Tailoring the structural, optical and paramagnetic properties of GO–PVA–AgNW nanocomposites by gamma irradiation

Abstract

Graphene oxide (GO)–polyvinyl alcohol (PVA)–silver nanowire (AgNW) ternary nanocomposites are of increasing interest as radiation-resistant materials, yet their structural and physicochemical response under high absorbed gamma doses remains largely unexplored. In this study, GO–PVA–AgNW nanocomposite films were exposed to high-dose γ-irradiation (250, 500, and 1500 kGy) from a ⁶⁰Co source, and the resulting morphological, structural, optical, thermal, and paramagnetic modifications were systematically investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, ultraviolet-visible (UV-vis) spectroscopy, thermogravimetric analysis (TGA), and electron paramagnetic resonance (EPR) spectroscopy. XRD and Raman analyses revealed a pronounced reduction in crystallite size (from 1.75 to 0.92 nm) accompanied by an increase in microstrain (from 0.115 to 0.217) and defect density with increasing dose. UV-vis spectroscopy showed a progressive red shift of the absorption edge and a decrease in the optical band gap from 5.07 (pristine) to 3.82 at 1500 kGy. TGA demonstrated enhanced thermal stability of the primary polymer backbone and an increase in residual carbonaceous content from 10.15% to 14.09%, indicating that crosslinking becomes the dominant process over chain scission at high absorbed doses. EPR measurements confirmed a dose-dependent increase in the concentration of carbon-centered π-radicals, together with a narrowing of the peak-to-peak linewidth and an evolution of the asymmetry factor toward unity, reflecting enhanced spin-exchange interactions and a more homogeneous radical distribution.

High-dose gamma irradiation (0–1500 kGy) of GO–PVA–AgNW nanocomposites: XRD, Raman, UV-vis, TGA and EPR reveal increasing defects/radicals, decreasing crystallite size and band gap, and improved thermal stability.

1. Introduction

In recent years, considerable attention has been directed toward ternary nanocomposites, as the integration of multiple fillers within a polymer matrix enables the formation of materials that display enhanced and often unconventional physicochemical characteristics.¹ Among these systems, nanocomposites incorporating metal-semiconductor hybrid fillers are of particular interest, one representative example being the GO–PVA–AgNW-based composite. PVA is a hydrophilic polymer with approximately 90% optical transparency² and is widely employed in electronics, energy-storage devices, and related applications owing to its favorable dielectric properties.^3,4

Nanomaterials constitute a field that has attracted considerable attention in recent years, both from a research and an application perspective.⁵ Among these materials, GO occupies a special place. GO has emerged as a highly versatile functional material. As an oxidized derivative of graphene, GO consists of carbon sheets decorated with oxygen-containing groups, which impart hydrophilicity, introduce a tunable band gap, and enhance its dielectric response. The attachment of oxygen functionalities transforms portions of the sp² carbon lattice into sp³-hybridized domains, creating structural defects that significantly modify the electronic and optical behavior of the material. Since these oxygen groups are reducible, the properties of GO can be deliberately tuned through controlled reduction, which has been shown to alter its band-gap characteristics.⁶ Owing to its rich functional chemistry, GO disperses effectively within polymer matrices such as PVA via hydrogen bonding, and the loading level of GO has been shown to strongly govern the resulting physicochemical, structural, electrical, and mechanical properties of GO/PVA nanocomposites.^7,8 Additionally, PVA chains may intercalate between GO layers, giving rise to new structural orientations within the composite.²

Incorporating silver-based nanostructures into polymer matrices represents another widely utilized strategy for tailoring nanocomposite performance. In particular, AgNWs, as one-dimensional conductive materials, exhibit two characteristic plasmon resonance peaks and are frequently applied in electronics and optoelectronic architectures.^9,10 The introduction of AgNWs into GO/PVA composites can modify the electronic energy landscape, generate additional potential states, and consequently influence the band gap and related physical parameters.¹¹ Such changes are crucial for tuning the optical and electrical behavior of nanocomposites in advanced optoelectronic systems.¹²

