Interconnected poly(dimethyldiphenylsiloxane)/silica networks for electrical actuation.

Prisacaru, Gheorghe ; Diaconu, Ilie ; Marta, Constantin 等

1. INTRODUCTION

Some technologies of actuators require electroactive materials capable to operate sometimes in special environmental conditions such as low temperatures and high humidity.

On the other hand, there is a growing demand in mechatronics, robotics and bioengineering, regarding polymer elastomers with high actuation strains and stresses, fast response time, good flexibility, high efficiency, stability and durability.

Silicone elastomers seem to be very promising in meeting these properties (Pelrine et al., 1998, 2000; Kornbluch et al., 2001; Trujillo et al., 2004). In order to achieve these requirements, high driving electric fields are needed for silicone actuators which may be an important disadvantage in certain applications.

The actuation electric field can be reduced by preparing new compounds based silicone with improved electromechanical properties.

First time, some attempts in this direction has been focused on the development of composites (Mazzoldi et al., 2004; Carpi & De Rossi, 2005; Gallone et al., 2007), which preserve, to some extent, the mechanical properties of silicone and increase the dielectric constant of the material. However, this approach led to limited results.

Recently, a blend composed of silicone and a high polarizable conjugated polymer showed remarkable increase of the electromechanical response (Carpi et al., 2008).

The goal of our paper is to determine some electromechanical parameters of a different type of electroactive dielectric material consisting of a series of new networks based on dimethyldiphenylsiloxane copolymer and tetraethylorthosilicats (TEOS) differing by copolymer composition and TEOS content. Electromechanical parameters, strain, apparent electrostrictive coefficient and response time, were determined from the thickness deformation of polymer films induced by d.c. driving electric field.

In our knowledge there are no reports in literature concerning the synthesis and electromechanical properties of these dimethyldiphenylsiloxane copolymer/silica networks.

2. EXPERIMENTAL

The synthesized dimethyldiphenylsiloxane copolymers having different contents in diphenylsiloxane units (between 6.7-21.6 % moles) were mixed in different ratios with tetraethylorthosilicate as crosslinker in presence of dibutyltindilaurate as catalyst. TEOS was used in large excess, thus in situ formation of silica network occurs concomitantly with crosslinking (Figure 1).

Interpenetrated silica networks were processed as films cast before crosslinking. Thickness induced strain of films (0.02-0.2 mm thick) was measured under ambient conditions using d.c. driving electric fields up to 60 MV/m. The measuring devices was composed of a micro Box Data Acquisition System, a MTN/EP080 Monitran displacement sensor and an electrode-film unit presented elsewhere (Diaconu et al., 2006).

[FIGURE 1 OMITTED]

3. RESULTS

All films showed compression in thickness direction under applied static electric field irrespectively of its sign (Figure 2).

For lower electric fields up to about 30 MV/m, the induced strains showed a quadratic dependence (Figure 3), whereas at higher values of the electric fields the strain presented a lower increase.

The quadratic dependences suggest the electrostatic nature of the strain responses. For the highest electric fields remarkable strains up to 25 % were found. From the slope of straight lines quite high values of the apparent electrostrictive coefficient up to about [10.sup.-14] [m.sup.2]/[V.sup.2] were determined (Table 1).

Contracting responses are quite quick and showed a steady strain for all films (Figure 4).

Electromechanical parameters depend both on the copolymer composition and the copolymer/silica ratio (Table 1). The induced strain and electrostrictive coefficient (M) decrease with diphenylsiloxane unit's content and increase with the copolymer/silica ratio decrease.

The response time (t) does not depend significantly on the copolymer composition and the copolymer/silica ratio.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

These dependences may be explained by the higher elastic stiffness of diphenylsiloxane units as compared to that of the dimethylsiloxane ones and by the higher positive effect of dielectric constant of silica than that of the negative effect of silica elastic stiffness respectively.

