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Original Article
DOI: 10.1016/j.jmrt.2019.09.074
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Available online 17 October 2019
Preparation, structural analysis, morphological investigation and electrical properties of gold nanoparticles filled polyvinyl alcohol/carboxymethyl cellulose blend
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M.A. Morsia,b,
Corresponding author
m.a_morsi@yahoo.com

Corresponding author at: Engineering Basic Science Department, Faculty of Engineering, Egyptian Russian University, Egypt.
, A.H. Orabyc, A.G. Elshahawyc, R.M. Abd El-Hadyc
a Physics Department, Faculty of Science, Taibah University, Al-Ula, Medina, Kingdom of Saudi Arabia
b Engineering Basic Science Department, Faculty of Engineering, Egyptian Russian University, Cairo, 11829, Egypt
c Physics Department, Faculty of Science, Mansoura University, Mansoura, 35516, Egypt
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Tables (3)
Table 1. Assignments of the FT-IR characterization bands of the pure PVA and pure CMC.
Table 2. The σdc, n, τ and τM values for PVA/CMC blend, the nanocomposite samples (0.02 and 0.16wt.%) and also for the nanocomposite sample (0.02wt.%) at different temperatures.
Table 3. Optical energy gap for pure PVA/CMC blend and the filled samples.
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Abstract

Polymer nanocomposite samples have been prepared through the solution casting method utilizing a polymer blend of polyvinyl alcohol and carboxymethyl cellulose (70/30wt.%) as organic host matrix and different concentrations of biosynthesized gold nanoparticles (Au NPs) by the leaf extract of green mint (Mentha Spicata L.) as inorganic nanofiller. The structural, optical and morphological properties of these samples have been through FT-IR, XRD, UV/Vis. and SEM techniques. The FT-IR spectra confirm the blend components are miscible by the formation of Hydrogen bond interaction and show the polymer-nanoparticle interactions. The XRD analyses depict the semicrystalline structure of these samples and the crystallinity degree decrease with increase of Au NPs content within the PVA/CMC structure. The TEM micrograph of biosynthesized Au NPs indicates that shapes of these NPs are spherical NPs and triangular/hexagonal nanoplates with average size range 5–29nm. The alternating electrical conductivity, electrical impedance, complex dielectric permittivity and electric modulus spectra of prepared samples have been investigated at 25°C in the frequency range (0.1Hz–20MHz). Thus, the variations in values of conductivity, dielectric permittivity and relaxation times indicate the feasibility of these materials as flexible nanodielectric of frequency tunable permittivity for radio/audio frequency operating microelectronic/conventional devices.

Keywords:
Au NPs
PVA/CMC
FT-IR
XRD
TEM
Dielectric parameters
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1Introduction

The blending process of some polar polymers has a significant interest in the modification/development of new polymeric material with significant properties that differ from the parent polymers [1–3]. This process is an effective method for the preparation of flexible polymeric matrix with a high degree of miscibility instead of new polymer synthesisation [4]. Among these polymers, the semicrystalline PVA and amorphous CMC are potentially non-toxic, hydrophilic and biodegradable [5,6]. Further, these polymers have the ability to form excellent flexible-type film through solution casting technique. Due to the high durability, good clarity, excellent electrical properties of thermally stable PVA, it has various applications like flexible water-soluble packing films, textile sizing, paper coating, polarizing sheets production and solid polymer electrolyte preparation [7–10]. On the other hand, the natural CMC polymer retains various desirable properties like low cost, water maintaining, suspension, emulsification and excellent film forming [11,12]. Thus, CMC is considered as a promising material for industrial and biotechnological applications such as paper making, textures, electrical elements, toilet products, printing and medicine [7,12]. The hydroxyl group (OH) within the chain backbone of PVA and CMC structures acts as a functional group for inter- and intra- hydrogen bonding. El-Sayed et al. [5] studied the optical, thermal and dielectric properties of CMC/PVA blends and concluded that these blends are miscible with each other and the composition (70/30wt.%) of PVA/CMC blend shows low absorption edge compared to other prepared samples. Also, they found that this composition exhibits higher dielectric constant. Saadiah et al. [6] investigated the structural, morphological and electrical conductivity of CMC/PVA blends. Further, they studied the interactions within the CMC/PVA through density functional theory (DFT) calculations. They depicted the formation of intermolecular forces of H-bonding within the CMC/PVA hybrid polymer between the OH group of PVA and COO group of CMC.

