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Developing chitosan-based composite nanofibers for supporting metal catalysts
Polymer 75 (2015) 168e177
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Developing chitosan-based composite nanofibers for supporting metal catalysts Linjun Shao a, Yuan Ren b, Zining Wang a, Chenze Qi a, **, Yao Lin b, * a b
Zhejiang Key Laboratory of Alternative Technologies for Fine Chemicals Process, Shaoxing University, Zhejiang Province, 312000, PR China Polymer Program, Institute of Materials Science & Department of Chemistry, University of Connecticut, Storrs, CT 06269, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 June 2015 Received in revised form 5 August 2015 Accepted 15 August 2015 Available online 19 August 2015
Using electrospinning technique to prepare chitosan (CS) nanofibers from an inexpensive solvent such as acetic acid has been a challenge, due to the lack of sufficient entanglements in these semi-rigid polyelectrolytes. Incorporating polymers such as poly(methacrylic acid) (PMAA) into the solution lowers the entanglement concentration of the polymers, and make all three morphology regimes (polymer droplets, beaded nanofibers and nanofibers with uniform thickness) accessible to electrospinning method. Uniform composite nanofibers are found at concentrations well above the entanglement concentration. The thickness and morphology of nanofibers can be regulated by addition of small amount of organic solvents such as dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). The composite nanofibers are subsequently crosslinked at an elevated temperature to improve their thermal and solvent stability. The mechanical properties of the crosslinked CS/PMAA fibers can be modulated by varying the amount and molecular weight of PMAA incorporated in the blend. The composite fiber materials provide a useful platform to support a variety of catalysts. As a demonstration, palladium catalyst has been immobilized on the crosslinked CS/PMAA nanofibers to carry out MizorokieHeck cross-coupling reactions of aromatic halides and acrylates. We found remarkable catalytic efficiency and stability in these materials. We also demonstrate that, by the “post-modification” of the nanofibers with ligands that chelate metal catalysts, a variety of metal catalysts can be incorporated into the fiber platform, with further improved stability and activity. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Chitosan Poly(methacrylic acid) Chain entanglement Electrospinning Composite nanofiber Palladium Catalysis
1. Introduction Chitosan, a derivative of chitin, is a versatile, low-cost biopolymer that has found applications in food, cosmetics, drug delivery and tissue engineering [1,2]. Recently, there has been increasing interest in the use of chitosan nanofibers (e.g., by electrospinning) as solid supports for immobilization of catalysts or enzymes, to take advantage of the metal-chelating and complexation properties of chitosan and the large accessible surface area of nanofibers [3e6]. In order to make continuous nanofibers, trifluoroacetic acid (TFA) was often used as the dispersion solvent for chitosan in the electrospinning process, but TFA is both expensive and toxic [7e9]. To prepare chitosan nanofibers in inexpensive and more environmentally friendly solvents, new methods have been
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (C. Qi), [email protected] (Y. Lin). http://dx.doi.org/10.1016/j.polymer.2015.08.031 0032-3861/© 2015 Elsevier Ltd. All rights reserved.
explored, e.g., by the addition of a second polymer as the cospinning agent with chitosan [10e12]. However, a general approach to obtain composite chitosan nanofibers with controlled morphology, based on mechanistic understanding of the process, is still much desired. Blending chitosan with another polymer for electrospinning may have a few advantages in making well-defined nanofibers as supporting materials for catalysts. First, a carefully selected polymer that is compatible with chitosan in the solution may improve the process to make nanofibers with controlled morphology. Chitosan is a cationic polymer and soluble in aqueous acidic solution, in which the eNH2 groups are protonated. However, the viscosity of the polyelectrolyte solution increases very rapidly with the concentration of chitosan, making it difficult to reach the minimum concentration required for the formation of continuous nanofibers by electrospinning. The blending of a polymer that improves chain interactions during the fast solvent evaporation of electrospinning process may facilitate the formation of continuous nanofibers with uniform sizes. Second, after the formation of nanofibers,
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crosslinking may be induced through the functional groups of the second polymer to enhance solvent and thermal stability of the composite nanofibers, while the chelating sites of chitosan can be kept intact for the subsequent immobilization of catalysts. This characteristic is desirable for developing materials to support catalytic reactions that may take place under harsh environmental conditions. Third, the composition and the molecular weight of the second polymer in the blend can be varied, to provide further controls over the physical properties of the nanofibers. Herein, we reported the use of a mixture of chitosan (CS) and poly(methacrylic acid) (PMAA) in acetic acid solution to prepare blended CS/PMAA nanofibers by electrospinning (see Scheme 1). PMAA interacts with chitosan in the solution and improves chain entanglements. The concentration, viscosity and composition of the polymer solution were found to play a central role in the formation of continuous, uniform nanofibers. After the formation of composite nanofibers, PMAA in the blend was readily crosslinked by simple thermal treatment to obtain solvent-resistant materials for supporting heterogeneous catalysts. As a demonstration, palladium catalyst has been immobilized on the surface of the crosslinked CS/ PMAA nanofibers to carry out MizorokieHeck cross-coupling reactions of aromatic halides and acrylates, and remarkable catalytic efficiency and stability were found in these materials. The mechanical properties of composite nanofiber mat can be modulated by using different molecular weights of PMAA in the blend and by varying the crosslinking temperature. The “post-modification” of the nanofibers with chelating groups, facilitates the attachment of a variety of metal catalysts on the fibers and further improves their stability. The general applicability of the approach has been demonstrated in the preparation of nanofibers from a mixture of poly(acrylic acid) (PAA) and chitosan. The result shows great promise in the use of a variety of synthetic polymers or biomacromolecules to blend with chitosan for the formation of composite nanofibers with controlled morphology and physical properties by simple electrospinning process. 2. Results and discussion 2.1. Nanofibers obtained from electrospinning of a mixture of CS and PMAA in solution Electrospinning is a simple technique to make continuing
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nanofibers from a variety of polymers dispersed in solution [13,14]. Under the same processing conditions (e.g., applied voltage, tip to target distance and the feed rate of the solution to the capillary tip), fiber morphology largely depends on the solution properties of the polymers such as solvent, viscosity, concentration and conductivity [15,16]. For many polymer/solvent systems, three morphology regimes can be observed as: polymer droplets, beaded nanofibers and nanofibers with uniform thickness, when the solution concentration is gradually increased. Defect-free, uniform nanofibers are usually found at concentrations well above the entanglement concentration (Ce) of the polymer, at which significant entanglement of polymer chains takes place in the solution [17]. Chitosan (Mn ¼ 2.0 105 in this study) can be readily dissolved in an acidic solution such as acetic acid, in which the eNH2 groups in the polymer chain are protonated (eNHþ 3 ). The specific conductance of chitosan in the acetic acid aqueous solution (CH3COOH:H2O ¼ 4:1 in weight, 20 C) decreases sharply as the concentration of chitosan increases (Fig. S1), suggesting it is a relatively weak polyelectrolyte at the conditions. The dynamic light scattering of the chitosan solution shows a characteristic reduced intensity from salt-free polyelectrolyte solution, and a convoluted correlation function from two time scales (Fig. S2). We found it difficult to obtain continuous nanofibers from the electrospinning of chitosan, due to the lack of sufficient entanglements in these semi-rigid polyelectrolytes. For example, only bead-like polymer droplets were obtained from the electrospinning of 4 wt% of chitosan in acetic acid solution (Fig. S3) [18,19]. And as the viscosity increases rapidly in this type of polyelectrolyte solution [20,21], controlling the electrospinning process became very difficult at chitosan concentration above 4 wt%. It is not feasible to obtain smooth nanofibers by simply increasing the concentration of chitosan in the acetic acid solution. Rather, we found that the addition of a second polymer that can interact with chitosan in the solution may resolve the chain entanglement issue during electrospinning process and provide a general approach to obtain composite chitosan fibers with controlled morphology. Fig. 1 shows how the conductivity and viscosity of the solution change with the increasing amount of PMAA (Mn ¼ 7.95 104, PDI ¼ 2.89) added into a 4 wt% chitosan solution, and how they are compared with the pure chitosan solution at the same total polymer concentration. Apparently, PMAA, a neutral polymer by itself at the pH condition, interacts with
Scheme 1. Preparation of stable chitosan/PMAA composite nanofibers from electrospinning.
