DOI: 10.1002/asia.201500035
Full Paper
Fluoropolymers
Fabrication of Fluoropolymer Microtubes via RAFT Copolymerization of N,N’-Methylene Bisacrylamide Gel Fibers and Fluoromonomer Qi Li,[a, b] Yi Wang,[a] and Liming Tang*[a] Abstract: Fluoropolymer microtubes with a smooth surface were fabricated in more than 70 % yield via reversible addition fragmentation chain transfer (RAFT) co-polymerization of N,N’-methylene bisacrylamide (MBA) gel fibers as both template and monomer, 2-(perfluoro-3-methylbutyl)ethyl acrylate (R-3420) as co-monomer, and pentaerythritol tetraacrylate (PET4A) as cross-linker. The resulting fluoropolymer microtubes were characterized fully by SEM, TEM, EDS, XPS, and FT-IR. The influence of the monomer composition on the yields and morphologies of the tubes were investigated in detail. The results indicated that polymer microtubes with
Introduction Fluoropolymers have frequently been applied as protecting materials in harsh environments or as modifiers for other polymers and inorganic materials because of their many unique properties, such as high temperature stability, excellent chemical resistance, low water sorption, low dielectric constant, and so forth.[1] Fluoropolymers with specific morphologies and microstructures have recently become important because of their improved performances and many potential applications.[1b, 2] For example, Chu coated a CF2-based fluoropolymer onto micro-sized pillar-type structures and acquired superhydrophobicity for the coated surfaces.[3] Kitamura fabricated a microstructure at fluoropolymer surface using ion beam irradiation and found that the irradiated surface covered with a large number of the regularly arranged protrusions was suitable for making a cell sheet and for harvesting cells.[4] Owing to their one-dimensional hollow structures, functionalities and structural diversities, polymer nano- and microtubes [a] Dr. Q. Li, Y. Wang, Prof. L. Tang Key Laboratory of Advanced Materials of Ministry of Education of China Department of Chemical Engineering Tsinghua University, Beijing 100084 (P.R. China) E-mail: [email protected] [b] Dr. Q. Li Science and Technology on Advanced High Temperature Structural Materials Laboratory Beijing Institute of Aeronautical Materials Beijing 100095 (China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201500035. Chem. Asian J. 2015, 10, 1363 – 1369
a smooth surface were obtained at suitable amounts of R3420 and PET4A. Because of the decreased solubility of MBA gel fibers, the wall thickness increased as more R-3420 was used. In the presence of PET4A, the solution polymerization could be facilitated and more R-3420 could be attached onto the tubes based on FT-IR analysis. The water contact angle and swelling ratio measurements both revealed the low hydrophilicity and high lipophilicity of the fluoropolymer microtubes, which made the sample able to absorb toluene selectively in a water/toluene two-phase system.
have many potential applications in separation,[5] drug delivery systems,[6] electronic devices,[7] and as catalyst carriers.[8] During the past two decades, many different techniques have been developed to fabricate polymer tubes, such as the template method, self-assembling method, electrostatic spinning method and so on.[9] The coating of a fluoropolymer layer on single-walled carbon nanotube field-effect transistors was found to improve drastically the key device characteristics.[10] However, fluoropolymer nano- and microtubes either formed from fluoropolymers or polymerized from fluoromonomers have not been investigated yet. Recently, we described a novel approach for fabricating polymer microtubes, which was implemented via reversible addition fragmentation chain transfer (RAFT) polymerization of N,N’-methylene bisacrylamide (MBA) xerogel fibers.[11] Polymer microtubes with improved morphologies could be obtained by using different acrylate co-monomers and the yields became higher with increasing the functionality of the co-monomers.[12] However, the performances and functions of the resulting tubes were quite limited by their simple compositions. The formation process of polymer microtubes has been proposed by us.[11, 12] Firstly, MBA monomers on the surface of MBA gel fibers participated in RAFT polymerization and crosslinked together to form frameworks of polymer microtubes. Meanwhile, a part of MBA molecules dissolved from the fibers and co-polymerized with the co-monomer in solution to form polymer networks.[13] As the reaction progressed, more and more MBA molecules dissolved from the fibers and polymerized onto the networks. Due to the living character of RAFT polymerization,[14] the growing networks could graft gradually
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Full Paper onto the frameworks via covalent binding and integrate together to form polymer microtubes. We also demonstrated that RAFT polymerization is critical for the formation of polymer tubes,[11a, c] because the active polymer chains and the slow growth rate increased intermolecular cross-linking and the interlacing of the polymer chains to form a cross-linked polymer layer covering the template.[13, 14b, c, 15] Under identical conditions, conventional radical polymerization gave only particles due to the dead full-length polymers formed from the beginning of the reaction and the enhancement of intramolecular cross-linking.[13, 16] Co-polymerization involving fluoromonomers is an effective approach to prepare fluoropolymers. In this article, fluoropolymer microtubes were prepared via RAFT co-polymerization by using MBA xerogel fibers as a template and monomer, 2-(perfluoro-3-methylbutyl)ethyl acrylate (R-3420) as a co-monomer, and pentaerythritol tetraacrylate (PET4A) as a cross-linker. The resulting tubes were characterized by various techniques, including SEM, TEM, EDS, XPS and FT-IR. The influence of monomer composition on the yields and morphologies of the tubes were investigated in detail. Because of the existence of fluorine atoms, the fluoropolymer microtubes possess high hydrophobility and low hydrophilicity, which made them suitable to absorb organic solvents from water. Compared to the literature results, we describe a facile method to fabricate fluoropolymer microtubes, which can have improved morphologies and specific properties. To the best of our knowledge, this is the first example concerning the preparation of fluoropolymer microtubes, and our approach may be extended to the fabrication of many other functional polymer nano- and microstructures.
Results and Discussion
Table 1. Compositions and yields of polymer microtubes. Sample
R-3420 [g]
PET4A [g]
Weight of product [g]
Yield [%]
1 2 3 4 5 6 7 8 9 10 11 12
0.0900 0.0900 0.0900 0.0900 0.0900 0.0900 0.0900 0 0.0300 0.0600 0.1200 0.1500
0 0.0006 0.0012 0.0025 0.0050 0.0075 0.0100 0.0012 0.0012 0.0012 0.0012 0.0012
0.1208 0.1434 0.1405 0.1360 0.1462 0.1394 0.1612 0.0625 0.0864 0.1034 0.1323 0.1490
61.7 73.1 71.4 68.6 72.8 68.6 78.4 58.5 63.1 61.9 58.3 58.3
Conditions: MBA gel fibers, 0.0900 g; AIBN, 0.0027 g; DBTTC, 0.013 g.
