Development of soft electrical conductive PDMS/CNT-Compo

ELASTOMERE UND KUNSTSTOFFE ELASTOMERS AND PLASTICS

22 KGK · 07-8 2020 www.kgk-rubberpoint.de

Planetary mixer • shear forces • disper-sion • percolation threshold • flexibility

In order to create electrical conductive polydimethylsiloxane/carbon nanotube-composites with high flexibility, it is ine-vitable to minimize the filler content. To hold conductivity at a minimum of CNT content, an appropriate filler dispersion has to be realized. This was achieved by applying shear forces during compoun-ding by modifying the mixing condi-tions to an optimum. The electrical per-colation threshold is determined to be at 0.9 wt.% of CNTs. An optimized roll mill step enabled to further increase the conductivity and lead to an orientation of CNTs. By adding a high-dispersible si-lica, a significant shift of the percolation threshold towards 0.5 wt.% of CNTs was observed. Adding silica amounts of ≥ 15.0 wt.% resulted in an increase in stiffness and density, which is contradic-ting for the application in lightweight, soft and flexible components.

Entwicklung von weichen elektrisch leitfähigen PDMS/CNT-Kompositen mit extrem geringem Gehalt an CNT

Planetenmischer • Scherkräfte • Disper-sion • Perkolationsschwelle • Flexibilität

Die Herstellung von elektrisch leitfähi-gen Polydimethylsiloxan/Carbon Nano-tube-Kompositen mit hoher Flexibilität erfordert die Reduzierung des Füllstoff-gehalts auf ein Minimum. Um bei solch geringen CNT-Gehalten die elektrische Leitfähigkeit zu erhalten, ist es wichtig eine ausreichende Dispersion zu errei-chen. Dies wurde durch den Eintrag von Scherkräften bei einem optimierten Mischprozess realisiert. Die elektrische Perkolationsschwelle liegt bei 0,9 Gew% CNT. Ein angepasster Walzschritt konnte die Leitfähigkeit erhöhen und führte zu einer Ausrichtung der CNTs. Die Zugabe von hoch-dispergierbarer Kieselsäure be-wirkte eine deutliche Erniedrigung der Perkolationsschwelle auf 0,5 Gew% CNT. Silica-Gehalte oberhalb von 15.0 Gew% bedingten einen Verlust der Flexibilität und sind auch auf Grund ihrer Dichte für den Einsatz als weiche oder flexible Leichtbaumaterialien nicht geeignet.

Figures and Tables: By a kind approval of the authors.

IntroductionElectrical conductive elastomers are wide spread and used for shielding pur-poses, as strain sensors or electrodes [1]-[5], for example. They mostly persist of a flexible polymer like silicone with incorporated conductive nanofiller parti-cles. This ensures to prepare compounds which are electrical conductive inside but insulating on their surface in order to prevent electric arcs between different components [6]. Within the elastomer, the filler particles build up conductive paths resulting in electrical conductivity of the material [7]. The corresponding minimum of filler content where a filler network is built and the electrical con-ductivity rises drastically is labelled by the percolation threshold. The formation of these conductive paths and therefore the percolation threshold depends on many factors such as the geometry, the intrinsic conductivity and the state of dispersion of the corresponding nanofill-ers [8], [9].

Carbon nanotubes (CNTs) are of great interest for the use as conductive filler particles, as they hold an outstanding electrical conductivity according to their sp2-hybridized structure [10]-[12]. The magnitude of conductivity of pristine CNTs is in the order of copper with up to 6.0 x 105 S/cm. By using multiwall carbon nanotubes (MWCNTs) in a polymeric ma-trix, a conductivity level of up to 4000 S/cm can be attained [13]. CNTs consist of rolled up graphene sheets, which leads to a high aspect ratio and enables to build up a conductive filler network at relatively low filler content [14],[15]. This is beneficial when thinking of applica-tions of electrical conductive elastomers where high flexibility, softness and low weight are required (e. g. medical devices like special electrodes). Especially regard-ing these applications, it is extremely important to decrease the percolation threshold as much as possible, as adding fillers to polymers results in contradict-ing reinforcement and density. As the percolation threshold depends not only