Polymer nanocomposite materials comprise the significant properties of the inorganic nanofiller and also the flexible polymer matrix [13,14]. Thus, these hybrid materials introduce a multifunctional advanced polymeric material for enormous technological uses such as in the development of the next fabrication of optoelectronic devices, solar cells, energy storage devices and the nanodielectric applications [4,14–16]. The choice of Au NPs in the of the nanocomposite preparation is due to its high chemical/mechanical stabilities, electrical conductivity, outstanding plasmonic activity, catalytic properties and antimicrobial activities [1,3,17,18]. Morsi et al reported the improvement of thermal and optical properties of PVP/CMC polymer blend due to the in situ formation of Au NPs within this polymeric matrix [11]. Abdelrazik et al. [19] found the enhancement of optical and thermal properties of PEO/PVP matrix at low content of Au NPs. Further, several authors reported that the electrical and dielectric properties of various polymer nanocomposite films containing different concentrations of Au NPs were significantly improved [20–22].

Various methods have showed the Au NPs can be prepared by the physical and chemical ways [1,3,11,17,23], but because of the usage of toxic chemicals, it becomes a mandate to use an alternative technique. Synthesis of NPs by plant extract emphasizes that usage of natural compounds has offered a simple, low cost, reliable and eco-friendly. Thus, a various plant extracts are already well-known to elaborate nanostructured materials such as Medicago sativa, Cinnamomum camphora, Avena sativa, Pelargonium graveolens, Coriandrum sativum, Emblica offcinalis and Carica papaya[24]. The mint (Mentha Spicata L.) is a popular herb and belongs to the family Lamiaceae that is a rich source of polyphenolic/flavonoids compounds (such as menthol, menthone, carvone…) and therefore could possess strong antioxidant properties [25]. Thus, this peppermint is widely utilized as ingredients in cosmetics, pharmaceutical products, and foods as a flavour enhancer. Also, it have both reducing and stabilizing agents involved in the process of the preparation of Au NPs from HAuCl4.3H2O solution and there is no need to add any other components to the reaction mixture. The Au NPs (10–300nm) were successfully obtained by the aqueous extract of Mentha piperta[26]. Also, Mubarak Ali et al. [24] used the Mentha piperta extract to prepare the spherical Au NPs (∼150nm).

The literature survey depicts that the biosynthesis process of Au NPs by the leaf extract Mentha Spicata L. has not prepared and the PVA/CMC blend matrix filled with these biosynthesized Au NPs has not been investigated for its electrical and dielectric properties so far. To fill this knowledge gap, the characterization of detailed structural, morphological, electrical, dielectric and optical properties of Au NPs incorporated into PVA/CMC blend are introduced for their suitable composition in the preparation of new nanodielectric for many ultimate applications such as the energy storage devices. Further, the biosynthesis of NPs becomes favourable because of their significant characteristic that is non-toxic, renewable and ease in preparation.

2Experimental details2.1Materials

The deionized water (DD) was utilized in the preparation of PVA/CMC blend and the nanocomposite films. The fine powder of PVA (Mw=35,000gmol−1, Laboratory Rasayan, Cairo, Egypt), CMC powder (Mw=250,000gmol−1, El Nasr Pharmaceutical Chemicals Co., Mansoura, Egypt), gold chloride trihydrate (HAuCl4.3H2O, purity 99.5%, Merck company, Germany) were used in the preparation of nanocomposite samples. The green mint leaves was obtained from Zagazig city, Sharkai governorate, Egypt.

2.2Preparation of the green mint extract and synthesis of gold NPs

For the preparation of mint extract, the protocol of MubarakAli et al. was used [24]. The green mint leaves (25g) were washed in DD to remove the dust particles and were cut into very small pieces. These pieces were boiled for 15min in Erlenmeyer flask (500mL) containing 200mL of DD. The obtained aqueous extract was filtered to have a clear extract and cooled for further processes. Then, 20mL of the extract solution (0.25g/mL) was added to 100mL of HAuCl4.3H2O solution (0.005M) at pH=6. This mixture was warmed for 10min and its colour turned to reddish-brown. This colour changing depicted that the plant extract behaved as a bioreducing agent for the gold ions. The obtained Au NPs solution was centrifuged and the deposited powder was washed for three times.

2.3Preparation of PVA/CMC/Au nanocomposite samples

The PVA (2.1g/100mL) and CMC (0.9g/150mL) powders were dissolved in DD at 80°C, separately. To obtain the PVA/CMC solution (70/30wt.%), these solutions of virgin polymers were mixed through magnetic stirrer for 6h to have a homogeneous solution of PVA/CMC blend at the same previous temperature. The required quantities of Au NPs (0.02, 0.04, 0.08 and 0.16 Au NPs) were mixed with the previously prepared PVA/CMC solution for 15min. Further, these nanocomposite solutions were immersed device in ultrasonic device for 10min to improve the NPs dispersion and homogeneous solutions were obtained. Subsequently, these solutions were cast onto a poly propylene dishes and it were put in oven for 5 days at 40°C. Finally, the materials had become a flexible-type freestanding polymer nanocomposite films with thickness (0.14–0.18mm).