Figure 1. Photos of the MBA organogel (a) and fluoropolymer gel (b).
of product reached up to 61.7 %, a value much higher than the yield (18.5 %) of sample prepared from sole MBA xerogel fibers.[11a] Figure 2 a,b shows the morphology of MBA xerogel fibers with their statistical average diameter of 2.994 mm (Supporting Information, Table S1). The SEM images of sample 1 in Fig-
Fabrication of fluoropolymer microtubes In our previous work, polymer microtubes were fabricated via RAFT polymerization of MBA xerogel fibers as a template and monomer source, and multifunctional acrylates as co-monomers in the presence of dibenzyl trithiocarbonate (DBTTC) as a RAFT reagent and azobisisobutyronitrile (AIBN) as an initiator. The acrylate co-monomers were found to promote the transformation of MBA xerogel fibers and facilitate the formation of polymer microtubes.[11a] Considering the various interesting properties of fluoropolymers, we herein attempted to fabricate fluoropolymer microtubes via RAFT co-polymerization of MBA xerogel fibers and fluoromonomer R-3420. Fluoropolymer microtubes were firstly prepared from MBA xerogel fibers and R-3420 at a weight ratio of 1:1 (Table 1, sample 1). MBA organogels formed in chloroform were used as both the template and monomer source. After immersing the MBA fibers in toluene containing R-3420 as the fluoromonomer, dibenzyl trithiocarbonate (DBTTC) as the RAFT agent, and AIBN as the initiator, the reaction solution was heated at 82 8C to initiate the RAFT polymerization. After polymerization, the MBA organogel turned into a polymer gel as seen from Figure 1. After being filtered and washed by ethanol, fluoropolymer microtubes were obtained. The yield Chem. Asian J. 2015, 10, 1363 – 1369
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Figure 2. (a,b) SEM images of MBA xerogel fibers; (c,d) SEM images of sample 1.
ure 2 c,d demonstrate that polymer microtubes with open ends were fabricated via RAFT co-polymerization of MBA xerogel fibers and R-3420. The statistical average outer diameter of the tubes is 2.868 mm (Supporting Information, Table S1), corresponding to the diameter of MBA xerogel fibers. Compared to the tubes without fluorine atoms, the fluoropolymer micro-
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Full Paper tubes have a quite smooth exterior surface (Figure 2 c,d), which should be owing to the low surface energy of fluoropolymers.[1a] Because of the low surface energy, the fluoropolymer networks preferred to spread over the tubes during grafting and integrate into a smooth surface. To promote the formation of fluoropolymer microtubes, the tetrafunctional acrylate monomer PET4A was used as a crosslinker. At a constant weight ratio of 1:1 for MBA xerogel fibers and R-3420, polymerizations at different amount of PET4A were carried out (see Table 1, samples 2–7). The yield could reach up to 73.1 % at the smallest amount of PET4A (sample 2) and changed little at higher amounts of PET4A (see also Figure S1 in the Supporting Information). The SEM images of the products (Figure 3) reveal that they are all of tubular structure with open ends. The tubes obtained at lower amounts of PET4A can maintain a rather smooth surface (Figure 3 a,b)). However, at higher amount of PET4A, some
out RAFT copolymerization at different amounts of R-3420 (Table 1, samples 3 and 8–12) in the presence of 0.0012 g PET4A. The yields ranged between 58.3 % and 71.4 % at different amounts of R-3420 (see Table 1, samples 3 and 8–12, Figure S4 in the Supporting Information). The SEM images of the resulting samples (Figure 4) show that they are all of tubular
Figure 4. SEM images of polymer microtubes obtained at different amounts of R-3420: (a) 0 g; (b) 0.0300 g; (c) 0.0600 g; (d) 0.1200 g; and (e) 0.1500 g.
Figure 3. SEM images of polymer microtubes at different amount of PET4A: (a) 0.0006 g; (b) 0.0012 g; (c) 0.0025 g; (d) 0.0050 g; (e) 0.0075 g; and (f) 0.01 g.
irregular particles are also noticed besides tubes (Figure 3 c–f)). The tube surfaces become rougher as more PET4A is used. Single polymer microtubes of samples obtained at different amounts of PET4A are shown in Figure S2 in the Supporting Information. The lower resolution image of sample 3 (see Figure S3 in the Supporting Information) shows that only polymer microtubes were obtained, with most of them having smooth surfaces and few of them having particles on the tube surfaces. At low amounts of PET4A, the solution polymerization of the monomers could be promoted because of the high functionality of PET4A. Therefore, more polymer chains could link into networks[17] to facilitate the formation of tubes. Because of the low cross-linked degree, the polymer networks were still soluble and could integrate together to achieve smooth surfaces. However, at high amounts of PET4A, the polymerization of monomers in solution resulted in the formation of rather dense networks or even insoluble particles, which are unable to integrate anymore because of their higher cross-linked degree and lower diffusion ability; accordingly, both particles and tubes with a rough surface were produced. To understand the influence of the fluoromonomer on the reaction and the morphology of the products, we also carried Chem. Asian J. 2015, 10, 1363 – 1369
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structure with open ends. In the absence of R-3420, both particles and tubes are noticed with the tubes covered by many nanosized particles (Figure 4 a, sample 8), which should be attributed to the formation of insoluble particles via polymerization of multifunctional monomers only. At low amounts of R3420 (0.0300 g, 0.0600 g and 0.0900 g, corresponding to samples 9, 10 and 3), only tubes with quite smooth surfaces are obtained and almost no particle is noticed in the samples (see Figure 4 b,c and Figure 3 b). However, at rather high amounts of R-3420 (0.1200 g and 0.1500 g), corresponding to samples 11 and 12), both particles and tubes are formed in the samples (Figure 4 d,e). As the initiator decomposed into radicals at high temperature, the radicals could initiate both solution monomers and MBA gel fibers. At the rather high concentration of R-3420, the radicals could have a higher chance to initiate solution monomers rather than MBA gel fibers. Due to the lack of sufficient reactive sites in the tubes, some of the solution polymers could not graft to the tubes but grow eventually into insoluble particles. From the insets in Figure 4, it is interesting to find that the wall thickness of the tubes is quite different for the samples. The statistical average wall thickness of a sufficient amount of tubes is plotted in Figure 5. The plot shows that the wall thickness increases gradually with increasing the amounts of R3420. This result could be explained by the solubility of MBA xerogel fibers in different reaction systems. At 82 8C, 0.2057 g MBA xerogel fibers could be dissolved in 100 g toluene, but only 0.1223 g MBA xerogel fibers was dissolved in 100 g mixture of R-3240 and toluene (at a weight ratio of 1.5:100 corresponding to sample 3). The low surface energy of R-3420 decreases the solution polarity and also the solubility of MBA xerogel fibers. Thus, at higher amounts of R-3420, the solubility of MBA xerogel fibers became lower, and therefore more MBA molecules polymerized in the tubes and resulted in thicker
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Figure 5. Wall thickness of polymer microtubes obtained at different amounts of R-3420.
Figure 7. XPS pattern of sample 3.
tubes. This result implies that the wall thickness of the tubes could be regulated by adjusting the solubility of MBA xerogel fibers by the solution composition. From the above results, fluoropolymer microtubes with a rather smooth surface could be obtained in more than 70 % yield at suitable amounts of R-3429 and PET4A (samples 2 and 3), indicating the effectiveness and controllability of this fabrication process.
Table 2. Weight percentages of components of sample 3. Component Weight percentage calcu- Weight percentage calculated lated from XPS data [%] from feeding composition [%] MBA PET4A R-3420 DBTTC AIBN
33.23 5.36 48.74 6.75 5.92
45.71 0.61 45.71 6.60 1.37
Characterization of fluoropolymer microtubes To confirm the presence of fluorine atoms in the tubes, sample 3 was further characterized by TEM. The TEM image in Figure 6 a displays a single polymer microtube with a hollow structure. The EDS spectrum of a selected area of the tube (Figure 6 a) confirms the existence of fluorine atoms in the tubes (Figure 6 b). The existence of Cu signals is attributed to the copper grids used to support the sample.
Figure 6. (a) TEM image of a single tube; (b) EDS spectrum of the tube in (a).
