Magnetic epoxy nanocomposites with superparamagnetic MnFe 2 O 4 nanoparticles

Magnetic epoxy nanocomposites with superparamagnetic MnFe2O4 nanoparticles Jiangnan Huang,1,3 Yonghai Cao,1,2,a Xi Zhang,3 Yutong Li,4 Jiang Guo,2 Suying Wei,3,a Xiangfang Peng,1,a Tong D. Shen,5,a and Zhanhu Guo2,a 1Laboratory of Polymer Processing Engineering of Ministry of Education, South China University of Technology, Guangzhou, Guangdong 510640 China 2Integrated Composites Laboratory (ICL), Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, TN 37996, USA 3Department of Chemistry and Biochemistry Lamar University, Beaumont, TX 77710 USA 4Magnetic Head Operation, Western Digital Corporation, Fremont, CA 94539 USA 5State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Hebei, 066004, China


I. INTRODUCTION
1][22] Especially, superparamagnetism, a unique feature of the magnetic nanocrystals, has been widely investigated because of its applications in ferrofluid technology, 23 magnetically guided site-specific drug delivery 24 and so on.6][27] Then each nanoparticle can be a single magnetic domain and shows supeparamagnetic behavior when the temperature is above the blocking temperature. 8The individual nanoparticles which have large constant magnetic moment and behave as a giant paramagnetic atom, can quickly respond to applied magnetic fields with negligible remanence (residual magnetism) and coercivity (the field required to bring the
AIP Advances 5, 097183 (2015)   magnetization to zero). 8MnFe 2 O 4 nanoparticles, as one of most common spinel ferrites (MFe 2 O 4 , M = Mn, Co, Ni, Cu, Zn Mg or Cd, etc.), are attractive for their potentials as magnetic resonance imaging contrast agents, 28 drug carrier, 29 microwave absorption, 30 water cleaner to remove lead and arsenic, 31 and radar absorbent. 32MnFe 2 O 4 shows excellent superparamagnetic due to its much higher saturation field (89 emu/g under applied magnetic field) 33 than paramagnetic materials such as Fe 3 O 4 (50.31emu/g). 34It is said that MnFe 2 O 4 is typical size-dependent superparamagnetic materials for its blocking temperature increase from 20 to 250 K with increasing mean size of the nanoparticles from 4.4 to 13.5 nm. 33,35,36For its superparamagnetic properties, MnFe 2 O 4 is considered to have potential technological application such as MRI contrast enhancement and magnetically guided drug delivery. 36poxy is a cured end product of epoxy resins, as well as one of the most important engineered polymers.The favorable physicochemical properties, such as high thermal stability, 37 little shrinkage, 38,39 high strength, and toughness, 40 which make epoxy become popular in science and industry areas.It is widely used in the fields of adhesives, 41 anti-corrosion coatings, 42 tissue substitutes, 43 electronics, 44 aerospace. 45and so on.However, epoxy is still lack of some kind of properties, such as low mechanical properties, flammability, magnetism, high permittivity, dielectric properties and so on, if it is supposed to be applied in industry areas.In order to improve the mechanical properties and introduce some new functionalities, for example, electrical conductivity, optical and magnetic properties, a variety of nanoparticles including carbon nanomaterials, [46][47][48][49][50] iron and iron oxide nanoparticles, 51,52 nanoclay 53 and organic nanoparticles 54,55 have been used to modify epoxy.However, it is common that the introduced multifunctional inorganic fillers would decrease the mechanical properties owning to the aggregate of the fillers in polymer matrix, which would lead to poor interfacial adhesion between the fillers and polymer matrix, 47 especially at high loading of fillers (>5.0 wt%), the mechanical properties dramatically dropped due to the aggregation of nanofillers. 41,47,51,56,57The reported tensile properties and application of epoxy nanocomposites were summarized in Table I.It  is found that, carbon nanomaterials, such as graphene, are the most popular nanoparticles to improve the mechanical properties, while iron oxide were used a lot for introducing magnetism properties.As shown in Table I, the nanocomposites show high Young's modulus and tensile strength with low loading of nanofillers (0.7∼4.0 wt%).What is more, the mechanical properties of nanocomposites were enhanced further when the nanofillers were modified by chemicals.That is because the interfacial adhesion was improved after the modification. 58In order to investigate high-quality epoxy nanocomposites, the processing parameters were obtained from the rheological properties.It is common that the shear stress and viscosity of the nanocomposites would increase with increasing the loading of nanoparticles. 57However, the shear stress and viscosity of the epoxy nanocomposites with low nanoparticle loadings such as less than 1.0 wt% were lower than that of pure epoxy.The reduced viscosity is called shear-thinning behavior, which was mainly caused by the break-down of the percolation structure. 59,60n this work, different loading levels of MnFe 2 O 4 nanoparticles were added into the epoxy resin.The rheological behaviors of the uncured pure epoxy and its nanocomposites (liquid phase), including the viscosity at steady state and the thermo-mechanical properties were studied.For all the cured samples (solid phase), the thermal stability and tensile mechanical properties were evaluated.The fracture surfaces of pure epoxy and its nanocomposites were comprehensively studied to explain the strengthening mechanism.The effects of the nanoparticle loading levels on the magnetic properties and dielectric properties of the epoxy nanocomposites were investigated as well.Different models were used to calculate the dielectric constant of the fillers.The magnetic moment was calculated and the effects of the epoxy matrix were disclosed.

