Introduction

Conducting polymers are attractive materials due to remarkable properties such as their simple synthesis, good environmental stability and electrical conductivity [1,2,3]. On the other hand, conducting polymers have some handicaps in their processing aspect because of insoluble, poor mechanical properties [1, 4, 5].

The formation of composites is one of the most useful ways in order to overcome these limitations [1, 2, 6,7,8]. Thus, physical, optical and mechanical properties of a polymer can be controlled and enhanced. The researchers synthesize conducting polymer composites used to enhance the mechanical, physical and optical properties of conducting polymers [9, 10].

Conductive polymer nanocomposites have a remarkable feature in nanoscience and technology because of improvement in the mechanical, electrical and thermal properties of polymers [11,12,13,14,15,16].

Poly(vinyl carbazole) (PVC) is the oldest known (since 1957) [17] and most widely characterized polymeric photoconductor. Poly(N-vinyl carbazole) (PNVC) is a more attractive photoconductive polymer because of wide using applications such as photorefractive materials, polymeric light-emitting diodes and photovoltaic devices [18]. Since the first successful use of PNVC in electrophotographic applications [19, 20], polymers with carbazole moieties have been often investigated. PNVC was used as a dielectric and heat-resistive material but these days it is used also in photoconductive material [21, 22].

Synthesized polymeric clay nanocomposites are widely used because of their unique properties such as availability, good swelling properties and low cost [23]. The application areas of the clay nanocomposites are optoelectronics devices, sensors, rechargeable batteries, etc. [23].

In this work, different weight percentages of nanoclay (NC) (2% and 5%) and N-vinyl carbazole were prepared by chemical polymerization. The polymerization was performed by using AIBN (azobisisobutyronitrile) as an initiator at 70 °C during 24 h. The obtained composites were characterized by spectroscopic, morphological, optical and dielectric measurements at high frequency.

In difference to the earlier works, this study provides a detailed investigation of the dielectric properties at high frequencies and spectroscopic and morphological characterization of PNVC–NC composites. PNVC–NC composites have been synthesized for the first time in this study. The novelty of this work is investigation of changes in dielectric parameters at high frequencies by doping nanoclay in nanoscale. It could be important for miscellaneous technological uses areas such as commercial and military area because the electromagnetic wave in the GHz ranges has been widely applied in wireless telecommunication systems area, radar and local area network, etc. [24].

Experimental part

Materials and instruments

N-Vinyl carbazole (Aldrich), azobisisobutyronitrile (AIBN) (Aldrich), nanoclay (Aldrich) and all the other chemicals used were of analytical grade and were used as received without any further purification. The samples have been characterized by SEM (Jeol 7001F), FTIR (Nicholet), UV–VIS–NIR (Shimadzu 3600) spectrophotometer, and dielectric analysis system (Fytronix FY-3000) and Vector Network Analyzer (Anritsu) in the frequency range of 200 MHz–20 GHz.

Preparation of PNVC and its composites

In this study, different weight percentages of nanoclay (2% and 5%) were used. NVC was placed into 20 mL of NMP and was sonicated 25 °C, and then radical polymerization of NVC was performed by using AIBN in the presence of clay at 70 °C. Because of removal of some soluble monomer and initiator contaminants, the mixture was transferred into ethanol. After that, the samples were dried under vacuum at 25 °C for 24 h.

Results and discussion

Spectroscopic characterization

The FTIR spectra of the homopolymer PNVC and its polymer composites PNVC + 2%NC and PNVC + 5%NC in powder form are shown in Fig. 1. The characteristic peaks observed for PNVC were ring deformation of the substituted aromatic structure at 719 cm−1, –CH2 rocking vibrations at 745 cm−1, out-of plane deformation of the vinylidene group at 1220 cm−1, –CH2 deformation of the vinylidene group at 1330 cm−1, ring vibrations of the N-vinyl carbazole moiety at 1450 cm−1, C=C stretching vibrations of the vinylidene group at 1680 cm−1 and aromatic C–H stretching vibrations at 3050 cm−1. These results suggest that PNVC was formed and convenient with the literature [25].

