Abstract
This study deals with the preparation and characterization of novel composites accomplished by filling hydroxyethylcellulose with several amounts of bentonite. Molecular modeling enabled understanding the conformational and physicochemical features, which are responsible for the chemical reactivity parameters. Rheological analyses are made to investigate the effect of the polymer loading on the shear flow behavior. The morphology and homogeneity of each system is explored via optical microscopy. The band gap of the samples is slightly reduced by the addition of the bio-filler in the cellulosic medium, as indicated by UV-VIS spectral data. The dielectric response of these materials is extracted from refractometry experiments at several wavelengths. The electric energy density was achieved based on the dielectric properties determined at high and low frequencies. The outcome of this study offers new ways to produce alternative dielectric eco-materials having a good potential of accumulating electrical energy, as demanded for capacitor devices.
1 Introduction
The field of polymer-based composites has expanded in many directions in agreement with the pursued application (1). In the context of pollution caused by the spreading of plastics all over the planet, a fresh perspective is arising for the use of materials that are biodegradable, as well as those that are abundant in the nature. As a result, a tremendous attention was given to cellulose and its derivatives (ethers or esters) due to its excellent coating formability, combined with remarkable mechanical, optical, and insulation properties (2,3). An inconvenience resided in the fact that the dielectric constant of this category of polymers is not as high as desired for certain applicative domains (i.e., electronics or energy) and this led to their reinforcement, rendering new products that mingle the benefits of each associated components (4,5). The performance is dependent on both polymer structure and the features of the incorporated filler.
Among the cellulosic materials, hydroxyethylcellulose (HEC) is a non-ionic modified natural polymer, characterized by good solubility in water, film flexibility, biodegradability, optical transparency, adhesion, and other features that make it suitable for uses ranging from pharmaceutics, agriculture, adhesives to medicine (6,7). However, the implications of this HEC in energy-related devices are not fully explored. Brige et al. (8) have studied the possibility to solve poor reversibility of Zn batteries by making a solid electrolyte derived from HEC. Azha et al. (9) employed a HEC-dextran mixture doped with NH4Br which acted as a path for ionic conduction and separation of the electrodes for energy source systems. Chavan et al. (10) evidenced a peculiar ionic conduction in NaPF6 salt filled HEC which is adequate for use as electrolyte separators in the Na ion batteries. Sudhakar et al. (11) prepared conductive composites from HEC and phosphoric acid as electrolytes for supercapacitors. Kim et al. (12) performed a KCl-assisted synthesis of HEC porous architectures with good electrochemical properties for supercapacitors. In any case, more developments towards green composite materials for energy storage could be achieved by utilization of bio-originating fillers are poorly described in literature. For instance, Bentonite (denoted as Bent) is a clay consisting mainly of montmorillonite (MMT), which is characterized by the ability of swelling and good absorption of moisture, which can improve the electrical features. This filler is incorporated in polymers for water purification (13), oil spill recovery (14), or for electronic applications (15,16). In a previous work, the utility of this filler in improving the dielectric performance of hydroxypropylmethyl cellulose for energy storage purposes (17) was analyzed. The theoretical studies pointed out that this bio-derived filler has good potential of improving the dielectric constant of a polymer leading to increase in discharge energy, as demanded for capacitors.
On the other hand, a dielectric material used in capacitor devices requires good thickness uniformity for adequate operability. So, it is relevant to check the solution properties of the composite prior to processing into solid films by shear flow testing. Such correlations are not commented in the literature and have implications on the solid film morphology also (18).
Starting from the depicted background, the study has the scope of preparation of new eco-friendly dielectric composites based on a HEC, in which variable amount of bentonite were incorporated. The sample’s homogeneity and microstructure were monitored by spectroscopy technique. The reinforcement impact on the refractivity and dielectric behavior is also discussed. The estimated discharge energy reveals that these materials represent a good alternative to the toxic dielectric components of current capacitors.
2 Materials and methods
The polymer matrix employed in this work is NATROSOL 250 HEC, which was manufactured by Ashland/Hercules.