on the geometry and conductivity of the nanofiller particles but to a great extent on their dispersion and distribution in the matrix, it is essential to enhance the filler dispersion. Unfortunately, fillers in the nanoscale, like CNTs, tend to agglom-erate due to van der Waals forces leading to high filler-filler-interactions [16]-[18]. Therefore, it is extremely challenging to break up CNT agglomerates adequate and to realize a homogeneous dispersion of the separated filler particles.

Incorporating CNTs into low viscous silicone rubber in order to produce elec-trical conductive materials has been test-ed, already. Referring to this, different studies demonstrated that the use of CNTs in polydimethylsiloxane (PDMS) composites provide high reinforcement

Development of soft electrical conductive PDMS/CNT-Compo-sites with extremely low CNT Content

Authors

Marvin Christopher Vincenzo Omelan, Astrid Diekmann, Ulrich Giese, Hannover, Germany Corresponding Author:Ulrich GieseDeutsches Institut für Kautschuk-technologie e. V.Eupener Straße 3330519 Hannover, Germany

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values and decreased electrical resistanc-es. In addition, filler alignments could be reached and result in an increase of elec-trical conductivity of up to 2.8 x 10-3 S/cm. A decrease of the percolation thresh-old could be realized by CNT functionali-zation ending up in CNT amounts be-tween 1.0 and 2.0 wt.% [19]-[28]. Re-garding flexible and lightweight com-posites with extremely low filler content, these CNT concentrations are still too high. In order to further reduce the filler content, an optimization of the mixing process is indispensable, as it is already possible to break up CNT agglomerates through the input of shear forces while compounding. Though, precaution has to be taken as applying high shear forces break up CNT agglomerates but can lead to tube damage and decrease in the as-pect ratio.

ApproachThe development of electrical conductive PDMS/CNT-composites with extremely low CNT content focusses the contradict-ing properties of electrical conductivity and reinforcement. In order to produce highly flexible and lightweight PDMS/CNT-compounds with high electrical conductivity, it is indispensable to reduce the content of CNTs as much as possible to prevent reinforcement on the one hand but to remain electrical conductivi-ty on the other hand. The approach in realizing this is to modulate the com-pounding process in order to break up CNT agglomerates and attain an appro-priate filler dispersion in the elastomer matrix by applying well-balanced shear forces. Additional CNT alignment could further decrease the percolation thresh-old as it leads to the formation of electri-cal paths at very low CNT amounts in tube direction. In order to realize CNT orientation, a supplementary milling step was implied and optimized in this study. Precaution must be taken here to prevent CNT fracture during mixing as this will decrease the aspect ratio of CNTs.

By increasing the viscosity a further enhancement of shear forces while com-pounding and subsequent an increase in dispersion due to agglomerate break-up is assumed. Therefore, silica was imple-mented as hybrid filler during the com-pounding, as silica is already a compo-nent of the PDMS matrix. Additional synergistic effects due to the insertion of the hybrid filler particles are to be ex-pected as well.

Experimental

MaterialsA two-component polydimethylsiloxane (Sylgard 184, Dow Corning Inc.) was used, which is a platinum-catalyzed, ad-dition-cured silicone rubber. The viscosi-ty of the base component is around 5000 cSt at 25°C and of about 110 cSt for the second component which contains the crosslinking agent (according to manu-facturers’ datasheet [30]). A ratio of 10:1 of base to curing agent is recommended. The electrical conductive filler particles were multiwall CNTs (Nanocyl7000, Na-nocylTM), which have a BET specific sur-face area of 270 m²/g measured by static gas adsorption and a purity of around 90% [31]. To increase the viscosity of the compound, high-dispersable silica parti-cles (Zeosil MP1165, Solvay) were added.

