Thermal resistance of magnetic polymeric composites basedon styrene, divinylbenzene, and Ni and Co particles

Simone Simplıcio • Elizabete Fernandes Lucas •

Marcos Antonio da Silva Costa • Luciana Cunha Costa •

Luiz Claudio de Santa Maria

Received: 10 July 2013 / Accepted: 16 February 2014 / Published online: 5 March 2014

� Akademiai Kiado, Budapest, Hungary 2014

Abstract Polymers based on styrene (STY) and divinyl-

benzene (DVB) are widely used for water treatment. The

chemical modification of these materials, with the intro-

duction of chemical groups, increases their selectivity for

certain types of contaminants. The incorporation of mag-

netic particles makes these systems useful for removing

contaminants from aquifers, due to their magnetic attrac-

tion of certain residues. In this work, STY–DVB resins

(unmodified, sulfonated, and impregnated with nickel and

cobalt particles) were analyzed by thermogravimetry. The

thermal stabilities of all the samples were compared,

showing that sulfonation reduces the thermal stability of

the resin, but the incorporation of metal particles restores

that stability, with the extent of this recovery depending on

the type of metal. This result shows that even when the

incorporation of metal particles does not involve removal

of contaminants by magnetic attraction, this inclusion is

still justified because it increases the material’s thermal

stability and also makes it more efficient for removing

certain types of non-metallic contaminants, as observed in

a previous study. Besides this, the thermogravimetric

analysis was highly useful to ascertain the changes caused

to the materials, including allowing inferences on the semi-

quantitative results of the degree of sulfonation and con-

firming that metal compounds are not only physical

mixtures.

Keywords Polymeric resins � Magnetic composites �Thermal degradation � Ash content

Introduction

Since the first synthetic organic polymer was produced,

there has been increasing research to obtain new materials

with technological potential in ion-exchange processes

[1–4]. Besides other uses, polymers are widely employed

for environmental remediation [5–10]. Normally, pollu-

tants are removed from industrial wastewaters by chemical

precipitation, flotation, oxi–reduction reaction, filtration,

and adsorption on activated charcoal. Although these

methods are widely used due to their low-operational cost,

they have low selectivity [5, 8, 9]. Hence, there is a need to

introduce new materials such as magnetic ion-exchange

(MIEX�) resins, which are based on a polymer matrix

containing magnetic microspheres. One of the most

important applications of these microspheres for environ-

mental remediation is to treat water for household use [11].

Porous resins can be characterized according to various

aspects, such as morphology, surface area, pore volume,

average particle diameter, density, and swelling capacity

[3, 6, 12, 13]. In the case of functionalized resins, the ion-

exchange capacity is a key feature to measure [12]. Mag-

netic resins require complementary characterization to

S. Simplıcio � M. A. da Silva Costa � L. C. de Santa Maria

Departamento de Quımica Organica, Instituto de Quımica,

Universidade do Estado do Rio de Janeiro, Rua Sao Francisco

Xavier 524, Maracana, Rio de Janeiro, RJ 20559-900, Brazil

E. F. Lucas (&)

Instituto de Macromoleculas, Universidade Federal do Rio de

Janeiro, Av Horacio Macedo, 2030, Cidade Universitaria,

Rio de Janeiro 21941598, Brazil

e-mail: elucas@ima.ufrj.br

L. C. Costa

Centro Setorial de Ciencias Biologicas e da Saude, Centro

Universitario Estadual da Zona Oeste, Av. Manuel Caldeira de

Alvarenga 1203, Rio de Janeiro 23070200, Brazil

123

J Therm Anal Calorim (2014) 117:369–375

DOI 10.1007/s10973-014-3703-9

confirm the incorporation of the metals. This can be done

by, for example, EDS as well as by measuring the magnetic

activity of the particles [3, 14, 15].

