Studies on the structural, thermal, and dielectric properties of fabricated Nylon 6,9/CaCu₃Ti₄O₁₂ nanocomposites

2.1 Synthesis of nano-CCTO ceramic

The CCTO nanoparticle was synthesized using the complex oxalate precursor route [36]. For the synthesis of CCTO nanocrystallites, chemicals such as TiCl₄ (titanium tetrachloride, 99.98%; Merck, Germany), CaCO₃ (calcium carbonate; BDH; A. R. grade, India), CuCl (cupric chloride; Fluka; proanalyse grade, India), oxalic acid (S.D. Fine Chemicals; analytical grade, Mumbai, India), and acetone (S.D. Fine Chemicals; analytical grade, Mumbai, India) were involved. In a typical preparation, titania gel was prepared from the aqueous TiOCl₂ (0.05 m) by adding NH₄OH (Merck, A. R. grade, Mumbai, India) (aqueous; at 25°C) until the pH reached 8.0, and NH₄Cl was washed off on a filter funnel with Whatman filter paper using double-distilled water. The washing was continued several times until all NH₄Cl content was fully removed. To this gel, 0.4 or 0.8 mole of oxalic acid (2 m solution) was added and then stirred continuously to form the clear solution. Then, the clear solution was kept warm (~40°C). CaCO₃ was added to the obtained clear solution, in aliquots, and stirred well to obtain a clear aqueous solution. The aqueous solution containing titanyl oxalic acid together with calcium titanyl oxalate remained clear without any precipitate formation. This solution was cooled at 10°C in which CuCl dissolved in acetone along with water (80:20 volume ratio) was then added and stirred continuously. Subsequently, the formed precipitate was then filtered and washed several times with acetone to make it chloride-free and then dried in air atmospheric temperature to evaporate acetone. The precursor was isothermally heated at approximately 700°C to get nanocrystallites (20–200 nm) of phase-pure calcium copper titanate (CCTO) as confirmed using XRD and transmission electron microscopy (TEM) studies.

2.2 Preparation of Nylon 6,9/CCTO nanocomposite

Nylon 6,9 having a molecular weight (M_w) of 60,000 g/mol was used as matrix material (purchased from Nanoglobal Technology, Mumbai, India). For the fabrication of nanocomposites, initially Nylon 6,9 granules were heated at 220°C until the Nylon 6,9 granules were thoroughly melted in a Brabender plasticorder (Model PLE331, Germany). The CCTO nanoparticles (0–58% by volume) were slowly added to the melted polymer material and mixed for approximately 30 min with each composite at the same temperature for the complete distribution of CCTO nanoparticles into the polymer matrix. The mixture obtained from the Brabender plasticorder was then hot pressed at the same temperature to obtain a sheet of 150 mm² with 0.5 mm thickness. To get flexible composites that could be made into a variety of shapes, the ceramic loading was restricted to a maximum of 58 vol%.

2.3 Characterization

To analyze the structure, size, and surface morphology of nano-CCTO, an XPERT-PRO diffractometer (XRD; Philips, The Netherlands), an FEI-Tecnai TEM (G-F30; Hillsboro, OR, USA) and an SEM (Cambridge Stereoscan S-360) were used, respectively. The thermal behavior of the composite was examined by TGA using TA Instrument (UK; Model TGA Q500). TGA was conducted in nitrogen atmosphere at a flow rate of 60 ml/min and a heating rate of 10°C/min. DSC (Mettler Toledo; Model DSC 821e, USA) was employed at a heating rate of 10°C/min under nitrogen atmosphere and a flow rate of 60 ml/min using aluminum pan to observe the glass transition and melting temperature of the composites. LCR meter (Model HP4194A, USA) was used for the capacitance measurements as a function of frequency (100 Hz–1 MHz).

3.1 XRD

Figure 1A–F shows the XRD patterns obtained for the as-synthesized CCTO nanoparticles, pure Nylon 6,9, and Nylon 6,9/CCTO nanocomposites with 10, 20, 38, and 58 vol% CCTO. The XRD pattern (Figure 1B) indicates that the as-synthesized CCTO nanoparticles are of a single phase with a cubic pervoskite-related structure [6]. The XRD analysis for the pure Nylon 6,9 (Figure 1C) revealed a crystalline structure associated with the broad peak of 2θ centered around 22.4. In the Nylon 6,9/CCTO nanocomposites (Figure 1D–F), the diffraction peaks pertaining to both the Nylon 6,9 and CCTO were unchanged only with a small variation in the intensity for Nylon 6,9. However, for the Nylon 6,9/CCTO-58 vol% nanocomposite (Figure 1F), the diffraction pattern pertaining to the Nylon 6,9 peak disappeared, which is due to the predominant amounts of CCTO present in the nanocomposite. The appearance of peak corresponding to pure CCTO was observed in Nylon 6,9/CCTO-58 vol% nanocomposite. The intensities of diffraction peaks were slightly increased with the increase in CCTO content in the composite.