CompoundingWith respect to the material, focus must be laid to the mixing process as the vis-cosity of PDMS contradicts the use of conventional internal mixers. Therefore, a special twin tool planetary mixer (LPV 1A40, PC Laborsystem) with a vessel vol-ume of appr. 0.6 L was used for the preparation of the compounds. The mix-er is equipped with two intermeshing rotors (twin tools), an additional scraper and a vacuum unit. An integrated bell jar offers to evaporate the mixture (-1000 mbar) while simultaneously stirring. The evaporation process removes potential volatile parts and contaminants from the surface of CNTs, which is beneficial to build up strong polymer-filler-interac-tions. As increased polymer-filler-interac-tions enhance to overcome filler-filler-in-teractions this contributes to the break-up of CNT agglomerates.

While optimizing the mixing strategy, a constant amount of 4.0 wt.% CNT were incorporated, which ensures being far above the estimated percolation thresh-old of about 1.0 – 2.0 wt.% of CNT. A subsequent milling process by means of a two roll mill (Co. Troester GmbH & Co. KG) was used to receive an orientation of CNTs in the rubber matrix for anisotropic properties. This process required higher viscosities of the compounds, which was realized by adding 10.0 wt.% silica (MP 1165) at 3.0 wt.% of CNTs for this pur-pose.

Estimation of the percolation thresh-old of the pure system was performed varying the CNT content in the range of 0 – 5.0 wt.% of CNTs (corresponding to

filler-polymer volume fractions of = 0.00 to 0.05) without the milling step, due to viscosity reasons. Investigations on hybrid systems were equally carried out without the milling process in order to compare the results with those of the pure CNT-system. Hereby, one hybrid se-ries was investigated with variation of the CNT amount between 0 – 5.0 wt.% and a constant silica concentration of 5.0 wt.%, whereas a second series was per-formed with a constant CNT concentra-tion of 3.0 wt.% and varied silica content from 0 to 20.0 wt.%.

The vulcanization was monitored by a rubber process analyzer (Monsanto RPA 2000) at 150°C. The curing was per-formed at 150°C in a vacuum heating press (KV 234.00) using a pressure of 280 bar for 14 minutes. As the PDMS is an addition-cured silicone rubber, no fur-ther vulcanization agents than the sec-ond component are necessary and curing at elevated temperatures merely speeds up the curing process.

CharacterizationThe dielectric spectroscopy (broadband analyzer BDS 40, Novocontrol GmbH) was carried out in a frequency range of 0.1 Hz to 10 MHz at room temperature on crosslinked cylinder samples (Ø 20 mm, d = 2 mm). To ensure contact, the samples were coated with gold on both sides (Polaron SC7640, Thermo VG Scien-tific). Rheological characterization of the non-cross-linked samples was performed using a frequency sweep of 0.5 Hz to 33 Hz at a temperature of 40 °C and a defor-mation of 1% (Monsanto RPA 2000). Rhe-ological measurements of cross-linked samples were carried out with a dynamic mechanical analyzer (type ARES, Rheom-etric Scientific) on strip samples (40 x 10 x 2 mm) using a variation of the load of 0.1 to 4% at a frequency of 1 Hz. Tensile tests were carried out on S2 specimen using a universal testing device (Zwick 1445, Co. Zwick Roell GmbH & Co. KG) with 0.5 N pre-load and 200 mm/min tensile speed. The illustrated values cor-respond to an average of 5-7 measure-ments each, using median curves for further analyses. The transmission elec-tron microscopic images (TEM) were tak-en using a LIBRA 120 microscope (Co. Carl Zeiss AG) with an acceleration voltage of 120 kV. Thin sections of the material were cut using a cryo ultramicrotome at a temperature of -140 °C (diamond knife, diatome 35°) and placed on a 600 mesh copper grid.



