Cure Kinetics of Samarium-Doped Fe₃O₄/Epoxy Nanocomposites

Abstract

To answer the question “How does lanthanide doping in iron oxide affect cure kinetics of epoxy-based nanocomposites?”, we synthesized samarium (Sm)-doped Fe₃O₄ nanoparticles electrochemically and characterized it using Fourier-transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy dispersive X-Ray analysis (EDX), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy analyses (XPS). The magnetic particles were uniformly dispersed in epoxy resin to increase the curability of the epoxy/amine system. The effect of the lanthanide dopant on the curing reaction of epoxy with amine was explored by analyzing differential scanning calorimetry (DSC) experimental data based on a model-free methodology. It was found that Sm³⁺ in the structure of Fe₃O₄ crystal participates in cross-linking epoxy by catalyzing the reaction between epoxide rings and amine groups of curing agents. In addition, the etherification reaction of active OH groups on the surface of nanoparticles reacts with epoxy rings, which prolong the reaction time at the late stage of reaction where diffusion is the dominant mechanism.

1. Introduction

The subject of magnetic nanoparticles has attracted significant interest, in particular, in the medical field and other high-tech applications [1,2,3]. Magnetite nanoparticle-filled polymers have been widely used in various applications such as electronic devices, nonlinear optic systems, sensors, magnetic filters, and photovoltaic solar cells [4]. Owing to their good properties and low price, magnetite (Fe₃O₄) nanoparticles have been studied most among the various kinds of magnetic nanoparticles [5,6,7].

Fe₃O₄ nanoparticles have been used in epoxy matrices to improve their final properties [8]. Dispersion state and interfacial interaction between Fe₃O₄ nanoparticles and epoxy matrix are two important parameters that affect the cross-linking reaction and final properties of epoxy nanocomposites [9,10]. Homogenous dispersion of nanoparticles in an epoxy matrix and strong interfacial interaction between Fe₃O₄ nanoparticles and polymer matrix are required to achieve a dense crosslinked network with improved final properties, in particular, in terms of mechanical performance [11,12]. However, magnetite nanoparticles have a high tendency to aggregate in the epoxy matrix [13]. It was shown that the success in curing the reaction of epoxy is highly dependent on the dispersibility of Fe₃O₄ nanoparticles in the resin matrix. However, the possibility of unmodified Fe₃O₄ nanoparticles to form agglomerates is high, hindering cross-linking reactions [14]. Many research works revealed that surface functionalization of Fe₃O₄ nanoparticles results in well-dispersed nanoparticles in the epoxy matrix [15,16]. Recently, it was shown that bulk modification of Fe₃O₄ nanoparticles by doping other metal ions can strongly affect cross-linking reaction of epoxy [17,18,19]. Furthermore, with an eye to possibly to enhance the properties of the Fe₃O₄ nanoparticles, doping of magnetite nanoparticles with other metal ions has been explored [20,21].

Lanthanides feature an f-electron shell, a property that only very heavy atoms have, which gives them unique optical and magnetic properties, making them interesting as dopants for other nanoparticles [22]. Besides introducing new properties, the doping of magnetite particles with small amounts of lanthanides leads to only minor alterations of physical and chemical characteristics. The doping of other metals/metal ions, such as zinc, manganese, copper, nickel, and cobalt, into the Fe₃O₄ nanoparticles enhances the availability of its surface sites [23,24,25]. Among lanthanides, samarium (Sm) has high magnetic properties and is relatively stable at room temperature. This rare-earth element is used in many applications such as microwave, electronic devices, optical lasers, and neutron absorbers in nuclear reactors [26,27]. Doping Fe₃O₄ nanoparticles with Sm atoms can increase the number of surface active sites and surface area of base nanoparticles, which may enhance the curability of epoxy resin. To the best of our knowledge, the effect of Sm-doped Fe₃O₄ nanoparticles on the curing reaction of epoxy resin has not been studied until now.

In the present study, Sm-doped Fe₃O₄ nanoparticles were fabricated via an electrochemical method. The synthesized samples were well characterized by Fourier-transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray analysis (EDX), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy analyses (XPS). Then, the epoxy-based film was reinforced with Sm-doped Fe₃O₄ nanoparticles to obtain an excellent corrosion protection coating. The cure potential of the epoxy-containing Sm-doped Fe₃O₄ nanoparticles was evaluated with dynamic differential scanning calorimetry (DSC) at different heating rates of 2.5, 5, 7.5, and 10 °C/min.