Various methods are employed to control the properties of materials, among which the modification of material characteristics through irradiation techniques is particularly favored. Such techniques include laser irradiation,¹³ proton beam irradiation,¹⁴ swift heavy ion irradiation,¹⁵ and others. Each of these methods offers particular advantages depending on the intended application. For the modification of nanocomposites, however, gamma irradiation is especially preferred. This is because gamma radiation, owing to its high penetration capability, ensures uniform energy deposition throughout the entire volume of the sample, which renders it particularly suitable for the homogeneous modification of bulk nanocomposite systems such as the one investigated in the present study. Gamma radiation interacts with matter primarily through Compton scattering, photoelectric absorption, and pair production, generating secondary high-energy electrons that ionize and excite the surrounding lattice. The resulting energy deposition drives bond scission, defect formation, and, in polymeric systems, competing crosslinking or degradation processes, with the balance between these mechanisms strongly dependent on the absorbed dose.¹⁶ This dose dependence has been well documented in inorganic thin-film systems: gamma irradiation of RF-sputtered WO₃ thin films has been reported to narrow the optical band gap while simultaneously enlarging the crystallite size,¹⁷ whereas high-dose gamma exposure, in the hundreds-of-kGy range, of nanocrystalline ZrO₂ thin films induces pronounced physico-chemical and surface-morphological changes that are absent at lower doses.¹⁸ These findings indicate that the absorbed-dose regime, rather than the mere presence of irradiation, is a decisive factor in determining which structural mechanisms dominate the material response.

Gamma irradiation is likewise among the effective approaches employed to modify the properties of carbon-based systems. Literature reports indicate that gamma radiation can partially reduce GO,^19,20 although the reduction mechanism does not follow a strictly linear dose-dependent trend.²¹ Importantly, gamma irradiation induces the formation of high-energy Compton electrons, which can directly interact with the oxygen-containing groups in GO, leading to both reduction and oxidative restructuring processes. This interaction plays a crucial role in reshaping the defect distribution and band-gap structure of GO. Additionally, gamma irradiation has been shown to influence the band-gap values of polymers and polymer-based nanocomposites.²² Investigations have shown that irradiation with gamma radiation at different doses can change the nonlinear optical properties of GO.²³ Earlier research has also demonstrated that the dielectric properties of GO/PVA-Ag/Gl nanocomposites are significantly modified under gamma exposure.²⁴

Previous works demonstrated that gamma irradiation at comparatively low absorbed doses (8, 25, and 50 kGy) can either generate or annihilate defects in GO–PVA–AgNW nanocomposites depending on the applied dose, without producing detectable morphological changes.²⁵ That study, however, was confined to a low-dose window in which the polymer matrix and the GO framework remain largely intact, leaving unresolved how the same ternary system responds once the absorbed dose is raised by more than an order of magnitude. Based on the behavior of other irradiated oxide and polymer systems,^17,18 pore formation, pronounced crystallite disruption, and irreversible oxidative restructuring are expected to progressively dominate over the reduction and defect-tuning behavior observed at low doses. Accordingly, in the present work “high-dose” irradiation refers to the 250–1500 kGy range, at least an order of magnitude above the doses examined in²⁵ and comparable to the high-dose regimes previously shown to induce severe structural reorganization in other nanocrystalline thin-film systems.¹⁸

In the present study, GO–PVA–AgNW nanocomposites were irradiated with gamma doses of 250, 500, and 1500 kGy, and the resulting structural, morphological, and optical modifications were systematically examined using XRD, SEM, Raman spectroscopy, UV-vis spectroscopy, and thermogravimetric analysis. While several studies have explored low-dose gamma effects on GO–PVA–AgNW composites²⁵ and on related oxide and polymer systems,^16–18 a systematic, multi-technique investigation of the intermediate-to-high dose regime, in which bond scission, pore formation, and defect-driven band-gap collapse compete with, and eventually dominate over, the radiolytic reduction processes reported at low doses, remains absent from the literature. The present work addresses this gap by correlating structural disorder (XRD, Raman), morphological evolution (SEM), optical band-gap narrowing (UV-vis, Tauc analysis), and thermal stability (TGA) within a single, internally consistent dose series, thereby providing a systematic, high-dose structure–property analysis of GO–PVA–AgNW ternary nanocomposites and offering new insight into radiation-matter interactions in ternary hybrid systems under conditions relevant to sterilization, aerospace, and nuclear-adjacent environments.