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Fig. 1. (A) The conductance and (B) the rotary viscosity of CS solutions (in black square) and CS/PMAA solutions (in red circle, an increasing amount of PMAA was added into a 4 wt% CS solution) as a function of the total polymer concentrations. A mixture of CH3COOH and H2O (4:1 in weight) was used as the solvent. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
chitosan in the solution and suppresses the ionization process (Fig. 1A). As a result, the viscosity only has a modest increase when an increasing amount of PMAA was added into a 4 wt% CS solution (Fig. 1B). Consequently, the mixture of chitosan and PMAA with varying compositions can be easily electrospun from the solution to access all three distinct morphology regimes: polymer droplets (Fig. 2A), beaded fibers (Fig. 2B) and uniform fibers (Fig. 2C). Continuous and relatively uniform nanofibers were obtained when a sufficient amount of PMAA has been mixed with CS in the solution (e.g., 4 wt% of CS and 4 wt% of PMAA), in which significant overlap of the polymer chains constrain the chain motion and cause entanglement coupling. The interaction of polymer chains in the solution was also evidenced by comparing the dynamic light scattering of the polymer mixtures with that of chitosan or PMAA itself (Fig. S4). Experimentally, the entanglement concentration, Ce, can be estimated by the minimum concentration required for electrospinning of beaded fibers. And usually, the polymer concentration needs to be well above the Ce to obtain uniform, bead-free fibers. To confirm this, we examined the fiber structures from different total concentration of polymer mixtures while keeping a 1:1 weight ratio between chitosan and PMAA (Fig. 3AeD). From that study, we estimated the Ce to be slightly above 4 wt% for the 1:1 CS/PMAA mixture in the solution. Continuing nanofibers were only obtained at a total polymer concentration 8 wt% or above (Fig. 3D). The concentration required for electrospinning of uniform, bead-free fibers was found to be at least 2 times of Ce, in agreement with the theory proposed by Colby and Long [17]. While the polymer concentration relative to Ce is one of the key parameters that facilitate the electrospinning of uniform CS/PMAA nanofibers, the addition of another co-solvent into the acetic acid solution may fine tune the interaction between the CS and PMAA in
solution and thus provide additional controls over the morphology of the resulting nanofibers. Fig. 4 shows the nanofibers obtained from electrospinning of the polymer solution with the addition of 12.5 wt% N,N-dimethylformide (DMF) or dimethyl sulfoxide (DMSO). We found that the nanofibers were significantly more uniform after the addition of DMF or DMSO into the acetic acid solution (Fig. 4), although the macroscopic solution properties such as surface tension, rotary viscosity and conductivity were in the same range (Table S1). The fiber diameter can now be varied by the choice of co-solvents, for example, for a mixture of 4 wt% of CS and 4 wt% of PMAA, the fibers were 187 ± 52 nm in width with DMF as co-solvent, and 457 ± 137 nm with DMSO as co-solvent. Thin, almost defect-free nanofibers were obtained from the mixture of acetic acid/H2O/DMF (5.4/1.6/1 in weight ratio). Under this solvent condition, well-defined CS/PMAA fiber mats can be prepared with varied compositions, e.g., CS/PMAA ratios ranging from 4:2 to 4:7 in weight (Fig. S5). Wide angle X-ray scattering (WAXS) patterns of CS/PMAA nanofibers were shown in Fig. 5A, and compared with that of CS and PMAA. Pure CS exhibited two scattering peaks (0.803 and 1.423 Å1) arising from their crystalline structures [22]. The CS crystalline domains were not found in the CS/PMAA composite fibers. The FT-IR of the fibers shows that the characteristic absorption from carbonyl group stretching vibration (1702 cm1 from PMAA) is significantly weakened in the CS/PMAA fiber, while a new peak at 1546 cm1 corresponding to stretching vibration of carboxylate anion was found (Fig. 5B) [23,24], indicating the interactions between the pendant groups of PMAA and CS, which should be responsible for the loss of crystalline structure in the fibers. To test the general applicability of our approach, polyacrylic acid (PAA) (Mn ¼ 2.98 104, PDI ¼ 2.29), a polymer similar to PMAA, was also synthesized, and the identical solvent condition was used
Fig. 2. SEM images of three different polymer morphologies obtained from electrospinning of a mixture of (A) 4 wt% CS and 0 wt% PMAA, (B) 4 wt% CS and 2 wt% PMAA, and (C) 4 wt% CS and 4 wt% PMAA in acetic acid solution. Solvent: CH3COOH/H2O ¼ 4/1 in weight.
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Fig. 3. SEM images of different fiber morphologies obtained from electrospinning of a mixture of CS and PMAA (1:1 in weight) with the total polymer concentration of (A) 4 wt%, (B) 5 wt%, (C) 8 wt% and (D) 9 wt%. Solvent: CH3COOH/H2O ¼ 4/1 in weight.
Fig. 4. SEM images of fibers obtained from electrospinning of (A) 8 wt% and (B) 9 wt% of CS/PMAA in acetic acid/H2O/DMF (5.4/1.6/1 in weight ratio); (C) 7 wt% and (D) 8 wt% of CS/ PMAA in acetic acid/H2O/DMSO (5.4/1.6/1 in weight ratio). CS/PMAA ratio was kept 1:1 in weight.
to prepare CS/PAA fibers by electrospinning (Fig. S6). The result confirms that blending a polymer interacting with chitosan facilitated the formation of smooth fibers with uniform thickness, presumably due to the improved entanglement of polymer chains during the process.
2.2. The stability and physical properties of CS/PMAA composite nanofibers The composite fibers can be crosslinked by simple thermal treatment to improve the solvent stability of the materials. The
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weight of PMAA in the blend. The tensile strength of CS/PMAA fiber mat reaches a maximum when the CS/PMAA weight ratio was 1:1 (Fig. 9). To examine the effect of molecular weight, additional PMAA samples with lower (Mn ¼ 786) or higher (Mn ¼ 1.63 105) molecular weights were synthesized and compared with the original PMAA with a Mn of 7.95 104. The higher molecular weight PMAA was found to greatly enhance the tensile strength of corresponding CS/PMAA fiber mat, while the electrospinning failed with the PMAA oligomer (Table 1).
2.3. Immobilization of palladium catalyst on crosslinked CS/PMAA (Pd-CS/PMAA) fibers for MizorokieHeck reactions
Fig. 5. (A) WAXS patterns, and (B) FT-IR spectra from CS (in red), PMAA (in black) and CS/PMAA composite fibers (in blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
crosslinking of CS/PMAA fiber mat was carried out at 180 C for 2 h, and the swollen behavior of fibers in acetic acid solution was tested afterward. Fig. 6 shows that, after crosslinking, the fibrous structure in the mat was largely retained even under extended incubation with acetic acid. FT-IR analysis on the samples shows that the chemical structures of fiber mats after thermal treatment at 180 C are similar, indicating the CS and PMAA were not oxidized in crosslinking process (Fig. S7). In contrast, the fiber mats that are insufficiently crosslinked (e.g., treated at 140 C for 2 h) swell easily in acetic acid solution (Table S2) and changed their surface morphology (Fig. S8). The thermo-stability of CS/PMAA fiber mat also improved after thermal treatment (Fig. 7). As shown in Fig. 7, the decomposition temperature of CS/PMAA fiber mats changed from 170 C to 247 C after crosslinking. The thermal treatment also affects the mechanical properties of CS/PMAA fiber mats (Fig. 8). The tensile strength of CS/PMAA fiber mat reaches a maximum (about 13 MPa) when treated at 160 C. Annealing at higher temperature made the fiber mat more brittle, as indicated by the decrease of the tensile strength (Fig. 8). The mechanical properties of CS/PMAA composite fibers can be further controlled by changing the amount and the molecular
The crosslinked CS/PMAA fiber mat holds great potential as a versatile material platform for catalysis, as the amine and carboxyl moieties on the CS/PMAA nanofibers are excellent chelating groups for transition-metal ions. As a demonstration, CS/PMAA fiber mat was immersed in the Pd2þ solution to chelate with the Pd2þ ion, and then hydrazine was used to reduce the Pd2þ ion into Pd0 nanoparticles. XRD pattern shows four diffraction peaks at 40.4 , 46.9 , 68.3 and 82.1 which correspond to the reflections from (111), (200), (220) and (311) planes of FCC Pd crystal respectively (Fig. S9), indicating the formation of Pd nanoparticles on the CS/ PMAA fiber mat. Fig. S10 shows that some larger Pd particles adsorbed on the fiber surface can be directly visualized by transmission electron microscopy (TEM). The catalytic performance of the Pd-CS/PMAA fiber mat was then evaluated by MizorokieHeck coupling reaction. MizorokieHeck reaction is a powerful method for the construction of the sp2esp2 carbonecarbon bonds [25e27]. The reaction conditions were adapted from our previous studies [28e30]. Reaction of aromatic iodides with n-butyl acrylate (Entry 2e6 in Table 2) shows that the Pd-CS/PMAA catalytic system is tolerant for both electron-rich and electron-deficient aromatic iodides to obtain the corresponding reaction products in good yields. The PdCS/PMAA fiber also showed excellent activity and stereo-selectivity for the mono-substituted acrylates (Entry 7 and 8 in Table 2). The NMR characterization of the reaction products is included in the SI (Figs. S13e20). One of the major challenges in applying homogeneous palladium catalyst in an industrial scale is the difficulty in the recovery of the catalyst and the use of potentially toxic phosphine ligands. On the other hand, palladium leaching is the problem for the heterogeneous palladium catalyst [31,32]. We found that Pd-CS/PMAA catalyst can be recycled a few times without losing catalytic activity significantly. Fig. 10 shows the change of catalytic activity of Pd-CS/ PMAA after recycling six times for the MizorokieHeck crosscoupling reaction of iodobenzene and n-butyl acrylate. The total palladium leaching was monitored by inductively coupled plasmaatomic emission spectroscopy (ICP-AES) during the process. It was found that palladium leaching was negligible for the first four cycles and the catalytic activity was largely retained even after Pd-CS/ PMAA was recycled for six times. Table 3 summarizes the dependence of the MizorokieHeck cross-coupling with the catalyst loadings. We found that the yield for the MizorokieHeck cross-coupling of iodobenzene with n-butyl acrylate gradually increased with catalyst loading. Excellent conversion and yield were obtained even using 1 mg Pd-CS/PMAA (metal loading ~2.90%) with PhI/Pd ratio of 2567/1. When the PhI/Pd ratio is approaching 513/1, the yield saturates at 98%. These results clearly demonstrated the Pd-CS/PMAA is a highly active and efficient heterogeneous catalyst for the MizorokieHeck reaction.
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Fig. 6. SEM images of CS/PMAA composite fibers (A) after annealing at 180 C for 2 h, and (B) after annealing at 180 C for 2 h and subsequent incubation in acetic acid solution (80 wt%) for 48 h.
Fig. 7. TGA curves of CS/PMAA composite nanofibers before (in black) and after (in red) thermal treatment at 180 C for 2 h. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. The effect of different polymer compositions in the blend on the tensile strength of the composite fibers.
Table 1 Effect of PMAA molecular weight on the tensile strength of CS/PMAA fiber mat. PMAA
Mn
Mw
PDI
Tensile strength (MPa)
PMAA-1 PMAA-2 PMAA-3
786 7.95 104 1.63 105
910 2.13 105 3.71 105
1.16 2.68 2.28
Electrospinning failed 6.92 ± 0.85 11.20 ± 3.74
2.4. “Post-modification” of CS/PMMA fibers to incorporate 2pyridylimine ligands as a versatile platform for immobilization of different metal catalysts with further improved recyclability
Fig. 8. The tensile strength of CS/PMAA fiber mats after annealing for 2 h at various temperatures.