Bent in sodium form is used as filler and was acquired from Thermo Scientific Chemicals and used as received.
The composites were made by the blending method. The protocol was the following: (1) the polymer powder (0.2078 g) was solved in 3 mL of water, (2) various quantities of Bent filler were dispersed in 2 mL of water by ultrasonication to achieve the gravimetric ratio of 5, 10, and 20 wt% in regard to the matrix, so that the sample codes are as follows: HEC, HEC/Bent 5, HEC/Bent 10, and HEC/Bent 20, (3) the HEC solution and Bent dispersions were blended and homogenized by mechanical stirring for 4 h, and (4) the wet samples were processed into films by spin coating technique (spinning time of 15 s and spinning speed of 100 Hz) and further dried at mild temperature in an oven.
The computations of the polymer and filler structures were realized with HyperChem to inspect the conformational and physicochemical features.
Rheology tests were undertaken on a Bohlin CS50 system (Malvern Instruments) at ambient temperature.
Optical microscopy (OM) data were registered on ADL 601 P Bresser system connected to a computer.
Scanning electron microscopy (SEM) images were collected on VeriosG4 UC system.
UV-VIS data for each film sample were registered on SPECORD 210 PLUS system.
Refractometry tests were conducted on a DR-M4 instrument at the selected wavelengths: 486, 589, and 670 nm.
Dielectric constant of the composite films was measured at variable frequency on broadband dielectric spectrometer, which is connected to an Alpha-A analyzer.
3 Results
3.1 Molecular modeling
The molecular modeling is advantageous for visualization of geometrical features of a polymer, for inspecting the conformational changes in the presence of other molecules, for predicting the interactions in the system, but also for estimation of basic properties that influence the performance of the designed material. Figure 1(a) depicts the conformation of HEC chain built of five structural units (SUs). The data are in agreement with the literature (19), which indicates that HEC molecules do not display elongated straight or helical conformation. The computations from Figure 1(b) reveal that in the presence of a Bent molecule, the HEC conformation easily changes since there are variations in certain bond lengths or dihedral angles in the polymer. This is as supported by the modification of the end-to-end chain distance, which is 38 Å for HEC and decreases to 31 Å for HEC/Bent system. This might be caused by the formation of hydrogen bonding between the hydroxyls from HEC and the functionalities from the Bent structure (see magnified image in Figure 1(b)). Such specific interactions in composite systems induce an increase in the energy barrier of rotation for macromolecules under external forces. Thus, by prediction of the conformational characteristics, it is possible to explicate the evolution of viscosity at variable shear rates. For instance, a polymer with a conformation that enables few chain entanglements renders a strong response to imposed deformation because many chains are oriented during shearing, hence the viscosity is suddenly reduced. Since the modeling data demonstrate that semi-flexible HEC chains are interacting with Bent molecules, the response to applied shear is expected to be affected by this aspect. Occurrence of hydrogen bonding will limit the mobility of cellulosic chains so that they will be less able to orient during shearing. In other words, a more stable system impedes macromolecules from orienting at small deformation rates. This will influence the shape of the flow curves as it will be explained in the next section related to rheological behavior of the samples.
Molecular conformation of (a) HEC chain made of five repeating units and (b) HEC in the presence of a molecule of Bent (green color) denoting the occurrence of hydrogen bonding.
After the geometry optimization of the polymer and filler structures, density functional theory (DFT) calculations are undertaken. Starting from DFT it was possible to extract information on the energy frontier orbitals, known as HOMO and LUMO, which impact the physicochemical descriptors, like energy gap (E g), ionization potential (I p), electron affinity (E af), electrophilicity index (ω), chemical hardness (η c), electro donating power (ω −), electro accepting power (ω +), and net electrophilicity (ω ±) (20). Also, molecular polarizability (α p) of samples was estimated. The extracted parameters are helpful for prediction of the impact of bio-filler on the chemical reactivity and/or bio-activity of the HEC composites. The 3D-structures at lowest free energy (in the tube shape) of the HEC and Bent together with their frontier orbitals are depicted in Figure 2(a) and (b).
