Dielectric, thermal, and swelling properties of calcium ion-crosslinked sodium alginate film

Dielectric, Thermal, and Swelling Properties of CalciumIon-Crosslinked Sodium Alginate Film

Seda Bekin,1 Shokat Sarmad,1 Koray G€urkan,2 G€okcen Yenici,1 G€on€ul Keceli,3 G€ulten G€urda�g1

1 Department of Chemical Engineering, Faculty of Engineering, Istanbul University, 34320, Avcilar, Istanbul,Turkey

2 Department of Electrical and Electronics Engineering, Faculty of Engineering, Istanbul University, 34320,Avcilar, Istanbul, Turkey

3 Department of Chemistry, Faculty of Engineering, Istanbul University, 34320, Avcilar, Istanbul, Turkey

In this study, Ca21-crosslinked sodium alginate (SA) gelfilms (SA-Ca/SA2-Ca) have been prepared, and theirstructural and thermal characterizations were investi-gated using fourier transform infrared spectroscopy, X-ray diffraction, differential scanning calorimetry, andthermogravimetric analysis, respectively. Dielectriccharacterization was performed at room temperature inthe frequency range of 12 Hz–100 kHz. The equilibriumswelling value (ESV) of gel films was determined both indistilled water and in 0.1 M sodium chloride (NaCl) solu-tion at room temperature. Sodium alginate films (SA/SA2) were also prepared for use as reference. The effectof the crosslinking of sodium alginate on the dielectricand thermal properties of gel films was investigated bycomparing the properties of gel films with those of SA/SA2 films. Although ESV of Ca21-crosslinked SA film indistilled water is about 350 gH2O/gpolymer, it decreasedinto one third in 0.1 M NaCl solution. The crosslinking ofSA did not significantly affect the thermal properties,but it decreased the b-relaxation associated with thepolar side groups. Frequency spectra of electric modu-lus, impedance and Cole-Cole plots confirmed thehigher conductance values of SA-Ca films at low fre-quencies than those of SA film due to the presence ofby-product of crosslinking. POLYM. ENG. SCI., 54:1372–1382, 2014. VC 2013 Society of Plastics Engineers

INTRODUCTION

Sodium alginate (SA) is a low cost, environmentally

safe, nontoxic, weak acidic, and an anionic, linear-chained

natural polysaccharide. In recent years, it has gained more

importance due to its excellent properties and gel-forming

ability with polyvalent metal ions or glutaraldehyde, and it

has been used in various applications such as controlled

drug delivery [1–3]; in the dehydration of some organic

solvents by pervaporation [4–7]; the removal of dyes or

heavy metal ions from aqueous solutions [8–10]; in textile

industry [11]; and in food industry [12]. SA is a binary

copolymer of b-(1–4)-linked-D-mannuronic acid (M) and

a-(1–4)-linked-L-guluronic acid (G) units with varying

contents of M and G blocks. These two blocks exist in SA

chains as homopolymeric MM and GG blocks and hetero-

polymeric MG or GM blocks [13]. The physical properties

of alginates in aqueous medium depend not only on the M/

G ratio in SA chain but also on the distribution of M and G

units along the polymer chain. The stiffness of alginate

chains as well as complex formation with calcium ions

depends on the chemical composition and sequence of the

blocks along the polymeric chains [12]. The reactivity of

homopolymeric blocks: GG, MM and heteropolymeric

GM/MG blocks increases in the following order: GM or

MG<MM<GG. This order is also valid for the flexibility

of the different blocks in SA chain. The complexes of SA

with Ca21 ion obtained at stoichiometric ratio of Ca21/

ACOOH are usually insoluble, and they can be used in the

forms of films, fibers, or capsules for various applications

in food industry and biomedicine [13]. It was reported that

the rigidity of these blocks decreases along in the order:

GG>MM>MG [13].