CS contains abundant amine and hydroxyl groups, so it is straightforward to carry out “post-modification” of the CS-based composite fibers to include specific ligands for immobilization of target metal catalysts and for improvement of the recyclability of the catalysts [33e35]. This significantly expands the potential applications of the CS-based composite fibers. As a demonstration, we modified the surface of CS/PMAA fibers by reacting them with the pyridine derivative 2-pyridinecarboxaldehyde. The formation of Schiff bond (C]N) and pyridylimine is confirmed by the absorption peaks in FT-IR spectra (Fig. S11 and 1715 cm1 for C]N and 751 cm1 for pyridinyl group). We then utilized the pyridine groups on the surface of 2-pyridylimine-CS/PMAA fibers to support different metal catalysts (Pd, Co, Ni) and compared their activities with those on CS/PMMA fiber without post-modification.
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Table 2 Pd-CS/PMAA fiber catalyzed MizorokieHeck reaction of aromatic iodides with acrylates.a Entry
Aromatic iodides
Acrylate
Conversion (%)b
Yield (%)b
1 2 3 4 5 6 7 8
PhI 4-F-PhI 4-Cl-PhI 4-Br-PhI 4-CH3PhI 4-NO2-PhI PhI PhI
CH2]CHCOO(CH2)3CH3 CH2]CHCOO(CH2)3CH3 CH2]CHCOO(CH2)3CH3 CH2]CHCOO(CH2)3CH3 CH2]CHCOO(CH2)3CH3 CH2]CHCOO(CH2)3CH3 CH2]CHCN CH2]CHCOOCH3
100 100 99 98 100 100 100 100
98 96 97 97 98 99 98 98
a b
(trans) (trans) (trans) (trans) (trans) (trans) (trans) (trans)
Reaction conditions: 0.7 mmol aromatic iodide and 1.4 mmol acrylate, 0.014 mmol Pd-CS/PMAA fiber, and 5.25 mmol K2CO3 in a solution of 3.0 mL DMAc at 110 C for 3 h. Cross-coupling conversions and product yields, based on the amount of aromatic iodide, were determined from GC/MS measurements.
Fig. 10. Reaction yield and palladium leaching measured after multiple recycling of the Pd-CS/PMAA catalyst. Reaction conditions: 0.7 mmol iodobenzene, 1.4 mmol n-butyl acrylate, 0.014 mmol Pd catalyst and 5.25 mmol K2CO3 in a solution of 3.0 mL DMAc at 110 C for 3.0 h.
Fig. 11. Reaction yield and palladium leaching measured after multiple recycling of the Pd-pyridylimine-CS/PMAA fiber catalyst. Reaction conditions: 0.7 mmol iodobenzene, 1.4 mmol n-butyl acrylate, 0.014 mmol Pd catalyst and 5.25 mmol K2CO3 in a solution of 3.0 mL DMAc at 110 C for 3.0 h.
Table 3 The effect of Pd-CS/PMAA loading on the conversion and yield of the reaction.a
PMAA fibers. It is clear that incorporating pyridine derivatives on the surface of composite fibers considerably improved the activity of all the metal catalysts investigated, presumably by enhancement of the effective loading of the catalysts on the fibers and preventing the leaching of catalysts in the reactions. We then closely examined the effect of catalyst loading and the reusability of Pd-pyridylimine-CS/PMAA fibers in MizorokeHeck reaction. Table S3 shows that, similar to Pd-CS/PMMA, excellent conversion and yield were obtained using Pd-pyridylimine-CS/ PMAA fiber catalyst with PhI/Pd ratio of 2567/1. Fig. 11 shows that the fibers can now be reused for at least fifteen times without significant loss of activity. Compared with Pd-CS/PMMA, the palladium leaching is considerably reduced. SEM studies show that the fiber structure of Pd-pyridylimine-CS/PMAA catalyst was retained after recycled for 15 times (Fig. S12). The remarkable catalytic efficiency and stability in these materials clearly
Pd-CS/PMAA (mg)
Molar ratio (PhI/Pd)
Conversionb (%)
Yieldb (%)
1 5 10 25 50
2567 513 256 102 51
89 98 98 100 100
88 98 98 98 98
a Reaction condition: iodobenzene: 0.7 mmol; n-butyl acrylate: 1.4 mmol; DMAc: 3.0 g; K2CO3: 5.25 mmol; reaction temperature: 110 C; reaction time: 3 h. b Cross-coupling conversions and product yields were determined from the GC/ MS measurements based on the amount of iodobenzene.
Table 4 summarizes the MizorokieHeck reaction of iodobenzene with n-butyl acrylate catalyzed by different metal catalysts (Pd, Co, Ni) supported on either CS/PMAA or pyridylimine-CS/
Table 4 Comparison of catalytic activities of CS/PMAA fiber and CS/PMAA-2-pyridylimine fiber supported transition metal catalyst.a,b Transitiona metal
Supporting material
Reaction time (h)
Conversion (%)
Yield (%)
Pd Pd Co Co Ni Ni
CS/PMAA fiber CS/PMAA-2-pyridylimine fiber CS/PMAA fiber CS/PMAA-2-pyridylimine fiber CS/PMAA fiber CS/PMAA-2-pyridylimine fiber
3 3 24 24 12 12
98 100 58 87 63 93
98 99 58 81 61 93
a Reaction condition: PhI: 0.7 mmol; n-Butyl acrylate: 1.4 mmol; K2CO3: 5.25 mmol; Pd fiber catalyst: 10 mg or Co fiber and Ni fiber catalyst: 50 mg; solvent: DMAc 3.0 g for Pd fiber catalyst and DMSO/ethylene glycol (2.0 g/1.0 g) mixture for Co and Ni fiber catalyst; reaction temperature: 110 C for Pd fiber catalyst and 145 C for Cu and Ni fiber catalysts. b Cross-coupling yields were determined from the GC/MS measurements based on the amount of aromatic halide subtrate.
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demonstrate the potential of utilizing the CS-based composite fibers as a versatile platform for supporting a variety of catalysts. Last, we compared the catalytic activities of the Pdpyridylimine-CS/PMAA and Pd-CS/PMMA fiber catalysts with PdCl2, a homogenous catalyst for Heck reaction. Often, faster kinetics is found in the reactions with homogeneous catalyst rather than that with heterogeneous catalyst, which involves biphasic system. Interestingly, at the same Pd loading, the reaction catalyzed by Pd-pyridylimine-CS/PMMA is almost as fast as that catalyzed by PdCl2, and significantly faster than Pd-CS/PMMA catalyzed reaction (Fig. 12). The improved catalytic activity of Pd-pyridylimine-CS/ PMMA may stem from the improved dispersion of Pd on the fibers and/or the specific interactions between Pd and pyridylimine ligands. An in-depth characterization of Pd-pyridylimine-CS/PMMA composite nanofibers and the modeling of metal-ligand interactions are the subject of future studies, which may elucidate the molecular mechanism of unusual catalytic activity and stability found in the system with the “post-modification”.
3. Conclusion We found that mixing PMAA or PAA with chitosan in acetic acid solution improves chain entanglements during the fast solvent evaporation of electrospinning process, and allow for the formation of continuous nanofibers. Crosslinking can be subsequently induced in the blend to enhance solvent and thermal stability of the nanofibers. The composition and the molecular weight of PMAA in the blend can be varied to provide further controls over the mechanical properties of the nanofibers. These composite nanofibers provide a useful platform to support a variety of catalysts by taking advantage of the chelating sites of chitosan, the large surface areas of nanofibers, and their stability even under harsh environmental conditions. We showed that palladium catalyst can be immobilized on the crosslinked composite nanofibers to carry out MizorokieHeck cross-coupling reactions efficiently. We further demonstrated that, by the “post-modification” of the nanofibers with ligands that chelate metal catalysts, a variety of metal catalysts can be incorporated into the fiber platform, with drastically improved stability and activity.