Energy-minimized structures for (a) HEC and (b) bentonite revealing the frontier molecular orbital surfaces, HOMO – represented by the uppermost green line, LUMO – represented by the lowest violet line as well as the HOMO-LUMO gap. Color signification: C – violet, O – red, H – gray, Al – yellow, Si – light blue, and Na – green.
The Bent compound exhibits a smaller value of E g compared to HEC, revealing a better stability. The small excitation energy might be linked to HOMO-LUMO gap. The energy of the HOMO and LUMO can be tuned by incorporation of bio-fillers, rendering novel materials with applicability in either (opto)electronics, where light-harvesting or electron traveling is needed, or biomedicine, where chemical reactivity and bioactivity are essential (21). The insertion of the Bent filler (with electron-rich and electron-deficient molecular groups) inside the donor HEC matrix renders a local acceptor character that affects frontier orbital energy levels.
Table 1 indicates the HOMO and LUMO values (extracted from semi-empirical calculations of examined composite systems) that define the sample’s reactivity. As the loading degree of HEC becomes higher, the magnitude of HOMO increases, while LUMO values decrease. On the other hand, the E g value of neat HEC is diminished with the addition of Bent, which slightly reduces the stability of the system, while increasing its reactivity. These aspects are reflected in smaller η c values. This represents a measure of the deformability of the molecular electron density and, thereby, the chemical reactivity. The I p and E af parameters are computed based on HOMO and LUMO (21), and the values from Table 1 show that upon HEC reinforcement, the I p is easily lowered and E af is increased. The values of electrophilicity index (denoting the system stabilization in energy under supplementary electronic charges) increase upon HEC loading with Bent, generating a larger stability of the system. This leads to the increase in the material’s chemical reactivity. In other words, the values of η c and ω show that addition of Bent in HEC determines a lower resistance toward electron cloud deformation (larger polarizability), which is indicative of better compatibility among the system’s components. The smaller E g of the samples containing more Bent molecules shows that the composites at higher loadings will display significant changes in electron density. These aspects can be correlated with the dielectric response of the HEC/Bent composites, since a larger polarizability is rendering an increase in the dielectric constant, as desired for the pursued scope.
Data for energy gap (E g), ionization potential (I p), electron affinity (E af), chemical hardness (η c), electrophilicity index (ω), and molecular polarizability (α p) for HEC and HEC system with 1, 2, and 5 molecules of bentonite
| HEC | Bent | HEC-Bent 1 | HEC-Bent 2 | HEC-Bent 5 | |
|---|---|---|---|---|---|
| HOMO (eV) | −11.62 | −13.65 | −11.35 | −11.21 | −10.96 |
| LUMO (eV) | −5.38 | −8.94 | −7.08 | −7.09 | −9.51 |
| E g (eV) | 6.25 | 4.71 | 4.27 | 4.11 | 4.05 |
| I p (eV) | 11.62 | 13.65 | 11.35 | 11.21 | 10.96 |
| E af (eV) | 5.38 | 8.93 | 7.09 | 7.10 | 9.51 |
| η c (eV) | 3.12 | 2.36 | 2.14 | 2.06 | 0.72 |
| ω (eV) | 11.56 | 27.09 | 19.87 | 20.37 | 72.39 |
| α p (Å3) | 48.73 | 62.72 | 127.05 | 189.00 | 374.83 |
The electrophilicity indices of the samples are illustrated in Figure 3, revealing that all these parameters are increasing as the Bent amount in HEC is higher. This also indicates a proper balance between the ω + (ability of accepting charges) and ω − (electron donor ability) as supported by the literature (22). The attained information on the global and local reactivity descriptors might clarify the design of composite materials with adequate chemical reactivity for achieving interfacial compatibility, and also has the desired biological activity – aspect of interest in medical devices. Thus, the clarification of structure–property correlation via molecular modeling is fruitful, projecting the desired electronic properties.
Electrophilicity descriptors for HEC/Bent systems.
