GO polymer nanocomposites

Materials Science & Engineering B 246 (2019) 62–75

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Materials Science & Engineering B journal homepage: www.elsevier.com/locate/mseb

Synthesis of graphite oxide nanoparticles and conductivity studies of PSF/ GO and PSAN/GO polymer nanocomposites S. Ningarajua, K. Jagadishb, S. Srikantaswamyb, A.P. Gnana Prakasha, H.B. Ravikumara, a b

T

⁎

Department of Studies in Physics, University of Mysore, Manasagangotri, Mysuru 570006, Karnataka, India Department of Studies in Environmental Science, University of Mysore, Manasagangotri, Mysuru 570006, Karnataka, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Microstructure Free volume Interface Conductivity Crystallinity

Graphite oxide (GO) nanoparticles were synthesized by modified Hummer method. DLS results suggest that the average size of GO nanoparticles is 140 nm. EDX study confirms the presence of oxygen containing functional groups in GO sheet. Polysulfone (PSF/GO) and Poly (styrene co-acrylonitrile) (PSAN/GO) polymer nanocomposites were prepared by solution casting technique. The ionic and electronic conductivity behaviors of PSF/ GO and PSAN/GO polymer nanocomposites were measured. The increased conductivity with increasing GO nanoparticles loading is due to the conductivity chain formation and increased mobility of charge carriers. The effect of crystallinity and free volume on electrical conductivity of PSF/GO and PSAN/GO polymer nanocomposites is explored. PSF/GO and PSAN/GO nanocomposites exhibit high electrical conductivity with low amount of (0.6 wt% and 1.0 wt%) GO loading is due to the formation of GO networks by the strong chemical interaction induced crystallinity of the polymer matrix. This is evident from FTIR and Raman spectroscopy studies.

1. Introduction Polymer-nanocomposites have attracted considerable scientific attention in the field of polymer nanotechnology for various technological/industrial applications. The properties of polymer nanocomposites exhibit completely different behavior than the bulk polymeric matrix. This is due to the excellent abilities of the nano-sized fillers and the increased surface area at the interface. The nanoparticles properties such as particle size, particle size distribution, dispersion state, geometric shape and surface properties are the other well known factors, which alter the material properties of the nanocomposites [1,2]. Depending on the chemical nature of the incorporating nanoparticles and the way in which they interact with the polymeric host matrix, the physical properties of polymer nanocomposites alter at different degrees [3]. The special properties of polymer nanocomposites often arise from interaction of its phases at the interfaces. Polymeric materials can be made conductive due to their conjugated bonds interact with the incorporation of appropriate nanoparticles. By adding conducting nanofillers of suitable concentration, the conductivity of polymeric material can be increased by several orders of magnitude. This improved conductivity depends on the chemical and physical nature of filling material. The incorporation of nanofillers introduces conducting nano-layers of nanofillers in the polymer matrix and hence

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exhibit improved electrical properties [4]. The formation of interface between the polymeric chains and nanofillers is the most important key parameter to provide better electric performance of the nanocomposites material [5]. The incorporation of inorganic/organic nanofillers proved to be an effective way of improving electrical properties of polymer nanocomposites [5]. Graphite oxide with two-dimensional structure can be used as a desirable candidate for the preparation of multifunctional polymer nanocomposites. Graphite oxide (GO) nanoparticles are wellknown for its excellent electrochemical properties. Graphite oxide nanofillers incorporated into the insulating polymeric matrix may improve the conducting behavior of polymer nanocomposites. Polysulfone (PSF) is a high performance engineering thermoplastic exhibits nanofiller dependent electrical properties [6]. The heat stability of PSF is also quite high compared to other polymeric materials. Because of its exceptional insulating properties, PSF is used as a dielectric material in the fabrication of capacitors [7]. Another polymer Poly (styrene co-acrylonitrile) (PSAN) is a copolymer of styrene and acrylonitrile exhibits superior electrical properties over polystyrene. Normally, Poly (styrene co-acrylonitrile) is a poor electric conductor and become conductive upon doping some organic/inorganic dopants. Due its molecular structure, styrene-based materials show unique characteristics such as durability, high performance and versatility of

Corresponding author. E-mail address: [email protected] (H.B. Ravikumar).

https://doi.org/10.1016/j.mseb.2019.06.002 Received 13 March 2018; Received in revised form 6 May 2019; Accepted 4 June 2019 Available online 10 June 2019 0921-5107/ © 2019 Elsevier B.V. All rights reserved.

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10 min. About 15 g of KMnO4 was added to the mixture kept at the reaction temperature below 5 °C. The resulting mixture was stirred constantly for more than 2 h. Then the solution was taken out from the ice bath and placed it over water bath and the temperature was raised up to 40 °C with continuous stirring. The addition of 200 mL deionized water to the solution mixture creates the homogenous suspension, the temperature of the suspension increased up to 98 °C for 60 min. The suspension was then stirred for more than 20 min using magnetic stirrer. The reaction was terminated by adding 15 mL of 30% hydrogen peroxide to the suspension gives brown colored solid particles. The reaction product was centrifuged, washed with deionized water and 5% HCl solution repeatedly. The final product was dried at 60 °C.

design [8]. The electrical conductivity of PSF and PSAN can be tailored to a specific requirement by the incorporation of suitable nanoparticles in the polymer matrix. The conducting nature of doped PSF and PSAN is thought to be due to the high physical and chemical interactions between dopants and isopropylidene, sulfonyl groups of PSF polymer matrix and nitrile groups of PSAN matrix respectively. The easy aggregation in aqueous solutions and interaction sites on carbon surfaces became the major shortcomings for PSF/graphite oxide and PSAN/ graphite oxide systems. Therefore, anti-aggregation and creation of additional surfaces in the polymeric matrix are necessary to achieve good interaction. In the present study, to investigate the influence of graphite oxide nano-fillers on electric properties of polymer nanocomposites, we have chosen Polysulfone (PSF) and Poly (styrene co-acrylonitrile) (PSAN) as the host polymer matrix. Aim of this study is to build conductive polymer nanocomposites of polysulfone (PSF) and poly styrene co-acrylonitrile (PSAN). From the literature survey, it has been found that the influence of Graphite oxide (GO) nanoparticles on electrical properties of PSF and PSAN is no where reported. Therefore, polymer nanocomposites of PSF/GO and PSAN/GO with different wt% of nanosized graphite oxide were prepared and their conductivity behavior is investigated. Graphite oxide (GO) nanoparticles were prepared by modified Hummer method, the microstructural characterization was performed by PALS. The main focus of this work is to discuss the role of crystallinity & free volume on electrical conductivity of PSF/GO and PSAN/GO polymer nanocomposites.