Results and Discussion

Optimization of Mixing StrategyWith respect to the material, focus must be laid to the mixing process as the vis-cosity of PDMS contradicts the use of conventional internal mixers. Therefore, a special compounding technique using a twin tool planetary mixer was chosen in this study as it is described above. To receive a high dispersion of CNTs during mixing, adequate shear forces have to be applied and it is indispensable to opti-mize the process parameters. A three stage mixing procedure was performed consisting of a constant degassing step (30 min), the compounding step and a constant homogenizing step of 90 s at a rotor speed of 150 rpm after adding the component containing the curing agent. During the compounding step the mix-ing time and the rotor speed were varied. Temperature and power remained the same over the complete mixing process within the varied parameters, respective-ly. While varying the mixing process, 4.0 wt.% CNT were incorporated, which en-sures being far above the estimated per-

colation threshold of about 1.0 – 2.0 wt.% of CNT.

The optimization of the process pa-rameters was initiated with a variation of mixing time in the range of 5 – 40 minutes where the rotor speed was set to 300 rpm. Figure 1 presents the results of dielectric measurements indicating a conductive percolated filler network in all samples. The PDMS sample without any filler particles is insulating itself (not shown here). An increase in conductivity is observed with increasing mixing time resulting in a maximum at 10 minutes. This indicates an improvement of filler dispersion due to the break-up of ag-glomerates as the amount of filler parti-cles remains the same over all experi-ments. With further increase of mixing time the conductivity decreases, imply-ing reagglomeration of CNTs and poten-tial damage of the tube structure.

With respect to the rotor speed, varia-tions from 100 up to 500 rpm were im-plemented (Fig 2). Comparing the con-ductivities at 1 Hz an increase in conduc-tivity with increasing rotor speed can be seen first which attains a maximum at a

rotor speed of 300 rpm. With further in-crease of rotor speed the conductivity decreases, showing the tendency of CNT reagglomeration again. Comparing the results of the mixing time series and the series of rotor speed, it can be estimated that the electrical conductivity is more dependent on mixing time than on rotor speed as the increase in conductivity due to the variation of mixing time is higher.

In addition to conductivity measure-ments, rheological and mechanical in-vestigations are appropriate to give in-formation on the effectiveness of com-pounding and the dispersion of CNTs in the matrix as they demonstrate poly-mer-filler-interactions and reinforce-ment factors. The formation of poly-mer-filler-interaction instead of filler-fill-er-interaction facilitates agglomeration break-up and therefore dispersion.

The frequency-dependent behavior of dynamic mechanical properties of CNT/PDMS composites mixed at different mixing time and rotor speed were meas-ured probing the uncured material (Fig. 3). When illustrating the dependency of the storage modulus G’ and the loss

Fig. 2: Electrical conductivity in dependency of a) frequency and b) rotor speed.

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Fig. 1: Electrical conductivity in dependency of a) frequency and b) mixing time.

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modulus G’’ as a function of rotor speed and mixing time at a frequency of 25 Hz, an increase of the storage modulus with increasing rotor speed and mixing time can be observed first, implying increas-ing material stiffness. As the amount of CNTs remains the same in all samples, the enhanced rheological properties can be attributed to improved CNT agglom-erate break-up. An optimum of 10 min-utes mixing time can be observed as in electrical measurements a mixing time of more than 10 minutes results in a de-crease of reinforcement implying reag-glomeration again. Concerning the rotor speed, the material stiffness reaches a plateau at approximately 300 rpm, re-vealing no further improvement at high-er rotor speed.

Tensile tests are shown in figure 4 where an increase of material stiffness can be observed using the aforemen-tioned optimal mixing conditions of 300 rpm (rotor speed) and 10 min (mixing time) with respect to the shapes of the stress-strain curves. Rotor speeds higher than 300 rpm and mixing times longer than 10 minutes tend to reduce stiffness

apparent by a shift of elongation at break and tensile strength to lower levels also confirming reagglomeration and me-chanical degradation of CNTs. Klingen-berg et al. [32] described the effect of stirring rate parameter and temperature on the filler flocculation and finally on the material stiffness. They also men-tioned that parameters like sliding forces and attractive forces are crucial for the flocculation behavior while stirring. As the addition of the curing agent was con-ducted at constant conditions, the en-hancement in dispersion is merely influ-enced by the variation of rotor speed and mixing time in this work.