To gain insights into the surface composition of the tubes, the XPS pattern of sample 3 was measured. As can be seen in Figure 7, sharp peaks were observed at 164, 285, 400, 531, 688, 835, and 977 eV, which are attributed to S2p, C1s, N1s, O1s, F1s, F auger and O auger, respectively. The weight percentages of the components in the sample could be calculated based on the elemental contents of the XPS measurement (Table 2), the calculation is given in the Supporting Information. For comparison, the weight percentages of the corresponding components were also calculated based on the feeding composition of the sample. From Table 2, the amount of PET4A Chem. Asian J. 2015, 10, 1363 – 1369
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units at the tube surface is much higher than the feeding amount of PET4A because of its high reaction capability. Meanwhile, both R-3420 units and MBA units are noticed at the tube surface with the content of MBA units being much lower than the feeding content of MBA xerogel fibers. Although the yield is only 71.4 %, the existence of the three monomer units at the surface provides a direct evidence that the solution polymerization products had grafted onto the tubes and integrated together to form the outer layer of the tubes. The rather high content of R-3420 units (close to 50 %) at the surface should make the sample suitable for special applications. The FT-IR spectra of samples 1, 3 and 8 were recorded and are compared in Figure S5 in the Supporting Information. In the spectra, the characteristic peaks of N¢H of amide appear at 3310, 1665, 1545 cm¢1, and the weak peaks at 3066 and 811 cm¢1 indicate the existence of residual double bonds, which should be attributed to the incomplete polymerization caused by steric hindrance. In addition, the spectra of samples 1 and 3 both show the characteristic peaks of R-3420 with CF2 at 1275 and 1250 cm¢1, and CF3 at 982 cm¢1, while these peaks are not found for sample 8. Therefore, fluorine atoms had been introduced into the polymer microtubes by using R3420 as the co-monomer. Taking the amide absorption peak at 1665 cm¢1 as an internal standard, the absorption peak of CF3 at 982 cm¢1 was compared for sample 1 and sample 3. This comparison indicates that the peak intensity of sample 3 is about 60 % larger than that of sample 1. The improved content of R-3420 in sample 3 should be attributed to PET4A, which could promote the solution polymerization and enhance the formation of the tubes.
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Full Paper Performances of fluoropolymer microtubes To understand the hydrophilicity of sample 3 (fluoropolymer microtubes) and sample 8 (common polymer microtubes), they were casted homogenously on glass plates to form coatings, and then the water contact angles on these coatings were measured (Figure 8). The water contact angles are 143.58 and
Figure 8. Water contact angle images on coatings of sample 3 (a) and sample 8 (b).
29.78, respectively, for the coatings of samples 3 and 8. The rather high water contact angle of sample 3 implies its high hydrophobicity or low hydrophilicity, which is attributed to the existence of many fluorine atoms at the tube surface. The swelling ratios of sample 3 and sample 8 in different solvents were measured next to understand their swelling capability. As can be seen in Figure 9, the swelling ratios in toluene,
Figure 10. (a) Colored toluene passing through sample 3 within 60 s; (b) Colored water isolated by sample 3 within 600 s.
The strong repulsion to water and high affinity to organic solvents of fluoropolymer microtubes should make the sample suitable to absorb organic solvents from water. A patch of sample 3 was firstly added into a vial containing water and the vial was sonicated for 1 h. The sample kept floating on the water surface (Figure 11 a) even after storage for several days,
Figure 11. (a) Sample 3 floating on the water surface for several days. (b) Colored toluene stratified above water; (c) Absorbance of toluene in (b) by a piece of sample 3; (d) Removal of saturated sample 3 shown in (c). Figure 9. Swelling ratios of sample 3 and sample 8 in different solvents.
ethanol, acetone and DMF all range between 17.0 and 22.2 for the two samples. In chloroform, the swelling ratio of sample 3 is obviously higher than that of sample 8. Because of the existence of fluorine atoms, sample 3 has a rather low swelling ratio of 1.0 in water. Without fluorine atoms, sample 8 has a rather high swelling ratio of 37.2 in water. Thus, the fluoropolymer microtubes have both high lipophilicity and low hydrophilicity. To investigate the selective adsorption capability, a suitable amount of sample 3 was filled in the middle of a pipette, and toluene (colored by SG3) and water (colored by methylene blue) were decanted into the pipette separately. Toluene could pass through the sample quickly, while water could not (Figure 10). Chem. Asian J. 2015, 10, 1363 – 1369
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which demonstrates its stable and lasting hydrophobility. If water and toluene (colored by SG3) were added into a vial, they stratified spontaneously into a two-phase system (Figure 11 b). When a piece of sample 3 was added into the vial, the upper blue toluene phase could be completely adsorbed within one second by the sample (Figure 11 c). The adsorbing sample could then be taken out as a gel by using a nipper (Figure 11 d). By contrast, sample 8 could absorb both toluene and water in the same system, suggesting its poor separation capability (see Figure S6 in the Supporting Information). From the above results we conclude that the fluoropolymer microtubes are both hydrophobic and lipophilic, which makes the sample suitable to absorb organic solvents from water.
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Full Paper Conclusions
Fabrication of fluoropolymer micro-tubes
In summary, we have found that fluoropolymer microtubes with a smooth surface could be fabricated in more than 70 % yield via RAFT co-polymerization of MBA xerogel fibers, R-3420, and PET4A. The tubular structures were confirmed by SEM and TEM. The existence of fluorine atoms in the tubes was proved by using EDS, XPS and FT-IR spectroscopies. Polymer microtubes with a smooth surface were obtained at suitable amounts of R-3420 and PET4A. The wall thickness of the tubes became thicker as more R-3420 was used. In the presence of PET4A, the solution polymerization was facilitated and more R3420 could graft onto the tubes. The water contact angle and swelling ratio measurements showed that fluoropolymer microtubes were both hydrophobic and lipophilic. We also demonstrated that the fluoropolymer microtubes could be used to adsorb toluene from water in the two-phase system. We believe that this facile preparation method could be applied to fabricate many other functional polymer tubes, considering the simple process and well-controlled tubular structures. The influence of the hollow structure and surface composition on the performance of the tubes is now under investigation.
Experimental Section Materials MBA (99 %, Alfa Aesar), R-3420 (analytical reagent, Daikin Co.), and pentaerythritol tetraacrylate (PET4A) (99 %, Sigma Aldrich) were used as received. Azobisisobutyronitrile (AIBN) (98 %, Shanghai No.4 Reagent & H. V. Chemical Co. Ltd.) was recrystallized from ethanol and dried under vacuum before use. Dibenzyl trithiocarbonate (DBTTC) was synthesized according to the literature.[18] 1,4-Bis(ptolylamino)-anthraquinone (SG3) (98 %, Tianjin Heowns Biochemical Technology Co. Ltd.) and methylene blue (98.5 %, Tianjin Guangfu Fine Chemical Research Institute) were used as received. All solvents were of analytical reagent grade and purchased from Beijing Chemical Works. The chemical structures of R-3420 and PET4A are shown below.
The preparation process of fluoropolymer microtubes was as follows. For preparing sample 3 in Table 1, 0.0130 g DBTTC, 0.0027 g AIBN, 0.0900 g R-3420, 0.0012 g PET4A, and 6.0000 g toluene were added into a vial and mixed uniformly by using ultrasound. The transparent solution was removed by a pipette and added into a vial containing 0.0900 g MBA xerogel fibers. After sealing and degassing by argon for 30 min, the vial was heated in an oil bath preheated to 82 8C. After polymerization for 48 h, the vial was cooled at room temperature and exposed to air to terminate the reaction. The resulting polymer gel was taken out and filtered. The solid product was washed with ethanol for three times and dried under vacuum to obtain fluoropolymer microtubes. The yield of product was calculated by dividing the weight of product by the total weight of monomers, AIBN and DBTTC. The compositions and the yields of the products are listed in Table 1.
