Self-standing elastomeric composites based on lithium ferrites and theirdielectric behavior

S. Soreto Teixeira,1 M. P. F. Graca,1 M. Dionisio,2 M. Ilc�ıkova,3 J. Mosnacek,3 Z. Spitalsky,3

I. Krupa,3,4 and L. C. Costa1

1I3N and Physics Department, University of Aveiro, 3810-193 Aveiro, Portugal2REQUIMTE, Departamento de Qu�ımica, Faculdade de Ciencias e Tecnologia, Universidade Nova de Lisboa,2829-516 Caparica, Portugal3Polymer Institute, Slovak Academy of Sciences, D�ubravsk�a cesta 9, Bratislava, Slovakia4Center for Advanced Materials, Qatar University, P.O. Box 2713, Doha, Qatar

(Received 25 October 2014; accepted 25 November 2014; published online 9 December 2014)

Lithium ferrite (LiFe5O8) is an attractive material for technological applications due to its physical

properties, which are significantly dependent on the preparation method and raw materials. In this

work, LiFe5O8 crystallites were obtained by controlled heat-treatment process at 1100 �C, of a ho-

mogeneous mixture of Li2O-Fe2O3 powders, prepared by wet ball-milling and using lithium and

iron nitrates as raw materials. The main goal was the preparation of a flexible and self-standing tick

composite film by embedding lithium ferrite particles in a polymeric matrix, taking advantage of

the good mechanical properties of the polymer and of the electrical and dielectric properties of the

ferrite. The selected polymer matrix was styrene-b-isoprene-b-styrene copolymer. To prepare the

composites, the lithium ferrite particles were chemically modified in order to functionalize their

surface. To analyse the influence of the particles surface modification, different composites were

made, with modified and unmodified particles. The structure of the obtained composites was stud-

ied by FTIR, XRD, TGA, and DSC techniques. The dielectric properties were analysed, in the fre-

quency range between 10 Hz and 1 MHz and in function of temperature in the range between

�73 �C and 127 �C. These properties were related with the structure and concentration of the par-

ticles in the matrix network. The composites with the modified particles present higher dielectric

constant, maintaining values of loss tangent sufficiently low (<10�2) that can be considered inter-

esting for technological applications. VC 2014 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4903735]

I. INTRODUCTION

Cubic lithium ferrites, such as LiFe5O8, belong to the

class of soft magnetic materials with high Curie temperature

(620 �C), square hysteresis loop, and high magnetization.1

These ferrites are very attractive materials for several appli-

cations, because of its low preparation cost.2 They are

mainly used as a substitute for the well-known yttrium iron

garnet (YIG), in mass-scale microwave devices for high-

density magnetic recording, magnetic fluids, ferrofluid tech-

nology, magnetocaloric refrigeration, magnetic resonance

imaging enhancement, and magnetically guided drug deliv-

ery.3–5 The lithium ferrite (LFO) is also a promising cathode

material in rechargeable Li-ion batteries.6,7

The lithium ferrite ceramics can be prepared by different

methods. It is known that preparation routes which involve

high temperatures are not desirable because there is a possibil-

ity of lithium volatilization. LFO particles can be synthesized

by solid state reaction, by using lithium and iron nitrates as

raw materials. The electrical and magnetic properties of this

ferrite prepared by this method were already published.8,9

In this article, the study of molecular dynamic of a thermo-

plastic elastomer, a triblock copolymer with polystyrene end

blocks and poly(styrene-isoprene) mid-block (SIS), commer-

cially known as kraton, with the inclusion of lithium ferrite par-

ticles to form a composite is presented. Before embedding the

lithium ferrite powders into the polymeric matrix, the surface

of particles were modified, first with silane and after with cho-

lesteryl chloroformate. This method evidenced to improve the

particles dispersion in the SIS polymer matrix.10

The ABA type copolymers, such as polystyrene-polyiso-

prene-polystyrene, are widely used in industry. In this work,

SIS polymer, which is a thermoplastic elastomer presenting

useful properties such as flexibility, high tensile strength,

and low swelling in water, was used because the mention

properties are required for actuation response under applied

electric field. Thus, this kind of combination provides the

soft and rubbery matrix of isoprene characteristic with the

glassy domain of styrene.11 The repetition of the styrene seg-

ment increases the modulus and the yield strength when

compared to that of polyisoprene alone. Furthermore, the

physical crosslinks give origin to a copolymer that exhibits

elastomeric properties of a cross-linked rubber that can be

easily obtained by conventional methods.12 The morphology

of SIS can be divided into three different types: lamellar, cy-

lindrical, and micellar, depending on the styrene content.

