40VP field-emission scanning electron microscope (SEM) was employed to examine the morphological structures. 2.5.

Evaluation of Ultrafiltration Performance

Membranes were tested using a dead-end filtration setup. An Amicon Stirred Ultrafiltration Cell (model 8010) was used without the stirring arm. Solutions were fed into the cell and filtered under the pressure of 40 pounds per square inch (psi) by using nitrogen. CraneMat Cu 414 support mats were placed under the membranes to prevent the pressure from deforming the membranes around the grooves in the base of the cell. Before using the support mats with membranes, they were tested to ensure that they had no impact on the flux and rejection properties of the membranes. Individual membranes were placed in the cell on top of support mat. Membranes were thoroughly wetted with water before taking measurements. RO water



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flux was then measured 3 times under the pressure of 40 psi.

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10 mL of separation

mixture/solution was then poured into the cell and 40 psi of nitrogen pressure was applied. Flux was determined by timing how long it took for the vessel to empty. Rejection was calculated by measuring the absorbance of the permeate solution and comparing with the Beer’s law equation calibrated from several concentrations of the feed solution. The equation %R = 100 × (1 ˗ Cp/Cf) was then used to calculate the rejection, where Cp is permeate concentration and Cf is feed concentration.

3.

Results and Discussion

Figure 1. A schematic showing the initiation of electrospun RC nanofiber membrane and the subsequent surface-grafting with poly(HEMA) or poly(AAS) chains via ATRP.

The preparation scheme is shown in Figure 1. There were abundant pores with visual pore size from hundreds of nanometers to a few microns from the SEM image of the pristine RC ENM.


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The hydroxyl groups on the RC nanofibers were reacted with 2-BIBB to introduce the Br atoms bonded with tertiary carbon atoms as the initiation sites, followed by the ATRP reaction with HEMA or AAS as monomer. Upon ATRP reaction, the water-insoluble poly(HEMA) chains would coat nanofibers thus the nanofiber diameters would be increased (i.e., the membrane equivalent/apparent pore size would be decreased), further leading to the reduction of membrane permeation; while the water-soluble poly(AAS) chains could move freely in water, after being dried, film-like structures would be formed among the pores. 3.1.

Initiation of Electrospun RC Nanofiber Membrane with 2-BIBB

Figure 2. FTIR spectra acquired from pristine RC ENM and 2-BIBB initiated RC membranes with different reaction times.

To determine the optimum initiation degree of RC membrane, the correlation of reaction time with initiation degree was studied by FTIR and XPS. Three samples were prepared with reaction times being 3, 6, and 9 h. In the acquired FTIR spectra (Figure 2), the bands centered at 3365 and 1726 cm−1 were attributed to the stretching vibrations of O−H and C=O groups, respectively.



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With increasing the reaction time from 3 to 9 h, the absorption intensity of 3365 cm−1 band decreased while that of 1726 cm−1 band increased, indicating that the reaction degree would become higher with prolonging the reaction time. The initiation with 2-BIBB was further studied by XPS, and the results are shown in Figure 3. The wide scan spectra showed that the membranes consisted of C, O, and Br atoms. From the Br 1S core-level spectra, the Br atom percentage was increased from 0.06 at% to 0.50 at% with the increase of reaction time from 3 to 9 h. Based upon the FTIR and XPS results, it was evident that the initiation degree could be improved upon prolonging the reaction time. However, the resulting membrane would become fragile (i.e., easy be break), if the reaction time was longer than 9 h. Furthermore, the experimental results indicated that the reaction time of 9 h appeared to be long enough for the subsequent ATRP reaction; therefore, the RC membrane initiated with 2-BIBB for 9 h (i.e., the RC-Br-9h membrane) was selected for the following studies.

Figure 3. Wide-scan and Br 1s core-level spectra of 2-BIBB initiated RC membranes with different reaction times of 3h (bottom row), 6 (middle row), and 9h (top row).


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3.2.