Characterization of fluoropolymer micro-tubes Scanning electron microscopy (SEM) images were obtained with the products sprayed by gold on a JSM-6301 field-emission scanning electron microscope with an operating voltage of 3 kV. Transmission electron microscopy (TEM) images and elemental distributions were obtained on a JEM-2010 transmission electron microscope equipped with an EDS spectrometer; the samples were dispersed by ultrasonication in ethanol and cast onto copper grids coated with a carbon film and measurements were done at an operating voltage of 20 kV. Infrared (IR) spectra were recorded on a Nicolet 560 Fourier-transform IR (FT-IR) spectrometer. The XPS pattern was measured on an ESCALAB 250Xi X-ray photoelectron spectrometer. Water contact angles on the surfaces of the powderlike products were measured on a JC2000C1 contact angle tester.
Measurement of swelling ratios A weighted polymer sample (w1) was immersed in an excess of solvent in a vial. After 24 h storage at 25 8C, the saturated sample was taken out, dried carefully by using filter paper and weighted (w2). The swelling ratio was calculated by the following equation: swelling ratio = (w2¢w1)/w1.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 21174079 and 20874055) and the National Basic Research Program of China (2014CB932202). Keywords: co-polymerization · hydrophobicity · microtubes · RAFT
Preparation of MBA xerogel fibers MBA xerogel fibers were prepared according to the literature.[19] MBA (0.0900 g) was added to 6.0000 g chloroform in a vial. The suspension in the vial was then heated at 82 8C for 10 min to obtain a transparent solution. By cooling the vial to room temperature for about 10 min, an MBA organogel was formed. After storage of the sealed gel at room temperature for 24 h, the gel was exposed to air to volatilize the solvent slowly for another 24 h and then dried under vacuum to obtain MBA xerogel fibers. Chem. Asian J. 2015, 10, 1363 – 1369
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[1] a) T. Imae, Curr. Opin. Colloid Interface Sci. 2003, 8, 307 – 314; b) A. Delimi, E. Galopin, Y. Coffinier, M. Pisarek, R. Boukherroub, B. Talhi, S. Szunerits, Surf. Coat. Technol. 2011, 205, 4011 – 4017. [2] a) B. Am¦duri, B. Boutevin, G. Kostov, Prog. Polym. Sci. 2001, 26, 105 – 187; b) A. Hirao, K. Sugiyama, H. Yokoyama, Prog. Polym. Sci. 2007, 32, 1393 – 1438; c) K. Yang, X. Huang, Y. Huang, L. Xie, P. Jiang, Chem. Mater. 2013, 25, 2327 – 2338; d) N. S. Bhairamadgi, S. P. Pujari, C. J. M. van Rijn, H. Zuilhof, Langmuir 2014, 30, 12532 – 12540. [3] D. Chu, A. Nemoto, H. Ito, Microsyst. Technol. 2014, 20, 193 – 200. [4] A. Kitamura, T. Kobayashi, A. Suzuki, T. Terai, Surf. Coat. Technol. 2011, 206, 841 – 844. Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper [5] a) H. J. Wang, W. H. Zhou, X. F. Yin, Z. X. Zhuang, H. H. Yang, X. R. Wang, J. Am. Chem. Soc. 2006, 128, 15954 – 15955; b) E. N. Savariar, K. Krishnamoorthy, S. Thayumanavan, Nat. Nanotechnol. 2008, 3, 112 – 117. [6] M. R. Abidian, D. H. Kim, D. C. Martin, Adv. Mater. 2006, 18, 405 – 409. [7] a) S. I. Cho, S. B. Lee, Acc. Chem. Res. 2008, 41, 699 – 707; b) A. N. Aleshin, Adv. Mater. 2006, 18, 17 – 27; c) H. Chang, Y. Yuan, N. Shi, Y. Guan, Anal. Chem. 2007, 79, 5111 – 5115; d) C. Zhou, Y. Shi, J. Luo, L. Zhang, D. Xiao, J. Mater. Chem. B 2014, 2, 4122 – 4129. [8] a) S. Ko, J. Jang, ChemInform 2007, 10, 7564 – 7567; b) H. Fan, X. Yu, Y. Long, X. Zhang, H. Xiang, C. Duan, N. Zhao, X. Zhang, J. Xu, Appl. Surf. Sci. 2012, 258, 2876 – 2882; c) W. Gao, S. Sattayasamitsathit, A. Uygun, A. Pei, A. Ponedal, J. Wang, Nanoscale 2012, 4, 2447 – 2453. [9] a) C. R. Martin, L. S. Van Dyke, Z. Cai, W. Liang, J. Am. Chem. Soc. 1990, 112, 8976 – 8977; b) S. Stewart, G. Liu, Angew. Chem. Int. Ed. 2000, 39, 340 – 344; Angew. Chem. 2000, 112, 348 – 352; c) M. Steinhart, J. Wendorff, A. Greiner, R. Wehrspohn, K. Nielsch, J. Schilling, J. Choi, U. Gçsele, Science 2002, 296, 1997; d) I. W. Hamley, Soft Matter 2005, 1, 36 – 43; e) S. Bao, M. Du, M. Zhang, H. Zhu, P. Wang, T. Yang, M. Zou, Chem. Eng. J. 2014, 258, 281 – 289. [10] S. Jang, B. Kim, M. L. Geier, P. L. Prabhumirashi, M. C. Hersam, A. Dodabalapur, Appl. Phys. Lett. 2014, 105, 122107. [11] a) Q. Li, L. Tang, Y. Xia, B. Li, Macromol. Rapid Commun. 2013, 34, 185 – 189; b) Q. Li, L. Tang, Y. Liang, Chem. J. Chin. Univ. 2013, 6, 1542 – 1546; c) Q. Li, L. Tang, J. Polym. Sci. Part A 2014, 52, 1862 – 1868.
Chem. Asian J. 2015, 10, 1363 – 1369
www.chemasianj.org
[12] Q. Li, L. Tang, Y. Jiao, Acta Polym. Sin. 2014, 8, 1135 – 1142. [13] a) T. Norisuye, T. Morinaga, Q. Tran-Cong-Miyata, A. Goto, T. Fukuda, M. Shibayama, Polymer 2005, 46, 1982 – 1994; b) Q. Liu, P. Zhang, A. Qing, Y. Lan, M. Lu, Polymer 2006, 47, 2330 – 2336. [14] a) D. Roy, J. T. Guthrie, S. Perrier, Macromolecules 2005, 38, 10363 – 10372; b) G. Moad, J. Chiefari, Y. K. Chong, J. Krstina, R. T. A. Mayadunne, A. Postma, E. Rizzardo, S. H. Thang, Polym. Int. 2000, 49, 993 – 1001; c) S. Perrier, C. Barner-Kowollik, J. F. Quinn, P. Vana, T. P. Davis, Macromolecules 2002, 35, 8300 – 8306; d) C. Boyer, M. H. Stenzel, T. P. Davis, J. Polym. Sci. Part A 2011, 49, 551 – 595. [15] J. Chiefari, Y. K. Chong, F. Ercole, J. Krstina, J. Jeffery, T. P. T. Le, R. T. A. Mayadunne, G. F. Meijs, C. L. Moad, G. Moad, Macromolecules 1998, 31, 5559 – 5562. [16] N. Ide, T. Fukuda, Macromolecules 1999, 32, 95 – 99. [17] J. Rosselgong, S. P. Armes, W. R. Barton, D. Price, Macromolecules 2010, 43, 2145 – 2156. [18] N. Aoyagi, T. Endo, J. Polym. Sci. Part A 2009, 47, 3702 – 3709. [19] Y. Xia, Y. Wang, K. Chen, L. M. Tang, Chem. Commun. 2008, 5113 – 5115.