2.4Characterization techniques

The complexation between PVA/CMC and Au NPs was investigated using Fourier transform infrared spectroscopy (Nicolet iS10, USA) in the wavenumber range (400–4000cm−1) The morphology of these nanocomposite films was investigated through scanning electron microscopy (SEM, JEOLJSM 6510LV) The X-ray diffraction (XRD) analysis of biosynthesized Au NPs and PVA/CMC/Au nanocomposite samples were measured using PANalytical X'pert Pro MPD diffractometer at a scan rate (0.05°/s) with Cu-Kα radiation (λ=1.5406Å) at voltage 30kV. The size and shape prepared Au NPs were studied through transmission electron microscope (TEM, JEOL-JEM-1011, Japan). The electrical and dielectric parameters of pure PVA/CMC and the filled films (0.02 and 0.16wt.% Au NPs) were measured by broad band dielectric spectroscopy (Novocontrol Turnkey Concept 40 System) in nitrogen atmosphere. The copper electrodes of diameter 2.5cm were prepared through using vacuum coating unit. Further, these parameters were measured in the temperature variation from 25 to 100°C for the nanocomposite film (0.02wt.% of Au NPs) and the temperature value was adjusted by the QUATRO Cryosystem. The absorbance spectra of the prepared samples were measured using a spectrophotometer (V-570 UV/VIS/NIR, Jasco, Japan) in the wavelength range (190–1000nm).

3Results and discussion3.1FT-IR spectroscopy

The FT-IR spectroscopy has been assured using to detect the functional groups of virgin polymers and its interactions with the prepared Au NPs. Fig. 1 indicates the FT-IR absorption spectra of virgin polymers (PVA and CMC), pure PVA/CMC blend, and the nanocomposite samples. Further, the FT-IR peaks and its assignments for the virgin polymers are listed in Tables (1 and 2)[2,27–30].

Fig. 1.

FT-IR spectra of pure PVA, CMC, PVA/CMC blend and the filled samples with different concentration of Au NPs.

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Table 1.

Assignments of the FT-IR characterization bands of the pure PVA and pure CMC.

Pure CMCPure PVA
Assignment  Wavenumber (cm−1Assignment  Wavenumber (cm−1
OH stretching  3402  OH stretching  3350 
CH2 asymmetric stretching  2922  CH2 asymmetric stretching  2942 
Carboxylate (COO) stretching group  1597  Stretching CO and CO from acetate group remaining from PVA  1735 
CH2 scissoring  1416  OH and CH bending vibration  1432 
OH bending  1322  CH2 out of plan bending vibration  1376 
CHOCH2 stretching  1062  CH2 bending  1250 
–  –  CO stretching motion  1094 
–  –  CH2 rocking vibration  845 
Table 2.

The σdc, n, τ and τM values for PVA/CMC blend, the nanocomposite samples (0.02 and 0.16wt.%) and also for the nanocomposite sample (0.02wt.%) at different temperatures.

  σdc−1cm−1n  τ (s)  τM (s) 
Pure blend  4.79×10−12  0.55  15.92×10−3  0.55×10−3 
0.02  4.13×10−12  0.58  47.66×10−3  0.05×10−4 
0.16  2.74×10−12  0.68  –  9.58×10−3 
T (oC)  PVA/CMC-0.02wt.% Au NPs 
25  4.13×10−12  0.58  47.66×10−3  0.05×10−4 
50  1.06×10−10  0.46  4.28×10−3  8.95×10−6 
75  4.04×10−9  0.29  1.24×10−3  1.004×10−6 
100  3.08×10−8  0.20  0.61×10−4  2.30×10−7 

Compared the FT-IR absorption spectra of virgin polymers (PVA and CMC) and the pure blend, there are a significant differences. The broadness of OH stretching vibration is increased and its position is shifted to 3454cm−1. The appearance of both the (CO) group of PVA and the (COO-) group of CMC in the spectrum of blend, where the CC group is appeared as a weak shoulder at 1710cm−1 and the absorbance of (COO) of blend is small with more broadness as compared to that of CMC polymer, indicates that the blend components are miscible. Further, the functional groups in the wavenumber region 1520–800cm−1 are significantly affected because of the blending process as the broadness of these functional groups is increased. Further, the absorbance of the functional group of blend at 1376cm−1 is largely decreased as compared to that the pure PVA and the functional group of PVA at 1250 is appeared as a weak shoulder at 1240cm−1 in the PVA/CMC spectrum. The crystalline peak of the PVA structure at 1094cm−1 is negatively affected due to blending with CMC. This peak is the characteristic alcohol peak and becomes broader because of the interference of the vibrational mode of this peak and that of the CHOCH2 group of CMC structure at 1062cm−1. All these observations depict the formation of intermolecular H-bonding within the hybrid polymer matrix (Scheme 1), where the PVA polymer a large tendency to hydrogen form bonding with other polymers containing highly electronegative groups such as CMC. It is worth mentioning that the presence of carbonyl functionalities within the PVA structure because of the residual acetate groups after the PVA preparation from the hydrolysis of polyvinyl acetate [23].