The preparation of calcium alginate films is a complex

process and it depends on the type of alginate, the extent

of reaction between alginate and calcium ions, the source

of calcium ion (chloride, phosphate, and lactate), and the

preparation method [14]. The crosslinking of SA with

polyvalent metal ions can improve the properties of film

such as water resistance, mechanical and barrier proper-

ties, cohesiveness, and rigidity [12]. The gel-forming

Correspondence to: G€ulten G€urda�g; e-mail: [email protected]

Contract grant sponsor: Scientific and Technical Research Council of

Turkey (TUBITAK); contract grant number: MAG/110M444; contract

grant sponsor: The Fund of Scientific Research Projects of Istanbul Uni-

versity; contract grant number: 29207.

DOI 10.1002/pen.23678

Published online in Wiley Online Library (wileyonlinelibrary.com).

VC 2013 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—2014

ability of alginates is closely related to the content and

length of G blocks [7, 15]. G blocks are mainly responsi-

ble for the gel formation with the egg-box structure,

whereas M blocks are responsible for the ion exchange

and sorption property of SA [16]. In SA chain, when two

G blocks are aligned side by side, a diamond shape hole

occurs. The size of this hole favors the cooperative bind-

ing of SA with Ca21 ions. When Ca21 ions got into con-

tact with SA solution, G blocks are so arranged that Ca21

ions exist between the two SA chains like eggs in an egg-

box [17]. The crosslinking of SA by Ca21 ions occurs

very fast, and it is completed in 10 min after addition of

SA to CaCl2 solution [18].

Because of the difference in the sizes of metal ions,

Ca21 ions form essentially bidentate linkages with the

oxygen of COO2 ions, whereas Zn21 ions constitute

preferentially monodentate bonds with hydroxyl group of

ACOOH group [16]. The affinity of SA toward different

divalent metal ions follows the order: Pb21>Cu21>Cd21>Ba21> Sr21>Ca21>Co21 5 Ni21>Mn21 [16].

Huang et al. [5] investigated the separation performance

of divalent or trivalent metal ion-crosslinked SA mem-

branes in the separation of water from ethanol/water and

isopropanol/water mixtures and determined that Ca21

divalent cation was the most effective for the separation

of 90 wt% aqueous ethanol and isopropanol mixtures.

Adam Rachocki et al. [19] investigated the electric con-

ductivity of the polymer films based on alginic acid and

benzimidazole to check their potential applications in

electrochemical devices [19]. Dielectric characterization

of SA hydrogels is of enormous interest for biotechnol-

ogy and medicine, where alginate beads are widely used

for immobilization of cells and enzymes, drug delivery,

and as microcarriers for cell cultivation [15]. Dielectric

measurements can provide important information about

the structural characteristics and microenvironment of

complex polymer systems, such as mobility of charged

groups and dipoles [20], the content and physical state

of water [21, 22], and enzyme activity [23]. Mishima

et al. [24] used dielectric measurements for online moni-

toring of the cell concentrations in the solutions with

yeast cells immobilized in Ca21-crosslinked alginate

beads [24]. Although the dielectric data of alginate solu-

tions and beads are available in the literature [25, 26],

the dielectric and electrical properties of SA and Ca21-

crosslinked SA films have not been reported yet. For

that reason, we investigated the effect of the crosslinking

of SA with Ca21 ions on the dielectric properties. The

effect of crosslinking on the swelling, thermal, and elec-

trical properties of Ca21-crosslinked SA films has been

investigated by comparing their properties with those of

SA film.

EXPERIMENTAL

Materials

SA with medium viscosity was purchased from Sigma.

The viscosity average molecular weight and sodium (Na)

content of SA were determined to be 225.600 g/mol by

viscosimetric analyses [27] and 7.2 wt% by using PANa-

lytical AXIOS X-ray fluorescence (XRF) spectrometer.

Calcium chloride (CaCl2.2H2O) was the product of

Across Organics. Sodium chloride (NaCl) was obtained

from Riedel-de Ha€en (Seelze, Germany). In the prepara-

tion of hydrogels, distilled water was used as well as for

the swelling measurements.

