Defense Date

2005

Document Type

Thesis

Degree Name

Master of Science

Department

Mechanical Engineering

First Advisor

Dr. Zoubeida Ounaies

Second Advisor

Dr. Karla Mossi

Abstract

PVDF, Poly(vinylidene fluoride), is a polymer that has been studied for over four decades due to its good electromechanical properties, stability, and durability in various environments. Currently, PVDF is the only commercially available piezoelectric polymer. PVDF is a polymorph, which indicates the presence of several crystalline phases such as α, β, γ, and δ-phase. Oriented β-phase PVDF exhibits ferroelectric properties and displays the largest piezoelectricity amongst the four phases, which makes it the most desirable phase. Preparing oriented β-phase PVDF is a multi-step process, which is cost intensive, due to the time, labor and energy utilized. The main goal of this work is to prepare oriented β-phase PVDF using the electrospraying technique in a one step process. During the electrospraying process a polymer jet is ejected. This jet disintegrates into droplets due to overwhelming surface tension, resulting in a sprayed coating on the collector substrate. Because of the combination of jet ejection and the high voltage applied between the needle tip and the substrate, the droplets can be stretched and the polymer chains can be oriented. Both the stretching and the high electric field are required for the transformation of α-phase to the oriented β-phase. This study proposes that by using the electrospraying technique it is possible to transform the α-phase to the β-phase in a one step process starting from solution. This research focuses on the processing and characterization of electrosprayed PVDF as well as electrosprayed PVDF/carbon nanotubes (PVDF/CNT) nanocomposites. The specific tasks are to determine the changes to the PVDF phases due to the electrospraying technique, and to determine the changes in the PVDF morphology due to the addition of carbon nanotubes to the polymer matrix.PVDF with two different molecular weights were electrosprayed using different solvents and parameters. Initial observations after electrospraying were that, high boiling point solvents resulted in the spraying of the solution and forming films, whereas a low boiling point volatile solvent such as acetone resulted in the spinning of the solution thus forming non-woven fiber mats. The thermal and electrical properties of the electrosprayed PVDF and PVDF/CNT composites are measured using several characterization techniques, including Modulated Differential Scanning Calorimetry (MDSC), Dielectric spectroscopy, Thermally Stimulated Current (TSC), Fourier Transform Infrared Spectroscopy (FT-IR), and X-Ray Diffraction (XRD). MDSC results show that electrosprayed PVDF has a lower melting point temperature than that of PVDF commercially available pellets. In addition, electrosprayed PVDF/CNT nanocomposites show a linear increase in the percentage of crystallinity with the increase of CNT concentration in the composite. Dielectric spectroscopy results indicate that by increasing the CNT concentration in the composite, the dielectric constant and the polymer conductivity increase.From the four characterizing techniques used, two of them, FT-IR and XRD, show that it is possible to transform α-phase to β-phase PVDF in a one-step process using electrospraying. The other two techniques, TSC and dielectric spectroscopy, show α-phase for the electrosprayed samples without CNT, and some β-phase formation with samples electrosprayed with CNT. These last two techniques; TSC and dielectric spectroscopy have results that differ from the FT-IR and XRD techniques. This contradiction may be a result of the small amounts of β-phase in the sample, which cannot be detected using these techniques. Another reason may be due to the difference in the probing levels between these techniques. XRD and FT-IR probe at the molecular level, whereas TSC and dielectric probe at a much larger scale, which may make it hard to detect small amounts of β-phase.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

June 2008

Included in

Engineering Commons

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