Improved thermal, mechanical, and electrochemical performance of bio-degradable starch/reduced graphene oxide nanocomposites

With the fast progress of electronic devices in this era, some unique substances with significant dielectric constant have garnered prominent research interest among scientists and researchers.^1,2 With the potential to be used in several fields, such as high energy density capacitors,¹ random access memory,³ and also different conversion applications,⁴ nanomaterials with potent dielectric properties have proclaimed eloquent regard. Their prime mechanical properties, outstanding flexibility together with the notable facility of fabrication, have acclaimed them to be a dominating contender as a fundamental component for future procreation of pliable electronic devices.^5,6 Generally, polymers reveal a low dielectric constant, and due to this, they are not highly suitable to be applied in capacitors or energy storage applications.⁷ One popular way to improve the dielectric performance is to agglomerate nanostructured fillers within the polymer.^8,9 The difference in electrical properties between the fillers and polymer is responsible for the improved dielectric constant of polymeric nanocomposites.^7–9 

The environment is alarmingly threatened by most of the polymers as they are not readily degenerative because most of them originate from synthetic processes.^10,11 Therefore, pondering their aptitude in device fabrication, along with their up-scaled economic feasibility and perpetuity, devices fabricated from biopolymers have attained extensive research attention compared to the synthetic polymer.^12–14 Non-decomposable devices are hazardous to the environment, and this problem can be addressed with organic biocompatible materials that are abundant in nature with environmental amiability also with cost-effectiveness.¹⁵ To be more exact, nature-derived polymers can be considered as the building blocks of bio-compatible and eco-friendly devices. More specifically, plasticized starch (PS), a plant-derived natural polysaccharide, is abundant in nature, cost-effective, and biodegradable and is a popular biopolymer.^16–18 The introduction of conducting nanofillers into the PS was found to significantly improve the mechanical, electrical, and dielectric performance of the polymer.^19,20 Graphene, a single atomic layer thin sheet of tightly bound hexagonal honeycomb carbon lattice, is considered as one of the most attractive nanofillers for polymer nanocomposites because of its unique remarkable chemical and physical properties.^21,22 Reduced graphene oxide (rGO) has gained substantial research attention as a nanofiller for polymer matrices due to its simple and scale-up production at low cost together with its outstanding tensile properties.^23,24

The 2D nature of rGO permits the manufacture of highly anisotropic nanocomposites with stratified structures, which consist of various characteristics among the directions of alignment.^25,26 rGO provides a substantial conductivity along the in-plane direction, and therefore, incorporation of rGO into the polymer matrix may enhance the conductivity of the nanocomposite.^25,27 In extension to alignment, the size of the graphene sheet also plays an important role in improving the conductivity of the nanocomposite.^28,29 Besides, the existence of a number of functional groups in rGO helps modifying the physical properties of the polymer-based nanocomposites.^28,30 For example, Yousefi et al.³¹ reported an outstanding dielectric constant of rGO/epoxy surpassing 24 000 at 1 Hz, and Fan et al.³² prepared a graphene/PVDF composite, which illustrates a dielectric constant of as high as 7940 at 100 Hz for 0.0177 vol. % of graphene in the composite. The self-assembled rGO sheets in polymer nanocomposites demonstrated morphological development from an anisotropic to a tiered structure as the graphene loadings were incremented gradually.^26,33

The effect of rGO on the structural, mechanical, and thermal properties of PS was reported in several articles;^20,25,34 however, the influence of rGO loading on the electrochemical performance of the PS matrix was never evaluated. Here, in this article, we report the synthesis of PS/rGO composites through a facile solution cast method. The effect of rGO on the structural, chemical, surface morphological, thermal, mechanical, and electrochemical properties of the nanocomposites was investigated. The incorporation of rGO into the PS matrix was found to significantly improve the thermal and mechanical properties of the polymer nanocomposite. The specific capacitances of PS were found to be improved from 2.51 and 42.25 F/g for the PS/rGO nanocomposite.

Natural flake graphite, potassium permanganate (KMnO₄, analytical grade reagent), sodium nitrate (NaNO₃, analytical grade reagent), concentrated sulfuric acid (H₂SO₄, analytical grade reagent, 98%), hydrogen peroxide (H₂O₂, analytical grade reagent, 30%), dimethyl sulfoxide (DMSO), hydrazine hydrate, glycerol, and vinegar were all purchased from the local. Food-grade potato starch was produced in the laboratory.

