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Conjugated microporous polymer with film and nanotube-like morphologies
Microporous and Mesoporous Materials 176 (2013) 25–30
Contents lists available at SciVerse ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Conjugated microporous polymer with film and nanotube-like morphologies Dazhi Tan a,⇑, Wannan Xiong a, Hanxue Sun c, Zhen Zhang a, Wei Ma a, Changgong Meng a, Wenjie Fan b,⇑, An Li c,⇑ a
Experimental Center of Chemistry, Faculty of Chemical, Environmental and Biological Science and Technology, Dalian University of Technology, Dalian 116024, PR China State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China c College of Petrochemical Technology, Lanzhou University of Technology, Lanzhou 730050, PR China b
a r t i c l e
i n f o
Article history: Received 3 November 2012 Received in revised form 8 March 2013 Accepted 9 March 2013 Available online 25 March 2013 Keywords: Conjugated microporous polymers Morphology Two-dimension Organic solvents storage
a b s t r a c t Conjugated microporous polymer (CMP) composed of alternative phenylene and ethynylene units with film and nanotube-like morphologies were synthesized by 1,3,5-triethynylbenzene with 1,4-dibromobenzene and 1,3,5-tribromobenzene with 1,4-diethynylbenzene, respectively. Our work reveals that the morphology of CMP networks is greatly affected by the structure of monomers. We also investigated the wettability of the as-prepared CMP samples and their adsorption performance for organic solvents. The CMP samples show good surface superhydrophobicity and large BET surface area and pore volumes, making them the promising absorbent materials for separation and selective adsorption of organic contaminants or oil spills from water. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Covalent organic polymers are nanoporous materials which are prepared solely from light elements (for example C, H and so on) and have been attracted considerable attention for various potential applications such as adsorption and separations, environmental protection, heterogeneous catalysis, and so on [1–4]. Several different classes of porous organic networks have been synthesized so far, including crystalline covalent organic frameworks (COFs) [5,6], crystalline, covalent triazine-based frameworks (CTFs) [7,8], hypercross-linked polymers (HCPs) [9], and polymers of intrinsic microporosity (PIM) [10,11]. In addition, a series of conjugated microporous polymers (CMPs) with the rigid structure have been reported by Cooper et al. [12–14]. Their works have shown that the pore sizes for the CMP networks can be fine tuned by changing the strut length of the monomers [15], which makes the CMP promising nanoporous medium for gas adsorption by combination with their large surface areas and high thermal stability [16,17]. Son and coworkers have reported the synthesis of microporous organic nanotubes through Sonogashira coupling [18,19]. In our previous study, we found that excellent hydrogen uptake can be achieved by doping lithium ions onto the CMP [20]. In addition to gas adsorption, more
⇑ Corresponding author. Tel.: +86 411 84379233; fax: +86 411 84675584. E-mail addresses: [email protected] (D. Tan), [email protected] (W. Fan), [email protected] (A. Li). 1387-1811/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2013.03.018
recently we reported for the first time the surface superhydrophobicity and superoleophilicity of the CMP as well as their excellent adsorption performance for organic solvents [21]. Due to their excellent selective adsorption performance, fast adsorption kinetics, good working capacity and recyclable use performance, the CMP has great advantages over those traditional absorbent materials such as active carbons, which makes them ideal candidates for selective adsorption and separation of oils or non-polar organic solvents from water. To achieve better adsorption performance as well as good adsorption selectivity in organics/water systems, the porosity and morphology of the CMP are two key factors that should be taken into account since both adsorption performance and adsorption selectivity (surface superhydrophobicity and superoleophilicity) are closely relative to these two factors [21]. By altering either the monomer ratio [22] or varying the solvents in polymerization system [23], we have recently prepared several kinds of CMPs with various porous properties and morphologies (e.g. spherical, tubular and plate-like structures). In a continuation of our previous studies [21–23], in this work we show that the morphology of the CMP can also be obviously affected by the structure of monomers. By respective employing 1,3,5-triethynylbenzene with 1,4-dibromobenzene and 1,3,5-tribromobenzene with 1,4-diethynylbenzene as monomers systems, the CMP composed of alternative phenylene and ethynylene units with film and nanotube-like morphologies were successfully synthesized. The wettability and adsorption performance of the as-prepared CMPs were also systematically investigated.