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4. Experimental procedures 4.1. Materials Chitosan (CS) (pharmaceutical grade, 95%, deacetylated, Mn ¼ 2.0 105) was purchased from Zhejiang Aoxing Biotechnology Co., Ltd (Zhejiang, China). Acetic acid, methacrylic acid, acetic acid and N,N-dimethylformide (DMF) (analytical grade) were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Palladium chloride (PdCl2) (Chemical Pure) was purchased from Zhejiang Metallurgical Research Institute (Zhejiang, China). Cobalt chloride hexahydrate (CoCl2$6H2O) was bought from Aladdin Chemistry Co., Ltd (Shanghai, China). Nickel nitrate hexahydrate (NiNO3$6H2O) was purchased from Shanghai Zhanyun Chemical Co., Ltd (Shanghai, China). Deionized water was used in all experiments. 4.2. Synthesis of poly(methacrylic acid) (PMAA) Methacrylic acid (20.0 g), benzoyl peroxide (BPO, 0.2, 2.0 and 20.0 g) and toluene (200 mL) were mixed in a two-neck, roundbottom flask equipped with a mechanical stirring and a condenser. The reaction was performed at 110 C for 5 h. On completion, the PMAA was separated by filtering, and dried under reduced pressure at 60 C for 12 h. 4.3. Preparation of CS/PMAA composite nanofibers CS (0.35 g) and PMAA (0.35 g) were dissolved in CH3COOH (5.40 g), DMF (1.00 g) and H2O (1.60 g) mixed solvent and stirred overnight to obtain homogeneous solution. The prepared chitosan solution was placed in a syringe (20 mL) bearing a capillary (0.8 mm inner diameter) as spinneret, which was connected with a high voltage power supply (GDW-a, Tianjin Dongwen High-voltage Power Supply Plant, China). A sheet of aluminum foil, connected to the ground, was placed under the syringe as collector. A voltage of 24 kV was applied to chitosan solution with the distance between the syringe tip and the collector being 15 cm. The feed rate of the polymer solution was kept at 1.0 mL/h by a micro-infusion pump (WZ-502C, Zhejiang Smiths Medical Instruments Co., Ltd. China). The resultant mats were dried under vacuum at room temperature to remove the residual solvent. The prepared fiber mats were then heated at 180 C for 2 h to prepare the crosslinked CS/PMAA fiber mats. 4.4. Swelling capacity of crosslinked CS/PMAA fiber mat The CS/PMAA fiber mats were soaked in a 50 wt% acetic acid aqueous solution for 48 h. The samples were then separated by filtration through a sintered filter funnel (pore size: 3e4 mm). The swelling capacity of the CS/PMAA fiber mat was calculated by the following equation:
Q¼
m2 m1 m1
(1)
where m1 and m2 are the mass of the dried and swollen fiber mat, respectively. The Q value was calculated as grams of acetic acid solution absorbed per gram of mat sample. Fig. 12. Kinetics of Heck reaction catalyzed by PdCl2 (in blue triangles), Pd-CS/PMAA (in red circles) and Pd-pyridylimine-CS/PMAA (in black squares). Reaction condition: 0.7 mmol iodobenzene, 1.4 mmol n-butyl acrylate, 0.014 mmol Pd catalyst and 5.25 mmol K2CO3 in a solution of 3.0 mL DMAc at 110 C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
4.5. Utilize crosslinked CS/PMAA nanofibers to support transition metal catalyst Palladium immobilization: 500 mg crosslinked CS/PMAA fiber mat was added to a round-bottom flask containing 25 mg PdCl2 and
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17 mg NaCl (molar ratio: 1/2) and 5 g H2O. The mixture was stirred overnight, and then 0.5 mL hydrazine (80 wt%) was added to reduce the Pd2þ into Pd0. The fiber mat turned black shortly, indicating the successful immobilization of palladium on the fibers. The palladium chelated chitosan fiber mat (Pd-CS/PMAA fiber) was filtered after the process and dried under a reduced pressure for 12 h. The Pd content in the Pd-CS/PMAA was 2.90 wt%, which was determined by inductively coupled plasma-atomic emission spectroscopy (ICPAES). Cobalt and nickel immobilization: 500 mg crosslinked CS/PMAA fiber mat was added to the CoCl2 or NiNO3 aqueous solution (10 mL, 50 mg CoCl2 or NiNO3). The mixture was stirred for 12 h. The crosslinked CS/PMAA fiber supported Co or Ni catalyst (Co-CS/ PMAA or Ni-CS/PMAA fiber) was collected by filtration and dried at reduced pressure for 12 h. The contents of Co and Ni in the catalyst were 4.51% and 4.73%, respectively. 4.6. Modification of CS/PMAA fiber with 2-pyridinecarboxaldehyde CS/PMAA fiber mat (500 mg) after thermal treatment was added into the 2-pyridinecarboxaldehyde aqueous solution (20 mL, 5 wt %). The mixture was stirred for 12 h and the CS/PMAA-2pyridylimine fiber was collected by filtration and wash with H2O and ethanol, and dried at room temperature for 12 h under reduced pressure. The immobilization of Pd, Co and Ni on CS/PMAA-2pyridylimine fibers was identical to that for CS/PMAA fibers. The contents of Pd, Co and Ni in the fiber catalyst were measured to be 2.96%, 5.58% and 4.75%, respectively. 4.7. MizorokieHeck cross-coupling reaction catalyzed by fiber catalyst Aromatic iodides (0.7 mmol), acrylate (1.4 mmol), Pd fiber catalyst (50 mg, Pd: 0.014 mmol), K2CO3 (5.25 mmol) and 3.0 g N,Ndimethylacetamide (DMAc) were added to a 20 mL tubular reactor equipped with a magnetic stir bar. The mixture was allowed to stir at 110 C and the reaction progress was monitored by TLC and GC/ MS analysis. After completion, the reaction mixture was cooled down to room temperature, and quenched with 10 mL of water and extracted three times with ethyl acetate (3 20 mL). The combined organic layer was washed with water, saturated brine, and then dried over anhydrous Na2SO4. Solvent was removed under a reduced pressure. The residue was purified by silica gel chromatography with a mixture of petroleum ether and ethyl acetate as eluting solvent to afford the cross-coupling products. All of the cross-coupling products were characterized by 1H-NMR spectroscopy and mass spectroscopy. The solvent for Co and Ni fiber catalyzed MizorokieHeck reaction was DMSO/ethylene glycol (2.0 g/1.0 g) mixture and the reaction was carried out at 145 C. The other process was same as that of Pd fiber catalyzed MizorokieHeck reaction. 4.8. Characterization The morphologies of the electrospun mats were characterized by a scanning electron microscope (SEM) (Jeol, Jsm-6360lv, Japan). The fiber samples were analyzed by Fourier transform infrared spectroscopy (FT-IR) in an attenuated total reflection mode (Nicolet, Nexus-470, USA). 1H-NMR spectra were recorded in CDCl3 (Bruker, AVANCE III 400 MHz, Switzerland) using TMS as the internal reference. Multiplicities are reported as: singlet (s), doublet (d), and multiplet (m). Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis was performed on a Leemann ICPAES Prodigy XP (Leeman Labs, USA). The rotary viscosity was tested
using NDJ-8S type rotary viscometer at room temperature (20 C) (Shanghai Pingxuan Scientific Instrument Co. Ltd, China). The weight-average molecular weights (Mw) and number-average molecular (Mn) were measured by GPC (Agilent PLgel 5 mm MIXED-C Column, USA) with N,N-dimethylacetamide as the mobile phase and relative to poly(methyl methacrylate) standards. The mechanical analysis was carried out on electronic fabric strength tester (YG065HC/PC, Shandong Lanzhou Electron Instrument Co., Ltd, China). The thermal stability of the fibers was analyzed by a thermogravimetric analyzer (TGA) (Beijing Scientific Instrument Factory, China) under nitrogen atmosphere. The heating ramp was from 20 to 650 C at a rate of 10 C/min. The conductivity was determined by conductivity meter (DDS-307, Shanghai Inesa Scientific Instrument Co. Ltd, China). The hydrodynamic radius of polymer solution was measured by dynamic Light Scattering (DLS) (ALV-CGS-3MD, ALV-Gmbh, Germany). Structural characterization was carried out by wide angle X-ray diffraction (WXRD) (D8 Advance, Bruker, Germany). Diffraction patterns of Pd-CS/PMAA were obtained by the method of X-ray powder diffraction (XRD) (Rigaku D, max-3BX, Japan). The dispersion of palladium nanoparticles on fibers were recorded by JEOL TEM (JEM-1011, Japan). Acknowledgments We greatly appreciate the financial supports from the Zhejiang Province Natural Science Foundation (LQ14E030003) and Zhejiang Nanomaterials and Chemistry Key Laboratory. L.S. is grateful for the support from the IMS at UCONN when he was a visiting scholar there. Y.L. acknowledges the support from University of Connecticut Research Foundation and a Senior Visiting Scholarship from Shaoxing University. We thank Prof. Richard Parnas for helpful inputs to the manuscript. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2015.08.031. References [1] M. Dash, F. Chiellini, R.M. Ottenbrite, E. Chiellini, Prog. Polym. Sci. 36 (8) (2011) 981e1014. [2] T. Kean, M. Thanou, Adv. Drug Deliv. Rev. 62 (1) (2010) 3e11. [3] C.K.S. Pillai, W. Paul, C.P. Sharma, Prog. Polym. Sci. 34 (7) (2009) 641e678. [4] S. Jana, A. Cooper, F. Ohuchi, M. Zhang, ACS Appl. Mater. Interface 4 (9) (2012) 4817e4824. [5] R. Xu, Q. Zhou, F.T. Li, B.R. Zhang, Chem. Eng. J. 222 (2013) 321e329. [6] F. Peirano, T. Vincent, F. Quignard, M. Robitzer, E. Guibal, J. Membr. Sci. 329 (1e2) (2009) 30e45. [7] M. Bradshaw, J.L. Zou, L. Byrne, K.S. Iyer, S.G. Stewart, C.L. Raston, Chem. Commun. 47 (2011) 12292e12294. [8] H. Kim, A. Nishida, H. Yamamoto, Macromol. Rapid Commun. 25 (18) (2004) 1600e1605. [9] K. Ohkawa, K.-I. Minato, G. Kumagai, S. Hayashi, H. Yamamoto, Biomacromolecules 7 (11) (2006) 3291e3294. [10] J.F. Zhang, D.Z. Yang, F. Xu, Z.P. Zhang, R.X. Yin, J. Nie, Macromolecules 42 (14) (2009) 5278e5284. [11] Y.Z. Zhang, B. Su, S. Ramakrishna, C.T. Lim, Biomacromolecules 9 (1) (2008) 136e141. [12] K. Sun, Z.H. Li, Express Polym. Lett. 5 (4) (2011) 342e361. [13] N. Bhardwaj, S.C. Kundu, Biotechnol. Adv. 28 (3) (2010) 325e347. [14] G.C. Rutledge, S.V. Fridrikh, Adv. Drug Deliv. Rev. 59 (14) (2007) 1384e1391. [15] Z.M. Huang, Y.Z. Zhang, M. Kotaki, S. Ramakrishna, Compos. Sci. Technol. 63 (15) (2003) 2223e2253. [16] L.J. Shao, J. Wu, Z.K. Xu, Chin. J. Polym. Sci. 27 (1) (2009) 115e120. [17] M.G. Mckee, G.L. Wilkeds, R.H. Colby, T.E. Long, Macromolecules 37 (5) (2004) 1760e1767. [18] M.Z. Elsabee, H.F. Naguib, R.E. Morsi, Mat. Sci. Eng. C Mater. 32 (7) (2012) 1711e1726. [19] M. Pakravan, M.C. Heuzey, A. Ajji, Polymer 52 (21) (2011) 4813e4824. [20] I. Dora, C. Michel, L. Leibler, Soft Matter 10 (2014) 1714e1722.