2. Materials and methods

2.1. Synthesis of GO, AgNWs, and GO–PVA–AgNW nanocomposites

GO was synthesized using a modified Hummers' method, in which graphite served as the carbon precursor and HNO₃, H₂SO₄, and KMnO₄ were employed as oxidizing agents (Fig. 1A). The complete step-by-step synthesis has been documented in our previous work.²⁵ Silver nanowires were produced via a modified polyol process carried out in an ethylene glycol medium using AgNO₃ as the silver source. Polyvinylpyrrolidone (PVP) was added as a capping and directing agent to promote anisotropic one-dimensional nanowire growth (Fig. 1B), following the methodology adapted from earlier reports.²⁵ The GO–PVA–AgNW nanocomposites were prepared through a solution-casting approach (Fig. 1C). PVA was dissolved in distilled water at ∼60 °C to form a clear polymer solution, while GO was dispersed in water and ultrasonicated to achieve exfoliation. The GO dispersion was gradually incorporated into the PVA solution under vigorous stirring and further sonicated for homogeneous mixing. Subsequently, a predetermined amount of AgNWs was added dropwise, followed by additional sonication to ensure uniform filler distribution. The final mixture was cast into Petri dishes and dried under ambient conditions for approximately one week, yielding smooth and crack-free composite films containing 0.8 wt% GO and 0.8 wt% AgNWs. The films were later subjected to gamma irradiation at doses of 250, 500, and 1500 kGy using a ⁶⁰Co gamma source.

Fig. 1. Schematic representation of the preparation of GO–PVA–AgNW nanocomposite films and their subsequent γ-irradiation. The fabrication process includes GO synthesis by the modified Hummers' method (A), AgNW synthesis via the modified polyol process (B), solution blending with PVA, film casting and ambient drying, followed by irradiation at doses of 250, 500, and 1500 kGy (C).

2.2. Characterization techniques

The structural, morphological, vibrational, and optical properties of the synthesized materials were investigated using a range of complementary techniques. X-ray diffraction (XRD) measurements were carried out with a Rigaku MiniFlex 600 diffractometer equipped with Ni-filtered Cu Kα radiation (λ = 1.5406 Å). Morphological observations were performed using low-vacuum scanning electron microscopy (Apreo 2S, 5 kV, 50 Pa), bright-field optical microscopy (Axio Imager.Z2m), and transmission electron microscopy (JEM-1400, 80–120 kV). Raman spectra were collected using a WITec Alpha 300 RS confocal Raman microscope with a 532 nm Nd:YAG laser focused through a 100×/0.9 objective at ∼1.4 mW power. Optical absorption spectra of all samples were recorded in the 190–1100 nm range using a Specord 250 Plus UV-vis spectrophotometer. Electron paramagnetic resonance (EPR) measurements were performed at room temperature using a Bruker ELEXSYS E580 spectrometer operating in the X-band region. TGA analyses were performed with the Netzsch STA 449 F3 (platinum–platinum rhodium thermocouple) differential scanning calorimeter. Gamma irradiation was performed with a ⁶⁰Co source providing an average photon energy of 1.25 MeV (individual gamma quantum energy ≈1.17 and 1.33 MeV).

3. Results and discussion

3.1. Electron microscopy analysis

Fig. 2 presents representative TEM micrographs of the individual fillers prior to their incorporation into the composite matrix, confirming the morphological quality of the as-synthesized materials. As shown in Fig. 2A, the synthesized GO exhibits a characteristic multilayered, sheet-like morphology with lateral dimensions of approximately 5 µm, consistent with efficient exfoliation during synthesis. The TEM image of the AgNWs (Fig. 2B) confirms their expected one-dimensional morphology, with an average length of 3–4 µm and a diameter in the range of 50–60 nm, corresponding to an aspect ratio of approximately 60–70. Alongside the nanowires, a minor population of spherical silver nanoparticles (AgNPs) is also observed; these arise from the isotropic nucleation and agglomeration of silver atoms that fail to align along the preferred (100) growth axis during the polyol-mediated anisotropic growth process, a byproduct commonly reported in polyol-based AgNW synthesis.