Scheme 1.

Formation of Hydrogen bond within the PVA/CMC matrix.

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From Fig. 1b, the vibrational bands of PVA/CMC matrix are significantly affected due to the Au NPs addition. The broadness of OH band and intensity of CH2 asymmetric stretching band are reduced. The absorbance of the functional bands in the wavenumber range 1800–800cm−1 decreases gradually with the increase of Au NPs content. The FT-IR spectra of the nanocomposite samples (0.08 and 0.16wt.% Au NPs) are found entirely varied in comparison to that of the pure PVA/CMC blend and the nanocomposite films (0.02 and 0.04wt.% Au NPs), where the vibrational bands at 1735, 1376 and 915cm−1 are absent. The band 915cm−1 refers to the syndiotactic structures of PVA matrix [9]. Furthermore, the absorbance of (COO) and (CHOCH2) groups is reduced. Thus, these considerable variations in the absorbance of all absorption bands indicate that there is the formation of nanoparticle-polymer interactions, such as hydrogen bonding or van der Waals interaction, between the (OH)/(COO) groups of PVA/CMC and the Au NPs (Scheme 2). This leads to a decrease in the crystallinity degree of the nanocomposite films and the formation of charge transfer complexes, where the PVA/CMC matrix acts as an electron donor and the Au NPs serves as an electron acceptor. These interactions influence the dynamics of entire chain structures of the nanocomposite samples.

Scheme 2.

Mechanism of interactions between Au NPs and the monomers of PVA and CMC.

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3.2SEM

Fig. 2 shows the SEM micrographs of pure PVA/CMC blend and the filled samples at magnification 2500 times. From Fig. 2a, the surface of blend is relatively rough, due to the semicrystalline nature of blend, without any phase separation as a result from the miscibility of PVA and CMC polymers. This micrograph corroborates the findings of Saadiah et al. [6]. From Fig. 4b–e, the Au NPs addition produces considerable modification in the surface morphology of blend. From Fig. 2b, the concentration of Au NPs (0.02wt.%) has slightly smooth surface and the surface roughness reduces with continuous increasing the nanofiller content. Also, the Au NPs are distributed uniformly with no aggregation (Fig. 2b,c). From Fig. 2d, there is low degree of agglomeration for the Au NPs that largely increases in the form of bright spots (Fig. 2e). It can be concluded that the nanocomposite films (0.02 and 0.04wt.% Au NPs) have relatively smooth surfaces with no aggregation of Au NPs and depicts the suitability to use them as insulator and substrate for the development of microelectronic devices. These morphological changes indicate nanoparticle-polymer interaction depicting good compatibility between the inorganic and organic constituents within the nanocomposite samples.

Fig. 2.

The SEM micrographs for the prepared samples.

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3.3XRD

The XRD patterns have been performed for the PVA/CMC blend, filled samples and the biosynthesized Au NPs, as shown in Fig. 3, to show the changes within the structure of nanocomposite samples. From Fig. 3b, the XRD patterns of Au NPs are in agreement with the results of its face-center cubic (FCC) structure [JCPDS file no. 04-0784] [11,17,31,32].

Fig. 3.

X-ray diffraction pattern for pure blend, blend with differ concentration of Au NPs and the prepared Au NPs.

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The semicrystalline PVA polymer exhibits broad diffraction peak at 19.50° that indicates to orthorhombic PVA (101) reflection plane [JCPDS file no. 41-1049] [8]. Similarly, Rajeh et al. [27] reported main diffraction peak of semicrystalline CMC polymer at 2θ=21.50°. Thus, the diffraction peak at 19.70° in the XRD pattern of PVA/CMC blend indicates its semicrystalline nature because of the existence of PVA with content (70wt.%) within the blend structure. It can be observed that the incorporation of NPs into the PVA/CMC matrix decreases the peak intensity and increases the main peak broadness. Also, there are no diffraction peaks attributed to the NPs phases, which indicates the dissolution of NPs within the polymeric matrix [19,20]. These observations depict that nanoparticle-polymer interactions break the PVA/CMC crystallites and the crystalline phases of the nanocomposite samples decreases. Further, electrostatic interaction of Au NPs with the PVA/CMC chains disturbs the crystalline phases of PVA/CMC within the filled samples. Thus, these PVA/CMC/Au nanocomposite samples are suitable for the solid polymer electrolyte preparation since the reduction of crystallinity degree is important for the improvement of electrical conductivity in solid ion dipolar complex.

