Effect of Trough and Dipping Parameters on Langmuir and Langmuir-Blodgett Thin Films of PVDF using MEK Solvent

Abstract

Poly vinylidene fluoride (PVDF) in the form of thin films finds multiple applications such as actuators for sound reduction, pressure sensors, piezo-actuators and reusable PVDF sensors to detect the structural faults due to high thermal stability, ultra sensitivity, high deformability and chemical resistance. The β-phase of PVDF homoploymer is the most promising phase of this material which can meet the hot spot for applications in piezoelectricity, pyroelectricity and ferroelectricity. Langmuir-Blodgett (LB) technique is useful for the formation of a highly organized assembly of amphiphilic molecules which can be transferred onto a solid substrate. The work presented in this thesis is aimed at synthesizing homopolymer PVDF LB films to expand their applications. PVDF swells up when dissolved in Methyl ethyl ketone (MEK) which is taken as the spreading solvent in the present work. To obtain good transfer characteristics the PVDF monolayers on the water surface were characterized for different compression rates and subphase temperature. The isotherm, hysteresis and barrier oscillation experiments show that the most stable PVDF films were obtained at 20 ˚C for 10 mm/min compression rate. These trough parameters were used for the deposition of the LB films. An important result to emerge from these experiments was that even though the MMA of the solid phase does not change significantly with the temperature range under consideration dissolution of the deposited film, the enhanced kinetic energy drastically reduces the elasticity of the solid phase at higher temperatures. The dipping experiments show that very good TR can be obtained for the first upstroke for 2 mm/min substrate speed. But down stroke results in partial desorption and reduced TR in subsequent upstrokes. This indicates that PVDF LB films are Z-Type in nature. But the Z-Type nature gets disrupted due to the partial dissolution of PVDF into water during downstrokes. To avoid this the subphase was changed after every single layer deposition. Transparent films of PVDF were synthesized by deposition of 1, 4 and 8 layers on the hydrophilic substrate followed by drying and annealing. The topographic analysis of the final films shows the emergence of few large crystallites which makes the films rough.