2.3. Preparation of PSF/GO nanocomposites films Polymer films of PSF, PSF/GO nanocomposites filled with different concentrations (0.2, 0.4, 0.6, 0.8 and 1 wt%) of 140 nm size graphite oxide were prepared by solution casting method. 5 g of PSF was added into 100 mL N methyl-2- pyrrolidone and stirred at 85 °C until a viscous transparent solution was obtained. Graphite oxide nanopowder dissolved in N methyl-2-pyrrolidone was added into the polymeric solution of PSF and stirred well to get homogeneous mixture. The solution was allowed to reach a suitable viscosity and the solution was then poured into the glass dish and kept for dry in the atmospheric air at room temperature. The prepared PSF/GO nanocomposites films were placed in Thermotek tubular oven at 80 °C for six hours to eliminate the residual solvent. The polymeric films of thickness about 0.5 mm and dimension of 1 cm × 1 cm were cut and kept in the desiccator for 48hrs at room temperature.

2. Experimental 2.1. Materials

2.4. Preparation of PSAN/GO nanocomposites films Graphite Oxide (GO) nanoparticles with particle diameter 140 nm were prepared by modified Hummer method. Elemental analyzer (EA; German Elementar Analysensysteme inc., D-63452 Hanau) and Dynamic Light Scattering were used to analyze particle size distribution of Graphite Oxide nanoparticles. Polysulfone (PSF) having molecular weight 35,000 and density 1.24 g/cm3, poly (styrene co-acrylonitrile) (PSAN) having molecular weight = 165, 000 and density 1.08 g/cm3 were procured from Sigma Aldrich, USA. The chemical structures of PSF and PSAN are as shown in Fig. 1a and b respectively.

Polymer films of PSAN and PSAN/GO nanocomposites filled with different concentrations of 140 nm size graphite oxide were prepared by solution casting method. 5 g of PSAN was added into 100 mL 2Butanone and stirred at 60 °C until a viscous transparent solution was obtained. Graphite oxide nanopowder dissolved in 2-Butanone was added into the polymeric solution of PSAN and stirred well to get homogeneous mixture. The solution was allowed to reach a suitable viscosity and the solution was then poured into the glass dish and kept for dry in atmospheric air at room temperature. The prepared PSAN and PSAN/GO nanocomposites films were placed in Thermotek tubular oven at 80 °C for six hours to eliminate the residual solvent. The polymeric films of about 0.5 mm thick and the dimension of 1 cm × 1 cm were cut and kept in the desiccator for 48hrs at room temperature. These samples were used for PALS, FTIR and SEM measurements.

2.2. Synthesis of graphite oxide (GO) nanoparticles by modified Hummer method Graphite oxide nanoparticles were synthesized using modified Hummer method [9,10]. Graphite powder (5 g) and NaNO3 (2.5 g) were mixed in the solution of H2SO4 (108 mL) and H3PO4 (12 mL) with 9:1 vol ratio. The solution was stirred continuously for uniform mixing. The mixture was then placed in an ice bath at 5 °C for the duration of

3. Measurements 3.1. Dynamic light scattering (DLS) studies

CH3

(a)

O

H

H

H

H

DLS experiments were carried out using Microtrac-nanotrac wave-w 3231 Instruments at a fixed scattering angle of 90° at room temperature. The dispersed graphite oxide (GO) nanoparticles were analyzed using DLS by measuring the average particle size. The background correction was taken by pure solvents and then the particle sizes with zeta potential of the dispersed nanoparticles were measured.

C

C

C

C

3.2. Energy dispersive X-ray (EDX) spectroscopy studies

H

C

O

O

C

S O

CH3

n

(b)

H

N

n

Elemental analysis (the percentage of the detected elements with respect to one another) of GO nanoparticles was carried out using Elemental analyzer (EA; German Elementar Analysensysteme inc., D63452 Hanau).

n

Fig. 1. Chemical structure of (a) PSF and (b) PSAN. 63

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3.3. Scanning electron microscopy studies The surface morphology of pure GO nanoparticles was characterized by means of scanning electron microscope (SEM) (ZEISS EVO 15, Germany) with an accelerating voltage of 15KV. The polymer nanocomposites were mounted on aluminum stubs and gold coated to avoid electrical charging during examination. All SEM images were taken at the magnification of 2KX. 3.4. X-ray diffraction studies X-ray diffraction spectra of GO nanoparticles, PSF, PSAN, PSF/GO and PSAN/GO polymer nanocomposites were recorded using Rigaku Mini Flex 11 diffractometer with Ni filtered Cukα radiation of wavelength 1.5406 Å with graphite monochromator. The X-ray scans were recorded in the 2θ range 6°–60° with the scan speed of 5ο/min in steps of 0.02°. The working voltage and current were 30 kV and 15 mA respectively.

Fig. 2. Particle size distribution of GO nanoparticles synthesized by modified Hummer’s method.

nanocomposites of dimension (1.2 cm × 1 cm × 0.47 mm) coated with silver paste on both sides for good electrical contact were sandwiched between two electrodes. The computer interfaced digital LCR meter in the frequency range 100 Hz to 5 MHz at room temperature was used to record the conductance, tanδ and capacitance data. From these recorded data, the value of AC conductivity of the polymeric material can be calculated using the formula,

3.5. Positron annihilation lifetime measurements Positron annihilation lifetime spectra of PSF, PSAN, PSF/GO and PSAN/GO polymer nanocomposites were acquired using fast-fast coincidence Positron Lifetime spectrometer. The consistently reproducible spectra were analyzed into three lifetime components with the help of computer program PATFIT-88 [11] with proper source and background corrections. The details of the experiment can be found elsewhere [12].

σ=

3.6. FTIR studies

Gd S/cm A

(1)

where G, d and A are the conductance, thickness and the area of cross section of the polymeric sample respectively.

FTIR spectroscopic experiments were carried out using Perkin Elmer Spectrum Version (model spectrum 2 series, NIOS2 Main software, USA) interfaced with Personal Computer (PC) for data processing. FTIR spectra of as received GO, PSF, PSAN, PSF/GO and PSAN/GO nanocomposites for different concentration were run at ambient temperature using KBr disk method at a wave number range of 4200–600 cm−1. By performing 4 scans at a resolution of 4 cm−1, the best scans were selected.

4. Results and discussion 4.1. Materials characterization of graphite oxide nanoparticles 4.1.1. Dynamic light scattering (DLS) results The advanced technique DLS in science research is used to investigate the distribution of size profile of small particles in the suspension or solutions. The particle size distribution of GO nanoparticles measured by DLS study is as shown in Fig. 2. The software used in DLS instruments typically displays the particle population at different diameters. The mean effective diameter of nanoparticles was considered as the size of GO nanoparticles. The size of GO nanoparticles in the suspension was distributed in different range starting from 60 to 250 nm. The mean diameter of GO nanoparticles synthesized by modified Hummer’s method was around 140 nm, in which size distribution of GO is more within 100 nm range. DLS can also be used for the study of stability of suspension by periodical measurements of a sample. This shows whether the nanoparticles aggregate over the time by seeing increased hydrodynamic radius of the particle. The suspension of GO nanoparticles measured periodically (1 hr) for the total period of 12 h exhibits high stability against the changes in its size distribution.