2. Materials and Methods

Iron (II) chloride (FeCl₂·4H₂O), iron(III) nitrate nonahydrate 99.9% (Fe(NO₃)₃·9H₂O), and samarium(III) nitrate (Sm(NO₃)₃·(H₂O)₂) were supplied by Sigma-Aldrich. Araldite LY 5052 epoxy resin and HY 5052 curing agent were purchased from MIS Hindustan Ciba-Geigy.

2.1. Synthesis of Sm-Doped Fe₃O₄ Nanoparticles

Sm³⁺-doped Fe₃O₄ nanoparticles were prepared through the cathodic electrodeposition (CED) procedure using a stainless steel cathode (316 L, 5 cm × 5 cm × 0.5 mm) inside two graphite anodes. The electrolyte 0.005 molar solution of iron(III) nitrate nonahydrate (2 g), iron(II) chloride (1 g), and samarium(III) nitrate (0.6 g) was prepared in water. Then, deposition occurred using a Potentiostat/Galvanostat, Model: NCF-PGS 2012 (Metrohm Autolab) at 25 °C and current density of 10 mA cm⁻² for 30 min followed by rinsing with deionized water several times. Finally, the dispersed Sm-Fe₃O₄ deposit in deionized water was centrifuged at 6000 rpm for 20 min, separated, and dried at 70 °C for 1 h.

2.2. Preparation of Epoxy/Sm-Doped Fe₃O₄ Nanocomposite

Epoxy nanocomposites were obtained by mixing 0.1 wt.% of Sm-doped Fe₃O₄ using a mechanical mixer at 2500 rpm for 15 min. Then, Sm-doped Fe₃O₄ in epoxy was further mixed by sonication for 5 min. Finally, the curing agent was added to EP/Sm-Fe₃O₄ nanocomposite in the stoichiometric ratio of 38/100 (curing agent/epoxy).

2.3. Characterization

The FTIR spectrum of Sm-doped Fe₃O₄ nanoparticles was obtained by Bruker Vector spectrometer, Coventry, UK, between 4000–400 cm⁻¹ wavelength. X-ray diffraction (XRD) of nanoparticles was performed by a Philips PW-1800 apparatus (Amsterdam, Netherlands) with Co Kα radiation. The micro- and nano-images of Sm-Fe₃O₄ nanoparticles were obtained by FESEM and EDX-Mapping (Mira 3-XMU, TESCAN, Brno-Kohoutovice, Czech Republic) at the voltage of 100 kV and TEM (Zeiss-EM10C-80 kV, Jena, Germany). A Thermo Fisher Scientific instrument XPS elemental analyzer (Waltham, MA, USA) determined the X-ray Photoelectron Spectroscopic properties of the nanoparticles.

The cure reaction of neat epoxy and EP/Sm-Fe₃O₄ nanocomposite was investigated by analyzing 12 mg of each sample using DSC (Perkin Elmer, DSC 4000, Waltham, MA, USA) at four different heating rates (2.5, 5, 7.5 and 10 °C·min⁻¹) in the temperature range of 15–300 °C under nitrogen atmosphere.

3. Results and Discussion

3.1. Characterization of Sm-Fe₃O₄ Nanoparticles

The FTIR spectrum of the prepared Sm–Fe₃O₄ nanoparticle is shown in Figure 1a. Two sharp bands can be observed at 562 cm⁻¹ and 628 cm⁻¹, which are ascribed to the splitting of the ν1 band of the Fe–O. A wide peak in the range of 415–443 cm⁻¹ is due to ν2 bands of the Fe–O and Sm–O [26]. The appearance of bands at 1648 and 3325 cm⁻¹ are attributed to the stretching and deformation vibrations of O–H groups on the surface of Sm–Fe₃O₄ nanoparticles.

Figure 1b shows the XRD pattern of Sm-Fe₃O₄ nanoparticles, which is the cubic spinel structure of Magnetite Fe₃O₄ [Joint Committee on Powder Diffraction Standards (JCPDS) 76-1849 and Inorganic Crystal Structural Database (ICSD) 28664]. A significant change was not observed in the XRD patterns of doped nanoparticles. The XRD pattern shows that doping Sm does not change the crystal structure of Fe₃O₄ nanoparticles.