There are techniques that can provide different morphologies

such as melting, shearing, different casting solvents,13 and

solvents using different polymer compositions14 which is the

most widely used.

In general, polymeric materials are very complex sys-

tems, when compared with low molecular weight compounds.

0021-8979/2014/116(22)/224102/8/$30.00 VC 2014 AIP Publishing LLC116, 224102-1

JOURNAL OF APPLIED PHYSICS 116, 224102 (2014)

A large number of macromolecular chains are responsible

for a great number of conformations with consequences at

the level of chain flexibility. Temperature has an important

influence in that flexibility, and consequently, this behaviour

is reflected in some other properties, such as the electrical

and dielectric, that can be studied by impedance spectros-

copy (IS).

From a practical point of view, impedance spectroscopy

can provide the measurement of the complex permittivity,

e�ðxÞ ¼ e0ðxÞ � i e00ðxÞ or derived quantities.15 One of those

quantities is the dielectric modulus,16 M¼ e�1, which is a

complex quantity M*(x)¼M0(x)þ iM00(x), being often

used in high conducting materials. The macroscopic proper-

ties, impedance Z*(x)¼Z0(x)� iZ00(x), or admittance,

Y¼ Z�1, that is, Y*(x)¼Y0(x)þ iY00(x) can also express the

properties of the material. The interrelations between these

quantities are simple when the sample geometric characteris-

tics are known.17

II. EXPERIMENTAL

A. Lithium ferrite powder preparation

LiFe5O8 powders were prepared by solid state method.

The raw materials were iron (III) nitrate (Fe(NO3)3.9H2O)

and lithium nitrate (LiNO3). After weighting the

Fe(NO3)3.9H2O and LiNO3 powders, taking into account the

stoichiometry of the lithium ferrite that presents a molar ratio

between the lithium and iron ions of 1:5, the powders were

homogenized in a planetary ball mill system (Fritsch-

Pulverisette 7.0), at 250 rpm for 1 h. Agata vessels and balls

were used with equal volume. After this first mixture, 10 ml

of ethanol was added, and this wet mixture was milled dur-

ing 3 h, at 500 rpm. In order to control the system tempera-

ture, every hour, the milling was stopped for 5 min. After

this process, the vessels with the powders were placed in a

furnace at 80 �C, for 24 h, to promote the evaporation of the

ethanol. After this procedure, several heat-treatments were

done at temperatures between 500 and 1200 �C.

The composite formation is described by several steps

that included the modification of the lithium ferrite particles,

first with 3-aminopropyl triethoxysilane, then with choles-

teryl chloroformate and the final preparation to obtain the

nanocomposites.

B. Raw materials

(3-aminopropyl) triethoxysilane, cholesteryl chlorofor-

mate (both Sigma-Aldrich, USA), triethylamine (Fluka,

Switzerland), solvents such as acetone (Analytical Grade)

and toluene (Sigma-Aldrich, USA) were used as received.

Tetrahydrofuran (THF; POCH, Poland) was dried over so-

dium wires and freshly distilled before reaction. The polymer

matrix styrene-b-isoprene-b-styrene (SIS; KRATON D1165,

KRATON, USA) was supplied in the form of pellets.

C. Modification of the lithium ferrite surface particles

The modification of LiFe5O8 particles was performed in

two steps according to Mrlik et al.18 Briefly, the particles

were modified with 3-aminopropyl triethoxysilane to

introduce amino functionality onto the particle’s surface that

enabled to anchor the cholesteryl chloroformate in the fol-

lowing steps.

1. Modification of LiFe5O8 particles with3-aminopropyl triethoxysilane

The LFO particles (0.19 g) were placed into the two-

neck round-bottom flask and 45 ml of toluene was added.

The mixture was stirred by using the mechanical glass stirrer

for 30 min at 110 �C. Then (3-aminopropyl)triethoxy silane

(6.64 ll, 3.778� 10�5 mol) was dissolved in 5 ml of toluene

added drop wise into the reaction flask. Further, the mixture

was refluxed for 8 h at 110 �C under argon atmosphere.

