Influence of clay nanofiller on electrical and rheological properties of conductive polymer composite

Materials Letters 58 (2004) 739 – 745 www.elsevier.com/locate/matlet

Influence of clay nanofiller on electrical and rheological properties of conductive polymer composite J.F. Feller *, S. Bruzaud, Y. Grohens Polymers and Processes Laboratory, South Brittany University, Rue de Saint-Maude´, 56 321 Lorient, France Received 25 June 2003; accepted 20 July 2003

Abstract The influence of melt dispersion of intercalted montmorillonite nanofiller (MT) on electrical and rheological properties of three conductive polymer composites (CPC): poly(ethylene)-carbon black (PE-CB), poly(propylene)-carbon black (PP-CB) and poly(ethylene-coethyl acrylate)-carbon black (EEA-CB) has been investigated as a function of the nature of both fillers and matrices. Even at very low content, i.e., less than 1.25% v/v, MT has an important effect on the percolation threshold and on the storage ( GV) and loss ( GW) moduli. The phenomena observed tend to show that CB aggregates can adsorb on MT platelets thus modifying both the level of CB dispersion and the flow properties. Added to EEA-CB, MT decreases the resistivity by 2.5 decades and decrease the storage modulus in the melt by 40% thus improving the conductivity and processing conditions. D 2003 Elsevier B.V. All rights reserved. Keywords: Conductive polymer composites; Nanofiller; Clay; Percolation threshold

1. Introduction Conductive polymer composites (CPC) resulting from the association at an insulating polymer matrix with conductive fillers exhibit several interesting features due to their resistivity variation with thermal [1 – 11], mechanical [11,12] or chemical [11,13 –18] solicitations. This versatility of CPC is used for ‘‘intelligent’’ applications such as selfregulated heating or vapor detection. However, this important sensitivity of CPC toward its environment also means that a good control of final properties is impossible if the numerous influent factors involved during the formulation and processing are not identified. The main significant factor is the filler distribution within the matrix, which can result from processing conditions (temperature, shearing, viscosity, and orientation), formulation (filler content, molecular weight and crystallinity of the polymer [2,10], solubility parameters, particle/particle and particle/macromolecule interactions [3,4]) and spatial parameters (shape factor of the conducting particles [8,19], exclusion domains in which particles cannot go [3 – 5,16]). Whatever the application, the percolation threshold, i.e., the volume fraction (/c) over which the CPC becomes conductive, is * Corresponding author. E-mail address: [email protected] (J.F. Feller). 0167-577X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.07.010

very sensitive to variations of any of the previously mentioned parameters. For many applications, it is useful to decrease the percolation threshold to both reduce the CPC cost and make the processing easier. This can be achieved with multiphase CPC matrices [3,5]. In the present work, we have investigated an original solution still few studied [20], which consists in introducing in the CPC, a non-conductive clay nanofiller (intercalated montmorillonite) to increase the exclusion volumes of the conductive filler (carbon black). Clays have been widely used in the last decade to design nanocomposites with increased mechanical, thermal, fireproofing and barrier properties due to their high specific area resulting from their nanometric dispersion [21,22]. Nanocomposites are constituted of a polymer material and lamellar mineral compound. Many routes leading to nanocomposites are described in the literature, in particular, the one consisting in the dispersion of layered mineral compound in organic polymer matrix. Several strategies can be considered to prepare polymer-layered mineral nanocomposites [23 – 28]. The three main processes are the in situ intercalative polymerization, the exfoliation from polymer in solution and the melt intercalation. In any case, the result from lamellar mineral is the synthesis of intercalated then exfoliated nanostructures (Fig. 1). The most often used nanofillers are modified layered clays such as montmorillonite, for instance. Because exfoliated nanocom-

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Fig. 1. Schematic representation of, from left to right: layered mineral, intercalated nanostructure and exfoliated nanostructure.

posites have higher phase homogeneity than the intercalated counterparts, the exfoliated structure is more desirable in enhancing the properties of the nanocomposites. However, it is not easy to achieve complete exfoliation of clays and, indeed with few exceptions, the majority of the polymer nanocomposites reported in the literature were found to have intercalated or partially exfoliated nanostructures. We have studied the influence of the melt incorporation of MT, on electrical and rheological properties of three different CPC: PE-CB, PP-CB and EEA-CB. The mechanisms involved in the CPC percolation threshold modification will also be discussed.