The oxygen atoms in magnetite (Fe₃O₄) form an inverse Spinel structure—a close-packed face-centered cubic sublattice, where Fe(II) are located in the octahedral sites and Fe(III) are occupying octahedral and tetrahedral sites [28], which can be described by (Fe₈³⁺)^tetr[Fe³⁺Fe²⁺]₈^octO₃₂. The close-packed plane is along the (111) axis, occupied by oxygen atoms. Those sites can also be occupied by the transition metal atoms [29]. Samarium ions, similarly to other lanthanide(III) ions, have six coordination sites and seven octahedral coordination sites, when crystallizing as Sm₂O₃ [30], making them suitable for being included in the inverse spinel structure at the octahedral sites.

Figure 2 shows the FESEM, EDS, mapping, and TEM images of Sm-Fe₃O₄ nanoparticles. The average particle size is about 20 nm. The EDX experiments proved the presence of nanoparticles, containing both Fe and Sm at approximately equal amounts (Figure 2b). Additionally, O (25.4%) and Au were found, the latter stemming from the TEM grids used.

The presence of Fe and Sm, O, and C in the Sm-Fe₃O₄ nanoparticles is confirmed by XPS analysis (Figure 3a), as the Fe2p peaks (Figure 3b) are clearly visible in terms of Fe2p_1/2 (710 eV) and Fe2p_3/2 peaks (720 eV), confirming the Fe(III)-oxidation state [31]. The Fe(II) in Fe₃O₄ is confirmed by the shoulder at 708 eV (Fe2p_1/2 peak) and at 721 eV (Fe2p_3/2 peak) [32].

Sm-3d_5/2 regions of Sm-Fe₃O₄ nanoparticles are shown in Figure 3c. The binding energy values (3d_5/2) of Sm(III) (1079 and 1107 eV) observed in the nanoparticles are in accordance with the standard values [Sm₂O₃ (1082 and 1108 eV) [33], thus this lanthanide is in its +3 oxidation state in Sm-doped Fe₃O₄. The varied coordination conditions in the crystal structure could explain the subtle differences observed.

3.2. Curing Analysis

Figure 4 displays nonisothermal DSC thermographs of neat epoxy and EP/Sm-Fe₃O₄ cured with a stoichiometric amount of amine curing agent at heating rates of 2.5, 5, 7.5, and 10 °C/min. One exothermic peak can be observed for both samples at different heating rates, which revealed that the presence of Sm-Fe₃O₄ nanoparticles in the epoxy matrix does not change the domination of the chemically controlled reaction mechanism [34,35].

The cure characteristics of EP and EP/Sm-Fe₃O₄ nanocomposite include T_Onset, T_Endset, T_p, ΔT, and ΔH_∞, which are the onset, endset, the exothermal peak temperature, temperature interval, and the enthalpy of complete cure, respectively, and are reported in

Table 1. Cure characteristics of EP and EP/Sm-Fe₃O₄ nanocomposite as a function of heating rate.

Sample	Heating Rate (°C/min)	TOnset (°C)	Tp (°C)	TEndset (°C)	ΔT (°C)	ΔH∞ (J/g)	ΔT*	ΔH*	CI
EP	2.5	25.7	90.4	159.9	134.2	329.6	N.a.	N.a.	N.a.
	5.0	37.4	101.7	171.7	134.4	336.2	N.a.	N.a.	N.a.
	7.5	40.7	110.3	180.7	140.0	344.1	N.a.	N.a.	N.a.
	10	41.4	119.2	225.3	183.9	404.3	N.a.	N.a.	N.a.
EP/Sm-Fe3O4	2.5	30.8	89.4	170.6	139.8	385.8	1.04	1.17	1.22
	5.0	35.7	101.5	175.2	139.5	363.7	1.04	1.08	1.12
	7.5	39.1	107.7	207.0	167.9	296.3	1.2	0.86	1.03
	10	40.1	115.1	226.0	185.9	403.4	1.01	1.01	1.02

N.a.: Not applicable.

. T_Onset, T_Endset, and T_p shifted towards higher temperatures by increasing heating rates from 2.5 to 10 °C/min to compensate for reducing curing time [36,37].