The modified particles were filtered, and washed by

100 ml toluene, 70 ml ethanol, and 70 ml acetone (three

times). The purified particles were dried in an oven, at

100 �C, and pressure of 300 mbar for 2 h.

2. Modification of LiFe5O8 particles with cholesterylchloroformate

The silane modified particles were placed into the three-

neck round-bottom, the flask was evacuated and back-filled

with argon (three times) and dry THF (200 ml) was added.

The cholesteryl chloroformate (0.02 g, 4.45� 10�5 mol) was

dissolved in the dry THF and added into reaction mixture fol-

lowed by addition of triethylamine (4 ml, 29 mmol). The reac-

tion mixture was stirred by using mechanical stirrer for 8 h.

The modified particles were filtered and washed with acetone

(three times, 50 ml) and dried in vacuum oven at 100 �C for

3 h. Then the pressure was decreased to 100 mbar for 1 h.

D. Composite preparation

The SIS was dissolved in toluene and the particles were

added. The solution was mixed by using high speed shear

blade mixing device at 1200 rpm for 2 h. After that the com-

posite solution was casted into the polytetrafluoroethylene

chambers where the solvent was evaporated at room temper-

ature and atmospheric pressure.

To completely remove the residual solvent, the compos-

ite was placed into the oven and the temperature was

increased to 80 �C at 10 �C step by day. Then the pressure

was decreased to 2 mbar by 100 mbar step by day. Finally,

the temperature was increased to 100 �C for 30 min and the

pressure kept at 2 mbar.

Two different concentrations of lithium ferrite were

made, 1% and 8% (% wt.), for modified and unmodified par-

ticles. The obtained samples, in square sheet geometry,

presents a thickness lower than 300 lm. At this stage, the

composites were ready to be analysed by Fourier Transform

Infrared (FTIR) spectroscopy, thermal gravimetric analysis

(TGA), differential scanning calorimetry (DSC), X-ray dif-

fraction (XRD), and IS.

E. Measurement techniques

The initial measurements by infrared spectroscopy

(FTIR) were performed with a Nicolet Impact 400 instru-

ment in ATR mode using a germanium crystal.

224102-2 Soreto Teixeira et al. J. Appl. Phys. 116, 224102 (2014)

TGA measurements were performed with a Mettler

Toledo TGA/SDTA 851 instrument in a nitrogen flow

(30 ml/min) using a heating rate of 10 �C min�1 in a temper-

ature range from 25 �C up to 800 �C. Indium and aluminium

were used for temperature calibration. Two parallel runs

were performed for each sample.

DSC was used for the determination of the glass transi-

tion temperature (Tg) in a Setaram DSC 131 calorimeter fit-

ted with a liquid nitrogen cooling accessory. Dry high purity

N2 gas was purged through the sample during measurements.

Samples were cooled to �40 �C and the thermograms were

collected in a subsequent heating run at 10 �C min�1.

XRD patterns data were obtained in a Siemens D5000

diffractometer (CuKa radiation, k¼ 1.54056 A) at 40 kV and

30 mA, with a curved graphite monochromator, an automatic

divergence slit (irradiated length 20.00 mm), a progressive

receiving slit (height 0.05 mm), and a flat plane sample

holder in a Bragg-Brentano parafocusing optic configuration.

Intensity data were collected by the step counting method

(step 0.02� in 1 s) in the 2h angle range of 10�–60�.For the electrical measurements, the samples were

inserted into two stainless steel electrodes (diameter 10 mm)

of a parallel plate capacitor. The electrical measurements

were carried out isothermally using an Alpha-N broadband

impedance analyzer (Novocontrol GmBH), in a range of fre-

quency from 10�1 up to 106 Hz. The temperature was

increased from �73 �C up to 127 �C in temperature steps of

10 �C. The sample cell (BDS 1200) was mounted in a cryo-

stat (BDS 1100) and exposed to a heated gas stream evapo-

rated from a liquid nitrogen dewar. The temperature control

was performed within 60.5 �C with the Quattro Cryosystem

from Novocontrol. The acquisition and control of data soft-

ware were performed by WinData and theoretical adjusts

were performed by WinFit from Novocontrol.

