Effect of gold nanoparticles on the physical properties of an epoxy resin

Citation: Fraga-López, F.; Carrillo, L.J.; Vázquez-Tato, M.P.; Seijas, J.A.; Meijide, F.; Vázquez Tato, J.; Jover, A. Effect of Gold Nanoparticles on the Physical Properties of an Epoxy Resin. Int. J. Mol. Sci. 2023,24, 5638. https://doi.org/10.3390/ ijms24065638 Academic Editor: Raphaël Schneider Received: 3 February 2023 Revised: 7 March 2023 Accepted: 9 March 2023 Published: 15 March 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). International Journal of Molecular Sciences Article Effect of Gold Nanoparticles on the Physical Properties of an Epoxy Resin F. Fraga-López 1,* , Lisbeth Jiménez Carrillo 2, María Pilar Vázquez-Tato 3, Julio A. Seijas 3, Francisco Meijide 4, JoséVázquez Tato 4and Aida Jover 4 1Departamento de Física Aplicada, Facultad de Ciencias, Universidad de Santiago de Compostela, Avda. Alfonso X El Sabio s/n, 27002 Lugo, Spain 2Laboratorio de Investigación y Tecnología de Polímeros, Universidad Nacional de Costa Rica, Heredia 86-3000, Costa Rica 3Departamento de Química Orgánica, Facultad de Ciencias, Universidad de Santiago de Compostela, Avda. Alfonso X El Sabio s/n, 27002 Lugo, Spain 4Departamento de Química Física, Facultad de Ciencias, Universidad de Santiago de Compostela, Avda. Alfonso X El Sabio s/n, 27002 Lugo, Spain *Correspondence: [email protected] Abstract: The effect of doping the bisphenol A diglycidyl ether (DGEBA)/m-xylylenediamine (mXDA) system with gold nanoparticles (AuNP) has been studied with differential scanning calorimetry (DSC), thermogravimetric analysis, dynamic mechanical analysis (DMA), and dielectric analysis (DEA). The evolved heat ( ∆ H t ), the glass transition temperature (T g ), and the associated activation energies of this relaxation process have been determined. Below a certain concentration of AuNPs (=8.5%, in mg AuNP/g epoxy matrix), T g decreases linearly with the concentration of AuNPs, but above it, T g is not affected. The degree of conversion α of this epoxy system was analyzed by the semiempirical Kamal’s model, evidencing that diffusion correction is required at high values of α . Activation energy values suggest that AuNPs can cause some impediments at the beginning of the crosslinking process (n-order mechanism). The slight difference between the initial decomposition temperature, as well as the temperature for which the degradation rate is at a maximum, for both systems can be accepted to be within experimental error. Mechanical properties (tension, compression, and bending tests) are not affected by the presence of AuNPs. Dielectric measurements show the existence of a second T g at high temperatures, which was analyzed using the Tsagarapoulos and Eisenberg model of the mobility restrictions of network chains bound to the filler. Keywords: bisphenol A diglycidyl ether (DGEBA); m-xylylenediamine; gold nanoparticles; epoxy resin; glass transition temperature; degree of conversion 1. Introduction Natural Epoxy resins are widely used for many applications due to their versatility [ 1 ]. Frequently, additives (fillers) are incorporated into the resin to modify properties such as viscosity, shrinking, thermal expansion, and anticorrosion or to improve their mechanical properties [ 2 ]. This strategy is an alternative to the development of a new resin [ 3 ], which is always a more expensive process. The kinetic features of epoxy nanocomposites with fillers have been reviewed recently by Irzhak [4]. The incorporation of nanoparticles as fillers of epoxy resins is a research field attracting growing interest. The addition of fillers can modify the electrical performance of epoxy resins, and they are often used in high-voltage apparatuses as insulators. The literature reports evidence that contrary effects may be obtained. Thus, silver nanoparticles increase the conductance of an epoxy resin, having been observed that, qualitatively, the samples display most of the expected behavior predicted by percolation theories [ 5 ]. Similarly, when insulator nanoparticles are added to the resin, a decrease in that property may Int. J. Mol. Sci. 2023,24, 5638. https://doi.org/10.3390/ijms24065638 https://www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2023,24, 5638 2 of 17 be observed. With respect to unfilled resin, in an epoxy-ZnO nanocomposite system, permittivity decreases with an increment of ZnO at low filler concentrations, but the opposite is observed above a threshold concentration [ 6 ]. Similar effects have been observed for SiO2and Al2O3fillers [7]. Akib et al. [ 8 ] performed the synthesis of metal nanoparticles (Au, Ag, and Cu) in an aqueous phase and their subsequent transfer to an epoxy resin. Close-packed particle arrangements (especially for gold nanoparticles, AuNP) and increments of the glass transition temperature of the epoxy resin due to the presence of nanoparticles were observed. Poly(methyl methacrylate)-coated AuNPs have been used for reinforcing hybrid membranes (prepared via the curing of bisphenol A diglycidyl ether (DGEBA) with poly(ethylene glycol)diamine) for the removal of heavy metal ions [ 9 ]. The dispersion of AuNPs into epoxy resins can create practical applications in the development of optical devices [10] and sensors. Lednickýand Bonyár [ 11 ] have prepared surface AuNP/epoxy nanocomposites for the successful label-free detection of DNA. AuNPs were used to decorate the surface of graphene/epoxy resin nanosheets to enhance the Raman sensitivity for detecting different concentrations of dopamine in human serum as a real sample [ 12 ]. Chen et al. [ 13 ] have used a direct synthesis of a AuNP monolayer on the surface of an epoxy resin SU-8 under UV exposure, providing a high specific surface area for applications of surface-enhanced Raman spectroscopy. The device was demonstrated to be useful in the analysis of malachite green (a commonly used biocide in aquaculture) under the limits