Fig. 2. TEM images of (A) graphene oxide and (B) silver nanowires.

Fig. 3 presents the SEM micrographs of the GO–PVA–AgNW nanocomposites. Fig. 3A–D correspond to the non-irradiated sample and those exposed to 250, 500, and 1500 kGy gamma doses, respectively. The AgNWs appear randomly oriented within the polymer matrix, consistent with a spatially uniform, non-aligned distribution. Alongside the nanowires, spherical silver particles are also visible, consistent with the AgNP population identified in the TEM analysis (Fig. 2B), originating from Ag species that did not align along the preferred growth direction during synthesis. With increasing gamma dose, brighter, contrast-enhanced regions progressively emerge within the matrix. Given the absence of any new diffraction peak in the corresponding XRD patterns, these bright regions are attributed to void/pore formation within the polymer matrix, most plausibly arising from radiolytic gas evolution accompanying chain scission and the release of adsorbed or bound water molecules under gamma exposure, rather than to the formation of a new crystalline phase. At 500 and 1500 kGy, bright contrast features are additionally observed in proximity to several AgNWs; while this may reflect localized interaction of Compton electrons with the metallic filler, consistent with irradiation-induced alterations reported for other silver-based nanostructures under high-dose gamma fields.²⁶ It is noteworthy that GO sheets are not distinctly resolved in the SEM images, which is attributed to their homogeneous dispersion within the PVA matrix and to the limited feasibility of higher-magnification imaging, as prolonged electron-beam exposure was found to induce localized degradation of the polymer matrix.

Fig. 3. SEM images of GO–PVA–AgNW nanocomposites: (A) initial, (B) 250 kGy, (C) 500 kGy, (D) 1500 kGy.

Fig. 4 displays the cross-sectional SEM images of the pristine and 1500 kGy-irradiated GO–PVA–AgNW samples. A clear morphological transformation is evident upon irradiation, with the 1500 kGy-irradiated sample exhibiting substantial structural reorganization within the matrix, consistent with bond scission, crosslinking, and radiolytic gas evolution induced by high-dose gamma exposure. These observations confirm that gamma irradiation induces significant physicochemical modifications in the composite microstructure.

Fig. 4. SEM images of cross-section area of GO–PVA–AgNW nanocomposites: (A) initial, (B) 1500 kGy.

3.2. X-ray diffraction analysis

Fig. 5 shows the XRD patterns of the pristine and gamma-irradiated GO–PVA–AgNW nanocomposites at different doses. Three main diffraction features are observed, corresponding to PVA and AgNWs. The broad reflection at 2θ = 19.88°, indexed to the (101) plane, is characteristic of the semicrystalline structure of PVA.²⁵ The additional peaks at 2θ = 38.49° and 45.21° are indexed to the (111) and (200) planes of metallic Ag, respectively, consistent with JCPDS card No. 04-0783, (ref. 27) confirming the crystalline nature of the embedded AgNWs. No distinct GO diffraction peak is detected in any of the patterns, which is expected given the homogeneous dispersion of GO within the PVA matrix and the inherently low intensity of the GO (001) reflection at the loading level used in this study.²⁸

Fig. 5. XRD pattern of GO–PVA–AgNW nanocomposites: (A) initial, (B) 250 kGy, (C) 500 kGy, (D) 1500 kGy.