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D. Tan et al. / Microporous and Mesoporous Materials 176 (2013) 25–30
2. Experimental section 2.1. Materials 1,3,5-tribromobenzene and 1,4-dibromobenzene were obtained from Aladdin, 1,4-diethynylbenzene was obtained from TCI, 1,3,5triethynylbenzene, tetrakis (triphenylphosphine) palladium and copper(I) iodide were all purchased from Alfa Aesar. All chemicals used had a purity of 97% or greater and used as received. Et3N was dried over KOH and toluene was distilled mixture with sodium metal. 2.2. Synthesis of the CMP-F and the CMP-P The CMP networks with film and nanotube structures were named as CMP-F and CMP-P, respectively. The CMP-F was synthesis via using Pd(II)/Cu(I)-catalyzed homocoupling polymerization as shown in Scheme 1. 1,3,5-triethynylbenzene (300 mg, 2 mmol) and 1,4-dibromobenzene (708 mg, 3 mmol), Tetrakis (triphenylphosphine) palladium(0) (100 mg) and copper(I) iodide (30 mg) were placed in 25 mL round-bottom flask which was degassed and nitrogen gas. Et3N (4 mL) and toluene (4 mL) were transferred to the flask by using a volumetric syringe. The mixture was heated to 80 °C and stirred for 12 h, the resulting polymer was filtered and washed with methylene dichloride, acetone, water and methanol for several times, then the polymer was further purified by Soxhlet extraction (methanol) for 3 days. The resulting product was dried at 100 °C for 24 h to a constant weight. The CMP-P was synthesized by the same method as CMP-F. 1,3,5tribromobenzene (630 mg, 2 mmol) and 1,4-diethynylbenzene (378 mg, 3 mmol), Tetrakis (triphenylphosphine) palladium(0) (100 mg) and copper(I) iodide (30 mg) were used in this case. 2.3. Characterization Fourier transform infrared spectroscopy (FTIR) spectra were recorded in the range of 4000–400 cm1 using the KBr pellet technique on Nicolet 6700 spectrum instrument. Solid-state 13C NMR
spectra were measured at the molecular level on an Avance Digital 500 MHz spectrometer. High-resolution imaging of the polymer morphology was achieved using a QUANTA 200FEG scanning electron microscope (FE-SEM) and Tecnai G2 Spirit Twin transmission electron microscope (TEM, FEI Company, USA). Polymer’s surface areas and pore size distributions were measured by nitrogen adsorption and desorption at 77.3 K using the ASAP 2020 volumetric adsorption analyzer. Water contact angle measurements for samples were performed on a contact angle meter (DSA100, Kruss Company, German), which was conducted by coating the CMP microgel particles on PDMS films which gives a macroscopically smooth surface for contact angle measurement. Organic solvent adsorption analyses were investigated by weight measurement. In the procedure, a weighted quantity of dry sample was immersed in an organic solvent, after adsorbing the organic solvent, the wet sample was separated from the solvent by filtering and weighing [21,24]. The weight gain of the samples was calculated using Eq. (1):
Weight gain ¼ ðW wet W dry Þ=W dry ðwt=wtÞ100%
ð1Þ
where Wdry and Wwet are the weights of the dried samples and the wet samples, respectively [21–23]. 3. Results and discussion 3.1. NMR and FTIR spectra CMPs are typically prepared by Sonogashira–Hagihara metalcatalyzed cross-coupling (or homocoupling) reactions. The molecular level structures of CMPs were investigated by 13C solid-state NMR measurements, and Fig. 1 shows the NMR spectra of CMP-F and CMP-P. We found from Fig. 1 that the NMR for CMP-F and CMP-P is the same, which demonstrates the same kinds of carbons in the polymers. The NMR results indicated that there are mainly there kinds of carbons in the networks. The observed appearance of resonance at d = 91 ppm can be ascribed to the AC„CA sites. The resonance at d = 132 ppm corresponds to sp2 carbons in the benzene rings (CAr–H), and 124 ppm is assigned to the sp2 benzene connect with alkynylene. The NMR spectra confirm the Sonogash-
Scheme 1. Route to conjugated organic polymers, showing a 2D network structure.
D. Tan et al. / Microporous and Mesoporous Materials 176 (2013) 25–30
Fig. 1. 13C solid-state NMR of the CMP samples, asterisks denote spinning sidebands.
ira–Hagihara reaction mechanism for the CMP networks we synthesized. FTIR spectra for the CMP samples are shown in Fig. 2. The infrared spectra for CMP-F and CMP-P are the same and show three characteristic adsorption regions: a first absorption band in the 1400–1650 cm1 regions and a second peak close to 3050 cm1, which are assigned to benzene stretch and the AArAH stretching respectively; a third peak close to 2200–2350 cm1, which corresponds to AC„CA sites stretching. The infrared and the NMR spectra of these two samples are quite similar and therefore demonstrate the structural similarity. 3.2. SEM and TEM observations SEM for CMP samples also provides useful morphological information to explain the observed porosity. The FE-SEM images show the morphologies of CMP-F and CMP-P networks, as shown in Fig. 3. The CMP-F powder displays uniform plate-shaped morphology, with a diameter of about dozens of micrometer. While the CMP-P powder reveals relatively hollow tube-like morphology, and it can be found that little spherical solid particles are attached to the pipes. The SEM images demonstrate that the morphologies
Fig. 2. FTIR spectra of the CMP samples.
