Influence of Surface Modified MWCNTs on the Mechanical, Electrical and Thermal Properties of Polyimide Nanocomposites
Polyamic acid, the precursor of polyimide, was used for the preparation of polyimide/multiwalled carbon nanotubes (MWCNTs) nanocomposite films by solvent casting technique. In order to enhance the chemical compatibility between polyimide matrix and MWCNTs, the latter was surface modified by incorporating acidic and amide groups by chemical treatment with nitric acid and octadecylamine (C18H39N), respectively. While the amide-MWCNT/polyimide composite shows higher mechanical properties at low loadings (<3 wt%), the acid-MWCNT/polyimide composites perform better at higher loadings (5 wt%). The tensile strength (TS) and the Young’s modulus (YM) values of the acid-MWCNT/polyimide composites at 5 wt% MWCNT loadings was 151 and 3360 MPa, respectively, an improvement of 54% in TS and 35% in YM over the neat polyimide film (TS = 98 MPa; YM = 2492 MPa). These MWCNT-reinforced composites show remarkable improvement in terms of thermal stability as compared to that for pure polyimide film. The electrical conductivity of 5 wt% acid modified MWCNTs/polyimide nanocomposites improved to 0.94 S cm− 1(6.67 × 10− 18S cm−1for pure polyimide) the maximum achieved so far for MWCNT-polyimide composites.
KeywordsMultiwalled carbon nanotubes Polyimide Nanocomposites Electrical properties Mechanical properties
After their discovery in 1991 by Iijima , carbon nanotubes (CNTs) have attracted considerable interest because of their unique as well as superior physical, electrical, magnetic, chemical stability, thermal conductivity and mechanical properties . Due to their exceptionally high aspect ratio and mechanical properties, incorporation of small amounts of CNTs into a polymer matrix is expected to enhance the properties of the resulting nanocomposites more than any existing material. The most critical issue of CNTs/polymer nanocomposites is the adhesion/compatibility between the nanotubes and polymer which ultimately controls the interface between the CNTs and the polymer matrix. Unfortunately pure CNTs are insoluble in any organic solvents and they tend to form agglomerates because of strong Van der Wall forces which results in negative effects on the properties of the resulting nanocomposites. As such achieving a high degree of dispersion of CNTs in any polymer matrix is quite a challenging task. Chemical functionalization is the simplest and widely accepted method to improve the compatibility between CNTs and polymer in which CNTs are treated with strong acids like nitric acid and sulphuric acid or combination of both. The functionalized CNTs contain carboxylic and hydroxyl functional groups and are soluble in most of the organic solvents .
Polyimide is a high performance specialty class of polymers of aromatic nature with chemical structure –R(CO)N–, which comes under the family of ladder polymers, due to their flexibility, high strength, superior thermal stability and dielectric properties. As a result it is used in applications ranging from adhesives, thermal resistant coatings, high performance composites, fibres, foams, membranes, mouldings and films . It can be used up to 500 °C for short duration and prolonged use between 200 and 350 °C without much deterioration in mechanical and other properties. Polyimides are produced by the condensation reaction of an aromatic dianhydride and aromatic diamine to form an oligomer of amic acid known as polyamic acid which act as a precursor for polyimide and can be further cyclized (imidization) by thermal means to produce polyimide. Out of the many types of aromatic polyimides produced worldwide, the largest market and most preferred polyimide for various applications is based on the pyromellatic dianhydride (PMDA) and 4,4′-oxydianiline (ODA) .
In recent times considerable effort has been devoted in the field of preparation and study of CNTs/polyimide nanocomposites from various angles ranging from highly conductive to super strong CNTs/polyimide nanocomposites. Addressing the problem of poor dispersion of CNTs in the polyimide matrix, Park et al.  used in situ polymerization technique to disperse unmodified single wall carbon nanotubes (SWCNTs) in polyimide. Ouanies et al.  studied the electrical properties of SWCNTs doped polyimide nanocomposites. Zhu et al.  described a method for achieving enhanced dispersion of multiwalled carbon nanotubes (MWCNTs) in the polyimide matrix by the acid treatment of MWCNTs. However, they reported a slight decrease in the thermal properties of the nanocomposites due to acid modification. Mo et al.  employed in situ technique to disperse functionalized MWCNTs in polyimide matrix and reported a maximum achievable strength of 165.5 MPa at 7 wt% loading of functionalized MWCNTs. Yuen et al. concluded that by using amino functionalized MWCNTs the electrical properties of the MWCNTs/polyimide nanocomposites are enhanced considerably; unfortunately, it resulted in decreased tensile strength (TS) due to a possibility of reaction between the amino functionalized MWCNTs with polyamic acid . Hu et al. also studied amino functionalized MWCNTs/polyimide nanocomposites using in situ polymerization method to disperse MWCNTs in the polyimide matrix .
Looking at the above scenario, it seems that it is difficult to achieve significant improvement in both the electrical as well as mechanical properties of the CNT/polyimide composites simultaneously. A new type of amino functional groups was therefore grafted on the MWCNTs by treating them with octadecylamine (C18H39N). The grafted functional group consists of a secondary amide attached to the MWCNTs along with a long alkyl chain. The long alkyl chain helped in preventing agglomeration and bundling of MWCNTs due to repulsive force between the alkyl chain, producing a stable and homogeneous dispersion of MWCNTs. We report here that these amino functionalized MWCNTs reinforced polyimide nanocomposites not only exhibit improved mechanical properties, but also the electrical properties much superior to the one reported so far.
