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Title: Processing, structure and ferroelectric properties of PVDF-based ferroelectric polymers
Author: Meng, Nan
ISNI:       0000 0004 7652 9605
Awarding Body: Queen Mary University of London
Current Institution: Queen Mary, University of London
Date of Award: 2017
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Polyvinylidene fluoride (PVDF) and its copolymer with trifluoethylene (PVDF-TrFE) have been widely investigated. This is largely attributed to their ferroelectric properties, which are present in a limited number of polymers. In comparison with the more widely used ferroelectric ceramics, the ease of their fabrication makes them attractive in flexible electronic devices. Despite many advances in their application, we are still lacking a complete fundamental understanding of the relationship between their structure and the functional properties. The melt-extrusion of PVDF revealed that the α-phase is predominantly formed in films. The ferroelectric β-phase PVDF was obtained by high temperature drawing of the α-phase of as-extruded films. It was observed that a minimum draw ratio of 3 is required to generate the β-phase. Chain mobility is crucial to the formation of β-phase. Too high chain mobility when drawing at temperatures above 100 °C can only orientate the pre-existing α-crystals without making the chain conformation change to form the β-crystals. Furthermore, the comparison between the produced α- and β-PVDF films is summarized. The α-PVDF films crystallized into spherulites with random orientation, while β-PVDF films displayed fibriliar structure showing preferred orientation of the polymer chains along the drawing direction. The overall crystallinity obtained from DSC data hardly varied, however, the drawn β-PVDF films had a lower melting temperature, which was also confirmed from the dielectric temperature spectra. The drawn β-PVDF films showed higher dielectric constant and larger remnant polarization compared with the as-extruded α-PVDF films, which is mainly ascribed to their higher β-phase content and preferred orientation. Highly aligned PVDF-TrFE films were processed using a melt extrusion processing route. Crystalline structure and orientation were optimized by controlling the melt extrusion conditions. XRD patterns suggested that there was nearly perfect alignment of the c-axis (polymer chain direction) along the extrusion direction in the optimized as-extruded films. SEM analysis confirmed the morphology of the crystalline phase, showing edge-on lamellae stacked perpendicular to the extrusion direction. DSC data indicated high crystallinity and well-ordered ferroelectric structure of the extruded films. FTIR spectroscopy revealed strong intermolecular dipole-dipole interaction in the extruded films. Accordingly, the optimized as-extruded PVDF-TrFE films exhibited a coercive field of 24 kV/mm, half of the commonly reported values for bulk films (~ 50 kV/mm) and a remnant polarization of 0.078 C/m2 which further increased to 0.099 C/m2 after annealing. This value is close to the theoretical limit (0.102 C/m2) assuming perfect in-plane c-axis orientation and 100% crystallinity. The typical limitations of PVDF - low crystallinity and indirect ferroelectric β-phase crystallization - and PVDF-TrFE - higher materials and processing costs and a low Curie point - are tackled by a simple and industrially viable melt blending approach. Despite the immiscible nature of PVDF and PVDF-TrFE, strong interactions exist between the two polymers when co-melt processed, which substantially affect the morphology and texture of the blends as well as their dielectric and ferroelectric properties. Surprisingly, minor amounts of PVDF-TrFE led to a significant increase in the β-phase content and preferred orientation of PVDF, well beyond the rule-of-mixtures. Moreover, the blends exhibited maximum increases in the dielectric constant of 80% and 30%, respectively compared with pure PVDF and PVDF-TrFE. The ferroelectric remnant polarization increased from 0.040 to 0.077 C/m2, while the coercive field decreased from 75 to 32 kV/mm with increasing PVDF-TrFE from 0 to 40 wt. %. The enhancement of properties is explained by the strong interactions at the interfaces between PVDF and PVDF-TrFE, which also suppresses the Curie transition of PVDF-TrFE, providing a potentially increased working temperature range for blended films, which is important in applications like non-volatile energy storage devices, ferroelectric field-effect transistors and touch sensors. Ferroelectric composites, integrating dielectric ceramic fillers with mechanically flexible polymers, are promising materials for flexible electronic applications. Numerous research works have demonstrated enhanced dielectric and ferroelectric properties of composite materials. However, the mechanisms responsible for these enhancements are not completely understood. Herein, PVDF and BaTiO3 (BTO) were used to study the effect of dielectric filler on the crystallization, phase transformation and dielectric properties of PVDF. The crystallization of α-PVDF was not affected by the presence of BTO particles, but small amounts of BTO (< 3 vol. %) made PVDF crystallize into larger spherulites. This is linked to crystallization kinetic studies, which showed that BTO acted as a nucleation agent for large full ring banded spherulites when its content was less than 1 vol. %. Furthermore, solid state drawing in the presence of BTO particles promoted the formation of β-PVDF with more pronounced crystalline orientation at high drawing temperatures (120 °C). The dielectric and ferroelectric properties were enhanced with BTO filling. The 100 °C oriented drawn PVDF tape exhibited a dielectric permittivity of 14 (100 Hz) and remnant polarization of 0.080 C/m2 (10 Hz), which increased to 20 and 0.095 C/m2, respectively, after filling with 5 vol. % BTO; neither resulting in high dielectric loss tangent (~ 0.02) nor obvious current leakage. Moreover, the coercive field decreased from 80 to 50 kV/mm with increasing BTO content from 0 to 5 vol. %.
Supervisor: Not available Sponsor: Chinese Scholarship Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Engineering and Materials Science ; Polymers ; Polyvinylidene fluoride