In order to investigate the morpholo-gy of the CNT agglomerates optically, transmission electron microscopy (TEM) was used. Figure 5 shows TEM-images of PDMS-composites with 4.0 wt.% CNTs. Therein, images a) and b) represent sam-ples mixed at 300 rpm for 10 minutes at different magnifications, which figured out to be the best mixing strategy. Dis-persed single CNTs and small clusters can be seen here where the diameter of the CNT agglomerates is less than 180 nm.

The average length of the nanotubes amounts around 210 nm. In contrast to this, a CNT/PDMS-composite mixed with a lower rotor speed of 100 rpm hold ag-glomerates of larger extend in the matrix (Fig. 5c).

Supplementary effort concerning en-hanced agglomerate break-up was real-ized using a subsequent milling step. This processing step requires a distinct viscosity of the compounds, which is not present in all samples. Therefore, sam-ples with 3.0 wt.% CNTs were mixed with an additional amount of 10.0 wt.% of silica (Zeosil MP1165) resulting in appro-priate processability. In order to optimize this processing step and to create maxi-mum shear forces the parameters mill time, nip and friction were modified. Figure 6 represents the results of dielec-tric measurements depending on time, nip and friction. This additional process-ing step indicates enhanced dispersion as the electrical conductivity increases with milling time. By increasing the roll nip and the friction, a high extensional flow arises, which results in higher shear forces and consequently in an improve-

Fig. 3: Storage (G’) and loss (G’’) modulus and loss factor (tan δ) in dependency of a) mixing time and b) rotor speed of uncured samples at 25 Hz.

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Fig. 4: Illustration of tensile tests in dependency of a) mixing time and b) as a function of rotor speed.

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ment in the break-up of CNT agglomer-ates. Ideal conditions are received using a milling time of 10 minutes at a roll nip of 0.5 mm with a friction of 1:1.5. Higher values result in a deterioration of con-ductivity, which is explained by fracture of the carbon nanotubes due to mechan-ical damage.

Using such a subsequent milling step, filler orientation of the CNTs can be ex-pected. Probing this, sample strips were removed during the rotation of the roll mill, stacked and vulcanized as samples of 6 mm thickness (Fig. 7). According to

Ponnamma et al. [26], a differentiation between the properties in the orientated direction and perpendicular to this were performed by executing a cutting step in rolling direction (B) and in the vertical direction (A).

Regarding the dielectric conductivity of compounds without milling and those which experienced an additional roll mill step, a significant increase in conductivi-ty towards 6 x 10-2 S/cm can be observed (Fig. 8a) when applying this subsequent processing step. Comparing the results in the roll mill direction and perpendicular

to that, additional benefit in conductivi-ty is observed in the oriented direction, which can be assigned to an alignment of CNTs facilitating the formation of con-ductive paths. Thereby, an additional in-crease in conductivity of about half a decade can be achieved.

A transmission electron microscopic image of a sample compounded by using ideal mixing conditions and subsequent milling prepared in orientated direction is shown in figure 8b. This indicates good dispersion as merely minor agglomer-ates of CNTs can be seen. A filler align-

Fig. 5: Transmission electron microscopic images of a PDMS/CNT-composite filled with 4.0 wt.% CNT, a + b) mixed at optimized conditions using 300 rpm and 10 min and c) compounded using 100 rpm.

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Fig. 6: Electrical conductivity depending on a) milling time b) roll nip and c) friction.

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Fig. 7: Vertical cut (A), circle stamp and meas-uring direction (A’) in CNT align-ment. Side cut (B) of specimen, circle stamp and measuring direc-tion (B’) perpen-dicular to the CNT orientation.

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ment can be observed for CNTs with high aspect ratios (> 100 length per diameter), which causes the rise in conductivity and is in good accordance with Rosca et al. [33]. Restrictively it is to remark, that the observed region of the sample is small due to the TEM-technology.