Received: January 8, 2015 Revised: March 13, 2015 Published online on April 20, 2015
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Full Paper
Fluoropolymers
Fabrication of Fluoropolymer Microtubes via RAFT Copolymerization of N,N’-Methylene Bisacrylamide Gel Fibers and Fluoromonomer Qi Li,[a, b] Yi Wang,[a] and Liming Tang*[a] Abstract: Fluoropolymer microtubes with a smooth surface were fabricated in more than 70 % yield via reversible addition fragmentation chain transfer (RAFT) co-polymerization of N,N’-methylene bisacrylamide (MBA) gel fibers as both template and monomer, 2-(perfluoro-3-methylbutyl)ethyl acrylate (R-3420) as co-monomer, and pentaerythritol tetraacrylate (PET4A) as cross-linker. The resulting fluoropolymer microtubes were characterized fully by SEM, TEM, EDS, XPS, and FT-IR. The influence of the monomer composition on the yields and morphologies of the tubes were investigated in detail. The results indicated that polymer microtubes with
Introduction Fluoropolymers have frequently been applied as protecting materials in harsh environments or as modifiers for other polymers and inorganic materials because of their many unique properties, such as high temperature stability, excellent chemical resistance, low water sorption, low dielectric constant, and so forth.[1] Fluoropolymers with specific morphologies and microstructures have recently become important because of their improved performances and many potential applications.[1b, 2] For example, Chu coated a CF2-based fluoropolymer onto micro-sized pillar-type structures and acquired superhydrophobicity for the coated surfaces.[3] Kitamura fabricated a microstructure at fluoropolymer surface using ion beam irradiation and found that the irradiated surface covered with a large number of the regularly arranged protrusions was suitable for making a cell sheet and for harvesting cells.[4] Owing to their one-dimensional hollow structures, functionalities and structural diversities, polymer nano- and microtubes [a] Dr. Q. Li, Y. Wang, Prof. L. Tang Key Laboratory of Advanced Materials of Ministry of Education of China Department of Chemical Engineering Tsinghua University, Beijing 100084 (P.R. China) E-mail: [email protected] [b] Dr. Q. Li Science and Technology on Advanced High Temperature Structural Materials Laboratory Beijing Institute of Aeronautical Materials Beijing 100095 (China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201500035. Chem. Asian J. 2015, 10, 1363 – 1369
a smooth surface were obtained at suitable amounts of R3420 and PET4A. Because of the decreased solubility of MBA gel fibers, the wall thickness increased as more R-3420 was used. In the presence of PET4A, the solution polymerization could be facilitated and more R-3420 could be attached onto the tubes based on FT-IR analysis. The water contact angle and swelling ratio measurements both revealed the low hydrophilicity and high lipophilicity of the fluoropolymer microtubes, which made the sample able to absorb toluene selectively in a water/toluene two-phase system.
have many potential applications in separation,[5] drug delivery systems,[6] electronic devices,[7] and as catalyst carriers.[8] During the past two decades, many different techniques have been developed to fabricate polymer tubes, such as the template method, self-assembling method, electrostatic spinning method and so on.[9] The coating of a fluoropolymer layer on single-walled carbon nanotube field-effect transistors was found to improve drastically the key device characteristics.[10] However, fluoropolymer nano- and microtubes either formed from fluoropolymers or polymerized from fluoromonomers have not been investigated yet. Recently, we described a novel approach for fabricating polymer microtubes, which was implemented via reversible addition fragmentation chain transfer (RAFT) polymerization of N,N’-methylene bisacrylamide (MBA) xerogel fibers.[11] Polymer microtubes with improved morphologies could be obtained by using different acrylate co-monomers and the yields became higher with increasing the functionality of the co-monomers.[12] However, the performances and functions of the resulting tubes were quite limited by their simple compositions. The formation process of polymer microtubes has been proposed by us.[11, 12] Firstly, MBA monomers on the surface of MBA gel fibers participated in RAFT polymerization and crosslinked together to form frameworks of polymer microtubes. Meanwhile, a part of MBA molecules dissolved from the fibers and co-polymerized with the co-monomer in solution to form polymer networks.[13] As the reaction progressed, more and more MBA molecules dissolved from the fibers and polymerized onto the networks. Due to the living character of RAFT polymerization,[14] the growing networks could graft gradually
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Full Paper onto the frameworks via covalent binding and integrate together to form polymer microtubes. We also demonstrated that RAFT polymerization is critical for the formation of polymer tubes,[11a, c] because the active polymer chains and the slow growth rate increased intermolecular cross-linking and the interlacing of the polymer chains to form a cross-linked polymer layer covering the template.[13, 14b, c, 15] Under identical conditions, conventional radical polymerization gave only particles due to the dead full-length polymers formed from the beginning of the reaction and the enhancement of intramolecular cross-linking.[13, 16] Co-polymerization involving fluoromonomers is an effective approach to prepare fluoropolymers. In this article, fluoropolymer microtubes were prepared via RAFT co-polymerization by using MBA xerogel fibers as a template and monomer, 2-(perfluoro-3-methylbutyl)ethyl acrylate (R-3420) as a co-monomer, and pentaerythritol tetraacrylate (PET4A) as a cross-linker. The resulting tubes were characterized by various techniques, including SEM, TEM, EDS, XPS and FT-IR. The influence of monomer composition on the yields and morphologies of the tubes were investigated in detail. Because of the existence of fluorine atoms, the fluoropolymer microtubes possess high hydrophobility and low hydrophilicity, which made them suitable to absorb organic solvents from water. Compared to the literature results, we describe a facile method to fabricate fluoropolymer microtubes, which can have improved morphologies and specific properties. To the best of our knowledge, this is the first example concerning the preparation of fluoropolymer microtubes, and our approach may be extended to the fabrication of many other functional polymer nano- and microstructures.
Results and Discussion
Table 1. Compositions and yields of polymer microtubes. Sample
R-3420 [g]
PET4A [g]
Weight of product [g]
Yield [%]
1 2 3 4 5 6 7 8 9 10 11 12
0.0900 0.0900 0.0900 0.0900 0.0900 0.0900 0.0900 0 0.0300 0.0600 0.1200 0.1500
0 0.0006 0.0012 0.0025 0.0050 0.0075 0.0100 0.0012 0.0012 0.0012 0.0012 0.0012
0.1208 0.1434 0.1405 0.1360 0.1462 0.1394 0.1612 0.0625 0.0864 0.1034 0.1323 0.1490
61.7 73.1 71.4 68.6 72.8 68.6 78.4 58.5 63.1 61.9 58.3 58.3
Conditions: MBA gel fibers, 0.0900 g; AIBN, 0.0027 g; DBTTC, 0.013 g.