3.7. Raman spectroscopy studies Raman spectra of PSF, PSAN, PSF/GO and PSAN/GO polymer nanocomposites were recorded using Laser Raman spectroscope (XPLORA PLUS from HORIBA Scientific, USA) equipped with a motorized sample stage. The wavelength of the excitation laser was 532 nm and the power of the laser was kept below 1 mW without noticeable sample heating. 3.8. Electrical conductivity measurements The electrical conductivity of PSF, PSF/GO and PSAN and PSAN/GO polymer nanocomposites as a function of Graphite Oxide nanoparticles wt% was measured by Keithley 2636A dual channel Source Meter. The samples of PSF, PSF/GO and PSAN and PSAN/GO nanocomposites of dimension (1.2 cm × 1 cm × 0.47 mm) coated with silver paste were sandwiched between two electrodes. The computer program Lab Tracer 2.0 was used to record the voltage and current data. From the recorded voltage and current data, the value of bulk resistance (Rb) is calculated. The electrical conductivity (σ) was obtained by the relation σ = t/RbA, where t and A are the thickness and the area of the contact respectively.

4.1.2. XRD results of graphite oxide (GO) XRD is the mostly widely used technique for the determination of crystallinity and crystal structure of the material under characterization. Fig. 3 shows the XRD spectrum of GO nanoparticles synthesized by modified Hummer’s method. The XRD spectra measured in a range of 2θ from 6° to 70°, which shows two characteristic diffraction peaks. The strong and high intense peak at 9.99° with hkl value 002 is due to the presence of graphite oxide. This confirms the introduction of oxygen functionalities by oxidation of graphite. This characteristic peak also suggests the exfoliation of graphite oxide layers from highly organized layer of graphite, which indicates the increase in d-spacing of crystal lattice. The additional peaks at 2θ values at 42.28°, 42.76° indicates a

3.9. AC conductivity measurements The AC conductivity of PSF, PSAN, PSF/GO and PSAN/GO nanocomposites as a function of Graphite Oxide nanoparticles wt% have been carried out using HIOKI 3532-50 Hi-tester (Japan) model. The prepared samples of PSF, PSAN and PSF/GO, PSAN/GO 64

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different nano-layers. The random orientation and wavy appearance of exfoliated graphite oxide is seen from the SEM images. 4.1.5. FTIR results of graphite oxide (GO) nanoparticles FTIR measurement was employed to investigate the chemical bonding in graphite after oxidation of graphite using modified Hummer’s method shown in Fig. 6. FTIR spectra of graphite oxide shows characteristic peaks in the range 1630 cm−1 to 1650 cm−1 corresponding to C]C bond and remains even after the oxidation of Graphite. The broad peak from 2880 to 3720 cm−1 due to the absorption of water molecules in GO are contributed by the O–H stretch of H2O molecules. This helps for the hydrophilic application of GO in various fields. The aromatic content appears at 660–680 cm−1 and epoxy groups at 1194.56 cm−1 indicates the complete oxidation of graphite to graphite oxide.

Fig. 3. XRD pattern of GO nanoparticles synthesized by modified Hummer’s method.

4.2. Effect of GO nanoparticles on free volume parameters of polymer nanocomposites

short range order in stacked graphitic layers. 4.1.3. Energy dispersive X-ray (EDX) spectroscopy results The elemental composition analysis of synthesized graphite oxide (GO) nanoparticles is used for qualitative and quantitative analysis of the elements present in the sample. EDX spectra of GO nanoparticles were synthesized by modified Hummer’s method is as shown in Fig. 4. During the process of preparation of GO nanoparticles via the modified Hummer method, the added mass of pure graphite was 5.0 g and the final weight of the obtained GO sheet was about 10.5 g. The theoretically estimated content of C element in GO sheet was about 47.61%, which is consistent with the experimental results of elemental analysis shown in Fig. 5. The elements like, C and O were observed with mass percentage 39.62% and 60.38% and weight percentage 68.47% and 31.53% respectively. This qualitative and quantitative analysis of oxygen indicates the presence of oxygen-containing functional groups embedded in the GO sheet.

4.2.1. Positron lifetime results of PSF/GO polymer nanocomposites: The free volume cavities are the open spaces exist in the amorphous domains of the polymers. The free volume holes provide pathways for thermal motion of the chain segments. The varieties of structural changes like first order phase transition, second order phase transitions like glass transition and relaxation processes in polymers and polymer nanocomposites are well described by considering the free volume as an internal material parameter [13]. As the free volume hole size and their concentration in PSF and PSF/ GO polymer nanocomposites are derived from third and long lifetime component viz., o-Ps lifetime (τ3), o-Ps intensity (I3) along with the intensity of positrons annihilating at the crystalline and amorphous interface region (I2) obtained by using PATFIT-88 program are reported in Table 1. Fig. 7a–c show the plots of o-Ps lifetime (τ3), free volume (Vf), o-Ps intensity (I3) and the intensity of trapped positrons annihilate at the crystalline and amorphous interface region (I2) as a function of GO concentration respectively. The o-Ps lifetime (τ3) and its intensity (I3) of as received PSF sample obtained by the analysis of lifetime spectrum using computer program PATFIT-88 [11] is about 2.052 ns and 17% respectively. The corresponding free volume hole sizes (Vf) of PSF is 102.47 Å3. From Fig. 7a, it is observed that the o-Ps lifetime (τ3) decreases initially as a function of GO concentration and reaches to 1.940 ns at 0.6 wt% of GO. This corresponds to the reduction of 10.52 Å3 in free volume size (Vf) from 102.47 Å3 to 91.95 Å3. Then there is an increase of 138 ps rise from 1.940 ns to 2.078 ns in o-Ps lifetime (τ3) at 1.0 wt% of GO loading in PSF/GO nanocomposites. This shows 12.98 Å3

4.1.4. SEM results of graphite oxide (GO) nanoparticles Scanning electron microscopy provides different structures and morphology of various micro/nanomaterials. Fig. 5 shows SEM images of typical graphite oxide nanoparticles with different sizes (50 and 200 nm) prepared directly from graphite. From SEM image it is clear that how the sheets are free from stalked structure of graphite. The exfoliated sheets like structure of GO were observed in SEM images with different magnification. When the graphite oxidised to GO, it loses its agglomerated structure and form material with high surface area. Graphite oxide consists of layered structure of graphene oxide that is strongly hydrophilic and intercalation of functional groups of the

Fig. 4. EDX spectra of GO nanoparticles synthesized by modified Hummer’s method. 65

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Fig. 5. SEM images of GO nanoparticles with different sizes (50, 200 nm) synthesized by modified Hummer’s method.