of the European Union, as well as in the analysis of Rhodamine 6G (detection limit 10 −10 M). The optical limiting threshold was also investigated in AuNP/epoxy resin systems [ 11 ]. Demir et al. [ 14 ] observed that the glass transition temperature of the DGEBA resin increases with the concentration of AuNPs, while thermal stability was slightly reduced and the effect on the mechanical properties was insignificant. Dong et al. [ 15 ] incorporated AuNPs into an epoxy matrix for photothermal curing, which enables the selective curing of an arbitrary shape within a liquid bath of epoxy, leaving the remainder unaltered but maintaining the mechanical properties of the cured epoxy. AuNPs were uniformly dispersed using poly(ethylene oxide) chains in an epoxy matrix based on DGEBA cured with a mixture of n-dodecylamine and m-xylylenediamine (mXDA). The composite exhibits photothermal effects, allowing for excellent shape memory properties [ 16 ]. A synergistic effect between multi-walled carbon nanotubes (MWCNT) and AuNPs and improved electromagnetic shield properties were observed for nanocomposites (epoxy-MWCNT-AuNP), with AuNPs being prepared by an impregnation method over organoclay [ 17 ]. Graphene/AuNP/epoxy composites were also prepared and used for the reduction of H 2 O 2 , with good linear response and high sensitivity, and for the catalytic reduction of 4-nitrophenol [18]. On the other hand, the microstructural perfection of the materials increases by minimizing the size of potential defects [ 2 ], with the size of the nano-filler being crucial for obtaining the good mechanical properties of brittle epoxy resins. Consequently, the experimental conditions for the synthesis and the dispersion control are critical parameters in the elaboration of these systems [ 3 ] when searching for the desired properties. AuNPs are commonly incorporated into the resin before starting the curing process, and only a few studies have been published in which the nanoparticles were obtained in situ within the resin [19]. The main aim of this paper was to perform a dielectric analysis of the DGEBA n= 0/m-XDA epoxy resin dopped with AuNPs that were incorporated in situ. This is a study to be conducted prior to any potential application of the new composite in electrical devices. However, as NPs could modify the curing process and the epoxy resin’s structure, the most important mechanical and thermodynamic properties must be determined. As the DGEBA/mXDA system has been an object of preferential attention during the last two decades, a comparison between both systems is feasible. For this last system, activation energies [ 20 , 21 ], optimum temperature range for the use of this material [ 22 ], transition glass temperature and the variation of the specific heat capacity [ 23 ], relaxation Int. J. Mol. Sci. 2023,24, 5638 3 of 17 times [ 24 ], and the effect of calcium carbonate as filler [ 25 ] have been determined. This large background of data facilitates any comparative study. The partial characterization of the system in the presence of AuNPs, which is closely related to the present contribution, has also been published [26–28]. 2. Results and Discussion Calorimetric scans were performed by DSC, operating in a. dynamic mode between 40 and 140 ◦ C at a heating ratio of 10 ◦ C/min, to determine the glass transition temperature (T g ) of resins prepared with different amounts of AuNPs. Figure 1shows the dependence of T g with the amount of AuNPs (T g ( ◦ C) = (121.4 ± 4.1) − (6.08 ± 0.79) × [AuNP, %]; r2= 0.9508 ); it remains constant above 8.5% AuNPs, the average value being 69.95 ±0.14 ◦C . Most remaining experiments were carried out at this concentration of 8.5% AuNPs. Int. J. Mol. Sci. 2023, 24, x FOR PEER REVIEW 3 of 18 decades, a comparison between both systems is feasible. For this last system, activation energies [20,21], optimum temperature range for the use of this material [22], transition glass temperature and the variation of the specific heat capacity [23], relaxation times [24], and the effect of calcium carbonate as filler [25] have been determined. This large background of data facilitates any comparative study. The partial characterization of the system in the presence of AuNPs, which is closely related to the present contribution, has also been published [26–28]. 2. Results and Discussion Calorimetric scans were performed by DSC, operating in a. dynamic mode between 40 and 140 °C at a heating ratio of 10 °C/min, to determine the glass transition temperature (Tg) of resins prepared with different amounts of AuNPs. Figure 1 shows the dependence of Tg with the amount of AuNPs (Tg(°C) = (121.4 ± 4.1) − (6.08 ± 0.79) × [AuNP, %]; r2 = 0.9508); it remains constant above 8.5% AuNPs, the average value being 69.95 ± 0.14 °C. Most remaining experiments were carried out at this concentration of 8.5% AuNPs. Figure 1. Dependence of the glass transition temperature (Tg) of the DGEBA/mXDA/AuNP nanocomposite with the amount of AuNPs. Often, an increase in Tg of epoxy resins has been observed when increasing the amount of added NPs [29–31]. It has been ascribed to the adsorption of polymers onto the particle’s surface, which reduces the polymer’s mobility and modifies the conformation of chain segments. Less common is the observation of a reduction in Tg. This is the case of an epoxy resin with various amounts of carbon black or carbon nanofibers, silver nanoparticles [32], and epoxy/alumina nanocomposites [33]. Complex behavior has been observed with the addition of CNT on CNT composites [34]. Liu et al. [33] speculated that the reduction in Tg is due to an uneven distribution of hardener molecules, thus leading to a stoichiometric imbalance between epoxy components. This is not the case for the present composite, as we have observed (Figure 2) that AuNPs are uniformly distributed in the epoxy resin. Dorigato et al. [32] have proposed that, simultaneously with the adsorption of