Fabrication of Vertical Array CNTs/Polyaniline Composite Membranes by Microwave-Assisted In Situ Polymerization
© Ding et al. 2015
Received: 15 October 2015
Accepted: 14 December 2015
Published: 24 December 2015
A vertical array carbon nanotubes (VACNTs)/polyaniline (PANi) composite membrane was prepared by microwave-assisted in situ polymerization. With microwave assistance, the morphology of PANi revealed a smaller diameter and denser connection. Meanwhile, thermogravimetric analysis showed improved thermal stability of microwave-assisted PANi for higher molecular weight. Focused ion beam thinning method was used to cut the VACNTs/PANi membrane into dozen-nanometer thin strips along the cross-sectional direction, and transmission electron microscopy observation showed seamless deposition of PANi between VACNT gaps, without damaging the vertical status of CNTs. Meanwhile, stronger conjugate interaction between the quinoid ring of PANi and VACNTs of the composite membrane were prompted by microwave-assisted in situ polymerization. By using nanoindentation technology, the VACNTs/PANi composite membrane showed exponential increasing of modulus and hardness. Meanwhile, the elasticity was also improved, which was proved by the calculated plastic index. The results can provide helpful guidance for seamlessly infiltrating matrix into CNT array and also demonstrate the importance of structural hierarchy for getting proper behavior of nanostructures.
KeywordsVertical array carbon nanotubes Polyaniline Composite Microwave assisted
Carbon nanotubes (CNTs) , with graphene composed of carbon atoms curling to a hollow tubular structure, possess an ultra-high aspect ratio, atomically smooth nanoscale pores for gas transport [2, 3], high mechanical strength , and unique electronic properties [5–7]. Vertical array CNT (VACNT) membranes have found many promising applications such as being supercapacitors , being compliant thermal interface materials , in selective gas transport , and being reinforcements in composites for enhanced thermal and mechanical properties .
Infiltrating a matrix material into a large-surface-area VACNT membrane has attracted great attention for obtaining a novel composite membrane with synergic properties and improved performances. Several routes, such as chemical vapor deposition (CVD)  and spin coating, can be used to deposit a matrix into the space between vertically aligned and dense carbon-packed CNTs. Hinds et al.  grew well-aligned multi-walled carbon nanotubes (MWCNTs) via CVD; then, a 50 wt% solution of polystyrene and toluene was spin-coated over the surface. However, spin coating may destroy CNT arrays because of its centrifugal force. Holt et al.  presented an approach for depositing silicon nitride into a MWCNT array via a low-pressure CVD method; then, the tubes were etched to prepare a seamless composite membrane. This method could successfully avoid destroying CNT arrays; however, silicon nitride was too brittle which limited the mechanical properties of the membrane. Miserendino  and Zhang  filled in the gaps of MWCNTs with parylene matrix via CVD and found that parylene could fully pad CNT gaps. Polymer materials exhibit good chemical stability and biocompatibility, easily filling implement, while choosing the appropriate polymer into VACNTs is a considerable challenge.
Polyaniline (PANi) is a widely studied low-cost electrically conducting polymer that exhibits facile synthesis and environmental stability [17, 18]. Over the last decades, the chemical synthesis approach has been used to obtain a wide variety of PANi with different morphologies and properties. Ramana  prepared a PANi-coated CNT composite thin film via an in situ rapid mixing chemical oxidative polymerization method. Polysulfone composite membranes created with PANi and functionalized multi-walled CNTs were synthesized using a non-solvent/solvent-induced phase separation technique in our research group . In addition, MWCNTs/PANi composite membranes were successfully fabricated by filtration and the flash welding method in our previous work . PANi can be synthesized through different approaches to obtain composites with other materials, such as in situ polymerization [22–25], electrochemical processes , microemulsion polymerization , and interfacial polymerization . As a novel energy, microwave can quickly absorb electromagnetic energy and generate rapid thermal effects through molecular dipole interaction . Microwave-assisted synthesis can be an effective strategy to control the structure and morphology of a polymer. A PANi nanofiber-coated graphite electrode was successfully fabricated by microwave-assisted chemical vapor-induced in situ polymerization, with mixed structures of emeraldine base and fully oxidized form .
In this study, PANi was deposited seamlessly into the space between VACNTs by microwave-assisted in situ polymerization. The structure and thermal stability of PANi were characterized by Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA), respectively. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to observe the morphology of PANi and VACNTs/PANi composite membrane. The structure and interaction of PANi and VACNTs were further explained by FTIR and Raman spectroscopy. In addition, the nanoscale mechanical properties of the VACNTs/PANi membrane were discussed according to the nanoindention measurement. The results of the investigation revealed that microwave could be an effective way for in situ polymerization and impregnation of PANi between the VACNT nanotube gaps.
The VACNTs (MWNT array with 3–10-nm diameter and 50-μm length, 98 wt%) were purchased from Chengdu Organic Chemical Co., Ltd., Chinese Academy of Sciences (Chengdu, China). Aniline (ANi) as the monomer, ammonium persulfate (APS) as the catalyst, and hydrochloric acid (HCl) (AR grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
To observe surface morphology easily, we prepared PANi on a cellulose membrane using the same synthesis process, and the product was coded as PANi-MW (powder or film, depending on the ways of sample collection). In addition, PANi polymerized by chemical oxidation at room temperature without microwave irradiation was synthesized for comparison, and the product was also coded as PANi (powder or film).
