Direct Electrospinning of Ultrafine Fibers with Interconnected

Subscriber access provided by University of Otago Library

Article

Direct electrospinning of ultrafine fibers with interconnected macro-pores enabled by in situ mixing microfluidics Wanjun Liu, Lei Zhu, Chen Huang, and Xiangyu Jin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11362 • Publication Date (Web): 30 Nov 2016 Downloaded from http://pubs.acs.org on December 10, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Direct electrospinning of ultrafine fibers with interconnected macro-pores enabled by in situ mixing microfluidics Wanjun Liu†, Lei Zhu†, Chen Huang, Xiangyu Jin* Wanjun Liu: [email protected] Lei Zhu: [email protected] Chen Huang: [email protected] †

These authors contributed equally to this work.

*

Correspondence: [email protected] (Xiangyu Jin)

Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, No. 2999 North Renmin Road, Shanghai 201620, China

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Porous ultrafine fibers are of great importance to various applications. Herein, we report a method to directly fabricate macro-porous ultrafine fibers by an in situ mixing microfluidics which allows for the simultaneous electrospinning of solution immediately after mixing. The formation mechanism of macro-pores should be attributed to the incomplete mixing coupled with nonsolvent-induced phase separation, which was elucidated by systematical investigation of various solvent systems and mixing solvents. The diameter of the macro-porous fibers can be tuned from 1.80 ± 0.40 µm to 6.75 ± 0.48 µm by adjusting the solution concentration and the feeding rate of mixing solvent. The results indicated that macro-porous fibers exhibited higher specific surface area (48.66 ± 8.30 m2 g-1), larger pore size (116.73 nm) and pore volume (0.169 ± 0.007 cm3 g-1) than conventional electrospun porous fibers, enabling the high oil absorption capacities of 95.68, 57.98, and 34.82 g g-1 for silicon oil, motor oil, and peanut oil, respectively. Our method has greatly expanded the solution scope for electrospinning from stable solution systems to unstable or sub-stable solution systems, thus provides intriguing opportunities for the investigation and fabrication of heterogeneous fibers by in situ mixing of various immiscible solvents/solutions. Our findings can serve as guidelines for the electrospinning of ultrafine fibers with interconnected macro-pores (> 50 nm).

KEYWORDS:

Electrospinning, ultrafine fibers, macro-pores, nonsolvent, phase

separation, oil cleanup

2


Page 2 of 29

Page 3 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Ultrafine fibers are promising materials for various applications, owing to their unique properties including high specific area, high porosity, and interconnected pores of their assembled nonwovens. However, it is still a vital challenge to directly fabricate ultrafine fibers with interconnected pores. The routine fabrication of porous ultrafine fibers is of great technological importance as it offers a method to tailor the roughness of fiber surface, increase the specific surface area, and enhance the porosity. Moreover, porous fibers hold promises for the advancements of various applications. For example, increasing the surface roughness would contribute to the improvement of cell attachment and proliferation1-5, and render the fiber mat with superhydrophobicity6-8. In addition, increasing the specific surface area and porosity would also be capable of enhancing the performance of absorption9-10, sensors11, catalysis12-13, etc. Up to now, various technologies including electrospinning14-15, centrifugal spinning16, solution/melt blowing17-18, and gyration method19-22 have been developed for the fabrication of ultrafine fibers, among them electrospinning has been widely recognized as a simple and versatile method to produce ultrafine fibers with controllable morphologies and functionalities14-15. Extensive efforts have been devoted to the electrospinning of porous fibers. Most methods involve using sacrificial templating or manipulating phase separation23-25. Although porous fibers with pores distributed both on the surface and in the core can be fabricated using sacrificial templating, harsh conditions for post treatment such as solvent extraction or high temperature calcination are inevitable, which usually results

3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

in the deformation and collapse of porous fibers26-29. In addition, the complete removal of sacrificial component(s) is also challenging30. Another widely used method is phase separation (e.g., thermally induced and vapor-induced phase separation23). By using highly volatile solvent such as tetrahydrofuran (THF), dichloromethane (DCM), chloroform, and acetone (ACE), porous fibers were successfully fabricated from a wide range of polymers (e.g., poly(methyl methacrylate) (PMMA)31-32, polystyrene (PS)31-33, poly(vinyl chloride) (PVC)34-35, poly(vinyl acetate)36, polyvinyl butyral37, poly(L-lactic acid) (PLLA)36, 38, poly(ɛ-caprolactone) (PCL)39-41, polycarbonate32, cellulose acetate42, ethyl cellulose43, polyvinylcarbazole44, poly(butylenes succinate)45, and polymethylsilsesquioxane46). However, the pores usually only exist on the surface of electrospun fibers because of thermally induced phase separation, and electrospinning process is pretty unstable and often interrupted by the problem of needle clogging due to the fast evaporation of highly volatile solvents and the requirement of high relative humidity. It has been reported that vapor-induced phase separation can facilitate the fabrication of fibers having interior pores. For example, PS23, 25, 30, 47-50, polyoxymethylene51, and PMMA52 fibers with interior pores were fabricated using low volatile solvents (e.g., N,N-dimethylformamide (DMF), N,N-dimethylacetamide) or a combination of low volatile and highly volatile solvents. In addition, polyacrylonitrile53, PLLA54-55, cellulose triacetate

56-57

, and polyethersulfone58 fibers with interior pores were obtained

using a ternary system of solvent/nonsolvent/polymer. This is a fascinating method to produce porous fibers, but this system is unstable due to the employment of nonsolvent and often results in needle clogging during electrospinning. Furthermore, Porous fibers were also fabricated under high relative humidity or using liquid nitrogen, water, or

4


Page 4 of 29

Page 5 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


other solvents as collectors47, 51, 59-62. Although ultrafine fibers with interior pores can be fabricated, it remains a challenge to routinely fabricate ultrafine fibers with interconnected macro-pores (> 50 nm). Herein, we propose a method that combines a modified electrospinning setup with nonsolvent-induced phase separation to directly fabricate ultrafine fibers with interconnected macro-pores (> 50 nm). The electrospinning setup was based on a microfluidic nozzle consisting of three channels that allows for liquid mixing from two input channels and simultaneous electrospinning of the resulting mixture from the other output channel. By varying the feeding rate of input fluids, ultrafine PS fibers with tunable macro-pore textures and distributions can be fabricated. To our knowledge, such a direct and versatile method to fabricate macro-porous fibers has not been reported previously, which can serve as guidelines for the electrospinning of macro-porous fibers for various applications.