polymers onto the particle surface, polymer–filler chemical interactions and the increment in viscosity reduce the crosslinking degree of the epoxy-net and drop Tg. However, according to Tsagaropoulos and Eisenberg [35], the addition of nanoparticles leads to the formation of tightly bound and loosely bound polymer chains around the Figure 1. Dependence of the glass transition temperature (T g ) of the DGEBA/mXDA/AuNP nanocomposite with the amount of AuNPs. Often, an increase in T g of epoxy resins has been observed when increasing the amount of added NPs [ 29 – 31 ]. It has been ascribed to the adsorption of polymers onto the particle’s surface, which reduces the polymer’s mobility and modifies the conformation of chain segments. Less common is the observation of a reduction in T g . This is the case of an epoxy resin with various amounts of carbon black or carbon nanofibers, silver nanoparticles [ 32 ], and epoxy/alumina nanocomposites [ 33 ]. Complex behavior has been observed with the addition of CNT on CNT composites [ 34 ]. Liu et al. [ 33 ] speculated that the reduction in T g is due to an uneven distribution of hardener molecules, thus leading to a stoichiometric imbalance between epoxy components. This is not the case for the present composite, as we have observed (Figure 2) that AuNPs are uniformly distributed in the epoxy resin. Dorigato et al. [32] have proposed that, simultaneously with the adsorption of polymers onto the particle surface, polymer–filler chemical interactions and the increment in viscosity reduce the crosslinking degree of the epoxy-net and drop Tg. Int. J. Mol. Sci. 2023,24, 5638 4 of 17 Int. J. Mol. Sci. 2023, 24, x FOR PEER REVIEW 4 of 18 filler particles. At very low filler concentrations, with the average interparticle distance being large, the percentage of the resin network close to the nanoparticles will also be low, and most of the resin will remain in the absence of the filler. Consequently, the T g value will be close to the unmodified resin. However, at large filler concentrations, the average interparticle distance is low, most of the resin network is tightly bound to nanoparticles, and most of the resin is in a continuous phase of restricted mobility, which will exhibit its own Tg value. The incorporation of more filler particles decreases the average interparticle distance, and loosely bound regions are gradually transformed into tightly bound ones. In other words, the mobility restrictions become progressively severe. At an intermediate concentration of the filler, the average interparticle distance will correspond to a “critical” value. The behavior shown in Figure 1 is consistent with this qualitative analysis, and above 8.5%, the further addition of AuNPs will not modify the status of the resin as the full network is already tightly bound to nanoparticles. Transmission electron microscopy (TEM) images show a fine distribution of the AuNPs in the epoxy composite (Figure 2), the size of the individual particles being less than 50 nm. They are mostly spherical in shape, which is expected from the reduction process followed [36]. Some agglomerates are also present, possibly formed during the preparation of the nanocomposite as a consequence of the attraction of the nanoparticles due to their high surface energy, as they do not have any surface treatment for helping the dispersion maintenance. Figure 2. TEM image of the composite DGEBA (n = 0)/mXDA/AuNP (8.5%). White circles: Examples of isolated AuNPs. Figure 2. TEM image of the composite DGEBA (n= 0)/mXDA/AuNP (8.5%). White circles: Examples of isolated AuNPs. However, according to Tsagaropoulos and Eisenberg [ 35 ], the addition of nanoparticles leads to the formation of tightly bound and loosely bound polymer chains around the filler particles. At very low filler concentrations, with the average interparticle distance being large, the percentage of the resin network close to the nanoparticles will also be low, and most of the resin will remain in the absence of the filler. Consequently, the T g value will be close to the unmodified resin. However, at large filler concentrations, the average interparticle distance is low, most of the resin network is tightly bound to nanoparticles, and most of the resin is in a continuous phase of restricted mobility, which will exhibit its own Tg value. The incorporation of more filler particles decreases the average interparticle distance, and loosely bound regions are gradually transformed into tightly bound ones. In other words, the mobility restrictions become progressively severe. At an intermediate concentration of the filler, the average interparticle distance will correspond to a “critical” value. The behavior shown in Figure 1is consistent with this qualitative analysis, and above 8.5%, the further addition of AuNPs will not modify the status of the resin as the full network is already tightly bound to nanoparticles. Transmission electron microscopy (TEM) images show a fine distribution of the AuNPs in the epoxy composite (Figure 2), the size of the individual particles being less than 50 nm. They are mostly spherical in shape, which is expected from the reduction process followed [ 36 ]. Some agglomerates are also present, possibly formed during the preparation of the nanocomposite as a consequence of the attraction of the nanoparticles due to their Int. J. Mol. Sci. 2023,24, 5638 5 of 17 high surface energy, as they do not have any surface treatment for helping the dispersion maintenance. The dynamic regime (10 ◦ C/min from − 30 ◦ C to 250 ◦ C) was used to determine the total enthalpy of curing ( ∆ H T ), the obtained value being ∆ H o = − 483 ± 5 J g −1 (Figure 3). This value is significantly lower than those reported by Paz et al. (= − 555 J/g) [ 37 ] and Fraga et al. [ 27 ] (= − 612 ± 6 J g −1 ) for the DGEBA/m-XDA system (without AuNPs). The curing reaction of the system with AuNPs is less exothermic, suggesting that nanoparticles can cause steric hindrances that reduce crosslinking, in agreement with the observed drop in Tg. Int. J. Mol. Sci. 2023, 24, x FOR PEER REVIEW 5 of 18 The dynamic regime (10 °C/min from −30 °C to 250 °C) was used to determine the total enthalpy of curing (ΔHT), the obtained value being ΔHo = −483 ± 5 J g−1 (Figure 3). This value is significantly lower than those reported by Paz et al. (=−555 J/g) [37] and Fraga et al. [27] (=−612 ± 6 J g−1) for the DGEBA/m-XDA system (without AuNPs). The curing reaction of the system with AuNPs is less exothermic, suggesting that nanoparticles can cause steric hindrances that reduce crosslinking, in agreement with the observed drop in Tg. Figure 3. DSC scans of the DGEBA/mXDA (blue line, 111.28 °C [27]) and DGEBA/mXDA/AuNP systems (117,46 °C. black line). 