The addition of Sm-Fe₃O₄ nanoparticles decreased T_Onset and T_p of epoxy/amine reaction, indicating that Sm doped magnetic nanoparticles accelerate cross-linking reaction. The surface activity of Sm-Fe₃O₄ nanoparticles can ascribe this increment in the system’s reactivity due to the presence of Sm³⁺ in the crystal structure of nanoparticles that catalyze the reaction between epoxy and amine curing agents [23,38]. However, T_Endset, ΔT increased for EP/Sm-Fe₃O₄ nanocomposite compared to neat epoxy, which means that at the late stage of cure reaction, the OH groups on the surface of nanoparticles participate in etherification reaction and prolong the cross-linking of epoxy reaction.

The effect of the etherification reaction of OH groups on the surface of Sm-Fe₃O₄ nanoparticles as well as the catalyzing effect of Sm³⁺, which acts as Lewis acid, increase total heat of cure (ΔH_∞) of EP/Sm-Fe₃O₄ nanocomposite in comparison to neat epoxy (Figure 5) [39].

Figure 6 shows the conversion (α) of curing reaction as a function of temperature, which is obtained from Equation (1) [40]:

$$\alpha =\frac{\mathsf{\Delta }{H}_T}{\mathsf{\Delta }{H}_{\infty }},$$

(1)

where ∆H_T is the enthalpy of reaction at a specific temperature.

In the initial stage of the curing reaction, cross-linking occurs rapidly until reaching gel point under the control of chemical reaction between the epoxy ring and amine groups of curing agent. In contrast, cross-linking occurs slowly at the late stage of cure, where diffusion is dominant. Additionally, Sm-Fe₃O₄ nanoparticles accelerate cross-linking of epoxy after vitrification, which indicated an acceleration of diffusion mechanisms due to the presence of OH groups on the nanoparticle surface [41].

Cure State of EP/Sm-Fe₃O₄

The effect of curability of Sm-Fe₃O₄ nanoparticles in epoxy/amine system are specified by Cure Index [42,43,44]:

$$CI=\mathsf{\Delta }{T}^{*}\times \mathsf{\Delta }{H}^{*},\mathsf{\Delta }{T}^{*}=\frac{\mathsf{\Delta }{T}_{\mathrm{nanocomposite}}}{\mathsf{\Delta }{T}_{\mathrm{Reference}}}and\mathsf{\Delta }{H}^{*}=\frac{\mathsf{\Delta }{H}_{\mathrm{nanocomposite}}}{\mathsf{\Delta }{H}_{\mathrm{Reference}}},$$

(2)

∆T is temperature window, within which curing occurs, with subscripts of “nanocomposite” and “Reference” for nanocomposite and blank epoxy systems, respectively. Similarly, ΔH_∞ of such systems is defined. The terms with asterisks are dimensionless in each case. Good, Poor, and Excellent curing reactions of nanocomposites occur at CI > ∆H*, CI < ∆T*, and ∆T* < CI < ∆H*, respectively [45,46,47]. The addition of Sm-Fe₃O₄ nanoparticles in the epoxy matrix resulted in a Good cure reaction, which means that Sm³⁺ participates in cross-linking of epoxy by catalyzing the reaction between epoxide rings and amine groups of curing agents. In addition, the active OH groups on the surface of nanoparticles react with epoxy polar groups that increase both ∆T and ∆H and result in Good CI [48,49,50].

3.3. Cure Kinetics of EP/Sm-Fe₃O₄

3.3.1. Calculation of Activation Energy

Isoconversional model-free Friedman (differential) and Kissinger–Akahira–Sunose (KAS, integral) were employed to obtain the apparent activation energies (E_α) of the curing reactions [51,52].