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of CMP networks synthesized are affected by the structure of monomers. TEM analysis gave further information of the CMPs morphologies, as shown in Fig. 4. The surface morphology of the CMP-F is quite different from that of CMP-P. Similar to SEM, the TEM images of CMP-F consists of planar structures and show platelet features; meanwhile CMP-P presents hollow tubular structure. In most cases, the CMP networks were synthesized under kinetic control, so it is difficult to obtain such unique structures in irreversible polymerization systems. In our previous study, we reported that the microstructure and morphology of CMP networks are greatly affected by the choice of polymerization solvent, and spherical, tubular and plate-like structures were received during the polymerization by using the different solvents [23]. In this work, based on the results of SEM and TEM analysis, it is clearly that the morphology of the CMP are also be obviously affected by the structure of monomers, which would be a better examples to show how to use diversity in monomer structure as a means to control the morphology and physical properties of CMP networks. In particular, the CMP-F shows an interesting planar shape, similar to that of graphene. We suggest that it consists of 2D microstructures composed of 1,3,5-substituted benzene nodes connected by struts containing one phenylene moiety and two ethynylene unit [25,26], which we will discuss later (see Fig. 5). There are two broad schemes for explaining morphology in amorphous polymers. Firstly, the CMP networks with same structure are very similar to some shape-persistent macrocycles. The 2D plate-like networks are thermodynamically unstable, and the monofilm is easy to curl. Secondly, the reagents 1,3,5-triethynylbenzene or 1,4-diethynylbenzene possess greater dimension than bromobenzene, and tend to prevent the laminated polymers from sphering. When the reactants polymerize and curl along the 1,3,5triethynylbenzene or 1,4-diethynylbenzene molecules, plate-like or hollow tube-like structures will form easily. The morphologies of the two CMPs demonstrate that the structures of monomers may play an inductive effect in the formation of 2D structures polymer. By respective employing 1,3,5-triethynylbenzene with 1,4-dibromobenzene and 1,3,5-tribromobenzene with 1,4-diethynylbenzene as monomers systems, the CMP composed of alternative phenylene and ethynylene units with film and nanotube-like morphologies were successfully synthesized. 3.3. Theoretical simulations of the CMP structures To explore the possible structures of the CMPs we synthesized at molecular level, we also performed theoretical simulations. The interlayer interactions in CMPs networks might be a proto-typical weakly interacting complex that involves p bonding. We used a computationally efficient approximation to density functional theory (DFT), the self-consistent charge density functional tightbinding (SCC-DFTB) scheme, complemented by the empirical London dispersion energy term (acronym DFTB-D) [27,28] to study the energy and geometry structure of CMPs we synthesized. We built a model to simulate the CMPs structure including two layers, with each layer having seven rings. There are 1344 atoms in total in the model. The full geometric optimizations were performed using the conjugate gradient algorithm until the residual forces were below 1 103 au; we also set the charge convergence criterion to 1 104 electrons. Fig. 5 shows the energetically optimal structure of the CMP model. We found that each single layer of the CMPs maintains a planar structure, and the two layers stacked in a typical staggered mode. The interlayer spacing is around 3.4 Å, characteristic for p–p stacked systems. For CMP-P, it would be natural to assume that it may also be composed of such 2D planar layers which are rolled-up into a tubular form, just as shown in TEM images (Fig. 4).
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Fig. 3. SEM images with a scale bar of 100 lm for (a) CMP-F, (b) CMP-P.
Fig. 4. TEM images for (a) CMP-F, (b) CMP-P.
Fig. 5. (a) Nitrogen adsorption (filled symbols) and desorption (empty symbols) isotherms of the CMP networks at 77.3 K, (b) pore size histograms for CMPs calculated after NLDFT models to adsorption data.