3.2 Rheology
The preparation of polymer composites involves two principal pathways, namely, melt compounding or solution casting. In both cases, it is important to know how the material behaves under deformation in the fluid phase. An adequate tool for elucidation of such aspects is rheology and this is why the response to shearing of the samples was tested. The viscosity dependence on the shear rate of all investigated solutions is shown in Figure 4(a). At 0.6 s−1, the viscosity exhibits variation with the system composition, revealing that the loading of HEC with Bent renders more viscous solutions. The neat polymer solution exhibits prevalent pseudoplastic behavior in the studied shearing range. This might be the result of orientation of cellulosic chains under the shearing forces. As the polymer is filled, a Newtonian zone is noticed at low shearing rates, which is continued with a shear thinning domain. The length of the viscosity plateau is augmented as the amount of Bent is increased from 5 to 20 wt% in the composite. It seems that the filler is gradually impeding the macromolecules through hydrogen bonding to orient at low deformation rates. This is reflected by the smaller changes in viscosity upon less intense shearing and in a less abrupt slope of the viscosity curves at the highest shear rates.
The variation in viscosity with (a) shear rate and (b) reciprocal of absolute temperature for the HEC and HEC/Bent samples.
In order to get information on the interactions among the system phases, the viscosity was measured at several temperatures (25–55°C) to extract the flow activation energy (E 0). The resulted plots are illustrated in Figure 4(b) and the impact of the temperature on the viscosity could be depicted by Arrhenius Eq. 1.
where η – the viscosity (zero shearing), ln C – pre-exponential factor, T – absolute temperature, and R – the gas universal constant.
From the slope of each plot represented in Figure 4(b), the E 0 parameter was evaluated and the attained values increase from 5.07 kJ·mol−1·K−1 for HEC sample to 7.73 kJ·mol−1·K−1 for HEC/Bent 20 sample. This means that the energetic barrier ascribed to the outset of the sample flow is enhanced upon HEC reinforcement. Thus, Bent interacts with the cellulosic chain segments via hydrogen bonding, so that the composite system requires a bigger amount of energy to flow.
Oscillatory rheological experiments are also undertaken to shed light on the evolution of the viscoelasticity with the sample composition. The frequency sweep tests for all the HEC-based solutions are illustrated in Figure 5. In the applied shear frequency interval, the samples display changes in the balance between the viscous (G″) modulus and elastic (G′) modulus. At small frequencies (under 1 Hz), the solutions present a dominant viscous, which is progressively converted in to an elastic flow. In the low frequency zone, where G″ is higher than G′, it appears that rheological moduli are dependent on the frequency by a power law, where the exponent of G′ is higher than that of G″. Such behavior is often noted for viscoelastic fluids (23). When the frequency further increases, the strain energy accumulated in the analyzed materials increases in comparison to unrecoverable viscous loss, so that the shear moduli cross at a specific frequency (denoted here by f*). According to Figure 5, this frequency (showing where G′ = G″) is affected by the amount of bio-derived filler introduced in HEC, i.e., it is reduced from 18.19 Hz for the neat polymer to 9.22 Hz for the composite containing 20 wt% filler. The f* parameter gives information on the relaxation properties. It indicates the rate at which the overall macromolecular chains are anisotropically deformed as a result of the applied shear. The re-installment of quiescent state imposes relaxation of the sample to gain the original global isotropic condition. The reorientation force of the polymer segments is impacted by Brownian motion, and the shape restoring of chains takes place via flexible sequences that act as internal springs. Thus, the variation in f* with the material’s composition reveals that the relaxation time increases from 0.054 s for HEC to 0.12 for HEC/Bent 20. Such transition from liquid-like behavior to solid-like behavior is often noted for many polymer systems (23,24).
Frequency sweep tests for (a) HEC, (b) HEC/Bent 5, (c) HEC/Bent 10, and (d) HEC/Bent 20 samples.
Preparation of polymer films based on their corresponding solutions via spin coating requires deep correlation with the rheological characteristics because during the processing inherent shear fields are accountable for the material leveling on the support. As shown by Acrivos (25), the thickness is related to viscosity as indicated in Eq. 2.
where h is the wet film thickness, h 0 is the initial fluid thickness, H is h/h 0, ω is the spin speed, t m is the spinning time, and υ is the kinematic viscosity.
