Electro-physical properties of jute fabrics in a 1 condition of high humidity - Effect of fabric 2 chemical composition and coating with Cu- 3 based nanoparticles 4

The electro-physical properties of raw and chemically modified jute fabrics were studied as complex phenomena of the interaction between the fabrics’ chemical composition, crystallinity, moisture sorption, COOH group content, structural characteristics, and frequency of the electric 34 field. At 80% relative air humidity, all chemically modified jute fabrics have 38-179% and 1.7-5.4 35 times higher dielectric loss tangent and effective relative dielectric permeability compared to 36 unmodified, respectively. To further improve these properties, fabrics were treated with CuSO 4 37 and Cu-based nanoparticles were in situ synthesized on their surface by reduction. A few single 38 Cu-based nanoparticles were observed across the alkali modified fabric’s surface, while single and 39 agglomerated nanoparticles were distributed over the oxidatively modified fabric’s surface. No 40 matter whether metallic Cu or copper oxide (Cu 2 O or CuO) nanostructures (or their mixtures) are 41 synthesized (proven by XRD), excellent fabrics’ effective relative dielectric permeability is 42 guaranteed. In other words, during the exploitation in specific conditions contributing to copper 43 reduction, the jute fabrics will be able to store 21-163 times more energy from an external electric 44 field than before the exploitation, which further extended their lifetime. On the other hand, with 45 increasing the total content of Cu after the reduction and formation of single and agglomerated Cu- 46 based nanoparticles, the movement of jute structural components’ molecules becomes difficult 47 resulting in lower energy dissipation within the chemically modified than within unmodified 48 fabric. Applied chemical modification and coating with Cu-based nanoparticles enables designing 49 fabrics with predictable electro-physical properties, which is very important from the application 50 point of view.


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The X-ray measurements were performed on a Rigaku Ultima IV diffractometer in a Bragg-115 Brentano configuration using CuKα radiation. The diffraction data were acquired over the 2θ 116 scattering angle (from 10° to 60°) with a step of 0.05° and an acquisition time of 0.5 °/min. The 117 obtained X-ray diffraction patterns were resolved into portions of cellulose I β , cellulose II lattice 118 (French 2014), and amorphous region using Gaussian and Lorentzian distribution function. The 119 conventional peak deconvolution involved curve fitting (by using a commercial software Peakfit 120 v4.12) to the observed pattern with the individual visible peaks plus a very broad peak for the 121 amorphous material (French 2020). The fabrics' crystallinity was calculated from the ratio of the 122 area of all crystalline peaks to the total area.

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Before measurements, the fabrics were conditioned at 80% relative humidity to find the connection 125 between the fabrics' moisture sorption and their electro-physical properties measured at the same 126 relative humidity.

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The jute fabric structural characteristics were characterized by the fabric thickness (measured on an 128 AMES 414-10 thickness tester under a pressure of 10 kPa), fabric weight (determined using ISO 129 3801 (1977) standard), and fabric porosity (Ivanovska et al. 2020       The frequency dependence of effective relative dielectric permeability ( ′ ) of unmodified and 293 chemically modified jute fabrics is given in Fig. 3. For all fabrics, the highest ′ was observed at Observing in parallel the tan δ (Fig. 2) and ′ (Fig. 3) of unmodified fabric, it can be concluded that 304 the interaction of dipole structures with the participation of hemicelluloses and lignin is strong 305 supporting the assumption that the untreated fabric's lowest ′ is at least partly caused by the 306 strengthening of the dipole interaction. Additionally, the untreated fabric's lowest ′ can be 307 attributed to its lowest content of cellulose (60.1%, Table 1) and carboxyl groups (207 μmol/g, Fig.   308 4) as well as the highest content of surface impurities (Fig. 5) responsible for lower accessibility of 309 functional groups. After the chemical modifications, these surface impurities were to some extend 310 removed, which together with the hemicellulose or lignin removal and increased cellulose content 311 lead to better accessibility of functional groups (including newly exposed cellulose hydroxyl groups 312 and newly formed carboxyl groups, Fig. 4). In the presence of moisture (80% RH), cellulose 313 hydroxyl groups, as well as newly formed carboxyl groups, contributed to an increased anionic 314 charge of modified fabrics thus increased their ′ , Fig. 3. Besides all the above mentioned, it is 315 known that the ′ as tan δ is also sensitive to fabric's moisture content.