2.1. Kinetic Analysis Calorimetric tests were carried out in the isothermal mode from 40 to 100 ° C in intervals of 10 °C, allowing for the measurement of partial enthalpies and the determination of the degree of conversion α of this epoxy system at each temperature, according to Equation (1): 𝛼=∆𝐻 ∆𝐻 (1) where ΔHt is the partial area under a DSC curve up to time t at the curing temperature T, and ΔHo is the total heat evolved during the curing reaction. The results are shown in Table 1. Table 1. Enthalpy of the reaction, ΔH(T), obtained by the isothermal mode and conversion degree, α, at different curing temperatures. Temperature (°C) −ΔH (J/g) α 40 334 ± 67 0.69 ± 0.14 50 360 ± 61 0.75 ± 0.12 60 412 ± 49 0.85 ± 0.10 70 422 ± 48 0.88 ± 0.10 80 458 ± 24 0.95 ± 0.06 90 452 ± 25 0.94 ± 0.06 100 451 ± 25 0.93 ± 0.06 It can be observed that α increases with T until 80 °C, and it remains constant above it, within experimental error. The maximum conversion observed (α = 0.95) is in -50 0 50 100 150 200 250 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 90.39ºC 481.2 J/g 84.52ºC 612.1 J/g 117.46ºC 111.28ºC Temperature ºC Heat Flow W/g Exo Figure 3. DSC scans of the DGEBA/mXDA (blue line, 111.28 ◦ C [ 27 ]) and DGEBA/mXDA/AuNP systems (117,46 ◦C. black line). 2.1. Kinetic Analysis Calorimetric tests were carried out in the isothermal mode from 40 to 100 ◦ C in intervals of 10 ◦ C, allowing for the measurement of partial enthalpies and the determination of the degree of conversion α of this epoxy system at each temperature, according to Equation (1): α=∆Ht ∆Ho(1) where ∆ H t is the partial area under a DSC curve up to time tat the curing temperature T, and ∆ H o is the total heat evolved during the curing reaction. The results are shown in Table 1. Table 1. Enthalpy of the reaction, ∆ H(T), obtained by the isothermal mode and conversion degree, α , at different curing temperatures. Temperature (◦C) −∆H(J/g) α 40 334 ±67 0.69 ±0.14 50 360 ±61 0.75 ±0.12 60 412 ±49 0.85 ±0.10 70 422 ±48 0.88 ±0.10 80 458 ±24 0.95 ±0.06 90 452 ±25 0.94 ±0.06 100 451 ±25 0.93 ±0.06 Int. J. Mol. Sci. 2023,24, 5638 6 of 17 It can be observed that α increases with Tuntil 80 ◦ C, and it remains constant above it, within experimental error. The maximum conversion observed ( α = 0.95) is in agreement with the observations made by Paz et al. [ 37 ] for this system without AuNPs. Simultaneously, the reaction rates, d α /dt, were measured. Figure 4shows an example of the dependence of dα/dt with αat 40 ◦C. Similar behavior is observed at other curing temperatures. Int. J. Mol. Sci. 2023, 24, x FOR PEER REVIEW 6 of 18 agreement with the observations made by Paz et al. [37] for this system without AuNPs. Simultaneously, the reaction rates, dα/dt, were measured. Figure 4 shows an example of the dependence of dα/dt with α at 40 °C. Similar behavior is observed at other curing temperatures. Figure 4. Experimental conversion rate (dα/dt) vs. conversion (α) at 40 °C of the curing reaction of the composite DGEBA/mXDA/AuNP, 8.5%. Fittings according to Kamal’s model (Equation (2)), with and without diffusion corrections, are also shown. Previous results were analyzed according to the semiempirical Kamal’s model (Equation (2)) with and without diffusion correction. 𝑑𝛼 𝑑𝑡= 󰇛𝑘  𝑘  𝛼  󰇜󰇛1𝛼󰇜  𝐹󰇛𝛼󰇜 (2) In this equation, k 1 and k 2 are kinetic constants, and m and n are the reaction orders corresponding to the autocatalytic and n-th order mechanisms, respectively. F(α) is equal to 1 when the diffusion correction is not required; otherwise, it is given by Equation (3) [38]. 𝐹󰇛𝛼󰇜=󰇟1𝑒 󰇛󰇜 󰇠  (3) Here, A 1 (diffusion coefficient) and α c (critical conversion) are empirical parameters that depend on the curing temperature. For low values of α, the effect of diffusion is negligible, 𝐹󰇛𝛼󰇜1, and the reaction is kinetically controlled. When α approaches α c and beyond that point, F(α) decreases, and the reaction becomes diffusion-controlled. All previous parameters were determined by the non-linear fitting of experimental data. The obtained values are shown in Table 2. Table 2. Kinetic constants, reaction orders for the autocatalytic and n-th order mechanisms, and values for A 1 and α c , according to Equation (3), at different curing temperatures. Parameter Temperature (°C) 40 50 60 70 80 k 1 /(10 −4 s −1 ) 0.094 ± 0.02 0.50 ± 0.03 1.90 ± 0.01 3.10 ± 0.02 8.90 ± 0.04 k 2 /(10 −4 s −1 ) 3.80 ± 0.10 9.10 ± 0.13 18.7 ± 0.09 30.4 ± 0.07 63.2 ± 0.23 m 1.25 1.20 1.50 1.40 1.53 n 1.72 1.78 1.51 1.59 1.49 m + n 2.97 2.98 3.01 2.99 3.02 Figure 4. Experimental conversion rate (d α /dt) vs. conversion ( α ) at 40 ◦ C of the curing reaction of the composite DGEBA/mXDA/AuNP, 8.5%. Fittings according to Kamal’s model (Equation (2)), with and without diffusion corrections, are also shown. Previous results were analyzed according to the semiempirical Kamal’s model (Equation (2)) with and without diffusion correction. dα dt = (k1+k2αm)(1−α)nF(α)(2) In this equation, k 1 and k 2 are kinetic constants, and mand nare the reaction orders corresponding to the autocatalytic and n-th order mechanisms, respectively. F( α ) is equal to 1 when