Fabrication and characterization of honeycombpatterned poly (n-Vinylcarbazole)/carbon nanotube composite films based on non-surface modified carbon nanotube

Nghiên cứu khoa học công nghệ Tạp chí Nghiên cứu KH&CN quân sự, Số Đặc san Hội thảo Quốc gia FEE, 10 - 2020 395 FABRICATION AND CHARACTERIZATION OF HONEYCOMB- PATTERNED POLY (N-VINYLCARBAZOLE)/CARBON NANOTUBE COMPOSITE FILMS BASED ON NON-SURFACE MODIFIED CARBON NANOTUBE Phung Xuan Thinh* Abstract: A micro-scale honeycomb-patterned conducting film was fabricated by using the Poly(N-vinylcarbazole)/ multiwalled carbon nanotube (PVK-MWCNT) composite solution. These composite solutions with different MWCNT concentrations were facially prepared by dispensing non-surface modified MWCNT into PVK solution under vigorous sonication for a long time. Analysis for the material characteristics and film morphology proved the interaction between PVK and non-surface modified multiwalled carbon nanotubes. DC conductivity of the patterned film is remarkable even at low non-surface modified MWCNT concentration. In addition, calcination of the honeycomb-patterned films was conducted at 150, 250, 400, and 490 oC to study the arrangement of MWCNTs in the patterned films and to measure the DC conductivity depending on the calcination temperature. DC conductivity of the patterned films was increased by increasing the concentration of MWCNTs in the composites and in the increased calcination temperature. This simple method is promising for the preparation of similar honeycomb-patterned conducting films. Keywords: Honeycomb pattern; Conducting film; Multiwalled carbon nanotubes; CNTs dispersion; Poly (N- vinylcarbazole). 1. INTRODUCTION It is well known that carbon nanotubes (CNTs) exhibit interesting mechanical (high tensile strength and stiffness), thermal, geometrical and electrical properties combined with a good chemical stability [1-3]. Carbon nanotubes exhibit semiconducting and metallic properties depending on their diameter and helicity [4]. With all possible applications of CNTs in the industry, the interaction between CNTs with other organic/inorganic moieties is not so appreciable [5, 6]. Several attempts have been made, one of the most often used strategies is to functionalize or incorporate such materials into a polymer backbone [7]. However, practical application of CNTs still faces the problem of good dispersion, as the substantial intertube van der Waals attraction makes them appear in bundles and affects their extraordinary properties. To overcome this disadvantage, functionalization methods have been adopted to chemically modify the surface properties of CNTs [8]. However, in these methods, functional groups are covalently linked to the surface of CNTs, making their mechanical and electrical properties change a lot, as compared with pristine tubes [9]. Therefore, the methods that can help disperse pristine CNTs in the composites are particularly interested because it retains the structural integrity of CNTs and their properties are hence not disrupted, which is important for the following applications. Honeycomb-patterned thin films have attracted considerable attention because of their potential use in microreactors, separation processes, electronics, photonics, and biotechnology [10, 11]. These films are fabricated by various approaches, such as lithography, use of colloidal crystals, self-assembly, and rod-coiled copolymers [11, 12]. In the self-assembly approach, the breath-figure method is the simple and useful technique for preparing honeycomb-patterned thin films [12]. In this way, the substrate is completely dispersed in organic solvents such as chloroform, and then the films were fabricated by casting these solutions under humid conditions [13, 14]. Hóa học – Sinh học – Môi trường Phung Xuan Thinh, “Fabrication and characterization modified carbon nanotube.” 396 Recently, studies indicated that the application of honeycomb-patterned thin films of conducting polymer/ polymer composites is great potential such as improving the performances of sensing devices, or improving the performance of solar cells and solar cell applications [13, 15, 16]. Therefore, with its enormous potential, the study for generating well-defined CNT and CNT-based materials architectures are not an exception. The combination of CNTs in selected polymer matrices is an effective strategy attracting the attention of research group in the world [17]. Among the different polymeric materials currently available, poly(N-vinylcarbazole) (PVK) has emerged as one of the more useful materials for electro-optically active applications, including light-emitting diodes and xerography. PVK is very useful in electronic devices because of its chemical and thermal stabilities and its excellent electrical properties [18]. Moreover, PVK is considered an ideal model of a nonconjugated photoconducting