EXPERIMENTAL SECTION Chemicals and Materials. PS (Mw=350 000 g mol-1), PCL (Mw=80 000 g mol-1), PMMA (Mw=350 000 g mol-1), and PVC (Mw=233 000 g mol-1) were purchased from Sigma Aldrich, Inc. THF, DCM, ACE, DMF, cyclohexane (CYH), 1-butanol (BuOH), n-hexane, methyl cyclohexane (MCH), and n-heptane were purchased from Shanghai Chemical Reagents Co., Ltd, China. All materials were used without further purification. Electrospinning. A polypropylene microfluidic nozzle consisting of three capillary channels (inner diameter (ID) 1 mm, length 15 mm) was connected with a conventional electrospinning setup (Figure 1). Two channels were used as inlets (Figure 1AI, II) to

5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 29

deliver polymer solution and mixing solvent by two syringe pumps (single-syringe infusion pump, KDS 100, KD Scientific Inc., Holliston, USA), respectively. The other channel used as outlet (Figure 1AIII) was connected by a stainless steel capillary (ID 0.7 mm, out diameter (OD) 1 mm, length 10 mm) to facilitate the charging of liquid and initiation

of

electrospinning

(high-voltage

direct-current

power

supply,

DW-P503-2ACDE, Tianjin Dongwen Co., Ltd., China). The temperature, relative humidity, collecting distance, feeding rate of polymer solution, applied voltage were kept at 25 ± 5 °C, 60% ± 5%, 15 cm, 1.5 mL h-1, 12 kV, respectively, which were determined according to our previous studies41,

43

. In addition, preliminary studies

indicated that electrospinning parameters including temperature, relative humidity, and collecting distance did not exerted obvious influence on the morphology of resultant fibers. Therefore, these parameters were kept constant for the entire study. In addition, the feeding rate of mixing solvent was varied from 0 to 4.5 mL h-1, further increase of the feeding rate resulted in the simultaneous electrospray of excessive mixing solvent(s) and electrospinning of PS fibers. Characterization. The surface morphology and cross section of the electrospun fibers were observed by scanning electron microscopy (SEM, TM-3000 and S-4800, Hitachi Ltd., Japan). The diameter of the electrospun fibers was measured using image analysis software (Adobe Acrobat X Pro 10.1.2.45) according to the SEM images. N2 physical adsorption–desorption isotherms (JW-BK132F, Beijing Science and Technology Co., China) were measured to determine the specific surface area, pore distribution, and total pore volume. Oil absorption. The oil absorption capacity was measured at 25 °C using the following procedures. 50 mL of oil-water mixture with a ratio of 1:1 was prepared and

6


Page 7 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


stored in a beaker. 0.2 g of the sorbent was put into the beaker to absorb oil for 1 hour, then the wet sorbent was transferred to a sieve and drained for around 30 min. The sorbent was weighted together with the sieve. The oil absorption capacity was calculated according to the following equation: =

(1)

Where Q is the oil absorption capacity (g g-1), m0 is the wet weight of the sorbent after oil absorption drained for around 30 min together with the sieve (g), m1 is the weight of the sieve (g), and m2 is the weight of the sorbent before absorption (g). Notes. For the whole study, the solvent ratio was the volume ratio, and the solution concentration was weight/volume (g mL-1). IME, SR, FR, Con., and PSS represent in situ mixing electrospinning, solvent ratio, feeding rate, concentration, and PS solution, respectively. Statistical analyses were conducted using at least 3 independent samples per experiment in order to determine the statistical significance at a p50 nm) were obtained when the feeding rate was equal to or higher than 1.5 mL h-1 except that when the THF/DMF ratio is 4:0 (pure THF), indicating the indispensable role of DMF in generating macro-pores. The interior macro-pores in the ultrafine fibers were further confirmed by imaging the cross sections of resultant fibers (Figure 3). Specifically, the fibers had interconnected macro-pores across the thickness of fibers when THF/DMF ratio was 3:1 and 1:1, while the fibers had a rough shell and macro-porous core when THF/DMF ratio was 1:3 and 0:4 (pure DMF). It should be noted that we failed to prepare the samples for cross sections due to needle clogging when the THF/DMF was 4:0. In addition, no macro-pores can be found when the feeding rate of CYH was 0.5 mL h-1, thus confirming that CYH as the nonsolvent

8


Page 8 of 29

Page 9 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


played an important role in generating macro-pore structures. Table 1. Typical properties of the solvents (√ and × represent soluble and insoluble, respectively). Solvents

THF

DCM

ACE

DMF

BuOH

CYH

MCH

n-hexane

n-heptane

66

39.75

56.12

153

117.7

80.719

100.934

68.7

98.4

26.4

28.12

23.7

35.2

24.6

24.38

23.17

17.9

19.6

√

√

×

√

×

×

×

×

×

Boiling point (℃) Surface tension (mN m-1) PS

(Swelling)

As macro-porous fibers have been produced by IME, we were wondering whether macro-porous fibers can be fabricated using conventional electrospinning setup, thus solutions with the same compositions as the resultant mixtures used in Figure 2 were prepared directly for conventional electrospinning (Figure S2 and S3). Unlike the porous fibers generated by IME strategy, the macro-pore structures disappeared except for Figure S2E,J,O. As CYH is miscible to THF and immiscible to DMF, laminate solutions (no solute precipitation) formed when the PS solutions containing no or less THF (detailed miscibility between different solvents, between solvents and PS solutions can be found in Table 2), therefore electrospinning were conducted immediately after stirring (Figure S2N-T and S3N-T), In addition, the fibers tended to have beads which should be attributed to the complete dispersion of CYH and thus the decrease of polymer entanglements63-64, while for IME bead-free fibers were produced when THF/DMF ratio was equal to or lower than 3:1 (Figure S1), confirming that the in situ mixing was incomplete.