Synthesis of Ca21-Crosslinked SA (SA-Ca) Gel Films

Both SA (SA/SA2) films and Ca21-crosslinked SA (SA-

Ca) gel films were prepared by solution casting and solvent

evaporation technique. To determine the effect of the cross-

linking on the determined property, two types of films were

synthesized: SA films at two different amounts, namely

thicknesses, of SA (SA and SA2) and their Ca21-cross-

linked counterparts (SA-Ca or SA2-Ca). For the preparation

of SA and SA2 films that were used as reference, SA solu-

tion of 1 or 2 wt% was prepared by dissolving 0.25 g or

0.50 g SA, respectively, in 25 mL distilled water at room

temperature by stirring with a magnetic stirrer. The solution

was poured into a glass Petri dish with inner diameter of

9.8 cm, and left to dry in an oven at 35�C for film forma-

tion for 3 days. Then, the films were further dried in a vac-

uum oven for 1 day at 35�C before the characterization and

the dielectric analysis to eliminate the effect of absorbed

water on the determined property.

SCHEME 1. Chemical structure of sodium alginate [13].

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—2014 1373

For preparation of mixed sodium/calcium alginate

(SA-Ca or SA2-Ca) gel films, 0.25 or 0.50 g of SA was

dissolved in 25 or 50 mL distilled water, respectively.

Then, 5 or 10 mL calcium chloride (CaCl2) solutions at

the same concentration (4.6 g/L) was added drop wise to

0.25 g SA/25 mL or 0.50 g SA/50 mL solution, respec-

tively, and the mixture was stirred with a magnetic stirrer

at room temperature. The amount of Ca21 ions added to

SA or SA2 solution is equal to bind 40 mol% of

ACOONa groups in both solutions. The content of

ACOONa group of SA was determined to be 3.13 3

1023 mol ACOONa per gram of SA by using the content

of Na. Afterward, the mixed sodium/calcium alginate

(SA-Ca or SA2-Ca) gel films were obtained by drying the

mixture in an oven at 35�C for 3 days long, which was

taken a glass petri dish, and then in vacuum oven at 35�Cfor 1 day under the same conditions with those of SA

films. The optical appearances of all the films were

recorded by a digital camera (Nikon, D90), and they are

given in Fig. 1.

Characterization of SA Gel Films

Optical Appearances of Gel Films. The optical appear-

ances of all the films were recorded by a digital camera

(Nikon, D90), and they are given in Fig. 1.

FTIR Analysis. All the films were characterized by

ATR technique between 400 and 4000 cm21 using Spec-

trum One, Perkin-Elmer (USA) FTIR equipment.

XRD Analysis. XRD patterns of the investigated films

were obtained by using DMAX-2200 X-ray diffractometer

(Rigaku Company, Japan) with Cu-Ka tube at 40 kV and 30

mA.

DSC Analysis. DSC thermograms of vacuum-dried films

were recorded by using the equipment DSC 131, Setaram.

DSC measurements were carried out with ca. 15 mg film

sample between 30 and 350�C at the heating rate of 10�C/

min under nitrogen gas atmosphere (40 mL N2 /min).

Before the measurements, the temperature and enthalpy

calibration were performed by four different metals,

namely In (220), Zn (480), Pb (360), and Sn (270), and an

organic chemical terphenyl (70) at five different heating

rate (2, 5, 10, 15, and 20�C/min). The numerals given in

parenthesis after the symbol of metals and terphenyl show

the final heating temperature in �C during the calibration of

DSC equipment.

TGA Analysis. The films (ca. 15 mg) were subjected to

thermogravimetric analysis (TGA) using Linseis STA PT

FIG. 1. Optical appearances of SA and SA-Ca films.