A. Materials
The used Epon 862 epoxy resin (bisphenol epoxy) and curing agent EpiCure W were obtained from Miller-Stephenson Chemical Company.The MnFe 2 O 4 nanoparticles (∼8 nm) were provided by Nano-structured and Amorphous Materials, Inc. Dimethylformamide (DMF, 99 %) was purchased from Sigma Aldrich.All the chemicals were used as-received without any further treatment.

B. Preparation of Epoxy Resin Nanocomposites
Pure epoxy resin and its nanocomposites with 1.0, 5.0, 10.0 and 20.0 wt% MnFe 2 O 4 nanoparticles loadings were prepared as follows.First of all, the nanoparticles were immersed into the epoxy resin (the total weight of epoxy resin and curing agent were 40.0 g) without any disturbance overnight to wet the nanoparticles completely.After that, in order to disperse the nanoparticles in the epoxy resin, the mechanically stirrer (Heidolph, RZR 2041) was used to stirred the MnFe 2 O 4 /epoxy resin liquid with a speed of 600 rpm at room temperature for 1 h.The curing agent W was added into the MnFe 2 O 4 /epoxy nanosuspensions to curing, the weight ratio of epoxy resin:curing agent was 100:26.5 as recommended by the company.The solution was then stirred at high-speed (600 rpm) at room temperature for 1 h to mix all the chemical.In order to remove the bubbles in the solution which were got from the high stirred, the MnFe 2 O 4 /epoxy nanosuspensions were stirred at a lower speed (200 rpm) at 70 o C in a water bath for 3∼4 h.Lastly, the nanosuspensions were slowly and carefully poured into the silicone rubber molds and cured at 120 o C in vacuum oven for 5 h, then cooled down naturally to room temperature.