A noticeable change in the intensity of the PVA peak is observed upon irradiation. At 250 kGy, the peak intensity increases, suggesting the formation of radiation-induced crosslinks that create more ordered local domains, leading to a sharper and narrower diffraction feature. At higher doses, however, the trend reverses: at 1500 kGy, the PVA peak becomes significantly weaker and broader. Rather than reflecting a scission-dominated loss of order, this behavior is attributed to a shift from regular, ordered crosslinking at low-to-moderate doses toward irregular, densely crosslinked domains at high dose – consistent with the pronounced increase in microstrain and with the near-complete insolubility of the 1500 kGy sample, which directly confirms extensive network crosslinking rather than chain shortening.

Crystallite sizes were calculated from the PVA (101) peak using the Scherrer equation:²⁹

1

where D is the crystallite size, K is the shape factor, λ is the X-ray wavelength, β is the FWHM of the peak, and θ is the Bragg angle.

As shown in the corresponding Table 1, the crystallite size initially increases after low-dose irradiation but decreases sharply at higher doses, corresponding to an approximately two-fold reduction at 1500 kGy relative to the pristine sample. This behavior is consistent with the increased probability of Compton scattering involving the high-Z Ag atoms present within the matrix under gamma radiation.³⁰ The high-energy secondary electrons generated in this process produce numerous free radicals, causing chain scission and disordered crosslinking within the PVA-rich regions. The resulting structural disruption reduces the average size of ordered crystalline domains.

Table 1. Crystallite size and microstrain values of GO–PVA–AgNW nanocomposites before and after gamma irradiation.

γ Irradiation doses	0 kGy	250 kGy	500 kGy	1500 kGy
Crystallite size (nm)	1.75	1.90	1.49	0.92
Microstrain	0.1152	0.1071	0.1353	0.2168

Microstrain in the PVA lattice was also evaluated using the Uniform Deformation Model:³¹

2

A slight decrease in microstrain is initially observed, which is attributed to the formation of low-density ordered crosslinks that partially heal microdefects. However, at 500 kGy the microstrain increases, and at 1500 kGy it rises sharply. This rise is caused by the generation of Compton electrons, extensive radical formation, bond rupture near AgNW-rich regions, and the formation of irregular crosslinked structures. At very high doses, both the high-energy photons and the secondary electrons contribute to severe lattice distortion, resulting in significantly elevated microstrain within the PVA-based matrix of the GO–PVA–AgNW nanocomposite.

3.3. Raman spectroscopy

Fig. 6 presents the Raman spectra of the pristine and gamma-irradiated GO–PVA–AgNW nanocomposites at 0, 250, 500, and 1500 kGy. All spectra exhibit the two characteristic carbon-related bands of GO: the D band at ∼1350 cm⁻¹, associated with disorder-activated breathing modes of sp² rings at defect sites, and the G band at ∼1590 cm⁻¹, corresponding to the E₂g phonon mode of ordered sp² carbon domains. In the pristine sample (Fig. 6a), the G band is more intense than the D band, indicating a relatively ordered sp² carbon structure. With increasing irradiation dose, the relative intensity of the D band progressively increases with respect to the G band, reflecting enhanced defect formation and progressive disruption of the sp² network under γ-irradiation.

Fig. 6. Raman spectra of GO–PVA–AgNW nanocomposites irradiated with γ-rays at doses of 0, 250, 500, and 1500 kGy.

The intensity of the band at 2917 cm⁻¹, assigned to C–H stretching vibrations of the PVA backbone,³² increases sharply at 250 kGy before progressively decreasing at 500–1500 kGy, a trend that closely parallels the dose dependence of the XRD crystallite size (Table 1), which likewise peaks at 250 kGy (1.90 nm) before decreasing to 0.92 nm at 1500 kGy. This behavior reflects a two-regime radiolytic response: at low dose, hydrogen abstraction from C–H bonds generates macroradicals that recombine into new C–C crosslinks, producing local chain compaction that enhances both the XRD crystallite size and the Raman C–H mode intensity, consistent with reports linking gamma-irradiation-induced crosslinking to increased crystallinity and Raman/XRD signal intensity in other polymer systems.³³ At higher doses, accumulated chain scission disrupts this order, consistent with the concurrent decrease in crystallite size and increase in microstrain (Table 1).