Fig. 6. FTIR spectra of GO nanoparticles synthesized by modified Hummer’s method.

increase in the free volume hole size (Vf) from 91.95 Å3 to 104.93 Å3. The incorporation of graphite oxide (GO) nanoparticles and their interaction with PSF polymer matrix introduces the changes in the polymeric nanostructure. The preface of the nano-sized filler distribute unevenly within the polymeric host may restrict the main chain segmental motion and the chain mobility of macromolecules and thus reduces the size of free-volume holes in the polymer [14]. The nanofillers normally distribute in the amorphous region of the polymer matrix. Since, due to their high surface area, they act as bridges over polymeric chains and hence affect their capability to change the conformation. Consequently, reduces inter and intra-chain free volume cavities with the increasing GO nanofiller loading. This is caused by the improved interfacial interactions between the surfaces of GO nanoparticles with PSF polymeric chains. The polymeric chains of PSF are attached on the surface of GO nanofillers through the formation of hydrogen bonding may restrict the movement of polymeric main chains [14]. However, at higher concentration of GO, due to its large surface area, the interaction with the polymer matrix is poor compare to the lower wt% of inorganic particles loading. This would create interfacial regions of finite thickness in the polymer matrix. These regions between the polymer matrix and the nanoparticles contribute to the enhancement of the overall free volume hole size of the polymer nanocomposites. The increased free volume hole size of PSF/GO nanocomposites at

Fig. 7. Plot of free volume parameters of PSF/GO nanocomposites (a) o-Ps lifetime (τ3) and free volume hole size (Vf) and (b) o-Ps intensity (I3) and (c) intensity of trapped positrons annihilation (I2) as a function of GO nanofiller concentration.

higher nanofiller wt% is mainly attributed to the poor interfacial adhesion between the components [15]. In addition, the packing modes of PSF molecular segments were

Table 1 The free volume parameters derived from the PALS results of PSF/GO polymer nanocomposites. τ3 (ns)

Sample name Pure PSF PSF + 0.2 wt% PSF + 0.4 wt% PSF + 0.6 wt% PSF + 0.8 wt% PSF + 1.0 wt%

GO GO GO GO GO

2.052 2.037 1.987 1.940 2.048 2.078

± ± ± ± ± ±

I3 % 0.019 0.020 0.021 0.023 0.020 0.020

17.00 17.25 17.42 17.87 17.30 17.01

66

Vf (Å)3

I2 % ± ± ± ± ± ±

0.19 0.21 0.23 0.24 0.21 0.20

47.35 48.70 49.18 53.70 47.82 47.34

± ± ± ± ± ±

0.60 0.66 0.62 0.81 0.62 0.60

102.47 ± 1.66 101.10 ± 1.72 96.35 ± 1.76 91.95 ± 1.88 102.16 ± 1.73 104.93 ± 1.75

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partly disarranged and a large amount of segmental chains were adsorbed on the surface of nanofillers, leading to the increase in mobility of the molecules. The covalent bonds and other interactions on the interface, limits the motion of the polymeric chains. This leads to the variation in packing density of the polymeric chains and hence enhances the degree of local ordered regions for the nanocomposites. This will further increase the free volume hole size of the nanocomposites due to the disruption of the polymer chain packing [16]. The increased loading of GO nanoparticles from 0.6 to 1.0 wt% perturbs the polymer chain packing. From Fig. 7b it is observed that the o-Ps intensity (I3) gradually increases with the increasing GO concentrations and reaches to the maximum values at 0.6 wt% of GO and thereafter decreases up to 1.0 wt % of GO loading in PSF/GO nanocomposites. The intensity of o-Ps annihilation (I3) corresponds to the amount of free volume or the number of voids present in the sample. The increased o-Ps intensity (I3) from 0.2 to 0.6 wt% of GO concentration is attributed to the formation of additional voids at the interface of PSF/GO polymer nanocomposites. The decreased value of o-Ps intensities (I3) suggests to the inhibition of o-Ps formation in the polymer matrix by providing an additional positron interaction mechanism at the interface [17]. The addition of nanoparticles may increase the dipole character of the polymer molecules and increases the localization of negative charge. The formation of o-Ps chemically inhibited in the favor of positron capture caused by the presence of cations in the nanofillers. The strong polarity of PSF molecule and the presence of polar groups in the filler will act as the Ps inhibitors. The variation of intensity of trapped positron annihilation (I2) with respect to GO nanofiller content is shown in Fig. 7c. The intensity I2 is usually ascribed to the concentration of the annihilated free positrons in the crystalline regions or at the crystalline-amorphous interface boundaries; therefore, the values of I2 are related to the defects density. The increased intensity of trapped positron annihilation (I2) up to 0.6 wt% of GO in PSF/GO nanocomposites indicates the formation of interfacial layer between PSF matrix and GO nanoparticles. This local negatively charged region will trap the positrons with a subsequent increase in its annihilation probability. The decreased I2 with the increased GO concentration above 0.6 wt% suggests the perturbed intermolecular forces in the nanoclusters vicinity leading to the variation in the free electron density [18].

Fig. 8. Plot of free volume parameters of PSAN/GO nanocomposites of (a) o-Ps lifetime (τ3) and free volume hole size (Vf) and (b) o-Ps intensity (I3) and (c) intensity of trapped positron annihilation (I2) as a function of filler concentration.

corresponding to the 18.57 Å3 decrease in free volume hole size (Vf) from 110.62 Å3 to 92.05 Å3 in PSAN/GO nanocomposites. The PALS parameters are the average values taking into the account of different dimensions of holes related to the phases and interfaces present in the materials. The o-Ps preferentially formed and localized within the free volume holes in amorphous polymers and within the interstitial free volumes in the semi crystalline polymers [19]. However, o-Ps formation occurs at the crystalline amorphous interface region in crystalline polymers and at the polymer-filler interface region in polymer nanocomposites [20]. The free volume properties are strongly affected by the amount and type of filler used in polymer nanocomposites. The possible interpretation for the decreased o-Ps lifetime (τ3) at 0.6 wt% and 1.0 wt% of GO content of PSF/GO and PSSAN/GO nanocomposites is as follows. The introduction of nano-sized filler distribute unevenly within the polymeric host may restrict the main chain segmental motion and the chain mobility of macromolecules and thus reduces the size of freevolume holes in the polymer [14]. The nanofillers are not rigid due to

4.2.2. Positron lifetime results of PSAN/GO polymer nanocomposites The positron lifetime parameters of PSAN and PSAN/GO nanocomposites are derived from the analysis of positron lifetime spectrum using PATFIT-88 program are reported in Table 2. Fig. 8a–c show the plots of o-Ps lifetime (τ3), free volume (Vf), o-Ps intensity (I3), the intensity of trapped positrons annihilate at the crystalline and amorphous interface region (I2) as a function of GO concentration respectively. The o-Ps lifetime (τ3) and its intensity (I3) of as received PSAN sample obtained by the analysis of lifetime spectrum using computer program PATFIT-88 [11] is about 2.136 ns and 19.26% respectively. The corresponding free volume hole sizes (Vf) of PSAN obtained are 110.62 Å3. From Fig. 8a, it is observed that the o-Ps lifetime (τ3) decreases continuously as a function of graphite oxide (GO) concentration and reaches to 1.941 ns at 1.0 wt% of GO. This reduction is about 195 ps

Table 2 The free volume parameters derived from the PALS results of PSAN/GO polymer nanocomposites. τ3(ns)

Sample name Pure PSAN PSAN + 0.2 wt% PSAN + 0.4 wt% PSAN + 0.6 wt% PSAN + 0.8 wt% PSAN + 1.0 wt%