In this work, rGO was prepared from the reduction of graphene oxide (GO). GO was synthesized from natural graphite by the modified Hummers method.³⁵ To synthesize GO, an appropriate amount of graphite powder, NaNO₃, and H₂SO₄ were mixed into a beaker followed by stirring at a constant temperature of 20 °C for a few hours. Then, de-ionized (DI) water was added to the mixture and then heated at 95 °C in an oil bath for a few hours. An aqueous solution of H₂O₂ was added to reduce the residual KMnO₄ from the solution. The obtained GO sheets were then separated via centrifugation at 6000 rpm for 20 min. The as-obtained GO was then mixed with hydrazine hydrate followed by heating at 100 °C for 6 h, yielding rGO.

To prepare potato starch, the cleansed potato was shredded and ground in DI water and the solution was placed in a beaker. Water was decanted from the beaker, and the potato starch was obtained behind the bottom of it. It was dried in an air-circulating oven for an hour leaving the starch powder.

To prepare the PS/rGO nanocomposite, 10.0 g starch powder, 3.0 g glycerol, and 3.0 ml vinegar were mixed. Since starch contains long shackles of glucose, vinegar was used to break the chains, and glycerol was used to plasticize the starch. The mixture was stirred constantly at 95 °C for 15 min in order to plasticize the starch. The solution was then diluted in dimethyl sulfoxide (DMSO) followed by ultrasonication for 30 min. To prepare the nanocomposite, 1.0 wt. % rGO fillers were added to the PS emulsion. The PS/rGO solution was then placed on a glass plate and heated at 60 °C, yielding nanocomposite films.

Once synthesized, required characterizations were performed to study the properties of the PS/rGO nanocomposite. The crystalline phase of the nanocomposites was determined by an x-ray diffractometer (3040XPert PRO, Philips) using monochromatic CuK_α radiation (λ = 1.54 Å). X-ray data were taken at a scanning speed of 2°/min, between 5° and 90°. FTIR spectra of rGO and PS/rGO were recorded in the range of 400–4000 cm⁻¹ via a Nicolet 6700 FTIR spectrometer (Thermo Fisher). The specimens were developed by the KBr disk method.

The mechanical strength of the nanocomposites was measured using a universal testing machine (Wance ETM 501) at room temperature using the standard procedure (specification ASTM D882-02). The data were taken for four samples, and the average values were reported.

The surface morphology of the nanocomposites (after the mechanical property measurements) was analyzed using a (JEOL JSM-7600F) field emission scanning electron microscope at an accelerating voltage of 5 kV.

The thermal decomposition behavior of the nanocomposites was observed by a thermal gravimetric analysis (TGA) apparatus (TG50, Shimadzu) under a nitrogen environment between 25 and 800 °C at a heating rate of 10 °C/min.

The electrochemical properties were studied by an electrochemical workstation (CorrTest CS310, China). The measurements were carried out in 0.1M KCl solution using a conventional three-electrode cell setup: the Ag/AgCl electrode was used as the reference electrode, polished glassy carbon electrode (GCE) with an electro-active material as the working electrode, and a platinum plate (1 × 1 cm²) as the counter electrode.

X-ray diffraction (XRD) measurements were conducted to study the structural properties of PS and PS/rGO and are presented in Fig. 1. The XRD spectrum of plasticized starch presents three main diffraction peaks at 2θ values of 17°, 18°, and 23°. After incorporation of rGO, the position of the peaks remained nearly unchanged; however, the height of the peaks got reduced, suggesting a lower degree of crystallinity for the PS/rGO nanocomposite. During the gelation of starch, the crystalline structure of starch helices got upset and developed amorphous thermoplastic starch.^19,36

The recrystallization of amylose in starch during the plasticization process is due to the lysophospholipids forming complexes with ingredients such as glycerol. Increasing the temperature of the mixture of starch and the plasticizer is necessary to make the starch iotas puff up and amylose disseminate to the starch iotas, while amylopectin is kept in.¹⁶ O–H bonds were broken down, and new interactions between starch and the plasticizer were formed. In this process, the rGO sheets efficiently stacked into starch chains and created hydrogen bonding with PS, which limited the mobility of polymer chains and reduced the re-crystallization of PS significantly.³⁷ 