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Polymer journal homepage: www.elsevier.com/locate/polymer
Developing chitosan-based composite nanofibers for supporting metal catalysts Linjun Shao a, Yuan Ren b, Zining Wang a, Chenze Qi a, **, Yao Lin b, * a b
Zhejiang Key Laboratory of Alternative Technologies for Fine Chemicals Process, Shaoxing University, Zhejiang Province, 312000, PR China Polymer Program, Institute of Materials Science & Department of Chemistry, University of Connecticut, Storrs, CT 06269, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 June 2015 Received in revised form 5 August 2015 Accepted 15 August 2015 Available online 19 August 2015
Using electrospinning technique to prepare chitosan (CS) nanofibers from an inexpensive solvent such as acetic acid has been a challenge, due to the lack of sufficient entanglements in these semi-rigid polyelectrolytes. Incorporating polymers such as poly(methacrylic acid) (PMAA) into the solution lowers the entanglement concentration of the polymers, and make all three morphology regimes (polymer droplets, beaded nanofibers and nanofibers with uniform thickness) accessible to electrospinning method. Uniform composite nanofibers are found at concentrations well above the entanglement concentration. The thickness and morphology of nanofibers can be regulated by addition of small amount of organic solvents such as dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). The composite nanofibers are subsequently crosslinked at an elevated temperature to improve their thermal and solvent stability. The mechanical properties of the crosslinked CS/PMAA fibers can be modulated by varying the amount and molecular weight of PMAA incorporated in the blend. The composite fiber materials provide a useful platform to support a variety of catalysts. As a demonstration, palladium catalyst has been immobilized on the crosslinked CS/PMAA nanofibers to carry out MizorokieHeck cross-coupling reactions of aromatic halides and acrylates. We found remarkable catalytic efficiency and stability in these materials. We also demonstrate that, by the “post-modification” of the nanofibers with ligands that chelate metal catalysts, a variety of metal catalysts can be incorporated into the fiber platform, with further improved stability and activity. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Chitosan Poly(methacrylic acid) Chain entanglement Electrospinning Composite nanofiber Palladium Catalysis
1. Introduction Chitosan, a derivative of chitin, is a versatile, low-cost biopolymer that has found applications in food, cosmetics, drug delivery and tissue engineering [1,2]. Recently, there has been increasing interest in the use of chitosan nanofibers (e.g., by electrospinning) as solid supports for immobilization of catalysts or enzymes, to take advantage of the metal-chelating and complexation properties of chitosan and the large accessible surface area of nanofibers [3e6]. In order to make continuous nanofibers, trifluoroacetic acid (TFA) was often used as the dispersion solvent for chitosan in the electrospinning process, but TFA is both expensive and toxic [7e9]. To prepare chitosan nanofibers in inexpensive and more environmentally friendly solvents, new methods have been
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (C. Qi), [email protected] (Y. Lin). http://dx.doi.org/10.1016/j.polymer.2015.08.031 0032-3861/© 2015 Elsevier Ltd. All rights reserved.
explored, e.g., by the addition of a second polymer as the cospinning agent with chitosan [10e12]. However, a general approach to obtain composite chitosan nanofibers with controlled morphology, based on mechanistic understanding of the process, is still much desired. Blending chitosan with another polymer for electrospinning may have a few advantages in making well-defined nanofibers as supporting materials for catalysts. First, a carefully selected polymer that is compatible with chitosan in the solution may improve the process to make nanofibers with controlled morphology. Chitosan is a cationic polymer and soluble in aqueous acidic solution, in which the eNH2 groups are protonated. However, the viscosity of the polyelectrolyte solution increases very rapidly with the concentration of chitosan, making it difficult to reach the minimum concentration required for the formation of continuous nanofibers by electrospinning. The blending of a polymer that improves chain interactions during the fast solvent evaporation of electrospinning process may facilitate the formation of continuous nanofibers with uniform sizes. Second, after the formation of nanofibers,
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crosslinking may be induced through the functional groups of the second polymer to enhance solvent and thermal stability of the composite nanofibers, while the chelating sites of chitosan can be kept intact for the subsequent immobilization of catalysts. This characteristic is desirable for developing materials to support catalytic reactions that may take place under harsh environmental conditions. Third, the composition and the molecular weight of the second polymer in the blend can be varied, to provide further controls over the physical properties of the nanofibers. Herein, we reported the use of a mixture of chitosan (CS) and poly(methacrylic acid) (PMAA) in acetic acid solution to prepare blended CS/PMAA nanofibers by electrospinning (see Scheme 1). PMAA interacts with chitosan in the solution and improves chain entanglements. The concentration, viscosity and composition of the polymer solution were found to play a central role in the formation of continuous, uniform nanofibers. After the formation of composite nanofibers, PMAA in the blend was readily crosslinked by simple thermal treatment to obtain solvent-resistant materials for supporting heterogeneous catalysts. As a demonstration, palladium catalyst has been immobilized on the surface of the crosslinked CS/ PMAA nanofibers to carry out MizorokieHeck cross-coupling reactions of aromatic halides and acrylates, and remarkable catalytic efficiency and stability were found in these materials. The mechanical properties of composite nanofiber mat can be modulated by using different molecular weights of PMAA in the blend and by varying the crosslinking temperature. The “post-modification” of the nanofibers with chelating groups, facilitates the attachment of a variety of metal catalysts on the fibers and further improves their stability. The general applicability of the approach has been demonstrated in the preparation of nanofibers from a mixture of poly(acrylic acid) (PAA) and chitosan. The result shows great promise in the use of a variety of synthetic polymers or biomacromolecules to blend with chitosan for the formation of composite nanofibers with controlled morphology and physical properties by simple electrospinning process. 2. Results and discussion 2.1. Nanofibers obtained from electrospinning of a mixture of CS and PMAA in solution Electrospinning is a simple technique to make continuing
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nanofibers from a variety of polymers dispersed in solution [13,14]. Under the same processing conditions (e.g., applied voltage, tip to target distance and the feed rate of the solution to the capillary tip), fiber morphology largely depends on the solution properties of the polymers such as solvent, viscosity, concentration and conductivity [15,16]. For many polymer/solvent systems, three morphology regimes can be observed as: polymer droplets, beaded nanofibers and nanofibers with uniform thickness, when the solution concentration is gradually increased. Defect-free, uniform nanofibers are usually found at concentrations well above the entanglement concentration (Ce) of the polymer, at which significant entanglement of polymer chains takes place in the solution [17]. Chitosan (Mn ¼ 2.0 105 in this study) can be readily dissolved in an acidic solution such as acetic acid, in which the eNH2 groups in the polymer chain are protonated (eNHþ 3 ). The specific conductance of chitosan in the acetic acid aqueous solution (CH3COOH:H2O ¼ 4:1 in weight, 20 C) decreases sharply as the concentration of chitosan increases (Fig. S1), suggesting it is a relatively weak polyelectrolyte at the conditions. The dynamic light scattering of the chitosan solution shows a characteristic reduced intensity from salt-free polyelectrolyte solution, and a convoluted correlation function from two time scales (Fig. S2). We found it difficult to obtain continuous nanofibers from the electrospinning of chitosan, due to the lack of sufficient entanglements in these semi-rigid polyelectrolytes. For example, only bead-like polymer droplets were obtained from the electrospinning of 4 wt% of chitosan in acetic acid solution (Fig. S3) [18,19]. And as the viscosity increases rapidly in this type of polyelectrolyte solution [20,21], controlling the electrospinning process became very difficult at chitosan concentration above 4 wt%. It is not feasible to obtain smooth nanofibers by simply increasing the concentration of chitosan in the acetic acid solution. Rather, we found that the addition of a second polymer that can interact with chitosan in the solution may resolve the chain entanglement issue during electrospinning process and provide a general approach to obtain composite chitosan fibers with controlled morphology. Fig. 1 shows how the conductivity and viscosity of the solution change with the increasing amount of PMAA (Mn ¼ 7.95 104, PDI ¼ 2.89) added into a 4 wt% chitosan solution, and how they are compared with the pure chitosan solution at the same total polymer concentration. Apparently, PMAA, a neutral polymer by itself at the pH condition, interacts with
Scheme 1. Preparation of stable chitosan/PMAA composite nanofibers from electrospinning.