III. RESULTS AND DISCUSSION

LiFe5O8 particles were covalently modified with choles-

teryl chloroformate to improve its interaction with SIS poly-

mer matrix. As was already proved, due to chemical and

sterical composition, the cholesteryl modification improves

interactions with aliphatic polymer chains when used for

modification of fillers in polymer composites.19 When used

in SIS matrix, it improved, namely, interactions with

polyisoprene chains of styrene-b-isoprene-b-styrene copoly-

mer.20 In addition, the cholesteryl modification was observed

to improve sedimentation stability and resistance to corro-

sion when used for modification of magnetic particles.21

LFO particles were modified in two steps, as shown in

the scheme presented in Figure 1.

In this process, first, the silanization was performed to

introduce amine functional groups on the particles surface.

The successful modification was confirmed by FTIR spec-

troscopy, as can be seen in Figure 2(a), where the neat par-

ticles did not exhibit any characteristic absorption bands,

while in the spectra of silane modified particles it was

showed a new absorption band centred at 1400 cm�1 that

corresponds to C-N vibration from (3-aminopropyl) triethox-

ysilane and absorption of C-H aliphatic at 1388 cm�1,

respectively. Other characteristic bands appear at

1165 cm�1, corresponding to the vibration of C-O bond and

Si-O vibrations at 1100 cm�1.

In the second step (Fig. 1), the silane modified LiFe5O8

particles were used for the reaction with cholesteryl chloro-

formate to obtain covalently bonded cholesteryl groups onto

the particles surface. The modification was confirmed by

FTIR spectra (Fig. 2(b)). It can be clearly seen that after

modification a new absorption band appears at

2800–3000 cm�1, corresponding to C-H vibration from cho-

lesteryl groups. New absorption bands also appear at around

1600 cm�1 and 1258 cm�1. The former is related to C¼O

vibrations and the latter is C-O-C stretching bands of choles-

teryl groups.

Figure 3 shows the XRD patterns of all samples and of

the lithium ferrite base powder (heat-treated at 1100 �C). The

main diffraction peaks of the lithium ferrite are visible in all

of the composite samples, being more intensive in the 8%

unmodified one. The existence of relative low intensity peaks

in the modified samples can be associated with the powders

surface functionalization. Assuming that amorphous matrix

will have similar intensity peak as unmodified 1 wt. %, it can

be seen that when modified particles are used, the peak inten-

sity is decreased, pointing to better dispersed particles.21 The

same behaviour is observed in the sample with 8 wt. %.

The TGA analysis of the composites samples (Figure 4)

shows that the decomposition of the polymer starts at tem-

peratures above 300 �C, indicating the maximum of the

working composite temperature. From this analysis, the

FIG. 1. Scheme containing the modifi-

cation of LiFe5O8 particles with triethyl-

amine and cholesteryl chloroformate.

224102-3 Soreto Teixeira et al. J. Appl. Phys. 116, 224102 (2014)

amount of lithium ferrite incorporated in the matrix is pre-

sented in Table I. As it can be seen, the mass quantities of

lithium ferrite embedded in the polymer matrix are the

expected one, i.e., 1% and 8%. Since the samples used for

the TGA analysis, in all compositions, were obtained from

the center of the dried sheets, the difference between mass

percentage of ceramic powder, obtained by TGA and the the-

oretical one indicates that the particles distribution in the

polymer network is not completely uniform.

The thermograms obtained by DSC measurements

(Figure 5) show that samples undergo no changes on their

thermal behaviour upon the treatment to which they were

submitted. Moreover, the only thermal phenomenon detected

is the heat flux step assigned to the signature of the glass

transition (Tg) that does not changes significantly upon sev-

eral thermal cycles. In all samples, it is visible two glass

transition temperatures, the first one detected at around

�59 �C and the second one at around 66 �C. It must be men-

tioned that those glass transition temperatures for the

different compositions were estimated by the on-set values.

These two glass transitions were obtained due to the compo-

sition of the copolymer (polyisoprene and polystyrene). Also

these glass transitions were observed by Ilc�ıkov�a et al.20 by

dynamic mechanical analysis, however at relatively higher

temperatures (Tg1¼�39.8 �C and Tg2¼ 110.7 �C). The dif-

ference in the glass transitions temperatures can be explained

by the existence of the plasticized effect which is related to

increasing of lithium ferrite particles mostly in the second

relaxation process.