The structural disorder in GO was further quantified using the I_D/I_G ratio and the Cançado model.^34,35 The lateral crystallite size (L_a) was calculated using:

3

where λ = 532 nm for the excitation laser. The defect-related parameters—defect spacing (L_D) and defect density (n_D)—were determined from:

4

5

The results (Table 2) show that the lateral crystallite size decreases with irradiation up to 500 kGy, consistent with an increasing number of structural defects and progressive disruption of sp² domains. At 1500 kGy, a modest increase in L_a (and a corresponding modest decrease in n_D) is observed relative to 500 kGy; given the relatively small magnitude of this change, it is interpreted with caution and is tentatively attributed to partial defect re-ordering or a reduction in oxygen-containing functional groups at the highest dose, rather than to a pronounced reversal of the irradiation-induced disorder trend.

Table 2. Lateral crystallite size, defect spacing, and defect density of GO domains in GO–PVA–AgNW nanocomposites at different γ-irradiation doses.

γ Irradiation doses	0 kGy	250 kGy	500 kGy	1500 kGy
L a (nm)	22.21	18.51	18.45	19.25
L D (nm)	12.91	11.78	11.77	12.02
n D (×1011)	1.94	2.33	2.34	2.25

3.4. Optical properties

Fig. 7 displays the UV-vis absorption spectra of the GO–PVA–AgNW nanocomposites irradiated at different γ-doses. In the pristine sample, two characteristic plasmon resonance peaks of AgNWs are clearly observed: a weak feature at 350 nm, commonly associated with the longitudinal/bulk-silver-like plasmon mode, and a stronger band at 380 nm, corresponding to the transverse plasmon mode arising from electron oscillation across the nanowire diameter. The presence of both features confirms that the AgNWs retain their one-dimensional morphology within the composite prior to irradiation.

Fig. 7. UV-vis spectra of GO–PVA–AgNW nanocomposites before and after γ irradiation.

Gamma irradiation induces a gradual red-shift of the absorption edge, accompanied by a systematic decrease in the intensity of the plasmon peaks. This attenuation becomes most pronounced at 1500 kGy. The reduction in plasmon intensity is attributed to (i) irradiation-induced crosslinking and morphological disorder in the PVA matrix, which enhances light scattering and reduces effective illumination of the nanowires, and (ii) modifications in the dielectric environment caused by newly formed defects and oxygen-containing groups, to which plasmon resonance is highly sensitive. Irradiated samples also show the emergence of an additional absorption band in the 275–300 nm region. This feature originates from n → π* electronic transitions of newly formed carbonyl groups produced by γ-induced chain scission of C–H and O–H bonds, followed by radical recombination and oxidation processes.³⁶ The increasing intensity of this band with dose confirms progressive formation of C O and conjugated C C groups in the matrix.

The optical band gap (E_g) was determined using the Tauc relation:³⁷

αhv = β(hv − E_g)ⁿ 6

where α is derived from the Beer–Lambert law, β is a constant related to the transition probability (Tauc parameter), reflecting the extent of structural order/disorder in the material, while n determines the nature of the optical transition (n = 1/2 for direct-allowed and n = 2 for indirect-allowed transitions).³⁸

To isolate the individual contribution of each filler to the optical band gap, control films of pure PVA and of 0.8 wt% GO/PVA were additionally prepared under identical solution-casting conditions and characterized under the same UV-vis measurement protocol. These control samples yielded E_g = 6.19 eV for pure PVA, 5.26 eV for 0.8 wt% GO/PVA, and 5.07 eV for the pristine (non-irradiated) 0.8 wt% GO–PVA–AgNW nanocomposite, indicating that the incorporation of GO introduces localized trap states that progressively narrow the band gap, consistent with the Mott–Davis model,²⁸ with a further narrowing contribution from the incorporation of AgNWs.

