polyaniline blends: Evaluation of mechanical and electromechanical properties

polyaniline blends: Evaluation of mechanical and electromechanical properties

Polymer Testing 27 (2008) 886–892 Contents lists available at ScienceDirect Polymer Testing journal homepage: www.elsevier.com/locate/polytest Mate...

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Polymer Testing 27 (2008) 886–892

Contents lists available at ScienceDirect

Polymer Testing journal homepage: www.elsevier.com/locate/polytest

Material Properties

Thermoplastic elastomer/polyaniline blends: Evaluation of mechanical and electromechanical properties G.M.O. Barra a, *, R.R. Matins a, K.A. Kafer a, R. Paniago c, C.T. Vasques b, A.T.N. Pires b a

´rio, 88040-900 Floriano ´ polis, Santa Catarina, Brazil Mechanical Engineering Department, LABMat, Federal University of Santa Catarina, Campus Universita ´rio, 88040-900 Floriano ´ polis, Santa Catarina, Brazil Chemistry Department, POLIMAT, Federal University of Santa Catarina, Campus Universita c Federal University of Minas Gerais, Physics Department, Belo Horizonte, Minas Gerais, Brazil b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 May 2008 Accepted 3 July 2008

Conducting polymer blends whose undiluted components have different properties are promising materials for several technological applications such as electromagnetic shielding, electrostatic charge dissipation, optoelectronic displays, chemical sensors, biosensors and electromechanical sensors. The aim of this study was to obtain and evaluate the electrical conductivity and electromechanical properties of a polystyreneblock-poly(ethylene-ran-butylene)-block-polystyrene copolymer (SEBS)/polyaniline doped with dodecylbenzenesulfonic acid (PAni.DBSA) blends. Electrically conductive elastomeric blends based on SEBS/PAni.DBSA were prepared through a solution casting method, at room temperature, after dissolving both components in toluene as a common solvent. SEBS/PAni.DBSA blends were also prepared through polymerization of aniline in the presence of SEBS solution, using a one-step in situ emulsion polymerization method. The protonation degree of the PAni.DBSA used in this study was determined from X-ray photoelectron spectroscopy (XPS) analysis by calculating the amount of different neutral and positive nitrogen species of the polyaniline chain from the properly curve-fitted N-1s core-level spectrum. The PAni.DBSA used to prepare solution casting blends had a 46% protonation degree, with almost all imine groups protonated and only a small amount of residual DBSA, as expected for an emeraldine salt. This protonation degree is responsible for the high conductivity (2.4 S cm1) and good solubility of this conducting polymer in toluene. The protonation degree of PAni.DBSA in the blends analyzed was higher than that of undiluted PAni.DBSA, due to an additional protonation during the casting or in situ processes. The insulator–conductor transition was not as sharp as those found in carbon black conducting systems and the percolation threshold was lower than 20 wt.% of PAni. DBSA. Above 20 wt.% PAni.DBSA content, the emulsion-polymerized systems had higher volume conductivity values than the corresponding solution casting blends. The mechanical properties such as tensile strength, elastic modulus and elongation at break of SEBS/PAni.DBSA blends were dependent on the blend preparation method. Blends were also characterized by electromechanical properties and optical microscopy. Ó 2008 Elsevier Ltd. All rights reserved.

Keywords: Polyaniline Electromechanical properties Protonation degree Thermoplastic elastomer

1. Introduction

* Corresponding author. E-mail address: [email protected] (G.M.O. Barra). 0142-9418/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2008.07.004

There is increasing interest in the development of electrical conducting polymeric materials with good mechanical properties and excellent processability. These materials are finding several technological applications including electromagnetic shielding, electrostatic charge