GO GO GO GO GO

2.136 2.104 2.060 2.033 1.996 1.941

I3 % ± ± ± ± ± ±

0.013 0.018 0.022 0.022 0.028 0.032

19.26 18.46 18.43 18.06 17.88 17.65

67

Vf (Å)3

I2 % ± ± ± ± ± ±

0.17 0.19 0.23 0.23 0.27 0.29

39.10 44.98 46.14 46.39 48.19 49.75

± ± ± ± ± ±

0.59 0.64 0.65 0.62 0.69 0.68

110.62 ± 1.17 107.53 ± 1.59 103.22 ± 1.90 100.68 ± 1.89 97.17 ± 2.36 92.05 ± 2.63

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their high surface area, the nanofillers acts as bridges over polymeric chains and hence limiting their capability to change the conformation. The nanofillers of GO mainly distributed in the amorphous region of the polymer matrix. Consequently, reduces inter and intra-chain free volume cavities (Vf) with the increasing GO nanofiller loading up to 1.0 wt % of GO loading. This is caused by the improved interfacial interactions between the surfaces of GO nanoparticles with PSAN polymeric chains. Therefore, the polymeric chains of PSAN are attached on the surface of GO nanofillers through the formation of hydrogen bonding and hence restrict the movement of polymeric main chains [14]. The o-Ps intensity (I3) in PSAN/GO nanocomposites gradually decreases with the increasing GO concentrations and reaches to the minimum values at 1.0 wt% of GO is as shown in Fig. 8b. The intensity of o-Ps annihilation (I3) corresponds to the amount of free volume or the number of voids present in the sample. The decreased value of o-Ps intensities (I3) suggests to the inhibition of o-Ps formation in the polymer matrix by providing an additional positron interaction mechanism at the interface [17]. The addition of nanoparticles may increase the dipole character of the polymer molecules and increases the localization of negative charge. The formation of o-Ps chemically inhibited in the favor of positron capture caused by the presence of negatively charged ions in the nanofillers. The strong polarity of PSAN molecule and the presence of polar groups in the filler will act as the Ps inhibitors. Fig. 8c shows the variation of intensity of trapped positron annihilation (I2) with respect to GO nanofiller loading. The increased intensity of trapped positron annihilation (I2) up to 1.0 wt% of GO in PSAN/GO nanocomposites indicates the formation of interfacial layer in PSAN matrix with GO nanoparticles. This local negatively charged region will trap the positrons with a subsequent increase in its annihilation probability.

Fig. 9. XRD spectra of (a) PSF and PSF/GO and (b) PSAN and PSAN/GO polymer nanocomposites as a function of GO nanofiller concentration.

4.3. X-Ray diffraction results of PSF/GO and PSAN/GO nanocomposites The XRD spectra of PSF, PSF/GO and PSAN, PSAN/GO nanocomposites are as shown in Fig. 9a and Fig. 9b respectively. It is possible to detect structural changes in polymer nanocomposites caused by the incorporation of GO nanoparticles. X-ray diffraction pattern of nanoparticle incorporated polymer nanocomposites contain both sharp as well as diffused peaks. The X-ray diffraction patterns of PSF and PSAN matrices exhibit sharp peaks correspond to the crystalline regions at 2θ = 19.82° and 2θ = 19.98° respectively. PSF and PSAN are amorphous and semi crystalline polymers; their crystallinity calculated by the ratio of the area under the crystalline peaks to the total area using Peak fit program is 20.84% and 26.35% respectively. The variation of crystallinity of PSF/GO and PSAN/GO nanocomposites as a function of GO nanofiller concentration is as shown in Fig. 10a and b respectively. The crystallinity of PSF/GO nanocomposites increases with the incorporation of GO nanofillers into PSF matrix and show maximum value of 49.23% at 0.6 wt% of GO loading. Thereafter, the crystallinity decreases to the minimum value of 16.17% at 1.0 wt% of GO. However, the crystallinity of PSAN/GO nanocomposites increases gradually as a function of GO nanofiller content and reaches to the maximum value of 51.68% at 1.0 wt% of GO. The incorporation of GO nanoparticles acts as the nucleating agents and increases the crystallinity up to 0.6 wt% of GO loading in PSF/GO and at 1.0 wt% of GO in PSAN/GO nanocomposites. The further addition (after 0.6 wt%) of GO in PSF/GO nanocomposites decreases the crystallinity due to the hindrance offered by the randomly oriented GO nanoparticles. The addition of more number of GO nanoparticles perturbs the polymeric chain packing and restricts the chain mobility, accompanies crystallization of PSF/GO nanocomposites. However, the decreased crystallinity after 0.6 wt% of GO indicates the weakening of crystallization due to the interaction of GO nanofillers with PSF matrix. The random distribution of GO nanofillers over the entire volume of PSF polymer matrix would facilitate the amorphous phase in the

Fig. 10. Plot of crystallinity of (a) PSF/GO and (b) PSAN/GO nanocomposites as a function of GO nanofiller concentration.

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Fig. 11. FTIR spectra of PSF/GO nanocomposites as a function of GO nanofiller loading.

Fig. 12. FTIR spectra of PSAN/GO nanocomposites of as a function of GO nanofiller loading.

polymer nanocomposites and hence reduces the crystallinity. The incorporation of GO nanofillers up to 1.0 wt% acts as the nucleating agents for increasing the degree of crystallinity and hence reduces the free volume hole size. The total results will depend on the crystallization behavior of the matrix and its interaction with nucleation agents.

the transmittance bands at pure PSAN is 3414, 3027, 2927, 2238, 1659, 1453, 758, and 698 cm−1. The prominent transmittance band at 3416 cm−1 is related to O–H stretching vibrations of adsorbed water molecules and hydroxyl group. The band at 2927 cm−1 is related to the CH2 stretching vibration, whereas the bending vibration of CH2 appears around 1453 cm−1 [21]. The out-of plane C–H bonds of the aromatic ring form intense peaks at 698 and 758 cm−1 [22,23]. The vibration band of C^N in PSAN appears at 2237 cm−1 [21]. The existence of styrene segment in PSAN is represented by 3027 cm−1. The continuous transmittance peak appeared at 1659 cm−1 corresponds to the stretching of carboxyl group (C]O). The FTIR spectra of PSAN/GO nanocomposites with varying GO wt % are shown in Fig. 12, which exhibit the characteristic peaks corresponding to both PSAN and GO nanofillers. The shifts in the wave number are tabulated in table 4. The bands in PSAN/GO nanocomposites at 3414 cm−1 shifted to 3440 cm−1, 2927 cm−1 shifted to 2925 cm−1, 1659 cm−1 shifted to 1602 cm−1. The shifting of broad peak from 3414 cm−1 to 3440 cm−1 is due to the absorption of water molecules in GO nanoparticles, which are contributed by the O–H stretch of H2O molecules. Therefore, strong chemical interaction between the carboxyl groups of GO and hydroxyl group of PSAN can be expected. However, a new band that appears at 1667 cm−1 at 0.2 wt% of GO and shifted to 1602 cm−1 at 1.0 wt% of GO is due to C]C backbone stretching indicates the chemical linking between GO and PSAN polymeric matrix. Based on the above observed variations, we can expect stable bonding (chemical linking) between the organic components of PSAN and GO nanoparticles. The possible microstructural changes may be understood by invoking the intra/intermolecular hydrogen bonding between PSF and GO and PSAN and GO molecules and these are shown in Figs. 13 and 14 respectively.