Figure 1. Photos of the MBA organogel (a) and fluoropolymer gel (b).
of product reached up to 61.7 %, a value much higher than the yield (18.5 %) of sample prepared from sole MBA xerogel fibers.[11a] Figure 2 a,b shows the morphology of MBA xerogel fibers with their statistical average diameter of 2.994 mm (Supporting Information, Table S1). The SEM images of sample 1 in Fig-
Fabrication of fluoropolymer microtubes In our previous work, polymer microtubes were fabricated via RAFT polymerization of MBA xerogel fibers as a template and monomer source, and multifunctional acrylates as co-monomers in the presence of dibenzyl trithiocarbonate (DBTTC) as a RAFT reagent and azobisisobutyronitrile (AIBN) as an initiator. The acrylate co-monomers were found to promote the transformation of MBA xerogel fibers and facilitate the formation of polymer microtubes.[11a] Considering the various interesting properties of fluoropolymers, we herein attempted to fabricate fluoropolymer microtubes via RAFT co-polymerization of MBA xerogel fibers and fluoromonomer R-3420. Fluoropolymer microtubes were firstly prepared from MBA xerogel fibers and R-3420 at a weight ratio of 1:1 (Table 1, sample 1). MBA organogels formed in chloroform were used as both the template and monomer source. After immersing the MBA fibers in toluene containing R-3420 as the fluoromonomer, dibenzyl trithiocarbonate (DBTTC) as the RAFT agent, and AIBN as the initiator, the reaction solution was heated at 82 8C to initiate the RAFT polymerization. After polymerization, the MBA organogel turned into a polymer gel as seen from Figure 1. After being filtered and washed by ethanol, fluoropolymer microtubes were obtained. The yield Chem. Asian J. 2015, 10, 1363 – 1369
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Figure 2. (a,b) SEM images of MBA xerogel fibers; (c,d) SEM images of sample 1.
ure 2 c,d demonstrate that polymer microtubes with open ends were fabricated via RAFT co-polymerization of MBA xerogel fibers and R-3420. The statistical average outer diameter of the tubes is 2.868 mm (Supporting Information, Table S1), corresponding to the diameter of MBA xerogel fibers. Compared to the tubes without fluorine atoms, the fluoropolymer micro-
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Full Paper tubes have a quite smooth exterior surface (Figure 2 c,d), which should be owing to the low surface energy of fluoropolymers.[1a] Because of the low surface energy, the fluoropolymer networks preferred to spread over the tubes during grafting and integrate into a smooth surface. To promote the formation of fluoropolymer microtubes, the tetrafunctional acrylate monomer PET4A was used as a crosslinker. At a constant weight ratio of 1:1 for MBA xerogel fibers and R-3420, polymerizations at different amount of PET4A were carried out (see Table 1, samples 2–7). The yield could reach up to 73.1 % at the smallest amount of PET4A (sample 2) and changed little at higher amounts of PET4A (see also Figure S1 in the Supporting Information). The SEM images of the products (Figure 3) reveal that they are all of tubular structure with open ends. The tubes obtained at lower amounts of PET4A can maintain a rather smooth surface (Figure 3 a,b)). However, at higher amount of PET4A, some
out RAFT copolymerization at different amounts of R-3420 (Table 1, samples 3 and 8–12) in the presence of 0.0012 g PET4A. The yields ranged between 58.3 % and 71.4 % at different amounts of R-3420 (see Table 1, samples 3 and 8–12, Figure S4 in the Supporting Information). The SEM images of the resulting samples (Figure 4) show that they are all of tubular
Figure 4. SEM images of polymer microtubes obtained at different amounts of R-3420: (a) 0 g; (b) 0.0300 g; (c) 0.0600 g; (d) 0.1200 g; and (e) 0.1500 g.
Figure 3. SEM images of polymer microtubes at different amount of PET4A: (a) 0.0006 g; (b) 0.0012 g; (c) 0.0025 g; (d) 0.0050 g; (e) 0.0075 g; and (f) 0.01 g.
irregular particles are also noticed besides tubes (Figure 3 c–f)). The tube surfaces become rougher as more PET4A is used. Single polymer microtubes of samples obtained at different amounts of PET4A are shown in Figure S2 in the Supporting Information. The lower resolution image of sample 3 (see Figure S3 in the Supporting Information) shows that only polymer microtubes were obtained, with most of them having smooth surfaces and few of them having particles on the tube surfaces. At low amounts of PET4A, the solution polymerization of the monomers could be promoted because of the high functionality of PET4A. Therefore, more polymer chains could link into networks[17] to facilitate the formation of tubes. Because of the low cross-linked degree, the polymer networks were still soluble and could integrate together to achieve smooth surfaces. However, at high amounts of PET4A, the polymerization of monomers in solution resulted in the formation of rather dense networks or even insoluble particles, which are unable to integrate anymore because of their higher cross-linked degree and lower diffusion ability; accordingly, both particles and tubes with a rough surface were produced. To understand the influence of the fluoromonomer on the reaction and the morphology of the products, we also carried Chem. Asian J. 2015, 10, 1363 – 1369
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structure with open ends. In the absence of R-3420, both particles and tubes are noticed with the tubes covered by many nanosized particles (Figure 4 a, sample 8), which should be attributed to the formation of insoluble particles via polymerization of multifunctional monomers only. At low amounts of R3420 (0.0300 g, 0.0600 g and 0.0900 g, corresponding to samples 9, 10 and 3), only tubes with quite smooth surfaces are obtained and almost no particle is noticed in the samples (see Figure 4 b,c and Figure 3 b). However, at rather high amounts of R-3420 (0.1200 g and 0.1500 g), corresponding to samples 11 and 12), both particles and tubes are formed in the samples (Figure 4 d,e). As the initiator decomposed into radicals at high temperature, the radicals could initiate both solution monomers and MBA gel fibers. At the rather high concentration of R-3420, the radicals could have a higher chance to initiate solution monomers rather than MBA gel fibers. Due to the lack of sufficient reactive sites in the tubes, some of the solution polymers could not graft to the tubes but grow eventually into insoluble particles. From the insets in Figure 4, it is interesting to find that the wall thickness of the tubes is quite different for the samples. The statistical average wall thickness of a sufficient amount of tubes is plotted in Figure 5. The plot shows that the wall thickness increases gradually with increasing the amounts of R3420. This result could be explained by the solubility of MBA xerogel fibers in different reaction systems. At 82 8C, 0.2057 g MBA xerogel fibers could be dissolved in 100 g toluene, but only 0.1223 g MBA xerogel fibers was dissolved in 100 g mixture of R-3240 and toluene (at a weight ratio of 1.5:100 corresponding to sample 3). The low surface energy of R-3420 decreases the solution polarity and also the solubility of MBA xerogel fibers. Thus, at higher amounts of R-3420, the solubility of MBA xerogel fibers became lower, and therefore more MBA molecules polymerized in the tubes and resulted in thicker
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Figure 5. Wall thickness of polymer microtubes obtained at different amounts of R-3420.
Figure 7. XPS pattern of sample 3.
tubes. This result implies that the wall thickness of the tubes could be regulated by adjusting the solubility of MBA xerogel fibers by the solution composition. From the above results, fluoropolymer microtubes with a rather smooth surface could be obtained in more than 70 % yield at suitable amounts of R-3429 and PET4A (samples 2 and 3), indicating the effectiveness and controllability of this fabrication process.
Table 2. Weight percentages of components of sample 3. Component Weight percentage calcu- Weight percentage calculated lated from XPS data [%] from feeding composition [%] MBA PET4A R-3420 DBTTC AIBN
33.23 5.36 48.74 6.75 5.92
45.71 0.61 45.71 6.60 1.37
Characterization of fluoropolymer microtubes To confirm the presence of fluorine atoms in the tubes, sample 3 was further characterized by TEM. The TEM image in Figure 6 a displays a single polymer microtube with a hollow structure. The EDS spectrum of a selected area of the tube (Figure 6 a) confirms the existence of fluorine atoms in the tubes (Figure 6 b). The existence of Cu signals is attributed to the copper grids used to support the sample.
Figure 6. (a) TEM image of a single tube; (b) EDS spectrum of the tube in (a).
