Effects of thickness on mechanical properties of conducting polythiophene films

Abstract

Owing to the rapid development of conducting polymers [1, 2], it has become possible to control the electrical property of polymers over the range from insulation to high conductivity. Some polymeric materials have successively been synthesized with conductivity similar to such metals as copper [3]. Polymers and electrical conductivity are no longer mutually exclusive. In addition, conducting polymers possess some other superior properties such as low mass density, ease of fabrication, flexibility in design, and resistance to corrosion. Therefore, they can be used in various applications as substitutes of metals [4]. The promise of combining these properties with good electrical properties in polymers has prompted extensive interest in the last two decades. However, most of the previously synthesized conducting polymers are brittle, insoluble, intractable, and often decomposed before melting [5]. The preparation of conducting polymers with high strength and good chemical stability remains still a great challenge. To date, there is still a lack of investigation on the relationship between the macroscopic properties and microstructures of conducting polymer films. Recently, polythiophene thin films with high conductivity and chemical stability have been synthesized by an electrochemical method [6, 7]. The mechanical properties of such kinds of films are experimentally investigated in this paper by using an electronic speckle pattern interferometry method and the SEM technique. Significant effects of thickness on the strength and Young’s modulus of films are found. The conducting polythiophene films used in this study were synthesized in a one-compartment cell with the use of an EG&G potentiostat model 283 under computer control [6, 7]. Two AISI 304 stainless steel sheets with a spacing of 5 mm are used as the working and counter electrodes. The anodic potential is referred to the third electrode made of Ag/AgC1. Polythiophene films were yielded by electrolysis of 0.03 M thiophene in freshly distilled boron trifluoride diethyl etherate at a constant potential of 1.3 V. The film thickness was controlled by adjusting the total charge passed in the cell. Thus, polythiophene films with different thicknesses ranging from one to hundreds of microns were successively grown/deposited on the base of a stainless steel plate by controlling the strength of the electric current and the synthesis time. These films behave like metal thin films and can be cut easily into various shapes of specimens. An electronic speckle pattern interferometry (ESPI) method [8] was used to measure the displacement of a film specimen under strain. When subjected to an externally applied force, a polythiophene film may undergo a considerable shape change, especially in the longitudinal direction due to the large ratio of the length L to the width W (L/W ≥ 10). The shape change of a film can be detected by the ESPI method. Several tensile stress-strain curves of polythiophene films of different thickness are shown in Fig. 1. Following the linear elastic deformation, plastic deformation occurs at a threshold stress, referred to as the yield stress. During plastic deformation, an evident strain-hardening curve is observed, that is, the applied stress has to be increased continuously for further development of plastic strain. The maximum stress that a polythiophene film of thickness less than 10 μm can bear is similar to that of an aluminum thin film. It can be seen from Fig. 1 that the strength of a polythiophene film exhibits an evident dependence upon its thickness. As the film thickness increases, the tensile strength decreases. This size effect of tensile strength is especially evident when the film thickness is less than 10 μm. However, the film strengths approach to a constant value when the thickness is larger than 15 μm. No evident difference has been observed in the yield stress among films when their thickness is larger than about 15 μm. In our measurement, films of thickness 4 μm possess the highest strength. The physical micromechanisms of such a size effect will be analyzed below. The elastic modulus of a film can also be obtained by the same experimental system, from which both strain