Figure 1:

XRD diffraction patterns: (A) CCTO JCPDS, (B) as-prepared CCTO nanoceramic, (C) pure Nylon 6,9, (D) Nylon 6,9+CCTO-5 vol%, (E) Nylon 6,9+CCTO-20 vol%, and (F) Nylon 6,9+CCTO-58 vol%.

3.2 TEM and SEM analyses

Figure 2 presents the bright-field TEM images of the phase-pure CCTO powders obtained from the thermal decomposition of the oxalate precursor. There are particles with curved edges and corners. The size of the particles, as measured by the intercept method from the micrographs, is in the range of 20–200 nm. Figure 3 shows the SEM image (cross-sectional view) of the broken sample of Nylon 6,9+CCTO-58 vol% nanocomposite. The SEM image clearly indicated that the CCTO nanoparticles are agglomerated in the Nylon 6,9 polymer matrix.

Figure 2:

Bright-field TEM images of CCTO nanocrystals with dimensions ranging from 20 to 200 nm.

Figure 3:

SEM micrograph of Nylon 6,9/CCTO-58 vol% nanocomposite.

3.3 DSC

Figure 4 shows the DSC thermograms recorded for the Nylon 6,9 and Nylon 6,9+CCTO composites (0–58 vol%) in nitrogen atmosphere (flow rate 60 ml/min) at a heating rate of 10°C/min. There are three main thermal events such as glass transition temperature (T_g), a cold crystallization, and a melting around occurring for both pure Nylon 6,9 and nanocomposites.

Figure 4:

DSC traces: (A) pure Nylon 6,9 and Nylon 6,9+CCTO-(B) 3, (C) 0, (D) 10, (E) 14, (F) 20, (G) 38, and (H) 58 vol%.

Nylon 6,9 had a double melting at the endothermic region with a weak endotherm in the range of 175°C–186°C combined with an exotherm at 179°C and a main endothermic range from 201°C to 215°C. After the addition of CCTO nanoparticles varying from 0 to 58 vol%, the T_g decreased with respect to the pure Nylon 6,9, and the value was reduced from 41°C to 40°C. The presence of CCTO nanoparticles induces the thermal interaction with the matrix material and reduces its T_g by changing the phase from one form to another form in crystalline structure. It is well established that the T_g either increases or decreases depending on the type of filler, its morphology, and the intercrystallite distance [6], [23], [24], [25], [26], [27], [28].

The crystallization is influenced by the length of the repeated units and by the fact that Nylon 6,9 is an even-odd polyamide. For crystallization, the ability of the chains to form hydrogen bond to crystallize by the part was played by the even-odd status. The n-type polyamides with even number for n>8 generally crystallize in the γ phase. Either α or β phase is possible in polyamides 4 and 6, whereas the α phase is their foremost structure. The m,n-type polyamides with even-even carbon atom numbers mainly crystallize in the α phase; however, the even-odd, odd-odd, and odd-even numbers crystallize generally in the γ phase. Due to high distortions during the production process, the α phase can occur [37], [38]. As reported [36], [39], Nylon 6,9 has a greater propensity than that of other polyamides to crystallize mainly in metastable form. The Nylon 6,9 also has a stronger tendency than Nylon 6 to crystallize in the high temperature stable form. For Nylon 6,9/CCTO-58 vol% composites, the crystallization temperature of Nylon 6,9 decreased from 182.29°C to 180.29°C due to the presence of a predominant amount of CCTO nanocrystallites in Nylon 6,9.