C. Characterizations
Rheological Behaviors of Liquid Epoxy Resin Nanosuspensions: The rheological behaviors of the epoxy resin nanosuspensions were tested by a rheometer (AR 2000ex, TA Instrumental Company) at 25 • C, the shear rate was from 0.1 to 500 rad/s.The testing samples were placed on a cone-and-plate geometry with a truncation of 64 µm and a diameter of 40 mm.The dynamic rheological tests, which was adjusted to be within the linear viscoelastic (LVE) range for these materials, were characterized with a sweeping frequency from 1 to 100 1/s at 1 % strain.The LVE range was determined by the strain-storage modulus (G') curve with a frequency of 1 rad/s at the strain range between 0.01 to 100 %.Before each frequency sweeping, the samples were placed between the cone and plate to equilibrate for about two minutes.
Thermal Characterization of Epoxy Nanocomposites: The differential scanning calorimeter (DSC, TA Instruments Q2000) tests were performed from 0 to 300 o C, the heating rate and nitrogen flow rate are 10 • C/min and 20 mL/min, respectively.The thermal stability of the cured MnFe 2 O 4 /epoxy nanocomposites was tested by a thermogravimetric analysis (TGA, Q-500, TA instruments).The experiments were performed from 30 to 70 • C, the heating rate and nitrogen flow rate were 10 • C/min and 60 mL/min, respectively.
Mechanical Characterization of Cured Epoxy Nanocomposites: The tensile strength and storage modulus of the cured MnFe 2 O 4 /epoxy nanocomposites was investigated by a unidirectional tensile testing machine (ADMET tensile strength testing system 2610) following ASTM standard (D 412-98a, 2002).The crosshead speed was 1.00 mm/min.
The dynamic mechanical analysis (DMA) were performed by an AR 2000ex (TA Instrumental Company) between 30 and 200 o C, the heating rate, constant frequency and strain were 2 • C/min, 1 Hz and 0.05%, respectively.The dimensions of the tested samples were 12 × 3 × 40 mm 3 .
Morphological Characterizations of Epoxy Nanocomposites surfaces: The morphology of the tensile failure surface of the MnFe 2 O 4 /epoxy nanocomposite samples were characterized by a field emission scanning electron microscope (SEM, JEOL JSM-6700F).The samples were coated by a thin gold layer before testing for better imaging.
Magnetic Property Measurements of Cured Epoxy Nanocomposites: The magnetic properties of the MnFe 2 O 4 /epoxy nanocomposites were investigated by Quantum Design in a 9 T physical properties measurement system (PPMS) at room temperature.
Resistivity and Permittivity Tests of Cured MnFe 2 O 4 /Epoxy Nanocomposites: The volume resistivity was measured by agilent 4339B high resistance meter, whose up-limit of resistivity measurement is 10 16 Ω.The diameter of testing samples was approximately 60 mm.The reported values represent the mean value of eight results with a deviation less than 10 %.

A. Rheological Behaviors of Epoxy Resin Nanosuspensions
The rheological behaviors were tested for pure epoxy resin and its nanosuspensions with different MnFe 2 O 4 nanoparticle loadings.As shown in Fig. 1(A), the shear stress increases with increasing the shear rate.However, the viscosity decreases with increasing the shear rate (Fig. 1(B)), mainly due to the common shear-thinning phenomenon in the pseudoplastic fluid. 57The power law model is employed to correlate the shear rate and shear stress, Eq (1) 63 : where σ is shear stress, K represents flow consistency index, r is shear rate, and n is flow behavior index.The Newtonian fluid is a kind of perfect fluid with n being equal to 1.For the pseudoplastic fluid, n is less than 1.The value of n is summarized in Table II.For all the samples, the value of n is less than 1, indicating that all the samples are pseudooplastic fluids, 59 which fitted the above shear-thinning phenomenon result.However, compared with pure epoxy, the value of n for the MnFe 2 O 4 /epoxy nanosuspensions is almost the same, indicating that the MnFe 2 O 4 nanoparticles have little influence on rheological behaviors of the nanosuspensions.The viscosity of the polymer nanosuspensions with 10.0 and 20.0 wt% MnFe 2 O 4 nanoparticles is higher than that of pure epoxy resin, which is probably caused by the nanoparticle agglomeration. 34However, the viscosity of the polymer nanosuspensions with 1.0 and 5.0 wt% MnFe 2 O 4 nanoparticles is lower than that of pure epoxy, which is also observed when the C 60 fullerenes was dispersed into polystyrene. 60It may be because that the degree of the nanoparticle dispersion was enhanced. 60,64The role of well dispersed nanoparticles behave as a solvent, which can cause a decreased viscosity. 65

B. Thermogravimetric Analysis of Cured Epoxy Resin and its Nanocomposites
The thermal decomposition curves of the cured pure epoxy and its MnFe 2 O 4 nanocomposites are shown in Fig. 2. For the pure epoxy, two degradation stages are observed during the decomposition.The sharp weight loss from 300 to 450 o C is due to the broken down chain (-CH 2 -CH(OH)-) in the polymer network structure. 57,66The second major weight loss occurred between 500 to 600 o C, which is caused by the degradation of benzene rings with high C-C bonding energy (614 KJ mol −1 of EC=C and 698 KJ mol −1 of 2EC-C). 67However, for the epoxy nanocomposites, three weight loss stages were observed, Figure 3