the diffusion correction is not required; otherwise, it is given by Equation (3) [38]. F(α)= [1+eA1(α−αc)]−1(3) Here, A 1 (diffusion coefficient) and αc (critical conversion) are empirical parameters that depend on the curing temperature. For low values of α , the effect of diffusion is negligible, F(α)≈ 1, and the reaction is kinetically controlled. When α approaches αc and beyond that point, F(α) decreases, and the reaction becomes diffusion-controlled. All previous parameters were determined by the non-linear fitting of experimental data. The obtained values are shown in Table 2. Table 2. Kinetic constants, reaction orders for the autocatalytic and n-th order mechanisms, and values for A1and αc, according to Equation (3), at different curing temperatures. Parameter Temperature (◦C) 40 50 60 70 80 k 1 /(10 −4 s −1 ) 0.094 ±0.02 0.50 ±0.03 1.90 ±0.01 3.10 ±0.02 8.90 ±0.04 k 2 /(10 −4 s −1 ) 3.80 ±0.10 9.10 ±0.13 18.7 ±0.09 30.4 ±0.07 63.2 ±0.23 m1.25 1.20 1.50 1.40 1.53 n1.72 1.78 1.51 1.59 1.49 m+n 2.97 2.98 3.01 2.99 3.02 A147.34 42.24 48.95 59.77 83.00 αc0.610 0.649 0.782 0.825 0.926 Int. J. Mol. Sci. 2023,24, 5638 7 of 17 It can be noticed that the experimental data conform to the proposed model. The critical conversion values are always above 60%, and they are comparable with those observed for the system without AuNPs for which values between 0.716 (50 ◦ C) and 0.978 (110 ◦ C) have been published [ 39 ]. A value of αc close to one means that the diffusion control only becomes important at the end of the crosslinking process. The global order of reaction m + n ∼ = 3 is constant in the range of curing temperatures studied. For this system without AuNPs, Paz et al. [ 37 ] obtained a reasonable correlation for a global order of 3, but the best fitting corresponds to a global order of 2.5 (m = 1 and n= 1.5). From the variation of k 1 and k 2 with temperature activation energies and pre-exponential factors (Arrhenius equation) were obtained. The values are shown in Table 3. For both mechanisms, the activation energies are much lower than those corresponding to the system without AuNPs. They are, however, within the range of published values for resins obtained with DGEBA and other secondary amines [ 40 – 42 ]. The comparison of values in Table 3suggests that the AuNPs hinder the beginning of crosslinking in the n-order mechanism, requiring higher activation energy. Table 3. Activation energies and pre-exponential factors (Arrhenius equation) for the n-th order and autocatalytic mechanisms. System nOrder Mechanism Autocatalytic Mechanism DGEBA/mXDA/AuNP (8.5%) Ea(kJ mol−1)100 ±3 63 ±4 A(s−1) 8.40 ±0.03 (×1011) 1.24 ±0.11 (×107) DGEBA/mXDA [37] Ea(kJ mol−1)61.03 45.14 A(s−1) 7.17 ×1071.68 ×106 2.2. Thermogravimetric Analysis Figure 5shows the mass loss for the DGEBA/mXDA/AuNP (8.5%) system as a function of temperature at different heating rates ( β ). The mass loss curves correspond to a single decomposition profile with well-defined initial and final temperatures, being of the C type [ 43 ]. This behavior is characteristic of epoxy materials in a nitrogen atmosphere [ 20 , 44 ]. In a solid state, these decomposition reactions are slow, being controlled by the rate of heat transfer from the reaction interface. Int. J. Mol. Sci. 2023, 24, x FOR PEER REVIEW 7 of 18 A1 47.34 42.24 48.95 59.77 83.00 αc 0.610 0.649 0.782 0.825 0.926 It can be noticed that the experimental data conform to the proposed model. The critical conversion values are always above 60%, and they are comparable with those observed for the system without AuNPs for which values between 0.716 (50 °C) and 0.978 (110 °C) have been published [39]. A value of αc close to one means that the diffusion control only becomes important at the end of the crosslinking process. The global order of reaction m + n ≅ 3 is constant in the range of curing temperatures studied. For this system without AuNPs, Paz et al. [37] obtained a reasonable correlation for a global order of 3, but the best fitting corresponds to a global order of 2.5 (m = 1 and n = 1.5). From the variation of k1 and k2 with temperature activation energies and pre-expo- nential factors (Arrhenius equation) were obtained. The values are shown in Table 3. For both mechanisms, the activation energies are much lower than those corresponding to the system without AuNPs. They are, however, within the range of published values for resins obtained with DGEBA and other secondary amines [40–42]. The comparison of values in Table 3 suggests that the AuNPs hinder the beginning of crosslinking in the n-order mechanism, requiring higher activation energy. Table 3. Activation energies and pre-exponential factors (Arrhenius equation) for the n-th order and autocatalytic mechanisms. System n Order Mechanism Autocatalytic Mechanism DGEBA/mXDA/AuNP (8.5%) Ea (kJ mol−1) 100 ± 3 63 ± 4 A(s−1) 8.40 ± 0.03 (×1011) 1.24 ± 0.11 (×107) DGEBA/mXDA [37] Ea (kJ mol−1) 61.03 45.14 A (s−1) 7.17 × 107 1.68 × 106 2.2. Thermogravimetric Analysis Figure 5 shows the mass loss for the DGEBA/mXDA/AuNP (8.5%) system as a function of temperature at different heating rates (β). The mass loss curves correspond to a single decomposition profile with well-defined initial and final temperatures, being of the C type [43]. This behavior is characteristic of epoxy materials in a nitrogen atmosphere [20,44]. In a solid state, these decomposition reactions are slow, being controlled by the rate of heat transfer from the reaction interface. Figure 5. Thermogravimetry experiments for the DGEBA/m-XDA/AuNP (8.5%) at different heating rates (β). 