As seen from Table 3, the band-gap values (E_g) of the samples decrease with increasing gamma irradiation dose. This reduction is attributed to bond scission, chain fragmentation, and the increased concentration of defects generated within the polymer matrix under γ-radiation, all of which lead to the formation of additional localized energy states. These defect-induced states appear near the valence and conduction band edges, thereby lowering the minimum energy required for electronic transitions. Furthermore, in the sample irradiated at 1500 kGy, the band gap decreases sharply, with E_g dropping from 5.07 eV to 3.82 eV. This pronounced reduction is directly related to the substantial increase in trap centers within the band gap. At such high doses, the number of radiation-induced defects rises significantly, leading to the formation of dense defect levels that markedly narrow the band gap.

Table 3. Band gap result of GO–PVA–AgNW nanocomposites depending on γ irradiation doses.

γ Irradiation doses	0 kGy	250 kGy	500 kGy	1500 kGy
Band gap Eg (eV)
GO–PVA–AgNW	5.07 ± 0.01	4.94 ± 0.01	4.84 ± 0.01	3.82 ± 0.01

3.5. TGA analysis

The TGA curves of pristine and γ-irradiated GO–PVA–AgNW nanocomposites are presented in Fig. 8. As can be seen, the mass loss of all samples occurs in a stepwise manner with increasing temperature.

Fig. 8. TGA curves of pristine and γ-irradiated GO/PVA/AgNW nanocomposites at different doses (0–1500 kGy).

The thermal degradation of the samples proceeds in three distinct stages. The initial stage occurs in the temperature range of 50–160 °C and is attributed to the removal of physically adsorbed moisture and weakly bound volatile compounds. The second stage corresponds to the main dehydration process, during which the elimination of water and the decomposition of side groups in the polymer chains take place. As a result, conjugated structures are formed, leading to a sharp mass loss. This stage occurs in the temperature range of 250–450 °C. The final stage is associated with the decomposition of polyene structures and appears as a small shoulder in the range of 450–550 °C. In this region, the stabilization of residual carbonaceous structures (char) and the contribution of composite components (GO and AgNWs) are observed.^39,40

As can be seen from the figure, the initial stage of thermal degradation is nearly identical for all samples. However, with increasing irradiation dose, the onset of the second degradation stage shifts toward lower temperatures. This behavior can be attributed to the formation of weak bonds and defect sites induced by irradiation-related chain scission processes, which are more easily broken and removed from the system at lower temperatures. In contrast, the degradation of the main polymer backbone shifts toward higher temperatures. This observation suggests an enhancement in the thermal stability of the primary chains due to the formation of radiation-induced crosslinks. The increased crosslink density restricts the mobility of polymer chains and improves the resistance of the network structure to thermal decomposition. Furthermore, an increase in the residual content is observed with increasing irradiation dose. The residue values for the pristine and irradiated samples were found to be 10.15%, 11.54%, 13.04%, and 14.09%, respectively. This increase indicates that, in addition to chain scission, crosslinking reactions also occur during irradiation. The higher char yield and residue content suggest that crosslinking is the dominant mechanism governing the structural modifications of the nanocomposites under the applied irradiation conditions.

3.6. EPR analysis

Fig. 9A and B present the EPR spectra of the pristine and γ-irradiated GO–PVA–AgNW nanocomposites. As shown in the spectra, the intensity of the extrema increases markedly upon irradiation. This is associated with the generation of new radicals within the matrix as the irradiation dose increases. To characterize the radicals generated under irradiation, the g-factor was calculated for the pristine and irradiated samples based on eqn (7):

7

where h = 6.62607 × 10⁻³⁴ J s and µ_BB = 9.27401 × 10⁻²⁴J/T are fundamental constants, ν is the fixed microwave excitation frequency of the spectrometer, and B is the resonance field.

Fig. 9. EPR spectra of (A) pristine and (B) γ-irradiated GO–PVA–AgNW nanocomposites at 250, 500, and 1500 kGy.

The g-factor values are presented in Table 4. It was determined that all samples exhibit a value of 2.001, corresponding to a carbon-centered π-radical delocalized over the sp² framework of GO/PVA.⁴¹ The invariance of the g-parameter indicates that the origin of the radicals generated by γ-irradiation does not change; only their concentration within the matrix increases.