polymer with strong electron-donor properties. Reactions with vinylcarbazole are easily undertaken and can be performed in bulk, solution, suspension, or precipitation [19]. In our earlier study [20], the honeycomb-patterned films were fabricated from PVK- MWCNT composite. This material was synthesized through the oxidative polymerization of N-vinylcarbazole monomer with ferric chloride in the presence of surface modified MWCNT. The film was shown optoelectronic properties remarkable that can be applied in many potential areas. In an effort to attempt preserving the structure and properties of pristine CNTs in the composite, in this study, MWCNT without any surface modification was dispersed at different concentrations in the composite by ultrasonication techniques. The resulting solution is then directly used to fabricate PVK-MWCNT honeycom- patterned films using breath-figure technique. The dispersion and obtained linking between MWCNT and PVK were characterized using Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible (UV-Vis) spectra, and thermogravimetric analysis (TGA). The ordered structures of the PVK-MWCNT polymer films were obtained and studied using scanning electron microscopy (SEM). Further to understand the self- assembly of MWCNTs in the ordered structures, calcination studies of the patterned films were conducted at 150, 250, 400, and 490 oC. The conductivity of the film depending on the concentration of MWCNT and calcinations temperature was also studied. 2. EXPERIMENTAL 2.1. Materials All the other reagents of the Poly(N-vinylcarbazole) (average Mw ~1,100,000 d=1.2 g/mL, powder, Aldrich), pristine MWCNTs (>90%, diameter =110-170 nm, length = 5-9 nm, Aldrich), methanol (≥ 99.8%, Aldrich), and chloroform (≥ 99 %, Aldrich) were used as-received without further purification. De-ionized (DI) water was used in this experiment. 2.2. Preparation of PVK-MWCNT composite and fabrication of the honeycomb pattern in PVK-MWCNT films In the preparation of PVK-MWCNT polymer composites, each predetermined percentage of PVK (1g) and 30 mL of chloroform (CHCl3) was introduced in a beaker. The mixture was sonicated at room temperature for 3 h to produce a homogenous dispersion of PVK in solvent. Then, the different concentrations (in wt %) of pristine MWCNTs were added in the mixture and further vigorously sonicated continuously for 6h until obtaining homogenous state. The target mass loading of MWCNTs in the composites varied from 1, 5, and 10wt %. The composites with 1, 5, and 10wt % of MWCNTs in PVK are termed in this report as PC-1, PC-5, and PC-10, respectively. Nghiên cứu khoa học công nghệ Tạp chí Nghiên cứu KH&CN quân sự, Số Đặc san Hội thảo Quốc gia FEE, 10 - 2020 397 To characterize the dispersion of MWCNT into PVK, after sonication, a part of each sonicated solution was centrifuged at 3000 rpm for 30 minutes to precipitate the free MWCNTs. The upper solution (supernatnat) is then collected. An excess amount of methanol was added to the supernatant to obtain a dark brown precipitate. After filtration, this precipitate was washed several times with distilled methanol and water, and dried in a vacuum oven for about 48 h at 60oC. The obtained products were used for characterizations of FTIR, UV-Vis, and TGA. To fabricate the honeycomb pattern in the obtained PVK-MWCNT polymer composites, the constant volumes (5mL) of each PVK-MWCNT composite solution was cast on a glass Petri dish. To obtain a highly ordered honeycomb-patterned structure, evaporated water was applied on the solution surface through an air pump with a flow rate of 0.6 L/min. The macroporous film was formed by the condensation and deposition of water droplets on the solution surface due to evaporative cooling. The temperature and relative humidity under which the ordered structures were conducted were 25oC and 60%, respectively. After complete evaporation of the solution under humid conditions, a dark gray film was obtained. The overall experimental scheme for formation of the honeycomb structure for the PVK-MWCNT composite with the assistance of water-droplet is introduced in figure 1 [20]. Figure 1. Schematic diagram showing the fabrication of honeycomb-patterned PVK- MWCNTs films, and treatment of the films by calcination. 