Fig. 1
figure 1

FTIR spectra of PNVC, PNVC + 2% NC and PNVC + 5% NC

Full size image

In addition of the PNVC composites, the Si–O–Si stretching frequency bond was formed in addition of clay at about 1027 cm−1 [26, 27].

Figure 2 shows that plot of (αhυ)2 plotted versus photon energy (hυ) of PNVC, PNVC + 2% NC and PNVC + 5% NC. The energy band gap value for PNVC and its composites was calculated by UV–Vis spectra. The band gap value decreases with increase of nanoclay due to the homogeneous structure of the nanoclay.

Fig. 2
figure 2

Evolution of (αhυ)2 plotted versus photon energy (hυ) of the PNVC and PNVC with different nanoclay concentrations and the energy band gap value of samples

Full size image

Morphological characterization

Morphological characterization of PNVC samples was carried out by scanning electron microscopy (SEM). The SEM pictures of PNVC (Fig. 3a) and PNVC composites (PNVC + 2%NC and PNVC + 5%NC) are shown in Fig. 3b, c, respectively. As can be seen in Fig. 3a, the various shapes were seen for the PNVC morphology [28] and the NC in different percentages was attached with PNVC (Fig. 3a, b). It is seen that the NC particles are fairly well dispersed in the PNVC matrix and layered structure is formed by the addition of nanoclay (Fig. 3c).

Fig. 3
figure 3

SEM picture of PNVC (a) and PNVC doped with 2% (b), 5% NC (c), respectively

Full size image

The TEM figure (Fig. 4) shows the structure of clay which is convenient to the literature. There is a homogenous distribution of the clay flakes [29].

Fig. 4
figure 4

TEM picture of NC with different magnifications

Full size image

Dielectric properties

For dielectric characterization, the samples were prepared in the form of pellets under 10 ton cm−2 pressure forming a circular disk with dimensions 13 mm in diameter and 1 mm in thickness. The samples were measured by vector network analyzer (VNA) by using coaxial cable method. The dielectric constant was estimated by Fytronix FY-3000 dielectric analyzer system in the frequency range of 200 MHz–20 GHz, and the measurement was made in this frequency range [30].

\(\varepsilon^{*}\) is the complex dielectric constant. The complex dielectric constant values were measured by VNA systems.

\(\varepsilon^{*}\) consists of the real ε′ and imaginary ε″ components, which represent the stored and dissipated energy components of the material, respectively [31]:

$$\varepsilon^{*} \left( \omega \right) = \varepsilon^{\prime } \left( \omega \right) + \varepsilon^{\prime \prime } \left( \omega \right)$$
(1)

Angular frequency ω is referred to by the radial frequency, ω = 2πf, where f is the frequency [32, 33].

The loss tangent \(\tan \delta\) is defined as the ratio of the imaginary part to the real part of dielectric constant.

$$\varepsilon^{\prime \prime } = \varepsilon^{\prime } \cdot \tan \delta$$
(2)

Dielectric parameters (ε′, ε′′, tanδ) and AC conductivity (σ) were measured at high frequencies. Initially, the real part of the dielectric constant showed an increase due to atomic interaction. As the frequency increased, the real (εʹ) part of dielectric constant decreased up to the critical frequency which is approximately 2.75 GHz and then minimum energy storage increases with increase of frequency. After 4 GHz, the real part of dielectric constant decreases with increase of frequency. PNVC and PNVC + 5% have highest and lowest values of the real (ε′) part of dielectric constant (Fig. 5), respectively. As can be seen from this, the real (ε′) part of dielectric constant was decreased by adding nanoclay.