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As it was listed in Tables 1 and 2, the alkali modified jute fabrics are characterized by a higher 317 amount of amorphous regions as well as lower porosity (i.e., lower spaces between the yarns within 318 the fabric) indicating that the absorbed water is most probably located as bound water. Namely, the 319 water molecules penetrate inside the fiber, break the secondary interactions between the cellulose 320 macromolecules, and after that, they are absorbed into the fibers by hydrogen bonds causing fibers' 321 swelling. All mentioned will contribute to the creation of favorable conditions for better mobility of 322 the fibers' cellulose chains, dipoles' displacement, and rotation enhancing the electric polarization 323 process (Cerovic et al. 2013), i.e., increasing ′ . This behavior is prominent for fabric A30 having 324 the highest moisture sorption value which is closely related to its chemical composition and 325 amorphous regions' expansion (i.e., decreased crystallinity). Because fabrics' electro-physical 326 properties refer to a "fiber-moisture-air" system, besides moisture sorption and crystallinity, the 327 fabrics' structural characteristics also affect their ′ . For example, a slightly higher ′ value was 328 noticed for the fabric A30 having slightly lower porosity as well as higher fabric thickness compared

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A comparative analysis of differently modified jute fabrics (Fig. 3) showed that the oxidatively 332 modified fabrics' ′ values are higher than that of alkali modified ones, which is especially 333 pronounced for fabric C90. About 2.7 times higher ′ (at 30 Hz) observed for the fabric C90 334 compared to that of A30 can be attributed to different factors acting in parallel. Namely, in the case 335 of high air humidity (80% RH), part of the absorbed water in the fabric is located as bound water 336 and the other part as bulk-free water. According to Saukkonen et al. (2015), adsorbed water 337 molecules are neither free to move around nor free to change their orientation, and consequently 338 their ′ are much lower than that of free water ( ′ = 81). For oxidatively modified jute fabrics 339 having higher crystallinity and fabric porosity (Tables 1 and 2) compared to the alkali modified, it 340 can be assumed that absorbed water is located as bulk-free water, and, therefore, it has a strong 341 influence on fabrics' ′ . In other words, the oxidatively modified fabrics' ′ increased as a 342 consequence of the higher number of water's polar groups present at the fabric surface as well as 343 between the yarns within the fabric. On the other hand, oxidatively modified jute fabrics have 344 considerably higher contents of carboxyl groups as compared to alkali modified, which is one more 345 factor affecting their higher ′ . In the investigation conducted by Simula (1999), the increased 346 content of carboxyl groups is responsible for higher ′ of the birch than that of pine kraft pulp.

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According to all the above discussed, it can be postulated that compared to alkali modifications (i.e., 348 hemicellulose removal), oxidative modifications (i.e., lignin removal) encourage the freedom of 349 water molecules to take part in polarization processes.

Electro-physical properties of decorated with Cu 2+ ions or Cu-412 based nanoparticles 413
Jute fabrics containing Cu 2+ ions and coated with Cu-based NPs were subjected to dielectric 414 measurements. By comparing the results presented in Figs. 3 and 7a, it is evident that CCu, A30Cu, 415 and C90Cu ′ values (at 30 Hz) are about 4.9, 6.7, and 4.6 times higher compared to C, A30, and 416 C90 ′ values, respectively. Additionally, the CuSO 4 treated fabrics' ′ increases as the Cu 2+ 417 uptake increases: CCu < A30Cu < C90Cu, Figs. 4 and 7a. On the other hand, the relation between 418 the total content of Cu after reduction (Fig. 4) and the fabrics' CuNPs, A30CuNPs, and C90CuNPs 419 ′ (Fig. 7b) was not established pointing out that some other factor/s influence/s the ′ of fabrics 420 coated with Cu-based NPs. Observing in parallel the SEM photographs (Fig. 5) and the results 421 presented in Fig. 7b, it seems that the NPs' agglomeration is the major reason for C90CuNPs lower 422 ′ compared to fabric A30CuNPs, which highest ′ is attributed to the presence of an evenly 423 distributed single Cu-based NPs. Surprisingly, the jute fabrics coated with Cu-based NPs have about 424 21-163 times higher ′ (at 30 Hz) than those treated with CuSO 4 , Fig. 7. This behavior is very 425 valuable since, during the exploitation in specific conditions contributing to copper reduction, the 426 jute fabrics will be able to store much more energy from an electric field than before the exploitation,