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Fig. 1. (A) The conductance and (B) the rotary viscosity of CS solutions (in black square) and CS/PMAA solutions (in red circle, an increasing amount of PMAA was added into a 4 wt% CS solution) as a function of the total polymer concentrations. A mixture of CH3COOH and H2O (4:1 in weight) was used as the solvent. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
chitosan in the solution and suppresses the ionization process (Fig. 1A). As a result, the viscosity only has a modest increase when an increasing amount of PMAA was added into a 4 wt% CS solution (Fig. 1B). Consequently, the mixture of chitosan and PMAA with varying compositions can be easily electrospun from the solution to access all three distinct morphology regimes: polymer droplets (Fig. 2A), beaded fibers (Fig. 2B) and uniform fibers (Fig. 2C). Continuous and relatively uniform nanofibers were obtained when a sufficient amount of PMAA has been mixed with CS in the solution (e.g., 4 wt% of CS and 4 wt% of PMAA), in which significant overlap of the polymer chains constrain the chain motion and cause entanglement coupling. The interaction of polymer chains in the solution was also evidenced by comparing the dynamic light scattering of the polymer mixtures with that of chitosan or PMAA itself (Fig. S4). Experimentally, the entanglement concentration, Ce, can be estimated by the minimum concentration required for electrospinning of beaded fibers. And usually, the polymer concentration needs to be well above the Ce to obtain uniform, bead-free fibers. To confirm this, we examined the fiber structures from different total concentration of polymer mixtures while keeping a 1:1 weight ratio between chitosan and PMAA (Fig. 3AeD). From that study, we estimated the Ce to be slightly above 4 wt% for the 1:1 CS/PMAA mixture in the solution. Continuing nanofibers were only obtained at a total polymer concentration 8 wt% or above (Fig. 3D). The concentration required for electrospinning of uniform, bead-free fibers was found to be at least 2 times of Ce, in agreement with the theory proposed by Colby and Long [17]. While the polymer concentration relative to Ce is one of the key parameters that facilitate the electrospinning of uniform CS/PMAA nanofibers, the addition of another co-solvent into the acetic acid solution may fine tune the interaction between the CS and PMAA in
solution and thus provide additional controls over the morphology of the resulting nanofibers. Fig. 4 shows the nanofibers obtained from electrospinning of the polymer solution with the addition of 12.5 wt% N,N-dimethylformide (DMF) or dimethyl sulfoxide (DMSO). We found that the nanofibers were significantly more uniform after the addition of DMF or DMSO into the acetic acid solution (Fig. 4), although the macroscopic solution properties such as surface tension, rotary viscosity and conductivity were in the same range (Table S1). The fiber diameter can now be varied by the choice of co-solvents, for example, for a mixture of 4 wt% of CS and 4 wt% of PMAA, the fibers were 187 ± 52 nm in width with DMF as co-solvent, and 457 ± 137 nm with DMSO as co-solvent. Thin, almost defect-free nanofibers were obtained from the mixture of acetic acid/H2O/DMF (5.4/1.6/1 in weight ratio). Under this solvent condition, well-defined CS/PMAA fiber mats can be prepared with varied compositions, e.g., CS/PMAA ratios ranging from 4:2 to 4:7 in weight (Fig. S5). Wide angle X-ray scattering (WAXS) patterns of CS/PMAA nanofibers were shown in Fig. 5A, and compared with that of CS and PMAA. Pure CS exhibited two scattering peaks (0.803 and 1.423 Å1) arising from their crystalline structures [22]. The CS crystalline domains were not found in the CS/PMAA composite fibers. The FT-IR of the fibers shows that the characteristic absorption from carbonyl group stretching vibration (1702 cm1 from PMAA) is significantly weakened in the CS/PMAA fiber, while a new peak at 1546 cm1 corresponding to stretching vibration of carboxylate anion was found (Fig. 5B) [23,24], indicating the interactions between the pendant groups of PMAA and CS, which should be responsible for the loss of crystalline structure in the fibers. To test the general applicability of our approach, polyacrylic acid (PAA) (Mn ¼ 2.98 104, PDI ¼ 2.29), a polymer similar to PMAA, was also synthesized, and the identical solvent condition was used
Fig. 2. SEM images of three different polymer morphologies obtained from electrospinning of a mixture of (A) 4 wt% CS and 0 wt% PMAA, (B) 4 wt% CS and 2 wt% PMAA, and (C) 4 wt% CS and 4 wt% PMAA in acetic acid solution. Solvent: CH3COOH/H2O ¼ 4/1 in weight.
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Fig. 3. SEM images of different fiber morphologies obtained from electrospinning of a mixture of CS and PMAA (1:1 in weight) with the total polymer concentration of (A) 4 wt%, (B) 5 wt%, (C) 8 wt% and (D) 9 wt%. Solvent: CH3COOH/H2O ¼ 4/1 in weight.
Fig. 4. SEM images of fibers obtained from electrospinning of (A) 8 wt% and (B) 9 wt% of CS/PMAA in acetic acid/H2O/DMF (5.4/1.6/1 in weight ratio); (C) 7 wt% and (D) 8 wt% of CS/ PMAA in acetic acid/H2O/DMSO (5.4/1.6/1 in weight ratio). CS/PMAA ratio was kept 1:1 in weight.
to prepare CS/PAA fibers by electrospinning (Fig. S6). The result confirms that blending a polymer interacting with chitosan facilitated the formation of smooth fibers with uniform thickness, presumably due to the improved entanglement of polymer chains during the process.
2.2. The stability and physical properties of CS/PMAA composite nanofibers The composite fibers can be crosslinked by simple thermal treatment to improve the solvent stability of the materials. The
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weight of PMAA in the blend. The tensile strength of CS/PMAA fiber mat reaches a maximum when the CS/PMAA weight ratio was 1:1 (Fig. 9). To examine the effect of molecular weight, additional PMAA samples with lower (Mn ¼ 786) or higher (Mn ¼ 1.63 105) molecular weights were synthesized and compared with the original PMAA with a Mn of 7.95 104. The higher molecular weight PMAA was found to greatly enhance the tensile strength of corresponding CS/PMAA fiber mat, while the electrospinning failed with the PMAA oligomer (Table 1).
2.3. Immobilization of palladium catalyst on crosslinked CS/PMAA (Pd-CS/PMAA) fibers for MizorokieHeck reactions
Fig. 5. (A) WAXS patterns, and (B) FT-IR spectra from CS (in red), PMAA (in black) and CS/PMAA composite fibers (in blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
crosslinking of CS/PMAA fiber mat was carried out at 180 C for 2 h, and the swollen behavior of fibers in acetic acid solution was tested afterward. Fig. 6 shows that, after crosslinking, the fibrous structure in the mat was largely retained even under extended incubation with acetic acid. FT-IR analysis on the samples shows that the chemical structures of fiber mats after thermal treatment at 180 C are similar, indicating the CS and PMAA were not oxidized in crosslinking process (Fig. S7). In contrast, the fiber mats that are insufficiently crosslinked (e.g., treated at 140 C for 2 h) swell easily in acetic acid solution (Table S2) and changed their surface morphology (Fig. S8). The thermo-stability of CS/PMAA fiber mat also improved after thermal treatment (Fig. 7). As shown in Fig. 7, the decomposition temperature of CS/PMAA fiber mats changed from 170 C to 247 C after crosslinking. The thermal treatment also affects the mechanical properties of CS/PMAA fiber mats (Fig. 8). The tensile strength of CS/PMAA fiber mat reaches a maximum (about 13 MPa) when treated at 160 C. Annealing at higher temperature made the fiber mat more brittle, as indicated by the decrease of the tensile strength (Fig. 8). The mechanical properties of CS/PMAA composite fibers can be further controlled by changing the amount and the molecular
The crosslinked CS/PMAA fiber mat holds great potential as a versatile material platform for catalysis, as the amine and carboxyl moieties on the CS/PMAA nanofibers are excellent chelating groups for transition-metal ions. As a demonstration, CS/PMAA fiber mat was immersed in the Pd2þ solution to chelate with the Pd2þ ion, and then hydrazine was used to reduce the Pd2þ ion into Pd0 nanoparticles. XRD pattern shows four diffraction peaks at 40.4 , 46.9 , 68.3 and 82.1 which correspond to the reflections from (111), (200), (220) and (311) planes of FCC Pd crystal respectively (Fig. S9), indicating the formation of Pd nanoparticles on the CS/ PMAA fiber mat. Fig. S10 shows that some larger Pd particles adsorbed on the fiber surface can be directly visualized by transmission electron microscopy (TEM). The catalytic performance of the Pd-CS/PMAA fiber mat was then evaluated by MizorokieHeck coupling reaction. MizorokieHeck reaction is a powerful method for the construction of the sp2esp2 carbonecarbon bonds [25e27]. The reaction conditions were adapted from our previous studies [28e30]. Reaction of aromatic iodides with n-butyl acrylate (Entry 2e6 in Table 2) shows that the Pd-CS/PMAA catalytic system is tolerant for both electron-rich and electron-deficient aromatic iodides to obtain the corresponding reaction products in good yields. The PdCS/PMAA fiber also showed excellent activity and stereo-selectivity for the mono-substituted acrylates (Entry 7 and 8 in Table 2). The NMR characterization of the reaction products is included in the SI (Figs. S13e20). One of the major challenges in applying homogeneous palladium catalyst in an industrial scale is the difficulty in the recovery of the catalyst and the use of potentially toxic phosphine ligands. On the other hand, palladium leaching is the problem for the heterogeneous palladium catalyst [31,32]. We found that Pd-CS/PMAA catalyst can be recycled a few times without losing catalytic activity significantly. Fig. 10 shows the change of catalytic activity of Pd-CS/ PMAA after recycling six times for the MizorokieHeck crosscoupling reaction of iodobenzene and n-butyl acrylate. The total palladium leaching was monitored by inductively coupled plasmaatomic emission spectroscopy (ICP-AES) during the process. It was found that palladium leaching was negligible for the first four cycles and the catalytic activity was largely retained even after Pd-CS/ PMAA was recycled for six times. Table 3 summarizes the dependence of the MizorokieHeck cross-coupling with the catalyst loadings. We found that the yield for the MizorokieHeck cross-coupling of iodobenzene with n-butyl acrylate gradually increased with catalyst loading. Excellent conversion and yield were obtained even using 1 mg Pd-CS/PMAA (metal loading ~2.90%) with PhI/Pd ratio of 2567/1. When the PhI/Pd ratio is approaching 513/1, the yield saturates at 98%. These results clearly demonstrated the Pd-CS/PMAA is a highly active and efficient heterogeneous catalyst for the MizorokieHeck reaction.
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Fig. 6. SEM images of CS/PMAA composite fibers (A) after annealing at 180 C for 2 h, and (B) after annealing at 180 C for 2 h and subsequent incubation in acetic acid solution (80 wt%) for 48 h.
Fig. 7. TGA curves of CS/PMAA composite nanofibers before (in black) and after (in red) thermal treatment at 180 C for 2 h. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. The effect of different polymer compositions in the blend on the tensile strength of the composite fibers.
Table 1 Effect of PMAA molecular weight on the tensile strength of CS/PMAA fiber mat. PMAA
Mn
Mw
PDI
Tensile strength (MPa)
PMAA-1 PMAA-2 PMAA-3
786 7.95 104 1.63 105
910 2.13 105 3.71 105
1.16 2.68 2.28
Electrospinning failed 6.92 ± 0.85 11.20 ± 3.74
2.4. “Post-modification” of CS/PMMA fibers to incorporate 2pyridylimine ligands as a versatile platform for immobilization of different metal catalysts with further improved recyclability
Fig. 8. The tensile strength of CS/PMAA fiber mats after annealing for 2 h at various temperatures.
