Processing mediated polymorphism in PVDF-based dielectric polymers and their application

Abstract

The ever-increasing demand in electronic devices, piezoelectric materials such as ceramics, polymers and polymer ceramic composites with enhanced piezoelectric and dielectric properties has become the focus of many research areas. Among the piezoelectric materials, polymers have advantages of flexibility, light weight and easy of processing as compared to ceramics. Among various polymers, PVDF is of special interest due to potential applications as sensors, capacitors and actuators etc. The piezoelectric properties of PVDF depend on the extent and orientation of different crystalline phases particularly, the electro-active - and - phases. However, obtaining these polar phases is not straight forward because they are thermodynamically unstable. The mechanistic origin of the piezoelectric properties, orientation and the extent of the piezoelectric polar phases can be tuned by various processing techniques. This thesis entitled “Processing mediated polymorphism in PVDF-based dielectric polymers and their application” systematically studies the effect of various processing methods on the crystalline structure, crystalline morphology, crystallization kinetics, mechanical properties, dielectric and piezoelectric properties in PVDF and PVDF based dielectric polymers. The thesis consists of 8 chapters. Chapter 1 introduces effect of various processing parameters like mechanical rolling, stretching, poling, solvent casting, electrospinning or a combination of these on the evolution of - PVDF. Further, this Chapter highlights the effect of blending, addition of nanoparticles like graphene oxide and CNT on the phase transformation of PVDF and relates with the dielectric and piezoelectric properties. In Chapter 2, the basic experimental techniques and principles involved in the processing and characterization of materials are discussed. In Chapter 3, the effect of shear rate and shearing temperature on the different polymorphs and crystalline morphology in PVDF has been investigated systematically by polarized optical microscopy coupled to a hot stage, Fourier transform infrared spectroscopy, differential thermal analysis, melt-rheology and dielectric relaxation spectroscopy. The piezoelectric properties of PVDF depend on the extent and orientation of - phase. Hence, the shear rate and shearing temperature are optimized to obtain maximum - phase in PVDF. The maximum amount of - phase was obtained for samples that were sheared at high temperature (220 °C) as compared to samples which were sheared at lower temperature (155 °C). It is further observed that the rate of crystallization increases after shear. Chapter 4 deals with the effect of shear, shear history, and addition of PMMA in inducing the β- polymorph and crystalline morphology in PVDF. The rheological measurements revealed that the induction time was significantly lower for blends with a higher PVDF content (≥80 wt %). The crystalline morphology observed from POM demonstrates that the growth rate of spherulites was greatly reduced with increasing PMMA content in the blends. FTIR results were used to determine the amount of β- phase in the blends before and after the shear history. The blends that were sheared at high temperature (220 °C) showed more β- phase than the blends that were sheared around the crystallization temperature. Chapter 5 delineates the effects of the amorphous content and various processing conditions (rolling, poling and rolling followed by poling) on the phase transformation, crystallographic texture, mechanical properties and the dielectric response of PVDF/PMMA blends. The highest β- phase content (>95%) was obtained for blends that were rich in the crystalline phase which were deformed up to a strain of 80%. Further, after rolling an increase in the degree of crystallinity and storage modulus was observed. The dielectric response revealed that the samples that were initially rolled followed by poled showed maximum dielectric constant and low dielectric loss as compared to the rolled or compression molded samples. Chapter 6 explains piezoelectric response in electrospun poly(vinylidene fluoride) fibers containing fluoro-doped graphene derivatives. The piezoelectric coefficient and β- phase fraction was enhanced upon the addition of GO, however, enhanced significantly in the case of GOF which was observed from PFM and FTIR results. The drastic enhancement in β- phase is due to the presence of highly electronegative fluorine. PVDF is one of the most inert fluoropolymers and difficult to functionalize using conventional chemical methods. To overcome some of its disadvantages in applications, several modification methods have been reported to incorporate desirable functionalities. Chapter 7 describes the effect of ozone treated and grafted copolymers on the crystalline structure or β- phase, dielectric behavior and piezoelectric coefficient in PVDF. The microstructures and phase transformations in PVDF were characterized by XPS, XRD, FTIR and DSC. The resulting hydroxyl modified PVDF (PVDF-OH) characterized by various techniques such as FTIR and XPS all confirmed hydroxylation of PVDF. By this approach, the amount of polar β- phase increases from 38 % (powder PVDF) to 96 % after grafting with PBSA. This phase transformation after ozonization and grafting was accompanied by an increase in dielectric permittivity and piezoelectric coefficient (d33). Chapter 8 sums up the significant findings from each chapter and highlights the outcome of the work.