Reports: DNI751049-DNI7: Curvature-Directed Crystallization of Polymer Dielectrics

Jodie Lutkenhaus, Texas A&M University

Capacitors based on polymer dielectrics are leading energy storage technologies because of their high reversibility and cyclability, fast response time, self-healing properties, and high power density. However, capacitor energy density – which is limited by dielectric breakdown strength (DBS) and loss - is generally low. Our scientific objective is to enhance and control the dielectric properties of popularly used polymer dielectrics (isotactic poly(propylene), poly(vinylidene fluroide), poly(ethylene terepthalate)) via curvature-directed crystallization, which is the crystallization of a polymer confined within a cylindrical nanoscale pore. Our rationale is that curvature-directed crystallization may produce oriented, homogeneous crystallites of improved dielectric properties (lower loss, higher breakdown strength) relative to bulk. Although confinement effects within polymer thin planar films are well-studied, less is known about the behavior of polymer dielectrics in confined cylindrical geometries.

The formation of isotactic poly(propylene) (iPP) nanotubes and nanowires of tunable diameter was demonstrated by melt-wetting the polymer into nanoporous anodic alumina, where pore diameter ranged from 15 to 200 nm. iPP was chosen as our first candidate material of study because it is widely used in commercial dielectric capacitors. Nanoporous anodic alumina was chosen as the porous template because it can be prepared in the lab with great precision. The pores are monodisperse in diameter, hexagonally packed, and do not intersect.

iPP powder was hot pressed into sheets at above iPP's melting temperature with a load of 4 metric tons. Anodic aluminum oxide (AAO) templates were formed using a two-step electrochemical oxidation method of aluminum foil. The AAO template was placed directly on top of the iPP sheet then sandwiched between two glass slides and fastened with binder clips. iPP was allowed to infiltrate the AAO by placing the samples under vacuum at 200oC for 20-24 hours. The samples were then immediately quenched on a steel plate to preserve the crystalline structure. The excess polymer was mechanically removed with sandpaper. When needed, the top layer of alumina was etched away using a 5 wt% sodium hydroxide solution. Also, when needed, the intermediate layer of aluminum was removed using a solution of copper (II) chloride and hydrochloric acid.

The crystallization process was analyzed using differential scanning calorimetry. Hoffman-Weeks analysis was used to identify the equilibrium melting temperature, Tmo, as a function of pore diameter. The Gibbs-Thomson equation predicts that Tmo will decrease from bulk values with 1/d. Our findings confirmed a linear relationship between Tm,bulko - Tmo and 1/d. A transition from hetero- to homogeneous crystallization was also observed as the pore diameter decreased. The crystallization kinetics were examined using Avrami analysis, and it was discovered that the Avrami exponent also decreased as the pore diameter decreased, suggesting a shift from 3D crystallization to 1D crystallization. This observation was supported by X-ray diffraction studies, which showed that polymer chains preferentially oriented along the a-axis, perpendicular to the pore wall. It was shown that iPP crystallizes into the α-phase. These results show that curvature does indeed influence the direction of crystallization, as well as their kinetics. At present, we are in the process of conducting broadband dielectric spectroscopy measurements to relate changes in dielectric properties with confinement-induced crystallization. We hypothesize that the dielectric properties will substantially change as pore diameter decreases as a consequence of the different crystal structure.

In the second half of this project, we have moved towards investigating another well-studied dielectric polymer, polycarbonate. Studies similar to that described for isotactic polypropylene are ongoing.

This project has directly supported Ms. Dariya Reid, a doctoral candidate, and indirectly supported Ms. Bridget Ehlinger, an undergraduate researcher.