To study alterations or formation of the chemical bonds in the polymer matrix due to the incorporation of the filler, FTIR analysis was conducted. The FTIR spectra of the PS and PS/rGO nanocomposite are presented in Fig. 2. For the PS, the peak at 1153 cm⁻¹ corresponds to the –C–O– stretching of the C–O–H group of the starch polymer. The peaks at 1109 and 991 cm⁻¹ were accredited to the C–O stretching bond of the C–O–C group.²² The broad peak between 2950 and 3450 cm⁻¹ corresponds to the hydroxyl groups.²⁰ Stretching and bonding of –OH groups occurred at 3352 and 1651 cm⁻¹, respectively. The peak at 2941 cm⁻¹ occurs due to the CH₂ groups in the polymer matrix.¹⁷ No new peak was observed in the PS/rGO nanocomposite, signifying that no new bonds were formed due to the addition of rGO in the polymer matrix. However, a little shift in the O–H peak together with a change in the peak height was observed, indicating the formation of hydrogen bonding between rGO and PS in the nanocomposite.³⁷ In the vicinity of starch, parts of carbon bonds kept and removed oxygen groups were substituted by oxygen atoms of hydroxyl groups. The more C–O–C bands in the region 1200–1500 cm⁻¹ in PS/rGO than PS mark the formation of C–O–C bonds in PS/rGO films. These additional bands can help enhancing the dielectric constant of the composite.¹³ 

Figure 3 shows the SEM images of fracture faces of the PS and PS/rGO nanocomposites. The surface of pure PS appeared to be smooth, and the surface roughness increases due to the addition of the rGO nanofiller. In the PS/rGO nanocomposite, rGO was found to be distributed uniformly in the PS matrix. A magnified image of the PS/rGO shows the presence of layer-structured rGO inside the polymer. Similar morphological features were also obtained for the rGO based polymer matrix.³⁸ 

The thermal stability of PS and PS/rGO films was measured by thermogravimetric analysis (TGA), as shown in Fig. 4. Thermal analysis was carried out to determine whether the inclusion of rGO could introduce any change in the nature of the thermal decomposition of the nanocomposite. The onset temperatures of the PS film and PS/rGO films are 242 and 250 °C, respectively. The onset temperature is the temperature where the first distinguishable heat is extricated and can be obtained by extrapolating the steepest portion of the curve.³⁹ Two major thermal events are studied in the TGA diagram. The weight loss before the onset temperature can be ascribed to the volatilization of water assimilated by the polymer and the plasticizer.³⁹ In the temperature range between 220 and 380 °C, weight loss occurred, which is mainly due to the decomposition of starch. In Fig. 3, we provide the TGA data for PS and PS/rGO and a little change in the TGA curve is observed due to incorporation of rGO. To make a quantitative analysis about the effect of rGO on the thermal stability of the PS nanocomposite, we estimate the initial decomposed temperature (IDT), the integral procedural decomposition temperature (IPDT), the temperature at 50% weight loss (T-50%), and the temperature at the maximum rate of mass loss (T_max) of the PS and PS/rGO nanocomposites, which are presented in Table I. From Table I, it is evident that the values of all the thermal stability parameters increase due to the incorporation of rGO, suggesting an improved thermal stability of the PS/rGO nanocomposite. The decomposition temperature of PS/rGO is larger than that of the PS film. Usually, improved interaction between the filler and the matrix is required for the improved thermal stability of the composites.^20,40 This increase in the decomposition temperatures of starch due to the addition of rGO can be attributed to the concealment of the mobility of PS chains originating from the strong hydrogen bonding interactions with rGO.

Figure 5 demonstrates the impact of the reduced graphene oxide nanofiller on the mechanical properties of the PS/rGO nanocomposites. Figure 5(a) demonstrates the effects of rGO on the tensile properties of the nanocomposites. The tensile strength of the nanocomposite increased from 20 to 42 MPa due to the incorporation of rGO in it. Besides, the extension at break of the PS/rGO composites was developed from 67% to 80% due to the incorporation of rGO. Figure 5(b) demonstrates the variation in the Young modulus of the PS and PS/rGO composites. The Young modulus varies from 40.8 to 61.3 MPa due to the introduction of rGO into the PS matrix.