0 200 400 600 800 1000 0 20 40 60 80 100 Mass (%) β=15 ºC min − 1 DGEBA /m-XDA DGEBA /m-XDA/AuNP (8.5%) Temperature ºC Figure 5. Thermogravimetry experiments for the DGEBA/m-XDA/AuNP (8.5%) at different heating rates (β). Int. J. Mol. Sci. 2023,24, 5638 8 of 17 At different heating rates, the initial decomposition temperatures (T i , determined at the 5% mass loss point) [ 45 ] are between 329 ◦ C and 364 ◦ C (Table 4), suggesting a high crosslinking density. This is expected behavior for thermoset materials [ 20 , 44 , 46 ]. Table 4also shows the residue after the decomposition process (determined at 850 ◦ C) and the temperature values of the inflection point of the mass loss curves, T m , at which the degradation rate is at a maximum. Such values are slightly higher than those of the pristine system [ 47 ]. However, by taking into account the common error in these determinations, which is around 3–5 K, the difference can be accepted to be within experimental error. Table 4. Degradation parameters at different heating rates of the system DGEBA/mXDA/AuNP (8.5%) nanocomposite. Heating Rate (β)◦C/min DGEBA/m-XDA/AuNP (8.5%) DGEBA/m-XDA [47] Ti◦C (5% Decomposition) % Residue (at 850 ◦C) Tm◦C Tm◦C 5 329.00 18.16 357.9 353.8 10 346.17 17.86 366.0 365.0 15 349.07 14.88 374.6 365.0 20 361.34 14.25 380.9 372.3 25 359.46 14.76 389.1 376.3 30 364.39 11.97 389.4 380.0 The values of T m and β may be used to estimate the activation energy of the thermodegradation reaction. According to Kissinger [48] the slope of the plot of lnβ/T2 mvs. T−1 m provides the value of the activation energy, E a . From the values of Table 4, a value of Ea= 169 kJ mol−1 (r 2 = 0.9683) is obtained. This value is considerably lower than those of 222 kJ mol −1 (r 2 = 0.9759) and 210 kJ mol −1 (r 2 = 0.9841) obtained by [ 47 ] and [ 20 ], respectively, for the system without AuNPs. Alternatively, other methods may be used for the estimation of the activation energy (see ref. [ 20 ] for a quick presentation). Among them, the Flynn–Wall–Ozawa method [ 49 , 50 ] is widely used, as it does not require the knowledge of the reaction order (as Kissinger’s does). Ozawa [ 51 ] has indicated that this method is more precise and simpler than Kissinger’s method. The method uses Doyle’s approximation [ 52 ], and consequently, it is limited to conversions in the interval of 11–26%. Table 5shows the obtained values together with those for the DGEBA/mXDA system [ 20 , 47 ]. Clearly, E a varies with α .Consequently, according to Vyazoykin and Sbirrazzuoli [ 53 ], as the method assumes that the activation energy is constant, systematic errors in E a may be introduced. Even so, all E a values for DGEBA/mXDA/AuNP (8.5%) are lower than for DGEBA/m-XDA. These results are in contrast with observations for other fillers and resins [54,55]. Table 5. Activation energies obtained by the Flynn–Wall–Ozawa method for DGEBA/m-XDA/AuNP (8.5%) and DGEBA/m-XDA [20,47] systems. DGEBA/m-XDA/AuNP (8.5%) DGEBA/m-XDA [47] DGEBA/m-XDA [20] α(%) * Ea(kJ/mol) R2Ea(kJ/mol) α(%) Ea(kJ/mol) 10 151 ±3 0.9897 202.7 5 148.6 13 156 ±2 0.9970 219.3 8 157.0 16 160 ±1 0.9992 229.2 11 188.4 19 160 ±1 0.9990 235.1 14 208.8 22 163 ±2 0.9989 238.7 17 220.9 25 163 ±1 0.9985 241.2 20 229.2 *αvalues for the DGEBA/m-XDA system were within 1% higher than the ones indicated here 2.3. Compression and Flexural Tests Figure 6shows examples of stress vs. strain curves obtained from compression tests for DGEBA (n= 0)/mXDA/AuNP (8.5%) and DGEBA (n= 0)/m-XDA systems. Curves are Int. J. Mol. Sci. 2023,24, 5638 9 of 17 typical of compression tests. At low deformations, an elastic region (linear dependence) is observed, followed by the plastic region and the densification regime. Table 6shows the Young’s modulus determined from the linear region of the stress–strain curves for both systems. The proximity of both values, together with their standard deviation, suggests that the gold nanoparticles do not alter the compression properties. Int. J. Mol. Sci. 2023, 24, x FOR PEER REVIEW 9 of 18 2.3. Compression and Flexural Tests Figure 6 shows examples of stress vs. strain curves obtained from compression tests for DGEBA (n = 0)/mXDA/AuNP (8.5%) and DGEBA (n = 0)/m-XDA systems. Curves are typical of compression tests. At low deformations, an elastic region (linear dependence) is observed, followed by the plastic region and the densification regime. Table 6 shows the Young’s modulus determined from the linear region of the stress–strain curves for both systems. The proximity of both values, together with their standard deviation, suggests that the gold nanoparticles do not alter the compression properties. Figure 6. Stress–strain curves from compression tests for DGEBA (n = 0)/mXDA /AuNP (8.5%) (□) and DGEBA (n = 0)/mXDA (■). Table 6. Young’s modulus and bending modules for DGEBA/mXDA/AuNP (8.5%) and DGEBA (n = 0)/mXDA systems. Property Young’s Modulus (MPa) Maximum Elasticity Limit (MPa) Compressive Strength (MPa) Stress at 5% Strain (MPa) Bending Modulus (MPa) DGEBA (n = 0)/m- XDA/AuNP (8.5%) 320 ± 4 60.4± 3.6 86 ± 13 87 ± 6 (2.22 ± 0.19) × 103 DGEBA (n = 0)/m-XDA 307 ± 27 58.7 ± 2.9 85 ± 11 76 ± 5 (2.01 ± 0.18) × 103 The modulus of elasticity in bending was determined by the three-point bending flexural test for both systems. Figure 7 shows typical experimental curves. From the curves, the stress at 5% strain and the bending modulus were determined (Table 6). When the experimental error is considered, the obtained values for both parameters of the two systems were not statistically different, suggesting that AuNPs do not alter bending properties and cannot be considered true reinforcement particles. Other studies involving nanoparticles report improvements in mechanical properties with the addition of small amounts of NPs. According to Zheng et al. [56], nanoparticles tend to occupy holes in the epoxy resin, resulting in a reduction in the total free volume and an increase in the cross-linking density. In the present case, DSC experiments suggest a small reduction in the cross-linking density in the presence of AuNPs, and consequently, a reinforcement of mechanical properties was not observed. < < < <<<<<<<<<<< 0 20 40 60 80 100 00.1 0.3 0.2 0.4 0.5 Stress (MPa) ΔL/L Figure 6. Stress–strain curves from compression tests for DGEBA (n= 0)/mXDA /AuNP (8.5%) (  ) and DGEBA (n= 0)/mXDA (). Table 6. Young’s modulus and bending modules for DGEBA/mXDA/AuNP (8.5%) and DGEBA (n= 0)/mXDA systems. Property Young’s Modulus (MPa) Maximum Elasticity Limit (MPa) Compressive Strength (MPa) Stress at 5% Strain (MPa) Bending Modulus (MPa) DGEBA (n= 0)/m-XDA/ AuNP (8.5%) 320 ±4 60.4±3.6 86 ±13 87 ±6(2.22 ±0.19) ×103 DGEBA (n= 0)/m-XDA 307 ±27 58.7 ±2.9 85 ±11 76 ±5(2.01 ±0.18) ×103 The modulus of elasticity in bending was determined by the three-point bending flexural test for both systems. Figure 7shows typical experimental curves. From the curves, the stress at 5% strain and the bending modulus were determined (Table 6). When the experimental error is considered, the obtained values for both parameters of the two systems were not statistically different, suggesting that AuNPs do not alter bending properties and cannot be considered true reinforcement particles. Other studies involving nanoparticles report improvements in mechanical properties with the addition of small amounts of NPs. According to Zheng et al. [ 56 ], nanoparticles tend to occupy holes in the epoxy resin, resulting in a reduction in the total free volume and an increase in the cross-linking density. In the present case, DSC experiments suggest a small reduction in the cross-linking density in the presence of AuNPs, and consequently, a reinforcement of mechanical properties was not observed. 2.4. Dynamic Mechanical Analysis Figure 8shows the behavior of the storage modulus, E’, and tan δ of the DGEBA (n= 0)/mXDA/AuNP (8.5%) nanocomposite at various frequency values. Int. J. Mol. Sci. 2023,24, 5638 16 of 17 12. Hussein, M.A.; El-Said, W.A.; Abu-Zied, B.M.; Choi, J.-W. Nanosheet composed of gold nanoparticle/graphene/epoxy resin based on ultrasonic fabrication for flexible dopamine biosensor using surface-enhanced Raman spectroscopy. Nano Converg. 2020 , 7, 15. [CrossRef] 13. Chen, Y.-J.; Chang, W.-H.; Li, C.-Y.; Yi, C.C.; Huang, C.-C.; Lin, C.-H. Direct synthesis of monolayer gold nanoparticles on epoxy based photoresist by photoreduction and application to surface-enhanced Raman sensing. Mater. Des. 2021 ,197, 109211. [CrossRef] 14. Demir, B.; Chan, K.Y.; Yang, D.; Mouritz, A.; Lin, H.; Jia, B.; Lau, K.T.; Walsh, T.R. Epoxy-gold nanoparticle nanocomposites with enhanced thermo-mechanical properties: An integrated modelling and experimental study. Compos. Sci. Technol. 2019 , 174, 106–116. [CrossRef] 15. Dong, J.; Firestone, G.E.; Bochinski, J.R.; Clarke, L.I.; Gorga, R.E. In situ curing of liquid epoxy via goldnanoparticle mediated photothermal heating. Nanotechnology 2017,28, 065601. [CrossRef] [PubMed] 16. Leonardi, A.B.; Puig, J.; Antonacci, J.; Arenas, G.F.; Zucchi, I.A.; Hoppe, C.E.; Reven, L.; Zhu, L.; Toader, V.; Williams, R.J.J. Remote activation by green-light irradiation of shape memory epoxies containing gold nanoparticles. Eur. Polym. J. 2015 ,71, 451–460. [CrossRef] 17. Angelov, V.A.; Batakliev, T.T.; Georgiev, V.F.; Ivanov, E.H.; Kotsilkova, R.K. Preparation and electromagnetic properties of epoxy/organoclay/MWCNT/gold nanocomposites. Bulg. Chem. Commun. 2020,52, 297–299. [CrossRef] 18. Zhang, Y.; Zhang, Q.; Lü, W. One-step synthesis of graphene/Au nanoparticle composite by epoxy resin: Electrocatalytic detection of H2O2and catalytic reduction of 4-nitrophenol. Mater. Res. Express. 2017,4, 105012. [CrossRef] 19. Yagci, Y.; Sangermano, M.; Rizza, G. Synthesis and Characterization of Gold − Epoxy Nanocomposites by Visible Light Photoinduced Electron Transfer and Cationic Polymerization Processes. Macromolecules 2008,41, 7268–7270. [CrossRef] 20. Fraga, F.; Rodríguez Núñez, E. Activation energies for the epoxy system BADGE n = 0/m-XDA obtained using data from thermogravimetric analysis. J. Appl. Polym. Sci. 2001,80, 776–782. [CrossRef] 21. Fraga, F.; Rodríguez Núñez, E. Lifetime predictions for the epoxy system BADGE n=0/m-XDA using kinetic analysis of thermogravimetry curves. J. Appl. Polym. Sci. 2002,83, 1692–1696. [CrossRef] 22. Fraga, F.; Nunez, E.R. Master curves and lifetime prediction for the epoxy system BADGE n = 0/m-XDA by thermogravimetric analysis. J. Appl. Polym. Sci. 2001,82, 461–466. [CrossRef] 23. Fraga, F.; Castro-Díaz, C.; Rodríguez-Núñez, E.; Martínez-Ageitos, J.M. Physical aging for an epoxy network diglycidyl ether of bisphenol A/m-xylylenediamine. Polymer 2003,44, 5779–5784. [CrossRef] 24. Fraga, F.; Castro-Díaz, C.; Rodríguez-Núñez, E.; Martínez-Ageitos, J.M. Kinetic analysis of relaxation process for the epoxy network diglycidyl ether of bisphenol A/m-xylylenediamine. J. Appl. Polym. Sci. 2005,96, 1591–1595. [CrossRef] 25. Fraga, F.; López, M.; Soto, V.H.; Rodríguez-Núñez, E.; Martínez-Ageitos, J.M.; Miragaya, J. Study of the physical aging of the epoxy system BADGE n = 0/m-XDA/CaCO3. J. Appl. Polym. Sci. 2009,113, 2456–2461. [CrossRef] 26. Fraga, F.; Miragaya, J.; Martínez-Ageitos, J.M.; Rodríguez-Núñez, E.; SotoTellini, V.H.; Jiménez Carrillo, L. Estudio calorimétrico de nanocompuestos de sistema epoxídico DGEBA n=0/m-XDA con nanopartículas de oro. Rev. Iberoam. Polím. 2010 ,11, 632–641. 27. Fraga, F.; Rodríguez-Núñez, E.; Martínez, A.; Miragaya, J. Calorimetric Study of Nanocomposites of Epoxy Network DGEBA n=0/m-XDA with Gold Nanoparticles. Int. J. Nanop. 2014,7, 100–111. [CrossRef] 28. Fraga, F.; Torres, C.; Rodríguez Núñez, E.; Martínez-Ageitos, J.M.; Vázquez Barreiro, E.C.; Suárez- Pereiro, F.; Miragaya, J. Characterization of an Epoxy Network with GoldNanoparticles Using Dielectric Analysis. Adv. Polym. Technol. 2018 ,37, 21730. [CrossRef] 29. Wetzel, B.; Haupert, F.; Zhang, M.Q. Epoxy nanocomposites with high mechanical and tribological performance. Compos. Sci Technol. 