Effect of thermal annealing duration on the temperature-dependent dielectric response of PVA-based GO/FeGaInS₄ composites

Thermal annealing plays a crucial role in tailoring the structural ordering and dielectric response of polymer-based nanocomposites. In this study, poly(vinyl alcohol) (PVA) nanocomposites containing 3 wt.% graphene oxide (GO) and 2 wt.% FeGaInS₄ were fabricated and systematically investigated to elucidate the effect of annealing duration on crystallinity, dielectric behavior, and electrical transport. The samples were annealed at 313 K for 1 h and 5 h, and their structural, dielectric, and electrical properties were evaluated. X-ray diffraction (XRD) confirmed the phase stability of FeGaInS₄, while Williamson–Hall analysis showed that short-term annealing (1 h) reduced the crystallite size (6.5 → 5.1 nm) and increased microstrain and defect density. Prolonged annealing (5 h) restored the crystallite size (~ 6.6 nm) and reduced defect density, consistent with lattice relaxation and partial polymer chain reorganization. X-ray diffraction (XRD) analysis confirmed the phase stability of FeGaInS₄ upon annealing, while Fourier-transform infrared spectroscopy (FTIR) revealed a progressive decrease in the crystallinity index (CI) from 69.6% in the as-cast state to 65.2% after 5 h of annealing, indicating that the presence of residual water within the polymer causes the PVA chains to swell under low-temperature conditions, thereby reducing the degree of crystallinity. Dielectric spectroscopy performed in the range 293–373 K and 120 Hz–1 MHz showed that the real part of permittivity (ε′) at 373 K and 1 kHz increased from ~ 75 to ~ 83 with longer annealing. At the same time, the dielectric loss tangent (tan δ) decreased from 3.0 (1 h) to 1.8 (5 h), with relaxation peaks shifting toward higher frequencies, reflecting improved dipolar and interfacial polarization stability. AC conductivity followed the frequency-dependent σ ~ ωˢ law and exhibited a marked increase from 1.6 × 10⁻⁷ to 4.8 × 10⁻⁷ S/cm at 353 K with increasing annealing duration. Moreover, the maximum barrier height (Wₘ) decreased significantly from 3.23 eV to 1.57 eV, while the density of localized states nearly doubled, suggesting reduced energy barriers and enhanced charge transport. Overall, prolonged thermal annealing was found to substantially improve crystallinity, dielectric stability, and carrier mobility in PVA/GO/FeGaInS₄ nanocomposites. These findings highlight the potential of such dual-filler polymer systems as promising candidates for advanced high-frequency electronic and sensing applications.