The thermal characterization of these types of materials

in general is not often performed. Nevertheless, the deg-

radation behavior as a function of the variation of tem-

perature is of great importance not only regarding potential

use of the material at a determined temperature [16], but

also for evaluation of degradative processes [17, 18] and

monitoring the modification reactions during the process of

obtaining the final materials [19].

Thermogravimetric analysis is defined as a continuous

process that involves measuring the variation in mass of a

sample as a function of temperature change (temperature

scan) or during a period of constant temperature (isother-

mal mode) [20]. It has been widely applied in fundamental

studies and also for industrial applications [21, 22].

Specifically to study ion-exchange resins, thermogravi-

metric analysis has been used to assess sulfonated resins in

the hydrogen, sodium, and calcium forms [23]; to deter-

mine kinetic parameters of the dehydration step and to

correlate these with the characteristics of reticulation, pore

formation, and exchange capacity [24]; or to monitor the

decomposition process as a function of the type of com-

plexed ion [25].

In this study, thermogravimetric analysis was used to

assess the stability of porous resins based on styrene and

divinylbenzene (STY–DVB) as a function of the sulfona-

tion reaction and type of metal particles incorporated, as

well for semi-quantitative characterization of the products

of the reactions obtained in each step of the synthesis

process.

Experimental

The copolymer materials based on STY–DVB used in this

study were synthesized as described in a previous work

[15, 26]. STY–DVB copolymers were synthesized by

aqueous suspension polymerization at 70 �C for 24 h, with

the aqueous phase composed of NaCl, gelatin, and 2-hy-

droxiethylcellulose; and the organic phase consisting of

STY:DVB (80:20 mol%), AIBN (6.2 9 10-3 mol in rela-

tion to total mols of monomers), and a diluent mixture

constituted of toluene and n-heptane. The n-heptane/

toluene ratios used as diluents were 90:10, 60:40, and

0:100 v/v, which produce the copolymers C1, C2, and C3,

respectively. The volumetric ratio between the aqueous and

organic phase was 3:1 (v/v). The sulfonation reaction of the

copolymers was carried out with acetyl sulfate and the

copolymer swollen in 1,2-dichloroethane, at a 90 �C for

24 h. The quantity of metal used experimentally to

impregnate the three sulfonated copolymers samples was

the same: 0.5 g of sulfonic resins immersed in 0.25 M

aqueous solutions containing salts of Ni2? or Co2?

(NiCl2�6H2O or CoCl2�6H2O, respectively). Table 1 pre-

sents the identification and data on the morphological and

magnetic characterization of these polymers. The desig-

nations C#, C#S, C#SCo, and C#SNi refer, respectively, to

STY–DVB, sulfonated STY–DVB, sulfonated STY–DVB

impregnated with cobalt, and sulfonated STY–DVB

impregnated with nickel.

The procedures used to get the characterization data are

summarized as following: the porosity was determined by

apparent density using the graduated cylinder method [27];

swelling degree in 1,2-dichloroethane [28]; surface area

Table 1 Characteristics of polymeric materials [15]

Material Morphological parameters Ion-exchange

capability/mmol g-1Magnetic parameters

A/m2 g-1 Vp/cm3 g-1 Dm/nm dap/g cm3 I/% Hc/Gauss Ms/emu g-1 Mr/emu g-1

C1 69.48 0.325 9.7 0.43 73 – – – –

C2 1.48 0.0031 3.3 0.60 166 – – – –

C3 1.53 0.0031 5.3 0.65 150 – – – –

C1S – – – – 73 4.9 – – –

C2S – – – – 110 5.3 – – –

C3S – – – – 117 4.5 – – –

C1SNi – – – – – – 217.74 1.3381 0.18469

C1SCo – – – – – – 181.08 0.57398 0.07767

C2SNi – – – – – – 295.60 1.5081 0.56403

C2SCo – – – – – – 213.95 1.3133 0.19034

C3SNi – – – – – 148.34 2.7952 0.52129

C3SCo – – – – – – 209.26 2.9383 0.36162

A specific area, Vp porous volume, Dm average diameter of porous, Dap apparent density, I swelling in 1,2-dichloroethane, Hc coercivity,