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 29

Table 2. The miscibility between different solvents, between solvents and PS solutions. (√, ×, #, and @ represent miscible, immiscible, laminate, and precipitated, respectively) THF

DMF

20% PS/THF

20% PS

20% PS/DMF

(THF/DMF 1:1) THF

√

√

√

√

√

DMF

√

√

√

√

√

BuOH

√

√

×@

×@

×@

CYH

√

×#

√

#→√

×#

MCH

√

×#

√

#→√

×#

n-hexane

√

×#

√

@→√

×#

n-heptane

√

×#

√

#→√

×#

We further investigated the miscibility between 20% PS solution (THF/DMF 1:3) and CYH at various ratios. When the mixing ratio between PS solution and CYH was 3:1, CYH can be fully dispersed by 20% PS solution (THF/DMF 1:3), which resulted in clear and homogeneous PS solution (Figure S4A-D). When the mixing ratio was equal to or lower than 1:1, turbid solutions formed after complete mixing with the aid of magnetic stirring or hand shaking. It should be noted that the turbid solution with a mixing ratio of 1:1 became transparent slowly and then laminar after 1 day standing (Figure S4E-H). While the turbid solutions with mixing ratios of 1:2 and 1:3 became turbidly laminar immediately after standing and turned into transparent laminar solutions after 1 day standing (Figure S4I-R), demonstrating the unique advantage of IME strategy in electrospinning of sub-stable and unstable solutions which contain discontinuous phase (e.g., CYH). We further investigated the dynamic mixing process between 20% PS solution (THF/DMF 1:3) and CYH with a mixing ratio of 1:1 by

10


Page 11 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


slowly adding equal volume of CYH into a bottle filled with PS solution. The resultant laminar liquid stayed transparent for a long time, indicating that the self-mixing and self-diffusion speed was relatively slow (Figure S5A-C). After slight shaking, turbid liquid formed on the upper layer with microspheres being observed (Figure S5D-F). After strong shaking and mixing, turbid solution without lamination formed which became transparent and then laminar gradually after standing, and the size of microspheres in the turbid solution decreased during standing process (Figure S5G-M). To verify the existence of CYH discontinuous phase and the composition of the microspheres existed in the turbid solutions as well, we further investigated the dynamic mixing process between THF/DMF mixture (1:3) and CYH with a mixing ratio of 1:1 by slowly adding equal volume of CYH into a bottle filled with THF/DMF mixture. Similarly, the resultant laminar liquid stayed transparent for a long time (Figure S5N). After strong shaking, the liquid can be mixed, but turned into new transparent laminar liquid immediately (Figure S5O). Contrarily, no turbid phenomenon was observed during the mixing process, indicating that the microspheres existed in the turbid solutions should be attributed to the slight precipitation of PS. It should be noted that the boundary of the laminar liquid marked by dashed line was difficult to identify by eye and optical microscope as THF, DMF, and CYH are transparent. However, we believe that during IME discontinuous phase of CYH exists in the mixing solutions when the THF/DMF ratio was equal to or lower than 3:1 and the feeding rate of mixing solvent of CYH was equal to or higher than 1.5 mL h-1 (Figure 2 and 3), ascribing to the incomplete mixing and immiscibility between DMF and CYH. As noted, macro-porous fibers were generated only when DMF existed in the PS solutions and the feeding rate was higher than 0.5 mL h-1, indicating the existence of DMF can facilitate

11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the discontinuous phase of CYH. As the mixing time was very short during IME, we can conclude that the incomplete mixing and the existence of discontinuous phase of CYH played a key role in the formation of macro-pores in the ultrafine fibers (Figure 1B). For example, when the two inlets were fed with 20% PS (THF/DMF 1:1) and CYH, the mixing microfluidics was not capable of completely mixing the PS solution and CYH due to the relatively low mixing speed, short mixing time and immiscibility between DMF and CYH, resulting in the formation of CYH-rich regions. As THF is more volatile than CYH and DMF, the fast evaporation of THF during IME further increased the size of CYH-rich regions, leaving macro-pores across the thickness of fibers after the evaporation of CYH (Figure 2J-L and 3E,F). In addition, the existence of THF can improve the pore uniformity by enhancing the dispersion of CYH in the resulting mixture as it is miscible with CYH (Figure 3E,F). For 20% PS solution (DMF), CYH was poorly dispersed as CYH is immiscible with DMF and PS, thus CYH tended to be encapsulated by PS solution (DMF), resulting in fibers having rough shells and macro-porous cores (Figure 3S,T). In summary, the generation of macro-porous fibers using IME strategy requires that the mixing solvent should be immiscible with at least one solvent in the polymer solution. In addition, the uniformity of macro-pore structure can be enhanced by the employment of miscible volatile solvent in the polymer solution and thus improving the dispersion of mixing solvent. Apart from THF/DMF solvent system, we confirmed that ultrafine fibers with macro-pores can also be produced from other binary solvent systems of DCM/DMF and ACE/DMF when the feeding rate of CYH was equal to or higher than 1.5 mL h-1 (Figure 4 and S6). In addition, we also investigated the effects of other mixing solvents instead of CYH on the morphology of electrospun fibers using 20% PS solution