C. Differential Scanning Calorimetry of Cured Epoxy Resin and its Nanocomposites
For the thermosetting polymers, the gelation, curing, vitrification and devitrification events can be investigated by DSC test. 71Generally speaking, the gelation and curing could be observed in the uncured samples.What is more, the exothermal peak represents the uncured samples.If the polymer is cured well, no exothermal peak would be observed.As shown in Fig. 3, no obvious exothermal peak is observed in all the samples, indicating that all the samples have been well cured.In addition, compared with cured pure epoxy, the glass transition temperature (T g ) of the epoxy nanocomposites decreased with increasing the loadings of MnFe 2 O 4 nanoparticles (5.0 ∼20.0 wt%), which is attributed to the enlarged free volume, arising from the large interface between the nanoparticles and epoxy at high nanoparticle loadings. 72The large interface would provide more space for the epoxy chain segments to move even at a lower temperature.However, compared with pure epoxy, there is a tiny increase in T g when the loading MnFe 2 O 4 nanoparticles is 1.0 wt%.It is said that  the nanoparticles could restrain the movement of the polymer chains, which would lead to a tiny enhancement of T g . 61If the low loading of the MnFe 2 O 4 nanoparticles was well distributed in epoxy resin, the restrictive behavior of the nanoparticles plays a leading role in influencing the movement of polymer chains, which lead to higher T g .

D. Dynamic Mechanical Properties of Cured Epoxy Resin and Its Nanocomposites
Dynamic mechanical analysis (DMA) measurement demonstrates the information of the storage modulus (G ′ ), loss modulus (G ′′ ), and loss factor (tan δ) of the samples in the testing temperature ranges.The data of storage modulus (G ′ ), which represents the elastic behavior or the energy storage in the materials, is the real part of the complex modulus, while the loss modulus (G ′′ ), which reflects the viscous behavior or the energy dissipation in the materials, is the imaginary part of the complex modulus. 73Fig. 4(A) and 4(B) show the G ′ and G ′′ vs. temperature of the cured pure epoxy resin and its nanocomposites with different MnFe 2 O 4 nanoparticle loading levels.During the low temperature (below 80 o C) (shown in Fig. 4(A)&4(B)), the polymer chains are "frozen" and the polymer cannot move, so the values of G ′ and G ′′ are almost the same for all the samples.It is found that the glass-transition temperature of polymer decreases with increasing the MnFe 2 O 4 nanoparticle loading, except 1.0 wt% naonocomposites.The reduce of the nanocomposites with high nanofiller loadings, especially for the nanocomposites with 20.0 wt% MnFe 2 O 4 loading, is mainly caused by the large interface between the nanoparticles and epoxy loaded nanoparticles, which was enlarge by the aggregation of nanofillers.As a result, the polymer chains become much easier to move.However, the a little improvement of T g in the epoxy nanocomposites with 1.0 wt% MnFe 2 O 4 loading, is attributed to the well dispersed MnFe 2 O 4 nanoparticles in the epoxy resin, which restricts the mobility of the main chains of the polymer. 46The similar effect of nanoparticles loadings on the stability of nanocomposites was also observed in the DSC section in Fig. 3 and Table III.
The loss factor (tan δ) is the ratio of the lost energy to total vibrational energy during a vibrational time in a cycle.Briefly, tan δ is the ratio of the G ′′ to the G ′ value.The peak of tan δ is often used to determine the glass transition temperature (T g ).As shown in Fig. 4(C), compared with that of pure epoxy, the T g of the epoxy nanocomposites decreased with increasing the MnFe 2 O 4 nanoparticle loading, except 1.0 wt% MnFe 2 O 4 /epoxy nanocomposites.The T g of most of the nanocomposites shifts towards a lower temperature as compared with that of pure epoxy.The explanation which was discussed before, is that the aggregate nanoparticles enlarge the interface between the nanoparticles and epoxy, which makes the polymer chains easier to move and then obtain a lower T g . 72While the nanoparticles are well dispersed in low loading (1.0 wt%), the restrictive behavior of the nanoparticle structure plays a leading role in influencing the T g , 46 which is consistent with the DSC test.