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dissipation, optoelectronic displays and sensors. Electromechanical sensors are important for application in robotics, touch-sensitive switches, sensors for measurement of vehicle weights to collect toll tax on roads and other remote control systems [1–3]. Electromechanical sensors obtained through blending of intrinsically conducting polymers and rubbers as the matrix, have been attracting interest due to their ease of processing, good flexibility and ability to absorb mechanical shocks. Some unsaturated rubbers such as ethylene–propylene– diene (EPDM) [4], styrene–butadiene copolymer (SBR) [5–8], nitrile rubber (NBR) [9,10] and thermoplastic rubbers including polystyrene-block-poly(ethylene-ran-butylene)block-polystyrene (SEBS) [11] and polyurethane [12] have been used as a matrix to produce conducting elastomeric blends. Of the intrinsically conducting polymers (ICP), polyaniline (PAni) has been extensively studied due to good environmental stability, straightforward polymerization, thermal stability and electrical properties which can be reversibly controlled through changing the oxidation state and protonation of the imine groups. Polyaniline is usually immiscible when blended with rubbers, and gross phase separation processes may restrict the formulation of compatible materials. However, the addition of compatibilizer to the blend allows the formation of the desired morphology in a subsequent processing step. For example, the compatibility of polycarbonate/polyaniline blends was improved after grafting sulfonic groups onto the main chain of the polycarbonate matrix [13]. In a recent study by our group, the phase separation in the SEBS/PAni blends was reduced through increasing the sulfonic group content in the insulating polymer chains [11]. On the other hand, the compatibility of conducting polymer blends was enhanced through inserting counter ions, such as functionalized protonic acids, in the polyaniline (PAni) chains making it possible to form blends with an insulating polymer. An example of such a functionalized acid is dodecylbenzenesulfonic acid (DBSA), which has long alkyl chains that increase the solubility of PAni.DBSA in toluene, xylene and chloroform and acts as a surfactant, inducing compatibility with polymer matrices with a similar structure [14]. Blending of PAni with insulating polymers through solution processing has been reported to be efficient when both polymers are soluble in a common solvent [15,16]. This study was focused on the preparation of SEBS/ PAni.DBSA blends through an in situ emulsion method using ammonium persulfate as the oxidant, and the solution casting technique. The electrical and mechanical properties, morphological and electromechanical sensitivity of conducting polymer blends were investigated. 2. Experimental 2.1. Materials Aniline (analytical grade, Merck) was distilled twice under vacuum and stored in a refrigerator. Ammonium peroxydisulfate (APS) (analytical grade, Merck), and dodecylbenzenesulfonic acid (DBSA) (technical grade, ProQuı´mica do Brazil) were used without purification. The

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polymer used in this study was a block copolymer with styrene hard segments and ethylene–butadiene rubbery segments. Polystyrene-block-poly(ethylene-ran-butylene)block-polystyrene copolymer (SEBS), commercially designated Kraton G1650, was kindly supplied by Shell Quı´mica (Brazil) and contains 29 wt.% styrene units. 2.2. Synthesis of PAni.DBSA The emulsion polymerization of aniline was performed as described by Gospodinova et al. [17]. Firstly, 3.28 g of DBSA were dissolved in 60 mL of toluene and 0.96 mL of aniline was added. The mixture was then stirred for 20 min, at 5  C, following which 2.28 g of APS dissolved in 20 mL water was slowly added, and the reaction mixture was stirred for 6 h. The reaction mixture was precipitated in acetone, and the precipitate was filtered, washed with acetone, and dried in an oven at room temperature. 2.3. Solution casting blend SEBS/PAni.DBSA blends were prepared by dissolving each pure component in toluene as a common solvent. Different compositions were then mixed and stirred for 3 h. The solution was cast on a glass plate to evaporate the solvent at room temperature, obtaining films with thicknesses of around 300–350 mm. 2.4. Preparation of SEBS/PAni.DBSA blends through an in situ process The preparation of SEBS/PAni.DBSA blends followed a procedure similar to that used for the synthesis of PAni. DBSA, adding SEBS previously dissolved in toluene at specific concentrations, to obtain blends with different component compositions. The reaction was carried out at 5  C for 6 h, and the SEBS precipitated in ketone and washed several times with water to remove unreacted monomers. All samples were dried in a drying chamber under vacuum before analysis. The proportion of PAni.DBSA in the blend was determined gravimetrically, considering that all SEBS was precipitated. The blends were dissolved in toluene and free-standing films with thicknesses of around 300–350 mm were prepared by casting on glass plates. Solvent evaporation was performed at room temperature for 24 h. 2.5. Characterization The tensile tests of undiluted SEBS and blends samples with thicknesses in the range 100–350 mm were performed using an EMIC DL 2000 analyzer (EMIC-Brazil), according to ASTM-D-882-95a for thin films at room temperature. The blends were kept under vacuum conditions before analysis. The modulus of elasticity, tensile strength at rupture and elongation were calculated from the stress– strain curves considering at least six analyses of each sample. The device used to measure the electrical–mechanical response consists of placing the specimen between two steel pistons fixed on an insulation cylinder, as schematized