12


Page 12 of 29

Page 13 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(THF/DMF 1:1). Mixing solvents including THF, DMF, BuOH, MCH, n-hexane, and n-heptane were selected based on the solubility/miscibility to PS, THF, and DMF (Table 1). Particularly, BuOH is a nonsolvent to PS, but it is miscible with THF and DMF. MCH, n-hexane, and n-heptane are also nonsolvent to PS, but they are miscible with THF and immiscible with DMF. It should be noted that n-hexane and n-heptane are linear molecules, while MCH is a ring molecule similar to CYH. As shown in Figure 5, when comparing with the fibers obtained from 20% PS solution (THF/DMF 1:1) (Figure 3D), THF exerted no obvious effect on the pore structure (Figure 5A-C), whereas DMF improved the surface roughness (Figure 5D-F). For BuOH, fibers with a rough surface and porous core were produced. The feeding rate of BuOH was limited up to 0.5 mL h-1 because of precipitation of PS and needle clogging under higher feeding rate (Figure 5G-I). In addition, macro-porous fibers were produced without needle clogging when MCH was used as the mixing solvent (Figure 5J-L). While n-hexane and n-heptane resulted in needle clogging possibly due to the higher volatility and complex molecule interactions, It should be noted much less needle clogging happened when n-heptane was employed as the mixing solvent possibly attributing to its higher molecular weight, thus macro-porous fibers were also successfully obtained (Figure 5M-O). Although macro-porous fibers were successfully electrospun, it would be of technological importance if the diameter of macro-porous fibers can be regulated. By keeping the THF/DMF ratio constant at 1:1, PS solutions with concentrations varying from 15% to 30% were further examined through IME. Similar to previous results, macro-porous fibers were obtained from all the concentrations when the feeding rate of CYH was equal to or higher than 1.5 mL h-1 (Figure 6 and S7). Macro-porous fibers

13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

with a diameter ranging from 1.80 ± 0.40 µm to 6.75 ± 0.48 µm can be produced by adjusting feeding rate of CYH and the concentration of PS solution (Figure 6Q-S). Specifically, the diameter of the porous fibers increased from 2.02 ± 0.34 µm to 5.88 ± 1.08 µm, from 2.13 ± 0.47 µm to 6.14 ± 0.86 µm, from 1.80 ± 0.40 µm to 6.75 ± 0.48 µm when the concentration of PS solution was elevated from 15% to 30% for CYH at feeding rate of 1.5 mL h-1, 3 mL h-1, and 4.5 mL h-1, respectively. However, we failed to generate macro-porous fibers using other polymers including PCL, PMMA, and PVC using IME strategy, yet we still believe that by carefully selecting the mixing solvent and designing of polymer solution with a better understanding of the role of molecule interactions, macro-porous fibers based on various polymers can be acquired. In order to quantify the pore structure in the fibers, the nitrogen adsorption isotherms of macro-porous fibers (Figure 2K, ES1), conventional electrospun porous fibers (Figure 3D, ES2), and commercial polypropylene (PP) meltblown fibers (ES3) were measured for comparison. As shown in Figure 7A, the maximal nitrogen adsorption of ES1, ES2, and ES3 were 105.31 cm3 g-1, 54.72 cm3 g-1, and 9.75 cm3 g-1, respectively, indicating the pores in the fibers of ES1 and ES2 significantly increase the nitrogen adsorption capability. The pore volume were 0.169 ± 0.007 cm3 g-1, 0.097 ± 0.010 cm3 g-1, and 0.018 ± 0.003 cm3 g-1, and the specific surface area were 48.66 ± 8.30 m2 g-1, 29.52 ± 7.10 m2 g-1, and 3.26 ± 0.58 m2 g-1 for ES1, ES2, and ES3, respectively (Figure 7B,C). We further confirmed that mesopores (2-50 nm) existed in the fibers of ES1 and ES2 (Figure 7D), while macro-pores around 116.73 nm only presented in ES1, resulting in its high pore volume and specific surface area (Figure 7E). We next demonstrated the application of macro-porous fibers for oil absorption (Figure S8). Three typical oils including motor oil, silicon oil, and peanut oil were

14


Page 14 of 29

Page 15 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


selected to test the different fibrous materials (ES1, ES2, and ES3). The basic properties of these oils were measured and summarized in Table 3. As expected, among the three kinds of oil absorption materials, the macro-porous fibers produced by IME (ES1) showed the highest oil absorption capacity of 95.68 ± 7.48 g g-1, 57.98 ± 4.19 g g-1, and 34.82 ± 2.44 g g-1 for silicon oil, motor oil, and peanut oil, respectively (Figure 7F). Particularly, ES1 exhibited 3.91, 3.83, and 2.74 times oil absorption capacities of the commercial PP meltblown fiber mat (ES3) for silicon oil, motor oil, and peanut oil, respectively. Among the three types of oil, ES1 performed the best absorption capacity for silicon oil, probably due to the existence of macro-pores in the fibers and the higher viscosity of silicon oil. Table 3. Typical properties of the oils used in this work. Oil types

Viscosity (mPa s) Density (g cm-3) Surface tension (mN m-1)

Motor Oil

131.8

0.8601

30.449

Silicon Oil

480.4

0.9566

19.769

Peanut Oil

61.5

0.9173

32.596

CONCLUSIONS We have demonstrated IME as a convenient and reliable method to directly fabricate macro-porous ultrafine fibers enabled by in situ mixing microfluidics that allows for the in situ mixing and simutaneous electrospinning. By systematical investigation of various solvent systems and mixing solvents, we propose that the formation mechanism of macro-pores (> 50 µm) should be attributed to incomplete mixing coupled with nonsolvent-induced phase separation. The results indicated that macro-porous fibers owned much higher specific surface area (48.66 ± 8.30 m2 g-1), larger pore size (116.73