Preparation of Electrospun RC Nanofiber Membranes Surface-Grafted with

Poly(HEMA) or Poly(AAS) Chains The initiated RC-Br-9h membrane was then reacted with HEMA and AAS for varied ATRP reaction times to investigate the relationship of reaction degree with reaction time. The FTIR spectra acquired from different membranes are shown in Figure 4. The bands centered at 3365, 2926, 1726, and 1463 cm−1 (Figure 4A) were attributed to the O−H stretching vibration, C−H stretching vibration (in methylene group), C=O stretching vibration (in ester group), and C−H deformation vibration (in methyl group), respectively. The bands centered at 3365 and 1556 cm−1 (Figure 4B) were attributed to the O−H stretching vibration and asymmetric COO− stretching vibration (in the AAS salt), respectively. All of the above mentioned bands increased with prolonging the ATRP reaction time, indicating the higher reaction degree accompanying the longer reaction time.

Figure 4. FTIR spectra acquired from the HEMA (A) and AAS (B) modified electrospun RC nanofiber membranes with different reaction times/degrees.



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Morphological structures of the HEMA and AAS modified RC membranes were then examined by SEM (Figure 5). The fiber diameters in the pristine RC membrane were ~500 nm; after surface-grafting with poly(HEMA), the fiber diameters increased and the membrane equivalent/apparent pore sizes decreased with prolonging the reaction time (e.g., the fiber diameters in the RC-HEMA-40 membrane increased to ~1.2 µm).

The poly(HEMA)

macromolecular chains were evidently coated on the fiber surfaces thus to make the fibers thicker. The weight of the HEMA-modified RC membrane increased by 2-4 times of the pristine RC membrane weight. With prolonging the reaction time to 60 min (i.e., the RC-HEMA-60 membrane), the poly(HEMA) chains might have reached the amount that some partially filled the pores in the membrane. It is necessary to note that poly(HEMA) can be swollen with water by a small degree while the polymer is not soluble in water.

For the AAS-modified RC

membranes, the fiber diameter did not vary distinguishably, while the pores were loosely filled with poly(AAS) chains. The weight of the AAS-modified RC membrane (at the ATRP reaction time of 30 min) only increased by 30% as compared to that of the pristine RC membrane; however, such a relatively small amount of poly(AAS) chains could completely fill/block the pores in the resulting membrane, as evidenced by the RC-HEMA-40 image in Figure 5. The following was the proposed explanations: the water-soluble poly(AAS) chains did not coat on (i.e., tightly attached to) the fiber surfaces; unlike the HEMA-modified membranes (in which the decrease of pore sizes by increasing the fiber diameters was observed), the AAS-modified membranes had water-soluble poly(AAS) chains that would partially fill the pores at the relatively low reaction degree. The different morphological structures of HEMA/AAS-modified RC membranes had important impacts on the ultrafiltration performance, which was studied by using nanoparticles (with diameters of ~40 nm) and BSA molecules (with sizes of ~10 nm). It is


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necessary to note that the pristine RC ENM even could not reject/remove 0.5 µm particles due to the large equivalent/apparent pore size.

Figure 5. SEM images showing the morphological structures of the pristine RC ENM, and the HEMA/SSA-modified RC membranes with different reaction times/degrees.

Figure 6 depicts the rejection fraction and water flux value of RC membranes before and after being surface-grafted with poly(HEMA) or poly(AAS) chains at different reaction times/degrees. For the HEMA-modified RC membranes, the rejection fraction of ~40 nm particles increased from ~9% (some of the particles may be blocked by small pores in the membrane) to >95%, when the reaction time was extended from 0 to 60 min (Figure 6A); hence, the ultrafiltration performance of RC ENM could be effectively improved upon the surface-grafting with poly(HEMA) via ATRP. However, even the RC-HEMA-60 membrane with the highest reaction degree could not reject any BSA. On the other hand, the AAS-modified RC membrane with a low reaction degree (i.e., the RC-AAS-15 membrane) could reject/remove more than 90% of 40 nm particles. Upon prolonging the reaction time, the BSA rejection fraction of the AASmodified RC membranes could be improved from ~3% to ~58%. It is necessary to note that