Summarizing the process optimiza-tion, an appropriate mixing strategy in compounding PDMS/CNT-composites is based on the use of a planetary mixer. A three stage mixing procedure turns out to be best, which consists of an initiating degassing step in order to remove vola-tile parts and oligomer residues. The fol-lowing compounding step gives opti-mum results at a rotor speed of 300 rpm and a mixing time of 10 minutes, where electrical conductivity and CNT disper-sion are enhanced and CNT fracture is inhibited. Using longer mixing times and higher rotor speed leads to reagglomera-tion effects and mechanical CNT dam-age. A subsequent homogenizing step of 90 s at a rotor speed of 150 rpm ensures to homogenize the compound after add-ing the curing agent. Applying an addi-

tional milling step at a roll nip of 0.5 mm with a friction of 1:1.5 for 10 minutes further increases the conductivity and dispersion and enables to produce nano-composites with CNT alignment.

Percolation ThresholdAs it is the approach of this study, to min-imize the percolation threshold as much as possible in order to create electrical conductive nanocomposites with an ex-tremely low CNT content for flexible, medical and lightweight applications, compounds of different CNT concentra-tions were analyzed to determine the electrical and mechanical percolation threshold. The percolation threshold was estimated in the pure CNT system and in the hybrid system, where the hybrid sys-tem contains 5.0 wt.% silica. The percola-tion threshold values and therefore the formation of a conducting filler network depend on the geometry (where the as-pect ratio of CNTs plays an important role), the intrinsic conductivity, and the state of dispersion of the corresponding nanofillers [8], [9].

The percolation threshold was esti-mated as absolute value of compounds consisting merely of PDMS/CNT for a start. Therefore, compounding was per-formed without the roll mill step due to viscosity reasons, as previously men-tioned. The initial electrical percolation threshold was calculated using equation (1), where the critical filler volume frac-tion Φ

c was incrementally varied [34] = 1 μ

eq. (1)

withσ = conductivity σ

o’ = conductivity threshold

Φ = filler volume fractionΦ

c = critical filler volume fraction

μ = critical exponent, depending on the system dimensionality (two dimensions: μ=1.33 and three dimensions μ=2) [35], [36].

Subject to this, an electrical percolation threshold of 0.9 wt.% CNT can be deter-mined at 1 Hz (Fig. 9a) which is at the lower range of literature data [19]-[28]

Fig. 8: Illustration of a) electrical conductivity and b) TEM image using optimized milling condi-tions.

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Fig. 9: Illustration of a) electrical percolation threshold and b) mechanical percolation threshold with variation of CNT content with and without 5.0 wt.% silica.

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and gives evidence of the requirement of an adequate mixing strategy.

For the estimation of the mechanical percolation threshold, dynamic deforma-tion-sweep tests were carried out on cured samples. To sense the Payne effect and the impact of occluded rubber [29], ΔG’ was used instead of G’ for further analyses. Hereby, a linear fit of the delta values of the storage modulus (ΔG’) at small deformations (ΔG’ = G’0.2% – G’4%) against filler content results in a mechanical percolation threshold of ap-proximately 2.2 wt.% of CNT (Fig. 9b).

To increase the viscosity in order to simultaneously increase the shear forces during compounding, a high-dispersible silica (Zeosil MP1165) was added, as sili-ca is already a component of the PDMS matrix. To hold comparability to previ-ously analyses regarding the percolation threshold, these compounds were pre-pared without the milling step as well. Thereby, various series of analyses were performed: At first, the CNT content was varied at a constant amount of silica

with regard to the percolation threshold. Secondly, the silica content was varied at a constant concentration of CNTs, in or-der to investigate the impact of the hy-brid filler on the elastomer compounds itself.