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3.4. Gas adsorption The pore structures of these CMP networks were investigated by N2 adsorption and desorption isotherms at 77.3 K, as shown in Fig. 6(a). Samples were degassed at 120 °C for 3 h under vacuum (105 bar) before analysis. According to the IUPAC classification, all these polymers gave rise to Type I nitrogen sorption isotherms with H4 hysteresis loops. A comparison of the BET surface areas and t-method micropore volume for the polymer networks is shown in Table 1. The surface areas of the CMP-F and CMP-P were calculated in the relative pressure (P/P0) range from 0.05 to 0.20, which results in apparent surface areas of 796 and 558 m2/g, respectively. The micropore surface areas derived using the t-plot method were 446 and 305 m2/g for CMP-F and CMP-P, respectively. The pore volumes, estimated from the amount of gas adsorbed at P/P0 = 0.99, were 0.456 and 0.323 cm3/g, while the micropore volumes derived from the t-plot method were 0.200 and 0.138 cm3/g, for CMP-F and CMP-P, respectively. Microporous pore volume distributions are calculated by NLDFT using carbon slit pores, as shown in Fig. 6(b). The peak is between 11 and 20 Å, which correspond to the shape-persistent macrocycles and the layer-by-layer structure. The structure of CMPs can be derived from graphite by replacing each C atom with a phenylene unit and each CAC bond by one phenylene and two ethynylene cross-links unit. 3.5. Organic solvents adsorption Recently, the creation and utilization of the microporous absorbents with surface superhydrophobicity for direct separation or selective adsorption of oils or hydrophobic organic solvents from water have generated extensive research interest, owing to the global scale of severe water pollution arising from oil spills and industrial organic pollutants [24,29]. Previously, we have reported the
surface superhydrophobicity of the CMPs and their excellent adsorption performance for organic solvents [21–23]. Motivated by our previous works, in this study we also investigated the wettability and adsorption performance of the as-prepared CMP-F and CMP-P since they have similar microporous structures and chemical compositions to the CMPs we reported previously. Fig. 7 shows the water contact angle (CA) measurements for these two samples. As expected, the CMP-F and the CMP-P exhibit surface superhydrophobicity with a water CA of 156° and 159°, respectively, which means that oils or non-polar organic solvents can be easily removed from water by the CMPs without adsorption of water, making them the promising absorbent materials for separation and selective adsorption of organic contaminants or oil spills from water. In addition, based on their high surface area and microporous characters, it is suggested that CMP-F and the CMP-P should also be ideal candidates as a porous media for gas adsorption, for example hydrogen storage and CO2 capture. The work in this direction is currently underway. In this study, we also investigated the adsorption capacity for organic solvents of these CMP networks, and various non-polar organic solvents were employed as shown in Fig. 8. Both the CMPs show good absorbencies with high surface and pore volume. The flexible hyper-cross-linked polymers display no significant swelling and good adsorption capacities in the range from 400 wt.% to 1200 wt.%, with maximum weight gain up to 12 times its weight. For CMP-F, the larger total pore volume of CMPs should lead to an increase in their adsorption abilities for organic solvents. We also found that the adsorption abilities of CMP-F and CMP-P are slightly lower than that of the CMP networks we reported previously [21–23], which might be related to the relatively compact
Fig. 7. Measurement of contact angle with water for the CMP-F (a) and CMP-P (b).
Fig. 6. The modeled structure of the CMPs carried out by SCC-DFTB.
Fig. 8. The weight gain of CMP samples for various organic solvents.
Table 1 The BET surface area, pore volume and t-method micropore volume physicochemical properties for CMP-F and CMP-P networks.