3.4 TGA

Figure 5 shows the thermogravimetric curve recorded for the pure Nylon 6,9 and the Nylon 6,9+CCTO nanocomposites (0–58 vol%) in nitrogen atmosphere (flow rate 60 ml/min) at a heating rate of 10°C/min. The decomposition started gradually at 250°C, and the first weight loss temperature of thermal degradation was 505°C, which was due to the presence of the matrix material. Thereafter, a slight weight loss was observed, which started from 506°C to 580°C, which was due to the presence of Nylon 6,9 materials that require more temperature to achieve loss, and interaction bonding appeared from pure Nylon 6,9 to 38 vol% CCTO content in the composite and Nylon 6,9 disappeared when the CCTO content increased to 58 vol%. With the addition of 3–38 vol% CCTO nanoparticles, the weight loss was the same for both steps and thereafter, leaving behind the CCTO nanoparticles as residue. The decomposition temperature of onset (temperature at 10% weight loss) [23] was increased for 3 vol% CCTO in the nanocomposites under study. The onset decomposition temperature accompanied by 10% weight loss was 454.64°C for Nylon 6,9+CCTO-3 vol% and 445.96°C for the Nylon 6,9+CCTO-58 vol%. For the pure Nylon 6,9, the onset decomposition temperature was 452°C. The decrease in onset temperature for the composite may lead to less stability of the composite at high temperature.

Figure 5:

TGA thermograph: (A) pure Nylon 6,9 and Nylon 6,9+CCTO-(B) 3, (C) 5, (D) 10, (E) 14, (F) 20, (G) 38, and (H) 58 vol%.

3.5 Frequency dependence of room temperature dielectric permittivity

The frequency dependence of the dielectric permittivity of the composite with different CCTO contents is shown in Figure 6A and B. As expected, the effective dielectric permittivity (ε_eff) increased with the increase in CCTO content in Nylon 6,9 at all frequencies under study. The ε_eff was obtained more than that of pure Nylon 6,9 but much lower than that of pure CCTO. The low-frequency dispersion increased with the increase in the CCTO content. The dielectric properties also partly originate from the electrode/sample contact effects, which depend on the surface resistivity of the samples [37], and the dielectric constant values vary with the electrode materials used [38]. Hence, to decipher the dielectric contribution, dielectric measurements were carried out using both silver and gold electrode samples, and the same is shown in Figure 7A and B. It is clearly visible that there is a difference in the dielectric behavior between the silver and gold electrode samples, indicating that the low-frequency dispersion is attributed to the space charge effect, which is due to the conductivity differences between the sample surface and the electrodes. The silver electrode samples show enhanced dielectric constant value compared to that of gold electrode samples (Figure 7A). Similarly, dielectric loss also varies with the electrode materials used (Figure 7B). From earlier studies, Nylon 6,9 was reported with three different polymorphisms designated as α, β, and γ, which were stable at room temperature, as discussed in Section 3.3 [36], [39], [40], [41]. The Nylon 6,9/CCTO nanocomposite had a low dielectric permittivity when compared to CCTO. This may be due to the following facts apart from the connectivity and particle size effects. At room temperature, the dielectric permittivity of pure Nylon 6,9 was 6 at 100 Hz and was reduced to 4 when the frequency was increased to 10 MHz. With the addition of CCTO into the polymer matrix, the dielectric permittivity was increased to 7 at 100 Hz for 3 vol% CCTO nanocomposites. The increase in the volume percentage of CCTO nanoparticles into the polymer matrix increased the dielectric permittivity to 25 for the maximum 20 vol% CCTO in the nanocomposite. Hence, a further increase in the CCTO content to 58 vol% has exhibited dielectric permittivity as high as 220 at 100 Hz, which is very high compared to that of PVDF/CCTO composite system [20] but less compared to that of PANI/CCTO nanocomposite system [16]. The dielectric loss for the pure Nylon 6,9 is 0.13 at 100 Hz and the composite dielectric loss is approximately 0.99 at 100 Hz.

Figure 6:

Frequency-dependent (A) permittivity and (B) dielectric loss (tan δ) of Nylon 6,9/CCTO nanocomposite as a function of volume percent of CCTO at room temperature.

Figure 7:

Frequency-dependent behavior of (A) dielectric constant and (B) dielectric loss of Nylon 6,9/CCTO-58 vol% composite using various electrode material at 300 K.