Benefits of CNT/silica hybrid systems have already been published on the basis of e.g. NR and NBR [37]-[40]. As expect-ed, additional dispersion can be observed with regard to both filler types resulting in improved properties and synergistic effects. Figure 10 represents the electri-cal conductivity depending on the CNT concentration varying from 0 – 5.0 wt.% with a constant silica content of 5.0 wt.%. All samples indicate percolated filler systems except the ones without CNTs. The PDMS elastomer is insulating itself which experiences a marginal in-crease in conductivity due to the addi-tion of 5.0 wt.% of silica. As high-dispers-ible silica are functionalized holding hy-droxyl groups on the surface [41], [42], the self-diffusing reaction of the proto-meric structures of the silica is accounta-

ble for the slight increase in conductivity. Overall, the contribution of silica to con-ductivity due to supplementary charge carrier effects is of minor extent. This implies that the shift of the electrical percolation threshold towards reduced CNT amounts of around 0.5 wt.% of CNTs (corresponding to filler-polymer volume fraction Φ = 0.005) is mainly based on enhanced CNT dispersion with respect to a silica amount of 5.0 wt.% (Fig. 9a). Syn-ergistic effects regarding a facilitated formation of the conducting filler net-work can be assumed as well, as the maximum electrical conductivity is in-creased to 2.5 x 10-1 S/cm.

The mechanical percolation threshold proves an improved CNT agglomerate break-up and associated filler dispersion with the addition of silica as the me-chanical percolation threshold is reduced to 2.0 wt.% of CNTs (corresponding to filler-polymer volume fraction Φ = 0.02) (Fig. 9b). Nevertheless, the impact on conductivity is more prominent than re-inforcing effects when incorporating sili-ca particles.

Regarding tensile tests at a constant amount of silica of 5.0 wt.%, a substan-tially increase in tensile strength and a moderate increase in elongation at break is shown with increasing CNT concentra-tion (Fig. 11a) implying an increase in re-inforcement and stiffness. This is under-lined by stress values at a constant elon-gation of 10% where an increase with in-creasing CNT amount is observed as well (verifying a decrease in flexibility).

Further investigations on the influ-ence of silica as hybrid filler for the quan-tification of its effect were made dealing with a constant CNT amount of 3.0 wt.% (corresponding to filler-polymer vol.-frac-tion Φ = 0.03) as the preceding results showed that this CNT content is satisfac-tory to ensure being far above the perco-

Fig. 10: Electrical conductivity in dependency of the CNT content at a constant silica amount of 5.0 wt.%.

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Fig. 11: Mechani-cal properties depending on the CNT content a) tensile strength and elongation at break and b) tensile strength at an elongation of 10%.

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Fig. 12: Proper-ties in dependen-cy of the silica content a) electrical conductivity and b) storage modulus of cured samples.

12

lation threshold. The amount of silica was varied from 1.0 wt.% to 20.0 wt.% in these analyses.

Regarding the electrical conductivity, the addition of silica to CNT filled PDMS mixtures results in an increase in con-ductivity with increasing silica content and to a broadening of the plateau (Fig. 12a). As the contribution of proton dif-fusion arising from functional groups on the silica surface is of minor impact at low silica amounts, increased disper-sion due to enhanced shear forces are reflected once more. Exceeding about 15.0 wt.% silica there is an abrupt rise in conductivity of over half a decade, which indicates an additional improve-ment due to synergistic effects. The for-mation of a filler network is substantial-ly promoted in these hybrid filler sys-tems ending up in an electrical conduc-tivity of 1.0 x 10-1 S/cm. As there is a much higher content of silica (15.0 – 20.0 wt.%) compared to the amount of CNTs (3.0 wt.%) in these samples, the filler network is mainly assigned to sili-ca particles and an additional impact of

proton diffusion regarding the conduc-tivity can be assumed here.