1374 POLYMER ENGINEERING AND SCIENCE—2014 DOI 10.1002/pen

1750 TGA analyzer in the temperature range of 25–

350�C at a heating rate of 10�C/min, with air flushed at

0.1 L/min to determine their thermal stability and decom-

position characteristics.

Determination of the Equilibrium Swelling Value

(ESV) and the Solubility of SA-Ca Films. The solubil-

ity and ESV of Ca21-crosslinked SA gel films were deter-

mined by gravimetric method using three replicates. A

given amount (Wd1) of dry gel film was immersed into

distilled water or in 0.1 M sodium chloride solution at

25�C for 24 h. The swollen gel films were removed from

the distilled water or salt solution gently wiped with a fil-

ter paper to remove excess water at the surface and

weighed (Ws). Finally, the swollen gel films were dried at

35�C for 2 days (1 day in oven and then 1 day in vacuum

oven). The dried gel films were reweighed (Wd2). The

solubility of the films in percentage was calculated

according to the equation:

Solubility ð%Þ5 1002 Wd2=Wd1ð Þ½ �3100 (1)

The ESV of the gel films was calculated by using the

equation:

ESV gH2O=gpolymer

� �5 Ws2Wd1ð Þ=Wd1 (2)

Dielectric Measurement. Dielectric property or the per-

mittivity of a material, in fact, is a complex unit which is

defined as following equation:

e�5e02je00 (3)

where e0, which is the real part of e*, represents energy

storage property or relative permittivity, and e00, which is

the imaginary part of e*, represents energy dissipation

property or dielectric loss. The ratio of dielectric loss to

relative permittivity gives the dissipation factor (D) or

loss tangent of material:

tan dð Þ5 e00

e0(4)

To measure complex permittivity of a material, parallel

plate method can be used. In this method, material with a

thickness of d is placed between two parallel metal plates

with a surface area of A. This dielectric cell structure can

be electrically modeled by a parallel RC network. Electri-

cal impedance (Z) of this network is defined as;

Z xð Þ5 R

11 xRCð Þ22j

xR2C

11 xRCð Þ25Zreal2jZimag

5jZ xð Þj/h xð Þ(5)

where x52pf is the angular frequency, Zreal and Zimag are

the real and imaginary parts, |Z(x)| is the magnitude and

h(x) is the phase of Z(x). The equations for the dielectric

constant, e0 and the dielectric loss, e00 are given below:

e05Zimag

xCO Z2real1Z2

imag

� � (6)

e005Zreal

xCO Z2real1Z2

imag

� � (7)

Here Co 5 eoA/d and eo is the permittivity of free

space. These equations require the measurement of modu-

lus (|Z(x)|) and the phase (h(x)) of impedance. Loss tan-

gent is defined as

tan dð Þ5D5Zreal

Zimag

51

xRC(8)

Thus, if C and D values can be measured directly, the

relative permittivity, dielectric loss, and the conductivity

(r) can be easily calculated using following equations:

e05C

CO(9)

e005De0 (10)

r5d

RA5eOe00x (11)

From dielectric studies, for a closer evaluation of con-

duction mechanism, electric modulus can also be ana-

lyzed. The electric modulus is basically the reciprocal of

permittivity;

M�51

e�5e0

e0ð Þ21 e00ð Þ21j

e00

e0ð Þ21 e00ð Þ25M01jM00 (12)

To calculate the complex dielectric constant, the elec-

tric modulus and the conductance, capacitance (C) and

loss tangent (D) of the dielectric cell which consists of

SA/SA2 or SA-Ca/ SA2-Ca films were measured at room

temperature in the frequency range 12 Hz–100 kHz by a

4-terminal LCR-meter (Instek LCR-819) equipped with a

copper-plated parallel plate-type cell. The films, which

were cut into size 2 3 3 cm2, were sandwiched between

two rectangular (2 3 3 cm2) copper plates. Measurement

mode of the LCR-meter was adjusted to capacitance

mode with equivalent parallel RC circuit, and the ampli-

tude of the measuring signal to 1 V. Capacitance and loss

tangent values were obtained from the averages of 10 sets

of measurements. The thicknesses of the films were meas-

ured by a micrometer screw gauge (“a” brand, 60.001

mm), and the measurements were taken from at four

TABLE 1. The thicknesses of SA/SA2 and SA-Ca/SA2-Ca films.