Fig. 5
figure 5

Frequency evolution of the real (εʹ) part of dielectric constant for PNVC and PNVC with different NC concentrations

Full size image

Firstly, the imaginary part of dielectric constant decreases with increase of frequency up to 2 GHz. After 2 GHz, the dielectric constant increases up to approximately 2.75 GHz which is a critical frequency. After 4 GHz, the imaginary part of dielectric constant increases with increase of frequency. PNVC and PNVC + 5% have highest and lowest values of the imaginary (ε′′) part of dielectric constant (Fig. 6), respectively. The atomic polarization behaviors in the high-frequency regions have been obtained for PNVC and PNVC doped with 2%, 5% NCs at 25 °C.

Fig. 6
figure 6

Frequency evolution of the imaginary (εʹʹ) part of dielectric constant for PNVC and PNVC with different NC concentrations

Full size image

The energy loss decreases up to 2 GHz. After critical frequency (approximately 2.75 GHz), the loss factor increases with increase of frequency. PNVC has high value of loss factor at high frequency, but adding nanoclay to PNVC, the loss factor decreases (Fig. 7). The minimum value of energy loss has reached around the critical frequency.

Fig. 7
figure 7

Frequency variation of the loss factor for PNVC and PNVC with different NC concentrations

Full size image

The AC conductivity value increases with increase of frequency in order of PNVC and PNVC with different NC concentrations as shown in Fig. 8. The AC conductivity exhibits increasing trends with frequency. PNVC has highest value of conductivity. Because of oxidation effect, the conductivity value decreases with adding nanoclay to PNVC. The conductivity values decrease with frequency due to the low mobility of nanoclay. The conductivity properties of NC-doped PNVC are changed with the nanosize of nanoclay particles.

Fig. 8
figure 8

Frequency variation of the conductivity for PNVC and PNVC with different NC concentrations

Full size image

The AC conductivity dependence of frequency can be expressed as the empirical Jonscher’s universal law [32] because of determination of AC conduction mechanism:

$$\sigma_{\text{AC}} \left( \omega \right) = A\omega^{s} ,$$
(3)

where A is a constant, ω is the angular frequency and s is the frequency exponent parameter.

The angular frequency exponent value s was calculated from the slopes of Fig. 9 [34]. The ln(σAC) versus ω plot is given in Fig. 9 for all samples. From the slopes two range of Fig. 9 for the first region and second frequency regions (Table 1), the angular frequency exponent values s can be calculated [34] because the frequency exponent is used to determine the electrical conduction mechanism of all samples. Frequency exponent “s” was restricted to be 0 < s < 1 which is known as Jonscher power law (JPL). JPL is proposed to exist at mega- to 3 GHz frequency range in the first region. The another power law for frequency-dependent conductivity is superlinear power law (SLPL) which is related to frequency exponent between one and two [35]. However, for PNVC and PNVC doped with nanoclays, the AC conductivity mechanism is in agreement with SLPL at the 3G to 18G Hz frequency (II. Region).

Fig. 9
figure 9

AC conductivity for PNVC and PNVC with different NC concentrations

Full size image
Table 1 S parameters of the samples
Full size table

Conclusion

In this work, PNVC/NC powder composites have been synthesized by chemical polymerization of N-vinyl carbazole in the presence of different weight percentages of nanoclay powder and AIBN as initiator at 70 °C for 24 h.

PNVC and PNVC with different NC concentrations have been investigated by chemical and morphological characterization using the FTIR and SEM, respectively. Frequency evolution of the real (ε′) and (ε′′) parts of dielectric constant, loss factor, AC conductivity behavior for PNVC and its composites have been determined by the impedance spectroscopy at room temperature.

Depending on the nanoclay doping, more compact materials were obtained. It has also been seen in SEM that the spacing is reduced with the addition of nanoclay. It was observed that the obtained composites by doping of nanoclay were contributed to the change in the conductivity values with increasing frequency.

All the samples show the atomic polarization behavior in the high-frequency regions at 25 °C.

PNVC and PNVC doped with nanoclays correspond to the AC conductivity behavior which is in agreement with JPL and SLPL at the first and second frequency regions, respectively.