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poly(AAS) chains possess negative charges, thus charge repulsion between the chains and the BSA molecules might play a role on rejection fraction; nevertheless, the size-based separation was expected as the primary mechanism, although the poly(AAS) chains could also adsorb BSA under neutral condition (i.e., at the pH value of 7). With further increase of the reaction time to 45 min, even water could not permeate through the membrane (i.e., the RC-AAS-45 membrane). Note that the tests were conducted at the maximum pressure of 60 psi on these membranes due to the limitation of our testing equipment. It is reasonable to speculate that the AAS-modified RC membrane with adequate reaction degree/time might be able to completely reject/remove BSA molecules with reasonably high water flux. The above experimental results clearly demonstrated that the ultrafiltration performance of surface-modified RC ENMs could be better improved with AAS than with HEMA, because poly(AAS) is soluble in an aqueous system and the macromolecular chains could partially fill the pores to form gel-like structures. Additionally, the Donnan effect might also contribute to the high rejection/removal of BSA, because both poly(AAS) chains and BSA molecules possess negative charges. The water flux values of surface-modified electrospun RC membranes were not very high as compared with those of some other ultrafiltration membranes, because the thickness of coated polymer barrier layers (in those ultrafiltration membranes) was merely a few microns. Based on the Hagen-Poiseuille equation of Flux = εr2∆p/8η∆x,50,51 the water flux is related to the membrane thickness; while the thickness of our membrane was approximately 60 µm. It might be possible to improve the water flux without distinguishably sacrificing the ultrafiltration performance by reducing the membrane thickness.


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Figure 6. Rejection fraction and water flux value of (A) the HEMA-modified RC membranes with different reaction times for the removal of ~40 nm nanoparticles (for comparison, the AASmodified RC membrane with reaction time of 15 min is also included), and (B) the AASmodified RC membranes with different reaction times for the removal of ~10 nm BSA molecules.

4.

Conclusions Poly(HEMA) and poly(AAS) were successfully grafted onto the nanofiber surfaces of RC

ENM via ATRP.

Three HEMA-modified and three AAS-modified RC membranes were

prepared with varied reaction degrees by adjusting the ATRP reaction time. The results showed that water-insoluble poly(HEMA) chains would coat on the surfaces of RC nanofibers to make the fibers thicker thus to decrease the membrane equivalent/apparent pore size. In contrast, water-soluble poly(AAS) chains did not coat on the nanofiber surfaces; instead, they would partially fill the pores to form gel-like structures. To test the ultrafiltration performance of surface-modified RC membranes, ~40 nm nanoparticles and ~10 nm BSA molecules were used. Note that the pristine RC membrane even could not reject any of 0.5 µm particles due to its relatively large equivalent/apparent pore size. With high reaction degree, the HEMA-modified



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RC membrane could reject more than 95% of the ~40 nm nanoparticles, while the AAS-modified RC membrane with low reaction degree could reject more than 90% of the ~40 nm particles. Therefore, the reject fractions of the RC membranes were significantly improved upon grafting of poly(HEMA) and poly(AAS) chains onto the nanofiber surfaces via ATRP. The watersoluble poly(AAS) chains could better improve the reject fraction than the water-insoluble poly(HEMA) chains at the same reaction degree, because the poly(AAS) chains could move freely in an aqueous environment thus partially fill the pores in the resulting membranes. With high reaction degree, the AAS-modified RC membrane could reject ~58% of BSA, while the HEMA-modified RC membranes could not reject any BSA. Therefore, the ATRP method was revealed as an effective approach to decrease the equivalent/apparent pore sizes and to improve the reject fractions of electrospun nanofiber membranes; and the resulting membranes could be utilized for water purification.

Acknowledgement: This research was sponsored by the US Air Force Civil Engineering Center under the Contract Number of FA4819-14-C-0004.


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Table of Contents Graphic


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Electrospun Regenerated Cellulose Nanofiber Membranes Surface

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