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in Fig. 1. The pistons were connected to a Keithley, Model 6220, current source to apply the current (1 nA–0.1 mA), and a Keithley, Model 6517, electrometer, also connected to the pistons, to measure the potential difference. The conductivity was calculated through Eq. (1), where d is the cylinder diameter (cm), w is the specimen thickness (cm), and I and V are the applied current and voltage measurement, respectively.



4wI

pVd2

(1)

Plots of the potential difference versus the applied current obeyed Ohm’s law, which allows the measurement of the specimen conductivity. The effect of conductivity as a function of compression force was then evaluated. A digital force gauge (MK Instruments) connected to a microprocessor was used to measure compression force. A continuous conductivity response curve with the application and removal of a mechanical load could thus be plotted. All specimens were electrically isolated from the mechanical testing fixtures and the total time for each assay was 300 s. The electrical conductivity of PAni.DBSA was determined using the four probe standard method with a Keithley, Model 6220, current source to apply the current and a Keithley, Model 6517, electrometer to measure the potential difference. The surfaces of the thin films were studied by optical microscopy, on an Olympus BX50 optical microscope at 200 magnification. The XPS measurements of the polymer samples were obtained using a VG ESCALAB 220i-XL spectrometer with an Al Ka X-ray source (1486.6 eV) operating at 150 W. The electron analyzer equipped with five Channeltrons was operated at a fixed pass energy of 50 eV for the survey spectra and 20 eV for the N-1s and C-1s spectra. To compensate for surface charging effects, all binding energies were referenced to the C-1s neutral carbon peak at 284.6 eV. The area ratio corrected by the sensitivity factor was used for quantitative analysis of the XPS data.

3. Results and discussion Emulsion polymerization of aniline in the presence of dodecylbenzenesulfonic acid (DBSA) is an interesting route through which to obtain PAni.DBSA via a one-step in situ method, maintaining good physico-chemical properties, such as thermal stability, high solubility, and good electrical conductivity when compared to the conventional synthesis of PAni.DBSA. An optimum electrical conductivity is reached when two nitrogen atoms per repeating PAni unit are protonated, which corresponds to half the number of nitrogen atoms in the macromolecular chain, i.e., 50% of all imine groups are protonated. The protonation degree of PAni.DBSA used in this study was determined from X-ray photoelectron spectroscopy (XPS) analysis by calculating the amount of different neutral and positive nitrogen species of the polyaniline chain from the properly curve-fitted N-1s corelevel spectrum. PAni.DBSA showed a protonation degree of 46%, with almost all imine groups protonated, as expected for an emeraldine salt. The high protonation degree is responsible for the elevated conductivity value of 2.4 S cm1 and the solubility in toluene and chloroform. As shown in Fig. 2, the PAni.DBSA showed peaks with binding energies (BE) at 398.2 eV and 399.6 eV which are related to imine (–N]) and amine (–NH–) groups, respectively. The peaks with binding energies at 400.8 eV and 402.0 eV are attributed to the positive nitrogen atom, according to reports in the literature [18,19]. The percentages of carbon, nitrogen, sulfur and oxygen were 81.0, 5.5, 3.0 and 10.5, respectively, with a C/N molar ratio of 14.7 and S/N molar ratio of 0.54. These values are very close to the theoretical molar ratios for PAni in the emeraldine salt oxidation state, indicating that almost all of DBSA molecules are bound to imine groups. The N-1s core-level spectra corresponding to the SEBS/ PAni.DBSA (70/30 w/w) blends prepared through in situ and solution casting methods reveal increases of 10% and 20% in the quantity of positive nitrogen, respectively, as

Fig. 1. Experimental device for the measurement of the electrical–mechanical tests.

