STUDY OF PIEZOELECTRICITY AND STORED POLARIZATION IN SOME CORONA CHARGED CYANO POLYMERS

Abstract

Ferroelectric polymers have attracted an immense interest in research because of their wide variety of applications in devices and transducers. These polymers have been used as sensors in electro-acoustic, electro-mechanical and thermal imaging applications. They have excellent potential for use in developing the intelligent materials, in the form of actuators and tactile sensors in robotics and as non-invasive cardiopulmonary sensors in the field of biomedical engineering. In general polymers offer several advantages over ceramic piezoelectric materials having low cost of production, flexibility, low density, low acoustic impedance and processibility in desired shape and size. Cyanopolymers, have recently emerged as a new class of amorphous piezoelectric materials because they contain -CN dipoles having a dipole moment — 3.5 D, higher than that found in poly ( vinylidene fluoride) and polyvinyl chloride. Many cyanopolymers such as, P(VDCN-VAc), P(AN-MA), P(AN-ALCN) and P(VDCN-MMA) have been investigated for piezo-, pyro- and ferroelectric properties to explore their use in device applications. Among these polymers poly (styrene - co - acrylonitrile) (SAN) and poly (acrylonitrile-co-butadiene-co-styrene) (ABS) have been paid less attention. Thus, the studies of these electroactive properties in the unstretched and the stretched corona poled films of SAN and ABS copolymers in relation to their structure form topic of the present thesis. Also, evaluation of polarization using thermally stimulated current experiments and its correlation to the piezoelectric properties has been been carried out in these polymers. The work presented in the thesis has been an attempt to obtain better understanding of the mechanisms of piezo-, pyro- and (i) ferroelectricity in SAN and ABS copolymer films. The research work incorporated in the thesis has been divided in various chapters as given below. Chapter I develops the concepts and hypothesis needed to understand the phenomena of piezo-, pyro- and ferroelectricity in polymers. Literature review on the subject and the theoretical models needed to understand these properties in ferroelectric polymers are also discussed here. A brief description of the scope of the present work and the areas of application of these materials have also been included. Chapter II deals with the details of preparation of solvent cast films of the SAN and ABS, techniques adopted for inducing piezoelectricity in these films and the description of the stretching and corona poling unit, and the experimental set-ups employed for thermally stimulated current (TSC), piezo-, pyro- and ferroelectric investigations. This chapter also carries a brief account of electrical conductivity and results of the TSPC studies in the unstretched and the stretched SAN and ABS films. Chapter III is concerned with the investigations of compositional, structural and mechanical properties of SAN and ABS films, The effects of annealing, stretching and corona poling on the crystallinity, glass transition temperature and Yeung's modulus of these films, studied using X-ray diffraction, differential scanning calorimetery (DSC) and tensile measurements, respectively are discussed in this chapter. The results of IR study on SAN and ABS films are also presented in this chapter. Chapter IV deals with the results of measurements of tensile piezo-electricity and bending piezoelectricity in these two polymert. The piezoelectric coefficient d31 of the SAN and ABS films prepared under different poling and stretching conditions has been measured. The poling parameters such as annealing (ii) temperature, poling field , poling temperature and stretching ratio have been optimized with respect to d31.The results of bending piezoelectric coefficient (3331 are also incorporated in this chapter. Chapter V presents the thermally stimulated current studies on these films under similar stretching and poling conditions carried out to obtain information about the nature of the frozen polarization which is found to be predominanttidipolar in nature. The TSC results are correlated with piezoelectric effect. An attempt has been made to understand the mechanism of piezoelectricity and to correlate it with the mechanical properties of these polymers by the application of modified Mopsik and Broadhurst theory....