E. Tensile Mechanical Property and Fracture Surface Analysis of Cured Nanocomposites
The tensile stress vs tensile strain curves of the cured epoxy and its nanocomposites with loaded MnFe 2 O 4 are shown in Fig. 5. Compared to the pure cured epoxy, the tensile stress of most epoxy nanocomposites is a little higher, which indicated that the additive of MnFe 2 O 4 nanoparticles or the polymer deformation between the MnFe 2 O 4 nanoparticles, which is similar to the results of vinyl ester nanocomposites, which were reinforced by alumina nanoparticles. 74While the tensile strain of the epoxy nanocomposites decreases with sequential increasing the MnFe 2 O 4 loadings.This is mainly caused by the agglomeration of MnFe 2 O 4 nanoparticles owing to the high mass ratio, which lead to the poor interaction between the nanoparticles and epoxy matrix. 57The agglomeration of MnFe 2 O 4 can be easily found in the fracture surfaces of the nanocomposites with 5, 10 and 20.0 wt% (Fig. 7(c) to 7(e)).The variation of storage modulus with MnFe 2 O 4 nanoparticles loading is shown in Fig. 6.The storage modulus increase gradually from 1.81 for the pure epoxy to 2.72 GPa for the nanocomposites with 20 wt% nanoparticles loading.The results of tensile stress, tensile strain and storage indicate that the stiffness of MnFe 2 O 4 /epoxy nanocomposites was improved but the toughness was reduced by the MnFe 2 O 4 nanocomposites, which were also observed in Fe@FeO /epoxy nanocomposites. 51What is more, the surface morphology of all samples in Fig. 7 verify that the experimental results of DSC, TGA and tensile strain, which were discussed previously, are all caused by the dispersion levels of MnFe 2 O 4 nanoparticles in the epoxy matrix.

F. Magnetic Property of the Cured Nanocomposites
Magnetization describes the response of magnetic materials to an applied external magnetic field. 75The saturation magnetization (M s ), which represents the maximum obtained magnetization of materials with the infinite applied external magnetic field, can be estimated by the extrapolated saturation magnetization, and is obtained from the intercept of M-H −1 at high field. 76,77The coercivity (H c , Oe) is the external applied magnetic field, which is the opposite to the original external applied magnetic field, and used for returning the external applied magnetic field magnetization of materials to zero, but the residual magnetization (M r ) will not be reduced to zero.The magnetic moment (m) is the moment of force, which is caused by magnetization.The magnetic properties can be described by the following Langevin Eq (2) 75 : where M represents magnetization (emu/g) in H (Oe), k B represents the Boltzmann constant, m represents the magnetic moment, T is the absolute temperature, and f (m) is the distribution function of magnetic moments, which is related to the M s and is described by Eq (3) 75 : According to Eq (2) and ( 3), the relationship between M and M s can be described by Eq (4) 75 : where x = aH, and a is related to the electron spin magnetic moment m of the individual molecule as described in Eq (5) 75 :  79 It is said that the magnetism which is determined by the dimensionality of the hydrogen bonding, was equally switchable, and the hydrogen bonding networks would cause the magnetic switching even at low temperature (300 K). 79