Ms saturation magnetizing, Mr permanent magnetizing

370 S. Simplıcio et al.

123

and pore volume distribution by nitrogen adsorption fol-

lowed the BET and BJH methods, respectively (Microm-

eritcs ASAP 2020 apparatus); the exchange capacity was

determined by acid–base titration using standardized

NaOH solutions (0.1 M) [29]; and magnetization curves

were obtained by using an AGGPAR 4500 vibrating sam-

ple magnetometer (VSM), calibrated against a cylindrical

nickel standard at room temperature (cycle time = 1 s) and

hysteresis (cycle time = 10 min).

The thermogravimetric analyses were carried out in a

TA Instruments (USA) Q50 analyzer. The samples were

heated to 700 �C at a rate of 20 �C min-1, under a nitrogen

atmosphere (flow of 100 mL min-1). The mass loss of the

sample was continuously monitored during the heating

process and was recorded on a graph of mass loss percent

as a function of temperature.

Results and discussion

STY–DVB resins

Thermogravimetric analysis under a nitrogen atmosphere

was used to evaluate the thermal resistance of the synthe-

sized materials. Figure 1 presents the TG and DTG curves

of the samples C1, C2, and C3, and Table 2 reports the

values of the principal degradation temperatures obtained

from these curves along with the residue content of these

unmodified copolymers. The curves show that the thermal

degradation occurs in a single stage, and the difference in

stability for the three samples is not significant. The

derivative of mass as a function of temperature provides

the mass loss rate (the peak of the curve indicates the

temperature of maximum mass loss velocity). Copolymer

C2 presented the lowest degradation onset temperature,

followed by C3 and C1, which was the most thermally

stable material. This behavior is probably related to the

diameter of the pores of each sample and the heterogeneity

of the sample: materials with larger pore diameters and

higher heterogeneity have greater thermal stability [30]. As

observed in a previous study [15] by electron microscopy,

the roughness of the samples increases inversely with the

thermal stability. The concentration of non-degraded

inorganic material (coke and inorganic residues) was very

low, indicating that approximately 99 % of the material

degraded up to the maximum temperature was organic.

Sulfonated STY–DVB resins

The TG and DTG curves can be seen, respectively, in

Fig. 2a, b for the three sulfonated resins, C1S, C2S, and

C3S. In relation to the original curves (Fig. 1), which

showed only one degradation stage, the sulfonated

copolymers presented three degradation stages, confirming

the chemical modification achieved.

The first stage was the loss of water with increasing

temperature, at around 80 �C. Between 250 and 380 �C

occurs the degradation related to the presence of the –SO3H

group in the copolymer’s structure, releasing SO and SO2; it

is known that the reverse reaction, desulfonation, is ther-

modynamically possible, so that sulfonation is one of the few

reversible aromatic electrophilic substitution reactions [31].

Finally, at DTG, peak maximum temperature around

420–430 �C occurs the decomposition of the polymeric

120

100

80

60

40

20

00 200 400 600

C1C2

C3

Mas

s/%

Temperature/°C

1.2

1.0

0.8

0.6

0.4

0.2

0.0

–0.20 200 400 600

Temperature/°C

Mas

s de

rivat

ive/

%/°

CC1

C2

C3

(a)

(b)

Fig. 1 TG (a) and DTG (b) curves of non-modified copolymers (C1,

C2, and C3)

Table 2 Thermal characteristics of the copolymers

Copolymer Tonset/�C Td/�C Content of

residue/%

C1 367 421 1.56

C2 345 414 1.84

C3 358 424 1.46

Tonset initial temperature of degradation, Td DTG peak maximum

temperature

Thermal resistance of magnetic polymeric composites 371

123

reticulum, which begins at lower temperatures, overlapping

with the degradation related to the sulfonic group. An

increase in the quantity of inorganic residue, due to the

introduction of inorganic components in the polymer and the

possible production of coking compounds, was observed.