CS contains abundant amine and hydroxyl groups, so it is straightforward to carry out “post-modification” of the CS-based composite fibers to include specific ligands for immobilization of target metal catalysts and for improvement of the recyclability of the catalysts [33e35]. This significantly expands the potential applications of the CS-based composite fibers. As a demonstration, we modified the surface of CS/PMAA fibers by reacting them with the pyridine derivative 2-pyridinecarboxaldehyde. The formation of Schiff bond (C]N) and pyridylimine is confirmed by the absorption peaks in FT-IR spectra (Fig. S11 and 1715 cm1 for C]N and 751 cm1 for pyridinyl group). We then utilized the pyridine groups on the surface of 2-pyridylimine-CS/PMAA fibers to support different metal catalysts (Pd, Co, Ni) and compared their activities with those on CS/PMMA fiber without post-modification.
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Table 2 Pd-CS/PMAA fiber catalyzed MizorokieHeck reaction of aromatic iodides with acrylates.a Entry
Aromatic iodides
Acrylate
Conversion (%)b
Yield (%)b
1 2 3 4 5 6 7 8
PhI 4-F-PhI 4-Cl-PhI 4-Br-PhI 4-CH3PhI 4-NO2-PhI PhI PhI
CH2]CHCOO(CH2)3CH3 CH2]CHCOO(CH2)3CH3 CH2]CHCOO(CH2)3CH3 CH2]CHCOO(CH2)3CH3 CH2]CHCOO(CH2)3CH3 CH2]CHCOO(CH2)3CH3 CH2]CHCN CH2]CHCOOCH3
100 100 99 98 100 100 100 100
98 96 97 97 98 99 98 98
a b
(trans) (trans) (trans) (trans) (trans) (trans) (trans) (trans)
Reaction conditions: 0.7 mmol aromatic iodide and 1.4 mmol acrylate, 0.014 mmol Pd-CS/PMAA fiber, and 5.25 mmol K2CO3 in a solution of 3.0 mL DMAc at 110 C for 3 h. Cross-coupling conversions and product yields, based on the amount of aromatic iodide, were determined from GC/MS measurements.
Fig. 10. Reaction yield and palladium leaching measured after multiple recycling of the Pd-CS/PMAA catalyst. Reaction conditions: 0.7 mmol iodobenzene, 1.4 mmol n-butyl acrylate, 0.014 mmol Pd catalyst and 5.25 mmol K2CO3 in a solution of 3.0 mL DMAc at 110 C for 3.0 h.
Fig. 11. Reaction yield and palladium leaching measured after multiple recycling of the Pd-pyridylimine-CS/PMAA fiber catalyst. Reaction conditions: 0.7 mmol iodobenzene, 1.4 mmol n-butyl acrylate, 0.014 mmol Pd catalyst and 5.25 mmol K2CO3 in a solution of 3.0 mL DMAc at 110 C for 3.0 h.
Table 3 The effect of Pd-CS/PMAA loading on the conversion and yield of the reaction.a
PMAA fibers. It is clear that incorporating pyridine derivatives on the surface of composite fibers considerably improved the activity of all the metal catalysts investigated, presumably by enhancement of the effective loading of the catalysts on the fibers and preventing the leaching of catalysts in the reactions. We then closely examined the effect of catalyst loading and the reusability of Pd-pyridylimine-CS/PMAA fibers in MizorokeHeck reaction. Table S3 shows that, similar to Pd-CS/PMMA, excellent conversion and yield were obtained using Pd-pyridylimine-CS/ PMAA fiber catalyst with PhI/Pd ratio of 2567/1. Fig. 11 shows that the fibers can now be reused for at least fifteen times without significant loss of activity. Compared with Pd-CS/PMMA, the palladium leaching is considerably reduced. SEM studies show that the fiber structure of Pd-pyridylimine-CS/PMAA catalyst was retained after recycled for 15 times (Fig. S12). The remarkable catalytic efficiency and stability in these materials clearly
Pd-CS/PMAA (mg)
Molar ratio (PhI/Pd)
Conversionb (%)
Yieldb (%)
1 5 10 25 50
2567 513 256 102 51
89 98 98 100 100
88 98 98 98 98
a Reaction condition: iodobenzene: 0.7 mmol; n-butyl acrylate: 1.4 mmol; DMAc: 3.0 g; K2CO3: 5.25 mmol; reaction temperature: 110 C; reaction time: 3 h. b Cross-coupling conversions and product yields were determined from the GC/ MS measurements based on the amount of iodobenzene.
Table 4 summarizes the MizorokieHeck reaction of iodobenzene with n-butyl acrylate catalyzed by different metal catalysts (Pd, Co, Ni) supported on either CS/PMAA or pyridylimine-CS/
Table 4 Comparison of catalytic activities of CS/PMAA fiber and CS/PMAA-2-pyridylimine fiber supported transition metal catalyst.a,b Transitiona metal
Supporting material
Reaction time (h)
Conversion (%)
Yield (%)
Pd Pd Co Co Ni Ni
CS/PMAA fiber CS/PMAA-2-pyridylimine fiber CS/PMAA fiber CS/PMAA-2-pyridylimine fiber CS/PMAA fiber CS/PMAA-2-pyridylimine fiber
3 3 24 24 12 12
98 100 58 87 63 93
98 99 58 81 61 93
a Reaction condition: PhI: 0.7 mmol; n-Butyl acrylate: 1.4 mmol; K2CO3: 5.25 mmol; Pd fiber catalyst: 10 mg or Co fiber and Ni fiber catalyst: 50 mg; solvent: DMAc 3.0 g for Pd fiber catalyst and DMSO/ethylene glycol (2.0 g/1.0 g) mixture for Co and Ni fiber catalyst; reaction temperature: 110 C for Pd fiber catalyst and 145 C for Cu and Ni fiber catalysts. b Cross-coupling yields were determined from the GC/MS measurements based on the amount of aromatic halide subtrate.
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demonstrate the potential of utilizing the CS-based composite fibers as a versatile platform for supporting a variety of catalysts. Last, we compared the catalytic activities of the Pdpyridylimine-CS/PMAA and Pd-CS/PMMA fiber catalysts with PdCl2, a homogenous catalyst for Heck reaction. Often, faster kinetics is found in the reactions with homogeneous catalyst rather than that with heterogeneous catalyst, which involves biphasic system. Interestingly, at the same Pd loading, the reaction catalyzed by Pd-pyridylimine-CS/PMMA is almost as fast as that catalyzed by PdCl2, and significantly faster than Pd-CS/PMMA catalyzed reaction (Fig. 12). The improved catalytic activity of Pd-pyridylimine-CS/ PMMA may stem from the improved dispersion of Pd on the fibers and/or the specific interactions between Pd and pyridylimine ligands. An in-depth characterization of Pd-pyridylimine-CS/PMMA composite nanofibers and the modeling of metal-ligand interactions are the subject of future studies, which may elucidate the molecular mechanism of unusual catalytic activity and stability found in the system with the “post-modification”.
3. Conclusion We found that mixing PMAA or PAA with chitosan in acetic acid solution improves chain entanglements during the fast solvent evaporation of electrospinning process, and allow for the formation of continuous nanofibers. Crosslinking can be subsequently induced in the blend to enhance solvent and thermal stability of the nanofibers. The composition and the molecular weight of PMAA in the blend can be varied to provide further controls over the mechanical properties of the nanofibers. These composite nanofibers provide a useful platform to support a variety of catalysts by taking advantage of the chelating sites of chitosan, the large surface areas of nanofibers, and their stability even under harsh environmental conditions. We showed that palladium catalyst can be immobilized on the crosslinked composite nanofibers to carry out MizorokieHeck cross-coupling reactions efficiently. We further demonstrated that, by the “post-modification” of the nanofibers with ligands that chelate metal catalysts, a variety of metal catalysts can be incorporated into the fiber platform, with drastically improved stability and activity.