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

nm) and pore volume (0.169 ± 0.007 cm3 g-1) than conventional electrospun porous fibers. The diameter of the macro-porous fibers can be tuned from 1.80 ± 0.40 µm to 6.75 ± 0.48 µm by adjusting feeding rate of CYH and the concentration of PS solution. The macro-porous fibers showed excellent performance of oil absorption of 95.68 ± 7.48 g g-1, 57.98 ± 4.19 g g-1, and 34.82 ± 2.44 g g-1 for silicon oil, motor oil, and peanut oil, respectively. Moreover, the development of IME has greatly expanded the solution scope for electrospinning from stable solution systems to unstable or sub-stable solution systems, thus provides intriguing opportunities for the investigation and fabrication of heterogeneous fibers by in situ mixing of various immiscible solvents/solutions. Our findings can serve as guidelines for the fabrication of macro-porous fibers for various applications. ASSOCIATED CONTENT Supporting Information SEM pictures (lower magnification) of the fiber surfaces fabricated using IME under different solvent ratios of THF/DMF and various feeding rates of mixing solvent of CYH; SEM pictures of the fiber surfaces fabricated by conventional electrospinning using the same components as the resultant mixtures used in Figure 2; SEM pictures (lower magnification) of the fiber surfaces fabricated using IME under different solvent systems and various feeding rates of mixing solvent of CYH; Photos and optical microscopic images of the solutions prepared by mixing CYH with 20% PS solution (THF/DMF 1:3) at various PS solution (PPS)/CYH ratios; Photos and optical microscopic images of the solutions or liquid prepared by slowly adding equal volume of CYH to PS solution (20%, THF/DMF 1:3) or THF/DMF mixture (1:3); SEM pictures (lower magnification) of the fiber surfaces fabricated using IME under different solution

16


Page 16 of 29

Page 17 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


concentrations and various feeding rates of mixing solvent of CYH; pictures of oil absorption. The Supporting Information is available free of charge on the ACS Publications website at DOI: AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected] (X. Jin).

Author Contributions W. Liu† and L. Zhu† contributed equally to this work. W. Liu and X. Jin conceived the original concept. W. Liu and L. Zhu designed, conducted the experiments, and analyzed the data. W. Liu wrote the manuscript. W. Liu, L. Zhu, C. Huang, and X. Jin revised the manuscript. Notes The authors declare that they have no competing interests. ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China (51403033) and the Fundamental Research Funds for the Central Universities from China (No. 14D310106 and NO. 2232014D3-15). W. Liu acknowledges the financial support from the program of China Scholarships Council (No. 201406630041).

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Schematic diagram of IME setup and formation mechanism of macro-porous ultrafine fiber. A) IME is enabled by the combination of a microfluidic nozzle with a conventional electrospinning setup. Two channels were used as the inlets (I, II) to deliver polymer solution and mixing solvent by two syringe pumps, respectively. The other channel was used as the outlet (III) for electrospinning; B) the incomplete mixing and the immiscibility between CYH and DMF facilitate the formation of discontinuous phase of CYH, resulting in the formation macro-porous ultrafine fibers.

18


Page 18 of 29

Page 19 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. SEM pictures of the fiber surfaces fabricated using IME under various feeding rates of mixing solvent of CYH. Solvent ratio of THF/DMF A-D) 4:0; E-H) 3:1; I-L) 1:1; M-P) 1:3; Q-T) 0:4 (20% PS solution).

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. SEM pictures of the fiber cross sections fabricated using IME under various feeding rates of mixing solvent of CYH. Solvent ratio of THF/DMF A-C) 3:1; D-F) 1:1; G-I) 1:3; J-L) 0:4 (20% PS solution).

20


Page 20 of 29

Page 21 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. SEM pictures of the fiber surfaces fabricated using IME under various feeding rates of mixing solvent of CYH. Solvent systems A-D) DCM/DMF; E-H) ACE-DMF; I-L) THF/DMF (20% PS solution, solvent ratio 1:1).

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. SEM pictures of representative fibers fabricated using IME under different mixing solvents. A-C) THF; D-F) DMF; G-I) BuOH; J-L) MCH; M-O) n-heptane (20% PS solution, THF/DMF 1:1, the feeding rates of the mixing solvents were 3 mL h-1 for THF, DMF, MCH, 0.5 mL h-1 for BuOH, and 1.5 mL h-1 for n-heptane).

22


Page 22 of 29

Page 23 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. SEM pictures of the fiber surfaces fabricated using IME under various feeding rates of mixing solvent of CYH. Solution concentrations A-D) 15%; E-H) 20%; I-L) 25%; M-P) 30%. Q-S) The influence of concentration of PS solution on the diameters of the macro-porous fibers under different feeding rates of mixing solvent(Q) 1.5 mL h-1, (R) 3 mL h-1, (S) 4.5 mL h-1 (PS solution, THF/DMF 1:1).

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure

7. Characterizations of

macro-porous fibers (ES1), conventional

electrospun porous fibers (ES2), and commercial PP meltblown fibers (ES3). A) The nitrogen adsorption isotherms; B) pore volume - pore diameter curve; C) specific surface area and pore volume; D) dV/dD - pore diameter curve; E) dV/dlogD - pore diameter curve; F) oil absorption capacities.

