New electrostriction materials based on organic polymers: A review
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New Electrostriction Materials Based on Organic Polymers: A Review V. V. Kochervinskiі Karpov Institute of Physical Chemistry, State Scientific Center of the Russian Federation, Moscow, 103064 Russia email: [email protected] Received March 23, 2009
Abstract—The nature of very high electrostriction strains in modified fluorinecontaining polymer ferroelec trics is considered. The main modification method is the irradiation of vinylidene fluoride (VDF) copolymers with trifluoroethylene (TrFE) by electrons with energies of several MeV. Such treatment leads to the transi tion of the ferroelectric phase to the relaxor state and paraelectric phase. These structural changes are caused by the formation of new functional groups in PVDF chains at radiolysis. The effect of the structure on the electrostriction strains is considered. It is shown that the increase in the electrostriction strain after irradiation is related to the increase in the amorphous phase fraction. This increased strain (all other factors being equal) in uniaxial films, in comparison with isotropic ones, indicates that the field of the anisotropic amorphous phase (mesomorphic state) regions plays an important role in the reaction. An analysis of the experimental data shows that the structural changes in the field (leading to electrostriction strains) are controlled to a great extent by the features of cooperative (segmental) mobility of amorphous phase chains. PACS numbers: 81.10.Fq DOI: 10.1134/S1063774509070062
INTRODUCTION The materials under consideration, which have a high piezoelectric modulus, can compete with con ventional inorganic piezoelectrics in a number of peculiar polymeric properties in some cases [1–3]. For example, their acoustic impedance is close to that of fluids and biological tissues. Hence, they can be used in medicine as energy converters [3] in the reception mode. When they are used in the emission mode, the electrostriction phenomenon (where the strain is pro portional to the squared electric field strength) is pre ferred. This operation mode of the polymers under study can be implemented, for example, in microro botics and in designing microvehicles. A separate field of technology deals with micropositioning objects. Special devices, called actuators, are used to this end. They are generally based on inorganic relaxor ferro electrics [4–7]. The discovery of the relaxor behavior of polymer ferroelectrics based on vinylidene fluoride (VDF) copolymer with trifluoroethylene (TrFE) and giant electrostriction in these compounds [8–11] revealed the prospects of using organic polymers to solve the abovementioned problems. The class of flexiblechain crystallizing polymers under consider ation is also of interest for some other important appli cations. For example, VDF/TrFE copolymers were found to exhibit a giant electrocaloric effect [12] which, as will be shown below, is directly related to electrostriction. Microcoolers based on the electroca loric effect can be used in computers. In this co
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