CMP-F CMPP
MultiPoint BET surface (m2/g)
t-Method micropore surface area (m2/g)
Total pore volume (cc/g)
t-Method micropore volume (cc/g)
Contact angle (°)
796 558
446 305
0.456 0.323
0.200 0.138
156 159
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microstructure and weaker swelling effect. The CMP-P with hollow tube structure demonstrates also showed good adsorption performance. Our results demonstrated that the microstructure and morphologies of CMP networks affect the adsorption performance for organic solvents moderately. 4. Conclusion 2D polymer structures were prepared by the cross-coupling polymerization of 1,3,5-triethynylbenzene with 1,4-dibromobenzene and 1,3,5-tribromobenzene with 1,4-diethynylbenzene, respectively. The networks polymerized alone 1,3,5-triethynylbenzene or 1,4-diethynylbenzene, and then polymer with film and nanotube-like morphologies were received. Our work shows that the morphology of CMP networks is great affected by the structure of monomers, which was confirmed by SEM and TEM studies combination with DFT simulations. Furthermore these CMP samples show good surface superhydrophobicity and large BET surface area and pore volumes, making them the promising absorbent materials for separation and selective adsorption of organic contaminants or oil spills from water. Acknowledgements This work was supported by the National High Technology 863 Program of China under (Grant No. 2012AA06A115), the Natural Science Foundation of China, China (Grant No. 51263012) and the Natural Science Foundation of Gansu Province, China (Grant No. 1107RJZA177). References [1] [2] [3] [4]
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Contents lists available at SciVerse ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Conjugated microporous polymer with film and nanotube-like morphologies Dazhi Tan a,⇑, Wannan Xiong a, Hanxue Sun c, Zhen Zhang a, Wei Ma a, Changgong Meng a, Wenjie Fan b,⇑, An Li c,⇑ a
Experimental Center of Chemistry, Faculty of Chemical, Environmental and Biological Science and Technology, Dalian University of Technology, Dalian 116024, PR China State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China c College of Petrochemical Technology, Lanzhou University of Technology, Lanzhou 730050, PR China b
a r t i c l e
i n f o
Article history: Received 3 November 2012 Received in revised form 8 March 2013 Accepted 9 March 2013 Available online 25 March 2013 Keywords: Conjugated microporous polymers Morphology Two-dimension Organic solvents storage
a b s t r a c t Conjugated microporous polymer (CMP) composed of alternative phenylene and ethynylene units with film and nanotube-like morphologies were synthesized by 1,3,5-triethynylbenzene with 1,4-dibromobenzene and 1,3,5-tribromobenzene with 1,4-diethynylbenzene, respectively. Our work reveals that the morphology of CMP networks is greatly affected by the structure of monomers. We also investigated the wettability of the as-prepared CMP samples and their adsorption performance for organic solvents. The CMP samples show good surface superhydrophobicity and large BET surface area and pore volumes, making them the promising absorbent materials for separation and selective adsorption of organic contaminants or oil spills from water. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Covalent organic polymers are nanoporous materials which are prepared solely from light elements (for example C, H and so on) and have been attracted considerable attention for various potential applications such as adsorption and separations, environmental protection, heterogeneous catalysis, and so on [1–4]. Several different classes of porous organic networks have been synthesized so far, including crystalline covalent organic frameworks (COFs) [5,6], crystalline, covalent triazine-based frameworks (CTFs) [7,8], hypercross-linked polymers (HCPs) [9], and polymers of intrinsic microporosity (PIM) [10,11]. In addition, a series of conjugated microporous polymers (CMPs) with the rigid structure have been reported by Cooper et al. [12–14]. Their works have shown that the pore sizes for the CMP networks can be fine tuned by changing the strut length of the monomers [15], which makes the CMP promising nanoporous medium for gas adsorption by combination with their large surface areas and high thermal stability [16,17]. Son and coworkers have reported the synthesis of microporous organic nanotubes through Sonogashira coupling [18,19]. In our previous study, we found that excellent hydrogen uptake can be achieved by doping lithium ions onto the CMP [20]. In addition to gas adsorption, more
⇑ Corresponding author. Tel.: +86 411 84379233; fax: +86 411 84675584. E-mail addresses: [email protected] (D. Tan), [email protected] (W. Fan), [email protected] (A. Li). 1387-1811/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2013.03.018
recently we reported for the first time the surface superhydrophobicity and superoleophilicity of the CMP as well as their excellent adsorption performance for organic solvents [21]. Due to their excellent selective adsorption performance, fast adsorption kinetics, good working capacity and recyclable use performance, the CMP has great advantages over those traditional absorbent materials such as active carbons, which makes them ideal candidates for selective adsorption and separation of oils or non-polar organic solvents from water. To achieve better adsorption performance as well as good adsorption selectivity in organics/water systems, the porosity and morphology of the CMP are two key factors that should be taken into account since both adsorption performance and adsorption selectivity (surface superhydrophobicity and superoleophilicity) are closely relative to these two factors [21]. By altering either the monomer ratio [22] or varying the solvents in polymerization system [23], we have recently prepared several kinds of CMPs with various porous properties and morphologies (e.g. spherical, tubular and plate-like structures). In a continuation of our previous studies [21–23], in this work we show that the morphology of the CMP can also be obviously affected by the structure of monomers. By respective employing 1,3,5-triethynylbenzene with 1,4-dibromobenzene and 1,3,5-tribromobenzene with 1,4-diethynylbenzene as monomers systems, the CMP composed of alternative phenylene and ethynylene units with film and nanotube-like morphologies were successfully synthesized. The wettability and adsorption performance of the as-prepared CMPs were also systematically investigated.