It is also visible that the heat flow variation, at Tg region,

is higher for the composites containing modified particles.

For further analysis of the glass transition phenomenon,

impedance spectroscopy measurements were performed,

since this method can provide information about the mobility

of the a-process that is behind the dynamic glass transition.

The temperature dependence of a relaxation exhibits the typ-

ical curvature of cooperative processes.

Figure 6(a) shows the temperature dependence of the

dielectric constant for samples with modified and unmodified

particles. For both concentrations in weight, the composites

containing modified particles present the highest values of

dielectric constant (e0), achieving at 300 K and 1 kHz a value

FIG. 3. XRD patterns of composite samples with 1 and 8 (% weight) modi-

fied and unmodified lithium ferrite particles and lithium ferrite base powder

(1100 �C heat-treated).

FIG. 4. TGA thermogram for composites with concentration in weight of

1% and 8% of modified and unmodified LiFe5O8 powder.

FIG. 2. (a) Fourier Transform Infrared (ATR, Ge) spectra of unmodified particles (LFO), triaminopropyl triethoxy silane (Silane), and particles modified by sil-

ane (LFO-silane); (b) Fourier Transform Infrared (ATR, Ge) spectra of cholesteryl chloroformate (cholesteryl chloroformate) and particles modified by silane

(LFO-cholesteryl chloroformate).

224102-4 Soreto Teixeira et al. J. Appl. Phys. 116, 224102 (2014)

of 3.65 (Fig. 6(b)). For the composites with unmodified par-

ticles, e0 increases with the rise of the concentration of

LiFe5O8 particles. At room temperature (RT) and 1 kHz,

LiFe5O8 ceramics exhibit a dielectric constant value of 54.8

The difference between those two values should be related to

the influence of the polymeric network. As expected, the pure

SIS copolymer network shows a smaller dielectric constant

value (�2.7 at 300 K—Fig. 6(b)). In our opinion, the mecha-

nism which leads the polarization is different for modified and

unmodified particles. In the modified ones, the polymeric net-

work has the main contribution for the polarization. On the

other hand, for unmodified particles, the dielectric behaviour

is mostly due to the particles contribution.

The dielectric losses (Table I) have similar values for

the samples with or without the surface modification process.

The slightly difference is due to the increase of the LiFe5O8

particles amount embedded in the polymeric matrix. In gen-

eral, the maximum of dielectric loss decreases with the tem-

perature as it can be seen in Figure 7, for the sample with

composition of 8% of modified particles.

Figure 8 shows the imaginary part of the complex per-

mittivity, as a function of frequency for a 8% lithium ferrite

unmodified particles dispersed in the polymeric matrix at

T¼ 97 �C.

A first analysis of these data was done by fitting the real

(e0) and imaginary (e00) parts of the permittivity with the

Havriliak–Negami (HN) function22

e� ¼ e1 þr

ixe0

þX

n

Den

1þ ixsHNnð ÞaHNn� �bHNn

; (1)

where n is the number of the relaxation processes, De is the

dielectric strength, aHN and bHN correspond to the shape pa-

rameters (0< aHN < 1; bHN < 1), sHN is the relaxation time,

related to the frequency of the loss peak maximum, and e1is the high frequency dielectric constant. It is known that at

higher temperatures, experimental data are influenced by a

low frequency conductivity contribution, and an additional

term r=ixe0 must be added. e0 represents the vacuum per-

mittivity and r the DC conductivity.23

In this figure it is also shown, for a representative tem-

perature, how the fitting process simulates the e00 spectrum

considering the sum of three relaxation processes and con-

ductivity. The process located at higher frequency is desig-

nated as b2 being assigned to a secondary, or sub-glass

relaxation due to relatively local reorientational motions of

dipoles. The process exhibiting a maximum at intermediate

frequencies is designated as a2 being attributed to the coop-

erative process underlying the dynamical glass transition

occurring at higher temperatures. The low frequency tail is

due to the conductivity.

For cooperative processes, the temperature dependence

for the relaxation time obeys the Vogel–Fulcher–Tamman

(VFT) law24

s ¼ s0 expB

T � T0

� �; (2)

where B is a constant, s0 is the relaxation time for infinite

temperature, and T0 is defined as the Vogel temperature,

30–70 �C below Tg. By replacing the VFT law in the activa-

tion energy expression, a new Eq. (3) was obtained, where Ris the ideal gas constant, 8.314 J/mol.