Film code Thickness of the film (mm)

SA 0.023 6 0.002

SA2 0.041 6 0.003

SA-Ca 0.031 6 0.004

SA2-Ca 0.047 6 0.004


different positions over the film surface using four films

and the mean values given in Table 1 were used in the

calculation of dielectric parameters. All measurements

were carried out by the films dried in vacuum oven at

35�C for 24 h just before the measurements to eliminate

the effect of water on the electrical properties of films.

RESULTS AND DISCUSSION

Characterization of the Films

Although the thickness of SA2 film (0.041 mm) is

nearly the twice that of SA film (0.023 mm), the ratio of

their crosslinked counterparts is 1.5 (Table 1). The cross-

linking slightly increased the thickness of the film, and

the increase in the thickness of the SA-Ca film is slightly

higher than that of SA2-Ca film. Although SA/SA2 films

are transparent, SA-Ca/SA2-Ca films are partly transpar-

ent as can be seen in Fig. 1 and had good flexibility.

After drying, no adhesion of both SA/SA2 and SA-Ca/

SA2-Ca gel films to glass Petri dish was observed, and

they could be peeled off from the Petri dish as one piece

(circular film) without any problem.

FTIR spectra of SA and SA-Ca film recorded by ATR

technique are given in Fig. 2. The peaks at 3265 cm21

and 2906 cm21 in FTIR spectrum of SA film are assigned

to stretching vibrations of OAH (mO-H) and CAH (mC-H)

bonds, respectively [2, 16, 28, 29]. The strong bands at

1595 and 1406 cm21 are attributed to the asymmetric and

symmetric stretching vibration of the carboxylate group

[6, 16, 28–30]. The bands at 1023 and 1079 cm21 are

attributed to the stretching vibration of CAO (mC-O) bond

[6, 28, 30, 31]. The peaks at 819, 886 ve 951 cm21 are

characteristic for polysaccharide structure, and they con-

firm the presence of guluronic and mannuronic acid units

[16]. In addition, the band at 819 cm21 is attributed to

Na-O bond in SA [31]. Although Roger et al. [18]

reported that the cosslinking of SA with calcium ions

shifts the FTIR band assigned to the symmetric stretching

of carboxylate group (mCOO2) to higher frequencies and

leads to an increase in its intensity, no difference between

the FTIR spectra of SA/SA2 and SA-Ca/SA2-Ca films

was determined in this study due to most probably incom-

plete crosslinking, namely the binding of only 40% of

FIG. 2. FTIR spectra of SA and SA-Ca film taken by ATR method. FIG. 3. XRD diffraction patterns of SA and SA-Ca films.

FIG. 4. DSC and TGA curves of SA and SA-Ca films.


ACOONa groups of SA by Ca21 ions. Thus, the charac-

teristic peaks of crosslinked SA films appeared at the

same wave numbers as those of SA film. FTIR spectra of

SA-Ca/SA2-Ca films prepared in this work are in good

agreement with those of Ca21-crosslinked SA obtained by

Li et al. [32]. In addition, Huang et al. [5] did not

observed any difference between the FTIR spectra of Co-

alginate and Na-alginate due to crosslinking of Na-

alginate with Co21 ion, and our finding is consistent with

the literature findings.

The X-ray diffraction patterns of SA and SA-Ca films

are shown in Fig. 3. There is a weak and broad diffrac-

tion peak at 2h 5 14.6� in the diffractogram of SA film,

and it indicates the amorphous structure of SA. There is

no apparent difference between the XRD patterns of SA

and Ca21-crosslinked SA (Sa-Ca/SA2-Ca) films. As the

intensity of the peak at 2h 5 14.6� in XRD pattern of SA-

Ca film is slightly higher than that of SA film, it can be

concluded that ionic crosslinking of SA leads to a slight

increase in the crystallinity.