4.4. FTIR results of PSF/GO and PSAN/GO polymer nanocomposites Fourier transform infrared spectroscopy (FTIR) is a well established technique for the investigation of chemical interaction between the functional groups of organic or inorganic fillers and the polymer side chain. In the present study FTIR is used to study the chemical interaction between GO nanoparticles and PSF polymer matrix. The FTIR spectrum of pure PSF is as shown in Fig. 11a. In the FTIR spectrum of pure PSF the prominent transmittance band at 3439 cm−1 is related to O–H stretching vibrations of adsorbed water molecules and hydroxyl group. The vibrational band at 1681 cm−1 corresponds to C]O stretching. The band at 1322 cm−1 is normally assigned to asymmetric stretching of sulfone group O]S]O. The band at 1238 cm−1 corresponds to the asymmetric stretching of aryl ether group C–O–C, while the peak at 1149cm1represents symmetric stretching of sulfone group O]S]O. The FTIR spectra of PSF/GO nanocomposites with varying GO wt% show the characteristic peaks corresponding to both PSF and GO nanofillers, which are shown in Fig. 11b–f respectively. In PSF/GO nanocomposites with 0.6 wt% of GO loading, the band at 3439 cm−1 shifted to 3385 cm−1 and 1681 cm−1 shifted to 1661 cm−1. This type of bands shifting indicates that the nanocomposite with 0.6 wt% of GO nanofiller exhibit better interaction than the other wt% of nanocomposites. At 1.0 wt% of GO loading, the band at 3385 cm−1 shifted to 3401 cm−1 and 1661 cm−1 shifted to 1667 cm−1. These shifts in the wave number are tabulated in table 3. These shifts suggests the reduced chemical interaction at 1.0 wt% of GO of GO loading in PSF/GO nanocomposites. The FTIR spectrum of pure PSAN is as shown in Fig. 12, that shows

4.5. Raman spectroscopy results of PSF/GO and PSAN/GO polymer nanocomposites Raman spectroscopy was employed to study the disorder and defect

Table 3 The shifts in the wave number in FTIR spectra of PSF/GO polymer nanocomposites. Sample

Pure PSF PSF + 0.2 wt% PSF + 0.4 wt% PSF + 0.6 wt% PSF + 0.8 wt% PSF + 1.0 wt%

GO GO GO GO GO

O–H stretchiest (cm−1)

C]O stretchiest (cm−1)

O]S]O asymmetric stretching (cm−1)

C–O–C asymmetric stretching (cm−1)

O]S]O symmetric stretching (cm−1)

3439.22 3411.53 3400.02 3385.97 3390.23 3401.09

1681.37 1650.77 1662.85 1661.17 1678.93 1667.29

1322.51 1322.77 1321.83 1320.94 1322.02 1322.74

1238.43 1239.52 1239.76 1239.99 1238.53 1238.21

1149.32 1148.15 1148.07 1149.98 1148.43 1148.87

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Table 4 The shifts in the wave number in FTIR spectra of PSAN/GO polymer nanocomposites. Sample Pure PSAN PSAN + 0.2 wt% PSAN + 0.4 wt% PSAN + 0.6 wt% PSAN + 0.8 wt% PSAN + 1.0 wt%

GO GO GO GO GO

O–H stretching (cm−1)

CH2 stretching (cm−1)

C^N stretching(cm−1)

C]O stretching (cm−1)

CH2 bending (cm−1)

3414.52 3027.70 3028.17 3413.87 3373.87 3440.92

2927.33 2925.35 2927.03 2926.36 2925.28 2926.02

2238.71 2236.65 2239.25 2240.34 2238.06 2238.88

1659.13 1673.56 1674.34 1667.90 1667.94 1602.61

1453.29 1453.37 1453.56 1453.47 1453.37 1453.38

spectra of PSAN/GO polymer nanocomposites is probably due to increased structural order upon GO nanofiller loading. The large difference in Raman intensity is observed for 0.6 wt% of GO loading in PSF/ GO and 1.0 wt% of GO in PSAN/GO polymer nanocomposites may be due to the agglomeration of the GO nanofillers. This is indicated by the larger shifts in the wave number and broadening of the Raman spectrum [29]. In the Raman spectra of PSAN/GO polymer nanocomposites, the peak positions of D, 2D and 2D1 bands slightly shifted towards the lower wave numbers for 1.0 wt% of GO nanofiller loading. These shifts are attributed to the improved interfacial interaction between the polymer and GO nanofillers by the stronger compressive forces associated with the polymer chains and GO nanofillers [30,31]. The relative intensity ratio of the D band to the 2D band is known as an index for determining the overall crystalline quality of the graphitic network and increases with long-range ordering in the polymeric materials [32]. The intensity ratio (ID/I2D) of PSF/GO and PSAN/GO polymer nanocomposites is shown in Fig. 16a and 16b respectively. The ratio of the intensity of disordered to order transition (ID/I2D) for pure PSF is 0.43. The intensity ratio (ID/I2D) increases from 0.43 to 1.14 for 0.6 wt% of GO nanofiller loading. This increased value of (ID/I2D) indicates the generation of surface defects due to functionalization and variation in van der Waals force of attraction between nanofillers aggregates in the polymer matrix [33]. In addition, the increased ratio of the intensity of disordered to order transition also indicates that biaxial stretching leads to the higher defect density on the surface of the GO nanofillers probably due to an intensive local friction effect during

levels of polymer nanocomposites [24]. Raman spectra of PSF, PSF/GO and PSAN, PSAN/GO polymer nanocomposites are as shown in Fig. 15a and b respectively. The Raman spectrum of pure PSF shows D, 2D and 2D1 bands at 1350 cm−1, 2632 cm−1 and 3798 cm−1 respectively [24–29]. In which, D band corresponds to the disordered and defect structure, 2D and 2D1 indicates the number of layers in the sample [25]. In the Raman spectra of PSF/GO polymer nanocomposites, the intensity of D, 2D and 2D1 peaks decreases up to 0.6 wt% of GO loading and increases thereafter. The reduced intensity of D, 2D and 2D1 peaks for 0.6 wt% of GO nanofiller loading in PSF/GO polymer nanocomposites can be attributed the increased structural order [26]. The increased intensity of D, 2D and 2D1 peaks for higher wt% of GO loading can be attributed to the increased structural disorder due to localized high concentration of GO nanofillers. The shifting in the wave number of 2D and 2D1 peaks towards the higher wave number suggests that more number of GO layers generated in PSF polymer matrix [27]. In addition, the shifts in the wave number of D, 2D and 2D1 bands slightly towards higher wave numbers indicate the possibility of carboxyl groups (–COOH) of GO nanofillers to be attached with OH group in the side chain of polymer molecules [28]. These carboxyl groups are extremely useful as they can easily react for attachment of various functionalities. Raman spectrum of pure PSAN shows D, 2D and 2D1 bands at 1401 cm−1, 2565 cm−1 and 3486 cm−1 respectively. In the Raman spectra of PSAN/GO polymer nanocomposites, the intensity of D, 2D and 2D1 peaks decreases continuously up to 1.0 wt% of GO nanofiller loading. This reduced intensity of D, 2D and 2D1 peaks in the Raman

OH O

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Fig. 13. The possible chemical interaction between PSF and GO nanoparticles. 70

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Fig. 14. The possible chemical interaction between PSAN and GO nanoparticles.