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2. Experimental section 2.1. Materials 1,3,5-tribromobenzene and 1,4-dibromobenzene were obtained from Aladdin, 1,4-diethynylbenzene was obtained from TCI, 1,3,5triethynylbenzene, tetrakis (triphenylphosphine) palladium and copper(I) iodide were all purchased from Alfa Aesar. All chemicals used had a purity of 97% or greater and used as received. Et3N was dried over KOH and toluene was distilled mixture with sodium metal. 2.2. Synthesis of the CMP-F and the CMP-P The CMP networks with film and nanotube structures were named as CMP-F and CMP-P, respectively. The CMP-F was synthesis via using Pd(II)/Cu(I)-catalyzed homocoupling polymerization as shown in Scheme 1. 1,3,5-triethynylbenzene (300 mg, 2 mmol) and 1,4-dibromobenzene (708 mg, 3 mmol), Tetrakis (triphenylphosphine) palladium(0) (100 mg) and copper(I) iodide (30 mg) were placed in 25 mL round-bottom flask which was degassed and nitrogen gas. Et3N (4 mL) and toluene (4 mL) were transferred to the flask by using a volumetric syringe. The mixture was heated to 80 °C and stirred for 12 h, the resulting polymer was filtered and washed with methylene dichloride, acetone, water and methanol for several times, then the polymer was further purified by Soxhlet extraction (methanol) for 3 days. The resulting product was dried at 100 °C for 24 h to a constant weight. The CMP-P was synthesized by the same method as CMP-F. 1,3,5tribromobenzene (630 mg, 2 mmol) and 1,4-diethynylbenzene (378 mg, 3 mmol), Tetrakis (triphenylphosphine) palladium(0) (100 mg) and copper(I) iodide (30 mg) were used in this case. 2.3. Characterization Fourier transform infrared spectroscopy (FTIR) spectra were recorded in the range of 4000–400 cm1 using the KBr pellet technique on Nicolet 6700 spectrum instrument. Solid-state 13C NMR
spectra were measured at the molecular level on an Avance Digital 500 MHz spectrometer. High-resolution imaging of the polymer morphology was achieved using a QUANTA 200FEG scanning electron microscope (FE-SEM) and Tecnai G2 Spirit Twin transmission electron microscope (TEM, FEI Company, USA). Polymer’s surface areas and pore size distributions were measured by nitrogen adsorption and desorption at 77.3 K using the ASAP 2020 volumetric adsorption analyzer. Water contact angle measurements for samples were performed on a contact angle meter (DSA100, Kruss Company, German), which was conducted by coating the CMP microgel particles on PDMS films which gives a macroscopically smooth surface for contact angle measurement. Organic solvent adsorption analyses were investigated by weight measurement. In the procedure, a weighted quantity of dry sample was immersed in an organic solvent, after adsorbing the organic solvent, the wet sample was separated from the solvent by filtering and weighing [21,24]. The weight gain of the samples was calculated using Eq. (1):
Weight gain ¼ ðW wet W dry Þ=W dry ðwt=wtÞ100%
ð1Þ
where Wdry and Wwet are the weights of the dried samples and the wet samples, respectively [21–23]. 3. Results and discussion 3.1. NMR and FTIR spectra CMPs are typically prepared by Sonogashira–Hagihara metalcatalyzed cross-coupling (or homocoupling) reactions. The molecular level structures of CMPs were investigated by 13C solid-state NMR measurements, and Fig. 1 shows the NMR spectra of CMP-F and CMP-P. We found from Fig. 1 that the NMR for CMP-F and CMP-P is the same, which demonstrates the same kinds of carbons in the polymers. The NMR results indicated that there are mainly there kinds of carbons in the networks. The observed appearance of resonance at d = 91 ppm can be ascribed to the AC„CA sites. The resonance at d = 132 ppm corresponds to sp2 carbons in the benzene rings (CAr–H), and 124 ppm is assigned to the sp2 benzene connect with alkynylene. The NMR spectra confirm the Sonogash-
Scheme 1. Route to conjugated organic polymers, showing a 2D network structure.
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Fig. 1. 13C solid-state NMR of the CMP samples, asterisks denote spinning sidebands.
ira–Hagihara reaction mechanism for the CMP networks we synthesized. FTIR spectra for the CMP samples are shown in Fig. 2. The infrared spectra for CMP-F and CMP-P are the same and show three characteristic adsorption regions: a first absorption band in the 1400–1650 cm1 regions and a second peak close to 3050 cm1, which are assigned to benzene stretch and the AArAH stretching respectively; a third peak close to 2200–2350 cm1, which corresponds to AC„CA sites stretching. The infrared and the NMR spectra of these two samples are quite similar and therefore demonstrate the structural similarity. 3.2. SEM and TEM observations SEM for CMP samples also provides useful morphological information to explain the observed porosity. The FE-SEM images show the morphologies of CMP-F and CMP-P networks, as shown in Fig. 3. The CMP-F powder displays uniform plate-shaped morphology, with a diameter of about dozens of micrometer. While the CMP-P powder reveals relatively hollow tube-like morphology, and it can be found that little spherical solid particles are attached to the pipes. The SEM images demonstrate that the morphologies
Fig. 2. FTIR spectra of the CMP samples.