The thermal properties of SA and SA-Ca films were

investigated by DSC and TGA techniques, and DSC and

TGA curves of the films are given in Fig. 4a and b. In

DSC curve of SA film, there are one endothermic peak at

86.5�C and two exothermic peaks at 223.3 and 236.2�C.

The former (endothermic) peak is attributed to the loss of

adsorbed moisture from the film which could not be com-

pletely removed during drying [30, 33].

In DSC curve of Ca21 -crosslinked SA film (Fig. 4a),

the exothermic peaks appear at 221.5 and 249.9�C.

Because of the stiffening effect of the crosslinking, the

second peak for the decomposition of SA shifts from

236.2�C to higher (�250�C) temperature; however, no

great difference was determined between the DSC curves

of linear-chained SA and crosslinked SA films. Figure 4b

shows the weight loss of SA and crosslinked SA films as

a function of temperature. These thermograms show that

SA undergoes thermal degradation in two-stages. A total

of 15 and 20% weight losses for SA2-Ca40 and SA films,

respectively, observed in the temperature range 50–150�Ccan be attributed to the loss of moisture [34–37]. Because

the polysaccharides have a strong affinity for water, and

SA contains two hydroxyl segments and one carboxylate

segment, the water molecules can be bound to SA by the

hydrophilic ACOONa and –OH groups. Because 40% of

ACOONa groups of SA is bound by Ca21 ions, SA-Ca

film adsorbs less amount of moisture than SA film, and

for that reason, a lower amount of weight loss due to the

loss of moisture was determined for SA-Ca film than that

for SA film. The second-stage of thermal degradation

observed in the temperature range 200–260�C is attrib-

uted to the decomposition of side chains of SA [34, 37].

Further decomposition of main chain of SA occurs at

450�C [34]. During the thermal decomposition of SA, the

carboxylate side chains are eliminated at first from the

polymer chain at the temperatures between 200 and

250�C, and then hydroxyl groups beginning from 300�C

[36]. For the second step of decomposition, the initial

decomposition temperature for SA and SA-Ca films

increases from 190.28 to 195.98�C, respectively. The

weight losses in the second step of thermal degradation

for SA1 and SA2-Ca films between the temperatures of

195 and 270 are 38.4% and 35.5%, respectively, and they

are nearly the same. It means that the weight loss in the

second step of decomposition occurs due to degradation

of SA chain, namely the elimination of carboxylate

groups by CO2 release [37]. It is known that at medium

temperatures, SA decomposes to Na2CO3 [30, 38]. In the

case of mixed sodium/calcium alginate, namely SA-Ca

film, it will decompose to Na2CO3 and CaCO3 which

their content depend on the crosslink density. The weight

loss values at 400�C for SA and SA2-Ca films are 72 and

58%, respectively, and the residue remaining after the

degradation of crosslinked SA film (42%) is higher than

that (28%) of SA film as expected since the former con-

tains Ca21 ions and they can oxidate to CaCO3 under air

atmosphere.

In consistent with the findings of DSC analysis, there

is no great difference between the TGA curves of SA and

SA-Ca films as expected because all the carboxylate

groups of SA are not crosslinked. This clearly shows that

the incomplete (40%) crosslinking of SA did not appa-

rently change the thermal properties.

Swelling Measurements

The equilibrium swelling value (ESV) and the solubil-

ity percentages determined both in distilled water and in

0.1 M NaCl solution are given in Table 2. Almost 50 and

8% of crosslinked gel films were dissolved in distilled

water and in 0.1 M NaCl solution, respectively, and the

rest remained in the crosslinked film. Although the ESV

of Ca21-crosslinked SA films in distilled water was �350

gH2O/gpolymer, it decreased to �100 gH2O/gpolymer in salt

solution due to the decrease in the driving force for swel-

ling, namely the difference between the osmotic pressures

of water in the gel film and that in the surrounding salt

solution.

