Study On Dielectric,photoelectric And Flexoelectric Properties Of PVDF-based Copolymers

The rapid development of science and technology has brought huge changes to the production and life of human society.The development and utilization of functional materials give a strong support for the development of science and technology.Among functional materials,in addition to the certain functional characteristics after being stimulated by electricity,sound,light,magnetism,etc.,the polymer electronic functional materials also have the advantages of light weight,good flexibility,easy to process and modify,and large-scale preparation.They have wide applications in energy storage,sensors,actuators and logic storage devices,and are very popular in the development of smart materials.Among them,PVDF-based materials have excellent performance in many electrical properties such as dielectric properties,photoelectric performance and flexoelectricity.However,at present there are still many mechanisms related to the dielectric properties of PVDF-based materials remaining unclear.The dielectric properties need to be further improved to meet the requirements of practical applications;the photovoltaic response performance of PVDF-based composite materials also faces the same problem.Although PVDF-based composite materials can have a photoelectric response of tens of millivolts,such photoelectric performance is not ideal enough and needs to be further improved.At the same time,their photoelectric response mechanisms are still unclear and needs to be continuously explored.The flexoelectric effect is an electromechanical coupling effect possessed by dielectric materials.The flexoelectric coefficient is 3-8 orders of magnitude more than that theoretically predicted in inorganic ceramic materials,and a certain theoretical basis is established based on inorganic ceramic materials.The flexoelectric effect can make materials have some apparent polarization without high-field polarizations.But there are still few studies on the flexoelectric effect of polymers,and the corresponding micromechanism is not clear.To solve the problems above,we selected PVDF-based copolymer material as the experimental object,then studied the effect of dielectric barrier discharge on the dielectric properties of P(VDF-CTFE)copolymer,fabricated and characterized the P(VDF-TrFE)composites with high photoelectric performance,and the flexoelectric effect of P(VDF-TrFE)copolymer and terpolymer was tested and discussed.The content of this dissertation is mainly divided into the following chapters:The Chapter 1 briefly introduces the development history,the crystal phase and the effects of preparation conditions on the crystal phase structure of PVDF-based materials.A brief introduction of the electrical properties of PVDF-based materials is given.Finally,the structure and electrical properties of several PVDF-based materials are presented.In Chapter 2,a DBD treatment was conducted to deal with P(VDF-CTFE)copolymer material on one side.The dielectric constant,polarization and breakdown strength of materials were characterized.It was found that the breakdown strength of the material is related to the way of connecting to the high voltage source.When the treatment surface is connected to the high potential side of the power source,the breakdown strength of the material would be greatly reduced.Through the analysis of crystal and surface structure,it is pointed out that hole injection is the main reason for dielectric breakdown,and DBD treatment destroys the chemical structure of the polymer surface,resulting in a reduction in breakdown strength.In Chapter 3,we prepared two pure organic composite systems of Schiff-base/P(VDF-TrFE)and spiropyran/P(VDF-TrFE)with high photoelectric responses more than 1 V.Under the green light and near infrared light,the composite materials of the Schiff base system can obtain photovoltaic response voltages of up to 39 mV and 279 mV respectively.After polarizing,the photovoltaic response of the sample under green light and near infrared light is greatly enhanced,which can reach 352 mV and 1386 mV respectively.Then a significant photothermal effect of Schiff-base composite material was found.The photovoltaic response of the polarized sample mainly comes from the photothermal-pyroelectric conversion.The non-polarized sample may have a certain spontaneous polarization during the preparation process,and its photovoltaic response should also come from the photothermal-pyroelectric conversion caused by the spontaneous polarization.Meanwhile,the photoelectric effect of Schiff-base molecules and photothermal-thermoelectric conversion due to uneven distribution of temperature of materials may also have a certain contribution.In spiropyran system,we use an external electric field to make ring-opening spiropyran in a certain orientation,and prepare pure organic composites with different orientations.In spiropyran composites,we obtained photoelectric response voltages of up to 362 mV and 1650 mV under green and near infrared light.At the same time,it is found that the orientation of spiropyran molecules has a certain effect on the light absorption and photoelectric response of the composites.In Chapter 4,we measured the flexoelectric coefficient of P(VDF-TrFE)(70/30),P(VDF-TrFE)(55/45)and P(VDF-TrFE-CTFE)using the three-point bending mode.It is found that the dielectric constant of the polymer material is not positively related to the flexoelectric coefficient,which indicates that the microscopic mechanism of the flexoelectric effect of the polymer material is different from that of the inorganic ceramic material.In addition,we further point out that the phase transition and relaxation process of the polymer would cause its flexoelectric coefficient to decrease sharply.Meanwhile we also indicate that the flexoelectrical response of the polymer mainly comes from the crystalline phase.