2. Experimental 2.1. Materials Poly(ethylene-co-ethyl acrylate) (EEA-CB) filled with 37% w/w (23% v/v) of Cabot Vulcan XC72 carbon black is LE 7704 from Borealis; EEA is a statistical copolymer resulting from radical polymerisation; low density poly(ethylene) (PE-CB) and isotactic poly(propylene) (PP-CB) filled with 32 w/w (19.6) and 40% w/w (25% v/v) of Cabot Vulcan XC72 carbon black, respectively, was provided by Premix; characteristics of the different polymers can be found in Table 1. The clay nanofiller GaramiteR 1958 (MT) was kindly supplied by Southern Clay Products. MT is composed of montmorillonite intercalated with an organo compound (IOC) which is a functional amino-alkyl ammonium short Table 1 Polymer composites characteristics

Tg (jC) Tm (jC) Tc,n (jC) DHm/DHl (J g
1) corrected from CB% X% crystallinity Density (at 25 jC) %Carbon black w/w

EEA-CB

PP-CB

PE-CB


33 F 3 99.5 F 0.5 83 F 0.5 63/303


10 F 3 164.4 F 0.5 115.4 F 0.5 104.9/200

– 101.7 F 0.5 86.1 F 0.5 90/303

20 0.925 F 0.05 37

52.4 0.893 F 0.05 40

27.7 0.93 F 0.05 32

chain. It is commonly used in paints as a tixotropic agent and in thermosets. MT has a density of about 1.6 and a moisture content close to 4%. MT has been incorporated into the CPC at 0.1, 0.5, 1.5 and 2% w/w. 2.2. Characterisation Sample processing was done in a first step using a Brabender 50 EHT internal mixer with contra rotating blades. All polymers were heated at 90 jC under vacuum before processing to get rid of the moisture. Polymers and fillers were blended in the melt at a processing temperature of 200 jC with a rotating speed of 40 rd min
1 for 15 min, to obtain about 40 g of CPC. In a second step, the melt CPC was pressed into 3-mm-thick sheets. Calorimetric measurements were made on a PerkinElmer Pyris 1 differential scanning calorimeter (D.S.C.) with Pyris V3.0 software for data collection and treatment. The calibration was done with indium and zinc. The base line was checked every day. Aluminium pans with holes were used and the samples mass was approximately 10 mg. All the temperatures measured from a peak extremum (Tc, Tm) are determined at less than F 0.5 jC and from a sigmoid (Tg) at less than F 1 jC. Electrical resistivity was measured by a four-probe technique described in a previous work [6]. Current is automatically adjusted for measurements under 2-V DC voltage. Silver paint was used to ensure good electrical contacts on samples of typical dimensions, 2  10  70 mm3. All measurements are done at 40 jC. Rheological properties of CPC were studied with a ThermoHaake RheoStress 1 rheometer with cone/plate geometry (diameter: 20 mm, conicity: 1j, gap: 60 Am) in dynamical mode at f = 1 Hz and c = 2%, after determination of the linear strain range.

3. Results and discussion 3.1. Influence of the clay on electrical properties As previously discussed in introduction, the determination of the percolation curve (evolution of resistivity with

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Fig. 2. Percolation curves of EEA-CB, PE-CB and PP-CB CPC.

carbon black content) is the first step for the validation of any CPC formulation. As /c depends not only on the surface chemistry of both matrix and filler but also on the polymer regions from which carbon black is excluded, its value is hardly predictable. Fig. 2 shows the percolation curves obtained for PE-CB, PP-CB and EEA-CB CPC which provide after fitting with Eq. (1), the values of /c, t and q0 recalled in Table 2. q ¼ q0 ð/
/c Þ
t

ð1Þ

q is the resistivity (V cm), q0 a constant, / the volume fraction, /c the volume fraction at the percolation threshold and t the critical exponent. These results show that PP-CB leads to both lower percolation threshold /c and critical exponent t. These findings would mean that in this system, CB is concentrated in exclusion zones resulting from crystallisation where carbon black is rejected in the amorphous zone, but not highly structured. In EEA-CB, unlike in the previous case, CB is more structured together with a strong association with the EA part of the copolymer and thus better dispersed in the matrix. PE-CB has an intermediate behaviour, less crystalline than PP but with a higher structured CB network.