In the process of spin coating, the solution is under the effect of deformation forces that might drive variations in the viscosity along the disk radius. This might introduce serious thickness non-uniformity if the material presents shear-dependent viscosity (25,26). The attained thickness profiles along the disk radius resulted for the as-deposited HEC and HEC/Bent systems are given in Figure 6(a). The shear thinning behavior of the samples generates shifts in the uniformity of the film thickness, while the zones of Newtonian flow render unchanged thickness profile. This is mainly remarked for the samples with high filling levels (e.g., HEC/Bent 10 and HEC/Bent 20), while for the neat sample and 5 wt%, the sample layers display obvious non-uniformities since the viscosity is reduced upon shearing. In order to make uniform films, it is desirable to apply during spin coating low shear rates so that the viscosity would not range significantly and lead to the desired thickness profile. Another aspect that intervenes during film deposition and affects H parameter is the spinning time (t sp). As noticed in Figure 6(b), when t sp is higher than 20 s, the H values of the samples tend to decrease and the variation is less sudden as the filler loading in HEC is larger. As a result, it is preferable to process the samples at bigger loadings and smaller spinning times to avoid oscillations in the thickness.
The influence of (a) radial position and (b) spinning time on wet film thickness for the HEC and HEC/Bent samples.
3.3 Morphology
The morphology of the neat HEC and HEC/Bent films was inspected by OM and SEM to investigate the surface topography at different scales. The registered OM images of all prepared materials are included in Figure 7(a)–(d). The neat sample presents a loose surface with no defects and tiny rare pores. After the filler incorporation in the cellulosic matrix, it is observed that the morphological aspect of the prepared films changes. The presence of the Bent is viewed as dark dots, which are uniformly distributed in HEC at the macro-scale. Randomly disposed small dots increase upon polymer filling, but no relevant agglomeration is remarked.
(a)–(d) Optical micrographs of the HEC and HEC/Bent samples; (e) and (f) SEM images of the HEC matrix and HEC/Bent 20 composites. The inset shows the cross-section SEM image of the composite.
Further examinations are conducted at finer scale by means of SEM. By comparing the neat polymer and the composite at highest Bent, loadings are investigated. According to Figure 7(e) and (f), the pristine sample continues to have at this magnification level a smooth surface morphology, while the presence of the filler in the cellulose ether is identified to be under the form of well-defined globular elements having a whitish color. Again, SEM scan of HEC/Bent 20 confirms the good distribution of Bent in the HEC. Moreover, the data recorded in cross-section of the composite film reveal that the filler is properly distributed in the bulk, so the materials are homogeneous even at highest reinforcement degree.
3.4 Absorption edges and refractometry properties
The interaction of the samples with electromagnetic waves of different energies is relevant for establishing the absorption limits and polarization mechanisms, which are of great interest in many applications. The spectra of HEC and HEC/Bent materials are recorded in a wide wavelength interval starting from 230–1,000 nm, as noted in Figure 8(a). The cellulosic film is characterized by excellent transparency in the visible and infrared domains, i.e., starting with 250 nm, the transmittance (T) is 70% and reaches 86% at 1,000 nm. After progressive introduction of Bent, the composite films display a reduction in transmittance. For instance, at 400 nm, the transmittance is 81% for the neat polymer and runs down to 61% for the HEC/Bent 20.
(a) Transmittance spectra, (b) dependence of the absorption coefficient on the photon energy (inset shows Urbach energy data), and (c) dependence of (αE)0.5 on the photon energy (inset shows band gap energy data) for HEC and HEC/Bent samples.
The spectral results were further utilized for computation of the absorption coefficient based on Eq. 3.
where α is the absorption coefficient and t o is the sample thickness.