3.4TEM

The TEM micrographs of prepared NPs and the nanocomposite sample (0.16wt.% Au NPs) are indicated in Fig. 4a,b. The shapes of biosynthesized Au NPs with average size range 5–29nm are spherical nanoparticles and triangular/hexagonal nanoplates (Fig. 4a). The presence of triangular/hexagonal shapes indicates the rapid adsorption of the functional groups of extract on the (111) facets of newly produced Au NPs [33]. From the TEM micrograph of the nanocomposite sample, the Au NPs with anisotropic structure of spherical, hexagonal triangular and truncated triangular are existed with average size range 5–38nm (Fig. 3b). The strong adsorption of the functional groups of the PVA/CMC blend-coated Au NPs on the selected facets of Au nuclei make this surface become hindered in growth process as the others grow fast. Further, the growth rate is decreased along the adsorbed surface (111) that may result in the formation of anisotropic plate form of Au NPs [34]. The increase in the size range of the nanocomposite sample is because of the existence of Au NPs with high content which assists the ability of Au NPs to move and aggregate. From Fig. 4c, the ring shape of selected area electron diffraction pattern of the prepared NPs confirm their (FCC) crystalline nature [11,17].

Fig. 4.

(a, b) The TEM images and histogram of particle size distribution of the prepared Au NPs and the nanocomposite sample (0.12wt.% Au NPs) and (C) the electron diffraction pattern for Au NPs.

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3.5Electrical properties3.5.1AC conductivity and impedance results

The spectra of real part σ′ of the complex AC electrical conductivity of pure PVA/CMC and the filled films (0.02 and 0.16wt.% Au NPs) at 25°C are shown in Fig. 5a. The σ′ values increase non-linearly with increase of frequency (f) and show DC plateau behavior over the audio frequency (AF) region (f <20kHz) and as well over the radio frequency (RF) region (f >20kHz), which are because of the semicrystalline nature of these samples. The behavior of σ′ has been reported previously [15,28,35]. The σ′ values of these films rapidly decrease with the decrease of f in due to the electrode polarization (EP) effect, where the accumulation of more and more charges exists at the electrode surfaces and interface of electrolyte. This results in a reduction in a number of mobile ions, and therefore, leads to the decrease of the ionic conductivity of the studied material. It is observed that in AF region, the σ′ value of pure PVA/CMC blend is large as compared to the values of filled samples, while in RF region it exhibits a reverse trend. The σ′ values follow the Jonscher's power law [36]:

σ′(ω)=σdc+n

Fig. 5.

Frequency dependent real part σ′ of the complex AC electrical conductivity, and also the real part Z′ and reactive part Z″ of complex impedance of PVA/CMC blend and the nanocomposite samples (0.02 and 0.16wt.% Au NPs) at 25°C.

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The values of exponent factor (n) from power law fit and dc electrical conductivity (σdc) for the investigated samples are listed in Table 2. From this table, the σdc values progressively reduce with the increase of Au NPs content within in the PVA/CMC matrix. For a dielectric material, the σdc value is controlled by the mobility of charge (μe), charge density (ne) and the mobile charge value as shown in the following relation:

σdc=e ne μe

Thus, the reduction in σdc values for the nanocomposite samples indicates that there is the decrease of the charge mobility because of the polymer-nanoparticle interactions. Therefore, the charge migration becomes slower and this makes the contribution of charge in electrical conductivity is less. Further, the incorporation of NPs produces charge transfer complexes (CTC) in which the movement of charges exists through coulomb forces and electric field [15]. Form this table, the n values are less than "1" indicating that the charge transportation within the polymeric matrix follows the hopping conduction mechanism [28]. These electrical conductivity results depict the suitability of the nanocomposite films for electrical insulation in the microelectronic devices as flexible polymeric nanodielectric and the development of next-fabrication flexible-type biodegradable green electronic.

The real part of impedance (Z′) of the investigated samples at 25°C is shown in Fig. 5b. The Z′ values in the AF range are significantly influenced by Au NPs, while these values are smaller as compared to that of the PVA/CMC blend in the RF region (inset figure). In the AF region, the Z′ values of the investigated films are around GΩ range that depicts their large electrical insulation of these dielectric materials at low f values, and subsequently, these samples may be utilized for the low power based microelectronic devices worked at AF range. From Fig. 5c, the reactive part of impedance (Z″) value of the nanocomposite samples is large as compared to the Z values implying the prevailing capacitive behavior of these dielectric films. Thus, these samples are favorable for the applications of capacitive devices.