The main degradation temperatures obtained from the peaks

of the DGT curves and the residue content of each sulfonated

resin are shown in Table 3. These results evidence the

chemical modification in the polymeric matrix, since the TG

and DTG curves revealed desulfonation stages and an

increase in residue content in relation to the unmodified

copolymer. In a previous work [15], the degree of incorpo-

ration of sulfonic groups was found to be related to the

materials’ swelling capacity and cation exchange capacity. It

is known that the concentration of sulfonic groups intro-

duced in the polymer matrix basically depends on two fac-

tors: surface area of the copolymers and their swelling

capacity in the sulfonation medium. However, the data pre-

sented in Table 1 reveal that the cation exchange capacity

did not vary according to swelling capacity. The variation in

mass loss related to the second degradation stage was smaller

for the copolymer C1S (15 %), which presented the lowest

swelling capacity. Samples C2S and C3S had similar

swelling capacities and mass loss percentages related to the

second degradation stage (*21 %). The mass loss at the

second stage was taken between the temperatures that de-

limitate the second DTG peak: from *200 to *360 �C.

Although the degradation of polymeric reticulum starts

during the sulfonic group degradation, we are assuming that,

since the degradation of polymer reticulum is almost the

same for the three samples (as shown in Fig. 1), the differ-

ences among the second stage of the three samples were

120

100

80

60

40

20

00 200 400 600

C1S

C2S

C3S

1.2

1.0

0.8

0.6

0.4

0.2

0.0

–0.20 200 400 600

Temperature/°C

Temperature/°C

C1SC2S

C3S

Mas

s de

rivat

ive/

%/°

CM

ass/

%(a)

(b)

Fig. 2 TG (a) and DTG (b) curves of modified copolymers (C1S,

C2S, and C3S)

Table 3 Thermal characteristics of sulfonated copolymers

Sulfonated resins Td0/�C Td1/�C Td2/�C Content of

residue/%

C1S 75 315 424 33

C2S 75 309 425 36

C3S 75 306 416 40

T initial temperature of degradation, Td DTG peak maximum

temperature

120

100

80

60

40

20

00 200 400 600

Temperature/°C

Mas

s/% C1

C1SC1SNi

C1SCo

C1

C1SC1SNi

C1SCo

1.2

1.0

0.8

0.6

0.4

0.2

0.0

–0.20 200 400 600

Temperature/°C

Mas

s de

rivat

ive/

%/°

C

(a)

(b)

Fig. 3 TG (a) and DTG (b) curves of copolymer C1 and the

materials C1S, C1SNi, and C1SCo

372 S. Simplıcio et al.

123

attributed to sulfonic group degradation. The residue con-

centration results for the sulfonated samples were very

similar, but the lowest residue content (Table 3) corre-

sponded to the smallest mass loss variation in the stage

associated with the degradation of the sulfonic groups and

the smallest swelling degree (Table 2). These results are

more mutually concordant than those for cation exchange

capacity, which were only related to the concentration of

accessible sulfonic groups.

Magnetized STY–DVB resins

The TG and DTG curves of the sulfonated copolymers

impregnated with nickel and cobalt indicated the presence

of other degradation steps in relation to the original and

sulfonated polymers, as well as a significant increase in the

concentration of residues, due to the introduction of the

metal particles.

The TG and DTG curves for the copolymers C1SMetal,

C2SMetal, and C3SMetal are shown in Figs. 3a, b, 4a, b,

and 5a, b, respectively.