TABLE I. Composites parameters comparison in 1 and 8% wt. of LFO: tan dmax, and dielectric constant at 1 kHz, Tg by DSC and IS, Ea, and fragility index

(m).

1%—modified 8%—modified 1%—unmodified 8%—unmodified

Composition, TGA (%) 1.62 7.83 1.40 8.67

Tan dmax (10�3), f¼1 kHz 4.9, T¼�38 �C 7.1, T¼�43 �C 4.7, T¼�43 �C 5.6, T¼�43 �C

7.4, T¼ 77 �C 6.8, T¼ 87 �C

e’f¼1 kHz, T¼300 K 3.1 3.6 3.0 3.1

Tg1,DSC ( �C) �58.80 6 0.09 �59.64 6 0.12 �59.67 6 0.13 �57.46 6 0.05

Tg2,DSC ( �C) 67.44 6 0.11 65.13 6 0.48 66.03 6 0.99 64.34 6 0.02

Tga2,IS ( �C) 6 DTga2 55.80 6 0.18

Eaa2 6 DEaa2 (kJ/mol) 390.8 6 0.2

Fragilitya2 (ma2 6 Dma2) 62.06 6 0.07

Eab2 6 DEab2 (kJ/mol) 31.2 6 1.0 52.4 6 1.3 21.3 6 2.2 56.7 6 2.9

Tga1,IS ( �C) 6 DTga1 �67.67 6 7.88 �63.16 6 10.62 �63.72 6 37.32 �63.24 6 0.02

Eaa1 6 DEaa1 (kJ/mol) 252.8 6 24.7 357.4 6 23.2 347.3 6 32.1 352.99 6 0.05

Fragilitya1 (ma1 6 Dma1) 64 6 13 89 6 10 87 6 59 88 6 1

FIG. 5. DSC thermograms showing Tg for each concentration and type of

LiFe5O8 particles in the polymeric matrix.

224102-5 Soreto Teixeira et al. J. Appl. Phys. 116, 224102 (2014)

Ea Tð Þ ¼ R@lns@ 1=Tð Þ ¼

RB

1� T0

T

� �2: (3)

The dynamic glass transition (main transition or a process) is

commonly accompanied by different secondary processes (band c processes).25 The dynamic in polymeric materials

involves a hierarchy of relaxation processes which goes from

very local motions, due to b, c processes which activation

energy usually varies between 21 and 63 kJ/mol and segmen-

tal mobility exhibiting cooperative, due to a process or even

relaxation processes involving large or complete polymeric

segments, Rouse dynamics or reputation.26 For all samples,

it can be observed three relaxation processes: one can be

assigned to a secondary relaxation and the others to coopera-

tive processes. The secondary relaxation, known as b pro-

cess, shows linear temperature dependence, Arrhenius

behavior in the relaxation map (Figure 9). The values of the

activation energy were similar and higher than activation

energy assigned to c processes (around 38 kJ/mol). These

secondary processes were called b1 and b2, respectively.

The a relaxation shows a non-linear dependence of

relaxation time as a function of the inverse of temperature

thus obeying the VFT equation. The activation energy of this

process, at Tg is higher than for secondary relaxation proc-

esses, around 100 kJ/mol. It is important to note that the

relaxation map (Figure 9) also includes the relaxation times

obtained from isochronal plot of the dielectric loss of a2,

which is an alternative procedure to extract the relaxation

times when the isothermal loss peaks are ill-defined, as it is

obtained for the a2-process. The agreement with the relaxa-

tion times obtained from the HN fits of the isothermal loss

curves is excellent, confirmed by according of both the data

adjusts for b-process: isochronal (fill dark squares) and iso-

thermal (open triangles) in Figure 9. This validates the use of

isochronal data especially when the respective loss peaks

maxima are poorly defined in the frequency domain or out of

the accessible frequency range.27–29

Analyzing the results of the molecular mobility studies,

for the samples with 8% in weight, modified and unmodified

FIG. 7. Tan d plot (composite with 8% wt. of modified particles).

FIG. 8. Dielectric loss curve and best fit, showing three relaxation processes

for the sample with 8% of lithium ferrite unmodified particles dispersed in

the polymeric matrix at T¼ 97 �C.

