Preparation, Characterization And Analysis On Atomic Oxygen Resistance Of POSS/PVDF Composites

The piezoelectric composites possess a number of superior properties compared with piezoelectric ceramics such as simple manufacturing process, low density, large elongation and good plasticity etc, which are being implemented in aerospace, automotive, ship industry and communication due to these advantages. PVDF as a frequently used piezoelectric material has been used widely in industry. The research work on PVDF is gradually progressed to mature with the development of technology. In recent years, PVDF as a high performance piezoelectric material has been used as transducers and components of control system in space equipments. But it is also found that the PVDF piezoelectric composites suffer functional failure due to the radiation environment in space, especially the atomic oxygen erosion. The radiation resistance of PVDF and its overall properties can be improved by incorporating the nano-scale polyhedral oligomeric silsesquioxane (POSS) into PVDF matrix. But the research on the physical, electrical properties and radiation resistance of POSS reinforced PVDF composites is still very limited so far.In this thesis, a solvent evaporation method was employed to fabricate the POSS/PVDF composite. The morphology, crystalline phase as well as thermal, mechanical, electrical properties of the composite were tested and analyzed. And the atomic oxygen erosion process was simulated using Monte Carlo method. The positive effect of POSS in PVDF matrix was researched and a number of reinforcing schemes were obtained, which can also be beneficial to the study of other POSS reinforced composites.First of all, Fluorinated-POSS was chosen among a series of POSS as the optimal candidate to reinforce the PVDF matrix. Different amounts of FP-POSS have successfully incorporated into the PVDF matrix by the solvent evaporation method. Morphological and crystalline analyses showed that FP-POSS was strongly compatible with the PVDF matrix and the size of Molecular group was altered by the addition of POSS. The composite obtained through low-temperature solvent evaporation possesses high percentage of β-phase that is ideal for piezoelectric films, and POSS tended not to agglomerate and crystallize when dispersed in the PVDF matrix.Secondly, the thermal properties of composite was analyzed by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) and the influence of POSS on thermal parameters of the composite was researched. A set of mechanical tests was carried out on the POSS/PVDF composite, including static and nano-mechanical testing and hardness using universal mechanical testing machine, hardness testing instrument and Nanotensile and Nanoindentation Testing. It was proven that with the introduction of a small amount of POSS, the overall strength and stiffness of the matrix can be improved remarkably. Specifically, the reinforcing mechanism of POSS to PVDF was discussed based on a variety of experiments. Along with several conclusions about conventional properties, the reinforcing effect of POSS was synthesized from different aspects such as the unique structure of POSS, ingredient, and other function agents.Besides, high voltage electric field was applied to the POSS/PVDF composite, and reorientation of the dipoles along the field direction was occurred and piezoelectric nanocomposites were derived. The morphology of the poled composites showed a smooth region influenced by the polarization and α-phase peak nearly disappeared. The electrical properties i.e. dielectric, piezoelectric and ferroelectric properties of POSS/PVDF composite were tested. The influence of POSS on the electrical properties of PVDF was researched. It was found that the composite exhibited typical ferroelectric features, the dielectric constant and loss of the composite was reduced with the addition of POSS, and the piezoelectric property was not obviously changed.Finally, the atomic oxygen erosion of POSS/PVDF composites was simulated using Monte Carlo method. The parameters used in simulation are based on long duration exposure facility by NASA, assumptions and empirical equations that has been verified during research of atomic oxygen(AO) erosion to other composites. The program is compiled by Matlab to simulate the interaction of protected PVDF with atomic oxygen. The PVDF was inherently prone to be eroded by atomic oxygen, but with the incorporation of POSS, the atomic oxygen resistance of PVDF was effectively increased. The role of POSS in improving the atomic oxygen resistance was demonstrated. A model of POSS/PVDF composite underwent1-5year of atomic oxygen erosion was constructed. The relationship between the initial defect size and the erosion effect was derived, which indicates that a very small initial defect can result in severe defect to the material, reducing the life of the material. With the reinforcement of POSS, the effects of AO influence and initial defect size on the substrate were analyzed.