In general, the impregnation of metal particles in the

polymer matrix improved the material’s thermal resistance,

by increasing the temperature of desulfonation and degra-

dation of the polymer reticulum. Like for the sulfonated

120(a)

(b)

100

80

60

40

20

00 200 400 600

Temperature/°C

Mas

s/%

1.2

1.0

0.8

0.6

0.4

0.2

0.0

–0.20 200 400 600

Temperature/°C

Mas

s de

rivat

ive/

%/°

C

C2C2SC2SNiC2SCo

C2C2S

C2SNiC2SCo

Fig. 4 TG (a) and DTG (b) curves of copolymer C2 and the

materials C2S, C2SNi, and C2SCo

120(a)

100

80

60

40

20

0

Mas

s/%

0 200 400 600

Temperature/°C

0 200 400 600

Temperature/°C

(b) 1.2

1.0

0.8

0.6

0.4

0.2

0.0

–0.2

Mas

s de

rivat

ive/

%/°

C

C3C3SC3SNiC3SCo

C3C3SC3SNiC3SCo

Fig. 5 TG (a) e DTG (b) curves of copolymer C3 and the materials

C3S, C3SNi, and C3SCo

Table 4 Thermal characteristics of copolymers containing metallic

particles

Resin Tonset/�C Residue/% Residue

differencea/%

Resistance

increasingb/%

C1SNi 440 62 29 87.9

C1SCo 451 56 23 69.7

C2SNi 466 60 24 66.7

C2SCo 446 54 18 50.0

C3SNi 461 65 25 62.5

C3SCo 462 68 28 70.0

a Difference between the residue percentages after and before metal

incorporation (residue % of C#SMe - residue % of C#S)b Percentage increasing of thermal resistance at 700 �C when com-

paring sulfonated copolymers and metal impregnated ones: (residue

% of C#SMe - residue % of C#S)/residue % of C#S 9 100

Thermal resistance of magnetic polymeric composites 373

123

resins, the exit of water from the materials could be

observed in the DGT curves at about 100 �C; for thermal

stability comparison among all copolymers, the water loss

step was considered. Some of the materials presented as

many as two degradation stages, above the onset temper-

ature. This can be explained by the generation of more

resistant materials during the degradation process.

Table 4 reports the degradation onset and residue per-

centage at 700 �C of the materials impregnated with the

metal particles. These results, compared with those in

Table 3, indicate that the anchorage of metal particles on

the sulfonated polymer matrix increased the material’s

thermal resistance. The thermal stability of the copolymers

impregnated with cobalt particles or nickel particles was

quite similar.

Difference between the TG residual mass percent values

results after (Table 4) and before (Table 3) the incorpora-

tion of the metals was calculated. The differences varied

from 18 % (C2SCo) to 29 % (C1SNi), corresponding to an

increasing in stability, at 700 �C, of 50 and 87.9 %,

respectively. By these results, it becomes clear that the

thermal resistance of the materials does not depend only on

the amount of metal added to the reaction (that was the

same in all reactions), but also on the characteristics of the

matrix: sample C2 having the lowest porous volume

(Table 1) presented also the lowest thermal resistance

increasing.

When comparing the increasing thermal resistance

(Table 4) with saturation magnetizing, Ms (Table 1), for

each copolymer matrix separately, it seems that there is a

relationship: the higher increasing in the thermal resis-

tance, the higher the saturation magnetizing. However, it

was not possible to establish a correlation between these

two results for samples constituted of different copolymer

matrix.

Conclusions

The resins based on STY–DVB presented variations of the

thermal stability as a function of their morphology: the

more homogeneous samples were more susceptible to

thermal degradation. The sulfonation of these samples

reduced their thermal stability and also modified the ther-

mal degradation mechanism. The mass loss percentages in

the temperature range associated with degradation of the

sulfonic groups were in accordance with the swelling

percentages of the unmodified resin, as obtained previ-

ously. The metal particle incorporation in the copolymer

matrix was confirmed by the thermal degradation profile.

The incorporation of metal in the sulfonated resins signif-

icantly increased their thermal resistance. This result shows

that even when the material will not be used for metal

removal by magnetic field action, the incorporation of

metal is justified by the increased thermal stability, besides

making the material more efficient in removing some types

of contaminants, as observed in a previous work.

Acknowledgements The authors thank the Brazilian Council for

Scientific and Technological Development (CNPq).

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