4. CONCLUSIONS

The investigated siloxane copolymer/silica networks show remarkable electromechanical parameters such as thickness strain, apparent electrostrictive coefficient and response time. These parameters depend both on the copolymer composition and copolymer/silica ratio. Further research is now developing to determine other parameters (effective compressive pressure, mechanical energy density and efficiency) and to synthesize new siloxane/silica networks in order to establish correlations between structure and properties so as to produce actuators with higher forces, motions, energy density, speed of response and stability.

5. REFERENCES

Carpi, F. & De Rossi, D. (2005). Improvement of electromechanical actuating performances of a silicone dielectric elastomer by dispersion of titanium dioxide powder, IEEE Transactions on Dielectrics and Electrical Insulation, 12, 4, August, 2005, pp.835-843, ISSN: 1070-9878

Carpi, F.; Gallone, G.; Galantini, F. & De Rossi, D. (2008). Silicone-Poly(hexylthiophene) Blends as Elastomers with Enhanced Electromechanical Transduction Properties. Advanced Functional Materials, 18, 2, January, 2008, pp.235-241, ISSN: 1616-301X

Diaconu, I.; Dorohoi, D. O. & Topoliceanu, F. (2006). Electrostriction of a Polyurethane Elastomer-Based Polyester. IEEE Sensors Journal 6, 4, August, 2006, pp. 876-880, ISSN: 1530-437X

Gallone, G.; Carpi, F.; De Rossi, D.; Levita, G & Marchetti, A. (2007). Dielectric constant enhancement in a silicone elastomer filled with lead magnesium niobate-lead titanati. Materials Science and Engineering:C, 27, 1, January, 2007, pp.110-116, ISSN: 0928-4931

Kornbluch, R.; Pelrine, R.; Pei, Q. & Shastri, S. (2001), Aplication of Dielectric Elastomer EAP Actuators, In: Electrostrictive polymer (ESP) Actuators as Artificial Muscles, Yoseph Bar-Cohen, pp. 457-495, Spie Press, ISBN 0-8194-4054-X, Bellingham, Washington 98227-0010

Mazzoldi, M.; Carpi, F.; Pioggia, G.; Ferro, M. & De Rossi, D. (2004). EAP Activity in Italy, WW-EAP Newsletter, 6, 2, 2004, 7-9, Available from: http://eap.jpl.nasa.gov

Pelrine, R. E.; Kornbluh, R. D. & Joseph, J. P. (1998). Electrostriction of Polymer Dielectrics with Compliant Electrodes as Means of Actuation. Sensors and Actuators A, 64, 1, January, 1998, pp. 77-85, ISSN: 0924-4247

Pelrine, R. E.; Kornbluh, R. D.; Pei, Q. & Joseph, J. P. (2000). High-Speed Electrically Actuated Elastomers with Strain Greater than 100%. Science, 287, 5454, February, 2000, pp.836-839, ISSN: 0036-8075.

Trujillo, R.; Mou, J.; Phelan, P. & Chan, D. (2004). Investigation of electrostrictive polymers as actuators for mesoscale devices. The International Journal of Advanced Manufacturing Technology, 23, 3-4, February, 2004, pp.176-182, ISSN: 0268-3768

Tab 1. Electromechanical parameters for some films with
different copolymer compositions and copolymer/silica
ratios

Sample MR * y(%) ** [DELTA]l/
code [l.sub.0] (%)

FSG16 0.64 6.7 6.0
FSG17 0.64 18.8 5.0
FSG20 0.64 21.6 0.268
FSG19 1.07 18.8 0.244

Sample E (MV/m) [tau] (s) M ([m.sup.2]/
code [V.sup.2])

FSG16 3.5 1.0 3.99x[10.sup.-15]
FSG17 3.5 1.25 8.62x[10.sup.-15]
FSG20 50 1.0 1.07x[10.sup.-18]
FSG19 3.5 1.25 1.64x[10.sup.-16]