By plotting $\ln \left[{\beta }_i{\left(d\alpha /dT\right)}_{\alpha ,i}\right]$ vs. 1/T_α from Equation (3), the value of E_α is obtained from the slope of Figure 7a,b [53,54].

$$\ln {\left[{\beta }_i{\left(\frac{d\alpha }{dT}\right)}_{\alpha ,i}\right]}_{\alpha }=\ln \left[f(\alpha )A_{\alpha }\right]-\frac{E_{\alpha }}{R{T}_{\alpha ,i}},$$

(3)

Additionally, plotting $\mathrm{l}\mathrm{n}\left({\beta }_i/T_{\alpha ,i}^2\right)$ vs. 1/T_α from Equation (4) for each α value gives a value for the E_α (Figure 7c,d).

$$\ln {\left[\frac{{\beta }_i}{T_{\alpha ,i}^{1.92}}{\left(\frac{d\alpha }{dT}\right)}_{\alpha ,i}\right]}_{\alpha }=\mathrm{Constant}-1.0008\left(\frac{E_{\alpha }}{R{T}_{\alpha ,i}}\right),$$

(4)

The apparent activation energy of neat epoxy and EP/Sm-Fe₃O₄ nanocomposite as a function of α based on both Friedman and KAS are shown in Figure 8. E_α was reduced for neat epoxy and its nanocomposite in α higher than 0.5 due to the participation of OH groups in epoxide ring-opening at a later stage of curing reaction revealing the autocatalytic mechanism of epoxy cure reaction [55,56]. The higher E_α values for EP/Sm-Fe₃O₄ nanocomposite compared to neat epoxy can be attributed to the higher viscosity of the epoxy system in the presence of Sm-Fe₃O₄ nanoparticles [57].

3.3.2. Determination of Reaction Model

Friedman Model

The Friedman method can be used to show the non-catalytic or autocatalytic reaction model using Equation (5). The plot of ln[Af(α)] vs. ln(1 − α) shows a maximum in α between 0.2 and 0.4 that suggests an autocatalytic reaction mechanism (Figure 9) [58].

$$\mathrm{l}\mathrm{n}\left[Af(\alpha )\right]=\mathrm{l}\mathrm{n}\left(\frac{d\alpha }{dt}\right)+\frac{E}{RT}=\mathrm{l}\mathrm{n}A+n\mathrm{l}\mathrm{n}\left(1-\alpha \right),$$

(5)

Malek Model

A more accurate Malek method was applied to determine the cross-linking reaction model through the shape and maximum points of y(α) and z(α) Malek master plots (Equations (6) and (7)) [59]. As can be observed in Figure 10 and

Table 2. Malek parameters of EP and EP/Sm-Fe₃O₄.

Sample	Heating Rate (°C/min)	αp	αm	αp∞
EP	2.5	0.447	0.075	0.497
	5	0.44	0.081	0.545
	7.5	0.482	0.087	0.553
	10	0.319	0.092	0.484
EP/Sm-Fe3O4	2.5	0.456	0.051	0.487
	5	0.582	0.049	0.551
	7.5	0.429	0.05	0.51
	10	0.418	0.041	0.505

, α_m = Max (y(α)) is lower than α_p = Max (z(α)) and αp^∞ (the conversion at the maximum point of DSC curves) is smaller than 0.632, which revealed that the two-parameter autocatalytic model could be used for both EP and EP/Sm-Fe₃O₄ nanocomposite [60,61].

$$y(\alpha )={\left(\frac{d\alpha }{dt}\right)}_{\alpha }exp\left(\frac{E_0}{R{T}_{\alpha }}\right)=Af(\alpha )$$

(6)

$$z(\alpha )={\left(\frac{d\alpha }{dt}\right)}_{\alpha }{T}_{\alpha }^2,$$

(7)

Therefore, the autocatalytic reaction model (f(α), Equation (8)) of neat epoxy and its nanocomposite were obtained by Friedman and Malek models.

$$f(\alpha )={\alpha }^m{(1-\alpha )}^{\mathrm{n}},$$

(8)

The reaction model parameters, including the pre-exponential factor (lnA), non-catalytic (n), and autocatalytic (m) reaction orders, were determined from Equations (9) and (10) and Figure 11 and reported in

Table 3. The kinetic parameters evaluated for the curing of EP and EP/Sm-Fe₃O₄ nanocomposite based on Friedman and KAS models at different heating rates.