Table 4. g-Factor, peak-to-peak linewidth (ΔH_pp), and asymmetry factor (F) of GO–PVA–AgNW nanocomposites as a function of γ-irradiation dose.

γ Irradiation doses	0 kGy	250 kGy	500 kGy	1500 kGy
g-Factor	2.0009	2.0012	2.0011	2.0013
ΔHpp (G)	2.88	2.79	2.54	2.10
F = I+/I−	0.8231	0.9022	0.9807	1.0489

Table 4 also presents the dose dependence of the peak-to-peak linewidth (ΔH_pp). The peak-to-peak linewidth (ΔH_pp) narrows monotonically with increasing dose, from 2.88 G in the pristine sample to 2.10 G at 1500 kGy. This narrowing indicates a transition toward the fast spin-exchange regime: as the radical population increases, frequent Heisenberg spin-exchange collisions average out the local dipolar field experienced by each spin, thereby narrowing the resonance. This is consistent with the concurrent increase in the overall EPR signal intensity, which indicates a dose-dependent, progressive increase in radical concentration without evidence of saturation up to 1500 kGy.

The asymmetry factor was also calculated for the samples and is presented in Table 4. F = I₊/I₋ (the ratio of the positive and negative derivative extrema) provides additional insight into the spatial environment of the radicals. The pristine sample yielded a value of F = 0.82, indicating that the radicals are not symmetrically distributed. However, with increasing irradiation dose, the F factor approaches a value of 1, indicating that with increasing irradiation dose the radicals evolve toward a symmetric distribution.

4. Conclusion

In this study, the effects of high-dose gamma irradiation (250–1500 kGy) on the morphological, structural, optical, thermal, and paramagnetic properties of GO–PVA–AgNW nanocomposites were systematically investigated. Electron microscopy results showed that gamma irradiation induces significant modifications in the microstructure of the nanocomposite, while matrix reorganization becomes more pronounced at higher irradiation doses. X-ray diffraction analyses demonstrated that low-dose irradiation promotes the formation of more ordered local structures, whereas higher doses lead to structural disorder accompanied by a decrease in crystallite size (from 1.75 nm to 0.92 nm) and a significant increase in microstrain (from 0.1152 to 0.2168). Raman spectroscopy results confirmed that gamma irradiation increases the number of defects in the GO structure, as evidenced by the decrease in lateral crystallite size (from 22.21 nm to 18.45 nm) and the increase in defect density (from 1.94 × 10¹¹ to 2.34 × 10¹¹ cm⁻²). UV-vis spectroscopy revealed a red shift of the absorption edge and a reduction of the optical band gap from 5.07 eV to 3.82 eV. These changes were attributed to the formation of irradiation-induced defects and localized energy states. Thermogravimetric analysis showed that gamma irradiation enhances the thermal stability of the primary polymer network and increases the amount of residual carbonaceous products (from 10.15% to 14.09%). The progressive increase in residue content with irradiation dose indicates that crosslinking becomes the dominant process over chain scission under high-dose irradiation conditions. EPR analyses revealed a significant increase in the concentration of radical centers with increasing irradiation dose. All samples exhibited a g-factor of approximately 2.001, indicating that the paramagnetic centers consist of carbon-centered π-radicals. The decrease in linewidth (from 2.88 G to 2.10 G) and the approach of the asymmetry factor toward unity (from 0.82 to 1.05) further suggest a more homogeneous distribution of radicals and enhanced spin-exchange interactions.

Author contributions

Conceptualisation, M. Mu.; methodology, M. Mu. and M. B. B.; software, G. E., M. C., M. Ma. and M. B. B.; validation, M. Mu. and M. B. B.; investigation, M. B. B., G. E., M. C., M. Ma., H. M., G. K., R. H. and F. M., R. A.; writing – original draft preparation, M. B. B.; writing – review and editing, M. B. B. and M. Mu.; visualisation, Z. A.; supervision, M. Mu.; project administration, M. Mu. All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting the findings of this study are available from the corresponding author upon reasonable request. All relevant data are included within the article.