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of CMP networks synthesized are affected by the structure of monomers. TEM analysis gave further information of the CMPs morphologies, as shown in Fig. 4. The surface morphology of the CMP-F is quite different from that of CMP-P. Similar to SEM, the TEM images of CMP-F consists of planar structures and show platelet features; meanwhile CMP-P presents hollow tubular structure. In most cases, the CMP networks were synthesized under kinetic control, so it is difficult to obtain such unique structures in irreversible polymerization systems. In our previous study, we reported that the microstructure and morphology of CMP networks are greatly affected by the choice of polymerization solvent, and spherical, tubular and plate-like structures were received during the polymerization by using the different solvents [23]. In this work, based on the results of SEM and TEM analysis, it is clearly that the morphology of the CMP are also be obviously affected by the structure of monomers, which would be a better examples to show how to use diversity in monomer structure as a means to control the morphology and physical properties of CMP networks. In particular, the CMP-F shows an interesting planar shape, similar to that of graphene. We suggest that it consists of 2D microstructures composed of 1,3,5-substituted benzene nodes connected by struts containing one phenylene moiety and two ethynylene unit [25,26], which we will discuss later (see Fig. 5). There are two broad schemes for explaining morphology in amorphous polymers. Firstly, the CMP networks with same structure are very similar to some shape-persistent macrocycles. The 2D plate-like networks are thermodynamically unstable, and the monofilm is easy to curl. Secondly, the reagents 1,3,5-triethynylbenzene or 1,4-diethynylbenzene possess greater dimension than bromobenzene, and tend to prevent the laminated polymers from sphering. When the reactants polymerize and curl along the 1,3,5triethynylbenzene or 1,4-diethynylbenzene molecules, plate-like or hollow tube-like structures will form easily. The morphologies of the two CMPs demonstrate that the structures of monomers may play an inductive effect in the formation of 2D structures polymer. By respective employing 1,3,5-triethynylbenzene with 1,4-dibromobenzene and 1,3,5-tribromobenzene with 1,4-diethynylbenzene as monomers systems, the CMP composed of alternative phenylene and ethynylene units with film and nanotube-like morphologies were successfully synthesized. 3.3. Theoretical simulations of the CMP structures To explore the possible structures of the CMPs we synthesized at molecular level, we also performed theoretical simulations. The interlayer interactions in CMPs networks might be a proto-typical weakly interacting complex that involves p bonding. We used a computationally efficient approximation to density functional theory (DFT), the self-consistent charge density functional tightbinding (SCC-DFTB) scheme, complemented by the empirical London dispersion energy term (acronym DFTB-D) [27,28] to study the energy and geometry structure of CMPs we synthesized. We built a model to simulate the CMPs structure including two layers, with each layer having seven rings. There are 1344 atoms in total in the model. The full geometric optimizations were performed using the conjugate gradient algorithm until the residual forces were below 1 103 au; we also set the charge convergence criterion to 1 104 electrons. Fig. 5 shows the energetically optimal structure of the CMP model. We found that each single layer of the CMPs maintains a planar structure, and the two layers stacked in a typical staggered mode. The interlayer spacing is around 3.4 Å, characteristic for p–p stacked systems. For CMP-P, it would be natural to assume that it may also be composed of such 2D planar layers which are rolled-up into a tubular form, just as shown in TEM images (Fig. 4).
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Fig. 3. SEM images with a scale bar of 100 lm for (a) CMP-F, (b) CMP-P.
Fig. 4. TEM images for (a) CMP-F, (b) CMP-P.
Fig. 5. (a) Nitrogen adsorption (filled symbols) and desorption (empty symbols) isotherms of the CMP networks at 77.3 K, (b) pore size histograms for CMPs calculated after NLDFT models to adsorption data.