To gain insights into the surface composition of the tubes, the XPS pattern of sample 3 was measured. As can be seen in Figure 7, sharp peaks were observed at 164, 285, 400, 531, 688, 835, and 977 eV, which are attributed to S2p, C1s, N1s, O1s, F1s, F auger and O auger, respectively. The weight percentages of the components in the sample could be calculated based on the elemental contents of the XPS measurement (Table 2), the calculation is given in the Supporting Information. For comparison, the weight percentages of the corresponding components were also calculated based on the feeding composition of the sample. From Table 2, the amount of PET4A Chem. Asian J. 2015, 10, 1363 – 1369
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units at the tube surface is much higher than the feeding amount of PET4A because of its high reaction capability. Meanwhile, both R-3420 units and MBA units are noticed at the tube surface with the content of MBA units being much lower than the feeding content of MBA xerogel fibers. Although the yield is only 71.4 %, the existence of the three monomer units at the surface provides a direct evidence that the solution polymerization products had grafted onto the tubes and integrated together to form the outer layer of the tubes. The rather high content of R-3420 units (close to 50 %) at the surface should make the sample suitable for special applications. The FT-IR spectra of samples 1, 3 and 8 were recorded and are compared in Figure S5 in the Supporting Information. In the spectra, the characteristic peaks of N¢H of amide appear at 3310, 1665, 1545 cm¢1, and the weak peaks at 3066 and 811 cm¢1 indicate the existence of residual double bonds, which should be attributed to the incomplete polymerization caused by steric hindrance. In addition, the spectra of samples 1 and 3 both show the characteristic peaks of R-3420 with CF2 at 1275 and 1250 cm¢1, and CF3 at 982 cm¢1, while these peaks are not found for sample 8. Therefore, fluorine atoms had been introduced into the polymer microtubes by using R3420 as the co-monomer. Taking the amide absorption peak at 1665 cm¢1 as an internal standard, the absorption peak of CF3 at 982 cm¢1 was compared for sample 1 and sample 3. This comparison indicates that the peak intensity of sample 3 is about 60 % larger than that of sample 1. The improved content of R-3420 in sample 3 should be attributed to PET4A, which could promote the solution polymerization and enhance the formation of the tubes.
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Full Paper Performances of fluoropolymer microtubes To understand the hydrophilicity of sample 3 (fluoropolymer microtubes) and sample 8 (common polymer microtubes), they were casted homogenously on glass plates to form coatings, and then the water contact angles on these coatings were measured (Figure 8). The water contact angles are 143.58 and
Figure 8. Water contact angle images on coatings of sample 3 (a) and sample 8 (b).
29.78, respectively, for the coatings of samples 3 and 8. The rather high water contact angle of sample 3 implies its high hydrophobicity or low hydrophilicity, which is attributed to the existence of many fluorine atoms at the tube surface. The swelling ratios of sample 3 and sample 8 in different solvents were measured next to understand their swelling capability. As can be seen in Figure 9, the swelling ratios in toluene,
Figure 10. (a) Colored toluene passing through sample 3 within 60 s; (b) Colored water isolated by sample 3 within 600 s.
The strong repulsion to water and high affinity to organic solvents of fluoropolymer microtubes should make the sample suitable to absorb organic solvents from water. A patch of sample 3 was firstly added into a vial containing water and the vial was sonicated for 1 h. The sample kept floating on the water surface (Figure 11 a) even after storage for several days,
Figure 11. (a) Sample 3 floating on the water surface for several days. (b) Colored toluene stratified above water; (c) Absorbance of toluene in (b) by a piece of sample 3; (d) Removal of saturated sample 3 shown in (c). Figure 9. Swelling ratios of sample 3 and sample 8 in different solvents.
ethanol, acetone and DMF all range between 17.0 and 22.2 for the two samples. In chloroform, the swelling ratio of sample 3 is obviously higher than that of sample 8. Because of the existence of fluorine atoms, sample 3 has a rather low swelling ratio of 1.0 in water. Without fluorine atoms, sample 8 has a rather high swelling ratio of 37.2 in water. Thus, the fluoropolymer microtubes have both high lipophilicity and low hydrophilicity. To investigate the selective adsorption capability, a suitable amount of sample 3 was filled in the middle of a pipette, and toluene (colored by SG3) and water (colored by methylene blue) were decanted into the pipette separately. Toluene could pass through the sample quickly, while water could not (Figure 10). Chem. Asian J. 2015, 10, 1363 – 1369
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which demonstrates its stable and lasting hydrophobility. If water and toluene (colored by SG3) were added into a vial, they stratified spontaneously into a two-phase system (Figure 11 b). When a piece of sample 3 was added into the vial, the upper blue toluene phase could be completely adsorbed within one second by the sample (Figure 11 c). The adsorbing sample could then be taken out as a gel by using a nipper (Figure 11 d). By contrast, sample 8 could absorb both toluene and water in the same system, suggesting its poor separation capability (see Figure S6 in the Supporting Information). From the above results we conclude that the fluoropolymer microtubes are both hydrophobic and lipophilic, which makes the sample suitable to absorb organic solvents from water.
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Full Paper Conclusions
Fabrication of fluoropolymer micro-tubes
In summary, we have found that fluoropolymer microtubes with a smooth surface could be fabricated in more than 70 % yield via RAFT co-polymerization of MBA xerogel fibers, R-3420, and PET4A. The tubular structures were confirmed by SEM and TEM. The existence of fluorine atoms in the tubes was proved by using EDS, XPS and FT-IR spectroscopies. Polymer microtubes with a smooth surface were obtained at suitable amounts of R-3420 and PET4A. The wall thickness of the tubes became thicker as more R-3420 was used. In the presence of PET4A, the solution polymerization was facilitated and more R3420 could graft onto the tubes. The water contact angle and swelling ratio measurements showed that fluoropolymer microtubes were both hydrophobic and lipophilic. We also demonstrated that the fluoropolymer microtubes could be used to adsorb toluene from water in the two-phase system. We believe that this facile preparation method could be applied to fabricate many other functional polymer tubes, considering the simple process and well-controlled tubular structures. The influence of the hollow structure and surface composition on the performance of the tubes is now under investigation.
Experimental Section Materials MBA (99 %, Alfa Aesar), R-3420 (analytical reagent, Daikin Co.), and pentaerythritol tetraacrylate (PET4A) (99 %, Sigma Aldrich) were used as received. Azobisisobutyronitrile (AIBN) (98 %, Shanghai No.4 Reagent & H. V. Chemical Co. Ltd.) was recrystallized from ethanol and dried under vacuum before use. Dibenzyl trithiocarbonate (DBTTC) was synthesized according to the literature.[18] 1,4-Bis(ptolylamino)-anthraquinone (SG3) (98 %, Tianjin Heowns Biochemical Technology Co. Ltd.) and methylene blue (98.5 %, Tianjin Guangfu Fine Chemical Research Institute) were used as received. All solvents were of analytical reagent grade and purchased from Beijing Chemical Works. The chemical structures of R-3420 and PET4A are shown below.
The preparation process of fluoropolymer microtubes was as follows. For preparing sample 3 in Table 1, 0.0130 g DBTTC, 0.0027 g AIBN, 0.0900 g R-3420, 0.0012 g PET4A, and 6.0000 g toluene were added into a vial and mixed uniformly by using ultrasound. The transparent solution was removed by a pipette and added into a vial containing 0.0900 g MBA xerogel fibers. After sealing and degassing by argon for 30 min, the vial was heated in an oil bath preheated to 82 8C. After polymerization for 48 h, the vial was cooled at room temperature and exposed to air to terminate the reaction. The resulting polymer gel was taken out and filtered. The solid product was washed with ethanol for three times and dried under vacuum to obtain fluoropolymer microtubes. The yield of product was calculated by dividing the weight of product by the total weight of monomers, AIBN and DBTTC. The compositions and the yields of the products are listed in Table 1.