2.3. Characterization The infrared (IR) spectra of the samples were obtained with an FTIR spectrometer (Perkin-Elmer Model 1600). The samples were prepared by cryogenically grinding the synthesized polymer with KBr (polymer: KBr = 1:20) and compressing the mixture on a disk. About 60 scans were signal-averaged at a resolution of 2 cm−1 from 4000 to 400 cm−1. The UV-Vis spectra of obtained PVK-MWCNT solution were further recorded using a Shimadzu UV-Vis-NIR spectrophotometer (UV-3101PC). The thermal properties of the samples were obtained by TGA (Perkin Elmer model TGA 7) in the range of 20 - 800 oC at 10 oC /min in a nitrogen atmosphere. The DC electric measurements of the obtained composite films were performed at room temperature using the four-probe technique with a Keithly 224 constant current source and a Keithly 617 digital electrometer. For the calcinations study, the honeycomb-patterned films of the PVK-MWCNT composites were placed for about 1 h at a predetermined temperature in an electric muffle furnace (Model C-FMD, Chang Shin Science Co.) in which the temperature is controlled by 1oC precision. The calcination temperatures were chosen as 150, 250, 400 and 490 oC. The films were then cooled in the oven to room temperature and removed carefully. The structures of the films before and after calcination were analyzed using SEM (COXEM CX-100) at room temperature. The DC electrical conductivity of the films after calcination was also measured at room temperature. Hóa học – Sinh học – Môi trường Phung Xuan Thinh, “Fabrication and characterization modified carbon nanotube.” 398 3. RESULTS AND DISCUSSIONS 3.1. Characterization of PVK-MWCNT polymer composites Figure 2a shows the FTIR spectra of PVK and the PC-1, PC-5, and PC-10 composites in the regions between 400 and 4000 cm-1. The spectra obtained for PVK and the PVK- MWCNT polymer blends are consistent with the infrared spectrum of PVK that has been reported in the literature [2023]. Ideally, pristine MWCNT does not show obvious absorption peaks in FTIR analysis. The presence of PVK in the PVK-MWCNT composites was supported by the appearance of FTIR peaks at 720, 745, 1223, 1327, 1450, 1625, and 3053 cm-1 in the region between 500 and 4000 cm-1. In addition, the presence of the peaks at 2960 cm-1, and 2850 cm-1 in the composite spectra can be assigned to asymmetric methyl stretching and asymmetric/ symmetric methylene stretching bands for MWCNTs, respectively. It is usually assumed that these groups are located at defect sites on the MWCNTs sidewall surface that may be caused during long- term sonication. Figure 2. FTIR (a), UV-vis (b) spectra, and TGA curves (c) of PVK and PVK-MWCNT. Figure 2b reports the UV-visible spectra of PVK and the PC-1, PC-5 and PC-10 composites. Absorbance peaks were observed at about 240, 262, 295, 320, and 340 nm, all of which are similar to the PVK peak reported in the literature [24]. Interestingly, the UV- visible spectra of PC-1, PC-5 and PC-10 showed only a slight blue shift of about 5 - 6 nm for the peaks between 260 and 295 nm. This shift in the composite absorbance bands may be attributed to the interactions between MWCNT and the PVK polymer. Figure 2c shows the thermogravimetric curves for PVK, PC-1, PC-5 and PC-10. PVK shows three different stages of thermal degradation: The first one from room temperature to 120oC probably the loss of moisture, the second from 120450oC probably due to the loss of dopant ions from the polymer matrix; the third step is a complete degradation of PVK which proceeds at about 450 oC. The thermal degradation process for PC-1, PC-5 and PC-10 is same as PVK, but with an increase in MWCNTs in PVK the thermal stability increases. The weight loss of PC-1, PC-5 and PC-10 greater than that of PVK recorded in 120-450 oC range may be attributed to the decomposition of the unstable-thermal components such as defects/dopant ions in the MWCNTs. Since MWCNTs are highly Nghiên cứu khoa học công nghệ Tạp chí Nghiên cứu KH&CN quân sự, Số Đặc san Hội thảo Quốc gia FEE, 10 - 2020 399 thermal stable, as shown in the figure, the remaining weights after 450 oC are corresponding to the initial MWCNT weights in the composites. This indicates that MWCNT was dispersed in the polymer network; and furthermore, the inclusion of MWCNTs in PVK has a positive influence on its thermal property. 3.2. Pattern formation at room temperature Figure 3. Photographs of pristine MWCNTs and PVK-MWCNT composites dispersion in chloroform at concentration of 5 mg mL-1 after 7 days sonication. The ordered films are formed by casting the homogeneous PVK-MWCNT composite solution under humid conditions. The homogeneous dispersion of the composite in chloroform may be due to the interaction of two factors: the formation of hydrogen bonding between the functional groups on MWCNTs surface and the ‘N’ atoms of PVK moieties [25], and the interactions between the polymer main chain and MWCNTs sidewalls through noncovalent electrostatic and van der Waals attraction after sonication for long times [8]. In the case of treated MWCNT, the presence of oxygen-containing functional groups helped modified MWCNT become more active, and therefore to be easier to engage in the bondings with PVK; Moreover, MWCNTs truncated during the chemical treatment will be dispersed more easily into the polymer solution. For the pristine case, there are not these factors leading to lower homogeneity. However, the dispersion state of CNTs into composite was stable even though after a long time of sonication treatment as shown in figure 3. Figure 4. SEM images: top-view (left) and cross-sectional view (right) of (a) PC-1, (b) PC-5, and (d) PC-10 films. Figure 5. SEM images of the films after calcination, (a) at 150 oC, (b) 250 oC , (c) 400 oC, and (d) 490 oC for PC-1 (left), PC-5 (center), and PC-10 (right). Hóa học – Sinh học – Môi trường Phung Xuan Thinh, “Fabrication and characterization modified carbon nanotube.” 