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3.4. Gas adsorption The pore structures of these CMP networks were investigated by N2 adsorption and desorption isotherms at 77.3 K, as shown in Fig. 6(a). Samples were degassed at 120 °C for 3 h under vacuum (105 bar) before analysis. According to the IUPAC classification, all these polymers gave rise to Type I nitrogen sorption isotherms with H4 hysteresis loops. A comparison of the BET surface areas and t-method micropore volume for the polymer networks is shown in Table 1. The surface areas of the CMP-F and CMP-P were calculated in the relative pressure (P/P0) range from 0.05 to 0.20, which results in apparent surface areas of 796 and 558 m2/g, respectively. The micropore surface areas derived using the t-plot method were 446 and 305 m2/g for CMP-F and CMP-P, respectively. The pore volumes, estimated from the amount of gas adsorbed at P/P0 = 0.99, were 0.456 and 0.323 cm3/g, while the micropore volumes derived from the t-plot method were 0.200 and 0.138 cm3/g, for CMP-F and CMP-P, respectively. Microporous pore volume distributions are calculated by NLDFT using carbon slit pores, as shown in Fig. 6(b). The peak is between 11 and 20 Å, which correspond to the shape-persistent macrocycles and the layer-by-layer structure. The structure of CMPs can be derived from graphite by replacing each C atom with a phenylene unit and each CAC bond by one phenylene and two ethynylene cross-links unit. 3.5. Organic solvents adsorption Recently, the creation and utilization of the microporous absorbents with surface superhydrophobicity for direct separation or selective adsorption of oils or hydrophobic organic solvents from water have generated extensive research interest, owing to the global scale of severe water pollution arising from oil spills and industrial organic pollutants [24,29]. Previously, we have reported the
surface superhydrophobicity of the CMPs and their excellent adsorption performance for organic solvents [21–23]. Motivated by our previous works, in this study we also investigated the wettability and adsorption performance of the as-prepared CMP-F and CMP-P since they have similar microporous structures and chemical compositions to the CMPs we reported previously. Fig. 7 shows the water contact angle (CA) measurements for these two samples. As expected, the CMP-F and the CMP-P exhibit surface superhydrophobicity with a water CA of 156° and 159°, respectively, which means that oils or non-polar organic solvents can be easily removed from water by the CMPs without adsorption of water, making them the promising absorbent materials for separation and selective adsorption of organic contaminants or oil spills from water. In addition, based on their high surface area and microporous characters, it is suggested that CMP-F and the CMP-P should also be ideal candidates as a porous media for gas adsorption, for example hydrogen storage and CO2 capture. The work in this direction is currently underway. In this study, we also investigated the adsorption capacity for organic solvents of these CMP networks, and various non-polar organic solvents were employed as shown in Fig. 8. Both the CMPs show good absorbencies with high surface and pore volume. The flexible hyper-cross-linked polymers display no significant swelling and good adsorption capacities in the range from 400 wt.% to 1200 wt.%, with maximum weight gain up to 12 times its weight. For CMP-F, the larger total pore volume of CMPs should lead to an increase in their adsorption abilities for organic solvents. We also found that the adsorption abilities of CMP-F and CMP-P are slightly lower than that of the CMP networks we reported previously [21–23], which might be related to the relatively compact
Fig. 7. Measurement of contact angle with water for the CMP-F (a) and CMP-P (b).
Fig. 6. The modeled structure of the CMPs carried out by SCC-DFTB.
Fig. 8. The weight gain of CMP samples for various organic solvents.
Table 1 The BET surface area, pore volume and t-method micropore volume physicochemical properties for CMP-F and CMP-P networks.
CMP-F CMPP
MultiPoint BET surface (m2/g)
t-Method micropore surface area (m2/g)
Total pore volume (cc/g)
t-Method micropore volume (cc/g)
Contact angle (°)
796 558
446 305
0.456 0.323
0.200 0.138
156 159
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microstructure and weaker swelling effect. The CMP-P with hollow tube structure demonstrates also showed good adsorption performance. Our results demonstrated that the microstructure and morphologies of CMP networks affect the adsorption performance for organic solvents moderately. 4. Conclusion 2D polymer structures were prepared by the cross-coupling polymerization of 1,3,5-triethynylbenzene with 1,4-dibromobenzene and 1,3,5-tribromobenzene with 1,4-diethynylbenzene, respectively. The networks polymerized alone 1,3,5-triethynylbenzene or 1,4-diethynylbenzene, and then polymer with film and nanotube-like morphologies were received. Our work shows that the morphology of CMP networks is great affected by the structure of monomers, which was confirmed by SEM and TEM studies combination with DFT simulations. Furthermore these CMP samples show good surface superhydrophobicity and large BET surface area and pore volumes, making them the promising absorbent materials for separation and selective adsorption of organic contaminants or oil spills from water. Acknowledgements This work was supported by the National High Technology 863 Program of China under (Grant No. 2012AA06A115), the Natural Science Foundation of China, China (Grant No. 51263012) and the Natural Science Foundation of Gansu Province, China (Grant No. 1107RJZA177). References [1] [2] [3] [4]
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