At different temperatures, the σ′, Z″ and Z″ spectra of the PVA/CMC/0.16wt.% sample are shown in Fig. 6. From Fig. 6a, there is a large improvement for the σ′ values with increasing the temperature (T) of the nanocomposite sample depicting its thermally activated electrical conduction behavior. Further, this implies that the increase of the free charge carrier's number and their mobility increases. The temperature dependent σdc values are investigated through power law fit to the AF region. At various T, the obtained values of n and σdc are listed in Table 2, where the σdcvalue improves with T depicting the thermally activated σdc behaviour of the nanocomposite sample. Also, the values of nreduce implying that charge hopping conduction mechanism is thermally activated process within the studied nanocomposite sample. The T dependent σdc value of PVA/CMC/0.16wt.% sample is plotted in Fig. 6b, where the linear behavior of this plot depicts the Arrhenius dependence of σdc. The activation energy (Ea) of this nanocomposite sample is determined through the Arrhenius's relation σ′=σ0exp (–Ea/kBT), respectively, as the σ0is the pre-exponential factor, T is the temperature and kB is the Boltzmann's constant. The obtained value of Eais 0.51eV which is significantly low. Thus, this suggests that the suitability of the nanocomposite samples in the solid electrolytes preparation as the potential barrier for the charge hopping is small. The temperature-dependent Z′ and Z″ spectra are shown in Fig. 6c,d. The values of Z′ and Z″ are largely decreased by when the temperature is increased. From these spectra, the prepared samples can be used as nanodielectrics in the electronic devices development such as antennas, capacitors and organic field effect transistors because the performance of such devices is greatly controlled by the electrical conductivities of these dielectric materials.

Fig. 6.

Frequency f dependent σ′, σdc, Z′, Z′′ of the PVA/CMC/0.16wt.% sample at different temperatures.

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3.5.2Dielectric spectra

The f dependent ε′ and ε″ spectra of the complex permittivity ε* for the investigated samples are obtained from the following equations [28]:

ε*(ω)=ε′jε″
ε′=tC/ε0 A
ε″=σ′/ω ε0
tanδ=ε″/ ε′
where, C is the measured capacitance, t is the film thickness, A the electrode cross-section area, ε0 is the permittivity of free space and tanδ is the dielectric loss tangent. Fig. 7a–c shows the ε′, ε″ and tanδ values of pure PVA/CMC and the filled films (0.02 and 0.16wt.% of Au NPs) at 25°C. These figures confirm the obtained electrical and impedance spectra. From these figures, the ε′ values of nanocomposite sample (0.02wt.% of Au NPs) are larger than that of the blend sample in the RF and AF regions. For the nanocomposite sample (0.16wt.% of Au NPs), the ε′ values are lower than that of the blend sample in the AF range and these values are higher in the RF range. Further, the ε″ values of the two nanocomposite samples are lower than that of pure blend. The ε′ values are high at low f because of the interfacial polarization (IP) effect [9] as a result from the difference in the conductivity and permittivity values of the blend matrix and the Au NPs, where the dielectric polarizability increases because the charges accumulate at the interfaces of various conductivity constituents within the filled samples. Moreover, it is noted that the ε′ and ε″ values of the investigated samples are almost frequency independent over the RF range, whereas these values increase with the decrease of f in the AF range. These linear variations over the AF range depict the f tunable dielectric behavior of these samples.

Fig. 7.

Frequency dependent ε′, ε″ and tanδ of pure blend and the nanocomposite samples (0.02 and 0.16wt.%) at 25°C.

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The obtained ε′ and ε″ spectra of the nanocomposite sample (0.16wt.% of Au NPs) are smaller than that of the pristine blend sample. This is a rebuttal for the existence of electrostatic interactions between the dipolar segments of PVA/CMC chain and the surface of Au NPs that hinder the dipolar ordering of polar blend, resulting in reduction for the dielectric permittivity values of this nanocomposite sample. Further, this reduction can be attributed to the nanoconfinement effect [9] that mainly results in the increase of hindrance by the NPs to the dipolar group motion of the polymeric chains with the time varying electric field. Therefore, the dipolar polarization of this sample decreases. From the literature survey, the same dielectric permittivity results were as well reported [9,15,35].

Fig. 7c indicates that the tanδ spectra show in the middle f region relaxation peak that is assigned to the local chain dynamics of PVA/CMC. This peak is shifted to lower frequencies in the spectra of nanocomposite sample (0.02wt.% Au NPs) and disappeared in the spectra of nanocomposite sample (0.16wt.% Au NPs). The dielectric relaxation time τ is determined through the relation τ=(2πfp)1, where the fp is the tanδ peak frequency and the τ values are listed in Table 2. The large τ value of the nanocomposite sample as compared to that of the PVA/CMC sample confirms that the significant electrostatic interactions between the functional dipolar groups of PVA/CMC and the NPs results in more hindrance to the segmental motion of polymeric chains. These observations infer that there is good dispersion for the Au NPs within the PVA/CMC matrix that results in increase for the nanofiller/polymer interactions.