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Table 1 Distribution of N species on the surface of PAni.DBSA and SEBS/PAni.DBSA blends

Fig. 2. XPS N-1s core-level spectra of PAni.DBSA obtained from emulsion polymerization in toluene. The proportion of each nitrogen component was found to be: ]N– (0.04), –NH– (0.50) and Nþ (0.46).

compared with undiluted PAni.DBSA. The absence of a peak at 398.2 eV (Fig. 3) indicates that all of the imine nitrogen (–N]) was protonated. Similar results have been reported by Leyva et al. in relation to the protonation degree of

Sample

S/N molar ratio

Proportion of N components ]N–

–NH–



S/N–Nþ

PAni.DBSA In situ Solution

0.54 0.95 0.55

0.04 0.00 0.00

0.50 0.45 0.50

0.46 0.55 0.50

0.08 0.40 0.05

PAni.DBSA in SBS/PAni.DBSA blends [13]. Table 1 summarizes the percentage of nitrogen species in PAni.DBSA in undiluted material and blend compositions. The excess of DBSA can be estimated by the difference of S/N molar ratio and the positive nitrogen (–Nþ–). When the value of this difference equals zero, it indicates that all imine groups are protonated. For the blends prepared through the in situ method the value of this difference was 0.40 indicating the presence of DBSA, although these samples had been washed several times with acetone to remove the excess DBSA. Therefore, the low value for this difference in the case of SEBS/PAni.DBSA prepared through solution casting indicates that almost all of the PAni present in the mixture was protonated, without the presence of excess DBSA. The emulsion-polymerized systems had significantly higher conductivity values than the corresponding solution casting blends, as shown in Fig. 4, in compositions above 15 wt.% This behavior suggests that the formation of an interconnected conducting PAni.DBSA phase in the SEBS matrix can be affected through the blend preparation method. The percolation threshold can be fitted to the scaling law of percolation theory [20,21], by Eq. (2):

sf ¼ c f  fp

t

(2)

Electrical Conductivity (S.cm-1)

where c is a constant, t a critical exponent, sf the conductivity, f the fraction of PAni.DBSA and fp the fraction of PAni.DBSA at the percolation threshold, expressed as a weight fraction. The values of the percolation threshold for the in situ and solution methods were 17.2 and 18.7, respectively.

1E-3 1E-4 1E-5 1E-6 1E-7 1E-8 (a) (b)

1E-9 1E-10 1E-11 1E-12 1E-13 0

10

20

30

40

50

PAni.DBSA content (wt.%) Fig. 3. XPS N-1s core-level spectra of SEBS/PAni.DBSA obtained through: (A) in situ emulsion polymerization and (B) solution casting method.

Fig. 4. Electrical conductivity of blends prepared through (a) in situ and (b) solution casting process.

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matrix, the dispersed phase is better interconnected, as can be seen in the optical micrograph of the insert in Fig. 5, increasing the electrical conductivity. The microstructure of both blends revealed typical phase separation with the presence of conducting polymer aggregates, where the dark phase corresponds to the PAni.DBSA. The relative conductivity [Ds] was calculated according Eq. (3), where [ss] is the electrical conductivity under compressive stress and [so] is the electrical conductivity for the original shape.

Fig. 5 shows the tensile strength and electrical conductivity of SEBS/PAni.DBSA blends as a function of PAni.DBSA content. On increasing the PAni.DBSA content in the blends, the tensile strength reduces significantly compared to the undiluted SEBS. For 5% and 30 wt.% of PAni.DBSA the tensile strength values for blends prepared through the in situ method were 18 and 7 MPa, respectively, which are 20% and 60% lower than the values for the pure SEBS. An analogous behavior was observed for the blends prepared through the solution process. However, the tensile properties of the films prepared through the in situ technique are lower than the values for the corresponding films obtained through the solution casting process (Table 2), due to DBSA-free molecules which act as a plasticizing agent. As expected, on increasing the amount of conducting polymer in the SEBS

Ds ¼

½ss  so 

(3)

so

For a filler content below the percolation threshold (around 20 wt.%), the conducting particles are separated by

and and

-in situ method -solution method -2

-4

20

-6 15 -8 10 -10

5

Log(conductivity) (S.c m-1)

Tensile Strength (MPa)

25

-12 0

10

20

30

40

50

PAni.DBSA (wt.%)

A

C

B

D

Fig. 5. Tensile strength and electrical conductivity of SEBS/PAni.DBSA blends as a function of PAni.DBSA content. Micrographs of blends prepared through: (A), (C) solution casting and (B), (D) in situ processes.