2003,63, 2055–2067. [CrossRef] 30. Goyanes, S.N.; König, P.G.; Marconi, J.D. Dynamic mechanical analysis of particulate-filled epoxy resin. J. Appl. Polym. Sci. 2003 , 88, 883–892. [CrossRef] 31. Medina, R.; Haupert, F.; Schlarb, A.K. Improvement of tensile properties and toughness of an epoxy resin by nanozirconiumdioxide reinforcement. J. Mater. Sci. 2008,43, 3245–3252. [CrossRef] 32. Dorigato, A.; Giusti, G.; Bondioli, F.; Pegoretti, A. Electrically conductive epoxy nanocomposites containing carbonaceous fillers and in-situ generated silver nanoparticles. Express Polym. Lett. 2013,7, 673–682. [CrossRef] 33. Liu, G.; Zhang, H.; Zhang, D.-J.; Zhang, Z.; An, X.-F.; Yi, X.-S. On depression of glass transition temperature of epoxy nanocomposites. J. Mater. Sci. 2012,47, 6891–6895. [CrossRef] 34. Barrau, S.; Demont, P.; Maraval, C.; Bernes, A.; Lacabanne, C. Glass Transition Temperature Depression at the Percolation Threshold in Carbon Nanotube–Epoxy Resin and Polypyrrole–Epoxy Resin Composites. Macromol. Rapid Commun. 2005 , 26, 390–394. [CrossRef] 35. Tsagaropoulos, G.; Eisenberg, A. Dynamic Mechanical Study of the Factors Affecting the Two Glass Transition Behavior of Filled Polymers. Similarities and Differences with Random Ionomers. Macromolecules. 1995,28, 6067–6077. [CrossRef] 36. Alex, S.; Tiwari, A. Functionalized Gold Nanoparticles: Synthesis, Properties and Applications-A Review. J. Nanosci. Nanotechnol. 2015,15, 1869–1894. [CrossRef] [PubMed] 37. Paz Abuín, S.; Pazos Pellín, M.; Núñez, L. Kinetic effects on the impurities in epoxy-resin/m-xylylenediamine reaction. J. Appl. Polym. Sci. 1990,41, 2155–2167. [CrossRef] Int. J. Mol. Sci. 2023,24, 5638 17 of 17 38. Chern, C.S.; Poehlein, G.W. A kinetic model for curing reactions of epoxides with amines. Polym. Eng. Sci. 1987 ,27, 788–795. [CrossRef] 39. Núñez, L.; Fraga, F.; Núñez, M.R.; Castro, A.; Fraga, L. Effects of Diffusion on the Kinetic Study of the System BADGE n=0/m-Xylylenediamine. J. Appl. Polym. Sci. 1999,74, 2997–3005. [CrossRef] 40. Sourour, S.; Kamal, M.R. Differential scanning calorimetry of epoxy cure: Isothermal cure kinetics. Thermochim. Acta 1976 , 14, 41–59. [CrossRef] 41. Riccardi, C.C.; Adabbo, H.E.; Williams, R.J.J. Curing reaction of epoxy resins with diamines. J. Appl. Polym. Sci. 1984 ,29, 2481–2492. [CrossRef] 42. Fraga, F.; Penas, M.; Castro, C.; Rodríguez-Núñez, E.; Martínez-Ageitos, J.M. Cure kinetics of a diglycidyl ether of bisphenol a epoxy network (n = 0) with isophorone diamine. J. Appl. Polym. Sci. 2007,106, 4169–4173. [CrossRef] 43. Hatakeyama, T.; Quinn, F.X. Thermal Analysis. In Fundamentals Andapplications to Polymer Science; Wiley: Oxford, UK, 1994. 44. Nuñez, L.; Fraga, F.; Nuñez, M.R.; Villanueva, M. Thermogravimetric study of the decomposition process of the system BADGE (n = 0)/1,2 DCH. Polymer 2000,41, 4635–4641. [CrossRef] 45. Haines, P.; Heal, G.; Laye, P.; Price, D.; Warrington, S.; Wilson, R. Principles of Thermal Analysis and Calorimetry; The Royal Society of Chemistry: Cambridge, UK, 2002. 46. Núñez-Regueira, L.; Villanueva, M.; Fraga-Rivas, I. Activation energies for the thermodegradation process of an epoxy-diamine system. J. Therm. Anal. Calorim. 2006,83, 727–733. [CrossRef] 47. Fraga Grueiro, L. Estudio Cinético, Dinamomecánico y Termogravimétrico Del Sistema Epoxidico BADGE(N=0)/m-XDA Mediante Las Técnicas de Análisis Termico: DSC, DMA y TGA. Construcción de Un Diagrama TTT. Ph.D. Thesis, Universidad de Santiago de Compostela, A Coruña, Spain, 2001. 48. Kissinger, H.E. Reaction Kinetics in Differential Thermal Analysis. Anal. Chem. 1957,29, 1702–1706. [CrossRef] 49. Ozawa, T. A new method of analyzing thermogravimetric data. Bull. Chem. Soc. Jpn. 1965,38, 1881–1886. [CrossRef] 50. Flynn, J.H.; Wall, L.A. General treatment of the thermogravimetry of polymers. JRes Natl Bureau Standards, A. 1966 ,70, 487–523. [CrossRef] 51. Ozawa, T. Estimation of activation energy by isoconversion methods. Thermochim. Acta 1992,203, 159–165. [CrossRef] 52. Doyle, C.D. Approximations to the Equation of Thermogravimetric Data. Nature 1965,207, 290–291. [CrossRef] 53. Vyazovkin, S.; Sbirrazzuoli, N. Isoconversional Kinetic Analysis of Thermally Stimulated Processes in Polymers. Macromol. Rapid Commun. 2006,27, 1515–1532. [CrossRef] 54. Zabihi, O.; Ghasemlou, S. Nano-CuO/Epoxy Composites: Thermal Characterization and Thermo-Oxidative Degradation. Int. J. Polym. Anal. Charact. 2012,17, 108–121. [CrossRef] 55. Zabihi, O.; Omrani, A.; Rostami, A. Thermo-Oxidative Degradation Kinetics and Mechanism of the System Epoxy Nanocomposite Reinforced with Nano-Al2O3. J. Therm. Anal. Calorim. 2012,108, 1251–1260. [CrossRef] 56. Zheng, Y.; Zheng, Y.; Ning, R. Effects of Nanoparticles SiO2 on the Performance of Nanocomposites. Mater. Lett. 2003 , 57, 2940–2944. [CrossRef] 57. Menard, K.P.; Menard, N. Dynamic Mechanical Analysis, Encyclopedia of Analytical Chemistry: Applications, Theory and Instrumentation; Wiley & Sons: Hoboken, NJ, USA, 2017. [CrossRef] 58. Chartoff, R.P.; Weissman, P.T.; Sircar, A. The Application of Dynamic Mechanical Methods to Tg Determination in Polymers: An Overview. Assignment of the Glass Transition, ASTM STP 1249; Seyler, R.J., Ed.; American Society for Testing and Materials: Philadelphia, PA, USA, 1994; pp. 88–107. 59. Li, G.; Lee-Sullivan, P.; Thring, R.W. Determination of activation energy for glass transition of an epoxy adhesive using dynamic mechanical analysis. J. Thermal. Anal. Calorim. 2000,60, 377–390. [CrossRef] 60. Fraga, F.; Salgado, T.; Rodríguez Añon, J.A.; Núñez Regueira, L. Determination of physical and structural parameters by DMA and DSC. Application to an epoxidic formulation. J. Therm. Anal. 1994,41, 1543–1550. [CrossRef] 61. Núñez, L.; Fraga, F.; Castro, A.; Fraga, L. Elastic moduli and activation energies for an epoxy/m-XDA system by DMA and DSC. J. Therm. Anal. Calorim. 1998,52, 1013–1022. [CrossRef] 62. Trihotri, M.; Dwivedi, U.K.; Malik, M.M.; Khan, F.H.; Qureshi, M.S. Study of low weight percentage filler on dielectric properties of MWCNT-epoxy nanocomposites. J. Adv. Dilectrics 2016,6, 1650024. [CrossRef] Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.