UNDERSTANDING PROCESSING INDUCED CHANGES IN THE STRUCTURAL HIERARCHY OF NYLON 11 AND ITS CLAY BLENDS, AND PVDF/PMMA BLENDS AND THEIR EFFECTS ON DIELECTRIC PROPERTIES
Polymer based dielectrics are typically known for their low cost to manufacture, and longer lifetimes compared with inorganic dielectrics. Within this class, biaxially oriented polypropylene (BOPP) is considered state of the art and takes a big portion of the market share. It is a semi-crystalline material known for its superior breakdown strength and low loss. However, having a low dielectric constant as well as low operating temperatures leaves much to be desired. Due to this, current work has been focused on understanding processing effect on polymers to develop an in-depth understanding of structure-process-property relationships. Commercially available polymers were melt-cast into capacitor grade films with thicknesses ≤ 20µm and having a thickness uniformity of 10% along its width.
In the first study, thin Nylon 11 films were prepared using film extrusion and the effect of processing on the structural and dielectric properties was studied. Capacitor grade thin films were cast using a film extruder and stretched uniaxially and real-time mechano-optical data was acquired at temperatures ranging from solid state all the way to partially molten state. These studies were supplemented with offline WAXS, SAXS and DSC to develop a complete understanding of processing induced structural changes and their relation to dielectric properties. When stretched in the solid-state, preferential crystalline and amorphous chain orientation levels increased as a function of strain while the long spacing remained constant due to lamellar slippage. Processing in the partially molten state led to the formation of new lamellae with larger long spacings. It was also observed that the dielectric properties of Nylon 11 were strongly dependent on the crystallinity and crystal phase, with higher crystallinity of the α’ phase giving higher electrical breakdown strength. When films were stretched to 3X solid and partially molten states, together with crystallinity the breakdown strength is increased, while the dielectric constant and loss decrease. Since crystalline regions are stiffer, they form effective barriers to hot carriers, retarding the electric breakdown.
In the second study, Nylon 11 was blended with Cloisitie 20A via twin screw extrusion and , the effect of organically modified clay on the orientation enhancement in Nylon 11 during melt casting was investigated. Nylon 11 was mixed with 1 and 3wt% Cloisite 20A using twin screw extrusion and they were cast into films with varying take up speeds. The addition of clay in Nylon 11 helped increase orientation levels substantially in melt cast films, both as a function of clay concentration as well as take-up speeds. This was primarily due to shear amplification effect caused by the movement of adjacent clay nanoparticles due to the shear flow gradient within the die. At low clay concentrations, the sub-Tm stretchability and electrical breakdown strength improves as the presence of clay reduces inter/intrachain hydrogen bonding. At higher clay concentrations, both orientation and electrical breakdown levels decrease. The latter is primarily caused by increased percolation path of charge carriers. Nevertheless, clay nanoplatelets were very effective in their role as melt processing aids, as they enhance orientation levels of Nylon 11 thin films by shear amplification effect where they increase local chain orientation of chains trapped between clay platelets while their orientation relaxation is suppressed.
In the third study, the effect of annealing on the structural hierarchy of PVDF/PMMA blends was investigated. PVDF was blended with PMMA in three concentrations: 50/50, 60/40 and 70/30 PVDF/PMMA using twin screw extrusion. Capacitor grade films (~20µm in thickness) were cast using a film casting line. Cast films were characterized structurally through offline birefringence measurements as well as WAXS, SAXS, IR Dichroism and DSC to develop an in-depth understanding of processing induced structural changes and their effect on overall properties. Films were also characterized dielectrically. Addition of PMMA aided the glassy quenching of the melt cast films, with the overall crystallinity decreasing with increasing PMMA content. It also opened up a rubbery region in the blend, providing an alternative processing window. Crystallinity in cast films was shown to correlate directly with breakdown strength. Systemic annealing experiments were done to enhance crystallinity and the effect of annealing induced structural changes on the dielectric properties were studied. It was found that upon annealing, the amorphous PMMA tends to undergo relaxation that is detrimental to the breakdown properties. As annealing progresses, the mismatch in the kinetics between crystallization/recrystallization and the amorphous relaxation cause density gradients within the bulk of the films and dictate the overall orientation levels in the films, as well as have implications on the dielectric properties.
This research was funded by the Office of Naval Research Grant #tN00014-17-1-2656.
- Doctor of Philosophy
- Materials Engineering
- West Lafayette