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Table 2 Mechanical properties of the blends prepared through in situ and solution casting processes Pani.DBSA content (wt.%)

In situ blends

0 2.5 5 10 15 20 30 40

Solution casting blends

s (MPa)

3 (%)

E (GPa)

s (MPa)

3 (%)

E (GPa)

22.1  0.1 – 17.1  0.4 13.5  0.5 11.7  0.2 10.8  0.3 8.7  0.5 6.2  0.9

535.3  0.8 – 530.7  1.3 525.9  1.6 523.1  2.1 499.7  2.6 470.3  2.1 372.5  2.7

95.2  0.9 – 99.3  1.2 110.5  1.8 115.7  1.1 120.8  1.4 125.3  1.3 128.5  1.9

22.1  0.2 21.0  0.3 18.1  0.5 15.4  0.7 14.3  0.9 13.1  0.5 10.5  0.6 8.3  0.9

535.2  0.7 537.5  1.1 548.7  1.5 545.5  1.0 540.1  1,7 534.2  1.9 507.0  2.0 404.9  2.4

95.2  0.4 101.2  1.8 108.9  2.1 120.8  1.1 125.6  2.0 130.3  2.2 135.2  2.5 137.6  2.8

a polymer matrix insulation layer and the electrical conductivity is low. Under these conditions, deformation of the blend specimen at constant temperature due to applied compressive stress can decrease slightly the PAni domain distances, and no significant changes in relative electrical conductivity are observed (Fig. 6). Above the percolation threshold, the conducting polymer particles are very close,

18

a

which contributes to an increase in the electrical conductivity of the blends. Under compressive stress, the contact between the conducting particles increases and, consequently, the relative electrical conductivity of the polymer blend increases significantly. For SEBS/PAni.DBSA samples (70/30) prepared through the solution method, there is a 15-fold increase in the relative conductivity with compressive stress up to 200 MPa, reaching a constant value above this pressure (Fig. 6a). For blends with a PAni.DBSA content of 40 wt.%, there was a 2-fold increase in the relative conductivity for the whole range of compressive

SEBS /PAni.DBSA (w/w) 80/20 70/30 60/40 50/50

Δσ

12 9 6 3 0 0

50

100

150

200

250

300

Electrical Conductivity (S.cm-1)

15

Compressive Stress (MPa)

1.2x10-4

a

1.0x10-4 8.0x10-5 6.0x10-5 4.0x10-5

Load Cycle one five ten

2.0x10-5 0.0 0

8

Electrical Conductivity (S.cm-1)

SEBS/PAni.DBSA(w/w) 80/20 70/30 60/40 50/50

10

6 4 2 0 0

100

150

200

250

Compressive Stress (MPa)

b

12

Δσ

50

50

100

150

200

250

Compressive Stress (MPa)

1.0x10-3

b

8.0x10-4 6.0x10-4 4.0x10-4 Load Cycle one five ten

2.0x10-4 0.0 0

50

100

150

200

Compressive Stress (MPa) Fig. 6. Relative conductivity (Ds) as a function of applied compressive stress for SEBS/PAni.DBSA blends prepared through: (a) solution and (b) in situ processes in different compositions.

Fig. 7. Electrical conductivity as a function compressive stress for (a) solution casting and (b) in situ method.

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stress applied. This behavior suggests that the PAni.DBSA agglomerate is compact enough to allow the limit value of relative conductivity, slightly influenced by the compressive stress applied. For 50 wt.% of conducting filler, an analogous behavior was observed, but the electrical conductivity increased only at values above 200 MPa, due to the higher conducting pathway for this blend composition. Similar results were obtained for the blends prepared through the in situ method, although for these blends the sensitivity to compression was less pronounced than for those obtained from the solution casting process. The dependence of the electrical conductivity on compressive stress during loading and unloading was measured for SEBS/PAni.DBDA (70/30) blends prepared through the different processes, as can be seen in Fig. 7. The irreversibility and hysteresis under mechanical pressure are due to the breakdown of the conductive network or plastic deformation of the matrix. Thus, a difference between the final and initial relative conductivities in the first cycle can be observed for the two conducting polymer systems. For the second to tenth cycles the relative conductivity practically returns to its previous value after the sample loading is removed due to the re-organization of the conducting pathway and elastic deformation of the matrix. 4. Conclusions Flexible and conducting SEBS/PAni.DBSA films can be obtained through solution casting or in situ methods. The method used to prepare the SEBS/PAni.DBSA blends affected the conductivity values and percolation threshold. Through the XPS analysis, it was possible to observe an increase in the proportion of positively charged nitrogen species in blends. The XPS data also indicated an excess of DBSA present in blends prepared through the in situ process, which acts as a plasticizing agent, reducing the Young’s modulus, the elongation and tensile stress at break point when compared to the blends obtained through the solution casting method. For all of the blend specimens studied, the relative electrical conductivity changed slightly as a function of compressive stress. However, for the SEBS/PAni.DBSA blend specimen with a 70/30 (w/w) composition the compressive