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4. Experimental procedures 4.1. Materials Chitosan (CS) (pharmaceutical grade, 95%, deacetylated, Mn ¼ 2.0 105) was purchased from Zhejiang Aoxing Biotechnology Co., Ltd (Zhejiang, China). Acetic acid, methacrylic acid, acetic acid and N,N-dimethylformide (DMF) (analytical grade) were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Palladium chloride (PdCl2) (Chemical Pure) was purchased from Zhejiang Metallurgical Research Institute (Zhejiang, China). Cobalt chloride hexahydrate (CoCl2$6H2O) was bought from Aladdin Chemistry Co., Ltd (Shanghai, China). Nickel nitrate hexahydrate (NiNO3$6H2O) was purchased from Shanghai Zhanyun Chemical Co., Ltd (Shanghai, China). Deionized water was used in all experiments. 4.2. Synthesis of poly(methacrylic acid) (PMAA) Methacrylic acid (20.0 g), benzoyl peroxide (BPO, 0.2, 2.0 and 20.0 g) and toluene (200 mL) were mixed in a two-neck, roundbottom flask equipped with a mechanical stirring and a condenser. The reaction was performed at 110 C for 5 h. On completion, the PMAA was separated by filtering, and dried under reduced pressure at 60 C for 12 h. 4.3. Preparation of CS/PMAA composite nanofibers CS (0.35 g) and PMAA (0.35 g) were dissolved in CH3COOH (5.40 g), DMF (1.00 g) and H2O (1.60 g) mixed solvent and stirred overnight to obtain homogeneous solution. The prepared chitosan solution was placed in a syringe (20 mL) bearing a capillary (0.8 mm inner diameter) as spinneret, which was connected with a high voltage power supply (GDW-a, Tianjin Dongwen High-voltage Power Supply Plant, China). A sheet of aluminum foil, connected to the ground, was placed under the syringe as collector. A voltage of 24 kV was applied to chitosan solution with the distance between the syringe tip and the collector being 15 cm. The feed rate of the polymer solution was kept at 1.0 mL/h by a micro-infusion pump (WZ-502C, Zhejiang Smiths Medical Instruments Co., Ltd. China). The resultant mats were dried under vacuum at room temperature to remove the residual solvent. The prepared fiber mats were then heated at 180 C for 2 h to prepare the crosslinked CS/PMAA fiber mats. 4.4. Swelling capacity of crosslinked CS/PMAA fiber mat The CS/PMAA fiber mats were soaked in a 50 wt% acetic acid aqueous solution for 48 h. The samples were then separated by filtration through a sintered filter funnel (pore size: 3e4 mm). The swelling capacity of the CS/PMAA fiber mat was calculated by the following equation:
Q¼
m2 m1 m1
(1)
where m1 and m2 are the mass of the dried and swollen fiber mat, respectively. The Q value was calculated as grams of acetic acid solution absorbed per gram of mat sample. Fig. 12. Kinetics of Heck reaction catalyzed by PdCl2 (in blue triangles), Pd-CS/PMAA (in red circles) and Pd-pyridylimine-CS/PMAA (in black squares). Reaction condition: 0.7 mmol iodobenzene, 1.4 mmol n-butyl acrylate, 0.014 mmol Pd catalyst and 5.25 mmol K2CO3 in a solution of 3.0 mL DMAc at 110 C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
4.5. Utilize crosslinked CS/PMAA nanofibers to support transition metal catalyst Palladium immobilization: 500 mg crosslinked CS/PMAA fiber mat was added to a round-bottom flask containing 25 mg PdCl2 and
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17 mg NaCl (molar ratio: 1/2) and 5 g H2O. The mixture was stirred overnight, and then 0.5 mL hydrazine (80 wt%) was added to reduce the Pd2þ into Pd0. The fiber mat turned black shortly, indicating the successful immobilization of palladium on the fibers. The palladium chelated chitosan fiber mat (Pd-CS/PMAA fiber) was filtered after the process and dried under a reduced pressure for 12 h. The Pd content in the Pd-CS/PMAA was 2.90 wt%, which was determined by inductively coupled plasma-atomic emission spectroscopy (ICPAES). Cobalt and nickel immobilization: 500 mg crosslinked CS/PMAA fiber mat was added to the CoCl2 or NiNO3 aqueous solution (10 mL, 50 mg CoCl2 or NiNO3). The mixture was stirred for 12 h. The crosslinked CS/PMAA fiber supported Co or Ni catalyst (Co-CS/ PMAA or Ni-CS/PMAA fiber) was collected by filtration and dried at reduced pressure for 12 h. The contents of Co and Ni in the catalyst were 4.51% and 4.73%, respectively. 4.6. Modification of CS/PMAA fiber with 2-pyridinecarboxaldehyde CS/PMAA fiber mat (500 mg) after thermal treatment was added into the 2-pyridinecarboxaldehyde aqueous solution (20 mL, 5 wt %). The mixture was stirred for 12 h and the CS/PMAA-2pyridylimine fiber was collected by filtration and wash with H2O and ethanol, and dried at room temperature for 12 h under reduced pressure. The immobilization of Pd, Co and Ni on CS/PMAA-2pyridylimine fibers was identical to that for CS/PMAA fibers. The contents of Pd, Co and Ni in the fiber catalyst were measured to be 2.96%, 5.58% and 4.75%, respectively. 4.7. MizorokieHeck cross-coupling reaction catalyzed by fiber catalyst Aromatic iodides (0.7 mmol), acrylate (1.4 mmol), Pd fiber catalyst (50 mg, Pd: 0.014 mmol), K2CO3 (5.25 mmol) and 3.0 g N,Ndimethylacetamide (DMAc) were added to a 20 mL tubular reactor equipped with a magnetic stir bar. The mixture was allowed to stir at 110 C and the reaction progress was monitored by TLC and GC/ MS analysis. After completion, the reaction mixture was cooled down to room temperature, and quenched with 10 mL of water and extracted three times with ethyl acetate (3 20 mL). The combined organic layer was washed with water, saturated brine, and then dried over anhydrous Na2SO4. Solvent was removed under a reduced pressure. The residue was purified by silica gel chromatography with a mixture of petroleum ether and ethyl acetate as eluting solvent to afford the cross-coupling products. All of the cross-coupling products were characterized by 1H-NMR spectroscopy and mass spectroscopy. The solvent for Co and Ni fiber catalyzed MizorokieHeck reaction was DMSO/ethylene glycol (2.0 g/1.0 g) mixture and the reaction was carried out at 145 C. The other process was same as that of Pd fiber catalyzed MizorokieHeck reaction. 4.8. Characterization The morphologies of the electrospun mats were characterized by a scanning electron microscope (SEM) (Jeol, Jsm-6360lv, Japan). The fiber samples were analyzed by Fourier transform infrared spectroscopy (FT-IR) in an attenuated total reflection mode (Nicolet, Nexus-470, USA). 1H-NMR spectra were recorded in CDCl3 (Bruker, AVANCE III 400 MHz, Switzerland) using TMS as the internal reference. Multiplicities are reported as: singlet (s), doublet (d), and multiplet (m). Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis was performed on a Leemann ICPAES Prodigy XP (Leeman Labs, USA). The rotary viscosity was tested
using NDJ-8S type rotary viscometer at room temperature (20 C) (Shanghai Pingxuan Scientific Instrument Co. Ltd, China). The weight-average molecular weights (Mw) and number-average molecular (Mn) were measured by GPC (Agilent PLgel 5 mm MIXED-C Column, USA) with N,N-dimethylacetamide as the mobile phase and relative to poly(methyl methacrylate) standards. The mechanical analysis was carried out on electronic fabric strength tester (YG065HC/PC, Shandong Lanzhou Electron Instrument Co., Ltd, China). The thermal stability of the fibers was analyzed by a thermogravimetric analyzer (TGA) (Beijing Scientific Instrument Factory, China) under nitrogen atmosphere. The heating ramp was from 20 to 650 C at a rate of 10 C/min. The conductivity was determined by conductivity meter (DDS-307, Shanghai Inesa Scientific Instrument Co. Ltd, China). The hydrodynamic radius of polymer solution was measured by dynamic Light Scattering (DLS) (ALV-CGS-3MD, ALV-Gmbh, Germany). Structural characterization was carried out by wide angle X-ray diffraction (WXRD) (D8 Advance, Bruker, Germany). Diffraction patterns of Pd-CS/PMAA were obtained by the method of X-ray powder diffraction (XRD) (Rigaku D, max-3BX, Japan). The dispersion of palladium nanoparticles on fibers were recorded by JEOL TEM (JEM-1011, Japan). Acknowledgments We greatly appreciate the financial supports from the Zhejiang Province Natural Science Foundation (LQ14E030003) and Zhejiang Nanomaterials and Chemistry Key Laboratory. L.S. is grateful for the support from the IMS at UCONN when he was a visiting scholar there. Y.L. acknowledges the support from University of Connecticut Research Foundation and a Senior Visiting Scholarship from Shaoxing University. We thank Prof. Richard Parnas for helpful inputs to the manuscript. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2015.08.031. References [1] M. Dash, F. Chiellini, R.M. Ottenbrite, E. Chiellini, Prog. Polym. Sci. 36 (8) (2011) 981e1014. [2] T. Kean, M. Thanou, Adv. Drug Deliv. Rev. 62 (1) (2010) 3e11. [3] C.K.S. Pillai, W. Paul, C.P. Sharma, Prog. Polym. Sci. 34 (7) (2009) 641e678. [4] S. Jana, A. Cooper, F. Ohuchi, M. Zhang, ACS Appl. Mater. Interface 4 (9) (2012) 4817e4824. [5] R. Xu, Q. Zhou, F.T. Li, B.R. Zhang, Chem. Eng. J. 222 (2013) 321e329. [6] F. Peirano, T. Vincent, F. Quignard, M. Robitzer, E. Guibal, J. Membr. Sci. 329 (1e2) (2009) 30e45. [7] M. Bradshaw, J.L. Zou, L. Byrne, K.S. Iyer, S.G. Stewart, C.L. Raston, Chem. Commun. 47 (2011) 12292e12294. [8] H. Kim, A. Nishida, H. Yamamoto, Macromol. Rapid Commun. 25 (18) (2004) 1600e1605. [9] K. Ohkawa, K.-I. Minato, G. Kumagai, S. Hayashi, H. Yamamoto, Biomacromolecules 7 (11) (2006) 3291e3294. [10] J.F. Zhang, D.Z. Yang, F. Xu, Z.P. Zhang, R.X. Yin, J. Nie, Macromolecules 42 (14) (2009) 5278e5284. [11] Y.Z. Zhang, B. Su, S. Ramakrishna, C.T. Lim, Biomacromolecules 9 (1) (2008) 136e141. [12] K. Sun, Z.H. Li, Express Polym. Lett. 5 (4) (2011) 342e361. [13] N. Bhardwaj, S.C. Kundu, Biotechnol. Adv. 28 (3) (2010) 325e347. [14] G.C. Rutledge, S.V. Fridrikh, Adv. Drug Deliv. Rev. 59 (14) (2007) 1384e1391. [15] Z.M. Huang, Y.Z. Zhang, M. Kotaki, S. Ramakrishna, Compos. Sci. Technol. 63 (15) (2003) 2223e2253. [16] L.J. Shao, J. Wu, Z.K. Xu, Chin. J. Polym. Sci. 27 (1) (2009) 115e120. [17] M.G. Mckee, G.L. Wilkeds, R.H. Colby, T.E. Long, Macromolecules 37 (5) (2004) 1760e1767. [18] M.Z. Elsabee, H.F. Naguib, R.E. Morsi, Mat. Sci. Eng. C Mater. 32 (7) (2012) 1711e1726. [19] M. Pakravan, M.C. Heuzey, A. Ajji, Polymer 52 (21) (2011) 4813e4824. [20] I. Dora, C. Michel, L. Leibler, Soft Matter 10 (2014) 1714e1722.
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