From Fig. 5, it is found that the addition of rGO simultaneously improved the tensile strength and elongation at break of PS, suggesting that rGO had a reinforcing effect on the PS matrix.⁴¹ Such enhancement in the mechanical performance may be attributed to the uniform dispersion of rGO within the PS matrix, resulting in strong interfacial interactions between the rGO and polymer matrix.⁴² As a result, when the tensile stress is applied to the nanocomposite, it becomes hard to separate the fillers from the matrix and may resist the transfer of applied force, causing a reduction in the loading stresses on the starch matrix.^43,44 Furthermore, the oxygen functional groups of rGO that form hydrogen bonding interactions with the hydroxyl groups of starch can help improving the tensile properties.^41,42,44

To explore the effect of rGO on the electrochemical performance of the nanocomposite, cyclic voltammetry (CV) was performed. Figures 6(a) and 6(b) represent the CV diagrams for PS and PS/rGO, respectively, at different sweep rates. The area of the CV curve was found to be increased with the scan rate. The CV curves for PS are almost rectangle in shape, whereas the shape becomes distorted when the rGO nanofillers are incorporated into it. Such distortion in the shape of the CV curves could be accredited to the pseudocapacitance and electric double-layer capacitance of rGO in the polymer matrix.^38,45 From the CV curve of the nanocomposite, it was found that the current rises sharply in the low-voltage regime, reaches a peak, and then drops sharply, suggesting improved electrochemical stability of the electrode materials.

Figure 6(c) represents the variation in the CV curve due to the incorporation of rGO. From the figure, it can be seen that the area of the quasi-rectangle CV curves of the PS/rGO nanocomposite is greater than that of pure PS, which indicates that the PS/rGO composite has a better capacitive performance compared to pure PS. This corroborates that the rGO nanofiller plays a significant role in improving the carriers’ transportation along the PS networking chains.^38,45

Figure 6(d) shows the galvanostatic charge–discharge (GCD) curves of PS and the PS/rGO nanocomposite at a constant current density of 0.1 mA/cm². The discharge time of the PS/rGO composite was found to be longer compared to that of the PS matrix. The specific capacitances (C_s) of PS and PS/rGO were calculated from the GCD diagram using the relation C_s = IΔt/mΔV,^35,40 where I, Δt, ΔV, and m are the current (A), discharging time (s), potential deviation (V), and the mass of the active materials (g), respectively. The specific capacitances of PS and PS/rGO were found to be 2.51 and 42.25 F/g, respectively.

A sharp IR drop was observed at the beginning of the discharging of the PS/rGO electrode. The IR drop is a measure of the internal resistance of the electrode materials. The observed smaller IR drop of the PS/rGO compared to that of the PS suggests that the incorporation of rGO reduces the internal resistance of the nanocomposite. A smaller internal resistance is a prerequisite for the synthesis of energy storage devices as it reduces the unwanted energy loss during charging–discharging processes.^35,46 Thus, PS/rGO with improved electrochemical performance can be considered as a suitable choice for the fabrication of energy storage devices.

Biodegradable and eco-friendly PS/rGO nanocomposites were synthesized via a simple, economic solution cast method. The structural, surface morphological, mechanical, and thermal properties of the composites were studied. The incorporation of rGO was found to create interfacial interaction with the PS matrix and thereby improve the thermal stability and mechanical strength of the nanocomposite. The effect of rGO on the electrochemical performance of the nanocomposites was also evaluated. rGO significantly reduces the internal resistance and enhances the specific capacitance of the nanocomposite. The PS/rGO nanocomposites with improved mechanical strength, enhanced thermal stability, and higher electrochemical performance synthesized from the natural, plant-derived polymer by an easy and economic method may offer an economic and versatile direction for the fabrication of sustainable and eco-friendly energy storage devices.

The authors gratefully acknowledge the financial support from the Ministry of Science and Technology, Government of Bangladesh, under Grant No. 39.00.0000.009.14.011.20/PHY’s-578.

The authors declare that they have no affiliations with or involvement in any organization or entity with any financial interest or nonfinancial interest in the subject matter or materials discussed in this article.

The data that support the findings of this study are available from the corresponding author upon reasonable request.