Fig. 15. Raman spectra of (a) PSF and PSF/GO and (b) PSAN and PSAN/GO polymer nanocomposites as a function of GO nanofiller loading.

Fig. 16. Raman intensity ratio of (a) PSF/GO and (b) PSAN/GO polymer nanocomposites as a function of GO nanofiller loading.

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respectively. The electrical conductivity of PSF at room temperature is 1.54 × 10−10 S cm−1, the conductivity values of PSF/GO nanocomposites initially increases as the concentration of GO increases and shows maximum value at 0.6 wt% of GO loading and then decreases up to 1.0 wt% of GO. The ionic conductivity of pure PSF polymer sample is (1.01 × 10−7 S/cm) and (2.32 × 10−3 S/cm) for 0.6 wt% GO incorporated PSF/GO nanocomposites at 5 MHz frequency and decreases thereafter. The increased AC conductivity is attributed to the increased mobile ions and charge carriers due to the increased crystallinity of the polymer nanocomposites with GO nanoparticles loading. The measured AC conductivity of PSF/GO nanocomposites at 0.6 wt % of GO nanoparticles loading is four orders of magnitude more (from 1.01 × 10−7 to 2.32 × 10−3 S/cm) and five orders of magnitude (from 1.54 × 10−10 S/cm to 3.92 × 10−5 S/cm) more in dc conductivity over pure PSF polymer matrix. This enhancement in both AC and DC conductivity is attributed to the charge transfer mechanism involved by the high density H+ and COO– vacancies associated with the defective grain boundary of GO nanoparticles in PSF polymer matrix. The charge carriers involved in both the cases are the holes associated with H+ vacancies [5]. The decreased ionic and electrical conductivity of PSF/ GO nanocomposites after 0.6 wt% is due to the hindrances for the charge transport through different chains and interfaces. There is a probability of strong interface dynamics at low filler loadings; the polymer chain entanglements inhibit the motion of charge carriers causing a reduction in the electrical conductivity of the system. In addition, the strong and stable bonding between the nanoparticle surface and the polymer chains lead to the free charge carriers to contribute to electrical conductivity in nanocomposites. The number density of charge carriers and applied frequency become dominating factor of the electrical conductivity of polysulfone (PSF/GO) nanocomposites. When more nanofiller are incorporated into the polymer matrix, the conductivity increases as a result of the increased number of charge carriers. The increased nanofiller concentration in the polymeric solution decreases the inter-ionic distance and ion-ion interactions become progressively more significant. At higher concentration of nanofillers, the formation of immobile ions aggregated regions will cause the decrease of the conductivity [37,38]. Also, the motion of the charge carriers is impeded at the crystalline amorphous interfaces. At higher nanofiller concentrations, the molecular aggregates generally tend to diffuse into amorphous regions of the polymer. The presence of aggregates in these regions affects of the crystalline – amorphous interface and hence reduces the conducting paths through the amorphous regions [5]. The variation of ionic and electrical conductivity of PSAN/GO nanocomposites as a function of GO wt% is as shown in Fig. 18a and b respectively. The measured AC and DC conductivity at 1.0 wt% of GO nanoparticles incorporated PSAN/GO nanocomposites is five orders of magnitude more (from 2.16 × 10−8 to 7.70 × 10−3 S/cm) and four orders of magnitude more (from 3.43 × 10−11 to 5.44 × 10−7 S/cm) over pure PSAN polymer matrix. Both AC and DC conductivity of nanocomposites gradually increases as the concentration of GO increases up to 1.0 wt%. The nanofillers are more likely to adhere to each other with the increased nanofiller loading and the interfacial area around nanoparticles is likely to overlap. As a result of overlapping, the charge carries travel through the bulk of the material much easier through the overlapping region. The increased concentration of GO nanofiller accumulates more and more positive charges in front of the cathode. In PSAN/GO system, the ionized species such as H+ and COO− may interact and form a charged GO surface associated with the local electric field. This space charge induced enhancement effect accelerates the transport of conduction ions and hence the conductivity. The increased number density of mobile ions leads to the conductivity chain formation through the aggregation of GO nanoparticles [39,40].

disentanglement [31]. The value of disordered to order transition (ID/I2D) ratio for PSAN polymer is 5.27 and decreases to 3.20 for PSAN/GO polymer nanocomposites with 1.0 wt% of GO nanofiller. The decreased value of (ID/ I2D) up to 1.0 wt% of GO nanofiller loading in PSAN/GO polymer nanocomposites and after 0.6 wt% of GO nanofiller loading in PSF/GO polymer nanocomposites indicates the decreased average size of the sp2 domains of GO created graphitic domains but more in number [34]. This is due to sp2 C atoms converted to sp3 C atoms at the surface of the GO nanofillers after fictionalization [35]. 4.6. AC and DC electrical conductivity (σ) results In nanoparticles incorporated polymer matrix, the electrical conductivity indicates the ease with which the charge carriers move under the influence of the applied electric field and the behavior is brought out by the interaction between ionic charge carriers and the polymer matrix. The carriers such as electric charge carriers in electronic conductors and cation and anion pairs in ionic conductors significantly contribute to the conductivity of the polymeric material. The contribution of metal oxides nanoparticles on conductivity of polymer nanocomposites depends on the processing, chemical doping, concentrations, impurity types and their interactions with the polymeric chains. The ion transport in polymer nanocomposites is considered to take place by a combination of ion motion coupled to the local motion of polymer segments and inter and intra -polymer transition between ion coordinating sites. The DC conductivity of polymer nanocomposites depends on the ability to transport charge carriers along the polymer backbone and the ability of carriers to hop between polymer chains through inter-polymer conduction [36]. The charge carrier mobility in polymer nanocomposites related to ordered and disordered nature of the solid state nanostructure of the polymer matrix. The variation of AC and DC electrical conductivity of PSF/GO nanocomposites as a function of GO wt% are shown in Fig. 17a and b

Fig. 17. (a) AC electrical conductivity and (b) DC electrical conductivity of PSF/GO polymer nanocomposites. 72

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Fig. 20. Plots of (a) crystallinity and free volume sizes and (b) crystallinity and electrical conductivity of PSAN/GO polymer nanocomposites.