As seen in Figure 8(b), the absorption coefficient is augmented upon incorporation of the bio-derived filler in HEC. Moreover, introduction of particles in polymers creates more electronic states in the gap, producing the limitation of the band gap and band tailing. In the domain of absorption edge, the radiation interaction with the composite generates excitation of electrons from lower energy to a state of upper energetic level. Therefore, the α parameter displays an exponential variation with photon energy (E), as described by Eq. (4) (27).
where E u is the Urbach energy.
At low E values, the absorption phenomenon is determined by the structural disorder reflected by the E u values. By linearization of Eq. 4 in the small photon energy range, a straight line was achieved and the reverse of the slope allows estimation of the Urbach energy. The latter reflects information on the width of the localized states created by the defects found upon reinforcement in forbidden gap. The resulted E u data are depicted in the inset of Figure 8(b) and they reveal that the Bent doping of HEC augments the structural disorder in the prepared eco-composites (increase in E u).
When the photon energy is close to the band gap, the α parameter follows the Tauc expression, as shown in Eq. 5 (28).
where D is a constant, E g is the band gap energy, and u is a parameter for electronic transitions.
During light absorption in a doped material, electrons might gain energy to jump, leading to transition in the band gap. Figure 8(c) illustrates the (αE)0.5 dependence on the photon energy and the obtained plots allow estimation of E g via extrapolating the linear region to intercept the x axis. The inset image in Figure 8(c) shows the band gap energy variation with the sample composition. The E g value of the neat polymer is high, i.e., 4.95 eV, which is slightly lower than that attained by means of molecular modeling. Furthermore, the insertion of Bent in HEC produces a reduction in E g magnitude to 4.55 eV for HEC/Bent 20 sample. This might be caused by the fact that the bio-derived filler makes extra other states in the forbidden gap limiting the E g.
The refractive index (n) is an optical feature useful for assessing the velocity of light propagation through a material. Figure 9(a) shows the energy dependence of the refractive index for HEC and HEC/Bent materials, indicating an increase in n as the incident radiation energy is bigger. Since n parameter is a function of the molecular polarizability, it appears that the filling of HEC enhances the polarization ability of the samples, as noted in the higher values of the refractive index.
(a) Refractive index dispersion curves of the HEC and HEC/Bent samples, (b) dielectric constant of samples at various frequency domains, (c) energy storage against electric field intensity for HEC, and (d) energy storage against electric field intensity for HEC/Bent 20 samples.
3.5 Dielectric properties and energy storage
The refractive index dispersion enables to make further correlation with the dielectric properties of the studied materials. The real (ε′) part of the electrical permittivity could be estimated using Eq. 6.
where k is the extinction coefficient and is derived from α parameter.
As indicated by the results from Figure 9(b), all samples present small variations in the ε′ at optical frequencies. It can be remarked that addition of Bent in HEC increases the dielectric constant. In this spectral range, only the electronic polarization mechanism occurs in the samples and it seems to be more efficient upon doping the cellulosic matrix. In order to see how the reinforced material behaves at lower frequencies, where atomic, ionic, and dipolar polarization mechanisms are acting aside from the electronic one, the broadband dielectric spectroscopy tests are undertaken. It was noted that as a result of more polarization processes, the dielectric constant is higher, i.e., at 100 Hz ε′ = 2.89 for the neat HEC and becomes ε′ = 8.79 at the highest loading of 20 wt%. In this frequency range, HEC/Bent 20 sample presents the biggest dielectric constant, which decreases as the electric field frequency is enhanced (Figure 9(b) – dashed line).
Dielectric breakdown of the filler was reported to be around 37.7 kV·mm−1 (29), while that of the matrix was estimated based on its relation with yield stress and permittivity (30). Knowing that the yield stress of HEC is 20 MPa (31), the dielectric breakdown strength for HEC was obtained to be 1.39 MV·mm−1. Furthermore, the storage of electric energy for the samples was evaluated at electric field intensities under the breakdown value, which for the composites was assessed to be ranging between 1.32 (5 wt%) and 1.11 MV·mm−1 (20 wt%). The energy density (U) can be described by the following relation (7):
where E f is the electric field intensity, while ε c and ε r are permittivities of the reinforced polymer and vacuum.