Designation	Heating Rate (°C/min)	Friedman			KAS
		m	n	lnA (s−1)	m	n	lnA (s−1)
EP	2.5	0.14	1.32	12.09	0.09	1.36	13.3
	5.0	0.29	1.38	12.71	0.24	1.42	13.9
	7.5	0.29	1.36	12.76	0.24	1.4	13.92
	10	0.25	1.69	12.57	0.2	1.74	13.71
EP/Sm-Fe3O4	2.5	0.16	1.58	14.5	0.13	1.62	15.39
	5.0	0.23	1.47	14.8	0.20	1.5	15.66
	7.5	0.34	1.72	15.05	0.30	1.75	15.9
	10	0.31	1.85	14.9	0.28	1.89	15.73

.

$$\mathrm{V}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e}\mathrm{I}=\mathrm{l}\mathrm{n}\left(\frac{d\alpha }{dt}\right)+\frac{E_{\alpha }}{RT}-\mathrm{l}\mathrm{n}\left[\frac{d(1-\alpha )}{dt}\right]-\frac{E_{\alpha }}{R{T}^{\prime }}=(n-m)\mathrm{l}\mathrm{n}\left(\frac{1-\alpha }{\alpha }\right),$$

(9)

$$\mathrm{V}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e}\mathrm{II}=\mathrm{l}\mathrm{n}\left(\frac{d\alpha }{dt}\right)+\frac{E_{\alpha }}{RT}+\mathrm{l}\mathrm{n}\left[\frac{d(1-\alpha )}{dt}\right]+\frac{E_{\alpha }}{R{T}^{\prime }}=(n+m)\mathrm{l}\mathrm{n}(\alpha -{\alpha }^2)+2\mathrm{l}\mathrm{n}A,$$

(10)

As can be observed, both n and m increased for EP/Sm-Fe₃O₄ nanocomposite compared to neat epoxy. An increment of n indicated the catalyzing effect of Sm³⁺ as a Lewis acid in the reaction between the epoxy ring and amine curing agent, and the enhancement of m is because of the reaction of OH groups on the surface of the Sm-Fe₃O₄ nanoparticles with epoxide rings. At a low heating rate (2.5 °C/min), the curing moieties have sufficient time to take part in cross-linking reactions at the early stage of curing reactions through chemically controlled reactions. At a high heating rate of 10 °C/min, the kinetic energy of the curing moieties is sufficiently high to facilitate a chemically control reaction as confirmed by a higher value of n. However, at medium heating rates (β = 5 and 7.5 °C/min), the n value may be decreased because of the absence of high kinetic energy and appropriate reaction time. The retardation effect of Sm-Fe₃O₄ nanoparticles on the cure reaction of epoxy is reflected in higher lnA values.

Validation of Isoconversional Methods

The validation of isoconversional methods (Friedman and KAS) is obtained by comparison with the experimental data and shown in Figure 12. Clearly, both KAS and Friedman approaches can predict the curing rate of the cross-linking reaction for neat epoxy and Sm-Fe₃O₄ nanoparticles incorporated epoxy systems.

4. Conclusions

Sm-doped Fe₃O₄ nanoparticle was synthesized through an electrochemical method to investigate their effect on the curability of epoxy/amine system. XPS results indicated that samarium is present in its +3 oxidation states in the structure of Fe₃O₄ lattice. The XRD pattern revealed that Sm³⁺ ions occupy the octahedral sites in the Fe₃O₄ crystal structure. DSC analysis at a different heating rate was performed to indicate Sm doping effect on the curing reaction of EP/Sm-Fe₃O₄ nanocomposites. It was found that the addition of Sm-Fe₃O₄ nanoparticles accelerates cross-linking reaction due to the catalyzing effect of Sm³⁺ in the crystal structure of Fe₃O₄ nanoparticles on the reaction between epoxy and amine curing agent, which is reflected in lower T_Onset and T_p. Obtaining Good CI by addition of Sm-Fe₃O₄ nanoparticles in epoxy matrix showed that Sm³⁺ participate in cross-linking of epoxy by catalyzing the reaction between epoxide rings and amine groups of the curing agent, and the etherification reaction of active OH groups on the surface of nanoparticles react with epoxy rings. The apparent activation energy determined by isoconversional Friedman and KAS methods indicated a complex curing reaction of epoxy in the presence of Sm-Fe₃O₄ nanoparticles, which caused an increase in average E_α value from 47.3 for neat epoxy to 52.6 kJ/mol. The autocatalytic reaction model was validated by experimental data. It can be concluded that doping Fe₃O₄ nanoparticles with Sm rare-earth element can enhance the surface active site of parent magnetic nanoparticles and enhance their dispersibility in the epoxy matrix and facilitate the cross-linking reaction of epoxy.