Dielectric Measurements

The dielectric properties are considered as an impor-

tant tool in studying structural transitions and molecular

TABLE 2. Percentage of solubility and equilibrium swelling values

(ESV) of crosslinked SA films in distilled water and in 0.1 M NaCl

solution.

Solubility (%) in ESV(gH2O/gpolymer) in

Film code

Distilled

water

NaCl solution

(0.1 M)

Distilled

water

NaCl solution

(0.1 M)

SA-Ca 56 6 2 5.9 6 0.2 375 6 12 104 6 2

SA2-Ca 49 6 1 9.3 6 0.3 315 6 10 107 6 3


mobility in polymers. Dielectric measurements were per-

formed at room temperature and the average humidity of

40%. The dielectric constant (e0), the dielectric loss (e00),and the conductivity (r) of the films as a function of fre-

quency are given in Fig. 5a–c. It is clear from the Fig. 5a

and b that both e0 and e00 of SA/SA2 film as well as SA-

Ca/SA2-Ca films decreased with the increase in fre-

quency. The dielectric constant (e0) indicates the amount

of the dipole alignment both induced and permanent. The

decrease in the dielectric constant with the increase in fre-

quency is due to the fact that the rotational motions of

the polar molecules of SA and SA-Ca films are not suffi-

ciently rapid as that of the change in the direction of elec-

tric field, and the polyelectrolyte molecules cannot keep

up with the increase in rate of change in the direction of

AC electric field. As a result, the polarization decreases

and reaches a constant value. It means that at higher fre-

quencies, the rotational motions of the molecules lag

behind the electric field. Therefore, the dielectric constant

decreases with the frequency [39, 40]. As seen from Fig.

5a that at the frequencies higher than 60 Hz, the dielectric

constant of SA film is higher than that of SA-Ca film

because crosslinking leads to decrease in the polarization

of the film. This behavior is displayed by many polymer

dielectrics and is resulting from the dielectric relaxation

phenomena of the polymers. Within the frequency range

FIG. 5. (a) Dielectric constant, (b) dielectric loss, and (c) conductance values of SA and SA-Ca films

toward frequency.


investigated in that work, the dielectric relaxation

involves the dipolar (rotational) polarization, which

depends on the molecular structure of the material. In the

case of SA polymer, the dielectric constant is due to the

b-relaxation induced by polar groups on the side chains

[41, 42]. This relaxation with a secondary mechanism is

associated with a dipole component such as polar side

chains of SA, namely ACOO2 groups [43]. Although the

b-relaxation process is resulting from the multiple relaxa-

tions and its origin is not well understood yet [42], it is

clear from Fig. 5a that an apparent decrease in the dielec-

tric constant of SA is observed at the frequencies higher

than �1 kHz. In addition, it can be said that SA film

polarizes more than crosslinked SA film as expected. The

loss graph (Fig. 5b) displays a shoulder at about 10 kHz,

which confirms the presence of a well known b-

relaxation process in polymers [41]. The crosslinking of

SA makes difficult the b-relaxation because important

part (40%) of carboxylate (ACOO2) groups is bound by

Ca21 ions. Figure 5c shows the variation in conductivities

of the films with frequency. The conductivity of the films

generally increased at high frequencies. Below 1 kHz, the

conductivity of SA film is lower than SA-Ca film, how-

ever, at 1 kHz conductivities are almost the same, and at

the frequencies higher than 1 kHz, the conductivity of SA

film is higher than that of SA-Ca film. It means that in

FIG. 6. (a) Real (M0) and (b) imaginary parts (M00) of the electric modulus of SA and SA-Ca films toward

frequency.