All CPC have been formulated with a CB content close to /c, 17.75% CB w/w (10% v/v), for PE and EEA and 7% CB w/w (3.6% v/v) for PP, to study the influence of MT nanofiller introduction on electrical properties. The results presented in Fig. 3 show rather different electrical behaviours as a function of the CPC matrix. Increasing MT content decreases EEA-CB resistivity of about 2.5 decades over 0.6% MT v/v, whereas for PP-CB and PE-CB, a large resistivity increase is observed up to 0.06% MT w/w, of 3 and 1.5 decades, respectively. Over 0.6% MT, PP-CB resistivity slightly increases (less than a decade) and PECB resistivity decreases of 1.5 decade. These rather important modifications with regard to the low contents of MT involved (0.06 to 1.28% v/v) suggest two types of mechanisms: interactions of MT with either CB

Table 2 Percolation model q0 and t coefficients /c q0 t

PP

PE

EEA

5.9 676.08 1.95

7.4 1288.25 2.57

11.5 10 000.00 2.58

Fig. 3. Influence of MT content on CPC resistivity near the percolation threshold.

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Fig. 6. Reinforcement effect of CB on PP, PE and EEA in the melt, fitted with the Guth and Gold model: GW/GW versus CB% v/v.

Fig. 4. Schematic representation of the CPC organisation at the nanometric scale.

or the polymer chains or changes in CB distribution due to steric or exclusion effects. For further interpretation, it must also be recalled that, in MT clay platelets can interact with CB or polymer chains through the functional amino-alkyl ammonium short chain (IOC) (Fig. 4). In the case of EEACB CPC, both mechanisms can be involved, but the decrease of resistivity observed over 0.5% v/v of MT as observed in Fig. 3, may be mainly attributed to a decrease of EEA < >CB interactions. This is likely to be due to the preferential association of MT with CB and subsequent aggregation of CB adsorbed on MT. For PE-CB CPC, the initial increase of resistivity reminds that in a first step, MT helps CB to disperse homogeneously into the matrix and that in a second step, at higher MT content, CB reaggregates. This could be explained by the formation of a percolation pathway of MT platelets on which CB aggregates are adsorbed. For PP-CB CPC, the same phenomenon takes place but the dispersive effect is not balanced by the

aggregation one, because of the lower CB content, and thus resistivity increases slightly with MT content. Moreover, it is interesting to notice that for about 1.25% MT v/v, the resistivity of all CPC differs from less than two decades, whereas in the absence of MT, the resistivity difference between PP-CB and EEA-CB was about seven decades. Thus, it appears that MT has a levelling effect on electrical properties of all CPC. 3.2. Influence of the clay nanofiller on rheological properties The rheological behaviour of the CPC has been studied in the melt in dynamic mode Figs. 5 and 6. An important reinforcing effect of CB in the CPC in the melt is evidenced by a large increase of GV/GV0 and GW/ GW 0 with CB content, respectively. Data have been interpreted using the Guth and Gold model [19,29] according to Eq. (2). G ¼ G0 ð1 þ a/ þ b/2 Þ

ð2Þ

with G0 the modulus of the matrix without CB and / the CB volume fraction. In this model, for spherical reinforcing particles, a = 2.5 and b = 14.1, whereas for rod-like particles, a = cf with c = 0.67 and b = df 2 with d = 1.62. / is the filler volume content and f is an adjustable parameter so-called aspect ratio. Assuming that particles can interact and form elongated aggregates, we have fitted experimental data with the previous model taking c = 0.67. For both GV/GV 0 and GW/GW 0 at 35 jC, the results are presented in Table 3. From this Table 3 Guth and Gold reinforcement model a and b coefficients

Fig. 5. Reinforcement effect of CB on PP, PE and EEA in the melt, fitted with the Guth and Gold model: GV/GV0 versus CB% v/v.

a b f

EEA-CB

PE-CB

PP-CB

5.5 109.2 8.2

5.2 97.6 7.8

7 176.8 10.4

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Fig. 7. Influence of MT% v/v on GV/G0V of PE-CB, PP-CB and EEA-CPC.