Characterization of fluoropolymer micro-tubes Scanning electron microscopy (SEM) images were obtained with the products sprayed by gold on a JSM-6301 field-emission scanning electron microscope with an operating voltage of 3 kV. Transmission electron microscopy (TEM) images and elemental distributions were obtained on a JEM-2010 transmission electron microscope equipped with an EDS spectrometer; the samples were dispersed by ultrasonication in ethanol and cast onto copper grids coated with a carbon film and measurements were done at an operating voltage of 20 kV. Infrared (IR) spectra were recorded on a Nicolet 560 Fourier-transform IR (FT-IR) spectrometer. The XPS pattern was measured on an ESCALAB 250Xi X-ray photoelectron spectrometer. Water contact angles on the surfaces of the powderlike products were measured on a JC2000C1 contact angle tester.
Measurement of swelling ratios A weighted polymer sample (w1) was immersed in an excess of solvent in a vial. After 24 h storage at 25 8C, the saturated sample was taken out, dried carefully by using filter paper and weighted (w2). The swelling ratio was calculated by the following equation: swelling ratio = (w2¢w1)/w1.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 21174079 and 20874055) and the National Basic Research Program of China (2014CB932202). Keywords: co-polymerization · hydrophobicity · microtubes · RAFT
Preparation of MBA xerogel fibers MBA xerogel fibers were prepared according to the literature.[19] MBA (0.0900 g) was added to 6.0000 g chloroform in a vial. The suspension in the vial was then heated at 82 8C for 10 min to obtain a transparent solution. By cooling the vial to room temperature for about 10 min, an MBA organogel was formed. After storage of the sealed gel at room temperature for 24 h, the gel was exposed to air to volatilize the solvent slowly for another 24 h and then dried under vacuum to obtain MBA xerogel fibers. Chem. Asian J. 2015, 10, 1363 – 1369
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[1] a) T. Imae, Curr. Opin. Colloid Interface Sci. 2003, 8, 307 – 314; b) A. Delimi, E. Galopin, Y. Coffinier, M. Pisarek, R. Boukherroub, B. Talhi, S. Szunerits, Surf. Coat. Technol. 2011, 205, 4011 – 4017. [2] a) B. Am¦duri, B. Boutevin, G. Kostov, Prog. Polym. Sci. 2001, 26, 105 – 187; b) A. Hirao, K. Sugiyama, H. Yokoyama, Prog. Polym. Sci. 2007, 32, 1393 – 1438; c) K. Yang, X. Huang, Y. Huang, L. Xie, P. Jiang, Chem. Mater. 2013, 25, 2327 – 2338; d) N. S. Bhairamadgi, S. P. Pujari, C. J. M. van Rijn, H. Zuilhof, Langmuir 2014, 30, 12532 – 12540. [3] D. Chu, A. Nemoto, H. Ito, Microsyst. Technol. 2014, 20, 193 – 200. [4] A. Kitamura, T. Kobayashi, A. Suzuki, T. Terai, Surf. Coat. Technol. 2011, 206, 841 – 844. Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper [5] a) H. J. Wang, W. H. Zhou, X. F. Yin, Z. X. Zhuang, H. H. Yang, X. R. Wang, J. Am. Chem. Soc. 2006, 128, 15954 – 15955; b) E. N. Savariar, K. Krishnamoorthy, S. Thayumanavan, Nat. Nanotechnol. 2008, 3, 112 – 117. [6] M. R. Abidian, D. H. Kim, D. C. Martin, Adv. Mater. 2006, 18, 405 – 409. [7] a) S. I. Cho, S. B. Lee, Acc. Chem. Res. 2008, 41, 699 – 707; b) A. N. Aleshin, Adv. Mater. 2006, 18, 17 – 27; c) H. Chang, Y. Yuan, N. Shi, Y. Guan, Anal. Chem. 2007, 79, 5111 – 5115; d) C. Zhou, Y. Shi, J. Luo, L. Zhang, D. Xiao, J. Mater. Chem. B 2014, 2, 4122 – 4129. [8] a) S. Ko, J. Jang, ChemInform 2007, 10, 7564 – 7567; b) H. Fan, X. Yu, Y. Long, X. Zhang, H. Xiang, C. Duan, N. Zhao, X. Zhang, J. Xu, Appl. Surf. Sci. 2012, 258, 2876 – 2882; c) W. Gao, S. Sattayasamitsathit, A. Uygun, A. Pei, A. Ponedal, J. Wang, Nanoscale 2012, 4, 2447 – 2453. [9] a) C. R. Martin, L. S. Van Dyke, Z. Cai, W. Liang, J. Am. Chem. Soc. 1990, 112, 8976 – 8977; b) S. Stewart, G. Liu, Angew. Chem. Int. Ed. 2000, 39, 340 – 344; Angew. Chem. 2000, 112, 348 – 352; c) M. Steinhart, J. Wendorff, A. Greiner, R. Wehrspohn, K. Nielsch, J. Schilling, J. Choi, U. Gçsele, Science 2002, 296, 1997; d) I. W. Hamley, Soft Matter 2005, 1, 36 – 43; e) S. Bao, M. Du, M. Zhang, H. Zhu, P. Wang, T. Yang, M. Zou, Chem. Eng. J. 2014, 258, 281 – 289. [10] S. Jang, B. Kim, M. L. Geier, P. L. Prabhumirashi, M. C. Hersam, A. Dodabalapur, Appl. Phys. Lett. 2014, 105, 122107. [11] a) Q. Li, L. Tang, Y. Xia, B. Li, Macromol. Rapid Commun. 2013, 34, 185 – 189; b) Q. Li, L. Tang, Y. Liang, Chem. J. Chin. Univ. 2013, 6, 1542 – 1546; c) Q. Li, L. Tang, J. Polym. Sci. Part A 2014, 52, 1862 – 1868.
Chem. Asian J. 2015, 10, 1363 – 1369
www.chemasianj.org
[12] Q. Li, L. Tang, Y. Jiao, Acta Polym. Sin. 2014, 8, 1135 – 1142. [13] a) T. Norisuye, T. Morinaga, Q. Tran-Cong-Miyata, A. Goto, T. Fukuda, M. Shibayama, Polymer 2005, 46, 1982 – 1994; b) Q. Liu, P. Zhang, A. Qing, Y. Lan, M. Lu, Polymer 2006, 47, 2330 – 2336. [14] a) D. Roy, J. T. Guthrie, S. Perrier, Macromolecules 2005, 38, 10363 – 10372; b) G. Moad, J. Chiefari, Y. K. Chong, J. Krstina, R. T. A. Mayadunne, A. Postma, E. Rizzardo, S. H. Thang, Polym. Int. 2000, 49, 993 – 1001; c) S. Perrier, C. Barner-Kowollik, J. F. Quinn, P. Vana, T. P. Davis, Macromolecules 2002, 35, 8300 – 8306; d) C. Boyer, M. H. Stenzel, T. P. Davis, J. Polym. Sci. Part A 2011, 49, 551 – 595. [15] J. Chiefari, Y. K. Chong, F. Ercole, J. Krstina, J. Jeffery, T. P. T. Le, R. T. A. Mayadunne, G. F. Meijs, C. L. Moad, G. Moad, Macromolecules 1998, 31, 5559 – 5562. [16] N. Ide, T. Fukuda, Macromolecules 1999, 32, 95 – 99. [17] J. Rosselgong, S. P. Armes, W. R. Barton, D. Price, Macromolecules 2010, 43, 2145 – 2156. [18] N. Aoyagi, T. Endo, J. Polym. Sci. Part A 2009, 47, 3702 – 3709. [19] Y. Xia, Y. Wang, K. Chen, L. M. Tang, Chem. Commun. 2008, 5113 – 5115.
Received: January 8, 2015 Revised: March 13, 2015 Published online on April 20, 2015
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