24


Page 24 of 29

Page 25 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES (1) Stevens, M. M.; George, J. H. Exploring and Engineering the Cell Surface Interface. Science 2005, 310, 1135-1138. (2) Baker, B. M.; Trappmann, B.; Wang, W. Y.; Sakar, M. S.; Kim, I. L.; Shenoy, V. B.; Burdick, J. A.; Chen, C. S. Cell-Mediated Fibre Recruitment Drives Extracellular Matrix Mechanosensing in Engineered Fibrillar Microenvironments. Nat. Mater. 2015, 14, 1262–1268. (3) Nerurkar, N. L.; Baker, B. M.; Sen, S.; Wible, E. E.; Elliott, D. M.; Mauck, R. L. Nanofibrous Biologic Laminates Replicate the Form and Function of the Annulus Fibrosus. Nat. Mater. 2009, 8, 986-992. (4) Grafahrend, D.; Heffels, K. H.; Beer, M. V.; Gasteier, P.; Möller, M.; Boehm, G.; Dalton, P. D.; Groll, J. Degradable Polyester Scaffolds with Controlled Surface Chemistry Combining Minimal Protein Adsorption with Specific Bioactivation. Nat. Mater. 2011, 10, 67-73. (5) Huang, C.; Ouyang, Y.; Niu, H.; He, N.; Ke, Q.; Jin, X.; Li, D.; Fang, J.; Liu, W.; Fan, C. Nerve Guidance Conduits from Aligned Nanofibers: Improvement of Nerve Regeneration through Longitudinal Nanogrooves on a Fiber Surface. ACS Appl. Mater. Interfaces 2015, 7, 7189-7196. (6) Miyauchi, Y.; Ding, B.; Shiratori, S. Fabrication of a Silver-Ragwort-Leaf-Like Super-Hydrophobic Micro/Nanoporous Fibrous Mat Surface by Electrospinning. Nanotechnology 2006, 17, 5151-5156. (7) Lin, J.; Cai, Y.; Wang, X.; Ding, B.; Yu, J.; Wang, M. Fabrication of Biomimetic Superhydrophobic Surfaces Inspired by Lotus Leaf and Silver Ragwort Leaf. Nanoscale 2011, 3, 1258-1262. (8) Tuteja, A.; Choi, W.; Ma, M.; Mabry, J. M.; Mazzella, S. A.; Rutledge, G. C.; McKinley, G. H.; Cohen, R. E. Designing Superoleophobic Surfaces. Science 2007, 318, 1618-1622. (9) Lin, J.; Shang, Y.; Ding, B.; Yang, J.; Yu, J.; Al-Deyab, S. S. Nanoporous Polystyrene Fibers for Oil Spill Cleanup.

Mar. Pollut. Bull. 2012, 64, 347-352.

(10) Zhang, Y.; Wei, S.; Liu, F.; Du, Y.; Liu, S.; Ji, Y.; Yokoi, T.; Tatsumi, T.; Xiao, F.-S. Superhydrophobic Nanoporous Polymers as Efficient Adsorbents for Organic Compounds. Nano Today 2009, 4, 135-142. (11) Ding, B.; Wang, M.; Wang, X.; Yu, J.; Sun, G. Electrospun Nanomaterials for Ultrasensitive Sensors. Mater. Today 2010, 13, 16-27. (12) Krogman, K. C.; Lowery, J. L.; Zacharia, N. S.; Rutledge, G. C.; Hammond, P. T. Spraying Asymmetry into Functional Membranes Layer-by-Layer. Nat. Mater. 2009, 8, 512-518. (13) Wang, Y.; Huang, H.; Gao, J.; Lu, G.; Zhao, Y.; Xu, Y.; Jiang, L. TiO2-SiO2 Composite Fibers with Tunable Interconnected Porous Hierarchy Fabricated by Single-Spinneret Electrospinning toward Enhanced Photocatalytic Activity. J. Mater. Chem. A 2014, 2, 12442-12448. (14) Huang, Z. M.; Zhang, Y. Z.; Kotaki, M.; Ramakrishna, S. A Review on Polymer Nanofibers by Electrospinning and Their Applications in Nanocomposites. Compos. Sci. Technol. 2003, 63, 2223-2253. (15) Li, D.; Xia, Y. Electrospinning of Nanofibers: Reinventing the Wheel? Adv. Mater. 2004, 16, 1151-1170. (16) Badrossamay, M. R.; McIlwee, H. A.; Goss, J. A.; Parker, K. K. Nanofiber Assembly by Rotary Jet-Spinning. Nano Lett. 2010, 10, 2257-2261. (17) Ellison, C. J.; Phatak, A.; Giles, D. W.; Macosko, C. W.; Bates, F. S. Melt Blown Nanofibers: Fiber Diameter Distributions and Onset of Fiber Breakup. Polymer 2007, 48, 3306-3316. (18) Medeiros, E. S.; Glenn, G. M.; Klamczynski, A. P.; Orts, W. J.; Mattoso, L. H. Solution Blow Spinning: A New Method to Produce Micro‐and Nanofibers from Polymer Solutions. J. Appl. Polym. Sci. 2009, 113, 2322-2330.