The structures of PANi and VACNTs/PANi composite were characterized by FTIR (Spectrum 100, PerkinElmer Co., Ltd., USA) and Raman (LabRam-1B, Horiba Jobin Yvon Co., Ltd., USA, laser 532 nm) spectroscopy, respectively. Thermal stability was characterized by TGA (Pyris 1, PerkinElmer Co., Ltd., USA) in nitrogen atmosphere with a 10 °C/min heating rate from 50 to 800 °C. The molecules of PANi were measured via gel permeation chromatography (GPC 50, Waters Co., Ltd., USA) and dissolved in N,N-dimethylformamide. Morphology characterizations were imaged using SEM (Quanta FEG450, FEI Co., Ltd., USA) and TEM (Tecnai G2 20 TWIN, FEI Co., Ltd., USA). Focused ion beam (FIB, Helios Nanolab 600, FEI Co., Ltd., USA) was used to prepare the samples with a thickness of approximately 70 nm for TEM observation. Nanoindentation (Agilent Nano G200) was performed to investigate the nanomechanical properties of the samples.
Results and Discussion
FTIR Spectroscopy of PANi
Raman Spectroscopy of PANi
Surface Morphology of PANi
GPC analysis of PANi and PANi-MW
1.08 × 105
5.09 × 105
SEM of VACNTs/PANi Composite
TEM of VACNTs/PANi Cross Section
To further reveal the PANi-occupied state between VACNT gaps, the cross section of the VACNTs and VACNTs/PANi composite membrane was analyzed by TEM observation.
As shown in Fig. 7b, the cross section of VACNTs was broken with loose nanotube distribution. FIB processing may lead to bundling phenomenon of VACNTs, showing a much bigger diameter. TEM morphology of the VACNTs/PANi cross section (Fig. 7c) showed continuous PANi distribution between the nanotube gaps, providing good support effect. The bright white spots in Fig. 7e are carbon nanotube pores, and the multiwall and the surrounding polymer structures could be observed in Fig. 7f and inset. The TEM images demonstrated that PANi coated VACNTs properly and did not leave any gap and cracks, and the nanotubes kept the vertical state during the FIB slicing process. The average gap width between the nanotubes was estimated to be 30 ± 5 nm, which could be derived from the histogram of the gap width between the nanotubes (counted from 324 individual CNTs) in Fig. 7d.
Structure of VACNTs/PANi Composite
As for the VACNTs, the characteristic Raman bands at ~1595 cm−1 (G band) and ~1300 cm−1 (D band) indicated graphitic nature and disorderness pertaining to VACNTs, respectively. The G band to D band intensity ratio of approximately 6.32 indicated the high crystallinity of the MWCNTs. However, in the case of the VACNTs/PANi composite, the disappearance of the characteristic G band indicated that the VACNTs were coated with PANi, while the vibration peak at 1472 cm−1 increased with VACNTs. Meanwhile, there was a considerable red shift in characteristic bands corresponding to C=C (from 1595 to 1586 cm−1) and C=N (from 1504 to 1472 cm−1) stretching of quinoid, consistent with the FTIR results. VACNTs were electron-rich molecules that form π-π interaction and CH-π interaction, and microwave irradiation offered the energy for the formation of a charge-transfer complex between the VACNTs and aniline. Aromatic structures, in general, were known to interact strongly with the basal plane of graphitic surface via π-stacking which was due to a charge transfer from the quinoid unit of PANi to VACNTs.
Nanoindentation of VACNTs/PANi Composite
The modulus and hardness of VACNTs/PANi were 3.354 ± 0.450 and 0.095 ± 0.0230 GPa, respectively, which improved more than four times in modulus compared with that of raw VACNTs and seven times compared with that of PANi, as seen in inset of Fig. 9. After PANi was deposited seamlessly into the gaps between VACNTs, the reinforcement effect on the composite membrane was evident. In addition, the plastic indexes (marked as η in Fig. 9), which were used to measure the relative plastic deformation during the indentation, were calculated according to the three nanoindentation curves. From the calculation, η 3 (VACNTs/PANi) was 0.756, which reduced 23 and 21 % compared with that of VACNTs and PANi-MW, respectively. The PANi acted as elastic links between the CNTs and increased the elastic recovery upon unloading. Therefore, VACNTs/PANi recovered a greater portion of the compressed length and absorbed a greater amount of energy. Seamless polymerization of PANi in VACNT gaps and stronger interaction effect can make up deficiencies of VACNTs in array stability, stiffness, and elastic recovery, so as to broaden the fields of applications, such as selective transport membrane and micro-electromechanical devices.
In summary, an effective method to polymerize PANi in situ seamlessly into VACNTs was successfully developed by microwave-assisted chemical bath deposition method. With microwave irradiation, an enhanced conjugate with a more quinoid structure of PANi could be found; a smaller diameter and denser connection morphology together with a higher molecular weight and narrower PDI could be realized, showing a better thermal stability. The morphology of VACNTs/PANi presented a uniform and seamless PANi deposition between VACNT gaps, and VACNTs maintained a highly aligned forest structure. Confirmed by FTIR and Raman spectra, the highly conjugated π-stacking between PANi and VACNTs are prompted by microwave-assisted in situ polymerization. Nanoindentation showed a significant improvement in modulus and hardness for the VACNTs/PANi membrane. In addition, the elasticity of VACNTs/PANi was improved in a remarkable degree for plastic index decreasing over 20 %, which can be attributed to seamlessly filling of PANi in the VACNTs and strong interaction effect between PANi and CNTs. The obtained results in this study will enhance the ability to design and validate the performance of the VACNTs/polymer composite membrane and its microstructure for many applications.
The authors are grateful to the National Natural Science Foundation of China (51373100 and 5140030478), the Natural Science Foundation of Shanghai (12ZR1446700), and the Innovation Project of Shanghai Municipal Education Commission (13YZ074).
The authors also show sincerely thanks to Dr. Lv of the Shanghai Institute of Microsystem and Information Technology for providing professional FIB sample thinning and Dr. Chen of Fudan University for TEM characterizing.
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