G. Dielectric permittivity
Epoxy can be used in the electronic device due to its excellent dielectric properties with a low and stable dielectric constant within a wide range of frequencies. 56So the dielectric properties of the MnFe 2 O 4 /epoxy nanocomposites were studied here.The real permittivity is a constant for certain materials, which represents the capacity of the stroging static electricity in the external electric field, while the imaginary permittivity (ε ′′ ) represents the attenuation of electromagnetic wave.The real permittivity (ε ′ ), imaginary permittivity (ε ′′ ) and dielectric loss (tan δ = ε ′′ /ε ′ ) as a function of the frequency (10 2 to 10 6 H z ) at room temperature for the cured epoxy and its nanocomposites with different MnFe 2 O 4 loadings were shown in Fig. 9.It is found that the ε ′ , ε ′′ and tanδ values of the epoxy nanocomposites show first a high value at 100 Hz and decrease with increasing the frequency, and then keep almost constant in high frequency range (above 1.5 × 10 6 H z ).For the cured pure epoxy, the ε ′ , Fig. 9(A), is observed to be almost constant around 4.25, indicating a stable dielectric performance of the cured epoxy upon frequency variation. 80The ε ′ increases with increasing the nanoparticles loading.The interfacial polarization of the fillers would cause the enhancement of real permittivity, 81 which was caused by the charge carriers blocked at the internal surface or interfaces between matrix and fillers. 82Similar phenomenon was also observed in the polypropylene-graphene nanoplatelet nanocomposite system. 83ig. 9(B) and 9(C) show the ε ′′ and tanδ of pure epoxy and its MnFe 2 O 4 nanocomposites with a loading of 1.0, 5.0, 10.0 and 20.0 wt%.The ε ′′ and tanδ of nanocomposites are higher than the pure epoxy, what is more, both values (ε ′′ and tanδ) of nanocomposites are enhanced with increasing the MnFe 2 O 4 loading, just as the increasing trend of ε ′ between pure epoxy and its MnFe 2 O 4 nanocomposites.This phenomenon means that the dielectric loss was enhanced with the MnFe 2 O 4 increasing loading, which is associated with the free charge motion difference, 84 indicating an interfacial polarization formed in the nanocomposites. 57he intrinsic permittivity of the MnFe 2 O 4 nanofillers were calculated by the following Bruggeman's equation ( 6) 85,86 : permittivity of MnFe 2 O 4 shown in Fig. 9(D), is almost constant at ∼7.9 in the whole measured frequency range, which is much higher than the pure epoxy (∼ 4.25).The higher real permittivity ε ′ of MnFe 2 O 4 than pure epoxy means that the MnFe 2 O 4 nanoparticles have better insulating property than pure epoxy materials.

IV. CONCLUSIONS
The polymer nanocomposites with MnFe 2 O 4 nanoparticles have been prepared and systematically studied.The rheological tests of the liquid epoxy nanosuspension systems show a tendency toward pseudoplastic behavior with increasing the MnFe 2 O 4 nanoparticle loading.The lower value of n in the epoxy nanosuspension indicates that the nanoparticles favor the pseudoplastic nature of the nanosuspensions.The MnFe 2 O 4 nanoparticles have a negative effect on the thermal stability of the polymer.No obvious exothermal peak found in all the samples indicates a well cured nanocomposite system.However, the observed decreased T g of the epoxy nanocomposites at high MnFe 2 O 4 loadings is mainly due to the large interface between the nanoparticles and epoxy.The tensile strength tests showed that 1.0 wt% MnFe 2 O 4 /epoxy owned a slightly higher tensile strength than that of pure epoxy, but it decreased sharply when the loading was increased, which might be due to the agglomeration of MnFe 2 O 4 nanoparticles.The storage modulus increase from 1.81 to 2.72 GPa with the increasing of nanoparticles loadings.There is no obvious agglomeration of nanoparticles in the 1.0 wt% MnFe 2 O 4 /epoxy sample, but serious agglomeration in high loading of MnFe 2 O 4 .Finally, the magnetic epoxy nanocomposites were achieved from the 5.0 and 10.0 wt% MnFe 2 O 4 nanoparticles.The coercivity of both samples with 5.0 and 10.0 wt% MnFe 2 O 4 loadings was almost the same, but compared to the pure MnFe 2 O 4 nanoparticles (H c = 14.94 O e ), the coercivity of two nanocomposites (H c = 44.70,43.90 O e ), was enhanced greatly.Also, the big difference in the saturation magnetization (M s ) of MnFe 2 O 4 nanoparticles and nanocomposites was observed.The higher loading of MnFe 2 O 4 had larger saturation magnetization.The magnetic moment (m) of the nanocomposites is a little higher than the m of MnFe 2 O 4 nanoparticles, which might be the impact of hydrogen bonding networks in the epoxy nanocomposites.Finally, the real permittivity increased with increasing the MnFe 2 O 4 nanoparticles loading.The calculated intrinsic permittivity of MnFe 2 O 4 was about 7.9 in the whole measured frequency range, which is much higher than the pure epoxy (4.25) at the same frequency.
(B).The tiny additional weight loss stage from 410 to 510 o C is caused by the degradation of another benzene rings of epoxy with the lower C-C bonding energy (566 KJ • mol −1 ) than the former one.68With increasing the loading of MnFe 2 O 4 nanoparticles, all peaks of the degradation stages become more and more apparent, indicating that the MnFe 2 O 4 nanoparticles decreased the stability of -CH 2 -CH(OH)-chain structure and C-C bonding with the energy of 614, 698 and 566 KJ mol −1 .This phenomenon can be also explained by the lower onset decomposition temperatures of epoxy nanocomposites.As shown in