Fig. 8a–c depicts the dielectric spectra of ε′, ε″ and tanδ for the nanocomposite sample (0.02wt.% Au NPs) at different temperatures, which gradually improve with the increase of temperature. This improvement is large in the AF range as compared to that of the RF range of these spectra (inset figures). The dc electrical conductivity and IP effect contributions at low f are the basic cause for relatively large raise of the ε′ values with increase of T. The relaxation peak of tanδ spectra moves towards high f with increasing T that increases the free volume and assists the polar chain segments orientation of the PVA/CMC macromolecules [12,28]. The temperature dependent τ values are listed in Table 2, which reduce with the increase of T that weakens the nanofiller-interactions and facilitates the polymer chain dynamics [12,35].

Fig. 8.

Frequency dependent ε′, ε″ and tanδ of the nanocomposite sample (0.02wt.%) at different temperatures.

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3.5.3Electric modulus spectra

Fig. 9a,b shows the complex electric modulus M*(ω) spectra of the investigated samples which are calculated by the following relation:

M*(ω)=1/ε*(ω)=M′+jM″=ε′/(ε′2″2)+″/′2+ε″2)

Fig. 9.

Frequency dependent (M′ and M″) of complex electric modulus and Argand plots of PVA/CMC and the nanocomposite samples (0.02 and 0.16wt.% Au NPs).

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The electric modulus spectra are important since it indicate the bulk response of the dielectric sample and doesn't depend on the effect of electrode polarization. The M′ spectra increase non-linearly with increase of f, while M″ spectra show relaxation peak in the AF range that is assigned to the PVA/CMC chain motion. The peak in high f is due to the re-orientation motion of polar groups of the PVA/CMC chains and this peak is disappeared in the spectra of two nanocmposite samples. The modulus relaxation time τMvalue is calculated from the following relation: τM=(2πfM)1, where fM is the f value indicating to the peak. The calculated τM value is listed in Table 2. The rise τM value of the nanocomposite sample (0.16wt.%) may be due to the formation of Au NPs aggregations and the low τM value of the nanocomposite sample (0.02wt.%) is due to the fine dispersion of NPs within the PVA/CMC structure. From Table 2, the τ value is higher than the τM value that is expected since most of the polymer nanocomposite samples show this pattern [15,35,37].

Fig. 9c indicates the Argand plots (M″ versus M′) for the investigated samples. The Argand plot for the pristine sample is deviated from the semicircular arc, which indicates that the relaxation processes are non‐Debye type (no single relaxation time) [27]. This can be due to the occurrence of different polarization mechanisms under the effect of external electric field, like ionic, electronic and dipolar polarization. Therefore, a various relaxation times exist. The Argand plots of the nanocomposite samples change to half semicircles that confirm the Debye type relaxation process of these filled samples. Further, the radii of these semicircles of these nanocomposite samples are largely decrease due to the increasing of ionic mobility resulting in the improvement of conductivity [38,39].

Fig. 10a,b shows the M′ and M″ spectra of nanocomposite sample at different temperatures. The M′ values reduce as the temperature of nanocomposite increases with the increase of T, while the relaxation peak of M″ spectra shows a progressive rasie of its height with a shift towards high f region. The T dependent values of τM are listed in Table 2 which indicates a reduction with the T increase. This reduction depicts the improvement of the polymeric matrix flexibility. Fig. 10c shows the Argan plots of the nanocomposite sample (0.02wt.%) at different temperatures, where the plots depict the relaxation processes are Debye type and these plots move toward the orgin with increasing T because of the increase of electrical conductivity.

Fig. 10.

Frequency dependent (M′ and M″) of complex electric modulus and Argand plots of the nanocomposite sample (0.02wt.% Au NPs) at various temperatures.

(0.34MB).
3.6UV/Vis. spectroscopy

Fig. 11 shows the UV/Vis. absorption spectra for the prepared Au NPs, pure PVA/CMC blend and the filled samples. The spectrum of Au NPs shows an absorption peak at 531nm which is referred to the surface plasmon resonance peak (SPR) of Au NPs [3,17–19]. Also, this peak is responsible for the transfer colors from yellow for the bulk gold chloride to pinkish-red for the prepared NPs. Thus, this peak confirms the successful preparation of Au NPs by the mint extract.

Fig. 11.

The UV/Vis. spectra of pure blend and the blend filled with different concentrations of Au NPs.

(0.17MB).