stress has a considerable influence on the electrical conductivity. The increase in the electrical conductivity with pressure may be a result of the re-organization of the electro-conductive polyaniline network structure. The relative conductivity is almost the same as its previous value after the sample loading is removed, which makes this material suitable for the development of pressure sensors. Acknowledgements The authors would like to thank CNPq for the financial support. References [1] M. Knife, V. Teteris, A. Kiploka, J. Kaupuzs, Sens. Actuat A – Phys. A 110 (2004) 142–149. [2] M. Knife, V. Tupureina, A. Fuith, J. Zavickis, V. Teteris, Mater. Sci. Eng. C – Biomim. 27 (2007) 1125–1128. [3] X. Wang, D.D.L. Chung, Sens. Actuat A – Phys. A 71 (1998) 208–212. [4] R. Faez, I.M. Martim, M.-A. De Paoli, M.C. Resende, J. Appl. Polym. Sci. 83 (7) (2002) 1568–1575. [5] F.G. Souza Jr., J.C. Pinto, G.E. de Oliveira, B.G. Soares, Polymer Test. 26 (6) (2007) 720–728. [6] F.G. Souza Jr., R.C. Michel, B.G. Soares, Polymer Test. 24 (8) (2005) 998–1004. [7] M.E. Leyva, B.G. Soares, D. Khastgir, Polymer 43 (26) (2001) 7505–7513. [8] F.G. Souza Jr., M. Almeida, B.G. Soares, J.C. Pinto, Polymer Test. 26 (5) (2007) 692–697. [9] W.E. Mahmoud, A.M.Y. El-Lawindy, M.H. El Eraki, H.H. Hassan, Sens. Actuat A – Phys. 136 (1) (2007) 229–233. [10] B.G. Soares, G.S. Amorim, F.G. Souza Jr., M.G. Oliveira, J.E.P. da Silva, Synth. Met. 156 (2006) 91–98. [11] G.M.O. Barra, L.B. Jacques, R.L. Ore´fice, J.R.G. Carneiro, Eur. Polym. J. 40 (2004) 2017–2023. [12] D.S. Vicentini, G.M.O. Barra, J.R. Bertolino, A.T.N. Pires, Eur. Polym. J. 43 (10) (2007) 4565–4572. [13] W.-J. Lee, Y.-Ju Kim, S. Kaang, Synth. Met. 113 (2000) 237–243. [14] C.Y. Yang, Y. Cao, P. Snith, A.J. Heeger, Synth. Met. 53 (1993) 293–301. [15] M.E. Leyva, G.M.O. Barra, B.G. Soares, Synth. Met. 123 (2001) 443–449. [16] E. Ruckenstein, L. Hong, Synth. Met. 66 (1994) 249–256. [17] N. Gospodinova, L. Terlemezyan, P. Mokreva, J. Stehjskal, P. Kratochvil, Eur. Polym. J. 29 (10) (1993) 1305–1309. [18] E.T. Kang, K.G. Neoh, K.L. Tan, Prog. Polym. Sci. 23 (1998) 277–324. [19] G.M.O. Barra, M.E. Leyva, M.M. Gorelova, B.G. Soares, M. Sens, J. Appl. Polym. Sci. 80 (2001) 556–565. [20] K. Levon, A. Margolina, Macromolecules 26 (1993) 4061–4063. [21] D. Stauffer, Introduction to Percolation Theory, Taylor and Francis, London, 1985.