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(19) Zhang, S.; Karaca, B. T.; VanOosten, S. K.; Yuca, E.; Mahalingam, S.; Edirisinghe, M.; Tamerler, C. Coupling Infusion and Gyration for the Nanoscale Assembly of Functional Polymer Nanofibers Integrated with Genetically Engineered Proteins. Macromol. Rapid Commun. 2015, 36, 1322-1328. (20) Mahalingam, S.; Edirisinghe, M. Forming of Polymer Nanofibers by a Pressurised Gyration Process. Macromol. Rapid Commun. 2013, 34, 1134-1139. (21) Xu, Z.; Mahalingam, S.; Basnett, P.; Raimi-Abraham, B.; Roy, I.; Craig, D.; Edirisinghe, M. Making Nonwoven Fibrous Poly(Ε-Caprolactone) Constructs for Antimicrobial and Tissue Engineering Applications by Pressurized Melt Gyration. Macromol. Mater. Eng. 2016, 301, 922-934. (22) Illangakoon, U. E.; Mahalingam, S.; Colombo, P.; Edirisinghe, M. Tailoring the Surface of Polymeric Nanofibres Generated by Pressurised Gyration. Surf. Innovations 2016, 4, 167-178. (23) Lu, P.; Xia, Y. Maneuvering the Internal Porosity and Surface Morphology of Electrospun Polystyrene Yarns by Controlling the Solvent and Relative Humidity. Langmuir 2013, 29, 7070-7078. (24) Fashandi, H.; Karimi, M. Pore Formation in Polystyrene Fiber by Superimposing Temperature and Relative Humidity of Electrospinning Atmosphere. Polymer 2012, 53, 5832–5849. (25) Zheng, J.; Zhang, H.; Zhao, Z.; Han, C. C. Construction of Hierarchical Structures by Electrospinning or Electrospraying. Polymer 2012, 53, 546-554. (26) Tran, C.; Kalra, V. Co-Continuous Nanoscale Assembly of Nafion-Polyacrylonitrile Blends within Nanofibers: A Facile Route to Fabrication of Porous Nanofibers. Soft Matter 2013, 9, 846-852. (27) Pal, J.; Sharma, S.; Sanwaria, S.; Kulshreshtha, R.; Nandan, B.; Srivastava, R. K. Conducive 3D Porous Mesh of Poly(Ε-Caprolactone) Made Via Emulsion Electrospinning. Polymer 2014, 55, 3970-3979. (28) Gupta, A.; Saquing, C. D.; Afshari, M.; Tonelli, A. E.; Khan, S. A.; Kotek, R. Porous Nylon-6 Fibers Via a Novel Salt-Induced Electrospinning Method. Macromolecules 2008, 42, 709-715. (29) Kim, C.; Ngoc, B. T.; Yang, K. S.; Kojima, M.; Kim, Y. A.; Kim, Y. J.; Endo, M.; Yang, S. C. Self‐ Sustained Thin Webs Consisting of Porous Carbon Nanofibers for Supercapacitors Via the Electrospinning of Polyacrylonitrile Solutions Containing Zinc Chloride. Adv. Mater. 2007, 19, 2341-2346. (30) Lin, J.; Ding, B.; Yu, J. Direct Fabrication of Highly Nanoporous Polystyrene Fibers Via Electrospinning. ACS Appl. Mater. Interfaces 2010, 2, 521-528. (31) Dayal, P.; Liu, J.; Kumar, S.; Kyu, T. Experimental and Theoretical Investigations of Porous Structure Formation in Electrospun Fibers. Macromolecules 2007, 40, 7689-7694. (32) Megelski, S.; Stephens, J. S.; Chase, D. B.; Rabolt, J. F. Micro-and Nanostructured Surface Morphology on Electrospun Polymer Fibers. Macromolecules 2002, 35, 8456-8466. (33) Casper, C. L.; Stephens, J. S.; Tassi, N. G.; Chase, D. B.; Rabolt, J. F. Controlling Surface Morphology of Electrospun Polystyrene Fibers: Effect of Humidity and Molecular Weight in the Electrospinning Process. Macromolecules 2004, 37, 573-578. (34) Lee, K. H.; Kim, H. Y.; La, Y. M.; Lee, D. R.; Sung, N. H. Influence of a Mixing Solvent with Tetrahydrofuran and N, N‐Dimethylformamide on Electrospun Poly (Vinyl Chloride) Nonwoven Mats. J. Polym. Sci., Part B: Polym. Phys. 2002, 40, 2259-2268. (35) Medeiros, E. S.; Mattoso, L. H. C.; Offeman, R. D.; Wood, D. F.; Orts, W. J. Effect of Relative Humidity on the Morphology of Electrospun Polymer Fibers. Can. J. Chem. 2008, 86, 590-599. (36) Kim, C. H.; Jung, Y. H.; Kim, H. Y.; Lee, D. R.; Dharmaraj, N.; Choi, K. E. Effect of Collector Temperature on the Porous Structure of Electrospun Fibers. Macromol. Res. 2006, 14, 59-65. (37) Lubasova, D.; Martinova, L. Controlled Morphology of Porous Polyvinyl Butyral Nanofibers. J. Nanomater. 2011, 2011, 1-6.

26


Page 26 of 29

Page 27 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(38) Wagner, A.; Poursorkhabi, V.; Mohanty, A. K.; Misra, M. Analysis of Porous Electrospun Fibers from Poly(L-Lactic Acid)/Poly(3-Hydroxybutyrate-Co-3-Hydroxyvalerate) Blends. ACS Sustainable Chem. Eng. 2014, 2, 1976-1982. (39) Nguyen, T.-H.; Bao, T. Q.; Park, I.; Lee, B.-T. A Novel Fibrous Scaffold Composed of Electrospun Porous Poly (Ɛ-Caprolactone) Fibers for Bone Tissue Engineering. J. Biomater. Appl. 2013, 28, 514-528. (40) Gulfam, M.; Lee, J. M.; Kim, J.-e.; Lim, D. W.; Lee, E. K.; Chung, B. G. Highly Porous Core–Shell Polymeric Fiber Network. Langmuir 2011, 27, 10993-10999. (41) Li, D.; Wu, T.; He, N.; Wang, J.; Chen, W.; He, L.; Huang, C.; Ei-Hamshary, H. A.; Al-Deyab, S. S.; Ke, Q.; Mo, X. Three-Dimensional Polycaprolactone Scaffold Via Needleless Electrospinning Promotes Cell Proliferation and Infiltration. Colloids Surf., B 2014, 121, 432-443. (42) Celebioglu, A.; Uyar, T. Electrospun Porous Cellulose Acetate Fibers from Volatile Solvent Mixture. Mater. Lett. 2011, 65, 2291-2294. (43) Park, J. Y.; Han, S. W.; Lee, I. H. Preparation of Electrospun Porous Ethyl Cellulose Fiber by THF/DMAc Binary Solvent System. J. Ind. Eng. Chem. (Seul) 2007, 13, 1002-1008. (44) Bognitzki, M.; Czado, W.; Frese, T.; Schaper, A.; Hellwig, M.; Steinhart, M.; Greiner, A.; Wendorff, J. H. Nanostructured Fibers Via Electrospinning. Adv. Mater. 2001, 13, 70-72. (45) Wu, Y.; Yu, J.-Y.; Ma, C. Electrospun Nanoporous Fiber.