The variation of dielectric permittivity at room temperature and 10 kHz for the Nylon 6,9/CCTO composite as a function of the volume percent of CCTO content is shown in Figure 8. The dielectric permittivity increased slowly up with the addition of 20 vol% CCTO nanocomposite, and with the further addition of CCTO, the dielectric permittivity was enhanced with more slope. Such behavior of the composite material could be described based on the percolation phenomenon, and this is called the percolation threshold [42], [43]. An increase in the dielectric permittivity of composites in the surrounding area of the percolation threshold was described by the well-known power law. It was broadly understood that, in the case of a percolative system, the ultrahigh dielectric permittivity of composites was not a direct concern of intrinsic dielectric permittivity value of fillers and host polymer as in the case of ceramic-polymer composites. The percolative system was very much concerned about the effect of interfacial polarization and high surface area due to the connection between the conductive particle clusters near the percolation threshold [44], [45], [46], [47], [48]. The interface between the matrix and filler is the most important in these systems, which can be made by controlling the variables such as the size and shape of fillers, the type of matrix, and the preparation process. The above-mentioned factors are interrelated with one another to develop a new material with high dielectric permittivity (k). The difference in the dielectric permittivity in the community of the percolation threshold could be explained based on the power law as given in Equation (1) [49], [50]:

Figure 8:

Both permittivity and loss to their corresponding volume percent of filler addition at room temperature.

where ε_eff and ε₁ are the dielectric permittivity of the composite and the matrix (6.4 at 100 Hz), respectively. f_c is the percolation threshold, f_CCTO is the volume fraction of the CCTO in the nanocomposite, and q is the critical exponent (q~1). Using Equation (1), there was a good understanding between the obtained and experimental values of ε_eff. The percolation threshold value was slightly more than that of the predicated value for the 58 vol% CCTO nanocomposites. The ε_eff obtained in the nanocomposite near the percolation threshold was 220 at 100 Hz (Figure 9). The ε_eff of the composite materials for higher frequency (10 kHz) was also measured using Equation (1) and was almost same as the experimental values (also plotted in Figure 9).

Figure 9:

Variation of ε_eff (100 Hz and 10 kHz) of Nylon 6,9/CCTO nanocomposite as a function of volume fraction.

Dark shaded denotes the fit of the experimental data using power law and cone denotes the experimental data.

3.6 Temperature and frequency dependence of dielectric permittivity

The dielectric properties of Nylon 6,9/CCTO-58 vol% nanocomposites were studied with temperature dependence as shown in Figure 10A and B. The dielectric permittivity of Nylon 6,9/CCTO-58 vol% nanocomposite at room temperature was 220 at 100 Hz, which was increased to 3845 at 150°C (100 Hz). The value at 1 kHz was approximately 134 at room temperature, which was increased to 659 when the temperature increased to 150°C. The increase in dielectric permittivity with the increase in temperature decreased with the increase in the frequency, as shown in the Figure 10A. The most visible change was noticed in dielectric permittivity at the low-frequency region (100 Hz–10 kHz). This may be due to the influence of the space charge effect, which was due to the conductivity differences between the sample surface and the electrodes, as discussed in Section 3.5. The dielectric loss for the Nylon 6,9/CCTO-58 vol% nanocomposite is shown in Figure 10B. The increase in the temperature may lead to the increase in the dielectric loss.

Figure 10:

Frequency-dependent (A) permittivity and (B) dielectric loss (tan δ) at various temperatures for Nylon 6,9+CCTO-58 vol% nanocomposite.

The variation of AC conductivity as a function of frequency for the composite material of different volume percent at room temperature is shown in Figure 11. The AC conductivity (σ) was derived from the dielectric data using Equation (2):

Figure 11:

Frequency-dependent AC conductivity of Nylon 6,9+CCTO nanocomposite as a function of volume percent of CCTO-dependent AC conductivity for different volume percent of CCTO.

where ε_o=8.85×10^-12 F/m is the dielectric permittivity of the free space and ω=2πf is the angular frequency. From the figure, the AC conductivity of the composite was increased with the increase in frequency as the CCTO content increased [49], [51]. This is due to the interfacial polarization and the increase in charge density of CCTO nanoparticles as a motion of polymer chain at low frequency. In the high-frequency region, AC conductivity was attributed to the electronic polarization to hop charge carrier over a small barrier of height [50]. The pure Nylon 6,9 has the conductivity of 4.96×10^-12 Scm^-1 at 100 Hz, which was increased to 1.18×10^-9 Scm^-1 when the CCTO content had increased to 58 vol% due to the better connectivity between the polymer and the CCTO in the composite and also to the increased charge density carriers. Thus, the increase in AC conductivity with the increase in CCTO content supports the hopping of the charge carrier conduction mechanism [26], [52].