The spectrum of virgin PVA/CMC blend shows absorbance peak at 195nm that is assigned to the n→π* transition [28,40,41]. Further, this spectrum exhibits a sharp absorption edge that gradually red shifted with an increase in absorbance in the filled samples when the Au NPs concentration increases, where the color of films is changed from transparent to pinkish-red color. These observations depict the complexation of Au NPs with the functional groups of blend structures (PVA and CMC) and also confirm the variation in the optical energy band gap (Eg) of the nanocomposite samples with a change of added Au NPs contents. These filled samples also show relatively weak absorbance peak at about 537nm and the intensity of this peak gradually increases with the continuous increase of Au NPs concentration within the PVA/CMC matrix. Also, the observed SPR peak is an evidence of the Au NPs existence within the PVA/CMC matrix. Fundamentally, the SPR is an oscillation of high-density free electrons occurring because of the effect of light radiation [19,22]. Thus, the improvement of SPR peak intensity for the filled samples is absolutely because of the increase of total surface area to volume ratio of the NPs, and this confirm the suitability of these flexible polymer nanocomposite samples as photosensors for many biomedical and optoelectronic applications. The red shift of SPR peak from 531nm for the prepared NPs to 542nm for the nanocomposite sample (0.16wt.%) implies the increase of Au NPs size as confirmed by the TEM micrographs, due to its presence with high content.

3.6.1Determination ofEg

The Eg values of the Au/PVA/CMC nanocomposite samples near the fundamental absorption edge have been determined using the following equation [28,42]:

(αhν)m=B(hνEg)

where, α is the absorption coefficient, is the photon energy, B is a constant and m=1/2 for indirect and 2 for direct allowed transitions. The α value has been obtained via the expression α=2.303 (Absorbance/L), where L is the sample thickness. Fig. 12a,b shows the (αhν)1/2 and (αhν)2 versus hν plots of the PVA/CMC/Au nanocomposite films. The straight‐line portions of these plots are extrapolated to zero absorbance to obtain the Eg values for direct and indirect transitions that are reported in Table 3. The estimated Eg value for pure PVA/CMC is consistent with the reported values in the literature [40]. Table 3 shows that the Eg values of filled samples gradually reduce with the increase of Au NPs concentrations that may be explained in terms of the formation of charge transfer complexes (CTCs) between the functional groups of PVA/CMC and the atoms of Au NPs [19,41] as indicated in the FT-IR analysis. This reduction presumes to increase with the degree of disorder for the localized state's generation in the material, where the incorporation of NPs may produce energy levels within the band gap of PVA/CMC matric which results in narrowing the Eg[20,22]. Thus, these nanocomposite samples are suitable materials for optoelectronic devices and electrochemical applications.

Fig. 12.

(αhν)2 and b (αhν)0.5 versus hν plots of pure PVA/CMC and the filled films.

(0.17MB).
Table 3.

Optical energy gap for pure PVA/CMC blend and the filled samples.

Samples  Eg (eV)
  indirect  direct 
Pure blend  5.14  5.76 
0.02wt.% Au NPs.  4.96  5.57 
0.04wt.% Au NPs.  4.85  5.44 
0.08wt.% Au NPs.  4.76  5.36 
0.16wt.% Au NPs.  4.10  4.82 
4Conclusion

The polymer nanocomposite samples composed of PVA/CMC matrix with different concentrations of Au NPs as nanofiller had been prepared by the solution casting technique. The FT-IR results confirmed the formation of strong interactions between the OH/COO groups of PVA/CMC and the Au NPs. From the SEM microghraphs, the dispersion of a small amount of Au NPs modified the rough surface morphology of the PVA/CMC blend to smooth for the nanocomposite samples and these NPs aggregates on the surface of nanocomposite samples with high content of Au NPs. These samples were semicrystalline and the incorporation of Au NPs reduced the degree of crystallinity as indicted in the XRD analysis. The TEM micrographs showed the size range increased from 5 to 29nm for the prepared NPs to 5–38nm for the nanocomposite sample (0.16wt.% Au NPs). The σdcvalues obtained from σ′ spectra in the low f region of the nanocomposite samples were lower than that of the pristine sample. The Au NPs addition with high content decreased the dielectric permittivity due to the nanoscale confinement effect that leads to an increase for the hindrance of polymeric chain motion. The UV/Vis. spectra depicted the presence of Au NPs within the filled samples by the SPR peak of Au NPs and the reduction of Eg values for the filled samples due to the formation of charge transfer complexes (CTCs) between the functional groups of PVA/CMC matrix and the atoms of Au NPs. From the structural properties and the dielectric spectra, these nanocomposite samples are suitable as insulator/substrate for AF operated electrical/electronics devices and as low dielectric permittivity nanodielectric for electronic devices of RF ranges.

Conflict of interest

The authors declare that they have no conflict of interest.

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