Text. Res. J. 2008, 78, 812-815.

(46) Luo, C. J.; Nangrejo, M.; Edirisinghe, M. A Novel Method of Selecting Solvents for Polymer Electrospinning. Polymer 2010, 51, 1654-1662. (47) McCann, J. T.; Marquez, M.; Xia, Y. Highly Porous Fibers by Electrospinning into a Cryogenic Liquid. J. Am. Chem. Soc. 2006, 128, 1436-1437. (48) Liu, W.; Huang, C.; Jin, X. Tailoring the Grooved Texture of Electrospun Polystyrene Nanofibers by Controlling the Solvent System and Relative Humidity. Nanoscale Res. Lett. 2014, 9, 350. (49) Pai, C.-L.; Boyce, M. C.; Rutledge, G. C. Morphology of Porous and Wrinkled Fibers of Polystyrene Electrospun from Dimethylformamide. Macromolecules 2009, 42, 2102-2114. (50) Liu, W.; Huang, C.; Jin, X. Electrospinning of Grooved Polystyrene Fibers: Effect of Solvent Systems. Nanoscale Res. Lett. 2015, 10, 237. (51) Kongkhlang, T.; Kotaki, M.; Kousaka, Y.; Umemura, T.; Nakaya, D.; Chirachanchai, S. Electrospun Polyoxymethylene: Spinning Conditions and Its Consequent Nanoporous Nanofiber. Macromolecules 2008, 41, 4746-4752. (52) Bae, H.-S.; Haider, A.; Selim, K. K.; Kang, D.-Y.; Kim, E.-J.; Kang, I.-K. Fabrication of Highly Porous PMMA Electrospun Fibers and Their Application in the Removal of Phenol and Iodine. J. Polym. Res. 2013, 20, 158. (53) Yu, X.; Xiang, H.; Long, Y.; Zhao, N.; Zhang, X.; Xu, J. Preparation of Porous Polyacrylonitrile Fibers by Electrospinning a Ternary System of PAN/DMF/H2O. Mater. Lett. 2010, 64, 2407-2409. (54) Qi, Z.; Yu, H.; Chen, Y.; Zhu, M. Highly Porous Fibers Prepared by Electrospinning a Ternary System of Nonsolvent/Solvent/Poly (L-Lactic Acid). Mater. Lett. 2009, 63, 415-418. (55) Zhang, K.; Wang, X.; Jing, D.; Yang, Y.; Zhu, M. Bionic Electrospun Ultrafine Fibrous Poly (L-Lactic Acid) Scaffolds with a Multi-Scale Structure. Biomed. Mater. 2009, 4, 035004. (56) Han, S. O.; Son, W. K.; Youk, J. H.; Lee, T. S.; Park, W. H. Ultrafine Porous Fibers Electrospun from Cellulose Triacetate. Mater. Lett. 2005, 59, 2998-3001. (57) Yoon, Y. I.; Moon, H. S.; Lyoo, W. S.; Lee, T. S.; Park, W. H. Superhydrophobicity of Cellulose Triacetate Fibrous Mats Produced by Electrospinning and Plasma Treatment. Carbohydr. Polym. 2009, 75,

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

246-250. (58) Wei, Z.; Zhang, Q.; Wang, L.; Wang, X.; Long, S.; Yang, J. Porous Electrospun Ultrafine Fibers Via a Liquid–Liquid Phase Separation Method. Colloid. Polym. Sci. 2013, 291, 1293-1296. (59) Seo, Y.-A.; Pant, H.; Nirmala, R.; Lee, J.-H.; Song, K.; Kim, H. Fabrication of Highly Porous Poly (Ε-Caprolactone) Microfibers Via Electrospinning. J. Porous Mater. 2012, 19, 217-223. (60) Guillen, G. R.; Pan, Y.; Li, M.; Hoek, E. M. Preparation and Characterization of Membranes Formed by Nonsolvent Induced Phase Separation: A Review. Ind. Eng. Chem. Res. 2011, 50, 3798-3817. (61) Nayani, K.; Katepalli, H.; Sharma, C. S.; Sharma, A.; Patil, S.; Venkataraghavan, R. Electrospinning Combined with Nonsolvent-Induced Phase Separation to Fabricate Highly Porous and Hollow Submicrometer Polymer Fibers. Ind. Eng. Chem. Res. 2011, 51, 1761-1766. (62) Wu, Y.; Clark, R. L. Controllable Porous Polymer Particles Generated by Electrospraying. J. Colloid Interface Sci. 2007, 310, 529-535. (63) McKee, M. G.; Layman, J. M.; Cashion, M. P.; Long, T. E. Phospholipid Nonwoven Electrospun Membranes. Science 2006, 311, 353-355. (64) Shenoy, S. L.; Bates, W. D.; Frisch, H. L.; Wnek, G. E. Role of Chain Entanglements on Fiber Formation During Electrospinning of Polymer Solutions: Good Solvent, Non-Specific Polymer–Polymer Interaction Limit. Polymer 2005, 46, 3372-3384.

28


Page 28 of 29

Page 29 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC graphic or Table of contents

29


Direct Electrospinning of Ultrafine Fibers with Interconnected

Recommend Documents