FIG. 7. Cole-Cole plots of SA and SA-Ca films.


crosslinked films, the mobility of ions decreases at high

frequencies. In other words, the crosslinking of SA film

leads to more compact structure, and subsequently limited

(suppressed) the mobility of ions which is more effective

at high frequencies. The similar behavior was observed in

the dielectric loss values as seen from Fig. 5b. Figure 6

shows the frequency dependence of the real (M0) and the

imaginary parts (M00) of the electric modulus of the films.

Electric modulus is the reciprocal of the complex permit-

tivity, and its imaginary part (M00) gives information about

the relaxation time and the conductivity of the material

investigated. Very small values of M0 at lower frequencies

indicate the removal of electrode polarization. In the stud-

ied frequency range, no relaxation peaks in M0 or M00 vs

frequency graphs were observed for SA or SA-Ca films.

If we could able to measure dielectric properties at fre-

quencies higher than 100 kHz or we could crosslinked all

ACOONa groups of SA, a relaxation peak would be

observed in modulus (M00) graphs of SA-Ca films in the

investigated frequency range. Although the peak values in

modulus (M00) and frequency graphs of SA and SA-Ca

films are not clearly seen, it can easily be said that the

peak frequency for the imaginary part of the electric mod-

ulus (M00) of Ca21-crosslinked SA films is about 100 kHz

and it is obviously lower than that of SA film. The shift

in peak frequency to lower values indicates the increase

in the relaxation time and the conductivity [44, 45]. It

means that the relaxation time of linear chained SA mole-

cules is shorter than that of crosslinked SA film due to

decrease in the mobility of crosslinked SA molecules. As

known, the relaxation time can be defined as the time

which is required for the molecules to return from the

polarized form due to the electric field to random oriented

(disordered) form when the electric field was removed

[46]. Figure 7 shows the Cole-Cole plots of SA and SA-

Ca films. The impedance spectra show a semicircle

behavior corresponding to the charge-transfer process.

The diameter of the semicircle represents the charge-

FIG. 8. (a) Real (Z0) and (b) imaginary part (Z00) of the impedance values of SA and SA-Ca films toward

frequency.


transfer resistance, and the higher the diameter the higher

the charge transfer resistance of the material is. SA-Ca/

SA2-Ca film has a lower charge-transfer resistance than

SA/SA2 film due to presence of the by-product of cross-

linking reaction between SA and CaCl2, namely, Na1 and

Cl2 ions. Figure 8 shows the variation of calculated real

(Z0) and imaginary (Z00) parts of impedance of the films

with frequency. Both Z0 and Z00 values decrease with the

increase in frequency. At low frequencies (Fig. 8b), SA/

SA2 film has higher Z00 values compared to SA-Ca/SA2-

Ca film, and it means that SA/SA2 is less conductive

than the crosslinked SA film.

CONCLUSIONS

SA and Ca21-crosslinked SA gel films were prepared

and characterized by FTIR, XRD, DSC, and TGA meth-

ods. The effects of crosslinking on the thermal and dielec-

tric behaviors of SA films were investigated. All the

films were transparent in appearance. No apparent differ-

ence was determined between the thermal properties of

SA and Ca21-crosslinked SA films. The increase in peak

intensity in XRD patterns of SA-Ca film in comparison to

that in SA film confirmed the slight increase in the crys-

tallinity or ordered structure of the film. Dielectric con-

stant and dielectric loss values of both SA and SA-Ca

films decreased with the increase in frequency. The cross-

linking of SA with Ca21 ions decreased the b-relaxation

associated with the carboxylate groups of SA. Frequency

spectra of electric modulus, impedance and Cole-Cole

plots confirmed the higher conductance values of SA-Ca

films at low frequencies than those of SA film due to the

presence of by-product of crosslinking reaction between

SA and CaCl2.

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Dielectric, thermal, and swelling properties of calcium ion-crosslinked sodium alginate film

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Transcript of Dielectric, thermal, and swelling properties of calcium ion-crosslinked sodium alginate film