400 Figure 4 shows SEM images obtained at 20 μm of the ordered patterns for PC-1, PC-5 and PC-10 polymer films at room temperature. The ordered structures formed in PC-1 are circular in nature with almost uniform pore-size. The increase of MWCNTs concentration in the composites not only increases the pore size but also increases the irregular in the pore arrangement as observed in PC-5 and PC-10 films. This may be due to the change of surface tension of the composite solution caused by the MWCNT presence in the polymer matrix. Compared with the films obtained in earlier results that using surface modified MWCNT [20], the regularity of pores in PC-5, PC-10 is lower, and the effect of MWCNTs concentration on the pore distribution is significantly more (considered in the same concentration range). 3.3. Calcination on the ordered structures in PVK-MWCNT films Figures 5 is the SEM images obtained at 20 μm for PC-1, PC-5 and PC-10 films calcinated at 150 and 250 oC; and for PC-5 and PC-10 films calcinated at 400 oC respectively (at low MWCNT concentration such as PC-1, it is impossible to prepare SEM samples after calcination at temperature more than 400 oC). At the ordered structures of the films do not seem to be affected and therefore the structures obtained in PC-1, PC-5 and PC-10 are same as that of the structures obtained at room temperature. The PVK is responsible and contributes to the formation of ordered structures of composite that containing MWCNT. The temperature has an effect only on the PVK since PVK is less thermally stable as compared to MWCNTs. At higher temperature as at 490oC, according to the thermogravimetric analysis, PVK will be disappeared completely, and skeleton frame of the ordered structures by MWCNT will be exposed visually and therefore the nature of the films were analyzed. Clearly, MWCNTs were distributed uniformly in the film even though without any chemical modification. This demonstrated to the effective dispersion of pristine MWCNTs in the composite under long-term sonication condition. 3.4. DC conductivity of the ordered structures in PVK-MWCNT films The DC conductivity of the PC-1, PC-5 and PC-10 films was measured after obtaining the ordered structures both at room temperature and also after calcination. The results are indicated in table 1. Table 1. DC conductivities of PC-1, PC-5, and PC-10 films before and after calcination at different temperatures. Materials At room temperature (S.cm-1) After calcination at different temperatures (S.cm-1) 150 oC 250 oC 400 oC PC-1 PC-5 PC-10 2.15x10-4 0.65x10-2 3.22 2.54x10-4 0.71x10-2 3.68 0.036 1.25 8.62 xxx xxx 325.5 xxx- film has broken and not possible to obtain conductivity It is similar to earlier study [20], it was not possible to obtain the conductivity data at 400 and 490oC after calcination of the films, because these films were very sensitive to handle and are easily breakable. Obviously, even at room temperature the conductivity values of the PC-5, PC-10 films were thousands times greater than the corresponding values of the films used modified MWCNT that noted in previous reports [20]. After calcination, the difference was becoming increasingly clear (it was increasing exponentially). This can be explained by the preservation of the structure and the unique properties of pristine MWCNT compared to modified MWCNT. At the higher calcination temperature, the structure of PVK gradually degraded, thus, the conductivity of the film Nghiên cứu khoa học công nghệ Tạp chí Nghiên cứu KH&CN quân sự, Số Đặc san Hội thảo Quốc gia FEE, 10 - 2020 401 will tend to rise and reach of MWCNT conductivity value. After calcination at 250oC, the value recorded for the film containing 10% wt/wt of non-modified MWCNT is 8.62 S.cm- 1[20], while this value is only 0.065 S.cm-1 for modified MWCNT case. After calcination at 400oC, these values are 325.5 S.cm-1 [20], and 0.255 S.cm-1 respectively for the non- modified- and modified MWCNT composite. The increase in DC conductivity of the patterned PVK-MWCNT composite films in room temperature and after calcination supports the uniform distribution of MWCNT in the PVK polymer composites. The honeycomb-patterned structures formed in the PVK- MWCNT composite films are due to PVK. During the formation of PVK-MWCNT polymer films, the polymer PVK wraps MWCNT. The study on the remained honeycomb structures after calcination provides the valuable information how MWCNTs are arra