PSAN/GO nanocomposites has been explored in the present study. It was previously reported that the charge carrier mobility in polymer nanocomposites occurs in both ordered and disordered regions [14]. The effect of crystallinity and free volume on electrical conductivity of the polymer nanocomposites are well documented [5,12,14,41–48]. The variation of crystallinity, free volume and electrical conductivity of PSF/GO and PSAN/GO polymer nanocomposites as a function on GO nanoparticle loading are as shown in Fig. 19a, b and Fig. 20a, b respectively. Both crystallinity and electrical conductivity show increasing trend up to 0.6 wt% of GO loading in PSF/GO nanocomposites and decreasing thereafter. Both crystallinity, electrical conductivity (σ) continuously increased and free volume show decreasing trend up to 1 wt% of GO nanoparticle loading in PSAN/GO nanocomposites. These changes suggest that the microstructure of PSF polymer undergo some significant change with the incorporation of 0.6 wt% GO nanoparticles and up to 1 wt% of GO nanoparticle loading in PSAN. The charge transfer in polymer matrix can be achieved by the formation of chemical linkages between filler and polymer or direct interaction between nanofillers themselves. The crystallinity of a polymer nanocomposite depends on the chemical and physical interaction between the filler and polymer matrix. The possible chemical interactions and probable covalent bonding between the nanofillers and polymer matrix presumably result in an increased crystallinity. The ordered packing in crystalline regions will facilitate the transfer of electrons more favorably than the disordered amorphous regions. These ordered regions play a vital role in charge transport in the polymeric nanocomposites [14]. The increased crystallinity at 0.6 wt% of GO in PSF/GO and up to 1 wt% of GO in PSAN/GO polymer nanocomposites is in concurrence with the understanding of proficient charge transport in polymer nanocomposites [40]. The availability of crystalline complex regions in the polymer nanocomposites facilitates the motion of the electric dipoles and offers orientation and relaxation of dipoles by the molecular motion [41]. In addition, the reduced free volume hole sizes up to 0.6 wt% of GO nanoparticle loading in PSF/GO and up to 1 wt% of GO in PSAN/GO nanocomposites can be attributed to the restricted chains mobility of polymer matrices due to the increased crystallinity of the

Fig. 18. (a) AC electrical conductivity and (b) DC electrical conductivity of PSAN/GO polymer nanocomposites.

Fig. 19. Plots of (a) crystallinity and free volume sizes, (b) crystallinity and electrical conductivity of PSF/GO polymer nanocomposites.

5. Effect of crystallinity on free volume and electrical conductivity of PSF/GO and PSAN/GO polymer nanocomposites As the crystallinity allows direct correlation with the charge carriers, the effect of crystallinity on electrical conductivity of PSF/GO and 73

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and hence decreases the free volumes. The increased crystallinity of the nanocomposites increases the better conductive network formation probability. 7. The reduced intensity of D peak in the Raman spectra of PSF/GO polymer nanocomposites for 0.6 wt% of GO nanofiller loading and 1.0 wt% of GO loading in PSAN/GO polymer nanocomposites can be attributed the increased structural order of the polymer nanocomposites. This is in agreement with the XRD results. The increased intensity of D peak after 0.6 wt% of GO nanofiller loading in PSF/ GO polymer nanocomposites suggests the increased structural disorderness due to localized high concentration of GO nanofillers. This is indicated by the increased free volumes in the PALS results of PSF/GO polymer nanocomposites.

polymer nanocomposites. It is believed that the influence of nanoparticles on ion motion arise exclusively from the impact of the nanoparticles upon the degree of amorphousity of the polymer matrix [45–47,49]. However, the influence of nanoparticles chemistry extended also to the ordered crystalline phases of polymeric hosts may support significant ionic conductivity. The crystalline morphology of PSF/GO nanocomposites at 0.6 wt% of GO incorporated sample plays an important role in H+ and COO– transport in the crystalline complex. The ordered environment is more favorable for H+ ion transport than the disorganized amorphous phase in the polymer nanocomposites. At higher wt% of nanofiller loading, the nanoparticle-nanoparticle interaction is more favorable than the polymer-nanoparticle interaction. The increased addition of nanofillers into the polymeric solution decreases the inter-ionic distance and hence the ion-ion interactions become progressively more significant. The formation of immobile ions aggregated regions will cause the decrease of conductivity [45,46]. The agglomeration of nanoparticles enhances and creates more hindrance for the migration of free electric charges and therefore impedes the electrical conductivity. The reduced chemical interaction between the nanoparticles rich phase and polymeric chains with the increased addition of GO nanofillers provides more mobility for the polymeric side chains. This is indicated by the reduced crystallinity, electrical conductivity and increased free volume sizes of PSF/GO nanocomposites after 0.6 wt% of GO nanofiller loading.

7. Conclusion The DC conductivity of PSF/GO polymer nanocomposites for 0.6 wt % of GO nanoparticles loading exhibit five orders of magnitude increase from 1.54 × 10−10 S/cm to 3.92 × 10−5 S/cm over pure PSF polymer matrix. The DC conductivity of PSAN/GO polymer nanocomposites for 1.0 wt% of GO nanoparticles loading exhibits four orders of magnitude increase from 3.43 × 10−11 to 5.44 × 10−7 S/cm. The electrical conductivity of PSF/GO and PSAN/GO polymer nanocomposites shows direct dependence on crystallinity and inverse relation with free volumes. Therefore, it is concluded that the ordered crystalline regions are more favorable for the conduction of electric charge carriers and ions than the disordered amorphous regions in PSF/GO and PSAN/GO polymer nancomposites.

6. Summary Based on the above experimental investigations following conclusions were drawn:

Acknowledgements

1. DLS results confirm the size distribution of GO nanoparticles in the suspension is in the range of 60 to 250 nm. In which, the distribution of GO is more in 100 nm range and the mean diameter of GO is around 140 nm. 2. The qualitative and quantitative analysis of GO nanoparticles by EDX indicates the presence of oxygen-containing functional groups embedded in the GO sheet. The theoretically estimated content of C element in GO sheet is about 47.61%, which is consistent with the experimental results of elemental analysis. 3. The transmittance peaks in the range of 1630 to 1650 cm−1 shows the presence of C]C bond after the oxidation of Graphite. The broad peak at 2880 to 3720 cm−1 are due to absorption of water molecules in GO contributed by the O–H stretch of H2O molecules. The aromatic content appears at 660 to 680 cm−1 and epoxy groups at 1194.56 cm−1 indicates the complete oxidation of graphite to graphite oxide. 4. PALS results reveal that the reduced free volume sizes up to 0.6 wt% of GO in PSF/GO and 1.0 wt% of GO in PSAN/GO nanocomposites is due to the restriction imposed on main chain segmental motion and the chain mobility of the macromolecules. This is due to strong chemical and physical interactions by the coordinating effect of graphite oxide ions in polymer nanocomposites. The higher wt% of GO nano-sized fillers boosts the large-scale segmental motion and the mobility of polymeric chains. This is indicated by the increased interfacial area of the polymer-nanoparticle interface by the nanoparticles agglomeration in the polymer matrix. 5. FTIR results of PSF/GO and PSAN/GO nanocomposites confirms the hydrogen bonding between the hydroxyl and carboxyl groups of GO nanofillers and isopropylidene and hydroxyl groups of PSF and PSAN polymeric matrices respectively. 6. PSF/GO and PSAN/GO nanocomposites exhibit high degree of crystallinity at 0.6 wt% and 1.0 wt% of GO loading respectively. The increased crystallinity of the polymer nanocomposites is helpful for fabricating conductive network at lower amount of GO loading due to selective aggregation of GO nanoparticles at amorphous regions

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