Based on the differences concerning the dielectric constant of the studied films, it was further selected for comparison of the results on the U parameter attained for the neat HEC and those for HEC containing the highest amount of bio-based filler. The changes in the energy storage with electric field intensity for these two samples are observed in Figure 9(c) and (d). Regardless of the filler presence, it was remarked that U parameter increases with electric field intensity and becomes lower at highest frequencies. More importantly, incorporation of Bent produces considerable improvement in the permittivity (especially at low frequencies), which is seen in a larger enhancement of the energy density of the eco-composites. The obtained data are comparable with those from literature on similar composites based on particular type of clay from the Bent class, namely, MMT. Ghosh et al. (32) prepared poly(vinylidene fluoride)/unmodified MMT composites and found that at 0.4 wt% filler, dielectric constant is enhanced to 28, the breakdown is 873 MV·m−1 and U reaches 24.9 J·cm−3. Wang et al. (33) used poly(vinylidene fluoride-co-hexafluoropropylene) as matrix and inserted Ag-decorated exfoliated MMT nano-sized platelets. The electrical characterization revealed that at 4 vol% loading, the energy density raises to 10.51 J·cm−3, which is ∼2.25 times larger in regard to that of the neat polymer. In a recent work, Cao et al. (34) inserted MMT sheets into polyamideimide and observed that the layered nanocomposites with 2D interfaces display average increase in permittivity (ε′ = 4) and energy density (U = 3.6 J·cm−3), but they present the benefit of reduced energy loss. The energy density results for our polymer composites are of the same order as those presented in the literature (32,33,34).
Impedance measurements were also undertaken and the obtained data are represented in Figure 10. It can be remarked that the real par of impedance (Z′) decreases as the frequency is increased up to 1 kHz, afterwards Z′ becomes independent of frequency. Analogous variation in Z′ was also reported in literature for polyethylene/carbon nanotube composites (35). Also, these materials have similar order of magnitude of real impedance as those of the studied HEC/Bent 20 film, namely, Z′ ∼ 104. The inset in Figure 10 illustrates the dependence of imaginary impedance (Z″) on the real part of impedance for the investigated composite. The shape of the attained inset plot is not typical semi-circle, which is noticed for the case of parallel combination of bulk resistance (movement of charges) and bulk capacitance (immobile macromolecular chains) (36). The digression from the semi-circular shape is indicative of the dominance of the material resistive component, so that the capacitor acts like a resistor and the resistance magnitude influences the charge transfer properties (37).
Variation in real part of impedance with frequency for HEC/Bent 20 sample and inset shows dependence of imaginary impedance on the real impedance.
4 Conclusion
The study pursued the effects introduced by the Bent filler in HEC matrix on the rheological, morphological, refraction, and dielectric properties of the attained composites with the purpose of making green dielectrics for energy storage. The hydrogen bonding interactions from the system at high loadings render an increase in the domain Newtonian flow in comparison to the neat polymer, this aspect favoring occurrence of uniform thickness of the films. Morphological examinations at the surface and bulk of the composite show a good homogeneity and suitable distribution of the Bent in the material. This bio-derived filler helps to increase HEC molecular polarizability, which is confirmed by higher values of refractive index and dielectric constant (at optical and microwave frequencies) upon HEC reinforcement. The estimated values of the storage energy prove that the incorporation of a bio-component in HEC is a good alternative for making eco-compatible dielectrics having adequate electrical properties as needed for capacitors.
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Funding information: This work was supported by a grant from the Ministry of Research, Innovation and Digitization, CNCS – UEFISCDI, project number PN-III-P1-1.1-TE-2021-0762, within PNCDI III.
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Author contributions: Andreea Irina Barzic: writing – original draft, writing – review and editing, methodology, investigation, and supervision; Iuliana Stoica: investigation, methodology, visualization, and validation; Mihai Asandulesa: investigation, formal analysis, and validation; Raluca Marinica Albu: investigation, resources, software, visualization, validation, and project administration; Bogdan Oprisan: investigation, visualization, and validation.
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Conflict of interest: Authors state no conflict of interest.
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Data availability statement: Not applicable.
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