table, it can be observed that whatever the CPC, the coefficients resulting from the fitting differ significantly from those obtained for spherical particles in the Guth and Gold model. Focussing on the shape factor f, we can see that the CB aggregates responsible for the reinforcement are more elongated in PP-CB than in EEA-CB and PE-CB. It is also interesting to notice that these results are in good agreement with those obtained from regressions on percolation curves (Table 2): more elongated objects (asymmetric fractal aggregates) lead to lower percolation thresholds and also to higher reinforcement. Moreover, it appears that, as the strength of interactions between the filler and the matrix tends to increase the percolation threshold, it also increases the reinforcement. Along with a strong reinforcement effect evidenced by the increase of the storage modulus, a strong increase of energy dissipation can be underlined, as attested by the curves of the loss modulus Fig. 6. The addition of MT to the CPC does not increase either storage or loss shear modulus as it is the case with CB alone and as it has been noticed by several authors for Poly(propylene)-MT over 1.3% w/w [30] and for Poly(Styrene-coIsoprene)-MT over 2.1% w/w [31]. At the contrary, it can be observed from Figs. 7 and 8 that for PE-CB and EEA-CB,

Fig. 8. Influence of MT% v/v on GW/GW0 of PE-CB, PP-CB and EEA-CPC.

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Fig. 9. GV/GW cross-over frequency evolution with CB% v/v for PE, PP and EEA.

both the storage and loss moduli decrease of about 20% when MT is added to the CPC. This effect is even more important with PP-CB for which the moduli decrease of about 60%. MT lowers the reinforcement effect of CB for all CPC which means that, making the hypothesis that CB can be adsorbed at MT surface, as previously stated from the electrical properties results, one can say that the reinforcement effect obtained with a percolated CB network is more important than that of a MT-CB one. Thus, it is interesting to notice that MT can enhance electrical properties without increasing the CPC complex moduli leading to an easier processing by current techniques as injection moulding or extrusion for instance. The frequency at which a crossover of GV and GW curves occurs is interesting to follow as a function of CB and MT content, respectively, in Figs. 9 and 10, since it represents a peculiar point sensitive to molecular mobility variations [32,33]. Thus, at low frequency, since large-scale molecular motions are allowed GW>GV, whereas at high frequency, these motions are hindered and the chains cannot relax, therefore, GV>GW. Generally, an increase of the crossover

Fig. 10. GV/GW cross-over frequency evolution with MT% v/v for PE-CB, PP-CB and EEA-CB.

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frequency (COF) means that the system is less cohesive due to either a decrease of molecular weight or to a plastification effect. In Fig. 9, excepted for neat EEA, the COF decreases with the CB content evidencing that CB reduces chain mobility at low frequency. In Fig. 10, the addition of MT has little effect on the PE-CB COF (slight increase), decreases EEA-CB COF and increase drastically PP-CB COF over 0.25% MT v/v. No crossover being observed for PP-CB in the experimental frequency range for higher MT amount. The plastification effect of MT already observed on GV at 1 Hz with PP-CB and PE-CB is confirmed, but for EEA-CB, the decrease of GV may be balanced by a more important decrease of GW leading to a decrease of COF with MT content.

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4. Conclusion Due to their heterogeneous nature resulting from the association at the nanometric scale of insulating polymer and conductive particles, conductive polymer composites (CPC) properties depend for a large part from the filler dispersion or exclusion and from macromolecules/particles interactions. The study of the percolation curves together with dynamic rheometry data provides interesting results concerning the aggregation process within the composite as a function of the nature of the matrix and fillers. The main results obtained show that few interactions between the polymer and CB particles and high crystallinity lead to low percolation thresholds (PP-CB). The reinforcement effect obtained by addition of CB is more important if CB aggregates are elongated and if CB aggregates interact with the polymer matrix. MT addition to the CPC tends to level the percolation thresholds, increasing the lower and decreasing the higher resulting certainly from adsorption of CB aggregates on MT platelets. MT also seems act as a plastifier toward the CPC which can be valuable to improve the CPC processing conditions.

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Acknowledgements The authors would like to thank A. Lepied, I. Pillin, H. Be´lle´gou, F. Peresse and A. Bourmaud for their contribution to this work. This project was supported by the French Ministry of Research and Technology.

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