097183 - 8 Huang 9 FIG. 5 .
FIG. 4. (A) Storage modulus (G ′ ), (B) loss modulus (G ′′ ) and (C) tanδ vs. temperature curves for (a) cured pure epoxy and its nanocomposites with the MnFe 2 O 4 loadings of (b) 1.0 wt%, (c) 5.0 wt%, (d) 10.0 wt% and (e) 20.0 wt%.enhanced the stiffness of materials.However, the tensile strain increases first and then decreases with increasing the nanoparticles loading from 1.0 to 20.0 wt% than that of pure cured epoxy.The slightly higher tensile strain in 1.0 wt% MnFe 2 O 4 /epoxy nanocomposites (83.2 MPa) than that of pure epoxy (79.8 MPa) was mainly due to the well disperse of nanoparticles in the epoxy matrix.The cured pure epoxy shows a smooth fracture surface (Fig.7(a)), while the 1.0 wt% MnFe 2 O 4 epoxy nanocomposites show a rough fracture surface without any aggregated MnFe 2 O 4 nanoparticles (Fig. 7(b)).The rough fracture surface is mainly caused by the matrix shear yielding

Fig. 8
Fig. 8 shows the room temperature hysteresis loops of pure MnFe 2 O 4 nanoparticles and its epoxy nanocomposites with 5.0 and 10.0 wt% MnFe 2 O 4 loadings, where the values of coercivity (H c ) and magnetization (M) can be obtained.The H c of pure MnFe 2 O 4 is about 14.96 Oe.After the MnFe 2 O 4nanoparticles are dispersed in the polymer matrix, the H c was enhanced greatly.For example, the H c of the epoxy nanocomposites with 5.0 and 10.0 wt% MnFe 2 O 4 loadings is almost the same, 44.7 and 43.9 Oe, respectively.The slightly reduced H c of the nanocomposites with 10.0 wt% MnFe 2 O 4 may be due to the increased inter-particle dipolar interaction.34It is found that the magnetization of all the samples could not reach saturation even at a high magnetic field.The calculated M s of the epoxy nanocomposites with 5.0 and 10.0 wt% MnFe 2 O 4 loadings and pure MnFe 2 O 4 nanoparticles are 1.84, 4.21, and 31.68 emu/g, respectively.The measured M s of commercial MnFe 2 O 4 in this work is a little lower than the reported M s of MnFe 2 O 4 (40 emu/g).35,36,78The magnetic moment (m) of the nanocomposites with 5.0 and 10.0 wt% MnFe 2 O 4 loadings and pure MnFe 2 O 4 are 1.373, 1.354 and 1.244 µ B , respectively.Compared with the m of pure MnFe 2 O 4 nanoparticles, the nanocomposites show a slight enhancement, which might be caused by the hydrogen bonding networks in the epoxy matrix.79It is said that the magnetism which is determined by the dimensionality of the hydrogen bonding, was equally switchable, and the hydrogen bonding networks would cause the magnetic switching even at low temperature (300 K).79

TABLE I .
The properties of epoxy nanocomposites.

TABLE II .
The rheological data of pure epoxy and the liquid epoxy nanosuspensions.

TABLE III .
Three onset decomposition temperature, weight loss at 700 o C in nitrogen condition of the cured pure epoxy and its nanocomposites.