FIG. 6. (a) Real part of the permittivity in dependence of the temperature (1 kHz); (b) dielectric constant (T¼ 300 K; f¼ 1 kHz) dependence of LFO particles

concentration in weight.

224102-6 Soreto Teixeira et al. J. Appl. Phys. 116, 224102 (2014)

particles, we obtained nearby values for activation energy of

b2 process, 55 kJ/mol. For the composite containing 1% in

weight of unmodified particles the b2 process needs less

energy, around 21 kJ/mol. This increment can be justified by

the fact of the lithium ferrite particles which are not aggre-

gated with the polymer in the matrix composite and the

dipoles from polymer (segmental particles) and lithium fer-

rite particles have more mobility, reaching easily the barrier

of energy.

The a1 and a2 processes have similar values of Tg

obtained by the DSC analysis and IS extrapolation.

However, by DSC, a1 glass temperatures are slightly higher,

�59.67 up to �57.46 �C. From the extrapolation of the VFT

equation to s¼ 100 s,30,31 a glass transition temperature for

this process is estimated to be in the range between �67.67

and �63.16 �C. Consequently, the dielectric results also evi-

denced the restricted mobility of the copolymer chains in

agreement with the previous DSC results, which revealed a

Tg increase, associated with a small and broad heat flux step.

Concerning the difference between the Tg values estimated

by IS and determined from DSC (see Table I), it is not sur-

prising since the relaxation time at the calorimetric glass

transition temperature is not 100 s, as it was assumed, for

many polymeric systems.32,33

Both compositions with 8% wt. of lithium ferrite par-

ticles have small Tg values which can be explained by the ex-

istence of the plasticized effect related to the increasing of

that particles. So that it leads to an increasing of the space

between the polymer chains, avoiding the packing of the

chains lowering the glass transition.

The second a process for all samples was very difficult

to adjust and just a few data were obtained. So, the error for

estimating the relaxation time for each temperature is higher

than using isochronal data method. As these systems have

Tg’s very similar, we made only one adjust using data points

from all a2 relaxation process compositions and isochronal

data for the composition holding 8% wt. modified particles

are also included.

Evaluating the activation energy, a2 relaxation process

has a higher value than the a1 relaxation process, due to the

motion of the different phases (polyisoprene and polystyrene

components) of the copolymer.

The temperature dependence of relaxation times allowed

also to estimate the fragility index (m) that is a measure of

the abruptness of a given rubber material to vitrify in a small

temperature range (how the flow properties vary on

approaching the glass transition). The fragility index is cal-

culated according to Angell’s equation34

m ¼ @ log s

@ Tg=T� �

T¼Tg

¼ Ea Tgð Þln10 RTgð Þ

; (4)

where Tg can be estimated by replacing s in Eq. (2) by 100 s.

One sees that for the a1 relaxation process, the compos-

ite containing 1% wt. of modified particles in the polymeric

matrix has a smaller value of fragility (m¼ 64) than the re-

spective composite with unmodified particles (m¼ 86)

(Table I). So, this means that the modification of lithium fer-

rite particles surface turns the material stronger and contrib-

utes to an improvement in the mechanical material

properties.

IV. CONCLUSIONS

Composites have two glass transition temperatures due

to the constitution of the copolymer: Tg1 (around �58 �C, by

DSC and �65 �C by IS) related to the polyisoprene phase

and Tg2 (around 66 �C, by DSC and 56 �C by IS). A compari-

son with SIS, which has higher Tg’s, show that the inclusion

of particles contribute to the plasticized effect, being more

evident in composites with modified particles.

According to the analysis of the dependence of the

relaxation time with the temperature, we can observe three

relaxation processes for all composites. There are two a-

processes which have a VFT behaviour, obtaining two Tg’s,

in agreement with the DSC results, and one b-process, which

has an Arrhenius behaviour. The activation energy of the

second a-process is slightly higher than for the first one sug-

gesting that the cooperative movements due to the polysty-

rene phase are smaller than to the polyisoprene phase. The

modification of the lithium ferrites, before the introduction

into the polymer matrix, makes the material mechanically

stronger. The fragility index for the same % in weight is

smaller in composites containing modified particles, con-

firming this behaviour. The particles with modificated sur-

face contribute to an increase of the dielectric constant,

making the material stronger, improving their mechanical

properties which is a request to be used in several electronic

applications.

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