Acid Modification of MWCNTs
Two grams of MWCNTs were treated with 65% (v/v) HNO3 for 48 h in a refluxing apparatus  with constant stirring to convert the MWCNTs to acid functionalized (MWCNTs-COOH). The treated material was washed several times with deionized water till washings were neutral to pH paper and dried at 100 °C for 12 h prior to use.
Amine Modification of Acid Modified
A 100 mL-flask was charged with 1 g of MWCNTs-COOH dispersed in 30 mL of anhydrous benzene (C6H6) to which 30 mL of thionyl chloride (SOCl2) was subsequently added and stirred at 70 °C for 8 h. After the reaction, the whole reaction mixture was filtered and the solid residue (MWCNTs-COCl) was washed several times with anhydrous THF and dried at 50 °C overnight. Approximately 0.5 g of MWCNTs-COCl was suspended in THF and was stirred in excess of octadecylamine (C18H39N) for about 90 h to produce amine modified MWCNTs (MWCNTs-CO–NH–C18H37), the whole reaction mixture was maintained at a temperature of 100 °C. After functionalization reaction, MWCNTs were washed with ethanol to dissolve excess octadecylamine and subsequently with deionized water followed by drying under vacuum prior to use .
Fourier Transform Infrared Spectroscopy
Functionalization of MWCNTs and conversion of polyamic acid to polyimide (imidization) was confirmed by the Fourier transform infrared spectra recorded on a Perkin Elmer spectrum BX FTIR spectrometer.
Scanning Electron Microscopy
The surface morphology of the as-produced, functionalized MWCNTs and the fractured surface of the nanocomposites was analysed on a Scanning Electron Microscope (Leo model: S-440).
Thermal stability of the nanocomposites was examined by using a Thermo gravimetric analyser (Mettler Toledo TGA/SDTA 851e*). The test was performed between 50 and 900 °C at a heating rate of 10 °C/min in air with a flow rate of 50 cc/min.
The tensile strength (TS) and Young’s modulus (YM) of the nanocomposites were measured on an Instron machine, Model 4411, at room temperature using ASTM-D882 test method. The test samples were cut into the strips of size 100 mm × 25 mm × 0.1 mm. The gauge length was kept as 50 mm, while the cross-head speed was maintained at 2 mm/min. The mechanical properties were evaluated using the built-in software in the machine. A minimum of five tests were performed for each composite sample and their average is reported.
The electrical conductivity of the composite films (100 mm × 25 mm × 0.1 mm) was measured by four-point contact method  using a Keithley 224 programmable current source for providing current, the voltage drop was measured by Keithley 197A auto ranging digital microvoltmeter. The values reported in text are averaged over five readings of voltage drops at different portions of the sample.
Result and Discussion
Figure 4b shows the FTIR spectrum of amide-MWCNTs reinforced polyimide composites. The spectrum matches closely with that of neat polyimide as shown in Fig. 4a. This is further confirmed by the absence of broad peak around the 3200 cm−1 due to –COOH functional groups which gets converted into polyimide. The IR spectra of both neat polyimide as well as MWCNT/polyimide composites also compare well with earlier reported data .
Surface Morphology of the MWCNTs
Thermal degradation temperature (Td) andT5of MWCNTs/polyimide nanocomposites
Acid-MWCNTs/polyimide (MWCNTs wt%)
Amide-MWCNTs/polyimide (MWCNTs wt%)
As in the case of TS, the YM of the amide-MWCNTs/polyimide is also higher (3.5 GPa) up to 3 wt% of MWCNT loadings as compared to only 2.75 GPa for acid-MWCNTs/polyimide nanocomposites. However, the value is much higher as compared to neat polyimide film (YM = 2.4 GPa). The study shows that the mechanical properties obtained with amide-MWCNTs/polyimide and that of acid-MWCNTs/polyimide composites are almost equivalent, the only difference being that these are achieved with only 3 wt% of MWCNT in case of former and with 5 wt% in case of the latter.
Multiwalled carbon nanotubes were successfully functionalized with acid and amino groups. The functionalization resulted in uniform dispersion of the tube during imidization process which resulted in strong bonding and thick coating of the matrix. The fracture behaviour of amine functionalized MWCNT nanocomposites show a strong bonding of the tubes with the matrix which resulted in ~50% improvement in the TS and 35% improvement in the YM over the neat polyimide films. The electrical conductivity of the MWCNT reinforced polyimide composites was 0.938 S cm−1(5 wt% acid-MWCNTs) and 1.032 S cm−1(5 wt% amide-MWCNTs) is the highest achieved so far for polyimide composites. High electrical conductivity together with high strength and improved thermal stability makes these nanocomposites a promising candidate for high-temperature EMI shielding materials.
The authors are grateful to Prof. Vikram Kumar, Director, NPL, and Dr. A.K. Gupta, Head Engineering Materials Division, for permission to publish the research work. The authors would like to thank Mr. R.K. Seth for carrying out the TGA studies, Mr. Jay Tawale for SEM observation and Ms. Chetna Dhand for carrying out the FTIR studies.