The formation of a filler network is proved by dynamic-mechanical analyses of cured compounds (Fig. 12b) where an increase of the storage modulus is ob-served especially at high silica amounts around 20.0 wt.%. Here, material stiff-ness is enhanced significantly verifying the formation of a filler network which mainly consists of silica particles. This is in good accordance to Wang [29] who found, that the filler network can appar-ently increase the effective filler volume due to occluded rubber in the filler ag-glomerates, resulting in high material stiffness. The occluded rubber in the filler agglomerates is losing its identity as originally polymer. Due to this, the effec-tive volume of the polymer (carrying the stresses applied to the sample) is re-duced by an apparently enlarged filler network resulting in increased modulus, which is substantially enhanced at high filler concentration. The occurrence of occluded rubber also illustrates ineffec-tive processing. This can be seen in par-

ticular regarding high silica amounts where inhomogeneous samples are pres-ent, indicated by high standard devia-tions. Having a look on the tensile strength (Fig. 13a - black) and elongation at break (red) of CNT/silica hybrid sys-tems, no distinct tendency of these prop-erties is observed but high standard devi-ations at high silica amounts are found implying such inhomogeneities.

Figure 13b represents the tensile strength of CNT/silica composites (3.0 wt.% CNT) with varying silica content at an elongation of 10%. By increasing the silica amount, the modulus of elasticity is increased, indicating higher stiffness at a constant cross-sectional area of the specimens. A significant increase in stiff-ness at about 20.0 wt.% of silica is ob-served, confirming the above mentioned results regarding the formation of a filler network.

Overall, high silica amounts cause re-inforcement and material stiffness, which is contradicting regarding light-weight and flexible applications and is therefore not recommendable.

Fig. 13: Mechani-cal properties in dependency of the silica content a) tensile strength and elongation at break and b) tensile strength at con-stant elongation of 10%.

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ConclusionBy optimizing the mixing process it is possible to control the applied shear forces during compounding with regard to an appropriate break-up of CNT ag-glomerates and filler dispersion in conse-quence, and to simultaneously inhibit CNT fracture. Thereby, a percolated filler network is built at low CNT amounts re-sulting in high electrical conductivity. The electrical percolation threshold amounts to 0.9 wt.% of CNTs. Inserting silica increases the viscosity of the PDMS/CNT-compounds and leads to a further decrease of the electrical percolation threshold towards 0.5 wt.% of CNTs (at 5.0 wt.% silica) with a conductivity level of 2.5 x 10-1 S/cm. This is attributed to an increase in shear forces and enhanced dispersion in consequence. A supple-mentary milling step at optimized condi-tions enables to further increase the fill-er dispersion and results in an alignment of the carbon nanotubes. Summarizing the process optimization, an appropriate mixing strategy in compounding PDMS/CNT-composites is based on the use of a planetary mixer. A three stage mixing procedure turns out to be best which consists of an initiating degassing step in order to remove volatile parts and oli-gomer residues. This is beneficial to build up strong polymer-filler-interactions and to overcome filler-filler-interactions in consequence. The following compound-ing step gives optimum results at a rotor speed of 300 rpm and a mixing time of 10 minutes, where electrical conductivi-ty and CNT dispersion are enhanced and CNT fracture is inhibited. Using longer mixing times and higher rotor speed leads to mechanical CNT fracture. A sub-sequent homogenizing step of 90 s at a rotor speed of 150 rpm ensures to ho-mogenize the compound after adding the curing agent. Applying an additional milling step at a roll nip of 0.5 mm with a friction of 1:1.5 for 10 minutes leads to further increase in conductivity and dis-persion and enables to produce nano-composites with CNT alignment.

By adding high amounts of silica (≥ 15.0 wt.%), a significant increase in elec-trical conductivity is observed, which is assigned not only to improved CNT dis-persion but to proton diffusion as well. In addition, dealing with high silica con-tents leads to inhomogeneities due to an impeded and ineffective mixing process. A significant increase in material stiff-ness occurs, which could be assigned to occluded rubber. Therefore, the filler net-

work apparently increases the effective filler volume. Overall, inserting high amounts of high-dispersible silica result not only in enhanced stiffness but in high density as well. This could be con-tradicting regarding the application as flexible and lightweight composites.

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